E. Alexander

The mechanism of polytype formation in vapour-phase grown ZnS crystals

Abstract Morphological, x-ray crystallographic and x-ray topographic studies on many specimens grown in this laboratory lead to a consistent description of polytypism in vapour-phase grown ZnS. Crystal growth begins with the emerging of hexagonal 2H needles from the polycrystalline substrate. The needles thicken during the growth and often expand into platelets. The growth of the needles takes place around a screw dislocation with a Burgers vector 2nc D, where C 0 is the interlayer distance along the c axis. Consequently the (00.1) layers form a set of interleaved helical surfaces. Usually n= 1, but occasionally n> 1 and in this case subsequent formation of a poly-type is possible. This takes place by means of stacking faults which are introduced while the specimens are cooling down in the growth tube. Guided by the helical topology the stacking faults expand onto many equidistant (00.1) layers and thus transform the structure. It is demonstrated that all known features of ZnS polytypes in vapour-phase gr...

Polar Properties of ZnS Crystals and the Anomalous Photovoltaic Effect

ZnS platelets grown from the vapor phase, bounded by (211), (011), and (111) faces, were used. The crystals showed birefringence bands perpendicular to the [111] axis. X‐ray rotation, oscillation, and Laue photographs showed that the bands are due to changes in crystal structure (cubic, hexagonal, and polytypes) and one‐dimensional stacking disorder. The polarity of the common [111] axis of the various bands was determined by several techniques. On the (211) faces, heating with H2O2 caused the appearance of triangular etch pits, aligned with bases parallel to the [011] direction and apexes pointing all in the same sense. The (111) face towards which the triangles pointed was attacked faster by the etchant than was the opposite one. The same (111) face became negatively charged if a (211) face was uniformly illuminated by 343 mμ radiation. Furthermore, the same face turned out to be a zinc plane, as revealed by comparing the intensity of two (111) reflections, using wavelengths at both sides of the Zn...

Vacuum Ultraviolet Spectrometer

A grazing incidence vacuum spectrometer is described. It is based on a 2‐m, 30 000‐lines/in., Siegbahn glass grating. Both plate holder and grating are held on arms fastened to the axis of the Rowland circle. The wavelength range extends up to 2300 A. Instrumental considerations for obtaining high resolution in the short wavelength range are discussed.

The effect of electric fields on the luminescence and conductivity of ZnS single crystals

Application of a direct electric field (of the order of 104Vcm) causes a temporary increase of the luminescence of ZnS crystals (“Gudden-Pohl effect”). It is demonstrated that the light pulse is always accompanied by a current pulse. Studies of the effect showed that both pulses appear only if the crystal has been optically excited. The sizes and shapes of the pulses were systematically studied during the excitation, afterglow and thermal glow. Kinetic equations were set up, governing the free and trapped electron concentrations in the crystal. Most coefficients of these equations were experimentally determined by conventional luminescence and photoconductivity methods. Three different ways by which the field might cause release of electrons from traps, were considered: (a) tunneling from traps; (b) diminishing the effective trap depths by the field and thereby enhancing thermal release; (c) impact ionization of traps by accelerated conduction-band electrons. The kinetic equations showed that with either of these mechanisms current and light pulses appear, as observed in the experiments. However, tunneling (a) would yield completely negligible effects at even much higher fields than actually applied and is in disagreement with the experimental results on the dependence of the pulse heights on the state of excitation of the crystal. Mechanism (b) can account for the order of magnitude of the effects observed only if local fields, higher than the average one by a factor of about twenty exist in the crystal. It does not explain the observed behaviour of the pulses during the afterglow. Impact ionization (c) is in complete accord with the results on the dependence of the pulse heights on the state of excitation of the crystal but it also seems to need somewhat higher fields than the average fields applied.

Gudden-Pohl and memory effects in an infrared stimulated phosphor

Abstract Gudden-Pohl effect during stimulation and enhancement of stimulability by the action of an electric field applied during stimulation are observed in Std. V I. Dependence of these effects on voltage and previous history, especially on previous field application is investigated together with the enhancement of stimulability by field application during fluorescence described previously. (17) It is shown that most of the features of electric field action observed may be understood on the basis of the term scheme deduced from thermal and optical evidence.

An Integrating Photometer for X‐Ray Intensity Measurements

X‐ray diffraction spots on films vary in area and are uneven in density. An instrument is described, which, after calibration against a standard intensity scale imprinted on the film, scans the spot by a light ray, and transforms transmitted light pulses into linear functions of intensity by electronic means. A figure, proportional to the intensity of the spot, is obtained on a mechanical counter. The integrating process is fully automatic, and measurement of one spot takes about a minute. Reproducibility depends on the relative intensity of the measured spot, and can be raised up to 1 percent.