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A donor-related resonance is observed in double-barrier resonant tunneling devices with Si donors incorporated in the quantum well. In high magnetic fields the resonance becomes dominant over the Is resonance associated with the ground state of a single donor. The bias position of the donor resonance, its magnetic field dependence, and large amplitude indicate unambiguously that the resonance is due to tunneling through the ground state of a shallow donor with two bound electrons (D- level).

We have studied fluctuations in the differential conductance of resonant tunnelling diodes which incorporate a δ-layer of donor impurities in the centre plane of the quantum well. The system is well-suited to studying this type of conductance fluctuation, as we can control the number and position of the impurity states and also vary their effective linewidth by the application of a magnetic field perpendicular to the current direction.

Electron tunnelling through donor-related states is discussed. This tunnelling process, which occurs well below the threshold voltage for conventional resonant tunnelling into the two-dimensional continuum states of the quantum well, reveals a new type of Fermi edge singularity effect which arises from the Coulomb interaction between the tunnelling electron on the localized site and the Fermi sea of electrons in the emitter layer. A new means of forming laterally confined resonant tunnelling devices is also described. By studying the effect of an applied magnetic field, the additional structure that appears in the current-voltage characteristics of these devices can be unambiguously associated with a lateral quantum mechanical confinement effect.

We have investigated resonant tunnelling in GaAs/(AlGa)As heterostructures which have been fabricated into square mesas 6 x 6 mum. A delta-layer of donors (n approximately 2 x 10(9) cm-2) has been incorporated at the centre of the quantum well which is 9 nm wide. The I(V) characteristics show a feature at approximately 70 mV, which is below the threshold for the main resonance and is due to resonant tunnelling through single donor states in the well. This feature is also present in large area mesas. At lower biases and at low temperatures we see a new set of resonances which, although they occur in all small area mesas, differ in detail between devices with regard to their strength and bias position. The form of the low-bias structure is strongly dependent on temperature, T, below 4 K where several very sharp steps appear, becoming sharper as T is decreased. We have also investigated the dependence of the new structure on magnetic field, B, parallel to the current direction. We attribute the new features to tunnelling through potential fluctuations on the mesoscopic scale due to donor clustering.

We have observed a Fermi-edge singularity in the tunneling current between a two-dimensional electron gas (2DEG) and a zero-dimensional localized state. A sharp peak in the tunnel current is observed when the energy of the localized state matches the Fermi energy of the 2DEG. The peak grows and becomes sharper as the temperature is decreased to our lowest temperature of 70 mK. We attribute the singularity to the Coulomb interaction between the tunneling electron on the localized site and the Fermi sea.

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.

We demonstrate that it is possible to observe transport through individual tunneling channels due to zero‐dimensional states in large area resonant tunneling devices (RTD). These localized states are found to be related to the presence of donor impurities in the vicinity of the quantum well but their binding energies are larger than that due to a single isolated hydrogenic donor. The states give rise to additional peaks in current voltage below the threshold for the main resonant peak. These peaks are visible in RTD with essentially any lateral dimension provided the current is measured with sufficient sensitivity.

We have studied tunnelling through Si donors incorporated in the quantum well of double barrier resonant tunnelling devices. In addition to a resonance associated with the ground state of a single donor (1s level), a novel donor-related resonance at a smaller binding energy is observed in high magnetic fields where it becomes dominant over the Is resonance. We attribute this novel feature to a D-minus state of a shallow donor.

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

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

(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.

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.

Photoluminescence spectroscopy is used to investigate the donor-assisted resonant tunnelling processes in double-barrier structures which incorporate a low-density delta -doped donor layer in the centre of the quantum well. A quantum well luminescence line corresponding to hole-neutral donor recombination is observed, along with two other lines at higher photon energy. The possible origin of these two lines is discussed.

Donor-assisted resonant tunneling in nominally symmetric GaAs/(AlGa)As large area double-barrier diodes is investigated. The log(I)–V characteristics are used to evaluate doping density in the quantum well and are investigated in connection with donor cluster-assisted resonant tunneling. The single-donor-related feature in the resonant-tunneling characteristics is used to detect the presence of donors in the quantum well, even at concentrations of the order of the lowest achieved so far in molecular beam epitaxy GaAs. Expected effects of the presence of donors in the quantum well on the log(I) vs V characteristics are discussed.