Two-dimensional Semiconductor Structures Research Articles

Ballistic spin resonance

The phenomenon of spin resonance has had far-reaching influence since its discovery 70 years ago. Electron spin resonance driven by high-frequency magnetic fields has enhanced our understanding of quantum mechanics, and finds application in fields as diverse as medicine and quantum information. Spin resonance can also be induced by high-frequency electric fields in materials with a spin-orbit interaction; the oscillation of the electrons creates a momentum-dependent effective magnetic field acting on the electron spin. Here we report electron spin resonance due to a spin-orbit interaction that does not require external driving fields. The effect, which we term ballistic spin resonance, is driven by the free motion of electrons that bounce at frequencies of tens of gigahertz in micrometre-scale channels of a two-dimensional electron gas. This is a frequency range that is experimentally challenging to access in spin resonance, and especially difficult on a chip. The resonance is manifest in electrical measurements of pure spin currents-we see a strong suppression of spin relaxation length when the oscillating spin-orbit field is in resonance with spin precession in a static magnetic field. These findings illustrate how the spin-orbit interaction can be harnessed for spin manipulation in a spintronic circuit, and point the way to gate-tunable coherent spin rotations in ballistic nanostructures without external alternating current fields.

Limit to two-dimensional mobility in modulation-doped GaAs quantum structures: How to achieve a mobility of 100 million

Considering scattering by unintentional background charged impurities and by charged dopants in the modulation doping layer as well as by GaAs acoustic phonons, we theoretically consider the practical intrinsic (phonons) and extrinsic (background and dopants) limits to carrier mobility in modulation-doped AlGaAs-GaAs two-dimensional (2D) semiconductor structures. We find that reducing background impurity density to ${10}^{12}\text{ }{\text{cm}}^{\ensuremath{-}3}$ along with a modulation doping separation of $1000\text{ }\text{\AA{}}$ or above will achieve a mobility of $100\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s}$ at a carrier density of $3\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }{\text{cm}}^{\ensuremath{-}2}$ for $T=1\text{ }\text{K}$. At $T=4(10)\text{ }\text{K}$, however, the hard limit to the 2D mobility would be set by acoustic phonon scattering with the maximum intrinsic mobility being no higher than $22(5)\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s}$. Detailed numerical results are presented as a function of carrier density, modulation doping distance, and temperature to provide a quantitative guide to experimental efforts for achieving ultrahigh 2D mobilities.

Exchange effects on electron scattering through a quantum dot embedded in a two-dimensional semiconductor structure

We have developed a theoretical method to study the scattering processes of an incident electron through an $N$-electron quantum dot (QD) embedded in a two-dimensional (2D) semiconductor. The generalized Lippmann-Schwinger equations including the electron-electron interaction in this system are solved for the continuum electron by using the method of continued fractions (MCF) combined with a 2D partial wave expansion technique. The method is applied to a one-electron QD case. Scattering cross sections are obtained for both the singlet and triplet couplings between the incident electron and the QD electron during the scattering. The total elastic scattering cross sections as well as the spin-flip scattering cross sections resulting from the exchange potential are presented. Furthermore, inelastic scattering processes are also studied using a multichannel formalism of the MCF.

Magnetized Plasma in Polar Semiconductors

Plasma excitations in solids have been intensively studied already at the time of the 1960’s. A significant interest was focused on doped polar semiconductors. In these systems, the nowadays text-book effect of coupling between plasmons and optical phonon modes can be clearly observed as demonstrated, for example, by the pioneering work of Mooradian and Wright [1] on inelastic light scattering in n-type GaAs samples. The properties of a plasma subjected to a static magnetic field are significantly modified. A magnetic field B applied to a metallic system introduces a characteristic frequency, namely the cyclotron frequency ωc =eB/m of motion of the carriers (with an effective mass m) in the plane perpendicular to the field direction. In polar-semiconductor plasmas, by changing the magnetic field the cyclotron frequency ωc can be made comparable to all important frequencies of the system, namely the plasma frequency ωp and the frequencies of longitudinal (LO) and transverse (TO) optical phonons. The physics of coupled plasmon phonon modes under these particular and interesting conditions has never been fully explored in Raman scattering experiments in 3D systems. On the other hand, there exist a number of works, concerning both Raman scattering and far-infrared magneto-spectroscopy which address the issue of coupling between the electronic and phonon excitations in twodimensional semiconductor structures. The physics of low energy excitations in these structures is far more complex because of the modification of the energy spectrum due to effects of spatial confinement. The results of high-magmetic-field (up to 28T) inelastic light scattering studies on coupled plasmon-LO-phonon modes in bulk, degenerate n-type GaAs will be presented. [2] The data analysis implies the use of a standard dielectric function approach. Results obtained for samples with high electron concentration are well understood. A strong interaction of coupled LO-phonon-plasmon modes with the collective cyclotron resonance excitations (Bernstein modes) is observed. In samples with lower electron concentration, an unexpected Raman scattering signal in the vicinity of the undressed optical phonon is observed at high magnetic fields. A field induced metal-insulator transition is shown to be observable in Raman scattering experiments in samples with lowest electron concentration. The plasmon-phonon modes which are characteristic of the metallic state are replaced by intradonor excitations in the case of the insulating state. It is very interesting, that the resonant magnetopolaron effect reflecting the interaction between the LO phonon and the 1s-2p intradonor excitation is observed in the insulating state but no coupling between LO-phonon and cyclotron resonance excitations is seen in the metallic state. This result allows us to discuss some aspects of an apparent controversy existing in two-dimensional systems regarding the fundamental problem of Frohlich interaction: whether it manifests itself as a resonant repulsion between the cyclotron resonance and LO-phononlike modes for a two-dimensional electron gas, or not.

Magnetic-impurity states of electron in two-dimensional semiconductor structures

Electron states on an attractive center of small-radius r c ≪ l (l = \(\sqrt {\frac{{c\hbar }}{{eH}}} \) is the magnetic length) located in a two-dimensional structure are investigated in a uniform magnetic field H applied perpendicularly to the structure surface. The spectrum of magnetic-impurity (MI) particle states with an arbitrary moment projection on the direction H for Landau bands 0 ≤ N < l 2/r 2c is derived in the approximation that mixing of Landau levels is weak. The dependence of the electron energy states on magnetic field, the layer thickness, and the impurity position are studied. It is shown that dimension lowering leads to a qualitatively different spectrum of MI states compared to the three-dimensional case [1]. A comparison of the obtained binding energy of the D − center with experimental data is performed.

Low-density spin-polarized transport in two-dimensional semiconductor structures: Temperature-dependent magnetoresistance of Si MOSFETs in an in-plane applied magnetic field

Low-density spin-polarized transport in two-dimensional semiconductor structures: Temperature-dependent magnetoresistance of Si MOSFETs in an in-plane applied magnetic field

Anomalous magnetoresistance of two-dimensional systems in the presence of spin-orbit scattering

A theory of weak localization in two-dimensional semiconductor structures and metal films is developed for spin relaxation by the Elliott-Yafet mechanism. The theory is valid in the entire range of classically weak magnetic fields. It is shown that effects due to spin-orbit interaction substantially modify magnetoresistance in both diffusive and ballistic regimes.

Frequency-dependent electrical transport under intense terahertz radiation

By using a formalism based on quantum transport equation, we study the current response of an electron gas strongly coupled to an intense terahertz laser. The effect of laser is treated exactly, which gives rise to the electron-photon sidebands. Our formalism includes up to the second order in electron-impurity scattering and to any order in optical transitions between the sidebands. Resonant scattering can occur when the frequency of the probing field equals a multiple of the terahertz laser frequency. The usual linear response theory is the zero-photon term in our formalism. This result can be applied to study the electrical transport in strongly coupled electron-photon systems. One such system is a two-dimensional semiconductor structure under an intense terahertz radiation.

Orbital effect of an in-plane magnetic field on quantum transport in chaotic lateral dots

We show that an in-plane magnetic field is able to break time-reversal symmetry of the orbital motion of electrons in two-dimensional semiconductor structures, due to the momentum-dependent inter-subband mixing, which results in supression of weak localization effect. Then, we analyze the influence of the in-plane field on weak localization correction and universal fluctuations of conductance in large-area chaotic semiconductor quantum dots.

Analysis of negative magnetoresistance: Statistics of closed paths. II. Experiment

It is shown that a new kind of information can be extracted from the Fourier transform of negative magnetoresistance in two-dimensional (2D) semiconductor structures. The procedure proposed provides the information on the area distribution function of closed paths and on the area dependence of the average length of closed paths. Based on this line of attack the method of analysis of the negative magnetoresistance is suggested. The method has been used to process the experimental data on negative magnetoresistance in 2D structures with different relations between the momentum and phase relaxation times. It is demonstrated this fact leads to distinction in the area dependence of the average length of closed paths.

Quantum point contacts on InGaAs/InP heterostructures

For the first time we have observed quantized conductance in a split gate quantum point contact prepared in a strained In0.77Ga0.23As/InP two-dimensional electron gas (2DEG). Although quantization effects in gated two-dimensional semiconductor structures are theoretically well known and proven in various experiments on AlGaAs/GaAs and also on In0.04Ga0.96As/GaAs, no quantum point contact has been presented in the InGaAs/InP material with an indium fraction as high as 77% so far. The major problem is the comparatively low Schottky barrier of the InGaAs (φB≈ 0.2 eV) making leakage-free gate structures difficult to obtain. Nevertheless this heterostructure—especially with the highest possible indium content—has remarkable properties concerning quantum interference devices and semiconductor/superconductor hybrid devices because of its large phase coherence length and the small depletion zone, respectively. In order to produce leakage-free split gate point contacts the samples were covered with an insulating SiO2layer prior to metal deposition. The gate geometry was defined by electron-beam lithography. In this paper we present first measurements of a point contact on an In0.77Ga0.23As/InP 2DEG clearly showing quantized conductance.

Green-function theory of plasmons in two-dimensional semiconductor structures: Zero magnetic field

A theoretical investigation has been made of the plasmon excitations in various two-dimensional (2D) semiconductor heterostructures in the framework of a Green-function (or response function) theory. The plasmon excitations in the periodic and nonperiodic systems are implicitly defined by the electromagnetic fields that are localized at and decay exponentially away from the interfaces. The Green-function theory generalized to be applicable to the 2D systems enables one to derive explicit expressions for the corresponding response functions (associated with the electromagnetic fields), which can in turn be used to calculate almost all physical properties of the systems at hand. A rigorous analytical diagnosis of the general results for all the systems investigated here leads one to reproduce exactly the previously well-established results obtained within a different theoretical framework. The elegance of the theory lies in its simplicity and the compact form of the desired results. The impact and relevance of the analytical results have been discussed briefly.

A Semiclassical Approach to the Electron Gas in Two-Dimensional Semiconductor Structures

A theory is developed of the density of states (DOS) of the two-dimensional electron gas (2D EG) in semiconductor heterostructures, taking into account the effect of disorder caused by some random field existing in the sample. For a smooth random field, the calculation is carried out within its Gaussian statistics and a semiclassical approach. A simple closed expression thus obtained includes the classical DOS and its quantum correction as well, which describe the DOS of the 2D EG in explicit dependence on the rms of the potential and of the force of the random field. The disorder effect is found to smear out the step-like singularity of the DOS at an unperturbed band edge of the ideal 2D EG into a tail deep below the band edge. A detailed treatment is given of the case when the disorder is due to remote ionized impurities, which are distributed randomly or correlated in the sample.

A Green-Function Theory of Plasmons in Two-Dimensional Semiconductor Structures: Finite Magnetic Field

A theoretical investigation has been made of the magnetoplasmon excitations in several two-dimensional (2D) semiconductor structures subjected to an applied magnetic field in the framework of a Green-function (or response function) theory. The applied magnetic field B→0is assumed to be oriented parallel to the interfaces and perpendicular to the direction of propagation (Voigt geometry). The material layers are characterized by frequency-dependent dielectric function and the quantum–size effects are ignored. The Green-function theory generalized to be applicable to the 2D systems enables us to derive explicit expressions for the corresponding response functions (associated with the electromagnetic fields) which can in turn be used to calculate almost all physical properties of the system at hand. A simple analytical diagnosis of the general results for all the systems investigated here leads us to reproduce exactly the previously well-established results, for the dispersion relations for plasmons and magnetoplasmons, obtained within a different theoretical framework. As examples, we have incorporated numerical results on the dispersion characteristics of magnetoplasmons in several geometries. It is found that the excitation spectrum in the Voigt geometry contains a complete magnetic-field-dependent gap within which no magnetoplasmons are allowed to propagate. The simplicity and the compact forms of the desired results give the present theory a width of interest. The implications of the analytical and numerical results have been discussed briefly.

Application of microwave detection of the Shubnikov–de Haas effect in two-dimensional systems

Microwave detection of the Shubnikov–de Haas (SdH) effect as a contact-free characterization technique for different types of two-dimensional semiconductor structures is explored in the low magnetic field region. The detection technique and the data analysis are described. The character and relevance of the single-particle relaxation time that can be detected by this technique are distinguished from the usual transport scattering time. The measured values of the carrier concentration and single-particle relaxation time agree with electrical measurements, while the problem of making contacts on the structure is avoided. Uncertainties in the analysis for the single-particle relaxation time are discussed. Cyclotron resonance, optically detected cyclotron resonance, and magneto-photoluminescence are applied as other contact-free techniques on the same samples. The results and suitability of these techniques are compared with the microwave detection of the SdH effect.

Observation of current filaments in GaAs/AlxGa1−xAs heterostructures using a time-resolved imaging technique

We developed a time-resolved optical beam induced current (TROBIC) technique, and performed time-resolved current imaging experiments on GaAs/AlxGa1−xAs heterostructures under high electric field conditions. These experiments are the first time-resolved imaging experiments of current patterns in a two-dimensional semiconductor structure. We attribute the current patterns observed in the TROBIC images to the formation of current filaments in the AlxGa1−xAs layer, parallel to the two-dimensional electron gas (2DEG). We show that even in samples where the two-dimensional electron gas and the contacts to the 2DEG are perfectly ohmic and homogeneous, current filaments can still develop in high electric fields. These temporal and spatial instabilities in the AlxGa1−xAs layer strongly affect the high-field transport properties of the heterostructure.

Quantum bound states in narrow ballistic channels with intersections.

A quantum-mechanical calculation is made of ballistic transport in intersecting narrow channels of finite length. The two-dimensional (2D) semiconductor structure we study consists of perpendicular channels, one of which connects to two reservoirs of 2D electron gas. These reservoirs serve as emitter and collectors when a potential difference is applied. At a single intersection with infinite leads, there are generally bound states that are well localized to the intersection area. In a structure with finite leads to emitter and collector, such localized states or quantum dots give rise to resonant tunneling. We show that in narrow ballistic channels with few stubs the bound states couple to each other. Therefore, the states at the different intersections combine as split bound states. The splitting is N-fold if there are N intersections. Here we study conductance in structures containing a few crossed-bar or T-shaped junctions and focus on the splitting of the resonance conductance below the first subband threshold. We argue that our results are in qualitative agreement with recent measurements. We also consider the spatial distribution of currents and show that a complicated flow pattern with vortex structures appears at higher energies.

Dielectric confinement effect for exciton and biexciton states in PbI 4-based two-dimensional semiconductor structures

We have studied the role of dielectric confinement for enhancing excitonic effects in layered PbI 4-based compounds. In particular, we demonstrate that unusually strong biexciton effects are readily observable (binding energy on the order of 50 meV) and that the large excitonic binding (∼ 300 meV) can be modified by chemically adjusting the dielectric constant of the alkylammonium barrier layers.

Dynamics of stimulated emission and optical gain in highly excited superlattices

The results of low temperature (4.2K) stimulated emission and optical gain dynamics in highly photoexcited Al xGa 1−xAs/As type I superlattices are presented. It was established that the increase of the pump quantum energy leads to the essential (more than one order) reduction of the stimulated emission intensity. The effect is explained in terms of quasiparticle system heating peculiarities in two-dimensional semiconductor structure.

High-field transport with hot phonons in degenerate semiconductors

The effect of phonons in non-thermodynamic equilibrium (hot-phonons) on steady-state transport in degenerate bulk and two-dimensional (2D) semiconductor structures is studied. The phonon-drift effects are taken to be negligible, and, the formulation is confined to electrons in a single parabolic band interacting with a non - drifted population of polar-optic phonons, restricted to interface modes in the 2D case. Hot phonons in bulk semiconductors lead not only to a decrease in the energy relaxation rate, but, also, to a decrease in the mobility as the carrier concentration n 3D is increased. Hot phonons in 2D systems, on the other hand, cause the energy relaxation rate and mobility to go through a minima as the electron concentration n 2D is increased. The electron drift velocity, contrary to the bulk case, is seen to increase with n 2D for n 2 D > 10 12 cm −2, and reflects the difference in the nature of electron density of states in the two cases.