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We investigate indium interdiffusion in InAs/(AlGa)As self-assembled quantum dots by studying the changes in the optical properties of the system induced by ion implantation and/or thermal annealing. Interdiffusion of In–Ga and In–Al atoms at the interface between the dot and the (AlGa)As barrier takes place in as-grown samples and is enhanced by the postgrowth treatments. In contrast to the proposed interdiffusion as the way for suppressing the optical emission from the wetting layer, we show that it drives the system towards a predominantly two-dimensional morphology.

By studying the Hall effect in a series of low resistivity Ga1−xMnxAs samples, accurate values for the hole density p, Mn concentration x, and Curie temperature TC are obtained over the range 0.015⩽x⩽0.08. The hole density corresponds to 90% of the Mn concentration at low x, and has a maximum value of 1.0×1027 m−3 when TC=125 K. These data allow the first meaningful comparison of mean field predicted Curie temperatures with experiment over a wide range of x. The theory is in qualitative agreement with experiment, but overestimates TC at large x and underestimates TC at low x.

We investigate the electrical and optical properties of GaAs/AlAs superlattices (SLs) in which a thin (⩽1.2 monolayers) InAs layer is inserted in the central plane of each GaAs quantum well. The InAs layer modifies the structure of the SL unit cell and provides an additional design parameter for tailoring the energy of the lowest miniband and the size of the minigap. We exploit this effect to enhance electron injection from a doped contact layer into the first miniband and to inhibit interminiband coupling.

We use an array of gate electrodes to control the electrostatic profile in a layer of self-assembled InAs quantum dots. In combination with magnetotunneling spectroscopy, this allows us to identify and measure the energy levels and wave functions associated with the ground and excited state of an individual quantum dot.

We have measured the magnetic domain patterns in Co/Pd multilayers of varying thickness using magnetic force microscopy in the presence of an external magnetic field applied perpendicular to the multilayers. We find that the domain patterns evolution is in qualitative agreement with existing theories for single layer thin films. Our results are in reasonable agreement with a theoretical model of domains appropriate to multilayer films.

Capacitance measurements are used to investigate a series of single-barrier n-i-n GaAs/AlAs/GaAs heterostructures incorporating a layer of self-assembled InAs quantum dots into the AlAs barrier. They reveal a low-density, excess positive charge in the AlAs barrier which we attribute to defects associated with quantum dot formation. The quantity of positive charge is proportional to the amount of AlAs deposited on top of the dots.

We adapt a multiple gating technology to characterize electrically an ensemble of “self-assembled” InAs dots embedded in a plane within an Al0.20Ga0.80As tunneling barrier. Although the μm-sized mesa incorporates several hundred dots, we find that only a few of them contribute to the current close to threshold. Gating allows us to probe the origin of the sharp current peaks, and we can classify these peaks into families in a simple way according to their gate voltage dependence.

We have studied the optical properties of (InGa)As self-assembled quantum dots grown on (311)B-oriented GaAs substrates. The luminescence linewidth is considerably narrower than that of similar samples grown on (100). The difference is explained in terms of the in-plane coupling of dots which is more significant in (311)B. In order to assess the device potential of (311)B (InGa)As dots, we have studied the properties of edge emitting lasers by extending the well-known technology for (100) to the (311)B devices.

Using magneto-tunneling spectroscopy, we observe the Zeeman spin splitting of the ground state of a single InAs quantum dot grown within AlAs. We obtain values for the g factor of different quantum dots between +0.52±0.08 and +1.6±0.2, with magnetic field applied in the plane of the dot. This value for the g factor is considerably different from that of bulk InAs (g=−14.8), and we explain this using a simple three band k⋅p calculation. Using the spin split states of the dot as a probe, we observe the complete spin polarization of the emitter accumulation layer.

We investigate a new type of magnetoresistance(MR) in which the resistivity of a near-surface two-dimensional electron gas is controlled by the magnetization of a submicron ferromagneticgrating defined on the surface of the device. We observe an increase in resistance of up to ∼1500% at a temperature of 4 K and ∼1% at 300 K. The magnitude and temperature dependence of the MR are well accounted for by a semiclassical theory. Optimization of device parameters is expected to increase considerably the magnitude of the room temperature MR.

We have investigated the photoluminescence spectrum of self-assembled InAs quantum dots embedded in a GaAs matrix in magnetic field B up to 23 T and under hydrostatic pressure up to 8 kbar. A strong anisotropy in the diamagnetic shift is found depending on whether B is applied parallel or perpendicular to the growth direction. In the former case, the spatial extent of the carrier wave function in the dot is estimated to be 60 A. The pressure coefficient for the dot emission line is (9.1±0.2) meV/kbar, about 20% smaller than for the Γ-point band gap in bulk GaAs.

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.

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 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.