Strain Effects on Rashba Spin-Orbit Coupling of 2D Hole Gases in GeSn/Ge Heterostructures

Isothermal Heteroepitaxy of Ge1-x Sn x Structures for Electronic and Photonic Applications

Epitaxy of semiconductor-based quantum well structures is a challenging task since it requires precise control of the deposition at the submonolayer scale. In the case of Ge_1-x Sn _x alloys, the growth is particularly demanding since the lattice strain and the process temperature greatly impact the composition of the epitaxial layers. In this paper, the realization of high-quality pseudomorphic Ge_1-x Sn _x layers with Sn content ranging from 6 at. % up to 15 at. % using isothermal processes in an industry-compatible reduced-pressure chemical vapor deposition reactor is presented. The epitaxy of Ge_1-x Sn _x layers has been optimized for a standard process offering a high Sn concentration at a large process window. By varying the N₂ carrier gas flow, isothermal heterostructure designs suitable for quantum transport and spintronic devices are obtained.

Tuning spin-orbit coupling and realizing inverse persistent spin helix by an extra above-barrier radiation in a GaAs/Al0.3Ga0.7As heterostructure

A persistent spin helix with equal strength of the Rashba and Dresselhaus spin-orbit coupling (SOC) is expected for future spintronic devices due to the suppression of spin relaxation. In this work we investigate the optical tuning of the Rashba and Dresselhaus SOC by monitoring the spin-galvanic effect (SGE) in a GaAs/Al_0.3Ga_0.7As two dimensional electron gas. An extra control light above the bandgap of the barrier is introduced to tune the SGE excited by a circularly polarized light below the bandgap of GaAs. We observe different tunability of the Rashba- and Dresselhaus-related SGE currents and extract the ratio of the Rashba and Dresselhaus coefficients. It decreases monotonously with the power of the control light and reaches a particular value of ∼-1, implying the formation of the inverse persistent spin helix state. By analyzing the optical tuning process phenomenologically and microscopically, we reveal greater optical tunability of the Rashba SOC than that of the Dresselhaus SOC.

Room-Temperature Negative Differential Resistance and High Tunneling Current Density in GeSn Esaki Diodes

Tunnel field-effect transistors (TFETs) are a promising candidate for low-power applications owing to their steep subthreshold swing of sub-60 mV per decade. For silicon- or germanium-based TFETs, the drive current is low due to the indirect band-to-band tunneling (BTBT) process. Direct-bandgap germanium-tin (GeSn) can boost the TFET performance since phonon participation is not required during the tunneling process. Esaki diodes with negative differential resistance (NDR) are used to characterize the BTBT properties and calibrate the tunneling rates for TFET applications. This work demonstrates high-performance GeSn Esaki diodes with clear NDR at room temperature with very high peak-to-valley current ratios of 15-53 from 300 K to 4 K. A record-high peak current density of 545 kA cm^-2 at 4 K is also reported for the tensile-strained Ge_0.925 Sn_0.075 device (strain ≈0.6 %). By applying tensile stresses to n-GeSn epitaxial films, the direct BTBT process dominates, leading to high tunneling rates. Hall measurements further confirm that more electrons populate in the direct Γ valley in the tensile-strained n-GeSn epitaxial films.

Tailoring Ultrafast and High-Capacity Sodium Storage via Binding-Energy-Driven Atomic Scissors

Controllably tailoring alloying anode materials to achieve fast charging and enhanced structural stability is crucial for sodium-ion batteries with high rate and high capacity performance, yet remains a significant challenge owing to the huge volume change and sluggish sodiation kinetics. Here, a chemical tailoring tool is proposed and developed by atomically dispersing high-capacity Ge metal into the rigid and conductive sulfide framework for controllable reconstruction of GeS bonds to synergistically realize high capacity and high rate performance for sodium storage. The integrated GeTiS₃ material with stable Ti-S framework and weak GeS bonding delivers high specific capacities of 678 mA h g^-1 at 0.3 C over 100 cycles and 209 mA h g^-1 at 32 C over 10 000 cycles, outperforming most of the reported alloying type anode materials for sodium storage. Interestingly, in situ Raman, X-ray diffraction (XRD), and ex situ transmission electron microscopy (TEM) characterizations reveal the formation of well-dispersed Na_x Ge confined in the rigid Ti-S matrix with suppressed volume change after discharge. The synergistically coupled alloying-conversion and surface-dominated redox reactions with enhanced capacitive contribution and high reaction reversibility by a binding-energy-driven atomic scissors method would break new ground on designing a high-rate and high-capacity sodium-ion batteries.

A Light-Hole Germanium Quantum Well on Silicon

The quiet quantum environment of holes in solid-state devices is at the core of increasingly reliable architectures for quantum processors and memories. However, due to the lack of scalable materials to properly tailor the valence band character and its energy offsets, the precise engineering of light-hole (LH) states remains a serious obstacle toward coherent optical photon-spin interfaces needed for a direct mapping of the quantum information encoded in photon flying qubits to stationary spin processors. Herein, to alleviate this long-standing limitation, an all-group-IV low-dimensional system is demonstrated, consisting of a highly tensile strained germanium quantum well grown on silicon allowing new degrees of freedom to control and manipulate the hole states. Wafer-level, high bi-isotropic in-plane tensile strain (<1%) is achieved using strain-engineered, metastable germanium-tin alloyed buffer layers yielding quantum wells with LH ground state, high g-factor anisotropy, and a tunable splitting of the hole sub-bands. The epitaxial heterostructures display sharp interfaces with sub-nanometer broadening and show room-temperature excitonic transitions that are modulated and extended to the mid-wave infrared by controlling strain and thickness. This ability to engineer quantum structures with LH selective confinement and controllable optical response enables manufacturable silicon-compatible platforms relevant to integrated quantum communication and sensing technologies.

Review of Si-Based GeSn CVD Growth and Optoelectronic Applications

GeSn alloys have already attracted extensive attention due to their excellent properties and wide-ranging electronic and optoelectronic applications. Both theoretical and experimental results have shown that direct bandgap GeSn alloys are preferable for Si-based, high-efficiency light source applications. For the abovementioned purposes, molecular beam epitaxy (MBE), physical vapour deposition (PVD), and chemical vapor deposition (CVD) technologies have been extensively explored to grow high-quality GeSn alloys. However, CVD is the dominant growth method in the industry, and it is therefore more easily transferred. This review is focused on the recent progress in GeSn CVD growth (including ion implantation, in situ doping technology, and ohmic contacts), GeSn detectors, GeSn lasers, and GeSn transistors. These review results will provide huge advancements for the research and development of high-performance electronic and optoelectronic devices.