Table of contents

Highly carbon doped p-type GaAs grown by MOVPE shows infrared absorption lines from localized vibrational modes of C_As acceptors and passivated H-C_As pairs. A comparison with samples grown by MOMBE shows that an unassigned line at 563 cm^-1 is due to a mode of H-C_As pairs, although an isotopic analogue from D-C_As pairs in plasma-treated samples has not been detected. The important new result is the demonstration that the line cannot be due to C_Ga donors, and so there is still no evidence for the amphoteric behaviour of carbon impurities in GaAs.

Thin film, <0.2 mu m, SOI layers were implanted with high doses of As and BF₂ to simulate actual source/drain formation of MOS devices. Electrical measurements and cross-sectional TEM (XTEM) were used to evaluate the degree of recrystallization achieved after furnace annealing BF₂-implanted samples showed complete recrystallization and a high degree of activation for all examined conditions. XTEM pictures of As-implanted samples showed polycrystalline structures for higher energies; electrical measurements showed increasing sheet resistivity with increasing implantation energy. Arsenic implantation at elevated substrate temperature in order to recrystallize the SOI film during implantation was also studied. XTEM showed no polycrystals or defects, and measured sheet resistivities were comparable to bulk values as indicated by SUPREM3 simulations. The results of this study are useful when converting an existing NMOS or CMOS process to SIMOX substrates.

Conductivity-type conversion and modification of the electrical properties of p- and n-type Hg_1-xCd_xTe epitaxial layers subjected to low energy (1.2-1.8 keV) ion-beam treatment have been investigated. The dependence of the conversion depth on the treatment parameters and the material characteristics has been studied. Chemical diffusion of mercury has proved to be the principal process that determines the conversion rate. The electron concentration in the treated Hg_1-xCd_xTe samples is found to be determined by the overall concentration of the background donor impurities and native donor defects. The latter seem to be generated during the post-growth thermal annealing.

Specific heat measurements have been performed on Hg_0.88Zn_0.12Te at low temperatures. C_v experimental results can be described by the interpolation of the C_vs of the end binaries HgTe and ZnTe. A comparison with results on HgCdTe show that the 0 K Debye temperature of HgZnTe is higher than that of HgCdTe having the same energy gap. Experimental C_v results can be modelled with a simple phonon density of states in these mixed crystals.

The authors report evidence obtained by secondary-ion mass spectroscopy (SIMS) and electrochemical profiling of the saturation of Zn doping and of the diffusion of Zn across p-n junctions in Ga_0.47In_0.53As grown in InP by organometallic vapour-phase epitaxy (OMVPE). The results are consistent with the 'kick-out' mechanism, in the context of which the authors propose that the diffusing species is probably a neutral Zn interstitial, I_Zn⁰. Accumulation of Zn at the interface with a highly n-doped layer indicates the possible formation of Zn-donor complexes.

The evolution of a titanium overlayer deposited on an oxidized GaAs(100) surface has been studied using synchrotron radiation photoemission spectroscopy. Experiments were carried out on clean GaAs surfaces which were subsequently oxidized in air to give a coverage of approximately one monolayer of oxide. These surfaces were prepared by the thermal decapping of a protective As overlayer in ultra-high vacuum (UHV). The subsequent deposition of submonolayer coverages of titanium resulted in the complete reduction of the native Ga and As surface oxides and the formation of a TiO_x (1<x<2) metallic overlayer. The inherent surface sensitivity and resolution of the photoemission technique allowed details of the sequence of oxide removal to be investigated. The authors study would suggest that the interface formed in this way is extremely abrupt with no significant interdiffusion across the metal-semiconductor interface detected by the photoemission studies. This observation is in sharp contrast with similar studies for the same metal deposited on an atomically clean GaAs surface and on a heavily oxidized GaAs surface, both which exhibit strong out-diffusion of Ga and As atoms into the metal overlayer.

The condensation process for thin polycrystalline CuInSe₂ films has been studied via effusion evaporation of the ternary substance in the Se-enriched and Se-free vapour phases. The film growth is shown to be in good agreement with a model of small nuclei. The temperature of the critical nucleus formation has been determined: p-type films generally grow at higher temperatures than n-type films. The authors discuss the influence of technological parameters on a point defect ensemble in the prepared films, resulting in a wide variation of their electrical and optical properties.

Barrier-doped ZnSe:Te-(CdSe)_m(ZnSe)_n short-period superlattice quantum wells (BDSPSQW) with isoelectronic Te elements have been fabricated by means of atomic layer epitaxy. Luminescence from BDSPSQW was strongly modified by spatially selective introduction of Te isoelectronic centres in the barrier. Free-quantum-well and self-trapped excitons can coexist over a wide range of temperatures because they are separated from each other spatially. Room-temperature luminescence was dominated by narrow recombination of quantum-well excitons. This phenomenon is discussed in terms of doping position and doping density, using the model of exciton extrinsic self-trapping.

The effect of the external longitudinal constant magnetic field H₀//E and the pumping field H₁ on the characteristics of Gunn domains (velocity, period, average field) under conditions of cyclotron parametric resonance (CPR) is theoretically investigated. The region of CPR parameters, in which the domain solutions exist, are defined, and the dynamics of general characteristics of domains with the change of the external magnetic field and the field of pumping is studied. It is shown that the properties of the domain solutions resonantly depend on detuning of CPR and are sharply sensitive to the change of the pumping amplitude. This allows one effectively to control the frequency and the intensity of the Gunn generation.

By using a variational-perturbative method, the authors have studied the excitons confined in wide quantum boxes, i.e. the size quantization of the exciton centre-of-mass. The exciton energies are significantly shifted to high energy. The oscillator strength is enhanced with decreasing box size.

The results of a study carried out on Pd-doped p⁺n Si junctions using deep-level transient spectroscopy (DLTS) are reported. Four deep levels associated with Pd at energy positions E_c-0.18 eV, E_c-0.22 eV, E_c -0.37 eV and E_c-0.59 eV are observed. These deep levels have been observed simultaneously in the same as-diffused Si:Pd samples for the first time, to the best of the authors knowledge. Detailed data on the emission rates, capture cross sections and their temperature dependence and deep-level distribution profiles of the palladium-related defects are presented. Isochronal annealing characteristics of these defects have also been studied up to 400 degrees C yielding some interesting new insights. The authors results cast doubts on some of the currently held models for the structure of some of these defects.

Various hydrodynamic approaches are used in this paper to illustrate hot-electron, non-equilibrium, and intervalley transfer effects in a two-valley semiconductor. These approaches include the single-valley, two-valley displaced Maxwellian, and the two-valley macro-kinetic hydrodynamic transport models. Results obtained from these transport models are compared with those from Monte Carlo simulations. It is found that the transport parameters and rate coefficients obtained from the displaced Maxwellian approach, in the case of no electron-electron scattering, significantly deviate from Monte Carlo results in hot-electron situations. In addition, the displaced Maxwellian distribution results in less pronounced velocity overshoot behaviour. Insufficient information on intervalley transfer in extreme non-equilibrium cases provided by the single-valley hydrodynamic model is also illustrated. Results clearly show that in non-equilibrium and hot-electron situations the single-valley approach actually provides a much better transport description for electrons in the two-valley semiconductor than the two-valley displaced Maxwellian hydrodynamic approach. Moreover, the previously developed macro-kinetic approach is shown to be in excellent agreement with the Monte Carlo method in extreme non-equilibrium and hot-electron situations.

A method of contactless characterization of semiconductors is developed which allows the charge carrier lifetime and diffusion coefficient to be determined locally with three-dimensional spatial resolution. The method is based on refraction of the probe beam that intersects the pump beam in the sample volume. In contrast to the conventional mirage-effect technique, the method discussed here uses the deeply penetrating and focused pump beam which allows the surface effect to be substantially weakened while determining the bulk characteristics. The method has been applied to Si wafers, showing a good quantitative agreement with data obtained independently by the conventional photoconductivity decay technique.

An investigation has been made of the dark current mechanisms of SWIR (2-3 mu m) HgCdTe photovoltaic detectors fabricated for optical fibre communication applications. The photodiode forward and reverse current-voltage (I-V) characteristics, as well as the temperature dependence of the zero-bias dynamic resistance (R₀), were measured in the temperature range of 78-300 K. A qualitative analysis of experimental results was carried out using relevant theories. It is found that the diode performance is mainly limited by surface channel leakage current originating from the p-side surface. Moreover, various current components flowing through the field-induced junction may make contributions to surface channel current and dominate in different temperature and bias regions. In the high- and intermediate-temperature regions, diffusion and generation-recombination current, respectively, take a dominant role. In the low-temperature region, the reverse bias characteristics are governed by some soft leakage current component, while the near zero and forward bias characteristics have the features of deep level-assisted tunnelling current. The reverse breakdown voltage of the bulk p-n junction is governed by an avalanche breakdown mechanism at low temperatures and by a thermal breakdown mechanism at high temperatures.

The authors investigate resonant interband tunnelling in asymmetrical double-barrier devices. The electrodes consist of degenerate p-type bulk InSb and the low-work-function metal Yb. A two-dimensional electron inversion well is separated from the bulk by a bandgap barrier and from the metal by a thin oxide barrier. Current peaks at forward biases and distinct structures in the reverse current-voltage characteristic at 4.2 K are explained in terms of transverse wavevector conservation in tunneling from the electron subbands into the valence band and vice versa. Concerning the bias position of the current peaks in the forward characteristic the role of wavevector conservation is proved quantitatively by measurements under quantizing magnetic fields. In the NDC region current peak-to-valley ratios up to 7.4 at 4.2 K and 2.0 at 77 K are achieved.

The I-V characteristics of Au/semi-insulating InP (100) under reversed bias have been measured between 230 K and 290 K. A simple model based on the thermionic field emission theory (TFE) is proposed to describe this system. Based on this model, the value of the bandgap obtained from the I-V characteristics is in reasonable agreement with the literature. At room temperature, the corresponding fitted values of the barrier height Phi _b and the series bulk resistance R_b are 0.68+or-0.05 V and 1.320⁷ Omega respectively.

The luminescence properties of ZnSe single crystals doped with isoelectronic substituent Mg by a simple diffusion procedure have been studied. It has been shown that Mg easily enters substitutionally on a Zn site forming Mg_xZn_1-xSe mixed crystals with a bandgap energy considerably larger than that of 'pure' ZnSe. The photoluminescence spectrum of such crystals is dominated by blue and violet emission, the efficiency of which is much higher than the efficiency of blue luminescence in ZnSe. Low-voltage blue-violet electroluminescence with maximum emission at 2.98 eV at T=45 K and 2.875 eV at room temperature in Mg_xZn_1-xSe mixed crystals has been demonstrated, which makes this material very promising for future application in constructing blue and violet light emitting diodes.

Impact and Auger excitation of threefold ionized rare earth ions by carriers are calculated for silicon and III-V semiconductors. The interaction between strongly localized f-electrons and local vibrations, which makes it possible to achieve energy balance in Auger processes, has been taken into account. It is shown that the probability of Auger excitation increases if there are additional localized electron states in the forbidden gap, which correspond to defects containing rare earth ions.

Uncertainty in electron energy levels due to carrier-carrier scattering is thought to cause line broadening in quantum well lasers. Calculations of this effect in the conduction and valence bands are presented. Approximate results applicable over a range of materials, temperatures and Fermi levels are given.

The authors have investigated the magnetotransport properties of quantum wires which were fabricated on high-mobility GaAs-GaAlAs modulation-doped heterostructures using laser holography and electron beam lithography. For low temperatures, regular Shubnikov-de Haas oscillations and an additional magnetoresistance peak are observed; the peak occurs each time the cyclotron diameter matches the wire width. For high temperatures (106 K<T<175 K), however, all these peaks are washed out and new magnetoresistance structures are revealed; these are due to the occurrence of magnetophonon scattering in the wire, scattering which is used to determine the 1D quantization energies.

Capacitance transient measurements under various reverse biases in the range 90<T<300 K have been made on n-Al_xGa_1-xAs with a composition close to the intervalley crossover. The key feature of the transients is the strong non-monotonic temperature dependence of transient amplitude. This was demonstrated to be a consequence of the fact that the electrical properties of the material are governed by deep donor states.

Transport and single-particle relaxation times are calculated for a two-dimensional electron gas in a back-gated GaAs-AlGaAs heterostructure, in which the electron wavefunction is deformed by the back gate. Coulomb scattering due to remote, spacer and channel ionized impurities is considered separately. It found that when the wavefunction is squeezed by the back gate, the spacer impurity scattering gives rise to the largest reduction in the ratio between those relaxation times. Channel impurity scattering-limited relaxation time increases as the wavefunction is more compressed. The temperature dependences of these quantities are also discussed.

The electrical properties of Pr-doped In_0.53Ga_0.47As LPE epitaxial layers have been studied. Layers prepared by adding a small amount of PrO₂ to the melt are n-type and they are insensitive to the temperature treatment. p-type epitaxial layers were grown when more than 1 mg of PrO₂ per 1 g of In was added to the melt. Annealing at a temperature of 400 degrees C results in drastic changes of the electrical parameters and influences the photoluminescent spectra of p-layers. Praseodymium can be activated in a thin near-surface layer independently of the protective gas or metal covering. Additional annealing of the activated layers at higher temperatures has no further influence on the parameters. After removing the thin surface layer by chemical etching the previous electrical parameters can be obtained.

The formation of transparent conducting CdO films by chemical bath deposition is reported for the first time. These films exhibit an optical transmittance and electrical conductivity much higher than those CdO films prepared by other physical vapour deposition techniques. They exhibit an optical transmittance of more than 90% and electrical conductivity of the order of 10³ Omega ^-1 cm^-1. The very high optical transmittance coupled with the high electrical conductivity exhibited by these films makes them potential candidates for application as low-cost large-area transparent conductors.