We have grown compositionally graded GexSi1−x layers on Si at 900 °C with both molecular beam epitaxy and rapid thermal chemical vapor deposition techniques. Triple-crystal x-ray diffraction reveals that for 0.10<x<0.53, the layers are totally relaxed. GexSi1−x cap layers grown on these graded layers are threading-dislocation-free when examined with conventional plan-view and cross-sectional transmission electron microscopy. Electron beam induced current images were used to count the low threading dislocation densities, which were 4×105±5×104 cm−2 and 3×106±2×106 cm−2 Eq. 2×106 cm−2 for x=0.23 and x=0.50, respectively. Photoluminescence spectra from the cap layers are identical to photoluminescence from bulk GexSi1−x.

A modulation-doped Si/GexSi1−x structure was fabricated in which a thin Si layer is employed as the conduction channel for the two-dimensional electron gas. The strained heterostructure is fabricated on top of a low threading dislocation density, totally relaxed, GexSi1−x buffer layer with a linearly graded Ge concentration profile. The mobility of the two-dimensional electron gas as determined from Hall measurements was 1600 cm2/V s at 300 K and 96 000 cm2/V s at 4.2 K. Recently, a 4.2 K mobility of 125 000 cm2/V s was observed from a similar sample.

Relaxed Si1−xGex buffers on Si have yielded record low-temperature mobilities for both electrons and holes in the Si–Ge system. We analyze various limitations on this mobility, including scattering from remote dopants, background impurities, interface roughness, alloy fluctuations, and the specific strain, morphology, and threading dislocations expected for relaxed alloy buffers. Comparison with experiments eliminates all but the first four as potential limitations on the mobility.

Articles you may be interested inMobility and metal-insulator transition of the two-dimensional electron gas in SiGe/Si/SiGe quantum wells

We have used the hot-carrier Shubnikov–de Haas effect to measure the power loss by hot two-dimensional holes in coherently strained Ge0.2Si0.8/Si heterostructures. The measured power loss versus carrier temperature data are best described by the two-dimensional formalism of P. J. Price [J. Appl. Phys. 53, 6864 (1982)] assuming weak screening. Excellent agreement with experiment is obtained only if scattering of the acoustic mode phonons by both the deformation potential and the piezoelectric coupling mechanisms are considered. We are therefore able to deduce a value for the piezoelectric constant for Ge0.2Si0.8 which is approximately 35% of that for InAs (e2pz≊0.22×108 dyne/cm2). In light of the fact that charge transfer effects are expected to be small in bulk (unstrained) GexSi1−x, the present observations are indicative of either a large strain induced change in ionicity or of scattering of acoustic phonons from ordered domains via the piezoelectric mechanism.