J. P. Thompson | Searcharxiv

arXiv2026

Growth optimization of shallow Ge quantum wells grown by molecular beam epitaxy

Shallow strained Ge quantum wells have gained recent attention for realizing scalable, high-performance hybrid superconductor/semiconductor-based qubits. Epitaxial superconducting contacts can improve the quality and consistency of the superconductor/semiconductor interface. The growth of Ge quantum wells by molecular beam epitaxy is then motivating due to the relative ease of integration with epitaxial superconductor growth. However, the performance of Ge quantum wells grown by molecular beam epitaxy (MBE) has been limited. To improve the properties of MBE-grown Ge quantum wells, the growth conditions were systematically optimized. Thick buffer layers are utilized to eliminate certain defects, and an optimal growth temperature is found. A peak hole mobility of 105,000 cm\textsuperscript{2}/Vs at 2 K is obtained in a 22-nm deep Ge quantum well, demonstrating that the Ge quantum wells grown in this study represent the highest mobilities for shallow MBE-grown samples. Mobility modeling indicates that the increase in mobility due to growth temperature optimization are likely due to a reduction in interface roughness scattering, and further improvements in mobility are expected through improved surface passivation.

cond-mat.mtrl-sci↗

arXiv2020

Integration of multi-layer black phosphorus into photoconductive antennas for THz emission

We report the fabrication, characterization, and modeling of photoconductive antennas using 40 nm thin-film flakes of black phosphorus (BP) as the photoconductor and hexagonal boron nitride (hBN) as a capping layer to prevent oxidation of BP. Dipole antennas were fabricated on oxidized high-resistivity Si substrates, and BP and hBN flakes were picked up and transferred onto the antenna inside a nitrogen glovebox. The transfer matrix technique was used to optimize the thickness of BP and hBN for maximum absorption. BP flakes were aligned with the armchair axis along the anode-cathode gap of the antenna, with crystal orientation measured using reflection anisotropy. Photocurrent imaging under illumination with 100 fs pulses at 780 and 1560 nm showed a bias-dependent maximum photocurrent localized to the antenna gap with a peak photoconductivity of 1 (2) S/cm in the linear regime of bias for excitation at 780 (1560) nm. Photocurrent saturation in bias (pump fluence) occurred at approximately 1 V (0.25 mJ/cm$^2$). Device performance was modeled numerically by solving Maxwell's equations and the drift-diffusion equation to obtain the photocurrent density in response to pulsed laser excitation, which was largely in qualitative agreement with the experimental observations. THz output computed from surface current density suggests that BP THz PCA performance is at least comparable to more traditional devices based on low-temperature-grown GaAs. These devices represent a step toward high-performance THz photoconductive antennas using BP.

physics.app-ph↗