Resonant tunneling in the silicon/silicon(1-x) germanium(x) material system.

摘要

We have investigated Si/Si{dollar}sb{lcub}rm 1-x{rcub}{dollar}Ge{dollar}sb{lcub}rm x{rcub}{dollar} hole resonant tunneling diodes (hole-RTDs) grown by molecular beam epitaxy. These have consisted of a Si{dollar}sb{lcub}rm 1-x{rcub}{dollar}Ge{dollar}sb{lcub}rm x{rcub}{dollar} quantum well between thin Si barriers. Growth and processing conditions have been studied, and temperature dependent and magnetotransport measurements have been carried out. Samples grown on relaxed Si{dollar}sb{lcub}rm 1-x{rcub}{dollar}Ge{dollar}sb{lcub}rm x{rcub}{dollar} buffer layers display large threading dislocation densities in cross-sectional TEM. These are avoided in fully pseudomorphic structures, where we obtain good I-V characteristics in samples with graded Ge concentration in the spacer layers. Structures grown at different temperatures indicate the importance of obtaining abrupt interfaces at the barriers. Short term annealing at low temperatures ({dollar}approx{dollar}500{dollar}spcirc{dollar}C) is shown to destroy the resonances. We believe this is due to monolayer interdiffusion at the barriers, destroying the abruptness of the interfaces.; Below T {dollar}approx{dollar} 70 K the I-V characteristics of the investigated structures are relatively temperature independent, with a largest peak-to-valley ratio of 2:1. At higher temperatures, the resonances are seen to disappear. Temperature dependent transport through strained Si/Si{dollar}sb{lcub}rm 1-x{rcub}{dollar}Ge{dollar}sb{lcub}rm x{rcub}{dollar} hole-RTDs show that the quenching of the negative differential resistance at room temperature is due to thermally assisted tunneling through higher resonant states.; We demonstrate a novel technique, angle-resolved magnetotunneling spectroscopy, to probe the dispersion relations and in-plane anisotropies of the valence band quantum well states. With a magnetic field parallel to the interface, we show that tunneling occurs through states removed from k = 0, and by rotating the field in the plane of the interfaces, large anisotropy in different crystal directions is detected, mirroring the anisotropies in the bulk valence band. The spectra of structures grown on relaxed Si{dollar}sb{lcub}rm 1-x{rcub}{dollar}Ge{dollar}sb{lcub}rm x{rcub}{dollar} buffer layers indicate inhomogeneous strain relaxation. A magnetic field perpendicular to the interfaces resolves some Landau level splitting. Most strikingly, however, is the similarity in the spectra with the case when the magnetic field is applied parallel to the interfaces. This indicates broadening of the levels, possibly due to scattering, and the importance of both 2-dimensional and 3-dimensional band structure effects.