M.L. Leadbeater

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.

Inter-Landau-level transitions of resonantly tunnelling electrons in tilted magnetic fields

The effect of a tilted magnetic field on the current-voltage characteristics of an n-type double barrier resonant tunnelling device based on GaAs/(AlGa)As is investigated. The component of magnetic field perpendicular to the tunnelling barriers quantizes the electronic states of the quantum well and emitter accumulation layer into discrete Landau levels. The effect of the component parallel to the barriers is to allow tunnelling transitions in which the Landau level index is not conserved. This gives rise to a spitting of the resonant peak in the current-voltage curve into a series of equally spaced sub-peaks. These features are explained using the sequential theory of resonant tunnelling and treating the effect of the transverse component of magnetic field in a perturbation approximation.

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.

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.

A new technique for directly probing the intrinsic tristability and its temperature dependence in a resonant tunneling diode

Abstract A new measurement technique employing a positively sloping load line has been used to probe the region of apparent bistability near a tunneling resonance in the electrical characteristic of a resonant tunneling diode. This technique is equivalent to using a voltage source and negative series resistance. The appearance of bistability is an artifact of the conventional measuring technique which uses a load line with negative slope. The complete characteristic is found to be a continuous Z shaped curve between 20 and 150 K, corresponding to tristability and in accordance with theoretical models based on the effects of charge accumulation in the central quantum well of the diode. The width of the tristable region passes through a maximum at 40 K and, at 150 K, disappears as the resonance broadens. Above this temperature the resonance develops a region of negative differential resistance (NDR). As the device is cooled below 20 K additional structure develops in the central arm of the Z, with some portions of the characteristic exhibiting five stable current states at temperatures below 15 K. At 4.2 K, the effect of an in plane magnetic field mimics that of increasing temperature.

Resonant magnetotunnelling spectroscopy : a direct probe of the complicated dispersion curves and negative mass behaviour of holes confined in a quantum well

Abstract We describe a novel magnetotunnelling spectroscopy technique for probing the complicated dispersion curves of hole states in the quantum well of p-type double barrier resonant tunnelling structures. Strong mixing between light and heavy hole states is observed. Some of the states clearly exhibit negative hole effective mass for motion in the plane of the quantum well. The results are contrasted with the simple case of resonant tunnelling of electrons in in-plane and tilted magnetic fields. For electron resonant tunnelling in tilted fields, we observe additional peaks in I ( V ). These are due to non-Landau level index-conserving transitions from the two-dimensional emitter accumulation layer to the quantum well.

The effect of the X conduction band minima on resonant tunnelling and charge build-up in double barrier structures based on n-GaAs/(AlGa)As

Abstract Hydrostatic pressure is used to investigate band structure effects in double barrier resonant tunnelling devices based on n-type (AlGa)As/GaAs. The pressure-induced increase in Γ-X tunnelling and scattering reduces the space charge build-up in the quantum well at resonance and allows us to study the transition from intrinsic bistability to ordinary negative differential conductivity in the I(V) characteristics. In the limit of fast Γ-X transfer the resonant peak in I(V) is lost.

The effect of conduction band anisotropy on hybrid magneto-electric states in resonant tunneling devices

Magneto-tunneling is investigated in a double barrier resonant tunneling device based on n-GaAs/(AlGa)As with a central GaAs well of width 120 nm. 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. Due to the ballistic motion of the electrons and the large electric field in the quantum well, the electrons reach high kinetic energies (∼ 1 eV) over part of their orbit. Rotation of the magnetic field in the (100) plane parallel to the barriers reveals the anisotropy of the Γ conduction band at these high electron energies.