E C Valadares

Cyclotron resonance of high-mobility GaAs/AlGaAs (311) 2DHGs

Cyclotron resonance of high-mobility (up to 300000 cm2 V-1 s-1 at 100 mK) GaAs/AlGaAs two-dimensional hole gases (2DHGs) grown on (311)A oriented substrates has been measured in magnetic fields up to 17 T and at temperatures down to 350 mK. The 2D hole density is in the range (1.0-2.0)*1011 cm-2. Two resonances are in general observed; at low magnetic fields the lower-field resonance dominates but a progressive transfer of oscillator strength takes place as the field increases. A self-consistent calculation of the Landau fan diagram for the higher-symmetry (100) direction is used to interpret the data and identify the observed transitions.

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.

Molecular beam epitaxy growth of GaAs/AlAs double-barrier resonant tunnelling devices on (311)A substrates

The authors investigate resonant tunnelling in p-type silicon-doped GaAs/AlAs double-barrier quantum well structures grown by molecular beam epitaxy on the (311)A GaAs surface. Their current-voltage characteristics compare favourably with structures grown on the conventional (100) orientation using beryllium as the acceptor.

High-magnetic-field studies of hole energy dispersion, cubic anisotropy and space charge build-up in the quantum well of p-type resonant tunnelling devices

Magnetic fields up to 41 T (B perpendicular to J) are used to investigate hole tunnelling in p-type double-barrier AlAs/GaAs structures and to probe the dispersion curves of the hole states of the quantum well. The high magnetic fields reveal the cubic anisotropy of these states. A detailed comparison is made between the experimental data and the theoretical hole dispersion curves. Hole space charge build-up is observed in the quantum well and is measured using the Landau quantization effect for B//J.

Self-consistent calculations of emitter accumulation layers in biased GaAs-AlxGa1-xAs heterostructures

A self-consistent numerical calculation is made of the potential profile in an accumulation layer with single subband in the lightly n-doped emitter contact of a vertical tunnel structure under an applied bias. A background of fixed minority acceptor charge is included to account for the pinning of the Fermi level of the two-dimensional electron gas to the band edge of the bulk emitter layer. The variation of the width of the accumulation layer and potential drop across the structure with a real electron density is found to be very similar to that given by the Fang-Howard one-parameter variational model. However, in the Fang-Howard approximation a thermodynamically stable state does not exist below a critical electron density (9.5*1010 cm-2 for GaAs).

High magnetic field studies of the confined hole states in quantum wells grown on novel substrate orientations: Evidence for a ‘camel's back’ structure in the energy subbands

Abstract Magnetic fields are used to examine the anisotropic in-plane energy dispersion of the quantum well states of an AlAs/GaAs/AlAs double-barrier diode grown on a (3 1 1)A substrate. The measurements reveal biaxial anisotropy in some subbands. These features are confirmed by a six-component envelope-function calculation of the subbands of a (3 1 1) valence band quantum well.

Sequential tunneling of holes in p-type semiconductor double-barrier heterostructures

We propose an extension of Bardeens transfer Hamiltonian formalism to treat the sequential tunneling of holes in p-type semiconductor heterostructures displaying weakly coupled QWs. Our discussion is aimed at modeling recent resonant magnetotunneling experiments on double-barrier heterostructures displaying an emitter accumulation layer and wide barriers.

Magnetotunnelling Spectroscopy to Measure the Electron and Hole ε(k) Dispersion Curves in the Quantum Well of Resonant Tunnelling Structures

We investigate the effect of a large magnetic field, B, applied parallel to the plane of the barriers on the resonant I(V) currents of n- and p-type double barrier structures. The shifts in the electron resonances with increasing B can be understood using the effective mass approximation. The results for the p-type structures allow us to probe the anomalous e(k‖) dispersion curves of the hole states in the quantum well.