Table of contents

Recent advances in the area of ZnSe-based blue-green laser diodes are reviewed. We will discuss the solutions to some long-standing problems which for many years stood in the way of successful development of these devices, such as the choice of the quantum well structure for the active region, doping of ZnSe and ohmic contacts. We also address the problems that are responsible for the still very short lifetimes of blue-green laser diodes at the present stage of development, which still need to be overcome before these devices can be applied in practical situations.

The properties of lattice defects in the tetrahedral Il-VI compounds, as revealed by optical and electron spin resonance spectroscopy, are reviewed. Particular attention is given to the A-centre, namely a complex of a cation vacancy and a shallow donor, as a model system. Cation and anion vacancies are treated but information about interstitials is scarce. Some of the older data are reinterpreted. The amount of firmly based information is small but is sufficient for tentative general ideas to be put forward.

This paper is devoted to the discussion of inelastic scattering of a high-energy conduction electron incident on an impurity in a semiconductor. On the one hand the rate of inelastic collisions of a hot electron samples the high-energy tail of the electron distribution, and thus may serve as a test of high-field transport theories. On the other hand the excited impurity may release its energy radiatively, and this is used in solid-state displays. We adopt a description of the impurity which emphasizes the structure of the excitable shell in a solid-state environment. Then we describe the direct and exchange paths between the initial and the final state, which we relate to spin selection rules. The scattering amplitudes are calculated to first order, with emphasis on the functional structure of each term. A crude computation of impact cross sections near the excitation threshold is performed, giving a rough magnitude of 10^-18 cm² which may be enhanced by exchange effects for a valence shell containing many electrons. More importantly, taking three examples we qualitatively discuss the factors affecting the cross section according to the structure of the impurity valence shell, and its location in the host lattice. The conclusions are of direct interest to device designers wanting to optimize the impact yield of hot-electron electroluminescence devices.

Two-dimensional structures containing the electron wall (the barrier transparent to phonons and impenetrable by electrons) and (or) the phonon wall (the barrier transparent to electrons and impenetrable by phonons) are proposed. The intrasubband (1 to 1) and intersubband (2 to 1) electron-polar optical phonon scattering rates are calculated for the quantum well divided by an electron wall or a phonon wall. Independent electron and phonon confinement caused by the electron and phonon walls is taken into account, and a radical change of the scattering rate is obtained. The electron wall increases, and the phonon wall decreases, the confined electron-confined optical phonon scattering rate by several times.

We have grown n-channel Si:SiGe quantum wells using gas source molecular beam epitaxy. The two-dimensional electron gas confined in the strained Si quantum well is weakly disordered and displays quantum interference in the form of a pronounced, low-field negative magnetoresistance. By fitting the data to the standard theory of weak localization, in the temperature range 50 mK to 6 K, we have been able to extract the product N_v alpha and the electron phase coherence length L_phi . Our results give a value for the product N_v alpha in the range 0.6+or-0.12, which is close to that expected for strong intervalley scattering and is similar to values obtained in Si MOSFETs. We show a power law dependence of L_phi with temperature, i.e. L_phi varies as T^-p2/. For T>0.7 K the value of the exponent p/2=0.75 which suggests that the dominant dephasing mechanism is electron-phonon scattering as observed in Si MOSFETs. However, for lower temperatures the variation is less pronounced (p/2=0.15). The weaker temperature dependence may be a precursor to a saturation in the value of L_phi as has recently been observed by other groups.

A theoretical study has been made of the interaction of longitudinal coherent bulk acoustic waves at 9.4 GHz with a 2DEG at a GaAs/AlGAs heterojunction in a strong magnetic field. A generalized transfer matrix formalism was used to consider such a dynamic interaction in a piezoelectric medium and hence to analyse the mode composition of the elastic fields in the layered dissipative structure. It is shown that although the incident wave is longitudinal, the main contribution to the signal detected by a CdS bolometer may come from the interference of the transverse acoustic wave forward scattered from the 2DEG and the transverse wave entering the bolometer after refraction and mode conversion of the primary beam at the Al/GaAs interface.

The effect of an inversion layer on the current-voltage characteristics of semiconductor grain boundaries is evaluated on the basis of a recently developed inversion layer and generalized interface state model. It is found that the characteristics are considerably different from those obtained under a depletion model. Initially, the logarithmic current density varies nonlinearly with voltage and then becomes linear after a critical value of the bias voltage which is a function of interface and bulk parameters. Such nonlinear variation of the logarithmic current density is consistent with the experimental results for a silicon bicrystal.

Optical measurements in the 8-14 mu m band on asymmetrically slab-doped InSb and InAs doping superlattices (a-nipis) at 10 K showed strong intersubband absorption features. The observed subband spectra are compared with a full self-consistent wavefunction calculation including the effects of non-parabolicity and the 'depolarization' screening of the optical transition. Absorption spectra are modelled with a multilayer calculation in which the well is taken to be an anisotropic oscillator and good qualitative agreement with the experimental data is found.

In this study the analysis of reflection anisotropy signals reveals that an unusual behaviour of the phase signal may occur in the presence of weakly anisotropic surfaces. It will be shown that the measured lineshape has to be considered with extreme care if a single-phase lock-in amplifier is used in the experimental set-up. It not only depends on the sample properties but is also influenced by the experimental conditions, in particular the signal phase, which is usually adjusted for maximum signal before spectra are taken and then fixed during measurement. For this purpose the reflection anisotropy spectra are compared with those of the phase for various semiconductor systems. Ideally one would expect the phase signal to change by 180 degrees whenever the reflection anisotropy signal changes sign. On the contrary it was found that the phase spectra, e.g. from the cubic semiconductor GaSb(100), exhibit continuous phase shifts near the zeros in the reflection anisotropy spectra when measuring the signal at twice the modulation frequency. The reflection anisotropy spectra may therefore be significantly different if measured with distinct phases. Theoretical considerations and further experiments reveal that this effect is likely to be caused by partial polarization, which is hard to avoid in any experimental set-up.

High quality epitaxial layers are found to behave like ordinary Fabry-Perot etalons and show typical interference effects in the broad band photoluminescence. We investigated these effects in a series of MOVPE-grown ZnSe/GaAs samples and present a quantitative analysis. This allows us to estimate the layer thickness and demonstrates that the interfaces are smooth on the scale of an optical wavelength.

We report comprehensive photoluminescence, Raman scattering and photoluminescence excitation data on GaN layers grown by MBE on GaAs and GaP substrates. The main photoluminescence feature of GaN layers corresponds to the recombination of free carriers from conduction and valence bands. It is supported by both photoluminescence and photoluminescence excitation spectroscopic data. The origin of impurity-assisted photoluminescence is discussed. We also report the Raman spectra on cubic GaN films.

The stability of In_0.75Ga_0.25P and GaSb_0.75P_0.25 random alloys with respect to decomposition and superstructure formation is discussed using a thermodynamic analysis in which the energy of elastic strain caused by mixing is considered as the enthalpy of mixing. We derive the stable, metastable and unstable regions for the random, decomposed and ordered states of these alloys. The possibility of ordering in the GaSb_0.75P_0.25 random alloy below 800 K is predicted.

It has been shown that the low-frequency 1/f^1.5 noise in GaAs and SiC, and occasionally in Si, exists in the temperature region where a generation-recombination (GR) noise predominates that is due to a local level. The 1/f^1.5 noise can be accounted for by energy level broadening, which inevitably occurs in any real crystal as a result of structural or doping inhomogeneities and defects. GR noise simulations were performed for GaAs and SiC, taking into account the effect of broadening. It has been shown that, to explain the experimental results, consideration must be given to the exponential energy dependence of the capture cross section sigma _n= sigma ₀ exp(-E₁/kT) and the dependence of the energy E₁ on the level position E₀. The simulated and experimental results are in good agreement.

We have investigated in detail the quasibound states of a quantum channel with a multiple double-bend discontinuity. We assume that our quantum system is of finite length and connected to two 2DEG reservoirs. The mode-matching technique has been employed. We have predicted that there are 2N resonant peaks of the conductance below the first transverse-mode energy for the quantum channel with an N double-bend discontinuity, which means the formation of one-electron miniband structure below the first transverse-mode energy for a lateral superlattice consisting of finite periods of double-bend structure. Our results clearly indicate that the 2N quasibound states of the channel with energies below the first transverse-mode energy are 2N-fold splitting quasibound states due to the coupling between the local bound states. Our analytical results from the nearest interactive model and our numerical results are qualitatively well consistent.

We explore the possibility of far-infrared and terahertz photovoltaic detection based on a quantum ballistic channel with an abrupt narrow end. By calculating the photoinduced intersubband transition in the channel and the transfer efficiency of electrons over the narrow end, we propose an optimized split-gate structure with maximum electron transfer efficiency. The detector responsivity, operating temperature for background-limited performance and noise equivalent power are evaluated. These detectors may be used in applications such as spectroscopy, infrared astronomy and space and environmental sensing.

The observation of a pronounced double peak structure in the equivalent parallel conductance of a metal-insulator-semiconductor (MIS) device prepared on a Bridgman-grown x approximately=0.2 p-type Hg_1-xCd_xTe crystal passivated by an anodic oxide layer is reported. The first peak, placed in the depletion part of the conductance-voltage (G-V) curves, has been attributed to the response of single-level interface traps distributed over the semiconductor gap supposing band-bending fluctuations. The second peak, placed at the onset of strong inversion, is due to shunting of the minority carrier flow by the electrons coming from the inverted surface area beyond the gate. An order of magnitude enhancement of the structure measured on capacitance-voltage (C-V) and G-V curves below T approximately=10 K has been attributed to hole freezeout, leading to a substantially slower sample response and to an exponential increase of its series resistance. The tunnelling from acceptor states near the Fermi level to interface traps and/or to light hole states in the valence band becomes a competing mechanism to majority carrier response in the depletion and accumulation regions in that case.

A measurement method based on the electron beam induced current (EBIC) phenomenon is described; it gives the lateral dopant diffusion in planer devices and allows an estimation of the junction depth. The method is particularly well suited for deep junctions and does not require any cleavage or removal of the field oxide on top of the structure. It is a quick and convenient method for device failure analysis or device reverse-engineering. The method has been checked with a numeric simulation and measurements on two bipolar transistors.

The high-frequency response of an in-plane gate transistor has been analysed by time-domain transmission measurements using a 20 GHz sampling oscilloscope. Application of a step pulse with 20 ps rise time to the gate of the transistor results in a drain-source current with a rise time of approximately 50 ps. Comparison with the estimated carrier transit time through the channel and with the RC time constant of the device reveals the dominant influence of the parasitic capacitance and the channel resistance on the measured response time.

Recently it has been observed that indium oxide/p-lnP and indium oxide/n-GaAs heterojunctions, where the indium oxide is prepared at lower reaction temperatures (<100 degrees C), have degraded electrical parameters. This has been attributed to the clustering of indium atoms at the interface. It is shown that rapid thermal annealing (600 degrees C, 20 s) improves electrical parameters of the contacts formed at lower reaction temperatures. The oxidation of GaAs and InP at lower reaction temperature and dissolution of oxidation products at higher annealing temperature and further oxidation of unoxidized indium atoms with the released oxygen atoms are used to explain the observations.

The effects of rapid thermal annealing on the electrical characteristics of Ni and Pd contacts on n-InP are investigated. Results show that annealing at temperatures up to 450 degrees C and for durations up to 100 s has little effect on the electrical parameters of these contacts. Contacts annealed at 600 degrees C showed some degradation. Contacts annealed at 450 degrees C and 600 degrees C for 100 s showed an aging effect. Different reaction processes at the interface and formation of an InP surface layer with no stoichiometry are used to explain the observations.

SbSBr, BiSBr, SbSBr:Co and BiSBr:Co single crystals were grown by the Bridgman technique. The grown single crystals crystallize in an orthorhombic structure and have an indirect band structure. The temperature dependence of the optical energy gap for the single crystals shows anomalies at the first and second-order phase transition points. Cobalt dopants are located at the T_d symmetry point in the host single crystals as Co²⁺ and Co³⁺ ions. Impurity optical absorption peaks due to the electron transitions between the energy levels of the Co²⁺ and Co³⁺ ions are observed.

Expressions for the field distribution in novel clamped or free-standing piezoelectric strained InGaAs/GaAs superlattices are given. The voltage for miniband formation can be selected by incorporating these structures into p-i-n diodes, without requiring strain compensation between wells and barriers. This feature is evidenced by the photocurrent spectra and photocapacitance versus voltage characteristics obtained from high quality 40-period (111)-oriented superlattices. Wannier-Stark localization and resonant transport of carriers have been observed in the piezoelectric superlattices. The In concentration, thickness of the superlattice period and strain relaxation are deduced from X-ray measurements.

Arsenic-rich InAs/lnAs_1-xSb_x strained layer superlattices (SLSs) grown on GaAs substrates by molecular beam epitaxy (MBE) are studied for their potential application as infrared emitters. The long-wavelength emission (4-11 mu m) is highly sensitive to superlattice design parameters and is accounted for by a large type-II band offset, greater than in previously studied antimony-rich InSb/lnAs_1-xSb_x SLSs. High internal PL efficiencies (>10%) and intense luminescence emission were observed at these long wavelengths despite large dislocation densities. Initial unoptimized InAs/lnAs_1-xSb_x SLS light emitting diodes gave approximately=200 nW of lambda =5 mu m emission at 300 K.

Polarization bistability with hysteresis in 1.3 mu m InGaAsP/lnP ridge waveguide bulk lasers has been achieved at room temperature by a built-in tensile strain with (i) temperature variation and (ii), for the first time, application of an external force. In both cases, four types of emission characteristics are observed: pure TE emission, TE/TM bistability, smooth TE/TM transition and pure TM radiation. It is demonstrated that the hysteresis width can be controlled by both temperature and strain, which opens the possibility to fabricate laser diodes with different switching properties. The physical mechanism responsible for the observed behaviour is discussed.