Table of contents

Evidence is presented that anomalies observed in the results of X-ray microanalysis of thin foils of aluminium-silver alloys are due to the presence of a surface film of differing composition from the matrix, in contradiction to the conclusions of a publication in this journal by Sawley et al. (see ibid., vol.10, p.1883 (1977)).

Sawley et al reply to these comments, indicating that surface films do nevertheless appear to have an effect on X-ray production out of proportion to their thickness.

Non-retarded van der Waals forces between a pair of macroscopic bodies have been measured as a function of separation in the range 2-30 nm for three systems: (a) mica/mica, (b) calcium stearate/calcium stearate, and (c) silver/mica. The values of the Hamaker constant, which is a measure of force strength, are slightly larger than those predicted by a simple version of Lifshitz theory, but the ratios of the Hamaker constants are in agreement with theory. Experimental reasons connected with the details of surface profile measurements, and theoretical reasons for the differences are discussed.

A study of the surface layers formed on CdTe single crystals, following immersion in a range of etchants, has been carried out using X-ray photoelectron spectroscopy. Most etchants lead to a layer of tellurium oxide on the surface and severely deplete the surface region of cadmium. The best procedures for obtaining stoichiometric surfaces are outlined.

On semiconductor surfaces adsorption enhancement due to the influence of an electron beam is often observed. This effect has usually been ascribed to the excitation or dissociation of adsorbed species; some evidence has, however, been brought to show that ionisation of molecules in the gas phase is responsible. The magnitude of the latter effect is calculated and it is shown that under the normal experimental conditions of Auger spectroscopy it only plays a role when the sticking coefficient for adsorption under normal conditions is lower than approximately 10^-6. For electron-beam-stimulated oxidation of GaAs it is further shown that excitation in the gas phase cannot be responsible for the enhancement. Only effects associated with adsorbed oxygen such as dissociation provide an explanation.

The epitaxial growth of GaAs_1-xSb_x crystals from solution has been studied at constant temperatures of 700 degrees C or lower. Single crystals of GaAs_1-xSb_x with compositions of x<0.14 and x>0.8 were obtained. Good compositional uniformity in the epitaxial layer along the growth direction was attained in spite of the peculiar phase diagram with a flat solidus curve.

The authors have made ultrasonic measurements at 10 MHz and between 190 and 270K on extruded samples of polypropylene and polyethylene terephthalate which have a wide range of crystallinity (0-0.6) and extrusion ratio (1-10) by the use of quartz transducers directly bonded on to sample discs cut parallel, normal and at 45 degrees to the extrusion direction and have thus obtained all five elastic moduli. The results for polypropylene are considered in both the aggregate and the Takayanagi (1966) models. The aggregate model reproduces the development of mechanical anisotropy fairly well. The increase in the ultrasonic Young's modulus along the extrusion direction is much less than that previously observed in low-frequency-modulus data. The Takayanagi model shows that the stiffening effect of intercrystalline bridges is much less important for the ultrasonic moduli than for the room-temperature static moduli because of the much higher amorphous modulus for the glassy state.

The interface chemistry of selenium evaporated onto brass in technical vacuum (10^-6 Torr) has been studied by X-ray photoelectron spectroscopy and argon ion profiling. The interface of thermally deposited layers is characterised by a chemical shift of 4 eV in the Se 3d spectra and the appearance of peak shifts and satellites in Cu 2p spectra. The latter arises due to the oxidation of a copper-rich surface layer and shifts in Se 3d spectra are attributed to the bonding of selenium to cupric oxide. Ion-plated interfaces are characterised by a slight reduction in Se 3d binding energy and the absence of Cu 2p shifts or satellites. High adhesion strength is attributed to the formation of Cu-Se interface bonds arising from oxygen displacement in the process of ion-plating.

The effects of adding up to 50 mole.% of tin to pyrolytic In₂O₃ selective coatings were investigated. Up to 10% doping the resistivity is found to decrease with the corresponding increase of carrier concentration. Further additions increase the resistivity and decrease the carrier concentration. X-ray diffraction and electron microscope observations indicated that (400) is the strongest peak. The tin additions cause the lattice parameter to decrease indicating that the decrease in resistivity is caused due to the replacement of In³⁺ ions by Sn⁴⁺ ions. The increase of resistivity at higher tin additions is due to poor crystallinity. The films with low resistivities had very low activation energies for electrical conduction.

A heterojunction solar cell, in which interface recombination and arbitrary energy barriers at the interface occur, is analysed to find the optimum structure. Thermionic emission theory is used; it is assumed that one type of carriers are majority carriers at the interface and that their concentration can be known a priori. It is shown that these assumptions do not seriously limit the applicability of the theory. The results are discussed in the case when one semiconductor is much more heavily doped than the other, which is a condition that certainly improves the efficiency of the solar cell.

Presents the results of a study of the photovoltaic effect of Cu_xS/CdS thin-film solar cells formed on chemically sprayed CdS films using a dipping method. The investigation centres on the study of the effects of the properties of the chemically sprayed CdS films on the photovoltaic response of the resulting cells. This includes the study of the parameters of film deposition, film thickness, grain-size and structure, spectral response and I-V characteristics. In general, good photovoltaic cells can be made using the chemically sprayed films. The photovoltaic response in the cells has been limited by the series resistance losses as well as grain-size effect which is responsible for the lowering of the shunting resistance observed in cells made on relatively thicker films. The cell performance is believed to be dependent on the copper content and the distribution within the cell.

When a metal and an insulator touch and become charged, the total charge on one surface divided by the apparent area of touching is not generally equal to the true charge density, but is less because the area of true contact is a fraction of the apparent area. The paper reports an experiment to determine the true contact charge density by contact measurements between a metal and an insulator in the form of a very soft rubber. The rubber is able to deform elastically around the asperities on the surface of a metal, so there is intimate contact between the two. For single contacts of smooth spheres on smooth flats of metal and rubber the true charge density on the rubber is independent of the metal work-function and is -(15+or-5) nC cm^-2, i.e. equivalent to about 1 electron per 10⁴ atoms of surface.

An expression for the temperature coefficients of resistance of thin metal films is derived wherein size effects, thermal strain and difference in the thermal expansion coefficients between film and its substrate are taken into account.

The dielectric properties of the solvents formamide, ethanediol and methanol have been studied over a frequency range of 10 MHz-70 GHz and a temperature range of 10-40 degrees C. It was found that formamide is best represented by a single Debye type dispersion, ethanediol by a double Debye and methanol by a Cole-Cole distribution of relaxation times.

Charge is deposited either on a free surface of a crystal sample from a corona discharge or else by contact on to an evaporated aluminium electrode on the surface. A high-resolution induction probe with a computer store is used to monitor the behaviour of charge on the surface. Charge deposited on the free surface decays away into the bulk with a mobility in the range 10^-13 to 10^-15 m² V^-1 s^-1. On the other hand, an aluminium electrode forms a blocking contact and apparent charge decay then occurs only by dielectric polarisation of the bulk. There is also evidence that charge can spread from the edge of the evaporated electrode on to the surrounding free surface of the crystal. Of most significance is the demonstration that the method permits clear detection of conductive channels in the octadecane crystals.

A method is outlined by which precision measurements may be made on the loss-angle of low-loss liquid dielectrics over a range of temperatures at microwave frequencies. Results are presented for the temperature variation of loss of six low-loss materials (n-hexane, n-heptane, cyclohexane, cycloheptane, cyclo-octane and isooctane) at two microwave frequencies ( approximately=9 and approximately=36 GHz). The data obtained are compared, wherever possible, with data obtained by other workers. A comprehensive list of references to such measurements is also given.

The transient current in polyethylene-terephthalate resulting from the application of a voltage step is studied in the temperature range 90-160 degrees C at field strengths ranging up to 3.2 MV cm^-1. The current is recorded in the time scale extending from approximately 50 mu s to 1 ms as measured from the edge of the voltage step. A prominent feature of the current, particularly below 140 degrees C, is its monotonic decay with time. On Fourier transformation of this data into the frequency domain a strong correspondence is found with low-field AC dielectric loss measurements. A specifically high-field conduction mechanism dominates the conduction with increasing temperature above 140 degrees C, and the current is then found to be only weakly dependent on the time.

Methods of improving the band-pass characteristics of NiO-doped glasses are described. Shifts in the peak transmittances caused by the introduction of NiO into eight different glass compositions (based on silicate, borate, phosphate and tellurite systems) and by the combination of borate and silicate glasses are reported. Further shifts in the peak transmittances, decreased half-widths and the elimination of unwanted transmittance in the 400-420 nm region were achieved by means of supplementary colourants, viz., CeO₂, TiO₂, V₂O₅, CuO or CoO. Possible means of developing new filter glasses with band-pass characteristics in the orange-red region are demonstrated.

The energy distributions of silver and gold atoms sputtered from an AgAu alloy by bombardment with a 6 keV Xe⁺ ion beam have been measured and compared with those from the pure elements. Total sputtering yields of the investigated samples have also been measured. The results show that in addition to collision cascades, spike effects contribute to the sputtering. The experimentally deduced binding energies of Ag and Au atoms in the AgAu alloy surface are lower than for pure elements which accounts for the higher sputtering yield of the AgAu alloy which the authors observed. The existing experimental data on composition changes of the AgAu surface during ion bombardment can be explained in the light of present measurements.

Examination under a 1 MeV electron microscope has shown that the tip radius is comparable in both cases, and in the region 5-10 nm. The AC method, however, produces more robust whiskers with larger cone angles and is probably more suitable for the manufacture of whiskers used in metal-insulator-metal diodes.

Djuric has discussed the electrostatic field produced when the conducting Earth rotates in (or with) its own magnetic field (see ibid., vol.9, p.2623 (1976)); he concluded that the electrostatic field plays a large part in the dynamics of magnetospheric particles. It is now shown that Djuric calculated the wrong field, and that the correct field is very much smaller; also, the magnetosphere is shielded from the electrostatic field by the conducting ionosphere.

Natural chromites taken from three Indian localities, Sukinda, Ladakh and Byrapur, have been studied by Mossbauer spectroscopy. Microscopical and chemical analyses have also been made. Of the four samples studied, three showed normal structure while one showed an inverse spinel structure with small particle size, which indicates that it crystallised at a higher temperature with a faster rate of cooling of the magma. This fourth sample had also suffered hydrothermal alterations. The Fe³⁺/Fe²⁺ ratio demonstrates a high oxygen fugacity at the time of formation of all the samples. The Mg/Fe²⁺ and Cr/Fe ratios suggest that the chromite from Sukinda was the earliest to be crystallised and that the samples from Byrapur were the latest.

Measurements of the thermal expansion of copper (NBS Standard Reference Material, SRM736) have been made between 300 and 700K. Agreement with the NBS calibration was obtained up to 630K within 0.5% but at 700K the difference amounted to 2.5%.