- Email: [email protected]
Growth characteristics of (100)HgCdTe layers in low-temperature MOVPE with ditertiarybutyltelluride
......... ClmYITAL OIROW'rH
ELSEVIER
Journal of Crystal Growth 166 (1996) 612-616
Growth characteristics of(100) HgCdTe layers in low-temperature MOVPE with ditertiarybutyltelluride K. Yasuda *, H. Hatano, T. Ferid, M. Minamide, T. Maejima, K. Kawamoto Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466, Japan
Abstract Low-temperature growth of (100)HgCdTe (MCT) layers in MOVPE has been studied using ditertiarybutyltelluride (DtBTe), dimethylcadmium (DMCd), and elementary mercury as precursors. MCT layers were grown at 275°C on (100)GaAs substrates. Growths were carried out in a vertical growth cell which has a narrow spacing between the substrate and cell ceiling. Using the growth cell, the Cd-composition (x) of MCT layers was controlled over a wide range from 0 to 0.98 by the DMCd flow. The growth rate of the MCT layers was constant at 5 I~m h-l for the increased DMCd flow. Preferential Cd-incorporation into MCT layers and an increase of the growth rate were observed in the presence of mercury vapor. The growth characteristics were considered to be due to the alkyl-exchange reaction between DMCd and mercury. The electrical properties and crystallinity of grown layers were also evaluated, which showed that layers with high quality can be grown at 275°C.
1. Introduction In the recent work on epitaxial growth of HgCdTe (MCT), metalorganic vapor phase epitaxy (MOVPE) has been one of the major growth techniques. MOVPE growth of MCT has been carried out in the growth temperature range from 350 to 425°C. The lower limit of the growth temperature is determined by the thermal stability of tellurium precursors. Decrease of the growth temperature is expected to have several advantages over the usual growth above 350°C. One advantage is a decrease of the mercury-vacancy formation during growth and another is a decrease of interdiffusion at the growth interface. These two effects are important not only for further improvements of infrared detectors but
* Corresponding author. Fax: +81 52 735 5442; E-mail: [email protected].
also for applications in new electronic devices where the controllability of the layer structure and electrical properties by extrinsic dopants is essential. The increase of dopant adsorption on the growth surface and the suppression of neutral vacancy-impurity complex formation are expected at low growth temperatures [1,2]. To decrease the growth temperature of MCT in MOVPE, several growth techniques have been studied previously. These are: photolysis [3], the use of thermally unstable Te precursors [4-6], and the precracking of precursors [7]. However, the growth characteristics and electronic properties of the layers have not been fully studied for a wide range of Cd composition. In this paper, we have studied the growth characteristics of MCT layers at a low growth temperature of 275°C using ditertiarybutyltelluride (DtBTe) as a Te precursor. The growths were carried out for wide range of Cd-compositions using a newly developed
0022-0248/96//$t5.00 Copyright© 1996Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00536-6
K. Yasuda et al. /Journal of Crystal Growth 166 (1996) 612-616
DtBTe DETe/.~ ~~____~~nts DMCd
Illll
~ I L
C~,;C~iling _-)~l-l--Susceptor 275*(3
=e
Fig. 1. Construction of the narrow-spacing MOVPE growth cell used for MCT growth.
growth cell. Crystallinity and electrical properties of grown layers were also evaluated.
2. Growth condition The growths were carried out in a vertical growth cell with a rotating susceptor, which was operated at atmospheric pressure. The structure of the growth cell is shown in Fig. 1. The growth cell was made of stainless steel (SUS316L) and with an inner diameter of 55 mm. This cell was designated to have a narrow spacing between the substrate surface and the cell ceiling. The narrow spacing was employed to suppress a convective flow of precursors over the substrate and to increase the flow velocity of precursors even at a low cartier flow rate. These factors were expected to improve the uniformity of the layer thickness and the compositional distribution over the growth surface. In this experiment, the spacing was varied from 4 to 20 mm which was controlled precisely within the reset error of + 50 pbm. The ceiling and wall of the growth cell were heated by an independent heater to prevent mercury condensation. The temperature of the ceiling and wall of the cell was set at 200°C, and the mercury supply line was set at 175°C. When optimization of the cell spacing was done at each of the growth conditions, efficient growth was achieved as shown later. The precursors used were DtBTe, diethyltelluride (DETe), dimethylcadmium (DMCd), and elementary mercury. DtBTe was used for growth at 275°C, and
613
DETe was used for growth at 425°C. DtBTe was purchased from Trichemical Co. Yamanashi, Japan. Precursors were supplied into the growth cell through independent supply lines as shown in Fig. 1. The total H 2 carrier flow in the growth cell was kept at 1.5 1 min -]. The substrates used were (100)GaAs which was degreased by organic solvents and etched by H2SO 4 : H 2 0 2 : H 2 0 = 5 : 1 : 1. MCT layers were grown on (100)CdTe buffer layers with thicknesses of 0.5 ixm which were grown at 425°C using DETe and DMCd. The Cd composition of the MCT layers was evaluated by electron probe microanalysis (EPMA) with an energy-dispersive mode. The crystallinity of the MCT layers was evaluated by X-ray double crystal rocking curve (DCRC) measurements. Diffraction from the (400) lattice plane was measured with a spot size of 1 X 1 mm 2 using CuKot as the X-ray source. Electrical characterization was carded out by Hall measurements (Van der Pauw) at 300 and 80 K.
3. Results and discussion Fig. 2 shows the growth rate of (100)HgTe layers at growth temperatures of 275 and 300°C as a function of cell spacing. The flow rates of the precursors were set to be constant at 1 X 10 - 4 mol min-~ for DtBTe and 2 X 10 - 4 mol m i n - 1 for mercury. At the growth temperature of 275°C, the growth rate shows the maximum value at a spacing of 10 mm and
5
.
.
.
.
i
4
.
.
/'/\
\
.
.
i
.
.
.
.
DtBTe: 1.0 × I 0"~mol/min Hg:2.0 × 10"~mol/min
o :275"c
• 300 c
x:
2
0
.
0
.
.
.
t
l0
.
.
.
.
I
.
20
.
.
.
30
Cell spacing [mm] Fig. 2. Growth rates of (100)HgTe layers at temperatures of 275 and 300°C as a function of cell spacing.
614
K. Yasuda et al. / Journal of C~. stal Growth 166 (1996) 612-616 1.0 ×
0.8
o
:~ 0.6 0.4
'
'
/
~
'
0.2
~
~
"
' -~ mil 1tg:6.6 x 104mol/mir I
2
3
4
5
DMCd flow rate [× 10-6mol/min] Fig. 3. Dependence of Cd composition x on the DMCd flow rate for MCT layers grown at 275°C.
decreases at a narrow spacing of 4 mm and at a wide spacing of 20 mm. At the higher growth temperature of 300°C, the growth-rate peak shifts to a narrower spacing of 7 ram. These results indicate that the decreases of growth rates at the narrow-space region of 4 mm are due to the increase of flow velocity, which causes insufficient decomposition of DtBTe. On the other hand, the decreases of growth rates at the spacings wider than 10 mm are considered to be due to an occurrence of convective flow over the substrate surface a n d / o r an increase of the wall deposition. These results show that efficient growth can be attained by optimizing the cell spacing for each of the growth conditions such as the substrate temperature, ceiling temperature, and carrier-flow rate. Fig. 3 shows the dependence of the Cd composition x on the DMCd flow rate for MCT layers grown at 275°C, where the DtBTe flow was kept at 5.0 X 10 -5 mol min -~, and the mercury flow was kept at 6.6 X 10 - 4 tool min -~. The x-value in the MCT layers increases abruptly from 0 to 0.98 with increasing DMCd flow from 0 to 4.2 X 10 6 mol min- 1. The growth rate of MCT layers was independent of the DMCd flow and was constant at 5 ~m h ~. The growth rate of CdTe measured without Hg flow and at a DtBTe flow of 5.0 X 10 -5 mol min- ~ and a DMCd flow of 2.0 X 10 -5 mol min ~ was only 2.3 I~m h -j . This indicates that the enhancement of Cd incorporation occurs in the presence of Hg, since the growth rate of MCT (x = 0.98) at a DMCd flow of
4.0 X 10 - 6 tool min- ~ is 5 ptm h - 1, which is higher than that of CdTe grown without a Hg flow. The constant growth rate for the increase of DMCd flow indicates that both of the effects, i.e. preferential incorporation of Cd and the suppression of Hg incorporation, occur in MCT growth at 275°C. The growth mechanism of MCT at the low growth temperature of 275°C is considered as follows. We assume that the surface kinetic reaction is governing the MCT growth (as has been observed previously for HgTe growth at 275°C [8] and CdTe growth with DETe [9,10]). The CdTe and HgTe are formed on the growth surface through the sticking of Cd and Hg species on Te species covering the surface since the Te species has the largest sticking efficiency among the three species [ 11,12]. At the same time, Cd precipitation also occurs on the growth surface through an alkyl-exchange reaction between DMCd and Hg [13]. This precipitated Cd also attaches to the surface Te species and forms CdTe on the growth surface. This CdTe and CdTe formed through sticking of Cd species occupy the large portion of the surface sites and prevent sticking of Hg species on the growth surface, since Cd species have a higher sticking efficiency than that of Hg species. This process results in preferential Cd incorporation and constant growth rate which is limited by the surface coverage rate of Te species - independent of the DMCd flow. If there was a wide space above the substrate, on the other hand, Cd precipitation would also occur on the wall of the growth cell where the temperature is usually lower than the substrate. This "wall-precipitated" Cd also consumes DtBTe by the formation of CdTe on the wall of the cell, since DETe can decompose in the presence of Cd [14]. This causes a depletion of the Cd and Te species in the gas phase above the substrate and results in a decrease of the MCT growth rate for the introduction of DMCd. This phenomenon has usually been observed in conventional growth cells where a wide space exists "up-stream" or around the substrate [15,16]. We have also previously observed the decrease of growth rate in a horizontal growth cell, where deteriorations of the composition and thickness uniformity also occurred if an exact adjustment of the growth condition was not done for each of the x-values. In contrast to this, the growth characteristics obtained
K. Yasuda et al. / Journal of Crystal Growth 166 (1996) 612-616
615
Table 1 Characteristics of (100)MCT layers grown on (100)GaAs substrates at 275°C Cd composition x
MCT thickness (p.m)
CdTe buffer thickness (p~m)
FWHM (arcsec)
80 K cartier density (cm -3)
1.00 0.93 0.87 0.44 0.00
2.1 3.2 3.3 2.6 4.65
0.5 8.0 5.0 5.0 8.0
338 253 222 424 227
1.2 1.5 7.4 4.6
X
1015 p a
X 1015 n X 1015 n × 10 ~° n
80 K Hall mobility (cm 2 V - i s - 1) 65 a 4.1 X 102 7.4 × l03 8.0 X 104
a Carrier density and mobility were measured at 300 K.
here show that an improvement of the MCT growth environment is obtained in this narrow-spacing growth cell. Decrease of wall deposition, efficient precursor transport to the growth surface, and suppression of convection flow over the substrate are contributing to the improvement. Compositional uniformities were evaluated by EPMA mapping over the growth-surface area of 10 X 15 mm 2. A high uniformity of the x-value was obtained for grown layers with x from 0.42 to 0.98 where the distribution of composition was within A x = +0.01. Further improvement in the compositional uniformity is expected since no attempt was made to optimize the growth conditions. MCT layers grown at 275°C were characterized by X-ray double crystal rocking curve method (DCRC) and by Van der Pauw-Hall measurements. Table 1 shows the results for layers grown on GaAs substrates for x-values from 0 (HgTe) to 1.0 (CdTe). For the layers grown on GaAs substrates, the full width at half maximum (FWHM) value of the DCRC ranged from 222 to 424 arcsec. On the other hand, FWHM values around 90 arcsec were obtained for layers grown on (100)CdZnTe substrates. These FWHM values for layers grown at 275°C are similar to those grown at the high growth temperature of 425°C. MCT layers grown on GaAs and CdZnTe substrates showed n-type properties after annealing at 250°C for 24 h in Hg-saturated conditions. The annealed layers showed electron densities of (1.57.4) X 10 ~5 cm -3 and mobilities of 4.1 X 102-7.4 X 10 3 c m 2 V i s - J depending on the Cd composition. Ga was detected by SIMS measurements for the MCT layers grown on the GaAs substrates. The Ga may also contaminate the layers grown on CdZnTe substrates, since the layers were grown using the
same susceptor which has been utilized for the growth on GaAs substrates. Although the MCT layers have high electron densities, the electron mobilities are comparable to high-quality layers. The above results indicate that high-quality MCT layers can be grown at a low growth temperature of 275°C using DtBTe.
4. Summary The growth characteristics of (100)HgCdTe (MCT) layers at the low growth temperature of 275°C in MOVPE were studied using DtBTe as tellurium precursor. Growths were conducted in a vertical narrow-spacing growth cell operated at atmospheric pressure. The Cd composition of MCT layers was controlled by DMCd flow. The growth rate of MCT layers was constant for an increase of the DMCd flow. Cd was preferentially incorporated into the MCT layers. Enhancement of the Cd incorporation in the presence of Hg was also observed. Crystallinities and electrical properties of layers grown at 275°C were also evaluated, which showed that high-quality MCT layers were grown at this low growth temperature using DtBTe.
References [1] M. Ekawa, K. Yasuda, T. Ferid, M. Saji and A. Tanaka, J. Appl. Phys. 71 (1992) 2669. [2] M. Ekawa, K. Yasuda, T. Ferid, M. Saji and A. Tanaka, J. Appl. Phys. 72 (1992) 3406. [3] S.J.C, Irvine, J.B. Mullin and J. Tunnicliffe, J. Crystal Growth 68 (1984) 188. [4] W.E. Hoke and P.J, Lemonias, Appl. Phys. Lett. 48 (1986) 1669.
616
K. Yasuda et al. / Journal of Crystal Growth 166 (1996) 612-616
[5] R. Korenstein, W.E. Hoke, P.J. Lemonias, K.T. Higa and D.C. Harris, J, Appl. Phys. 62 (1987) 4927. [6] J.D. Parsons and L.S. Lichtmann, J. Crystal Growth 86 (1988) 222. [7] P.-Y. Lu, C.-H. Wang, L.M. Williams, S.N. Chu and C.M. Stiles, Appl. Phys. Lett. 49 (1986) 1372. [8] K. Yasuda, M. Ohno, J. Yamaguchi, T. Onakado, T. Fetid and H. Hatano, Ext. Abstr. US Workshop Phys. Chem. MCT (1993) p. 27. [9] K. Yasuda, M. Ekawa, N. Matsui, S. Sone, Y. Sugiura, A~ Tanaka and M. Saji, Jpn. J. Appl. Phys. 29 (1990) 479. [10] I.B. Bhat, N.R. Taskar and S.K. Ghandhi, J. Vac. Sci. Technol. A 4 (1986) 2230. [11] K. Yasuda, S. Sone, M. Ekawa, Y. Sugiura, N. Matsui, A.
[12] [13] [14] [15] [16]
Tanaka and M. Saji, Tech. Digest 1st. Int. Meeting on Advanced Processing and Characterization Technologies, Tokyo (1989) p. 131. S. Sone, M. Ekawa, K. Yasuda, Y. Sugiura, M. Saji and A. Tanaka, Appl. Phys. Lett. 56 (1990) 539. A.H. McDaniel, W.G. Xie and R.F. Hicks, Ext. Abstr. US Workshop Phys. Chem. MCT (1993) p. 25. I.B. Bhat, N.R. Taskar and S.K. Ghandhi, J. Electrochem. Soc. 134 (1987) 196. I.B. Bhat, J. Crystal Growth 117 (1992) 1. H. Takada, T. Murakami, M. Suita, K. Yasumura, Y. Endo, K. Takahashi and M. Nunoshita, J. Crystal Growth 117 (1992) 44.
ELSEVIER
Journal of Crystal Growth 166 (1996) 612-616
Growth characteristics of(100) HgCdTe layers in low-temperature MOVPE with ditertiarybutyltelluride K. Yasuda *, H. Hatano, T. Ferid, M. Minamide, T. Maejima, K. Kawamoto Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466, Japan
Abstract Low-temperature growth of (100)HgCdTe (MCT) layers in MOVPE has been studied using ditertiarybutyltelluride (DtBTe), dimethylcadmium (DMCd), and elementary mercury as precursors. MCT layers were grown at 275°C on (100)GaAs substrates. Growths were carried out in a vertical growth cell which has a narrow spacing between the substrate and cell ceiling. Using the growth cell, the Cd-composition (x) of MCT layers was controlled over a wide range from 0 to 0.98 by the DMCd flow. The growth rate of the MCT layers was constant at 5 I~m h-l for the increased DMCd flow. Preferential Cd-incorporation into MCT layers and an increase of the growth rate were observed in the presence of mercury vapor. The growth characteristics were considered to be due to the alkyl-exchange reaction between DMCd and mercury. The electrical properties and crystallinity of grown layers were also evaluated, which showed that layers with high quality can be grown at 275°C.
1. Introduction In the recent work on epitaxial growth of HgCdTe (MCT), metalorganic vapor phase epitaxy (MOVPE) has been one of the major growth techniques. MOVPE growth of MCT has been carried out in the growth temperature range from 350 to 425°C. The lower limit of the growth temperature is determined by the thermal stability of tellurium precursors. Decrease of the growth temperature is expected to have several advantages over the usual growth above 350°C. One advantage is a decrease of the mercury-vacancy formation during growth and another is a decrease of interdiffusion at the growth interface. These two effects are important not only for further improvements of infrared detectors but
* Corresponding author. Fax: +81 52 735 5442; E-mail: [email protected].
also for applications in new electronic devices where the controllability of the layer structure and electrical properties by extrinsic dopants is essential. The increase of dopant adsorption on the growth surface and the suppression of neutral vacancy-impurity complex formation are expected at low growth temperatures [1,2]. To decrease the growth temperature of MCT in MOVPE, several growth techniques have been studied previously. These are: photolysis [3], the use of thermally unstable Te precursors [4-6], and the precracking of precursors [7]. However, the growth characteristics and electronic properties of the layers have not been fully studied for a wide range of Cd composition. In this paper, we have studied the growth characteristics of MCT layers at a low growth temperature of 275°C using ditertiarybutyltelluride (DtBTe) as a Te precursor. The growths were carried out for wide range of Cd-compositions using a newly developed
0022-0248/96//$t5.00 Copyright© 1996Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00536-6
K. Yasuda et al. /Journal of Crystal Growth 166 (1996) 612-616
DtBTe DETe/.~ ~~____~~nts DMCd
Illll
~ I L
C~,;C~iling _-)~l-l--Susceptor 275*(3
=e
Fig. 1. Construction of the narrow-spacing MOVPE growth cell used for MCT growth.
growth cell. Crystallinity and electrical properties of grown layers were also evaluated.
2. Growth condition The growths were carried out in a vertical growth cell with a rotating susceptor, which was operated at atmospheric pressure. The structure of the growth cell is shown in Fig. 1. The growth cell was made of stainless steel (SUS316L) and with an inner diameter of 55 mm. This cell was designated to have a narrow spacing between the substrate surface and the cell ceiling. The narrow spacing was employed to suppress a convective flow of precursors over the substrate and to increase the flow velocity of precursors even at a low cartier flow rate. These factors were expected to improve the uniformity of the layer thickness and the compositional distribution over the growth surface. In this experiment, the spacing was varied from 4 to 20 mm which was controlled precisely within the reset error of + 50 pbm. The ceiling and wall of the growth cell were heated by an independent heater to prevent mercury condensation. The temperature of the ceiling and wall of the cell was set at 200°C, and the mercury supply line was set at 175°C. When optimization of the cell spacing was done at each of the growth conditions, efficient growth was achieved as shown later. The precursors used were DtBTe, diethyltelluride (DETe), dimethylcadmium (DMCd), and elementary mercury. DtBTe was used for growth at 275°C, and
613
DETe was used for growth at 425°C. DtBTe was purchased from Trichemical Co. Yamanashi, Japan. Precursors were supplied into the growth cell through independent supply lines as shown in Fig. 1. The total H 2 carrier flow in the growth cell was kept at 1.5 1 min -]. The substrates used were (100)GaAs which was degreased by organic solvents and etched by H2SO 4 : H 2 0 2 : H 2 0 = 5 : 1 : 1. MCT layers were grown on (100)CdTe buffer layers with thicknesses of 0.5 ixm which were grown at 425°C using DETe and DMCd. The Cd composition of the MCT layers was evaluated by electron probe microanalysis (EPMA) with an energy-dispersive mode. The crystallinity of the MCT layers was evaluated by X-ray double crystal rocking curve (DCRC) measurements. Diffraction from the (400) lattice plane was measured with a spot size of 1 X 1 mm 2 using CuKot as the X-ray source. Electrical characterization was carded out by Hall measurements (Van der Pauw) at 300 and 80 K.
3. Results and discussion Fig. 2 shows the growth rate of (100)HgTe layers at growth temperatures of 275 and 300°C as a function of cell spacing. The flow rates of the precursors were set to be constant at 1 X 10 - 4 mol min-~ for DtBTe and 2 X 10 - 4 mol m i n - 1 for mercury. At the growth temperature of 275°C, the growth rate shows the maximum value at a spacing of 10 mm and
5
.
.
.
.
i
4
.
.
/'/\
\
.
.
i
.
.
.
.
DtBTe: 1.0 × I 0"~mol/min Hg:2.0 × 10"~mol/min
o :275"c
• 300 c
x:
2
0
.
0
.
.
.
t
l0
.
.
.
.
I
.
20
.
.
.
30
Cell spacing [mm] Fig. 2. Growth rates of (100)HgTe layers at temperatures of 275 and 300°C as a function of cell spacing.
614
K. Yasuda et al. / Journal of C~. stal Growth 166 (1996) 612-616 1.0 ×
0.8
o
:~ 0.6 0.4
'
'
/
~
'
0.2
~
~
"
' -~ mil 1tg:6.6 x 104mol/mir I
2
3
4
5
DMCd flow rate [× 10-6mol/min] Fig. 3. Dependence of Cd composition x on the DMCd flow rate for MCT layers grown at 275°C.
decreases at a narrow spacing of 4 mm and at a wide spacing of 20 mm. At the higher growth temperature of 300°C, the growth-rate peak shifts to a narrower spacing of 7 ram. These results indicate that the decreases of growth rates at the narrow-space region of 4 mm are due to the increase of flow velocity, which causes insufficient decomposition of DtBTe. On the other hand, the decreases of growth rates at the spacings wider than 10 mm are considered to be due to an occurrence of convective flow over the substrate surface a n d / o r an increase of the wall deposition. These results show that efficient growth can be attained by optimizing the cell spacing for each of the growth conditions such as the substrate temperature, ceiling temperature, and carrier-flow rate. Fig. 3 shows the dependence of the Cd composition x on the DMCd flow rate for MCT layers grown at 275°C, where the DtBTe flow was kept at 5.0 X 10 -5 mol min -~, and the mercury flow was kept at 6.6 X 10 - 4 tool min -~. The x-value in the MCT layers increases abruptly from 0 to 0.98 with increasing DMCd flow from 0 to 4.2 X 10 6 mol min- 1. The growth rate of MCT layers was independent of the DMCd flow and was constant at 5 ~m h ~. The growth rate of CdTe measured without Hg flow and at a DtBTe flow of 5.0 X 10 -5 mol min- ~ and a DMCd flow of 2.0 X 10 -5 mol min ~ was only 2.3 I~m h -j . This indicates that the enhancement of Cd incorporation occurs in the presence of Hg, since the growth rate of MCT (x = 0.98) at a DMCd flow of
4.0 X 10 - 6 tool min- ~ is 5 ptm h - 1, which is higher than that of CdTe grown without a Hg flow. The constant growth rate for the increase of DMCd flow indicates that both of the effects, i.e. preferential incorporation of Cd and the suppression of Hg incorporation, occur in MCT growth at 275°C. The growth mechanism of MCT at the low growth temperature of 275°C is considered as follows. We assume that the surface kinetic reaction is governing the MCT growth (as has been observed previously for HgTe growth at 275°C [8] and CdTe growth with DETe [9,10]). The CdTe and HgTe are formed on the growth surface through the sticking of Cd and Hg species on Te species covering the surface since the Te species has the largest sticking efficiency among the three species [ 11,12]. At the same time, Cd precipitation also occurs on the growth surface through an alkyl-exchange reaction between DMCd and Hg [13]. This precipitated Cd also attaches to the surface Te species and forms CdTe on the growth surface. This CdTe and CdTe formed through sticking of Cd species occupy the large portion of the surface sites and prevent sticking of Hg species on the growth surface, since Cd species have a higher sticking efficiency than that of Hg species. This process results in preferential Cd incorporation and constant growth rate which is limited by the surface coverage rate of Te species - independent of the DMCd flow. If there was a wide space above the substrate, on the other hand, Cd precipitation would also occur on the wall of the growth cell where the temperature is usually lower than the substrate. This "wall-precipitated" Cd also consumes DtBTe by the formation of CdTe on the wall of the cell, since DETe can decompose in the presence of Cd [14]. This causes a depletion of the Cd and Te species in the gas phase above the substrate and results in a decrease of the MCT growth rate for the introduction of DMCd. This phenomenon has usually been observed in conventional growth cells where a wide space exists "up-stream" or around the substrate [15,16]. We have also previously observed the decrease of growth rate in a horizontal growth cell, where deteriorations of the composition and thickness uniformity also occurred if an exact adjustment of the growth condition was not done for each of the x-values. In contrast to this, the growth characteristics obtained
K. Yasuda et al. / Journal of Crystal Growth 166 (1996) 612-616
615
Table 1 Characteristics of (100)MCT layers grown on (100)GaAs substrates at 275°C Cd composition x
MCT thickness (p.m)
CdTe buffer thickness (p~m)
FWHM (arcsec)
80 K cartier density (cm -3)
1.00 0.93 0.87 0.44 0.00
2.1 3.2 3.3 2.6 4.65
0.5 8.0 5.0 5.0 8.0
338 253 222 424 227
1.2 1.5 7.4 4.6
X
1015 p a
X 1015 n X 1015 n × 10 ~° n
80 K Hall mobility (cm 2 V - i s - 1) 65 a 4.1 X 102 7.4 × l03 8.0 X 104
a Carrier density and mobility were measured at 300 K.
here show that an improvement of the MCT growth environment is obtained in this narrow-spacing growth cell. Decrease of wall deposition, efficient precursor transport to the growth surface, and suppression of convection flow over the substrate are contributing to the improvement. Compositional uniformities were evaluated by EPMA mapping over the growth-surface area of 10 X 15 mm 2. A high uniformity of the x-value was obtained for grown layers with x from 0.42 to 0.98 where the distribution of composition was within A x = +0.01. Further improvement in the compositional uniformity is expected since no attempt was made to optimize the growth conditions. MCT layers grown at 275°C were characterized by X-ray double crystal rocking curve method (DCRC) and by Van der Pauw-Hall measurements. Table 1 shows the results for layers grown on GaAs substrates for x-values from 0 (HgTe) to 1.0 (CdTe). For the layers grown on GaAs substrates, the full width at half maximum (FWHM) value of the DCRC ranged from 222 to 424 arcsec. On the other hand, FWHM values around 90 arcsec were obtained for layers grown on (100)CdZnTe substrates. These FWHM values for layers grown at 275°C are similar to those grown at the high growth temperature of 425°C. MCT layers grown on GaAs and CdZnTe substrates showed n-type properties after annealing at 250°C for 24 h in Hg-saturated conditions. The annealed layers showed electron densities of (1.57.4) X 10 ~5 cm -3 and mobilities of 4.1 X 102-7.4 X 10 3 c m 2 V i s - J depending on the Cd composition. Ga was detected by SIMS measurements for the MCT layers grown on the GaAs substrates. The Ga may also contaminate the layers grown on CdZnTe substrates, since the layers were grown using the
same susceptor which has been utilized for the growth on GaAs substrates. Although the MCT layers have high electron densities, the electron mobilities are comparable to high-quality layers. The above results indicate that high-quality MCT layers can be grown at a low growth temperature of 275°C using DtBTe.
4. Summary The growth characteristics of (100)HgCdTe (MCT) layers at the low growth temperature of 275°C in MOVPE were studied using DtBTe as tellurium precursor. Growths were conducted in a vertical narrow-spacing growth cell operated at atmospheric pressure. The Cd composition of MCT layers was controlled by DMCd flow. The growth rate of MCT layers was constant for an increase of the DMCd flow. Cd was preferentially incorporated into the MCT layers. Enhancement of the Cd incorporation in the presence of Hg was also observed. Crystallinities and electrical properties of layers grown at 275°C were also evaluated, which showed that high-quality MCT layers were grown at this low growth temperature using DtBTe.
References [1] M. Ekawa, K. Yasuda, T. Ferid, M. Saji and A. Tanaka, J. Appl. Phys. 71 (1992) 2669. [2] M. Ekawa, K. Yasuda, T. Ferid, M. Saji and A. Tanaka, J. Appl. Phys. 72 (1992) 3406. [3] S.J.C, Irvine, J.B. Mullin and J. Tunnicliffe, J. Crystal Growth 68 (1984) 188. [4] W.E. Hoke and P.J, Lemonias, Appl. Phys. Lett. 48 (1986) 1669.
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K. Yasuda et al. / Journal of Crystal Growth 166 (1996) 612-616
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