Table of contents

Small-angle neutron scattering is used to study the precipitation behaviour of dissolved oxygen in Czochralski-grown dislocation-free single crystals of silicon doped with antimony. The results are compared with those from undoped silicon. The precipitation of oxygen in silicon is very different for the Sb-doped material as compared with undoped silicon. Sb doping appears to slow down the rate of precipitation of oxygen in silicon into large cushion-shaped SiO₂ precipitates.

Calcium contamination of silicon layers grown by ultra high vacuum vapour phase epitaxy has been studied using in situ secondary ion mass spectrometry. Calcium and barium concentrations present as a contamination of the native oxide on silicon wafers as supplied by the manufacturer were measured and found to vary between different manufacturers. The efficiency of wet chemical cleaning in removing the calcium contamination has also been characterized. The relative contamination of the silicon surface after desorbing the various oxides in hydrogen or UHV is compared and a mechanism by which calcium can contaminate epitaxial growth in a hydrogen ambient is proposed.

ZnSe has been deposited on (100) GaAs by molecular beam epitaxy under a uv irradiance of 4 W cm^-2. At this level, the ZnSe growth rate can be severely reduced or stopped altogether. The authors show for the first time that the reduction in growth rate is a strong function of layer thickness for epilayers less than approximately 2000 AA thick. The growth rate under laser irradiation is found to be directly proportional to the Se flux, demonstrating that Se is the minority surface species even under high VI-II flux ratios. Furthermore they present observations indicating that the electron stimulated desorption of Se by RHEED beams and the photostimulated desorption are both due to the accumulation of photoinduced holes at the growth surface.

The authors report the use of an electrochemical iodine cell to dope epitaxial ZnSe grown by molecular beam epitaxy over a range of carrier concentrations from 10¹⁶ to 10¹⁹ cm^-3. The doping levels throughout the layers have been measured by electrochemical CV profiling and staircase doping structures have been used to calibrate the doping level in terms of the cell flux. Photoluminescence and Hall data confirm the growth of well-behaved n-type ZnSe.

A series of p-type GaAs layers grown by molecular beam epitaxy at temperatures below 600 degrees C directly on n⁺-type substrate was investigated by means of deep-level transient spectroscopy. The presence of nine hole-emitting traps was revealed, the Arrhenius plots of which are given. A comparison is made between layers not intentionally doped and layers isoelectronically doped with In. The trap suppression by In doping was found to be strong for most of the traps especially in layers grown at lower temperatures. The contribution of native defects and impurities to the observed trap spectra is discussed. Most of them were related to impurities, partly to transition metals. The so-called B level known as a native defect, was also found.

The violet-blue emission band due to the incorporation of Zn in GaN has been studied and has been shown to possess a complex behaviour involving both donor-acceptor (D-A) pair and self-activated (S-A) pair properties. A Zn concentration dependence and temperature dependence of this emission has been shown. An attempt to present and analyse the transition process by a suitable model is reported.

In doped semiconductors the collective influence of the carriers on the polar modes modifies the electron-phonon interaction. The author considers a continuum theory for the coupled phonon-plasmon modes and using appropriate boundary conditions at the interfaces derives the dispersion relation for coupled phonon-plasmons in GaAs/Ga_0.7Al_0.3As superlattices. As a consequence of the overlap of the coupled phonon-plasmon dispersion relations for bulk GaAs and bulk Ga_0.7Al_0.3As the allowed superlattice coupled phonon-plasmons are split into different regions characterized by guided, interface and travelling modes. The author writes down the Hamiltonian for the coupled phonon-plasmon system and carries out quantization of the corresponding Coulomb potential. He considers intersubband energy relaxation rates in GaAs/Ga_0.7Al_0.3As superlattices via the emission of the coupled modes and explores the variation in rate with well width. At electron densities easily achievable in semiconductor superlattices these intersubband transition rates are greater than the corresponding rates via solely LO phonon emission.

The relaxation of holes in GaAs-AlGaAs quantum wells by emission of acoustic phonons via deformation potential coupling is calculated. Using the 4*4 Luttinger Hamiltonian and the Bir-Pikus Hamiltonian to describe the hole dispersion and the coupling to acoustic phonons the authors take into account the coupling of light and heavy holes. They find a distinct angular dependence of the hole relaxation which is mainly due to the anisotropy of the phonon dispersion and the coupling mechanism. The results confirm the predominance of parity-conserving processes and incomplete hole-spin relaxation derived from luminescence experiments.

Interband magneto-optical absorption has been studied on InGaAs/InGaAsP superlattice structures. In addition to the well known E1-HH1, E1-LH1, E2-HH2 confined state transitions, the observation of the strong transitions from the valence band continuum to the E1 and E2 states allows one to make reliable estimates of the band discontinuities in this system, giving Delta E_c=0.377 Delta E_g. Application of a magnetic field both perpendicular and parallel to the superlattice layer planes allows one to deduce the superlattice dispersion, showing a large reduction in the electron mass along the superlattice direction for the E2 band due to the formation of zone-folded minibands. The electron mass of the motion in the layer planes for the same band also shows a reduction over the equivalent bulk alloy.

Transient photoconductivity has been used to study the recombination of free carriers in GaInAs/InP multiple quantum well material. The sublinear increase in the initial amplitude of the photoconductivity with excitation intensity is attributed to the carrier density dependence of the mobility. This nonlinearity is taken into account when the time constant associated with the decay of the free carriers is obtained from the photoconductive decays. For low excess carrier densities, in the non-degenerate regime, the recombination was consistent with a kinetic radiative recombination model which included free carriers, excitons and photon recycling. The time constants, deduced from the experimental data, were in good agreement with the temperature dependence predicted by the model. When the excess carrier density was high enough to make the material degenerate, the initial decay, while the material remained degenerate, was exponential with a relatively short time constant. Even when the wells were full there was no evidence of Auger recombination. An upper limit of 3*10^-29 cm⁶ s^-1 could therefore be placed on the Auger coefficient.

The Hall and the Shubnikov-de Haas effects have been studied in a quasi-two-dimensional inversion layer adjacent to the grain boundary in a Hg_1-xCd_xTe bicrystal with a bulk x value of x=0.23. For magnetic fields perpendicular to the grain boundary interface a complicated pattern of Shubnikov-de Haas oscillations and well developed quantum Hall plateaux are observed which result from an asymmetric double potential well at the grain boundary. A cosine dependence is found for the Shubnikov-de Haas oscillation period of the ground subband in tilted fields, whereas the first excited subband shows remarkable deviations from conventional two-dimensional behaviour. It is shown how these deviations can be related to the spatial extension of the wavefunction. The diamagnetic Shubnikov-de Haas effect confirms the complex subband structure of the system, including the occupation of a large number of subbands.

The magnetotransmission of a p-type Hg_0.766Cd_0.234Te bicrystal containing a single grain boundary with an inversion layer has been investigated in the submillimetre wavelength range. For the first time the cyclotron resonance lines belonging to the various electric subbands of a quasi-two-dimensional carrier system at a grain boundary could be detected. The measured cyclotron masses and the subband densities determined from Shubnikov-de Haas experiments are compared with theoretical predictions and it is found that the data can be explained very well within the framework of a triangular well approximation model which allows for non-parabolic effects.

Excitation of bulk deep levels in Schottky barriers by electrical bias pulses of amplitudes Delta U_r<kT/q is analysed. Spatial resolution of the order of a few interatomic distances can be achieved if potential fluctuations in any of the crystal planes are negligible. The concept of an average density of bound states is applied to the latter case of a uniform distribution of shallow dopants. Some experimental aspects of profiling defects are pointed out. An example of a true exponential response under the small-signal excitation conditions of Al/GaAs diodes is shown. The magnitude of the response is a superlinear function of Delta U_r in the region where Delta U_r>kT/q.

The authors report investigations of the low temperature optical properties of AlInAs-InP single heterojunctions and short period superlattices grown by atmospheric pressure MOCVD. Strong luminescence is observed and allows photoluminescence excitation measurements. Bandgaps are accurately determined from luminescence excitation and optical absorption spectra, and cross-checking optical data with structural properties yields a precise value (413 meV) of the conduction band offset in this system. This value deviates significantly from the prediction of the offset transitivity rule.

It is demonstrated that among commercial Al_xGa_1-xAs lasers there are three types of double heterostructure which differ in their capacitance (C) versus voltage (V) behaviour at 77 K. This is explained by the difference in extent of their anisotype heterointerface. It is also shown that the following properties of these double heterostructures can be determined: (1) pP heterojunction quality (determined from C(V) profiling data (N(W) dependences) at 300 K) and Np heterojunction quality (determined from C(V) dependences at 77 K); (2) the position of the pn junction, the magnitude of its displacement (if there is any) from the anisotype heterojunction and the layer of the structure in which it is displaced (determined from the spectral photocurrent response measurements at 300 and 77 K). All the mentioned techniques are non-destructive.

The authors estimate, by reviewing the available photoluminescence data, the band alignments at heterojunctions of Zn(Cd)S(Se) semiconductors. The photoluminescence peak energies of ZnSe-ZnS, CdS-ZnS and CdSe-ZnSe strained layer superlattices (SLS), with a range of well and barrier widths, have been calculated using Kronig-Penney theory and assumed values of the band offsets which yield acceptable fits to both the experiments and data taken from the literature. For ZnSe-ZnS SLS, fair agreement is obtained between calculation and experiment by assuming, in line with previous work, a negligible conduction band offset. At the same time, good estimates of the photoluminescence peak energies of CdS-ZnS and CdSe-ZnSe superlattices are obtained by assuming a negligible valence band offset in each case. The authors construct a band alignment diagram which predicts that CdSe-CdS and CdS-ZnSe are type-II SLS with electron confinement in the CdS layers; these predictions are borne out by experiment.

In low-dimensional systems dynamical nuclear polarization is a sensitive tool for the study of electron-nuclear interaction. The authors report on Overhauser-shift experiments on the 2DEG of Al_xGa_1-xAs/GaAs heterostructures. The conduction electron spin resonance has been electrically detected via the associated change of the magnetoresistance R_xx, and dynamical nuclear polarization (the Overhauser effect) has been achieved by saturating the conduction electron spin resonance (CESR). In order to investigate the nuclear spin relaxation the samples were embedded in an RF coil and the temporal decay of the Overhauser shift was followed with and without additional NMR irradiation. Different spin-lattice relaxation times of ⁶⁹Ga, ⁷¹Ga and ⁷⁵As could be detected and small deviations from a pure Korringa behaviour have been found.

The inhomogeneous broadening of the near-gap emission (bound excitons (BE) and conduction-band to acceptor (CA)) in semiconductor alloys is reanalysed using the Markoff statistical theory for fluctuations of alloy composition. The authors give the exact relationship between the linewidth and the Bohr radius of the bound particle. The results of their theory indicate that even in the best GaAlAs samples there is still a significant contribution from other broadening mechanisms. They also show that the linewidth ratio of the CA to BE emission lines may provide a good estimate of the natural band offset in the alloy.

Absorption spectra of strained InGaAs/InP quantum wells exhibit a single, unusually large exciton feature. 60% quenching is obtained for 4.3 mW Nd:YAG effective pump power at a wavelength of 1.064 mu m. A simple two-level formula fits well the behaviour at the exciton peak, and absorption spectra are reasonably approximated by a standard two-dimensional exciton model. Saturation intensities calculated with these techniques range around 200 W cm^-2. Finally the Kramers-Kronig relations are used to calculate the associated refractive index change.

A detailed study of the 1.32 eV photoluminescence emission present in p-type GaAs was made at various temperatures and excitation intensities in conjunction with the effects of electron irradiation and infrared absorption measurements. The authors find that the binding energy of the acceptor responsible for the 1.32 eV emission is not the same as that of the Ga_As^-/Ga_As^-- transition as commonly believed. It is proposed instead that the acceptor responsible for this luminescence is a gallium antisite double acceptor-effective mass donor pair complex, Ga_As-donor. The activation energy of this complex acceptor is determined to be approximately 197 meV.

Photoluminescence and Raman spectroscopy have been used to determine composition and strain in pseudomorphic In_xGa_1-xAs/Al_xGa_1-yAs quantum wells. Series of MBE-grown samples, comprising bulk-relaxed In_xGa_1-xAs reference layers and In_xGa_1-xAs/Al_yGa_1-yAs quantum wells of different thicknesses, were considered. Photoluminescence transitions in the strained structures were analysed using a semiempirical model including strain and composition effects in the In_xGa_1-xAs well. The GaAs-like longitudinal optical LO phonon Raman shift allowed an independent strain estimation. The agreement between photoluminescence and Raman results confirms the assumption that there is no relaxation in the wells. The effects of uncertainties in the stress deformation potentials and phonon coefficients on strain determination have been discussed. The authors' results show that photoluminescence is an adequate tool to characterize InGaAs strained layers.

Phonon energies in InAs_1-xSb_x ternary alloys, grown on GaAs by molecular beam epitaxy, have been studied by Raman scattering. The microstructure for this alloy system depends strongly on the growth temperature. For growth temperatures above 400 degrees C, transmission electron microscopy (TEM) shows the alloy epilayer to be homogeneous. Raman spectra of these homogeneous InAsSb alloys show a strong InAs-like longitudinal optical (LO) phonon line, as well as an InSb-like LO line, throughout the composition range. The frequency of the InAs-like LO phonon varies linearly with composition. For growth temperatures below 400 degrees C and compositions near the middle of the range, an interleaved platelet structure, arising from phase separation, is observed in TEM. Effects of phase separation in these alloys have been observed in the Raman spectra.

The authors present an optical study of multiple quantum wells of CdTe/Cd_1-xMn_xTe grown with a low manganese concentration in the barrier layers, typically x approximately=0.05. The structures grow pseudomorphically to the substrate so that the strains in the CdTe and CdMnTe layers are precisely known. In an attempt to determine the valence band offset the energy difference between the light- and heavy-hole excitons has been carefully analysed. They find that in these quantum wells with small barrier heights, and hence with small confinement energies, the exciton binding energies may account for a substantial fraction of the splitting. Thus a precise determination of the valence band offset would need a very accurate calculation of these binding energies. The transition from a type I to a type II band structure with an applied magnetic field has been investigated. Various simulations show that the large decrease of energy expected for the heavy-hole excitonic gap at this transition can be almost completely compensated by a simultaneous decrease of the exciton binding energy, which explains why no significant features at the transition were observed in their samples.

Optical and photoelectrical properties of p-Hg_0.6Cd_0.4Te are investigated. The optical transitions detected are interpreted in the framework of a qualitative model including bound and free excitons as well as interband transitions via levels. A consistent description of the luminescence, absorption, photoconductive and transport behaviour is given.

Silicon dioxide films were deposited onto InP substrates in the ranges of 350-550 degrees C temperature, and 3-15 Torr pressure by means of the rapid thermal low-pressure chemical vapour deposition (RTLPCVD) technique. SiO₂ films were deposited using a variety of O_x: SiH₄ ratios in the range of 5:1-50:1, and deposition rates of 15-50 nm s^-1. The high temperature and high rate deposition was obtained without creating any damage to the InP substrate surface, and resulted in highly densified SiO₂ layers with good thickness control and low stress. The main parameters of the as-deposited and annealed films, such as deposition rate, density, refractive index, wet and dry etch rates, stresses, film microstructure, and the SiO₂/InP interface quality are reported. These parameters were studied as a function of the process variables, which include deposition time, temperature, pressure, O₂:SiH₄ ratio, gas flow rates and chamber geometry.

AlGaAs/GaAs heterojunction bipolar transistors grown on InP substrates have been characterized at microwave frequencies for the first time. For emitter dimensions of 3*5 mu m², current gain cut-off frequencies f_T of 24 GHz and maximum oscillation frequencies f_max of 13 GHz have been obtained. A comparison is also made with the previously reported DC characteristics for MBE-grown material.

InGaP/GaAs single-heterojunction bipolar transistors (HBTs) and double-heterojunction bipolar transistors (DHBTs) grown by organometallic vapour phase epitaxy are reported. A current gain beta =40 was obtained for 90 mu m diameter HBT devices at a base-collector bias of 0 V. The base carbon-doping concentration for the devices was 2*10¹⁹ cm^-3 and the sheet resistivity ( rho _s) of the base layer was 600 Omega / Square Operator . For the DHBTs, a current gain beta =27 was obtained for a base-collector bias of 2 V. The carbon doping concentration in these devices was 8*10¹⁸ cm^-3 with rho _s=1400 Omega / Square Operator . This represents the first successful fabrication of DHBTs for this material system.

Modulation-doped In_0.3Ga_0.7As/In_0.29Al_0.71As heteroepitaxial layers were grown on GaAs substrates by molecular beam epitaxy using compositionally graded buffer step layers. The misfit strain in the x=0.3 layer, which contains the two-dimensional electron gas channel, is relaxed by 96% as determined by X-ray diffraction. Transmission electron microscopy reveals that the active layer is dislocation-free. The sheet electron density of the channel is of the order 10¹² cm^-2 and the mobility is 9340 cm² V^-1 s^-1 at room temperature and 42290 cm² V^-1 s^-1 at cryogenic temperatures. The effective mass of the electrons, m*=0.066m₀, is greater by about 20% than that expected of this ternary alloy, due principally to the nonparabolic correction at the Fermi level. Shubnikov-de Haas oscillations measured at 1.6 K as a function of magnetic field show that the quantum relaxation time is 8.8*10^-14 s while the classical relaxation time determined from the measured mobility and calculated m* is 1.5*10^-12 s.

Measurements of sheet resistance and Hall coefficient between 0.3 K and 30 K have been used to infer two-dimensional weak localization and hole-hole interaction effects in Si:B delta-doped layers. It is found that the renormalized screening parameter in the interaction theory F* approximately 0.9 for samples of sheet concentrations 1.8*10¹³ cm^-2 to 7.6*10¹³ cm^-2 and is only weakly dependent on carrier concentration, in agreement with theory. The weak localization correction provides evidence for spin-orbit scattering, which appears to become increasingly important as the sheet concentration is increased.

Upper limits have been obtained for the size of any asymmetry in the power dissipation between the current entry and exit points of a Si MOSFET in a quantizing magnetic field. Measurements have been made for power inputs between 0.3 and 2000 mu W and for filling factors between 4 and 32. These show that if the total power dissipated I²R_H is shared between the current entry and exist points in proportions (50+or-x)%, then for example at 8 mu W, x<or=1.35 for i=8. No evidence was obtained for any additional dissipation close to the electron entry point near to quantum Hall breakdown.

A practical method proposed to determine the transfer length of the quantum field effect directional coupler (QFED) is reported. Calculation shows that the conductance of the QFED oscillates between 0 and 2e²/h with the Fermi energy of the electron reservoir connected to the device, and the transfer length can be easily obtained by locating the extrema of the conductance from the curve of conductance versus Fermi energy.

Quasi-Schottky diodes on n-Ga_0.47In_0.53As were fabricated by employing MOCVD-grown GaAs assist layers. Rectifying I-V characteristics were measured, yielding an apparent Schottky barrier of 0.61 eV. The mechanism of current transport in both directions is found to be dominated by a tunnelling component, presumably via states in the highly dislocated assist layer. In the reverse direction the current density is 0.18 A cm^-2 at a 2 V bias. Annealing of samples up to 310 degrees C leads to a redistribution of traps near or at the heterointerface. Consequently the quasi-Schottky barrier is lowered.

Capacitance-voltage (C-V) profiles have been measured for single delta -doped GaAs(Si) samples at various 2D sheet electron concentrations. The effect of temperature and incident light on the profiles has been monitored. In a heavily doped sample the authors have observed persistent photoconductivity, which they believe confirms the existence of the DX centre in delta -doped GaAs(Si). They have also noted a freeze-out of the equivalent 3D electron distribution with decreasing temperature, leading to an effective increase in the full width at half maximum (FWHM) of the C-V profile which can be attributed to electron trapping by DX centres. Using deep level transient spectroscopy they have observed a trap with activation energy approximately 350 meV in the heavily doped sample that exhibited persistent photoconductivity; this trap was not present in the lower doped delta -layers.