Table of contents

Advances made in the growth and properties of CSi and CSiGe pseudomorphic strained layers are reviewed. The solubility of C in Si is small (3.5*10¹⁷ atoms/cm³ near the melting point). However, high-quality strained layers of the alloys with considerably larger C concentrations have been grown using MBE, CVD and solid-phase epitaxy methods. A careful control of the growth rate and temperature is necessary to avoid formation of silicon carbide. In high-quality layers, most of the C atoms occupy lattice positions of the Si or SiGe host crystals and a substitutional alloy is formed although the equilibrium volume of C atoms is only 30% of that of Si. The formation and stability of alloys of atoms with large differences in size is a topic of fundamental interest. Experimental and theoretical investigations have focused on the microscopic structure of substitutional Si_1-x-yGe_xC_y alloys. C compensates the compressive strain produced by Ge in the pseudomorphic layers grown on a Si substrate. From Raman studies of the microscopic strain in substitutional Si_1-x-yGe_xC_y alloys it has been concluded that Si-Si bonds experience a considerable local deformation even in strain-compensated alloys. The pair interaction of substitutional C atoms in an Si lattice and the possibility of forming ordered alloys have been studied theoretically. It has been found that the interaction of pairs of substitutional C atoms is attractive for special atomic configurations. Information available on electronic properties is rather meagre. Recent theoretical work shows that the bandgap of the alloy should decrease with C concentration. Experiments to confirm this have not yet been performed. Using strain-compensated alloys it is possible to grow symmetrically strained superlattices without the need of growing buffer layers. Si_1-x-yGe_xC_y strained layers are likely to be very useful for passive applications such as buffer layers. Considerably more work is required to determine their utility for active device applications.

We theoretically study the subband structure of single Si delta -doped GaAs inserted in a quantum well and subject to an electric field applied along the growth direction. We use an efficient self-consistent procedure to solve simultaneously the Schrodinger and Poisson equations for different values of electric field and temperature. We thus find the confining potential, the subband energies and their corresponding envelope functions, the subband occupations, and the oscillator strength of intersubband transitions. Opposite to what is usually the case when dealing with the quantum-confined Stark effect in ordinary quantum wells, we observe an abrupt drop of the energy levels whenever the external field reaches a certain value. This critical value of the field is seen to depend only slightly on temperature. The rapid change in the energy levels is accompanied by the appearance of a secondary well in the confining potential and a strong decrease of the oscillator strength between the two lowest subbands. These results open the possibility to design devices for use as optical filters controlled by an applied electric field.

The energy spectra of parabolic quantum dots with two interacting electrons in a magnetic field are obtained. The transitions in the ground-state energy of the system are shown. The effective-mass Hamiltonian is solved using the 1/N expansion method and good agreement is obtained when our results are tested against the exact diagonalization method.

The conductance of a GaAs/GaAlAs quantum dot with only one spin-degenerate Landau level populated exhibits Aharonov-Bohm type oscillations when plotted as a function of magnetic field. The conductance oscillations are due to resonant transmission through the Landau level edge states. A closely related oscillatory behaviour may be studied when the quantum dot is influenced by an external gate voltage, which changes the carrier density as well as the area of the quantum dot. The oscillation period in both magnetic field and gate voltage is analysed using a model where the fluxoid, i.e. the line integral of the canonical momentum rather than just the enclosed magnetic flux, is quantized.

Plasmons have been investigated in highly doped GaAs/Al_0.2Ga_0.8As multiple quantum well structures using the far-infrared techniques of dispersive Fourier transform spectroscopy and polarized oblique-incidence reflection spectroscopy. The plasma effective masses were obtained from comparison with van der Pauw measurements, and found to be considerably enhanced over their bulk values. This enhancement has been compared with that obtained using non-parabolicity theory. Good agreement is obtained, but the results suggest that the theory slightly underestimates the effect.

We report on electron and hole capture times in n-type GaAs/AlAs/Al_0.3Ga_0.7As double-barrier quantum well (DBQW) structures in an electric field applied perpendicular to the layers. We have measured the time-dependent photoluminescence (PL) originating from the GaAs wells and the Al_0.3Ga_0.7As barrier layers. The experimental capture times are obtained from least-squares fits of appropriate model functions to the observed PL transients. We have theoretically determined electron capture times for the investigated DBQWs, by calculating the electron wavefunctions and taking into account various scattering processes, including impurity scattering, optical phonon-assisted tunnelling and intervalley scattering. We find evidence that electron capture occurs by Gamma -X intervalley transfer via X-point subbands localized in the AlAs layers.

Results are presented for room-temperature photoreflectance measurements on four GaAs/Al_xGa_1-xAs single-quantum-well structures, with x=0.2 and x=0.3, and nominal GaAs quantum well widths of 50 AA and 100 AA. However, the photoreflectance spectra of all the samples display pronounced Franz-Keldysh oscillations in the neighbourhood of both the GaAs and Al_xGa_1-xAs band edge energies which overlap with, and partially obscure, the photoreflectance features from the quantum well. The spectra are fitted with a model incorporating an approximate lineshape form for the Franz-Keldysh oscillations together with third-derivative functional forms. This has enabled the single-quantum-well transition energies to be determined as well as the DC electric field responsible for the Franz-Keldysh oscillations. In order to identify these transitions, the results are compared with a theoretical model for the single-quantum-well structures, which incorporates parabolic effective masses, and good agreement is obtained.

Shubnikov-de Haas (SdH) and Van der Pauw Hall effect measurements on a lattice-mismatched In_0.65Ga_0.35As/In_0.52Al_0.48As one-side-modulation-doped single quantum well grown by metalorganic chemical vapour deposition have been carried out to investigate the electrical properties of an electron gas and to determine the subband carrier densities, the subband energies and the wavefunctions in a strained In_0.65Ga_0.35As quantum well. Transmission electron microscopy measurements show that the In_0.65Ga_0.35As/In_0.52Al_0.48As heterointerfaces have no misfit dislocations. The SdH measurements at 1.5 K and fast Fourier transformation of the data show that a two-dimensional electron gas occupied three subbands in the well. The electronic subband energies and the wavefunctions in the quantum well have been calculated by a self-consistent method using the transfer matrix method based on the envelope-function approximation. The self-consistent calculation was performed taking into account exchange-correlation effects together with strain effects.

First, the admittance associated with the depletion region of Schottky diodes produced on phosphorus-doped a-Si:H films was analysed and characterized through capacitance (conductance) measurements as a function of DC bias, frequency of AC bias, and temperature. Then, the gap states within the depletion region were studied by double-beam photocapacitance spectroscopy, which is a sensitive and rapid survey technique. Two distributions of localized gap states were measured with average positions located at 0.75 eV and 0.95 eV respectively from the band edge.

The diffusion from CoSi₂ layers, implanted with As and B ions, into the underlying Si substrate has been studied by a high-resolution carrier delineation technique. In the early stages of diffusion the junction shape follows the silicide/silicon interface for B, while it is deeper near the CoSi₂ grain boundaries for As. The different behaviour is related to the different diffusion mechanisms of As and B in the silicide layer. Using a two-step anneal or a thin silicide diffusion source a laterally uniform junction has also been obtained for As-implanted CoSi₂. The diffusion coefficients of arsenic and boron in silicon have been measured by this technique. The measured diffusivity values for boron and arsenic are very close to the data reported in the literature.

The effect of hydrogenation on a Pd/p-GaAs diode in hydrogen gaseous ambient has been studied by I-V and C-V measurements. Hydrogenation has been found to improve the ideality factor of the diode. Reverse C-V characteristics show that the number of shallow and deep acceptors is reduced on hydrogenation. The forward C-V characteristics has shown the presence of interfacial deep donors at E_c-1.05 eV and E_c-0.88 eV, the density of which decreased on hydrogenation. The observed results are discussed in terms of the interaction of the diffusing hydrogen with the interface and bulk states of the Pd/GaAs structure.

The electric field dependence of the emission rates of two radiation-induced hole traps in epitaxially grown p-GaAs, H beta ₄ (E_v+0.2 eV) and H beta ₅ (E_v+0.3 eV), was studied using deep-level transient spectroscopy (DLTS). Both defects exhibit a significant emission enhancement at electric fields above 5*10⁴ V cm^-1. With the enhancement being much more pronounced for H beta 5. Neither of the two defects shows an emission enhancement that is characteristic for a Coulombic well. When modelling the experimental results according to the Poole-Frenkel distortion of a square well, the H beta 4 can be adequately presented by a well with radius 10-20 AA, which is typical of the potential range of a point defect. Using the same formalism, however, the H beta 5 has to be represented by extraordinary wide wells with radii of 35 AA and 162 AA at low- and high-field conditions, respectively. We conclude that the shape of the H beta 5 potential well changes with increasing electric field.

The temperature behaviour of GaAs/GaAlAs DQW-GRINSCH high-power laser diodes is calculated by means of a numerical model. The model includes a microscopic description of gain and spontaneous radiative recombination, a phenomenological description of interface and Auger recombination, and includes a pumping-current-dependent leakage. Based on the model, the temperature dependences of the macroscopic parameters of threshold current, external differential efficiency and wavelength are calculated. The resulting numerical values for these parameters are in excellent agreement with our experiments. Spontaneous radiative recombination is shown to be the dominant loss mechanism.

Although processes in the polysilicon layers are very important in CMOS-compatible bipolar technology, there remains a particular need for improved modelling of diffusion in this field. A simulation model for dopant codiffusion in polycrystalline silicon is developed, taking into account the grain itself, as well as the grain boundary, and the exchange between these two regions.

Magnetically confined plasma reactive ion etching has been used, with SiCl₄ with a small amount of O₂ as process gases, to etch quantum wires of GaAs multiple quantum wells with AlAs barriers. The impact of adding oxygen is discussed. Vertical and smooth sidewalls were obtained for these structures with a high aspect ratio. Photoluminescence of etched quantum wires shows 1D subbands.

The electrical properties and thermal stability of WN_x/GaAs contacts where WN_x is formed by reactive sputtering of W are investigated. The results show that contacts have good electrical properties and thermal stability provided that the partial pressure of nitrogen gas during sputtering does not exceed certain limits. Excess nitrogen in WN_x film degrades contact properties, The formation of W and W₂N phases, trapping of excess nitrogen atoms at the interface and the possible formation of GaAs_xN_1-x, compound at the interface, with x being a function of annealing temperature, were used to explain the observation.

Antidot superlattices with a period of a=500 nm and different etch depths were fabricated in two-dimensional hole gases confined in surface-side as well as in substrate-side modulation-doped Ge/SiGe heterostructures by holographic lithography and reactive ion etching and were then studied by magnetotransport at T=4.2 K. Even though the elastic mean free paths are smaller than or comparable to the superlattice period, typical features of locally ballistic transport around groups of antidots, such as commensurability oscillations and additional, non-quantized Hall plateaus, are observed in both types of samples at sufficient etch depth. These geometry-related effects are found to be rather robust against inhomogeneities and defects brought about by imperfect fabrication.

Raman scattering and photoluminescence (PL) under near-UV excitations were used to assess the 6H-SiC films epitaxied on 6H-SiC by low pressure vertical chemical vapour deposition (LPV-CVD). Raman linewidths and relative intensities with respect to the PL emissions were used to characterize the quality of the 6H-SiC films. Ti-related PL and outgoing resonance Raman scattering were studied. Samples grown by LPV-CVD with different growth parameters were compared on the basis of their Raman-PL spectra. This study showed that a combination of Raman and PL measurements can be used as a convenient method to assess the 6H-SiC/6H-SiC homoepitaxial structures.