Strain Relaxation and Dislocations in SiGe/Si Structures

Epitaxially grown lattice mismatched structures are of increasing importance for microelectronic and optoelectronic devices and circuits fabricated using a variety of semiconducting materials. For some applications, the strain in the epitaxial layers is intentionally relieved by the introduction of misfit dislocations, and the film has an in-plane lattice parameter close to that of the bulk material. For example, a fully relaxed Si(sub0.7)Ge(sub0.3) layer is required as a buffer layer for high mobility field effect transistors having strained Si or strained SiGe channels. In this case it is necessary to control the growth conditions and the design of the structure so as to minimize the density of dislocations threading through device layers grown on top of the relaxed buffer layer. The need to prevent strain relaxation in thin layers or to control the density and distribution of defects in intentionally relaxed structures has lead to extensive research on strain relaxation mechanisms and on the properties of the defects which are required to relieve the strain. This article will focus on recent work on strain relaxed SiGe/Si structures. Two different relaxation mechanisms occurring at low mismatch strain and low growth temperature will be discussed, and the characteristics of step-graded buffer layer strcutres will be reviewed. Effects of dislocations on the electronic properties of relaxed SiGe layers will also be discussed.