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The influence of growth atmosphere on the self-selection of the grains during ZnO crystal growth

Paweł Skupiński 1Krzysztof Grasza 1,2Elżbieta Łusakowska 1Wojciech Paszkowicz 1Anna Reszka 1Rafał Jakieła 1Andrei Avdonin 1Emil Tymicki 2Katarzyna Racka-Dzietko 2B. J. Kowalski 1Andrzej Mycielski 1

1. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland
2. Institute of Electronic Materials Technology (ITME), Wólczyńska 133, Warszawa 01-919, Poland



    The self-selection of grains during crystal growth is a very useful phenomenon which enables one to obtain large size single crystals without using a seed during the first stage of the growth process. In this work the ZnO crystals were grown by chemical vapour transport (CVT). The hydrogen gas was used as the transport agent. Initially the growth processes were carried out in the zinc-rich growth atmosphere (a mixture of zinc vapour and molecules composed of oxygen and hydrogen). The overabundance of one of the crystal forming components, the zinc,  results in a diffusion barrier for the second crystal forming component, that is oxygen bound with hydrogen. Such condition ensures stable growth of the crystals. However, the self-selection of crystal grains in such case is poor. The number of grains in the grown crystals is strongly dependent on the first stage of growth process, the spontaneous seeding. To avoid the problems with the randomly appearing seeds in this first stage of the growth process, a specially prepared piece of crystal (seed) mounted inside the growth ampoule was used. This approach allowed us to demonstrate that the Zn-terminated (0001) surface of the crystallization front gives the highest growth rate and the largest single grain volume [1].
    In order to achieve an improvement of the self-selection, the influence of a variety of chemical reagents added to the growth atmosphere, on the growth conditions was investigated.  Crystals obtained as the result of the most successful experiments were polycrystalline but with largest single grains exceeding a half of the area of the crystallization front. Even in the case of spontaneous seeding, where more than ten seeds appeared at the beginning of the growth process, in 80% of the experiments one large grain was found at the crystallization front of the resulting crystal at the end of the growth. The halogens and their components with manganese, when added to the growth atmosphere were found to give much better and reproducible results. In this case the largest grains of the crystal were oriented along directions other than [0001].
    These observations were confirmed in experiments carried out with two differently oriented seeds, placed together into the same growth ampoule. These experiments allowed to compare the growth rate for various crystallographic directions in different growth ambiences. We believe that in the CVT growth of ZnO the improvement of the self-selection by addition of halogens and their components is a result of combining of two phenomena: (i) the increase of the diffusion barrier and (ii) the passivation of some of the ZnO crystallographic surfaces by the added components. The increase of the diffusion barrier caused by the presence of added components made it possible to improve the growth stability. The grains growing in the crystallographic directions which are blocked by the added components are being quickly overgrown by the other more favourably oriented grains. This passivation effect is responsible for the decreasing of the number of  grains in the crystal.
    In our report, an analysis of the  influence of growth atmosphere composition on the self-selection of the crystal grains will be presented. The findings will be supported by microscopic investigations, X-ray diffraction and secondary ion mass-spectrometry studies of the crystals.

[1] P. Skupinski, K. Grasza, A. Mycielski, W. Paszkowicz, E. usakowska, E. Tymicki, R. Jakiea, B. Witkowski, phys. stat. sol. (b), 247 (2010) 1457
This work was partially supported by the National Science Centre in Poland, grant UMO-2011/01/D/ST7/02657


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Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 3, by Paweł Skupiński
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-12 17:08
Revised:   2013-05-06 12:52