Table of contents

The strain and crystalline quality of heteroepitaxial In_xGa_1-xAs on GaAs substrates, nominally with x=0.12, are measured as a function of layer thickness and substrate orientation by RBS channelling with 2.5 MeV alpha particles. The layers are grown by low pressure MOVPE on substrates with surfaces oriented (i) exactly on a (100) plane and (ii) misoriented by 2 degrees from the (100) plane towards the (110) axis. Layers of thickness around the predicted critical value (particularly those on misoriented substrates) exhibit remarkably low chi _mix values, indicating a high crystalline quality, The strain in each layer, measured by finding the angular displacement between the (112) axial channels of the epilayer and the substrate, shows no sign of relaxation until the thickness is many times the critical thickness. For layers which show some strain relaxation, the authors evaluate the concentration of misfit dislocations at the substrate/layer interface from the channelled spectra. These measurements confirm that channelling, although it provides an unambiguous measurement of layer strain, is not as sensitive as other techniques for the detection of the onset of dislocation formation.

The diffusion and codiffusion of boron and arsenic in monocrystalline silicon during rapid thermal annealing (RTA) have been investigated. Samples were characterized by secondary-ion mass spectroscopy (SIMS). Experimental results give depth profiles and are used to adjust the parameters of the process simulation programs.

Recharging of defects in the space-charge region of a diode generates in general a current transient which often is used for the characterization of the defects involved. The current of such transients is distributed between a measurable external current and an internal displacement current. The relative magnitude of the currents is described by the so-called D factor. The nature of the D factor is discussed and equations for the theoretical deviation are given. Furthermore, measurement techniques for the experimental determination of the D factor are described. Excellent agreement is found between theory and experiments. Finally, a new measurement technique which avoids displacement currents is presented. The use of this technique may imply further applications of current transient methods.

The conductance G of narrow channels is calculated for the case that an impurity is present in the channels. The channels are modelled as the electron waveguides, and the impurity is assumed to be a barrier. The effect of impurities on G distorts the quantization of conductance in a quasi-one-dimensional constriction. The resonant tunnelling associated with bound states in a crossbar structure is reduced by the presence of the impurity. The sharp peaks in G associated with resonant tunnelling are still apparent for a crossbar structure containing an impurity. Weak impurity potentials do not have a pronounced effect on the conductance. For strong impurity potentials, however, both the magnitude and position of the peaks in G are affected by the presence of the impurity. The position of the impurity plays a role in reducing the resonant tunnelling in the crossbar constriction. In addition to the peaks in G associated with resonant tunnelling, there are deep anti-resonances that are insensitive to the presence of the impurity. The current density may be strongly affected by the impurity. A complex flow pattern containing vortices appears when the impurity is inserted.

Impurity avalanche breakdown in n-type GaAs layers prepared by the LPE, VPE and MBE methods has been studied in detail using voltage-controlled current-voltage (I-V) characteristics and luminescence measurements together with computer simulations. Scholl's model of the first-order non-equilibrium phase transition in a two-level system was found to be adequate to describe the main experimental results. The threshold voltage for breakdown depends on the initial free-electron concentration. The hysteresis width diminishes with decreasing binding energy of electrons in the excited states of donors. The changes in luminescence spectra observed during impurity breakdown reflect changes in the charge state of shallow impurities. The band corresponding to D-A transitions disappears together with the decrease of the exciton radiative recombination rate. In the post-breakdown part of the I-V characteristics another region of nonlinear behaviour with hysteresis and sharp increase of instabilities was observed. Its origin has not yet been determined unambiguously.

The ionization dynamics of shallow donors in stepped electric fields is investigated in the nanosecond time-scale by monitoring the current transients in InP and GaAs epitaxial layers. Electron multiplication in the conduction band is analysed within the rate equation formalism, taking into account the ground state as well as the first excited state of the donor atom. Various ionization and recombination mechanisms, which can influence the dynamics of impact ionization, are considered. The dependence of the impact ionization and excitation coefficients on electric field strength has been deduced from a comparison of the experimental and theoretical current transients.

Careful measurements have been made of the temperature coefficient of the barrier height ( alpha ) and the Richardson constant (A**) for Schottky diodes consisting of Au and Cu contacts on GaAs. The metals were deposited at a pressure of approximately 10^-10 Torr so that there is no possibility of an oxide film between metal and semiconductor. The values obtained for alpha were (-2.3+or-0.3)*10^-4 eV K^-1 and (-3.2+or-0.3)*10^-4 eV K^-1 for Au and Cu, respectively, the corresponding values of A** being 1.5*10⁴ A m^-2 K^-2 and 16*10⁴ A m^-2 K^-2. The values for Al/GaAs diodes obtained previously were (-3.8+or-0.3)*10^-4 eV K^-1 and 0.41*10⁴ A m^-2 K^-2. The variation of alpha between metals implies that the interface states which determine the barrier height depend on the metal, and the substantial difference between the values of A** suggest that the band structure of the metal plays a role in determining the reflection coefficient for electron at the interface.

A generalized method of DC and high-frequency analysis for microwave transit time diodes in mixed tunnelling and avalanche mode, which can be applied to any type of diode structure is reported. Taking a purely field-dependent tunnel generation rate for electrons, the same is computed for holes from a simulated energy band diagram within the depletion layer of the diode. The method has been applied to a variety of Si, GaAs and InP diode structures. The results show a substantial degradation of IMPATT properties due to phase distortion caused by the tunnelling current.

Indium-doped Cd_xHg_1-xTe grown by low-temperature metal-organic chemical vapour deposition (MOCVD) is highly resistive. Indium is also known to react with HgTe and with Cd_xHg_1-xTe to give InTe. The equilibrium distribution of indium tellurides resulting from indium doping of Cd_xHg_1-xTe has been calculated for a range of temperatures from 373 to 633 K and shows that the major component is the mixed-valence InTe, with a few per cent of In₂Te₃. These results suggest that self-compensation in In-doped Cd_x-Hg_1-xTe is due to the formation of InTe, but that this self-compensation will decline with increasing temperature and increasing Hg content.

Studies have been made of the room temperature deposition of hydrogenated amorphous carbon and hydrogenated amorphous silicon-carbon in a magnetron discharge system at low ion energy (E_i approximately 30 eV), high density (n_e approximately 10¹¹ cm^-3) and low pressure ( approximately 5 mTorr) with CH₄/Ar and CH₄/SiH₄/Ar gas mixtures respectively. A gas flow rate modulation technique was introduced to investigate the effect of low energy Ar ion bombardment immediately after the deposition of ultra-thin films. The plasma created a highly reactive environment and the deposition process had high gas conversion efficiency and good uniformity. Both carbon and silicon-carbon films were smooth with good adhesion. Absorption from the C-CH₃ mode and some from the C=CH₂ mode were found in the IR spectra of the carbon film. The IR spectra of the silicon-carbon film were mainly contributed by the SiH_x, Si-CH₃ and CH_n related modes, and showed higher carbon incorporation than those obtained from the conventional discharge system. The strengths of the SiC and CH_n modes and their ratio could be increased by increasing the methane to silane partial pressure ratio or the effective RF power. The index of refraction decreased with the increasing carbon incorporation. The film with alternating deposition of ultra-thin (20-75 AA) a-Si_xC_1-x film and Ar plasma bombardment cycles showed that the post-deposition, low energy ion bombardment does not change the film thickness but is sufficient to crack the hydrocarbon bonds and promote the formation of Si-CH_n bonds on the top few tens of monolayers of the ultra-thin film.

Chemical deposition of good quality ZnS thin films from aqueous solutions containing zinc sulphate, NH₃/NH₄Cl buffer (pH 10), triethanolamine and thioacetamide is described. It is shown that the presence of these films on glass substrates offers various advantages in the case of chemically deposited metal sulphide thin films: (i) improved adhesion of Bi₂S₃ thin films, which is vital for growing films of thickness >0.2 mu m for semiconductor applications and in metal sulphide thin film (MSTF) photography based on photoaccelerated chemical deposition; (ii) improved adhesion of Cu_xS films, offering the choice of different shades of colours for the same integrated transmittance in the visible region of approximately 20% in solar control coatings and the possibility of multiple-dip deposition of Cu_xS films to enhance the film thickness, enabling the realization of solar absorptance of approximately 90%; and (iii) reduced reflectance in the visible light which can reduce the glare from PbS solar control coatings and improve the solar absorptance in thicker PbS coatings and offer better contrast in MSTF photography.

A complex model for low-temperature PEMOCVD of A₂B₆ semiconductor layers is considered as follows: (i) metal-organic compound (MOC) molecules together with the carrier gas flux arrive at the reaction zone of a horizontal reactor where a gas discharge is maintained by means of an external inductor; (ii) molecular dissociation and convective diffusion of atoms of material to be deposited occur in the reaction zone; (iii) the atoms are captured directly at chemical absorption sites and included in the surface migration and nucleation processes. Thus, the modelling problem consists of three parts which are connected with quantitative descriptions of the discharge, the convective diffusion and the surface processes respectively. Each of these parts has been treated both analytically and numerically and the dependence has been obtained of layer properties on initial technological parameters such as total gas pressure, discharge power gas flux velocity and ratio of MOC partial pressures. The dependence is compared with experimental results.

(111) silicon slices have been machined by a highly stiff single-point diamond turning machine to a finish of R_a approximately 1 nm. The associated subsurface damage was found by Rutherford backscattering of 1.5 MeV He⁺ ions to be approximately 170 nm. It is suggested that this residual damage consists mainly of dislocations.

n-type modulation-doped Si/SiGe heterostructures were grown on different types of partly relaxed SiGe buffer layers, which are required in this material system to obtain a large enough conduction band offset. The samples were characterized by secondary-ion mass spectroscopy, X-ray rocking analysis, transmission electron microscopy, Rutherford backscattering and temperature-dependent Hall measurements. The highest Hall mobilities of 173000 cm² V^-1 s^-1 at 1.5 K were found in a sample grown on a thick, linearly graded SiGe buffer layer deposited at 750 degrees C. Such layer sequences reach room-temperature mobilities around 1800 cm² V^-1 s^-1. Mainly at lower temperatures, a strong reduction of the Hall mobility is found if either a conventional buffer layer without Ge grading is used, or if the modulation-doped SiGe barrier of the active layers begins to relax with respect to the strained Si channel.

The authors investigate resonant tunnelling in p-type silicon-doped GaAs/AlAs double-barrier quantum well structures grown by molecular beam epitaxy on the (311)A GaAs surface. Their current-voltage characteristics compare favourably with structures grown on the conventional (100) orientation using beryllium as the acceptor.

Resonant tunnelling experiments under hydrostatic pressures up to 15 kbar in a double-barrier structure with a two-dimensional emitter pre-well are reported. At room temperature the structure shows two transitions in its tunnelling characteristics as a function of pressure. The authors associate these transitions with the enhancement of the Gamma -X tunnelling in the collector and emitter barriers. At lower temperatures these features are less pronounced. In addition they estimate the pressure coefficient for the decrease in the energy of the X minimum in relation to the Gamma minimum in Al_0.6Ga_0.4As.

A 10% SnO₂/ZnO:p-Si diode (3 mm diameter) has been prepared using an electron beam vacuum evaporation technique. Dark I-V characteristics reveal the values of 0.72 eV, 130 Omega and 1.2 for the barrier height, series resistance and the diode quality factor respectively.

The optical constants n and k for single crystals of CdSnP₂ and CdSiAs₂ ternary compounds have been measured using laser ellipsometric techniques in the range of photon energies from 1.9 to 2.6 eV at T=300 K. It has been found that the refractive index dispersion dn/d(h(cross) omega ) changes its sign in the spectral range from positive to negative for CdSnP₂ and has a singularity for CdSiAs₂, but at the same time the extinctive index dispersion dk/d(h(cross) omega ) remains positive. A figure for effective birefringence Delta n₍₁₁₂₎=0.022 at h(cross) omega =1.96 eV has been determined for the natural ₍₁₁₂₎ facet of CdSnP₂ single crystal.

A process for reactive ion etching of GaAs and InP is described using a mixture of ClCH₃/X, where X=H₂, He, O₂, Ne or Ar. The ClCH₃/H₂ process is shown to etch GaAs and InP with anisotropic etch rates of up to 80 and 300 nm min^-1, respectively. For the ClCH₃/O₂ process an etch rate of 11 nm min^-1 is obtained for GaAs with no appreciable etching of InP. This simple combination of gases leads to the realization of selective InP over GaAs or GaAs over InP etching. When compared with CH₄/H₂ RF plasma etching in the same reactor, a significant reduction in the measured applied RF power density and self-bias is observed.