Extremely high room-temperature two-dimensional hole gas mobility in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures

High mobility holes in a strained Ge quantum well grown on a thin and relaxed Si0.4Ge0.6/LT-Si0.4Ge0.6/Si(001) virtual substrate

“…Some degradation of the mobility may also occur because the carriers are confined against the rougher top interface of the Ge QW, since the present structures are MOD only from above the QW rather than from bottom side. 2 By contrast, at room temperature the mobility is mainly limited by phonon scattering and will not be significantly affected by the low TDD and high quality interfaces. 9,10 Indeed, the measured room-temperature Hall mobility exceeds the value of 1110 cm 2 V −1 s −1 reported for the similar 7.5 nm Ge QW structure grown on the thicker VS mentioned previously.…”

“…Hence, we might have expected the low TDD and very low rms surface roughness measured in these samples to result in a higher low-temperature hole mobility than we measure, given that a 2DHG mobility of 15 770 cm 2 V −1 s −1 was obtained in a similar 7.5 nm Ge QW sample grown on much thicker ͑1100 nm͒ SiGe/Si͑001͒ VS that had a much higher TDD of ϳ3 ϫ 10 7 cm −2 and a much larger rms surface roughness of ϳ7 nm. 2,9 We can speculate that the reason for the lower than expected low-temperature 2DHG mobility could be the proximity of the Ge QW to the Si 0.4 Ge 0.6 / LT-Si 0.4 Ge 0.6 region of the VS, which could contain some amount of misfit dislocations and point defects that could be sources of remote scattering for the holes in the Ge QW. Some degradation of the mobility may also occur because the carriers are confined against the rougher top interface of the Ge QW, since the present structures are MOD only from above the QW rather than from bottom side.…”

“…9,10 Indeed, the measured room-temperature Hall mobility exceeds the value of 1110 cm 2 V −1 s −1 reported for the similar 7.5 nm Ge QW structure grown on the thicker VS mentioned previously. 2 However, conventional resistivity and Hall effect measurements only yield the averaged density and mobility of carriers that exist not only in the QW but also in the other parallel conducting layers, e.g., the doped supply layer, the buffer layer, the substrate, and their interfaces. The transport properties of the various carriers can be separated using the technique of mobility spectrum analysis.…”

“…The temperature dependence of Hall mobility and carrier density clearly indicates a two-dimensional hole gas in the Ge QW. At room temperature, which is more relevant for electronic devices applications, the samples show a very high Hall mobility of 1235 cm 2 In recent years significant progress has been made toward increasing the room-temperature carrier mobility in a two-dimensional hole gas ͑2DHG͒ by using modulation doped ͑MOD͒ heterostructures with a compressively strained Ge quantum well ͑QW͒ that acts as a channel for mobile carriers. By growing with an intermediate relaxed SiGe buffer on an underlying Si͑001͒ substrate, the strain narrows the band gap of Ge and causes the appearance of a QW in the valence band.…”

“…Indeed, very high 2DHG mobilities in the range 2400-3100 cm 2 V −1 s −1 with carrier densities of 5 -41ϫ 10 11 cm −2 have become routinely achievable in 7.5-25 nm thick Ge QWs. [1][2][3] Until now, very high mobility holes were only obtained in strained Ge QWs grown on relatively thick ͑1-8 m͒, high Ge content, Si 1−x Ge x buffers. 2,3 However, the low thermal conductivity of undoped SiGe is a drawback for nanoscale metal oxide semiconductor field effect transistor ͑MOSFET͒ and modulation doped semiconductor field effect transistor ͑MODFET͒ device applications.…”