The effect of alloy scattering on the mobility of holes in a Si 1−x Ge x quantum well is considered from a theoretical perspective. Issues relating to the definition and value of the alloy potential are discussed, and results are presented showing how the alloy scattering component of the overall mobility varies with temperature, carrier density and alloy concentration. Screening is allowed for and found to be important at low temperatures. By considering other scattering mechanisms such as interface impurities, surface roughness and acoustic phonons, we argue that alloy scattering is not a significant limiting factor as regards device applications, contrary to recent claims.
A detailed comparison is made between theory and experiment for the low-temperature mobility of holes in gated oxide, coherently strained Si/SiGe heterostructures. We conclude that the mobility is mainly limited by interface impurities, conventional surface roughness and strain fluctuations; by contrast, we argue that alloy scattering is comparatively weak. Comments regarding possible mobility degradation due to oxide formation are also made.
The 4 K Hall mobility has been measured in a top-gated, inverted, modulation-doped Si/Si0.8Ge0.2 structure having a Si:B doping layer beneath the alloy. From comparisons with theoretical calculations, we argue that, unlike an ordinary enhancement-mode SiGe p-channel metal–oxide–semiconductor structure, this configuration leads to a decrease of interface roughness scattering with increasing sheet carrier density. We also speculate on the nature of the interface charge observed in these structures at low temperature.
The hole mobility has been measured in a metal-oxide semiconductor field-effect transistor featuring a 6 nm Si 0.5 Ge 0.5 conduction channel. Physical characterization and analysis of the electrical properties confirm that the channel is fully strained. At a total sheet carrier concentration, p s , of 10 12 cm −2 , the 300 K mobility of 220 cm 2 V −1 s −1 is double that of a Si control device (and at 4 K has a peak value of 1800 cm 2 V −1 s −1 ). This improvement is largely maintained up to p s ∼ 10 13 cm −2 , despite the onset of conduction in a parallel Si channel at the oxide interface. Comparing the measurements with calculations suggests that the mobility is affected more strongly by interface roughness than by alloy scattering.
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