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Transient capacitance and admittance spectroscopy techniques have been used to investigate the perpendicular electron transport properties of GaAs/AlGaAs multi-quantum wells. Two transport processes have been studied at weak electric fields: thermionic transport over the barriers and tunnelling processes through the barriers. Purely exponential time dependences have been observed in both cases and the transport rates could therefore be directly measured and compared with theory. The conduction band offset at the GaAs/AlGaAs interface was determined using both measurement techniques and compared with previously published data.

Uniformly Si doped GaAs/Al0.33Ga0.67As multilayer structures have been studied by deep level transient spectroscopy (DLTS) and photocapacitance measurements. DLTS spectra showed five peaks which are related to defects in the GaAs layers. The concentration of these defects decreased with increasing layer thickness. An additional peak, which has been observed with forward bias filling pulses, is suggested to be related to defects near the surface, most probably due to defect accumulation in multilayers. Their emission and capture properties as well as photoionization cross sections have been studied. Evidence is provided that the emission and filling processes of these deep levels are modified due to the energy quantization in the conduction band and the carrier transport through the quantum structures. No DX center related DLTS peaks or other features like persistent photoconductivity effects have been observed in any of our samples.

Acceptorlike states with energy levels in the lower part of the band gap have been observed by photocapacitance measurements in Si-doped molecular-beam-epitaxial-grown AlxGa1−xAs (x=0.30–0.59). The microscopic structure of these defects is still unclear. Their concentration, however, can exceed the net donor concentration. The energy positions of the acceptorlike states as well as their photoionization cross sections of holes and electrons have been studied for different x. Due to their large concentration, these states may disturb experiments performed on DX centers which are often assumed to be the main electronic levels in these materials.