A INTRODUCTION

The binary alloys of silicon and germanium form a continuously variable system with a wide range of energy gaps and thus optical properties. Since the pioneering work of Braunstein et al [1] it has been known that for the lowest lying conduction bands the crossover from the ?(6) states to the L(4) states occurs for a germanium content as high as 85%. Consequently, the conduction band structure of bulk or relaxed SiGe alloys is silicon like for a wide range of alloy compositions.

For epitaxially grown Si _1-xGe _x films the band structure is drastically altered by the built-in strain which is fixed by the lattice constant of the substrate as long as the growth is pseudomorphic (lattice matched). Since the lattice mismatch between Si and Ge is about 4%, the critical thickness for pseudomorphic growth, which depends also on the growth conditions, decreases quite rapidly with increasing Ge content. For a pseudomorphic SiGe layer grown along the [001] direction, the biaxial strain causes a tetragonal distortion which shifts and splits the valence and conduction band edges and thus alters the energy gap.

In Section B experimental findings on the indirect minimum bandgap are summarized and compared with theoretical results for the bulk alloys and for Si _1-xGe _x films grown on Si substrates. Experimental data on the direct energy gaps are...