- Email: [email protected]
Achievement of low p-type carrier concentration for MOCVD growth HgCdTe without an annealing process
,outwuo, CRYSTAL QROWTH ELSEVIER
Journal of Crystal Growth 184/185 (1998) 122881231
Achievement of low p-type carrier concentration for MOCVD growth HgCdTe without an annealing process K. Matsushita
*,l ,
K. Shigenaka’, A. Kamata
Materials and Devices Research Laboratories, Toshiba Corporation, I Komukai. Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan
Abstract The electrical characteristics of as-grown epitaxial HgCdTe layers grown by metalorganic chemical vapor deposition were investigated. We propose a new cap layer in order to obtain as-grown HgCdTe layers with low p-type carrier concentration. The new cap layer called the “double-cap layer” consists of a graded Cd content Hg, -,Cd,Te layer with x = 0.24.7 and a CdTe cap layer which was continuously grown at low temperature on the graded HgCdTe layer. The carrier concentration of HgCdTe layers with the new cap layer was about 3 x 1Or6 cm- 3. The p-type carrier concentration has been reduced further to 2 x 1015 cm- 3 (for x = 0.22) by lowering the growth temperature and increasing the
partial pressure of Hg. This carrier concentration is lower than the estimated Hg vacancy concentration considerations of the thermal equilibrium conditions. 0 1998 Elsevier Science B.V. All rights reserved. PACS:
from
81.15.Gh; 73.90. + f; 61.72.51
Keywords:
MOCVD;
HgCdTe;
Electrical
properties;
p-Type conduction;
The electrical characteristics of HgCdTe depend strongly on Hg vacancies, which act as acceptors. These vacancies are mainly generated during the cooling processes in metalorganic chemical vapor deposition (MOCVD) [l] and their concentration is difficult to control during the growth. The carrier concentration in HgCdTe has often been held to below 1016 cmW3 by a thermal equilib-
*Corresponding author. Fax: + 81 44 548 5955; e-mail: [email protected]. 1Present address: Microwave Solid-State Department, Komukai Works, Toshiba Corporation 1, Komukai, Toshibacho, Saiwai-ku, Kawasaki 210, Japan. 0022-0248/98/$19.00 0
rium anneal under a Hg atmosphere [2,3]. Therefore, annealing processes are necessary to obtain low carrier concentration p-type layers which are grown by MOCVD [2,4], MBE as well as by LPE. It has been reported that the CdTe cap layer prevents Hg atoms from dissociating and changing the conduction type of HgCdTe, but the relationship between the partial pressure of Hg in the growth of HgCdTe and the as-grown electrical properties was not mentioned [l]. In this study, the electrical properties of asgrown HgCdTe layers with a new cap layer which suppresses the Hg vacancy creation after HgCdTe layer growth were investigated.
1998 Elsevier Science B.V. All rights reserved.
PII SOO22-0248(97)00620-9
Cap layer
K. Matsushita
et al. /Journal
qf Crystal Growth 1841185 (1998) 1228-1231
The epitaxial layers were grown on Cd, _,Zn,Te (y = 0.045) (2 1 l)B substrates by MOCVD under atmospheric pressure, using diisopropyltelluride (DIPTe), dimethylcadmium (DMCd), and elemental Hg as source materials [S]. The temperature of the Hg reservoir was precisely controlled to within f 1°C. The substrates were etched in a 2% Brmethanol solution for 1 min and rinsed in deionised water. The HgCdTe layers were grown to a thickness of 4 urn over a period of 2 h on 0.05 urn thick CdTe buffer layers. The thin buffer layer effects the electrical properties of the HgCdTe layer [6]. The growth temperature for HgCdTe was 360 or 350°C. The electrical properties of as-grown HgCdTe layers were measured by Hall effect at 77 K under a 0.5 T magnetic field after removing the surface by a 0.7 urn etch in a Br-methanol solution. The Cd content of the HgCdTe layers, estimated from Fourier transform infrared (FTIR), was between 0.17 and 0.35. Effects of cap layers on electrical properties of HgCdTe layers were investigated by comparing the following post growth procedures. The schematic structures of HgCdTe and cap layers are shown in Fig. 1. For (A), no cap layer was formed on the HgCdTe. The layers were cooled down to the substrate temperature of 200°C under the equivalent Hg partial pressure to that of the HgCdTe growth. For (B), a single CdTe cap layer was subsequently formed on the HgCdTe layer at 360°C without the Hg supply. The thickness of the cap layer was 0.3 urn. For (C), the double cap layers were fabricated onto the HgCdTe layer in succession. The first layer was a Hg, _,Cd,Te layer in which the Cd
1229
content was made to increase gradually up to 0.7 by lowering the substrate temperature. The second layer was a CdTe layer grown at a low-temperature of 320°C without the Hg supply on the graded Cd content layers. The first graded Cd content Hg,_,Cd,Te layer was grown for 3 min on Hg,.sCdO,,Te. This layer is automatically obtained just by decreasing the substrate temperature from 360 to 320°C while maintaining the source material flow rates. The content of Cd gradually increased from 0.2 to 0.7. The second CdTe cap layer was grown at 320°C without supplying Hg flow. The sample was cooled down to room temperature. The total thickness of the double cap layers was about 0.5 urn. The e&t ofcap layers. Table 1 lists the electrical results for as-grown HgCdTe layers with or without cap layers. The HgCdTe layers (A) without cap layer and (B) with the single CdTe cap layer are also shown to have p-type conductivity with about 1017 cme3 carrier concentration. On the other hand, the HgCdTe layer with (C) the double cap layer indicated a low of lOi cmm3 with p-type conduction. The carrier concentration of HgCdTe layers depended on the post growth procedures. It is considered that the Hg vacancies are mainly created at the end of the growth and the carrier concentration were determined during the cooling process. These phenomena are explained by defect formation reactions and the mass action relations are shown below [7,8]: Hg,.sCdo,2Te
*
CVH,~~I + HgW + 2h ,
KVHg” = [V,,,J[h’]2P,,
(1)
= 1.58 x 106’
x exp( - 2.24e I//k T) cm - ’ atm,
I
(A)
CdTe cap layer
CdTe
CdTe buffer layer
I
I (B)
(2)
< Graded Cd content cap layer
I
cc>
Fig. 1. The structures of HgCdTe layers; (A) without cap layer, (B) with CdTe single cap layer; (c) with graded Cd content HgCdTe layer and CdTe cap layer.
Table 1 Electrical properties of HgCdTe layers with various cap layers obtained by Hall effect measurement (at 77 K): (A) without cap layer, (B) with CdTe single cap layer, (C) with the double cap layers. Growth parameters: T, = 360°C; Pup = 3.5 x 1O-3 (atm) Carrier
cont. (cm-‘)
(A) 9.3 x 1Or6 (p) (B) 1.7 x 10” (p) (C) 3.2 x 1OL6 (p)
Mobility 410 330 428
(cm’ V-t
s-‘)
x 0.22 0.22 0.23
1230
K. Matsushita et al. 1 Journal ofCiysta1 Growth 184/185 (1998) 1228-1231
where VHgSS indicates a doubly negative ionized vacant lattice site of Hg which generates two holes in Hg, _,Cd,Te as shown in Eq. (1) and KvHBSS is the equilibrium constant for the Hg vacancies, which is a function of the equilibrium temperature. The concentration of Hg vacancies is determined by the equilibrium temperatures and the partial pressure of Hg. If the cap layer was not deposited, the carrier concentration of HgCdTe was about 101’ cmm3, which is due to the dissociation of Hg atoms from the surface during the cooling from the growth temperature of 360°C to room temperature. Even when a single CdTe cap layer was formed, the dissociation of Hg atoms occurred because of the lack of the partial pressure of Hg after the HgCdTe layer growth. However, when the double cap layers were deposited, low carrier concentration layers were obtained. The reason is probably that the growth temperature was lowered from 360 to 320°C so that the Hg atoms did not dissociate from the HgCdTe layer, and the CdTe layer grown at low temperature effectively suppressed any further dissociation of Hg atoms. According to Eqs. (1) and (2) the concentration of Hg vacancies at 320°C is less than half as much as at 360°C. Then the generation of the Hg vacancies decreased at the cooling stage. Consequently, the electrical characteristics of the as-grown HgCdTe layer with the double cap layer were revealed. This double cap layer enables the electrical properties of the HgCdTe layer to be controlled without annealing. When the cap layer was not formed or a single CdTe layer was formed, the Hall concentrations were not reduced by either lowering the growth temperature or increasing the partial pressure of Hg during the growth. Controlling carrier concentration. Fig. 2 shows the Hall concentration at 77 K as a function of the Cd content of Hg,_,Cd,Te. All samples had the double cap layers. As a result of lowering the substrate temperature and increasing the partial presthat the carrier sure of Hg, it is obvious concentration is reduced into the range of 2 x 1O1’ to 2 x lO”j cmm3. The HgCdTe layers became ntype and showed high mobility in the range from 2~10~ to 4x105cm2V-‘s-’ when the Cd content was less than 0.2 as shown by open circle symbols. The ratio of electron-to-hole mobility in
p-type region 1 k
1014
I
,
I
I
0.20
%\I
,
0.25
I
I
I
0.30
x : Hg,.,Cd,Te Fig. 2. The composition
dependence of Hall concentration on cadmium in HgCdTe. (A) Tsub = 36O”C, P,, = 3.5 x lO-3 atm, (0,O) Tsub = 35O”C, PHg = 6.3 x 10e3 atm. Solid line shows calculated n to p conversion and dashed line shows minimum of carrier concentration.
HgCdTe is so large that the contribution of the minority carrier to the Hall concentration is not negligible in the p-type HgCdTe layer. The Hall concentration is given by following equation: C.C. = (p + nb)‘/(p - nb’),
(3)
where b is the ratio of electron-to-hole mobility and is assumed to be b = 300 [9]. It should be noted that no residual impurity contamination is considered. In Fig. 2, the solid line shows the boundary of carrier type conversion, and the dashed line is the minimum Hall concentration. The sample which was plotted with the solid circle marked in Fig. 2 had the lowest hole concentration value in HgCdTe (for x = 0.20). The concentration of Hg vacancies was estimated to be about 5 x 1015 cme3 with this growth condition. Fig. 3 shows the equilibrium phase diagram of Hg,,,CdO,,Te. The isohole concentration lines for Hg,,sCdO,,Te calculated by Vydyanath et al. [lo] are also shown in the figure. The hole concentration of HgCdTe is determined by the partial pressure of Hg and the temperature of isothermal annealing in Hg atmosphere. The partial pressures of Hg and the reciprocal temperatures for this MOCVD experiment were plotted as square symbols in Fig. 3. If the MOCVD growth of HgCdTe occurred under conditions of thermal equilibrium,
K. Matsushita
et al. J Journal ofCysta1
1.5
1.6
1 03/T Fig. 3. P-T Hg,.sCda.,Te. periment.
1231
atoms at the end of growth and maintained the characteristics of the HgCdTe layers obtained during the growth process. Using cap layers and optimizing the growth temperature and the partial pressure of Hg, the hole concentration of as-grown Hg,,7sCd0.zzTe layers was reduced to as low as 2 x 1015 cm-3. This method enables Hg vacancies in HgCdTe to be controlled during the MOCVD epitaxial growth, and yields low p-type carrier concentration HgCdTe layers without the post-growth annealing process.
T (“C)
‘-1.4
Growth 1841185 (1998) 1228-1231
1.7
1.0
(K’)
curve and isohole concentration The square symbols were conditions
The authors would like to thank Mr. M. Kuroda, Dr. Y. Ikawa an Mr. M. Azuma for their encouragement throughout this work.
lines in of this ex-
References Cl1 S.J. C Irvine, J.S. Gough, J. Giess, M.J. Gibbs, A. Royle,
the carrier concentration expected from the isohole concentration would be more than lOi cmm3. However, the Hall concentration for the as-grown layer with double cap layer was of the order of 1016 cm-3. The difference between the expected Hall concentration and the measured Hall concentration means that the growth of HgCdTe by MOCVD can be considered to proceed under conditions which are far from the thermal equilibrium condition. In conclusion, low carrier concentrations in MOCVD grown HgCdTe layers have been achieved without post-growth annealing processes by using graded Cd content and low-temperature grown CdTe double cap layers. It is possible that these cap layers prevented the dissociation of Hg
CA. Taylor, G.T. Brown, A.M. Keir, J.B. Mullin, J. Vat. Sci. Technol. A 7 (1989) 285. PI S. Barton, P. Capper, C.J. Jones, N. Metcalfe, N.T. Gordon, Semicond. Sci. Technol. 10 (1995) 56. c31 B. Li,X.Q. Chen, J.H. Chu, J.Y. Cao, J.Q. Zhu, D.Y. Tang, Thin Solid Films 278 (1996) 1. M W.L. Ahlgren, E.J. Smith, J.B. James, T.W. James, R.P. Ruth, E.A. Patten, R.D. Knox, J.-L. Staudenmann, J. Crystal Growth 86 (1988) 198. L. Sugiura, F. Nakata, K. Hirahara, J. c51 K. Shigenaka, Electron. Mater. 22 (1993) 865. et al., to be published. C61 K. Matsushita J. Electrochem. Sot. 128 (1981) 2609. c71 H.R. Vydyanath, PI J.B. Mullin, A. Royle, J. Giess, J.S. Gough, S.J.C. Irvine, J. Crystal Growth 77 (1986) 460. c91 MC. Gold, D.A. Nelson, J. Vat. Sci. Technol. A 4 (1986) 2040. C.H. Hiner, J. Appl. Phys. 65 (1989) [lOI H.R. Vydyanath, 15.
Journal of Crystal Growth 184/185 (1998) 122881231
Achievement of low p-type carrier concentration for MOCVD growth HgCdTe without an annealing process K. Matsushita
*,l ,
K. Shigenaka’, A. Kamata
Materials and Devices Research Laboratories, Toshiba Corporation, I Komukai. Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan
Abstract The electrical characteristics of as-grown epitaxial HgCdTe layers grown by metalorganic chemical vapor deposition were investigated. We propose a new cap layer in order to obtain as-grown HgCdTe layers with low p-type carrier concentration. The new cap layer called the “double-cap layer” consists of a graded Cd content Hg, -,Cd,Te layer with x = 0.24.7 and a CdTe cap layer which was continuously grown at low temperature on the graded HgCdTe layer. The carrier concentration of HgCdTe layers with the new cap layer was about 3 x 1Or6 cm- 3. The p-type carrier concentration has been reduced further to 2 x 1015 cm- 3 (for x = 0.22) by lowering the growth temperature and increasing the
partial pressure of Hg. This carrier concentration is lower than the estimated Hg vacancy concentration considerations of the thermal equilibrium conditions. 0 1998 Elsevier Science B.V. All rights reserved. PACS:
from
81.15.Gh; 73.90. + f; 61.72.51
Keywords:
MOCVD;
HgCdTe;
Electrical
properties;
p-Type conduction;
The electrical characteristics of HgCdTe depend strongly on Hg vacancies, which act as acceptors. These vacancies are mainly generated during the cooling processes in metalorganic chemical vapor deposition (MOCVD) [l] and their concentration is difficult to control during the growth. The carrier concentration in HgCdTe has often been held to below 1016 cmW3 by a thermal equilib-
*Corresponding author. Fax: + 81 44 548 5955; e-mail: [email protected]. 1Present address: Microwave Solid-State Department, Komukai Works, Toshiba Corporation 1, Komukai, Toshibacho, Saiwai-ku, Kawasaki 210, Japan. 0022-0248/98/$19.00 0
rium anneal under a Hg atmosphere [2,3]. Therefore, annealing processes are necessary to obtain low carrier concentration p-type layers which are grown by MOCVD [2,4], MBE as well as by LPE. It has been reported that the CdTe cap layer prevents Hg atoms from dissociating and changing the conduction type of HgCdTe, but the relationship between the partial pressure of Hg in the growth of HgCdTe and the as-grown electrical properties was not mentioned [l]. In this study, the electrical properties of asgrown HgCdTe layers with a new cap layer which suppresses the Hg vacancy creation after HgCdTe layer growth were investigated.
1998 Elsevier Science B.V. All rights reserved.
PII SOO22-0248(97)00620-9
Cap layer
K. Matsushita
et al. /Journal
qf Crystal Growth 1841185 (1998) 1228-1231
The epitaxial layers were grown on Cd, _,Zn,Te (y = 0.045) (2 1 l)B substrates by MOCVD under atmospheric pressure, using diisopropyltelluride (DIPTe), dimethylcadmium (DMCd), and elemental Hg as source materials [S]. The temperature of the Hg reservoir was precisely controlled to within f 1°C. The substrates were etched in a 2% Brmethanol solution for 1 min and rinsed in deionised water. The HgCdTe layers were grown to a thickness of 4 urn over a period of 2 h on 0.05 urn thick CdTe buffer layers. The thin buffer layer effects the electrical properties of the HgCdTe layer [6]. The growth temperature for HgCdTe was 360 or 350°C. The electrical properties of as-grown HgCdTe layers were measured by Hall effect at 77 K under a 0.5 T magnetic field after removing the surface by a 0.7 urn etch in a Br-methanol solution. The Cd content of the HgCdTe layers, estimated from Fourier transform infrared (FTIR), was between 0.17 and 0.35. Effects of cap layers on electrical properties of HgCdTe layers were investigated by comparing the following post growth procedures. The schematic structures of HgCdTe and cap layers are shown in Fig. 1. For (A), no cap layer was formed on the HgCdTe. The layers were cooled down to the substrate temperature of 200°C under the equivalent Hg partial pressure to that of the HgCdTe growth. For (B), a single CdTe cap layer was subsequently formed on the HgCdTe layer at 360°C without the Hg supply. The thickness of the cap layer was 0.3 urn. For (C), the double cap layers were fabricated onto the HgCdTe layer in succession. The first layer was a Hg, _,Cd,Te layer in which the Cd
1229
content was made to increase gradually up to 0.7 by lowering the substrate temperature. The second layer was a CdTe layer grown at a low-temperature of 320°C without the Hg supply on the graded Cd content layers. The first graded Cd content Hg,_,Cd,Te layer was grown for 3 min on Hg,.sCdO,,Te. This layer is automatically obtained just by decreasing the substrate temperature from 360 to 320°C while maintaining the source material flow rates. The content of Cd gradually increased from 0.2 to 0.7. The second CdTe cap layer was grown at 320°C without supplying Hg flow. The sample was cooled down to room temperature. The total thickness of the double cap layers was about 0.5 urn. The e&t ofcap layers. Table 1 lists the electrical results for as-grown HgCdTe layers with or without cap layers. The HgCdTe layers (A) without cap layer and (B) with the single CdTe cap layer are also shown to have p-type conductivity with about 1017 cme3 carrier concentration. On the other hand, the HgCdTe layer with (C) the double cap layer indicated a low of lOi cmm3 with p-type conduction. The carrier concentration of HgCdTe layers depended on the post growth procedures. It is considered that the Hg vacancies are mainly created at the end of the growth and the carrier concentration were determined during the cooling process. These phenomena are explained by defect formation reactions and the mass action relations are shown below [7,8]: Hg,.sCdo,2Te
*
CVH,~~I + HgW + 2h ,
KVHg” = [V,,,J[h’]2P,,
(1)
= 1.58 x 106’
x exp( - 2.24e I//k T) cm - ’ atm,
I
(A)
CdTe cap layer
CdTe
CdTe buffer layer
I
I (B)
(2)
< Graded Cd content cap layer
I
cc>
Fig. 1. The structures of HgCdTe layers; (A) without cap layer, (B) with CdTe single cap layer; (c) with graded Cd content HgCdTe layer and CdTe cap layer.
Table 1 Electrical properties of HgCdTe layers with various cap layers obtained by Hall effect measurement (at 77 K): (A) without cap layer, (B) with CdTe single cap layer, (C) with the double cap layers. Growth parameters: T, = 360°C; Pup = 3.5 x 1O-3 (atm) Carrier
cont. (cm-‘)
(A) 9.3 x 1Or6 (p) (B) 1.7 x 10” (p) (C) 3.2 x 1OL6 (p)
Mobility 410 330 428
(cm’ V-t
s-‘)
x 0.22 0.22 0.23
1230
K. Matsushita et al. 1 Journal ofCiysta1 Growth 184/185 (1998) 1228-1231
where VHgSS indicates a doubly negative ionized vacant lattice site of Hg which generates two holes in Hg, _,Cd,Te as shown in Eq. (1) and KvHBSS is the equilibrium constant for the Hg vacancies, which is a function of the equilibrium temperature. The concentration of Hg vacancies is determined by the equilibrium temperatures and the partial pressure of Hg. If the cap layer was not deposited, the carrier concentration of HgCdTe was about 101’ cmm3, which is due to the dissociation of Hg atoms from the surface during the cooling from the growth temperature of 360°C to room temperature. Even when a single CdTe cap layer was formed, the dissociation of Hg atoms occurred because of the lack of the partial pressure of Hg after the HgCdTe layer growth. However, when the double cap layers were deposited, low carrier concentration layers were obtained. The reason is probably that the growth temperature was lowered from 360 to 320°C so that the Hg atoms did not dissociate from the HgCdTe layer, and the CdTe layer grown at low temperature effectively suppressed any further dissociation of Hg atoms. According to Eqs. (1) and (2) the concentration of Hg vacancies at 320°C is less than half as much as at 360°C. Then the generation of the Hg vacancies decreased at the cooling stage. Consequently, the electrical characteristics of the as-grown HgCdTe layer with the double cap layer were revealed. This double cap layer enables the electrical properties of the HgCdTe layer to be controlled without annealing. When the cap layer was not formed or a single CdTe layer was formed, the Hall concentrations were not reduced by either lowering the growth temperature or increasing the partial pressure of Hg during the growth. Controlling carrier concentration. Fig. 2 shows the Hall concentration at 77 K as a function of the Cd content of Hg,_,Cd,Te. All samples had the double cap layers. As a result of lowering the substrate temperature and increasing the partial presthat the carrier sure of Hg, it is obvious concentration is reduced into the range of 2 x 1O1’ to 2 x lO”j cmm3. The HgCdTe layers became ntype and showed high mobility in the range from 2~10~ to 4x105cm2V-‘s-’ when the Cd content was less than 0.2 as shown by open circle symbols. The ratio of electron-to-hole mobility in
p-type region 1 k
1014
I
,
I
I
0.20
%\I
,
0.25
I
I
I
0.30
x : Hg,.,Cd,Te Fig. 2. The composition
dependence of Hall concentration on cadmium in HgCdTe. (A) Tsub = 36O”C, P,, = 3.5 x lO-3 atm, (0,O) Tsub = 35O”C, PHg = 6.3 x 10e3 atm. Solid line shows calculated n to p conversion and dashed line shows minimum of carrier concentration.
HgCdTe is so large that the contribution of the minority carrier to the Hall concentration is not negligible in the p-type HgCdTe layer. The Hall concentration is given by following equation: C.C. = (p + nb)‘/(p - nb’),
(3)
where b is the ratio of electron-to-hole mobility and is assumed to be b = 300 [9]. It should be noted that no residual impurity contamination is considered. In Fig. 2, the solid line shows the boundary of carrier type conversion, and the dashed line is the minimum Hall concentration. The sample which was plotted with the solid circle marked in Fig. 2 had the lowest hole concentration value in HgCdTe (for x = 0.20). The concentration of Hg vacancies was estimated to be about 5 x 1015 cme3 with this growth condition. Fig. 3 shows the equilibrium phase diagram of Hg,,,CdO,,Te. The isohole concentration lines for Hg,,sCdO,,Te calculated by Vydyanath et al. [lo] are also shown in the figure. The hole concentration of HgCdTe is determined by the partial pressure of Hg and the temperature of isothermal annealing in Hg atmosphere. The partial pressures of Hg and the reciprocal temperatures for this MOCVD experiment were plotted as square symbols in Fig. 3. If the MOCVD growth of HgCdTe occurred under conditions of thermal equilibrium,
K. Matsushita
et al. J Journal ofCysta1
1.5
1.6
1 03/T Fig. 3. P-T Hg,.sCda.,Te. periment.
1231
atoms at the end of growth and maintained the characteristics of the HgCdTe layers obtained during the growth process. Using cap layers and optimizing the growth temperature and the partial pressure of Hg, the hole concentration of as-grown Hg,,7sCd0.zzTe layers was reduced to as low as 2 x 1015 cm-3. This method enables Hg vacancies in HgCdTe to be controlled during the MOCVD epitaxial growth, and yields low p-type carrier concentration HgCdTe layers without the post-growth annealing process.
T (“C)
‘-1.4
Growth 1841185 (1998) 1228-1231
1.7
1.0
(K’)
curve and isohole concentration The square symbols were conditions
The authors would like to thank Mr. M. Kuroda, Dr. Y. Ikawa an Mr. M. Azuma for their encouragement throughout this work.
lines in of this ex-
References Cl1 S.J. C Irvine, J.S. Gough, J. Giess, M.J. Gibbs, A. Royle,
the carrier concentration expected from the isohole concentration would be more than lOi cmm3. However, the Hall concentration for the as-grown layer with double cap layer was of the order of 1016 cm-3. The difference between the expected Hall concentration and the measured Hall concentration means that the growth of HgCdTe by MOCVD can be considered to proceed under conditions which are far from the thermal equilibrium condition. In conclusion, low carrier concentrations in MOCVD grown HgCdTe layers have been achieved without post-growth annealing processes by using graded Cd content and low-temperature grown CdTe double cap layers. It is possible that these cap layers prevented the dissociation of Hg
CA. Taylor, G.T. Brown, A.M. Keir, J.B. Mullin, J. Vat. Sci. Technol. A 7 (1989) 285. PI S. Barton, P. Capper, C.J. Jones, N. Metcalfe, N.T. Gordon, Semicond. Sci. Technol. 10 (1995) 56. c31 B. Li,X.Q. Chen, J.H. Chu, J.Y. Cao, J.Q. Zhu, D.Y. Tang, Thin Solid Films 278 (1996) 1. M W.L. Ahlgren, E.J. Smith, J.B. James, T.W. James, R.P. Ruth, E.A. Patten, R.D. Knox, J.-L. Staudenmann, J. Crystal Growth 86 (1988) 198. L. Sugiura, F. Nakata, K. Hirahara, J. c51 K. Shigenaka, Electron. Mater. 22 (1993) 865. et al., to be published. C61 K. Matsushita J. Electrochem. Sot. 128 (1981) 2609. c71 H.R. Vydyanath, PI J.B. Mullin, A. Royle, J. Giess, J.S. Gough, S.J.C. Irvine, J. Crystal Growth 77 (1986) 460. c91 MC. Gold, D.A. Nelson, J. Vat. Sci. Technol. A 4 (1986) 2040. C.H. Hiner, J. Appl. Phys. 65 (1989) [lOI H.R. Vydyanath, 15.