Table of contents

This article reviews some recent results concerning the effects of isochronal annealing and the attempts to model the kinetics of local reconstruction mechanisms of layers with different degrees of inhomogeneity such as implanted unannealed and annealed silicon layers. The kinetics of the reconstruction mechanisms of the damaged layers and the annihilation of defects are studied via some electronic, optical and thermal-wave techniques. In order to determine the activation energy of the local annealing recovery mechanism the 'law of mass action' (LMA) has been used. This showed that the kinetics of annihilation of the damage layer are consistent with a local annealing process in which the observed variation of the electric, optical and thermal parameters could be described by a relaxation-type relationship. The activation energy of the recovery process of the ion implantation damage is found to be between 0.1 and 0.8 eV. On the other hand, the range of annealing temperature from 400 to 600 degrees C is the critical range where the layer reconstruction takes place.

A direct method for calculating local vibrational mode (LVM) frequencies from defect complexes in zinc blende structure semiconductors is discussed. Interatomic interactions are described by an extension of the Keating model. Sets of force constants or eleven pure host crystals, including homopolar and III-V semiconductors, are found by fitting to available phonon dispersion relations. Simulations of isolated impurities in III-V hosts are reported and, from these, some trends are deduced for the changes in force constants needed for reproduction of experimental measurements. The use of the method in the interpretation of experimental results from defect complexes is illustrated by detailed calculations of (i) isotopic fine structures of the C_As line and C_As-(As)_interstitial lines in GaAs and (ii) line patterns associated with carbon in Al_xGa_1-xAs.

GaAs samples doped with boron up to (B)=3*10¹⁸ cm^-3 have been analysed by local vibrational mode infrared absorption spectroscopy and by SIMS. A calibration has been established indicating that an integrated absorption of 1 cm^-2 in the LVM line at 517 cm^-1 due to ¹¹B_Ga corresponds to 11.0+or-0.5*10¹⁶ boron atoms per cm³. This calibration agrees with and extends a recent calibration proposed by Alt and Maier.

The formation of CoSi₂ from an amorphous Co₇₀Ti₃₀ alloy film on Si(100) has been studied by Auger depth profiling, Rutherford backscattering spectrometry, cross-section transmission electron spectroscopy and X-ray diffraction. The solid-state reaction starts around 400 degrees C with the diffusion of Co out of the alloy into the substrate, forming an interface layer consisting of CoSi and CoSi₂. The simultaneous diffusion of Si into the Co-depleted region of the film is limited for annealing temperatures below 600 degrees C. After removing the remaining top layer by selective etching, an extra heat treatment is performed to transform the mixed silicide layer completely into CoSi₂. The latter is found to be partially aligned with the substrate. The amount of epitaxial CoSi₂ increases with the use of higher post-anneal temperatures.

Epitaxial films of CaF₂ have been grown on Si(100) and Si(111). The substrate temperatures required for epitaxy, determined by Rutherford backscattering of 340 keV protons, are compared with those previously reported, as are the results of examination of the surface morphologies by scanning electron microscopy. Initial investigations of the electrical properties of the insulating films are also discussed.

Tungsten (W) films were deposited onto InP in a cold wall, rapid thermal low-pressure chemical vapour deposition (RT-LPCVD) reactor, from a tungsten hexafluoride (WF₆) gas reduced by hydrogen (H₂). W films of thickness 50-450 nm were deposited in the temperature range 350-550 degrees C, pressure range 0.5-4.5 Torr at deposition rates up to 4 nm s^-1 with an apparent activation energy of about 1.12 eV. The film stress varied depending upon the deposition pressure, from low compressive for deposition at 0.5 Torr to moderate tensile for deposition at about 4.5 Torr. The films were aged at temperatures as high as 300 degrees C for about 800 h and exhibited an excellent mechanical stability. Post-deposition sintering of the W films at temperatures up to 600 degrees C led to reduction of the resistivity with a minimum value of about 55 mu Omega cm as a result of heating at 500 degrees C. Conditions for both selective and blanket deposition were defined, and a dry etching process for further geometrical definitions of the films was developed, providing etch rates of 40-50 nm min^-1. This report reflects the first attempt to deposit W films onto III-V semiconductor at a very high rate by means of RT-LPCVD.

Transitions at energies of 1.31 eV and 1.41 eV have been observed in Zn₃P₂ at room temperature (T=295 K) by using surface photovoltage (SPV) spectroscopy. Results obtained from absorption and photoconductivity measurements are also discussed here. The polarity of the PSV signal has been analysed. Two acceptor levels were found to be at 0.29 eV and 0.19 eV above the valence band.

Auger recombination (AR) in quantum well (QW) semiconductors is re-investigated taking into account a realistic band structure instead of the usual parabolic approximation. Since direct AR is negligible in QWS phonon-assisted AR is treated. The theoretical results are in good agreement with experimental investigations. In contrast, the description of QWS by a reduced heavy-hole mass is insufficient. The influence of AR on the temperature dependence of semiconductor lasers is discussed.

A new method of determining generation lifetime is suggested. By reading out several values of high-frequency MOS capacitance with equal time intervals from a capacitance-time transient curve of a pulsed MOS capacitor, the generation lifetime of minority carriers can be determined. This method involves straight line regression and is suitable for computer-aided measurement.

Phonon-drag thermopower is calculated in a semiconductor superlattice with temperature gradient parallel and perpendicular to the superlattice axis. Umklapp processes associated with the superlattice periodicity are considered. Numerical results are presented for parameters characteristic of a GaAs/GaAlAs superlattice. Effect of inclusion of umklapp processes is to suppress the magnitude of thermopower parallel to the superlattice axis and to change its sign for Fermi energies near the top of the miniband. The behaviour of thermopower in the plane of the layer is found to be similar to that of a two-dimensional electron gas.

A study of circular flat samples in connection with the van der Pauw and Wasscher methods and the magnetoresistance skewness effect for galvanomagnetic measurements is reported. The results concern the position of the four-contact system and the orientation of the magnetic field with respect to the principal resistivity directions on the sample plane. As is shown, the necessary information is provided by the sample behaviour. Furthermore, in the case of isotropic material, a simple formula has been found, from which the zero-field resistivity can be determined by a single measurement.

The carrier dynamics in highly excited In_1-xGa_xAs/InP/In_1-xGa_xAs_yP_1-y/InP structures, with x=0.47 and y=0.9, have been investigated by the picosecond dynamic grating technique. The carrier recombination time tau _R and the ambipolar diffusion coefficient D_a at a non-equilibrium carrier density of about 10¹⁹ cm^-3 have been determined for layers of InGaAs, InP and InGaAsP on InP substrates. The influence of many-body effects and reabsorption of emitted photons on the carrier diffusion at high excitation levels are discussed.

Persistent photoconductivity (PPC) has been observed in Ga_xIn_1-xP/InP:Fe (0<or=x<0.18) in a large temperature range from room temperature down to 77 K. The relationships between PPC and sample structure, material parameters, temperature and excitation time have been experimentally investigated. The results have been explained by the assumption that a macroscopic potential barrier between the film and the substrate separates the charge and thus delays recombination.

Electrical properties of heterojunctions consisting of amorphous silicon carbide (a-SiC) on p-type crystalline silicon (c-Si) have been investigated by measuring their current-voltage (I-V) and capacitance-voltage (C-V) characteristics, as well as the temperature dependence of their current (I). The I-V characteristics of a-SiC/c-Si(p) heterojunctions exhibit a low turn-on voltage and a high reverse breakdown voltage (about 140 V). It was found that a model of an abrupt anisotype crystalline heterojunction is valid for the a-SiC/c-Si(p) structure. This model, for values of forward-bias voltage lower than a critical value V_T, predicts that at low temperatures (T<250 K) recombination is the dominant transport mechanism, whereas at high temperatures (T>250 K) recombination-diffusion becomes the dominant transport mechanism. For higher voltages (V>V_T), the tunnelling process dominates in the whole temperature range studied. From I-V measurements of a-SiC/c-Si(p) heterojunctions it was concluded that the conductivity of undoped a-SiC is n-type. Finally, from C-V measurements, the value of electron affinity of a-SiC was obtained, for the first time, and it was found to be chi ₁=4.12+or-0.04 eV.

Measurements of the degree of polarization of the luminescence as a function of energy give information about the relaxation of the magnetic moments of the photoexcited carriers. A study of this effect in undoped and modulation-doped samples and samples that are modulation n-doped with additional acceptors, in magnetic fields both perpendicular and parallel to the layers, allows the different contributions of hole and electron relaxation to be analysed. The experiments show that, contrary to bulk material, the relaxation time for the hole magnetic moment must be taken into account in explaining the polarization behaviour. This conclusion is substantiated by the observation that in undoped samples a very high field (10 T) parallel to the layers and perpendicular to the light propagation (Hanle configuration) is needed to depolarize the luminescence.

A variety of magnetotransport measurements have been performed on InSb-CdTe heterojunctions containing high mobility ( approximately 20000 cm² V^-1 s^-1) two-dimensional electron gases (2DEGS) with low area carrier densities (N_s approximately 1.8-4.2*10¹¹ cm^-2). The experimental data have been compared with self-consistent calculations of the energy levels carried out using the 'three-band' model of narrow-gap semiconductors. The 2DEGS in the samples exhibit well resolved quantum Hall plateaux and Shubnikov-de Haas oscillations at temperatures below 5 K; using these data and the persistent photoconductivity (PPC) effect, the subband occupancies have been evaluated as functions of N_s. Parallel-field magnetoresistance measurements show that only two subbands are occupied, even at the highest carrier densities, and indicate the importance of intersubband scattering. The self-consistent calculations predict the number of occupied subbands correctly, and are in agreement with the experimental subband occupancies when the effects of a small amount of interdiffusion are taken into account. The low-field magnetoresistance of the heterojunctions is at first positive and then negative; this is characteristic of weak localization in the presence of spin-orbit scattering.

GaAs epitaxial layers grown by molecular beam epitaxy (MBE) at low temperature (LT) have been characterized by electron spin resonance (ESR) infrared absorption, magnetic circular dichroism (MCD) and optically detected magnetic resonance (ODMR) via MCD. The data prove that As_Ga antisite defects contribute to the below-gap infrared absorption of LT MBE GaAs layers. Furthermore it is found that ESR-spectra of LT GaAs epitaxial layers are very similar to those observed in fast neutron irradiated bulk GaAs crystals.

Resonant enhancement of LO and TO phonon Raman scattering is observed in GaAs when the energy of the exciting light h(cross) omega _p coincides with a donor-to-acceptor transition. Strong additional Raman satellites arise on the high-energy side of the resonance curve (1.495<h(cross) omega _p<1.510 eV). These satellites are ascribed to localized phonons associated with specific donor-acceptor pairs. This interpretation is substantiated by the observation that their Raman shifts increase with decreasing donor-acceptor separation R, according to Delta E varies as exp(-2R/a_D).

Polycrystalline films of ZnS with a slight excess of S have been grown on (100) Si by congruent sublimation from a single Knudsen cell. Intense blue emission is observed at 460 nm from room-temperature photoluminescence studies, and is ascribed to S-Zn vacancies acting as self-activated (SA) centres. The emission is quenched by the addition of Zn from a second Knudsen cell, which also causes the growth of a luminescent peak centred on 678 nm. No reduction in the SA luminescence is seen when co-sublimating ZnS and AgS, implying that it is not possible to remove the S-vacancy completely. Quenching of the blue emission occurs when Mn, instead of Zn, is added to the ZnS films, providing direct evidence of Mn incorporation into Zn vacancies. At the optimum Mn concentration, the intensity of the Mn emission at 580 nm is comparable to the blue emission (460 nm) from the undoped ZnS films.

The authors report an investigation of the exciton photoluminescence (PL) spectra of high-purity nominally n-type GaAs and InP layers, grown by metalorganic vapour phase epitaxy. The results are discussed in terms of exciton-polariton kinetics. The ratio of the integrated emission intensities of neutral donor-bound exciton (D⁰, X) transitions and free exciton-polariton (FX) transitions r_DF=I(D⁰, X)/I(FX) shows a sublinear dependence upon neutral donor concentration n_D⁰ deduced from 77 K Hall measurements. This relationship is particularly useful for estimating carrier concentrations in high-purity materials, which cannot be characterized by Hall measurements due to depletion.

The optical properties of ZnSe bulk crystals grown by the seeded physical vapour phase transport technique (SPVT) were studied by low-temperature photoluminescence and room-temperature stimulated emission measurements. Large strain-free ZnSe single crystals with a diameter as large as 5 cm have been grown uniformly without twinning by the SPVT technique. A variety of ZnSe samples, the as-grown and post-growth treated as well as intentionally doped crystals, were prepared in this study. Low-temperature photoluminescence measurements showed that near band edge I₁^d line and its LO phonon replicas dominated the spectra, and that donor-acceptor pair emission signatures were absent for SPVT as-grown samples; the intensity of the I₁^d, line decreased considerably and the I₃ line became predominant after Zn extraction treatment. The pressure dependence on the bound exciton emission lines suggested that the I₃ line might originate from excitons bound to a donor-like deep centre rather than to ionized shallow donors. The room-temperature optically pumped stimulated emission and blue lasing from the as-grown samples demonstrated that SPVT ZnSe single crystals have the quality sufficient to develop low-threshold, high-power output blue lasers.

The experimental results of hot electron energy and momentum relaxation rates in GaAs/Al_xGa_1-xAs multiple quantum wells, grown by molecular beam epitaxy (MBE) are presented. The energy relaxation rates are measured at lattice temperatures of T=4.2 K and T=77 K and up to a maximum electron temperature of T_e=270 K. The results are compared with a theoretical model involving non-equilibrium interface phonon production. The experimental energy relaxation time of tau _eff approximately=0.55 ps is shown to be in disagreement with the calculations. This discrepancy is attributed to the uncertainty in the hot phonon lifetime. The authors also present the results of high field, parallel transport, drift velocity measurements. The results indicate a drift velocity saturation at V_d=7*10⁶ cm s^-1 at 300 K and V_d=1.5*10⁷ cm s^-1 at 77 K. The saturation is followed by current instabilities.

A theory of two-photon magnetoabsorption has been given for GaAs/GaAlAs quantum wells for circularly polarized radiation. Numerical results are given for the specimen of Frohlich et al. (1988 Phys. Rev. Lett. vol.61 p.1878). A good fit is obtained between experimental and calculated values.

An InGaP etch-stop layer was used to control the mesa height in a ridge waveguide InGaAs/AlGaAs strained multiple quantum well laser grown by organometallic vapour phase epitaxy. These 950 nm lasers exhibited internal quantum efficiencies of eta _i=0.99 and very low internal losses of alpha _i=2.6 cm^-1. For a cavity length of 500 mu m, a threshold current of 12 mA was obtained. The high performance of these lasers and the improved uniformity of their characteristics demonstrate the utility of InGaP etch-stop layers in improving process yield without detrimental effects.