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 german ium 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). Geome tric magnetoresistance technique was used to measure the mobility as a func tion of the applied field up to 300 V/cm for lattice temperatures from 13 t o 200 K. We observe that at high fields the mobility decreases at a faster rate in the germanium channels compared to silicon channels. Numerical calc ulations based on standard transport theories for low-field transport and n onlinear transport at high fields are also presented. We propose that the f aster mobility decrease in germanium is due to local heating of confined ph onons owing to acoustic mismatch with the cladding layers. We also present results based on the transient response of the lattice heating effects in b oth silicon and germanium systems as a function of the duty cycle of the ap plied electric field, which confirms the role of the local phonon temperatu re in the germanium quantum well.