Table of contents

The theory of recombination-induced heating is presented. The main mechanisms involved in the excitation and relaxation of local vibrations of defects are considered. It is shown that the high-energy 'tail' of the defect distribution function by vibration energies dramatically increases in the presence of excess carriers in semiconductors due to multiphonon recombination via deep defects. An example of numerical calculation of the defect distribution function is given. The theory is used to calculate the rate of recombination-stimulated reactions and to interpret experimental data on inelastic sputtering of solid surfaces by ion bombardment. It is shown that the defect heating should change the non-radiative recombination rate in the case of highly doped semiconductors or large concentrations of excess carriers.

The form of the high-field spin-resolved Shubnikov-de Haas resistance maxima in a high-mobility GaAs-(Ga,Al)As heterojunction 2D electron gas is shown to be strongly dependent on current, illumination time and the nature of the voltage contacts. The effects of increasing current and illumination time are very similar, with both reducing the influence of the contact nature. The similarity is attributed to a common modification of the edge potential in the device due to a change in the device electrostatics. At saturation illumination the current dependence is found to be wholly absent. The edge-bulk scattering length calculated from the theory of the current dependence of the maxima is found to be strongly dependent on the type of voltage contact used, indicating that this value does not reflect a fundamental bulk property. We also observe contact-specific fine structure in the N=1 high-field maximum, which is attributed to fluctuations in the transmission coefficients of the voltage probes.

Electroreflectance measurements have been made to clarify the resonant coupling between the states in an asymmetric triple quantum well structure, where the triple quantum well structure consists of three kinds of quantum well with different widths, and their widths are designed so that the ground states of the three wells in the conduction band should be at resonance under a certain electric field. The electroreflectance spectra reveal six types of optical transition, which indicate a clear resonant coupling between the wells at about 17 kV cm^-1. The transition energies are found to agree well with the calculated results.

Shubnikov-de Haas (SdH) and van der Pauw Hall effect measurements on In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al$ d0_.48As one-side modulation-doped quantum wells grown by metalorganic chemical vapour deposition have been carried out to investigate the magnetotransport properties of an electron gas and to determine the subband energies and wavefunctions in a single quantum well. Transmission electron microscopy measurements show that the In_0.53 Ga_0.47As/In_0.52Al_0.48As heterointerfaces have a perfect lattice match. The SdH measurements at 1.5 K have demonstrated clearly the existence of a quasi-two-dimensional electron gas in the quantum well. The fast Fourier transformation results for the SdH data indicate clearly the occupation of two subbands in the In_0.53Ga_0.47As single quantum well. Electron energy subbands in the quantum well were calculated by a self-consistent method taking into account exchange-correlation effects and making use of the experimentally determined carrier density.

The first results are presented of a deep-level transient spectroscopy (DLTS) study of deep levels in p-type silicon wafers with different oxygen contents and thermal pretreatments. The DLTS results are correlated with those of a structural characterization using cross-sectional transmission electron microscopy (TEM). The silicon oxide precipitate density and distribution correlates with the occurrence of minority carrier traps in the substrate. After irradiation with the fission products of a ²⁵²Cf source, an order of magnitude increase of the leakage current is observed. At the same time, new DLTS peaks are introduced. While the creation of the divacancy-related centre (V₂) is not affected by the presence and the status of oxygen, this is not the case for the second dominant peak related to an interstitial carbon-interstitial oxygen complex (C_iO_i) in p-type Czochralski substrates. In general the concentration of this trap scales with the residual interstitial oxygen content.

Two-dimensional structures with different thicknesses of confined optical phonon and electron quantum wells are proposed. The confined electron-polar optical phonon scattering rates in these structures when an electron quantum well is localized inside a phonon one are calculated. The independent electron and phonon confinement allows the scattering rates by confined and interface phonons to change significantly. The effect of independent electron and phonon confinement is demonstrated for an AlAs/GaAs/AlAs structure.

Semiconducting chalcogenide glasses, AI_xTe_100-x with the atomic percentage of aluminium (x) varying between 15 and 25, have been prepared by the vacuum sealed-melt quenching technique. The I-V characteristics of these samples have been measured at four different temperatures using a custom-built PC-based system. All the compositions studied are found to exhibit a current-controlled negative resistance irreversible switching behaviour. Furthermore, it is observed that there is an increase in the switching field 'E_t' of AI_xTe_100-1 glasses with increasing Al content. The threshold field for a given composition is found to decrease with increasing temperature.

The distribution of damage in Si-implanted GaAs has been studied using differential reflectance (DR) spectroscopy. This novel and relatively simple optical technique has high sensitivity and allows the study of damage distribution in a dose range of 1*10¹¹-3*10¹⁴ cm^-2, which is difficult to access by other techniques. The effects of substrate temperature (30-300 degrees C) and crystal orientation ((100), (110) and (111) directions) on the damage profiles have also been measured. As expected, the overall damage was found to increase with increasing ion dose and to decrease with increasing temperature. The measured damage profiles, obtained by successively etching the sample and measuring the DR signal, have been compared with TRIM simulations. The magnitude and shape of the damage profiles were found to be very sensitive to crystal orientation.

The characteristics of an optically pumped three-level laser using intersubband transitions in asymmetric quantum wells are discussed and an optimization of the band structure is proposed. The possibility of including the inverted laser medium in a Bragg mirror microcavity or in a two-dimensional photonic bandgap is also analysed.

An (AlGa)As/GaAs heterojunction bipolar transistor with a double-barrier resonant-tunnelling structure (DBRTS) inserted between the base and the collector has been investigated. Peaks due to resonant-tunnelling electrons have been observed in the device characteristics at 4.2 K. We have extracted the energy levels in the quantum well from the bias voltages for the peaks. The highest measured level corresponds to an energy greater than the barrier height.

The band bending at the Si:SiO₂ interface in the metal-oxide-semiconductor system is often evaluated using a low-frequency capacitance-voltage measurement. Here we discuss the effect of commonly used approximations for the oxide capacitance and midgap gate voltage on the derived gate voltage dependence of the surface Fermi energy, and show that they can result in significant systematic errors. Using both high- and low-frequency capacitance-voltage data, a simple self-consistent algorithm is presented that yields a more accurate surface Fermi energy versus gate voltage characteristic. Hence better estimates of quantities such as fixed oxide charge density, interface state capture cross section and density of interface states can be obtained.

A procedure for rapidly extracting the minority generation lifetime from the C-t transient data of a metal-oxide-semiconductor (MOS) capacitor is proposed. The interpretational method combines ease of application with a relatively high level of accuracy.

The performance of Hg_1-xCd_xTe infrared photoconductors is strongly dependent on the semiconductor surface conditions and, in particular, the degree to which the surface contributes to recombination of photogenerated excess carriers. Although published photoconductor fabrication processes based on bulk Hg_1-xCd_xTe address this issue by fully passivating both major surfaces (i.e. front and back) with anodically grown native oxide, passivation of the sidewalls is neglected. In this paper it is shown both theoretically and experimentally that leaving the sidewalls unpassivated can result in approximately a factor of two reduction in responsivity for long-wavelength infrared (LWIR) detectors used in high-resolution thermal imaging systems. Detector arrays are typically fabricated on x=0.23 Hg_1-xCd_xTe representing a cut-off wavelength of 9.4 mu m and use individual element sizes of approximately 50*50 mu m². We describe in detail for the first time a device technology which enables the fabrication of Hg_1-xCd_xTe photoconductor arrays such that the entire surface of the semiconductor is effectively passivated, including the sidewalls. Of particular interest is the fact that this improved device technology is compatible with present-day Hg_1-xCd_xTe epitaxial growth processes. This is in contrast to current photoconductor technology which is primarily based on bulk Hg_1-xCd_xTe. Experimental results are presented which compare device performance of LWIR detectors fabricated using the improved photoconductor technology with current published photoconductor technology. These results clearly indicate that detectors fabricated on liquid phase epitaxially (LPE) grown x=0.23 Hg_1-xCd_xTe material using the improved photoconductor device technology achieve much higher responsivities and detectivities. Furthermore, it is shown that only a fully passivated device structure is capable of exploiting any future improvements in bulk minority carrier lifetime as it approaches the Auger recombination limit.

The electrical and metallurgical properties of Au-Zn contacts to p-type InP, prepared by vacuum evaporation of Au-Zn (10 wt%) alloy, were investigated as a function of deposition parameters and annealing conditions. The deposition rate of Au-Zn alloy was found to be the major factor that strongly influences the properties of as-deposited and annealed p-InP/Au-Zn contacts. At a substrate temperature of about 150 degrees C, when a high deposition rate of about 16-25 nm s^-1 was used, excellent homogeneous ohmic contacts could be formed after annealing with specific contact resistance values of about 1*10^-8 Omega m² (for N_A-N_D=(0.8-1.0)*10²⁴m^-3). In contrast, a low deposition rate of about 0.1 nm s^-1 resulted in a large scattering of resistance values and non-ohmic behaviour of contacts. In this case, a much smaller amount of zinc and its non-uniform distribution in the Au-Zn layer, as well as strong InP/Au-Zn interfacial reactions, were detected by AES and SEM techniques. The electrical and metallurgical properties of slow-deposited Au-Zn contacts could be improved by use of a lower substrate temperature (in the range of 30-50 degrees C) during contact deposition.

The flatband potential of n-GaAs/non-aqueous electrolyte contacts has been measured as a function of (i) the redox energy level of the electrolyte (over 2 V in acetonitrile and over 1.3 V in methanol) and (ii) the crystallographic orientation, considering the (100), (111)As, (111)Ga and (110) faces. The experimental conditions have been selected to prevent oxide formation. A strong Fermi level pinning (FLP) is observed for the (100) and both (111) faces due to the same surface state distribution for a given solvent. For acetonitrile (methanol), this distribution is localized near one-third of the gap (0.1 eV) from the valence band edge. In comparison with the (100) face, both (111) faces exhibit a flatband potential negatively shifted by 0.5-0.7 V. This shift can be correlated with the piezoelectric property of the (111) direction. On the contrary, for the (110) face there is no FLP. The flatband potential is independent of the redox level when this latter is varied over all the GaAs gap. This means that the interactions between the redox species and the GaAs surface are sufficiently weak to hold the ideal character of the (110) surface, i.e. without surface states as for the cleaved surface in UHV conditions.

A method for calculating diffusion enhancement due to dopant-defect pairing in ion-implanted silicon is described. The number of dopant-defect pairs is calculated from a general defect clustering model. The assumption that a net dopant flux arises from dopant-defect pair gradients leads to the calculation of an initial enhanced diffusion coefficient. This diffusion enhancement is shown to be consistent with measured transient diffusion.

Undoped and phosphorus-doped microcrystalline silicon carbide ( mu c-SiC:H) films were deposited by plasma-enhanced chemical vapour deposition (PECVD) with high hydrogen dilution and high RF power density. The doped films obtained have a bandgap up to 2.0 eV and electrical dark conductivity up to 17 S cm^-1. The thermoelectric power (TEP) measurements and the behaviour of the conductivity and Hall mobility as functions of temperature provided information on electrical transport phenomena. Structural and optical measurements confirmed properties of this material for application in LED and solar cell technology.

We demonstrate experimentally the existence of unpolarized spin states of composite fermions in the fractional quantum Hall effect regime. The modulus of the effective Lande g-factor was reduced using hydrostatic pressure and the transport properties of the first and second hierarchy of composite fermion states were studied in an AlGaAs/GaAs heterojunction in perpendicular magnetic fields. The activation energy of states at filling factor, nu =2/3 and 2/5 show a variation with effective Lande g-factor that is consistent with a spin-unpolarized ground state at these filling factors. The rapid destruction of the energy gap in the first composite fermion hierarchy at nu =2/5 follows the pressure dependence of the effective Lande g-factor of free electrons in GaAs. We show that structure in sigma _xx, initially absent at ambient pressure, develops at even filling factor fractions corresponding to metallic states at nu =3/8 and nu =5/12 under reduced Lande g-factor conditions. Consistent behaviour is observed independent of the carrier density in the dilute electron system at 4.4*10¹⁰ and 6.8*10¹⁰ cm^-2.

A direct excite-probe semiconductor lifetime determination in the picosecond regime has been made for the first time in the far infrared. We have used an RF-linac-pumped free-electron laser to determine the relaxation rate associated with intersubband absorption in GaAs/AlGaAs quantum wells having a subband separation smaller than the optical phonon energy. The measurement yields a relaxation lifetime of 40+or-5 ps. This is compared with a variety of other results obtained with less direct techniques.

In this letter the liquid phase epitaxial growth of In_0.71Ga_0.29As_0.68P_0.32 ( lambda _g=1.32 mu m) single quantum well structures lattice matched to (001) InP substrates is reported. The electrical and optical confinement was formed either by InP or In_0.88Ga_0.12As_0.26P_0.74 ( lambda _g=1.05 mu m). Low-temperature photoluminescence was employed to prove the samples capable of displaying quantum size effects. Energy upshifts up to 125 meV were measured for InP-clad quantum wells of about 50 AA thickness. All multilayered stacks originate from step-cooling growth cycles in an ordinary linear slider boat. Streamlining of the experimental conditions has led to a conspicuous decrease in the FWHM values of the blueshift signals.

A new technique has been developed, which allows the controlled, partial disordering of GaAs/AlGaAs multiple quantum wells, using thermal diffusion of aluminium and/or gallium vacancies created by etching the surface oxides. Spectral measurements by photoluminescence and optical absorption indicate that the disordering can result in a blue shift of the effective band edge by up to 40 nm without broadening of the excitonic resonances.

We report on optical reflection measurements on ternary alloy (CdMg)Te thin films grown by molecular beam epitaxy. Data have been obtained in the spectral range between 1.2 eV and 2.8 eV, and at temperatures between 300 K and 77 K. The reflection coefficient as a function of energy E has been theoretically described taking into account multiple reflections at the interfaces involved. The dispersion relation n²(E)=a+bE²/(1-(E/c)²) has been used. A comparison with the experimental data has yielded the refractive index as a function of energy. The value of the refractive index n is given as a function of energy for the ternary alloy (CdMg)Te for different Mg concentrations and different temperatures. These results are essential for designing efficient waveguides for semiconductor laser structures using (CdMg)Te as a base material.

Flexible reflecting membranes with an electrostatically controlled surface shape have been fabricated in a silicon wafer by means of bulk micromachining. A typical membrane has a square aperture with a side length of 10 mm. The membrane is formed by a 500 nm thick tensile stressed silicon nitride diaphragm coated with a 200 nm thick reflecting aluminium layer to form a flexible mirror. The mirror has an initially plane surface with a mean square deviation out of plane approximately lambda /10 for lambda =633 nm. The high initial optical quality and the possibility of electrostatic control of the reflecting surface make the mirror useful for various on-chip electro-optical applications.