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Study of low Te-doped GaSb single crystals
Journal of Crystal Growth 135 (1994) 290—296 North-Holland
~
CRYSTAL
GROWTH
Study of low Te-doped GaSb single crystals B. ~tèpánek, Z. Sourek, V. Sestáková, J. ~esták and J. Kub Institute of Physics, Academy of Sciences of the Czech Republic, Cukrocarnickd /0, 162 00 Prague 6, Czech Republic Received 22 August 1993; manuscript received in final form 24 September 1993
The distribution of tellurium concentration in (111> GaSb single crystals grown using the Czochralski method without encapsulant in an atmosphere of flowing hydrogen was studied. No facets were formed in undoped GaSh, while in the case of Te-doped crystals the facets have appeared at the centre of the GaSb bowl. The formation of facets is dependent on the temperature conditions in the growth surface (its shape) and on the level and the nonhomogeneity of dopant concentration near to the growth interface. It follows that it is difficult to grow single crystals which would have a homogeneous concentration of tellurium in the grown plane of (111) GaSb.
1. Introduction Gallium antimonide (GaSb) is a very important material for many semiconductor devices, such as light diodes, laser diodes, etc. [1]. One of the shortcomings of GaSb single crystals is their high level of concentration of residual acceptors. Their concentration reaches a value of about 1.7 x 1017 cm3 [2] and was identified as VGaGasb complexes [3] with doubly ionizable structure. In order to compensate for these residual acceptors, it is necessary to add to the starting GaSb melt a certain amount of a donor dopant, such as tellurium. The concentration of dopants, however, changes during the growth of single crystals in the solid material as a result of the distribution coefficient kCff [4] of tellurium in GaSb, which is about 0.32 [5]. For this reason the concentration of tellurium increases from the beginning to the end of the grown crystal. The increase, along the growth direction in the crystals grown using the Czochralski method, is approximately described by the Pfann equation for the low starting concentration [2,61.Other authors [7,8] also showed the distribution of carrier concentration in a horizontal direction, i.e. in the growth plane. They described the nonuniformity of the carrier con-
centration on the wafers cut across the pulling axis. The inhomogeneous distribution of dopants is due to the presence of a faceted region which forms an impurity core in pulled crystals [91. These observations, however, were carried out for crystals having too high a level of tellurium for the crystals to be n-type conductors [101from the very beginning. On the other hand, if the starting concentration of tellurium is low (<5 x i0~atoms cm3 in the melt), the GaSb crystals are of p-type [51in their top part and the concentration NA ND will become lower toward the tail of the crystal (NA being the residual acceptor concentration and ND the concentration of donors). In a certain portion of the GaSb, the concentration of the residual acceptors would be compensated for and after that the last part of crystals would become n-type with increasing concentration ND NA. This would be valid under the assumption that all the tellurium atoms are ionized, which is likely, assuming the low concentration of dopant. The distribution of tellurium concentration in the growth direction and in the growth plane was reported [4—11],but the condition was not studied in more detail in the case of the interface between p-type and n-type conductivity. The aim of our investigation is to examine the
0022-t)248/94/$07.00 © 1994 — Elsevier Science B.V. All rights reserved SSDI 0022-0248(93)E0378-K
—
—
B. Stépdnek et al.
/ Study of low Te-doped
GaSb single crystals
291
carrier concentration distribution in the portion of GaSb crystals where the interface between pand n-type occurs with respect to the preparation
mechanically-chemically polished and the damaged layer was removed by chemical etching in an acid solution (20 CH3COOH + 9 HNO3 + 1 HF).
of homogeneous wafers with as low carrier concentration as possible.
The thermoelectric power was measured on the wafers to determine the boundary of p- and n-type conductivity. In order to map the conductivity type, the wafers were cut into small pieces and the distribution of carrier concentration was estimated by the Van der Pauw method. The quality of the crystals was characterized by X-ray topography. We employed the method of double-crystal spectrometer in the parallel position and we used reflection (Bragg) geometry [14]. This topographic technique is highly sensitive, especially to fine local misorientations of the lattice and to small changes of the lattice parameter. The Cu Ka radiation was used for all X-ray experiments. A nearly perfect asymmetrically cut (111) oriented GaSb single crystal a disloca2 with was used as a tion density lower than i0~cm monochromator. The asymmetrical 511 reflection in the first crystal was used, so that the horizontal angular divergence of the beam impinging on the samples was 0.8 arc sec. All topographs were taken in a nearly symmetrical 333 reflection by samples and were recorded by a photo-plate which was located parallel to the sample surface. A nearly perfect GaSb sample gives the full width of the diffraction curve at a
2. Experimental procedure The complete Czochralski apparatus for the growth of GaSb single crystals and the growth procedure were described in detail in our previous papers [12,13]. Single crystals were grown using the Czochralski technique without encapsulant in an atmosphere of flowing hydrogen. Tellurium was added in an elementary form to the starting GaSb material in a batch (of about 170 g). In order to grow Te-doped crystals which would not be fully compensated for in the first portion, i.e., which would have p-type conductivity, the concentration of tellurium in therespecting melt was chosen to be (1—6) x 1017 atoms cm3, the Pfann equation. The crystals were grown in the <111) direction, i.e., along the b-axis toward the melt. The length of the crystals was about 60 mm and their diameter was about 20 mm. Finally, the crystals were cut nearly perpendicular to the growth direction to prepare samples of about I mm thickness. After being cut from the crystal ingot, the wafers for X-ray measurements were
A-A
~ ~
(a)
BB
~
~1Omm
:~ n—type
Fig. 1. (a) Schematic view of the p-type and the n-type conductivity regions in the whole grown crystal GaSb—Te12. (b) Schematic view of the p-type and the n-type conductivity regions in selected wafers (see (a)) which were examined by X-ray measurement.
292
B. Stbpánek et al.
/ Study of low Te-doped
half maximum (FWHM) of 6.0 ±0.3 arc sec for the 333 reflection, which is in agreement with the theorettcal value.
r T -5
-31
17 1
- -
a-t
0e
J °
. -
~10 °~ HaS concentration
—
The profile of conductivity, perpendicular to
the growth direction, was found from the p- and n-type boundary on each sample (fig. 1). In the starting portion of the crystal (for x <0.1, where x is the solidified fraction from the Pfann equation) the carrier concentration was homogeneous in all wafers. Then, almost at the centre of the crystal (x 0.1), the inhomogeneity of the Te concentration appeared to be exhibiting n-type conductivity. When the crystal was grown larger, the n-type portion increased and vice versa (see fig. la). In the case of a constant diameter of the crystal body, the central n-type part increased and the following part was fully n-type; however, the homogeneity of the Hall concentration was not good enough. The Te concentration was higher at the centre of the crystal than on the edge. It was possible to fit the increasing Te concentration measured on the edge of the crystal by the Pfann equation over the whole length of the GaSb crystal, but the centre of the n-type portion, where the Te concentration was the highest, did not correspond to this equation. The discrepancy of the values was difficult to describe by a distribution equation. The Hall concentration was four to six times higher than on the edge of the wafers (fig. 2). For the study of this effect, three types of GaSb crystals were chosen: (a) undoped GaSb (GaSb-6); (b) low Te-doped GaSb (GaSb—Te12) (mentioned above); (c) heavily Te-doped GaSb (GaSb—Te18) with a concentration of tellurium in the starting melt of 3. 4.5 The x 1018 atoms cm was intentionally undoped crystal GaSb-6
GaSb single crystals
•
• •
t —
-
p~
2
--
0
14
~+-
±-
20
30
.
-
40 50 ength of the orysto~ traim .
Fig. 2. The longitudinal distribution of carrier concentration in crystal GaSb—Te12. (a) (x x) measured on the edge of the wafers; (b)
— —•) measured at the centre of the wafers.
(.—
_____ ______
4 _______ _____
1jrnm1 and the X-ray topograph in fig. 3 shows a standard image of a rather homogeneous single crystal with 2. a dislocation density of the order of 102 cm
Fig. 3. X-ray topograph of the crystal GaSh-b taken in the 333 reflectiondirection with CuofKa The are magnification the radiation. incident beam indicated. and the
B. S~tt’páneket al.
/
Study of low Te-doped GaSb single crystals
The crystal GaSb—Te12 is representative of the ingot with an intermediate dopant concentration which differs from the top to the tail of the crystal. Fig. 4 shows topographs of wafers cut from various parts of this crystal. The quality of the crystal is not satisfactory and differs depend-
_
ing on the spatial dopant concentration distribution. Moreover, facets which manifest themselves [10,111 by parallel growth striations in the wafers cut in the way described above occur in most of the wafers. Their extent differs from wafer to wafer.
~:
~
293
~.
__
-~
..~
~.
__
Fig. 4. X-ras topographs ot er~st:iI(jaSb—Te 12 taken on the 333 reflection with ~ Ka radiation. The magnification and the direction of the incident beam are indicated. From the top of the crystal the wafers are: (a) 10 mm; (b) 17 mm; (c) 26 mm; (dl 48 mm.
294
B. Stépánek et al.
/
Study of low Te-doped GaSb single crystals
Fig. 4a was taken from the wafer 10 mm from the top of the crystal, which was still completely p-type. Circular growth striations are clearly visible. No region with parallel growth striations was observed in the whole wafer. The wafer 17 mm from the top, shown in fig. 4b, was cut from that part of the ingot where an island of n-type conductivity was already detected. The topograph reveals a facet with a completely different morphology of striations. Fig. 4b depicts the part of this wafer with the boundary of the facet where the growth striations change from circular to parallel ones. The wafer 26 mm from the top (see fig.
~ _____
~ ____________ _______
4c) exhibits a very damaged part of the ingot with a high dislocation density; however, the growth striations in the facet were detected in a large area of the sample. In the wafer 48 mm from the top (almost at the crystal tail), where the crystal is nearly completely n-type, the number of lattice defects decreased with respect to the previous wafer and a large faceted area occurred (see fig. 4d). Fig. Sa shows a part of the wafer cut from the crystal GaSb—Te18 where the starting Te concentration was so high that the whole crystal has n-type conductivity. This crystal was nearly dislocation free, but the faceted region is present again, see fig. 5a, where it is exposed in such an angular setting that the resolution is the same for both regions. Figs. Sb and Sc represent two expositions of the wafer which are exposed on different flanks of the rocking curve (always in the half maximum) and differs by 7.5 arc sec in angular setting. Turning this wafer upside down and repeating the same procedure did not reveal any contrast between faceted and non-faceted regions and an FWHM of 6.0 arc sec was measured again. This means that the inclination a of the lattice in facet with respect to that in the nonfaceted region and the relative difference of the lattice parameters ia/a (~1a a~~) contribute to the contrast in figs. Sb and Sc by nearly the same amount. We estimate that a 0.8 arc sec and ~.ta/a X ~ In all cases, the facets observed by X-ray topography coincide with the regions on the wafer surface which are visualized by etching in different ways compared with their surrounding and could be easily seen optically.
=
—
=
=S
(°)
-~
4. Discussion and conclusion
n Fig. 5. X-ray topographs of the crystal GaSb—Te18 taken on the 333 reflection with Cu Ka radiation. The arrow indicates . the direction of the incident beam. (a) The magnification is given in the scale. (h), (c) The angular difference between (b) and (c) is 7.5 arc sec.
The measurement of Hall concentration on the samples from the Te-doped GaSb single crystals has shown that it would be very difficult to prepare wafers with homogeneous concentration of tellurium. The idea to grow low-Te-doped crystals where the type of conductivity would be changed abruptly in the whole plane of the grown crystal is almost unachievable. The undoped crys-
B. Stépánek et a!.
/ Study of low Te-doped GaSb single crystals
tals have shown a relatively good homogeneity in the Hall concentration and the difference between the edge and the centre of the wafers is small (of about 25%) in accordance with X-ray topography (no facets observed), but in the case of Te-doped GaSb the concentration has changed 4—8 times. The difference may stem from the growth conditions. The formation of facets is a good indicator of the sequence of the crystal growth process. From the point of view of X-ray topography, we understand by facet formation a macroscopic part of the crystal, where variations in the morphology of striations with respect to the nonfaceted region (NFR) are observable. Parallel growth striations indicate a planar solid/ liquid (S/L) interface in the facet, contrary to the circular ones in the NFR. Facets in this sense display many times higher Te concentration and a different character of etched surface with respect to non-faceted region. The higher concentration of Te in a certain range of the wafer causes the change from p- to n-type conductivity. In the undoped crystal, no facets were observed. The facets can be created either when the growth has a convex character or when the stirring of the melt is insufficient. It is necessary to notice that the formation of facets is conditional for the existence of impurities in the melt. Facets in the crystals are created around the centre of growth rotation, where the melt has the largest degree of supercooling because the growth surface has a convex shape towards the temperature gradient. The growth is probably initiated by a dislocation and is enhanced by supercooling [151. However, the edge of the crystal grows continuously because the supercooling is lower in cornparison with the central portion which grows in steps. As mentioned above, the crystals were grown by seed and without crucible rotation. The origin of the facets could be explained by poor stirring conditions of the melt near the centre of growth rotation and consequently producing a higher value of keff and/or causing a step-by-step growth mechanism from the supercooled melt at the centre of rotation, in contrast to the continuous growth mechanism at the periphery [161.
295
The sharp facet boundary and the growth striation indicate that the latter reason is more relevant. The following equation can partly describe the condition when facets are created:
keff= k*
+
(1
—
k* k*) exp( —~)
‘
(1)
where keff is the effective distribution coefficient, k * the interface distribution coefficient and i f6/D where 6 1.6Dh/’OpL’or 1/2, f is the growth rate, D the liquid diffusion coefficient, x.’ the viscosity of the liquid and the velocity of rotation. In a limit of poor stirring, 6 —s ~ and kCff —s 1, while for perfect stirring conditions, 6 0 and keff —~k Thus, at the centre of the grown crystals, keff will be higher than on the edge. The facet appears at a definite part of the top of the crystal. This could be explained by an increased conduction of heat out of the crystal centre and consequently a higher melt supercooling around the rotation centre. In this light we summarize: (i) The formation of facets is due to the inhomogeneous concentration of dopant impurities in the melt and/or to the complementary mechanism of the growth connected with the local supercooling. (ii) With the Te concentration increment, the crystal lattice parameters increase owing to the presence of a large-diameter tellurium atom in the facets. This effect can be observed very well, especially in the case of high concentration of tellurium. The distribution of the carrier concentration at the interface between the p- and n-type conductivity was examined and interpreted in terms of the inhomogeneity observed. The idea of preparing low Te-doped homogeneous crystal wafers is achievable only at a certain region of the top of the crystal. =
=
i’
~‘.
Acknowledgements The authors would like to thank Mrs. V. MI~kováand Mr. V. Charvát for their technical assistance.
296
B. Stbpánek et al.
/
Study of low Te-doped GaSb single crystals
References [1] M. Astles, A. Hill, A.J. Williams, P.J. Wright and M.J. Young, J. Electron. Mater. 15 (1986) 41. [2] B. Stèpánek and V. Sestáková, Thermochim. Acta 209 (1992) 285. [3] Y.J. van der Meulen, J. Phys. Chem. Solids 28 (1967) 25. [4] S. Miyazawa, S. Kondo and M. Naganuma, J. Crystal Growth 49 (1980) 670. [5] V. ~estáková and B. ~tépánek, Thermochim. Acta 198 (1992) 213. [6] WA. Sunder, R.L. Barns, T.Y. Kometani, J.M. Parsey, Jr. and R.A. Laudise, J. Crystal Growth 78 (1986) 9. 17] AK. Chin and WA. Bonner, AppI. Phys. Letters 40 (1982) 248. 181 Y. Ohmori, K. Sugii, S. Akai and K. Matsumoto, J. Crystal Growth 60 (1982) 79.
[9] J.B. Mullin, in: Compound Semiconductors, Vol. 1, Eds. R.K. Willardson and H.L. Goering (Reinhold, New York, 1962) ch. 41. [101 J. Doerschel and U. Geissler, J. Crystal Growth 121 (1992) 781. 111] NV. Quang and M. Kumagawa, Japan. J. AppI. Phys. 20 (1981) 817. 1121 F. Moravec and Y. Tomm, Crystal Res. Technol. 22 (1987) K30. [13] F. Moravec, V. Sestáková, B. Stépánek and V. Charvát. Crystal Res. Technol. 24 (1989) 275. 114] Z. Sourek and R. Bubáková, Phys. Status Solidi (a) 70 (1982) 641. 1151 W.P. Alired and R.T. Bate, J. Electrochem. Soc. 108 (1961) 258. [161 J.C. Brice, The Growth of Crystals from the Melt (North-Holland, Amsterdam, 1965) p. 83.
~
CRYSTAL
GROWTH
Study of low Te-doped GaSb single crystals B. ~tèpánek, Z. Sourek, V. Sestáková, J. ~esták and J. Kub Institute of Physics, Academy of Sciences of the Czech Republic, Cukrocarnickd /0, 162 00 Prague 6, Czech Republic Received 22 August 1993; manuscript received in final form 24 September 1993
The distribution of tellurium concentration in (111> GaSb single crystals grown using the Czochralski method without encapsulant in an atmosphere of flowing hydrogen was studied. No facets were formed in undoped GaSh, while in the case of Te-doped crystals the facets have appeared at the centre of the GaSb bowl. The formation of facets is dependent on the temperature conditions in the growth surface (its shape) and on the level and the nonhomogeneity of dopant concentration near to the growth interface. It follows that it is difficult to grow single crystals which would have a homogeneous concentration of tellurium in the grown plane of (111) GaSb.
1. Introduction Gallium antimonide (GaSb) is a very important material for many semiconductor devices, such as light diodes, laser diodes, etc. [1]. One of the shortcomings of GaSb single crystals is their high level of concentration of residual acceptors. Their concentration reaches a value of about 1.7 x 1017 cm3 [2] and was identified as VGaGasb complexes [3] with doubly ionizable structure. In order to compensate for these residual acceptors, it is necessary to add to the starting GaSb melt a certain amount of a donor dopant, such as tellurium. The concentration of dopants, however, changes during the growth of single crystals in the solid material as a result of the distribution coefficient kCff [4] of tellurium in GaSb, which is about 0.32 [5]. For this reason the concentration of tellurium increases from the beginning to the end of the grown crystal. The increase, along the growth direction in the crystals grown using the Czochralski method, is approximately described by the Pfann equation for the low starting concentration [2,61.Other authors [7,8] also showed the distribution of carrier concentration in a horizontal direction, i.e. in the growth plane. They described the nonuniformity of the carrier con-
centration on the wafers cut across the pulling axis. The inhomogeneous distribution of dopants is due to the presence of a faceted region which forms an impurity core in pulled crystals [91. These observations, however, were carried out for crystals having too high a level of tellurium for the crystals to be n-type conductors [101from the very beginning. On the other hand, if the starting concentration of tellurium is low (<5 x i0~atoms cm3 in the melt), the GaSb crystals are of p-type [51in their top part and the concentration NA ND will become lower toward the tail of the crystal (NA being the residual acceptor concentration and ND the concentration of donors). In a certain portion of the GaSb, the concentration of the residual acceptors would be compensated for and after that the last part of crystals would become n-type with increasing concentration ND NA. This would be valid under the assumption that all the tellurium atoms are ionized, which is likely, assuming the low concentration of dopant. The distribution of tellurium concentration in the growth direction and in the growth plane was reported [4—11],but the condition was not studied in more detail in the case of the interface between p-type and n-type conductivity. The aim of our investigation is to examine the
0022-t)248/94/$07.00 © 1994 — Elsevier Science B.V. All rights reserved SSDI 0022-0248(93)E0378-K
—
—
B. Stépdnek et al.
/ Study of low Te-doped
GaSb single crystals
291
carrier concentration distribution in the portion of GaSb crystals where the interface between pand n-type occurs with respect to the preparation
mechanically-chemically polished and the damaged layer was removed by chemical etching in an acid solution (20 CH3COOH + 9 HNO3 + 1 HF).
of homogeneous wafers with as low carrier concentration as possible.
The thermoelectric power was measured on the wafers to determine the boundary of p- and n-type conductivity. In order to map the conductivity type, the wafers were cut into small pieces and the distribution of carrier concentration was estimated by the Van der Pauw method. The quality of the crystals was characterized by X-ray topography. We employed the method of double-crystal spectrometer in the parallel position and we used reflection (Bragg) geometry [14]. This topographic technique is highly sensitive, especially to fine local misorientations of the lattice and to small changes of the lattice parameter. The Cu Ka radiation was used for all X-ray experiments. A nearly perfect asymmetrically cut (111) oriented GaSb single crystal a disloca2 with was used as a tion density lower than i0~cm monochromator. The asymmetrical 511 reflection in the first crystal was used, so that the horizontal angular divergence of the beam impinging on the samples was 0.8 arc sec. All topographs were taken in a nearly symmetrical 333 reflection by samples and were recorded by a photo-plate which was located parallel to the sample surface. A nearly perfect GaSb sample gives the full width of the diffraction curve at a
2. Experimental procedure The complete Czochralski apparatus for the growth of GaSb single crystals and the growth procedure were described in detail in our previous papers [12,13]. Single crystals were grown using the Czochralski technique without encapsulant in an atmosphere of flowing hydrogen. Tellurium was added in an elementary form to the starting GaSb material in a batch (of about 170 g). In order to grow Te-doped crystals which would not be fully compensated for in the first portion, i.e., which would have p-type conductivity, the concentration of tellurium in therespecting melt was chosen to be (1—6) x 1017 atoms cm3, the Pfann equation. The crystals were grown in the <111) direction, i.e., along the b-axis toward the melt. The length of the crystals was about 60 mm and their diameter was about 20 mm. Finally, the crystals were cut nearly perpendicular to the growth direction to prepare samples of about I mm thickness. After being cut from the crystal ingot, the wafers for X-ray measurements were
A-A
~ ~
(a)
BB
~
~1Omm
:~ n—type
Fig. 1. (a) Schematic view of the p-type and the n-type conductivity regions in the whole grown crystal GaSb—Te12. (b) Schematic view of the p-type and the n-type conductivity regions in selected wafers (see (a)) which were examined by X-ray measurement.
292
B. Stbpánek et al.
/ Study of low Te-doped
half maximum (FWHM) of 6.0 ±0.3 arc sec for the 333 reflection, which is in agreement with the theorettcal value.
r T -5
-31
17 1
- -
a-t
0e
J °
. -
~10 °~ HaS concentration
—
The profile of conductivity, perpendicular to
the growth direction, was found from the p- and n-type boundary on each sample (fig. 1). In the starting portion of the crystal (for x <0.1, where x is the solidified fraction from the Pfann equation) the carrier concentration was homogeneous in all wafers. Then, almost at the centre of the crystal (x 0.1), the inhomogeneity of the Te concentration appeared to be exhibiting n-type conductivity. When the crystal was grown larger, the n-type portion increased and vice versa (see fig. la). In the case of a constant diameter of the crystal body, the central n-type part increased and the following part was fully n-type; however, the homogeneity of the Hall concentration was not good enough. The Te concentration was higher at the centre of the crystal than on the edge. It was possible to fit the increasing Te concentration measured on the edge of the crystal by the Pfann equation over the whole length of the GaSb crystal, but the centre of the n-type portion, where the Te concentration was the highest, did not correspond to this equation. The discrepancy of the values was difficult to describe by a distribution equation. The Hall concentration was four to six times higher than on the edge of the wafers (fig. 2). For the study of this effect, three types of GaSb crystals were chosen: (a) undoped GaSb (GaSb-6); (b) low Te-doped GaSb (GaSb—Te12) (mentioned above); (c) heavily Te-doped GaSb (GaSb—Te18) with a concentration of tellurium in the starting melt of 3. 4.5 The x 1018 atoms cm was intentionally undoped crystal GaSb-6
GaSb single crystals
•
• •
t —
-
p~
2
--
0
14
~+-
±-
20
30
.
-
40 50 ength of the orysto~ traim .
Fig. 2. The longitudinal distribution of carrier concentration in crystal GaSb—Te12. (a) (x x) measured on the edge of the wafers; (b)
— —•) measured at the centre of the wafers.
(.—
_____ ______
4 _______ _____
1jrnm1 and the X-ray topograph in fig. 3 shows a standard image of a rather homogeneous single crystal with 2. a dislocation density of the order of 102 cm
Fig. 3. X-ray topograph of the crystal GaSh-b taken in the 333 reflectiondirection with CuofKa The are magnification the radiation. incident beam indicated. and the
B. S~tt’páneket al.
/
Study of low Te-doped GaSb single crystals
The crystal GaSb—Te12 is representative of the ingot with an intermediate dopant concentration which differs from the top to the tail of the crystal. Fig. 4 shows topographs of wafers cut from various parts of this crystal. The quality of the crystal is not satisfactory and differs depend-
_
ing on the spatial dopant concentration distribution. Moreover, facets which manifest themselves [10,111 by parallel growth striations in the wafers cut in the way described above occur in most of the wafers. Their extent differs from wafer to wafer.
~:
~
293
~.
__
-~
..~
~.
__
Fig. 4. X-ras topographs ot er~st:iI(jaSb—Te 12 taken on the 333 reflection with ~ Ka radiation. The magnification and the direction of the incident beam are indicated. From the top of the crystal the wafers are: (a) 10 mm; (b) 17 mm; (c) 26 mm; (dl 48 mm.
294
B. Stépánek et al.
/
Study of low Te-doped GaSb single crystals
Fig. 4a was taken from the wafer 10 mm from the top of the crystal, which was still completely p-type. Circular growth striations are clearly visible. No region with parallel growth striations was observed in the whole wafer. The wafer 17 mm from the top, shown in fig. 4b, was cut from that part of the ingot where an island of n-type conductivity was already detected. The topograph reveals a facet with a completely different morphology of striations. Fig. 4b depicts the part of this wafer with the boundary of the facet where the growth striations change from circular to parallel ones. The wafer 26 mm from the top (see fig.
~ _____
~ ____________ _______
4c) exhibits a very damaged part of the ingot with a high dislocation density; however, the growth striations in the facet were detected in a large area of the sample. In the wafer 48 mm from the top (almost at the crystal tail), where the crystal is nearly completely n-type, the number of lattice defects decreased with respect to the previous wafer and a large faceted area occurred (see fig. 4d). Fig. Sa shows a part of the wafer cut from the crystal GaSb—Te18 where the starting Te concentration was so high that the whole crystal has n-type conductivity. This crystal was nearly dislocation free, but the faceted region is present again, see fig. 5a, where it is exposed in such an angular setting that the resolution is the same for both regions. Figs. Sb and Sc represent two expositions of the wafer which are exposed on different flanks of the rocking curve (always in the half maximum) and differs by 7.5 arc sec in angular setting. Turning this wafer upside down and repeating the same procedure did not reveal any contrast between faceted and non-faceted regions and an FWHM of 6.0 arc sec was measured again. This means that the inclination a of the lattice in facet with respect to that in the nonfaceted region and the relative difference of the lattice parameters ia/a (~1a a~~) contribute to the contrast in figs. Sb and Sc by nearly the same amount. We estimate that a 0.8 arc sec and ~.ta/a X ~ In all cases, the facets observed by X-ray topography coincide with the regions on the wafer surface which are visualized by etching in different ways compared with their surrounding and could be easily seen optically.
=
—
=
=S
(°)
-~
4. Discussion and conclusion
n Fig. 5. X-ray topographs of the crystal GaSb—Te18 taken on the 333 reflection with Cu Ka radiation. The arrow indicates . the direction of the incident beam. (a) The magnification is given in the scale. (h), (c) The angular difference between (b) and (c) is 7.5 arc sec.
The measurement of Hall concentration on the samples from the Te-doped GaSb single crystals has shown that it would be very difficult to prepare wafers with homogeneous concentration of tellurium. The idea to grow low-Te-doped crystals where the type of conductivity would be changed abruptly in the whole plane of the grown crystal is almost unachievable. The undoped crys-
B. Stépánek et a!.
/ Study of low Te-doped GaSb single crystals
tals have shown a relatively good homogeneity in the Hall concentration and the difference between the edge and the centre of the wafers is small (of about 25%) in accordance with X-ray topography (no facets observed), but in the case of Te-doped GaSb the concentration has changed 4—8 times. The difference may stem from the growth conditions. The formation of facets is a good indicator of the sequence of the crystal growth process. From the point of view of X-ray topography, we understand by facet formation a macroscopic part of the crystal, where variations in the morphology of striations with respect to the nonfaceted region (NFR) are observable. Parallel growth striations indicate a planar solid/ liquid (S/L) interface in the facet, contrary to the circular ones in the NFR. Facets in this sense display many times higher Te concentration and a different character of etched surface with respect to non-faceted region. The higher concentration of Te in a certain range of the wafer causes the change from p- to n-type conductivity. In the undoped crystal, no facets were observed. The facets can be created either when the growth has a convex character or when the stirring of the melt is insufficient. It is necessary to notice that the formation of facets is conditional for the existence of impurities in the melt. Facets in the crystals are created around the centre of growth rotation, where the melt has the largest degree of supercooling because the growth surface has a convex shape towards the temperature gradient. The growth is probably initiated by a dislocation and is enhanced by supercooling [151. However, the edge of the crystal grows continuously because the supercooling is lower in cornparison with the central portion which grows in steps. As mentioned above, the crystals were grown by seed and without crucible rotation. The origin of the facets could be explained by poor stirring conditions of the melt near the centre of growth rotation and consequently producing a higher value of keff and/or causing a step-by-step growth mechanism from the supercooled melt at the centre of rotation, in contrast to the continuous growth mechanism at the periphery [161.
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The sharp facet boundary and the growth striation indicate that the latter reason is more relevant. The following equation can partly describe the condition when facets are created:
keff= k*
+
(1
—
k* k*) exp( —~)
‘
(1)
where keff is the effective distribution coefficient, k * the interface distribution coefficient and i f6/D where 6 1.6Dh/’OpL’or 1/2, f is the growth rate, D the liquid diffusion coefficient, x.’ the viscosity of the liquid and the velocity of rotation. In a limit of poor stirring, 6 —s ~ and kCff —s 1, while for perfect stirring conditions, 6 0 and keff —~k Thus, at the centre of the grown crystals, keff will be higher than on the edge. The facet appears at a definite part of the top of the crystal. This could be explained by an increased conduction of heat out of the crystal centre and consequently a higher melt supercooling around the rotation centre. In this light we summarize: (i) The formation of facets is due to the inhomogeneous concentration of dopant impurities in the melt and/or to the complementary mechanism of the growth connected with the local supercooling. (ii) With the Te concentration increment, the crystal lattice parameters increase owing to the presence of a large-diameter tellurium atom in the facets. This effect can be observed very well, especially in the case of high concentration of tellurium. The distribution of the carrier concentration at the interface between the p- and n-type conductivity was examined and interpreted in terms of the inhomogeneity observed. The idea of preparing low Te-doped homogeneous crystal wafers is achievable only at a certain region of the top of the crystal. =
=
i’
~‘.
Acknowledgements The authors would like to thank Mrs. V. MI~kováand Mr. V. Charvát for their technical assistance.
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Study of low Te-doped GaSb single crystals
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