Session R15 - Semiconductor Heterostructure: Electronic Properties.
MIXED session, Friday morning, March 21
Room 2201, Conv. Center

[R15.01] Energy loss spectra for a quasi-two dimensional electron gas

A theoretical investigation is made of the fast-particle energy loss to a quasi-two dimensional electron gas (Q2DEG) within the linear response theory. The formalism is based on the dielectric response theory which proceeds in two steps. The first step consists in evaluating the work done by the polarization field of the sample on the electron (responsible for the polarization) along its classical trajectory in which the electron is regarded as an external time-dependent potential that causes transitions in the target. The first, classical step is complemented by a suitable quantal description of the multiple excitations emitted or absorbed by the electron. The essential result of the second step is accomplished by confining ourselves within the Bohm-Pines random-phase approximation, so that exchange-correlation effects are ignored. One-dimensional self-consistent solutions of the Schrdinger and Poisson equations are used to find the electronic states in GaAs- Ga1-x AlxAs quantum well. Analytical closed-from expressions for the energy-loss spectra for inelastically scattered coherent electron beam are obtained in terms of the exact inverse dielectric function[1]. We consider three geometries: the fast-particle moving parallel to, specularly reflected from, and shooting through the Q2DEG. Detailed numerical results on the collective excitaions and energy-loss spectra will be presented and the observation of the predicted loss spectra through the high-resolution electron energy loss spectroscopy (HREELS) will be discussed.

\beginenumerate \item M.S. Kushwaha and F. Garc!a-Moliner, Phys. Lett. A 205, 217 (1995). \endenumerate

[R15.02] Compressibility in the Double-Layer Electron Systems

We have examined the compressibility matrix in coupled double-layer electron systems. Underappropriate circumstances the compressibility matrix of this system becomes singular. The singularity is manifested experimentally by a discontinuity in the dependence of charge transferred between layers on bias voltage. The compressibility matrix can be expressed in terms of second derivatives of the electronic energy with respect to charge density in the two layers. We have evaluated it by computing the interaction energy for two coupled electronic layers using the STLS approximation, treating interlayer and intralayer correlations fully self-consistently. In our numerical work we have found it useful to do a coupling constant integration for the interlayer interaction alone. We find that the compressibility singularities tend to occur only when at least one of the layers has quite low electron density and comment on the implications for recent experimental work.

[R15.03] Interlayer charge transfer in bilayer systems

As the front layer density in a bilayer system is depleted by the application of a biased front-gate, the density in the back layer increases due to the exchange term in the energy. We study bilayer electron or hole systems in double quantum well structures of 140 Åwide GaAs wells separated by 30 Åwide AlAs barriers. The structures have layer densities in the range of 3x10^10 cm^-2 to 7x10^10 cm^-2. For samples with these parameters, a self-consistent calculation using the local-density approximation predicts that when a low enough front layer density is reached, all of the remaining charge will abruptly transfer to the back layer. Experimentally, we do not observe this discontinuous charge transfer. We discuss possible origins for this discrepancy.

[R15.04] Diffusion Monte Carlo study of the phase diagram of coupled electron--electron and electron--hole layers

We have determined the phase diagram of symmetric electron bilayers at zero temperature by means of Diffusion Monte Carlo simulations within the fixed-node approximation. We show that for small values of the distance between layers, interlayer correlations stabilize crystalline phases at much higher densities than for a single, isolated layer. These densities are now experimentally attainable in GaAs/AlGaAs heterostructures by means of heavy holes. A further decrease of the interlayer distance results in enhanced screening that destroys crystalline order. A similar study for the case of symmetric electron--hole layers is made by using trial wavefunctions of the BCS type in order to explicitly take into account the excitonic instability of the particles.

[R15.05] Scattering and Momentum Conservation at a Metal/Semiconductor Interface probed by BEEM

The verification of scattering or parallel momentum nonconservation at a metal/semiconductor (M/S) interface is fundamentally important to novel hot electron device structures and photodetectors. In epitaxial M/S structures it is often assumed that parallel momentum is conserved across the interface, whereas for non-epitaxial systems it is generally presumed not to be. The direct verification of momentum conservation at such an interface has been difficult previously, primarily due to the presence of other sources of scattering. This talk will describe experiments using ballistic-electron-emission microscopy (BEEM) which provide strong evidence of momentum conservation at the Au/Si interface. A scattering model is developed to describe the data, which requires the presence of multiple electron reflections within the Au layer. Data which independently verifies the contribution of these reflections will also be presented.

[R15.06] Band Structure Characterization of AlGaInP Lasers by Model-Solid Analysis of Pressure-Photoluminescence Experiments

We report studies of the different \Gamma- and X- energy gaps, and band offsets, in AlGaInP quantum well lasers using high pressure photoluminescence (PL) interpreted via the model-solid theory (MST).(C.G. Van de Walle, Phys. Rev. B39, 1871 (1989).) A diamond-anvil cell was employed to measure two complex (Al_0.6Ga_0.4)_0.5In_0.5P-Ga_0.4In_0.6P devices at 7K over the range 0-70kbar. The active well widths were 125Åand 30Åboth compressively strained by 0.9%. A detailed analysis fit calculated MST energies to the pressure-PL data. We found that the \underbarnon-linear pressure shifts of the observed transitions could be described easily within the MST by allowing the lattice constants to follow Murnaghan's equation. An important consequence of our work is a realistic set of deformation potentials (constrained tightly by the pressure-PL data) for InP, AlP, and GaP. Using these deformation potentials, we apply the MST to compute the band offsets, and the bandgap energies for arbitrary AlGaInP compositions and pressures. Our MST analysis is a significant refinement over purely empirical fitting schemes employed in previous pressure-PL studies of heterostructures.

[R15.07] Conductance Resonances in a Two-Dimensional Electron Gas Interferometer

In a two-dimensional electron gas system, we use a quantum point contact and a circular reflector gate to confine electron waves in a Fabry-Perot type cavity. Below 1.5 K, quantum interference oscillations are observed as the cavity dimensions are tuned. The width of the quantum point contact may be adjusted, allowing the behavior of the interferometer conductance resonances to be examined in both the multi-mode and the tunneling regimes. In the latter limit, we observe periodic oscillations with finesse, which provide a means to study the nature of electronic transport and interference in nanostructure devices.

This work was funded at Harvard by ONR grants N00014-95-1-0104 and N00014-95-0866, and at UCSB by AFOSR Grant No. F49620-94-1-0158

[R15.08] Coherent Plasmon Generation in Steady State Non-equilibrium Quantum Well Structures.

We show theoretically the possibility of coherent plasmon generation (plasma instabilities) in quantum well structures as well as superlattices. A certain population inversion, maintained by appropriate extraction-injection arrangements or by photo excitation, drives the system to plasma instability. We employ the self-consistent ground state in the Hartree approximation and the RPA response formalism. Strong coherent plasma wave growth is predicted due to flat dispersion of plasma frequency with the in-plane wave number for small wave numbers. The ensuing dipole type induced charge density oscillations of such bounded systems emit electromagnetic radiation, and appropriately designed devices could thus lead to sources for THz radiation. Work supported by U.S. Army Research Office.

[R15.09] Non-Abelian Geometric Phases and Conductance of Spin \frac32 Holes.

We discuss nonabelian geometric phases in a system of two dimensional (2D) holes in mesoscopic semiconductor structures. The nonabelian setting arises naturally in the case of light and heavy holes described by the 4\times 4 (J=\frac32) matrix Luttinger Hamiltonian. Such a system may exhibit effects in which, e.g.\/ one member of a multiplet evolves into another upon completion of a cycle of variation of parameters. In the case of p-doped semiconductors, both doublets of degenerate states exhibit nonabelian properties. We predict conductance oscillations due to this geometric phase, arising from both abelian as well as intrinsically nonabelian effects. A geometry which isolates the nonabelian interference effects is proposed and studied.

[R15.10] Quantitative Study of a Two-Subband Occupied 2D Electron System by Fourier Decomposition of Commensurability Oscillations

We have used Fourier decomposition of commensurability oscillations to quantitatively study ballistic mean-free-path of each subband in a two-dimensional electron system (2DES) with two electric subbands occupied. Lateral one dimensional periodic potentials were created on a 2DES confined to a 45nm-wide GaAs quantum well by depositing a metal gate on top of lithographically patterned PMMA strips with periods from 300nm to 1200nm. By Fourier decomposition of commensurability oscillations, we can separate the oscillations for each individual subbands. From the disappearance of upper subband commensurability oscillations at the modulation period of 1200nm, we conclude that the upper limit of upper subband ballistic mean-free-path is about 3.8\mum. We also deduced the scattering times from the exponential decay of commensurability oscillations for both lower and upper subbands and compared them with scattering times deduced from mobility, magnetic focusing experiment, and Shubnikov-de Haas oscillations.

[R15.11] Can a Resonant Quantum-Well State be Used as the initial and/or Final State of a Golden-Rule Calculation for a Transition Rate due to Photon or Phonon Emission or Absorption?

According to conventional wisdom, the answer would be `no'. A resonant quantum-well state is an eigen-state with a complex eigen-energy (imaginary part negative), due to tunneling through a potential barrier into a reservoir. Such a state is obtained by demanding that only an `out-going' wave (i.e., away from the tunneling barrier) exists in the reservoir. Its wave function cannot be normalized. Therefore, it cannot be used in the way stated in the title. We have investigated a way to change the answer to `yes', so far with partial success. (I.e., `yes', but with some undesirable features.) An idea to improve it is currently being explored, and will be reported at the meeting. Our main trick is to introduce a suitable optical potential in the reservoir, in order to make the resonant state normalizable, and yet with a complex energy very close to the original value. The Fermi golden rule is then generalized, in order to accommodate a non-hermitian Hamiltonian. Other non-resonant reservoir states should make negligible contributions to the transition rate, if this trick is to work.

[R15.12] Geometric Resonance and Spin Configuration in a periodically modulated 2D Electron System

It is now well known that the geometric resonance in two-dimensional electron system (2DES) is a unique technique to establish Fermi wave vector k_f both for the band electrons and for the composite fermions (CF). In the present work we explore the spin configuration and spin transition of a Fermi system by means of geometric resonance in tilted magnetic fields. A periodically modulated 2DES is realized by a high-resolution e-beam lithography and lift-off technique on high mobility MBE and MOCVD GaAs/AlGaAs heterostructures. We achieve a modulation period (a) as small as 1000Å\ on a square mesa of large area 1mm\times1mm. Sharp magnetoresistance maxima and minima near B=0 reveal geometric resonance up to m=6 of the electron orbit 2R_c=2\hbar k_f/eB with a multiple of the period, ma. Angular dependent data near B=0 will be presented together with preliminary results of CF geometric resonance near \nu=1/2 and \nu=3/2.

[R15.13] Electron correlation and charge transfer instability in bilayered two dimensional electron gas

We prove that the predicted charge transfer state in symmetric bilayers of two dimensional electron gases is always unstable at zero bias voltage, due to interlayer interactions and/or tunneling. This is most easily seen by resorting to a pseudospin formalism and considering coherent states obtained from the charge transfer state through rotations of the pseudospins. Diffusion Monte Carlo simulations rule out the presence of a pseudospin--polarized phase at zero separation and tunneling. Evidently, the charge transfer state is stabilized by a sufficiently strong gate voltage, as found in recent experiments. We show that a simple model, in which the layers are strictly two dimensional, is able to account quantitatively for such experimental findings, when intralayer correlation is properly included.

Part R of program listing