Electrical transport in strained silicon quantum wells on vicinal substrates

This thesis deals with the electrical transport studies of strained Si quantum wells grown on tilted Si substrates. Magnetotransport measurements at very low temperatures are used to investigate the high electron mobility, scattering processes and modified band structure for four different substrate orientations (2, 4, 6 and 10 deg.) and in two different directions of transport. We first discuss the morphology of the tilted system with the aid of, atomic force and optical microscopy. A clear change of surface morphology of tilted layers in comparison with the (001) type surfaces is explained by the degree of tilt in the system. The electron mobility and in-plane effective mass becomes anisotropic, which scale roughly with the tilt angle. The mobility anisotropy is shown to be the result of extra scattering when electrons travel across the steps common to vicinal surfaces. The extra scattering has characteristics similar to interface roughness scattering, as inferred from the trend that the transport (τt) and quantum scattering (τq) times follow. As the tilt angle grows, it is found that τt/τq→1 in the direction perpendicular to the steps. Electrons in tilted channels of multivalley semiconductors can involve a new interband scattering mechanism due to a one dimensional minigap opening in the conduction band. This effect, known from bulk Si MOSFETs, is investigated in strained Si for the first time in this thesis. First, the effect of applied electric fields on electron conduction is considered. Shubnikov-de Haas oscillations in the magnetoresistance data indicate a remarkably different electron scattering behaviour in tilted samples with increasing fields in directions parallel and perpendicular to the tilt direction. An FFT analysis of the data produces extra peaks in the electron density spectra. By clear contrast, flat samples grown under similar conditions do not show any unusual features. The difference is attributed to the existence of a minigap in the conduction band of the tilted samples. We also investigated the effect of reduced dimensionality on the properties of the tilted 2DEG in strained Si. We argue that, in pseudo-1D wires aligned parallel or perpendicular to the tilt direction, the 1D nature of the minigap can be more readily discerned. The FFT spectra of SdH oscillations taken under different accelerating fields in the wires fabricated along the tilt direction show that extra peaks are better resolved, indicating that electrons scatter mainly in the direction of the minigap. A similar analysis on wires aligned in the orthogonal direction show a similar but considerably attenuated peak. The presence of an extra peak also in this direction can be explained by the higher mobility in this direction as well as incomplete confinement. In a attempt to demonstrate a practical device application, we also employed anodic oxides grown electrochemically to form a MOS system. First, the quality of anodic oxide is assessed. The oxidation is shown to cause no damage to the electron mobility owing to the low temperature processing, but further improvement in the process is necessary to lower interface state density and threshold voltage. Next, by modulating the Fermi energy in the tilted channel within possible limits imposed by the oxide performance, we investigate the transport properties as a function of electron concentration. It is shown that the electron density and four terminal resistance of the channel exhibit an anomalous characteristic, which may be attributed to the minigap. (author)