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 μm , 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 Si1−xGex VS and Si0.2Ge0.8 buffer layer, using Molecular Beam Epitaxy. The Si1−xGex VS is grown by deposition of 100 nm Ge with a subsequent high-temperature annealing step, followed by a 100 nm thick Si0.2Ge0.8 buffer layer. The resulting two-dimensional hole gas reaches a hole mobility of over 8⋅104cm2V−1s−1 with a corresponding sheet carrier density of 5.7⋅1011cm−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.