Showing papers by "Peter H. Beton published in 1993"

Probing the wave function of quantum confined states by resonant magnetotunneling.

Abstract: We have measured the low-temperature (4.2 K) current-voltage characteristics I(V) of a GaAs/(AlGa)As double-barrier resonant tunneling diode in which the quantum well is intentionally 6 doped with Si donors. A peak in I(V) at low voltage is observed and attributed to resonant tunneling of electrons from two-dimensional free-electron-like states into the fully localized bound states of the shallow donors. The magnetic-field dependence of this peak is fundamentally different from that of the main resonance. We show that the Fourier spectrum of the shallow-donor wave function may be deduced from the variation of the peak amplitude with magnetic field

Transport in sub-micron resonant tunnelling devices

Abstract: We have investigated the I ( V ) characteristics of a sub-micron gated GaAs/ (AlGa)As resonant tunnelling diode. We find that, relative to a large area device, there is additional structure at both high and low source-drain bias. The low bias features are shown to be due to the presence of impurities in the quantum well. This is confirmed by experiments on δ-doped large area devices. Using the gate on the sub-micron devices we are able to control the current through a single donor state. The high voltage structure occurs within the main resonance and only in forward bias where the peak-to-valley ratio (PVR) is poor. We ascribe the poor PVR to a lateral variation of the resonance condition and discuss the high voltage structure in these devices, and in similar two-terminal devices, in the light of this hypothesis.

High magnetic field studies of resonant tunneling via shallow impurities in δ-doped quantum wells

Abstract: We have observed a new sub-threshold peak in the I(V) characteristics of double-barrier resonant tunneling structures. By investigating a range of intentionally δ-doped and control layers we can relate this peak to resonant tunneling through the bound state of a shallow donor impurity in the quantum well. We have studied the dependence of the voltage position of this peak on the magnitude of a magnetic field applied either parallel or perpendicular to the plane of the quantum well. In the parallel orientation the donor peak shifts slightly to lower voltage whereas the threshold of the continuum resonance shows the usual large quadratic shift to higher voltage. The amplitude of the donor-related peak is reduced in the presence of a magnetic field. We present a theoretical model to explain this behaviour and show how the results can be used to probe the spatial form of the impurity wavefunction in the quantum well.

Sulphide passivation of (AlGa)As/GaAs modulation-doped heterostructures

Abstract: We have measured the low-temperature magnetoresistance of a range of GaAs/(AlGa)As modulation-doped heterostructures which have been passivated by immersion in ammonium sulphide solution. Values for sheet density and mobility of the electrons before and after illumination are extracted from our data. The electron density decreases as the degree of passivation is increased. In addition parallel conduction (for those wafers in which it is originally observed) is eliminated by the action of passivation. This implies that the Fermi level is pinned at a lower energy at the surface of the passivated samples as compared with the as-grown samples. The passivated samples show a dependence of electron mobility on density which is different to that of the as-grown samples and cannot be understood within standard models.

Nonlocal magnetoresistance of diffusive wires in high magnetic fields

Abstract: We have used a nonlocal configuration of measurement probes to study the magnetoresistance of diffusive n+-GaAs wires at high magnetic fields where ωcτ>1. At low temperatures we observe universal conductance fluctuations (UCF). For ωcτ>1 there is a large increase in the Lee-Stone correlation field but the amplitude is unchanged in strong disagreement with the theoretical prediction. We conclude that the UCF in this regime cannot be scaled in terms of a single parameter, the phase coherence length. At higher temperatures, where the UCF are quenched, we observe qualitatively new magnetoresistance oscillations which have the same periodicity as Shubnikov-de Haas oscillations but differ from them in many important respects. In particular, the new oscillations disappear at low temperatures. This indicates the importance of dissipation in the measurement of nonlocal resistance. There is excellent agreement between the temperature dependence of the new oscillations and a simple model.

Effect of Si δ doping and growth temperature on the I(V) characteristics of molecular‐beam epitaxially grown GaAs/(AlGa)As resonant tunneling devices

Abstract: (AlGa)As–GaAs–(AlGa)As double barrier resonant tunneling diodes with different δ‐doping levels in the GaAs quantum well (QW) together with undoped control samples have been investigated. A new subthreshold peak in the I(V) characteristics is observed and assigned to resonant tunneling through the bound state of a shallow donor impurity in the QW. By comparing the I(V) characteristics for wafers grown at different temperatures between 480 and 630 °C, the effects of Si segregation from contact layers into the well can be identified. This constitutes a new technique of assessment of donors which is sensitive to areal doping densities as low as 107 cm−2. The effect of low growth temperature (480 °C) and δ doping on peak/valley ratios of the resonances on the I(V) characteristic is also assessed.

Quantum Dot Fabrication by Optical Lithography and Selective Etching

Abstract: We have developed a process for the fabrication of 100 nm scale dots using conventional optical lithography together with selective wet etching. Resonant tunnelling diode (RTD) a n d quantum well based dots have been fabricated from GaAs/AlAs heterostructures on (100) and (111)B GaAs substrates. The GaAs is selectively etched by NH4OH/H2O2 to fabricate deep sub-micron structures. The selective nature of the etch facilitates control of the vertical dimension of the dots, and the preferential nature uncouples the etch front geometry from the photoresist mask geometry. The suitability of these dots for probing with a scanning tunnelling microscope will be discussed.

Resonant Tunnelling via the Bound States of Shallow Donors

Abstract: We have performed a systematic study of GaAs/(AlGa)As resonant tunnelling diodes in which the quantum well is intentionally δ-doped with Si donors. By comparing the I(V) of such devices with control samples we are able to identify a peak at low voltage which is due to resonant tunnelling through the localised bound states of the Si donors. To eliminate this peak from the control samples the MBE wafers must be grown at lower temperature (550°C) and with much wider spacer layers (200 A) than are typically used for resonant tunnelling diodes. Our work shows that the presence of intentional and/or unintentional donors may dominate I(V) close to the threshold for resonant tunnelling. The possibility of using this technique to assess MBE material will be discussed