**2.2 Technology for mechanical machining of fianite crystals**

Fianite lend itself to a machining considerably readily, similarly to sapphire crystals. Dislocation densities in ZrO2 – (8-20) mol% Y2O3 crystals have been measured:

in central parts – 103 сm-2 in periphery – 105сm-2

Followed by annealing (21000С, vacuum) dislocation density decreased to 102 -103сm-2.

*Pre-epitaxial treatment of surface of the substrates.* With the purpose to guarantee optimal physical-chemical state of fianite substrates various techniques and conditions of preepitaxial treatment have been studied. Treatment at 1000-1400С temperatures in air during 1-4 h was used as one of such techniques. The high-temperature annealing provides a relief of stress occurred in the surface layer at mechanical treatment, removal of impurities from the surface and increasing of phase and structural perfection.

The effect of high-temperature annealing on surface quality of the substrates has been studied. In Fig.3a scratches occurred in course of polishing of the substrate by ACM 1/0 diamond paste are apparently observable. The following annealing (12500 С in air) did not result in smoothing of the relief as a whole but caused re-structuring of the surface layer (Fig. 3b) and flatten the relief in micro-locations at scratch residues and results in 2-3-fold decrease of high bump- valley drops (Fig. 3, right side)

Fig. 3. AFM image of fianite substrate (left) and surface profile (right) after chemicalmechanical polishing (a) and subsequent high temperature annealing (b).

The studies of the effect of thermal treatment of the substrates on roughness of polished surface have shown that high-temperature annealing (1250-14000 С) conducted following chemo-mechanical treatment promoted an increase of structural and phase homogeneity of the surface of zirconia-based crystal substrates.

Half-width of the rocking curve (HWRC) is another parameter featuring quality of a substrate surface. The HWRC values significantly decreased due to the annealing.

Fianite in Photonics 137

The laser technique has the following advantages. It is: contactless, nondestructive, express, rather easily automated, technological and easy in use. So it is convenient for the use in commercial production with hundred percent inspections of the products and for the

**Development of the laser control equipment.** Draft-scheme of apparatus for laser control of


Fast horizontal scanning of the whole wafer is provided by its rotation and the radial movement of the light from motionless laser (the optical system is used). At tomographic investigations of bulky crystals a periodical crystal movement along vertical axis with a

Fianite has a number of advantages over other dielectric materials as a substrate and buffer

A wide spectral range (260–7500 nm) of the fianite transparency completely covering absorption and emission ranges of AIIIBV compounds and its solid solutions makes "semiconductor-on-fianite" structures promising for the development of various optoelectronic devices with advanced characteristics, including photodiodes with Schottky

Thin films of fianite and related solid solutions such as Zr(Ce)O2 can be used as **insulating layers** (alternative to SiO2, SiC, Si3N4) in development of Si-, Ge- and GaAs-based "semiconductor-dielectric" multilayer structures. Fianite is also good **gate dielectric** for Sias well as for AIIIBV –based devices (including GaN-based) due to its high dielectric constant value (25…29.7). Thin fianite films are a barrier for diffusion of impurities and provide significant (up to 1000-fold and even more) decrease of current loss in highly-integrated devices [14, 15].Due to high chemical inertness fianite films can also be used as **protective** 

The first epitaxial Si films on YSZ were grown in [6]. The first successful results on epitaxial MOCVD growth of various AIIIBV compounds (GaAs, InAs, InGaAs, AlGaAs, GaAsN and GaN) on YSZ are presented in a number of studies [10, 16, 17], InN on YSZ – in [21, 22]. In [17, 18] «capillary epitaxy technique» - the new effective way of heteroepitaxy – was developed. It has been shown that the use of capillary forces in the method positively influences both on the mechanism of epitaxial growth, and on

solution of some scientific and technological problems.


given step (for example 0.5 or 1 cm) is added.

about one minute.

**compounds epitaxy** 

**coatings.** 

sapphire and fianite wafers and bulky crystals is shown in Fig. 4.


Operational characteristics of the apparatus can be further improved [23, 24].

**3. Fianite as a substrate and buffer layer for Si, Si-Ge and AIIIBV**

barrier, photoresistors, emitting and laser diodes, avalanche photodetectors.

layer for the epitaxy of Si and AIIIBV compounds [6-12, 16-18].


The laser control apparatus has the following characteristics:

Fig. 4. Draft-scheme of the apparatus for laser control of sapphire and fianite wafers and bulky crystals

The technique for production of epi-ready substrates has been developed. The epi-ready substrates of 2" and 3" diameter have been manufactured from zirconia-based crystals. The polished surface was mirror-flat, scratches, etching pits, fractures and other defects are absent. The profilograms showed no noticeable deviations from R2 – 5 nm height. Typical roughness values for the epi-ready polished surface of fianite are less than 0.5 nm.

#### **2.3 New laser technique for express-monitoring of the defects in a volume of wafers and crystals - Development of the technique and equipment**

**The Laser technique** is based on a laser emission at the wavelength, which coincides with the region of transparency of sapphire and fianite, for example, radiation of СО2-laser in the middle IR range ( ~ 5.4 μm).

Such laser radiation readily penetrates through the flawless regions of fianite and sapphire, and scatters on impurity inclusions within fianite volume and micro-bubbles within sapphire volume. The scattered part of radiation passes through the filter and is registered by a photodetector arranged perpendicularly to the direction of the laser beam. Subsequent computer processing of the signal provides information on the defects.

Besides information on presence or absence of the defects in a volume of wafers and crystals, it is also possible to observe its two- or three-dimensional distribution over square of a plate and volume of a crystal.

This technique allows:


The data obtained can be sorted and saved in a computer memory for the subsequent analysis of reasons of the defect formation in bulk crystals with the purpose of the technology improvement.

Fig. 4. Draft-scheme of the apparatus for laser control of sapphire and fianite wafers and

roughness values for the epi-ready polished surface of fianite are less than 0.5 nm.

**and crystals - Development of the technique and equipment** 

computer processing of the signal provides information on the defects.

by special treatment (thus increasing the yield);

**2.3 New laser technique for express-monitoring of the defects in a volume of wafers** 

**The Laser technique** is based on a laser emission at the wavelength, which coincides with the region of transparency of sapphire and fianite, for example, radiation of СО2-laser in the

Such laser radiation readily penetrates through the flawless regions of fianite and sapphire, and scatters on impurity inclusions within fianite volume and micro-bubbles within sapphire volume. The scattered part of radiation passes through the filter and is registered by a photodetector arranged perpendicularly to the direction of the laser beam. Subsequent

Besides information on presence or absence of the defects in a volume of wafers and crystals, it is also possible to observe its two- or three-dimensional distribution over square


The data obtained can be sorted and saved in a computer memory for the subsequent analysis of reasons of the defect formation in bulk crystals with the purpose of the


The technique for production of epi-ready substrates has been developed. The epi-ready substrates of 2" and 3" diameter have been manufactured from zirconia-based crystals. The polished surface was mirror-flat, scratches, etching pits, fractures and other defects are absent. The profilograms showed no noticeable deviations from R2 – 5 nm height. Typical

bulky crystals

middle IR range ( ~ 5.4 μm).

of a plate and volume of a crystal.

This technique allows:

technology improvement.

The laser technique has the following advantages. It is: contactless, nondestructive, express, rather easily automated, technological and easy in use. So it is convenient for the use in commercial production with hundred percent inspections of the products and for the solution of some scientific and technological problems.

**Development of the laser control equipment.** Draft-scheme of apparatus for laser control of sapphire and fianite wafers and bulky crystals is shown in Fig. 4.

The laser control apparatus has the following characteristics:


Fast horizontal scanning of the whole wafer is provided by its rotation and the radial movement of the light from motionless laser (the optical system is used). At tomographic investigations of bulky crystals a periodical crystal movement along vertical axis with a given step (for example 0.5 or 1 cm) is added.

Operational characteristics of the apparatus can be further improved [23, 24].

#### **3. Fianite as a substrate and buffer layer for Si, Si-Ge and AIIIBV compounds epitaxy**

Fianite has a number of advantages over other dielectric materials as a substrate and buffer layer for the epitaxy of Si and AIIIBV compounds [6-12, 16-18].

A wide spectral range (260–7500 nm) of the fianite transparency completely covering absorption and emission ranges of AIIIBV compounds and its solid solutions makes "semiconductor-on-fianite" structures promising for the development of various optoelectronic devices with advanced characteristics, including photodiodes with Schottky barrier, photoresistors, emitting and laser diodes, avalanche photodetectors.

Thin films of fianite and related solid solutions such as Zr(Ce)O2 can be used as **insulating layers** (alternative to SiO2, SiC, Si3N4) in development of Si-, Ge- and GaAs-based "semiconductor-dielectric" multilayer structures. Fianite is also good **gate dielectric** for Sias well as for AIIIBV –based devices (including GaN-based) due to its high dielectric constant value (25…29.7). Thin fianite films are a barrier for diffusion of impurities and provide significant (up to 1000-fold and even more) decrease of current loss in highly-integrated devices [14, 15].Due to high chemical inertness fianite films can also be used as **protective coatings.** 

The first epitaxial Si films on YSZ were grown in [6]. The first successful results on epitaxial MOCVD growth of various AIIIBV compounds (GaAs, InAs, InGaAs, AlGaAs, GaAsN and GaN) on YSZ are presented in a number of studies [10, 16, 17], InN on YSZ – in [21, 22]. In [17, 18] «capillary epitaxy technique» - the new effective way of heteroepitaxy – was developed. It has been shown that the use of capillary forces in the method positively influences both on the mechanism of epitaxial growth, and on

Fianite in Photonics 139

 Fianite is very promising material for the development of semiconductor-on-fianite structures for various optoelectronic devices with enhanced characteristics. It has broad band of optical transparency (260 to 7500 nm), which completely overlaps the absorption and emission bands of Si and AIIIBV compounds and their solid solutions Application of thin layers of fianite on Si and GaAs instead of its monolithic substrates allows avoiding spatial limitations of the structures and decreasing the net cost. At the same time, the structures on "fianite/Si" and "fianite/GaAs" episubstrates have better

heat conductivity in comparison with the structures on monolithic substrates.

The first studies on silicon epitaxy on fianite single crystal substrates have been carried out in France and USA [7, 8]. Silicon films on fianite substrate were deposited by chloride and hydride epitaxy at 900…1100oC. The films obtained were of polycrystalline structure and, consequently, featured with poor electrophysical parameters. However, at the same time it was shown that silicon-on-fianite structures sustaining actually all advantages of silicon-on-

At the epitaxy of Si on fianite a formation of SiO2 intermediate layer between the film and the substrate was observed [7, 8]. Subsequent annealing of the structure leaded to the increase of SiO2 layer thickness. It was demonstrated [8] that the layer can improve

Smoothens over negative effect occurring due to a difference of linear expansion

Acts as a barrier for metal impurities diffusing from the substrate and forming deep

The formation of SiO2 intermediate layer at high-temperature epitaxy is associated with peculiar properties of fianite. In contrast to the other dielectrics, fianite features with a unique peculiarity as a solid electrolyte: starting from 6500C it becomes actually oxygen-transparent due to high mobility of oxygen. The reason for significant mobility of oxygen in fianite crystals is an occurrence of oxygen vacancies due to Zr+4 to Y+3 cation substitution at formation of the solid solution. High mobility of oxygen in fianite crystals is determined by an occurrence of oxygen vacancies at ZrO2(HfO2) – R2O3 (here: R - Y, Gd…Yb) solid solutions formation due to Zr+4(Hf+4) to R+3 cation substitution. The process results in the oxygen non-stoichiometric ZrO2 (HfO2) based phase [4]. Because of high mobility of oxygen at high-temperature epitaxy (900…10000С), which was used in [6-8] the formation of ether SiO2 continuous layer or its islets

The phenomenon occurs even at the epitaxy initial stages when a continuous epitaxial film is forming. It was shown [9] that the formation of SiO2 layer or isles at the initial stage of molecular-beam epitaxy on fianite results in 3-dimensional mechanism of growth, formation of structural defects and hindered the synthesis of Si films of single crystal structure. The occurrence of oxide SiO2 isles at the initial epitaxy stages and polycentric growth of Si layers were shown possible to avoid only by using a set of techniques, those which prevent diffusion of oxygen from the substrate to the film at initial stage of the process. In particular,

properties of silicon-on-fianite epitaxial structure because its formation:

Improves insulation of the integrated circuit elements (ICE) based on Si;

Removes mechanical stress in the layer-substrate interface;

between the substrate and the film was shown to be inevitable.

**3.1 Silicon-on-fianite epitaxial structures** 

sapphire are free from its principal drawbacks.

coefficients between fianite and silicon;

levels in silicon;

quality of AIIIBV epitaxial films, and also reduces the minimum thickness of a continuous layer [17, 18].

An application of fianite as either monolithic substrate or buffer layer in "semiconductor-ondielectric" technology is of peculiar importance for micro- and opto-electronics. The technology allows improving such characteristics of integrated circuits as operation speed, critical operational temperature and radiation resistance. Due to a decrease of loss of current and stray capacitance energy consumption of the devices is decreasing. Moreover, the devices based on "semiconductor-on-dielectric" structures are more reliable, especially under extreme operational conditions. Currently, "silicon-on-insulator" structures are one of the most dynamically developing directions in the field of semiconductor material science. However, electrophysical and operational parameters of the devices, as well as its radiation resistance and reliability significantly suffer because of structural imperfection of silicon layers. In case of "silicon-on-sapphire" structures the imperfection is determined, in particular, by a difference in crystallographic structure of silicon and sapphire, as well as by autodoping of a silicon film by aluminum penetrating from the sapphire substrate in concentrations up to 1018–1020сm-3. Considering crystal-chemical and physical characteristics of fianite, the material is more preferential for the epitaxy of Si as an alternative substrate in comparison with sapphire.

In comparison with the other dielectrics, there are the following merits of fianite in application as **a substrate material and buffer layer** for Si and AIIIBV compounds epitaxy:


Fig. 5. Correlation of the lattice parameters of Si and fianite (ZrO2)0,85·(Y2O3)0,15.


quality of AIIIBV epitaxial films, and also reduces the minimum thickness of a continuous

An application of fianite as either monolithic substrate or buffer layer in "semiconductor-ondielectric" technology is of peculiar importance for micro- and opto-electronics. The technology allows improving such characteristics of integrated circuits as operation speed, critical operational temperature and radiation resistance. Due to a decrease of loss of current and stray capacitance energy consumption of the devices is decreasing. Moreover, the devices based on "semiconductor-on-dielectric" structures are more reliable, especially under extreme operational conditions. Currently, "silicon-on-insulator" structures are one of the most dynamically developing directions in the field of semiconductor material science. However, electrophysical and operational parameters of the devices, as well as its radiation resistance and reliability significantly suffer because of structural imperfection of silicon layers. In case of "silicon-on-sapphire" structures the imperfection is determined, in particular, by a difference in crystallographic structure of silicon and sapphire, as well as by autodoping of a silicon film by aluminum penetrating from the sapphire substrate in concentrations up to 1018–1020сm-3. Considering crystal-chemical and physical characteristics of fianite, the material is more preferential for the epitaxy of Si as an alternative substrate in comparison with sapphire.

In comparison with the other dielectrics, there are the following merits of fianite in application as **a substrate material and buffer layer** for Si and AIIIBV compounds epitaxy:

 Similarly to Si, Ge and AIIIBV compounds it is of cubic structure (in contrast to hexagonal of sapphire) and has low mismatch by its lattice parameters with these compounds. In

particular, the mismatching of fianite (15% Y2O3) with silicon is ~5,3 % (Fig. 5); it is possible to alter fianite cubic lattice constant in solid solutions by varying ratio of the main (zirconium or hafnium dioxide) and stabilizing oxides (yttria, rare earth oxides from gadolinium to lutetium and alkaline-earth oxides) that allows an optimum matching between substrate and cubic lattice of semiconductor films thus improving its structural perfection. For example, the values of lattice parameter mismatching between Si and fianite crystals of (ZrO2)100 – *<sup>x</sup>* × (Y2O3)*x* compositions are 5.7, 5.3% and 4.4% at *x* =

Fig. 5. Correlation of the lattice parameters of Si and fianite (ZrO2)0,85·(Y2O3)0,15.

semiconductor only. Elevated temperature is not critical for the substrate.

of a film (typical for sapphire) hindering the film parameters;

 Fianite is characterized by low cation diffusion up to 1000–1200°С temperatures that reduces interdiffusion of substrate and film impurities and uncontrollable self-doping

 Due to its excellent stability at elevated temperatures, the upper limit of the corresponding structure operational temperatures depends on physical properties of a

High resistivity - 1012 Ohm•cm at 300 K;

9, 15 and 21, respectively;

layer [17, 18].


#### **3.1 Silicon-on-fianite epitaxial structures**

The first studies on silicon epitaxy on fianite single crystal substrates have been carried out in France and USA [7, 8]. Silicon films on fianite substrate were deposited by chloride and hydride epitaxy at 900…1100oC. The films obtained were of polycrystalline structure and, consequently, featured with poor electrophysical parameters. However, at the same time it was shown that silicon-on-fianite structures sustaining actually all advantages of silicon-onsapphire are free from its principal drawbacks.

At the epitaxy of Si on fianite a formation of SiO2 intermediate layer between the film and the substrate was observed [7, 8]. Subsequent annealing of the structure leaded to the increase of SiO2 layer thickness. It was demonstrated [8] that the layer can improve properties of silicon-on-fianite epitaxial structure because its formation:


The formation of SiO2 intermediate layer at high-temperature epitaxy is associated with peculiar properties of fianite. In contrast to the other dielectrics, fianite features with a unique peculiarity as a solid electrolyte: starting from 6500C it becomes actually oxygen-transparent due to high mobility of oxygen. The reason for significant mobility of oxygen in fianite crystals is an occurrence of oxygen vacancies due to Zr+4 to Y+3 cation substitution at formation of the solid solution. High mobility of oxygen in fianite crystals is determined by an occurrence of oxygen vacancies at ZrO2(HfO2) – R2O3 (here: R - Y, Gd…Yb) solid solutions formation due to Zr+4(Hf+4) to R+3 cation substitution. The process results in the oxygen non-stoichiometric ZrO2 (HfO2) based phase [4]. Because of high mobility of oxygen at high-temperature epitaxy (900…10000С), which was used in [6-8] the formation of ether SiO2 continuous layer or its islets between the substrate and the film was shown to be inevitable.

The phenomenon occurs even at the epitaxy initial stages when a continuous epitaxial film is forming. It was shown [9] that the formation of SiO2 layer or isles at the initial stage of molecular-beam epitaxy on fianite results in 3-dimensional mechanism of growth, formation of structural defects and hindered the synthesis of Si films of single crystal structure. The occurrence of oxide SiO2 isles at the initial epitaxy stages and polycentric growth of Si layers were shown possible to avoid only by using a set of techniques, those which prevent diffusion of oxygen from the substrate to the film at initial stage of the process. In particular,

Fianite in Photonics 141

situated in vicinity of the substrate was heated to T = 1200оС. With the purpose to avoid destruction of germane on evaporators (Ti) following pre-epitaxial annealing of the sources and substrates the sublimating pumps were switched off and the growth was carried out at pumping-down using only diffusion- and for-pumps. It is worth to note that the gas filling up to such high pressure (~10-3 torr) is impossible in MBE installations with electron-beam heating. The pressure in the cell was tentatively assigned by ionization vacuum gage indications. Nevertheless, this peculiarity in GeH4 pressure measurement did not impede the controlled growth of Ge films at 700-750oC temperature of the substrate. Ge films were continuous and homogeneous. Solid solution GeSi with up to 80% Si content on fianite substrates (111) and (100) also was obtained. Vacuum annealing at 1250 С during 10 min was used as pre-epitaxy treatment. The growth was carried out under 5·10-4 torr germane pressure and at600o С substrate temperature. Simultaneously, Ta plate positioned in vicinity of the substrate was heated to 1200оС. At fig. 7 X-ray diffraction pattern of Ge film (0.3 μm thickness) on fianite substrate (111) is shown, Ge(111) 27.3o and YSZ(111) 30.0opeaks are apparent. Heteroepitaxial Ge films obtained show high structural perfection. The half-width

 Fig. 7. Spectra of /2- scanning (a) and rocking curve (b) of Ge layer on (111) fianite.

The surface morphology of the Ge epitaxial layers grown on (100) and (111) fianite substrates (fig. 8a) as well as the peaks of Raman scattering near 300 cm-1 (fig. 8b) are identical to those of bulk Ge Therefore, it is possible to conclude that there are no stains in

Fig. 8. Surface morphology (a) and Raman spectrum (b) of Ge film on fianite; Тs=700ºC,

a b

100 200 300 400 500 600 700 800

см-1

of the X-ray curve for Ge film of 0.3 μm thickness was 0.31 (fig. 7).

the Ge/fianite layer.

t=60 min (AFM).

high structural perfection of the Si-on-fianite films was achieved by using low-temperature (T<650OC) molecular-beam epitaxy [9].

#### **3.2 Ge and GeSi films on fianite substrates**

Growth of Ge and Ge-Si heterostructures on fianite substrates was carried out using the installation shown in Fig. 6.

Fig. 6. Draft-scheme of *HWCVD* installation for Si1-xGex growth : 1 - diffusion pump, 2 - forepump, 3 - liquid nitrogen trap, 4 - getter-ion pump, 5 – high-vacuum shuttle, 6 – charging flange for sources, 7- charging flange for substrate, 8 – system for supply of GeH4

The growth cell comprised a stainless steel cylinder of 290 mm inner diameter and 360 mm length. There were two charging flanges in the cylinder faces, one for 3 pairs of current leads and the other for current leads for a substrate.

Base pressure in the chamber ~ 1·10-8 torr was maintained by pump-down using two hetero-ionic pumps. High-vacuum gate was used for isolation of the growth cell and the pumps from other parts of the vacuum system. Forepumping of the chamber was performed using diffusion pump. The diffusion pump allowed to exhaust any gas (including GeH4) both in atomic and molecular state. FM-1 oil with low vapor pressure was used as a pressure fluid. There was a nitrogen trap above the diffusion pump preventing reverse diffusion of the oil from preevacuation and diffusion pumps into the growth cell. The (100) and (111) oriented fianite single crystal plates were used as substrates. Silicon atomic beam was maintained by sublimation of the element single crystal (high-resistance) in form of 4x4x90 mm ingot sections. The sources were mounted on the cooled current leads. There was Ta plate of 80×5×0,5 mm size istalled in one of the sources position.

Before the epitaxial growth the sources and substrates were subjected to 10 min annealing at 1350 and 1250оС, respectively, then temperature of the source was increased to 1380оС, as the substrate temperature was decreased to assigned values (600-7000С) and the buffer layer was grown. The pressure in the cell corresponded to basic one.

In order to grow Ge layers the cell was filled with GeH4 up to 1·10-3 – 5·10-6 torr, the pressure was maintained constant by a system of the gas feeding. Simultaneously the Ta plate

high structural perfection of the Si-on-fianite films was achieved by using low-temperature

Growth of Ge and Ge-Si heterostructures on fianite substrates was carried out using the

3

7

5

1 2

Fig. 6. Draft-scheme of *HWCVD* installation for Si1-xGex growth : 1 - diffusion pump, 2 - forepump, 3 - liquid nitrogen trap, 4 - getter-ion pump, 5 – high-vacuum shuttle,

4

6

8

There was Ta plate of 80×5×0,5 mm size istalled in one of the sources position.

was grown. The pressure in the cell corresponded to basic one.

6 – charging flange for sources, 7- charging flange for substrate, 8 – system for supply of GeH4

The growth cell comprised a stainless steel cylinder of 290 mm inner diameter and 360 mm length. There were two charging flanges in the cylinder faces, one for 3 pairs of current leads

Base pressure in the chamber ~ 1·10-8 torr was maintained by pump-down using two hetero-ionic pumps. High-vacuum gate was used for isolation of the growth cell and the pumps from other parts of the vacuum system. Forepumping of the chamber was performed using diffusion pump. The diffusion pump allowed to exhaust any gas (including GeH4) both in atomic and molecular state. FM-1 oil with low vapor pressure was used as a pressure fluid. There was a nitrogen trap above the diffusion pump preventing reverse diffusion of the oil from preevacuation and diffusion pumps into the growth cell. The (100) and (111) oriented fianite single crystal plates were used as substrates. Silicon atomic beam was maintained by sublimation of the element single crystal (high-resistance) in form of 4x4x90 mm ingot sections. The sources were mounted on the cooled current leads.

Before the epitaxial growth the sources and substrates were subjected to 10 min annealing at 1350 and 1250оС, respectively, then temperature of the source was increased to 1380оС, as the substrate temperature was decreased to assigned values (600-7000С) and the buffer layer

In order to grow Ge layers the cell was filled with GeH4 up to 1·10-3 – 5·10-6 torr, the pressure was maintained constant by a system of the gas feeding. Simultaneously the Ta plate

(T<650OC) molecular-beam epitaxy [9].

installation shown in Fig. 6.

**3.2 Ge and GeSi films on fianite substrates** 

and the other for current leads for a substrate.

situated in vicinity of the substrate was heated to T = 1200оС. With the purpose to avoid destruction of germane on evaporators (Ti) following pre-epitaxial annealing of the sources and substrates the sublimating pumps were switched off and the growth was carried out at pumping-down using only diffusion- and for-pumps. It is worth to note that the gas filling up to such high pressure (~10-3 torr) is impossible in MBE installations with electron-beam heating. The pressure in the cell was tentatively assigned by ionization vacuum gage indications. Nevertheless, this peculiarity in GeH4 pressure measurement did not impede the controlled growth of Ge films at 700-750oC temperature of the substrate. Ge films were continuous and homogeneous. Solid solution GeSi with up to 80% Si content on fianite substrates (111) and (100) also was obtained. Vacuum annealing at 1250 С during 10 min was used as pre-epitaxy treatment. The growth was carried out under 5·10-4 torr germane pressure and at600o С substrate temperature. Simultaneously, Ta plate positioned in vicinity of the substrate was heated to 1200оС. At fig. 7 X-ray diffraction pattern of Ge film (0.3 μm thickness) on fianite substrate (111) is shown, Ge(111) 27.3o and YSZ(111) 30.0opeaks are apparent. Heteroepitaxial Ge films obtained show high structural perfection. The half-width of the X-ray curve for Ge film of 0.3 μm thickness was 0.31 (fig. 7).

Fig. 7. Spectra of /2- scanning (a) and rocking curve (b) of Ge layer on (111) fianite.

The surface morphology of the Ge epitaxial layers grown on (100) and (111) fianite substrates (fig. 8a) as well as the peaks of Raman scattering near 300 cm-1 (fig. 8b) are identical to those of bulk Ge Therefore, it is possible to conclude that there are no stains in the Ge/fianite layer.

Fig. 8. Surface morphology (a) and Raman spectrum (b) of Ge film on fianite; Тs=700ºC, t=60 min (AFM).

Fianite in Photonics 143

Therefore fianite is apparently in advance as a substrate for InN epitaxy as compared to sapphire. A new effective method of heteroepitaxy, capillary epitaxy, was proposed in [17]. It allows us in

The investigations showed that continuous GaAs layers on fianite can be obtained only in a very narrow range of epitaxial conditions. In particular, temperature range of 550-600°C is necessary. The minimum thickness of a continuous layer was 1.5-2.0 µm. The epitaxial films had polycrystalline structure and rough surface. Structural and electrical properties of GaAs films could be improved using capillary epitaxy. The essence of this method is that a thin (less than 50 nm) film of an III-group element is initially deposited on the fianite surface and then saturated with a V-group component with the formation of a thin continuous epitaxial III-V layer. After this procedure, the film growth continues to obtain the necessary thickness

The use of capillary forces in the first (heteroepitaxial) stage of GaAs film formation led to improvement of epitaxial quality. Electron microscopy of the GaAs films at the initial growth stages showed that the transition from the standard MOCVD growth to capillary epitaxy leads to a change in the growth mechanism. Three-dimensional island mechanism changes to the two-dimensional one with propagation of the growth steps (Fig. 9, A). This process is similar to graphoepitaxy [27, 28] from aqueous solutions with addition of surfactants, where an increase in the substrate wettability also significantly improves the

In both cases, the height of the crystallization medium (melt or solution) decreases in the initial stage due to the capillary forces. This effect impedes growth of epitaxial nuclei in the direction normal to the substrate surface and facilitates their growth in the tangential direction. As a result, the substrate orienting role increases and a transition to the layer-bylayer growth mechanism occurs with a decrease in the growth step height. As a result, the minimum height of the continuous layer decreases and the film structural quality is improved. It has been shown that the use of capillary force in this method has a positive influence on both the mechanism of epitaxial growth and the quality of AIIIBV epitaxial films. It also reduces the minimum thickness of a continuous layer [17, 19]. Virtually the same approach has now begun to be used with success in the works of other authors in

The use of capillary epitaxy made it possible to decrease minimum thickness of a continuous GaAs/fianite film to 25 nm and to improve its structural quality and surface morphology.

**3.3.2 Study of impurities content in GaAs-on-fianite films using mass-spectrometry** 

Mass-spectrometry analysis using single crystal GaAs standard curve has shown concentration of the impurities in GaAs-on-fianite films grown using the capillary epitaxy technique to be in the range of 51016–51017сm–3 (Tab. 2). Layerwise mass-spectrometry analysis of the GaAs/fianite structures has shown uniform distribution of the impurities in GaAs film. Somewhat increase of Ca, Na and Cr concentrations in the film-substrate

The technique was also efficient for growing other AIIIBV compounds on fianite.

interface seems to be associated with a formation of oxides in the interface.

particular to obtain films of AIIIBV compounds on fianite using the MOCVD approach.

**3.3.1 GaAs on fianite films - MOCVD capillary epitaxy of III-V on fianite** 

under conventional epitaxial conditions.

quality of graphoepitaxial layers [27] (Fig. 9, B).

order to obtain AIIIN films on various substrates [29].

**analysis** 

#### **3.3 Epitaxial films of AIIIBV on-fianite**

Crystallochemical and physical properties of fianite are favorable not only for silicon but also for АIIIBV compounds epitaxy (Table 1).


Table 1. Some properties of fianite crystals and АIIIBV compounds.

First successful results on growth of АIIIBV compound epitaxial films on fianite substrates were presented in [10, 17]. GaAs, InAs, GaN and other АIIIBV semiconductor compound films have been grown on fianite, as well as on silicon and gallium arsenide with fianite buffer layer substrates by means of metal-organic Chemical Vapor Deposition (MOCVD). A new efficient epitaxy technique – "capillary epitaxy" has been suggested. The technique allowed synthesizing of АIIIBV compounds films by MOCVD on fianite substrates. Samples of structurally perfect sub-micron (up to 0.1 μ) epitaxial films of АIIIBV compounds have been obtained using this technique. The samples demonstrated high electrophysical parameters [16, 17, 19, 25, 26]. In [20] GaN epitaxial films have been grown on fianite substrates by MOVP technique. It was observed that the epitaxial growth of GaN on fianite significantly depends on conditions of initial stage of the process.

In [12, 21, 22] fianite substrates were successfully tested for growth of InN heteroepitaxial films. InN films of cubic structure have been grown on (001) fianite substrates by plasmastimulated molecular-beam epitaxy (RF–MBE) at 400–490 OC temperature. The lattice mismatch of InN and fianite at (001) plane is very low (less than 2.3%), in contrast to 17% for InN – sapphire and more than 10% for InN – GaAs. Due to this fact, InN films grown on (001) fianite substrate were superior InN films grown on sapphire [12] and (001) GaAs substrates by its crystallographic perfection [22].

Crystallochemical and physical properties of fianite are favorable not only for silicon but

5.141(x=10) 5.157(x=15) 5.198(x=21)

GaAs Cubic (sphalerite) 5.65 1283 5.4 1.43 GaP Cubic (sphalerite) 5. 445 1467 4.7 2.26

GaN Cubic (sphalerite) 4.52 1700 3.9 3.2

InN Cubic (sphalerite) 4.98 1200 4.4 0.67

First successful results on growth of АIIIBV compound epitaxial films on fianite substrates were presented in [10, 17]. GaAs, InAs, GaN and other АIIIBV semiconductor compound films have been grown on fianite, as well as on silicon and gallium arsenide with fianite buffer layer substrates by means of metal-organic Chemical Vapor Deposition (MOCVD). A new efficient epitaxy technique – "capillary epitaxy" has been suggested. The technique allowed synthesizing of АIIIBV compounds films by MOCVD on fianite substrates. Samples of structurally perfect sub-micron (up to 0.1 μ) epitaxial films of АIIIBV compounds have been obtained using this technique. The samples demonstrated high electrophysical parameters [16, 17, 19, 25, 26]. In [20] GaN epitaxial films have been grown on fianite substrates by MOVP technique. It was observed that the epitaxial growth of GaN on fianite

In [12, 21, 22] fianite substrates were successfully tested for growth of InN heteroepitaxial films. InN films of cubic structure have been grown on (001) fianite substrates by plasmastimulated molecular-beam epitaxy (RF–MBE) at 400–490 OC temperature. The lattice mismatch of InN and fianite at (001) plane is very low (less than 2.3%), in contrast to 17% for InN – sapphire and more than 10% for InN – GaAs. Due to this fact, InN films grown on (001) fianite substrate were superior InN films grown on sapphire [12] and (001) GaAs

a=3.54

(wurtzite) a=3.186; c=5.178 1700 5.6 ; 7.8 3.4

**Tm,°C (melting point)** 

**Therm.Exp.Coeff. 10–6 deg-1**

2800 11.4 (15–1000°C)

c=5.70 1200 12.7 0.7

**Eg, eV** 

 **on-fianite** 

**type a, Å** 

Table 1. Some properties of fianite crystals and АIIIBV compounds.

significantly depends on conditions of initial stage of the process.

substrates by its crystallographic perfection [22].

**3.3 Epitaxial films of AIIIBV**

(ZrO2)100–x (Y2O3)x

also for АIIIBV compounds epitaxy (Table 1).

Cubic (fluorite)

(wurtzite)

**Crystal Lattice** 

GaN Hexagonal

InN Hexagonal

Therefore fianite is apparently in advance as a substrate for InN epitaxy as compared to sapphire. A new effective method of heteroepitaxy, capillary epitaxy, was proposed in [17]. It allows us in particular to obtain films of AIIIBV compounds on fianite using the MOCVD approach.
