High-mobility Two-dimensional Hole Gas Research Articles

Lateral Mn5Ge3 spin-valve in contact with a high-mobility Ge two-dimensional hole gas

Abstract Ge two-dimensional hole gases (2DHG) in strained modulation-doped quantum-wells represent a promising material platform for future spintronic applications due to their excellent spin transport properties and the theoretical possibility of efficient spin manipulation. Due to the continuous development of epitaxial growth recipes extreme high hole mobilities and low effective masses can be achieved, promising an efficient spin transport. Furthermore, the Ge 2DHG can be integrated in the well-established industrial complementary metal-oxide-semiconductor (CMOS) devices technology. However, efficient electrical spin injection into a Ge 2DHG—an essential prerequisite for the realization of spintronic devices—has not yet been demonstrated. In this work, we report the fabrication and low-temperature magnetoresistance (MR) measurements of a laterally structured Mn5Ge3/Ge 2DHG/ Mn5Ge3 device. The ferromagnetic Mn5Ge3 contacts are grown directly into the Ge quantum well by means of an interdiffusion process with a spacing of approximately 130 nm, forming a direct electrical contact between the ferromagnetic metal and the Ge 2DHG. Here, we report for the first time a clear MR signal for temperatures below 13 K possibly arising from successful spin injection into the high mobility Ge 2DHG. The results represent a step forward toward the realization of CMOS compatible spintronic devices based on a 2DHG.

Direct Microwave Spectroscopy of Andreev Bound States in Planar Ge Josephson Junctions

We demonstrate microwave measurements of the Andreev-bound-state (ABS) spectrum in planar Josephson junctions (JJs) defined in Ge high-mobility two-dimensional hole gases contacted by superconducting platinum germanosilicide (PtSiGe). The JJs and readout circuitry are located on separate chips and inductively coupled via flip-chip bonding. For a device with 350-nm junction length, the spectroscopic signatures were consistent with the short-junction limit, with an induced superconducting gap Δ∗≈48μeV and transmission τ≈0.94. The interaction between the highest-transmission ABS and the resonator was well described by a Jaynes-Cummings model with a vacuum Rabi splitting of approximately 6 MHz. A device with a junction length of 1μm showed an ABS spectrum consistent with a long-junction model. Time-resolved monitoring of the readout resonator in the dispersive regime revealed gate-voltage tunable-junction parity fluctuations on the time scale of seconds. Our work indicates a viable path toward hybrid quantum devices based on planar Ge. Published by the American Physical Society 2024

Radiation-induced resistance oscillations in a 2D hole gas: a demonstration of a universal effect

We report on a theoretical study about the microwave-induced resistance oscillations and zero resistance states when dealing with p-type semiconductors and holes instead of electrons. We consider a high-mobility two-dimensional hole gas hosted in a pure Ge/SiGe quantum well. Similarly to electrons we obtain radiation-induced resistance oscillations and zero resistance states. We analytically deduce a universal expression for the irradiated magnetoresistance, explaining the origin of the minima positions and their 1/4 cycle phase shift. The outcome is that these phenomena are universal and only depend on radiation and cyclotron frequencies. We also study the possibility of having simultaneously two different carriers driven by radiation: light and heavy holes. As a result the calculated magnetoresistance reveals an interference profile due to the different effective masses of the two types of carriers.

Detection of anomalous Hall voltages in ultrahigh-mobility two-dimensional hole gases generated by optical spin orientation

By combining optical spin orientation and an externally applied longitudinal electric field, transverse charge accumulation has been detected in very high-mobility two-dimensional hole gases by measuring the transverse voltage drop across simple Hall devices. Our results indicate intrinsic band-structure (rather than extrinsic skew scattering) derived spin-orbit coupling as the underlying mechanism of this spin-polarized transport effect.

Observation of microwave-induced resistance oscillations in a high-mobility two-dimensional hole gas in a strained Ge/SiGe quantum well

Microwave-induced resistance oscillations (MIRO) have been extensively studied for more than a decade but, until now, have remained unique to GaAs/AlGaAs-based 2D electron systems. Here, we report on the first observation of MIRO in a 2D hole gas hosted in Ge/SiGe quantum well. Our findings confirm that MIRO is a universal phenomenon and demonstrate that microwave photoresistance can be utilized to probe the energy spectrum and the correlation effects of 2D holes in Ge/SiGe quantum wells.

Optically induced charge conversion of coexistent free and bound excitonic complexes in two-beam magnetophotoluminescence of two-dimensional quantum structures

We report on extensive polarization-resolved photoluminescence (PL) studies of a variety of excitonic complexes formed in high-quality symmetric GaAs quantum wells containing a high-mobility two-dimensional (2D) hole gas in a broad range of magnetic fields from 0 to 23 T and under two-beam illumination, allowing for dynamical control of the hole concentration beyond the point of conversion from $p$- to $n$-type structures. We have demonstrated charge conversion between positive and negative complexes bound to acceptors in the well, differing from the charge conversion of free trions due to charge reflection symmetry breaking by a fixed impurity, leaving a qualitative trace (exchange splitting) in the PL spectrum. The effect of switching between the electron and hole gases (in the same well) on different emission lines has also allowed us to distinguish the (direct and cyclotron-satellite) emission lines from positive trions moving almost freely in the quantum well and bound to nearby ionized acceptors in the barrier, thus demonstrating their coexistence in high-quality structures.

Electrical characterisation of deep level defects in Be-doped AlGaAs grown on (100) and (311)A GaAs substrates by MBE

The growth of high mobility two-dimensional hole gases (2DHGs) using GaAs-GaAlAs heterostructures has been the subject of many investigations. However, despite many efforts hole mobilities in Be-doped structures grown on (100) GaAs substrate remained considerably lower than those obtained by growing on (311)A oriented surface using silicon as p-type dopant. In this study we will report on the properties of hole traps in a set of p-type Be-doped Al0.29Ga0.71As samples grown by molecular beam epitaxy on (100) and (311)A GaAs substrates using deep level transient spectroscopy (DLTS) technique. In addition, the effect of the level of Be-doping concentration on the hole deep traps is investigated. It was observed that with increasing the Be-doping concentration from 1 × 1016 to 1 × 1017 cm-3 the number of detected electrically active defects decreases for samples grown on (311)A substrate, whereas, it increases for (100) orientated samples. The DLTS measurements also reveal that the activation energies of traps detected in (311)A are lower than those in (100). From these findings it is expected that mobilities of 2DHGs in Be-doped GaAs-GaAlAs devices grown on (311)A should be higher than those on (100).

Intrinsic photoinduced anomalous Hall effect

We have measured the intrinsic photoinduced anomalous Hall effect (PI-AHE) in very high mobility two-dimensional hole gases. In this new effect, similarly to both the spin and anomalous Hall effects, carriers with different spins are deflected to opposite sides of the sample by the spin–orbit interaction, thereby producing in this case a photoinduced voltage transverse to the applied current. We have been able to measure the intrinsic PI-AHE in finite magnetic field in symmetrically and asymmetrically doped (3 1 1)-grown quantum wells but not in (1 0 0)-grown wells.

Magnetotransport in Zener tunneling regime in a high-mobility two-dimensional hole gas

In a high-mobility two-dimensional hole gas (2DHG) in C-doped (001) $\text{GaAs}/{\text{Al}}_{0.4}{\text{Ga}}_{0.6}\text{As}$ quantum wells, we observe sharp features in the differential magnetoresistance, which we interpret as the Zener-tunneling peak and valley associated with the commensuration transition of Landau orbits. Comparison of data with that in $\text{GaAs}/{\text{Al}}_{0.3}{\text{Ga}}_{0.7}\text{As}$ 2D electron gas suggests that the amplitude of Zener oscillations in a 2DHG is strongly damped. The data indicate the significant role of band structures in nonlinear transport in 2D electronic systems.

Cyclotron mass of two-dimensional holes in (100) oriented GaAs∕AlGaAs heterostructures

Microwave cyclotron resonance of low-density high-mobility two-dimensional hole gases of different densities in (100) oriented GaAs∕AlGaAs heterostructures has been measured at 4.2K. The measurements show that the hole effective mass depends strongly on the confining potential. For holes in a 20nm quantum well, the cyclotron mass decreases from 0.48me at density p=6.9×1010∕cm2 to 0.29me at p=0.8×1010∕cm2. The cyclotron mass for holes confined at a heterojunction is fairly insensitive to the density and has a value of approximately 0.5me.

Features of the Shubnikov–de Haas oscillations of the conductivity of a high-mobility two-dimensional hole gas in a SiGe∕Ge∕SiGe quantum well

The Shubnikov–de Haas oscillations in a two-dimensional hole gas in a quantum well of pure germanium in a SiGe∕Ge∕SiGe heterostructure with a hole concentration pH=5.68×1011cm2 and mobility μ=4.68×104cm2V−1s−1 are investigated in magnetic fields up to 15 T at temperatures from 40 mK to 4 K. The observed deviation from the known relation describing the conductivity oscillations in the Shubnikov–de Haas effect are explained by additional broadening of the Landau levels due to the existence of a nonuniform distribution of the concentration of charge carriers, and, accordingly, of their energy, in the plane of the two-dimensional gas. It is assumed that the latter is due to natural atomic-step variations of the well width. The effective hole mass (m*=0.112m0) is determined from the temperature dependence of the oscillation amplitude, and its dependence on magnetic field is used to determine the quantum scattering time and the value of the carrier concentration fluctuations.

Carbon-doped high-mobility two-dimensional hole gases on (110) faced GaAs

Carbon-doped high-mobility two-dimensional hole gases grown on (110) oriented GaAs substrates have been grown with hole mobilities exceeding 106cm2∕Vs in single heterojunction GaAs∕AlGaAs structures. At these high mobilities, a pronounced mobility anisotropy has been observed. Rashba induced spin-splitting in these asymmetric structures has been found to be independent on the transport direction.

Conductance anisotropy of high-mobility two-dimensional hole gas at GaAs/(Al,Ga)As( [formula omitted])A single heterojunctions

We investigate the anisotropy of mobility of two-dimensional hole gases formed at modulation-doped single GaAs/(Al,Ga)As heterojunctions on GaAs( 1 1 3 )A in dependence on the surface morphology arising during molecular-beam epitaxy growth. A regular step distribution along the [ 3 3 2 ̄ ] direction results in nearly two times higher mobility compared to the [ 1 ̄ 1 0] direction. The magnetoresistance at low magnetic fields exhibits features, which allow to determine an effective electronic correlation length for hole scattering which correlates with the step distribution obtained from AFM images. This correlation length governs the mobility anisotropy. In addition, we estimate the amplitude of the potential modulation by means of the low-field positive magnetoresistance in high-mobility two-dimensional hole gases.

Hole-Hole Interaction Effect in the Conductance of the Two-Dimensional Hole Gas in the Ballistic Regime

On a high-mobility two-dimensional hole gas (2DHG) in a GaAs/GaAlAs heterostructure we study the interaction correction to the Drude conductivity in the ballistic regime, k(B)Ttau/ variant Planck's over 2pi >1. It is shown that the "metallic" behavior of the resistivity (drho/dT>0) of the low-density 2DHG is caused by the hole-hole interaction effect in this regime. We find that the temperature dependence of the conductivity and the parallel-field magnetoresistance are in agreement with this description, and determine the Fermi-liquid interaction constant Fsigma0 which controls the sign of drho/dT.

Weak-localization-like temperature-dependent conductivity of a dilute two-dimensional hole gas in a parallel magnetic field.

We have studied the magnetotransport properties of a high mobility two-dimensional hole gas (2DHG) in a 10 nm GaAs quantum well with densities in the range of (0.7-1.6) x 10(10) cm(-2) on the metallic side of the zero-field "metal-insulator transition." In a parallel field well above B(c) that suppresses the metallic conductivity, the 2DHG exhibits a conductivity Delta(g)(T) approximately (1/pi) (e(2)/h)lnT reminiscent of weak localization for Fermi liquids. The experiments are consistent with the coexistence of two phases in our system: a metallic phase and a weakly insulating Fermi liquid phase.

Optical investigation of high-mobility dilute two-dimensional hole gases in GaAs (3 1 1)A quantum structures

We have investigated the photoluminescence (PL) and inelastic light-scattering spectra of dilute two-dimensional (2D) hole gases of ultra-high mobility. The samples were GaAs–Al x Ga 1− x As (3 1 1) A quantum structures, modulation-doped with Si. We find these samples not to deplete fully on illumination. The PL displays both excitonic and 2D hole gas transitions. We observe inelastic light-scattering, resonating on these intersubband transitions. Our measured intersubband excitation energies are in agreement with current calculations. These resonance enhancements are consistent with an electron-like dispersion of the first-excited hole subband at zero magnetic field.

Magnetophotoluminescence of positively-charged excitons in GaAs quantum wells

We have studied the low-temperature photoluminescence of high-mobility two-dimensional hole gases in GaAs quantum wells in magnetic fields up to 50 T . We have observed both spin states of the neutral ( X 0) and of the singlet state of the positively charged exciton ( X +), allowing us to determine their g-factors and the binding energy of X +. We find that the g-factor of the X + is larger than that of X 0 by a factor of 1.6. In very high fields, we expect the low-energy spin-state of X + to be bound only because of the Zeeman interaction.

Enhanced g factors of a one-dimensional hole gas with quantized conductance

We have measured the transport properties of quasi-one-dimensional ballistic constrictions defined on a high-mobility two-dimensional hole gas. The conductance quantization of the constrictions is changed from even to odd multiples of ${\mathrm{e}}^{2}$/h as a function of the magnetic field in the plane of the heterojunction, as spin splitting causes the subbands of the one-dimensional (1D) channel to cross. We calculate the in-plane g factors of the 1D subbands using the magnetic fields at which crossing occurs and find that they increase as the number of occupied 1D subbands decreases. We attribute this enhancement of the g factor to Coulomb interactions.

Strain relaxation of Ge1−xSix buffer systems grown on Ge (001)

We have investigated the strain relaxation behavior of biaxial tensile strained Ge1−xSix buffer systems grown on Ge (001) by a high-resolution x-ray reciprocal space mapping technique. The molecular beam epitaxy grown structures contain a linearly graded buffer, followed by a uniform buffer and a modulation-doped heterostructure with a high mobility two-dimensional hole gas in a Ge channel. Our quantitative measurements of the in-plane strain show that the lower part of the graded buffer is completely strain relaxed, while the top part of this region and the uniform alloy buffer are partly strain relaxed showing a linear increase of strain towards to surface.

Growth and properties of high-mobility two-dimensional hole gases in Ge on relaxed Si/SiGe, Ge/SiGe buffers and Ge substrates

High-mobility two-dimensional hole channels were grown pseudomorphically on Ge substrates and strained on relaxed graded buffers both on Si and for the first time on Ge substrates. Low temperature mobilities of up to 27000 cm 2/V · s and effective masses down to 0.12 m 0 are observed for the Ge/SiGe/Ge hole channel.