Table of contents

The first successful pseudomorphic Ge_xSi_1-x strained layers on Si were grown in Germany in 1975. The extensive work that has been done on the production, properties and application of Ge_xSi_1-x strained layers and superlattices over the last fifteen years is described in this review. Values of critical layer thickness of the strained layers predicted by equilibrium theories are discussed and compared with experimental results. For x<0.6 the observed values are much larger than the theoretical values. The possible causes of this discrepancy are investigated and it is concluded that the layers must be metastable due to an energy barrier for the production of dislocations. Critical layer thicknesses of superlattices are also discussed and symmetrically strained superlattices are described. Results of band structure calculations of the strained layers and superlattices are given. Strain causes a large reduction in the fundamental indirect band gap of the Si/Ge alloy. Calculated band line-ups at the strained layer-substrate interface are compared with experiment. Modifications in the band structure due to strain have considerable effect on the optical and transport properties of the strained layers. Observed luminescence, photoconductivity and mobility are compared with predictions, taking strain into account. In the case of superlattices, band structure is further modified by zone folding. Superlattice structures which can produce a direct band gap are discussed. Raman scattering has proved a valuable tool for characterizing strained layers and superlattices. In the case of Ge_xSi_1-x strained layers, the Raman spectrum shows phonon lines shifted by strain. In the case of superlattices, additional lines due to zone folded LA phonons are observed. Raman measurements may be used to determine strain, thickness of a period and quality of interfaces. This review places emphasis on the use of Si/Ge strained in layers in Si-technology-based integrated circuits. The devices which have been fabricated to exploit properties of these layers include double heterostructure bipolar transistors, optical detectors, modulation doped field-effect transistors and mixed tunnelling avalanche transit time diodes. Progress made in the last 18 months has been rapid; a transistor with a cut-off frequency of 75 GHz was recently fabricated and this device has considerable promise.

Non-equilibrium impurity diffusion of dopants has been realized in monocrystalline silicon through controlled surface injection of self-interstitials and vacancies. By varying the parameters of the surface oxide layer during the boron/phosphorus diffusion process, it was possible to obtain, for the first time, quantum-size diffusion profiles and p-n junctions with dimensions that could be controlled over the 10-220 AA range.

The photolysis of NH₃ and SiH₄ by the 185 nm radiation of a low-pressure Hg lamp is used to deposit silicon nitride films at temperatures ranging from 50 up to 420 degrees C. The deposition kinetics are analysed by considering the gas phase and surface reactions of radicals (H, NH₂, SiH₃, SiH₂) involved in the photochemical process. Radical densities and flux to the walls in the deposition chamber are computed from numerical solutions of one-dimensional spatially resolved balance equations for creation, reaction and diffusion of each species. The film composition is analysed by X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectrometry (SIMS). The thickness and refractive index are obtained by ellipsometry. Finally the electrical properties are derived from current-voltage I(V) and capacitance-voltage C(V) characteristics. The optimum dielectric properties are obtained for deposition temperatures between 250 and 350 degrees C and for SiH₄ concentration in NH₃ of about 1%.

Explicit formulae for superlattice subband energies and effective masses in terms of the effective masses and layer widths of the constituent materials and the band offset are presented.

The authors present an evaluation for the calculation of the effective g-factor and the effective mass of conduction band electrons in pseudomorphic strained layers. They apply this evaluation to some important heterostructure systems and show that effective mass is mainly isotropically shifted whereas the g-factor exhibits anisotropic splitting. They show that these effects, being attributed to the internal strains induced by lattice mismatch, may be used to characterize heterostructures.

A novel very efficient technique for charge current calculations in two-dimensional Monte Carlo particle simulation is proposed. The technique obtains accurate results by using a significantly smaller number of particles than the commonly accepted technique. The proposed technique permits the two-dimensional Monte Carlo particle simulation to run on personal computers instead of on supercomputers. The results of field-effect transistor simulation are presented.

Electrical conductivity measurements combined with transmission electron microscopy observations were performed on a p-type Si bicrystal containing a Sigma =3 coincidence site lattice grain boundary. An increased conductivity observed in the surroundings of the grain boundary is due to the formation of a potential well for the holes. The potential well is found to be due to the segregation of impurity atoms, which saturate existing free bonds in the core of the observed dislocations at the interface.

It is found that the AC surface photovoltage (SPV) increases after X-ray irradiation (8 kGy(Si)=0.8 Mrad (Si)) in an air-free dry-oxidized and hydrogen-annealed n-type silicon (Si) wafer with resistivity 0.1 Omega m. Since the AC SPV is known to increase as the depletion layer becomes wider, the present phenomenon means that negative interface-trapped charge due to X-ray-induced interface traps must be dominant in the wafer. This result is also supported by the fact that the AC SPV in an as-oxidized n-type Si wafer with native high interface trap density (D_it) is higher than that in a hydrogen-annealed wafer with resultant low D_it.

Hall and Shubnikov-de Haas measurements have been used to determine the depletion behaviour of a gated GaAs/(Al,Ga)As double quantum well structure. A Fourier transform analysis of the Shubnikov-de Haas oscillations has enabled the carrier density in each well to be found as a function of gate bias, and good agreement with a theoretical model based on the numerical solution of the coupled Poisson and Schrodinger equations has been found. Despite the absence of a second maximum in the transconductance characteristics, these results show that depletion of the wells is sequential. These data have enabled the low-field Hall measurements to be analysed to give the dependence of mobility on carrier density in each well; in the near-surface well, a dependence characteristic of remote impurity scattering is observed, but the much steeper variation found for the lower well suggests that localization occurs here.

An experimental study and theoretical analysis of the Shubnikov-de Haas oscillations in Cd_3-x-yZn_xMn_yAs₂ single crystals is presented for samples with various compositions. The contribution of spin-dependent scattering of the conduction electrons on magnetic ions to the total relaxation time is deduced from the waveshape analysis. The formula for the relaxation time corresponding to this scattering mechanism is derived and compared with the experimental data.

The authors present an investigation into the electrical and magnetic properties of a symmetric (AlGa)As/GaAs resonant tunnelling double barrier structure in which a superlattice (SL) is included between one of the heavily doped contacts and the adjacent barrier. With the emitter electrons confined to the miniband states of the SL (forward bias), the threshold voltage is found to decrease and the peak current density to increase. Moreover, a significant increase in phonon-assisted tunnelling is apparent with three LO-phonon satellite peaks observed in I(V). With a magnetic field applied parallel to the tunnel current direction, magneto-quantum oscillations in I(B) are observed. In forward bias only, two series of oscillations are detected in and past the valley current region with high values of the fundamental magnetic field, B_f. These two series are attributed to k_{perpendicular to} non-conserving scattering events with and without the involvement of an LO phonon. These observations suggest that an increase in penetration of the emitter wavefunction into the emitter barrier produces an increase in LO-phonon-mediated tunnelling and novel structure in I(B).

A study of the transient photoconductive response of semiconductor MQW systems in the nanosecond regime has shown that the initial amplitude increases sublinearly with excitation intensity and exhibits saturation. The authors show that the origin of this behaviour can be understood on the basis of the density dependence of the electron mobility in the presence of the subband structure induced by quantum confinement. Analysis of the experimental results for a GaInAs/InP MQW system along these yields a measurement of electron mobility in an electron-hole plasma and its dependence on plasma density.

A new DC extraction algorithm based on an improved MOS transistor current model has been proposed to determine the gate voltage dependence of transistor parameters from measured I-V characteristics. Experimental results obtained with long and short channel MOSFETS show that the parameters extracted using the algorithm are in good agreement with the theoretical values.

The authors have studied the electronic energy structure of pseudomorphic Ge_m/Si_n superlattices by using the empirical tight-binding method. Effects of the band offset, sublattice periodicity and the lateral lattice constant on the transition energies have been investigated. They found that Ge_mSi_n superlattices grown on Ge (001) can have a direct band gap, if m+n=10 and m=6. However, optical matrix elements for in-plane and perpendicular polarized light are negligible for the transition from the highest valence band to the lowest conduction band state at the centre of the superlattice Brillouin zone.

A numerical method is developed to determine the subband energies and electron concentrations in shallow etched quantum wires from magnetotransport data. Solving the two-dimensional Schrodinger equation self-consistently, the results are compared with the electron densities and subband spacings obtained from a harmonic oscillator model. Due to the non-parabolic wire potential, the numerically calculated electron densities are systematically smaller than the values obtained by a harmonic oscillator model.

The electrical properties of Cu-Si(100) Schottky barriers formed by the deposition of Cu on lightly doped (10¹⁴-10¹⁵ cm^-3) n-type and p-type Si(100) have been studied in the temperature range 80-295 K using current-voltage and capacitance-voltage techniques. Deep-level transient spectroscopy was used to determine Cu-related energy levels in Si and X-ray photoemission spectroscopy to monitor the reaction between Cu and Si. In both n-type and p-type Si(100), no Cu-related levels deeper than approximately 0.1 eV below (above) the conduction-band edge (the valence-band edge) and with concentrations above 10¹¹ cm^-3 were observed after copper deposition or after silicide formation. Silicide formation was found to have very little or no effect on the barrier height and its temperature dependence. For both the as-deposited Cu and the reacted Cu₃Si silicide phase, the temperature coefficient of the n-type barrier height was found to be almost equal to that of the indirect band gap in Si. The p-type barrier height did not exhibit a temperature dependence. These results suggest that the Fermi level at the interface is pinned relative to the valence-band edge. These results deviate significantly from the predictions of models of Schottky-barrier formation based on Fermi-level pinning in the centre of the semiconductor indirect band gap.

The influence of Si pre-treatment on the impedance of Co/n-Si Schottky barriers is investigated. Some classical as well as some recently developed cleaning methods are applied. Only these treatments which contain a HF dip produce frequency dependence of the impedance. This frequency dependence can be removed by a sputter etch. The results allow the authors to exclude some common causes of frequency dependence previously reported. The results point to F, adsorbed at the Si surface, to be responsible for the frequency dependence. Additional experiments show that Cl produces analogous effects.

The author shows that the shorter the laser pulse the greater the enhancement of nonlinear optical response in semiconductor superlattices which can be expected even for systems far from resonant transitions to higher subbands. The mechanism responsible for this enhancement is analogous to Landau-Zener transitions.

The design and optimization of double drift low-high-low indium phosphide IMPATT diodes have been studied using computer simulation programs for the static and high frequency analyses of the devices. A maximum efficiency of 28% has been obtained with a breakdown voltage of 104.5 V. The device properties have also been studied for various current densities, which involves the effect of mobile space charge in the depletion layer. The admittance characteristics and the spatial distribution of high frequency negative resistivity and reactance in the depletion layer of the optimized low-high-low DDR diode have been studied and compared with those of the flat profile DDR diode. The profiles of negative resistivity in the depletion layers of both low-high-low DDR and flat profile DDR exhibit two peaks in the middle of two drift layers separated by a minimum in the avalanche zone. These profiles provide considerable physical insight regarding the source of microwave power generation in these devices.

Theoretical studies of the AC photothermal and AC electrothermal responses of semiconductor p-n junction devices with regard to photogenerated or applied voltages are presented, which cover the entire set of optical, electronic, thermal and geometric parameters involved in the operation of a conventional p-n junction device. In the theoretical analyses, the coupling between DC and AC components of the time-dependent transport equations of the minority carriers is considered, and the thermalization due to the minority carriers across the space-charge region and due to the equivalent series resistance effect are also taken into account.

The effect of annealing on SnO₂/a-SiC:H(p⁺-type)/a-Si:H/a-Si:H(n⁺-type)/Al structure has been investigated. Hydrogenated amorphous silicon carbide and amorphous silicon thin films were prepared by the DC magnetron sputtering technique. p⁺-type amorphous silicon carbide (100 AA), intrinsic amorphous silicon (4100 AA) and n⁺-type amorphous silicon films (420 AA) were deposited on a glass substrate coated with a transparent SnO₂ layer. The depositions of amorphous silicon carbide and amorphous silicon films were made in separate chambers. An aluminum metal contact was attached to the n⁺-type amorphous silicon film under a vacuum of 10^-7 Torr. The dark and illuminated current-voltage characteristics were measured before and after annealing in the temperature range of 23-175 degrees C, under reverse biases of 0.5, 1.0, 1.5 and 2.0 V. A significant increase of the efficiency was observed after annealing, at 2.0 V reverse bias.

Modern single-poly electrically erasable programmable read-only memories (EEPROM) use a stacked silicide/polysilicon (polycide) floating gate as a charge storage element. In these devices the ability to retain stored charge relies on the quality of the top silicon dioxide, thermally grown on the silicide, which is known to be affected by the oxidation procedure. The authors have studied silicon dioxide films, thermally grown in a dry oxygen atmosphere on WSi₂, both in terms of composition and electrical properties. Concentration depth profiles were obtained by means of Auger electron spectroscopy (AES), while electrical characteristics were obtained on poly-Si/SiO₂/WSi₂ capacitors, from C-V plots, I-V measurements and ramped voltage stress. To characterize the oxide reliability, the endurance and memory retention at high temperature have been measured on single-poly EEPROM cells. The results show that the oxide thermally grown on tungsten silicide can meet the requirements for single-poly PROM device fabrication. The functionality of memory cells has been demonstrated, both with endurance and retention measurements.

n-ZnCdS/p-CdTe polycrystalline thin film solar cells were fabricated by laser evaporating CdTe onto sprayed ZnCdS films. The cells were characterized by studying current-voltage, capacitance-voltage and spectral response measurements. A maximum efficiency of 7.6% was observed for a cell area of 1 cm².

Recently, III-V/II-VI single and double heterostructures have attracted much attention for device applications. The first demonstration of metal organic vapour phase epitaxy (MOVPE) of a GaAs/ZnSe/GaAs double heterostructure is reported. The influence of the ZnSe layer on the top GaAs layer is discussed from photoluminescence and reflectivity results.

The durability of GaAs oxide layers to chlorine gas exposure was compared for the cases of photo-oxidation and thermal oxidation at room temperature. The GaAs oxide layer formed by photo-oxidation was found to be more durable to chlorine gas exposure, hence a photo-oxidized GaAs layer is suited to use as an inorganic mask. The time required to locally remove the oxide layer by simultaneous irradiation with chlorine molecules and an electron beam (EB) for patterning depended on the conditions of formation of the oxide layer, and was inversely proportional to the EB current density for the fixed GaAs oxide layer. The etch rate of GaAs increased with increasing EB density under constant chlorine irradiation.

High resolution X-ray diffraction has been used to investigate the relative tilt between epilayer and substrate lattice planes of different semiconductor heterostructures. All epilayers were grown on (001) GaAs substrates misoriented by 2 degrees towards the next (011) direction. Results on the amount of the relative tilt and the direction of the maximum relative tilt are presented. The authors find that for heterostructures with small misfit (f<0.001) the direction relative tilt coincides with the direction defined by the miscut substrate. In heterostructures with large misfit (f>0.05) an angle of about 90 degrees between the direction of maximum relative tilt and the direction of the miscut substrate has been observed.

The authors have studied the influence of a weak magnetic field on the energy levels in a lateral surface superlattice fabricated on GaAs-GaAlAs heterostructures. In the transition region between a two-dimensional electron gas and a multiple quantum wire system, a new set of equidistant magnetoresistance oscillations is observed These structures can be explained in terms of a locally modulated density of states, which leads to a charge transfer between regions of high and low electron mobility.

Photothermal spectroscopy experiments have been used for the first time to study non-radiative processes in mesoporous silicon layers of varying porosity. Data on low porosity material can be explained by a model which involves interference between thermal waves reflected at the porous silicon film-silicon substrate interface. High porosity material, which exhibits strong red photoluminescence, cannot be fitted to this model. These materials seem to indicate the existence of another critically damped wave-like excitation in addition to the thermal waves.