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“Contactless” growth of uniform Cd0.8Zn0.2Te monocrystals from the vapour
Materials Letters 58 (2004) 1781 – 1783 www.elsevier.com/locate/matlet
‘‘Contactless’’ growth of uniform Cd0.8Zn0.2Te monocrystals from the vapour Z. Golacki, J.Z. Domagala, K. Swiatek, A. Szczerbakow * Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, pl-02-668, Warsaw, Poland Received 1 July 2003; received in revised form 24 October 2003; accepted 30 October 2003
Abstract Cd0.8Zn0.2Te monocrystals of about 12 mm in size were produced by self-selecting vapour growth (SSVG). The growth process leads to compositional uniformity of solid solutions due to small temperature differences acting in the growth space, and the influence of any foreign material in the phases of growing and cooling a crystal is avoided. The crystals were characterised by means of X-ray diffraction and lowtemperature luminescence. Variation in the ZnTe molar fraction x below 0.002 (0.2%) and the ‘‘rocking curve’’ half-width around 30 arcsec were determined. D 2004 Elsevier B.V. All rights reserved. Keywords: Crystal growth; Electronic materials; Semiconductors; Vapour growth; Solid solutions; II – VI; Cadmium compounds; Zinc compounds
The efforts to produce compositionally uniform and structurally perfect monocrystals of (Cd,Zn)Te are in part directed towards the ‘‘contactless’’ growth from vapour, where valuable results were obtained with a system designed initially for cylindrical CdS crystals [1] and, much later, adapted to (Cd,Zn)Te [2,3]. In particular, compositional uniformity was achieved by the introduction of separate sources of CdTe and ZnTe [2]; nevertheless, the ranges of compositional variations in monocrystals grown in these systems [2,3] were not published. Data of this kind were presented for a near equilibrium process of selfselecting vapour growth (SSVG), which delivers faceted crystals [4]. The key feature of this process is to avoid a contact of the growing crystal to the ampoule walls by control of radial temperature differences [5]. The SSVG was applied to Cd1 xZnxTe with ZnTe molar fraction xg0.04 [6] and the level of compositional variations was identified by measurements of lattice constants and lowtemperature luminescence. The employed analytical procedures were sensitive enough to measure variations in x in the range below 0.002 (0.2%) despite the fact that the accuracy in the determination of the absolute x-values was worse. Monocrystallinity of the product was confirmed, neither
* Corresponding author. Tel.: +48-22-843-6601x2961; fax: +48-22843-0926. E-mail address: [email protected] (A. Szczerbakow). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.10.052
precipitations of a foreign phase nor twins were found, but—in contrast to pure CdTe—signs of undesired mosaicity emerged; this caused doubts about the possibility to achieve a high level of structural quality at larger ZnTe contents. The subject of this paper is SSVG of Cd0.8Zn0.2Te led under the conditions of minimum total vapour pressure with a capillary reaching the space of room temperature, as it was used for Cd(Te,Se) [7]. (The capillary protects against an excess of Te or metal, which could stop the material transport in the vapour phase or cause precipitations in the crystals.) The choice of the x-value was justified by the mentioned problem of structural quality as a function of ZnTe content and by applicability to g-ray detectors [8]. A standard tube furnace of the radius 35 mm with three separately supplied heating segments of total length 550 mm was used for the crystal growth. Flat temperature profile with a shallow minimum at 890 jC in the central part was created, as required in SSVG [4]. Physical mixtures of 80% CdTe and 20% ZnTe crushed to the grains of 1 – 3 mm were used as the source materials. Both tellurides were obtained from the elements of the purity grade 5 N. Faceted crystals reaching around 12 mm in size were grown within 14 days including the introductory homogenisation period of about 3 days. The homogenisation is characterised by decrease of partial vapour pressures of both solution components CdTe and ZnTe until the creation of a uniform solid solution. Naturally, the period depends on the grain size of the starting mixture. Completeness can be recognised by the
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appearance of numerous crystallites with flat, shiny facets. Next, under stable conditions, selection and growth of larger crystals begins. The growth conditions are determined by maximum entropy of the uniform solution, and the dominating thermodynamic force is that created by the temperature difference of few Centigrade [5]. The second law of thermodynamics delivers the simple conclusion that, under isothermal conditions, no separation of a solution (including a solid one) occurs, while the maximum separation caused by an ‘‘almost negligible’’ temperature difference of a few degrees was the subject of theoretical estimations [9,10]. The structural properties of the produced crystals were analysed on cleaved (110) planes by high resolution X-ray diffractometry (Philips X’Pert diffractometer MRD equipped with an X-ray mirror, a 4 bounce 220 asymmetric Ge monochromator and a 3 bounce Ge analyser). The lattice ˚ with a measureconstant ranged from 6.4040 to 6.4034 A 4 ˚ ment error Da = F 110 A. Assuming obedience to Vegard’s rule, the ZnTe content was found from the measured lattice constants. The published data allowed to calculate consistent values of x for the average lattice constant ˚ . The relation a = 6.4822 0.3792x [11] and the 6.4037 A formula a = 6.4820 0.3784x obtained from the lattice constant of CdTe 6.482 [12] and measured by us for ZnTe 6.1036 [13] led to x = 0.207. Due to the slope a versus x, the measured spread of the lattice constant indicates variability in x of vdiffr = 0.0016 (0.16%) with an error Dx = F 0.0003 (0.03%). The slight change in the average proportion ZnTe/ CdTe (in comparison to the starting mixture) may be caused by condensation of small amount of the source material condensed in the capillary beyond the crystallisation zone— probably, this part contained dominantly the more volatile CdTe. Additional tests of compositional uniformity were carried out by plotting diffraction curves in the triple axis
Fig. 1. Diffraction curve of 220 reflection in the triple axis geometry (x/2h mode).
Fig. 2. Reciprocal space map of symmetrical 220 reflection obtained with the CuKa1 radiation of k = 0.15406 nm. X and Y represent normalized reciprocal space vectors (X is in the direction parallel to the surface, Y is in the direction perpendicular to the surface, both given in k/2d units, d denotes the interplanar spacing).
mode to examine the distribution of the lattice constant value, as shown in Fig. 1. The height and the smoothness of the peak demonstrate uniformity in the local range (X-ray beam spot of about 1 mm2 and X-ray penetration depth of about 10 Am). Curves of this kind were taken at 6 points along a segment of 5 mm and they did not differ from that shown in Fig. 1. Reciprocal lattice map with symmetrical reflection 220 was plotted to check the possible spread of orientation (mainly by mosaicity) and, simultaneously, to verify uniformity of the lattice constant. The pattern presented in Fig. 2 contains no perceptible signs of imperfections and the presence of the streak extending in the Y-direction shows that the cleaved
Fig. 3. Photoluminescence spectrum taken at one point on a split plane of a Zn0.20Cd0.80Te crystal at 6 K. The peaks are marked by: D0X-donor-bound exciton, A0X-acceptor-bound exciton, eA0-recombination of free electron with a hole bound on a neutral acceptor, DAP-recombination of a donor – acceptor pair.
Z. Golacki et al. / Materials Letters 58 (2004) 1781–1783
surface is smooth. The corresponding full-width at half maximum (FWHM) values of the 220 rocking curve in the double crystal mode (without the analyser) varied in the range of 25 –34 arcsec, while the theoretical value calculated for an ideal crystal—involving the apparatus function of the diffractometer—was 17 arcsec. Further analyses were performed by means of lowtemperature photoluminescence (PL). As an example, a spectrum taken at 6 K is presented in Fig. 3. It consists mainly of emission lines from donor- and acceptor-bound excitons and donor – acceptor pairs. A comparison of intensity between the emission lines originating from donor- and acceptor-bound excitons seems to indicate domination of the donor states. The well-resolved peak at 1.72 eV, which is attributed to the emission of a donor-bound exciton (D0X), proves suitable for composition monitoring. Such spectra were taken at 10 points along and across a 25-mm2 area of the split plane of a crystal, on which the X-ray analyses were earlier performed. The energy of the D0X emission peak was measured in the range of 1.7183 –1.7201 eV. The literature data [14] allow to calculate the slope of the function describing photon energy versus molar fraction at x = 0.2 to be 0.93 eV per x = 1 (0.0093 eV/%); thus, the determined spread of the energy values is due to the variation in the ZnTe content within vlumin = 0.0019 (0.19%). It seems to be regular that the PL measurements indicate slightly stronger variations in composition than lattice constants measurements. Analogous discrepancies emerged in the analyses of Cd0.96Zn0.04Te, where the x-variation measured by X-ray diffraction was found to be vdiffrg0.001, while the PL gave vlumin = 0.0014 [6]. Similar differences appeared in Cd(Te,Se) with vdiffr = 0.0024 and vlumin = 0.0034 [7], as well as Cd(Te,S) with vdiffr = 0.0013 and vlumin = 0.0015 [15]. Since the luminescence technique seems to be more sensitive to disturbances than the X-ray diffraction, we consider the latter to be more accurate; nevertheless, the PL remains appreciated as a verification technique which also describes preliminarily the point defect status. In conclusion, the SSVG process demonstrated its ability to deliver uniform Cd0.8Zn0.2Te crystals. Not only exclusion of the influence by any foreign material (contact only with the polycrystalline source material) is advantageous-homogenisation of the CdTe – ZnTe solution is here spontaneous, thus no external control is necessary to achieve compositional uniformity. Superior structure quality of the crystals described here to those with smaller ZnTe content does not confirm the supposed worsening with growing x-value; nevertheless, this dependence cannot be excluded. The mosaic observed at a lower x-value [6] might be caused
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by a factor of different character—for instance, by instabilities in the vapour transport and condensation, since sensitivity to fluctuations and disturbances is more probable in the case of strongly diluted Zn in the vapour phase. The data concerning the vapour pressures of ZnTe [16] and CdTe [17] allow to estimate the quotient of the partial vapour pressures pZn/pCd to be about 20 times smaller from the molar fraction quotient in the solid xZnTe/xCdTe, and undesired effects may occur, especially, at low ZnTe contents in the solid phase. More clearness in the problem of structural quality as a function of x should be achieved by experiments on (Cd,Zn)Te solid solutions of the compositions lying closer to ZnTe.
Acknowledgements This work was partly funded by KBN (Polish State Committee for Scientific Research) grant 7 T08A 006 20. The Philips X’Pert MRD diffractometer was founded by KBN.
References [1] E.V. Markov, A.A. Davydov, Izv. Akad. Nauk SSSR, Neorg. Mater. (Russ.) 7 (1971) 575. [2] A.A. Melnikov, A.S. Sigov, K.A. Vorotilov, A.A. Davydov, L.I. Topalova, N.V. Zhavoronkov, J. Cryst. Growth 197 (1999) 666. [3] M. Fiederle, T. Feltgen, M. Rogalla, J. Meinhardt, J. Ludwig, K. Runge, K.W. Benz, Nucl. Instrum. Methods Phys. Res., Sect. A, Accel. Spectrom. Detect. Assoc. Equip. 434 (1999) 152. [4] A. Szczerbakow, Cryst. Growth Des. 1 (2001) 183. [5] A. Szczerbakow, J. Cryst. Growth 257 (2003) 31. [6] A. Szczerbakow, J. Domagala, D. Rose, K. Durose, V.Y. Ivanov, A.R. Omeltchouk, J. Cryst. Growth 191 (1998) 673. [7] A. Szczerbakow, J. Domagala, Z. Golacki, K. Swiatek, Cryst. Res. Technol. 34 (1999) 53. [8] T.E. Schlesinger, J.E. Toney, H. Yoon, E.Y. Lee, B.A. Brunett, L. Franks, R.B. James, Mater. Sci. Eng., R Rep. 32 (2001) 103. [9] A. Szczerbakow, Cryst. Res. Technol. 29 (1994) 543. [10] A. Szczerbakow, J. Cryst. Growth 151 (1995) 384. [11] M. Schenk, I. Ha¨hnert, L.T.H. Duongh, H.H. Niebsch, Cryst. Res. Technol. 31 (1996) 665. [12] D.J. Williams, in: P. Capper (Ed.), Properties of Narrow Gap Cadmium-Based Compounds, INSPEC, London, 1994, p. 399. [13] A. Szczerbakow, J. Domagala, Z. Golacki, V.Y. Ivanov, M. Leszczynski, Cryst. Res. Technol. 33 (1998) 875. [14] K. Saito, A. Ebina, T. Takahashi, Solid State Commun. 11 (1972) 841. [15] K. Durose, A.J.W. Yates, A. Szczerbakow, J. Domagala, Z. Golacki, K. Swiatek, J. Phys., D, Appl. Phys. 35 (2002) 1997. [16] R. Reynolds, D.G. Stroud, D.A. Stevenson, J. Electrochem. Soc. 114 (1967) 1281. [17] M.R. Lorentz, J. Phys. Chem. Solids 23 (1962) 1449.
‘‘Contactless’’ growth of uniform Cd0.8Zn0.2Te monocrystals from the vapour Z. Golacki, J.Z. Domagala, K. Swiatek, A. Szczerbakow * Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, pl-02-668, Warsaw, Poland Received 1 July 2003; received in revised form 24 October 2003; accepted 30 October 2003
Abstract Cd0.8Zn0.2Te monocrystals of about 12 mm in size were produced by self-selecting vapour growth (SSVG). The growth process leads to compositional uniformity of solid solutions due to small temperature differences acting in the growth space, and the influence of any foreign material in the phases of growing and cooling a crystal is avoided. The crystals were characterised by means of X-ray diffraction and lowtemperature luminescence. Variation in the ZnTe molar fraction x below 0.002 (0.2%) and the ‘‘rocking curve’’ half-width around 30 arcsec were determined. D 2004 Elsevier B.V. All rights reserved. Keywords: Crystal growth; Electronic materials; Semiconductors; Vapour growth; Solid solutions; II – VI; Cadmium compounds; Zinc compounds
The efforts to produce compositionally uniform and structurally perfect monocrystals of (Cd,Zn)Te are in part directed towards the ‘‘contactless’’ growth from vapour, where valuable results were obtained with a system designed initially for cylindrical CdS crystals [1] and, much later, adapted to (Cd,Zn)Te [2,3]. In particular, compositional uniformity was achieved by the introduction of separate sources of CdTe and ZnTe [2]; nevertheless, the ranges of compositional variations in monocrystals grown in these systems [2,3] were not published. Data of this kind were presented for a near equilibrium process of selfselecting vapour growth (SSVG), which delivers faceted crystals [4]. The key feature of this process is to avoid a contact of the growing crystal to the ampoule walls by control of radial temperature differences [5]. The SSVG was applied to Cd1 xZnxTe with ZnTe molar fraction xg0.04 [6] and the level of compositional variations was identified by measurements of lattice constants and lowtemperature luminescence. The employed analytical procedures were sensitive enough to measure variations in x in the range below 0.002 (0.2%) despite the fact that the accuracy in the determination of the absolute x-values was worse. Monocrystallinity of the product was confirmed, neither
* Corresponding author. Tel.: +48-22-843-6601x2961; fax: +48-22843-0926. E-mail address: [email protected] (A. Szczerbakow). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.10.052
precipitations of a foreign phase nor twins were found, but—in contrast to pure CdTe—signs of undesired mosaicity emerged; this caused doubts about the possibility to achieve a high level of structural quality at larger ZnTe contents. The subject of this paper is SSVG of Cd0.8Zn0.2Te led under the conditions of minimum total vapour pressure with a capillary reaching the space of room temperature, as it was used for Cd(Te,Se) [7]. (The capillary protects against an excess of Te or metal, which could stop the material transport in the vapour phase or cause precipitations in the crystals.) The choice of the x-value was justified by the mentioned problem of structural quality as a function of ZnTe content and by applicability to g-ray detectors [8]. A standard tube furnace of the radius 35 mm with three separately supplied heating segments of total length 550 mm was used for the crystal growth. Flat temperature profile with a shallow minimum at 890 jC in the central part was created, as required in SSVG [4]. Physical mixtures of 80% CdTe and 20% ZnTe crushed to the grains of 1 – 3 mm were used as the source materials. Both tellurides were obtained from the elements of the purity grade 5 N. Faceted crystals reaching around 12 mm in size were grown within 14 days including the introductory homogenisation period of about 3 days. The homogenisation is characterised by decrease of partial vapour pressures of both solution components CdTe and ZnTe until the creation of a uniform solid solution. Naturally, the period depends on the grain size of the starting mixture. Completeness can be recognised by the
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Z. Golacki et al. / Materials Letters 58 (2004) 1781–1783
appearance of numerous crystallites with flat, shiny facets. Next, under stable conditions, selection and growth of larger crystals begins. The growth conditions are determined by maximum entropy of the uniform solution, and the dominating thermodynamic force is that created by the temperature difference of few Centigrade [5]. The second law of thermodynamics delivers the simple conclusion that, under isothermal conditions, no separation of a solution (including a solid one) occurs, while the maximum separation caused by an ‘‘almost negligible’’ temperature difference of a few degrees was the subject of theoretical estimations [9,10]. The structural properties of the produced crystals were analysed on cleaved (110) planes by high resolution X-ray diffractometry (Philips X’Pert diffractometer MRD equipped with an X-ray mirror, a 4 bounce 220 asymmetric Ge monochromator and a 3 bounce Ge analyser). The lattice ˚ with a measureconstant ranged from 6.4040 to 6.4034 A 4 ˚ ment error Da = F 110 A. Assuming obedience to Vegard’s rule, the ZnTe content was found from the measured lattice constants. The published data allowed to calculate consistent values of x for the average lattice constant ˚ . The relation a = 6.4822 0.3792x [11] and the 6.4037 A formula a = 6.4820 0.3784x obtained from the lattice constant of CdTe 6.482 [12] and measured by us for ZnTe 6.1036 [13] led to x = 0.207. Due to the slope a versus x, the measured spread of the lattice constant indicates variability in x of vdiffr = 0.0016 (0.16%) with an error Dx = F 0.0003 (0.03%). The slight change in the average proportion ZnTe/ CdTe (in comparison to the starting mixture) may be caused by condensation of small amount of the source material condensed in the capillary beyond the crystallisation zone— probably, this part contained dominantly the more volatile CdTe. Additional tests of compositional uniformity were carried out by plotting diffraction curves in the triple axis
Fig. 1. Diffraction curve of 220 reflection in the triple axis geometry (x/2h mode).
Fig. 2. Reciprocal space map of symmetrical 220 reflection obtained with the CuKa1 radiation of k = 0.15406 nm. X and Y represent normalized reciprocal space vectors (X is in the direction parallel to the surface, Y is in the direction perpendicular to the surface, both given in k/2d units, d denotes the interplanar spacing).
mode to examine the distribution of the lattice constant value, as shown in Fig. 1. The height and the smoothness of the peak demonstrate uniformity in the local range (X-ray beam spot of about 1 mm2 and X-ray penetration depth of about 10 Am). Curves of this kind were taken at 6 points along a segment of 5 mm and they did not differ from that shown in Fig. 1. Reciprocal lattice map with symmetrical reflection 220 was plotted to check the possible spread of orientation (mainly by mosaicity) and, simultaneously, to verify uniformity of the lattice constant. The pattern presented in Fig. 2 contains no perceptible signs of imperfections and the presence of the streak extending in the Y-direction shows that the cleaved
Fig. 3. Photoluminescence spectrum taken at one point on a split plane of a Zn0.20Cd0.80Te crystal at 6 K. The peaks are marked by: D0X-donor-bound exciton, A0X-acceptor-bound exciton, eA0-recombination of free electron with a hole bound on a neutral acceptor, DAP-recombination of a donor – acceptor pair.
Z. Golacki et al. / Materials Letters 58 (2004) 1781–1783
surface is smooth. The corresponding full-width at half maximum (FWHM) values of the 220 rocking curve in the double crystal mode (without the analyser) varied in the range of 25 –34 arcsec, while the theoretical value calculated for an ideal crystal—involving the apparatus function of the diffractometer—was 17 arcsec. Further analyses were performed by means of lowtemperature photoluminescence (PL). As an example, a spectrum taken at 6 K is presented in Fig. 3. It consists mainly of emission lines from donor- and acceptor-bound excitons and donor – acceptor pairs. A comparison of intensity between the emission lines originating from donor- and acceptor-bound excitons seems to indicate domination of the donor states. The well-resolved peak at 1.72 eV, which is attributed to the emission of a donor-bound exciton (D0X), proves suitable for composition monitoring. Such spectra were taken at 10 points along and across a 25-mm2 area of the split plane of a crystal, on which the X-ray analyses were earlier performed. The energy of the D0X emission peak was measured in the range of 1.7183 –1.7201 eV. The literature data [14] allow to calculate the slope of the function describing photon energy versus molar fraction at x = 0.2 to be 0.93 eV per x = 1 (0.0093 eV/%); thus, the determined spread of the energy values is due to the variation in the ZnTe content within vlumin = 0.0019 (0.19%). It seems to be regular that the PL measurements indicate slightly stronger variations in composition than lattice constants measurements. Analogous discrepancies emerged in the analyses of Cd0.96Zn0.04Te, where the x-variation measured by X-ray diffraction was found to be vdiffrg0.001, while the PL gave vlumin = 0.0014 [6]. Similar differences appeared in Cd(Te,Se) with vdiffr = 0.0024 and vlumin = 0.0034 [7], as well as Cd(Te,S) with vdiffr = 0.0013 and vlumin = 0.0015 [15]. Since the luminescence technique seems to be more sensitive to disturbances than the X-ray diffraction, we consider the latter to be more accurate; nevertheless, the PL remains appreciated as a verification technique which also describes preliminarily the point defect status. In conclusion, the SSVG process demonstrated its ability to deliver uniform Cd0.8Zn0.2Te crystals. Not only exclusion of the influence by any foreign material (contact only with the polycrystalline source material) is advantageous-homogenisation of the CdTe – ZnTe solution is here spontaneous, thus no external control is necessary to achieve compositional uniformity. Superior structure quality of the crystals described here to those with smaller ZnTe content does not confirm the supposed worsening with growing x-value; nevertheless, this dependence cannot be excluded. The mosaic observed at a lower x-value [6] might be caused
1783
by a factor of different character—for instance, by instabilities in the vapour transport and condensation, since sensitivity to fluctuations and disturbances is more probable in the case of strongly diluted Zn in the vapour phase. The data concerning the vapour pressures of ZnTe [16] and CdTe [17] allow to estimate the quotient of the partial vapour pressures pZn/pCd to be about 20 times smaller from the molar fraction quotient in the solid xZnTe/xCdTe, and undesired effects may occur, especially, at low ZnTe contents in the solid phase. More clearness in the problem of structural quality as a function of x should be achieved by experiments on (Cd,Zn)Te solid solutions of the compositions lying closer to ZnTe.
Acknowledgements This work was partly funded by KBN (Polish State Committee for Scientific Research) grant 7 T08A 006 20. The Philips X’Pert MRD diffractometer was founded by KBN.
References [1] E.V. Markov, A.A. Davydov, Izv. Akad. Nauk SSSR, Neorg. Mater. (Russ.) 7 (1971) 575. [2] A.A. Melnikov, A.S. Sigov, K.A. Vorotilov, A.A. Davydov, L.I. Topalova, N.V. Zhavoronkov, J. Cryst. Growth 197 (1999) 666. [3] M. Fiederle, T. Feltgen, M. Rogalla, J. Meinhardt, J. Ludwig, K. Runge, K.W. Benz, Nucl. Instrum. Methods Phys. Res., Sect. A, Accel. Spectrom. Detect. Assoc. Equip. 434 (1999) 152. [4] A. Szczerbakow, Cryst. Growth Des. 1 (2001) 183. [5] A. Szczerbakow, J. Cryst. Growth 257 (2003) 31. [6] A. Szczerbakow, J. Domagala, D. Rose, K. Durose, V.Y. Ivanov, A.R. Omeltchouk, J. Cryst. Growth 191 (1998) 673. [7] A. Szczerbakow, J. Domagala, Z. Golacki, K. Swiatek, Cryst. Res. Technol. 34 (1999) 53. [8] T.E. Schlesinger, J.E. Toney, H. Yoon, E.Y. Lee, B.A. Brunett, L. Franks, R.B. James, Mater. Sci. Eng., R Rep. 32 (2001) 103. [9] A. Szczerbakow, Cryst. Res. Technol. 29 (1994) 543. [10] A. Szczerbakow, J. Cryst. Growth 151 (1995) 384. [11] M. Schenk, I. Ha¨hnert, L.T.H. Duongh, H.H. Niebsch, Cryst. Res. Technol. 31 (1996) 665. [12] D.J. Williams, in: P. Capper (Ed.), Properties of Narrow Gap Cadmium-Based Compounds, INSPEC, London, 1994, p. 399. [13] A. Szczerbakow, J. Domagala, Z. Golacki, V.Y. Ivanov, M. Leszczynski, Cryst. Res. Technol. 33 (1998) 875. [14] K. Saito, A. Ebina, T. Takahashi, Solid State Commun. 11 (1972) 841. [15] K. Durose, A.J.W. Yates, A. Szczerbakow, J. Domagala, Z. Golacki, K. Swiatek, J. Phys., D, Appl. Phys. 35 (2002) 1997. [16] R. Reynolds, D.G. Stroud, D.A. Stevenson, J. Electrochem. Soc. 114 (1967) 1281. [17] M.R. Lorentz, J. Phys. Chem. Solids 23 (1962) 1449.