T.J. Foster

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.

Submicrometer resonant tunnelling diodes fabricated by photolithography and selective wet etching

A new process based on photolithography and selective wet etching has been used to fabricate small area resonant tunnelling diodes. The low‐temperature current‐voltage (I‐V) characteristics of diodes with conducting widths less than 0.1 μm show additional peaks due to 1D lateral quantum confinement. We observe a pronounced asymmetry in I‐V which we explain in terms of tunnelling from emitter states which have a different degree of lateral quantum confinement.

Edge effects in a gated submicron resonant tunneling diode

We have investigated the current‐voltage [I(V)] characteristics of a gated GaAs/(AlGa)As resonant tunneling diode. As the negative gate voltage is progressively increased I(V) becomes asymmetric. In particular the peak‐to‐valley ratio in forward bias is decreased from ≂20 to ≂1, but in reverse bias remains constant ≂20. This arises from a lateral variation of the voltage drop across the emitter tunnel barrier, which in forward bias leads to a smearing of the resonance. We discuss the relationship between our experiment and the low peak‐to‐valley ratios of two‐terminal submicron resonant tunneling diodes observed by other groups.

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.

Observation of the transition to an insulating state consistent with a Wigner solid in a high-density 2D hole gas

Abstract We have observed a high-field transition into a high-resistance state at filling factor v just below 2/7 in a high -density, high-mobility two-dimensional hole gas (2DHG). The state shows clear threshold behaviour consistent with a strongly pinned Wigner solid. The transition is at lower filling factor than the value of ∼0.37 recently reported by Santos et al. [7] for a low -density 2DHG, and higher than the value of ∼0.22 usually found in equivalent 2D electron systems. Our experiment shows no indication of a re-entrant behaviour of the insulating phase. We believe that this may be the highest-carrier-density Wigner-transition observed.

Investigations of double barrier resonant tunneling devices based on (AlGa)As/GaAs

A series of electrical measurements on double barrier resonant tunneling structures is described. It is shown that the bistability effect in the current–voltage characteristics of a double barrier structure can be removed by connecting a suitable capacitor to the external circuit. These measurements cast serious doubt on the recent interpretation of the bistability as an intrinsic space‐charge effect. The temperature dependence of the tunneling current at voltages below threshold is also investigated. An expression for the temperature and voltage dependence of the thermally activated resonant tunneling process is compared with experiment. Finally, the effect of hydrostatic pressure (up to 11 kbar) on I(V) characteristics is studied. It is found that, whereas the pressure has little effect on the low‐temperature I(V) characteristics, the peak/valley ratio at room temperature is significantly reduced. This behavior is attributed to the effect of the higher conduction band minima.

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.

Electrical characterization of single barrier GaAs/GaN/GaAs heterostructures

A modified molecular beam epitaxy technique has been used to grow single barrier GaAs/GaN/GaAs heterostructures of both n‐ and p‐type. The temperature‐dependent I(V) measurements show thermally activated conduction over the barrier above about 200 K. A Type I band alignment is indicated by significant offsets in both the conduction and valence bands at the GaAs/GaN heterointerface.

Observation of correlated v = 1 quantum Hall and insulating states in strongly coupled p-type double quantum wells

We present magnetotransport results for strongly coupled p-type double quantum wells which demonstrate the existence of a correlated v = 1 quantum Hall state in the limit of weak tunnelling. Our results are in good agreement with the finite-temperature phase transition predicted for this state. We also observe an activated insulating behaviour for v < 1, which may be evidence of a correlated bilayer Wigner state.

Asymmetry in the I(V) characteristics of a gated resonant tunnelling diode

The authors have investigated the I(V) characteristics of a GaAs/(AlGa)As resonant tunnelling diode in which the effective cross-sectional area, A, may be varied using a gate. As A is progressively reduced I(V) becomes asymmetric. In particular the peak to valley ratio in forward bias is decreased from a value approximately=20 to approximately=1, but in reverse bias remains constant at approximately=20. They propose that this arises from a lateral variation of the voltage drop across the tunnel barriers which leads to a smearing of the resonance. As A is reduced they also observe additional maxima and minima in I(V).