High-mobility Two-dimensional Hole Systems Research Articles

Commensurability Oscillations of Hole-Flux Composite Fermions

We report the observation of commensurability oscillations of hole-flux composite fermions near a filling factor of ν=1/2 in a high-mobility two-dimensional hole system confined to a GaAs quantum well and subjected to a weak, strain-induced, unidirectional periodic potential modulation. The oscillations, which are consistent with ballistic transport of fully spin-polarized composite fermions in a weak periodic effective magnetic field, are surprisingly strong and exhibit up to third-order minima. We extract a ballistic mean-free path of about 0.1 μm for the hole-flux composite fermions.

High mobility two-dimensional hole system on hydrogen-terminated silicon (111) surfaces

We have realized a two-dimensional hole system (2DHS), in which the 2DHS is induced at an atomically flat hydrogen-terminated Si(111) surface by a negative gate voltage applied across a vacuum cavity. Hole densities up to 7.5 × 1011 cm−2 are obtained, and the peak hole mobility is about 104 cm2/Vs at 70 mK. The quantum Hall effect is observed. Shubnikov-de Haas oscillations show a beating pattern due to the spin-orbit effects, and the inferred zero-field spin splitting can be tuned by the gate voltage.

Gate control of low-temperature spin dynamics in two-dimensional hole systems

We have investigated spin and carrier dynamics of resident holes in high-mobility two-dimensional hole systems in $\text{GaAs}/{\text{Al}}_{0.3}{\text{Ga}}_{0.7}\text{As}$ single-quantum wells at temperatures down to 400 mK. Time-resolved Faraday and Kerr rotation as well as time-resolved photoluminescence spectroscopy are utilized in our study. We observe long-lived hole-spin dynamics that are strongly temperature dependent, indicating that in-plane localization is crucial for hole-spin coherence. By applying a gate voltage, we are able to tune the observed hole $g$ factor by more than 50%. Calculations of the hole $g$ tensor as a function of the applied bias show excellent agreement with our experimental findings.

Magnetic ordering effects in a Mn-modulation-doped high mobility two-dimensional hole system

We have studied the magnetotransport properties of a manganese (Mn)-modulation-doped high mobility two-dimensional hole system in a strained InAs quantum well (QW) structure. At precisely $T=600\text{ }\text{mK}$ a phase transition from paramagnetism to ferromagnetism can be observed by a change of the low-field magnetotransport behavior and hysteresis. In the magnetically ordered phase a superposition of positive magnetoresistance and weak antilocalization was detected in the longitudinal resistance ${R}_{xx}$ and in the Hall resistance ${R}_{xy}$ a superposition of normal, anomalous, and planar Hall effects demonstrating spontaneous magnetization in the QW plane was detected. From extensive analysis of the temperature and magnetic field dependence of the Shubnikov--de Haas oscillations we deduce the effective mass, transport, and quantum-scattering times. The latter indicates presence of small-range scattering potential. From corrections to the Drude conductivity we determine the impurity interaction time, which is significantly reduced in the ferromagnetic phase indicating interaction of the two-dimensional free holes' spin with the localized magnetic moments of 5/2 from Mn ions.

Transport and Percolation in a Low-Density High-Mobility Two-Dimensional Hole System

We present a study of the temperature and density dependence of the resistivity of an extremely high quality two-dimensional hole system grown on the (100) surface of GaAs. For high densities in the metallic regime (p > or approximately4x10;{9} cm;{-2}), the nonmonotonic temperature dependence ( approximately 50-300 mK) of the resistivity is consistent with temperature dependent screening of residual impurities. At a fixed temperature of T=50 mK, the conductivity versus density data indicate an inhomogeneity driven percolation-type transition to an insulating state at a critical density of 3.8x10;{9} cm;{-2}.

Undoped high mobility two-dimensional hole-channel GaAs∕AlxGa1−xAs heterostructure field-effect transistors with atomic-layer-deposited dielectric

The authors have fabricated undoped p-channel GaAs∕AlxGa1−xAs heterostructure field-effect transistors with nearly ideal drain current-voltage characteristics, using atomic-layer-deposited Al2O3 as the dielectric, and measured their transport properties. At 0.3K, the densities and mobilities of the two dimensional holes can be tuned up to 2.9×1011∕cm2 and 6.4×105cm2∕Vs, respectively. The variable density high mobility two-dimensional hole system provides a large parameter space for the study of two-dimensional physics.

Growth and subband structure determination of high mobility hole gases on (0 0 1) and (1 1 0) GaAs

We introduce high mobility two-dimensional hole systems (2DHSs) in AlGaAs/GaAs heterosystems grown by molecular beam epitaxy (MBE). Utilizing a carbon filament source, two-dimensional hole gases (2DHGs) were fabricated on the (0 0 1) and (1 1 0) crystal plan. These samples exhibit low-temperature mobilities beyond 10 6 cm 2/V s at densities varying from 0.6 to 2.3×10 11 cm −2. In order to explore the subband structure as a function of carrier density, aluminum top gates were deposited on the samples. The carrier density was found to show a hysteretic behavior when tuned with an external electric field. The origin of this effect will be discussed. It opens a way to adjust the hole density in a wide range within a single sample in the absence of an externally applied electric field.

High mobility two-dimensional hole system in GaAs∕AlGaAs quantum wells grown on (100) GaAs substrates

We report on the transport properties of a high mobility two-dimensional hole system (2DHS) confined in GaAs∕AlGaAs quantum wells grown by molecular-beam epitaxy on the (100) surface of GaAs. The quantum wells are modulation doped with carbon utilizing a resistive filament source. At T=0.3K and carrier density p=1×1011cm−2, a mobility of 106cm2∕Vs is achieved. At fixed carrier density p=1011cm−2, the mobility is found to be a nonmonotonic function of the quantum well width. The mobility peaks at 106cm2∕Vs for a 15-nm well and is reduced for both smaller and larger well widths for these (100) samples. The mobility anisotropy is found to be small. Mobility along [01¯1] is approximately 20% higher than along the [011] direction. In addition, the low-temperature carrier density is found to have low sensitivity to light. The hole density increases by only ∼10% after exposure to red light at T=4.2K. In structures designed for a lower carrier density of 3.6×1010cm−2, a mobility of 800000cm2∕Vs is achieved at T=15mK.

OSCILLATORY AND VANISHING RESISTANCE STATES IN MICROWAVE IRRADIATED 2D ELECTRON SYSTEMS

Giant-amplitude oscillations in dc magnetoresistance of a high-mobility two-dimensional electron system can be induced by millimeterwave irradiations, leading to zero-resistance states at the oscillation minima. Following a brief overview of the now well-known phenomenon, this paper reports on aspects of more recent experiments on the subject. These are: new zero-resistance states associated with multi-photon processes; suppression of Shubnikov-de Haas oscillations by high-frequency microwaves; and microwave photoconductivity of a high-mobility two-dimensional hole system.

Pressure dependence of the cyclotron resonance modes in high mobility two-dimensional hole systems in GaAs–(Ga,Al)As heterojunctions

We have performed the first cyclotron resonance studies of high mobility two-dimensional hole systems (2DHS) under pressure. Experiments were carried out using millimetre wave radiation on GaAs–(Ga,Al)As heterojunctions. Pressures were applied using a clamp cell specially designed for millimetre wave measurements. Our data at ambient pressure show collectively coupled resonances arising from interactions between the two hole systems with different effective masses formed by the spin-split halves of the lowest heavy-hole subband. The application of pressure appears to completely suppress the collective effect and only a resonance corresponding to the lighter cyclotron mass from the heavy-hole spin-split subband is observed.

Zero-field spin splitting in GaAs/AlGaAs 2D holes

From Fourier analysis of the low-magnetic-field Shubnikov–de Haas oscillations, we quantitatively determine the spin splitting, in the limit of zero magnetic field, for high-mobility two-dimensional hole systems confined to GaAs/AlGaAs heterostructures. The spin-splitting can be tuned continuously from large values to nearly zero in a square quantum well whose charge distribution can be controlled from being asymmetric to symmetric via the application of a front-gate bias. In a triangular well with an asymmetric potential, we always observe substantial spin splitting. In the latter system, our measured commensurability oscillations, induced by a one-dimensional periodic potential, reveal two frequencies, providing clear evidence for spin-resolved ballistic transport.

Collective cyclotron modes in high-mobility two-dimensional hole systems in GaAs - (Ga, Al)As heterojunctions: II. Experiments at magnetic fields of up to forty Tesla

The cyclotron resonance of very high-mobility holes in GaAs - (Ga, Al)As heterojunctions grown on (111), (311) and (100) substrates has been studied in high magnetic fields of up to 40 T. As the temperature is increased from to , the cyclotron resonance is found to shift to lower magnetic fields, the size of shift depending on the cyclotron frequency and the substrate orientation. These observations may be explained using the model of interacting hole subsystems developed by Cole et al.

Collective cyclotron modes in high-mobility two-dimensional hole systems in GaAs - (Ga, Al)As heterojunctions: I. Experiments at low magnetic fields and theory

The cyclotron resonance of very high-mobility two-dimensional holes in GaAs - (Ga, Al)As heterojunctions grown on (111), (311) and (100) substrates has been studied over the frequency range 30 to 200 GHz. Although the presence of two hole spin subbands in the samples suggests that two cyclotron resonances should be observed, in practice only a single resonance occurs for a wide range of conditions (temperature, field) and sample properties (hole density, mobility). Furthermore, the cyclotron resonance spectra often exhibit a strong temperature dependence. In the case of a single, sharp cyclotron resonance, the resonance field may shift by as much as 20% when warming the sample from 1.4 to 4.2 K. In the case of spectra containing multiple cyclotron resonances, similar changes in temperature shift the resonance positions together to form a single cyclotron absorption. This behaviour is explained in terms of two interacting hole subsystems with different effective masses formed by the two spin subbands. An analytical expression for the contribution to the high-frequency conductivity due to coupled cyclotron motion of the two hole systems is derived and shown to encompass previous theories developed for more restricted ranges of conditions. The expression predicts the complex behaviour of the experimental spectra very well, and enables hole masses, hole - lattice scattering rates and hole - hole scattering rates to be extracted. Comparisons between theory and data also show that a reactive interaction dominates the coupling between the spin subsystems at low temperatures. This is the first of two papers dealing with correlated hole cyclotron resonance; the second shows that the model derived in this work can also be used to treat cyclotron resonance data recorded at very high magnetic fields .