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Two-dimensional hole gases in SiGeSn alloys

Two-dimensional hole gases are demonstrated in modulation doped Si x Ge1−x−y Sn y quantum wells (QWs), which are embedded in Si0.2Ge0.8 barrier layers. The modulation doped QW structures are fabricated with molecular beam epitaxy on a thin (100 nm) virtual SiGe substrate on a (001) oriented Si substrate. The virtual substrate (VS) concept utilizes the Si diffusion into an as- grown thin, strain relaxed Ge layer during a following annealing step. The lateral lattice spacing of the SiGe-VS could be varied by the annealing temperature in the range between 830 °C and 860 °C. Half-hour anneal at 848 °C results in nearly strain free growth for the following Si0.2Ge0.8 barrier layer. Boron doping above an undoped 10 nm spacer on top of the 15 nm QW provides a reservoir for hole transfer from the barrier to the well. Electrical conductivity, sheet hole density ps and mobility are measured as function of temperature. In all investigated Si x Ge1−x−y Sn y channels the Hall measurements show the typical freeze out of holes outside the QW. Alloy scattering dominates the low-temperature mobility by adding Sn or Si to the Ge reference well. A linear relationship for the charge transfer from the modulation doping into the undoped Si x Ge1−x−y Sn y channel as function of the lattice mismatch between the channel material and the matrix material could be found at low-temperatures (8 K). An analytical model for this charge transfer confirms the nearly linear relationship by considering the triangular shape of the potential in modulation doped QW structures.