Table of contents

Properties of well known (e.g. III-V) and less well known (e.g. II-IV-V₂) semiconductors are reviewed, with emphasis on the optical nonlinearity of the lesser known materials. Diamond-derived structures are discussed. Possible techniques for assessing the usefulness of new materials, and for crystal growth are also discussed.

¹⁸O(200 keV, 2.0*10¹⁷ cm^-2) has been implanted at 6 K into an 'as implanted' SIMOX wafer (silicon implanted with ¹⁶O at 200 keV, 1.8*10¹⁸ cm^-2 and 500 degrees C). In situ Rutherford backscattering measurements at 6 K, 77 K and room temperature indicate approximately 15% excess oxygen above the stoichiometric value for SiO₂ in the buried-oxide layer. Additional room-temperature measurements were carried out off-line by infrared and secondary ion mass spectrometries. A comparison of the original SIMOX material with the sample implanted with ¹⁸O at 6 K leads to the conclusion that during warm up the ¹⁸O concentration depth profile remains essentially unchanged with the maximum total concentration remaining above the original ¹⁶O value. The infrared measurements indicate that all of the oxygen atoms are bound to silicon atoms.

The construction and operation of an instrument is described that measures excess-carrier lifetimes (1 mu s to 400 mu s) in semiconductors by generating electron-hole pairs with a pulsed flashlamp and contactless probing of the photoconductivity decay with microwaves. This instrument was especially designed for measurements in the thin top layer of SIMOX structures. For this purpose UV radiation is required with a short penetration depth. This made the construction of a special UV filter necessary that transmits below 400 nm and blocks all longer wavelength radiation up to the silicon band gap wavelength.

The differential mode of the grazing-incidence diffraction is employed for the characterization of a thin Ga_1-xAl_xAs/GaAs (001) heterostructure. Using a high resolution detector system a conventional X-ray source is appropriate to record the fine structure of the diffraction curve. The quality of experimental curves permits the authors to detect the thickness of both the nondiffracting top part and the perfect crystalline portion of the layer. Their quantitative determination results from simulation of the curve by means of the dynamical and the kinematic theory. Both yield nearly the same crystalline thickness; however, they differ somewhat for the top part. A quantitative interpretation of the real structure of sample is impossible due to the limited experimental resolution.

The mechanical stress in RF-sputtered Al₂O₃ films on silicon substrates has been investigated under different deposition conditions by measuring the net radius of curvature of the bent silicon wafer. At the beginning of the deposition, remarkably high tensile stresses are found within the film. As the film thickness increases, the stress converts to compressive and reaches further a saturation about -3.5*10⁸ N m^-2. An interpretation is proposed to explain both the film thickness and power density dependences of the stress. Structural changes are believed to be responsible for the observed stress transition. Infrared transmission spectra and refractive index measurements are also performed by layer characterization.

The authors show that the electric field strongly influences the electron emission from the effective-mass, metastable, excited states of DX centres in Al_0.55Ga_0.45As, while up to the field of 4.5*10⁵ V cm^-1 no influence on the emission from the group, localized state of DX centres has been detected. Such behaviour is inconsistent with the small lattice relaxation models of the DX centre and can be understood if the DX centres are bistable donors and/or of the negative-U type.

An AC surface photovoltage (SPV) is observed in n-type silicon (Si) wafers treated with RCA solutions (alkaline and acid rinse) and aqueous hydrofluoric acid (HF) solution. The AC SPV increases with exposure time in air at room temperature, indicating that the negative charge increases. Aluminium (Al) in the native oxide contributes to this negative charge. The AC SPV saturates after (2-6)*10³ min (1-4 days) and the saturation value depends on the concentration of Al, provided that the wafer surface is not strongly inverted. After HF dipping, relatively low AC SPVS appear after 20 days. This is because it takes time not only for the negative charge to arise at the surface. but also for the positive charge to decrease, as has been reported in p-type Si wafers.

The influence of the subband-Landau level coupling resulting from the tilted configuration of the valleys on the intraband magneto-optical transitions in PbTe and Si parabolic quantum wells is discussed. The calculations are performed in the framework of the one-band effective-mass approximation for the case of normal magnetic field. in the range of the strong field the absorption spectrum consists of two hybridized cyclotron-intersubband modes. It has features similar to that observed for the case when the magnetic field but not the valley is tilted.

Pd/n-GaAs Schottky diodes were fabricated by using either spin-on or evaporated Pd sources. Initial electrical characteristics of both types of Schottky diode revealed little difference in their respective diode parameters. Capacitance-voltage (C-V) measurements gave a barrier height of 0.85+or-0.01 eV for spin-on Pd diodes as compared with a barrier height of 0.88+or-0.01 eV for evaporated Pd devices. The ideality factor for both diodes was found to be close to unity. Elevated temperature aging studies at temperatures up to 200 degrees C for 1000 h, produced little or no change in diode parameters. On the other hand annealing at 250 degrees C for times up to 1000 h resulted in a gradual degradation of diode parameters. This degradation in diode parameters was significantly more rapid during a 400 degrees C anneal. Using a failure criterion of a sixfold increase in reverse saturation current J_s^riv, an Arrhenius plot was constructed by plotting log time to failure versus 1000/T, where T is the aging temperature. The resulting plots were linear indicating a single failure mechanism being dominant at all temperatures. The room temperature (300 K) median life, for a sixfold increase in J_s^riv, of Pd/n-GaAs Schottky diodes from spin-on and evaporated Pd sources was found to be 10⁵ h and 4*10³ h, respectively.

A theoretical investigation of double-heterojunction bipolar transistors (DHBTs) based on the layered N⁺-N-n-N-N⁺ heterostructures in the III-V material system with acceptor delta-layer near an emitter N-n interface has been undertaken in an attempt to determine the peculiarities of their current-voltage characteristics. It has been shown that intervalley scattering in narrow-gap n-type base layers results in negative difference conductance.

A realistic and accurate computer method for high frequency numerical analysis under small signal conditions of a double avalanche region (DAR) (n⁺p nu np⁺) IMPATT diode is described. The method is used to determine microwave properties of several silicon DAR diodes under various operating conditions. The results indicate the existence of discrete and widely separated negative conductance bands in the DAR diode which can provide selective tuning in addition to wide frequency coverage for the device. The magnitude of DAR negative conductance, which is found to be smaller by an order of magnitude than that for the corresponding double drift diode, can be enhanced by the introduction of suitable asymmetry in the doping concentration of n and p of the DAR diode structure. The common drift layer for both types of charge carriers in the case of a DAR diode would result in space charge wave cancellation which may make it possible to push the input current to a high value and reduce the noise generation. Thus, a DAR diode may provide an appreciable amount of microwave power with low noise level over several frequency bands covering a wide frequency range.

A Monte Carlo simulation of electron transport in a GaAs/AlAs superlattice is presented. The simulation is based on realistic superlattice band structure which is calculated by a method which matches complex empirical pseudopotential bulk band structure at the heterointerfaces. The structure considered here is a (GaAs)₁₂/(AlAs)₁₂ superlattice in which significant Gamma -X mixing is observed. The pseudopotential band structure method has been used to obtain the wavevector locations and effective masses of the six lowest sets of conduction band valleys. These valleys were then used as the basis of ensemble Monte Carlo simulations of electron transport and relaxation in the superlattice under the action of phonon scattering. The drift velocity of electrons in the superlattice as a function of electric field applied parallel to the layers is predicted using the simulation and compared with bulk GaAs. The drift velocity exhibits a much broader and lower maximum than bulk GaAs and the negative differential mobility at higher fields is also much weaker. Simulations have also been carried out to study the thermalization of electrons which are initially in excited states of the system. The results show that the rates at which the initial valley empties and the ground valley becomes occupied are strongly dependent on the initial valley.

The authors present the first realistic account of the electronic structure and optical properties of the finite-length (five-period) Si-Ge superlattice grown on a Ge(001) substrate. Firstly, their theory gives a fresh account of the strong optical transitions reported in recent experiments and reveals several novel features concerning the localization of superlattice states. Secondly, they show that the wavefunctions are strongly modulated by reflections whose characteristic length is that of the five-well structure. Thirdly they demonstrate that the states originating from the Ge buffer form localized resonances resulting from strain-induced momentum mixing at the interface.

The shallow electron traps introduced by As implantation in thermal SiO₂ were investigated using the avalanche injection technique. In oxygen-deficient oxides the trapping behaviour was dominated by a shallow trap with a capture cross section of 1*10^-15 cm². The supply of oxygen leads to a decrease in the density of this trap and in a fully oxidized network it is completely annihilated. In the latter case all As atoms are incorporated in the SiO₂ network as deep electron traps.

The magneto-optical properties of Fe³⁺ have been measured in InP, GaAs and GaP. In these semiconductors both photoionization and excitonic transitions were detected and unambiguously assigned to the neutral charge state of the iron acceptor (A⁰:Fe³⁺:3d⁵). In all three crystals, many sharp lines were measured which had not been detected previously. They could be assigned to phonon replicas of the excitonic transitions. In addition, the magnetic circular dichroism of the optical absorption (MCDA) of semi-insulating InP:Fe samples revealed a hitherto unknown diamagnetic defect. The optical transitions of this defect are superimposed on those of Fe³⁺. Therefore, a reliable measurement of the Fe³⁺ concentration with conventional optical absorption is difficult. Fe³⁺ is distributed nearly homogeneously across a semi-insulating InP:Fe water. The optical detection of the electron spin resonance (ODESR) lines of Fe³⁺ in InP show an unusual intensity behaviour due to fast spin lattice relaxations connecting states with Delta m_s=+or-2.

Natural and photoinduced gyrotropy (optical activity) in chalcogenide vitreous semiconductor films under interband excitation has been discovered and studied. This phenomenon arises together with the photoinduced anisotropy, and both of them lead to rotation of the polarization plane of light, exciting interband transitions. The mechanism of these phenomena is referred to the interaction of polarized light with gyrotropic and anisotropic 'native' defects in the films.

Spatially resolved photoluminescence measurements with a resolution of 4 mu m, have been carried out in the active p-region of n-p GaAlAs light-emitting single heterojunctions, having undergone 2000 h strong current ageing. The degraded material exhibits a characteristic pattern of dark-line defects (DLDS) well revealed in cathodoluminescence topographs and identified as dense dislocation networks in easy-glide crystallographic planes. When the scanning spot goes from undergraded to degraded zones containing DLDS, the dominant near-bandgap photoluminescence peak due to donor-to-acceptor recombination is gradually replaced by an initially weak satellite, 15 meV above, attributed to unintentionally present impurities, like carbon. This striking behaviour reveals that a redistribution of impurities or defects, connected with the gettering activity of dislocations in DLDS, occurs during current ageing. Such segregation effects induced by the stress field around dislocations have been, indeed, often observed in growth or anneal processing.

It is shown that large negative threshold voltages and high leakage currents can occur in n-channel polycrystalline silicon thin film transistors during the plasma hydrogenation process. This is a result of sodium contamination, with the sodium ions residing in both the silicon dioxide and the grain boundaries of the polycrystalline silicon, close to the silicon-silicon dioxide interface. The sodium ions in the grain boundaries act like donor centres, and a strong electron channel is formed in the device. It is essential that film transistors are passivated against sodium ions before studying the effects of plasma hydrogenation on device characteristics.

The extent of interdiffusion in wide-bandgap II-VI superlattices of Zn-ZnSe, ZnSe-CdSe and ZnS-CdS grown at low temperature is assessed by measuring the photoluminescence spectra and X-ray diffraction profiles of samples annealed in the temperature range from 300 to 550 degrees C. It is shown that although Zn-ZnSe superlattices are stable against anion interdiffusion over the entire temperature range, the common-anion systems ZnSe-CdSe and ZnS-CdS show evidence of disordering at anneal temperatures greater than 400 and 450 degrees C respectively. These results confirm that the common-anion binary superlattices are stable against interdiffusion at low growth temperature.

Reflectance spectra (RS) of semiconductor quantum wells (QWS) have been measured with Fourier transform spectroscopy (FTS) from 77 K to room temperature. Room temperature excitons and higher lying energy transitions have been successfully detected in various types of QWS. The result demonstrates that the measurement of RS by FTS is very useful for investigating excitonic transitions in QW structures.

DX centres in Al_0.38Ga_0.62As doped with Te have been studied by deep level transient spectroscopy (DLTS) as a function of uniaxial stress applied along the (100), (110) and (111) directions. No splitting nor broadening of the DLTS peaks was observed for stress up to 10 kbar. However, the DLTS peak positions and heights were found to vary nonlinearly with stress, depending on the stress direction. The results have been analysed in terms of existing models.

The use of F-based (SF₆/O₂, CF₄/O₂ and CHF₃/O₂) electron cyclotron resonance discharges for dry etching of TiN_x films deposited onto InP by low pressure (3-10 Torr), rapid thermal metalorganic chemical vapour deposition is described. Etch rates of 50-100 AA min^-1 are obtained for low DC biasing (-100 V) of the sample, giving rise to anisotropic features, infinite selectivity of etching TiN_x over InP, and minimal damage to the semiconductor. SF₆/O₂ mixtures offer the fastest etch rates, while CHF₃/O₂ discharges produce significant polymer deposition under high pressure (20 mTorr) or high microwave power (>200 W) etching conditions.