Finite layer thickness and disorder effects in Raman scattering from ultrathin Ge films

here is considerable interest in the properties of structures composed of multiple layers of different compositions of the Si1-xGex system because of speculation that such structures can have significant new properties. For example, it has been claimed theoretically that Si4.Gem superlattices can support direct optical transitions near or at the fundamental gap, and there have been several experimental claims of the observation of such transitions in Si4Ge4 and Si4Ge4 superlattices grown on several different types of substrates.1,2 Recently, it has been shown that electrons confined to strained Si layers grown on high-quality, relaxed, Si0.7Ge0.3 buffer layers can display enhanced mobilities with published values now in excess of 105 cm2, V-1 - s-1 at low temperatures.3,4 A major problem in achieving such interesting new properties in semiconductor structures is the characterization of the individual layers that are combined, to create the structures. Of particular interest is the composition of the layers, especially with regards to interdiffusion across their interfaces, the magnitude of the strain in the layers and the atomic perfection of the layers.

© 1992 Optical Society of America