Buried-channel semiconductor heterostructures are an archetype material platform for the fabrication of gated semiconductor quantum devices. Sharp confinement potential is obtained by positioning the channel near the surface; however, nearby surface states degrade the electrical properties of the starting material. Here, a 2D hole gas of high mobility (5 × 10 5 cm 2 V −1 s −1 ) is demonstrated in a very shallow strained germanium (Ge) channel, which is located only 22 nm below the surface. The top-gate of a dopant-less field effect transistor controls the channel carrier density confined in an undoped Ge/SiGe heterostructure with reduced background contamination, sharp interfaces, and high uniformity. The high mobility leads to mean free paths ≈ 6 µm, setting new benchmarks for holes in shallow field effect transistors. The high mobility, along with a percolation density of 1.2 × 10 11 cm −2 , light effective mass (0.09m e ), and high effective g-factor (up to 9.2) highlight the potential of undoped Ge/SiGe as a low-disorder material platform for hybrid quantum technologies.

Superconductors and semiconductors are crucial platforms in the field of quantum computing. They can be combined to hybrids, bringing together physical properties that enable the discovery of new emergent phenomena and provide novel strategies for quantum control. The involved semiconductor materials, however, suffer from disorder, hyperfine interactions or lack of planar technology. Here we realise an approach that overcomes these issues altogether and integrate gate-defined quantum dots and superconductivity into germanium heterostructures. In our system, heavy holes with mobilities exceeding 500,000 cm2 (Vs)−1 are confined in shallow quantum wells that are directly contacted by annealed aluminium leads. We observe proximity-induced superconductivity in the quantum well and demonstrate electric gate-control of the supercurrent. Germanium therefore has great promise for fast and coherent quantum hardware and, being compatible with standard manufacturing, could become a leading material for quantum information processing.

We report density-dependent effective hole mass measurements in undoped germanium quantum wells. We are able to span a large range of densities (2.0 − 11 × 10 11 cm −2 ) in top-gated field effect transistors by positioning the strained buried Ge channel at different depths of 12 and 44 nm from the surface. From the thermal damping of the amplitude of Shubnikov-de Haas oscillations, we measure a light mass of 0.061me at a density of 2.2 × 10 11 cm −2 . We confirm the theoretically predicted dependence of increasing mass with density and by extrapolation we find an effective mass of ∼ 0.05me at zero density, the lightest effective mass for a planar platform that demonstrated spin qubits in quantum dots.Holes are rapidly emerging as a promising candidate for semiconductor quantum computing.[1-3] In particular, holes in germanium (Ge) bear favorable properties for quantum operation, such as strong spin-orbit coupling enabling electric driving without the need of microscopic objects,[1-3] large excited state splitting energies to isolate the qubit states, [4] and ohmic contacts to virtually all metals for hybrid superconducting-semiconducting research [5][6][7][8][9]. Furthermore, undoped planar Ge quantum wells with hole mobilities µ > 5 × 10 5 cm 2 /Vs were recently developed[10] and shown to support quantum dots [11,12] and single and two qubit logic,[3] providing scope to scale up the number of qubits.

We investigate the structural and quantum transport properties of isotopically enriched 28 Si/ 28 SiO 2 stacks deposited on 300-mm Si wafers in an industrial CMOS fab. Highly uniform films are obtained with an isotopic purity greater than 99.92%. Hall-bar transistors with an oxide stack comprising 10 nm of 28 SiO 2 and 17 nm of Al 2 O 3 (equivalent oxide thickness of 17 nm) are fabricated in an academic cleanroom. A critical density for conduction of 1.75 × 10 11 cm −2 and a peak mobility of 9800 cm 2 /Vs are measured at a temperature of 1.7 K. The 28 Si/ 28 SiO 2 interface is characterized by a roughness of = 0.4 nm and a correlation length of = 3.4 nm. An upper bound for valley splitting energy of 480 μeV is estimated at an effective electric field of 9.5 MV/m. These results support the use of wafer-scale 28 Si/ 28 SiO 2 as a promising material platform to manufacture industrial spin qubits.

In this paper, we have experimentally and numerically studied the nonradiative intersubband (ISB) relaxation in n-type Ge/SiGe quantum well (QW) systems. Relaxation times have been probed by means of pump-probe experiments. An energy balance model has been used to interpret the experimental differential transmission spectra and to assess the relevance in the nonradiative relaxation dynamics of both electron and lattice temperature as well as of the carrier density. The comparison between experimental data and theoretical simulation allowed us to calibrate the interaction parameters which describe the electron-optical phonon scattering in two-dimensional (2D) Ge systems. Characteristic relaxation times has been calculated and compared with those of GaAs QWs as a function of the 2D electron density, of the subband energy separation, and of the lattice and electronic temperature. We found that ISB relaxation times for the Ge/SiGe systems are generally shorter than that previously calculated when the electron distribution was neglected. Nonetheless, our main result is that the relaxation time in Ge/SiGe QW systems is longer than 10 ps, also for transition energies above the Ge optical phonon energy, up to 300 K. Furthermore, we obtained that the relaxation times are at least one order of magnitude longer than in GaAs-based systems.

Asymmetric quantum well systems are excellent candidates to realize semiconductor light emitters at far-infrared wavelengths not covered by other gain media. Group-IV semiconductor heterostructures can be grown on silicon substrates, and their dipole-active intersubband transitions could be used to generate light from devices integrated with silicon electronic circuits. Here, we have realized an optically pumped emitter structure based on a three-level Ge/Si0.18Ge0.82 asymmetric coupled quantum well design. Optical pumping was performed with a tunable free-electron laser emitting at photon energies of 25 and 41 meV, corresponding to the energies of the first two intersubband transitions 0 → 1 and 0 → 2 as measured by Fourier-transform spectroscopy. We have studied with a synchronized terahertz time-domain spectroscopy probe the relaxation dynamics after pumping, and we have interpreted the resulting relaxation times (in the range 60 to 110 ps) in the framework of an out-of-equilibrium model of the intersubband electron–phonon dynamics. The spectral changes in the probe pulse transmitted at pump–probe coincidence were monitored in the range 0.7–2.9 THz for different samples and pump intensity and showed indication of both free carrier absorption increase and bleaching of the 1 → 2 transition. The quantification from data and models of the free carrier losses and of the bleaching efficiency allowed us to predict the conditions for population inversion and to determine a threshold pump power density for lasing around 500 kW/cm2 in our device. The ensemble of our results shows that optical pumping of germanium quantum wells is a promising route toward silicon-integrated far-infrared emitters.

We present an energy-balance model of the electronic intersubband relaxation in optically excited n-type Ge/SiGe quantum wells with absorption resonance in the THz range. To this aim, the energy relaxation rates of the electron system due to interactions with both nonpolar optical and acoustic phonons are calculated. The time dependence of the relative differential transmission is also evaluated and compared with experimental data from recent pump-probe measurements. The energy relaxation rates due to acoustic and optical phonon are investigated for different electron temperatures, set by the pump beam intensity. We find that the relaxation dynamics strongly depends on the intersubband energy spacing when this is close to the optical phonon energy. Finally, our results evidence that in this material system the time dependence of the depolarization shift may have a strong influence on the relative differential transmission signal.

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