dewetting application to cdte single crystal growth on earth
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Journal of Crystal Growth 261 (2004) 590–594
*Corresp
7682-5249.
0022-0248/
doi:10.101
Letter to the Editors
Dewetting application to CdTe single crystal growth on earth
Nicolas Chevaliera, Pierre Dusserrea, Jean-Paul Garandeta, Thierry Duffarb,*aDTEN/SMP/LESA, Commissariat "a l’Energie Atomique, 38054 Grenoble, France
bEPM-Madylam, ENSHMG, BP 95, F-38402 Saint Martin d’H!eres, France
Received 12 November 2002; accepted 24 October 2003
Communicated by D.T.J. Hurle
Abstract
The dewetting technique, based on the application of a gas pressure in order to prevent crystal crucible contact during
Bridgman growth, has been applied to the growth of CdTe. After optimization of the set up and growth parameters,
two CdTe single crystals, 14mm in diameter and 50mm in length, have been obtained, the first one from a twinned seed
and the other one from a polycrystalline seed. These results show that the dewetting technique has a great potential for
the growth of II–VI single crystals.
r 2003 Elsevier B.V. All rights reserved.
PACS: 81.05.Dz
Keywords: A1. Dewetting; A2. Bridgman technique; A2. Single crystal growth; B1. CdTe
1. Introduction
The technological potential of CdTe is verylarge (IR, X- and g-ray detectors, photorefractivesensors) but commercial applications are limitedbecause it is very difficult to grow single crystals,which are mandatory for most of the applications.CdTe is a difficult material in terms of crystalgrowth because of its very low stacking faultenergy (facilitating twin occurrence), critical re-solved shear stress (dislocations are easily multi-plied) and its low thermal conductivity (meaningthat temperature fluctuations cannot be easily
onding author. Tel.: +33-4-7682-5213; fax: +33-4-
address: [email protected] (T. Duffar).
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6/j.jcrysgro.2003.10.094
smoothed out). Presently, the yield of single crystalgrowth is limited to 10%; however the industrialneed is more and more toward large highstructural quality single crystals, which meanshigh purity, no grains or twins, low dislocationdensity and a homogeneous chemical composition.Among the available growth techniques, theBridgman or VGF processes give the best results,but the yield is limited by the inherent contactbetween the crucible and the crystal.
However, the growth of crystals, by the Bridg-man or VGF techniques, without contact withthe crucible has been observed by almost allthe investigators who grew crystals under micro-gravity conditions. An explanation of the phenom-enon has been given in the case of rough andsmooth crucibles [1]. This phenomenon is called
d.
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dewetting. As a consequence of the suppressionof crystal-crucible interactions, most of the cry-stals grown in space have shown a better structuralquality than crystals obtained on the earth underthe same conditions. It has recently been discov-ered that the dewetting phenomenon can also beobtained on the earth, provided that a gaspressure, more or less equal to the hydrostaticpressure in the liquid, is applied between thecrucible and the crystal [2]. On this basis, GaSbsingle crystals have been obtained reproducibly,with a structural quality close to the initial seed,in closed silica ampoule where the gas pressurewas controlled by controlling the temperature ofthe gas volume situated at the cold side of theampoule [3].
Obviously the dewetting process is a goodcandidate for the growth of CdTe single crystalsas it can be expected that the crystal quality mightbe dramatically improved. Indeed, CdTe crystalgrowth experiments under microgravity conditionshave shown such an improvement in the presenceof dewetting [4,5]. The present paper deals with theground-based application of the dewetting processto the growth of CdTe single crystals.
2. Experimental set-up and method
The crystal growth equipment is based on theclassical vertical gradient freeze process but hasthree heating zones (cylindrical furnaces) and avacuum tight silica ampoule containing the CdTesample and the single crystal seed. In the previousexperiments on GaSb single crystal growth bythe dewetting technique [3], an optical windowplaced between the two upper furnaces per-mitted observation of the liquid-crystal-cruciblemeniscus. However it rapidly appeared that,due to the high melting point of CdTe, thisgenerated a thermal perturbation in the sampleand it was decided to put the two furnaces in directcontact, without window or thermal insulation inbetween.
The CdTe samples are 13.95–14mm in diameterand some single crystal seeds have been used. Thesilica ampoule (inner diameter 14mm, length500mm) is filled with a polycrystalline CdTe feed
(about 80 g) and with a little piece of pure Cd(about 15mg) to control the Cd partial pressurewithin the ampoule and thus to prevent the CdTedecomposition at high temperature. Silica piecesare introduced in order to fix the positions of thevarious sample parts (Fig. 1). The ampoule isevacuated to 10�6mbar, then filled with argonat about 850mbar and sealed. A second, con-centric, silica tube is placed around the ampoule inorder to avoid projection of toxic material in caseof ampoule cracking. Five thermocouples areplaced between the two silica tubes, along theampoule.
In order to satisfy simultaneously a partial seedmelting, a thermal gradient of 10�C/cm in thesample and to get a totally solidified crystal atthe end of the cooling phase, an optimisedinitial temperature setting was selected: 1100�C,915�C and 890�C respectively for the threefurnaces. A thermal soak of 1 h ensures thermalhomogenisation. Applying a cooling rate of 2�C/hgives a growth rate varying between 3 and1.870.1mm/h during the process. The wholeprocess lasts 47 h.
3. Experimental results
Two CdTe crystal growth experiments havebeen performed during the initial stage ofthis study, with a non-optimised thermal profile.They gave relatively bad results; dewetting wasnot noticeable (shiny surface of the samples,showing contact with the silica), poly-crystalswere obtained and deposition of solid particleson the ampoule walls occurred, preventing thesample from moving in the ampoule. However,a number of interesting results were obtainedon these samples: characterisation of the solidparticles on the walls, by X-ray diffractionand electron microscopy, showed they consis-ted of stoichiometric CdTe. This showed thatthe effect of the pure Cd inserted in the am-poule is sufficient and that the stoichiometry ofthe sample is maintained. IR microscopy of thesesamples has shown inclusions 10–20 mm indiameter, both at the surface and inside thesamples. The IR transmission curve, for these
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Un-molten part
Fig. 2. The first CdTe sample processed with the improved thermal conditions.
Silica additional parts
Security tube
Processed sample
Unmolten feedor seed
Silica ampoule
TC 1
TC 2
TC 3
TC 4
TC 5
Pure Cd
Fig. 1. Details of the set-up and picture of the silica ampoule after processing of a CdTe sample.
N. Chevalier et al. / Journal of Crystal Growth 261 (2004) 590–594592
samples, is classical and gives a forbidden gapwidth of 1.4770.2 eV. IR macroscopy shows thatthe mechanical stresses are not uniformly distrib-uted inside the samples.
The thermal field and growth parameter set wasthen improved and set to the values given inSection 2. A solidification experiment, on aninitially totally polycrystalline sample, has been
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Un-molten seed
Twins
Growthdirection
Fig. 3. Structural quality of the beginning of the second CdTe crystal, after cutting, lapping and chemical etching.
N. Chevalier et al. / Journal of Crystal Growth 261 (2004) 590–594 593
performed and satisfactory results have beenobtained (Fig. 2):
* No solid deposition on any part of the ampoule.* The sample was totally free to move inside the
crucible and its unpolished aspect was anindication that dewetting occurred, on most ofthe sample surface.
* The sample broke along a {1 1 1} cleavageplane, on 3
4th of its surface, thus showing that
it reached a single crystal structure at this point.
Characterisation of this sample is on-going inorder to carefully determine the behaviour of theinitial grains along the solidification axis.
For the second experiment, a single crystal seed,with twins, has been used. The initial cooling ratewas decreased to 0.7�C/h in order to limitmechanical stresses during solidification, and a5�C/h cooling phase has been added, to avoid toolarge thermo-elastic stresses in the still plasticmaterial. Consequently, the experiment took 173 hto perform. Dewetting has also been obtained, atleast partially, and in spite of the twins in the seed,a single crystal has been obtained at the beginningof the growth (Fig. 3). A chemical attack (100mllactic acid, 8ml nitric acid, 4ml fluorhydric acid)along the remaining part of the sample did notreveal other grains or twins. Thus it appears thatthis sample is a single crystal over its full length.More in depth characterisation is under way.
4. Conclusion
The objective of this research was to study thedewetting crystal growth conditions necessary toget contactless growth of CdTe in silica ampoules,in order to grow single crystals. The approach hasessentially been experimental with a carefulcharacterisation of the thermal field inside andaround the sample. After calibration of the set-up,growth conditions have been optimised, typically athermal gradient of 10�C/cm and a growth rate of2mm/h, in order to get the dewetting phenomenonon the CdTe samples.
Analyses performed on samples obtained under anon-optimised thermal process that prevented thedewetting phenomenon to occur, have shownclassical inclusion sizes and IR transmission curvesclose to standard material. After improvement ofthe thermal process, the results obtained during thetwo first experiments are encouraging. The deposi-tion of solid CdTe particles has been prevented,thus allowing the samples to move freely within theampoule. These samples have an unpolishedexternal surface aspect typical of dewetting. Com-plementary analysis is under way in order todefinitively assess the quality of the grown crystals,preliminary characterisation of the two samplesshows that a single crystal has been obtained from atwinned seed and that the structural quality of thesample grown from a polycrystalline seed has beendramatically improved. Therefore the dewetting
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crystal growth process, as a variant of the verticalgradient freeze technique, is a very promising toolfor the growth of CdTe single crystals with a highyield on the earth.
Acknowledgements
The authors express their gratitude to Dr.Pellicari, CEA LETI/LIR, and Dr. M. Fiederle,University of Freiburg, Germany, for the CdTeseed and feed materials provided for this study andtheir help in the characterisation of the samples.This study has been partly funded by theEuropean Space Agency, under the MAP contractnumber MAP-99-035.
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