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Photoluminescence characterization of sulphur-doped GaSb grown by liquid phase electroepitaxy
j. . . . . . . .
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 158 (1996) 1-5
Photoluminescence characterization of sulphur-doped GaSb grown by liquid phase electroepitaxy J. Nov{tk a,*, M. Ku~era a, S. Lauer b K.W. Benz b a Institute of Electrical Engineering, Slovak Academy of Sciences, 842 39 Bratislava, Slovak Republic b Kristallographisches Institut, Universiti~t Freiburg, Hebelstrasse 25, D-79104, Freiburg, Germany
Received 1 July 1995
Abstract
Low temperature (5 K) spectra of liquid phase electroepitaxially grown sulphur-doped GaSb have been examined. Native acceptor-related transition appears at 777 meV in undoped GaSb and it is shifted by 4-6 meV to lower energy in sulphur-doped layers. The dominant sulphur-related transition S~ lies at 731-733 meV in different samples. The intensity of this transition is higher than the intensity of the native acceptor-related transition and their ratio increases with the amount of Sb2S 3 in the growth melt. Both photoluminescence and photoreflectance measurements have shown that the energy band gap of these sulphur-doped samples (E c = 732 meV at 295 K) is a little higher in comparison with other published values.
1. I n t r o d u c t i o n
The use of GaSb and related I I I - V compound materials in heterostructures and devices is of current scientific and technological interest. It is well-known that some donors introduce deep levels commonly known as DX centres in binary or ternary I I I - V semiconductors. These deep donor levels can be produced in GaSb by incorporation of group-six elements (Te, Se, and S) [1]. It was found that trap densities in Te- and Se-doped GaSb are at least two orders of magnitude lower than a shallow donor concentration, and these levels do not exhibit DX-like nature. On the other hand, the sulphur-doped GaSb exhibits deep levels with DX-like behaviour and the trap concentration is comparable to that of shallow donors [2]. Therefore, the preparation of G a S b : S
* Corresponding author.
and study of its electrical and optical properties is very interesting and it may contribute to explaining the origin and behaviour of DX centres. Although the optical and electrical properties of GaSb are strongly dependent on growth conditions, the undoped GaSb is always p-type regardless of the growth technique. The dominant residual acceptor has been attributed to the native defects caused by antimony deficiency, usually due to the Ga antisite or Ga antisite defect in combination with the Ga vacancy. The low temperature photoluminescence (PL) spectra of undoped GaSb exhibit about 20 transitions in the energy interval of 6 8 0 - 8 1 0 meV [3,4], but only a part of these transitions have been associated with specific impurities or defects. However, the residual acceptor is generally associated with the dominant transition at energy near 777 meV as it is generally accepted [3-5]. The problem of a suitable sulphur source for epitaxial growth of sulphur-doped GaSb was suc-
0022-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00343-6
2
J. Noufk et al./Journal of Crystal Growth 158 (1996) 1-5
cessfully solved only for the molecular beam epitaxy technique. The donor concentration in different samples between 7.5 × 1016 and 3.3 X 1017 cm -3, and the native acceptor concentration between 1.2 X 1016 and 8 X 1016 cm -3 was reported [6]. A disadvantage of this technology lies in the need to use semi-insulating substrates (in this case GaAs) with a different lattice constant for the Hall measurements, because semi-insulting gallium antimonide is not available. Consequently, the accuracy of the measured Hall parameters may be influenced by strain and lattice imperfections related to defects. In this paper we report preparation, electrical properties and photoluminescence spectra of sulphur-doped GaSb epitaxial layers grown by liquid phase electroepitaxy (LPEE). This technique makes it possible to prepare very uniform and thick epitaxial layers, and consequently, to exclude the problems resulting from the lack of GaSb semi-insulating substrates.
mental results, the growth temperature must not reach or exceed the melting point of the Sb2S 3. Only in this case is it possible to prevent an uncontrolled dissociation of Sb2S 3, and to prevent the formation of other sulphur compounds in the melt. Under these conditions n-type sulphur-doped GaSb layers with good electrical parameters can be grown. More details about the LPEE growth procedure have been reported previously [7]. The presence of sulphur in the growth solution has a substantial influence on the growth process and on the electrical and optical properties of the grown layers (see Table 1). The growth velocity decreases from the values of more than 25 / z m / h in undoped or very low-doped GaSb (the amount of Sb2S 3 was only a few mg at total weight of the melt ca. 14g) down to 15 / z m / h for the highest amount of Sb2S 3 in the melt. The layers grown with the highest amount of Sb2S 3 have shown a small lattice expansion (Aa/a = 9.6 X 10-4), measured by high resolution four crystal X-ray diffractometry.
2. Experimental procedure 3. Results and discussion All samples were grown in a vertical LPEE equipment at a constant temperature near 550°C and current density J = 5 A cm 2. Standard quality 6N gallium and antimony were used for the melt preparation. As a sulphur source, 5N antimony sulphide was used. An advantage of this compound used as a sulphur source for doping GaSb lies in the possibility to add sulphur to the melt in a relatively high amount and find the growth conditions providing successful incorporation of sulphur into GaSb with good uniformity and reproducibility. The amount of Sb2S 3 was varied from 0 to 124 mg at the total weight of the melt near 14 g. As it follows from our experi-
The low temperature ( T = 5 K) photoluminescence spectra were obtained with a 0.25m Digikrom monochromator and a cooled PbS detector. The excitation source was the 632.8 nm line of a 50 mW H e - N e laser. The normally incident laser beam was focused onto the sample surface in a spot with a power density of 1 W cm -2. For the photoreflectance measurement a standard experimental setup was used. Fig. 1 shows a comparison of the PL spectra of three GaSb samples. The KNF-1 sample is a typical undoped GaSb with p-type conductivity. Its domi-
Table 1 Properties of the epitaxial GaSb layers used in this study (T = 295 K) Sample
KNF-1 KNF-3 KNF-8 KNF-9 SL-1
Sb 2S 3 (mg)
Conductivity type
0 16.6 77 + 1.1 Te 124 124+ 1.5 Te
p p n n n
Hall concentration
Hall mobility
(cm- 3)
(cm2/V - s)
7.8 x 1016 3 × 1016 5 X 1017 1 x 1016
655 514 2382 1944 3158
8.6X 1016
J. Nov6k et a l . / Journal of Crystal Growth 158 (1996) 1-5
1,0
A I KNF-1
$1
GaSb" S T=5K
10
tt
i! it
ii
0,8
ti i
:3
0,6 $2
,,_1
< z (9
,,_1
t KNF-3 i
\'
0,4
.4. i
n
:!" i
"i:i
10 x 1 4 FE
0,2
]
I-2
/
0,0
"~ " ~ 0 0,66 0,68 0,70 0,72 0,74 0,76 0,78 0,80 0,82
ENERGY (eV) Fig. 1. Low temperature photoluminescence spectra (T = 5 K) of three GaSb epitaxial layers with different sulphur dopings.
nant transition band ( e - A ) considered as a native acceptor-related transition is at 777 meV and its phonon replica (A-LO) at 748.3 meV [3,4]. Samples KNF-3 and KNF-9 were grown with a medium and a high amount of Sb2S 3 ( K N F - 3 : 1 6 . 6 mg, p-type, p = 3 × 1016 c m - 3 ; K N F - 9 : 1 2 4 mg, n-type, n = 1 × 1016 c m 3). The incorporation of sulphur leads to a compensation of the native acceptor and besides the previous transitions, the bound exciton BE 4 at 796 meV and two additional peaks S~ at 733 and an unresolved peak S 2 near 706 meV were identified. The intensity of the dominant S~ transition is higher as the intensity of the native acceptor-related transi-
3
tion and their ratio increases with the overcompensation of the native acceptor and with the transition from p-type to n-type conductivity. The native acceptor-related transition is at all sulphur-doped samples shifted about 4 - 6 meV to the lower energies and it lies between 771-773 meV (at different samples)• The free exciton (FE) transition is seen only on the KNF-9 sample and it lies at 812.5 meV. The energy band gap of this sample can be obtained by adding the free exciton binding energy ( ~ 1.1 meV) to the free exciton transition energy. In this way E G = 813.6 meV. Consequently, the main sulphur-related donor (peak S] in the photoluminescence spectra) can be estimated to occupy an energy level about 81 meV below the bottom of the conduction band (Table 2). With the aim to prepare very interesting semiconductor material for electrical and DLTS study of the deep levels, we have grown gallium antimonide codoped with sulphur and tellurium. As it follows from Table 1, tellurium doping dominates and n-type material with relatively very high electron Hall mobility was grown. On the contrary to very good electrical properties, the low temperature PL spectra of these co-doped epitaxial layers consist of one very broad peak with a maximum at an energy of 773 meV (Fig. 2). The position of this maximum is probably determined by the native acceptor level and is shifted by 4 meV to a lower energy as it is in all other GaSb sulphur-doped samples. We have not observed a transition at 807.4 meV (denoted as D and attributed to Te) which was reported by Chen and Su [8]. Since ,,~e have not observed a free exciton transition line at all samples, the determination of the activation energy of the dominant sulphur-related transmission S~ is hindered by the uncertainty of the energy gap of the samples measured at temperature T = 5 K. Therefore, we have used the room-temperature photoreflectance measurement to determine E G
Table 2 Comparison of selected PL transitions and peaks, T = 5 K Peak S i
Peak A Sample
Energy (meV)
FWHM (meV)
Relative intensity
Energy (meV)
FWHM (meV)
Relative intensity
KNF-I KNF-3 KNF-9
777 771 773
7.9 14 16
1 0.405 0.176
731 733
25 19
1 1
4
J. Novfk et al./Journal of Crystal Growth 158 (1996) 1-5 i
1,0
:"'"
GaSb T=5K
,
E
or following Ref. [9]
A
EG = 0.810 - 3.78 X 10 4 T 2 / ( T +
:. / KNF-8 ..' (: Te,S) /
0,8
~ 0,6 < Z (~ 0,4 CO -.I
":
0,2
..:"
,
I
,
I
/
A - LO
,
I
,
I
I
i
I
0,68 0,70 0,72 0,74 0,76 0,78 0,80 0,82 E N E R G Y (eV) Fig. 2. Comparison o f PL spectra of an undoped p-type GaSb (KNF-1) and n-type GaSb co-doped with S and Te (KNF-8).
at 295 K and known formulas for its temperature dependence to determine E~ at 5 K. Temperature dependence of the GaSb energy gap can be expressed following Chen and Su [8] EG = 0.812-- 4.2 X 10
4T2/(Z-}- 140),
200
GaSb : S
(1)
4o
SL-1 100 ~
..
20
/
i 0f
/
-100 ""-""
:'"
o -20
94).
(2)
As it follows from our room-temperature photoreflectance measurements (see Fig. 3), the energy gap of all GaSb sulphur-doped samples is E G = 732 meV at T = 295 K. This Ea value is a little higher as can be estimated from both expressions (from (1): E G = 728 meV, from (2): E G = 726 meV) or as was published by Iyer et al. E~ = 729 meV [3]. Using Ea from photoreflectance results at 295 K and the average value estimated from both formulas, we obtain 813 meV for E G at 5 K. This value is in good agreement with that obtained from the free exciton transition (sample KNF-9). In addition, using EG(0) = 0.8137 eV in (1), we obtain an excellent agreement of both estimated E G values (EG(5 K) = 813.6 meV from PL and E~(295 K) = 732 meV from PR) with temperature dependence of EG described by this expression.
4. Conclusions In conclusion, we have studied properties and photoluminescence spectra of the sulphur-doped GaSb grown by LPEE. This technique allows effective incorporation of sulphur and overcompensation of native acceptor. From the photoluminescence measurements at 5 K, it follows that the native acceptor-related transitions appear at 777 meV in undoped GaSb and it is shifted by 4 - 6 meV to lower energy in sulphur-doped layers. The S~ peak in the PL spectra is the dominant sulphur-related transition and lies at 731-733 meV (at different samples). The relative intensity of this transition depends on the amount of sulphur in the growth melt. Both photoreflectance and photoluminescence measurements have shown that the energy band gap of these samples E G = 732 meV at 295 K is a little higher in comparison with other published values.
I Eg=0.732 eV 200 . . . . . . - .... -40 0,68 0,70 0,72 0,74 0,76 0,78 0,80 0,82 ENERGY (eV)
Fig. 3. Room-temperature photoreflectance spectra of GaSb doped with sulphur (sample KNF-9) and co-doped with sulphur and tellurium (SL-1).
References [1] P.S. Dutta, K.S.R. Koteswara Rao, K.S. Sanguni and H.L. Bhat, Appl. Phys. Leu. 65 (1994) 1412. [2] I. Poole, M.E. Lee, I.R. Cleverley, A.R. Parker and K.E. Singer, Appl. Phys. Lett. 57 (1990) 1645.
J. Novfk et al./ Journal of Crystal Growth 158 (1996) 1-5
[3] S. Iyer, S. Hedge, K.K. Bajaj, A. Abul-Fadl and W. Mitchel, J. Appl. Phys. 73 (1993) 3958. [4] M. Lee, D.J. Nicholas, K.E. Singer and B. Hamilton, J. Appl. Phys. 59 (1986) 2895. [5] E.T.R. Chidley, S.K. Heywood, A.B. Henriques, N.J. Mason, R.J. Nicholas and P.J. Walker, Semicond. Sci. Technol. 6 (1991) 45.
5
[6] M.E. Lee, I. Poole, W.S. Truscott, I.R. Cleverley, K.E. Singer and D.M. Rohling, J. Appl. Phys. 68 (1990) 131. [7] J. NovLk, M. Klaus and K.W. Benz, J. Crystal Growth 139 (1993) 206. [8] S.C. Chen and Y.K. Su, J. Appl. Phys. 66 (1989) 350. [9] J. Camassel and D. Auvergene, Phys. Rev. B 12 (1975) 3258.
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 158 (1996) 1-5
Photoluminescence characterization of sulphur-doped GaSb grown by liquid phase electroepitaxy J. Nov{tk a,*, M. Ku~era a, S. Lauer b K.W. Benz b a Institute of Electrical Engineering, Slovak Academy of Sciences, 842 39 Bratislava, Slovak Republic b Kristallographisches Institut, Universiti~t Freiburg, Hebelstrasse 25, D-79104, Freiburg, Germany
Received 1 July 1995
Abstract
Low temperature (5 K) spectra of liquid phase electroepitaxially grown sulphur-doped GaSb have been examined. Native acceptor-related transition appears at 777 meV in undoped GaSb and it is shifted by 4-6 meV to lower energy in sulphur-doped layers. The dominant sulphur-related transition S~ lies at 731-733 meV in different samples. The intensity of this transition is higher than the intensity of the native acceptor-related transition and their ratio increases with the amount of Sb2S 3 in the growth melt. Both photoluminescence and photoreflectance measurements have shown that the energy band gap of these sulphur-doped samples (E c = 732 meV at 295 K) is a little higher in comparison with other published values.
1. I n t r o d u c t i o n
The use of GaSb and related I I I - V compound materials in heterostructures and devices is of current scientific and technological interest. It is well-known that some donors introduce deep levels commonly known as DX centres in binary or ternary I I I - V semiconductors. These deep donor levels can be produced in GaSb by incorporation of group-six elements (Te, Se, and S) [1]. It was found that trap densities in Te- and Se-doped GaSb are at least two orders of magnitude lower than a shallow donor concentration, and these levels do not exhibit DX-like nature. On the other hand, the sulphur-doped GaSb exhibits deep levels with DX-like behaviour and the trap concentration is comparable to that of shallow donors [2]. Therefore, the preparation of G a S b : S
* Corresponding author.
and study of its electrical and optical properties is very interesting and it may contribute to explaining the origin and behaviour of DX centres. Although the optical and electrical properties of GaSb are strongly dependent on growth conditions, the undoped GaSb is always p-type regardless of the growth technique. The dominant residual acceptor has been attributed to the native defects caused by antimony deficiency, usually due to the Ga antisite or Ga antisite defect in combination with the Ga vacancy. The low temperature photoluminescence (PL) spectra of undoped GaSb exhibit about 20 transitions in the energy interval of 6 8 0 - 8 1 0 meV [3,4], but only a part of these transitions have been associated with specific impurities or defects. However, the residual acceptor is generally associated with the dominant transition at energy near 777 meV as it is generally accepted [3-5]. The problem of a suitable sulphur source for epitaxial growth of sulphur-doped GaSb was suc-
0022-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00343-6
2
J. Noufk et al./Journal of Crystal Growth 158 (1996) 1-5
cessfully solved only for the molecular beam epitaxy technique. The donor concentration in different samples between 7.5 × 1016 and 3.3 X 1017 cm -3, and the native acceptor concentration between 1.2 X 1016 and 8 X 1016 cm -3 was reported [6]. A disadvantage of this technology lies in the need to use semi-insulating substrates (in this case GaAs) with a different lattice constant for the Hall measurements, because semi-insulting gallium antimonide is not available. Consequently, the accuracy of the measured Hall parameters may be influenced by strain and lattice imperfections related to defects. In this paper we report preparation, electrical properties and photoluminescence spectra of sulphur-doped GaSb epitaxial layers grown by liquid phase electroepitaxy (LPEE). This technique makes it possible to prepare very uniform and thick epitaxial layers, and consequently, to exclude the problems resulting from the lack of GaSb semi-insulating substrates.
mental results, the growth temperature must not reach or exceed the melting point of the Sb2S 3. Only in this case is it possible to prevent an uncontrolled dissociation of Sb2S 3, and to prevent the formation of other sulphur compounds in the melt. Under these conditions n-type sulphur-doped GaSb layers with good electrical parameters can be grown. More details about the LPEE growth procedure have been reported previously [7]. The presence of sulphur in the growth solution has a substantial influence on the growth process and on the electrical and optical properties of the grown layers (see Table 1). The growth velocity decreases from the values of more than 25 / z m / h in undoped or very low-doped GaSb (the amount of Sb2S 3 was only a few mg at total weight of the melt ca. 14g) down to 15 / z m / h for the highest amount of Sb2S 3 in the melt. The layers grown with the highest amount of Sb2S 3 have shown a small lattice expansion (Aa/a = 9.6 X 10-4), measured by high resolution four crystal X-ray diffractometry.
2. Experimental procedure 3. Results and discussion All samples were grown in a vertical LPEE equipment at a constant temperature near 550°C and current density J = 5 A cm 2. Standard quality 6N gallium and antimony were used for the melt preparation. As a sulphur source, 5N antimony sulphide was used. An advantage of this compound used as a sulphur source for doping GaSb lies in the possibility to add sulphur to the melt in a relatively high amount and find the growth conditions providing successful incorporation of sulphur into GaSb with good uniformity and reproducibility. The amount of Sb2S 3 was varied from 0 to 124 mg at the total weight of the melt near 14 g. As it follows from our experi-
The low temperature ( T = 5 K) photoluminescence spectra were obtained with a 0.25m Digikrom monochromator and a cooled PbS detector. The excitation source was the 632.8 nm line of a 50 mW H e - N e laser. The normally incident laser beam was focused onto the sample surface in a spot with a power density of 1 W cm -2. For the photoreflectance measurement a standard experimental setup was used. Fig. 1 shows a comparison of the PL spectra of three GaSb samples. The KNF-1 sample is a typical undoped GaSb with p-type conductivity. Its domi-
Table 1 Properties of the epitaxial GaSb layers used in this study (T = 295 K) Sample
KNF-1 KNF-3 KNF-8 KNF-9 SL-1
Sb 2S 3 (mg)
Conductivity type
0 16.6 77 + 1.1 Te 124 124+ 1.5 Te
p p n n n
Hall concentration
Hall mobility
(cm- 3)
(cm2/V - s)
7.8 x 1016 3 × 1016 5 X 1017 1 x 1016
655 514 2382 1944 3158
8.6X 1016
J. Nov6k et a l . / Journal of Crystal Growth 158 (1996) 1-5
1,0
A I KNF-1
$1
GaSb" S T=5K
10
tt
i! it
ii
0,8
ti i
:3
0,6 $2
,,_1
< z (9
,,_1
t KNF-3 i
\'
0,4
.4. i
n
:!" i
"i:i
10 x 1 4 FE
0,2
]
I-2
/
0,0
"~ " ~ 0 0,66 0,68 0,70 0,72 0,74 0,76 0,78 0,80 0,82
ENERGY (eV) Fig. 1. Low temperature photoluminescence spectra (T = 5 K) of three GaSb epitaxial layers with different sulphur dopings.
nant transition band ( e - A ) considered as a native acceptor-related transition is at 777 meV and its phonon replica (A-LO) at 748.3 meV [3,4]. Samples KNF-3 and KNF-9 were grown with a medium and a high amount of Sb2S 3 ( K N F - 3 : 1 6 . 6 mg, p-type, p = 3 × 1016 c m - 3 ; K N F - 9 : 1 2 4 mg, n-type, n = 1 × 1016 c m 3). The incorporation of sulphur leads to a compensation of the native acceptor and besides the previous transitions, the bound exciton BE 4 at 796 meV and two additional peaks S~ at 733 and an unresolved peak S 2 near 706 meV were identified. The intensity of the dominant S~ transition is higher as the intensity of the native acceptor-related transi-
3
tion and their ratio increases with the overcompensation of the native acceptor and with the transition from p-type to n-type conductivity. The native acceptor-related transition is at all sulphur-doped samples shifted about 4 - 6 meV to the lower energies and it lies between 771-773 meV (at different samples)• The free exciton (FE) transition is seen only on the KNF-9 sample and it lies at 812.5 meV. The energy band gap of this sample can be obtained by adding the free exciton binding energy ( ~ 1.1 meV) to the free exciton transition energy. In this way E G = 813.6 meV. Consequently, the main sulphur-related donor (peak S] in the photoluminescence spectra) can be estimated to occupy an energy level about 81 meV below the bottom of the conduction band (Table 2). With the aim to prepare very interesting semiconductor material for electrical and DLTS study of the deep levels, we have grown gallium antimonide codoped with sulphur and tellurium. As it follows from Table 1, tellurium doping dominates and n-type material with relatively very high electron Hall mobility was grown. On the contrary to very good electrical properties, the low temperature PL spectra of these co-doped epitaxial layers consist of one very broad peak with a maximum at an energy of 773 meV (Fig. 2). The position of this maximum is probably determined by the native acceptor level and is shifted by 4 meV to a lower energy as it is in all other GaSb sulphur-doped samples. We have not observed a transition at 807.4 meV (denoted as D and attributed to Te) which was reported by Chen and Su [8]. Since ,,~e have not observed a free exciton transition line at all samples, the determination of the activation energy of the dominant sulphur-related transmission S~ is hindered by the uncertainty of the energy gap of the samples measured at temperature T = 5 K. Therefore, we have used the room-temperature photoreflectance measurement to determine E G
Table 2 Comparison of selected PL transitions and peaks, T = 5 K Peak S i
Peak A Sample
Energy (meV)
FWHM (meV)
Relative intensity
Energy (meV)
FWHM (meV)
Relative intensity
KNF-I KNF-3 KNF-9
777 771 773
7.9 14 16
1 0.405 0.176
731 733
25 19
1 1
4
J. Novfk et al./Journal of Crystal Growth 158 (1996) 1-5 i
1,0
:"'"
GaSb T=5K
,
E
or following Ref. [9]
A
EG = 0.810 - 3.78 X 10 4 T 2 / ( T +
:. / KNF-8 ..' (: Te,S) /
0,8
~ 0,6 < Z (~ 0,4 CO -.I
":
0,2
..:"
,
I
,
I
/
A - LO
,
I
,
I
I
i
I
0,68 0,70 0,72 0,74 0,76 0,78 0,80 0,82 E N E R G Y (eV) Fig. 2. Comparison o f PL spectra of an undoped p-type GaSb (KNF-1) and n-type GaSb co-doped with S and Te (KNF-8).
at 295 K and known formulas for its temperature dependence to determine E~ at 5 K. Temperature dependence of the GaSb energy gap can be expressed following Chen and Su [8] EG = 0.812-- 4.2 X 10
4T2/(Z-}- 140),
200
GaSb : S
(1)
4o
SL-1 100 ~
..
20
/
i 0f
/
-100 ""-""
:'"
o -20
94).
(2)
As it follows from our room-temperature photoreflectance measurements (see Fig. 3), the energy gap of all GaSb sulphur-doped samples is E G = 732 meV at T = 295 K. This Ea value is a little higher as can be estimated from both expressions (from (1): E G = 728 meV, from (2): E G = 726 meV) or as was published by Iyer et al. E~ = 729 meV [3]. Using Ea from photoreflectance results at 295 K and the average value estimated from both formulas, we obtain 813 meV for E G at 5 K. This value is in good agreement with that obtained from the free exciton transition (sample KNF-9). In addition, using EG(0) = 0.8137 eV in (1), we obtain an excellent agreement of both estimated E G values (EG(5 K) = 813.6 meV from PL and E~(295 K) = 732 meV from PR) with temperature dependence of EG described by this expression.
4. Conclusions In conclusion, we have studied properties and photoluminescence spectra of the sulphur-doped GaSb grown by LPEE. This technique allows effective incorporation of sulphur and overcompensation of native acceptor. From the photoluminescence measurements at 5 K, it follows that the native acceptor-related transitions appear at 777 meV in undoped GaSb and it is shifted by 4 - 6 meV to lower energy in sulphur-doped layers. The S~ peak in the PL spectra is the dominant sulphur-related transition and lies at 731-733 meV (at different samples). The relative intensity of this transition depends on the amount of sulphur in the growth melt. Both photoreflectance and photoluminescence measurements have shown that the energy band gap of these samples E G = 732 meV at 295 K is a little higher in comparison with other published values.
I Eg=0.732 eV 200 . . . . . . - .... -40 0,68 0,70 0,72 0,74 0,76 0,78 0,80 0,82 ENERGY (eV)
Fig. 3. Room-temperature photoreflectance spectra of GaSb doped with sulphur (sample KNF-9) and co-doped with sulphur and tellurium (SL-1).
References [1] P.S. Dutta, K.S.R. Koteswara Rao, K.S. Sanguni and H.L. Bhat, Appl. Phys. Leu. 65 (1994) 1412. [2] I. Poole, M.E. Lee, I.R. Cleverley, A.R. Parker and K.E. Singer, Appl. Phys. Lett. 57 (1990) 1645.
J. Novfk et al./ Journal of Crystal Growth 158 (1996) 1-5
[3] S. Iyer, S. Hedge, K.K. Bajaj, A. Abul-Fadl and W. Mitchel, J. Appl. Phys. 73 (1993) 3958. [4] M. Lee, D.J. Nicholas, K.E. Singer and B. Hamilton, J. Appl. Phys. 59 (1986) 2895. [5] E.T.R. Chidley, S.K. Heywood, A.B. Henriques, N.J. Mason, R.J. Nicholas and P.J. Walker, Semicond. Sci. Technol. 6 (1991) 45.
5
[6] M.E. Lee, I. Poole, W.S. Truscott, I.R. Cleverley, K.E. Singer and D.M. Rohling, J. Appl. Phys. 68 (1990) 131. [7] J. NovLk, M. Klaus and K.W. Benz, J. Crystal Growth 139 (1993) 206. [8] S.C. Chen and Y.K. Su, J. Appl. Phys. 66 (1989) 350. [9] J. Camassel and D. Auvergene, Phys. Rev. B 12 (1975) 3258.