RIETVELD ANALYSIS OF POLYTYPIC (Zn,X)Se SOLID SOLUTION

W. Paszkowicz¹, R. Minikayev¹, F. Firszt², S. Łęgowski², H. Męczyńska², and
A. Marasek²

¹Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46,
02-668 Warsaw, Poland

²Institute of Physics, N. Copernicus University, ul. Grudziądzka 5, 87-100 Toruń, Poland

ZnSe based solid solutions are components of heterostructures which are of interest in optoelectronic technologies as components of blue-green lasers and UV-visible photodetectors. The occurrence of crystal defects leads to structure degradation of Mg-containing layers. Studies of polytypism in this kind of crystals may lead to a better understanding of phase relationships and defect nature.

Higher polytypes occur for those tetrahedrally bonded semiconducting compounds, for which the formation energies of wurtzite (2H) and sphalerite (3C) polymorphs do not differ much. For semiconducting solid solutions, the polytypes appear close to the transition point between the wurtzite and sphalerite phases, and their formation may be influenced by the thermal history of the crystal (in annealed crystals, the contribution of higher polytypes is markedly reduced). At the vicinity of the transition, an enhanced tendency to form stacking faults is observed. An X-ray and electron microscopic study [1] has allowed for systematising the conditions at which the higher polytypes (4H and 8H) form in bulk Zn_1xMg_xSe crystals. Additional substitution of Zn atoms by Be atoms was expected to cause an increase of the bandgap value as well as to improve the resistance against defect propagation.

In this work, Rietveld refinement was performed for Bridgman-grown (ZnSe based) crystals exhibiting higher polytypic structures 4H and 8H and for polytypic mixtures involving these phases. Phase analysis[1] has indicated the polytypes occurring in some of those crystals. In Zn_0.83Mg_0.17Se and Zn_0.84Mg_0.12Be_0.04Se crystals 8H phase with a contribution of 3C phase have been identified, while for Zn_0.82Mg_0.18Se a 'pure' 4H polytypic structure has been found. For (Zn,Mg,Be)Se, the polytypism is observed for a specific Mg and Be content.

[1] W. Paszkowicz, P. Dlużewski, Z.M. Spolnik, F. Firszt, and H. Męczyńska, J. Alloys Compds 286 (1999) 224

[2] M. Aoki, H. Yamane, M. Shimada, T. Kajiwara J. Alloys Compds. 364 (2004) 280