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Raman scattering from arrays of GaAs quantum cylinders with radii down to 30 nm has been observed. The spectra show an enhanced TO phonon intensity due to a change in the first-order selection rules. In addition, the smallest cylinders exhibit an extra spectral feature between the TO and LO phonons. The behaviour of this additional peak as a function of the scattering geometry, the cylinder size and the variation of the medium surrounding the cylinders is consistent with a theoretical model of the surface phonons of a cylinder.

For part II see ibid., vol.5, p.36 (1990). In parts I and II models are derived in which diffusion of growth species and surface kinetics are combined to describe the growth rate and depletion effect in horizontal metal organic vapour phase epitaxy reactors. Some examples are given in order to show the validity of the models. It is demonstrated that, using the analytical expressions as derived earlier, the experimental growth results for GaAs in the diffusion-limited region can be described well using values for the diffusion coefficient at room temperature D₀ and the thermal diffusion factor alpha _T of trimethylgallium (TMG) of 0.7 cm² s^-1 and 1.0, respectively. The low-temperature growth of GaAs, which is kinetically limited, is modelled using data on the decomposition of TMG. The models are not restricted to the use in MOVPE systems as is shown in the case of the growth of Si from SiH₄, where the calculated growth rates in both the diffusion-limited region as well as in the kinetic-limited region can be accurately described using kinetical data for the decomposition of SiH₄. On the same basis, also the doping of GaAs with SiH₄ is described.

On the basis of the two-zone hybrid model of doped semiconductors suggested earlier (see Phys. Status Solidi (b), vol.156, p.487 (1989)) have analysed the behaviour of the Fermi level and the degree of ionisation of the impurity band over a wide temperature range. The calculation of the density of states is performed using a modified Matsubara-Toyozawa method (1960) in which the so-called multiple occupancy corrections are fully taken into account. The results obtained are compared with the corresponding ones of a simpler delta model which is a good approximation of the well known Kane and Morgan density of states relations.

The far infrared (FIR) photoconductive response of silicon-doped GaAs/AlGaAs multi-quantum wells (MQWS) at 4.2 K reveals evidence of several transitions from the ground state to higher excited states of the confined impurity (e.g. 1s-3d₊₁, 1s-3p₊₁, 1s-3d₊₂, 1s-4d₊₂ etc.). Assignments are made by comparison with the bulk case and with available theory. Linewidths of the 1s-2p₊ transition indicate that in the most favourable case minimum redistribution of the silicon dopant occurs which permits observation of higher state transitions.

The authors have investigated the tunnelling processes between two independently contacted two-dimensional electron gas systems on GaAs-GaAlAs-GaAs heterostructures. This 2D-2D system consists of an inversion layer and an accumulation layer which are separated by a GaAlAs barrier of only 200 AA. On shifting the two systems energetically with respect to each other, resonant tunnelling between the subbands on both sides of the barrier is evident as a series of peaks in the derivative of the tunnelling current. Self-consistent calculations were performed to determined the subband energies from the data obtained by tunnelling spectroscopy. The authors find that these results are in excellent agreement with the subband energies determined by far-infrared transmission measurements.

Minority carrier transport equations in heavily doped silicon with a non-parabolic density of states are proposed for the first time. The derivation is based on a minimum of assumptions and approximations without any artificial quantities like 'apparent bandgap narrowing' and 'effective mobility'. Two coupled first-order differential equations are obtained for the calculation of the minority carrier current. It is shown that the band tail has only little influence on the steady-state transport due to the small contribution of the charge carriers in the localised states, but it does affect the transient-state transport. It is predicted that the measured Auger coefficient is a decreasing function of doping due to band tail states.

The author presents a novel family of semiconductor multilayer structures, isotope heterostructures. The simplest isotope heterostructure is created by epitaxial growth of chemically pure layers of pure or deliberately mixed isotopes of one chemical element that can be doped after growth by exposure to thermal neutrons (neutron transmutation doping (NTD)). He shows that isotope multilayer structures offer selective doping using NTD. The advantages of isotope multilayers include but are not limited to the complete absence of autodoping and dopant impurity induced intermixing, choice of dopant concentration through the choice of the neutron fluence after growth, perfect homogeneity of doping, fixed dopant concentration ratios in the various layers given by the isotope abundances and possibly doping to concentrations above solubility limits. The element germanium is used as an example for isotope multilayer formation because NTD can produce shallow acceptors, shallow donors or no doping depending only on the choice of the appropriate Ge isotopes. This allows an almost limitless number of possible combinations of selectively doped and pure layers. Isotope heterostructures, especially isotope homo and hetero superlattices of Ge, Si and compound semiconductors, offer an abundance of novel properties enabling new physics and new devices.

The authors have investigated new Schottky diodes on MOCVD-grown n-type InP, by introducing a few monolayers of GaInP, GaInAs ternary compound between the metal and the semiconductor. In order to improve the contact behaviour, they have modified on the one hand the experimental conditions e.g. the nature of the metal, the temperature and the duration of the contact thermal annealing and on the other hand, the thickness of the ternary epilayer. Electrical characterisations (current-voltage, capacitance-voltage) were performed and analysed in a conventional way. Optimised parameters of this study were found to be Pt/Ti/Pt/Au/GaInP(200-300 AA) with a contact thermal annealing of 430 degrees C for 2 min. The best diodes exhibit a barrier height of 0.58+or-0.02 eV with an ideality factor of 1.03. The metallurgical behaviour of the contact was studied by Auger electron spectroscopy and by STEM, which revealed a strong diffusion of the Pt into the ternary compound layer after annealing. This study on n-type InP Schottky diode has led to the formation of a MESFET: the preliminary results are presented here.

The effect of electron-phonon interaction on resonant tunnelling has been calculated. This interaction gives a phonon shoulder in the current-voltage characteristic also seen by Goldman and co-workers. The scattering also leads to a decreased peak-to-valley ratio, which can be rather large for II-VI materials. The model used is a microscopic model based on perturbation theory.

In calculations of recombination rates and optical gain in semiconductors it is necessary to relate the quasi-Fermi level positions for electrons and holes at a given injection level. Usually this is done by requiring the volume densities of electrons and holes to be equal but when this charge neutrality condition is applied to a quantum well a charge imbalance is implied in the barrier regions. A more satisfactory approach is to require the whole active region, comprising wells and barriers, to be neutral overall; nevertheless, the band bending implied by both of these models is inconsistent with the use of spatially constant band edge energies. The authors have explored the range of validity of these two neutrality conditions and their influence upon the calculated gain-current characteristic for a 25 AA wide GaAs quantum well with Al_0.35Ga_0.65As barriers. While the choice of neutrality condition influences the individual electron and hole densities at a given quasi-Fermi level separation the radiative recombination rate, which depends upon the (np) product, is not significantly changed. Similarly, the gain as a function of spontaneous current is not strongly affected, though the calculation of other recombination currents which depend primarily upon the density of only one type of carrier, such as recombination via traps and Auger recombination, are influenced by the choice of charge neutrality condition.

Silicon molecular beam epitaxy layers are characterised by photoluminescence. Low-energy ion doping with indium during growth has been exploited to obtain an In-doped silicon layer on a phosphorus-doped silicon substrate. The samples were grown over a range of growth temperatures, 550-850 degrees C, and the photoluminescence spectrum of the well known In-related shallow bound exciton line was detected for doping concentrations from 1*10¹⁶ to 1*10¹⁸ cm^-3. Undoped silicon layers on boron-doped silicon substrates were used to study the effect of growth temperature. A previously unreported line at 884 meV is also studied.

The thermal stability of strained Si_1-xGe_x layers grown epitaxially by molecular beam epitaxy on Si(100) and capped by a Si top layer was studied using Rutherford backscattering spectrometry. Diffusion coefficients were evaluated from the Ge scattering profiles of a 50 nm SiGe film, capped with 50 nm Si, for anneal temperatures between 850 degrees C and 1010 degrees C. The diffusion coefficient, calculated from the increase in signal in the tail of the Ge profile, proved to be comparable with the value for Ge in bulk Si. An enhanced decrease in signal at the centre of the Ge profile indicated a faster diffusion within the SiGe layer. This was confirmed by analysis of the FWHM of the Ge profile, from which a diffusion coefficient was derived which was up to 20 times higher.

The authors describe the application of ballistic electron emission microscopy to measure electrical barriers with nm spatial resolution. They show that the interface between Au and chemically treated CdTe is non-uniform and that problems of reproducibility are associated with small patches at the interface where the barriers are low.

The authors report the breakdown of the nearly dissipationless quantum Hall effect into a set of distinct, quantised dissipative states in a wide, high-quality GaAs/AlGaAs sample. They found 35 dissipative stages on one plateau and nine on another plateau which have longitudinal voltage drops accurately quantised in units of h(cross) omega _c/e to within our +or-0.6% measurement uncertainty. This voltage quantisation implies that the energy dissipation per carrier is quantised in units of the Landau level spacing h(cross) omega _c.

The collector-emitter offset voltage V_CE,offset of heterojunction bipolar transistors (HBTS) stems mainly from the asymmetry between the emitter and collector junctions in such devices. Based on device physics, they have developed analytical models for predicting V_CE,offset of InAlAs/InGaAs and AlGaAs/GaAs single and double heterojunction bipolar transistors (SHBTS and DHBTS). Reasonable agreement is found when the present models are compared with data taken from InAlAs/InGaAs SHBTS and DHBTS grown by molecular beam epitaxy. Comparisons between the offset voltages of InAlAs/InGaAs and AlGaAs/GaAs HBTS are also included.

A method is proposed for the determination of the thickness of a thin semiconductor layer etched-off with either a dry or wet method, in order to determine the etching rate. Special features of the continuous electrochemical carrier concentration, C-V depth profiling and the vapour phase epitaxy are involved. Special multilayered structures with 'marking' layers are to be grown, used and measured twice in this method. 2 nm resolution is obtainable in the thickness determination of the removed layer for GaAs epitaxial structures.

Silicon dioxide films on silicon have been deposited by microwave plasma-enhanced MOCVD process using TEOS and oxygen. The structural characterisation of the films have been carried out by measurements of refractive index and etch rate and analysis of IR absorption and X-ray photoelectron spectra. MOS capacitors fabricated using deposited oxides have been used to characterise the electrical behaviour of the films. Results indicate a quality suitable for VLSI application.

A new photoelectrode system TiO₂-In₂O₃ for photo-assisted electrolysis of water is described. The mixed oxide electrodes have been synthesised by solid-state sintering. These sintered electrode discs exhibited enhanced photovoltage and photocurrent of 0.78 V, 14 mA cm^-2 whereas for TiO₂ they were 0.8 V, 5 mA cm^-2 respectively. X-ray diffraction and scanning electron microscopy investigations of the mixed oxide electrodes have been carried out to monitor their bulk and surface characteristics. EDAX analysis has been done to explore the chemical stoichiometry. These studies indicate microcrystalline-like surface characteristics and a cation doping ('In' up to approximately 2%) for the TiO₂-In₂O₃ electrodes. The Mott-Schottky plots (1/c²-V) in 1 M NaOH solution and temperature variation of electronic conductivity (In sigma -1/T) for TiO₂ as well as TiO₂-In₂O₃ have been evaluated. These investigations suggest decrease in band gap and slight shift in the flat-band potential of the mixed oxide electrode compared with TiO₂. The relative enhancement in hydrogen evolution under photo-assisted mode of electrolysis of TiO₂ and mixed oxide (TiO₂-In₂O₃) electrodes has been found to be from 1.2 to 1.8 ml h^-1. The probable causes of the enhancements in photoactivity and hydrogen evolution have been suggested as: (i) 'In' cation doping in TiO₂ lattice; (ii) increased surface area with In₂O₃ inclusion; (iii) decrease in the band gap and enhanced range of absorption of mixed oxide electrode; and (iv) catalytic activity of In₂O₃.