E.S. Alves

Sequential tunneling due to intersubband scattering in double‐barrier resonant tunneling devices

Magnetoquantum oscillations in the tunnel current of double‐barrier n‐GaAs/(AlGa)As/GaAs/(AlGa)As/GaAs resonant tunneling devices reveal evidence of sequential tunneling in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the GaAs quantum well. The sequential tunneling arises from intersubband scattering between two quasi‐bound states of the well. Near this resonance, the charge buildup in the well can be estimated from the magnetoquantum oscillations.

Electrical and spectroscopic studies of space-charged buildup, energy relaxation and magnetically enhanced bistability in resonant-tunneling structures

Abstract Photoluminescence measurements and magnetoquantum oscillations in the differential capacitance are used to measure space-charged buildup and study electron thermalization in a double-barrier resonant tunneling structure based on n-type (AlGa)As. The intrinsic bistability observed in the I(V) characteristics is also seen in the linewidth and photon energy of the photoluminescence. The spectroscopic data reveal clearly the importance of intersubband transitions in the voltage range at which electrons tunnel resonantly into the second bound state of the quantum well. A novel field-induced enhancement of the intrinsic bistability effect is reported for B ‖ J .

Observation of space-charge bulk-up and thermalisation in an asymmetric double-barrier resonant tunnelling structure

By means of a study of magnetoquantum oscillations in the differential capacitance, the authors have observed the thermalisation of the space charge stored dynamically in the quantum well of an asymmetric double-barrier resonant tunnelling heterostructure based on n-GaAs/(AlGa)As. Fourier analysis of the oscillations was used to monitor the charge build-up in both the emitter accumulation layer and in the well. The storage time of an electron in the well was found to be approximately=0.5 mu s. The resonant tunnelling is truly sequential rather than coherent.

Charge build-up and intrinsic bistability in an asymmetric resonant-tunnelling structure

Intrinsic bistability is observed in an asymmetric resonant-tunnelling structure based on n-GaAs/(AlGa)As, incorporating a thin emitter barrier and a thick collector barrier. The resonant charge build-up in the quantum well which gives rise to the bistability is monitored by the Landau level structure in the magneto-capacitance.

The oscillatory magnetoresistance of electrons in a square superlattice potential

The transverse magnetoresistance of a two-dimensional electron gas in an n-type GaAs/(AlGa)As heterostructure subjected to a square superlattice potential is investigated. Magneto-oscillations are observed at low field (B ≤ 0.4 T) with period Δ(1/B) = ea/2kF, where a = 145 nm is the superlattice constant. At higher fields the magneto-resistance is dominated by Shubnikov-de Haas oscillations. A comparison is made with experiments on a one-dimensional superlattice.

Hybrid magneto-electric states in resonant tunnelling structures

Double barrier resonant tunnelling structures with wide undoped quantum wells are used to study quantum ballistic transport in the presence of a magnetic field B. The structures are based on n−GaAs/(AlGa)As with well widths of 60 and 120 nm. At B=0, the wider well structure (120 nm) shows as many as 70 resonances in I(V). With B applied in the plane of the barriers (B·J) these resonances evolve into hybrid magneto-electric states. At sufficiently large B, the electron orbits no longer extend to the second barrier and tunnelling occurs into cycloidal interface states which are localised near the emitter barrier. A theoretical model for the observed resonances based on the quantisation of the hybrid and cycloidal orbits is presented. Ballistic path lengths of at least 400 nm are observed.

Evidence for sequential tunnelling and charge build-up in double barrier resonant tunnelling devices

Abstract Magnetoquantum oscillations in the tunnel current of double barrier n-GaAs/(AlGa)As/GaAs/ (AlGa)As/GaAs resonant tunnelling devices are shown to contain both resonant and non-resonant contributions. The former demonstrates the build-up of charge in the GaAs quantum well when electrons are resonant with the first subband of the well. A theoretical analysis of resonant tunnelling is in excellent agreement with the experimental data. Evidence of sequential tunnelling is found in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the well. Near this resonance, the charge build-up in the well is estimated from the magnetotunnelling oscillations.

Resonant tunnelling studies of magnetoelectric quantisation in wide quantum wells

Resonant tunnelling in n-type GaAs-(AlGa)As double-barrier heterostructures with wide quantum wells is investigated as a function of magnetic field applied in the plane of the tunnel barriers. The evolution of the resonances in the current-voltage characteristics with magnetic field is used to study the transition from electric to magnetic confinement of electrons in the quantum well.

Hot‐electron magnetospectroscopy in resonant tunneling devices: A probe of conduction‐band anisotropy

We report a series of magnetotransport measurements on two double barrier resonant tunneling devices based on n‐GaAs/(AlGa)As. The central GaAs wells have widths of 60 and 120 nm. Many well‐defined quantum resonances are observed in the conductance even at room temperature and up to 70 resonances are observed at liquid helium temperatures. The existence of these resonances indicates that a significant fraction of the conduction electrons travel ballistically over distances >0.3 μm. When a magnetic field is applied in the plane of the well (B⊥J) tunneling takes places into two distinct types of quantum states: traversing states in which the electrons interact with both barriers and skipping states in which the electrons only interact with the emitter barrier. Rotation of the magnetic field in the (100) plane parallel to the barriers reveals the anisotropy in the Γ conduction band at high electron energies.

Magnetic field and capacitance studies of intrinsic bistability in double-barrier structures

Abstract Intrinsic bistability is observed in the current-voltage and capacitance-voltage characteristics of double-barrier resonant tunnelling structures. Analysis of the C(V) measurements and the magneto-oscillations in both I(V) and C(V) is used to determine the resonant charge build-up in the well which gives rise to the intrinsic bistability.