Table of contents

FIR transmission spectra of HgCdMnTe are investigated over a wide spectral range (from 40 cm^-1 up to 400 cm^-1) at different temperatures (from 5 K up to 300 K). It is shown that different transverse and longitudinal acoustic, local and resonant modes are activated by disordering and manganese impurities in the range of low values of single-phonon state densities of HgCdTe. FIR spectra seem to be a good probe for characterization of the quality of semimagnetic crystals.

The transport length of minority carriers has been determined by various EBIC methods on low-doped n-type (AlGa)As. Transport lengths of 12 mu m and 6 mu m were concluded from EBIC investigations as a function of primary energy of the electron beam at T=300 and 80 K, respectively. The hole diffusion length obtained by EBIC profiles using an electron beam scan along the surface is 3.8 mu m at room temperature. Based on an impurity gradient in the growth direction observed by capacitance-voltage measurements the authors explain the long transport length as being due to carrier acceleration in the electric field.

As, P and B ions were implanted into TiSi₂ and CoSi₂ thin layers and subsequently diffused into the underlying silicon substrate either by a rapid thermal process or by a long annealing in a furnace. For all the species, shallow junctions (20-400 nm depth) were obtained with a high dopant concentration (10¹⁹-10²⁰ cm^-3) at the silicide/silicon interface. In the case of As, the comparison between the amount of diffused substitutional arsenic and the carrier profile indicates the presence of inactive substitutional atoms at the silicide/silicon interface. These As atoms form coherent precipitates at the silicide/silicon interface as evidenced by transmission electron microscopy cross sections. Precipitates were also found for P and B dopants, but for this latter species they were located into the silicon substrates at 30 nm from the interface. The precipitate formation has been associated with the high tensile stress induced by the silicide layer on the surface silicon region and to its influence on the solid solubility of the dopant. The precipitate density decreases if the silicide thickness is reduced, i.e. if the tensile stress produced by silicide layer becomes lower. These precipitates are easily dissolved during high-temperature annealing after removal of the silicide layer.

The formation of epitaxial CoSi₂ thin films by reactive deposition of cobalt onto silicon (001) substrates at temperatures around 600 degrees C is described. When the deposition rate is below a certain critical value for a particular substrate temperature (for example below 0.02 nm s^-1 at 600 degrees C), epitaxial disilicide formation can be achieved. Deposition rates above this critical value lead to the production of polycrystalline disilicide. A mechanism is described to explain the influence of the deposition rate on the formation of epitaxial material.

The surface and volume crystal structure of PbTe and PbTe:Cr films deposited by laser-assisted deposition (LAD) on KCI substrates were investigated by reflectance electron and X-ray diffraction. It is found that the crystal structure of the films changes along their thickness. In the initial stage of growth, formation of the metastable phase or phases of the material (depending on the substrate temperature) takes place. At a certain thickness the growth with the metastable phases changes to growth with the stable phase. Thus films with comparable thicknesses of the sublayers with metastable and stable phases represent heterophase junctions. The growth mechanism of these films was semiquantitatively interpreted on the grounds of Sirota's considerations concerning the conditions for the realization of Ostwald's step law.

The absorption of single GaAs quantum wells in Ga_1-xAl_xAs or GaAs/AlAs short-period superlattices is studied in optical waveguiding experiments at low and at room temperatures. The authors show that a complete characterization and determination of the guide parameters is essential in order to determine accurately the optical properties of quantum wells or superlattices embedded in the guide. The waveguiding configuration allows one to obtain the quantum well absolute absorption coefficients in both polarization directions, parallel and perpendicular to the plane of the layers, and to determine the exciton binding energies and the associated oscillator strengths. New results are presented for an enlarged quantum well in a superlattice. The authors show that the excitonic binding energies depend on the extension of the wavefunction in the superlattice and are different for heavy- and light-hole excitons. The experimental values obtained for a quantum well in an alloy agree well with recent theoretical calculations.

The refractive index related to the excitonic polariton in a GaAs quantum wire is calculated numerically. The maximum value of the real part of the refractive index is as large as 5.6 when the width of the GaAs wire is 5 nm. This large refractive index results from the large binding energy of the exciton confined in a quasi-one-dimensional structure. The electromagnetic field of the excitonic polariton transmitted in such a quantum wire structure is therefore squeezed significantly because the wavelength in the structure is reduced by the refractive index increase. The estimated full width at half-maximum (FWHM) of the light power related to the excitonic polariton is typically as small as 67 nm, although the transmission loss is fairly high. However, the transmission loss can be reduced by selecting an appropriate wavelength, while keeping the FWHM of the power profile small. These results indicate the possibility of making extremely small optical devices.

Study of the excitonic spectra of a GaAs epilayer, via the 'luminescence self-absorption' technique, shows that heavy deuteration of the material leads to strong accumulation of deuterium at the epilayer/substrate interface and to the creation of an electric field. This causes full quenching of the free exciton and an Urbach tail extending below the fundamental edge by an energy of the order of the exciton Rydberg. This corresponds to a maximum attained field around the ionization value (5.7*10³ V cm^-1), and a fixed charge at the interface equal to about 0.1% of the locally accumulated deuterium.

A comparative study of the temperature dependence of the absorption edge has been performed with thin single-crystal wafers of bulk GaAs and multilayer quantum-size GaAs/Ga_0.3Al_0.7As structures with the well and barrier thicknesses L_a=L_b approximately=135 Å in the temperature range T=4-300 K. The total number of periods was N>20. The authors consider the quantity K*, which is proportional to the integral absorption coefficient calculated as a product of the line halfwidth and the optical density at the exciton band maximum. In bulk samples K* shows a sharp increase by nearly an order of magnitude, up to T* approximately= 110 K, and then remains roughly constant. The temperature T* may be considered as critical for the excitonic polariton-mechanical exciton transition. A similar behaviour of K* was observed for superlattices with a slightly increasing integral absorption up to a temperature of T* approximately=20 K. This fact is considered as evidence for the polariton nature of light absorption at T

The authors report experimental and theoretical investigations of excitons in CdTe/Cd_1-xMn_xTe single quantum wells with high manganese content x=0.20 and various well widths. The Zeeman splitting of the excitonic transitions is measured by photoluminescence excitation in an external magnetic field up to 7.5 T. They have developed an extended model describing the Zeeman splitting of quantum well excitons, including the exchange interaction, the intrinsic g-factor of electron and hole and the Coulomb interaction in an external magnetic field. The calculation allows the interpretation of both light- and heavy-hole excitons. From the Zeeman splittings they deduce the natural valence band offset Q_v in the strain-free limit. Due to the small; valence band offset, a magnetic field-induced type I to type II transition occurs for the heavy-hole state. They show that the main contribution to this transition comes from the excitonic effects, which nearly compensate for the large decrease of the confinement energy. In wide quantum wells with high manganese content they found an enhanced paramagnetic exchange interaction, which is discussed in terms of interface effects.

The electrical and optical properties of niobium-doped silicon have been investigated using junction space-charge techniques. Three energy levels were observed at E_c-0.293 eV (A level), E_c-0.583 eV (B level) and E_v+0.163 eV (C level). The corresponding changes in enthalpy for the capture of electrons into the A and B levels are 4 meV and -7 meV, respectively, whereas the change in enthalpy for the capture of holes into the C level is 22 meV. Using these data, changes in Gibbs free energies as a function of temperature were calculated for all three levels. Good agreement of Gibbs free energy changes with optical threshold energies of the A and B levels suggests that lattice relaxation effects should be small.

The band-related impurity level (BRIL) model that the authors formulated earlier is applied to the centre in a binary semiconductor doped with rare-earth elements. Owing to the very weak hybridization of the 4f electrons with the host states the authors can obtain analytical expressions for a two-level system consisting of a deep level E_f far below the top of the valence band and a quasi-vacancy level E_v in the gap. The ground state of the system is investigated by taking into account the correlation effects inside the 4f-shell of the rare-earth ion.

In n-type AlGaAs layers, the evolution of high frequency capacitance with time due to electron emission from DX centres has been computer simulated. The electron density profiles during the emission and capture processes are also presented. The effects of switching voltage amplitude and DX level position in the gap have been considered. A critical discussion of the signals obtained, the intrinsic sources of non-exponentiality and the reliability of the DX parameters extracted, are presented. The artifacts due to the DLTS processing of the capacitance transients generated were also considered. The subject of alloy broadening has been addressed, and it has been unambiguously shown that intrinsic broadening may only account for less than half the measured broadening in Al_0.3Ga_0.7As:Si.

The effect of photon recycling (or self-excitation) in semiconductors is established theoretically for the minority carrier lifetime, the minority carrier diffusion coefficient and an electric field parameter, for a wide range of physical situations. These include absorption coefficients, reflection coefficients at the boundary of the semiconductor (and therefore the refractive indices), semiconductor layer thickness and internal quantum efficiency. The new features of this work are: the inclusion of the dependence on the incidence angle of the reflection coefficients, an improved but fairly simple derivation of the corrections to the observable quantities due to photon recycling, and the effect of photon recycling on the electric field. As already known, the most important effects are on the lifetime, whereas the diffusion coefficient is relatively insensitive to photon recycling. It is pointed out here that the effect can, however, be significant if the absorption length exceeds the diffusion length. In this case the effect on the electric field is also important and increases with the reflection coefficient.

The authors present an analysis, in the frequency domain, of the carrier recombination kinetics most frequently encountered in semiconductor systems under quasistatic excitation. The analysis can be used to interpret carrier lifetime distributions obtained in frequency-resolved spectroscopy experiments. The effect of trapping on the lifetime distribution is analysed in detail and the authors discuss how frequency-resolved spectroscopy can be used to obtain the major trap parameters.

The authors present a novel hydrodynamic approach to the study of DC and AC hot-carrier transport in semiconductors. To this end use is made of a total-energy scheme which incorporates simultaneously the kinetic and potential energy associated with different conduction band minima. Furthermore, convective and diffusive contributions are considered by including the variance of velocity-velocity and velocity-energy fluctuations. Together with static characteristics, a small-signal analysis under spatially homogeneous conditions of the most important response functions of the electron system (e.g. differential mobility, diffusivity, velocity-noise spectrum, etc) is developed consistently. The validation of the present approach is supported by an excellent comparison with Monte Carlo results carried out for the case of n-Si at 300 K.

Using non-parabolic ellipsoidal X and L valleys to represent the conduction band of Si, the authors have developed a Monte Carlo simulation for the study of electron transport properties in this material, under both low and high electric field conditions. Employing a simple model for the characterization of the impact ionization processes they have obtained the ionization coefficient and the probability of electron ionization. The results highlight the importance of the L valleys in very high transport phenomena, and compare favourably with other experimental and theoretical data.

A large number of sharp structures are observed in the 4.2 K magnetoresistance of n-GaAs biased above impurity breakdown in a regime where current flow is filamentary. Most of the structures cannot be attributed to spectral properties of the semiconductor such as impact excitation of shallow donors or the magnetoimpurity effect. Experimental results give evidence that these structures are caused by a redistribution of the filamentary current flow when one filament border is swept across an imperfection in the material.

The field contrast mode of a scanning electron microscope has been used to image high-field domains and current filaments in GaAs/Al_xGa_1-x As multi-quantum wells. In samples where negative differential resistance and current instabilities occur, high-field domains form. However, these domains are shown to be either stationary or propagating slowly along the samples, in stark contrast to the case in bulk material. In samples where n-type negative differential resistance, caused by either real-space transfer or intervalley transfer, is inhibited, high-field domains do not form. In the case of moving domains they are shown to annihilate before reaching the anode, probably by a lateral dissipative mechanism. In samples where s-type negative differential resistance occurs current filaments are shown to form, which leads to the irreversible destruction of the material.

Photocurrent spectroscopy (PCS), photovoltage spectroscopy (PVS) and electrolyte electroreflectance spectroscopy (EER) have been compared as techniques for compositional characterization of n-Ga_1-xAl_xAs epitaxial layers (x<or=0.3, N_d=10¹⁸ cm^-3) in 0.1 mol 1^-1 KOH. The donor densities and flatband potentials of the samples in 0.1 mol 1^-1 KOH have also been studied by admittance measurements in the frequency range 500 Hz to 10 kHz, both in the dark and under illumination. The donor densities obtained in this way are compared with values derived from mercury probe capacitance measurements on the same samples. Values of the flatband potential (E_fb) measured in the dark showed a systematic variation with aluminium content consistent with the changes in bandgap energy. The displacement of E_fb caused by illumination was found to switch from negative to positive as the Al content was increased from 0% to 30%, demonstrating that the photoinduced surface charge is sensitive to bulk composition. The bandgaps (E_g) derived by analysis of photovoltage and photocurrent spectra are compared with E_g values derived by three-point fits of the EER spectra at E₀. The latter values were found to depend on the DC bias applied to the samples and this is attributed to the effect of the inhomogeneous field in the space-charge region.

The Cd_xHg_1-xTe avalanche photodiodes present hole or electron majority multiplication, depending on the x values, with a ratio of ionization factors k= alpha _h/ alpha _e maximum for x=0.6 and minimum for x=0.4. This behaviour is explained by a simulation of the multiplication phenomenon. Monte Carlo electron trajectories are averaged to obtain ionization factors alpha _h and alpha _e. This model introduces a linear relationship between the logarithm of k and the ionization energy difference for holes and electrons: Delta E_i=E_ih-E_ie. The authors compute this energy difference in an empirical tight binding band structure and show that its variation versus x agrees with experimental k(x) variations. The hole multiplication maximum at x=0.6 is explained as the effect of transitions issued from the split-off band at Gamma .

It is shown that photoconductivity (PC) of thin epitaxial Cd_xHg_1-xTe/CdTe layers in the stationary crossed fields has a complicated non-monotonic behaviour as a function of magnetic field. The analytical expressions for PC and the photoelectromagnetic (PEM) effect, including diffusion, gradient and photoconductive components, are presented and qualitative physics is discussed. The experiments were carried out on liquid-phase epitaxial Cd_xHg_1-xTe/CdTe films of p-type conductivity with x approximately=0.2 at liquid-nitrogen temperature. On the basis of the combined analysis of PC and PEM effects a set of the recombination and diffusion parameters was determined: the volume lifetime, the diffusion length, the surface recombination velocities and the internal electric field due to the composition gradient. Also, it is shown that from the PC measurements in different electric fields one can obtain information about variation of the parameters across the layer.

Thin films of ZnTe are prepared under different conditions by a vacuum evaporation technique. Dark- and photoconductivity and measured at different temperatures. The effect of doping on conductivity is studied for BaF₂ (7 wt.%), PbCl₂ (6 wt.%) and In (6 wt.%) dopants. The spectral response of photoconductivity are intensity variation of photocurrent in ZnTe films of different thickness is studied. The mechanism of conduction in those films is properly explained by considering different models. The variation of drift barrier height with film thickness, and hence the particle size, is studied. An exponential distribution of traps or impurity states within the bands is suggested for the films.

The results of investigations into in-plane photoconductivity in GaAs/Ga_0.7Al_0.3As multiple quantum wells are presented. The results indicate clearly that the experimental technique can be used as a reliable and sensitive tool to determine the purity of the material and the excitonic transitions that occur in the quantum well structures. It is shown that the photoconductivity is a strong function of the excitation intensity and the modulation frequency. The effects of DC background illumination and the application of longitudinal electric fields are also studied and the results are compared with a model based on hot-electron percolation in quantum well structures.

The effect of electron-beam-induced damage on the leakage current between a back gate and an ohmic contact in a GaAs/AlGaAs modulation-doped heterostructure grown on a conducting substrate was investigated. Enhanced leakage currents were observed for certain values of the electron-beam energy. The experimental data are shown to be in qualitative agreement with the theory of Gruen, and device implications of this damage, pertaining to back-gating, are discussed.

Tunnelling processes in thin double-barrier GaAs/AlAs diodes have been studied under hydrostatic pressure. Gamma -X tunnelling strongly influences the current-voltage (I-V) characteristics of samples having barriers ten monolayers thick. Non-resonant tunnelling through transverse X valleys predominates at T=300 K, whereas at low temperature it proceeds by longitudinal X valleys. Indirect Gamma -X tunnelling is found to be inversely proportional to barrier thickness. The I-V characteristics of samples having barriers six and five monolayers thick are controlled by elastic Gamma - Gamma tunnelling. It is shown that extremely thin barrier thicknesses do not necessarily imply better electrical features.

The diode performance of Au/n-Si and Au/n-GaAs systems has been investigated within the temperature range of 100 K to 300 K. From the results of photovoltage and I-V measurements, the calculated solar cell efficiency of the Au/n-Si diode degrades significantly from the ideal case (n factor equal to 1). The change of the efficiency of the Au/n-GaAs system with temperature, however, adheres closely to the ideal case. The difference between the results of these two systems is explained by the recombination current effect, which appears to be more significant in the Au/n-Si system.

The authors present the first theoretical predictions concerning the intensity-dependent (nonlinear) optical properties of certain highly excited Si-SiGe semiconductor microstructures. They use a simplified model that can only be applied to a limited range of effects. However, they show that this model correctly accounts for the relevant experimental results available for bulk GaAs and some GaAs-GaAlAs microstructures. They find that in the range of applicability of the theory the nonlinear refractive index in certain Si-SiGe quantum well structures is comparable to that in analogous GaAs-based microstructures.

Simulations of apparent doping profiles as obtained from C-V measurements at pn or Schottky diodes are used to analyse data measured at a Si-SiGe-Si single quantum well device. C-V characteristics are simulated by numerically solving Poisson's equation for a multilayer structure using the algorithm of Rimmer, Missous and Peaker (1991). Some refinements are given to improve the precision and stability of this method. The valence band offset for a SiGe layer (4% Ge content) on a Si substrate is deduced from temperature-dependent C-V measurements to be Delta E=25-30 meV. The main accuracy-limiting factor for a band offset determination from C-V measurements is the influence of doping inhomogeneities, but the error can be reduced by taking into account C-V profiles measured at different temperatures.

The boron acceptor in 6H-SiC was investigated using electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). The hyperfine interactions with ¹¹B could be determined precisely for the two quasi-cubic and the hexagonal sites. The microscopic model suggested from the EPR and ENDOR results is as follows: boron occupies a silicon site and is a negatively charged ligand to an adjacent carbon atom on which most of the unpaired spin density is located. At low temperatures the symmetry of the two quasi-cubic site defects is monoclinic, while the hexagonal site defect has C_3V symmetry about the hexagonal axis. At about 50 K the two quasi-cubic site defects experience a thermally activated motion of the hole at the adjacent carbon about the hexagonal crystal axis and the apparent defect symmetry also becomes C_3V. The boron acceptor should be regarded as a boron-induced carbon acceptor.

Commercial silicon diodes were investigated using electrically detected electron paramagnetic resonance (EDEPR) and photovoltaic detected electron paramagnetic resonance (PDEPR) between 3.5 and 300 K. At room temperature a single EDEPR line was observed for 1N4007 diodes, which is generally believed to be due to surface defects. At temperatures between 3.5 K and 100 K the authors have found a new anisotropic hyperfine split EDEPR spectrum. DLTS measurements on these diodes yield three deep levels. On the basis of theoretical calculations the EDEPR is explained to be most probably due to a (P_i-C_Si) pair defect. The dependence of the EDEPR signals upon temperature, microwave power and illumination was investigated and is discussed.

It is explained in physical terms why the nonlinear refraction due to real excitation is enhanced by the factor 2T₁/T₂ (T₁ is the lifetime and T₂ is the dephasing time) compared with that due to virtual excitation. In this explanation it is assumed that only changes in electron state occupation probabilities lead to changes in the refractive index, regardless of the nature, real or virtual, of the excitation. This assumption is verified for a two-level model using an exponentially growing pulse envelope, leading to the concept of generalized state filling, i.e. filling by a mixture of real and virtual excitation. To establish this concept a generalization of Fermi's Golden Rule for the transition rate is required. The required generalization is derived using the equation of motion for the density matrix. The derivation adds insight to the range of validity of the standard Fermi Golden Rule result. The concept of generalized state filling is extended to arbitrary pulse shapes. To illustrate this concept, an expression is derived for the change in the occupation probabilities of a two-level system illuminated near resonance by a pulse with exponential leading and trailing edges. The physics contained in this algebraic result is discussed and much insight into the relationship between real and virtual excitation is obtained. The algebra shows that the time constants T₁, and T₂/2 that govern the response for real excitation are both replaced by the same time constant for virtual excitation, namely the time constant determining the rate of change of the pulse intensity.

The temperature distribution in a GaAs/(Al,Ga)As multiple quantum well structure was measured with high lateral and temporal resolution while passing through the characteristic hysteresis loop of a thermally induced absorptive optical bistability. In a pump and probe beam experiment, the local transmission spectra of the quantum well structure were recorded and the temperature distribution in the sample was deduced from the spectral positions of characteristic exciton resonances. These resonances remain sufficiently sharp even far above room temperature to guarantee an accuracy of better than 3 K The experiment was conducted under both steady-state and dynamical conditions. The steady-state results were compared with numerical calculations, showing good agreement. The dependence of the relative width of the characteristic hysteresis on the environmental temperature of the bistable sample was investigated. A simple relaxation-time model of the bistability was shown to describe nicely the observed behaviour if the bleaching of the lowest exciton resonance due to high laser intensities is considered. Based on this dependence, a miniature temperature-sensing device using an optical fibre to guide the laser beam is presented.

A multiquantum well (MQW) GaAs/GaAlAs system is studied by a non-conventional technique of thermally detected optical absorption (TD-OA) in which the sample temperature variation is measured by a germanium thermometer at 0.5 K. In such a sample, for which the holder and quantum well materials are the same, the QW transitions give rise to temperature minima. The latter are interpreted in terms of intensity variations of the energy reflected at the interfaces of the wells close to the excitonic recombinations. The TD-OA spectra corresponding to the n=1 heavy-hole and light-hole excitonic transitions in the wells are examined, together with the spectra obtained by conventional photoluminescence. Doublet lines involving QW excited levels, a component of which is due to a transition only allowed by valence band mixing, are also detected as in photoluminescence excitation.

Photoluminescence and Raman spectroscopy have been used to study a delta -doped In_0.1Ga_0.9As:Si/GaAs strained quantum well with the doping spike placed at the centre of the 30 nm wide well. In this structure the potential confining the electron gas is given by a superposition of the V-shaped potential well induced by the doping spike and barriers introduced by the heterointerfaces. Both luminescence and intersubband Raman spectra provide information on the energy spacing and population of the electron subbands. A comparison with results of self-consistent subband calculations is made. Raman spectra excited in resonance with the E₀+ Delta ₀ bandgap of the In_0.1Ga_0.9As quantum well show intense single-particle intersubband transitions, which dominate over the signal from collective spin-density and charge-density excitations. Possible mechanisms for the occurrence of such strong single-particle intersubband Raman signals are discussed.

Photomodulation techniques have been used to study the fundamental transitions of partially strained In_xGa_1-xAs/GaAs (x=0.07 and 0.16) epilayers. The strain-induced splitting of the valence band was observed. The identification of the split valence bands was confirmed using linearly polarized photoreflectance under a total internal reflection mode. The temperature dependence of two bandgap energies E₀₁ (heavy-hole band) and E₀₂ (light-hole band) was studied over a temperature range from 15 to 300 K. The parameters that describe the temperature dependence of E₀₁ and E₀₂ as well as the broadening function are evaluated.

In recent years there has been an increasing trend towards 'dry' and 'cold' processing. The oxidation of silicon in an oxygen plasma allows the controlled growth of thin, high-quality films of silicon dioxide at temperatures down to room temperature in a clean vacuum environment. In this review the various experimental systems which have been reported in the literature over the last twenty-eight years are compared and contrasted. Physical properties of the oxides are found to be very similar to thermally grown films. The reported kinetics of plasma oxide growth are reviewed, together with the electrical and electronic properties of the grown oxide layers. Inductively coupled plasma oxidation is shown to have advantages over the other techniques, and some recent results obtained using this method are highlighted. The current and possible future applications of plasma oxidation for novel silicon and silicon-based device processing are discussed.

In the present work the gas dynamics in the growth zone of SiC crystals is investigated. It is shown that the propagation of SiC vapour from the growth cavity walls towards the lids is effected by diffusion. On this basis the calculation of the concentration distribution of SiC vapour (n), the equilibrium vapour concentration (n_s) and the supersaturation ( alpha =((n-n_s)/n_s)*100%) in the crystal growth zone at different radial and axial temperature gradients is carried out by solving the Laplace equation in cylindrical coordinates for a stationary case corresponding to the conditions of crystal growth. The results obtained are compared with the available experimental data; this makes it possible to explain some of the peculiarities observed during SiC crystal growth from the vapour phase by the sublimation method.

An (In,Ga)As surface enriched in elemental arsenic by etching in H₂SO₄:H₂O₂:H₂O=1:1:125 has been investigated by means of ellipsometry, angle-resolved photoelectron spectroscopy (ARXPS) and scanning electron microscopy (SEM). The deconvolution of the As 3d peak reveals the existence of elemental As on top of InGaAs. During storage in air arsenic is oxidized, as indicated by the growing intensity of the As 3d (oxide) peak. At the same time the ellipsometric angles are shifted appreciably nearer to their substrate values. Regarding the ARXPS and SEM results, this shift has been explained by a decreasing thickness of a rough As surface layer. Using known optical constants of the components, the ellipsometric data could be fitted assuming a rough As layer of 15 nm thickness together with a 1 nm thick dense As layer on top of the freshly etched sample. After 120 h storage in air both of the layers were reduced in thickness to about the half of their initial values and covered with As oxide. The ellipsometric data after etching in H₂SO₄:H₂O₂:H₂O and a plasma-stimulated deposition of SiO₂ are in agreement with the presence of a persistent As-rich layer.

Selective deposition of W(Zn) metallization, for formation of diffused ohmic contacts onto InP-based material, was realized by means of rapid thermal, low-pressure metalorganic chemical vapour deposition (RT-LPMOCVD). The W(Zn) layers were deposited using a reactive gas mixture that contained diethylzinc (DEZn), WF₆, H₂ and Ar, at temperatures of 450 to 550 degrees C and pressures in the of 2-3 Torr. Uniform and continuous layers of W(Zn), 30 to 120 nm thick, were obtained. These layers contained Zn at concentrations higher than 1*10¹⁸ cm^-3, which was subsequently in-diffused into the underlying semiconductor layers to form highly doped semiconductor layers as thick as 0.2 mu m. As a result, the specific contact resistance of the W(Zn)/In_0.53Ga_0.47As contact was reduced to a minimum value of 5*10^-6 Omega cm². The W(Zn) films were found to be mechanically stable with a small compressive stress of 5*10⁸ dyn cm^-2, and were dry etched at rates of up to 90 nm min^-1.

Nuclear radiation detectors constructed from GaAs Schottky barrier structures require control of barrier height if high resolution is to be reproducibly obtained. The authors have therefore studied the variation of device barrier height after treatment with common etching solutions. The heights of Au and Al Schottky barriers on respective samples of n- and p-type liquid-phase epitaxial GaAs were found from reverse bias I-V measurements subsequent to etching with aqua regia and 3H₂SO₄:H₂O₂:H₂O (3:1:1). An increase in barrier height observed after the 3:1:1 etch was found to be a consequence of sulphur contamination. This result is consistent with those reported for deliberate sulphur passivation of GaAs. Low diode ideality factors, from 1.007 to 1.064, were measured for layers with carrier concentrations in the range 4*10¹⁴ cm^-3 to 2.5*10¹⁶ cm^-3. A comparison was made between samples etched in the 3:1:1 solution at 5 degrees C and at 100 degrees C. C-V measurements gave anomalously large barrier heights for those samples etched at 5 degrees C, indicating the formation of a significant interfacial layer prior to Schottky barrier metallization, while X-ray photoelectron spectroscopy measurements confirmed the presence of a large oxide layer. No such anomalies were observed for the samples etched in 3:1:1 at 100 degrees C.

The electrodeposition of CdTe on Cd_xHg_1-xTe is carried out from aqueous solutions at 55 degrees C and the film growth is monitored using in situ ellipsometry. The measurements reveal that, at a growth potential of -0.55 V versus SCE, a thin (120 AA) Te layer is initially formed on the surface followed by the growth of the CdTe film (1.8 mu m) which appears to be of uniform composition for most of its thickness. Further analysis of the film using the techniques of Raman spectroscopy, differential scanning calorimetry (DSC) and energy dispersive X-ray analysis (EDAX) confirm the excess tellurium in the electrodeposited film. The EDAX measurements after electrodeposition also reveal the presence of 2-8% Hg in the film depending on the depth of analysis. The presence of Hg in the film can only be explained by the diffusion of Hg from the substrate through the electrodeposited layer.

Cyclotron resonance of high-mobility (up to 300000 cm² V^-1 s^-1 at 100 mK) GaAs/AlGaAs two-dimensional hole gases (2DHGs) grown on (311)A oriented substrates has been measured in magnetic fields up to 17 T and at temperatures down to 350 mK. The 2D hole density is in the range (1.0-2.0)*10¹¹ cm^-2. Two resonances are in general observed; at low magnetic fields the lower-field resonance dominates but a progressive transfer of oscillator strength takes place as the field increases. A self-consistent calculation of the Landau fan diagram for the higher-symmetry (100) direction is used to interpret the data and identify the observed transitions.

This letter presents the first calculation of the exciton binding energy (E_x) and oscillator strength (OS) for quantum dots in type II semiconductor systems. These structures consist of a spherical dot of one semiconductor embedded in a second semiconductor. As in the type II exciton systems in quantum wells, the electron is confined in one semiconductor and the hole in the other due to band line-ups in the two materials which make this arrangement energetically favourable. The author uses a variational calculation where the wavefunction of the particle confined to the dot is a spherical Bessel function and the wavefunction of the other particle is a generalized Fang-Howard wavefunction allowing correlation with the confined particle. He has considered the GaAs/AlAs system where the electron is confined in the X state in the AlAs while the hole is confined in the GaAs dot which is indirect in both real and k space for a dot radius of less than 56 AA.

Electroreflectance spectroscopy is a useful diagnostic tool for potential and field distribution in semiconductors, and spectra from experiments performed on samples taken from highly doped (near-degenerate) wafers of III-V materials have been compared with theory. Satisfactory modelling has been achieved using recent advances in the understanding of the Moss-Burstein (band-filling) effect, and the accurate modeling of bandgap narrowing and other heavy doping properties is also demonstrated.

A theoretical approach to the interpretation of the physical mechanisms of electron transitions in delta -doped structures is proposed. Spatial variations of electric field and effective bandgap within the delta region are taken into account in order to simulate differential photoreflectance spectra from Si delta doping in GaAs. A good qualitative agreement between the theoretical results and recent experimental data confirms the validity of the present model. This indicates that photoreflectance spectra from delta -doped structures may be described in terms of the common three-dimensional two-particle Franz-Keldysh effect.

The authors have performed detailed investigations of an asymmetrically doped GaAs/AlAs/GaAs tunnel diode in which a 2D accumulation layer forms at the emitter/barrier interface under forward bias. The systematic application of hydrostatic pressure at 77 K leads to the development of a well resolved peak in dl/dV in forward bias above 2 kbar. The peak shifts approximately linearly downwards in voltage as the pressure is increased in constant steps. Comparison with self-consistent theoretical calculations enables them to unambiguously identify the peak as arising from resonant tunnelling between the electron states in the accumulation layer and the longitudinal X-point subbands of the AlAs barrier. To their knowledge this is the first time that quantitative agreement between theory and experiment has been achieved and places our interpretation beyond question. The conclusions have important implications for the optimization of microwave diodes based on such structures.

The valence band offsets in Si/Si_1-yGe_y/Si strained-layer heterostructures with 0<y<0.14 have been determined by CV measurements and from the analysis of Kroemer. Good agreement is obtained with theoretical predictions of values in the range 0 to 0.1 eV. Extracted interface charge densities are consistent with values deduced from parallel transport measurements in 2DHG structures.

Electron beam lithography (EBL) with a low accelerating voltage ( approximately 2 kV) was utilized for the fabrication of nanostructures. A resolution of 30 nm was achieved for both sparse and dense lines. The high resolution resulted from the low aberrations of the electron optics system of the field emission scanning electron microscope used as an EBL machine and from the preferred small-angle forward scattering characteristic of the low-energy exposing electrons. By comparison with 50 kV EBL, the authors show a large reduction in the proximity effect and demonstrate a 60 nm spacing between two large exposed areas. Moreover, it is shown that the critical dose at 2 kV is more than an order of magnitude less than that at 50 kV exposures.