A procedure for the fabrication of two-dimensional carrier (electron a
nd hole) gases in modulation doped GeSi/Si heterostructures is present
ed, The best 4.2 K mobilities measured for the two-dimensional electro
n and hole gases are 180 000 cm2/V s and 18 000 cm2/V s, respectively.
Recently, two-dimensional hole gases with mobilities as high as 55 00
0 cm2/V S have been obtained. The carrier gases are fabricated on top
of relaxed, compositionally graded GexSi1-x. buffer layers with low th
reading dislocation densities (almost-equal-to 10(6) cm-2). Experiment
al evidence indicates that the function of the graded buffer is to pro
mote dislocation propagation while suppressing nucleation. A comparati
ve analysis is carried out for two dimensional electron gases in GeSi/
Si/GeSi and in AlGaAs/GaAs structures. Although molecular beam epitaxy
is used to grow the samples, the principle discussed here is independ
ent of growth technique.