Solid State Recrystallization of II-VI Semiconductors : Application to Cadmium Telluride, Cadmium Selenide and Zinc Selenide R. Triboulet, J. Ndap, A. El Mokri, A. Tromson Carli, A. Zozime To cite this version: R. Triboulet, J. Ndap, A. El Mokri, A. Tromson Carli, A. Zozime. Solid State Recrystallization of II-VI Semiconductors : Application to Cadmium Telluride, Cadmium Selenide and Zinc Selenide. J. Phys. IV, 1995, 05 (C3), pp.C3-141-C3-149. 10.1051/jp4:1995312. jpa-00253678 HAL Id: jpa-00253678 https://hal.archives-ouvertes.fr/jpa-00253678 Submitted on 1 Jan 1995 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE IV Colloque C3, supplCment au Journal de Physique 111, Volume 5, avril 1995 Solid State Recrystallization of 11-VI Semiconductors: Application to Cadmium Telluride, Cadmium Selenide and Zinc Selenide R. Triboulet, 3.0. Ndap, A. El Mokri, A. Tromson Carli and A. Zozime CNRS, Laboratoire de Physique des Solides de Bellevue, 1 place Aristide Briand, F 92195 Meudon cedex, France Abstract : Solid state recrystallization (SSR) has been very rarely used for semiconductors. It has nevertheless been proposed, and industrially used, for the single crystal growth of cadmium mercury telluride according to a quench-anneal process. The reasons of the interest of this process, in specific cases of 11-VI semiconductors crystal growth, will be analyzed (particular phase diagrams, phase transitions in the solid state close to the melting point, high temperature contaminations..etc.. make the use of traditional melt growth techniques unfavourable) and illustrated refering to CdHgTe literature. Original results related to the binary compounds CdTe, CdSe and ZnSe will be presented. 1. INTRODUCTION Solid state recrystallization (SSR) has been very rarely used for semiconductors, except for cadmium mercury telluride, and more for metals according to a rather different process, generally preceded by plastic deformation. Several reasons can suggest its use for the growth of semiconductors, among which high temperature contamination risks or particular features of the phase diagrams, such as a large merence between liquidus and solidus curves (CdHgTe case), or the existence of phase transitions in the solid state (CdTe and ZnSe cases), or high melting points (CdSe case), making it =cult the use of melt- growth techniques. The cadmium mercury telluride alloys provide a good example of a pseudobinary phase diagram, with a large difference in liquidus and solidus compositions along a given thermal tie line, in which the slow moving interface of a near-equilibrium growth process will invariably result in a steady variation in the composition of the crystal. The features of the HgTe-CdTe phase diagram make it so di£licult the growth of homogeneous crystals that almost all the kinds of growth techniques used for semiconductor materials have been applied to MCT. The QuenchIRecrystallization, or Cast-Recrystallise-Anneal (CRA), process has been the widest used industrially for the single crystal growth of cadmium mercury telluride [l- 41. But even if it is still used now, it has been progressively replaced by the Travelling Heater Method, Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1995312 C3-142 JOURNAL DE PHYSIQUE IV which has proved to be the only viable MCT bulk growth technique that simultaneously meets the material requirements of large area, uniformity, low defects and well-dehed orientation. Other methods are the epitaxial growth techniques at low temperature, like LPE, MOCVD or MBE. Cadmium telluride and zinc selenide are good examples of iono-covalent semiconductor compounds in which phase transitions in the solid state make the crystals obtained from melt-growth techniques defective. It has been frequently stressed that the bigb ionicity of the chemical bond of some II-VI compounds favors the hexagonal structure. Note that, among the ZI-VI compounds, CdTe and ZnSe present a rather high ionicity, as it can be shown in table 1 in which the ionicities of several 11-VI compounds are gathered together with the structure of their crystalline lattice. table 1 Comuound 1onicitv5 Structure CdS 0.74 Hex. CdSe 0.64 Hex. CdTe 0.55 Cub. twin. ZnS 0.77 Cub.Hex. ZnSe 0.63 Cub.twin. ZnTe 0.49 Cub. HgTe 0.50 Cub. Hex. = hexagonal; cub. = cubic; twin. = twinned The tendency of CdTe to hexagonal structure has been emphasized by several authors, through various experimental investigations. Leibov et al. [6] have reported a phase transition in the 893-920 OC temperature range from a study of thermodynamic properties of CdTe by electromotive forces. The existence of a hexagonal structure at high temperature in CdTe has been also reported by several authors [7-91. Triboulet [lo] has reported a phase transition in the solid state, in the CdTe-MnTe system, between a high temperature hexagonal phase and a low temperature cubic one, with a transition line converging in the vicinity of the melting point of CdTe. This should mean that CdTe could extremely easily oscillate between hexagonal and cubic structures just at its melting point. Ivanov [ll] has reported recently the existence of two phases transitions in the solid state in CdTe at high temperature, at - 980 OC and 900 "C, separating three Werent phases, a,P and y, ftom the high temperature y phase to the "low" temperature a one. Very recently, Rosen [12] has reported also a sharp drop in electric conductivity near 1060°C in CdTe, that could be associated with an allotropic phase transformation, following the statement of Kendall [13] pointing out that electrical properties, such as electric conduct~ty,were closely dependent on the structural symmetry of the substance. All these features could explain why the growth of twin-free CdTe crystals is difEcult fkom the melt. The situation is yet clearer in the ZnSe case. With its high ionicity of 0.63, ZnSe presents a phase transition in the solid state, which has been studied by Kulakov et al. [14]. A reversible phase transition of the first kind has been found at 1425 + 10°C, with a heat of 0.3 kcal mol-l, which was thought to be a transition between the 2H wurtzite form of ZnSe, at high temperature, and the room temperature 3C cubic one. Crossing the transition line during the cooling process induces a very high density of twins, as observed by numerous authors. The existence of phase transitions in the solid state in CdTe and ZnSe justifies the use of low temperature growth, and mainly of SSR as will be seen later. Another sigmiicant example of the interest of SSR for the growth of semiconductor compounds is the case of materials with a melting point higher than the softening temperature of silica, preventing ftom using classical melt-growth techniques. This will be briefly iUustrated in the case of CdSe. Beside the detrimental consequences of high temperature melt-growth, contamination has been also found to occur from the high temperatures used during some growth processes in the case of CdTe [15]. Table 2 displays results of electrical measurements on CdTe crystals grown respectively first by a "low temperature" solution growth technique using Te as the solvent, called "cold THM", in which the material remains at a temperature lower than its melting point over the whole process, second by SSR using such CTHM crystals as source material, and third by vertical Bridgman. Although only 5N elements were used in Cold THM and 6N+ in Bridgman, the purest crystals are the Cold THM ones. The electrical characteristics of the SSR samples show a weak contamination, beside the CTHM ones, as the result of the long annealing process. table 2 Name growth elements N~ N A N~/N~p300~ pmax E~ method Tgrowth (cm-3) (cm-3) cm2Ns cm2Ns meV CTH59 CTHM 5NTe, 6NCd 3.4~1014 1.5x1013 0.04 840 41,400 14 TG = 780°C (28K) CTRE SSR 5N Te, 6N Cd 7.6~1014 2.2~10~~0.03 780 9,000 11.2 TG = 990°C (22K) CT64 Bridgman 6N+CdandTe 2.6x1015 1.6x1015 0.7 800 12,500 9 TG = 1100°C (36K) 2. WHAT "LOW TEMPERATURE" CRYSTAL GROWTH TECHNIQUE TO BE USED ? All the previous considerations justify the use of low temperature growth, for which several techniques can compete, among which solution growth, vapour phase growth and solid state recrystallization. C3-144 JOURNAL DE PHYSIQUE IV In the case of the growth of CdTe and CdZnTe by THM, which has been the most popular solution growth method during the last decade, the crystallographic properties of the crystals suffer from their large stoichiometric deviation, arising from the Te-rich growth conditions. Some mosalc structure is found as the result of these growth conditions, as shown for instance by the multipeak spectra of the rocking-curves measured in X-ray double *action [15]. Moreover, the Te-rich growth conditions together with the retrograde shape of the solidus line lead also to the unavoidable presence of Te precipitates that are detrimental to the use of the crystals as substrate. In the ZnSe case, due to the low ZnSe solubility in both Zn and Se, it has been shown necessary to have recourse to heterosolvent growth [16]. But the solvent used, either In or PbC12, gives rise to residual precipitates in the crystals.