Table of contents

A correlation between electron microscopy, secondary-ion mass spectroscopy (SIMS) and electrical measurements is reported for AuGeNi alloyed contacts to InP. Changes in the morphology and in the composition of the material at the interface show strong trends as a function of the initial alloy temperature and of subsequent heat treatment to simulate accelerated device operation. Dramatic changes in the electrical properties of the contacts are shown to follow closely and consistently the changes in morphology. The principal factor controlling the stability of the contact appears to be the thickness and integrity of a new AuGeP phase at the interface. Growth of large particles of Au₂P₃ occurs at high alloy temperatures or after prolonged heat treatment and is strongly associated with contact degradation. There was no evidence for preferential Ge diffusion. Various models for current transport across the metal-semiconductor interface are discussed as well as the implications for the control of contact resistance.

The effect of radiation damage on neutron-transmutation-doped silicon of initially low- and high-resistivity materials has been studied by electrical property measurements. The materials were irradiated with a moderate thermal (0.45-1.0*10¹⁸ neutrons cm^-2) and fast (0.45-8.1*10¹⁶ neutrons cm^-2) neutron fluences. After irradiation, a small reduction of carrier mobility of the initially low-resistivity materials was observed, while the reduction of conductivity was large. The annealing behaviour and temperature dependence of mobility of the materials indicate that the mechanism of carrier scattering is dominated by ionised impurities acting as point defects. The conductivity of those materials was recovered after annealing at 560 degrees C for 60 min. In the range of annealing temperatures below 600 degrees C, the carrier mobility of initially high-resistivity materials decreased together with the conductivity. The last result can tentatively be explained by a defect cluster model for fast-neutron-induced lattice damage.

The 'rigid-well' approximation for the local vibrational mode frequency of a light impurity in a crystal in which only the impurity itself is regarded as moving may be regarded as the leading term in an expansion in powers of the ratio of the impurity mass to the host mass or masses. It is found that the retention of only one further term provides an analytic approximation that is quantitatively useful over a much wider range of cases of interest. This approximation assumes a particularly simple form if the contributions from the motion of second and further neighbours of the impurity are negligible. Various applications are presented.

The Hall effect and transverse magnetoresistance in the intrinsic nondegenerate semiconductors InSb and GaAs have been investigated according to the scattering processes of carriers in solids. These scattering processes include the acoustic phonon scattering, piezo-electric scattering and ionised-impurity scattering. The energy band structure of carriers is assumed to be nonparabolic. Results show that the Hall angle, the Hall coefficient, and the transverse magnetoresistance depend strongly on the DC magnetic field due to the energy-dependent relaxation time. The holes in semiconductors play a major role in the electrical transport phenomenon for the piezo-electric scattering and ionised-impurity scattering, while for the acoustic phonon scattering the conduction electrons in semiconductors play the major role in the electrical transport phenomenon. However, the numerical results for the piezo-electric scattering and ionised-impurity scattering are shown to be rather smaller than those for the acoustic phonon scattering in the intrinsic semiconductors. Therefore, the conduction electrons play a dominant role in the carrier transport phenomenon in the intrinsic semiconductors. A comparison between experimental work and theory for the Hall effect and transverse magnetoresistance in InSb has been made. The effect of nonparabolicity in InSb has also been discussed in comparison with the numerical results for the parabolic band structure.

An analysis of electronic structure and scattering in GaAs/AlGaAs superlattices suggests the possibility of a new class of electronic device exploiting the properties of hot electrons in the higher sub-bands. In particular it is shown that the electron mass in the higher sub-bands can be very low, leading to high-sub-band mobility, and the inter-sub-band scattering can be strongly suppressed, leading to long energy relaxation times and very long quasi-ballistic path lengths. It is suggested that these ideas can be readily adapted to achieve high transport factors in hot-electron transistors by replacing the conventional homogeneous-base region by an appropriate superlattice.

The scattering of electrons by charged impurities in a semiconductor is conventionally seen as caused by relatively infrequent two-body collisions. In practice, electrical screening of the long-range Colomb interaction is usually invoked in order to eliminate the many distant interactions and this may often be a good approximation. In principle, however, the problem of the interaction with the many distant impurities should have a solution independent of screening strength. This problem is particularly acute in the case of scattering by remote impurities in modulation-doped structures. One approach which has been used in the past to reconcile Brooks-Herring and Conwell-Weisskopf formulations is to constrain scattering to be effected only by nearest neighbours. This implies weighting the scattering probability associated with a particular impurity with the possibility that there exists no nearer impurity. This procedure introduces a statistical cut-off to the interaction-an effect which is termed as statistical screening. This idea is applied to the case of impurity scattering in modulation-doped structures, and it is shown that the scattering potential of remote impurities in the barrier layer can be drastically reduced. Quantitative illustrations for the case of GaAs show that statistical screening can account for the very high mobilities that have been observed in modulation-doped structures.

Contrary to some expectations for n-ZnSe/n-GaAs heterostructures, optical and electrical measurements indicate that the GaAs side of the junction is depleted of electrons, while the ZnSe side accumulates them. The low T conductance of the n-ZnSe exhibits two-dimensional metallic behaviour: activated conduction is observed from 1.4 to 6 K, and a logarithmic variation of the conductance with temperature is observed between 14 and 45 K. A 0.75 V photovoltage under 5 mW cm^-2 of white light with the GaAs negative is consistent with the theoretical predictions of Ihm and Cohen.

The commonly used formalism for the ionised remote impurity scattering of electrons in a 2D electron gas contains the following inconsistency. The potential of the donors (e.g. the Si donors in the AlGaAs in the case of a GaAs-AlGaAs heterojunction) is accounted for twice: first in the self-consistent calculation of the envelope function and secondly in the calculation of the scattering rate. Instead, only the scattering as due to the fluctuations in the average field of the donors should be accounted for. The authors have developed expressions for this scattering amplitude and have applied them to a few GaAs-AlGaAs samples with very high mobilities. These samples posed the problem of a serious underestimation of the experimental mobility by the theory. This problem now can be resolved and a better agreement between experiment and theory has been obtained.

The impedance associated with recombination of free electrons and free holes in the space charge layer has been calculated. It has been shown that this impedance corresponds to a parallel-equivalent circuit of a capacitance and a resistance in parallel with the space charge layer capacitance. It has been calculated that the recombination capacitance has almost the same voltage dependence as the space charge layer capacitance, and that the recombination resistance is inversely proportional to the recombination rate in the space charge layer. The impedance associated with recombination in the space charge layer has been compared with the surface recombination impedance and with experimental results obtained at illuminated GaAs electrodes.

Cadmium sulphide thin films showing photo-to-dark conductivity ratio up to 10⁹ and photoconductivity up to 3 Omega ^-1 cm^-1 for white illumination approximately 300 W m^-2 can be prepared from chemical baths containing thiourea and triethanolamine complex of cadmium ions. The photocurrent decay time depends on the bath temperature and the duration of storage and it ranges from a few seconds to 10⁴ s per decade. The optical transmission of the films also varies significantly: from about 10% to 70-80% (above the band-gap absorption), depending on the deposition conditions. The high activation energy approximately=1 eV for dark conductivity as well as the high photosensitivity suggest the nearly stoichiometric nature of the films. The implications of these characteristics in various opto-electronic applications are discussed.

The structures in the complex dielectric function of an isoelectronic substitutional dilute alloy have been analysed in terms of extra and missing wavevector conserving transitions. Wavevector non-conserving transitions have not been pointed out for an s conduction band and a p valence band with NaCl structure. The transitions are related to the modification of the host density of states due to the impurity potential. The lowest-energy direct transitions give rise to a structure which can be used to determine or to confirm the band gap energy. The other transitions serve to investigate critical points in the Brillouin zone.

Curves showing the anomalous Hall coefficient versus temperature have been obtained for thick low-doped n-type Hg_0.8Cd_0.2Te samples grown by the solid state recrystallisation technique. These curves have a lower value of the Hall coefficient at 77 K than at very low temperature, contrary to classical predictions. These results are modelled in terms of a p-type core sandwiched between two n-type surface regions and, using this, the curves are analysed to obtain the properties of the n-type surface region.

The IR absorption method is investigated in detail with respect to a routine application in semiconductor industry laboratories. The experimental and analytical procedures necessary for an accurate determination of the residual carbon acceptor (C_As) concentration also in commercial wafers are given. A specific technique to eliminate the interference fringes arising from multiple reflections inside the wafer is presented. At a measurement temperature of 77 K the detection limit for carbon is 6*10¹³ cm^-3 in wafers of 5 mm thickness. The IR method is also applicable to commercial wafers giving a detection limit of about 6*10¹⁴ cm^-3.

This paper analyses the influence of Joule self-heating on the electrical conduction process in bulk glassy semiconductors, when non-switching square voltage pulses are applied; the dependence of the heating time constant on the ratio between steady-state current increase and the initial current; and the relationship between steady-state current increase, applied voltage and ambient temperature.