Strained Ge Channels with High Hole Mobility Grown on Si Substrates by Molecular Beam Epitaxy

Strained modulation-doped quantum wells (QW) offer a huge potential for semiconductor device applications due to their high mobility. The material Ge is particularly interesting here, exhibiting the highest bulk hole-mobility of all known semiconductors. However, the growth for Ge QW structures is quite complex and a special virtual substrate (VS) technique is needed. The VS is commonly grown with thicknesses of over
$1\ \mu \mathrm{m}$
, making it difficult for integration with other devices on a single chip. In this paper, we report on the growth of a 15 nm thick strained Ge QW on top of a
$\mathbf{Si_{1-x}}\mathbf{Ge_{x}}$
VS and
$\mathbf{Si_{0.2}}\mathbf{Ge_{0.8}}$
buffer layer, using Molecular Beam Epitaxy. The
$\mathbf{Si_{1-x}}\mathbf{Ge_{x}}$
VS is grown by deposition of 100 nm Ge with a subsequent high-temperature annealing step, followed by a 100 nm thick
$\mathbf{Si_{0.2}}\mathbf{Ge_{0.8}}$
buffer layer. The resulting two-dimensional hole gas reaches a hole mobility of over
$\boldsymbol{8\cdot 10^{4}\text{cm}^{2}\mathrm{V}^{-1}\mathrm{s}^{-1}}$
with a corresponding sheet carrier density of
$\boldsymbol{5.7\cdot 10^{11}\text{cm}^{-2}}$
at 8 K. The Ge QW is further analysed, and comparing it to a sample with a higher VS thickness, a possible limitation of the mobility due to background doping is being discussed. These results show that complementary metal-oxide-semiconductor (CMOS) compatible device integration of the Ge QW is possible, thin buffer layers suffice for the mobilities achieved and background doping limits the low-temperature mobility.