Non-homogeneous electrical transport through silicon-on-sapphire thin films: Evidence of the internal stress influence

Abstract

The non-homogeneous electrical parameters of carrier transport in silicon on sapphire (SOS) films are investigated through galvanomagnetic experiments using “MOS-Hall” thin (0.65 μm) SOS devices with low n-type dopings ( n 0 ⋍ 10 15 to 10 16 cm −3 ). Capacitance-voltage (C-V), Hall and magnetoresistance measurements are performed on the gate-controlled depleted or “active” layers of the film. Experimental results and theoretical calculations show that longitudinal magnetoresistance (LMR) measurements yield the accurate determination of anisotropy and carrier drift mobility. On the contrary, it is underlined that the (previous) characterization methods, exclusively based on C-V or Hall experiments could produce inconsistent and misleading results in SOS. This is partly due to the fact that the usual 6 valley conduction band model used for bulk silicon is no longer valid in thin SOS films, for which the high stress at Si-sapphire interface requires the adoption of a 4 valley model throughout the film; this implies a high anisotropy for carrier mobility and explains why, in SOS films, the Hall mobility is 40% lower than the drift mobility, and the LMR is higher than the transverse magnetoresistance. C-V, Hall and LMR measurements then give fully consistent results allowing one to describe the variation of the carrier scattering mechanisms across the film and to determine the exact profiles for carrier mobilities and concentration as well as for the anisotropy coefficient.