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. Vapour growth can lead to crystals of high perfection, but growth rates are fkustratingly low and, moreover, the single-crystal yield is variable, due to uncontrolled convection regimes in the growth vessels due to gravity effects. Moreover, due to the low thermal conductivity of both CdTe and ZnSe, the initial growth conditions are lost after a limited thickness. Akhekyan et al. [17] have reported ZnSeTe, ZnCdSe and ZnCdS crystals grown from the vapour phase, 30-50 mm in diameter, but only 20 mm high, which was the maximum dimension achieved after growth for 8Oh. Nevertheless, vapour growth is used industrially at Eagle Picher for CdTe and ZnSe according to the sublimation and physical transport technique (SPVT) [IS]. Here also, the growth of a 300 g CdTe crystal, of limited thickness, takes about six weeks ! It stems from these considerations that solution growth and vapour phase growth do not really meet the substrates requirements of large area and high crystalline perfection for both CdTe and ZnSe . In order to overcome the disadvantages of these techniques, solid state recrystallization appears very promising as a method without any crystal sue limitation, allowing the benefit of low temperature growth and the use of perfectly stoichiometric source material grown from the vapour phase (cf the ZnSe part). However well-defjned crystalline orientations cannot be selected using SSR, which is not a real limitation because of the very large volumes that are reachable by this technique.

3. CdHgTe SSR GROWTH

Although THM has been shown to meet the material requirements of large area, good uniformity, low defects and well-defined orientation [19], SSR has, for long, been one of the most successll technique for growing HgCdTe (MCT) crystals, and is always exploited industrially due to its process simplicity and cost effectiveness. In this process, the MCT melt is quenched to a well-cast polycrystalline ingot, having microstructural inhomogeneities due to dendritic freezing. This ingot is subsequently annealed just below the solidus temperature (in the range 650-680°C for 20% Cd) for homogeneization at microscopic scale and grain growth. A very heand highly uniform dendritic distribution is desirable in the cast so as to obtain a fairly high uniformity of composition aRer recrystallization. Most of the efforts on improving the SSR technique have therefore been concentrated on reforming the casting procedure itself, that is made dicult because of the poor thermal conductivity not only of liquid MCT but also of the silica ampoule in which it is included. The main developments have been gas quenching processes, that are normally performed by forcing compressed/cooled airlgas onto the bottom of the ampoule, and the incremental quenching [20].Very recently, a novel gas quenching procedure, called horizontal casting, has been shown to result in a marked improvement in compositionnal uniformity and crystalline perfection of the SSR grown MCT (x = 0.22) crystals [21]. Most of the time, casting is simply achieved in switching off the fiunace power after having brought the liquid temperature just some degrees above the liquidus. In such a case, the fine MCT dendritic microstructure is constituted of grains of - 0.1 mm size. The hegrain size favors recrystallization and minimizes the distances across which a boundary must travel in order to completely eliminate the grain. The crystallizationlanneal process occurs in isothermal conditions at a temperature just below the solidus temperature for periods of five to ten days, or in a thermal gradient which is expected to induce directionnal crystallization. It has been shown necessary to maintain the mercury pressure in the ampoule at a value lower than the equilibrium vapour pressure of the compound at that temperature in order to increase the crystallization rate. The explanation proposed was that the interdiffusion occurs by mean of mercury vacancies. The significance of this parameter will be emphasized also in the CdTe and ZnSe cases.

4. CdTe SSR GROWTH

In order to obtain large crystals of CdTe, and of its alloys with Zn, needed as substrate for MCT, the SSR growth of these materials has been studied. Energetically, this process is favored by the multi- grain state of the material, which is thermodynamically unstable. The use of SSR for CdTe has been made possible in obtaining, with parameters intentionally maladjusted during the CdTe growth by THM, a regular fine grain size structure, as shown in Fig. la. Ingots presenting such a microstructure have been used for SSR experiments and heated up under vacuum to the crystallization temperature Tc at about 4 OC/h in order to avoid the thermal strains due to an abrupt thermal change which could be a source of dislocations. In a fist experiment, Tc was very close to the meIting point, at 1060 "C. In spite of a sigdcant increase in the grain size, the crystals presented many gas bubbles. In a second experiment, Tc has been reduced to 990 OC and kept two months at this value. A cross-section of this ingot shows a grain occupying more than halfthe section, demonstrating the efficiency of the process (Fig. lb).

Fig. 1 a and b. Cross-sectional views of a CdTe ingot : (a) fine grain structure resulting fiom an intentional maladjustment of the CTHM growth parameters; (b) same ingot after solid state recrystallization. C3-146 JOURNAL DE PHYSIQUE IV

To our knowledge, SSR has never been used before for CdTe. The off-stoichiometric source material that was used, as a result of a THM growth in Te-rich conditions, is likely the reason why the CdTe SSR growth has been successfid, while it failed up to now. The presence in the material of Cd vacancies, or Te interstitials, allows to increase the crystallization rate, as mercury vacancies do (or Te interstitials ?) in the MCT case. It seems that the migration of the grain boundaries is made easier by the presence of Te precipitates decorating them Some kind of "THM process" at microscopic scale could be suggested. In the CdZnTe case, the presence of Zn, strengthening the lattice of CdTe as shown elsewhere [22], has been found to inhibit the SSR growth.

5. CdSe SSR GROWTH

CdSe has a high melting point of 1239°C which prevents fiom using classical melt-growth techniques, like the Bridgman one, in silica ampoules. It stems fiom the Cd-Se binary phase diagram that Cd and Se cannot be easily used as the solvent for CdSe [23]. Low temperature growth by SSR provides a good way to overcome these difliculties. After pre-synthesis of a Cd + Se mixture at 1100°C, the resulting boule, of microdendritic structure, is submitted to SSR at this same temperature in the same ampoule during several weeks. A significant increase in the grain size, sufficient for cutting crystals dedicated to some physical studies, is obtained 1241.

6. ZnSe SSR GROWTH

The ZnSe-based heterostructures, used for blue electroluminescent devices, lasers and LED'S, are usually grown on GaAs substrates.The short lifetime of the lasers is partially due to the mismatch of the epitaxial layers with the substrate. Even the small mismatch of 0.3 % between GaAs and ZnSe gives rise to a dislocation density at the substrate~layerinterface of about lo7 - 108 cm-2, which is excessive for acceptable usefd times. The growth of large, high quality ZnSe bulk crystals is thus quite topical in order to achieve the ZnSe homoepitaxy, but the difliculties in the ZnSe growth, mentionned above, make the production of ZnSe substrates confidential and extremely costly. We have used CVD grown polycrystalline ZnSe boules as source material for the SSR experiments. Such a material is produced industrially by chemical vapor deposition for making high-quality, low-cost aared transmitting materials, such as output windows for high energy lasers. The CVD process consists in the reaction of selenium hydride on zinc vapors at high temperature. As the result, a very fine grain structure is obtained, with micrograins of size 20 to 30 pm . In order to achieve the SSR experiments under different pressure regimes, the SSR was restricted to temperatures I 1000°C (compared to the melting point of ZnSe of 1520 OC). For higher temperatures under vacuum, the crystals were found to flow or to be transported to the cold end of the tubes. A significant result is the influence of the stoichiometry of the crystals on the crystallization rate. Large single crystals (- 8 cm3) have been obtained under selenium pressure, in agreement with the observations made on CdHgTe (low mercury pressures found to increase the crystallization rate) and CdTe (SSR successfid on Terich crystals). The crystallization rate has been found to be significantly reduced under zinc vapor pressure. Low temperature photoluminescence measurements performed on such crystals do not reveal any contamination coming fiom the SSR process. On the contrary, the Y and Z lines, respectively at 2.44 eV and 2.60 eV, that dominate the spectra of as-grown samples, disappear in the spectra of the annealed SSR samples which are then dominated only by excitonic emissions (fiee exciton at 2.804 eV, two very weak DOX lines at 2.8012 and 2.7974 eV, a AOXline at 2.7932 eV, and the prominent AOXline at 2.7836 eV with several phonon replica). The identiiication of some of these lines is in progress. The ZnSe crystal quality was investigated by double crystal X-ray dZEaction. The Saction occurred respectively on a (3 11) Si plane, for the first reflection, and on a (400) ZnSe plane, for the second reflection,The measurement was achieved through a large aperture of 2 x 4 mm2. A rocking-curve with a X-ray fdl width half maximum (FWHM) value of 20 arc sec., as shown in Fig. 2, was measured after orientation, cutting and mechano-chemical polishing on a (100) plane, indicating the excellent crystalline quality. The experimental conditions ((311) silicon and (400) ZnSe *action planes) are estimated to produce a rocking-curve FWHM broadening of about 6 arc sec.. X-ray topography was also achieved by reflection using the method of Lang. Remarkably uniform images (Fig. 3) are obtained for a X-ray beam divergence 160 sec.. X-ray topography has been also camed out by double *action on the small surface delimited by the aperture of 1 x 8 mm2 and by scanning a larger surface of the crystal by moving it in fiont of this aperture. Here also remarkably uniform images are obtained, as shown on figures 4a and 4b, for a X-ray beam divergence of only 5 sec.

7. CONCLUSIONS

Different reasons making the use of high temperature melt-growth of some 11-VI semiconductors not desirable have been described and illustrated with reference to some of these compounds : large separation of solidus and liquidus curves in the HgCdTe case, existence of phases transitions in the solid state for CdTe and ZnSe, melting point higher than the softening temperature of silica for CdSe, and high temperature contamination illustrated in the CdTe case. The interest of solid state recrystallization among the techniques of low temperature growth, mainly solution growth and vapour growth, has been discussed, with reference to CdTe and ZnSe. The SSR growth of HgCdTe has been described fiom results reported in the literature. SSR has been successfidly applied to CdTe, CdSe and ZnSe. A significant influence of the stoichiomet~yofthe crystals on the crystallization rate has been demonstrated. Large ZnSe crystals of high crystallographic perfection have been obtained, as shown by a rocking-curve halfwidth lower than 20 arc sec., and by very uniform X-ray topographies recorded with beam divergences of 60 arc sec and 5 arc sec, respectively by the method of Lang and by double Saction. Furthermore, the high purity of the ZnSe crystals has been shown fiom photoluminescence measurements. Solid state recrystallization, which combines the advantages of low temperature growth with the possibility of growing very large crystals, without any size restriction, appears as a very promising technique which could renew the crystal growth of some topical II-VI compounds. C3-148 JOURNAL DE PHYSIQUE IV

Fig. 2 - X-ray rocking-curve measured on a ZnSe crystal grown by Solid State Recrystallization.

Fig. 3 - X-ray topography recorded by the method of Lang on a ZnSe crystal grown by Solid State Recrystallization. (x 10, divergence of the X ray beam I 60 sec).

Fig. 4a and b - X-ray topography images recorded by X-ray double &action on a ZnSe crystal grown by Solid State Recrystallization (x 10) . Images recorded through an aperture of 8x 1 mm2 (a), and by scanning of the aperture on the surface (b). Acknowledgements. The authors are pleased to warmly acknowledge Dr. C. Ard, from U-VI Incolporated, for kindly providing the excellent ZnSe source material, and Dr. T. Nguyen Duy, from Societb Anonyme de Tel~communications,for giving usefid informations on the HgCdTe SSR process and for valuable discussions.

References

[l] S.G. Parker and H. Kraus, US Patent No. 1,284,419 (1969). [2] RE. Johnson, US Patent No 1,284,419 (1969). [3] M.J. Brau, US Patent 3,656,944 (1973). [4] M.J. Brau and RA. Reynolds, US Patent No 3,849,205 (1974). [5] F. Bailly, J. Physique, 27, (1966)335. [6] V.A. Leibov, Yu. M. Ivanov & A.V. Vanyukov, Inst. Stali. Splavov, Moscow, 1976, Deposited Doc. 1976, Viniti 1606-76. [7] E.KendaIl and J. Hendom, Phys. Lett., 8 (1964)237. [8] J.Appell, Naturfos., 9 (1954) 265. [9] P. Hoschl and C. Konak, Czechoslov. J. Physics, 13 (1963) 850. [lo] RTriboulet, MateriaZsForum, 15 (1991) 30. [l 11 Y.M. Ivanov, Moscow Steel and AUoys Institute, private communication. [12] G. J. Rosen, PhD thesis, 1994. [13] E.J.M. Kendall, Phys. Letters, 84 (1964) 237. [14] M.P. Kulakov, V.D. Kulakovskii, I.B. Savchenko and A.V. Fadeev, Sov.Phys.-Solid State, 18 (1976) 526. [15] A. El Mokri, R Triboulet, A. Lusson, A. Tromson-Carli, G. Didier, J. Clystal Growth, 138 (1994) 168. [16] R Triboulet, Semicond. Scz. Technol. 6 (1991) A18-23. [17] A.M. Akhekyan, V.I. Kozlovskii, Yu.V. Korostelin, A.S. Nasibov, Yu.M. Popov and P.V. Shapkin, Sov. J. Quantum Electron. 15 (1985) 737. [IS] J.L. Boone, G. Cantwell, W.C. Harsch, J.E. Thomas and B.A. Foreman, J. Crystal Growth, 139 (1994) 27. [19] RTriboulet, T. Nguyen Duy and A. Durand, J. Vac. Sc. Technol.,A3 (1985) 95. [20] L. Colombo, A.J. Syllaios, RW. Perlaky and M.J. Brau, J. Vacuum Scz. Technol. A3 (1985) 95. [2 11 RK Sharma, V.K Singh, N.K Nayyar, S.R Gupta and B.B. Sham, J. Crystal Growth, 13 1 (1993) 565. [22] K Guergou* RTriboulet, A. Tromson-Carli and Y. Marfaing, J. Crystal Growth, 86 (1988) 61. [23] A. Reisman, M. Berkenblit and M. Witzen, J. Phys. Chem, 66 '1962) 2210. [24] R. Triboulet and G. Didier, to be published.