We describe a tunable electron paramagnetic resonance (EPR) spectrometer designed to operate at frequencies between 160 and 525 GHz and magnetic fields of up to 20 T. To operate in such a broad frequency range we use a very stable optically pumped far infrared laser. The performance of the spectrometer has been measured with solid and liquid samples. This allows us to outline the potential uses of the spectrometer.

Photoluminescence and electroluminescence measurements on InGaN/GaN quantum well (QW) structures and light emitting diodes suggest that QWs with gross fluctuations in width (formed when, during growth, the InGaN is exposed unprotected to high temperatures) give higher room temperature quantum efficiencies than continuous QWs. The efficiency does not depend on the growth temperature of the GaN barriers. Temperature-dependent electroluminescence measurements suggest that the higher efficiency results from higher activation energies for defect-related non-radiative recombination in QW samples with gaps. At high currents the maximum quantum efficiency is similar for all samples, indicating the droop term is not dependent on QW morphology.

The conduction bands in GaAs and InP have been studied very accurately through the cyclotron resonance over a wide range of energies using the photoconductive detection technique. Pronounced band non-parabolicity has been measured in GaAs and InP and the band anisotropy has been measured in GaAs.Values for the band-edge masses of 0.0660 mo and 0.07927 mo respectively have been determined. A five-level k e p model of the band structure in the presence of a magnetic field has been used to describe the data where resonant and non-resonant polaron contributions to electron energies are included in the theory. The nonparabolicity and spin-doublet splitting of the cyclotron resonance are well described by the employed formalism in both materials and the theory also gives a reasonable account of the anisotropy of the band in GaAs. The fit to experimental data allowed us to determine the matrix element of momentum 0 between the higher conduction and valence bands in both materials.

The cyclotron resonance (CR) of the two-dimensional electron gas (2D EG) in GaAs-(Ga, A1)As heterojunctions has been studied in the resonant-polaron regime for 2D carrier densities N, in the range (0.8-5.4) && 10" cm . A reflectivity technique has allowed the CR to be recorded at energies up to 35.63 meV, within the GaAs reststrahlen band, and a calculation of the dielectric response of the complete heterostructure has enabled the effective masses to be reliably evaluated from the line shapes and positions of the resonances. The results indicate that the 2D electrons are coupling to the LO phonon (36.7 meV), in agreement with theoretical predictions. At low carrier densities, the resonant-polaron contribution to the effective mass becomes apparent at cyclotron energies above 25 meV, and increases in size as the LO-phonon energy is approached: however" this mass enhancement is removed rapidly as N, is increased, indicating the importance of Landau-level occupancy and screening in the 2D EG. Close to the LO-phonon energy, large shifts in the resonance position, which are several times the linewidth in size, are produced by varying N, . This large N, dependence explains previous convicting reports of "enhanced" or "reduced" polaron effects in the 2D electron gas. A comparison of the experimental results with existing memory-function calculations of the polaron contribution to the effective mass indicates that the greater part of the N, dependence can be ascribed to Landau-level occupancy effects.

The efficiency of light emitting diodes (LEDs) remains a topic of great contemporary interest due to their potential to reduce the amount of energy consumed in lighting. The current consensus is that electrons and holes distribute themselves through the emissive region by a drift-diffusion process which results in a highly non-uniform distribution of the light emission and can reduce efficiency. In this paper, the measured variations in the external quantum efficiency of a range of InGaN/GaN LEDs with different numbers of quantum wells (QWs) are shown to compare closely with the predictions of a revised ABC model, in which it is assumed that the electrically injected electrons and holes are uniformly distributed through the multi-quantum well (MQW) region, or nearly so, and hence carrier recombination occurs equally in all the quantum wells. The implications of the reported results are that drift-diffusion plays a far lesser role in cross-well carrier transport than previously thought; that the dominant cause of efficiency droop is intrinsic to the quantum wells and that reductions in the density of non-radiative recombination centers in the MQW would enable the use of more QWs and thereby reduce Auger losses by spreading carriers more evenly across a wider emissive region. V