arXiv subjects

M. C. Tam

Publications and source records attributed to M. C. Tam.

8 recordsLinked to original sources

arXiv2026

Magnetotransport Spectroscopy of Strongly Rashba-Split Hole Subbands Reveals Many-Body Interactions

We report the results of magnetotransport experiments carried out on low-disorder 2D hole gases (2DHG) in the strongly correlated liquid regime, hosted in dopant-free (100) GaAs/AlGaAs single heterojunctions. Over a wide range of 2DHG densities (from 0.7 $\times$ 10$^{15}$/m$^2$ to $2 \times 10^{15}$/m$^2$), Fourier analysis of low-field (B < 1 T) Shubnikov-de Haas oscillations reveals two spin-orbit-split heavy-hole (HH) subbands with distinct effective masses contributing to transport. Surprisingly, the lighter-mass HH subband exhibits a parabolic dispersion with Fermi wavevector below the anticrossing between the heavy-hole and light-hole subbands, while the heavier HH subband is non-parabolic throughout. Quantitative comparison with numerical calculations based on the Luttinger model reveals that both effective masses are enhanced by a common factor ($\approx$ 2.3), which we attribute to many-body interactions. This common scaling factor has a very weak dependence on the 2DHG density, likely due to band hybridization. Our measured hole masses are compared with published cyclotron resonance and magnetotransport values. We propose a cohesive framework reconciling the long-standing three-way discrepancy between Luttinger theory, magnetotransport, and cyclotron resonance measurements of density-dependent effective masses in partially spin-orbit-polarized heavy-hole systems in GaAs.

cond-mat.mes-hall↗

arXiv2026

Enhanced effective masses, spin-orbit polarization, and dispersion relations in 2D hole gases under strongly asymmetric confinement

The dispersion of Rashba-split heavy-hole subbands in GaAs two-dimensional hole gases (2DHGs) is difficult to access experimentally because strong heavy-hole-light-hole mixing produces non-parabolicity and breaks the usual correspondence between carrier density and Fermi wave vector. Here we use low-field magnetotransport (B < 1 T) to reconstruct the dispersions of the two spin-orbit-split heavy-hole branches (HH-, HH+) in undoped (100) GaAs/AlGaAs single heterojunction 2DHGs operated in an accumulation-mode field-effect geometry. The dopant-free devices sustain out-of-plane electric fields up to 26 kV/cm while maintaining mobilities up to 84 m$^2$/Vs and exhibiting a spin-orbit polarization as large as 36%. Fourier analysis of Shubnikov-de Haas (SdH) oscillations resolves the individual HH-/HH+ subband densities; fitting the temperature dependence of the corresponding Fourier amplitudes yields both branch-resolved SdH effective masses over the same magnetic field window. SdH regimes in which reliable subband parameters can be extracted are delineated. Over 2DHG densities (0.76-1.9) $\times$ 10$^{15}$ /m$^2$, the HH- mass is nearly density independent ($\approx 0.34m_e$), implying a near-parabolic HH- dispersion below the first LH+/HH- anticrossing, whereas HH+ exhibits strong non-parabolicity with an effective mass that increases with density. Combining the extracted dispersions yields a transport-based determination of the spin-orbit splitting energy $Δ_\text{HH}$ between HH and HH+ as a function of in-plane wave vector. Parameter-free Luttinger-model calculations reproduce the qualitative trends but underestimate both masses by a common factor $\approx$ 2, suggesting a many-body renormalization of the heavy-hole mass in this strongly asymmetric regime.

cond-mat.mes-hall↗

arXiv2026

Electroluminescence in dopant-free GaAs/AlGaAs single heterojunctions: 2D free excitons, H-band, and the tidal effect

Bright electroluminescence (EL) from dopant-free ambipolar lateral p-n junctions in GaAs/AlGaAs single heterointerface (SH) heterostructures is used to probe neutral free excitons arising from two-dimensional electron and hole gases (2DEGs and 2DHGs). The EL spectra reveal both the heavy-hole neutral free exciton (X$^0$) and the high-energy free exciton of the H band (HE). A combination of transition energies, lifetimes, spatial emission profiles, and temperature dependences points to a predominantly two-dimensional character for these excitons at the SH. For X$^0$, the EL peak energies (1515.5-1515.7 meV) lie slightly above the corresponding bulk GaAs photoluminescence (PL) line at 1515.3 meV, while time-resolved measurements yield markedly shorter lifetimes for EL than for PL (337 ps vs. 1610 ps), consistent with recombination in a confined interfacial layer. The HE exciton exhibits a Stark blueshift under forward bias below threshold, and its energies and lifetimes (down to 575 ps) are tuned by the topgate voltage; above threshold, HE emission is quenched in favor of X$^0$. Finally, the tidal effect $-$ a form of pulsed EL generated by swapping the topgate voltage polarity in ambipolar field-effect transistors $-$ produces an X$^0$ line at the same energy as in the lateral p-n junction and reproduces the characteristic nonmonotonic frequency dependence of the brightness previously observed in quantum-well heterostructures, again indicating a 2D-like origin. Taken together, these results show electrically generated and controllable 2D-like excitons (HE and X$^0$), thereby bridging 2D exciton physics and 2DEG/2DHG platforms in dopant-free GaAs/AlGaAs SH devices.

cond-mat.mes-hall↗

arXiv2025

Pulsed Electroluminescence in a Dopant-free Gateable Semiconductor

We report on a stable form of pulsed electroluminescence in a dopant-free direct bandgap semiconductor heterostructure which we coin the tidal effect. Swapping an inducing gate voltage in an ambipolar field effect transistor allows incoming and outgoing carriers of opposite charge to meet and recombine radiatively. We develop a model to explain the carrier dynamics that underpins the frequency response of the pulsed electroluminescence intensity. Higher mobilities enable larger active emission areas than previous reports, as well as stable emission over long timescales.

cond-mat.mes-hall↗

arXiv2025

Optimizing GaAs/AlGaAs Growth on GaAs (111)B for Enhanced Nonlinear Efficiency in Quantum Optical Metasurfaces

This study is an optimization of GaAs and Al$_{0.55}$Ga$_{0.45}$As growth on GaAs (111)B substrates with a surface misorientation of 2° towards [$\bar{2}$11], aiming to enhance surface morphology. For quantum optical metasurfaces (QOMs) the change from (001) growth to (111) growth will increase the efficiency of the nonlinear process of spontaneous parametric downconversion (SPDC). This work identifies key factors affecting surface roughness, including the detrimental effects of traditional thermal oxide desorption, which prevented growth of smooth surfaces. A novel Ga-assisted oxide desorption method using Ga flux "pulses" was developed, successfully removing oxides while preserving surface quality. In-situ characterization techniques, including reflection high energy electron diffraction (RHEED) and a novel technique called Diffuse Laser Scattering (DLS), were employed to monitor and control the oxide removal process. Mounting quarter wafers with sapphire substrates as optical diffusers improved surface uniformity by mitigating temperature gradients. The uniformity of the growths was clearly visualized by an in-house technique called black box scattering (BBS) imaging. Indium (In) was tested as a surfactant to promote step-flow growth in GaAs, which resulted in atomic-scale flatness with root mean square (RMS) roughness as low as 0.256 nm. In combination with a higher growth temperature, the RMS roughness of Al$_{0.55}$Ga$_{0.45}$As layers was reduced to 0.302 nm. The optimized growth methods achieved acceptable roughness levels for QOM fabrication, with ongoing efforts to apply these techniques to various devices based on GaAs/AlGaAs heterostructures with (111) orientation.

cond-mat.mtrl-sci↗

arXiv2024

Novel 3D Reciprocal Space Visualization of Strain Relaxation in InSb on GaAs Substrates

This study introduces the Reciprocal Space Polar Visualization (RSPV) method, a novel approach for visualizing X-ray diffraction-based reciprocal space data. RSPV allows for the precise separation of tilt and strain, facilitating their individual analysis. InSb was grown by molecular beam epitaxy (MBE) on two (001) GaAs substrates $\unicode{x2014}$ one with no misorientation (Sample A) $\unicode{x2014}$ one with 2° surface misorientation from the (001) planes (Sample B). There is a substantial lattice mismatch with the substrate and this results in the generation of defects within the InSb layer during growth. To demonstrate RSPV's effectiveness, a comprehensive comparison of surface morphology, dislocation density, strain, and tilt was conducted. RSPV revealed previously unobserved features of the (004) InSb Bragg peak, partially explained by the presence of threading dislocations and oriented abrupt steps (OASs). Surface morphologies examined by an atomic force microscope (AFM) revealed that Sample B had significantly lower root mean square (RMS) roughness. Independent estimates of threading dislocation density (TDD) using X-ray diffraction (XRD) and electron channelling contrast imaging (ECCI) confirmed that Sample B exhibited a significantly lower TDD than Sample A. XRD methods further revealed unequal amounts of $α$ and $β$ type threading dislocations in both samples, contributing to an anisotropic Bragg peak. RSPV is shown to be a robust method for exploring 3D reciprocal space in any crystal, demonstrating that growing InSb on misoriented GaAs produced a higher-quality crystal compared to an on-orientation substrate.

cond-mat.mtrl-sci↗

arXiv2021

Effects of biased and unbiased illuminations on dopant-free GaAs/AlGaAs 2DEGs

Illumination is performed at low temperature on dopant-free two-dimensional electron gases (2DEGs) of varying depths, under unbiased (gates grounded) and biased (gates at a positive or negative voltage) conditions. Unbiased illuminations in 2DEGs located more than 70 nm away from the surface result in a gain in mobility at a given electron density, primarily driven by the reduction of background impurities. In 2DEGs closer to the surface, unbiased illuminations result in a mobility loss, driven by an increase in surface charge density. Biased illuminations performed with positive applied gate voltages result in a mobility gain, whereas those performed with negative applied voltages result in a mobility loss. The magnitude of the mobility gain (loss) weakens with 2DEG depth, and is likely driven by a reduction (increase) in surface charge density. Remarkably, this mobility gain/loss is fully reversible by performing another biased illumination with the appropriate gate voltage, provided both n-type and p-type ohmic contacts are present. Experimental results are modeled with Boltzmann transport theory, and possible mechanisms are discussed.

cond-mat.mes-hall↗

arXiv2021

Non-adiabatic single-electron pump in a dopant-free GaAs/AlGaAs 2DEG

We have realized quantized charge pumping using non-adiabatic single-electron pumps in dopant-free GaAs two-dimensional electron gases (2DEGs). The dopant-free III-V platform allows for ambipolar devices, such as p-i-n junctions, that could be combined with such pumps to form electrically-driven single photon sources. Our pumps operate at up to 0.95 GHz and achieve remarkable performance considering the relaxed experimental conditions: one-gate pumping in zero magnetic field and temperatures up to 5K, driven by a simple RF sine waveform. Fitting to a universal decay cascade model yields values for the figure of merit $δ$ that compare favorably to reported modulation-doped GaAs pumps operating under similar conditions. The devices reported here are already suitable for optoelectronics applications, and with further improvement could offer a route to a current standard that does not require sub-Kelvin temperatures and high magnetic fields.

cond-mat.mes-hall↗