Thermodynamic aspects of (Te,S)-double-doped GaSb crystal growth

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Thermodynamic aspects of (Te, S)-double-doped GaSb crystal growth V. gestfikovfi, B. gtrpfinek, J. gestfik, P. Hubfk and V. gmid Institute of Physics, Semiconductor Department, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 162 O0Prague 6 (Czech Republic) (Received December 2, 1992)

Abstract A series of GaSb single crystals double doped with tellurium and sulphur were grown using the Czochralski method

without encapsulant in an atmosphere of flowing hydrogen. The Hall carrier concentration of these crystals was measured and compared with the calculated values. Very good agreement appeared for the sulphur concentration Cs for total amounts of dopants (Te and S) smaller than 12 at.%. If Cs exceeds a value of 12 at.%, the theoretical and practical values of the Hall concentration differ from each other. It seems that this effect was caused by the creation of a Te-S solid solution and by the subsequent elimination of dopants from the GaSb lattice. This assumption is supported by measurements of dislocation density. In the case of Cs >~12 at.% the dislocations were uniformly distributed on the surface of GaSb(l I 1) samples because the so-called "glide phenomena" could be suppressed by the formation of some precipitates of the Te-S solid solution which might generate dislocations in the whole volume of the GaSb single crystals.

I. Introduction Gallium antimonide belongs to a large family of III-V semiconductor compounds. GaSb crystals have been used for producing many semiconductor devices, such as light diodes and laser diodes. The GaSb starting single-crystalline material must be of high quality so that it can be used as the final material for the fabrication of these devices [ 1]. GaSb also appears convenient for the investigation of an interesting deep metastable centre--the DX centre [2]. This defect is often observed in different III-V semiconductors, mainly in ternary alloys. Nevertheless, the study of the DX centre in a binary compound allows us to separate the effect of alloying on energy levels and other properties of the centre [3]. GaSb is, up to now, the only binary compound where the DX centre has been detected at atmospheric pressure and under non-degenerate doping conditions [4, 5]. Recently, we have shown [5] that in Czochralskigrown GaSb doped with sulphur a DX-like centre dominates in Hall measurements between 77-300 K. However, if a deep level is the main source of free carriers in a wide temperature range, the evaluation of capacitance in deep-level transient spectroscopy (DLTS) studies of such a level becomes complicated. It is given by a transient non-exponentiality due to a high deep-level concentration [6]. This non-exponentiality is avoided if samples are measured where the concentra0921-5107/93/$6.00

tion of shallow impurities is high in comparison with that of deep levels (ten times or more). Since the DX centre is difficult to observe in p-type material, it is necessary to prepare n-GaSb crystals doped both with sulphur and with some shallow donor impurity. Tellurium can be used as the codopant because it creates a shallow donor level in GaSb at atmospheric pressure [7]. The growth and conditions of Te-doped GaSb single crystals have been reported in many papers [8-11 ] but the literature devoted to the growth of S-doped GaSb is rather scant. Only a few authors [12-15] were engaged in such work and the conditions of crystal growth and of the quality of these crystals have been described either insufficiently or with some inaccuracies [12]. However, papers on double doping of Te and S have been entirely absent. The codoping procedure has been described very well in the case of GaAs single crystals (In-Si) [16] but nobody has attempted to prepare double-doped GaSb single crystals with n-type conductivity. The goal of our investigation is to prepare (Te, S)double-doped GaSb single crystals with n-type conductivity and to describe their quality. We used the Czochralski technique without encapsulant in an atmosphere of flowing hydrogen. This method can produce dislocation-free GaSb single crystals with high structural quality since only small temperature gradients appear in the growth apparatus [17]. In addition, using this method, the preparation of starting © 1993 - Elsevier Sequoia. All rights reserved

V. ~¢estdkov6et al. /

Thermodynamic aspects of GaSb crystal growth

material, doping and the whole growth procedure are very simple and it is possible to control them easily and to reproduce the whole process.

2. Crystal growth The method of crystal preparation is described in previous papers [18, 19] in detail. Hydrogen very well purified by a .palladium purifier flowed through a quartz ampoule appended to the Czochralski apparatus. A small amount of sulphur was placed in the ampoule which was heated to a temperature of about 190 °C. The GaSb crystals were grown in an atmosphere of flowing hydrogen which also served as a transport agent for sulphur. The starting amount of polycrystalline GaSb (Spurmetalle Freiberg, Germany) was about 170 g. Tellurium was added as dopant in an elementary form and sulphur was used in the form of Sb2S 3 compound. The single crystals were grown in the (111) direction. Tellurium concentration (charge in the melt) ranged between 4.79 × 1 0 t7 and 4.21 × 1018 atoms cm -3 and the concentration of sulphur was (1.25x101v) (4.90 x 1019) atoms cm -3. The crystals were cut into many wafers perpendicular to the growth direction and the Hall concentration was measured by the van der Pauw method with an accuracy of about 20%.

3. Results and discussion

The grown crystals were always single crystalline, but in the case of high concentrations of sulphur the growth was disturbed and either some twins or some parasitic grains appeared. If the sulphur concentration in the starting melt was lower than about 1 x 1019 atoms cm 3 the GaSb was up to g = 0.9 (g is the solidifying fraction) single crystalline. For GaSb melt having a starting sulphur concentration below 1 x 1018 atoms cm- 3, the grown crystals were fully single crystalline up to the end. The carrier concentrations were always measured close to the seed, i.e. for g-~ 0, and in some cases also in the end part of the crystals. We compared the measured Hall concentration of these samples with the theoretical values calculated in the following way. Dopant concentrations in the crystal were determined from starting charges in the melt and from the Pfann equation using distribution coefficients keff(si=0.06 [20] and keff/Te) = 0.32 [21]. The dopant concentrations were used to evaluate Hall electron concentrations at 300 K by solving the charge neutrality equation. The Fermi-Dirac distribution and Sagar's two-band model

15

were assumed [22, 23]. Into the calculation we inserted a ratio of mobilities in the L and F bands equal to 1/6, a density of states in F and L minima of 2.6 x 10 j7 cm -3 and 1.04x 1019 cm 3 respectively [23] and an energy separation between the two minima of 85 meV [24]. The Te and S donor levels were assumed to be lying 10 meV below the L minimum [22] and 65 meV below the F minimum [24]. Fully occupied acceptor levels with a concentration of 1.7x 1017 c m 3 were assumed for all samples [25, 26]. Taking into account a scatter in the literature values of the above quantities, we estimated the resulting inaccuracy of the calculated Hall concentration to be not greater than 40%. The results of a comparison of the theoretical and experimental Hall concentrations are shown in Fig. 1 and Table 1. We found that the agreement or disagreement of the theoretical and experimental values is related to the sulphur concentration in the total amount of dopants. For g = 0 this concentration can be expressed as

CS

( ms/ ms)k~ls! (S)_I(To:,X 100 (ms/Ms)kef f (mTe/Mxc)k~,,-t

--

•

(1)

We can classify our crystals into two groups: the first group includes crystals with C s < 12 at.%; in the second group Cs> 12 at.%. The plot of experimental Hall concentration nil,,, ws'. theoretical Hall concentration nH,t for g ~ 0 and a temperature of 300 K is shown for both groups in Fig. 1. Data obtained from all measured samples (with different tellurium concentrations covering approximately one order of magnitude) are presented. In spite of a large scatter it can be seen that for Cs < 12 at.% the relation nHx ~ nH.t is roughly fulfilled.

nH,e(measured ) .1017 [crn 3] 4.0 /s

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Fig. 1. Comparison of experimental and theoretical values of the Hall concentration in the starting portion (g~ 0) of GaSb crystals at 300K: x - - - , Cs<12 at.%; [] , Cs>12at.%. Negative values indicate p-type conductivity and correspond to hole concentrations.

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V. ~est6kovd et al.

/

Thermodynamic aspects of GaSb crystal growlh

TABLE t. Comparison of calculated and measured Hall concentrations ( T= 300 K) in the GaSb crystals for an almost constant Te concentration and various concentrations of S Crystal

Concentration of Te in the melt (atoms cm- ~)

Concentration of S in the doping charge (atoms cm --~)

('s (at.%)

TeS-107

2.08 x 10 j~

6.31 x 1017

5.39

TeS-105

1.96 x 10 ~8

6.25 × 1()j~

38.50

TeS-103

1.87 x 10 Is

4.90 x l(I ~9

83.73

Solidified fraction g

Calculated Hall concentration (cm ~'/

Measurcd Hall concentration ~ (cm-~)

0.0 0.95 0.0 0.70 0.0 0.70

2.64 x 1()i7 1.48 x 10 t~ 2.47 × 1017 5.67x 1017 2.89 x 1017 6.08 x t 0 ~7

3.0 x 10 l: 4.0 x It) ~ 1.6 x 1()17 -0.5 x 1017 -0.8 × 1017 Inhomogeneous - -

aNegative values indicate p-type conductivity and correspond to hole concentration.

In contrast, for C s > 12 at.% the relation between nil. ~ and nil, t is quite different--the experimental values are much smaller than the theoretical values and the difference increases with an increasing theoretical concentration. T h e effect of the sulphur concentration is evident also if in the melt the concentration of tellurium does not change (see Table 1). For high values of Cs the measured Hall concentration is lower than the calculated value. T h e r e f o r e it seems that the ratio of sulphur to tellurium is the determining factor because this effect was observed in the case of several different concentrations of tellurium. We analysed the quaternary system G a - S b - S - T e in six binary systems: G a - S b , G a - T e , G a - S , Sb-Te, Sb-S and Te-S. It seems that the most important systems mentioned above are the Sb-Te, T e - S and S-Sb pseudobinary phase diagrams [27]. T h e s e systems are closest to the reality appearing in (Te, S)-double-doped GaSb crystals. It should be noted that the transformation of the quaternary system to a ternary system is very rough but this simplification can be the first step in approaching the understanding of what occurred with the dopants in GaSb. We constructed the ternary system S b - S - T e (see Fig. 2). T h e used concentrations of the doping elements are as follows: for S in Sb, 0 . 0 0 0 0 4 - 0 . 0 1 6 7 ac% S; for S in Te, 2.43-87.73 at.% S; and for Te in Sb, 0 . 0 0 0 8 7 - 0 . 0 0 7 2 7 at.% Te. From the phase diagrams, it becomes evident in the case of the S-Sb system and the region of used concentrations of the elements that sulphur exists in the form of SbzS 3 below its melting temperature, i.e. sulphur is probably bound in the GaSb structure without creating any second phase. Tellurium can form two solid solutions with antimony from a concentration of about 3 at.% Te. However, the used concentration was markedly lower than this value; thus the larger amount of tellurium either became a solute in the GaSb struc-

ture or was free. T h e T e - S pseudobinary system shows the boundary line of solid solution at a sulphur concentration of about 15 at.%. For this reason, if the concentration of sulphur exceeded this value, T e - S solid solution started to occur. Thus no free atoms of tellurium and sulphur existed in the G a - S b system because they were bound in the second phase. Their ionization was not active and therefore the flee-carrier concentration was low and the Hall measurement showed no d o n o r doping level. This assumption can adequately explain the behaviour of dopants (S and Te) in GaSb. T h e reality in the quaternary system is more complicated and therefore our simplification should be considered only as a rough approach. It is necessary to add that, if the concentration of tellurium increased without exceeding the sulphur concentration above the value of 12 at.%, the calculated contribution of dopants was the same as that obtained from the measurements. However, as soon as the sulphur concentration exceeded 12 at.%, independently of the tellurium concentration, the calculated and measured concentrations showed different values (the theoretical value was higher than the experimental value). In the case of very high sulphur concentrations (C s > 35 at.%) either the crystals showed a low doping level or sulphur and tellurium did not act as n-type dopants. T h e measurement of dislocation density can also confirm our simple explanation. In the case of very low concentrations of sulphur the etch pit density (EPD) measured by chemical etching decreased rapidly along the growth direction, i.e. from the beginning to the end of the crystals. In the middle part of the crystals the E P D was already about 1 - 1 0 c m - 2. However, if the concentration of sulphur reached the value C s -~ 12 at.% the E P D decreased very slowly. T h e lowest E P D was in the end portion of the crystals, showing a value of about 102 cm -2. In the case of very high sulphur

K

~esthkov6 et al.

I

Thermodynamic aspects of GaSb crystalgrowth

tion of the second phase of Te-S solid solution might cause this effect, producing active defect centres; in addition, the effect is a function of sulphur concentration C s in the total amount of dopants (Te and S). For this reason it is possible to relate directly the Te-S-Sb ternary phase diagram and the difference between the measured and calculated contributions of dopants (Te and S). It is worth mentioning that our simplification is not so bold and, at least in the first approximation, it could explain roughly open questions of (Te, S) double doping of GaSb single crystals.

concentrations (Cs ~ 80 at.%) the EPD was almost the same (102-10; cm -t) in the whole crystal and the single-crystalline growth usually changed in the end part of GaSb into either polycrystalline or twin growth. It was observed that the (111) samples cut from the (111) crystals showed a very low concentration of dislocatioris and a non-uniform distribution of etch pits on the Ga side surface [9, 10, 17, 18]. The highest EPD was on the edges of the samples and the centre was almost dislocation free. This effect is connected with the so-called "glide phenomena" [9]. However, the samples cut from the (Te, S)-double-doped crystals with concentrations of sulphur Cs > - 12 at.% did not show such an inhomogeneous spreading of EPD on the surface. The higher the value of Cs, the higher was the homogeneity of the EPD in the GaSb samples in (111} orientation. It seems that the so-called "glide phenomena" are suppressed because of another mechanism which causes a uniform creation of dislocations in the whole volume of crystals. We suppose that the forma-

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4. Conclusion

A series of GaSb single crystals double doped with tellurium and sulphur were grown using the Czochralski technique without encapsulant. The measured and calculated Hall electron concentrations

per ~0

cent sulphur 60 80

S 100

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Thermodynamic aspects q/GaSh cry:s'talgrowt/i

were compared. T h e agreement of these values was very good up to a concentration of sulphur in the total amount of dopants (Te and S) C s -< 12 at.% but in the case of C s >- 12 at.% the values were different. To explain this effect we used the most suitable Te-S binary phase diagram from the quaternary system G a - S b - T e - S where a T e - S solution appeared from about 15 at.% S. It is possible that a second phase occurred in the GaSh crystals, eliminating the dopants from the GaSb lattice. T h e high dislocation density in the GaSb crystals and its homogeneity on the surface of GaSb samples in the case of Cs >- 12 a1.% confirmed this assumption. If the sulphur concentration was lower than 12 at.% the calculated and measured values were equal and, in addition, the dislocation density was not uniformly spread on the surface of samples, i.e. the highest E P D was on the edges and the centres of the samples were almost dislocation free. For this reason, in the case of Cs >~ 12 at.%, we suppose that the so-called "glide p h e n o m e n a " were suppressed by another mechanism which could be represented by the formation of the T e - S solid solution producing defects and decreasing the doping level. This simplification could be considered as a first approach to the explanation of the technological problem concerning the growth of (Te, S)-double-doped GaSb single crystals.

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