Table of contents

The potential distribution in Hall effect experiments on two-dimensional electron systems (2DES) is imaged with a scanning polarization optical microscope utilizing the internal electro-optic effect. Outside the quantum Hall regime (QHR), Hall potential profiles vary linearly between the two edges. In the QHR, the observed Hall potential profile is nonlinear. Its shape depends on magnetic field and current, indicating inhomogeneous current transport in the interior of the 2DES. In the plateau centre, the Hall potential drop is concentrated in the middle of the channel. The results are interpreted in terms of electron density inhomogeneities. These lead to the observed potential distributions because of the very strong dependence of the conductivity on the local electron density.

We report a determination of the conduction-band offset ratio O_c of the In_xGa_1-xAs/GaAs strained-layer single quantum wells by low-temperature modulation spectroscopy. Various modulation reflectance measurements include contactless electroreflectance (CER), photoreflectance (PR) and piezoreflectance (PZR) are carried out on four different samples at room temperature and 20 K. The samples have In composition x=0.05 (sample A), 0.11 (sample B), 0.14 (sample C) and 0.21 (sample D) respectively. A number of allowed and forbidden excitonic transitions have been observed. The light-hole transitions were unambiguously identified by comparing the CER spectra with different modulation strengths. The origins of the various spectral features have been assigned by comparison with an envelope-function calculation taking into account the effects of strain. Good agreement between experiments and theory is achieved by taking Q_c=0.46+or-0.05, 0.50+or-0.10, 0.54+or-0.07 and 0.63+or-0.05 for samples A, B, C and D respectively. It has been shown that O_c depends linearly with the In composition x between 0.05 and 0.21.

We have used a Monte Carlo simulation to study the temperature dependence of the electron impact ionization coefficient in silicon. On the basis of the results of the Monte Carlo calculations, we propose an empirical model for the impact ionization coefficient as a function of electric field or average electron energy, which is applicable over a wide range of lattice temperatures.

A theoretical analysis is made of self-generated current oscillations under DC bias in modulation-doped GaAs/Al_xGa_1-xAs multilayer structures. With applied fields parallel to the layers, real-space transfer of electrons into the Al_xGa_1-xAs barriers can lead to negative differential conductivity and oscillations, depending crucially upon the slope of the load line and upon lattice temperature. We find by numerical simulations that the oscillations are relatively robust against weak disorder introduced by fluctuations of the well and barrier widths, the doping concentration and the alloy fraction of the barrier.

Using photoluminescence (PL) and electroluminescence (EL) measurements and calculated energy band diagrams, we have studied the mechanisms of radiation recombination in InAsSb/InAsSbP double heterostructures forming the basis of lasers emitting at 3.6 mu m. Up to four PL lines are observed, which can be attributed to band-to-acceptor transitions in the active region, cladding layer and substrate and band-to band transitions in the substrate. In EL experiments emission from the active region and the InAs/InAsSbP interface was detected. The possibility of creating a tunable radiation source emitting at two wavelengths is demonstrated.

A mechanism of formation of inhomogeneities in the alloy Cd_xHg_1-xTe during either post-growth cooling or low-temperature annealing is proposed, based on a diffusion instability in the interacting system which includes cations in lattice sites, mercury interstitials and cation vacancies. The principal points of the mechanism are: (i) vacancies follow local variations of alloy composition, being always in local equilibrium; (ii) their concentration depends superlinearly on local composition. The uniform distribution of composition and the native dopants becomes unstable under a condition that is likely to be satisfied at temperatures below 200 degrees C in samples enriched with mercury interstitials. We show that the instability should result in the formation of a 'layered' doping profile where the concentration of mercury interstitials varies by a factor of 2-3 while the variation of composition is of the order of 10%. Possible consequences of this effect for the electrophysical properties of the alloy are discussed.

A quantitative electron paramagnetic resonance (EPR) study of vanadium-related defects in semi-insulating and co-doped p-type and n-type bulk CdTe:V shows the V_Cd donor to be the dominant electrically and optically active defect in semi-insulating photorefractive material. The effective trap density of 4*10¹⁵ cm^-3 for the V_Cd⁰+/ defect, determined by quantitative EPR spectroscopy, is in agreement with the value obtained from photorefractive measurements on the same crystals. A comparison of the total V content with the V_Cd donor concentration indicates the existence of additional V-related defects not observed in EPR. Surprisingly the neutral donor state, V_Cd⁰, is not observed by EPR in the n-type material. No other V-related defects were detected in the n- or p-type material.

Modulation-doped In_0.5Al_0.5As/In_0.5Ga_0.5As heterostructures on compositionally graded In_{x0<or=x<or=0.5}Ga_1-xAs buffer layers on (001) GaAs substrates were prepared by molecular beam epitaxy. An investigation of samples is reported, which have different initial In compositions x₀=0, 0.12, 0.18, 0.24, 0.5 in the linearly graded buffer. The sample with x₀=0.18 shows the highest electron mobility and the highest electron density, which are 9.3*10³ cm² V^-1 S^-1 and 3*10¹² cm^-2 at 300 K respectively, of the investigated samples. The smallest photoluminescence linewidth of 18 meV was measured from both samples with x₀=0.12 and 0.18. A small lattice tilt of 1.4 mrad between the relaxed In_0.5Ga_0.5As layer and substrate and a smooth surface was achieved for x₀=0.18.

Here we report studies carried out on heavily tin-doped and undoped indium oxide films. Structural, electronic transport and compositional properties of the sprayed films deposited with different doping levels of tin (0-110 at.% in solution), were investigated. X-ray diffraction, room-temperature Hall effect measurements, conductivity measurements at low temperatures and x-ray photoelectron spectroscopy were used for these studies. The films were polycrystalline and the preferred growth showed systematic changes with dopant incorporation from (400) to (222). The carrier mobility was very sensitive to these changes in orientation of preferred growth as well as the grain size. The high carrier concentration ( approximately 10²⁰ cm^-3) implied that the films belong to the class of degenerate semiconductors. Such a high carrier concentration was a result of increased non-stoichiometry and increased dopant incorporation. The conductivity measurements at low temperatures show that with increased Sn incorporation, electronic transport is governed by ionized impurity scattering and acoustic lattice scattering.

Deep-level transient spectroscopy measurements were performed on a Ge_0.4Si_0.6/Si single quantum well grown by molecular beam epitaxy. The results show hole traps labelled H₁ and H₂. H₁ is a deep level in Si whose activation energy measured from the valence band and capture cross section are 0.3 eV and 3*10^-22 cm² respectively. When an electric field was applied to the quantum well, an anomalous behaviour was observed for H₂ only. The activation energy of H₂ increased and then saturated as the applied electric field increased. Such behaviour can be explained by identifying H₂ as an emission from interface states to a bound state in a quantum well. The depth profile of H₂ also showed a spatially confined behaviour which strongly supports the idea that H₂ is an interface state. Calculations using a variational method were performed to determine the bound state energy levels of the quantum well in a constant electric field. Comparing the results of experiments with those of the calculations, H₂ corresponds to an interface state whose energy level lies at 0.45 eV above the valence band edge of the well; the capture cross section of H₂ is 2.1*10^-18 cm².

The anomalous capacitance properties in Al_xGa_1-xAs graded p-n junctions governed by the DX centres are studied both theoretically and experimentally. In order to give a quantitative description of these properties a new model is proposed which takes into account the Debye tail effects in the case of free electron freeze-out into deep dopant levels. On the basis of the model we have created a new capacitance method for determination of the dopant impurity ionization energy using C-V curves measured at different temperatures. We applied this method to the DX centres in Al_xGa_1-xAs with 0.2<x<0.7. The obtained results agree with Hall-effect data. We found that the energetic position of the DX level with respect to the L minimum of the conduction band is about 160 meV.

A method of calculating the theoretical capacitance-voltage characteristics of heavily doped silicon/insulator/silicon capacitors is presented. Unlike previously published models the capacitance-voltage characteristics are derived using Fermi-Dirac statistics and energy bandgap reduction to account for the degenerate doping levels. The method of computing the ideal characteristics for the case of degenerate doping is outlined, and the capacitance-voltage behaviour is compared with the results obtained using a Maxwell-Boltzmann distribution. The two solutions are compared as a function of doping concentration and insulator thickness to determine the errors which result when Maxwell-Boltzmann statistics are assumed. The significance of these simulations to integrated capacitors in analogue MOS circuits is discussed. Furthermore, the simulations obtained from the complete Fermi-Dirac solution are compared with measured capacitance-voltage characteristics of heavily doped n-type polysilicon/oxide/polysilicon capacitors from two different 1 mu m analogue CMOS processes. It is demonstrated how the simulations can be used to estimate the electrically active doping concentration at the two oxide/polysilicon interfaces. The influence of fixed oxide charge on the capacitance-voltage behaviour is also examined.

Zn_1-xMg_xSe mixed crystals were grown by the high-pressure Bridgman method in the x range 0.06 to 0.285. X-ray investigations show that with increasing Mg content the transition from a sphalerite structure to wurtzite occurs at x=0.185+or-0.03. After annealing in liquid zinc or zinc vapour, investigated crystals exhibit n-type conductivity as well as blue-violet and orange photoluminescence in the temperature range from 40 K to room temperature.

ZnSe/ZnMnSe MQW structures are grown by MBE. In situ RHEED control allows one to lock the growth cycle on the phase of the RHEED oscillations so that lattice plane completion is achieved independent of beam flux fluctuations and other irregularities. The band-edge resonant optical properties of the structures are dominated by sharp and pronounced excitonic features. The influence of strain and confinement on these excitons, their localization and interaction with phonons are discussed.

We measure the electrical conductivity in the dissipative regime between adjacent integral quantum Hall plateaus. Variable-range-hopping transport is found, in accordance with a recent theory. The characteristic temperature for hopping conduction follows a power-law dependence on filling factor. Conductivity-peak broadening with increasing current is studied and a power-law dependence is observed.