- Email: [email protected]
Bulk vapour growth of CdTe
,. . . . . . . .
C R Y S T A L G R O W T H
ELSEVIER
Journal of Crystal Growth 146 (1995) 65-68
Bulk vapour growth of CdTe Krzysztof Grasza a,b a Institute of Physics, Polish Academy of Sciences, ALLotnikdw 32/46, PL-02-668 Warsaw, Poland b Institute of Electronic Materials Technology, W6lczytiska 133, PL-01-919 Warsaw, Poland
Abstract Growth of CdTe crystals was performed reproducibly in 130 Torr hydrogen atmosphere by a method of self nucleation and growth with no contact between crystal and ampoule wall [K. Grasza et al., J. Crystal Growth 123 (1992) 519]. The growth temperature was 960°C and the furnace translation rate 4 mm/day. Large CdTe single crystals up to 15 c m 3 have been grown using this method. The way to increase the size of crystals and results of growth of crystals up to 3.5 cm in length and 5.5 cm diameter are presented.
1. Introduction Crystals grown by the method of directional crystal growth with no contact between crystal and ampoule wall [1-7] are characterized by a low density of dislocations (below 10 4 cm -2) and a wide range of ohmic resistance [8]. The growth of CdTe crystals from the vapour is very difficult because of the low thermal conductivity of this material at high temperature. Usually the crystals grown from the vapour are small, up to 3 cm 3 [9-16]. Reports describing the growth of larger CdTe crystals are scarce [17-19]. We have been successful in growing single crystals by the method of self nucleation and growth with no contact between crystal and ampoule wall. These are among the largest available CdTe crystals grown from the vapour phase. 2. Experimental procedure The CdTe crystals were grown by a novel method of crystal growth described in detail in
our earlier work [8]. Four growth runs were performed in a vertical configuration with hydrogen inside the ampoule. The construction of the silica glass ampoules used for growth of crystals was similar to that presented in Refs. [1,2] (Fig. 1). The ampoules were up to 15 cm in length and had inner diameters of 2.3, 2.5, 3.2 and 5.5 cm. A crystal holder of 3-5 cm in length was placed inside the ampoule with a slightly smaller outer diameter than the inner diameter of the growth ampoule. Through this small gap ( < 0.5 mm) excess components and impurities are passed to a reservoir behind the crystal holder. The steady mass flow through the gap prevents contact between the crystal and the ampoule wall. The distance between the ampoule end and the crystal holder ranged from 3.5 up to 5 cm, depending on details of axial temperature profile and diameter of the furnace used. A typical temperature profile is shown in Fig. 2a. A parabola-like temperature profile with a rather steep temperature gradient in the growing crystal was applied, which leads to large radial temperature gradients and
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00510-9
K. Grasza /Journal of Crystal Growth 146 (1995) 65-68
66
order
growth is ensured by slow movement of the furnace. The growth temperature was 960°C and the growth rate was 4 mm/day. The ampoules were cooled down quickly at rates of 900, 300 or lO0°C/h.
3. Results and discussion
Fig. 1. Three stages of forming a seed on the crystal holder.
convex growth interfaces. Growth was performed from nearly stoichiometric material obtained by heating the CdTe material in dynamic vacuum [8]. Prior to closing the ampoules were filled with 130 Torr of hydrogen. At the beginning of crystal growth the source material was compacted in the ampoule end. On changing the temperature field in the ampoule, the source material becomes conically shaped, with a sharp tip that adhers to the surface of the crystal holder (Fig. la). This cone-forming process is a result of increasing radial gradients and steeper isotherms. The next step in the growth procedure is separation of the monocrystalline tip of the CdTe cone from the rest of the source material by a small change of the temperature field. The last stage is the growth of a crystal by physical vapour transport. Stable
a 96o
hof "g'°'
I
~ i
The crystallographic orientation of the crystals was dependent on the orientation of the monocrystalline tip of the cone adhering to the crystal holder. The growth directions of the bulk CdTe single crystal were close to (100) in the three growth runs and in only one case close to (110). All the crystals obtained were twinned in 30-80 vol% of the crystal, and were covered by flat crystallographic faces of {110}, {111} or {100} orientation. The facets were formed in the early stages of crystal growth. Also twinning was observed in the very early stage of crystal growth. The observed increase of the size of facets on crystals grown in hydrogen atmosphere can be understood by considering the effect of an inert gas on the gradient of gaseous species above the crystal. The lack of inert gas makes the interface uniform and the phase boundary tends to be isothermal. The inert gas causes diffusion barriers, which are an obstacle to the free flow of vapour phase species. When the same translation
hot region
=
E
• 760 \gecond stage (crysfo,[ growfh/
disfonce
(cm}
Fig. 2. Self nucleation and growth of 3.5 cm long CdTe crystal with no contact with the ampoule wall. (a) Furnace temperature profile in the first (seeding) and second (growth) stage. (b) CdTe load in ampoule removed from the furnace at the m o m e n t when the seed is forming on the crystal holder.
I~ Grasza/Journal of Crystal Growth 146 (1995) 65-68
speed of the furnace is used for ampoules with an inert gas, concentration gradients of mass flow around the growing face are possible, and the microscopic growth mechanism can enforce the observed growth anisotropy. The use of hydrogen as an inert gas for stabilization of diffusive transport of crystal forming components appears to be successful also in increasing the ohmic resistivity of the crystals. Our previous paper [8] reports 108 f~.cm for p-type crystals grown at the relatively low temperature of 850°C with no inert gas present in the ampoule. Ongoing investigations [20] of the electrical properties of the crystals grown in this work show, that uniform resistivity at a level of 109 f l . c m is possible in these undoped crystals. The diffusive barrier of hydrogen facilitates the stop chiometric condensation of the crystal forming components at the growth interface and improves the growth stability. The hydrogen atmosphere makes the growth temperature dependent on the amount of the inert gas. It is now less dependent on the stoichiometry of the source material. Thus, more perfect repeatability of the growth process is possible. This is an important result, because of the dynamic character of growth with no contact
67
of the crystal with ampoule wall. In this method, lack of optimised consistency of the growth temperature and translation rate leads to losses of material in the cold end of the ampoule or to unstable growth. The optimal volume of the source material for self nucleation applied in our experiments is the half of the volume of the ampoule between its hot end and crystal holder. When crystals of bigger size are grown, the diameter of the ampoule or the axial dimensions have to be increased. It is more convenient for the crystal grower to increase the diameter of the crystal and grow crystals in contact with the ampoule wall [18] or to grow short crystals [19] by the "contactless" method. We were successful in increasing the diameter up to 5.5 cm for growth of short (1.5 cm) crystals (Fig. 3). Most problematic is increasing the length of the crystal, in this method of growth with no contact with ampoule wall, since it is extremely difficult to perform the nucleation procedure prior to growth of the bulk crystal. The increase of the axial dimensions of the ampoule demands an increase of the furnace "hot region" (temperature plateau in Fig. 2a). The increased "hot region" of the furnace makes it impossible
Fig. 3. CdTe crystals grown at 960°C in an atmosphere of hydrogen with no contact between the crystal and ampoule wall.
68
IK Grasza /Journal of Crystal Growth 146 (1995) 65-68
to grow a long C d T e crystal with no contact with the a m p o u l e wall, if the growth t e m p e r a t u r e is high [21]. T h e p r o b l e m can be solved by dividing the process on (1) the nucleation in the furnace zone with long " h o t region", and (2) the growth of the crystal with no contact with the a m p o u l e wall in furnace with short " h o t region". A 3.5 cm long C d T e crystal was successfully grown by this m e t h o d . In Fig. 2a the m o m e n t of nucleation in such a process is shown. In Fig. 2b the C d T e load in the a m p o u l e r e m o v e d from the furnace at the m o m e n t w h e n the seed is forming on the crystal holder is shown.
4. Summary C d T e growth experiments by the m e t h o d of self nucleation and growth with no contact between the crystal and a m p o u l e wall were perf o r m e d in a h y d r o g e n a t m o s p h e r e at 960°C, with a furnace translation rate of 4 m m / d a y . It was found, that 130 T o r r of h y d r o g e n enclosed in a cold a m p o u l e e n s u r e d reproducibility of the growth conditions and stable growth. T h e optimal growth conditions increased the probability of growth of crystals with a stable s o l i d - v a p o u r interface of (100) crystallographic orientation. T h e p r e s e n c e of an inert gas in the growth a m p o u l e increased the probability of fiat facet growth. T h e application of this m e t h o d to growth large crystals of 5.5 cm d i a m e t e r is possible in a simple o n e - z o n e furnace. In o r d e r to increase the length of the C d T e crystals o b t a i n e d by self nucleation and growth with no contact with the a m p o u l e wall, it is necessary to use a two-zone furnace and to separate the p r o c e d u r e of self nucleation f r o m that of bulk growth.
Acknowledgement T h e a u t h o r is grateful to E. Lusakowska for crystallographic orientation of the crystals.
References [1] E.V. Markov and A.A. Davydov, Izv. Akad. Nauk SSSR, Neorg. Mater. 7 (1971) 575. [2] E.V. Markov and A.A. Davydov, Izv. Akad. Nauk SSSR, Neorg. Mater. 11 (1975) 1755. [3] Z. Golacki, M. G6rska, J. Makowski and A. Szczerbakow, J. Crystal Growth 56 (1981) 213. [4] Z. Gotacki, Z. Furmanik, M. G6rska, A. Szczerbakow and W. Zahorowski, J. Crystal Growth 60 (1982) 150. [5] Z. Golacki, M.G6rska, T. Warmifiski and A. Szczerbakow, J. Crystal Growth 74 (1986) 129. [6] L.A. Klinkova and S.A. Erofieieva, Izd. Akad. Nauk SSSR, Neorg. Mat 24 (1988) 223. [7] L.A. Klinkova and S.A. Erofieieva, Izd. Akad. Nauk SSSR, Neorg. Mat. 24 (1988) 397. [8] K. Grasza, U. Zuzga-Grasza, A. Jfdrzejczak, R.R. Gai~zka, J. Majewski, A. Szadkowski and E. Grodzicka, J. Crystal Growth 123 (1992) 519. [9] N. Yellin, D. Eger and A. Shachna, J. Crystal Growth 60 (1982) 343. [10] P. Buck and R. Nitsche, J. Crystal Growth 48 (1980) 29. [11] Z. Golacki, J. Majewski and J. Makowski, J. Crystal Growth 94 (1989) 559. [12] N. Yellin and S. Szapiro, J. Crystal Growth 69 (1984) 555. [13] N. Yellin and S. Szapiro, J. Crystal Growth 73 (1985) 77. [14] N. Yellin, A. Zemmel and R. Tenne, J. Electron. Mater. 14 (1985) 85. [15] K. Mochizuki, J. Crystal Growth 51 (1981) 453. [16] K. Mochizuki, J. Crystal Growth 53 (1981) 355. [17] H. Kuwamoto, J. Crystal Growth 69 (1984) 204. [18] J.L. Boone, G. Cantwell, W.C. Harsch, J.E. Thomas and B.A. Foreman, J. Crystal Growth 139 (1994) 27. [19] K. Durose, A. Turnbull and P. Brown, Mater. Sci. Eng. B 16 (1993) 96. [20] M. Laasch, R. Schwarz, W. Joerger, C. Eiche, M. Fiederle, K.W. Benz and K. Grasza, J. Crystal Growth 146 (1995) 125. [21] K. Grasza, J. Crystal Growth 128 (1993) 609.
C R Y S T A L G R O W T H
ELSEVIER
Journal of Crystal Growth 146 (1995) 65-68
Bulk vapour growth of CdTe Krzysztof Grasza a,b a Institute of Physics, Polish Academy of Sciences, ALLotnikdw 32/46, PL-02-668 Warsaw, Poland b Institute of Electronic Materials Technology, W6lczytiska 133, PL-01-919 Warsaw, Poland
Abstract Growth of CdTe crystals was performed reproducibly in 130 Torr hydrogen atmosphere by a method of self nucleation and growth with no contact between crystal and ampoule wall [K. Grasza et al., J. Crystal Growth 123 (1992) 519]. The growth temperature was 960°C and the furnace translation rate 4 mm/day. Large CdTe single crystals up to 15 c m 3 have been grown using this method. The way to increase the size of crystals and results of growth of crystals up to 3.5 cm in length and 5.5 cm diameter are presented.
1. Introduction Crystals grown by the method of directional crystal growth with no contact between crystal and ampoule wall [1-7] are characterized by a low density of dislocations (below 10 4 cm -2) and a wide range of ohmic resistance [8]. The growth of CdTe crystals from the vapour is very difficult because of the low thermal conductivity of this material at high temperature. Usually the crystals grown from the vapour are small, up to 3 cm 3 [9-16]. Reports describing the growth of larger CdTe crystals are scarce [17-19]. We have been successful in growing single crystals by the method of self nucleation and growth with no contact between crystal and ampoule wall. These are among the largest available CdTe crystals grown from the vapour phase. 2. Experimental procedure The CdTe crystals were grown by a novel method of crystal growth described in detail in
our earlier work [8]. Four growth runs were performed in a vertical configuration with hydrogen inside the ampoule. The construction of the silica glass ampoules used for growth of crystals was similar to that presented in Refs. [1,2] (Fig. 1). The ampoules were up to 15 cm in length and had inner diameters of 2.3, 2.5, 3.2 and 5.5 cm. A crystal holder of 3-5 cm in length was placed inside the ampoule with a slightly smaller outer diameter than the inner diameter of the growth ampoule. Through this small gap ( < 0.5 mm) excess components and impurities are passed to a reservoir behind the crystal holder. The steady mass flow through the gap prevents contact between the crystal and the ampoule wall. The distance between the ampoule end and the crystal holder ranged from 3.5 up to 5 cm, depending on details of axial temperature profile and diameter of the furnace used. A typical temperature profile is shown in Fig. 2a. A parabola-like temperature profile with a rather steep temperature gradient in the growing crystal was applied, which leads to large radial temperature gradients and
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00510-9
K. Grasza /Journal of Crystal Growth 146 (1995) 65-68
66
order
growth is ensured by slow movement of the furnace. The growth temperature was 960°C and the growth rate was 4 mm/day. The ampoules were cooled down quickly at rates of 900, 300 or lO0°C/h.
3. Results and discussion
Fig. 1. Three stages of forming a seed on the crystal holder.
convex growth interfaces. Growth was performed from nearly stoichiometric material obtained by heating the CdTe material in dynamic vacuum [8]. Prior to closing the ampoules were filled with 130 Torr of hydrogen. At the beginning of crystal growth the source material was compacted in the ampoule end. On changing the temperature field in the ampoule, the source material becomes conically shaped, with a sharp tip that adhers to the surface of the crystal holder (Fig. la). This cone-forming process is a result of increasing radial gradients and steeper isotherms. The next step in the growth procedure is separation of the monocrystalline tip of the CdTe cone from the rest of the source material by a small change of the temperature field. The last stage is the growth of a crystal by physical vapour transport. Stable
a 96o
hof "g'°'
I
~ i
The crystallographic orientation of the crystals was dependent on the orientation of the monocrystalline tip of the cone adhering to the crystal holder. The growth directions of the bulk CdTe single crystal were close to (100) in the three growth runs and in only one case close to (110). All the crystals obtained were twinned in 30-80 vol% of the crystal, and were covered by flat crystallographic faces of {110}, {111} or {100} orientation. The facets were formed in the early stages of crystal growth. Also twinning was observed in the very early stage of crystal growth. The observed increase of the size of facets on crystals grown in hydrogen atmosphere can be understood by considering the effect of an inert gas on the gradient of gaseous species above the crystal. The lack of inert gas makes the interface uniform and the phase boundary tends to be isothermal. The inert gas causes diffusion barriers, which are an obstacle to the free flow of vapour phase species. When the same translation
hot region
=
E
• 760 \gecond stage (crysfo,[ growfh/
disfonce
(cm}
Fig. 2. Self nucleation and growth of 3.5 cm long CdTe crystal with no contact with the ampoule wall. (a) Furnace temperature profile in the first (seeding) and second (growth) stage. (b) CdTe load in ampoule removed from the furnace at the m o m e n t when the seed is forming on the crystal holder.
I~ Grasza/Journal of Crystal Growth 146 (1995) 65-68
speed of the furnace is used for ampoules with an inert gas, concentration gradients of mass flow around the growing face are possible, and the microscopic growth mechanism can enforce the observed growth anisotropy. The use of hydrogen as an inert gas for stabilization of diffusive transport of crystal forming components appears to be successful also in increasing the ohmic resistivity of the crystals. Our previous paper [8] reports 108 f~.cm for p-type crystals grown at the relatively low temperature of 850°C with no inert gas present in the ampoule. Ongoing investigations [20] of the electrical properties of the crystals grown in this work show, that uniform resistivity at a level of 109 f l . c m is possible in these undoped crystals. The diffusive barrier of hydrogen facilitates the stop chiometric condensation of the crystal forming components at the growth interface and improves the growth stability. The hydrogen atmosphere makes the growth temperature dependent on the amount of the inert gas. It is now less dependent on the stoichiometry of the source material. Thus, more perfect repeatability of the growth process is possible. This is an important result, because of the dynamic character of growth with no contact
67
of the crystal with ampoule wall. In this method, lack of optimised consistency of the growth temperature and translation rate leads to losses of material in the cold end of the ampoule or to unstable growth. The optimal volume of the source material for self nucleation applied in our experiments is the half of the volume of the ampoule between its hot end and crystal holder. When crystals of bigger size are grown, the diameter of the ampoule or the axial dimensions have to be increased. It is more convenient for the crystal grower to increase the diameter of the crystal and grow crystals in contact with the ampoule wall [18] or to grow short crystals [19] by the "contactless" method. We were successful in increasing the diameter up to 5.5 cm for growth of short (1.5 cm) crystals (Fig. 3). Most problematic is increasing the length of the crystal, in this method of growth with no contact with ampoule wall, since it is extremely difficult to perform the nucleation procedure prior to growth of the bulk crystal. The increase of the axial dimensions of the ampoule demands an increase of the furnace "hot region" (temperature plateau in Fig. 2a). The increased "hot region" of the furnace makes it impossible
Fig. 3. CdTe crystals grown at 960°C in an atmosphere of hydrogen with no contact between the crystal and ampoule wall.
68
IK Grasza /Journal of Crystal Growth 146 (1995) 65-68
to grow a long C d T e crystal with no contact with the a m p o u l e wall, if the growth t e m p e r a t u r e is high [21]. T h e p r o b l e m can be solved by dividing the process on (1) the nucleation in the furnace zone with long " h o t region", and (2) the growth of the crystal with no contact with the a m p o u l e wall in furnace with short " h o t region". A 3.5 cm long C d T e crystal was successfully grown by this m e t h o d . In Fig. 2a the m o m e n t of nucleation in such a process is shown. In Fig. 2b the C d T e load in the a m p o u l e r e m o v e d from the furnace at the m o m e n t w h e n the seed is forming on the crystal holder is shown.
4. Summary C d T e growth experiments by the m e t h o d of self nucleation and growth with no contact between the crystal and a m p o u l e wall were perf o r m e d in a h y d r o g e n a t m o s p h e r e at 960°C, with a furnace translation rate of 4 m m / d a y . It was found, that 130 T o r r of h y d r o g e n enclosed in a cold a m p o u l e e n s u r e d reproducibility of the growth conditions and stable growth. T h e optimal growth conditions increased the probability of growth of crystals with a stable s o l i d - v a p o u r interface of (100) crystallographic orientation. T h e p r e s e n c e of an inert gas in the growth a m p o u l e increased the probability of fiat facet growth. T h e application of this m e t h o d to growth large crystals of 5.5 cm d i a m e t e r is possible in a simple o n e - z o n e furnace. In o r d e r to increase the length of the C d T e crystals o b t a i n e d by self nucleation and growth with no contact with the a m p o u l e wall, it is necessary to use a two-zone furnace and to separate the p r o c e d u r e of self nucleation f r o m that of bulk growth.
Acknowledgement T h e a u t h o r is grateful to E. Lusakowska for crystallographic orientation of the crystals.
References [1] E.V. Markov and A.A. Davydov, Izv. Akad. Nauk SSSR, Neorg. Mater. 7 (1971) 575. [2] E.V. Markov and A.A. Davydov, Izv. Akad. Nauk SSSR, Neorg. Mater. 11 (1975) 1755. [3] Z. Golacki, M. G6rska, J. Makowski and A. Szczerbakow, J. Crystal Growth 56 (1981) 213. [4] Z. Gotacki, Z. Furmanik, M. G6rska, A. Szczerbakow and W. Zahorowski, J. Crystal Growth 60 (1982) 150. [5] Z. Golacki, M.G6rska, T. Warmifiski and A. Szczerbakow, J. Crystal Growth 74 (1986) 129. [6] L.A. Klinkova and S.A. Erofieieva, Izd. Akad. Nauk SSSR, Neorg. Mat 24 (1988) 223. [7] L.A. Klinkova and S.A. Erofieieva, Izd. Akad. Nauk SSSR, Neorg. Mat. 24 (1988) 397. [8] K. Grasza, U. Zuzga-Grasza, A. Jfdrzejczak, R.R. Gai~zka, J. Majewski, A. Szadkowski and E. Grodzicka, J. Crystal Growth 123 (1992) 519. [9] N. Yellin, D. Eger and A. Shachna, J. Crystal Growth 60 (1982) 343. [10] P. Buck and R. Nitsche, J. Crystal Growth 48 (1980) 29. [11] Z. Golacki, J. Majewski and J. Makowski, J. Crystal Growth 94 (1989) 559. [12] N. Yellin and S. Szapiro, J. Crystal Growth 69 (1984) 555. [13] N. Yellin and S. Szapiro, J. Crystal Growth 73 (1985) 77. [14] N. Yellin, A. Zemmel and R. Tenne, J. Electron. Mater. 14 (1985) 85. [15] K. Mochizuki, J. Crystal Growth 51 (1981) 453. [16] K. Mochizuki, J. Crystal Growth 53 (1981) 355. [17] H. Kuwamoto, J. Crystal Growth 69 (1984) 204. [18] J.L. Boone, G. Cantwell, W.C. Harsch, J.E. Thomas and B.A. Foreman, J. Crystal Growth 139 (1994) 27. [19] K. Durose, A. Turnbull and P. Brown, Mater. Sci. Eng. B 16 (1993) 96. [20] M. Laasch, R. Schwarz, W. Joerger, C. Eiche, M. Fiederle, K.W. Benz and K. Grasza, J. Crystal Growth 146 (1995) 125. [21] K. Grasza, J. Crystal Growth 128 (1993) 609.