**2. The technological processes of MCT HES growth on (0130GaAs substrates**

Fig. 1 showed the scheme of MCT HES included ZnTe and CdTe buffer and adsorber layers in sequence grown on (013)GaAs substrate by MBE.

**Figure 1.** The scheme of MCT HES on (013) GaAs substrate.

The technology of growth MCT HES includes the following processes:


#### **2.1. The preepitaxial preparation of surface substrate**

The GaAs substrate surface before epitaxial growth must be atomically smooth and clean.

For this purpose the preepitaxial substrate surface preparation included the chemical etch‐ ing process and thermal cleaning process in ultra-high vacuum at 500600 0 С.

been reached in development of growth MCT HES by MBE on GaAs and Si large in diame‐ ter substrate. The decision of fundamental physical and chemical investigations and techno‐ logical developments allows to fabricate high quality MCT HES on GaAs substrate. Such MCT HES are widely used for developments and production of different formats linear and

In this chapter the results of studies of technological processes at growth MCT HES on (013)GaAs by MBE, the developments of HES design and fabrication on its basis of high quality IR detectors of different applications for IR radiation registration in spectral long

Fig. 1 showed the scheme of MCT HES included ZnTe and CdTe buffer and adsorber layers

**2. The technological processes of MCT HES growth on (0130GaAs**

matrix IR detectors sensitive in separate spectral IR ranges.

in sequence grown on (013)GaAs substrate by MBE.

**Figure 1.** The scheme of MCT HES on (013) GaAs substrate.

**•** the growth of ZnTe and CdTe buffer layers;

**•** the preepitaxial preparation of surface substrate;

**•** the growth of MCT absorber layer with special design.

**2.1. The preepitaxial preparation of surface substrate**

The technology of growth MCT HES includes the following processes:

The GaAs substrate surface before epitaxial growth must be atomically smooth and clean.

wavelength range (LWIR) 8-11 μm.

134 Photodiodes - From Fundamentals to Applications

**substrates**

We used (013)GaAs substrates 2" and 3" in diameter which initially prepared as epiready. Nevertheless it is necessary to remove the defects surface layer which prevents the epitaxial growth of high quality MCT HES. The study of chemical etching of GaAs in sulfuric acid etch‐ ant [1] allows to determine optimal conditions for preparation GaAs surface. Fig. 2 represents the density of luminous points which appeared after chemical etching of GaAs substrates [2].

**Figure 2.** The dependence of number luminous points on etching depth (013) GaAs.

It is clear that the density of luminous point changes with etching depth increases more that one order of magnitude from initial values reaches maximum at 10-15 μm. So we deter‐ mined the optimal etching depth for epitaxial growth MCT HES which is equal to ~20 μm.

It was determined that carbon contamination does not evaporate at thermal treatment in ul‐ tra-high vacuum. The presence of 0,06 monolayer carbons coating on the GaAs surface is disturb epitaxy [3-5]. It necessary to create a continence protective layer more than monolay‐ er thickness which must not adsorbs carbon and desorbs at low temperatures. It was found by SIMS that at treatment of etching GaAs surface in HCl solution in spirit lead to decrease of carbon on the surface less 0.5 % monolayer [6]. At this procedure the arsenic oxides and gallium oxides were removed at room temperature from the GaAs substrate leading to stoi‐ chiometric surface with elemental arsenic coating.

We used the procedure of final etching in boiling HCl solution in isopropyl alcohol for for‐ mation elemental arsenic coating.

So, the chemical procedure of GaAs surface preparation before the growth includes the etch‐ ing in H2SO4/ H2O2/H2O (3:1:1) mixture at 35-40 0 C during 6-10 min. and final and in boil‐ ing HCl solution in isopropyl alcohol during 10 min.

perature. The curves 3 and 4 shows the intensity distribution along diffraction strikes in [11]

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

sorption of volatile arsenic oxides [1]. The following sharp increasing of diffraction reflec‐

the formation atomic smooth GaAs surface. The further temperature increasing or exposing

**Figure 4.** The dependences ψ and Δ on temperature at thermal heating and cooling for 3 samples (001)GaAs. The

The typical changing of ellipsometric parameters ψ and Δ were measured at thermal heating

The dependence of ψ on substrate temperature showed has reversible character because of that determined by GaAs optical constants. The dependence of Δ on temperature showed non-monotone character that determined by the GaAs surface roughness. The increasing of

(001)GaAs surface. Further changing Δ was determined by desorption arsenic oxides [1] and

С was conditioned by desorption of adsorbed gases from

С and cooling to room temperature (see Fig. 4).

tion intensity with formation strikes was observed at temperature more 540 0

observed at GaAs substrate thermal heating from 20 0

during 0.5 hour of GaAs substrate 570580 0

tion reflections connected with roughening surface.

rows show the direction of changing of ellipsometric parameters.

(001)GаAs up to 580 <sup>0</sup>

Δ at thermal heating to ~ 150 0

¯1] azimuths respectively. The sharp increasing of diffraction reflection intensity was

С to 300 0

С leads to transfer diffraction strikes to diffrac‐

С that connected with de‐

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С that means

137

and [01

**Figure 3.** The diffraction reflections intensity dependences on temperature at thermal heating (001)GaAs: curves 1 and 3 in [011] azimuth, curves 2 and 4 in [01¯1] azimuth.

After chemical etching GaAs substrates are attached to special holder and then loaded into loading-unloading vacuum chamber of ultra-high MBE set.

The procedure of thermal heating of GaAs substrate in vacuum are developed using (001) GaAs together with monitoring by high energy electron diffraction (HEED) and single wavelength automatic ellipsometer (AE) LEF-755. Usually before the thermal treatment it is seen weak diffraction patent from (100)GaAs in [11] and [01 ¯1] azimuths (weak diffraction reflection). The increasing of brightness of diffraction reflection is observed at increasing temperature GaAs substrate up to 250 300 0 С. The diffraction background is practically disappeared at temperatures more than 500 0 С with appearance diffraction strikes and su‐ perstructures 2×1 and 3×1 types.

Fig. 3 showed the typical dependences of diffraction reflection intensity of (100)GaAs sur‐ face (curves 1 and 2 for [11] и [01 ¯1] azimuths) normalized to diffraction background on tem‐ perature. The curves 3 and 4 shows the intensity distribution along diffraction strikes in [11] and [01 ¯1] azimuths respectively. The sharp increasing of diffraction reflection intensity was observed at GaAs substrate thermal heating from 20 0 С to 300 0 С that connected with de‐ sorption of volatile arsenic oxides [1]. The following sharp increasing of diffraction reflec‐ tion intensity with formation strikes was observed at temperature more 540 0 С that means the formation atomic smooth GaAs surface. The further temperature increasing or exposing during 0.5 hour of GaAs substrate 570580 0 С leads to transfer diffraction strikes to diffrac‐ tion reflections connected with roughening surface.

**Figure 4.** The dependences ψ and Δ on temperature at thermal heating and cooling for 3 samples (001)GaAs. The rows show the direction of changing of ellipsometric parameters.

**Figure 3.** The diffraction reflections intensity dependences on temperature at thermal heating (001)GaAs: curves 1

After chemical etching GaAs substrates are attached to special holder and then loaded into

The procedure of thermal heating of GaAs substrate in vacuum are developed using (001) GaAs together with monitoring by high energy electron diffraction (HEED) and single wavelength automatic ellipsometer (AE) LEF-755. Usually before the thermal treatment it is

reflection). The increasing of brightness of diffraction reflection is observed at increasing

Fig. 3 showed the typical dependences of diffraction reflection intensity of (100)GaAs sur‐

¯1] azimuths (weak diffraction

С. The diffraction background is practically

С with appearance diffraction strikes and su‐

¯1] azimuths) normalized to diffraction background on tem‐

and 3 in [011] azimuth, curves 2 and 4 in [01¯1] azimuth.

136 Photodiodes - From Fundamentals to Applications

temperature GaAs substrate up to 250 300 0

disappeared at temperatures more than 500 0

perstructures 2×1 and 3×1 types.

face (curves 1 and 2 for [11] и [01

loading-unloading vacuum chamber of ultra-high MBE set.

seen weak diffraction patent from (100)GaAs in [11] and [01

The typical changing of ellipsometric parameters ψ and Δ were measured at thermal heating (001)GаAs up to 580 <sup>0</sup> С and cooling to room temperature (see Fig. 4).

The dependence of ψ on substrate temperature showed has reversible character because of that determined by GaAs optical constants. The dependence of Δ on temperature showed non-monotone character that determined by the GaAs surface roughness. The increasing of Δ at thermal heating to ~ 150 0 С was conditioned by desorption of adsorbed gases from (001)GaAs surface. Further changing Δ was determined by desorption arsenic oxides [1] and optical constant temperature dependence. Then we observed the sharp increasing Δ con‐ nected with desorption of gallium oxides [1]. The ellipsometric parameter Δ was increased at cooling to room temperature that сonnected with the changing of GaAs optical constants. The ellipsometic measurements of (001)GaAs surface at thermal treatments are in a good agreements with REED measurements.

that determined by interference with period *d*<sup>0</sup> <sup>=</sup> *<sup>λ</sup>*

ZnTe, φ - angle of incidence of AE laser beam on GaAs surface (see Fig. 5).

optical constants because of inclusion in layer volume at ZnTe growth.

The deviation from optimal conditions leads to surface roughness or adsorption to changing

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

**Figure 6.** The calculation data of changing of ellipsometric parameters Ψ и Δ at ZnTe growth on GaAs: a – roughness changes from 0 nm up to 5 nm А; b – absorption factor k=0.05; c – roughness changes from 0 nm up to 2,5 nm, ab‐

sorption factor k=0.03.

2 *n* <sup>2</sup> −sin*ϕ*

where n – refractive index of

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139

These data given the understanding of technological process of preparation atomic smooth and clean (001)GaAs surface by thermal heat treatments. The technological process of prepa‐ ration (013)GaAs atomic smooth and clean surface is analogous ones. We observed analo‐ gous changing of REED pattern and ellipsometric parameters ψ and Δ at thermal heat treatments of (013)GaAs in vacuum.

#### **2.2. The growth of ZnTe/CdTe buffer layer**

The buffer layer on (013)GaAs was fabricated by sequence growth of ZnTe and CdTe layers. At first the 20 – 300 nm ZnTe layer was grown on atomic smooth and clean GaAs surface from sep‐ arate molecular beam Te2 and Zn. This procedure is necessary to growth of only one (013) ori‐ entation. At CdTe growth on GaAs surface there was observed the growth of mixture orientation [7] that determined with large lattice mismatch 14,6 % between CdTe и GaAs.

**Figure 5.** The changing of ellipsometric parameters Ψ and at growth of high quality ZnTe layer at optimal growth con‐ ditions. The points – experimental data. The rows show the direction of changing Ψ and Δ at increasing ZnTe thickness.

The optimization of ZnTe growth was carried out with monitoring technological processes by AE *in situ*. At growth of high quality ZnTe in optimal conditions the undamped periodic changing of ellipsometric parameters Ψ and Δ (ligth radiation AE λ=6328 Å) are observed that determined by interference with period *d*<sup>0</sup> <sup>=</sup> *<sup>λ</sup>* 2 *n* <sup>2</sup> −sin*ϕ* where n – refractive index of

ZnTe, φ - angle of incidence of AE laser beam on GaAs surface (see Fig. 5).

optical constant temperature dependence. Then we observed the sharp increasing Δ con‐ nected with desorption of gallium oxides [1]. The ellipsometric parameter Δ was increased at cooling to room temperature that сonnected with the changing of GaAs optical constants. The ellipsometic measurements of (001)GaAs surface at thermal treatments are in a good

These data given the understanding of technological process of preparation atomic smooth and clean (001)GaAs surface by thermal heat treatments. The technological process of prepa‐ ration (013)GaAs atomic smooth and clean surface is analogous ones. We observed analo‐ gous changing of REED pattern and ellipsometric parameters ψ and Δ at thermal heat

The buffer layer on (013)GaAs was fabricated by sequence growth of ZnTe and CdTe layers. At first the 20 – 300 nm ZnTe layer was grown on atomic smooth and clean GaAs surface from sep‐ arate molecular beam Te2 and Zn. This procedure is necessary to growth of only one (013) ori‐ entation. At CdTe growth on GaAs surface there was observed the growth of mixture orientation [7] that determined with large lattice mismatch 14,6 % between CdTe и GaAs.

**Figure 5.** The changing of ellipsometric parameters Ψ and at growth of high quality ZnTe layer at optimal growth con‐ ditions. The points – experimental data. The rows show the direction of changing Ψ and Δ at increasing ZnTe thickness.

The optimization of ZnTe growth was carried out with monitoring technological processes by AE *in situ*. At growth of high quality ZnTe in optimal conditions the undamped periodic changing of ellipsometric parameters Ψ and Δ (ligth radiation AE λ=6328 Å) are observed

agreements with REED measurements.

138 Photodiodes - From Fundamentals to Applications

treatments of (013)GaAs in vacuum.

**2.2. The growth of ZnTe/CdTe buffer layer**

The deviation from optimal conditions leads to surface roughness or adsorption to changing optical constants because of inclusion in layer volume at ZnTe growth.

**Figure 6.** The calculation data of changing of ellipsometric parameters Ψ и Δ at ZnTe growth on GaAs: a – roughness changes from 0 nm up to 5 nm А; b – absorption factor k=0.05; c – roughness changes from 0 nm up to 2,5 nm, ab‐ sorption factor k=0.03.

Fig. 6 shows the results of calculations of changing of ellipsometric parameters Ψ and Δ at surface roughness and changing optical constants. The ellipsometric variation of Ψ and Δ in Ψ-Δ plane is little by little move to decreasing Δ at relief evolution (see Fig.6 a). The ellipso‐ metric curve amplitude is decreased at growth of weakly adsorption layer с k=0,05 (see Fig. 6b). Fig. 6c shows the changing of Ψ и Δ at developing roughness and absorption factor (k) of growing ZnTe layer.

These data were used for determination of growth mechanism ZnTe on (013) GaAs at growth conditions. The behavior of ellipsometric parameters Ψ and Δ at 2 D and 3D growth is similar as shown in Fig. 6a and Fig. 6b respectively.

Fig. 7 represents the experimental data of the ZnTe growth on (013) GaAs at different tem‐ peratures and different molecular fluxes of zinc and tellurium (JZn/JTe2).

**Figure 8.** The variation of ellipsometric parameters Ψ и Δ at CdTe growth (001)GaAs for 3 samples: squares, triangular and crosses– experimental data; solid lines – calculation data. Figures near curves – CdTe layer thickness. In insert – the dependence of adsorption factor on thickness for case of essential differences (triangular) and good agreement

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

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141

In case of good agreement of experimental and calculated data Ψ and Δ (curves 1 and 3) al‐

The differences of experimental and calculated Ψ and Δ data at initial stage (up to 200 nm) we observed at growth at non optimal growth condition (curve 2). REED pattern shows the presence of (001) and (111) orientation. At further growth the experimental and calculated Ψ

The observed differences of experimental and calculated Ψ and Δ data свидетельствуют, that optical constants o CdTe for this case are differed from analogous data for bulk CdTe. Really the calculation (see Fig. 8 in insert triangular) shows that adsorption coefficient high‐ er at initial growth stage and reached the bulk data at thickness 200 nm. The higher values of k are explained by poor crystalline performance at initial stage of CdTe on GaAs [8]. Re‐ fractive index n = 3 does not depend on growth condition of CdTe growth. These studies

The optimal conditions for growth of high quality CdTe on (013)ZnTe/GaAs substrate were de‐ termined during investigation of technological processes at different growth temperature and molecular cadmium and tellurium fluxes(JCd/JTe2) relationships with monitoring by AE *in situ*.

lows to determine the growth rate and the thickness of growing CdTe layer.

determined necessity of ZnTe growth on GaAs before CdTe growth.

(squares) between experiment and calculation respectively.

and Δ data becomes near one another.

**Figure 7.** The phase diagram of ZnTe growth mechanism: triangular – 2D growth; points – 3 D growth; solid curve – the boundary of growth conditions between 2D and 3 D growth.

Following these data we determined the optimal conditions for growth of high quality ZnTe layer on GaAs. The temperature of substrate is situated in range 2800 С - 295<sup>0</sup> С at molecular fluxes relations JZn/JTe2 in range 3 - 8.

The studies of CdTe growth on (001) and (013)ZnTe/GaAs substrates were carried out with monitoring by AE *in situ*. Fig. 8 shows the spiral variation of experimental and calculated ellipsometric parameters Ψ and Δ in Ψ-Δ plane at CdTe growth on (001) GaAs.

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE http://dx.doi.org/10.5772/50822 141

Fig. 6 shows the results of calculations of changing of ellipsometric parameters Ψ and Δ at surface roughness and changing optical constants. The ellipsometric variation of Ψ and Δ in Ψ-Δ plane is little by little move to decreasing Δ at relief evolution (see Fig.6 a). The ellipso‐ metric curve amplitude is decreased at growth of weakly adsorption layer с k=0,05 (see Fig. 6b). Fig. 6c shows the changing of Ψ и Δ at developing roughness and absorption factor (k)

These data were used for determination of growth mechanism ZnTe on (013) GaAs at growth conditions. The behavior of ellipsometric parameters Ψ and Δ at 2 D and 3D growth

Fig. 7 represents the experimental data of the ZnTe growth on (013) GaAs at different tem‐

**2-d growth**

**240 260 280 300 320 340 Substrate temperature 0C,** 

**Figure 7.** The phase diagram of ZnTe growth mechanism: triangular – 2D growth; points – 3 D growth; solid curve –

Following these data we determined the optimal conditions for growth of high quality ZnTe

The studies of CdTe growth on (001) and (013)ZnTe/GaAs substrates were carried out with monitoring by AE *in situ*. Fig. 8 shows the spiral variation of experimental and calculated

С - 295<sup>0</sup>

С at molecular

of growing ZnTe layer.

140 Photodiodes - From Fundamentals to Applications

is similar as shown in Fig. 6a and Fig. 6b respectively.

**1**

fluxes relations JZn/JTe2 in range 3 - 8.

the boundary of growth conditions between 2D and 3 D growth.

**1,5**

**2**

**2,5**

**J Zn/Te2**

**, arb.un.**

**3**

**3,5**

**4**

**4,5**

**5**

**5,5**

peratures and different molecular fluxes of zinc and tellurium (JZn/JTe2).

**3-d growth**

layer on GaAs. The temperature of substrate is situated in range 2800

ellipsometric parameters Ψ and Δ in Ψ-Δ plane at CdTe growth on (001) GaAs.

**Figure 8.** The variation of ellipsometric parameters Ψ и Δ at CdTe growth (001)GaAs for 3 samples: squares, triangular and crosses– experimental data; solid lines – calculation data. Figures near curves – CdTe layer thickness. In insert – the dependence of adsorption factor on thickness for case of essential differences (triangular) and good agreement (squares) between experiment and calculation respectively.

In case of good agreement of experimental and calculated data Ψ and Δ (curves 1 and 3) al‐ lows to determine the growth rate and the thickness of growing CdTe layer.

The differences of experimental and calculated Ψ and Δ data at initial stage (up to 200 nm) we observed at growth at non optimal growth condition (curve 2). REED pattern shows the presence of (001) and (111) orientation. At further growth the experimental and calculated Ψ and Δ data becomes near one another.

The observed differences of experimental and calculated Ψ and Δ data свидетельствуют, that optical constants o CdTe for this case are differed from analogous data for bulk CdTe. Really the calculation (see Fig. 8 in insert triangular) shows that adsorption coefficient high‐ er at initial growth stage and reached the bulk data at thickness 200 nm. The higher values of k are explained by poor crystalline performance at initial stage of CdTe on GaAs [8]. Re‐ fractive index n = 3 does not depend on growth condition of CdTe growth. These studies determined necessity of ZnTe growth on GaAs before CdTe growth.

The optimal conditions for growth of high quality CdTe on (013)ZnTe/GaAs substrate were de‐ termined during investigation of technological processes at different growth temperature and molecular cadmium and tellurium fluxes(JCd/JTe2) relationships with monitoring by AE *in situ*.

not changed at stationary stage of CdTe growth at JCd/JTe2 = 3,5. A large excess of cadmium at JCd/JTe2 = 27 it was observed weak decreasing of ellipsometric parameterΔ. In condition of stoichoimetric relationship JCd/JTe2=1 it was observed sharp decreasing of ellipsometric pa‐

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

It well-known that a large dislocation density in CdTe/ZnTe interface [9] was formed and decreased with increasing of CdTe thickness [10]. The growth of CdTe layers with different thickness were carried out at optimal condition. Fig. 10 shows the dependence of full width

It is clear that FWHM sharply decreased at increasing of CdTe thickness reaching practically

So we determined the optimal conditions for growth of high quality CdTe layer on

The growth of MCT layer was carried out on (013)CdTe/ZnTe/GaAs substrates from separate molecular sources of elemental Cd, Te и Hg. The original construction of molecular sources ant their unique location in vacuum chamber allows to grow MCT layer with high uniformity over

**Figure 11.** The MCT composition uniformity over the surface area of 3" in diameter GaAs substrate measured by

transmission spectra. The mean value <XCdTe>=0.2164 and standard deviation δ<XCdTe>=0,0036.

С - 295<sup>0</sup>

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143

С at molecu‐

rameter Δ that determined by sharp surface roughening.

stationary values less 3 angle minutes at 6 μm.

**2.3. The growth of MCT layers**

of half maximum (FWHM) of rocking curves on CdTe layer thickness.

(013)ZnTe/GaAs. The temperature of substrate is situated in range 2800

lar fluxes relations JCd/JTe2 in range 5 - 7. The thickness of CdTe layer is 5 – 7 μm.

the surface area of 3" in diameter GaAs substrate without rotation (see Fig. 11).

The AFM measurements showed that grown CdTe surface roughness less than 10 нм.

**Figure 9.** The changing of ellipsometric parameter Δ at CdTe growth.

**Figure 10.** The dependence of FWHM on CdTe thickness.

Fig. 9 represents the changing of ellipsometric parameter Δ at CdTe growth at temperature 2900 С and constant molecular tellurium flux. The ellipsometric parameter Δ practically does not changed at stationary stage of CdTe growth at JCd/JTe2 = 3,5. A large excess of cadmium at JCd/JTe2 = 27 it was observed weak decreasing of ellipsometric parameterΔ. In condition of stoichoimetric relationship JCd/JTe2=1 it was observed sharp decreasing of ellipsometric pa‐ rameter Δ that determined by sharp surface roughening.

It well-known that a large dislocation density in CdTe/ZnTe interface [9] was formed and decreased with increasing of CdTe thickness [10]. The growth of CdTe layers with different thickness were carried out at optimal condition. Fig. 10 shows the dependence of full width of half maximum (FWHM) of rocking curves on CdTe layer thickness.

It is clear that FWHM sharply decreased at increasing of CdTe thickness reaching practically stationary values less 3 angle minutes at 6 μm.

So we determined the optimal conditions for growth of high quality CdTe layer on (013)ZnTe/GaAs. The temperature of substrate is situated in range 2800 С - 295<sup>0</sup> С at molecu‐ lar fluxes relations JCd/JTe2 in range 5 - 7. The thickness of CdTe layer is 5 – 7 μm.

The AFM measurements showed that grown CdTe surface roughness less than 10 нм.

#### **2.3. The growth of MCT layers**

**Figure 9.** The changing of ellipsometric parameter Δ at CdTe growth.

142 Photodiodes - From Fundamentals to Applications

**Figure 10.** The dependence of FWHM on CdTe thickness.

2900

Fig. 9 represents the changing of ellipsometric parameter Δ at CdTe growth at temperature

С and constant molecular tellurium flux. The ellipsometric parameter Δ practically does

The growth of MCT layer was carried out on (013)CdTe/ZnTe/GaAs substrates from separate molecular sources of elemental Cd, Te и Hg. The original construction of molecular sources ant their unique location in vacuum chamber allows to grow MCT layer with high uniformity over the surface area of 3" in diameter GaAs substrate without rotation (see Fig. 11).

**Figure 11.** The MCT composition uniformity over the surface area of 3" in diameter GaAs substrate measured by transmission spectra. The mean value <XCdTe>=0.2164 and standard deviation δ<XCdTe>=0,0036.

It is clear that MCT composition mean value <XCdTe>=0.2164 and standard deviation δ<XCdTe>=0,0036. So during the growth of MCT layer we used for monitoring technological process AE *in situ*.

This dependence allows to determine the growth rate and MCT composition. These data al‐ lows in further to provide monitoring of MCT thickness and MCT composition and its changing at stationary growth stage *in situ*. Note that (013) substrate orientation is appropri‐ ate for MCT layer growth of different composition without essential changing technological conditions. This circumstance allows to grow MCT HES with special design to improve the

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

Fig. 13 shows the MCT HES in which there is graded wide gap layer on the boundaries of absorber layer. Widegap layers created built-in electric fields in which non-equilibrium car‐ rier's drive back into the volume from surfaces with high recombination velocities and acted as passivating coatings. Wide gap layers lead to the essential increase of minority lifetime. In insert it is shown the variation of growth temperature measured by polarized pyrometer. It

variation of MCT composition less than 0,001 mole fraction of CdTe. This data were sup‐ ported by measurement of transmission spectra with layer-by-layer chemical etching which

**Figure 13.** The distribution of MCT composition throughout the thickness measured by AE *in situ*. Open circles – MCT composition measured by transmission spectra at layer-by-layer chemical etching. In insert – variation of growth tem‐

We suggested the novel MCT HES allows to decide the problem of high sequence resistance

The construction of absorber layer for p-type MCT HES used for matrix photovoltaic (PV) FPA includes additional MCT layer with high conductivity. The additional MCT layer with

for matrix focal plane arrays (FPA) based on p-type absorber layer.

perature during MCT growth.

C that gives the

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145

parameters of detectors and to simplify their technological process of fabrication.

is seen that the variation of growth temperature lies in interval 186 -188 0

in a good agreement with ellipsometric measurement of MCT composition *in situ*.

It is necessary to remark that the MCT growth conditions differ from molecular regime. There is exist high mercury pressure 10-3 - 10-4 torr at growth MCT layer because low stick‐ ing coefficient. The tellurium adsorbed on the growth surface as diatomic molecules with formation solid phase MCT through reaction with cadmium and mercury. There is the pos‐ sibility to formation solid phase of tellurium at growth temperatures 185-190 0 С for case of mercury deficit for some reasons. The thermodynamic analysis reveals the possibility of ex‐ isting two solid phases – MCT and tellurium. Consequently, the processes on the growing surface determined by crystallization tellurium with formation of high quality MCT layer or solid phase of tellurium which leads to defect structure [11,12]. So we determined the opti‐ mal growth condition with the purpose to decrease appearance defects as minimal as possi‐ ble. In opposite case there is possibility of irreversible decreasing of surface roughness and crystalline perfection of epitaxial structure.

At initial stage of MCT growth the changing of ellipsometric parameters Ψ and Δ in Ψ-Δ plane represents by convergent spiral curve (Fig. 12).

**Figure 12.** The evolution of ellipsometric parameters Ψ and Δ in Ψ-Δ plane at initial stage of growth: triangular – ex‐ perimental data; dotted line –calculated data. The цифры – MCT thickness.

This dependence allows to determine the growth rate and MCT composition. These data al‐ lows in further to provide monitoring of MCT thickness and MCT composition and its changing at stationary growth stage *in situ*. Note that (013) substrate orientation is appropri‐ ate for MCT layer growth of different composition without essential changing technological conditions. This circumstance allows to grow MCT HES with special design to improve the parameters of detectors and to simplify their technological process of fabrication.

It is clear that MCT composition mean value <XCdTe>=0.2164 and standard deviation δ<XCdTe>=0,0036. So during the growth of MCT layer we used for monitoring technological

It is necessary to remark that the MCT growth conditions differ from molecular regime. There is exist high mercury pressure 10-3 - 10-4 torr at growth MCT layer because low stick‐ ing coefficient. The tellurium adsorbed on the growth surface as diatomic molecules with formation solid phase MCT through reaction with cadmium and mercury. There is the pos‐

mercury deficit for some reasons. The thermodynamic analysis reveals the possibility of ex‐ isting two solid phases – MCT and tellurium. Consequently, the processes on the growing surface determined by crystallization tellurium with formation of high quality MCT layer or solid phase of tellurium which leads to defect structure [11,12]. So we determined the opti‐ mal growth condition with the purpose to decrease appearance defects as minimal as possi‐ ble. In opposite case there is possibility of irreversible decreasing of surface roughness and

At initial stage of MCT growth the changing of ellipsometric parameters Ψ and Δ in Ψ-Δ

**Figure 12.** The evolution of ellipsometric parameters Ψ and Δ in Ψ-Δ plane at initial stage of growth: triangular – ex‐

perimental data; dotted line –calculated data. The цифры – MCT thickness.

С for case of

sibility to formation solid phase of tellurium at growth temperatures 185-190 0

process AE *in situ*.

144 Photodiodes - From Fundamentals to Applications

crystalline perfection of epitaxial structure.

plane represents by convergent spiral curve (Fig. 12).

Fig. 13 shows the MCT HES in which there is graded wide gap layer on the boundaries of absorber layer. Widegap layers created built-in electric fields in which non-equilibrium car‐ rier's drive back into the volume from surfaces with high recombination velocities and acted as passivating coatings. Wide gap layers lead to the essential increase of minority lifetime. In insert it is shown the variation of growth temperature measured by polarized pyrometer. It is seen that the variation of growth temperature lies in interval 186 -188 0 C that gives the variation of MCT composition less than 0,001 mole fraction of CdTe. This data were sup‐ ported by measurement of transmission spectra with layer-by-layer chemical etching which in a good agreement with ellipsometric measurement of MCT composition *in situ*.

**Figure 13.** The distribution of MCT composition throughout the thickness measured by AE *in situ*. Open circles – MCT composition measured by transmission spectra at layer-by-layer chemical etching. In insert – variation of growth tem‐ perature during MCT growth.

We suggested the novel MCT HES allows to decide the problem of high sequence resistance for matrix focal plane arrays (FPA) based on p-type absorber layer.

The construction of absorber layer for p-type MCT HES used for matrix photovoltaic (PV) FPA includes additional MCT layer with high conductivity. The additional MCT layer with high conductivity must be fabricate by intentional doping of more wide gap layer or grow‐ ing more narrow layer than absorber ones during the growth MCT HES.

The MCT composition in this layer is slightly more than for absorber layer. These high con‐ ductivity layer decreases of sequence resistance and serves as short wavelength cut off filter for cooling PV FPA. It was determined by photoconductive measurement the minimal height of barrier layer 0,05 mole fraction CdTe. Fig. 15 illustrates a novel MCT HES MBE with variation of MCT composition on boundaries of absorber layer. In Fig. 15 the MCT composition profile with narrowgap layers at the interface and widegap layer at the surface is shown. There is a barrier layer as wide gap layer between narrowgap and absorber layers.

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

Electrical characteristics were determined from Hall measurements at 77 K by Van der Paw method. As grown MCT HES MBE with construction represented in Fig. 12 had ntype conductivity. The electron concentration and mobility and minority carrier lifetime

spectively. For conversion of as-grown n-type MCT HS's MBE to p-type annealing in heli‐

The graded widegap layer at the boundaries of MCT absorber layer allows to improve IR

We studied the influence of widegap layers at the boundaries of absorber layer on minority lifetime by numerical calculation and supported this effect by experimental investigations.

The distribution of non-equilibrium carrier was determined by decision of one dimension‐ al diffusion equation taking into account the generation-recombination processes and built-in electric in graded widegap layer. The dependence of hole current *j <sup>p</sup>* on the thick‐ ness position *y* in homogeneous MCT *n*-type in approximation of low generation ex‐

(*y*)−*μp*(*y*)*kBT*

*d*

*dy <sup>p</sup>*(*y*) (1)

– intrinsic carrier concentration, *T* -

annealing hole concentration and mobility were (5 - 20)×1015 cm-3 and 400 - 700 cm2

**3.1. The graded widegap layer at the boundaries of MCT absorber layer**

detectors and simplify the technology of their development and fabrication.

*d dy* ln*ni* 2

/V sec and 2-10 μs for composition ХCdTe~0,20-0,22 re‐

С and low mercury vapor pressure. After

http://dx.doi.org/10.5772/50822

147

/V s

Narrowgap layer sharply decreases diodes series resistance.

**2.4. Electrical parameters MCT HES**

were 1014-1015 cm-3 and over 105 cm2

for composition ХCdTe~0,20-0,22.

pressed by following [13]:

*j*

temperature, *k <sup>B</sup>* – Boltzmann constant.

*<sup>p</sup>*(*y*)=*μp*(*y*)*p*(*y*)*kBT*

where *μ <sup>p</sup>*- hole mobility, *p* – carrier concentration, *n <sup>i</sup>*

The equation of non-equilibrium hole in MCT n-type expresses by

um atmosphere was carried out at 200-2500

**3. The design and parameters IR detectors**

**Figure 14.** Novel construction of MCT HES for matrix PV FPA.

Fig. 14 shows the MCT composition distribution throughout the thickness for novel MCT HES for large format matrix PV FPA.

MCT HES includes the following layer which is growing in sequence technological process:


**Figure 15.** MCT composition profile throughout the thickness with narrowgap+widegap layer at the interface and wi‐ degap at the surface measured by AE *in situ*. In insert – variation of MCT composition in MCT volume.

The MCT composition in this layer is slightly more than for absorber layer. These high con‐ ductivity layer decreases of sequence resistance and serves as short wavelength cut off filter for cooling PV FPA. It was determined by photoconductive measurement the minimal height of barrier layer 0,05 mole fraction CdTe. Fig. 15 illustrates a novel MCT HES MBE with variation of MCT composition on boundaries of absorber layer. In Fig. 15 the MCT composition profile with narrowgap layers at the interface and widegap layer at the surface is shown. There is a barrier layer as wide gap layer between narrowgap and absorber layers. Narrowgap layer sharply decreases diodes series resistance.

#### **2.4. Electrical parameters MCT HES**

high conductivity must be fabricate by intentional doping of more wide gap layer or grow‐

Fig. 14 shows the MCT composition distribution throughout the thickness for novel MCT

MCT HES includes the following layer which is growing in sequence technological process:

**•** the barrier excluded cross-talking between high conductivity layer and absorber ones;

**Figure 15.** MCT composition profile throughout the thickness with narrowgap+widegap layer at the interface and wi‐

degap at the surface measured by AE *in situ*. In insert – variation of MCT composition in MCT volume.

**•** the high conductivity layer n-type with In doping up to more than n=5\*1016cm-3;

**•** graded wide gap layers on the boundaries of absorber layer.

ing more narrow layer than absorber ones during the growth MCT HES.

**Figure 14.** Novel construction of MCT HES for matrix PV FPA.

HES for large format matrix PV FPA.

146 Photodiodes - From Fundamentals to Applications

Electrical characteristics were determined from Hall measurements at 77 K by Van der Paw method. As grown MCT HES MBE with construction represented in Fig. 12 had ntype conductivity. The electron concentration and mobility and minority carrier lifetime were 1014-1015 cm-3 and over 105 cm2 /V sec and 2-10 μs for composition ХCdTe~0,20-0,22 re‐ spectively. For conversion of as-grown n-type MCT HS's MBE to p-type annealing in heli‐ um atmosphere was carried out at 200-2500 С and low mercury vapor pressure. After annealing hole concentration and mobility were (5 - 20)×1015 cm-3 and 400 - 700 cm2 /V s for composition ХCdTe~0,20-0,22.

## **3. The design and parameters IR detectors**

#### **3.1. The graded widegap layer at the boundaries of MCT absorber layer**

The graded widegap layer at the boundaries of MCT absorber layer allows to improve IR detectors and simplify the technology of their development and fabrication.

We studied the influence of widegap layers at the boundaries of absorber layer on minority lifetime by numerical calculation and supported this effect by experimental investigations.

The distribution of non-equilibrium carrier was determined by decision of one dimension‐ al diffusion equation taking into account the generation-recombination processes and built-in electric in graded widegap layer. The dependence of hole current *j <sup>p</sup>* on the thick‐ ness position *y* in homogeneous MCT *n*-type in approximation of low generation ex‐ pressed by following [13]:

$$j\_p(y) = \mu\_p(y) p(y) k\_B T \frac{d}{dy} \text{Im} \, n\_i^2(y) - \mu\_p(y) k\_B T \frac{d}{dy} p(y) \tag{1}$$

where *μ <sup>p</sup>*- hole mobility, *p* – carrier concentration, *n <sup>i</sup>* – intrinsic carrier concentration, *T* temperature, *k <sup>B</sup>* – Boltzmann constant.

The equation of non-equilibrium hole in MCT n-type expresses by

$$\frac{k\_B T}{e} \frac{d}{dy} j\_p(y) - G(y) + \frac{p(y) - p\_o(y)}{\tau(y)} = 0 \tag{2}$$

er layer is *x CdTe* = 0.2. The MCT composition on the surface is varied. The electron concentra‐ tion is *n*=4 1014 cm-3 at 77 К. The surface recombination velocity at interface with CdTe and at

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

and lifetime in 2-х MCT HES for *С <sup>d</sup>* determination. We calculate using (4) *С <sup>d</sup>* = 40-80 с/cm2

Follow *τ eff* calculation we found that the graded widegap layer leads to decrease of the in‐ fluence of surface recombination because of existing built-in electrical field (see Fig. 16).

**Figure 16.** The effective lifetime τ*eff* in MCT HES. The dislocation density- 5 107 cm-2, *C <sup>d</sup>*=60 s/cm2. The surface recombi‐

We compared the calculated lifetime *τ calc* and experimental lifetime *τ exp* at 77 K measured by

**τexp μs**

 3.8 1.4 1.4 4 2.5 1.1 1.4 5.3 8.6 1.2 0.9 2.3 1.3 0.75 1.5 8

photoconductive relaxation in MCT HES with graded widegap layers (Table 1).

= 0.05 for suppression of surface recombination which

**nd opt ×107 cm-2**

**τcalc μs**

cm/с). We measured the dislocation densities

http://dx.doi.org/10.5772/50822

149

cm/с and *s =* (0 - 107

cm-2 and lifetime 0.4-0.8 μs.

the MCT surface is *s <sup>0</sup>* = 105

at measured *n <sup>d</sup>* (4 -6) 10<sup>7</sup>

nation velocity: 1 - 103 cm/s, 2 - 105 cm/s.

**Table 1.**

It is needed only *Δx Cd = x Cd <sup>s</sup> - x Cd <sup>b</sup>*

compared to analogous ones in [16].

**Sample n**

**×1014 cm-3**

where *e* – electron charge, *p <sup>o</sup>*- equilibrium hole concentration. The generation rate is ex‐ pressed by equation

$$G(y) = \alpha(y) \text{Exp}\left(-\bigwedge\_{0}^{y} \alpha(y^{\otimes}) dy^{\otimes}\right) \tag{3}$$

where F - photon flux of radiation, *α* - absorbtion factor, *τ* - the recombination time of charge carriers. At homogeneous MCT composition this time *τ <sup>A</sup>* is determined by Auger processes *A1* [14]. For calculation of recombination time *τ <sup>d</sup>* limited by dislocation density *n <sup>d</sup>* we used the expression of empirical model [15]:

$$
\tau\_d = \frac{C\_d}{n\_d} \tag{4}
$$

where где *C <sup>d</sup>* – fitting parameter depended chemical composition of semiconductor, tech‐ nology of fabrication and dislocation nature.

So the recombination time in (2) is detemined by expression:

$$\frac{1}{\tau} = \frac{1}{\tau\_A} + \frac{1}{\tau\_d} \tag{5}$$

The boundary conditions are expresses by [11]:

$$\mathbf{j}\_p(0) = -es\_o \mathbf{\sf I} p(0) - p\_o(0) \mathbf{\sf I} \mathbf{\sf I} j\_p(L\_s) = es\_L \mathbf{\sf I} p(L\_s) - p\_o(L\_s) \mathbf{\sf I} \tag{6}$$

where *L*- MCT layer thickness, *s o, s <sup>L</sup>* – the recombination velocity at interface *y=*0 and at the surface *y=L*.of MCR absorber layer.

The equation 2 is decided by difference method. Further the effective lifetime *τ eff* is deter‐ mined from the following equation through the non-equilibrium carrier Δp as:

$$\pi\_{\varepsilon\circ} \int\_{0}^{L} G(y) dy = \int\_{0}^{L} \Delta p(y) dy \tag{7}$$

The calculations were done for photoconductor fabricated on basis of MCT HES with MCT distribution throughout the thickness represented in Fig. 12. The thickness of absorber layer and widegap layers is equal to *L*=10 and 1 μm respectively. The MCT composition in absorb‐ er layer is *x CdTe* = 0.2. The MCT composition on the surface is varied. The electron concentra‐ tion is *n*=4 1014 cm-3 at 77 К. The surface recombination velocity at interface with CdTe and at the MCT surface is *s <sup>0</sup>* = 105 cm/с and *s =* (0 - 107 cm/с). We measured the dislocation densities and lifetime in 2-х MCT HES for *С <sup>d</sup>* determination. We calculate using (4) *С <sup>d</sup>* = 40-80 с/cm2 at measured *n <sup>d</sup>* (4 -6) 10<sup>7</sup> cm-2 and lifetime 0.4-0.8 μs.

Follow *τ eff* calculation we found that the graded widegap layer leads to decrease of the in‐ fluence of surface recombination because of existing built-in electrical field (see Fig. 16).

**Figure 16.** The effective lifetime τ*eff* in MCT HES. The dislocation density- 5 107 cm-2, *C <sup>d</sup>*=60 s/cm2. The surface recombi‐ nation velocity: 1 - 103 cm/s, 2 - 105 cm/s.

It is needed only *Δx Cd = x Cd <sup>s</sup> - x Cd <sup>b</sup>* = 0.05 for suppression of surface recombination which compared to analogous ones in [16].

We compared the calculated lifetime *τ calc* and experimental lifetime *τ exp* at 77 K measured by photoconductive relaxation in MCT HES with graded widegap layers (Table 1).


*kBT e*

pressed by equation

148 Photodiodes - From Fundamentals to Applications

the expression of empirical model [15]:

nology of fabrication and dislocation nature.

The boundary conditions are expresses by [11]:

*j*

surface *y=L*.of MCR absorber layer.

So the recombination time in (2) is detemined by expression:

*<sup>p</sup>*(0)= −*eso p*(0)− *po*(0) , *j*

*d dy <sup>j</sup>*

*<sup>p</sup>*(*y*)−*G*(*y*) +

*G*(*y*)=*α*(*y*)Fexp( − *∫*

*τ<sup>d</sup>* = *Cd nd*

1 *<sup>τ</sup>* <sup>=</sup> <sup>1</sup> *τA* + 1 *τd*

*p*(*y*)− *po*(*y*)

where *e* – electron charge, *p <sup>o</sup>*- equilibrium hole concentration. The generation rate is ex‐

0

where F - photon flux of radiation, *α* - absorbtion factor, *τ* - the recombination time of charge carriers. At homogeneous MCT composition this time *τ <sup>A</sup>* is determined by Auger processes *A1* [14]. For calculation of recombination time *τ <sup>d</sup>* limited by dislocation density *n <sup>d</sup>* we used

where где *C <sup>d</sup>* – fitting parameter depended chemical composition of semiconductor, tech‐

where *L*- MCT layer thickness, *s o, s <sup>L</sup>* – the recombination velocity at interface *y=*0 and at the

The equation 2 is decided by difference method. Further the effective lifetime *τ eff* is deter‐

0

The calculations were done for photoconductor fabricated on basis of MCT HES with MCT distribution throughout the thickness represented in Fig. 12. The thickness of absorber layer and widegap layers is equal to *L*=10 and 1 μm respectively. The MCT composition in absorb‐

*L*

mined from the following equation through the non-equilibrium carrier Δp as:

*G*(*y*)*dy* = *∫*

*τeff ∫* 0

*L*

*y*

*<sup>τ</sup>*(*y*) =0 (2)

*α*(*y* ©)*d y* ©) (3)

*<sup>p</sup>*(*L* )=*esL p*(*L* )− *po*(*L* ) (6)

*Δp*(*y*)*dy* (7)

(4)

(5)

Here *τ exp* – experimental lifetime and *τ calc* - calculated lifetime determined by Auger and dislocation recombination lifetime for *С <sup>d</sup>* =60 с/cm<sup>2</sup> и *n <sup>d</sup>*=4 107 cm-2. *n <sup>d</sup> opt* - fitting dislocation density for calculated and experimental data lifetime. It is apparently clear a good agree‐ ment between calculated and experimental lifetime.

The testing of influence of graded widegap layers on lifetime was checked experimentally at measurement non contact super high frequency conductivity relaxation before and after chemical etching moving of upper graded widegap layer. Fig. 16 represents the temperature dependences of MCT HES with graded widegap layers at the boundaries of absorber layers (curve 1) and after chemical etching of upper graded widegap layer (curve 2). One can see that the experimental data of measurement lifetime supported the calculation ones at tem‐ peratures lower than 150K. It means that the presence of graded widegap layers at bounda‐

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

One important spurious component of p-n junction dark current is leakage current (LC) which limited the threshold characteristics. At low carrier concentration inside p-n junction volume LC current is mainly the surface LC determined by the carrier generation-recombi‐ nation, tunneling, ohmic conductivity and etc. The surface LC can be expressed by the fol‐

where Is – saturation current; Rs – sequence resistance; Rsh – shunt resistance; V – bias volt‐ age; IT ~ exp[-4(2m)1/2Eg 3/2/3qħ*E*] for triangular potential barrier; β = 1 at diffusion current

respectively yet in case of changing of MCT composition from XCdTe = 0.22 up to XCdTe = 0.3. It means that the presence of widegap layers at the boundaries of active layer suppress effec‐ tively surface LC. It is necessary to notice that these widegap layers eliminate the influence

Sequence resistance (Rs) is other parameter which influence on p-n characteristics and deter‐ mine the IR detector operating frequency range, operating point of heterodyne IR detector and analogous ones of different pixels of FPA etc. In last case it is equivalent of increasing of cross-taking and noise current. Really, Rs reaches several units (MWIR) or tens kilo-ohms (LWIR) for IRD with p-type MCT absorber layer with optimal values p77К ≤ 1016 cm−3, μ77К =

creasing Rs due to narrow gap layer at the interface between absorber and buffer layers of MCT HES. But the presence only narrow gap layer leads to decreasing of quantum efficien‐ cy (QE). This problem was solved by the special MCT composition with the growing se‐ quent narrow gap and wide gap layers at interface (Fig. 14). The numerical calculation of QE for different MCT composition distribution at the interface and with wide gap layer at the

/В×с and thicknesses less than 10 μm and pixel size 20-40 μm. We suggested de‐

It is clear that DC or/and (G-R)C decrease exponentially with Eg and falls 108

of surface on minority lifetime as remembered earlier.

**3.3. The role of high conductivity layer in MCT HES**

surface (Fig. 19) was carried out [18,19].

I = Is exp q(V−IRs) / *β*kT) + 1 + (V−IRs)/ Rsh + IT (8)

~ exp(-Eg/2kT); Eg – band gap; k – Boltzmann constant; T- temperature.

~ exp(-Eg/kT); β = 2 at generation-recombination current ((G-R)C))

and 105 times

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151

ries of absorber layer is very important for cooled IR detectors.

**3.2. Surface leakage**

lowing equation [17]:

(DC) component ID~ni <sup>2</sup>

component IGR~ ni

400-600 cm2

Fig. 17 demonstrates the calculation of changing lifetime at etching layer-by-layer upper graded widegap layer at the surface MCT HES. It is seen that *τ eff* sharply decreases at com‐ pletely moving.graded widegap layer from the surface.

**Figure 17.** The changing of effective lifetime τ*eff* on etching graded wide gap layer thickness d for MCT HES: *x Cd <sup>s</sup>* = 0.3, *n <sup>d</sup>* = 5 107 cm-2. The surface recombination velocity: 1 - 103 cm/s, 2 - 105 cm/s.

**Figure 18.** The temperature dependences of lifetime for n-type MCT HES with MCT composition in absorber layer 0,215 mole fraction CdTe: 1) – with graded wide gap layers; 2) – after chemical moving of upper graded wide gap layer.

The testing of influence of graded widegap layers on lifetime was checked experimentally at measurement non contact super high frequency conductivity relaxation before and after chemical etching moving of upper graded widegap layer. Fig. 16 represents the temperature dependences of MCT HES with graded widegap layers at the boundaries of absorber layers (curve 1) and after chemical etching of upper graded widegap layer (curve 2). One can see that the experimental data of measurement lifetime supported the calculation ones at tem‐ peratures lower than 150K. It means that the presence of graded widegap layers at bounda‐ ries of absorber layer is very important for cooled IR detectors.

#### **3.2. Surface leakage**

Here *τ exp* – experimental lifetime and *τ calc* - calculated lifetime determined by Auger and

density for calculated and experimental data lifetime. It is apparently clear a good agree‐

Fig. 17 demonstrates the calculation of changing lifetime at etching layer-by-layer upper graded widegap layer at the surface MCT HES. It is seen that *τ eff* sharply decreases at com‐

**Figure 17.** The changing of effective lifetime τ*eff* on etching graded wide gap layer thickness d for MCT HES: *x Cd <sup>s</sup>*

**Figure 18.** The temperature dependences of lifetime for n-type MCT HES with MCT composition in absorber layer 0,215 mole fraction CdTe: 1) – with graded wide gap layers; 2) – after chemical moving of upper graded wide gap layer.

и *n <sup>d</sup>*=4 107

cm-2. *n <sup>d</sup> opt* - fitting dislocation

= 0.3,

dislocation recombination lifetime for *С <sup>d</sup>* =60 с/cm<sup>2</sup>

150 Photodiodes - From Fundamentals to Applications

ment between calculated and experimental lifetime.

pletely moving.graded widegap layer from the surface.

*n <sup>d</sup>* = 5 107 cm-2. The surface recombination velocity: 1 - 103 cm/s, 2 - 105 cm/s.

One important spurious component of p-n junction dark current is leakage current (LC) which limited the threshold characteristics. At low carrier concentration inside p-n junction volume LC current is mainly the surface LC determined by the carrier generation-recombi‐ nation, tunneling, ohmic conductivity and etc. The surface LC can be expressed by the fol‐ lowing equation [17]:

$$\mathbf{I} = \mathbf{I}\_s \mathbf{I} \exp\left\{ \mathbf{V} - \mathbf{I} \mathbf{R}\_s \right\} / \beta \mathbf{k} \,\mathrm{T} \,\mathrm{I} \,\mathrm{I} + \,\mathrm{I} \,\mathrm{I} + \,\mathrm{\left(V - \mathbf{I} \mathbf{R}\_s\right)} / \mathbf{R}\_{\mathrm{sh}} + \mathrm{I}\_\mathrm{T} \tag{8}$$

where Is – saturation current; Rs – sequence resistance; Rsh – shunt resistance; V – bias volt‐ age; IT ~ exp[-4(2m)1/2Eg 3/2/3qħ*E*] for triangular potential barrier; β = 1 at diffusion current (DC) component ID~ni <sup>2</sup> ~ exp(-Eg/kT); β = 2 at generation-recombination current ((G-R)C)) component IGR~ ni ~ exp(-Eg/2kT); Eg – band gap; k – Boltzmann constant; T- temperature.

It is clear that DC or/and (G-R)C decrease exponentially with Eg and falls 108 and 105 times respectively yet in case of changing of MCT composition from XCdTe = 0.22 up to XCdTe = 0.3. It means that the presence of widegap layers at the boundaries of active layer suppress effec‐ tively surface LC. It is necessary to notice that these widegap layers eliminate the influence of surface on minority lifetime as remembered earlier.

#### **3.3. The role of high conductivity layer in MCT HES**

Sequence resistance (Rs) is other parameter which influence on p-n characteristics and deter‐ mine the IR detector operating frequency range, operating point of heterodyne IR detector and analogous ones of different pixels of FPA etc. In last case it is equivalent of increasing of cross-taking and noise current. Really, Rs reaches several units (MWIR) or tens kilo-ohms (LWIR) for IRD with p-type MCT absorber layer with optimal values p77К ≤ 1016 cm−3, μ77К = 400-600 cm2 /В×с and thicknesses less than 10 μm and pixel size 20-40 μm. We suggested de‐ creasing Rs due to narrow gap layer at the interface between absorber and buffer layers of MCT HES. But the presence only narrow gap layer leads to decreasing of quantum efficien‐ cy (QE). This problem was solved by the special MCT composition with the growing se‐ quent narrow gap and wide gap layers at interface (Fig. 14). The numerical calculation of QE for different MCT composition distribution at the interface and with wide gap layer at the surface (Fig. 19) was carried out [18,19].

We found that Rs is equal to approximately several Ohms for MCT HS type 3 and at the same time several hundreds Ohms for MCT HS type 3. It means that the presence of narrow +wide gap layer allows to decrease Rs without IRD performance degradation. Really the typ‐ ical characteristics of photodiodes with maximum wavelength at λ<sup>p</sup> = 7 μm and cut-off λco =

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

As mentions above there is the problem of sequence resistance at developments of different

For IR detector operated at high frequency the limiting frequency is determined by Rs×C,

creasing of hole concentration in absorber layer more 2×10<sup>16</sup> cm-3 leads to decreasing of threshold diode parameters at forward bias, decreasing electrons diffusion length and corre‐ spondingly decreasing of quantum efficiency. The bulk MCT p-type with the thickness ~ 1 mm and hole concentration 2×10<sup>16</sup> cm-3 used for IR PV detectors (10.6 μm) operated at fre‐ quencies more than 1 GHz and has a high quantum efficiency when MCT provides wave‐ length cut off more 12 μm. For MCT HES p-type a sequence resistance is about kΩ unit that

For large format PV FPA it is necessary to create the same condition (the same bias voltage) for central and edge diodes. At high sequence resistance there is the possibility of so called "debiasing substrate" or "boublik" effects that observed by this time for 128× 128 PV FPA.

The problem of sharp decreasing of sequence resistance is decided by growing MCT HES

The numerical calculation of n+-p diodes current of matrix PV FPA based on MCT HES with ptype absorber layer were carried out. For this n+-p junction the minority charge carriers (elec‐ trons) are collected by p-n junction, while the excess holes moves to base contact at periphery

where *G <sup>p</sup>* –generation rate; *U <sup>p</sup>*- recombination velocity (cm-3/s), q- electron charge, *J <sup>p</sup>* – hole

where *μ <sup>p</sup>* – hole mobility, *р* – hole concentration, *Е*- electric intensity, *D* p – hole diffusion

For 2-D FPA the stationary equation for homogeneous hole current through the thickness *d*

and taking into account *G <sup>p</sup>* - *U <sup>p</sup>* = *J* <sup>S</sup> */d* at low generation rate (Δ*р*<<*p*) is described by

where Rs – sequence resistance and С – p-n junction capacity. For IR FPA n<sup>+</sup>

at С ~ 1 – 10 pF gives receiving radiation at frequencies lower than 1 GHz.

with high conductivity layer which does not influence on threshold PV FPA.

∂ *p*

of FPA. The continuity equation of hole current is described by following expression:

<sup>∂</sup> *<sup>t</sup>* <sup>=</sup>*Gp* <sup>−</sup>*Up* <sup>−</sup> <sup>1</sup>

, S(λp) = 3,5 А/Вт, D\* \*(λp, 500 K, 1200

http://dx.doi.org/10.5772/50822

153

*<sup>q</sup>* ∇ ⋅ *J*<sup>p</sup> (9)

*Jp* =*qμp pE* −*qDp*∇ *p* (10)


9,1 μm were as following: Rs ~1 Ом, R0А=100 Ohm×cm2

Hz, 1 Hz)= 6,5×1011 cmHz1/2W-1.

PV type IR detectors.

current density.

coefficient.

The hole current density is expressed by

**Figure 19.** MCT composition (X) distribution throughout the thickness with graded widegap layer at the surface and different layer at the interface: type 1– widegap (solid line); type 2 – narrowgap (dotted line); type 3 – narrow gap + widegap (pointed line).

The calculated A/W sensitivities (Sj ) normalized to λсo is presented in Fig. 19.

**Figure 20.** The calculated (solid 1,2,3) and experimental (dashed 1',2', 3') A-W of diodes sensitivity (Sj ) normalized to λc. Numerals are mean number MCT type on fig.19. Crosshatched region is deviation for type 2.

Apparently clear essential decreasing Sj for MCT HES type 2 especially near λсo in compar‐ ing with MCT HES's type 1 and type 3. We fabricated testing diodes on the basis of three types MCT HES and measures V-dependences, differential resistances on bias voltage and spectral responses at 77K. The experimental data is in good agreement with calculated ones. We found that Rs is equal to approximately several Ohms for MCT HS type 3 and at the same time several hundreds Ohms for MCT HS type 3. It means that the presence of narrow +wide gap layer allows to decrease Rs without IRD performance degradation. Really the typ‐ ical characteristics of photodiodes with maximum wavelength at λ<sup>p</sup> = 7 μm and cut-off λco = 9,1 μm were as following: Rs ~1 Ом, R0А=100 Ohm×cm2 , S(λp) = 3,5 А/Вт, D\* \*(λp, 500 K, 1200 Hz, 1 Hz)= 6,5×1011 cmHz1/2W-1.

As mentions above there is the problem of sequence resistance at developments of different PV type IR detectors.

For IR detector operated at high frequency the limiting frequency is determined by Rs×C, where Rs – sequence resistance and С – p-n junction capacity. For IR FPA n<sup>+</sup> -p type the in‐ creasing of hole concentration in absorber layer more 2×10<sup>16</sup> cm-3 leads to decreasing of threshold diode parameters at forward bias, decreasing electrons diffusion length and corre‐ spondingly decreasing of quantum efficiency. The bulk MCT p-type with the thickness ~ 1 mm and hole concentration 2×10<sup>16</sup> cm-3 used for IR PV detectors (10.6 μm) operated at fre‐ quencies more than 1 GHz and has a high quantum efficiency when MCT provides wave‐ length cut off more 12 μm. For MCT HES p-type a sequence resistance is about kΩ unit that at С ~ 1 – 10 pF gives receiving radiation at frequencies lower than 1 GHz.

For large format PV FPA it is necessary to create the same condition (the same bias voltage) for central and edge diodes. At high sequence resistance there is the possibility of so called "debiasing substrate" or "boublik" effects that observed by this time for 128× 128 PV FPA.

The problem of sharp decreasing of sequence resistance is decided by growing MCT HES with high conductivity layer which does not influence on threshold PV FPA.

The numerical calculation of n+-p diodes current of matrix PV FPA based on MCT HES with ptype absorber layer were carried out. For this n+-p junction the minority charge carriers (elec‐ trons) are collected by p-n junction, while the excess holes moves to base contact at periphery of FPA. The continuity equation of hole current is described by following expression:

$$\frac{\partial \lrcorner p}{\partial t} = G\_p - \mathcal{U}\_p - \frac{1}{q} \nabla \cdot \mathcal{J}\_p \tag{9}$$

where *G <sup>p</sup>* –generation rate; *U <sup>p</sup>*- recombination velocity (cm-3/s), q- electron charge, *J <sup>p</sup>* – hole current density.

The hole current density is expressed by

**Figure 19.** MCT composition (X) distribution throughout the thickness with graded widegap layer at the surface and different layer at the interface: type 1– widegap (solid line); type 2 – narrowgap (dotted line); type 3 – narrow gap +

**Figure 20.** The calculated (solid 1,2,3) and experimental (dashed 1',2', 3') A-W of diodes sensitivity (Sj

ing with MCT HES's type 1 and type 3. We fabricated testing diodes on the basis of three types MCT HES and measures V-dependences, differential resistances on bias voltage and spectral responses at 77K. The experimental data is in good agreement with calculated ones.

λc. Numerals are mean number MCT type on fig.19. Crosshatched region is deviation for type 2.

) normalized to λсo is presented in Fig. 19.

) normalized to

for MCT HES type 2 especially near λсo in compar‐

widegap (pointed line).

The calculated A/W sensitivities (Sj

152 Photodiodes - From Fundamentals to Applications

Apparently clear essential decreasing Sj

$$J\_p = q\mu\_p pE - qD\_p \nabla \text{ } p\tag{10}$$

where *μ <sup>p</sup>* – hole mobility, *р* – hole concentration, *Е*- electric intensity, *D* p – hole diffusion coefficient.

For 2-D FPA the stationary equation for homogeneous hole current through the thickness *d* and taking into account *G <sup>p</sup>* - *U <sup>p</sup>* = *J* <sup>S</sup> */d* at low generation rate (Δ*р*<<*p*) is described by

$$\frac{d^2\boldsymbol{\varrho}}{d\boldsymbol{x}^2} + \frac{d^2\boldsymbol{\varrho}}{d\boldsymbol{y}^2} = \rho\_s \boldsymbol{J}\_S \tag{11}$$

where *φ*(x,y) – the potential in absorber layer, *ρ* <sup>S</sup> – surface resistance ρS =1/*qμ <sup>p</sup> pd*.

The surface current density *J* S=*I×N*, where *I* – diode current; *N* – density of surface state. For ideal n-p junction the current is expressed by

$$I = I\_{ph} + I\_S \left( e^{\frac{qV}{kT}} - 1 \right) \tag{12}$$

**Figure 22.** The experimental image at homogeneous radiation of central part of 128× 128 PV FPA n-p type.

high level doping (Fig. 14) or narrow gap layer (Fig. 15) during growing MCT HES.

For decreasing or elimination of "debiasing substrate" or "boublik" effect we suggested

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

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155

**Figure 23.** The numerical calculation data of potential distribution in 640×512 PV FPA p-type on dependence of resist‐

In case of fabricating high conductivity layer by doping during the growth the thickness d and MCT composition can be chosen for creating cooled short wavelength cur off filter.

ance of high conductivity layer. The photocurrent of diodes is 20 nA.

where Iph – photocurrent; *V* = -(φd-*φ*(x,y)) and *φ* d – potential at diode from multiplexer and boundary condition *φ*(x,y)=0 at the base contact.

Fig 21 shows the calculation data of distribution of diodes current for the case of appearance of positive voltage bias at central part of FPA due to voltage drop because of large summed current in absorber layer.

**Figure 21.** The numerical calculation data of changing of output diodes current at homogeneous radiation of central part of 128× 128 FPA.

This data demonstrates "debiasing substrate" or "boublik" effect which means the breaking of central diodes that observed experimentally (Fig. 22).

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE http://dx.doi.org/10.5772/50822 155

*d* 2 *φ d x* <sup>2</sup> +

ideal n-p junction the current is expressed by

154 Photodiodes - From Fundamentals to Applications

boundary condition *φ*(x,y)=0 at the base contact.

current in absorber layer.

part of 128× 128 FPA.

*d* 2 *φ*

where *φ*(x,y) – the potential in absorber layer, *ρ* <sup>S</sup> – surface resistance ρS =1/*qμ <sup>p</sup> pd*.

*I* = *I ph* + *IS*

The surface current density *J* S=*I×N*, where *I* – diode current; *N* – density of surface state. For

(*e qV*

where Iph – photocurrent; *V* = -(φd-*φ*(x,y)) and *φ* d – potential at diode from multiplexer and

Fig 21 shows the calculation data of distribution of diodes current for the case of appearance of positive voltage bias at central part of FPA due to voltage drop because of large summed

**Figure 21.** The numerical calculation data of changing of output diodes current at homogeneous radiation of central

This data demonstrates "debiasing substrate" or "boublik" effect which means the breaking

of central diodes that observed experimentally (Fig. 22).

*d y* <sup>2</sup> <sup>=</sup>*ρsJS* (11)

*kT* −1) (12)

**Figure 22.** The experimental image at homogeneous radiation of central part of 128× 128 PV FPA n-p type.

For decreasing or elimination of "debiasing substrate" or "boublik" effect we suggested high level doping (Fig. 14) or narrow gap layer (Fig. 15) during growing MCT HES.

**Figure 23.** The numerical calculation data of potential distribution in 640×512 PV FPA p-type on dependence of resist‐ ance of high conductivity layer. The photocurrent of diodes is 20 nA.

In case of fabricating high conductivity layer by doping during the growth the thickness d and MCT composition can be chosen for creating cooled short wavelength cur off filter. The results of numerical calculation of potential distribution in the absorber layer for 640×512 PV FPA is shown in Fig. 23 with the following parameters: *I <sup>d</sup>* =20 nA; А=6.25×10-6 cm2 ; *ρ <sup>s</sup>*=2 kΩ/ (*р*=8×1015 cm-3, *μ <sup>р</sup>*=400 cm2 /(V×s); *d*=10 μm); *n-p* junction be‐ tween *n*-type layer and barrier layer is ideal with the density of saturation current *J* S = 1.6×10-7 A/cm<sup>2</sup> ; ground potential to base contact.

The good values of voltage drop lower 70-110 mV reached at resistance 10-100 Ω/ of high conductivity layer thickness 3 μm doping by In up to (1-5)×1016 cm-3.

#### **3.3. MCT HES p-P design**

The special dual layer absorber construction of MCT HES p-P type (p in narrowgap part of absorber layer; P in widegap of absorber layer) (see Fig.24) allows to decrease dark cur‐ rent and photocurrent that leads to increase of wavelength cut off in range 8-12 μm or operating temperature [20].

**Figure 25.** The scheme of n+-P-p diode fabricated in p-P MCT HES.

*dz* (ln *ni* 2

+ 5*L* <sup>n</sup>

=*np*0(exp(*qV* / *kT* )−1) on n+-p junction borders

=0 on the planar borders of absorber layer at z = 0 и z = H.

0

coefficient; *L* n- electron diffusion length, *ni*

=0at *r* = 0 and *r* = *r* <sup>j</sup>

*z*

mobility and lifetime is expressed by

∂2*n* ′ <sup>∂</sup>*<sup>r</sup>* <sup>2</sup> <sup>+</sup>

**1.** ∂*n* ′ ∂*r*

**2.** *n* ′

**3.** ∂*n* ′ ∂ *z*

sensitivity at 78 К.

1 *r* ∂*n* ′ ∂*r* + ∂2*n* ′ <sup>∂</sup> *<sup>z</sup>* <sup>2</sup> <sup>−</sup> *<sup>d</sup>*

where*g*(*z*)=*α*(*z*)*Q*exp(− *∫*

conditions are the following:

The diffusion current is determined from decision of stationary continuity equation for ex‐ cess electron in *р* range. The valence band location in p-P absorber layer is permanent (com‐ mon anion rule). The *р*-range is quasi neutral. The current in n+ range does not take into account. The stationary continuity equation in cylindrical coordinates at constant electron

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

<sup>∂</sup> *<sup>z</sup>* <sup>−</sup>( *<sup>d</sup>* <sup>2</sup>

Q – the density of flux of radiation, *<sup>n</sup>* ′ - excess electron concentration, *Dn*- electron diffusion

The incident radiation from back side of diode has wavelength for maximal ampere-watt

*d z* <sup>2</sup> ln *ni*

2 (*z*) +

*α*(*t*)*dt*) *g*(*z*)- generation function and α(z) – adsorption coefficient;

1 *L <sup>n</sup>* <sup>2</sup> ) <sup>⋅</sup>*<sup>n</sup>* ′


<sup>=</sup> <sup>−</sup> *<sup>g</sup>*(*z*) *Dn*

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157

(13)

(*z*) ) <sup>⋅</sup> <sup>∂</sup>*<sup>n</sup>* ′

**Figure 24.** The MCT distribution throughout the thickness in p-P MCT HES: Δx – barrier between narrowgap and wide‐ gap parts of absorber layer; z<sup>j</sup> – position of n+-P junction.

The quantum efficiency *η* and *R* <sup>0</sup> *A* product (*R* 0 – differential resistance at 0 bias voltage; A diode area) was numerical calculated for p-P MCT HES taking into account only diffusion current in which the lateral diffusion current contribution becomes essential [21,22].

Fig. 25 shows the scheme of PV diode for calculation *η* and *R* <sup>0</sup> *A.* The MCT of narrow gap p and wide gap P layers is equal to 0.22 mole fraction CdTe and 0.22+Δx mole fraction CdTe respectively. N*<sup>+</sup> -p* junction located in P layer.

**Figure 25.** The scheme of n+-P-p diode fabricated in p-P MCT HES.

The results of numerical calculation of potential distribution in the absorber layer for 640×512 PV FPA is shown in Fig. 23 with the following parameters: *I <sup>d</sup>* =20 nA;

tween *n*-type layer and barrier layer is ideal with the density of saturation current *J* S =

The good values of voltage drop lower 70-110 mV reached at resistance 10-100 Ω/ of high

The special dual layer absorber construction of MCT HES p-P type (p in narrowgap part of absorber layer; P in widegap of absorber layer) (see Fig.24) allows to decrease dark cur‐ rent and photocurrent that leads to increase of wavelength cut off in range 8-12 μm or

**Figure 24.** The MCT distribution throughout the thickness in p-P MCT HES: Δx – barrier between narrowgap and wide‐

The quantum efficiency *η* and *R* <sup>0</sup> *A* product (*R* 0 – differential resistance at 0 bias voltage; A diode area) was numerical calculated for p-P MCT HES taking into account only diffusion

Fig. 25 shows the scheme of PV diode for calculation *η* and *R* <sup>0</sup> *A.* The MCT of narrow gap p and wide gap P layers is equal to 0.22 mole fraction CdTe and 0.22+Δx mole fraction CdTe

current in which the lateral diffusion current contribution becomes essential [21,22].

– position of n+-P junction.

respectively. N*<sup>+</sup> -p* junction located in P layer.

/(V×s); *d*=10 μm); *n-p* junction be‐

; *ρ <sup>s</sup>*=2 kΩ/ (*р*=8×1015 cm-3, *μ <sup>р</sup>*=400 cm2

; ground potential to base contact.

conductivity layer thickness 3 μm doping by In up to (1-5)×1016 cm-3.

А=6.25×10-6 cm2

1.6×10-7 A/cm<sup>2</sup>

**3.3. MCT HES p-P design**

156 Photodiodes - From Fundamentals to Applications

operating temperature [20].

gap parts of absorber layer; z<sup>j</sup>

The diffusion current is determined from decision of stationary continuity equation for ex‐ cess electron in *р* range. The valence band location in p-P absorber layer is permanent (com‐ mon anion rule). The *р*-range is quasi neutral. The current in n+ range does not take into account. The stationary continuity equation in cylindrical coordinates at constant electron mobility and lifetime is expressed by

$$\frac{\partial^2 n'}{\partial r^2} + \frac{1}{r} \frac{\partial n'}{\partial r} + \frac{\partial^2 n'}{\partial z^2} - \frac{d}{dz} \{ \ln[n\_i^2(z)] \} \cdot \frac{\partial n'}{\partial z} - \left( \frac{d^2}{dz^2} \text{ln} \left[ n\_i^2(z) \right] + \frac{1}{L\_n z} \right) \cdot n' = -\frac{\mathcal{g}(z)}{D\_n} \tag{13}$$

where*g*(*z*)=*α*(*z*)*Q*exp(− *∫* 0 *z α*(*t*)*dt*) *g*(*z*)- generation function and α(z) – adsorption coefficient;

Q – the density of flux of radiation, *<sup>n</sup>* ′ - excess electron concentration, *Dn*- electron diffusion coefficient; *L* n- electron diffusion length, *ni* - intrinsic carrier concentration. The boundaries conditions are the following:


The incident radiation from back side of diode has wavelength for maximal ampere-watt sensitivity at 78 К.

The diode current I is determined after equation decision by integration of *j <sup>N</sup>* =*qDn* ∂*n* ′ ∂ *z* and *j <sup>L</sup>* =*qDn* ∂*n* ′ <sup>∂</sup>*r* on planar and lateral surfaces of n+-p junction. The *<sup>R</sup>* <sup>0</sup> *<sup>A</sup>* product is deter‐ mined from dark diffusion current *I* as *R*0*A*<sup>=</sup> *kT qI <sup>A</sup>*, where *A*=*πrj* 2 - area of *n+-p junction*. The quantum efficiency*<sup>η</sup>* <sup>=</sup> *Ip qQA* is determined by calculated photocurrent *<sup>I</sup>* p.

**Figure 27.** The diffusion current dependences on position of n+-p junction in P region (a) and relationship rj

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

The linear 288×4 PV FPA has been fabricated by planar technology described in [23] using MCT HES with graded widegap layer at absorber layers boundaries represented in Fig. 13. The FPA has 288 channels of four pixels, on which time delay and integration (TDI) is per‐ formed through the readout integrated circuit (ROIC). The size of each pixel is 25 μm (scan direction) per 28 μm (cross-scan direction). The in-scan pitch is 43 μm and the cross-scan

keV and a dose ~3×1013 cm–2 into *p*-MCT structures. The spectral response of one of the ele‐

Current-voltage characteristics of 30 diodes have been measured at random with the help of the microprobe device cooled by the liquid nitrogen vapour. The typical dark current was

A defect diode can be detected by its high dark current, and the deselection function of the ROIC allows us to switch such a diode off. The time delay integration is performed over the entire 4-diode channel thus, the presence of a single defect diode does not influence the

**4. The technology and parameters of IR detectors**

pitch is 28 μm. The diodes were formed by the implantation of B+

ment in the array is shown in Fig. 28, (STD = 0.1 μm).

equal to 5.3 nA at the reverse bias voltage 150 mV.

**4.1. Linear LWIR 288х4 FPA**

/*L* n (b).

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159

ions with the energy ~50

**Figure 26.** The *R*0*A* and η для 1D and 3D diodes on Δ*x* at *z* <sup>j</sup> =8 μm, *L* <sup>n</sup>=25 μm and *r* <sup>j</sup> = 5 μm.

Fig. 26 represents the dependence of *R* <sup>0</sup> *A* and *η* on Δ*x.* The Δ*x* increasing leads to decreas‐ ing of diffusion current and changes the relationship between volume and lateral compo‐ nents from comparing *R* <sup>0</sup> *A* and *η* for 3D and 1D diodes.

Fig. 27 (a, b) shows the influence of n+-p position and relationship rj /*L* n on diffusion current.

So the anyone using suggested model could be carried out the calculation and/or taking the data in Fig. 25, 26 to determine the MCT HES p-P construction (Δ*x,* rj and poison n+-p junc‐ tion) which allows to fabricate IR detector with definite low diffusion current.

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE http://dx.doi.org/10.5772/50822 159

**Figure 27.** The diffusion current dependences on position of n+-p junction in P region (a) and relationship rj /*L* n (b).

#### **4. The technology and parameters of IR detectors**

#### **4.1. Linear LWIR 288х4 FPA**

The diode current I is determined after equation decision by integration of *j*

<sup>∂</sup>*r* on planar and lateral surfaces of n+-p junction. The *<sup>R</sup>* <sup>0</sup> *<sup>A</sup>* product is deter‐

*qQA* is determined by calculated photocurrent *<sup>I</sup>* p.

*qI <sup>A</sup>*, where *A*=*πrj*

=8 μm, *L* <sup>n</sup>=25 μm and *r* <sup>j</sup>

Fig. 26 represents the dependence of *R* <sup>0</sup> *A* and *η* on Δ*x.* The Δ*x* increasing leads to decreas‐ ing of diffusion current and changes the relationship between volume and lateral compo‐

So the anyone using suggested model could be carried out the calculation and/or taking the data in Fig. 25, 26 to determine the MCT HES p-P construction (Δ*x,* rj and poison n+-p junc‐

= 5 μm.

/*L* n on diffusion current.

2

*j <sup>L</sup>* =*qDn* ∂*n* ′

158 Photodiodes - From Fundamentals to Applications

quantum efficiency*<sup>η</sup>* <sup>=</sup> *Ip*

mined from dark diffusion current *I* as *R*0*A*<sup>=</sup> *kT*

**Figure 26.** The *R*0*A* and η для 1D and 3D diodes on Δ*x* at *z* <sup>j</sup>

nents from comparing *R* <sup>0</sup> *A* and *η* for 3D and 1D diodes.

Fig. 27 (a, b) shows the influence of n+-p position and relationship rj

tion) which allows to fabricate IR detector with definite low diffusion current.

*<sup>N</sup>* =*qDn*


∂*n* ′ ∂ *z* and

> The linear 288×4 PV FPA has been fabricated by planar technology described in [23] using MCT HES with graded widegap layer at absorber layers boundaries represented in Fig. 13. The FPA has 288 channels of four pixels, on which time delay and integration (TDI) is per‐ formed through the readout integrated circuit (ROIC). The size of each pixel is 25 μm (scan direction) per 28 μm (cross-scan direction). The in-scan pitch is 43 μm and the cross-scan pitch is 28 μm. The diodes were formed by the implantation of B+ ions with the energy ~50 keV and a dose ~3×1013 cm–2 into *p*-MCT structures. The spectral response of one of the ele‐ ment in the array is shown in Fig. 28, (STD = 0.1 μm).

> Current-voltage characteristics of 30 diodes have been measured at random with the help of the microprobe device cooled by the liquid nitrogen vapour. The typical dark current was equal to 5.3 nA at the reverse bias voltage 150 mV.

> A defect diode can be detected by its high dark current, and the deselection function of the ROIC allows us to switch such a diode off. The time delay integration is performed over the entire 4-diode channel thus, the presence of a single defect diode does not influence the

channel operation. Additionally, there is an option in the ROIC to use the average dark cur‐ rent value for deselected defect diodes.

The experimentally measured I-V curves of photodiodes has been modelled with the help of the carrier balance equations approach [24,25], assuming the presence of in-gap donor-type

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

the Shockley-Read-Hall (SRH) generation/recombination as two current mechanisms. The other relevant current mechanisms, that do not involve the trap levels, have been taken into account additively. The modelling has shown that at a small reverse bias (less than –0.25 V) the dark current is limited by the diffusion current and the SRH current outside the *n*–*p*junction. At the reverse bias larger than –0.25 V, the dark currents were determined by the tunnelling and thermal generation from the trap levels. Interband tunnelling as well as the other recombination mechanisms do not contribute substantially at the operational bias val‐ ues. The modelling has shown that these heterostructures are of the *n <sup>+</sup> -n* – *-p* type, with the *n-p* junction shifted into *n-*region that is characterized by long carrier lifetimes and low con‐ centration of recombination centres (due to the compensation of the Hg vacancies). The use of the varyband potential at the surface of the heteroepitaxial MCT structure allows us to increase the effective carrier lifetime by means of diminishing the influence of the surface

Experimentally, our average diodes have shown current-voltage characteristics that are practically limited by the diffusion current mechanism for ideal diodes. Such characteristics

The multiplexer was designed using the 1.0-μm CMOS technology with two polysilicon and two metallic layers. The multiplexer provides a bidirectional TDI scanning, random pixel deselecting, anti-blooming and background skimming, and testing analogue part of a circuit without connection to photodiodes. The output charge capacity of the multiplexer exceeds

recombination, as well as by suppressing the surface leakage currents [26].

make possible to realize the FPA operating in a BLIP regime.

2.5 pC at the nonlinearity lower than 2%.

**Figure 30.** The photo view 288×4 PV FPA hybrid assembly.

≈0.7*Е <sup>g</sup>* and taking into account the trap-assisted tunnelling and

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161

trap level with the energy *E <sup>t</sup>*

**Figure 28.** The typical spectral response of one of the sensitive element in 288×4 PV FPA.

In Fig. 29, the typical I-V curvе and differential resistance *R* versus bias voltage at 77 K are shown. The values of *R <sup>0</sup>*, *R max*, and of the product *R* <sup>0</sup> *A* were equal to 1.6×107 Ω, 2.1×10<sup>8</sup> Ω, and 70 Ω cm<sup>2</sup> , respectively.

**Figure 29.** The typical current (1 – the photocurrent,2 - the dark current) and the differential resistance (3) depend‐ ence on the bias voltage.

The experimentally measured I-V curves of photodiodes has been modelled with the help of the carrier balance equations approach [24,25], assuming the presence of in-gap donor-type trap level with the energy *E <sup>t</sup>* ≈0.7*Е <sup>g</sup>* and taking into account the trap-assisted tunnelling and the Shockley-Read-Hall (SRH) generation/recombination as two current mechanisms. The other relevant current mechanisms, that do not involve the trap levels, have been taken into account additively. The modelling has shown that at a small reverse bias (less than –0.25 V) the dark current is limited by the diffusion current and the SRH current outside the *n*–*p*junction. At the reverse bias larger than –0.25 V, the dark currents were determined by the tunnelling and thermal generation from the trap levels. Interband tunnelling as well as the other recombination mechanisms do not contribute substantially at the operational bias val‐ ues. The modelling has shown that these heterostructures are of the *n <sup>+</sup> -n* – *-p* type, with the *n-p* junction shifted into *n-*region that is characterized by long carrier lifetimes and low con‐ centration of recombination centres (due to the compensation of the Hg vacancies). The use of the varyband potential at the surface of the heteroepitaxial MCT structure allows us to increase the effective carrier lifetime by means of diminishing the influence of the surface recombination, as well as by suppressing the surface leakage currents [26].

Experimentally, our average diodes have shown current-voltage characteristics that are practically limited by the diffusion current mechanism for ideal diodes. Such characteristics make possible to realize the FPA operating in a BLIP regime.

The multiplexer was designed using the 1.0-μm CMOS technology with two polysilicon and two metallic layers. The multiplexer provides a bidirectional TDI scanning, random pixel deselecting, anti-blooming and background skimming, and testing analogue part of a circuit without connection to photodiodes. The output charge capacity of the multiplexer exceeds 2.5 pC at the nonlinearity lower than 2%.

**Figure 30.** The photo view 288×4 PV FPA hybrid assembly.

channel operation. Additionally, there is an option in the ROIC to use the average dark cur‐

**2 4 6 8 10 12**

In Fig. 29, the typical I-V curvе and differential resistance *R* versus bias voltage at 77 K are

**Figure 29.** The typical current (1 – the photocurrent,2 - the dark current) and the differential resistance (3) depend‐

Ω, 2.1×10<sup>8</sup> Ω,

 **Wavelength,** m**m**

rent value for deselected defect diodes.

160 Photodiodes - From Fundamentals to Applications

**0,0**

, respectively.

**Figure 28.** The typical spectral response of one of the sensitive element in 288×4 PV FPA.

shown. The values of *R <sup>0</sup>*, *R max*, and of the product *R* <sup>0</sup> *A* were equal to 1.6×107

**0,2**

**0,4**

**0,6**

**Response, arb.un.**

and 70 Ω cm<sup>2</sup>

ence on the bias voltage.

**0,8**

**1,0**

The 288×4 PV FPA was fabricated by hybrid assembling of the photosensitive array and the ROIC, with the help of indium bumps group welding at 120ºС. After hybridization, the total height of In bumps was equal to ~10 μm [27]. In Fig. 30, a photo of the 288×4 PV FPA hybrid assembly is presented.

The measurement of PV FPA parameters were carried out in a cryostat at 77 K, the input signal was coming through the GaAs substrate (FOV 32º, 295 K). The integration time was 20 μs. Typical values of responsivity and detectivity at the maximum of the spectral sensitiv‐

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

tively. The example of a 576×610 thermal image by PV FPA (FOV = 32º, F = 1/1.6) is

We developed the technology of fabricating 320×256 (320×240) and 320×240 PV FPA operat‐ ed in wavelength ranges 8-12 μm at 77 K. The MCT HES composition distribution through‐

The NEDT histogram is shown in Fig. 32. The average NETD value is about 9 mK.

out the thickness used for FPA is analogous presented in Fig. 14.

V/W and 2.13×1011 cm×Hz1/2×W–1 at STD 6.7% and 15.3%, respec‐

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163

ity were equal to 2.27×108

**4.2. Matrix LWIR 320х256 PV FPA**

**Figure 33.** The photo view 320×240 PV.

**Figure 34.** Spectral responsivity 320×240 PV FPA.

presented in Fig. 31.

**Figure 31.** An example of the thermal image 288×4 PV FPA.

**Figure 32.** The histogram of NETD.

The measurement of PV FPA parameters were carried out in a cryostat at 77 K, the input signal was coming through the GaAs substrate (FOV 32º, 295 K). The integration time was 20 μs. Typical values of responsivity and detectivity at the maximum of the spectral sensitiv‐ ity were equal to 2.27×108 V/W and 2.13×1011 cm×Hz1/2×W–1 at STD 6.7% and 15.3%, respec‐ tively. The example of a 576×610 thermal image by PV FPA (FOV = 32º, F = 1/1.6) is presented in Fig. 31.

The NEDT histogram is shown in Fig. 32. The average NETD value is about 9 mK.

#### **4.2. Matrix LWIR 320х256 PV FPA**

The 288×4 PV FPA was fabricated by hybrid assembling of the photosensitive array and the ROIC, with the help of indium bumps group welding at 120ºС. After hybridization, the total height of In bumps was equal to ~10 μm [27]. In Fig. 30, a photo of the 288×4 PV

FPA hybrid assembly is presented.

162 Photodiodes - From Fundamentals to Applications

**Figure 31.** An example of the thermal image 288×4 PV FPA.

**Figure 32.** The histogram of NETD.

We developed the technology of fabricating 320×256 (320×240) and 320×240 PV FPA operat‐ ed in wavelength ranges 8-12 μm at 77 K. The MCT HES composition distribution through‐ out the thickness used for FPA is analogous presented in Fig. 14.

**Figure 33.** The photo view 320×240 PV.

**Figure 34.** Spectral responsivity 320×240 PV FPA.

The topology 320×256 (320×240) FPA is matrix with pixel pitch 30 μm in X и Y directions. Fig. 33 shows photo view of 320×240 FPA (photosensitive pixels array in left corner).

**4.3. Heterodyne LWIR detector**

described by the expression (1)

The HF PD threshold heterodyne detection power (Pt

where Pd – threshold power at baseband detection;

In Fig. 37 the calculation solid curves (1-3) of Pt

W/Hz1/2; 2 - Pd = 10-12 W/Hz1/2; 3 - Pd = 10-11 W/Hz1/2.

Pg – heterodyne power, ωs – frequency of detected radiation,

ωg – heterodyne frequency, η(ωs- ωg) – quantum efficiency.

*Pt* <sup>=</sup> *Pd* 2 2*Pg* +

ℏ*ω<sup>s</sup>*

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

**Figure 37.** The dependences of Pt on Pg of HF PD for detection 10,6 µm wavelength radiation at η = 0,5: 1 - Pd = 10-13

imum value at essentially smaller Pg for PD's with the minimal Pd values. Experimental Pt values for PD's based on bulk p-type HgCdTe (dashed line) has analogous dependence on Pg. But at Pg> 10-3 W Pt begin increases that connected with changing optimal condition oper‐ ation because of PD heating. So it is necessary to fabricate PD with smaller Pd values for fa‐ vorable operation condition. And it is maybe important for multi-channel system having low heterodyne radiation power. We fabricated HF PD on the basis of p-type MCT HES (see

The dashed line is typical experimental data. It is clear that theoretically Pt

) taking into account real parameters

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165

*<sup>η</sup>*(*ω<sup>s</sup>* <sup>−</sup>*ωg*) (14)

dependences on Pg for different Pd are shown.

values reach min‐

The typical spectral response is present in Fig. 34.

The low dark current is equal to 1.5-2 nA and is constant up to reverse bias voltage 200 mV for best diodes. The product R0 *A*=40 Ohm*×*cm2 for such diode (optical area А=8×10-6 cm2 ) is compatible to the best literature data for *n <sup>+</sup> -p* photodiodes [17].The readout integrated cir‐ cuit 320×256 design is based on a silicon CMOS technology. The charge capacity is very large - more then 20 pC. This multiplexer can operate with LWIR photodiodes for spectral range to 14 μm, that have large dark and background current. Developed multiplexer oper‐ ate with two formats: 320×256 and 320×240 elements. IR FPA was fabricated by hybrid cold welding by indium bumps of photodiode array and silicon ROIC under pressure. The F/1,6 NEDT and thermal images with the help of LWIR 320×240 FPA are shown in Fig 35, 36.

**Figure 35.** NEDT histogram of 320×240 PV FPA.

**Figure 36.** Thermal images 320×240 PV FPA.

#### **4.3. Heterodyne LWIR detector**

The topology 320×256 (320×240) FPA is matrix with pixel pitch 30 μm in X и Y directions.

The low dark current is equal to 1.5-2 nA and is constant up to reverse bias voltage 200 mV

compatible to the best literature data for *n <sup>+</sup> -p* photodiodes [17].The readout integrated cir‐ cuit 320×256 design is based on a silicon CMOS technology. The charge capacity is very large - more then 20 pC. This multiplexer can operate with LWIR photodiodes for spectral range to 14 μm, that have large dark and background current. Developed multiplexer oper‐ ate with two formats: 320×256 and 320×240 elements. IR FPA was fabricated by hybrid cold welding by indium bumps of photodiode array and silicon ROIC under pressure. The F/1,6 NEDT and thermal images with the help of LWIR 320×240 FPA are shown in Fig 35, 36.

for such diode (optical area А=8×10-6 cm2

) is

Fig. 33 shows photo view of 320×240 FPA (photosensitive pixels array in left corner).

The typical spectral response is present in Fig. 34.

164 Photodiodes - From Fundamentals to Applications

for best diodes. The product R0 *A*=40 Ohm*×*cm2

**Figure 35.** NEDT histogram of 320×240 PV FPA.

**Figure 36.** Thermal images 320×240 PV FPA.

The HF PD threshold heterodyne detection power (Pt ) taking into account real parameters described by the expression (1)

$$P\_t = \left[\frac{P\_d^2}{2P\_\mathcal{g}} + \frac{\hbar\omega\_s}{\eta\{\omega\_s - \omega\_\mathcal{g}\}}\right] \tag{14}$$

where Pd – threshold power at baseband detection;

Pg – heterodyne power, ωs – frequency of detected radiation,

ωg – heterodyne frequency, η(ωs- ωg) – quantum efficiency.

In Fig. 37 the calculation solid curves (1-3) of Pt dependences on Pg for different Pd are shown.

**Figure 37.** The dependences of Pt on Pg of HF PD for detection 10,6 µm wavelength radiation at η = 0,5: 1 - Pd = 10-13 W/Hz1/2; 2 - Pd = 10-12 W/Hz1/2; 3 - Pd = 10-11 W/Hz1/2.

The dashed line is typical experimental data. It is clear that theoretically Pt values reach min‐ imum value at essentially smaller Pg for PD's with the minimal Pd values. Experimental Pt values for PD's based on bulk p-type HgCdTe (dashed line) has analogous dependence on Pg. But at Pg> 10-3 W Pt begin increases that connected with changing optimal condition oper‐ ation because of PD heating. So it is necessary to fabricate PD with smaller Pd values for fa‐ vorable operation condition. And it is maybe important for multi-channel system having low heterodyne radiation power. We fabricated HF PD on the basis of p-type MCT HES (see Fig. 15) with thickness of absorber ~ layer 10 μm. The calculations and experimental data were shown that it is necessary to grow wide gap layer between high conductivity narrow gap layer and absorber to reach limiting PD and HF PD parameters [23]. The QE of PD on the basis HgCdTe HS MBE without antireflection coating η≅ 0.65. The Rs< 10 Ω and Ro×A = 100130 Ω cm2. The Sv (λ=15.0 μm) and D\* (λ=15.0 μm) were (57)×105 V W-1 and (68) ×1010 cm Hz1/2 W-1 at FOV=300 and Tbackground 295К.

**4.4. Matrix LWIR 128×128 PV FPA**

MCT HES (see Fig. 24). N+

We fabricated 128×128 PV FPA by planar low temperature technology on the basis of р-P

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

MCT P-layer. The photodiode parameters were as following: λc = 10,6 μm; dark current 0,8 nA at V= −200 mV at 77K. The diodes pitch was 40 μm. The diode size was 17×17 μm. The wavelength cut off was 10,3μm. The diode design and band diagram was shown in Fig. 39.

+

contact

r j

<sup>S</sup> / <sup>N</sup> <sup>=</sup> Is / <sup>I</sup>*<sup>У</sup>* ×(k×Q / <sup>q</sup>)1/2 (15)

heterojunction

layer surface graded layer

Fermi level <sup>E</sup>

implantation and located into

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167


GaAs substrate n

p-type active

(à)

g

where Is – the signal photocurrent, IУ – total current, equal to sum of dark current and back‐ ground current (neglecting the signal current), k – the charging of accumulating capacitance

The relation S/N of FPA based on P-p NCT HES does not depend on barrier height because of decreases at presence of potential barrier at the same manner of photo- and dark currents at diffusion approximation. Nevertheless, in real diode frequently there is existed an excess 1/f noise and dark current caused by generation and tunneling processes inside spacecharge region. In this situation large photocurrent through high-resistance p-absorber to ground bus leads to large differences of voltage biases of FPA photodiodes at the center and periphery. It appears as an additional noise increase. Moreover ROIC added noise according to In = Vn/Rd, where In – noise current, Vn – bias noise voltage, Rd – differential resistance

(b)

The ratio signal to noise (S/N) of hybrid FPA expressed by well-known equation

surface graded layer

**Figure 39.** Photodiode base on p-P MCT HES : (a) design and (b) band diagram.

coefficient, Q – charge capacitance, q – electron charge.

illumination

CdTe buffer layer


The specification of single element HF PD and PC is the following:

#### **Table 2**

HF PD and PC mounted into LN2 cooled Dewar (Fig. 38).

**Figure 38.** The view HF PD into LN2 Dewar.

#### **4.4. Matrix LWIR 128×128 PV FPA**

Fig. 15) with thickness of absorber ~ layer 10 μm. The calculations and experimental data were shown that it is necessary to grow wide gap layer between high conductivity narrow gap layer and absorber to reach limiting PD and HF PD parameters [23]. The QE of PD on the basis HgCdTe HS MBE without antireflection coating η≅ 0.65. The Rs< 10 Ω and Ro×A =

> Element size, µm 250 100×100 λmax, µm 10 15 λ0,1, µm 11 19 Pd (λmax), W/Hz1/2 2×10-13 1×10-13 Pt (λmax), W/Hz 10-19 10-18 operating temperature, K 77-78K frequency range, GHz ≥ 1

**PD PC**

V W-1 and (68) ×1010

100130 Ω cm2. The Sv (λ=15.0 μm) and D\* (λ=15.0 μm) were (57)×105

and Tbackground 295К.

The specification of single element HF PD and PC is the following:

HF PD and PC mounted into LN2 cooled Dewar (Fig. 38).

**Figure 38.** The view HF PD into LN2 Dewar.

cm Hz1/2 W-1 at FOV=300

166 Photodiodes - From Fundamentals to Applications

**Table 2**

We fabricated 128×128 PV FPA by planar low temperature technology on the basis of р-P MCT HES (see Fig. 24). N+ - P junctions were fabricated by B<sup>+</sup> implantation and located into MCT P-layer. The photodiode parameters were as following: λc = 10,6 μm; dark current 0,8 nA at V= −200 mV at 77K. The diodes pitch was 40 μm. The diode size was 17×17 μm. The wavelength cut off was 10,3μm. The diode design and band diagram was shown in Fig. 39.

**Figure 39.** Photodiode base on p-P MCT HES : (a) design and (b) band diagram.

The ratio signal to noise (S/N) of hybrid FPA expressed by well-known equation

$$\mathbf{S}/\mathbf{N} = \mathbf{I}\_s/\mathbf{I}\_\mathbf{y} \times (\mathbf{k} \times \mathbf{Q}/\mathbf{q})^{1/2} \tag{15}$$

where Is – the signal photocurrent, IУ – total current, equal to sum of dark current and back‐ ground current (neglecting the signal current), k – the charging of accumulating capacitance coefficient, Q – charge capacitance, q – electron charge.

The relation S/N of FPA based on P-p NCT HES does not depend on barrier height because of decreases at presence of potential barrier at the same manner of photo- and dark currents at diffusion approximation. Nevertheless, in real diode frequently there is existed an excess 1/f noise and dark current caused by generation and tunneling processes inside spacecharge region. In this situation large photocurrent through high-resistance p-absorber to ground bus leads to large differences of voltage biases of FPA photodiodes at the center and periphery. It appears as an additional noise increase. Moreover ROIC added noise according to In = Vn/Rd, where In – noise current, Vn – bias noise voltage, Rd – differential resistance which may be essential values at small Rd. So, in real FPA values S/N is usually lower than calculated one by formula (1). This difference is increase with the increase total diode cur‐ rent. The potential barrier at P-p MCT HES leads to decrease of total diode current of FPA and eliminates negative phenomena described previously. We showed from measurement of noise spectrum that frequency cut off of 1/f noise for photodiode on the basis of P-p MCT HES is equal up to less than 10 Hz. These 1/f values are essentially lower than for FPA based on MCT epitaxial structure without P-p heterojunction. It means that S/N of FPA on the ba‐ sis P-p heterojunction will approach to values given by equation 16.

**•** quantum efficiency value was taken to be 0.7;

without P-p barrier with λc in range 10,5-10,7 мm at T=77 K.

of wavelength cut off at constant operating temperature.

**•** the accumulation time was chosen from the above condition of charging of accumulation

LWIR Photodiodes and Focal Plane Arrays Based on Novel HgCdTe/CdZnTe/GaAs Heterostructures Grown by MBE

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169

In the same Figure measured S/N values are given for few FPA on the basis of MCT layer

The experimental S/N values of FPA on the basis of P-p DLHJ with barrier more closely correspond to calculated dependence in comparison with FPA on the basis of MCT layer without P-p barrier. This can be explained as follows. At temperatures near 77 K lower S/N ratio of FPA on the basis of MCT layer without P-p barrier is associated to excess currents (generation and tunneling current) and to presence of 1/f noise. When the tem‐ perature increases in range 77 -130K the diodes differential resistance of FPA diodes de‐ creases and total current increases. The first leads to increase of ROIC noise and diodes noise. The second leads to additional noise determined by strong mutual coupling of pho‐ todiodes. All this factors decrease the S/N ratio faster than calculated one. It means that FPA for operating at elevated temperatures must be fabricated on the basis of P-p DLHJ with barrier. The behavior of S/N will analogous ones which given in Fig.4 for at increase

The technology of fabrication of mercury cadmium telluride (MCT) heterostructure (HES) at growth by molecular beam epitaxy (MBE) was developed. The MBE ultra vacuum set allows to grow high quality n-type MCT HES with monitoring in real time. Thermal treatments are used for fabrication high quality p-type MCT HES for LWIR photovoltaic (PV) devices.

We suggested different MCT HES design with graded widegap layers, high conductivity

We demonstrated the four cooled LWIR detectors for spectral range 8-11 μm based on

**•** linear 288×4 PV FPA on the basis MCT HES with graded widegap layers on the bounda‐

**•** matrix 320×256(240) PV FPA on the basis MCT HES with high conductivity layer (grow‐

**•** matrix 128×128 PV FPA on the basis MCT HES with p-P absorber layer which successfully

**•** one elements heterodyne detector on the basis MCT HES with high conductivity layer

**•** 1/f noise was zero;

capacitance.

**5. Conclusion**

layer and p-P structures.

ries of absorber layer;

ing by doping) to eliminate "debiasing" effect;

(growing narrowgap layer) operating at GHz frequencies.

The parameters of these devices are limited by background radiation.

operated at elevated temperature;

these MCT HES:

128×128 hybrid FPA was package inside cooled cryostat with ZnSe window. We meas‐ ured the FPA parameters in range 77 – 300K, at FOV 450 , black body temperatures 300 - 500К and background temperature 295K. The temperature at measurement maintained with accuracy 0,5К. We measured the black body's diode signal and noise. The signal storage time satisfied by k = 0,8. The S/N experimental (squares) data of FPA on the basis of P-p MCT HES are given in Fig. 40.

**Figure 40.** The ratio of S/N on temperature. Squares – FPA on p-P MCT HES. Triangles – FPA on p-type MCT HES. Solid curve – calculation.

In the same Figure the calculated S/N dependence is shown for analogous FPA without barrier at the same measurements regime. The S/N calculation was carried out in follow‐ ing approximations:


which may be essential values at small Rd. So, in real FPA values S/N is usually lower than calculated one by formula (1). This difference is increase with the increase total diode cur‐ rent. The potential barrier at P-p MCT HES leads to decrease of total diode current of FPA and eliminates negative phenomena described previously. We showed from measurement of noise spectrum that frequency cut off of 1/f noise for photodiode on the basis of P-p MCT HES is equal up to less than 10 Hz. These 1/f values are essentially lower than for FPA based on MCT epitaxial structure without P-p heterojunction. It means that S/N of FPA on the ba‐

128×128 hybrid FPA was package inside cooled cryostat with ZnSe window. We meas‐

500К and background temperature 295K. The temperature at measurement maintained with accuracy 0,5К. We measured the black body's diode signal and noise. The signal storage time satisfied by k = 0,8. The S/N experimental (squares) data of FPA on the basis

70 80 90 100 110 120 130

T,K

**Figure 40.** The ratio of S/N on temperature. Squares – FPA on p-P MCT HES. Triangles – FPA on p-type MCT HES. Solid

In the same Figure the calculated S/N dependence is shown for analogous FPA without barrier at the same measurements regime. The S/N calculation was carried out in follow‐

**•** the carrier collection area at fixed temperature was determined by geometric size of n-re‐

, black body temperatures 300 -

sis P-p heterojunction will approach to values given by equation 16.




ured the FPA parameters in range 77 – 300K, at FOV 450

of P-p MCT HES are given in Fig. 40.

168 Photodiodes - From Fundamentals to Applications

0,01

**•** the dark current was calculated by diffusion model;

**•** the velocity of surface recombination on boundaries was zero;

gion and carrier diffusion length but was no more than pixel size;

0,1

S/N, arb. un.

curve – calculation.

ing approximations:

1

**•** the accumulation time was chosen from the above condition of charging of accumulation capacitance.

In the same Figure measured S/N values are given for few FPA on the basis of MCT layer without P-p barrier with λc in range 10,5-10,7 мm at T=77 K.

The experimental S/N values of FPA on the basis of P-p DLHJ with barrier more closely correspond to calculated dependence in comparison with FPA on the basis of MCT layer without P-p barrier. This can be explained as follows. At temperatures near 77 K lower S/N ratio of FPA on the basis of MCT layer without P-p barrier is associated to excess currents (generation and tunneling current) and to presence of 1/f noise. When the tem‐ perature increases in range 77 -130K the diodes differential resistance of FPA diodes de‐ creases and total current increases. The first leads to increase of ROIC noise and diodes noise. The second leads to additional noise determined by strong mutual coupling of pho‐ todiodes. All this factors decrease the S/N ratio faster than calculated one. It means that FPA for operating at elevated temperatures must be fabricated on the basis of P-p DLHJ with barrier. The behavior of S/N will analogous ones which given in Fig.4 for at increase of wavelength cut off at constant operating temperature.

#### **5. Conclusion**

The technology of fabrication of mercury cadmium telluride (MCT) heterostructure (HES) at growth by molecular beam epitaxy (MBE) was developed. The MBE ultra vacuum set allows to grow high quality n-type MCT HES with monitoring in real time. Thermal treatments are used for fabrication high quality p-type MCT HES for LWIR photovoltaic (PV) devices.

We suggested different MCT HES design with graded widegap layers, high conductivity layer and p-P structures.

We demonstrated the four cooled LWIR detectors for spectral range 8-11 μm based on these MCT HES:


The parameters of these devices are limited by background radiation.
