Abstract: The distribution of Schottky barrier heights over the contact area in Au/III-V semiconductor (GaAs, InP, AlxGa1-xAs, InxGa1-xAs) diodes was determined using ballistic electron emission microscopy. Samples which received a chemical pretreatment in aqueous HF or HCl solutions showed changes in the barrier height distribution. In some cases, short rinses in deionized water could remove these effects. Additional XPS measurements and our former work on Si enabled us to propose a model wherein negatively charged species containing F or Cl at the interface are assumed to be responsible for these changes in barrier height distribution. However, in some cases, these effects were shadowed by more drastic influences due to the chemical processing such as changes in the stoichiometry of the surface region.

Abstract: 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.

Abstract: 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.

Abstract: The conduction band offset of the type II heterostructure CdS/ZnSe is determined from photoluminescence data of single quantum wells. The cubic quantum well samples have been grown by compound-source molecular-beam epitaxy. Photoluminescence spectra were measured at low temperatures and evaluated by fitting an effective mass model to the transition energies. A conduction band offset of (0.80±0.1) eV and an effective electron mass for cubic CdS of (0.18±0.05)m0 were determined.

Abstract: A microscopic study of 0.1 µm recessed gate -doped AlInAs/GaInAs HEMTs has been performed by using a semiclassical Monte Carlo device simulation. The geometry and layer structure of the simulated HEMT is completely realistic, including recessed gate and -doping configuration. The usual T-gate technology is used to improve the device characteristics by reducing the gate resistance. For first time we take into account in the Monte Carlo simulations the effect of the T-gate and the dielectric used to passivate the device surface, which affects considerably the electric field distribution inside the device. The measured Id-Vds characteristics of a real device are favourably compared with the simulation results. When comparing the complete simulation with the case in which Poisson equation is solved only inside the semiconductor, we find that even if the static I-V characteristics remain practically unchanged, important differences appear in the dynamic and noise behaviour, reflecting the influence of an additional capacitance.

Abstract: Heterojunction MOSFETs (HMOSFETs), grown on a virtual substrate of SiGe and having a strained silicon channel, have been fabricated. A conventional silicon MOS process was used, including dry thermal oxidation and high temperature source-drain annealing. Good transistor I-V characteristics were obtained for devices having drawn gate lengths between 150 nm and 10 µm and an extrinsic transconductance of 220 mS mm-1 is reported for the 150 nm gate length device. Technology computer aided design (TCAD) is used to determine that the bulk low field mobility of the strained silicon which forms the channel is 1500 cm2 V-1 s-1, while the source-drain series resistance is 1.5 mm. Good agreement between simulated and experimental I-V data is obtained.

Abstract: Recent achievements in controlling the electronic spectrum of InAs-based quantum dots (QDs) formed by self-organization phenomena during the initial stages of strained layer epitaxy are reviewed. Three different ways to exercise this control are discussed, based on variation of QD size with the amount of QD material deposited, tuning of the electronic levels in QDs by changing the matrix bandgap, and electronic coupling of neighbouring QDs vertically stacked in the growth direction. Possibilities to prevent thermal evaporation of carriers out of QD states and to tune the emission wavelength in the range 0.85-1.3 µm on GaAs substrates and up to 2 µm on InP substrates are demonstrated.

Abstract: A dye-sensitized photoelectrochemical cell made from a nano-porous composite film consisting of tin(IV) and zinc oxides generates exceptionally high photocurrents at an optimum mixing ratio of the two oxides. It is suggested that this phenomenon originates from ballistic transport of electrons injected by the excited dye molecules along the interconnected chains of tin(IV) oxide nanocrystallites to a zinc oxide crystallite.

Abstract: When prepared by the sol-gel dip-coating technique, doped tin oxide films need several superimposed layers to reach the expected low sheet resistance. In this paper we show that successive depositions increase the uniformity and homogeneity of multi-layered sol-gel tin oxide films. The final thickness is always found to be lower than a multiple of the initial first layer thickness, indicating that densification occurs. A strong improvement of physical properties is jointly observed in this multilayered configuration. All parameters (atomic density, refractive index, electrical resistivity, optical transmission and reflectivity) evolve towards bulk material values. This is explained by the fact that the sol, in which is dipped the substrate, fills in the pores or cracks remaining in the previously deposited layers. The quality of the sol-gel tin oxide stacked deposits obtained in such a way is evidenced by the M-line spectroscopy experiments presented in this paper. It is shown that multilayered coatings are able to guide light over some millimetres.

Abstract: 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.

Abstract: Results of photoluminescence and photoconductivity measurements in epitaxial films are presented. The photoluminescence peak energy and intensity show several anomalous behaviours. The peak energy changes with temperature exhibiting an inverted S-shape dependence, where it decreases, then increases with increasing temperature in the range 40-100 K and finally decreases with increasing temperature. The intensity shows a temperature dependence similar to that of amorphous semiconductors and disordered superlattices. A blue shift of the photoluminescence energy with increasing excitation intensity is observed. A large Stokes shift between the photoluminescence peak position and the band edge transition energy is found; it decreases with decreasing indium content. A persistent photoconductivity effect has been detected up to room temperature with a stretched-exponential function for its decay rate. All these observations can be explained in a consistent way by alloy potential fluctuations, and these clearly indicate the existence of compositional fluctuations. These two related effects thus appear to constitute the mechanism for the widely observed localized excitons in InGaN-based devices.

Abstract: We report for the first time, epitaxial growth of high-quality ZnS films on sapphire and silicon substrates, using pulsed laser deposition. X-ray diffraction results show that at all growth temperatures from 200°C to 680°C, epitaxial wurtzite (002) ZnS films have been successfully grown on (1012) sapphire and (001) silicon substrates. X-ray diffraction data yield full width at half maximum 2theta values of 0.13° for as-grown samples, compared with 28 values or 0.09° and 0.08° for the bare sapphire and silicon substrates respectively.

Abstract: Transmission electron microscopy of GaAs/InxGa1-xAs/GaAs double heterostructures has enabled the onset and subsequent development of misfit dislocations to be followed for increasing strained-layer thicknesses, from sub- to supercritical. It has been observed that misfit segments are introduced into threading dislocations when the strained-layer thickness is close to, but below, the critical thickness predicted by the Matthews-Blakeslee (M-B) model. Analysis shows that threading dislocations may be able to glide to form interfacial misfit dislocation segments even though the critical thickness predicted by the M-B model has not been reached. It has also been observed that the total dislocation density rises slowly as the layer thickness increases above its critical value, until a sudden increase occurs. It is suggested that the sudden increase in dislocation density is associated with a different mechanism of misfit dislocation formation, which dominates the global relaxation of the structure.

Abstract: We propose a lateral effective potential which allows a straightforward calculation of electron and hole states in V-groove quantum wires. This effective potential is used together with a suitable coordinate transformation which results in two decoupled one-dimensional Schrodinger equations which are readily solved. The energy levels and wave functions calculated by this method are in close agreement with several previous results, which indicates that this method may be valuable for the study of physical properties in V-shaped quantum wires which require analytically calculated wave functions. Also, the proposed effective potential gives rise to a shallow vertical quantum well which is found in some V-groove quantum wire structures.

Abstract: High-resolution photoinduced transient spectroscopy and the modulated photocurrent technique are compared in terms of possible application to the investigation of defect levels in semi-insulating monocrystalline materials. After a description of the theoretical and experimental aspects, the advantages, drawbacks and limitations of each method are discussed. The two techniques complement each other and their potentialities are exemplified by the measurements of trap parameters in the same samples of semi-insulating Cr-doped and undoped GaAs. From these results we deduce a possible model for the properties of the Cr defect in GaAs.

Abstract: The stress-induced leakage current (SILC) of a 42 nm SiO2 layer is investigated during constant gate voltage stress of metal-oxide-semiconductor capacitors The density of bulk electron traps generated during the electrical stress is extracted from the SILC contribution, assuming a trap-assisted tunnelling mechanism It is shown that a fixed critical value for the density of traps is reached at breakdown or soft breakdown of the SiO2 layer, independent of the gate voltage stress A physical model based on the formation of a percolation path between the bulk electron traps randomly generated during the stress is proposed to link SILC to time-dependent dielectric breakdown in ultra-thin gate oxides The validity of this model with respect to positive and negative stress polarities is discussed It is also shown that this model allows us to predict the reliability of ultra-thin gate oxide layers at low applied gate voltage stress

Abstract: 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.

Abstract: Dark carrier transport mechanisms in Hg1-xCdxTe (x = 0.19-0.265) photodiodes in the temperature range 70-150 K and PbTe1-ySy/Pb1-xSnxTe (x = 0.2, y = 0.03) heterojunctions at T = 80 K are discussed. Two major current mechanisms were included into balance equations for the p-n junction: trap-assisted tunnelling (TAT) and Shockley-Reed-Hall generation-recombination processes for a defect trap level in the gap. Other current mechanisms (band-to-band tunnelling, bulk diffusion, etc) were taken into account as additive contributions. For TAT Anderson's matrix element of the impurity ionization was used and the tunnelling rate characteristics were calculated in the k-p approximation with constant barrier field. Using donor and acceptor concentrations in n- and p-type regions of the diode, trap level concentration, trap level energy and the in-junction trap level lifetimes as fitting parameters, a relatively good agreement with the experimental data for HgCdTe and PbSnTe diodes with large zero-resistance-area products R0A (e.g. for the 10 µm spectral region R0A>10 cm2) was obtained, which allows one to deduce the parameters of mercury-cadmium telluride and lead-tin telluride photodiodes from the parameters of the materials used for the diode fabrication. For the diodes with poor R0A characteristics (R0A<1 cm2 for the 10 µm spectral region) the agreement with the experimental data is not so good, which is apparently caused by the presence of several types of traps with different energy positions inside the gap and with different properties.

Abstract: In this paper, we report the growth of very pure InAs epitaxial layers of high quantum efficiency, by introducing the rare-earth element Gd into the liquid phase during LPE growth. We find that the carrier concentration of InAs layers can be effectively reduced to . Also, the peak photoluminescence (PL) intensity of such layers can be considerably increased by between ten- and 100-fold compared with untreated material. We attribute this behaviour to the gettering of residual impurities and corresponding reduction of non-radiative recombination centres in the presence of Gd. Four intense sharp lines dominated the low temperature (4 K) photoluminescense spectra of Gd-treated InAs layers. The strongest two of these were found to originate from (a) bound excitons, and (b) donor-acceptor recombination, whereas the remaining two, (c) and (d), were associated with defect-related recombination. The linewidth (FWHM) of the exciton peak (a) was reduced to only 3.8 meV, which is narrower than for undoped epitaxial InAs grown by MBE or MOVPE.

Abstract: Selective epitaxial growth of Ge dot structures is investigated to obtain highly regular dot arrays with excellent size uniformity in the nanoscale. The dot structures are grown in patterned fine windows ranging in diameter from 650 to 90 nm on Si(100) substrates covered with masks. The dimensions and number of the dots grown in a window significantly change depending on window size, growth temperature and time and the thickness of the Si buffer layer. These growth characteristics are considered to be induced by the Stranski-Krastanov growth mode and migration mechanism of Ge atoms on Si substrates. It is noted that Ge dots whose diameter is much smaller than the pattern size are formed with high uniformity and that the position is precisely controlled by SEG. The Ge dot structures are found to give rise to prominent luminescence with well separated phonon replicas and the energy position systematically changes when the growth conditions are varied.

Abstract: We present a model for electron injection from thermal reservoirs, which is applied to particle simulations of one-dimensional mesoscopic conductors. The statistics of injected carriers is correctly described from non-degenerate to completely degenerate conditions. The model is validated by comparing Monte Carlo simulations with existing analytical results for the case of ballistic conductors. An excellent agreement is found for the average and noise characteristics, in particular, the fundamental unities of electrical and thermal conductances are exactly reproduced.

Abstract: Optically detected magnetic resonance (ODMR) was used for study of defects in n-type 6H-SiC. Four ODMR spectra related to spin S = 1 centres were observed. Two of these centres, labelled a and b, have a trigonal symmetry with the symmetry axis along the c-axis of the hexagonal crystal. For the other two centres, labelled c and d, the symmetry is lower (C1h) and the principal axis z of the g- and D-tensor is about 71 degrees off the c-axis. Based on the symmetry axes, the annealing behaviour and the intensity, these spectra are suggested to originate from different configurations of the paired centre between a silicon vacancy and a nearest-neighbour point defect (either a carbon vacancy or a silicon antisite), occupying different inequivalent sites in the 6H-SiC. These defects are non-radiative and act as efficient recombination channels in the material.

Abstract: The dislocation structure and photoluminescence of partially relaxed Si1-xGex layers on Si(001) substrates were studied to reveal the contribution from dislocations localized in different regions of the heterostructure (SiGe layer, SiGe/Si interface, Si substrate) to the dislocation-related PL. The D1 and D2 lines were ascribed to products of dislocation reactions in intersection sites. The known dependence of the D4 line spectral position on the Ge content is not observed, which is explained by the effect of elastic strain in the SiGe/Si heterostructure.

Abstract: From the static current-voltage properties of irradiated silicon p-i-n diodes we develop a semi-quantitative model based on established relaxation semiconductor theory for forward and reverse bias. The properties of such diodes when used as photodiodes or ionized particle detectors are also described and analysed. The basic properties of this semi-insulating, relaxation semiconductor device are described together with possible applications. The same behaviour is observed in semi-insulating GaAs and other compound semiconductor diodes so that the analysis is also suitable for them and devices incorporating such diodes in substrates, such as MESFETS.

Abstract: The metal-organic vapour phase epitaxy (MOVPE) growth of GaN on 6H-SiC with a conductive AlGaN buffer layer has been studied. With only 6% Al incorporation, a continuous 200 nm thick AlGaN layer n-type doped in the 1018 cm-3 range was obtained. A crack-free 1.5 µm thick GaN layer was subsequently grown. The conductivity of the buffer was checked by I(V) measurements with Ni/Au contacts deposited on the back of SiC and on the GaN surface. The layer exhibits strong low temperature (10 K) band edge luminescence at 3.465 eV related to donor bound exciton with a ratio to deep level about 103 and an FWHM of 3.6 meV. Only threading dislocations are evidenced by TEM with a density of ~109 cm-2. The FWHM of the (0002) line of GaN in rocking curve scan by x-ray diffraction is 150 arc sec. These properties are similar to the ones of GaN layers deposited on an insulating AlN buffer layer.

Abstract: We investigate the origin of the broad luminescence observed around 2.7-2.9 eV in heavily Mg-doped GaN through first-principles pseudopotential calculations. We find that a defect complex composed of an Mg interstitial and an N vacancy gives rise to optical transition levels around 2.8 eV above the valence band maximum, suggesting that the blue luminescences are caused by deep-donor-to-valence-band transitions. The formation of the - complex is enhanced by hydrogenation and is more preferable in p-type samples grown under Ga-rich conditions.

Abstract: We have fabricated and characterized a uniformly n-doped silicon double-dot structure. The electrical behaviour could be changed between that of a multiple tunnel junction and that of a double dot by applying appropriate gate voltages. The double-dot characteristics observed can be attributed to the geometry of the structure, and it is shown that the influence of the multiple tunnel junctions can be entirely eliminated. In the double-dot regime, characteristic charging diagrams were obtained by independently sweeping two sidegate voltages. Using a classical capacitance equivalent circuit the hexagonal lattice of the conductance resonances in the charging diagram was modelled and single-electron charging in the geometrical double dot is concluded from the match between model and experimental data.

Abstract: 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 1-12 eV is close to a quantum energy of the YAG:Nd laser used ( eV)

Abstract: When optical microprobe spectroscopies, based on Raman or luminescence, are used to study materials properties (such as composition or strain) that vary within the volume of material excited by the microprobe, it is necessary to convolute the material property variation with the response function of the microprobe to obtain the observable spectrum. A method for this convolution, in the general case in which the material shows significant absorption of the incident light, is suggested and is shown to give acceptable interpretation of test experiments on alumina and silicon single crystals.