- Email: [email protected]
Liquid phase epitaxial growth of CdTe in the CdTe-CdCl2 system
Journal of Crystal Growth 43 (1978) 13 16 © North-Holland Publishing Company
LIQUID PHASE EPITAXIAL GROWTH OF CdTe IN THE CdTe—CdCI2 SYSTEM J. SARAIE, M. KITAGAWA, M. ISHIDA and T. TANAKA Department of Electronics, Kyoto University, Kyoto, Japan Received 2 September 1977
CdTe layers with mirror-like surfaces have been obtained by liquid phase epitaxial technique on CdTe substrates. CdC12 was used as a solvent. Growth temperature was as low as 55O—65O~C.Dependence of the growth rate on the substrate orientation and the growth temperature was studied.
1. Introduction
properties of the substrates. Bi, Ga, In, Sn, Pb and Sb are studied as well [1—10], but there are also some disadvantages. Some of them form compounds with a chalcogen element and some of them have deep impurity levels in the energy gap and significantly affect the electrical properties. Tai et a!. [111 have shown the possibility of using CdCI2 as a solvent for the growth of CdTe which is famous as a good flux for the sintering of CdS powder. CdCl2 has been used for the growth of CdTe bulk crystal [121 and for the LPE of CdSe and ZnTe [13]. In this paper we report the experimental results of LPE of n-CdTe in the CdTe—CdCl2 system.
For the liquid phase epitaxial growth (LPE) of the 111—V compound semiconductors Ga and Sn, metal solvents are successfully used. For the LPE of II VI compounds, the component metal and chalcogen solvents are studied [1 3], but there are some disadvantages. The electrical properties of the epitaxial layer are determined by these solvents because of the strong tendency of selfcompensation of II VI cornpounds and these solvents often change the electrical I
1100
1
0 PRESENT WORK
°
~91 ~
—
H TAI et
~
Fig. I shows the phase diagram of the CdTe— CdC12 system bybyTai al. [111 and by us. Ourpseudobinary results were obtained theettechnique of dif-
1000
ferential thermal analysis. This system has the following advantages from the point of view of LPE. (1) It is a simple eutectic system and CdCl2 has no other compounds with Cd and Te.
900
\ ~ 800
\
(2) The solubility of CdTe in CdCl2 is large compared with that in metal solvents. (3) The low temperature growth is possible. (4) This system only includes component elements
~ 700 600 500
56~ 505°
4O0~
(Cd Te) and donor (Cl), so that it is and adequate for shallow the growth of element highly doped n-type material. (5) CdC12 is soluble in water and it is very easy to
508°
~O00~
20
‘.0
60
80
CdTe
100
detach the substrates from the boat after growth. One disadvantage is that the vapour pressure of CdCl2 is comparatively high (10 Torr at 65 6°C),so
CdCI2 MOLE FRACTION of CdCI2
Fig. 1. Phase diagram of the CdTe—CdC12 system. 13
14
J. Saraie et al.
QUARTZ PLUG
/ LPEgrowth of CdTe in the Cd Te-CdCl2 system
WAFER NOTCH CHARGE SUBSTRATE
COOLING RATE. Rc
___
~
OUARTZ AMPOULE GRAPHITE BOAT
~
~ROWTH”
/
BACK
Fig. 2. Schematic drawing of the growth ampoule. 0
that we must use a closed growth system. Another disadvantage is that commercially available CdC12 includes crystallization water of ~ molecules and a careful process of dehydration is inevitable.
2. Experimental technique A tipping method (Nelson method) was employed. The growth was carried out in a closed system as shown in fig. 2. It shows a cross section of a growth ampoule loaded with a graphite boat. The CdTe wafer is inserted at the end of the boat in order to achieve full saturation of CdTe in the CdC12 melt. The mass density of CdTe is fairly large compared with that of CdCl2 (6.2 and 4.1, respectively) so that CdTe tends to gather at the lower portion of the solution. If the substrate is not horizontal during growth, the growth conditions are not the same on the whole area due to this concentration gradient. A notch at the center of the boat makes it possible to maintain the furnace horizontal during the growth after it is tipped and the substrate is covered with solution. The substrate wafers were cut out from p-type single crystal ingot doped with phosphor prepared by Bridgrnan technique. They were polished mechani-
~
___
I
TIPPING
ii~~:
30
60
DECANTATION 70
TIMEImIn)
Fig. 3. Temperature program for LPE.
cally and etched chemically by “E-solution” (HNO3: H20 : K2Cr2O7 = 10 ml: 20 ml: 4 g). CdCl2 .5/2 H20 was heated to 150°Cin vacuo for about 1 h and dehydrated thoroughly just before weighing. The quartz ampoule and the graphite boat were evacuated and heated to 1100°C for about 2 h for baking. Undoped CdTe powder (synthesized in our laboratory) and CdC12 were weighed according to the phase diagram and mounted in the graphite boat as a charge together with the substrate and the wafer for saturation in dry N2 atmosphere. Usually the amount of CdC12 is 1.5 g. The ampoule was evacuated and heated to 150°Cfor degassing and sealed at 1 X 10 6 Torr. Fig. 3 shows a temperature program. The ampoule was held at TH for 30 mm and the solution was saturated with CdTe. In order to melt-back the substrate the temperature was raised by 3 5°C and held at that temperature TG for about 10 mm. After growth the solution was decanted from the substrate by tipping the furnace and the ampoule was pulled out from the furnace and air-quenched.
GROWN LAYER SUBSTRATE GROWN LAYER
(on BACK SURFACE) Fig. 4. Stained cross section of the grown layer on the (lll)Te substrate. The thickness of the grown layer is 30 ~m.
J. Saraie et al. / LPE growth of CdTe in the CdTe-Cd Cl
2 system
3. Experimental results
15
substrates. The thickness decreases with decreasing TG.
Epitaxial layers with mirror-smooth surfaces were obtained under the conditions that TG = 550 650°C, cooling rate Rc = 0.1 0.5°C/mm,L~sT=20°C.Fig. 4 shows the stained cleaved plane and it reveals the uniform layer thickness and flat boundary between the grown layer and the substrate. Smooth layers were obtained on the substrates with several orientations we tried, that is, on (11 l)cd, (11 l)Te, (110) and (100) surfaces. Fig. 5 shows the dependence of the growth rate on the substrate orientation. In the case of the cooling rate of 0.5°C/mm the order is as follows: (100)> (110)> (11 l)Te > (11 l)cd. But in the case of 0.125°C/min the orientation dependence is hardly recognized. The layer thickness is shown in fig. 6 as a function of TG. These results are on the (11 l)Te and (11 l)cd 0 R
~3 0
-
0 5°C/mn
~ R,~0 125°C/mm =
~2O
600°C 20°c
0
~i.o
0 A
(100)
A
A
(110) (111)~(111)~c~ ORIENTATION
Fig. 5. Dependence of the growth rate on the substrate orientation. ____________________
~ ~ie
J~. RcO.5C/rrun ~T= 20°C E 60L o,/~U L ~-~s~” ~
80
2.0
‘.~
1
~ 0
-
2O~ O
~ 550
I
600
650
0
GROWTH TEMPERATU~~C)
Fig. 6. Dependence of the layer thickness and the growth rate on the growth temperature, TG.
All the samples show n-type conductivity. The study of electrical properties will be reported later together with the vapour pressure effect of the excess component element added to the solution.
4. Discussions The fact that the mirror-smooth layers are obtamed implies that CdC12 is a suitable solvent for the LPE of CdTe, although only n-type layers can be obtamed. The growth rate is determined by solute diffusion and surface free energy of the substrate. One extreme is the diffusion limited case, that is, the growth rate is limited by the solute diffusion and does not depend on the substrate orientation. The other extreme is the surface kinetics limited case and the growth rate depends on the substrate orientation. In our case the growth is diffusion limited under the cooling rate of 0.125°C/mm and the growth rate is independent of the substrate orientation. As for the (100) orientation, it seems to be applied up to the cooling rate of 0.5°C/mm,but as for (110) and (111) orientation surface free energy affects the growth rate. The surface free energy is the largest on (11 l)cd and the smallest on (100). Sangster [14] has proposed a model of the crystal growth on account of the surface structure of the seeds with different orientations. He reported the orientation dependence of the growth rate, on account of the results of the melt growth of lnSb to be (211) > (100) > (110) > (111). Our results agree well with this order. The surface free energy cannot be determined by the substrate itself but by the cornbination with the solvent. Sangster’s model does not consider the existence of the solvent and it is applicable mainly to melt growth. In our case CdCl2 exists as a molecule in the solution and the number of the atoms of Cd and Te are the same and the solution includes about 30 mole% of CdTe, that is, this solution is not so dilute. The growth conditions appear to resemble that of melt growth. It may be the reason why our results agree with those of Sangster’s. The decrease of the thickness of the growth layer with decreasing TG is explained from the shape of the
16
J. Saraie et al. /LPE growth of CdTe in the CdTe-CdC1
liquidus of the phase diagram. The liquidus is upward convex in the relating region and it means that the amount of CdTe precipitating from the same amount of CdCl2 solution decreases with decreasing TG under the same z~T.The solid line in fig. 6 is the calculated thickness simply assuming that CdTe, precipitating from the solution including 1.5 g CdC12 under the same L~T on by the asubstrate with an This area 2,(20°C),grows but it is reduced factor of 0.36. of 1 cmis considered to be due to the simultaneous factor growth on the back and side surfaces. The tendency of the experimental results is explained well by this solid line.
5. Conclusions We have obtained the epitaxial layers with mirrorsmooth surfaces at growth temperatures as low as 550 650°C on (100), (110), (11 l)’re and (11 l)cd oriented substrates. CdC1 2 is a suitable solvent for the LPE of n-type CdTe. The orientation dependence of the growth rate was observed at the cooling rate of 0.5°C/mm and the order is (lOO)>(l10)>(1ll)Te > (11 i)Cd. Any dependence was hardly recognized at the cooling rate of 0.125°C/mm and it is the diffusion limited case. The dependence of the layer thickness on the growth temperature is well explained by the phase diagram.
2 system
Acknowledgement The authors wish to thank H. Hayashi for his assistance in this work.
References [1] M. Rubinstein, J. Crystal Growth 3/4 (1968) 309. [2] H. Ishida and K. Tanaka, in: Proc. 3rd Conf. on Solid State Devices, Tokyo, 1971, Oyo Buturi (J. Japan. Soc. App!. Phys.) 41(1972) Suppl., p. 117. [3] S. Fujita, PhD Thesis, Kyoto University (1975). [4] R. Wagner and M.R. Lorentz, J. Phys. Chem. Solids 27 (1966) 1749. [51R. Widmer, D.P. Bortfeld and H.P. Kleinknecht, J. Crystal Growth 6 (1970) 237. [6] T. Tamura, T. Moriizumi and T. Takahashi, Japan. J. App!. Phys. 10 (1971) 813. [7] 5. Fujita, K. Itoh, S. Arai and T. Sakaguchi, Japan. J. Appi. Phys. 10 (1971) 516. [8] H. Tai and S. Hori, J. Japan Institute Metals 34 (1970) 843. [9] H. Tai and S. ion, J. Japan Institute Metals 38 (1974) 451. [10] Kanamori, T. Ota and K. Takahashi, J. Electrochem. Soc. 122 (1975) 1117. [11] H. Tai and S. Hori, J. Japan Institute Metals 40 (1976) 722. [12] T. Taguchi, J. Shirafuji and Y. Inuishi, Japan. J. Appl. Phys. 13 (1974) 1169. [13] A.V. and R.L. Tsiulyanu, J. Crystal GrowthSimashkevich 35 (1976) 269. [14] R.C. Sangster, in: Compound Semiconductors, Vol. 1 (Reinhold, 1962) p. 241.
LIQUID PHASE EPITAXIAL GROWTH OF CdTe IN THE CdTe—CdCI2 SYSTEM J. SARAIE, M. KITAGAWA, M. ISHIDA and T. TANAKA Department of Electronics, Kyoto University, Kyoto, Japan Received 2 September 1977
CdTe layers with mirror-like surfaces have been obtained by liquid phase epitaxial technique on CdTe substrates. CdC12 was used as a solvent. Growth temperature was as low as 55O—65O~C.Dependence of the growth rate on the substrate orientation and the growth temperature was studied.
1. Introduction
properties of the substrates. Bi, Ga, In, Sn, Pb and Sb are studied as well [1—10], but there are also some disadvantages. Some of them form compounds with a chalcogen element and some of them have deep impurity levels in the energy gap and significantly affect the electrical properties. Tai et a!. [111 have shown the possibility of using CdCI2 as a solvent for the growth of CdTe which is famous as a good flux for the sintering of CdS powder. CdCl2 has been used for the growth of CdTe bulk crystal [121 and for the LPE of CdSe and ZnTe [13]. In this paper we report the experimental results of LPE of n-CdTe in the CdTe—CdCl2 system.
For the liquid phase epitaxial growth (LPE) of the 111—V compound semiconductors Ga and Sn, metal solvents are successfully used. For the LPE of II VI compounds, the component metal and chalcogen solvents are studied [1 3], but there are some disadvantages. The electrical properties of the epitaxial layer are determined by these solvents because of the strong tendency of selfcompensation of II VI cornpounds and these solvents often change the electrical I
1100
1
0 PRESENT WORK
°
~91 ~
—
H TAI et
~
Fig. I shows the phase diagram of the CdTe— CdC12 system bybyTai al. [111 and by us. Ourpseudobinary results were obtained theettechnique of dif-
1000
ferential thermal analysis. This system has the following advantages from the point of view of LPE. (1) It is a simple eutectic system and CdCl2 has no other compounds with Cd and Te.
900
\ ~ 800
\
(2) The solubility of CdTe in CdCl2 is large compared with that in metal solvents. (3) The low temperature growth is possible. (4) This system only includes component elements
~ 700 600 500
56~ 505°
4O0~
(Cd Te) and donor (Cl), so that it is and adequate for shallow the growth of element highly doped n-type material. (5) CdC12 is soluble in water and it is very easy to
508°
~O00~
20
‘.0
60
80
CdTe
100
detach the substrates from the boat after growth. One disadvantage is that the vapour pressure of CdCl2 is comparatively high (10 Torr at 65 6°C),so
CdCI2 MOLE FRACTION of CdCI2
Fig. 1. Phase diagram of the CdTe—CdC12 system. 13
14
J. Saraie et al.
QUARTZ PLUG
/ LPEgrowth of CdTe in the Cd Te-CdCl2 system
WAFER NOTCH CHARGE SUBSTRATE
COOLING RATE. Rc
___
~
OUARTZ AMPOULE GRAPHITE BOAT
~
~ROWTH”
/
BACK
Fig. 2. Schematic drawing of the growth ampoule. 0
that we must use a closed growth system. Another disadvantage is that commercially available CdC12 includes crystallization water of ~ molecules and a careful process of dehydration is inevitable.
2. Experimental technique A tipping method (Nelson method) was employed. The growth was carried out in a closed system as shown in fig. 2. It shows a cross section of a growth ampoule loaded with a graphite boat. The CdTe wafer is inserted at the end of the boat in order to achieve full saturation of CdTe in the CdC12 melt. The mass density of CdTe is fairly large compared with that of CdCl2 (6.2 and 4.1, respectively) so that CdTe tends to gather at the lower portion of the solution. If the substrate is not horizontal during growth, the growth conditions are not the same on the whole area due to this concentration gradient. A notch at the center of the boat makes it possible to maintain the furnace horizontal during the growth after it is tipped and the substrate is covered with solution. The substrate wafers were cut out from p-type single crystal ingot doped with phosphor prepared by Bridgrnan technique. They were polished mechani-
~
___
I
TIPPING
ii~~:
30
60
DECANTATION 70
TIMEImIn)
Fig. 3. Temperature program for LPE.
cally and etched chemically by “E-solution” (HNO3: H20 : K2Cr2O7 = 10 ml: 20 ml: 4 g). CdCl2 .5/2 H20 was heated to 150°Cin vacuo for about 1 h and dehydrated thoroughly just before weighing. The quartz ampoule and the graphite boat were evacuated and heated to 1100°C for about 2 h for baking. Undoped CdTe powder (synthesized in our laboratory) and CdC12 were weighed according to the phase diagram and mounted in the graphite boat as a charge together with the substrate and the wafer for saturation in dry N2 atmosphere. Usually the amount of CdC12 is 1.5 g. The ampoule was evacuated and heated to 150°Cfor degassing and sealed at 1 X 10 6 Torr. Fig. 3 shows a temperature program. The ampoule was held at TH for 30 mm and the solution was saturated with CdTe. In order to melt-back the substrate the temperature was raised by 3 5°C and held at that temperature TG for about 10 mm. After growth the solution was decanted from the substrate by tipping the furnace and the ampoule was pulled out from the furnace and air-quenched.
GROWN LAYER SUBSTRATE GROWN LAYER
(on BACK SURFACE) Fig. 4. Stained cross section of the grown layer on the (lll)Te substrate. The thickness of the grown layer is 30 ~m.
J. Saraie et al. / LPE growth of CdTe in the CdTe-Cd Cl
2 system
3. Experimental results
15
substrates. The thickness decreases with decreasing TG.
Epitaxial layers with mirror-smooth surfaces were obtained under the conditions that TG = 550 650°C, cooling rate Rc = 0.1 0.5°C/mm,L~sT=20°C.Fig. 4 shows the stained cleaved plane and it reveals the uniform layer thickness and flat boundary between the grown layer and the substrate. Smooth layers were obtained on the substrates with several orientations we tried, that is, on (11 l)cd, (11 l)Te, (110) and (100) surfaces. Fig. 5 shows the dependence of the growth rate on the substrate orientation. In the case of the cooling rate of 0.5°C/mm the order is as follows: (100)> (110)> (11 l)Te > (11 l)cd. But in the case of 0.125°C/min the orientation dependence is hardly recognized. The layer thickness is shown in fig. 6 as a function of TG. These results are on the (11 l)Te and (11 l)cd 0 R
~3 0
-
0 5°C/mn
~ R,~0 125°C/mm =
~2O
600°C 20°c
0
~i.o
0 A
(100)
A
A
(110) (111)~(111)~c~ ORIENTATION
Fig. 5. Dependence of the growth rate on the substrate orientation. ____________________
~ ~ie
J~. RcO.5C/rrun ~T= 20°C E 60L o,/~U L ~-~s~” ~
80
2.0
‘.~
1
~ 0
-
2O~ O
~ 550
I
600
650
0
GROWTH TEMPERATU~~C)
Fig. 6. Dependence of the layer thickness and the growth rate on the growth temperature, TG.
All the samples show n-type conductivity. The study of electrical properties will be reported later together with the vapour pressure effect of the excess component element added to the solution.
4. Discussions The fact that the mirror-smooth layers are obtamed implies that CdC12 is a suitable solvent for the LPE of CdTe, although only n-type layers can be obtamed. The growth rate is determined by solute diffusion and surface free energy of the substrate. One extreme is the diffusion limited case, that is, the growth rate is limited by the solute diffusion and does not depend on the substrate orientation. The other extreme is the surface kinetics limited case and the growth rate depends on the substrate orientation. In our case the growth is diffusion limited under the cooling rate of 0.125°C/mm and the growth rate is independent of the substrate orientation. As for the (100) orientation, it seems to be applied up to the cooling rate of 0.5°C/mm,but as for (110) and (111) orientation surface free energy affects the growth rate. The surface free energy is the largest on (11 l)cd and the smallest on (100). Sangster [14] has proposed a model of the crystal growth on account of the surface structure of the seeds with different orientations. He reported the orientation dependence of the growth rate, on account of the results of the melt growth of lnSb to be (211) > (100) > (110) > (111). Our results agree well with this order. The surface free energy cannot be determined by the substrate itself but by the cornbination with the solvent. Sangster’s model does not consider the existence of the solvent and it is applicable mainly to melt growth. In our case CdCl2 exists as a molecule in the solution and the number of the atoms of Cd and Te are the same and the solution includes about 30 mole% of CdTe, that is, this solution is not so dilute. The growth conditions appear to resemble that of melt growth. It may be the reason why our results agree with those of Sangster’s. The decrease of the thickness of the growth layer with decreasing TG is explained from the shape of the
16
J. Saraie et al. /LPE growth of CdTe in the CdTe-CdC1
liquidus of the phase diagram. The liquidus is upward convex in the relating region and it means that the amount of CdTe precipitating from the same amount of CdCl2 solution decreases with decreasing TG under the same z~T.The solid line in fig. 6 is the calculated thickness simply assuming that CdTe, precipitating from the solution including 1.5 g CdC12 under the same L~T on by the asubstrate with an This area 2,(20°C),grows but it is reduced factor of 0.36. of 1 cmis considered to be due to the simultaneous factor growth on the back and side surfaces. The tendency of the experimental results is explained well by this solid line.
5. Conclusions We have obtained the epitaxial layers with mirrorsmooth surfaces at growth temperatures as low as 550 650°C on (100), (110), (11 l)’re and (11 l)cd oriented substrates. CdC1 2 is a suitable solvent for the LPE of n-type CdTe. The orientation dependence of the growth rate was observed at the cooling rate of 0.5°C/mm and the order is (lOO)>(l10)>(1ll)Te > (11 i)Cd. Any dependence was hardly recognized at the cooling rate of 0.125°C/mm and it is the diffusion limited case. The dependence of the layer thickness on the growth temperature is well explained by the phase diagram.
2 system
Acknowledgement The authors wish to thank H. Hayashi for his assistance in this work.
References [1] M. Rubinstein, J. Crystal Growth 3/4 (1968) 309. [2] H. Ishida and K. Tanaka, in: Proc. 3rd Conf. on Solid State Devices, Tokyo, 1971, Oyo Buturi (J. Japan. Soc. App!. Phys.) 41(1972) Suppl., p. 117. [3] S. Fujita, PhD Thesis, Kyoto University (1975). [4] R. Wagner and M.R. Lorentz, J. Phys. Chem. Solids 27 (1966) 1749. [51R. Widmer, D.P. Bortfeld and H.P. Kleinknecht, J. Crystal Growth 6 (1970) 237. [6] T. Tamura, T. Moriizumi and T. Takahashi, Japan. J. App!. Phys. 10 (1971) 813. [7] 5. Fujita, K. Itoh, S. Arai and T. Sakaguchi, Japan. J. Appi. Phys. 10 (1971) 516. [8] H. Tai and S. Hori, J. Japan Institute Metals 34 (1970) 843. [9] H. Tai and S. ion, J. Japan Institute Metals 38 (1974) 451. [10] Kanamori, T. Ota and K. Takahashi, J. Electrochem. Soc. 122 (1975) 1117. [11] H. Tai and S. Hori, J. Japan Institute Metals 40 (1976) 722. [12] T. Taguchi, J. Shirafuji and Y. Inuishi, Japan. J. Appl. Phys. 13 (1974) 1169. [13] A.V. and R.L. Tsiulyanu, J. Crystal GrowthSimashkevich 35 (1976) 269. [14] R.C. Sangster, in: Compound Semiconductors, Vol. 1 (Reinhold, 1962) p. 241.