**4. IR Material growth Techniques for HgCdTe**

Hg1-xCdxTe (MCT) is the most widely used infrared (IR) detector material in military appli‐ cations, compared to other IR detector materials, primarily because of two key features: it is a direct energy band gap semiconductor and its band gap can be engineered by varying the Cd composition to cover a broad range of wavelengths. The direct band gap of MCT allows for a high absorption of IR radiation, yielding high quantum efficiency in a relatively thin detector structure. As the Cd mole fraction, x, increases, the energy gap for MCT increases linearly from a semimetal (HgTe) to a wide band gap semiconductor (CdTe).

The ability to tune the band gap of MCT enables IR detectors to operate in the wavelength bands ranging from SWIR to VLWIR (0.7-30 microns). For low-cost high-performance detec‐ tors, the MCT material must be produced on large diameter wafers with low defect densities and reproducible stoichiometric properties. These requirements are satisfied by a host of crystal growth techniques ranging from high temperature, melt grown bulk crystals, to low temperature, multilayer epitaxial layers.

Depending on the detector architecture, the crystal growth strategy could utilize any of the following techniques: Bulk Crystal Growth, Liquid Phase Epitaxy (LPE), Metal-organic Chemical Vapor Deposition (MOCVD), and Molecular Beam Epitaxy (MBE). The sections below will highlight each of these growth techniques with references to publications that will provide additional coverage.

### **4.1. Bulk Crystal Growth**

**Figure 2.** Detectivity curves for various commercially available photon and thermal IR detectors. Calculated detectivi‐

Another frequently quoted figure of merit for a photodiode is its R0A product, where R0 is the dynamic resistance of the photodiode and is equal to the slope of the I-V curve at the zero bias voltage point. This FOM is independent of the junction area, except when the di‐

Thermal detectors require a temperature change to produce a signal and do not generally need cooling, in contrast to photo detectors which are cooled to minimize noise. Absorbed radiation causes a temperature change that alters a temperature sensitive property of the de‐ tector which can be measured externally. A few examples include: electrical resistance in a bolometer, thermal expansion of Golay cells, and polarization in pyroelectric materials. Since these detectors depend on temperature changes resulting from incident radiation, they must be thermally isolated from their surroundings and have low thermal capacities for fast response to the radiation. In the case of a bolometer, the FOM is its thermal time constant

where *Cth* is the thermal capacity of the detector, *Rth* is the thermal resistance and *Gth* is the thermal coupling of the detector to its surroundings. The interaction of the bolometer with

*Gth* <sup>=</sup> *Cth Rth* (6)

mensions are comparable to the minority carrier diffusion length.

*<sup>τ</sup>th* <sup>=</sup> *Cth*

ties are indicated by dashed lines [7].

154 Optoelectronics - Advanced Materials and Devices

which is defined as:

Bulk crystal growth of MCT continues to play an important role in producing IR detector materials for photoconductive arrays, despite the progress made with various epitaxial thin film deposition techniques. Bulk growth process is typically used for large area single detec‐ tors for applications such as spectrometry. However, for photovoltaic arrays there are chal‐ lenges associated with crystal grain boundaries, which are electrically active and contribute to line defects. Also there are limitations in the ingot diameter, which makes bulk growth suitable for only quad or single detector arrays. Several methods have been developed for growing MCT bulk crystals: Solid State Recrystallization (SSR), Traveling Heater Method (THM), Bridgman, Czochralski, Slush Growth, and Zone Melting [9-14]. This section will cover SSR and THM techniques.

The general challenge with melt grown MCT is to maintain a relatively high Hg vapor pres‐ sure during growth; otherwise, it is difficult to control the stoichiometry of the grown crys‐ tal. Also, the large separation between the liquidus and solidus compositions (see Figure 3) across a constant thermal tie line can result in a steady variation in the composition of a moving growth interface.

convection and diffusion, the solid material is dissolved at the high temperature interface

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Crystal growth occurs in the 500-700 °C range, lower than the temperature used for Solid State Recrystallization growth method. The lower growth temperature used in THM re‐ duces the incidence of antisite defects, resulting in crystals with more reproducible composi‐ tion and higher resistivity. Also, the lower temperatures reduce contamination from the crucible walls and decrease the evaporation of the constituent species. One successful imple‐ mentation of THM resulted in crystals up to 5 cm in diameter [15]. The perfect quality of

LPE growth method offers,in comparison with bulk growth techniques, lower growth temperatures, shorter growth times, multilayered device structures, and better composi‐ tional homogeneity over large substrate areas. The versatility of LPE as a production tool for high performance device quality MCT epitaxial layers, with different Cd mole frac‐ tions and excellent compositional uniformity, is discussed in [16-20]. Today, detector ar‐ rays prepared from LPE based materials exhibit best performance, and majority of

LPE is a solution growth technique that involves the controlled precipitation of a solute dis‐ solved in a solvent onto a single crystal substrate. For LPE growth of MCT, bulk grown CdZnTe single crystal substrates are suitable, since they are thermodynamically compatible

Both Te-solution growth (420–500 °C) and Hg-solution growth (360–500 °C) are used with equal success in a variety of configurations. The design of the Te-rich LPE system can be configured to allow the melt to contact the substrate by either sliding, tipping or dipping techniques. A sliding boat system uses a small melt volume and is adaptable for changing composition, thickness and doping. Tipping and dipping systems can be scaled up for large melts to provide thick, uniform layers. Both the tipping and dipping designs are being used for Te and Hg-rich solutions, while only the sliding technique is used for Te-rich solutions. The major difference between the Hg and Te-rich solvents is that in the former case, the vapor pressure of Hg over the melt is much higher than in the latter case. The Hg partial pressure curves in Figure 4 indicate that at 500 °C and a Cd mole fraction of 0.1, the Hg partial pressure over Te-saturated MCT is 0.1 atm, while that of Hg-saturated MCT is 7 atm [23]. Te-rich solu‐ tions saturated with Hg vapor allow for small volume melts that do not appreciably deplete

This is because the solubility of Cd in Te is high. On the other hand, the limited solubility of Cd in Hg requires the volume of Hg-rich melts to be much larger than Te melts, in order to mini‐ mize melt depletion during growth in the 360–500 °C temperature range. Unfortunately, the larger melt volume in Hg-rich LPE precludes the use of the slider boat approach and makes an open tube growth impossible [24]. For these reasons, it would not be surprising if many more manufacturers are pursuing LPEgrowth from Te-rich melts rather than from Hg-rich melts.

crystals grown by this method is achieved at the cost of a low growth rate [11].

and nearly lattice matched. The solvent can either be Hg [21] or Te-rich [22].

during growth in the temperature range 420-500 °C using the slider technique.

and deposited at the low temperature interface of the zone.

*4.1.3. Liquid Phase Epitaxy (LPE)*

military IR applications use this technology.

**Figure 3.** *T* - *x* phase diagram for the pseudo-binary CdTe-HgTe [14].

#### *4.1.1. Solid State Recrystallization*

The SSR technique is used to alleviate the compositional variation at the growth interface in ternary systems, such as MCT, where the solidus and liquidus lines are widely separat‐ ed. In the basic technique, the three high purity elements of MCT are cleaned and loaded into a thick walled, small diameter quartz ampoule that is evacuated, sealed and placed into a furnace. The ampoule is heated to approximately 950 °C and the melt is mixed by rocking the furnace. The MCT ampoule is removed from the furnace and rapidly quenched to produce a uniform composition.

The rapid quenching produces a dendritic structure in the MCT that is reduced by an ex‐ tended (several days) recrystallization step, at temperatures just below the melting point. Grain growth occurs during the re-crystallization step and remaining compositional inho‐ mogeneities are removed. On the down side, the high melt temperatures and very high Hg vapor pressures used in SSR to produce MCT can cause the ampoule to explode. Also, very long annealing times are required and the resulting crystals are small. The typical diameter of the ingot is limited to about 2.0 cm in order to control impurity segregation in the crystal.

#### *4.1.2. Traveling Heater Method*

In the THM method, a solvent zone is created between a solid seed and the feed stock mate‐ rial. In the case of MCT, the crystal is grown by passing the solvent zone (e.g. Te rich) through a polycrystalline MCT rod having a composition that is to be replicated in grown crystal. The motion of the molten interface is produced by the slow movement of the heater along the charged crucible. Crystallization takes place at the advancing seed-solvent inter‐ face and dissolution of feed material occurs at the solvent-feed phase boundary. Through convection and diffusion, the solid material is dissolved at the high temperature interface and deposited at the low temperature interface of the zone.

Crystal growth occurs in the 500-700 °C range, lower than the temperature used for Solid State Recrystallization growth method. The lower growth temperature used in THM re‐ duces the incidence of antisite defects, resulting in crystals with more reproducible composi‐ tion and higher resistivity. Also, the lower temperatures reduce contamination from the crucible walls and decrease the evaporation of the constituent species. One successful imple‐ mentation of THM resulted in crystals up to 5 cm in diameter [15]. The perfect quality of crystals grown by this method is achieved at the cost of a low growth rate [11].

#### *4.1.3. Liquid Phase Epitaxy (LPE)*

**Figure 3.** *T* - *x* phase diagram for the pseudo-binary CdTe-HgTe [14].

quenched to produce a uniform composition.

The SSR technique is used to alleviate the compositional variation at the growth interface in ternary systems, such as MCT, where the solidus and liquidus lines are widely separat‐ ed. In the basic technique, the three high purity elements of MCT are cleaned and loaded into a thick walled, small diameter quartz ampoule that is evacuated, sealed and placed into a furnace. The ampoule is heated to approximately 950 °C and the melt is mixed by rocking the furnace. The MCT ampoule is removed from the furnace and rapidly

The rapid quenching produces a dendritic structure in the MCT that is reduced by an ex‐ tended (several days) recrystallization step, at temperatures just below the melting point. Grain growth occurs during the re-crystallization step and remaining compositional inho‐ mogeneities are removed. On the down side, the high melt temperatures and very high Hg vapor pressures used in SSR to produce MCT can cause the ampoule to explode. Also, very long annealing times are required and the resulting crystals are small. The typical diameter of the ingot is limited to about 2.0 cm in order to control impurity segregation in the crystal.

In the THM method, a solvent zone is created between a solid seed and the feed stock mate‐ rial. In the case of MCT, the crystal is grown by passing the solvent zone (e.g. Te rich) through a polycrystalline MCT rod having a composition that is to be replicated in grown crystal. The motion of the molten interface is produced by the slow movement of the heater along the charged crucible. Crystallization takes place at the advancing seed-solvent inter‐ face and dissolution of feed material occurs at the solvent-feed phase boundary. Through

*4.1.1. Solid State Recrystallization*

156 Optoelectronics - Advanced Materials and Devices

*4.1.2. Traveling Heater Method*

LPE growth method offers,in comparison with bulk growth techniques, lower growth temperatures, shorter growth times, multilayered device structures, and better composi‐ tional homogeneity over large substrate areas. The versatility of LPE as a production tool for high performance device quality MCT epitaxial layers, with different Cd mole frac‐ tions and excellent compositional uniformity, is discussed in [16-20]. Today, detector ar‐ rays prepared from LPE based materials exhibit best performance, and majority of military IR applications use this technology.

LPE is a solution growth technique that involves the controlled precipitation of a solute dis‐ solved in a solvent onto a single crystal substrate. For LPE growth of MCT, bulk grown CdZnTe single crystal substrates are suitable, since they are thermodynamically compatible and nearly lattice matched. The solvent can either be Hg [21] or Te-rich [22].

Both Te-solution growth (420–500 °C) and Hg-solution growth (360–500 °C) are used with equal success in a variety of configurations. The design of the Te-rich LPE system can be configured to allow the melt to contact the substrate by either sliding, tipping or dipping techniques. A sliding boat system uses a small melt volume and is adaptable for changing composition, thickness and doping. Tipping and dipping systems can be scaled up for large melts to provide thick, uniform layers. Both the tipping and dipping designs are being used for Te and Hg-rich solutions, while only the sliding technique is used for Te-rich solutions.

The major difference between the Hg and Te-rich solvents is that in the former case, the vapor pressure of Hg over the melt is much higher than in the latter case. The Hg partial pressure curves in Figure 4 indicate that at 500 °C and a Cd mole fraction of 0.1, the Hg partial pressure over Te-saturated MCT is 0.1 atm, while that of Hg-saturated MCT is 7 atm [23]. Te-rich solu‐ tions saturated with Hg vapor allow for small volume melts that do not appreciably deplete during growth in the temperature range 420-500 °C using the slider technique.

This is because the solubility of Cd in Te is high. On the other hand, the limited solubility of Cd in Hg requires the volume of Hg-rich melts to be much larger than Te melts, in order to mini‐ mize melt depletion during growth in the 360–500 °C temperature range. Unfortunately, the larger melt volume in Hg-rich LPE precludes the use of the slider boat approach and makes an open tube growth impossible [24]. For these reasons, it would not be surprising if many more manufacturers are pursuing LPEgrowth from Te-rich melts rather than from Hg-rich melts.

elements with high solubilities [20], but layers grown from Te-rich solutions are not [26]. Group VB dopants have low solubility and are not fully active electrically. Group IIIB ele‐ ments, indium in particular, are easily incorporated from both solutions. Indium doping from Te-rich melts, however, has the advantage that the segregation coefficient is near unity.

From a device perspective, its performance is dependent on the electrical, optical and me‐ chanical characteristics of the epitaxial layers. For IR FPA, the dislocation count controls the number of defective pixels. Typical etch pit (chemical etched decoration of dislocations) densities for LPE layers grown from a Te-rich melt onto a CdZnTe substrate are in the

mal to the epilayer surface. Compositional uniformity of x=0.223 +/- 0.001 has been demon‐

In general, MOCVD growth of MCT depends on transporting the elements Cd and Te (and dopants In, I and As) at room temperature as volatile organometallics. These species react with Hg vapor in the hot gas stream above the substrate or catalytically on the substrate surface at about 400 °C. The pyrolytic nature of the reaction requires that only the substrate be heated to ensure efficient deposition. In practice, the growth of MCT by MOCVD using dimethyl cadmi‐ um (DMCd) and diethyl telluride (DETe) is accomplished by two processes: a) CdTe synthesis from DMCd and DETe, and b) formation of HgTe from DETe and Hg at the heated substrate.

The challenge with this growth technique is to control the composition of the epitaxial layer and achieve uniformity over large surface areas. MOCVD-MCT composition is influenced by substrate temperature, DMCd and Hg partial pressures. Compositional control and layer uniformity are addressed using the inter-diffused multilayer process (IMP) technique in which very thin layers (0.1-0.2 μm) of HgTe and CdTe are deposited sequentially. These lay‐ ers, with high diffusion coefficients, inter-diffuse during growth at about 400 °C to form a homogeneous ternary epilayer with a composition that is controlled by the thickness ratio of

The preferred precursor for Te is di-isopropyl telluride (DiPTe) that allows for a reduction in the MCT growth temperature from 400 °C to 350 °C. DiPTe in conjunction with DMCd can allow the deposition CdTe to occur at lower temperatures (300 °C). Doping for MOCVD-MCT layers is straightforward using Group III metals for P-type doping and Group VII hal‐ ogens for N-type doping. The main morphological problem for MOCVD are macro defects called hillocks, which are caused by preferred (111) growth, nucleated from a particle or pol‐ ishing defect. Hillocks can cause clusters of defects in focal plane arrays. Orientations 3-4 off

The MOCVD technique is used to manufacture high-quality, large-area infrared focal plane ar‐ rays for many applications [30]. Consequently, there is a renewed interest in using MOCVD because of its ability to: achieve low surface defect densities, deposit MCT films on large area-

(100) are used primarily to reduce both the size and density of hillocks.

reproducibility in composition of x=0.226 +/- 0.0033 has been demonstrated [28].

range [27]. These defects are associated with threading dislocations that are nor‐

. For a series of 200 growth runs, the run-to-run

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3-7E4/cm2

HgTe:CdTe layers [29].

strated over areas ranging from 43-54 cm2

*4.1.4. Metal-Organic Chemical Vapor Deposition (MOCVD)*

**Figure 4.** Partial pressure of Hg along the three-phase curves for various MCT solid solutions [23].

Despite the lower Hg vapor pressure in a Te-rich melt, the partial pressure of Hg must still be controlled in the growth system in order to obtain compositional uniformity, reproduci‐ bility, and stability. One way of controlling the Hg partial pressure under Te-rich conditions is to carry out the entire LPE growth process in a sealed ampoule. The disadvantages of this approach are low production levels because of the necessity of sealing the ampoule for each growth run and difficulty for in-situ preparation of multilayer growth that is necessary for advanced IR device structures.

In an open tube LPE-MCT, Te-rich system, the Hg partial pressure can be controlled by [25]: a) implementing an external Hg source to replenish the depleted Hg from the growth cham‐ ber, b) using chunks of HgTe near the melt as a solid source for Hg vapor, or c) using a high inert gas overpressure to minimize Hg loss.

As discussed above, the growth of MCT from Hg-rich melts is not as popular as growth from Te-rich solutions because of the low solubility of Cd and Te in Hg below 600 °C, and the high vapor pressure of Hg. On the other hand, LPE growth from a Hg-rich melt offers the following advantages: excellent surface morphology; high purity source material; good control over N- and P-type doping levels; very good compositional and thickness uniformi‐ ty over large surface areas; and no need for post-growth anneals.

LPE layers grown from Te-rich melts are P-type due to the Hg vacancies induced during the growth process. These unintentionally doped layers can be converted to N-type by appro‐ priate annealing schedules in Hg vapor. Layers grown from Hg-rich melts are usually Ntype. LPE layers grown from Hg-rich solutions are intentionally doped with group VB elements with high solubilities [20], but layers grown from Te-rich solutions are not [26]. Group VB dopants have low solubility and are not fully active electrically. Group IIIB ele‐ ments, indium in particular, are easily incorporated from both solutions. Indium doping from Te-rich melts, however, has the advantage that the segregation coefficient is near unity.

From a device perspective, its performance is dependent on the electrical, optical and me‐ chanical characteristics of the epitaxial layers. For IR FPA, the dislocation count controls the number of defective pixels. Typical etch pit (chemical etched decoration of dislocations) densities for LPE layers grown from a Te-rich melt onto a CdZnTe substrate are in the 3-7E4/cm2 range [27]. These defects are associated with threading dislocations that are nor‐ mal to the epilayer surface. Compositional uniformity of x=0.223 +/- 0.001 has been demon‐ strated over areas ranging from 43-54 cm2 . For a series of 200 growth runs, the run-to-run reproducibility in composition of x=0.226 +/- 0.0033 has been demonstrated [28].

### *4.1.4. Metal-Organic Chemical Vapor Deposition (MOCVD)*

**Figure 4.** Partial pressure of Hg along the three-phase curves for various MCT solid solutions [23].

advanced IR device structures.

158 Optoelectronics - Advanced Materials and Devices

inert gas overpressure to minimize Hg loss.

ty over large surface areas; and no need for post-growth anneals.

Despite the lower Hg vapor pressure in a Te-rich melt, the partial pressure of Hg must still be controlled in the growth system in order to obtain compositional uniformity, reproduci‐ bility, and stability. One way of controlling the Hg partial pressure under Te-rich conditions is to carry out the entire LPE growth process in a sealed ampoule. The disadvantages of this approach are low production levels because of the necessity of sealing the ampoule for each growth run and difficulty for in-situ preparation of multilayer growth that is necessary for

In an open tube LPE-MCT, Te-rich system, the Hg partial pressure can be controlled by [25]: a) implementing an external Hg source to replenish the depleted Hg from the growth cham‐ ber, b) using chunks of HgTe near the melt as a solid source for Hg vapor, or c) using a high

As discussed above, the growth of MCT from Hg-rich melts is not as popular as growth from Te-rich solutions because of the low solubility of Cd and Te in Hg below 600 °C, and the high vapor pressure of Hg. On the other hand, LPE growth from a Hg-rich melt offers the following advantages: excellent surface morphology; high purity source material; good control over N- and P-type doping levels; very good compositional and thickness uniformi‐

LPE layers grown from Te-rich melts are P-type due to the Hg vacancies induced during the growth process. These unintentionally doped layers can be converted to N-type by appro‐ priate annealing schedules in Hg vapor. Layers grown from Hg-rich melts are usually Ntype. LPE layers grown from Hg-rich solutions are intentionally doped with group VB In general, MOCVD growth of MCT depends on transporting the elements Cd and Te (and dopants In, I and As) at room temperature as volatile organometallics. These species react with Hg vapor in the hot gas stream above the substrate or catalytically on the substrate surface at about 400 °C. The pyrolytic nature of the reaction requires that only the substrate be heated to ensure efficient deposition. In practice, the growth of MCT by MOCVD using dimethyl cadmi‐ um (DMCd) and diethyl telluride (DETe) is accomplished by two processes: a) CdTe synthesis from DMCd and DETe, and b) formation of HgTe from DETe and Hg at the heated substrate.

The challenge with this growth technique is to control the composition of the epitaxial layer and achieve uniformity over large surface areas. MOCVD-MCT composition is influenced by substrate temperature, DMCd and Hg partial pressures. Compositional control and layer uniformity are addressed using the inter-diffused multilayer process (IMP) technique in which very thin layers (0.1-0.2 μm) of HgTe and CdTe are deposited sequentially. These lay‐ ers, with high diffusion coefficients, inter-diffuse during growth at about 400 °C to form a homogeneous ternary epilayer with a composition that is controlled by the thickness ratio of HgTe:CdTe layers [29].

The preferred precursor for Te is di-isopropyl telluride (DiPTe) that allows for a reduction in the MCT growth temperature from 400 °C to 350 °C. DiPTe in conjunction with DMCd can allow the deposition CdTe to occur at lower temperatures (300 °C). Doping for MOCVD-MCT layers is straightforward using Group III metals for P-type doping and Group VII hal‐ ogens for N-type doping. The main morphological problem for MOCVD are macro defects called hillocks, which are caused by preferred (111) growth, nucleated from a particle or pol‐ ishing defect. Hillocks can cause clusters of defects in focal plane arrays. Orientations 3-4 off (100) are used primarily to reduce both the size and density of hillocks.

The MOCVD technique is used to manufacture high-quality, large-area infrared focal plane ar‐ rays for many applications [30]. Consequently, there is a renewed interest in using MOCVD because of its ability to: achieve low surface defect densities, deposit MCT films on large arealow cost substrates such as GaAs and control N- and P-type doping levels. In comparison to an MBE system, the overall maintenance and operational costs of an MOCVD system is lower.

ers grown on bulk CZT substrates exhibit characteristics comparable to those prepared by

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The main challenge of MBE-MCT technology is to grow very high quality layers on lowcost, large- area substrates. The issues that complicate MBE growth on alternative large-area substrates are: lattice mismatch, nucleation phenomena, thermal mismatch, and contamina‐ tion [36-37]. Sapphire, Si, and GaAs are some of the low-cost, large-area materials that have been successfully employed as substrates for MCT epitaxial growth [38-41]. However, ap‐ propriate buffer layers of CdTe or CZT are required on the alternative substrates before

The best MBE-MCT layers grown on buffer/Si substrates achieved thus far exhibit defect densities of 2-5x106 cm−2. Novel thermal cycle annealing schedules have been used to fur‐ ther reduce the defect density.More effort is necessary to reduce this defect density by at least an order of magnitude to make MBE based materials for many military applications. The ability to grow MCT on large diameter Si wafers will enable low cost, large format

The SWIR band (0.9-2.5 um) bridges the spectral gap between the visible and thermal bands in the electromagnetic spectrum. In this spectral band, the primary phenomenology of inter‐ est is the reflectance signature of the target, manifested as either its variations in brightness

Infrared imaging in the SWIR band offers several advantages: can detect reflected light, offer‐ ing more intuitive, visible-like images; better suited for imaging in adverse environments and weather conditions, including fog, dust, and smoke; can also see in low light conditions, and use eye safe 1550 nm illumination that is totally undetectable by regular night vision equip‐ ment; and can generate digital video outputs and thus offering more advantages than tradi‐ tional image intensifier night vision equipment. Under low light conditions, the sensitivity of

For SWIR imaging, InGaAs is one of the widely used detector materials due to its low dark current. The detector material can be prepared using any of the following techniques: Mo‐ lecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), liquid phase epitaxy (LPE), hydride-transport vapor phase epitaxy (VPE), and atomic layer epitaxy (ALE). InGaAs layers are typically grown on lattice matched InP substrates using an alloy

The spectral response typically covers 0.9-1.7μm at room temperature. By increasing the composition to x=0.82, InGaAs is able to extend its cutoff to 2.6 μm. However, the crystal

the focal plane array is ultimately determined by the R0 A product of the photodiode.

LPE. MBE technology is now routinely used for multicolor detector arrays.

MCT films are deposited.

infrared focal plane arrays.

or spectral reflectance, or both.

composition of x = 0.53.

**5. SWIR Detector Technologies**

**5.1. Inx Ga1-x As Detector Array Development**

#### *4.1.5. Molecular Beam Epitaxy (MBE)*

Thin film deposition by MBE enables the growth of large area epilayers with sophisticated multilayer structures having abrupt and complex compositions and doping profiles. Growth of MBE-MCT is carried out under an ultra-high vacuum environment with Knudsen-type ef‐ fusion source cells charged with Hg, Te2, and CdTe [31-32]. MBE-MCT deposition tempera‐ ture plays a critical role in the introduction of extended defects. Typically, growth is carried out at 180 °C–190 °C on (211) CdZnTe substrates.

The low growth temperature and the ability to rapidly shutter the sources are key features that allow MBE to produce sharp interfaces for multilayered IR devices that operate in two or three different spectral bands. The ultra-high vacuum growth chamber allows for in-situ analytical tools to monitor and control the MCT growth process and evaluate the properties of the grown layers [33-34].

At the lower temperature range, a Hg-rich condition prevails at the substrate because the sticking coefficient of Hg increases as the temperature is reduced. The condition with excess Hg results in the formation micro-twins that are detrimental to the performance of the MCT IR focal plane array. Typical etch pit densities (EPD) of material grown under such Hg-rich conditions are high (106 –107 cm–2). If the growth temperature is raised to about 190 °C, then a deficiency of Hg leads to the formation of voids in the MCT layer.

Hg is incorporated in the film only by reacting with free Te, thus the MCT composition is contingent on the Te to CdTe flux ratio. The structural perfection of the film depends strong‐ ly on the Hg to Te flux ratio and growth is usually restricted to a tight temperature range. By optimizing the Hg to Te flux ratio, the concentration of voids is about 100 cm–2 which may be attributable to dust particles or substrate related imperfections. The EPD values for epilayers grown under these conditions are in the low 105 cm–2 ranges.

Indium is the most widely used N-type extrinsic dopant in MCT epitaxial layers and is well activated. At low Indium doping levels, Hg vacancies can compensate some of the N-type impurities and affect dopant control. P-type dopants, such as Arsenic, are less conveniently incorporated into the epilayer. Significant efforts are being expended to improve the incor‐ poration of As and Sb during the MBE process and to reduce the temperature required for activation. The metal saturation conditions cannot be reached at the temperatures required for high-quality MBE growth. The necessity to activate acceptor dopants at high tempera‐ tures diminishes the gains of low-temperature deposition. Nearly 100% activation has been achieved for a 2 × 10E18 cm−3 As concentration, with as low as 300 °C activation anneal, fol‐ lowed by a 250 °C stoichiometric anneal [35].

Because of its various advantages, MBE-MCT technology is becoming more attractive than the other epitaxial technologies and is required for the fabrication of IR detectors with ad‐ vanced architectures. The MBE-MCT technology has developed to the point where MBE lay‐ ers grown on bulk CZT substrates exhibit characteristics comparable to those prepared by LPE. MBE technology is now routinely used for multicolor detector arrays.

The main challenge of MBE-MCT technology is to grow very high quality layers on lowcost, large- area substrates. The issues that complicate MBE growth on alternative large-area substrates are: lattice mismatch, nucleation phenomena, thermal mismatch, and contamina‐ tion [36-37]. Sapphire, Si, and GaAs are some of the low-cost, large-area materials that have been successfully employed as substrates for MCT epitaxial growth [38-41]. However, ap‐ propriate buffer layers of CdTe or CZT are required on the alternative substrates before MCT films are deposited.

The best MBE-MCT layers grown on buffer/Si substrates achieved thus far exhibit defect densities of 2-5x106 cm−2. Novel thermal cycle annealing schedules have been used to fur‐ ther reduce the defect density.More effort is necessary to reduce this defect density by at least an order of magnitude to make MBE based materials for many military applications. The ability to grow MCT on large diameter Si wafers will enable low cost, large format infrared focal plane arrays.
