Low- and high-field transport properties of modulation-doped Si/SiGe and Ge/SiGe heterostructures: Effect of phonon confinement in germanium quantum wells

In this paper we report experimental studies carried out on two-dimensional electrons in strained silicon and two-dimensional holes in strained germanium channel modulation doped heterostructures, to understand their low- and high-field transport properties. Hall measurements were done to determine the low-field Ohmic mobility as a function of temperature (13-300 K). Geometric magnetoresistance technique was used to measure the mobility as a function of the applied field up to 300 V/cm for lattice temperatures from 13 to 200 K. We observe that at high fields the mobility decreases at a faster rate in the germanium channels compared to silicon channels. Numerical calculations based on standard transport theories for low-field transport and nonlinear transport at high fields are also presented. We propose that the faster mobility decrease in germanium is due to local heating of confined phonons owing to acoustic mismatch with the cladding layers. We also present results based on the transient response of the lattice heating effects in both silicon and germanium systems as a function of the duty cycle of the applied electric field, which confirms the role of the local phonon temperature in the germanium quantum well.