Table of contents

The purpose of this paper is to simulate capture events into quantum well structures of the kinds studied in luminescence experiments in order to understand the carrier dynamics, to extract local electron and hole capture times and to relate microscopic carrier processes to the luminescence signal. To do this, a quantum mechanical model for charge capture by a quantum well has been incorporated into a self-consistent Monte Carlo simulation of carrier transport in quantum well laser diodes. The capture model makes use of the established technique of modifying Fermi's golden rule to calculate a dimensionless capture probability instead of a capture rate. This model has been used to simulate time-resolved photoluminescence experiments on a variety of InGaAsP-based structures which have unstrained quantum wells. Local electron and hole capture times have been extracted from the transport data for simulations performed at 300 K, and these times oscillate as a function of well width. Wells 85-90 Å wide permit fast electron capture into a second electronic subband (0.56 ps capture time) and hole capture into a second light hole band (0.44 ps). This may be contrasted with a 75 Å well for which electron capture into the single conduction subband exceeds 1.7 ps, and the peak hole capture time is 1.1 ps.

Strained-balanced /InAlAs quantum well structures have been shown to generate high carrier density, high-mobility layers suitable for power field effect transistor (FET) applications. Doped channel devices allow higher carrier densities than modulation-doped structures but with reduced carrier velocities due to increased ionized impurity scattering. The amount of ionized impurity scattering may be reduced by the introduction of compositional grading in quantum wells which results in an increasing depth of the quantum well on the opposite side to the delta-doped plane. The grading is achieved by the use of a number of steps, where each step has a fixed value of x. An ensemble Monte Carlo simulation has been used to calculate the velocity-field characteristics of the conduction electrons confined within the quantum well in such a structure. Here the envelope wavefunctions for the confined electrons are calculated using a self-consistent Poisson-Schrödinger solver. Our velocity-field characteristics show good agreement with experiment for the three-step structure but the agreement is noticeably worse for lattice-matched and five-step structures. We have shown that a correlation between the low-field mobility and saturation velocity observed experimentally for GaAs/AlGaAs and quantum wells and theoretically for GaAs/AlGaAs quantum wells is also valid for the predicted results in these structures. A similar explanation to that provided for the GaAs/AlGaAs quantum wells is shown to hold here.

A series of calculations have been performed on group-III nitrides (GaN, AlN and InN) in both zinc-blende and wurtzite structures. Three different levels of computation have been performed in an integrated programme of study: first-principles total energy calculations, semi-empirical pseudopotential calculations and calculations. Bandstructures are obtained from each method in a consistent manner, and used to provide effective masses and parameters for planned work on the electronic structure of alloys and quantum well heterostructures.

Interface morphology of a strained SiGe/Si multilayer has been investigated by means of non-specular x-ray reflection. Depending on the azimuth of the scattering plane, two types of interface pattern have been observed, namely an asymmetrical random staircase and symmetrical islands superimposed on it. The distribution of the scattered intensity in reciprocal space has been simulated using a structure model of self-similar interface patterns and the distorted-wave Born approximation. The parameters of the interfaces following from the fit of the measured and simulated data were compared with the results of high-resolution x-ray diffractometry and transmission electron microscopy, and a good correspondence has been achieved.

The isochronal annealing and electric-field-enhanced emission properties of three defects (EHe203, EHe584 and EHe211), observed in low-energy He-ion bombarded n-Si, were studied using deep level transient spectroscopy. EHe203 ( eV) and EHe584 ( eV) were thermally stable up to C after which their removal was accompanied by the introduction of a secondary defect EHe211 ( eV). EHe211 was thermally stable at 650C. The emission rate of EHe203 was significant for electric fields above , and was enhanced by over 3 orders of magnitude for a corresponding three-fold increase in electric field. The emission rate of EHe211 was only weakly field dependent over the electric field range studied, while that of EHe584 remained constant for electric fields between and . EHe584 has been proposed to be an acceptor-type defect. It was found that square wells of radii 57 Å and 40 Å described the potentials induced by EHe203 and EHe211, respectively, reasonably well. Alternatively, Gaussian potential wells with Å and eV (EHe203) and Å and eV (EHe211) could be used to fit our experimental data.

Photoexcited carrier and space charge field subnanosecond dynamics has been investigated in bulk vanadium-doped and Cl- (or As-) co-doped CdTe crystals by using a degenerate four-wave mixing technique. Time-resolved measurements of picosecond grating decay at various illumination intensities revealed peculiarities of defect transformation with doping or under illumination. Novel features in carrier generation and dynamics were observed for the first time and allowed parameters of the defects to be extracted. A fast electron capture and its saturation with increasing intensity of illumination were observed in CdTe:V and attributed to a Cd divacancy, which is known as a deep double acceptor. Numerical modelling by using the two-deep-trap model allowed us to extract the concentration and recombination activity of Cd divacancies. An enhanced electron generation rate by factor of 3-4 was found in Cl-co-doped CdTe:V crystals in spite of a decreased density of donor vanadium states after the co-doping. We attribute this additional channel of electron generation to photoexcitation of the DX centre, which activation energy of 1-1.2 eV is close to a quantum energy of the YAG:Nd laser used ( eV).

We present absorption spectra as a function of temperature for CdTe quantum dots in doped glasses. We calculate the absorption spectra considering the transition energies from a spherical model. We consider also the inhomogeneous broadening due to the quantum-dot size distribution and the homogeneous broadening, due mainly to electron-phonon interactions. The samples grown with a two-step heat treatment present very thin size distributions, with a standard deviation of 5.8%, which is comparable to the best CdSe samples in the literature. Also, the results show that the homogeneous width increases as the quantum-dot size decreases, indicating an enhancement of the electron-phonon interaction for small dots.

We compare the electrical properties of an AlGaAs thin layer and a GaAs-AlAs short-period superlattice, both being doped uniformly with silicon. The carrier thermal activation is interpreted within the framework of the multidonor splitting of the deep silicon state, the so-called DX centre. We focus on the scattering mechanisms involved in the two structures. In addition to classical scattering processes, alloy scattering is introduced to account for the behaviour of the Hall mobility with temperature in the alloy. We utilize the persistent electron photoionization effect in investigating the increase of the Hall mobility with the free carrier concentration. We find that interface roughness scattering cannot account alone for the experimental behaviour in the superlattice and we suggest that an additional anisotropic scattering process is probably responsible for this discrepancy owing to the two-semiconductor structure of the pseudo-alloy.

Attenuated total reflection (ATR) spectra of hydrophobic and hydrophilic Si wafers, Si wafers with thermally grown layers and bonded Si wafers were investigated. It was found that the surface of the as-prepared hydrophobic wafer is terminated by hydrogen and water molecules while the IR spectra of the hydrophilic wafer demonstrate only the presence of water molecules at the surface. ATR spectra of Si wafers covered by a thermally grown layer exhibit a number of the strong absorption bands. The analysis of ATR spectra and the single-transmission spectra allows these modes to be assigned to combinational phonon bands in . The use of a double-bonded Si structure allows the influence of the outer surfaces to be excluded. The wafer bonding leads to the appearance of siloxane and hydroxyl groups at the buried interface whose absorption bands were observed in ATR spectra.

A novel method, based on an infrared absorption and neutron irradiation technique, has been developed for the determination of interstitial oxygen in heavily boron-doped silicon. The new procedure utilizes fast neutron irradiated silicon wafer specimens. On fast neutron irradiation, the free carriers of high concentration in silicon can be trapped by the irradiated defects and the resistivity increased. The resulting calibration curve for the measurement of interstitial oxygen in boron-doped silicon has been established on the basis of the annealing behaviour of irradiated boron-doped CZ silicon.

Boron-doped, float zone silicon has been hydrogenated at C and then quenched to room temperature. Infrared absorption measurements of the samples in their as-quenched state and following anneals at C reveal the vibrational mode from H-B pairs, together with the line that we have assigned to the vibrational mode of isolated hydrogen molecules . Annealing leads to irreversible increases in the concentrations of H-B pairs and decreases in the concentration of centres. The results imply that molecules diffuse to boron acceptors and that there is subsequent dissociation of these molecules with the formation of H-B pairs. The measurements confirm an earlier proposal that `hidden hydrogen' present in such samples is in the form of isolated molecules.

Dynamical electrical breakdown in the depleted region of diodes has been investigated in the nanosecond timescale. A persistent conductivity due to Si-related DX centre impact ionization in high electric fields was observed after the onset of the breakdown. From the present investigation it follows that DX centre breakdown begins at electric fields of ( and is related to impact ionization of DX centres by free hot carriers.

We have measured the temperature dependence of the spectral features near the direct bandgap of a film, grown by molecular beam epitaxy on a (001) GaAs substrate, in the temperature range between 15 and 300 K using contactless electroreflectance. The parameters that describe the temperature variations of the energy and broadening function of the band-edge exciton have been evaluated.

Gallium antimonide and related compounds are promising materials for fabricating monolithic vertical cavity semiconductor lasers operating at telecommunications wavelengths. With that aim active layers based on multiquantum wells have been evaluated by means of a separate-confinement laser diode structure grown on a GaSb substrate by molecular beam epitaxy. Owing to optimization of the growing process, for the well/barrier structure, laser emission at 1.4 m has been obtained at 80 K with a threshold current as low as 15 mA for a 640 m long and 15 m wide mesa stripe structure. At room temperature laser emission occurred at 1.55 m with a pulsed threshold current density of 4 kA according to the measured characteristic temperature of 50 K. In a first attempt such an active layer has been included in a 1.5 m microcavity involving antimonide Bragg mirrors.

Slicing of GaAs wafers from ingots causes a damage layer which must be completely removed by polishing. For this purpose a non-destructive control of the machining process is very desirable. Laser-induced surface acoustic waves are demonstrated to be a promising method to characterize the state of the surface. The dispersion of this wave mode has been used as a measure of the thickness of the damage layer. The investigations were performed on wafers which were stepwise polished to remove the damage layer. In this way the damage layer was found to have a thickness of about . This result is in good agreement with investigations performed with positron annihilation. The damage layer was found to have a considerably lower elastic modulus (by up to about 18%) than the bulk material of GaAs. With increasing polishing depth the modulus of the damage layer approaches that of the bulk material. The elastic modulus allows us to estimate the defect volume in the damage layer by applying a theory of heterogeneous materials. A defect volume fraction of 0.48% was estimated for the as-sawn state of the wafer. It decreases to 0.05% for a polishing depth of and goes to zero at a depth of .

The effect of self-adjustment of the cavity mode in vertical cavity surface-emitting lasers containing three-period InGaAs-GaAs vertically-coupled quantum dots has been observed. The effect originates from a strong modulation of the refractive index near the gain peak, caused by excitons in quantum dots. The possibility of single quantum dot lasing is demonstrated.

We have studied the effects of the two-dimensional electron gas (2DEG) and indium surface segregation in four pseudomorphic AlGaAs/InGaAs/GaAs high electron mobility transistor structures using room-temperature phototransmittance (PT) and photoluminescence (PL) measurements. The PT spectra from the InGaAs modulation-doped quantum well channel can be accounted for by a lineshape function which is the first-derivative of a step-like two-dimensional density of states and a Fermi level filling factor. A detailed lineshape fit makes it possible to evaluate the Fermi energy, and hence the concentration of 2DEG in addition to the energies of the intersubband transitions. The lowest-lying intersubband transition has been confirmed by a comparison of the PT and PL spectra. From the difference of intersubband transition energies, the surface segregation effects of indium atoms are demonstrated.

The polarized photoluminescence from localized interface states in a GaAs/AlGaAs single heterojunction have been studied as a function of magnetic field in Faraday geometry. A theoretical model of carrier trapping into these interface states has been developed. The effect of magnetic field on the spectra is discussed in terms of the field-dependent carrier trapping into the tail of localized states.

The electrical characteristics of Cr/- and Fe/liquid-encapsulated Czochralski (LEC)n-GaAs Schottky barrier diodes (SBDs) annealed at temperatures from 100 to C for 5 min and from 350 to C for 1 min have been investigated as a function of annealing temperature, with the use of current-voltage techniques. For Cr/n-GaAs SBDs, the Schottky barrier height and ideality factor n values range from 0.57 eV and 1.10 (for the as-deposited sample) to 0.80 eV and 1.10 (for C annealing). The ideality factor values remain approximately unchanged up to C and increased to 1.28 at C . For Fe/n-GaAs SBDs, the and n values range from 0.60 eV and 1.06 (for the as-deposited sample) to 0.78 eV and 1.08 (for C annealing). After C annealing, while the n values of the Fe/n-GaAs SBDs remain approximately unchanged between 1.10 and 1.12 up to C annealing, the value decreased with increasing temperature and became 0.75 at C (n = 1.11) and 0.73 at C (n = 1.20). It has been seen that the Cr/- and Fe/LECn-GaAs contacts are thermally stable under annealing up to C.

Gain characteristics of injection lasers based on self-organized quantum dots (QDs) were studied experimentally for two systems: InGaAs QDs in an AlGaAs matrix on a GaAs substrate and InAs QDs in an InGaAs matrix on an InP substrate. A ground-to-excited state transition was observed with increasing threshold gain. An empirical equation is proposed to fit the current density dependence of the QD gain. This fitting equation is shown to be valid for both the ground and excited state lasing in the systems under study in the 77-300 K temperature range. The effect of QD surface density on gain characteristics is calculated analytically.