Heterostructures of Si 0.80 Ge 0.20 /Si (100) were grown by pulsed laser deposition consisting of alternating 10 nm SiGe and silicon layers for a total of 10 periods. The presence of alloy clustering at SiGe/Si interfaces was investigated by parallel transport in transistor structures. Photoluminescence line broadening indicated the presence of Ge-rich clusters (20-30 nm) in layers grown at a substrate temperature of 600 • C using a excimer laser at 248 nm wavelength.
The 2D clusters were modeled, assuming a spherical shape and a potential of the form δ U where δ is the composition variation and U is the disorder potential. A scattering matrix element was derived which included intraband scattering. Transport effects were calculated using Monte Carlo techniques. Of interest was lateral transport in a field effect transistor configuration. Therefore, velocity versus field curves were calculated where electron motion is confined to the 2D-like Si/SiGe interface layers. Using concepts of charge control, current versus voltage relationships were derived. The derived models showed that for the case of a 250 µm conducting channel lateral length with 2D clusters evenly placed at every 10 nm, the transconductance increase is 25% and is a function of both cluster size and separation.
In order to measure the effect of clustering, high electron mobility transistors were fabricated with two conducting 2D channels consisting of Si/SiGe/Si grown on high resistivity (100) silicon. Using 0.5 µm gates, a transconductance of 125 mS/mm was obtained in transistors without alloy clustering which decreased to less than 80 mS/mm when alloy clustering was present. The present investigation has related the presence of 2D nanoclusters to device performance and has also shown agreement between the experimental results and theoretical calculations.