In this paper, we report a Hall mobility of one million in a germanium two-dimensional hole gas. The extremely high hole mobility of 1.1 × 106 cm2 V−1 s−1 at a carrier sheet density of 3 × 1011 cm−2 was observed at 12 K. This mobility is nearly an order of magnitude higher than any previously reported. From the structural analysis of the material and mobility modeling based on the relaxation time approximation, we attribute this result to the combination of a high purity Ge channel and a very low background impurity level that is achieved from the reduced-pressure chemical vapor deposition growth method.

Enhanced electron cooling is demonstrated in a strained-silicon/superconductor tunnel junction refrigerator of volume 40 um^3. The electron temperature is reduced from 300 mK to 174 mK, with the enhancement over an unstrained silicon control (300 mK to 258 mK) being attributed to the smaller electron-phonon coupling in the strained case. Modeling and the resulting predictions of silicon-based cooler performance are presented. Further reductions in the minimum temperature are expected if the junction sub-gap leakage and tunnel resistance can be reduced. However, if only tunnel resistance is reduced, Joule heating is predicted to dominate.Comment: 3 pages, 5 figure

The vibrations of a single-crystal germanium (Ge) membrane are studied in air and vacuum using laser vibrometry, in order to determine mechanical properties such as Q-factors, tensile stress, anisotropy, and robustness to shock. Resonance modes up to 3:2 are identified, giving a residual stress measurement of 0.22 GPa, consistent with the value obtained from x-ray relaxation studies. The membrane is found to be isotropic, with Q-factors ranging from around 40 at atmospheric pressure to over 3200 at mbar, significantly lower than those found in polycrystalline Ge micromechanical devices. The robustness to shock is explained through the high resonance mode frequencies and the dissipation mechanism into the substrate, which is a direct consequence of having a high quality film with low residual tensile stress, giving the potential for such films to be used in optoelectronic devices.

We demonstrate significant modification of the electron-phonon energy loss rate in a many-valley semiconductor system due to lattice mismatch induced strain. We show that the thermal conductance from the electron system to the phonon bath in strained n + Si, at phonon temperatures between 200 mK and 450 mK, is more than an order of magnitude lower than that for a similar unstrained sample.The interaction between electrons and phonons is one of the most important parameters of a semiconductor system, because it dictates the room temperature electron mobility and electron-phonon (e-ph) energy loss. 1 In the low temperature (sub-1 K) regime e-ph coupling typically dominates the total thermal coupling from electrons to the environment. Therefore, e-ph coupling is one of the key parameters of interest in low temperature bolometer and microcooler applications. 2 In single-valley semiconductors the electron-phonon interaction can be typically described by deformation potential or piezoelectric coupling constants, which means that the dependence of the e-ph matrix elements on the electronic variables, such as momentum, can be typically ignored. 3 The situation is quite different in metals where momentum dependency must be included explicitly.Simply setting the e-ph matrix elements to a constant is not the whole story even for deformation potential coupling in semiconductors. Certain lattice perturbations can lift the valley degeneracy so that the valley degree of freedom plays an important role in low temperature e-ph energy loss. 4,5 Mean-field theory predicts that the e-ph energy loss rate is strongly enhanced in many-valley semiconductors in comparison to single-valley ones due to lack of screening. 5 This suggests that if one is able to depopulate asymmetric valleys of a many-valley semiconductor then a strong reduction in e-ph energy loss rate should be observed. In this letter, we will experimentally test this prediction by using strain to depopulate the four inplane valleys of epitaxial n + Si films [see Fig. 1(a)]. Sub-1 K e-ph energy loss experiments of strained samples are compared to experimental data of Ref. 4 and our nonstrained control sample. We find that qualitatively the experiments agree with theory and e-ph thermal conductance is indeed reduced in the strained sample. The effect is, however, smaller than ideally expected.Strained silicon epi-layers were produced using a RP-CVD epitaxial growth system. Initially, a linearly graded