The parallel-field magnetoresistance of the accumulation layers formed at the interface between n-Hg,_,Cd,Te with x=0.2 and x=0.3 and an anodic oxide film has been studied as a function of total surface carrier density. The surface carrier density was varied using a persistent negative photoconductivity, with a lifetime in excess of hours at 77 K, caused by excitation of electrons across the anodic oxide band gap (around 3.4 eV): the electrons then recombine with ionised deep traps in the oxide. A series of strong oscillations, known as the 'diamagnetic Shubnikov-de Haas effect' (DSDH) and corresponding to the depopulation of the surface sub-bands, were observed in the parallel-field magnetoresistance. With the comprehensive data obtained in this work, and the re-interpretation of several previous results from different systems, we have been able to show that these oscillations in any accumulation-layer-type system occur at well defined fields as a function of total surface carrier density and thus that the DSDH is a remarkably 'universal' effect.

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A description is given of the use of the Shubnikov -de Haas effect, parallel -field magnetoresistance and cyclotron resonance to give a rather complete characterisation of (Hg,Cd)Te photodetectors. The Shubnikov -de Haas and parallel -field magnetoresistance enable one to distinguish the separate contributions to conductivity from surface layers and bulk which bedevil normal characterisation methods such as the Hall effect. In addition, the surface carrier densities can be very accurately measured, and the number of populated surface subbands deduced. Cyclotron resonance is used to make an accurate determination of the x -value of the bulk material, and to measure the effective masses of the surface electrons, thus indicating whether the surface composition differs from that in the bulk. It is shown that these measurements are not only useful in the characterisation of detectors and material, but are of fundamental interest in the study of the two -dimensional electron gas: the surface electrons exhibit many novel modes of behaviour, such as enhanced or reduced polaronic effects and skipping orbits, the latter of which have only been previously observed in metals.