The Mechanisms of Relaxation in Strained Layer GeSi/Si Superlattices: Diffusion vs. Dislocation Formation
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THE MECHANISMS OF RELAXATION IN STRAINED LAYER GeSi/Si SUPERLATflCES: DIFFUSION VS. DISLOCATION FORMATION.
F.K. LeGoues, S.S. Iyer, K.N. Tu, and S.L. Delage IBM T.J. Watson Research Center, Yorktown Heights, New York 10598, USA
ABSTRACT SiýGe,_1 strained layer superlattices are known to be metastable in that they can be grown fully commensurate with layer thickness higher than the equilibrium, calculated T. at which dislocation formation becomes energetically favorable. In this paper, we describe the mechanism of relaxation in such multilayers. Both plane-view and cross-sectional transmission electron microscopy (TEM) were used to examine the formation of dislocation at the different interfaces. RBS was used to follow interdiffusion. We found two competing mechanisms for relaxation: The preferred mode for relaxation is the creation of dislocation networks at each of the interfaces. This process can be stopped or considerably inhibited by the difficulty of forming new dislocations in samples which are perfectly commensurate after growth; Some dislocations appear necessary in order to generate more dislocations during annealing. When this is not the tase, the only possible way to attain relaxation is through diffusion. In such a case, stress-enhanced diffusion is observed, with a diffusion coefficient 200 times higher than expected.
L INTRODUCTION Recently, a considerable amount of work has been published on strained-layer semiconductor superlattices. The interest in these structures comes from the possibility of modifying the band structure and enhancing mobility for specific multilayers compos-
itions and thickness. This has indeed been proven in the Si1Ge,, /Si case'. More recently, the SiGe superlattices have received renewed interest due to the demonstration of an heterojunction bipolar transistor'. Van de Merve proposed3 , based on equilibrium calculations and using the idea that, at a critical thickness T, it will be energetically more favorable to create dislocations than to strain the layer as a whole. In semiconductors, the theories generally predict value of T. much lower than experimentally observed, because of the difficulty of forming dislocations at the low growth temperatures that can now be attained by MBE and CVD 45..6. These metastable strained layer superlattices may however relax during thermal annealing'. Two mechanisms compete during this relaxation process: The first one is the introduction of dislocations at the interfaces. The second one is interdiffusion between the layers. As was pointed out by Matthews and Blakeslee'. 9.'0 , the interface between the substrate as a whole is inherently different from the individual interfaces in the superlattice because two types of misfits need to be con-
sidered: one is between the substrate and the superlattice as a whole, and the other one is between individual layers in the superlattice. It was indeed shown experimentally that there can be dislocation formation at the substrate/superlattice interface even in cases when the individual layers are c
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