Table of contents

In polycrystalline semiconductors the trapping of charge at the grain boundaries has a decisive influence on the electrical transport properties through the formation of electrostatic potential barriers. By proper materials processing many interesting device applications can be realised, which exploit the electrical activity of these interfaces. In this review, the authors describe both the origin and the consequences of the charge capturing at grain boundaries. Special emphasis is given to polycrystalline compound semiconductors, where they summarise the present knowledge on the interface microstructure and its electrical properties. The model of a double Schottky barrier is shown to provide a quantitative basis for understanding the wide range of electrical phenomena in this class of materials. The steady-state current-voltage characteristic becomes highly non-linear through the interplay between the applied bias and the occupation of the defect states at the interface and in the depletion regions. For large potential barriers, high doping levels and elevated bias, large electric fields build up in the depletion regions. This triggers minority carrier generation through impact ionisation by hot majority carriers and strongly enhances the non-linearities in the charge transport. The dynamic electrical properties are probed by AC admittance or pulse measurements and can be traced back to the finite relaxation times of the trapped electron and hole charges. Comparing the experimental results with the theoretical predictions allows one to obtain valuable information on the electronic grain boundary parameters. The relationship between the observed electrical properties and the electronic structure of the junctions is discussed in detail, with ZnO varistors providing the majority of the experimental data. First indications for a general picture of the grain boundary electronic structure appropriate for all compound semiconductors are presented.

The dependence of the cleaved facet reflectivity R₀ and hence the mirror loss alpha _m0 for the fundamental TE₀ mode oscillation of BH InGaAsP/InP laser diodes (LD) emitting at 1.3 and 1.55 mu m wavelengths, upon the laser diode parameters, has been studied.

The magneto-optical spectrum of the shallow donors in ultra-high-purity MOCVD InP is studied. The central cell structure on the 1s-2p₊ and 1s-3p₊ lines shows that the dominant donor contaminant is silicon but significant amounts of sulphur, germanium and another donor (probably selenium) are also present. Strong lines from negatively charged donor ions (D^- states) can be observed, indicating that the samples are relatively uncompensated. The main spectral features are very similar to those found with high-purity VPE GaAs but, because MOCVD InP is slightly purer, more lines are observed. The appearance of certain higher order lines causes the interpretation of the GaAs data to be changed in some cases. Theory can predict the observed line positions on the revised interpretation to an accuracy of about 0.3%. The 1s-2p lines are broader than expected in comparison with GaAs.

A large number of sharp peaks are observed in the derivative magnetoresistance of high-purity n-GaAs and InP at 4 K with electrical field biases slightly less than those required to produce impact ionisation of the shallow donors. The peaks are caused by inelastic scattering of the few electrons in the conduction band by the neutral donors. As series of peaks are observed with compensated samples whose peaks move rapidly to higher magnetic field with increasing bias voltage. These peaks are believed to arise from an impact excitation process whereby the impurity is left in an excited state after a collision and subsequently ionises by the absorption of a phonon. The second set of peaks observed is due to the magneto-impurity effect. A wide range of excited states are involved in high-purity materials, and in the case of InP, nearly one hundred peaks can be detected. Fine structure is found on the fundamental (highest field) peak of the strongest series and is correlated with the central-cell structure observed on the far-infrared magneto-optical transitions between the shallow donor states in the same material.

Hot electron magnetophonon resonances (MPR) in n-type Si were investigated at a temperature T=80 K in pulsed high magnetic fields up to 40 T. Although the oscillation profile is very complicated due to the overlap of several different intervalley MPR series, the assignment of each peak has become possible owing to the high resolution measurements in high magnetic fields. In the longitudinal magnetoresistance for B//(100) and B//(110) configuration, the dominant series of the observed MPR oscillation peaks M=2-19 (B//(100)) or N=3-19 (B//(110)) were assigned as the intervalley transitions from the excited states to the ground state of different valleys with the lighter electron mass by emitting a f-110 Sigma ₃ phonon with a momentum of (2 pi h(cross)/a) (0.85, 0.85, 0). The electronic field dependence of the oscillation amplitude of each series and the damping factor were investigated in electric fields up to 300 V cm^-1. The damping factor as a function of electric field was found to have a minimum in the warm electron range around 200 V cm^-1.

There is a high charge density in the well of a double barrier structure when it is at resonance. This alters the potential profile through the diode and may cause it to have two stable states for a given applied bias. The authors have studied this effect both analytically and numerically. The analytical work extends the earlier calculations of Sheard and Toombs (1988). Their main result is a condition that must be satisfied for bistability to occur: the potential due to the trapped charge must shift the centre of the resonance by more than its width. This requires the resonance to be strong, which can be achieved by designing the barriers to be asymmetric at zero bias, but symmetric when the bias is near that required for resonance. The build-up of charge and the range of bistability can be increased by making the exit barrier more opaque, but the current through the diode decreases. These results are supported by numerical calculations, in which the Poisson and Schrodinger equations are solved self-consistently. The analytical and numerical calculations agree quantitatively, but they are only able to explain the experiments qualitatively. This is because their numerical model has a severe flaw, which plagues all simple quantum-mechanical simulations: it omits scattering which is vital in treating the contacts, particularly the accumulation layer from which charge is injected into the barrier.

The soft threshold lucky drift model of impact ionisation in semiconductors due to Marsland (1987) has been developed by replacing an approximation with a more accurate power series approximation. The model has been fitted to measure ionisation coefficients and the new results give better agreement to the soft threshold lucky drift model due to Ridley (1987), especially for InP and GaAs. Further agreement for Si is obtained when the same experimental data is used. The variation of mean free paths and the 'softness of threshold' factor due to the choice of different experimental data and different ionisation threshold energies is estimated in silicon. The lucky drift model with the power series approximation is also fitted to ionisation coefficients measured in Al_xGa_1-xAs and Ge and a chemical trend in the mean free path is observed for all the materials reported.

A theoretical calculation of the subband energy structure of inversion layers in narrow gap semiconductors is presented. The role of boundary conditions imposed on the electron wavefunctions is examined in detail, both at the insulator-semiconductor interface and inside the semiconductor, where the degeneracy of the two-dimensional states with the bulk valence band plays an important part. The method used is briefly compared with previously published calculational models. Self consistently calculated subband energies and spin splittings for narrow gap HgCdTe are presented and compared with the experimental results.