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This article reviews the pioneering investigations of the luminescence and photoelectric phenomena in type II heterojunctions based on the GaInAsSb/GaSb system. This system is remarkable because it is possible to create and study heterojunctions with both staggered and broken-gap alignment, depending on the alloy composition. Type II heterojunctions differ from type I in the existence of adjacent dual quantum wells for electrons and holes on both sides of the interface. Simultaneous confinement of electrons and holes in these wells causes unique optical and electrical properties of such heterojunctions and greatly modifies the characteristics of optoelectronic devices. The review considers the photo- and electroluminescence spectra of GaInAsSb/GaSb heterojunctions with staggered band alignment. The importance of tunnelling-assisted transitions through the interface in the radiative recombination of confined carriers is shown. The influence of these transitions on the structure and polarization characteristics of the luminescence spectra is considered. A new mechanism of photocurrent gain in isotype n-N heterojunctions due to hole confinement at the type II interface is discussed. Unusual asymmetric electrical properties of type II heterojunctions with broken-gap band alignment are demonstrated and discussed in connection with their energy band diagrams. Novel in light sources and photodetectors for the 1.6-4.7 mu m spectral range based on the GaInAsSb/GaSb system are briefly reviewed.

We study the mobility of a two-dimensional electron gas (2DEG) which forms at an inverted GaAs/AlAs interface, in a structure where a back gate and a front gate are both used to modify the density of the 2DEG. The availability of two different control voltages provides a special opportunity to gain insight into the scattering mechanisms. We find that we can induce a significant change in the mobility, at a fixed density, by changing the interface roughness scattering, which depends on the perpendicular electric field acting on the 2DEG. We also find different mobility changes for current flowing in different directions in the plane of the 2DEG. We conclude that the roughness of the interface where the 2DEG resides is anisotropic, and it is responsible for the consistent anisotropy of the mobility in the (100) plane. We also develop a theoretical framework to calculate scattering due to anisotropic roughness.

The electronic states of GaAs/AlGaAs quantum well wires under an applied magnetic field are studied by using the empirical pseudopotential method. The energy spectra are calculated as a function of the magnetic field strength. The results show a strong non-parabolicity and field-dependent subband effective mass. The formation of bulk-like Landau levels and edge (skipping) states results in a rapid increasing of density of states at the subband bottom reflecting the change of the 1D subband structures to 2D-like Landau levels. The asymptotic behaviours of the Landau levels in the weak and strong magnetic field limits are also obtained by a simple effective mass analysis and used to interpret the results of the microscopic calculation.

By coupling an ensemble Monte Carte simulator with a one-dimensional Poisson solver, we analyse anisotropic electron transport in homogeneous Si structures and submicrometre n⁺nn⁺ diodes. The model employed for the conduction band in the simulation approximates the real band by means of non-parabolic ellipsoidal X and L valleys. An energy limit for the energy that the electron can reach in the X valleys has been established. Firstly, in order to check the validity of the band model over a wide range of electric fields a study of anisotropic electron transport in bulk Si has been performed. On the basis of bulk Si results, the electron transport properties in both structures have been thoroughly explained by the analysis of the static characteristics. Special attention is paid to the origin of the anisotropic effects, which are explained in terms of the different occupation of the X valleys. The L valleys are found to have an influence for the highest fields.

We derive the formal expression for the dielectric response of composite semiconducting materials and then show that the inverse dielectric function can be approximately deduced in the particular case of a two-dimensional electron gas (2DEG) embedded between two media. In given situations, these surrounding media contribute noticeably to the non-local dielectric response. This allows us to propose a general formulation for the actual scattering potentials acting against the motion of the 2DEG in transport experiments. The improvement brought about by our model is pointed out in the particular case of ionized impurity scattering.

We present magnetotransport measurements (up to 7 T) performed at very low temperatures (down to 20 mK) on a GaAs sample containing two parallel delta -doped layers whose carrier concentration can be varied by means of a gate electrode. With increasing negative gate voltage the resistance becomes more strongly temperature-dependent, indicating a more localized electron system. The magnetoresistance is found to be strongly anisotropic. When the field is parallel to the layers we find a large positive magnetoresistance which we attribute to orbital shrinking of the strongly localized donor wavefunction. In contrast, in the perpendicular orientation, we observe a strong negative magnetoresistance at low fields whose origin remains unclear, and the quantum Hall effect at larger fields. At low gate voltages both delta -layers are in the quantum Hall state whereas at larger negative voltages the layer adjacent to the gate becomes insulating. In the case of strong depletion the high-ohmic sample shows reproducible conductivity fluctuations as a function of either the gate voltage or the magnetic field. The fluctuations diminish at higher temperatures and larger measuring currents.

We report electron paramagnetic resonance measurements for various charge states of chromium substituting for silicon in single crystal epitaxial layers of 6H-SiC. The EPR spectra of Cr³⁺(3d³) with S=3/2 in the A^--state on three lattice sites (a hexagonal and two quasi-cubic ones) were observed. The hyperfine structure of the isotope ⁵³Cr and the superhyperfine structure of the ligand ²⁹Si and ¹³C were measured. For the neutral A⁰-state of Cr⁴⁺(3d²), a spin triplet EPR spectrum was found with the fine structure parameter D=3.35 GHz and g/sub ///=2.0, g_{perpendicular to} =2.01. Additional anisotropic EPR lines were detected and described by the effective spin 1/2 and g-factors 1.77 and 3.7 when the magnetic field was applied parallel and perpendicular to the c-axis, respectively. In p-type epitaxial layers produced by boron diffusion into chromium-doped samples, new EPR spectra were observed with parameters close to the ODMR data for deep boron centres.

Thermally induced defects in oxygen-rich silicon are the thermal donors (TDDS) and the so-called NL10 defects. These defects are formed by annealing at temperatures around 450 degrees C. Whereas the TDDS have a unique nature, some NL10 defects probably have different structures in differently doped silicon samples. NL10 defects were studied in P-doped silicon with electron paramagnetic resonance (EPR) and Hall effect measurements yielding the result that NL10 is a donor in this n-type silicon with a level deeper than 65 meV below the conduction band minimum. P is, however, not part of the defect. Electron nuclear double resonance (ENDOR) experiments have been performed on NL10 defects in B- and C-doped Si, while they were not possible in P-doped Si because of a short electron spin-lattice relaxation time. No ENDOR lines, except the 'distant' ENDOR lines of ¹¹B,¹³C and ²⁹Si respectively, were detected. This is in contrast to what was found previously in Al-doped Si. To explain this it is proposed that neither B nor C may be part of the NL10 defect in B- and C-doped Si respectively. In ¹⁷O-diffused B-doped Si a weak and rather broad ENDOR line was found which is not found in ¹⁶O-containing Si and which is attributed to ¹⁷O. In this case oxygen has much smaller quadrupole interactions than those found previously for NL10 centres in Al-doped Si and for TDDS. The identical EPR spectra of NL10 centres do not reveal structural differences in differently doped Si, for which our investigations have given some evidence.

We have made non-local measurements of a two-dimensional electron gas (2DEG) formed at the interface of a GaAs/Ga_xAl_1-xAs heterojunction. The magnetoresistance was measured on a standard Hall bar that had a gate placed between the current and voltage probes. We observed non-local resistance peaks at the location of the Shubnikov-de Haas oscillation maxima. The magnitude of these peaks depends on the gate voltage applied to the semiconductor between the current and voltage probes. We also observe 'Aharanov-Bohm'-like oscillations, presumably due to a localized impurity near the gate.

We consider electron dynamics in two-band semiconductor superlattices within the envelope function approximation. We develop a numerical method, based on the properties of the periodic continued fractions, to find the dispersion relation inside allowed minibands and to obtain the envelope functions. As an application, we concentrate on GaAs sawtooth-doped superlattices, consisting of periodic alternating n- and p-type delta -doped sheets separated by undoped material. Results are compared with one-band semiconductor predictions, and we find that coupling of bands in the host semiconductor indeed gives rise to relevant effects, especially for higher minibands.

Calculations of the optical absorption of short-period strain-symmetrized (Si)_n/(Ge)_10-n, superlattices, with n=3-7, are presented. The calculations are based on an empirical tight-binding model in the three-centre representation, which includes third-neighbour and spin-orbit interactions. These strained-layer superlattices (SLS) exhibit a direct gap. Their absorption coefficient, alpha ( omega ), is evaluated close to the bandgap, taking into account only direct transitions. The influence of the symmetry, the lattice constant in the growth plane and the composition of the SLS on the absorption coefficient is examined. It is found that among these materials the most appropriate for use in devices, such as receivers, is the strain-symmetrized (Si)₄/(Ge)₆ SLS.

We investigate the electrical properties of bonded wafers using deep-level transient spectroscopy (DLTS). A directly bonded silicon wafer and bonded silicon on insulator (SOI) wafers with different interfacial oxide thicknesses are evaluated. In a directly bonded wafer, one unstable trap and two traps (E_c-0.16 eV, E_c-0.24 eV) are detected. They exist within about 20 mu m of the bonded interface in both the active layers and the substrates. In the bonded SOI wafers, traps with the same activation energies as those in the directly bonded wafer are observed, and their densities depend on the thickness of the bonded interfacial oxide. The density of the trap with the energy level of E_c-0.16 eV increases with the interfacial oxide thickness, and the density of the trap with E_c-0.24 eV decreases.

The I-line (0.9650 eV), T-line (0.9351 eV) and several other luminescence centres created by annealing Czochralski silicon at 450 degrees C are all shown to incorporate one hydrogen atom, from observed deuterium-hydrogen isotope structure, and mainly one carbon atom deduced from carbon isotope structure. It is also demonstrated that hydrogen is a common contaminant in silicon and that it can be readily introduced by thermal treatments at 450 degrees C in the presence of water vapour or gaseous hydrogen. With increasing hydrogen content, the luminescence from the centres decreases, implying that if more than one hydrogen atom is captured by a centre it becomes optically inactive.

Samples of CZ silicon doped with zinc and annealed in the temperature range 500-700 degrees C produce intense photoluminescence at liquid helium temperatures. The majority of the luminescence occurs in two vibronic bands with zero-phonon lines at 1129.57(5) meV and 1090.47(5) meV denoted Zn_A and Zn_B respectively. In both cases, split ground and excited states are observed. Isotope substitution experiments reveal that one zinc atom is incorporated in each of the defects. The spectra do not occur for FZ silicon unless oxygen is deliberately introduced in addition to zinc. We conclude that the defects are complexes of zinc and oxygen atoms which are produced during the annealing process but which dissociate for anneal temperatures above 700 degrees C. Uniaxial stress and magnetic field perturbation data show that the Zn_A and Zn_B defects are very similar in their structural and electronic properties. The spectra are attributed to electron-hole recombination at Zn-O defects, but the details of the electron and hole configurations and the precise nature of the Zn-O defects have not yet been determined.

Time-resolved photoluminescence spectra of low-temperature MOCVD-grown layers of In_0.16Ga_0.84As and In_0.47Ga_0.53As have been measured with subpicosecond temporal resolution. The analysis of transients monitored at different photoluminescence energies allowed a separate estimation of both the electron and hole trapping times in these materials. For both layers the hole trapping times were about 34 ps, while the electron trapping times were equal to 18 and 60 ps for the layers of In_0.16Ga_0.84As and In_0.47Ga_0.53As respectively.

Although contact metallization causes stress in the underlying material, the magnitude of the resulting change in refractive index is often not appreciated. We investigate here the distribution of the change in refractive index due to stress caused by Ti/Pt/Au contact layers in weakly guiding InGaAsP waveguides operating near 1.3 mu m. Polarization-resolved photoluminescence and beam propagation calculations, combined with a simple stress model, are applicable to general processing-induced stress in InP-based waveguides and to waveguide design.

A lateral hot-electron phototransistor (LHEPT) utilizing intersubband absorption of radiation is proposed and considered theoretically. The principle of operation of the LHEPT is associated with two-dimensional or quasi-one-dimensional electron gas heating due to absorption of radiation in the base. The photoelectric performance of the LHEPT is evaluated using the proposed model. It is shown that the LHEPT exhibits high responsivity owing to high-efficiency near-ballistic hot-electron transport in the LHEPT base and weak electron energy relaxation at low temperatures. LHEPTs can be used for detection of infrared radiation as tunable photodetectors.

Experimental photoresponses and electrical characteristics of metal/ beta -FeSi₂/Si structures are presented. Three kinds of samples are compared: two with a thin epitaxial silicide layer (180 AA), prepared by two different methods, and one with a thick polycrystalline silicide layer (2500 AA). The rectifying behaviour and the photoelectric response of the three kinds of samples are different. In the thin samples these properties are governed by those of the beta -FeSi₂/Si interface, whereas for thick samples bulk mechanisms dominate. Analysis of the photocurrent in one kind of thin sample shows that two contributions exist. Their intensities follow similar temperature behaviours but the two transition thresholds do not. These considerations allow assignment of the initial and final states of the transitions, and the upper threshold is shown to correspond to an internal photoemission effect at the beta -FeSi₂/Si interface. The conduction band offset is deduced from the difference between the two thresholds. The valence band discontinuity is less than 50 meV between 360 K and 260 K, whereas it changes sign when the temperature decreases below 260 K, the two bandgaps becoming nested within each other. These properties are also discussed for the other kinds of sample and related to the mechanisms which are responsible for the electrical characteristics.

A range of electrical characterization techniques have been applied to thin film, thin buried oxide SIMOX SOI substrate. Some capacitance-voltage results were seen to exhibit frequency dispersion due to high concentrations of surface states but have been analysed to yield buried oxide thickness and doping levels. The ability to deplete from both interfaces of the buried oxide allowed us to determine both the density and the position of the positive charge centroid present in the oxides. The current-voltage characteristics were seen to be ohmic at low voltages (<50 V) but there is evidence that Fowler-Nordheim conduction is the dominant mechanism at higher voltages. Analysis of Fowler-Nordheim data yielded low barrier heights to electron emission in the range of 0.8 to 1.9 eV which were explained as the effect of field intensification at interfacial asperities. The I-V plots also indicated that high levels of electron trapping were occurring in the oxides and this was studied further using constant voltage stressing. The role in the trapping of excess Si and Si islands present in the implanted oxides is discussed at length.

The endurance of textured polysilicon floating gate (TPFG) EEPROM cells under repeated WRITE/ERASE cycles is studied as a function of the surface roughness. Electron tunnelling from non-planar surfaces is modelled for current injection from a single tip, and the corrugated surface is taken as a distribution of separated tips. A simple degradation model, based on charge trapping at the polysilicon/dielectric interface, is also introduced, and is able to lead to significant conclusions about the reliability of the memory cells.

We analyse an electro-optical device consisting of a stack of undoped alternating layers of narrow- and wide-gap materials (e.g. GaAs/AlGaAs) together with a series resistor under constant voltage bias. The device depends crucially on the photoconductivity of the narrow-gap layers with a width of the order of a carrier mean free path. The Franz-Keldysh effect, the charge separation of photoexcited carriers in a static field, and the ballistic part of the current are responsible for the resulting very large electro-optical nonlinearity. In a Fabry-Perot cavity a room temperature electro-optical bistability is obtained at light intensities smaller than 10 mW cm^-2.