Quantum transport quality of a processed undoped Ge/SiGe heterostructure

A degraded mobility of $5.2\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ but a long quantum scattering time of 2.3 ps at the hole density of $2.25\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ were obtained from a two-dimensional hole gas in a processed undoped Ge/SiGe heterostructure Hall-bar field-effect device. The heterostructure was grown by the reduced pressure chemical vapor deposition method and the device fabrications were compatible with well-established semiconductor technology. The percolation density of $0.69\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ indicated a very low disorder potential landscape experienced by holes in the strained Ge quantum well. In addition to integer quantum Hall effects (IQHEs) of consecutive filling factors from $\ensuremath{\nu}=8$ down to 1, we also observed the quantum states at filling factors between $\ensuremath{\nu}=1$ and 2, and between $\ensuremath{\nu}=2$ and 3 at 1.71 K. The observation of quantum Hall effects at the inferior mobility confirmed the mobility/quality dichotomy in the ultraclean undoped Ge/SiGe heterostructure. The study explicitly indicated that the device fabrication process likely compromised the transport mobility, whereas the quantum quality was less influenced, and established the heterostructure as an ideal platform for quantum device implementations.