A INTRODUCTION

Advances in the epitaxial growth of modulation-doped Si/SiGe structures have resulted in two-dimensional electron (2DEG) and hole gases (2DHG) which exhibit carrier mobilities which are at least by a factor of 10 higher than those reported in Si-MOSFET structures at low lattice temperatures. Essentially three kinds of Si/SiGe heterostructure have been investigated by magnetotransport methods: (i) electrons confined in biaxially tensile strained Si-layers with record low temperature mobilities reported up to 800 000 cm ²/V s, (ii) holes in biaxially compressively strained Ge quantum wells with mobilities up to about 55 000 cm ²/V s, and (iii) holes in SiGe quantum wells grown pseudomorphically on Si, with mobilities up to about 20 000 cm ²/V s. For the growth of structures (i) and (ii) on top of (001) Si, graded and constant composition SiGe buffers are grown sufficiently thick to provide for a partially relaxed 'virtual' SiGe substrate layer for the subsequent growth of strained Si or Ge quantum wells.

For the tensile strained Si layer (i), the lowest conduction band state consists of a two-fold valley degenerated band with transverse masses of about 0.19 m _o and longitudinal masses of about 0.9 m _o The latter accounts for a strong confinement of the electron wave function. In the 2DEG systems, the electron effective g-factor is about 3.4 for low magnetic fields. However, in compressively strained Ge layers (ii), 2D hole effective masses down...