Table of contents

Czochralski-grown (cz) silicon samples have been implanted at room temperature with low doses (10⁹-10¹¹ cm^-2) of ¹¹B and ²⁸Si using energies between 0.38 and 3.0 MeV. Deep-level transient spectroscopy has been applied for sample analysis, and four levels approximately 0.18, approximately 0.23, approximately 0.35 and approximately 0.43 eV below the conduction band edge (E_c) are resolved. The concentrations of these levels have been determined as a function of ion dose and sample depth. The total concentration of electrically active defects amounts to less than 10% of the vacancy concentration predicted by Monte Carlo simulations. The depth profiles of the levels at E_c-0.18, E_c-0.23 and E_c-0.43 eV are mainly confined to the damage peak region, and evidence is obtained for substantial surface-enhanced annihilation of migrating monovacancies. However, the depth distribution of the E_c-0.35 eV level exhibits a pronounced leading surface tail and is broader than expected from energy deposition calculations. The identity of this latter level is not well established but the authors' argue that the generation process involves diffusion of interstitial carbon. Finally, in contrast to that for cz samples irradiated with MeV electrons the strengths of the E_c-0.23 and E_c-0.43 eV levels deviate from a one-to-one proportionality. Similar deviations have previously been observed in ion-implanted float-zone samples and are attributed to strain accommodated by the lattice in the damage peak region.

The diffusion of aluminium in single-crystal silicon has been studied in the temperature range 1000-1290 degrees C. A low implantation dose (1*10¹⁴ Al cm^-2) was used to avoid dopant precipitation and a high energy (6.0 MeV) to reduce the influence, if any, of the surface. The experimental profiles, measured by spreading resistance, have been fitted taking into account the escape of Al from the Si surface into the ambient atmosphere. A pre-exponential value of 8.88 cm² s^-1 and an activation energy of 3.44 eV have been found in this experiment. When aluminium is implanted through a capping layer, a reduction of the residual amount has been observed at medium energy (300 keV), while no difference has been measured at high energy (6 MeV).

Si(111) surfaces misoriented by 4 degrees towards (112) have been studied by scanning tunnelling microscopy. Topographs of surfaces, produced by annealing, followed by rapid quenching, reveal a temperature-dependent transition from a structure consisting of wide irregular terraces separated by small irregular clusters of double steps to a single highly ordered phase of narrow, evenly spaced terraces separated by single monatomic steps. The quenched-in phases are stable at room temperature.

In_xGa_1-xP(x=0.436-0.546)/GaAs heterostructures grown on (001) GaAs substrates by molecular beam epitaxy have been studied using transmission electron microscopy (TEM) and cathodoluminescence (CL) techniques. TEM study of plan-view and cross-sectional samples of the epitaxial layers revealed no evidence of CuPt-type atomic ordering in the InGaP layers, but two types of composition modulation in the InGaP layers were observed. Different strain relaxation mechanisms for the layers in tension and compression were also observed. CL analysis was carried out for both strained and relaxed InGaP layers. The dependence of CL emission energy and of peak width on the misfit strain is discussed and correlated with the TEM study. Biaxial compressive strain increases the CL emission energy and reduces the emission peak width. Biaxial tensile strain relaxation near to the microcracks was also studied by CL.

InGaAs layers of varying nominal epilayer thickness grown at different temperatures on GaAs substrates by molecular beam epitaxy are analysed by atomic force microscopy (AFM). This relatively new technique allows high-resolution imaging and analysis of surface morphology and roughness of semiconducting materials. The surface roughness and morphology of InGaAs grown at substrate temperatures ranging from 450 degrees C to 530 degrees C are analysed as the nominal epilayer thickness is increased from 100 AA to 800 AA. Over an area of 100 mu m², an abrupt increase in the surface roughness indicates an acceleration of surface roughening mechanisms related to the process of strain relaxation. At higher nominal InGaAs thicknesses, surface corrugations were observed. The results indicate that thicker pseudomorphic InGaAs films on GaAs substrates can be grown at lower temperatures.

A density-of-states approach is used to describe the influence of external electric and magnetic fields applied perpendicular to the layers of a semiconductor superlattice. Effects of Wannier-Stark localization and interacting Stark ladders are investigated for null magnetic field. Special attention is paid to extremely shallow multiple quantum well structures. Furthermore, it is shown that the density of states in high magnetic and electric fields reveals the interplay between electric and magnetic confinement effects.

A new fast transient method is developed to study free and bound exciton formation and decay dynamics in semiconductors. The method uses two time-scales, slow and fast. In the slow time-scale the excitonic system, after its creation, is allowed to evolve freely in time, while in the fast one the detection of the state of the system due to tunnel dissociation of excitons is performed in a fast swept electric field. The principle of the proposed method is demonstrated by measuring free and bound exciton lifetimes in the n region of silicon p⁺nn⁺ structures. diffusion to p⁺n and nn⁺ junctions is found to limit the free-exciton lifetime at temperatures lower than 14 K, whereas at higher temperatures exciton thermal dissociation is found to dominate.

The author points out the advantages of the full quantum-mechanical treatment of the asymmetric coupled double well (ACDQW) in an electric field. The author believes that a clearer physical picture of resonance tunnelling emerges from this approach than from the approximate one which treats the ACDQW as a composite of two isolated wells placed in contact. In an electric field energy levels of the first two bound states first approach each other but do not cross, and later, with increasing field, their separation again increases. Resonance effects occur at the value of the field at which the levels are closest to each other. At the same time the occupation probabilities of the two wells are equal. Furthermore, it is shown that the double well with a thin barrier behaves as coupled even at fields far from resonance. This claim is substantiated by an approximate calculation of the absorption coefficient in the wide well and a comparison of the calculated values with experiment (Leo et al.).

A method which permits simultaneous determination of the values of the free energy of ionization, the capture cross section and the concentration of a trap in a semiconductor is presented. The method depends on an accurate fit of a set of analytical expressions to the experimental data obtained by studying the variation of transient capacitance amplitudes with the duration of a filling pulse bias in a junction experiment. The analytical expressions used are based on the rigorous theoretical model of Pons. Various criteria which must be observed to ensure reliable results are carefully worked out in detail. The effectiveness of the present method is demonstrated on two copper-induced electron traps in gallium arsenide phosphide and a laser radiation-induced electron trap in silicon.

Deep-level transient spectroscopy (DLTS) has been employed to study grown-in point defects in vapour phase epitaxial layers of GaAs_0.6P_0.4:Te. It was found that for the Ga-rich vapour phase three electron traps T1 (0.20 eV), T2 (0.18 eV) and T3 (0.38 eV) are formed. The traps T1 and T2 were found to be donor-related and are tentatively identified as Te_AsGa_AsTe_As and O_iTe_As complexes, respectively. The trap T3 was found to be dependent on the dislocation density and it is possibly associated with either an As or P vacancy generated by the increase in misfit dislocations. A detrimental effect of these defects on the radiative recombination efficiency is shown. Under an excess of the group V elements in the vapour phase, only one electron trap T4 (0.18 eV) was observed.

The authors' consider the effect of carrier resonance scattering in slightly doped compensated semiconductors on the carrier capture by neutral shallow impurities, resulting in the formation of H^--like centres. The energy dependence of the capture coefficient alpha ( epsilon ) exhibits a weak maximum at E_i, the carrier affinity to the H^--like centre. The photoconductivity and photo Hall effect are studied for Si:B samples with a boron concentration N_A<10¹⁶ cm^-3 and a degree of compensation K<10^-3 at liquid helium temperature in 'heating' electric fields. The obtained energy dependence of the hole capture coefficient for neutral boron atoms is in agreement with the authors' calculations.

The charge pumping technique is used to determine the increase of interface state density due to hot carrier stressing in n-channel MOSFETs. It is seen that the interface trap created by hot hole injection is different from that due to hot electron injection. Hot hole induced interface traps are mostly acceptor type with energy distribution in the upper half of the silicon bandgap. In the case of hot electron injection, the interface traps have an energy distribution throughout the silicon bandgap, similar to that of the unstressed device. For the same increase of interface state density, the shift of threshold voltage is lower for hot hole injection that for hot electron injection. However, the maximum transconductance degradation for hot hole injection is higher than that for hot electron injection. The variation of charge pumping current with gate offset voltage has been used to identify the type of interface traps created by hot carrier injection.

A method has been formulated to explain field-enhanced non-equilibrium transients of pulsed MOS capacitors. In the author's theory the Coulombic emission of trap centres is assumed to be field dependent. The one-dimensional Poole-Frenkel model is used to describe the Coulombic emission probability of trap centres, which is dependent on the applied electric field. On the other hand, unlike previous theories, the contribution of carrier generation from non-Coulombic emission of trap centres is also considered. A different equation is founded to describe the change of the width of the space charge region with time, and then the MOS capacitance and gate current are related to the width of the space charge region. In such a manner the non-equilibrium capacitance-time and current-time transients are given. The comparison between theory and experiment has been made and shows that the author's theory can well explain the experimental results.

The authors' present the results of a study into the potential benefits of including heterostructures within single-drift impact avalanche transit-time (IMPATT) diodes. One particular heterostructure device, which exploits the change in saturated drift velocity in going from one material to another (the double-velocity avalanche transit-time (DOVATT) diode), is examined in detail using a series of GaAs/Al_xGa_1-xAs devices grown by molecular beam epitaxy. Contrary to claims in the literature, they conclude that there is little prospect for realizing high-performance DOVATT diodes using this materials system. The authors' note, however, that a novel In_xGa_1-xAs heterojunction technique to reduce series resistance appears promising and is deserving of further study.

A time-dependent theory of a double quantum well resonant interband tunnel transistor is presented. The theory is based on a system of three linear differential equations for time-dependent resonant currents and charges stored in the wells. First the stationary resonant current-voltage curve is investigated. then the general solution for the non-stationary tunnelling current is obtained by the perturbation method. The current is found to be an ultra-sensitive function of the time-dependent small external voltage which is applied between the emitter and the second quantum well for detuning the resonances of quantum levels. Two nonstationary cases are investigated. For a step-function impulse of external voltage the characteristics of the response current are found. For a harmonic external voltage the frequency dependence of the response current is found. Matched and unmatched systems of barriers are investigated and found to exhibit significantly different behaviours.

The authors' have performed self-consistent electronic structure calculations for a two-dimensional electron gas confined at a GaAs/Al_xGa_1-xAs interface in the presence of a parallel magnetic field. Their study has focused on the evolution of the subband states with increasing field strength and the depopulation of higher subbands as a result of diamagnetic energy shifts. Significant differences have been found between the self-consistent results and those obtained by the commonly used perturbative method. These differences can be attributed to two limitations of the perturbative formulation: (i) neglect of the magnetic field dependence of the self-consistent potentials and (ii) neglect of the modification of the density of states from the zero-field behaviour. A comparison of both theoretical formulations with data derived from magnetic depopulation experiments is also made.

A theoretical study of the electronic structure of periodically delta -doped GaAs is presented. When the doping period is reduced, minibands are formed, and the photoluminescence spectrum presents a broad emission band at energies above the GaAs band edge. The experimentally obtained photoluminescence lineshape is well reproduced by the theory. The calculated cut-off energy in the photoluminescence spectrum is in excellent agreement with the experimental value. The Fermi surface is studied as a function of the doping period. The gradual change in the Fermi surface which occurs when the spacing between the dopant sheets decreases, provides a clear-cut description of the crossover from two- to three-dimensional electronic structure in delta -doped GaAs. The response of the system to a magnetic field, applied in the plane of the layers, is considered, and the condition of magnetic breakdown is studied.

The results of numerical modelling of Schottky barrier DLTS spectra in the case of non-exponential relaxation of the barrier capacitance are presented. The authors' consider the following factors capable of inducing non-exponential relaxation: comparable densities of deep and shallow electronic traps, a substantial electric field dependence of the carrier capture cross sections of deep electronic traps and a non-uniform spatial distribution of electronic traps in a semiconductor. The effect of these on a DLTS spectral band may manifest itself in the band broadening mainly due to a shift of the high-temperature 'shoulder' to higher temperatures, in the appearance of dependences of the shape and position of the DLTS spectral band on the bias voltage applied to the barrier structure, and/or in a non-monotonic character of the above dependences. Since this behaviour of the DLTS band does not follow from the conventional DLTS technique models, the algorithms commonly used to determine the deep trap recombination parameters face some complications. A number of suggestions on how to overcome the complications are made. The complex behaviour of the DLTS band predicted by calculations for the metal-n-GaP Schottky barriers is illustrated by experimental data.

Microwave plasma discharges of CF₄, SF₆ and CHF₃ at high pressures (0.5-1.5 Torr) have been used for rapid etching of silicon, silicon dioxide and silicon nitride suitable for single-wafer etching applications. Etch rates and selectivities have been studied as a function of pressure, gas flow and the amount of O₂ as the additive gas. Extremely high etch rates of 2.0-3.6 mu m min^-1 and 5.0 mu m min^-1 for silicon, obtained respectively in an CF₄ and a CF₄+20% O₂ plasma can be useful for deep-trench etching applications. Formation of a sidewall passivation layer in silicon at high CF₄ pressure ( approximately=1.2 Torr) gives rise to good etching anisotropy. The variation of etch selectivity between SiO₂ and Si₃N₄ has been studied as a function of gas flow rates in SF₆ plasma. The formation of a thin fluorocarbon polymer film on the etching surface in a CHF₃ plasma, which has resulted in selective etching of SiO₂ over Si₃N₄, has been observed by X-ray photoelectron spectroscopy.

Highly doped n-type (n=2*10¹⁹ cm^-3) and p-type (p=7*10¹⁹ cm^-3) layers of GaAs were isolated by selective implantation of F⁺ or H⁺ ions, followed by annealing at 500-550 degrees C for 60 s. The resistance of the implanted regions was measured as a function of time for long-term aging at 200 degrees C. For both initial conductivity types and both types of implant species this resistance was stable for the duration of the aging ( approximately 1200 h). The measurement temperature dependence of the resistance of the implanted regions was characterized by a single activation energy in all cases, with values of 0.74 and 0.82 eV respectively for F⁺ isolation of p⁺ or n⁺ GaAs, and 0.44 and 0.53 eV respectively for H⁺ isolation of p⁺ or n⁺ GaAs.

Admittance spectroscopic assessment of Au/p-InP structures has revealed two new hole defects HD1 and HD2, following 1 MeV electron radiation of fluence 10¹⁶ cm^-2. These defects were found to be different from the previously reported hole defects H3 and H4, and were responsible for the removal of most carriers, hence the increase of the series resistance and the degradation of solar cell efficiency. The defect HD1 showed a recovery threshold annealing temperature of approximately 70 degrees C and was completed by 100 degrees C after 10 minutes annealing. This annealing behaviour corresponded to the recovery of the removed carriers.

Infrared absorption measurements of heavily carbon-doped AlAs have revealed a localized vibrational model at 630 cm^-1 which appears as a Fano profile. Raman scattering measurements show a corresponding line at 634 cm^-1: possible reasons for the difference in frequency are discussed. The line is assigned to C_As acceptors and a comparison is made with a two-parameter Keating cluster model. Additional weaker lines at 615 cm^-1, 633 cm^-1 and 645 cm^-1 have been observed in absorption, and possible interpretations have been considered.

The authors' report the highest 4 K hole mobility (9300 cm² V^-1 s^-1) observed so far in a Si/SiGe/Si heterostructure. It is tentatively concluded that this improvement over previous results is due to a reduction in interface charge and that the mobility is now limited by interface roughness scattering.

By applying a shallow etching technique, the electrons in a remote doped GaAs/AlGaAs quantum well and the photogenerated holes are separated into adjacent lateral regions. The dominant photoluminescence of these carriers is shown to be indirect in real space. In addition, a spatially direct luminescence is observed. Its origin depends on the type of structuring and is discussed for wires, dots and antidots in comparison with unstructured samples.