- Email: [email protected]
Sulphur doping of GaSb grown by atmospheric pressure MOVPE
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JOU.NACO.
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ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 183 (1998) 69-74
Sulphur doping of GaSb grown by atmospheric pressure MOVPE J. Novak":",
s. Hasenohrl", M. Kucera",
K. Hjelt b , T. Tuomi"
a Institute
of Electrical Engineering, Slovak Academy of Sciences. Dubraiska cesta 9. SK- 842 39 Bratislava. Slovak Republic "Optoelectronics Laboratory. Helsinki University ofTechnology, Po. Box llOO. FIN-02I50 Espoo, Finland
Reccived 15 June 1997
Abstract Sulphur-doped GaSb epitaxial layers are grown on GaSb and GaAs substrates by atmospheric pressure MOVPE. Trimethylgallium and trimethylantimony are used as the Ga and Sb sources, respectively. Hydrogen sulphide (1% HzS diluted in hydrogen) is employed as the sulphur source. The mole fraction of HzS in the reactor ranging from 6.1 x 10- 6 to 1.2 X 10- 4 results in the hole concentrations from 2.8 x 1017 to 2.5 X 1018 cm- 3 , respectively. Low temperature (T = 5 K) photoluminescence measurements show a sulphur-related transition S1 near 732 meV indicating that sulphur is successfully incorporated into GaSb. The PL spectra of the samples grown with a HzS mole fraction larger than 6.2 x 10- 5 consist only of a native acceptor transition A and a sulphur-related transition S1.The position of S1 transition is independent of the HzS mole fraction. The ratio of the intensity of the sulphur-related transition S1 to the that of the transition A increases from 0 up to 1.5 with increasing HzS mole fraction. PACS: 81.15.Gh Keywords: Epitaxial growth; MOVPE; Sulphur doping
1. Introduction
The study of GaSb and related compound materials is of current scientific and technological interest due to their applications as optoelectronic materials. It is well known that some deep donor levels can be produced in GaSb by the incorporation of group VI elements (Te, Se and S). It is known that
* Corresponding author. Fax: +421 7375 806; e-mail: [email protected]. 0022-0248/98/$19.00 QJ 1998 Elsevier Science B.V. All rights reserved PH S0022-0248(97)00396-5
the density of the deep levels in sulphur-doped GaSb is comparable with that of the shallow donors [1]. The ratio of the concentration of the deep levels to that of the shallow donor levels formed by sulphur is at least two orders of magnitude larger in the S-doped GaSb than in Te- and Se-doped one, which makes the S-doped GaSb especially interesting material. The incorporation of sulphur into epitaxial GaSb is hindered by the absence of a suitable sulphur source. This problem has been sufficiently solved only for molecular beam epitaxy (MBE) by
70
J. Novak et al. / Journal of Crystal Growth 183 (/ 998) 69- 74
using an electrochemical cell [2J and for liquidphase electroepitaxy (LPEE) by using antimonide sulphide as the sulphur source [3]. In both cases the overcompensation of the native acceptor by sulphur was achieved and the growth of n-type epitaxial layers was reported. It was shown that the low-temperature photoluminescence (PL) spectra of GaSb : S have a typical sulphur-related transition at about 732 meV . The intensity of this transition is larger than that of the native acceptor-related one, and their intensity ratio depends on the amount of incorporated sulphur [4]. Hydrogen sulphide (H 2S) has been used in low-pressure metalorganic vapour-phase epitaxial (MOVPE) growth as an n-type dopant source, but no incorporation of sulphur was observed [5]. In this work we have studied the influence of sulphur from hydrogen sulphide on the growth of GaSb by atmospheric pressure MOVPE and measured electrical and optical properties of the grown layers.
2. Epitaxial growth All GaSb epitaxial layers were grown using an MOVPE technique in a horizontal atmospheric pressure reactor at a temperature of 600°C. The source materials for the deposition of GaSb were trimethylgallium (TMGa) and tr imethylantimony (T M Sb). They were kept at - 10°C in temperature-controlled baths. The TMGa mole fraction in the input gas stream was 8.35 x 10- 5 and it was kept constant in all experiments. The input V/IlI ratio was 1.1. The growth rate of GaSb was about 4 um /h , and the total gas veloci ty in the reactor was 8.8 cm /s. Hydrogen sulphide (1 % H 2S diluted in hydrogen) was used as the sulphur source. The mole fraction of H 2S in the input gas stream was varied between 6 x 10 - 6 and 1.2 x 10-4-. Values of H 2S flow with corresponding mole fractions ofH 2S in the input gas st ream for various samples are given in Table 1. Palladium diffused hydrogen was used as the carrier gas. The epitaxial layers were grown on <1 0 0) oriented semi-insulating GaAs substrates (for electrical characterisation) and on (l 00) tellurium-doped GaSb substrates. The GaAs substrates were first degreased by successively imm ersing in trichloro-
Table 1 Value s of H zS flows with corr espon ding mole fractions of HzS in the input gas stream for GaSb : S layers grown on GaSb substra tes Sample no.
H zS flow (ml min" ')
HzS mole fraction
SG ASB06 SGASB1 2 SGASB13 SGASB14 SGASB15
1 5 7 10
6.1 x 10 x 4.3 x 6.2 x 1.2 x
20
10- 6 lO- 5 10- ; 10- 5 10- 4
ethylene, acetone and methanol. After a 10 min water rinse the substrates were etched in a solution of H 2 S0 4 : H 2 0 Z : H 20 (5: 1 : 1) for 8 min. Finally, th e substrates were rinsed in water for 20 min and blown dry with nitrogen. The G aSb substrates were cleaned in trichloroethylene, acetone a nd methanol. After rin sing in wa ter , they were etched in HF : H z0 2 (1 : 1) for 1 min and th en oxidised in water. The oxide was remo ved by immersion in HCI for 1 min . Finally, they were washed in methanol, dried with nitrogen, and put into th e N j-filled glo ve bo x. To desorb the residual oxide layer , the GaSb substrate was annealed at 650°C for 8 min. It wa s done prior to growth in the reactor during the TMSb flow . From the previous works [6-9J we know that the optimum growth conditions as to the electrical properties and surface quality are: the V/ III input ratio is between 1 and 1.1 and th e growth rate is in the range from 2 to 8 urn/h. The undoped GaSb epitaxial layers prepared at these conditions on GaAs substrates have a Hall hole-mobility larger than 800 crrr' IV s and a hole concentration ranges from 10 16 to 10 1 7 cm- 3 •
3. Results and discussion The surface of the samples ligh tly d oped with sulph ur was mirror like . The surface morphology of sam ples grown at H zS flow of 10 ml/min (sample SGASB4 on GaAs substrate and SGASB14 and SGASB1 5 on GaSb substrate) was not as excellen t a nd the ir surface was partially dim. Ho wever, X-ray
1. Novak et al. / Journal of Crystal Growth 183 (1998) 69-74
(see Fig. 2). We assume that this behaviour is a consequence of the overcompensation of the native acceptor by sulphur atoms. At large values of the H 2S mole fraction, the hole concentration rapidly increases with increasing the H 2S mole fraction. It is well known that sulphur in GaSb creates deep donor levels and therefore n-type sulphur-doped layers can be grown [2,3]. In our case we suppose that hydrogen sulphide acts as a two-type dopant source. It creates a deep donor level related to sulphur (see photoluminescence results below) and a shallow level with an acceptor-like behaviour. There are several possible reasons for the existence of this kind of an acceptor level. Firstly, our source of hydrogen sulphide may be contaminated with some element or compound which acts as an acceptor. However, this explanation is not very probable one, because also other researchers have observed similar behaviour when employing hydrogen sulphide as a source of sulphur [5]. Another possibility is the incorporation from organometallics. The acceptor-like behaviour of sulphur can be also related to an increase of native defects as a result of very large amount of sulphur. Sulphur could also give rise to an acceptor level similar to [VGaGaSbTesbJ complex, one observed in tellurium-doped GaSb proposed by Hjelt and Tuomi [9J and by Dutta [10]. It is worth noticing that the
Table 2 Growth parameters and electrical properties measured at 300 K of GaSb: S layers grown on GaAs substrates Sample no.
HzS flow HzS mole (ml min - I) fraction
SGASBI SGASB2 SGASB7 SGASB5 SGASB6 SGASB4
0 1 2 3 5 10
0 6.1 x 10- 6 1.2 x 10- 5 i.s x io " 3.0xl0- s 6.2 x 10- 5
Hole cone. Hole mobility (cm- 3 ) (cmZV- 1s- 1) 8.6 X 101 6 2.8xl0 17 2.2x 101 7 5.0xl0 17 4.6x 101 7 2.5 X ]018
730 715 730 630 590 290
diffraction measurements show that the crystalline quality and the lattice constant do not depend on the H 2S amount in the growth gas. Electrical properties of the GaSb epitaxial layers grown on the GaAs substrates measured using the Van der Pauw method are summarised in Table 2. Fig. 1 shows the dependence of the room-temperature hole mobility and hole concentration on the H 2S mole fraction in the reactor for the samples grown on the GaAs substrates. The Hall holemobility for the small values of the H 2S mole fraction is nearly constant with a slight tendency to increase with increasing H 2S mole fraction. At the same time, the hole concentration slightly decreases
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J Novak et al. / Journal of Crystal Growth 183 (1998) 69-74
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samples for electrical measurements were grown on GaAs substrates. The thickness of the epitaxial layers was about 3 urn to exclude the influence of interface defects on the measured electrical parameters. In spite of this the surface of the epitaxial layers grown at large values of H 2S flow (> 10 ml/min) were smooth but grayish. The reason for the peculiar doping behaviour observed remains unclear at present and it certainly needs more experimental work to explain it. The low-temperature PL spectra were measured at 5 K. The excitation source was the 632.8 nm line of a 50 mW He-Ne laser. The laser beam was focused at normal incidence to a spot, at which its intensity was 1 W em - 2. Luminescent radiation was filtered by a quarter-meter monochromator and detected by a cooled PbS detector using a standard lock-in technique. Fig. 3 shows the photoluminescence spectra of two lightly doped GaSb samples (SGaSb06 and SGaSb2) which were grown under the same conditions on a GaAs and a GaSb substrate. The PL spectrum of a sample grown on the GaSb substrate exhibits a dominant transition
0.76
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energy leV] Fig. 3. Comparison of the low-temperature photoluminescence spectra of GaSb epitaxial layers grown on GaAs and GaSb substrates.
A at 777 meV, which is considered to originate from a native acceptor, its phonon replica (A-LO) at 748.3 meV, and a free-exciton transition (FE) at 810 meV. Bound exciton transitions (probably BE 4 at 798.6 meV and BE 2 or BE 3 at 805.8 meV) dominate the PL spectrum of the sample grown on the GaAs substrate and their intensity is four times that of the native acceptor transition A at 778 meV. The transitions BE t-BE 4 are usually attributed to the decay of an exciton bound to an unidentified neutral acceptor, (A0, X). As seen from Fig. 3 there is an essential and big difference in the luminescence properties of GaSb grown on GaSb and GaAs substrates. Fig. 4 depicts a PL spectrum detected in the range from 720 to 820 meV of the GaSb sample SGASB06 prepared on a GaSb substrate and lightly doped with sulphur (H 2S mole fraction was 6 x 10- 6). This doping level has practically no influence on the electrical properties of the layer. However, in the PL spectrum in addition to a typical sulphur-related transition Sl at about 732 meV six other transitions are observed [4]. The free-exciton
1. Novak et 01. ! Journal of Crystal Growth 183 (1998) 69-74
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transition at 810.2 meV is an indication of a high optical quality of the sample and it is observed only in the samples having the least amount of incorporated sulphur. The bang-gap energy EG = 811.3 meV of this sample is obtained by adding the free-exciton binding energy (1.1 meV) to the free exciton transition energy. Two bound-exciton transitions BE 2 at 803.4 meV and BE 4 at 797 meV are also clearly observable in Fig. 4. The native acceptor related transition A located at 778 meV dominates the PL spectrum. Its phonon replica A-LO is observed at 749 meV. A typical donor-acceptor transition B is clearly seen at 758.7 meV. These results show that the lightly doped epitaxial layers are of good electrical and optical quality and are fully comparable with other published results [11, 12]. The layers were used as reference samples in the subsequent study of highly sulphur doped GaSb layers. The incorporation of sulphur leads to a drastic change in the PL spectra. Fig. 5 shows the PL spectrum of the homoepitaxial samples grown with a H 2S mole fraction larger than 6.2 x 10- 5 . It consists of only two transitions: a native acceptor peak A and a sulphur-related peak St. The native accep-
Fig. 5. The photoluminescence spectra of four GaSb layers grown with different hydrogen sulphide mole fraction.
tor transition A is shifted in all sulphur doped samples from the starting position of 778 meV to lower energies with increasing H 2S mole fraction. A largest shift observed was 16 meV. The intensity of the sulphur-related transition St increases compared to the intensity of the native acceptor transition and their ratio changes from 0 up to 1.5 when the H 2S mole fraction reaches its largest value. The position of the peak S, is practically independent of the H 2S mole fraction, and it lies at about 732 meY. Using the band-gap energy EG = 811.3 meV we can estimate that the main sulphurrelated level is located about 80 meV below the bottom of the conduction band. The effect of sulphur doping on the PL spectra of the heteroepitaxial samples is somewhat different from that of the homoepitaxial ones. Remarkable is that the sample SGaSb6 is half shiny (front part) and half grayish (back part), which is caused by the change of the V/III-ratio along the flow direction of the growth gas. The electrical and optical properties of this sample depend drastically on the surface morphology. The shiny part of the sample SGaSb6 exhibits a weak, but clear St transition at 730 meV and a native acceptor-related transition at about 776 meV, whereas the dim part exhibits only one broad transition centered at 740 meY. The PL spectra of the sample grown with the largest H 2S flow (SGaSb4) consists also of this one very broad transition at 740 meV. It is interesting to notice
74
J Novak et al. / Journal of Crystal Growth 183 (1998) 69-74
that in the heteroepitaxial samples this deep transition band attributed to large doping with sulphur is located at an energy somewhat different from that observed in the homoepitaxial samples. This may be connected to the strain and dislocations caused by the large difference in the lattice constants of GaSb and GaAs.
4. Conclusions We have studied the influence of sulphur from hydrogen sulphide on the electrical and optical properties of GaSb layers grown by atmospheric pressure MOVPE. The growth at large values of the H 2S mole fraction results in deterioration of the morphology and samples having a dim surface are grown. The low-temperature PL measurements show that the electronic transition related to the native acceptor defect dominates only in samples grown at small values of the H 2S mole fraction. With increasing H 2S mole fraction the intensity of the transition associated with the incorporated sulphur at about 732 meV becomes dominant and sulphur is successfully incorporated into GaSb. The electrical characterisation of the heteroepitaxial samples grown on a GaAs substrate show that the deep donor level related to sulphur is overcompen-
sated by an unidentified shallow acceptor level. The source and nature of this acceptor is not yet known.
References [1] 1. Poole, M.E. Lee, 1.R. Claverly, A.R. Parker, KE. Singer, App!. Phys. Lett. 57 (1990) 1645. [2] I. Poole, M.E. Lee, KE. Singer, 1.E.F. Frost, T.M. Kerr, G.E.C. Wood, D.A. Andrews, W.J.M. Rothwell, G.1. Davies, J. App!. Phys. 63 (1988) 395. [3] 1. Novak, M. Klaus, KW. Benz, 1. Crystal Growth 139 (1994) 206. [4] J. Novak, M. Kucera, S. Lauer, KW. Benz, 1. Crystal Growth 158 (1996) 1. [5] C. von Eichel-Streiber, M. Behet, M. Heuken, K Heime, J. Crystal Growth 170 (1997) 783. [6] M. Sopanen, T. Koljoncn, H. Lipsanen, T. Tuomi, 1. Crystal Growth 145 (1994) 492. [7] T. Koljonen, M. Sopanen, H. Lipsanen, T. Tuomi, J. Electron. Mater. 24 (1995) 1691. [8] K. Hjelt, T. Tuomi, in: J. Novak, A. Schlachctzki (Eds.), High Technology, vol. 11, Heterostructure Epitaxy and Devices, NATO ASI Series 3. Kluwer, Dordrecht, 1996, 37. [9] K Hjelt, T. Tuomi, 1. Crystal Growth 170 (1997) 794. [10] P.S. Dutta, V. Prasad, H.L. Bhat, V. Kumar, J. Appl. Phys. 80 (1996) 2847. [11] M. Lee, D.J. Nicholas, KE. Singer, B. Hamilton, J. Appl. Phys. 59 (1986) 2895. [12] S. Iyer, S. Hegde, K.K. Bajaj, A. Abul-Fadl, W. Mitchel, J. Appl. Phys. 73 (1993) 3958.
JOU.NACO.
~
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 183 (1998) 69-74
Sulphur doping of GaSb grown by atmospheric pressure MOVPE J. Novak":",
s. Hasenohrl", M. Kucera",
K. Hjelt b , T. Tuomi"
a Institute
of Electrical Engineering, Slovak Academy of Sciences. Dubraiska cesta 9. SK- 842 39 Bratislava. Slovak Republic "Optoelectronics Laboratory. Helsinki University ofTechnology, Po. Box llOO. FIN-02I50 Espoo, Finland
Reccived 15 June 1997
Abstract Sulphur-doped GaSb epitaxial layers are grown on GaSb and GaAs substrates by atmospheric pressure MOVPE. Trimethylgallium and trimethylantimony are used as the Ga and Sb sources, respectively. Hydrogen sulphide (1% HzS diluted in hydrogen) is employed as the sulphur source. The mole fraction of HzS in the reactor ranging from 6.1 x 10- 6 to 1.2 X 10- 4 results in the hole concentrations from 2.8 x 1017 to 2.5 X 1018 cm- 3 , respectively. Low temperature (T = 5 K) photoluminescence measurements show a sulphur-related transition S1 near 732 meV indicating that sulphur is successfully incorporated into GaSb. The PL spectra of the samples grown with a HzS mole fraction larger than 6.2 x 10- 5 consist only of a native acceptor transition A and a sulphur-related transition S1.The position of S1 transition is independent of the HzS mole fraction. The ratio of the intensity of the sulphur-related transition S1 to the that of the transition A increases from 0 up to 1.5 with increasing HzS mole fraction. PACS: 81.15.Gh Keywords: Epitaxial growth; MOVPE; Sulphur doping
1. Introduction
The study of GaSb and related compound materials is of current scientific and technological interest due to their applications as optoelectronic materials. It is well known that some deep donor levels can be produced in GaSb by the incorporation of group VI elements (Te, Se and S). It is known that
* Corresponding author. Fax: +421 7375 806; e-mail: [email protected]. 0022-0248/98/$19.00 QJ 1998 Elsevier Science B.V. All rights reserved PH S0022-0248(97)00396-5
the density of the deep levels in sulphur-doped GaSb is comparable with that of the shallow donors [1]. The ratio of the concentration of the deep levels to that of the shallow donor levels formed by sulphur is at least two orders of magnitude larger in the S-doped GaSb than in Te- and Se-doped one, which makes the S-doped GaSb especially interesting material. The incorporation of sulphur into epitaxial GaSb is hindered by the absence of a suitable sulphur source. This problem has been sufficiently solved only for molecular beam epitaxy (MBE) by
70
J. Novak et al. / Journal of Crystal Growth 183 (/ 998) 69- 74
using an electrochemical cell [2J and for liquidphase electroepitaxy (LPEE) by using antimonide sulphide as the sulphur source [3]. In both cases the overcompensation of the native acceptor by sulphur was achieved and the growth of n-type epitaxial layers was reported. It was shown that the low-temperature photoluminescence (PL) spectra of GaSb : S have a typical sulphur-related transition at about 732 meV . The intensity of this transition is larger than that of the native acceptor-related one, and their intensity ratio depends on the amount of incorporated sulphur [4]. Hydrogen sulphide (H 2S) has been used in low-pressure metalorganic vapour-phase epitaxial (MOVPE) growth as an n-type dopant source, but no incorporation of sulphur was observed [5]. In this work we have studied the influence of sulphur from hydrogen sulphide on the growth of GaSb by atmospheric pressure MOVPE and measured electrical and optical properties of the grown layers.
2. Epitaxial growth All GaSb epitaxial layers were grown using an MOVPE technique in a horizontal atmospheric pressure reactor at a temperature of 600°C. The source materials for the deposition of GaSb were trimethylgallium (TMGa) and tr imethylantimony (T M Sb). They were kept at - 10°C in temperature-controlled baths. The TMGa mole fraction in the input gas stream was 8.35 x 10- 5 and it was kept constant in all experiments. The input V/IlI ratio was 1.1. The growth rate of GaSb was about 4 um /h , and the total gas veloci ty in the reactor was 8.8 cm /s. Hydrogen sulphide (1 % H 2S diluted in hydrogen) was used as the sulphur source. The mole fraction of H 2S in the input gas stream was varied between 6 x 10 - 6 and 1.2 x 10-4-. Values of H 2S flow with corresponding mole fractions ofH 2S in the input gas st ream for various samples are given in Table 1. Palladium diffused hydrogen was used as the carrier gas. The epitaxial layers were grown on <1 0 0) oriented semi-insulating GaAs substrates (for electrical characterisation) and on (l 00) tellurium-doped GaSb substrates. The GaAs substrates were first degreased by successively imm ersing in trichloro-
Table 1 Value s of H zS flows with corr espon ding mole fractions of HzS in the input gas stream for GaSb : S layers grown on GaSb substra tes Sample no.
H zS flow (ml min" ')
HzS mole fraction
SG ASB06 SGASB1 2 SGASB13 SGASB14 SGASB15
1 5 7 10
6.1 x 10 x 4.3 x 6.2 x 1.2 x
20
10- 6 lO- 5 10- ; 10- 5 10- 4
ethylene, acetone and methanol. After a 10 min water rinse the substrates were etched in a solution of H 2 S0 4 : H 2 0 Z : H 20 (5: 1 : 1) for 8 min. Finally, th e substrates were rinsed in water for 20 min and blown dry with nitrogen. The G aSb substrates were cleaned in trichloroethylene, acetone a nd methanol. After rin sing in wa ter , they were etched in HF : H z0 2 (1 : 1) for 1 min and th en oxidised in water. The oxide was remo ved by immersion in HCI for 1 min . Finally, they were washed in methanol, dried with nitrogen, and put into th e N j-filled glo ve bo x. To desorb the residual oxide layer , the GaSb substrate was annealed at 650°C for 8 min. It wa s done prior to growth in the reactor during the TMSb flow . From the previous works [6-9J we know that the optimum growth conditions as to the electrical properties and surface quality are: the V/ III input ratio is between 1 and 1.1 and th e growth rate is in the range from 2 to 8 urn/h. The undoped GaSb epitaxial layers prepared at these conditions on GaAs substrates have a Hall hole-mobility larger than 800 crrr' IV s and a hole concentration ranges from 10 16 to 10 1 7 cm- 3 •
3. Results and discussion The surface of the samples ligh tly d oped with sulph ur was mirror like . The surface morphology of sam ples grown at H zS flow of 10 ml/min (sample SGASB4 on GaAs substrate and SGASB14 and SGASB1 5 on GaSb substrate) was not as excellen t a nd the ir surface was partially dim. Ho wever, X-ray
1. Novak et al. / Journal of Crystal Growth 183 (1998) 69-74
(see Fig. 2). We assume that this behaviour is a consequence of the overcompensation of the native acceptor by sulphur atoms. At large values of the H 2S mole fraction, the hole concentration rapidly increases with increasing the H 2S mole fraction. It is well known that sulphur in GaSb creates deep donor levels and therefore n-type sulphur-doped layers can be grown [2,3]. In our case we suppose that hydrogen sulphide acts as a two-type dopant source. It creates a deep donor level related to sulphur (see photoluminescence results below) and a shallow level with an acceptor-like behaviour. There are several possible reasons for the existence of this kind of an acceptor level. Firstly, our source of hydrogen sulphide may be contaminated with some element or compound which acts as an acceptor. However, this explanation is not very probable one, because also other researchers have observed similar behaviour when employing hydrogen sulphide as a source of sulphur [5]. Another possibility is the incorporation from organometallics. The acceptor-like behaviour of sulphur can be also related to an increase of native defects as a result of very large amount of sulphur. Sulphur could also give rise to an acceptor level similar to [VGaGaSbTesbJ complex, one observed in tellurium-doped GaSb proposed by Hjelt and Tuomi [9J and by Dutta [10]. It is worth noticing that the
Table 2 Growth parameters and electrical properties measured at 300 K of GaSb: S layers grown on GaAs substrates Sample no.
HzS flow HzS mole (ml min - I) fraction
SGASBI SGASB2 SGASB7 SGASB5 SGASB6 SGASB4
0 1 2 3 5 10
0 6.1 x 10- 6 1.2 x 10- 5 i.s x io " 3.0xl0- s 6.2 x 10- 5
Hole cone. Hole mobility (cm- 3 ) (cmZV- 1s- 1) 8.6 X 101 6 2.8xl0 17 2.2x 101 7 5.0xl0 17 4.6x 101 7 2.5 X ]018
730 715 730 630 590 290
diffraction measurements show that the crystalline quality and the lattice constant do not depend on the H 2S amount in the growth gas. Electrical properties of the GaSb epitaxial layers grown on the GaAs substrates measured using the Van der Pauw method are summarised in Table 2. Fig. 1 shows the dependence of the room-temperature hole mobility and hole concentration on the H 2S mole fraction in the reactor for the samples grown on the GaAs substrates. The Hall holemobility for the small values of the H 2S mole fraction is nearly constant with a slight tendency to increase with increasing H 2S mole fraction. At the same time, the hole concentration slightly decreases
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J Novak et al. / Journal of Crystal Growth 183 (1998) 69-74
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5x10 17
0.0
5x10 18
hole concentration [cm-3 ] Fig. 2. Room temperature Hall hole mobility as function of hole concentration for GaSb epitaxial layers grown on GaAs substrates.
samples for electrical measurements were grown on GaAs substrates. The thickness of the epitaxial layers was about 3 urn to exclude the influence of interface defects on the measured electrical parameters. In spite of this the surface of the epitaxial layers grown at large values of H 2S flow (> 10 ml/min) were smooth but grayish. The reason for the peculiar doping behaviour observed remains unclear at present and it certainly needs more experimental work to explain it. The low-temperature PL spectra were measured at 5 K. The excitation source was the 632.8 nm line of a 50 mW He-Ne laser. The laser beam was focused at normal incidence to a spot, at which its intensity was 1 W em - 2. Luminescent radiation was filtered by a quarter-meter monochromator and detected by a cooled PbS detector using a standard lock-in technique. Fig. 3 shows the photoluminescence spectra of two lightly doped GaSb samples (SGaSb06 and SGaSb2) which were grown under the same conditions on a GaAs and a GaSb substrate. The PL spectrum of a sample grown on the GaSb substrate exhibits a dominant transition
0.76
0.78
0.80
0.82
energy leV] Fig. 3. Comparison of the low-temperature photoluminescence spectra of GaSb epitaxial layers grown on GaAs and GaSb substrates.
A at 777 meV, which is considered to originate from a native acceptor, its phonon replica (A-LO) at 748.3 meV, and a free-exciton transition (FE) at 810 meV. Bound exciton transitions (probably BE 4 at 798.6 meV and BE 2 or BE 3 at 805.8 meV) dominate the PL spectrum of the sample grown on the GaAs substrate and their intensity is four times that of the native acceptor transition A at 778 meV. The transitions BE t-BE 4 are usually attributed to the decay of an exciton bound to an unidentified neutral acceptor, (A0, X). As seen from Fig. 3 there is an essential and big difference in the luminescence properties of GaSb grown on GaSb and GaAs substrates. Fig. 4 depicts a PL spectrum detected in the range from 720 to 820 meV of the GaSb sample SGASB06 prepared on a GaSb substrate and lightly doped with sulphur (H 2S mole fraction was 6 x 10- 6). This doping level has practically no influence on the electrical properties of the layer. However, in the PL spectrum in addition to a typical sulphur-related transition Sl at about 732 meV six other transitions are observed [4]. The free-exciton
1. Novak et 01. ! Journal of Crystal Growth 183 (1998) 69-74
A
1.0
GaSb:S(MOVPE) SGaSb06 T= 5K
0.8
1.0
~ 0.4
\
1\ \
I
(])
c
5x
\\
0.4
....J
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B
BE,
Q..
0.2
\ \..,
....J Q..
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I \
='= til c
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ro (])
A
::i ~ 0.6 >.
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:::J
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GaSb:S
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........ 0.6 ~
73
0.68
0.70
0.72
074
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energy [ev]
0.2
00 '--~--L_~-'-~_.L-~--L_~--' Qn O.M O.M o.n O.W O.~
energy (eV) Fig. 4. Photoluminescence of a sample SGASB06 showing the various transitions observed in good-quality material.
transition at 810.2 meV is an indication of a high optical quality of the sample and it is observed only in the samples having the least amount of incorporated sulphur. The bang-gap energy EG = 811.3 meV of this sample is obtained by adding the free-exciton binding energy (1.1 meV) to the free exciton transition energy. Two bound-exciton transitions BE 2 at 803.4 meV and BE 4 at 797 meV are also clearly observable in Fig. 4. The native acceptor related transition A located at 778 meV dominates the PL spectrum. Its phonon replica A-LO is observed at 749 meV. A typical donor-acceptor transition B is clearly seen at 758.7 meV. These results show that the lightly doped epitaxial layers are of good electrical and optical quality and are fully comparable with other published results [11, 12]. The layers were used as reference samples in the subsequent study of highly sulphur doped GaSb layers. The incorporation of sulphur leads to a drastic change in the PL spectra. Fig. 5 shows the PL spectrum of the homoepitaxial samples grown with a H 2S mole fraction larger than 6.2 x 10- 5 . It consists of only two transitions: a native acceptor peak A and a sulphur-related peak St. The native accep-
Fig. 5. The photoluminescence spectra of four GaSb layers grown with different hydrogen sulphide mole fraction.
tor transition A is shifted in all sulphur doped samples from the starting position of 778 meV to lower energies with increasing H 2S mole fraction. A largest shift observed was 16 meV. The intensity of the sulphur-related transition St increases compared to the intensity of the native acceptor transition and their ratio changes from 0 up to 1.5 when the H 2S mole fraction reaches its largest value. The position of the peak S, is practically independent of the H 2S mole fraction, and it lies at about 732 meY. Using the band-gap energy EG = 811.3 meV we can estimate that the main sulphurrelated level is located about 80 meV below the bottom of the conduction band. The effect of sulphur doping on the PL spectra of the heteroepitaxial samples is somewhat different from that of the homoepitaxial ones. Remarkable is that the sample SGaSb6 is half shiny (front part) and half grayish (back part), which is caused by the change of the V/III-ratio along the flow direction of the growth gas. The electrical and optical properties of this sample depend drastically on the surface morphology. The shiny part of the sample SGaSb6 exhibits a weak, but clear St transition at 730 meV and a native acceptor-related transition at about 776 meV, whereas the dim part exhibits only one broad transition centered at 740 meY. The PL spectra of the sample grown with the largest H 2S flow (SGaSb4) consists also of this one very broad transition at 740 meV. It is interesting to notice
74
J Novak et al. / Journal of Crystal Growth 183 (1998) 69-74
that in the heteroepitaxial samples this deep transition band attributed to large doping with sulphur is located at an energy somewhat different from that observed in the homoepitaxial samples. This may be connected to the strain and dislocations caused by the large difference in the lattice constants of GaSb and GaAs.
4. Conclusions We have studied the influence of sulphur from hydrogen sulphide on the electrical and optical properties of GaSb layers grown by atmospheric pressure MOVPE. The growth at large values of the H 2S mole fraction results in deterioration of the morphology and samples having a dim surface are grown. The low-temperature PL measurements show that the electronic transition related to the native acceptor defect dominates only in samples grown at small values of the H 2S mole fraction. With increasing H 2S mole fraction the intensity of the transition associated with the incorporated sulphur at about 732 meV becomes dominant and sulphur is successfully incorporated into GaSb. The electrical characterisation of the heteroepitaxial samples grown on a GaAs substrate show that the deep donor level related to sulphur is overcompen-
sated by an unidentified shallow acceptor level. The source and nature of this acceptor is not yet known.
References [1] 1. Poole, M.E. Lee, 1.R. Claverly, A.R. Parker, KE. Singer, App!. Phys. Lett. 57 (1990) 1645. [2] I. Poole, M.E. Lee, KE. Singer, 1.E.F. Frost, T.M. Kerr, G.E.C. Wood, D.A. Andrews, W.J.M. Rothwell, G.1. Davies, J. App!. Phys. 63 (1988) 395. [3] 1. Novak, M. Klaus, KW. Benz, 1. Crystal Growth 139 (1994) 206. [4] J. Novak, M. Kucera, S. Lauer, KW. Benz, 1. Crystal Growth 158 (1996) 1. [5] C. von Eichel-Streiber, M. Behet, M. Heuken, K Heime, J. Crystal Growth 170 (1997) 783. [6] M. Sopanen, T. Koljoncn, H. Lipsanen, T. Tuomi, 1. Crystal Growth 145 (1994) 492. [7] T. Koljonen, M. Sopanen, H. Lipsanen, T. Tuomi, J. Electron. Mater. 24 (1995) 1691. [8] K. Hjelt, T. Tuomi, in: J. Novak, A. Schlachctzki (Eds.), High Technology, vol. 11, Heterostructure Epitaxy and Devices, NATO ASI Series 3. Kluwer, Dordrecht, 1996, 37. [9] K Hjelt, T. Tuomi, 1. Crystal Growth 170 (1997) 794. [10] P.S. Dutta, V. Prasad, H.L. Bhat, V. Kumar, J. Appl. Phys. 80 (1996) 2847. [11] M. Lee, D.J. Nicholas, KE. Singer, B. Hamilton, J. Appl. Phys. 59 (1986) 2895. [12] S. Iyer, S. Hegde, K.K. Bajaj, A. Abul-Fadl, W. Mitchel, J. Appl. Phys. 73 (1993) 3958.