I. T. Steinberger

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...

The density dependence of the Quasi-free electron state in fluid xenon and krypton.

Abstract Photoinjection measurements show that in liquid and gaseous krypton and xenon V 0 is negative. V 0 has a minimum (−0.77 eV for Xe and −0.53 eV for Kr) near the density of the mobility maximum, becoming zero at densities of the order of 10 20 atoms/cm 3 .

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...

Periodic slip processes and the formation of polytypes in zinc sulphide crystals

Inherent deformations of ZnS polytype regions are determined by measuring angles between linear markings. The deformations are correlated to the crystallographic structures of the polytypes as well. The relations clearly indicate that polytype formation occurred by a periodic slip process after crystal growth had been completed. Further observations substantiating this model are quoted.

Reflection and Absorption Spectra of the Higher π → π* Transitions of Solid Benzene

Reflection and absorption spectra of C6H6 and C6D6 were obtained at temperatures near 4.2°K, in the wavelength region of 1700 A   50 000 cm−1, strong system), respectively. The possible assignments of the individual bands within each system are discussed on the basis of the present results, taking into account conclusions derived from spectra of benzene embedded in rare‐gas matrices. Semiem...

Optical Constants of Solid Xenon in the Vacuum‐uv Region

The reflectance spectrum in the range 1180–1600 A solid xenon was measured near 4°K. The Kramers–Kronig inversion technique was used to evaluate the real part e′ and the imaginary part e″ of the dielectric constant. A new self‐consistent approximation method was developed and used to evaluate the contribution of absorption bands lying beyond the range of measurements to the phase of the reflection. The e″ vs photon energy E curve thus obtained exhibited all the details of the best thin‐film absorption measurements, and it also showed two hitherto unknown peaks, at 8.76 and 8.93 eV, respectively. Resolution of the e″ vs E curve into a sum of absorption bands was an inherent part of the new approximation method. From this resolution, the oscillator strengths f of the transitions were determined. The largest f value, namely 0.16, was found for the lowest‐energy peak at 8.378 eV, as compared with f = 0.256 for the corresponding 3P1 resonance line in xenon gas. The f value for this band was also computed on th...

The energy V0 of the quasi-free electron in gaseous, liquid and solid methane

Abstract Photoinjection experiments show that the energy V 0 of the quasi-free electron in methane is negative at all densities from the dilute gas to the solid with a minimum roughly coinciding with the mobility maximum. The results are in qualitative agreement with the Springett—Jortner—Cohen model and with Basak and Cohens relationship between the mobility and V 0 .

Extrinsic photoconductivity in xenon-doped fluid argon and krypton

Abstract Monochromated synchrotron radiation from the DORIS storage ring at DESY was used to excite photoconduction in xenon-doped fluid argon for densities ranging from 0.7 × 10 22 to 2.1 × 10 22 cm −3 , in xenon-doped fluid krypton from 1.3 × 10 22 to 1.6 × 10 22 cm −3 as well as in solid Xe/Ar. The measurements yielded directly the ionization energy E G i of the impurity in the dense medium. Using previous experimental results on the energy V 0 of the conduction electron in conjunction with E G i the polarization energy P + i of a hole trapped at a xenon atom was also determined for the respective density ranges. These experimental P + i values do not agree with theoretical predictions. Combining the E G i values with results from absorption spectra by Messing et al. led to the determination of the binding energy and effective mass of the Wannier—Mott impurity exciton and indicated that previous assignments of absorption bands in xenon-doped solid argon have to be revised.

The stacking faults and partial dislocations involved in structure transformations of ZnS crystals

Abstract Morphological and structural studies of ZnS crystals grown from the vapour phase lead to the following conclusions: (a) A relatively small number of Shockley partials and associated stacking faults is sufficient to transform, by periodic slip process, the parent 2H structure to polytypes not having ‘1’ in their Zhdanov symbol, or to 3C. (b) The stacking faults are introduced by mechanical and thermal stresses, and by thermal agitation, (c) Shockley dislocation dipoles and multipoles take part in the structure transformations. A possible mechanism for the formation of dipoles of Shockley partials is proposed. It is shown that a ‘sandwiched’ stacking fault of the type ABACABA, together with its associated elementary dipole, should be a common fault in the hep and wurtzite structures, especially if the energy a of the intrinsic stacking fault is negative. From considerations of the total energy of the system: intrinsic stacking fault and Shockley partial, it is estimated that |a|<600 erg/cm2 in 2H ZnS.

The energies of excess electrons in helium

Abstract The energy V 0 of conduction electrons in helium gas was measured by photoinjection up to a density of 3.6 × 10 21 atoms/cm 3 and analysed together with results by Broomall, Johnson and Onn obtained for higher densities, including the liquid phase. The data could be well approximated by the model of Springett, Jortner and Cohen using a hard-core radius of 0.075 nm.