The Stability of Si-Si 1-x Ge x Strained Layer Heterostructures.
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THE STABILITY OF Si-Sil_xGex STRAINED LAYER HETEROSTRUCTURES. D.C.HOUGHTON*t, J-M.BARIBEAU*, K.SONG* and D.D.PEROVICt. *National Research Council of Canada, Ottawa, Ontario, KIA OR8, CANADA. tUniversity of Toronto, Toronto, Ontario, Canada, M5S 1A4. tBritish Telecom Research Labs, Martlesham Heath, Ipswich, IP5 7RE, UK. ABSTRACT The structural stability of strained layer superlattices (SLS's) is addressed using an equilibrium model and then compared to the stability of single strained layers. Relaxation mechanisms are described for various superlattice geometries. The application of a critical thickness/strain criterion to define stability limits was found to be very useful in predicting the detailed relaxation process. The competition between relaxation by misfit accommodation at the base of the SLS and at individual strained interfaces is considered for the initial condition of full coherency and after partial relaxation. Experimental data for the Si-Ge strained layer system are presented; as-grown by MBE and after annealing in 0 0 the temperature range 500 C - 900 C. The extent of relaxation and the detailed dislocation structure within the SLS's were determined by X-ray rocking curve analysis, Nomarski interference microscopy and transmission electron microscopy. The abrupt changes in relaxation behaviour indicate that rigid boundaries between stable and metastable structures do exist, as predicted by the equilibrium models. INTRODUCTION Strained layer epitaxy has recently attracted considerable attention, due to the discovery of novel optoelectronic properties in strained semiconductors and the developments in heteroepitaxy, notably GaAs on Si. Many theoretical suggestions have been put forward [1-6] over the past 25 years to describe the stability of such strained heterostructures. If working devices are to be fabricated from strained material, their structural stability during anneal cycles similar to those experienced during implant activation and contact sintering must be determined. Although experimental data exist for strain relaxation in several material systems, their interpretation is hampered by the poor resolution of many experimental techniques in detecting small strains [7]. The precise point at which plastic relaxation begins is difficult to identify in epilayers grown at low temperatures, since large barriers to nucleation and propagation of misfit dislocations exist [5]. Annealing studies are thus useful since the introduction of thermal energy allows the structure to approach equilibrium. The upper limit to structural stability is an important parameter since the position of this boundary determines the driving force available for plastic relaxation processes. In this work an expression is derived for the stability of strained multilayers using an energy minimization approach [1,2]. Experimental data are presented which confirm the abrupt transition between stable and metastable structures defined by a critical thickness/strain criterion. THE STABILITY OF STRAINED LAYER SUPERIATTICES. Two poss
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