The properties of (Pb, Ge)Te single crystals grown from vapour phase

Journal of Crystal Growth 135 (1994) 565—570 North-Holland

o~’~o,

CRYSTAL GROWT H

The properties of (Pb,Ge)Te single crystals grown from the vapour phase M. Leszczynski

a,b

A. Szczerbakow

C

and G. Karczewski

Centro Nazionale Ricerca e Sciluppo del Materiali, CNRSM, SS7per Mesagne km 7300, 1-72 100 Brindisi, Italy “High Pressure Research Center Unipress, Polish Academy of Sciences, UI. Sokolowska 29/37, P1-01142 Warsaw, Poland Institute of Physics, Polish Academy of Sciences, AlLotnikow 32/46, P1-00668 Warsaw, Poland °

Received 17 July 1993; manuscript received in final form 2 November 1993

Single crystals of Pb

1 ~Ge~Te solid solutions were grown from the vapour phase over the whole composition range. The properties of the samples were examined using electrical methods (Hall concentration and mobility), microprobe and X-ray single and powder diffractometry at 20—400°C.Except for the case of x = 0, the samples were p-type with increasing hole concentration for higher GeTe content. The crystals with low GeTe content exhibited high crystallographic perfection, which decreased as the value of x increased. This was caused by a transformation from cubic to rhombohedral structure when the crystals were cooled after the growth process. For crystals with x <0.5, the rhombohedral angle was smaller than the one measured using powder diffractometry. This can be attributed to the blocking of the phase transition by the cubic cell elongation in different (111) directions.

1. Introduction Narrow gap IV—VI semiconductors attract much interest because of their unique physical propertiesorand possible applications as temperainfrared detectors lasers [1], also tunable with ture [2] or pressure [31.The most interesting features of these compounds are: high values of dielectric constant, high values of effective charge, L-point band inversion and a tendency to polymorphic temperature and pressure phase transitions. (Pb,Ge)Te solid solutions exhibit two kinds of phase transitions. The first one, of the second order, from cubic to rhombohedral structure occurs when the temperature is lowered and is related to the TO-phonon softening [4]. The second one, of the first order, from cubic to orthorhombic structure is pressure induced and is a kind of metal—insulator transformation [5,61. The recent interest in IV—VI compounds has

been concentrated on epitaxial layers [7,8] grown on lattice mismatched substrates (GaAs, BaF2 and others). This paper shows that is possible to grow high quality crystalsrange. of a 2 size over(Pb,Ge)Te the whole single composition few mm The growth of these solid solutions for x 0.25— 0.80 from the melt is impossible because of the character of the liquid—solid equilibrium [9]. Growth from the vapour phase must be performed below the temperature of incongruent melting and above the temperature of decomposition to PbTe-rich and GeTe-rich solid solutions. An additional problem is the phase transition from the cubic to the rhombohedral phase that occurs when the crystals are cooled after the growth process. This phenomenon was studied with a special interest. X-ray diffraction examinations were performed in situ at 20—400°Cin order to obtain more information about the influence of this phase transition on crystal perfection. And vice versa, as a temperature of phase transition

0022-0248/94/$07.00 © 1994 — Elsevier Science B.V. All rights reserved SSDI 0022-0248(93)E0474-L

=

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/ Properties of (Pb,Ge) Te single crystals grown from

depends on the defect concentration [10], this value could give additional information about the real structure of the crystal.

2. Crystal preparation The crystal growth was based on the processes of sublimation described by Harman and McVittie as horizontal unseeded vapour growth (HUVG) [111 and by Stöber et al. as self-seeded vapour growth (SSVG) [121. The method was successfully applied in crystal growth of IV—VI compounds, their solid solutions [13—171,as well as of some of Il—VI compounds [18—201.The particular feature of the procedure is that the material transport is driven by radial temperature differences, which are determined by thermal radiation phenomena [12,211. The details of the growth method can be found in the above-mentioned papers. The model which served for the growth procedure design is given in ref. [211. However, in order to increase the growth rate, not only radial, but also longitudinal temperature differences were introduced by applying an asymmetrical temperature profile on the furnace. This was desirable, as the growth temperature was lower than in previous experiments on other materials and the rate of vapour transport was decreased. This modification made the growth further from the equilibrium condition and a stronger tendency to create composition differences between the crystals and the source had to be taken into account, As described previously [211,the vapour growth of IV—VI crystals is very sensitive to the deviation from stoichiometry especially an excess of chalcogen (here: Te) slows down dramatically the material transport and may result in porous crystals. The requirements concerning the metal/ chalcogen ratio cannot be fulfilled by weighing, particularly for such a strongly non-stoichiometric compound as GeTe. The most reliable procedure is based on the relatively easy preparation of Te-saturated (but free of elementary tellurium) binary compounds PbTe and GeTe followed by subsequent additions of small amounts of elementary metal. —

the vapourphase

The synthesis of Te-saturated compounds was performed separately for PbTe and GeTe starting from the pure elements by melting them in a vacuum-sealed ampoule. The ratio of Te : metal was 1.002: 1. The melt was cooled down and the ampoule end in the region of the reaction products was placed for three days in a furnace (600°C). The remaining elementary tellurium evaporated and condensed in the cold end of the ampoule. The source material was prepared by weighing portions of crushed (1—3 mm grain size) PbTe and GeTe in the ratio required for the crystal. The total weight of the source material was 5—10 g. Then, 1 mg of PbTe per 1 g of total weight was added. The mixture was loaded into the silica ampoule (about 12 mm diameter and 100 mm length), which was then evacuated down to approximately 5 x 10 Torr and sealed. After melting the mixture at 1000°C, the ampoule was kept in a horizontal position and cooled down in air. The homogenization and crystal growth were carried out in a 36 mm furnace, which had an isothermal (660°C)section 150 mm long and a 50 mm long zone with temperature increasing up to 685°C.The ampoule was placed in the furnace in the position for which the source material was at the maximum of temperature and the major part of the ampoule was in the isothermal section. For the first 3—5 days no phenomena on the material surface could be observed. Later on, the creation of small, bright crystals was observed on the cooler part of the source material and within the next 5—7 days spontaneous selected growth of several crystals resulted. Then, the crystals grew at a rate of 0.3—0.5 mm per day until they reached a few mm size and the growth rate decreased. The whole process of homogenization, crystal selection and growth lasted 20—30 days and was interrupted by rapidly cooling the ampoule in air. The 3—8 mm crystals had mirrorlike, well-shaped faces, as has already been reported [21]. .

3. Crystal characterization The crystals were examined using electrical methods, microprobe and X-ray diffraction. The

/ Properties of (Pb, Ge)Te single crystals grown from

M. Leszczynski et al.

carrier concentration was measured with the Hall method in a magnetic field of 1 T. The mobility measurements were performed using the Van der Pauw method whilst employing a constant current of 0.1 A. The results, together with microprobe and Xray diffraction data, are gathered in table 1. The change from n to p character was observed for all samples with GeTe content. This could be attributed to the presence of overstoichiometric tellurium. This finding is in agreement with previously published papers [221. An increase of carrier concentration and decrease of mobility were observed as the GeTe content increased, The microprobe measurements had an accuracy of about 1% as a result of using PbTe and GeTe calibration standards. The powder diffractometry was carried out in the Bragg—Brentano configuration. The Lorentz function was used to fit the diffraction peaks. This allowed one to obtain 0.001 A accuracy in the lattice constant determination, which corresponded to about 0.2% accuracy in the evaluation of x. The error (1%) given in table 1 is the maximum scatter for various crystals grown in the same process. As can be seen from the table, the maximum deviation from the expected (from weighing) value of x was 2%. The rhombohedral angle was derived from the peak splitting (e.g. (440) and (440)) which is zero for a cubic structure. The powder diffraction measurements were performed on the source material and the grown

the vapour phase

567

crystals crushed in a mortar. For both cases the results were the same within an accuracy of 0.001 A. This means that stoichiometry during crystal growth was preserved. The single crystals were examined using the Bond [231 method, in which the X-ray beam was collimated to about 1 arc mm, and using double crystal diffractometer, where the beam was monochromatized by a symmetrical (220) Cu Ka, reflection from a GaAs crystal. The measurements were performed with a variable beam size from 0.1 mm up to 2 mm (different slits placed before the sample) and with different slits placed before the counter (co—2~scan). The latter enabled us to resolve the lattice constant variation and the lattice plane bending with an accuracy of about 3 arc mm. The faces of the crystals turned out to be of (100), (110) and (111) type. The surfaces of the crystals were mirror-like, but in the case of crystals of x 0.65, 0.1—1 mm micro-crystallites could be observed in an optical microscope. The lattice constant of PbTe single crystal at 21°Cwas measured as 6.4610 ±0.0002 A (previously published value: 6.462 A [241) and of GeTe (powder and single crystal) as 5.968 A (previously published values 5.984 A for GeTe saturated with Ge, 5.966 A for Te saturated GeTe [91).These values were used for an evaluation of the GeTe content in other samples. The lattice constants of all samples as measured in powder and single crystal diffractometry were the same within the accuracy claimed in

Table 1 Carrier concentration and mobility of Pb

1 ,Ge~Tecrystals as a function of GeTe content measured with a microprobe and X-ray

diffraction

3)

p. (cm2/Vs)

x(%) Weighed

ti-probe

X-ray

p(cm 300 K

77 K

300 K

77 K

0 25 35 50 65 80 100

0 21 32 44 60 77 100

0 25 37 48 65 78 100

8X10’7(n) lxlO’8 lxlO’8 2x10’8 8x10’9 1x102’ lx102’

8x10’7(n) lxlO’8 2x10’8 3x10’8 8x10’9 1x102’ 2x102’

1x103 3x102 3x102 3x102 5x101 3x10° 3x10’

1x104 8x103 1x103 4x102 1x102 5x10° 5x10’

±1

±1

±10%

±10%

±10%

±10%

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M. Leszczynski et a!.

/ Properties of (Pb,Ge) Te single crystals grown from the vapourphase

same

position with an accuracy of about 1 arc mm. When the crystals were rotated and other faces examined, the measured angle between the faces was as in cubic, not rhombohedral, strucc

48%

ture slit the to within was placed an accuracy before of the about counter 2 arcand mm.the If w—2~scan was performed, still no splitting was observed. For crystals with x 0.65 the results were as

7% C (1)

could be expected. The measurements with a narrow slit before the counter gave two well-resolved peaks in exactly the same positions as in powder diffractometry. For a wide aperture de-

25%

0%

tector, -1000

-500

0

500

1000

angle (arc see) Fig. 1. Rocking curves of (220) CuKa, reflection from Pb 1 ~Ge~Te single crystals at various temperatures: (a) 20°C; (b) 100°C,(c) 195°C.

powder diffraction. However, when comparing the values of rhombohedral angles obtained in powder and single crystal examinations, discrepancies for x 0.5 were observed. The single crystal rocking curves, as shown in fig. 1 (no slit before the counter), were not split and had a smaller half-width than the peak splitting in powder diffractometry. The half-width depended very slightly on the beam size, which means that the grain size was smaller than 0.1 mm or larger than 2 mm. In order to check the latter possibility the crystal was scanned, but the peak remained at the

the peak shape and position depended very strongly on the beam size and crystal position. The narrowest peaks were of order 8 arc mm, whereas the broadest were of about 3°and consisted of number of peaks. This means that for x 0.65 the crystals had a mosaic structure of 0.01—0.1 mm crystallites disoriented with respect to each other. The most important results of the X-ray diffraction measurements are given in table 2.

4. The high temperature measurements In order to observe the influence of the phase transition on the crystal perfection, the X-ray measurements, as described above, were performed at temperatures 20—400°C.However, the

Table 2 Splitting of peaks in powder diffractometry, the corresponding rhombohedral angle and full width at half maximum of peaks in single crystal diffractometry (double crystal); the last column contains the maximum possible rhombohedral angle as measured in single crystal diffraction x X-ray (%)

90°-a powder (arc mm)

~lt9 (220)—(2~0) powder (arc mm)

FWHM (220) monocrystalline (arc mm)

~lf9 (440)_(4z10) powder (arc mm)

FWHM (440) monocrystalline (arc mm)

90°-a mono-crystalline maximum (arc mm)

0 25 37 48 65 78 100 ±1

0 19 34 40 44 51 64 ±1

0 7 12 15 16 19 23 ±1

3 6 9 15 20—100 20—150 20—200 ±10%

0 18 32 38 40 48 59 ±2

6 15 26 35 40—150 40—200 40—200 ±10%

0 14 28 35 44 51 64 ±2

M. Leszczynski et al.

/ Properties of (Pb, Ge) Te single crystals grown from the vapour phase

For

GeTe crystals the rocking curves broadened irreversibly at a temperature of about 300°C, i.e. at a temperature slightly lower than the phase transition.

048 90.0

~

89.6

.9.0.9~7

,..

/ / I P /~ / ;,./

P 0.65

•‘°

,~~/078

~/ ..~

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569

er~ ~

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5. Discussion This paper shows that growth of (Pb,Ge)Te crystals is possible over the whole composition range. The phase transition, which occurs during

~. g.%

~

post-growth cooling, the crystals only for high (above 50%)detonates GeTe content. For these 88.8 0

100

~200

•400

temperature (°C) Fig. 2. Values of rhombohedral angles for Pb

1_~Ge~Tecrystals versus temperature. The numbers in the figure correspond to x

crystals of x 37%—78% could only be heated up to 200°C, because of their decomposition at higher temperatures. Fig. 2 shows the change of rhombohedral angle as measured with powder diffractometry. The phase transitions (a 90°)for crystals of x 0.25, 0.37, 0.48 and 1.0 (values from X-ray diffraction) can be seen. The dependence of rhombohedral angle on temperature was continuous, indicating the second order character of this phase transition. This agrees with the work of Hohnke et al. [9], but opposes the earlier one of Clarke [25], who found non-continuity of this dependence for (Sn,Ge)Te samples. The dependence of critical temperature on the composition turned out to be almost linear (which it is not for low temperature range, i.e. for lower GeTe content [9]). For all samples the peak shape remained the same after temperature examinations in the range 20—200°C. Especially interesting was a full reversibility of rocking curves for single crystals of x 0.25—0.48, which were undergoing the phase transition. This confirmed an assumption that these crystals have rather internal strains, not low angle boundaries. Fig. 1 shows the peak narrowings as the temperature was raised. =

=

=

=

crystals the phase transition takes place at higher temperatures and the strains are high enough to be relaxed and to form low angle boundaries. For lower GeTe content, it seems that the phase transition to rhombohedral structure was .

blocked by strains resulting from four equivalent (111) directions, in which the cubic cell can elongate. The blocking of this phase transition in a highly strained PbTe—PbGeTe superlattice has already been reported [7]. The parts of the crystal transformed into a new phase form a highly strained structure with a typical correlation length smaller than 0.1 mm and rhombohedral angle smaller than in the powder. This angle was not well defined and presumably varied from zero to the maximum values given in table 2. The magnitude of these strains was about 0.1% and it seems that it was too small to push dislocations and form low angle boundaries at phase transition temperatures of about 100°C. The GeTe content could be measured with a very high accuracy using X-ray diffraction. However, as can be seen from table 1, the x values were higher than those obtained with a microprobe. This can be either caused by a larger error of the microprobe, or the positions of the diffraction peaks were influenced by the strains in the crystals. The latter possibility has been pointed out recently in our paper [26]. As mentioned above, except PbTe, the samples were Te-rich. This was confirmed by the low temperature of the phase transition for GeTe (350°C),i.e., a temperature similar to the critical temperature of Te-saturated crystals, and about 60°lower than for Ge-saturated samples [9]. Also

570

M. Leszczynski eta!.

/ Properties of (Pb,Ge)Te single crystals grown from

the lattice constant of GeTe was measured to be similar to the lattice constant of Te-saturated i

samp’es

~ L~J’

References [1] [2] [3] [4]

H. Preier, Semicond. Sci. Technol. 5 (1990) S12. H. Preier, AppI. Phys. 20 (1979) 189. G. Martinez, Phys. Rev. B 8 (1973) 4693. W. Jantsch, in: Dynamical Properties of IV—VI Compounds, Springer Tracts in Modern Physics, Vol. 99 (Springer, Berlin, 1983). [5] T. Suski, J. Karpinski, K.L.J. Kobayashi and K.F. Komatsubara, J. Phys. Chem. Solids 42 (1981) 479. [6] T. Suski, Mater. Sci. 11(1985) 3. [7] G. Bauer and H. Clemens, Semicond. Sci. Technol. 5 (1990) S122. [8] H. Clemens, P. Ofner, H. Krenn and G. Bauer, J. Crystal Growth 84 (1987) 571. [9] D.K. Hohnke, H. Holloway and S. Kaiser, J. Phys. Chem. Solids 33 (1972) 2053. [10] H. Kawamura, in: Proc. Intern. Conf. on Physics of Narrow-Gap Semiconductors, Warsaw, 1977, Ed. J. Rauluszkiewicz (PWN, Warsaw) p. 122.

the vapourphase

[11] T. Harman and J.P. McVittie, J. Electron. Mater. 3 (1974) 843. [12] D. Stöber, B.O. Hildmann, H. Bbttner, S. Scheib, K.H. Bachem and M. Binnewies, J. Crystal Growth 121 (1992) 565. [13] I. Kasai, D.R. Daniel, H. Maier and H.D. Wurzinger, J. Crystal Growth 23 (1974) 201. [14] H. Maier, D.R. Daniel and H. Preier, J. Crystal Growth 35 (1976) 121. [15] W. Lo, G.P. Montgomery and D.E. Sweets, J. Appl. Phys. 47 (1976) 267. [16] W. Lo, J. Electron. Mater. 6 (1977) 39. [17] H. Preier, R. Herkert and H. Pfeiffer, J. Crystal Growth 22 (1974) 153. [181 E. Kaldis, J. Crystal Growth 5 (1969) 276. [191 Z. Golacki, J. Majewski and J. Makowski, J. Crystal Growth 94 (1989) 559. [20] A. Szczerbakow and Z. Golacki, Mater. Sci. Eng. B16 (1993) 68. [21] A. Szczerbakow, J. Crystal Growth 82 (1987) 709. [22] N.Kh. Abrikosov and L.E. Schelimova, in: Poluprovodnikovyie Materialy na Osnove Soedineneiy AIV—BVI (Nauka, Moscow 1975) p. 57 (in Russian). [23] W.L. Bond, Acta Cryst. 13 (1960) 814. [24] R. Dalven, Infrared Phys. 9 (1969) 141. [25] R. Clarke, Phys. Rev. B 18 (1978) 4920. [26] M. Leszczynski, J. AppI. Cryst. 26 (1993) 280.