Table of contents

The theory of electron transport in semiconductors is traditionally formulated in terms of the semiclassical Boltzmann equation. In nonlinear transport such an equation must be solved without linearization with respect to the external driving fields. This task is practically impossible by analytical means, but for many years a Monte Carlo numerical technique has been successfully applied to all sorts of problems in semiconductor electron transport. In this paper a new approach to Monte Carlo simulation of electron transport in semiconductors, which has recently appeared in the literature, is reviewed. In the traditional Monte Carlo approach a direct simulation of the electron motion is realized, where all possible events (scattering processes) occur with the same probability as in the 'real' world. On the contrary, in the new approach, called the weighted Monte Carlo technique, events occur with arbitrary probabilities, and the weight of the particle in the simulation is accordingly modified in such a way as to maintain an unbiased result. In this way it is possible to emphasize, during the simulation, the analysis of the effect of rare events that in standard simulations would occur too rarely. The traditional Monte Carlo approach is recovered as a special case of this new more general technique. Applications of the weighted Monte Carlo technique to the evaluation of high-energy tails of the distribution function and of the electron current through a potential barrier are presented. A generalization of the method to quantum electron transport is also reviewed.

GaAs layers grown by the molecular beam epitaxy (MBE) method at low temperatures (200 degrees C) and also MBE samples grown at 300 degrees C highly doped with Be or Si show high concentrations of As antisite-related defects in the optical absorption and in the magnetic circular dichroism of the absorption (MCDA). With optical detection of EPR it is shown that these antisite-related defects have properties similar to those of the EL2 defects except for the EL2 bleaching characteristics and the so called zero phonon line. Their spin-lattice relaxation time is strongly reduced compared with that of EL2⁺. These differences may be related to their high concentrations, which are of the order of approximately 10¹⁹-10²⁰ cm^-3. In MBE samples grown at higher temperatures (325 degrees C, 400 degrees C) a new As antisite-related defect was detected with a reduced ⁷⁵As hyperfine splitting as compared with that of EL2⁺. This new As antisite-related defect has properties very similar to those of another As antisite-related defect previously detected in horizontal Bridgman n-type GaAs. A comparison of four different As antisite-related defects with similar reduced ⁷⁵As hyperfine splittings is presented.

The problem of the binding energy of an exciton R^ex formed by carriers from certain spin subbands has been solved for hexagonal diluted magnetic semiconductors (DMS) with the energy bands spin-split by the exchange field G. It has been shown that in the case of H//c₆ (H is the external magnetic field, c₆ is an axis of the sixth order) the field G//H can affect considerably the effective masses only in the hole B and C subbands (the influences are different for various spin states), while the effective masses are changed in all the whole subbands when H perpendicular to c₆. As a result, dependences of R^ex on G arise and lead, in certain cases, to considerable changes in the band splittings of the exciton transition spectra. Quantitative estimations of the effect were carried out for DBS band parameters corresponding to Cd_1-xMn_xS and Cd_1-xMn_xSe.

The electronic band structures of GaAs/(GaAl)As superlattices under a uniform electric field along the growth axis are calculated on the basis of a microscopic tight-binding (TB) description. Finite sets of quantum wells are considered in order to apply the TB method to a system without translational symmetry. The main effect of the electric field appears as diagonal elements of the TB matrix, resulting in modulation of miniband dispersion and localization of wavefunctions (Wannier-Stark effect). The calculated results are found to be in good agreement with the experimental observation by electroreflectance measurements.

The degradation of the photovoltaic properties of CdS-Cu₂S solar cells stored in the dark and under illumination has been investigated. Unencapsulated devices maintained in air at room temperature degraded much less rapidly in the dark than those maintained under constant illumination. Examination using the ESCA technique showed that the illuminated devices developed a higher concentration of surface oxygen. The degradation, which is attributed to the formation of surface oxide with consequent loss of copper from the Cu₂S layer could be retarded significantly by replacing the ambient air with argon. However, if cells were stored in argon at 50 degrees C instead of 22 degrees C, copper diffused from the Cu₂S into the CdS, leading to degradation by this process.

Traces of transition metals (Fe, Cr and Mo) were inadvertently introduced into buried oxide silicon-on-insulator structures during oxygen implantation in a medium current implanter. The redistribution of these metal impurities during post-implantation annealing between 900 and 1300 degrees C was studied by secondary-ion mass spectroscopy. The microstructure of the as-implanted and annealed specimens was characterized by transmission electron microscopy (cross-sectional and plan view). Auger electron spectroscopy was used to investigate the oxygen depth profiles. A correlation is shown between the density and depth distribution of Fe, Cr and Mo and the microstructural changes that occur upon annealing. The redistribution of Fe and Cr was found to be strongly influenced by damage recovery whereas Mo remained immobile during heat-treatment. These results are discussed on the basis of diffusion mechanisms, radiation-enhanced diffusion, supersaturation of silicon interstitials, surface segregation, metal impurity precipitation and dislocation trapping.

The influence of traces of transition metals on the electrical properties of the top silicon layer of SIMOX (separation by implanted oxygen) structures was investigated using spreading resistance. A comparative study of silicon containing Fe, Cr and Mo with non-contaminated silicon, implanted and annealed under the same conditions was performed. The resistivity of the top silicon layer was found to be lower for samples which contained transition metal impurities, which is primarily attributed to the n-type doping effects of Cr retained in the top layer.

An isolated quantum dot is modelled by a potential box with delta-profiled, penetrable potential walls embedded in a large outer box with infinitely high potential walls which represent the work function with respect to a vacuum. The authors calculate the energies E and the density of states of the electron gas in the quantum dot as functions of the strengths of the potential walls of the inner box. As the strengths of the opposite potential walls increase pairwise from small to large values, the density of states shows smooth crossover from the three-dimensional E^1/2 form to the quasi-two-dimensional (Q2D) staircase and further down to the Q1D sawtooth and the delta-function peaks of the Q0D electron gas.

Continuous magnetic fields applied parallel to layer interfaces are used to examine the in-plane energy dispersion and anisotropy of the quantum well states of double barrier AlAs/GaAs resonant tunnelling diodes. The devices are grown on substrates with different orientations using molecular beam epitaxy. Measurements on a device grown on a (311)A substrate reveal the pronounced biaxial symmetry of the confined hole states in a quantum well of this orientation, including a marked saddle-shaped structure for one of the subbands. The spectra are compared with those of similar devices grown on a (100) surface.

The authors report an investigation of contact resistance to high-mobility 2DEG structures ( approximately 1*10⁶ cm² V^-1 s^-1) as a function of temperature (30-300 K) and magnetic field (0-0.6 T). The transmission line model (TLM) and the two-layer TLM of Look (1988, 1989) are used to evaluate to evaluate the contact resistance. The calculated specific contact resistance shows a strong temperature dependence, with both thermionic and tunnelling behaviour. The vestiges of an interface remain below the contact region. Using the two-layer TLM analysis, the degree of parallel conduction in the samples is obtained.

Conductance-frequency measurements down to temperatures of 100 K have been performed on both p-type and n-type <100> silicon oxidized in dry oxygen at 900 degrees C. The metal electrode capacitors used were not given a post-metallization anneal in forming gas. This has allowed measurements of the intrinsic density of states, capture cross section, and surface potential fluctuations to within 0.06 eV of the band edges. Two peaks in the defect density at energies of 0.3 eV and 0.85 eV above the valence band are clearly visible above an asymmetric background, which rises rapidly towards the conduction band edge. The capture cross section is near constant at approximately 10¹⁶ cm² across the gap and independent of temperature. The surface potential fluctuations reveal a peak value of approximately 70 meV centred at 0.4 eV above the valence band superimposed on a constant background of approximately 40 meV. The authors attribute the peaks in the density of states to the amphoteric trivalent silicon P_b centres. The probable causes of the asymmetric background are either the tail of a defect peak centred around the conduction band edge, or states descending from the conduction band induced by stress within the oxide.

Peak and valley current densities of double quantum well (DQW) resonant interband tunnel (RIT) diodes with well widths of 20, 30, 40 and 60 AA are experimentally investigated and compared with those of non-resonant heterojunction tunnel diodes. For the 40 AA well diode peak current density at resonance increases by an order of magnitude in comparison with the 20 AA and 30 AA well diodes, and is of the same order of magnitude as for the non-resonant tunnel diodes. The valley current for the 40 AA well diode is an order of magnitude smaller than in the non-resonant diodes. Peak-to-valley (P/V) current ratios for the 20 AA well diodes are the same as for the non-resonant diodes. A theoretical analysis of the observed phenomenon is presented, along with a discussion of the optimization of the P/V ratio, which is based on the properties of double resonance and double antiresonance in a DQW three-barrier system. The analysis predicts the possibility to increase the peak resonant current and P/V ratio by orders of magnitude. For this purpose, new DQW RIT structures with three matched barriers are suggested.

Polycrystalline silicon thin film transistors formed with undensified gate oxides have shown instability phenomena not observed for TFTS formed with oxides annealed at 600 degrees C or above. These instabilities are related to ionized water in the oxide, with positive (H⁺) and negative (OH^-) ions drifting on the application of gate bias to produce both negative and positive threshold voltage shifts. The mobility for the OH^- ions is found to be lower than that for the H⁺ ions, as might be expected. In addition, kinks in the subthreshold characteristic are sometimes seen, and these are shown to be due to a sidewall effect. The water diffuses in from the atmosphere laterally under the gate electrode, and this gives rise to a gate length dependence of the effects. From this dependence a diffusion coefficient approximately 3*10^-12 cm² s^-1 is deduced at room temperature. This is some eight orders of magnitude higher than that for fused silica, and confirms the porosity of the material.

A method of reconstruction of two-dimensional lattice deformation fields in crystals with periodically distorted near-surface regions is advanced. The method is based on computer processing of triple-crystal X-ray diffractometry data. It has high spatial resolution ( approximately 10^-2 mu m) and sensitivity to lattice parameter variation of the order of 10^-7. Deformation profiles in distorted Si crystals are reconstructed.

A range of values exists in the literature for compositional dependence of bandgap in Al_xGa_1-xAs below the direct-indirect alloy composition and the way this is apportioned between the conduction and valence band edges. The authors compare results obtained from studies of compositionally graded quantum-well structures with consensus values and find that a self-consistent picture emerges only if a larger bandgap dependence than is still widely quoted in the literature is used. The preferred relation for bandgap dependence is Delta E_g(x)=1.427x+0.041x² (eV) with a conduction-to-valence band split ratio of about 60:40, yielding Delta E_c=0.88x and Delta E_v=0.55x.

GaAs grown by liquid phase epitaxy (LPE) and free from deep levels is well suited for studying vanadium-related levels in bulk GaAs. LPE growth of GaAs:V was carried out and it is found from deep-level transient spectroscopy (DLTS) that vanadium gives rise to an electron trap at 0.19+or-0.01 eV below the conduction band edge with a capture cross section of 4*10^-16 cm². This could be a level different from the electron trap at 0.14 eV reported for LEC-grown GaAs:V due to the formation of different complexes under different growth conditions.

When large currents are passed through a high-quality quantized Hall resistance device, the voltage drop along the device is observed to assume discrete, quantized states when plotted against the magnetic field. These quantized voltage states are interpreted as occurring when electrons are excited to higher Landau levels and then return to the original Landau level. The quantization is found to be a function of magnetic field, and consequently can be more difficult to verify and determine than previously suspected.

The I-V characteristics of SiO_x films deposited in an RF magnetron system with low pressure ( approximately 6 mTorr) SiH₄/O₂/Ar gas mixtures have been studied. With the enhanced surface processes under the high flux, low energy ion bombardment, the substrate temperature (<50 degrees C) was lower than in other conventional plasma deposition processes (>300 degrees C). The film deposited under the optimum conditions without annealing has a very smooth surface, low leakage current (<10^-10 A for F<6 MV cm^-1) and high breakdown field strength (9-10 MV cm^-1). It is found that the breakdown voltage is strongly correlated with the initial injection current. The film roughness, porosity and stoichiometry are the important factors for the electrical properties and can be controlled. The effects of RF power, deposition rate, and the oxygen to silane partial pressure ratio are investigated and discussed.