CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 3 183-185
Communications Self-Selecting Vapor Growth of Monocrystals: An Alternative in the Area of Wide-Gap II-VI Solid Solutions Andrzej Szczerbakow* Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, pl-02-668 Warszawa, Poland Received December 13, 2000
ABSTRACT: Self-selecting vapor growth (SSVG) allows the achievement of uniform composition with structural perfection of solid solution crystals. Proven usefulness of this process for IV-VI semiconducting compounds and their solid solutions is presently extended to the II-VI wide-gap semiconductors. Continuous variations of properties by continuous variations of composition are fundamental attributes of solutions (including solids). Hence, control over material parameters by molar fractions of the solid solution components can be widely applied to meet precise requirements of solid-state physics and electronics. Band gaps of semiconductors and lattice constants of substrates for epitaxy are typical parameters set in this way. Current development of green and blue light optoelectronics determines new tasks concerning wide-gap semiconducting compounds and their solid solutions, including those of the II-VI type. In the latter case, most advantages are expected from thin films and bulk monocrystals of pseudobinary solid solutions composed from cadmium or zinc tellurides, selenides, or sulfidesseither with mixed cation or anion. Demand for compositionally uniform and structurally perfect monocrystals of wide-gap solid solutions of the type II-VI is not fully covered by crystallization from the liquid phase (melt) that partly fulfills requirements concerning cadmium and zinc tellurides, but not for sulfides or selenides. An analogous problem appeared in the 70s with IV-VI semiconductors, and several centers in the U. S. and Germany and, later on, in Poland, developed a few similar (but differently defined) methods of crystal growth from the vapor in sealed silica ampules under almost isothermal conditions. Well-faceted monocrystals of IV-VI compounds1-5 and their solid solutions1,3,6-11 were obtained. The sizes exceeded 10 or even 15 mm, and the compositions of the solid solution crystals were uniform. In a range of cases, full conversion of the material contained in the ampule to one perfect monocrystal was reported. The first results of practical value were achieved empirically; still, the basic effects were studied in the next decades. The main conclusion from the studies was that the key factor of the procedures defined as horizontal unseeded vapor growth1 or, later on, self-seeded vapor growth,4 was neither hori* To whom correspondence should be addressed: phone: (+4822) 843 66 01 ext. 2961; fax: (+4822) 843 09 26; e-mail:
[email protected]; personal page: http://info.ifpan.edu.pl/∼szczer/.
zontal position of the ampule, nor character of seeding, but rather formation of heat exchange conditions causing spontaneous selection and growth of crystals. This selfselected vapor growth5 or, better expressed, self-selecting vapor growth (SSVG),12 should not be considered as a technique, but rather as a phenomenon that often emerges on polycrystalline source material resting loosely in a sealed ampule, where perfect, but small, crystals appear. An effect of this kind occurs with different intensity in many systems, including chemical transport. The task of an operator is providing conditions for the phenomenon to change from such “vapor recrystallization” with crystals smaller than 2 mm to the growth of much larger crystals. A good example of a large crystal produced this way was PbSe of 60 g.4 Spontaneous selection progresses if larger crystals are better cooled by heat emission and grow faster. Obviously, a growing crystal ought to create the coolest site inside the ampule to avoid escape of the material to competitive “coolers”. Such coolers exist usually at the ampule ends, and introduction of hotter, border zones in the furnace9 can ensure desired protection. More complete rules of controlling the effect were given after the observation that a growing crystal must be cooled by heat radiation to far distant, much cooler zones of the furnace. In particular, a model of bodies having and not having direct contact with the furnace chamber wall allowed the estimation3,4 of radial temperature differences in the growth furnaces, which create the thermodynamic driving force of the process. These differences were observed earlier and interpreted as temperature gradients.1-2 By calculation, cooling of a crystal by a few single degrees was found possible for a typical system with temperatures around 1100 K. The minimum solid angle of seeing the cool zones in the furnace chamber from a crystal, which allows a crystal to grow, was estimated to be 0.02 steradian.13-14 A horizontal SSVG system with asymmetrical temperature profile of the furnace chamber is schematically shown in Figure 1. It demonstrates the creation of a temperature difference between the bottom of the ampule heated by
10.1021/cg0102812 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/31/2001
184
Crystal Growth & Design, Vol. 1, No. 3, 2001
Figure 1. Horizontal system of SSVG with asymmetrical temperature profile. Main heat flows are marked with arrows. Ω, solid angle, under which a cooler zone is seen from a crystal; ∆T, relative height of a hotter, border zone.
conduction and a crystal cooled by heat radiation on the right side. On the overheated left side, no crystal growth occurs, but escape of the material follows. Owing to continuous evaporation from the bottom, the contact of crystallized material with silica glass remains loose, and no foreign material causes strains. Moreover, the temperature field of SSVG is advantageous in another respect: almost isothermal conditions exist inside the ampule and the natural tendency to increase the entropy leads to incomparably uniform distribution of solution components, even if their volatilities differ considerably. Separation of solution components by evaporation-condensation led under near equilibrium is essentially weaker than that caused by conventional distillation with totally irreversible condensation. Maximum separation as a function of a small temperature difference was estimated with a model of ideal solution,15 and experimental data remain much below the calculated values. However, another kind of separation may appear if vapor transport disturbances are negligible. In such a case, phase transitions of solid-vapor and vapor-solid may dominate and cause a very small separation effect, where a component that conveys more heat is transported faster.16 The theoretical study concerned purely physical transport, but main conclusions apply to dissociative sublimation of the considered materials as well. High degree of uniformity appears in every near equilibrium system; for example, it was noticed after driving (Pb, Sn)Te and (Pb, Ge)Te of the IV-VI type17 and (Cd, Zn)Te of the II-VI type18 into the slightly cooler end of a sealed ampule. In a such configuration, however, the ampule material affects (more or less) the structural properties of a crystal. On the basis of a sophisticated seeding system, a different concept of “contactless” growth producing cylindrical CdS crystals from the vapor phase was earlier realized in Russia.19 Characteristic features of that procedure were reviewed,20 but it should be added that despite efficiency of the method in production of very large and perfect crystals of pure widegap II-VI and also narrow-gap IV-VI semiconductors,21,22 compositional gradients ought to be taken into consideration if solid solutions are crystallized. The gradients may occur because of fairly large temperature differences employed in the process, as it was identified in (Pb, Sn)Te.22 In contrast, the SSVG is driven by much smaller thermodynamic forces, and formation of any property that increases chemical potential (for instance, compositional gradient or structural defects) is less intensive. Typical for SSVG, “mild” growth conditions allow application to sensitive materials, and, for example, crystal growth of (Pb, Ge)Te in compositional range of metastability23 would be less probable by another process. On the other hand,
Communications small thermodynamic forces have a disadvantage in slow growth; typical, linear growth rates remain below 5 mm per day. Extension of SSVG to the II-VI compounds and their solid solutions was the subject of systematic experiments in the next period, and encouraging results were obtained for pure compounds of cubic structure CdTe24,25 and ZnTe.26 More detailed investigation of CdTe revealed that not only are monocrystals selected on the initial step, but also improvement of the structural properties follows in the next phases.12 No precipitation of a foreign material (e.g., Te or Cd) was found. Similar, introductory results for hexagonal CdSe were obtained as well.27 Tests with hexagonal CdS were omitted, since much earlier described CdS crystal growth in a vertical system with the product of sizes above 15 mm28 could be considered an SSVG process. In the range of solid solutions, CdTe-based systems of regular structure were the subject of experiments. Cd1-xZnxTe (x = 0.04) monocrystals of excellent uniformity and free from twins were grown, but undesired mosaic emerged.12 Most probably, low volatility of the minor component ZnTe could cause vapor transport fluctuations resulting in local instabilities in the vapor-solid-phase transition. Improvement in the vapor transport conditions should allow avoidance of this disturbance. Besides the (Cd, Zn)Te with mixed cation, Cd(Te, Se) and Cd(Te, S) crystals with mixed anion were grown. A high degree of structural perfection and compositional uniformity was observed in CdTe1-xSex (x = 0.04).29 Thus, the ability of the latter material to compete with (Cd, Zn)Te (as substrate for epitaxy with customized lattice constant) could be anticipated. Compositional uniformity of CdTe1-xSx (x = 0.04) crystals was satisfying as well, but twins and low angle boundaries appeared, caused by lattice properties rather than by the growth conditions. Still, the Cd(Te, S) crystals allowed finding direct evidence of some phenomena influencing action of CdS-CdTe solar cells.30 Introductory experiments on hexagonal CdSe1-xSx (x = 0.15) confirmed the SSVG action in the solid solution system, but worsening of structural properties in comparison to pure CdSe was observed.27 Up to the present, less attention was paid to II-VI solid solutions containing ZnSe and ZnS, but known properties of these compounds allow the assumption of the possible extension of SSVG in this direction; no fundamental problems should appear, but technical modifications will be necessary because of lower volatility of the latter compounds. Self-selecting vapor growth proved exceptionally useful in reconnaissance experiments on crystallization of nonstandard IV-VI and II-VI solid solutions, but a satisfying increase in the sizes of crystals was achieved only for IVVI compounds and not to the same degree for the II-VI compounds. Despite some dissimilarity between the behavior of IV-VI and II-VI compounds in an SSVG process (which is mainly due to the structure types), a difference in engaged efforts seems to be the main cause of distance in the achieved sizes. At present, a good starting position exists for further improvement in SSVG, since not only traditional, horizontal configuration is in use, but also vertical systems allowing better controllable, recycled crystallization are being developed.12,26
References (1) Harman, T. C.; McVittie, J. P. J. Electron. Mater. 1974, 3, 843-54. (2) Maier, H.; Daniel, D. R.; Preier, H. J. Cryst. Growth 1976, 35, 121-126. (3) Szczerbakow, A. J. Cryst. Growth 1987, 82, 709-716. (4) Sto¨ber, D.; Hildmann, B. O.; Bo¨ttner, H.; Schelb, S.; Bachem, K.-H.; Binnewies, M. J. Cryst. Growth 1992, 121, 656-664.
Communications (5) Szczerbakow, A. Cryst. Res. Technol. 1993, 28, K77-80. (6) Preier, H.; Herkert, R.; Pfeiffer, H. J. Cryst. Growth 1974, 22, 153-158. (7) Kasai, I.; Daniel, D. R.; Maier, H.; Wurzinger, H.-D. J. Cryst. Growth 1974, 23, 201-206. (8) Kennedy, C. A.; Linden, K. L. Proc. IEEE 1975, 63, 27-32. (9) Lo, W.; Montgomery, G. P.; Swets, D. E. J. Appl. Phys. 1976, 47, 267-271. (10) Lo, W. J. Electron. Mater. 6, 1977, 6, 39-48. (11) Szczerbakow, A.; Berger, H. J. Cryst. Growth 1994, 139, 172-178. (12) Szczerbakow, A.; Domagala, J.; Rose, D.; Durose, K.; Ivanov, V. Yu.; Omeltchouk, A. R. J. Cryst. Growth 1998, 191, 673678. (13) Szczerbakow, A.; Barthel, U.; Siche, D.; Muehlberg, M. Patent PRL 138 487. (14) Szczerbakow, A.; Barthel, U.; Siche, D.; Mu¨hlberg, M. Patent DDR DD 220 348 A1. (15) Szczerbakow, A. Cryst. Res. Technol. 1994, 29, 543-547. (16) Szczerbakow, A. J. Cryst. Growth 1995, 151, 384-386. (17) Parker, S. G.; Pinnel, J. E.; Johnson, R. E. J. Electron. Mater. 1974, 3, 731-746. (18) Ben-Dor, L.; Yellin, N. J. Cryst. Growth 1985, 71, 519-524. (19) Markov, E. V.; Davydov, A. A. Izv. Akad. Nauk SSSR Neorg. Mat. (Russ.) 1971, 7, 575-579.
Crystal Growth & Design, Vol. 1, No. 3, 2001 185 (20) Brinkman, A. W.; Carles, J. Prog. Cryst. Growth Charact. Mat. 1998, 37, 169-209. (21) Golacki, Z.; Furmanik, Z.; Gorska, M.; Szczerbakow, A.; Zahorowski, W. J. Cryst. Growth 1982, 60, 150-152. (22) Golacki, Z.; Gorska, M.; Warminski, T.; Szczerbakow, A. J. Cryst. Growth, 1986, 74, 129-134. (23) Leszczynski, M.; Szczerbakow, A.; Karczewski, G. J. Cryst. Growth 1994, 135, 565-570. (24) Golacki, Z.; Majewski, J.; Makowski, J. J. Cryst. Growth 1989, 94, 559-60. (25) Szczerbakow, A.; Golacki, Z. Mater. Sci. Eng. 1993, B16, 6870. (26) Szczerbakow, A.; Domagala, J.; Golacki, Z.; Ivanov, V. Yu.; Leszczynski, M. Cryst. Res. Technol. 1998, 33, 875-879. (27) Szczerbakow, A., unpublished data. (28) Kaldis, E. J. Cryst. Growth 1969, 5, 376-390. (29) Szczerbakow, A.; Domagala, J.; Golacki Z.; Swiatek, K. Cryst. Res. Technol. 1999, 34, 53-57. (30) Durose, K.; Sadler, J. R. E.; Yates, A.; Szczerbakow, A. 28th IEEE Photovoltaic Specialists Conference, Anchorage, September 17-22, 2000, IEEE Conf. Proceeding, in press.
CG0102812
