Table of contents

In III-V semiconductors a common mode of vacancy migration is by nearest-neighbour hopping. In the presence of Zn diffusion this mode is greatly enhanced. A consequence is the production of antisite defects, which usually become antistructure pairs as the vacancy returns to its original sublattice. Many important deep-level defects involve vacancies during their creation. One should expect to find neutral antistructure pairs in the vicinity of Zn-O luminescence centres, EL2 defects, etc. Much of the mystery of these centres can be attributed to the dipolar interaction of these antistructure pairs with the nominal defects.

Phonons bound to sulphur in GaP have been studied by Raman scattering techniques as a function of the sulphur impurity concentration. A new structure appears at a binding energy higher than that of the phonon bound to an isolated donor and is tentatively assigned as originating from sulphur complexes.

Photoluminescence excitation (PLE) spectra taken in the ODMR mode with tunable laser excitation are discussed. Experimental data are shown for the 1.911 eV Cu-related defect-bound exciton in GaP, illustrating the power of this ODMR-PLE technique in establishing the important connection between an ODMR signal and the defect state experiencing the microwave resonance.

A general correlation of high-energy excitations of negative-U centres with metastable structural changes in glassy semiconductors is shown to follow from a recent theory identifying the negative-U centres with self-trapped electron and hole pairs. Some related effects characteristic of glassy semiconductors are considered. Correlations between the selftrapped pair excitations and low-temperature anomalies in the glasses are discussed.

It is shown that modified overlap interaction, covalent interaction and polar energy introduced by Harrison (1980, 1981) allows the nearest-neighbour spacing in semiconductors to be determined accurately. It is also shown that the force constants of the diamond and zincblende structure semiconductors result from the same covalent and overlap interactions, and are obtained in terms of Herman-Skillman atomic potentials in accordance with experiment. A simple approximation from first principles is proposed for the determination of the local distortion around an impurity atom in a host lattice. The local distortion is calculated for various impurities in GaAs and GaP and is found to be in accordance with the experimentally known systems.

Multiphonon transitions are discussed semiclassically using an extension of the configuration-coordinate diagram to a complex plane of the configuration coordinate. The author considers the tunnelling regime, complementary to the region of applicability of the Landau-Zener formula, where the crossing point of the potential curves is classically inaccessible. An expression is obtained for the vibrational matrix element in terms of a contour integral, which can be evaluated analytically for a wide class of adiabatic potential curves. He obtains a general expression for the multiphonon transition rate at T=0, and this is used to calculate the transition rate between non-intersecting Morse curves.

Low-dose proton- and helium-implanted silicon was studied by deep-level transient spectroscopy. By comparing the spectra as well as the defect level concentration profiles, five electron traps and one hole trap (after proton implantation at room temperature) and one dominant electron trap (after proton implantation at 80K) were identified to be hydrogen-related. Two of the hydrogen-related defect levels produced at room temperature represent different charge states of the same defect with a structure probably containing two hydrogen atoms. The electron trap produced by proton implantation at 80K is a donor level located at about E_c-0.2 eV. The defect is tentatively identified as a vacancy-hydrogen complex or a hydrogen atom in a single interstitial site and anneals out before reaching room temperature.

The axial nature of isoelectronic defects in Si is modelled by a uniaxial stress acting on the crystal's valence band and conduction band extrema. The model provides an approximate description of the energy spacings and relative luminescence intensities of the lowest-energy bound exciton transitions. Experimental data on the effects of uniaxial stresses and magnetic fields can be fitted using three parameters for each, the value of the parameters being predictable to well within a factor of two from the properties of perfect Si.

The authors report a study of the intense acoustic paramagnetic resonance spectra observed in semi-insulating or illuminated n-type chromium-doped gallium arsenide. The experimental data are compared with a Jahn-Teller model for Cr^2- ions occupying substitutional sites but subject to a large trigonal distortion as well as to random lattice strains (Cr²-II). Despite the lack of a detailed knowledge of this lattice strain distribution, a satisfactory fit to the major experimental features is obtained after noting that the four equivalent directions for the trigonal strain are not magnetically equivalent for an arbitrary magnetic field direction. The origin of this large trigonal distortion present at a small fraction of the Cr sites is, however, unexplained.

The local symmetry and transient properties of the ODMR signals related to the oxygen centre in GaP have been studied in detail. The associated impurity centre was observed to have an orthorhombic or lower symmetry and one order of magnitude difference of lifetimes between the 0.841 eV emission and the ODMR signals of this emission were found. Possible recombination processes involving the Jahn-Teller effect are discussed.

Effective-mass sum-rule methods for estimating overlap integrals between conduction and heavy-hole band states for Auger recombination are reviewed critically and their fundamental weakness identified. Detailed calculations of such overlap integrals are presented for GaAs and InP using the full-zone 15-band k.p and non-local pseudopotential methods. The agreement between these two calculation methods is very good considering the empirical element in each. It is found that the effective-mass sum-rule method overestimates the modulus squared of the overlap integrals by at least an order of magnitude. The four-band approximation to the k.p method underestimates the modulus squared of the overlap integral by a similar factor. Errors in the pseudopotential method due to the orthogonality hole have been estimated and found to be small and the reason for this identified. The significance of the results is discussed in relation to recent experimental work on Auger recombination in GaInAsP.

The donor impurities in InSb are identified by the observation of central cell structure in the donor optical excitation spectra. Components of the (000) to (010) (1s to 2p_-) transition are resolved by studies of the far-infrared photoconductivity and transmission in high magnetic fields. Lines due to Se, Te and Sn impurities are identified with suitably doped samples. Spectra of the residual donors in zone-refined, Czochralski pulled, and liquid-phase epitaxial material are compared with these results. The ordering of the central cell components of different impurities is described by a theory based on a tight-binding calculation and from a comparison of atomic energy levels of host and impurity atoms.

Far-infrared laser spectroscopy is used to study the central-cell structure from the shallow donors in GaAs and InP. In the case of GaAs up to five donor species can be found in VPE material and one of these has a negative chemical shift. It is suggested that tellurium is the shallowest residual donor found in LPE material. With InP a variety of VPE, MOCVD and bulk (LEC) samples are studied. It is believed that up to twelve shallow donor species are commonly present in high purity in InP. One of the donors found in bulk-grown material has a substantial negative chemical shift. Samples back-doped with silicon, sulphur, tin and germanium are studied and, on the basis of these back-doping experiments, four of the common residual donors are identified. Hydrostatic pressure is used as an additional variable in certain of these experiments to improve the experimental resolution, both by increasing the separation of the central-cell components and by acting to sharpen the lines. In the case of InP the increase in separation of the individual donors is too great to be accounted for by the increase in mass, indicating either an increase in the dielectric constant or an interaction with donor states resonant with the conduction band.

Well resolved structure in the neutral donor bound exciton (D⁰X) recombination in InP, observed at high magnetic fields and low temperatures, is presented. This structure is explained using a model of a donor bound exciton in a cubic semiconductor in a magnetic field in which p-like zero-field excited states are forced to lower energy than the s-like states which form the degenerate zero-field ground states. Account is taken of the degeneracy and anisotropy of the valence bands and diamagnetic effects in second-order perturbation theory. The selection rules for the electric dipole transitions for magnetic fields along the (100), (111) and (110) directions are found to be in good overall agreement with the observed donor bound exciton spectra in InP at magnetic fields of 9.7 T. High-magnetic field transitions of the (D⁰X) complex to both the 1s neutral donor final state, and to the 2s, 2p_-1, 2p₀ and 2p_-1 excited states (the 'two-electron' transitions) are analysed.

The authors report the results of Zeeman spectroscopy of the 704.45 meV and 707.0 meV photoluminescence zero-phonon lines of InP:V. The luminescence is shown to be due to the ³T₂ to ³A₂ transition of V³⁺ (3d²) in a tetrahedral crystal field. The excited state of the transition has tetragonal symmetry, which is ascribed to a static Jahn-Teller effect. A very good fit to the Zeeman splitting patterns and anisotropies is obtained for both the 704.45 meV and 707.0 meV lines using the spin Hamiltonian for the ³T₂ excited state, with a term to describe the axial distortion.

Absorption spectra of n-type InP:Cr samples consist mainly of two bands associated with two sets of zero-phonon lines (ZPL). The first band at about 0.8 eV and the associated ZPL around 0.756 eV are due to Cr²⁺ at an indium site. The second band has a zero-phonon structure around 0.886 eV which coincides exactly with the zero-phonon structure of the chromium-related luminescence in InP; it is shown that the nature of this transition is the same as that of the 1.03 eV transition in GaP:Cr, the origins of which are still not definitively established.

Results are presented of high-resolution luminescence studies from individual dislocations and related defects in ZnSe and InP performed in a transmission electron microscope. In the case of ZnSe unusual luminescence bands (Y at 2.60 eV and S at 2.52 eV) originally observed in photoluminescence studies are attributed to dislocations. In some instances, complete quenching of the excitonic transitions was observed to correlate with the presence of Y emission from complex dislocation tangles. In the case of individual screw dislocations this quenching of the exciton luminescence was found to be variable; for example reduction of the exciton signal was not always observed. For InP, donor-exciton-related transitions were quenched at individual screw dislocations. Donor-acceptor pair/free-to-bound and deep level (band C) transitions were unaffected. For the case of InP, unlike ZnSe, no dislocation-related luminescence was observed within the system detection limit (0.7-4.0 eV).

A review of experimental data on the time dependence of luminescent decay in solids shows very few examples of the classically expected exponential dependence and none of the bimolecular law. The prevailing long-term time dependence obeys power laws in time with exponents between -1 and -2. It is suggested that the widely applicable power-law dependence is the result of many-body interactions dominating the dynamics of charge release from trapping centres. A simple model involving either one or two sets of such trapping centres can explain most of the observed features of transient response under short excitation. A qualitative physical mechanism is proposed for the power-law response.

Using junction-space-charge techniques, a deep centre with excited states has been studied in undoped MOVPE n-type GaP grown under high phosphorus pressure. The ground-state and excited-state energy positions have been determined to be 0.82 and 0.16 eV below the conduction-band edge respectively. The photoionisation cross section for electrons shows a pronounced photo-thermal excitation behaviour. The kinetic properties of electron excitation and recombination are analysed using a model for two-step excitation. A method for determining the thermal activation energy of the dominant excited state is described. Comparing the results with those published previously, a possible relation of the centre to Ni(d⁹) is discussed.

The detailed structure of porous Si (PS) layers formed in p-type wafers with resistivities 0.01-25 Omega cm has been investigated using reflectance, transmission, ellipsometry and photoluminescence techniques. Marked differences were observed in the optical properties of PS formed in degenerate or non-degenerate Si and these results are correlated with the results of other techniques. The optical techniques together with effective medium modelling have been shown to be useful non-destructive methods for either assessment of PS density or detection of unsuspected phases. The degenerate PS layers consistently showed good retention of the single-crystal characteristics of the starting wafer, only c-Si and voids being detected. For these samples, good agreement was obtained between optical and gravimetric densities. However, the non-degenerate PS had much greater variability, with greater loss of crystallinity and significant incorporation of oxygen, due to partial oxidation having occurred on or immediately after anodisation. Oxide fractions have been determined both optically and gravimetrically, with up to 50% oxide being detected in some samples. Non-degenerate PS samples with high oxygen concentrations appeared to be in the form of a chemical mixture, SiO_x, from interpretation of the optical constants. Photoluminescence measurements together with the other techniques indicated a complex mixture of phases in the latter samples-voids, alpha -Si:O (and/or alpha -Si:H), an unknown amorphous phase and silicon oxide. This complex structure probably contributes to the observed instability of thick non-degenerate PS layers when heated in UHV as part of the cleaning procedure prior to epitaxial growth, all degenerate samples being able to withstand heat treatment.

The valence-alternation-pair model for non-crystalline SiO₂ and its application to various defect properties of this material are briefly reviewed. It is shown that this model, which assumes the existence of pairs of over- and under-coordinated oxygen atoms, enables a consistent explanation of the origin and nature of interface states, fixed oxide charges, and radiation defects of the microelectronic Si-SiO₂ interface and an interpretation of the role of its most important impurities, i.e. hydrogen and sodium.

The scattering of conduction electrons from the GaAs-Ga_1-xAl_xAs heterojunction is calculated for a number of alloy concentrations. It is shown that the excitation of evanescent states derived from the higher conduction minima play a major part in the scattering process. Substantial deviations from effective-mass theory are found. It is shown how the numerical results may be incorporated into an evaluation of quantum well and superlattice systems.