R. Weil

Structure of CdZnTe films on glass

Polycrystalline Cd1−xZnxTe films were grown on glass substrates over the full range of compositions (0 < x < 1) by metal–organic chemical vapour deposition at 480 °C. The films (~5 µm thick) showed uniform texture oriented along the 1 1 1 direction, perpendicular to the substrate, independent of the film composition. The dependence of the lattice parameter of cubic Cd1−xZnxTe on the composition followed Vegards law. The thick Cd1−xZnxTe films were shown to be of a single phase and structurally stable. The average grain size in the thick films was in the range 3–5 µm. The dominant imperfections in the films were twins (mostly Σ = 3) and dislocations. The x-ray diffraction (XRD) FWHM parameter reached a maximum at x = 0.5. Transmission electron microscopy (TEM) in situ heating in the range 200–400 °C caused plastic deformation in the grains without causing ordering effects. Optical absorption and low-temperature photoluminescence measurements confirmed the XRD and TEM results.

Deuterium separation in formaldehyde by an intense pulsed CO2 laser

The first CO2 laser enrichment of deuterium in the gaseous phase of formaldehyde is reported. Irradition with pulses from a ∼20‐MW CO2 TEA laser focused strongly onto the sample yields highly efficient selective photodissociation of the formaldehyde molecule. A deuterium enrichment factor of approximately 40 is found for formaldehyde gas at an initial pressure of 20 Torr, after irradiation with 300 laser pulses.

Crystallization in fluorinated and hydrogenated amorphous silicon thin films

The amorphous‐to‐crystalline (AC) transition of amorphous Si thin films containing fluorine or hydrogen is studied by transmission electron microscopy. The AC transition can be described quantitatively by the incubation time prior to the onset of crystallization t0. This parameter is found to decrease exponentially with temperature with an activation energy of 1.7 eV for a‐Si:F and 3.1 eV for a‐Si:H:D. It is found that during the crystallization process in a‐Si:F the crystallites organize as dendrite single crystals oriented along the 〈110〉 axis perpendicularly to the film surface. a‐Si samples that had been covered by Pd or Al crystallize at appreciably lower temperatures. In the case of Al lower activation energies of 0.7 eV for hydrogenated and 0.4 eV for fluorinated a‐Si are measured. In the case of Pd/a‐Si:H,F for both kinds of a‐Si an activation energy of 1.7 eV is found.

Ferroelectric properties of Cd1−xZnxTe solid solutions

Measurements of the spontaneous polarization P, x‐ray diffraction, birefringence, dielectric constant at different frequencies, and specific heat Cp of the Cd0.9Zn0.1Te alloy are presented. The results demonstrate that this system exhibits a diffuse, second‐order ferroelectric transition. The transition is of order‐disorder type as deduced from the dielectric measurements. It is found that: (a) The birefringence is proportional to P, as expected from a system with a piezoelectric paraelectric phase; (b) the heat capacity is given by Cp=(Tc/C)‖(PdP/dT)‖, where C is the Curie constant. One of the main phenomena observed in these solid solutions is the instability of the ferroelectric phase: Once the neighborhood of the transition temperature is reached, the transition disappears upon subsequent cooling. A hypothesis for this instability is offered in terms of a two‐state configuration‐coordinate diagram.

Dispersion and attenuation of the eigenwaves for light propagation in heiicoidal liquid crystals

Abstract The dispersion curves and attenuation factors of the Bloch eigenwaves in heiicoidal liquid crystals (chiral smectic C and cholesterics) are calculated using the 4 × 4 characteristic matrix method. Four possible types of eigenmodes are seen to exist in the medium. The reflection peaks, which correspond to odd parts of the full pitch, are total reflection peaks, while those corresponding to even parts of the full pitch are composed of three branches at high incidence angles, where the outer branches are selective reflection and the central one is a total reflection peak. Selective reflection regions occur only at the edges of the Brillouin zones, while total reflections can occur anywhere inside the zones. The former are interpreted according to the coupled mode theory as resonant Bragg reflections, with the latter as exchange Bragg reflections. When the tilt direction exceeds the propagation direction, the central branch of the even sequence of peaks starts to be due to resonant Bragg reflections....

Arrhenius parameters and the compensation effect in crystallization and diffusion in amorphous Si:H(F) in the presence or absence of metal contacts

A kinetic many‐body electron‐related model of crystallization in a‐Si:H(F) in the presence and absence of metal contacts is proposed. The model expresses the parameters of the formation of crystalline nuclei in terms of atomic and electronic material characteristics. The model is based on the kinetic electron‐related theory of thermally activated rate processes in solids. The model considers picosecond atomic and electronic phenomena occurring in the nanometer vicinity of strongly fluctuating Si atoms executing diffusionlike jumps over energy barriers to more ‘‘ordered’’ positions which are associated with the formation of crystalline nuclei. The influence of random picosecond fluctuational heatings (coolings) in the nanometer vicinity of hopping atoms on the nucleation (crystallization) rate is considered. The following main results are obtained. (i) The Arrhenius‐like equations for the nucleation time tN linking it with the Si self‐diffusion in the metal/semiconductor interface are found from the kineti...

Unusual room-temperature intermixing and oxidation in copper deposited on a fluorinated amorphous silicon system

The room‐temperature behavior of Cu thin films on crystalline and amorphous silicon (c‐Si and a‐Si) substrates was studied by Auger electron spectroscopy. All samples, except Cu/ a‐Si:F, behaved as usual metal on silicon systems. It was found that F greatly enhances Cu‐Si intermixing which proceeds up to the sample surface. After prolonged room‐temperature exposure to air, this system settles into the unusual configuration: Cu/SiO2/a‐Si:F. The effect is explained in terms of the permeability of Cu3Si to oxygen and the low solubility of Cu in SiO2.

Laser‐induced structural instabilities in amorphous materials

Amorphous materials irradiated by cw lasers undergo structural changes, which depend quasiperiodically on the irradiation time. The changes in the transmitted light intensity are related to the changes of the Raman line shape. The structural changes between metastable states are favored by the photogenerated free carriers.

Correction to the elastic constants of AISb

The elastic constants of AISb are recalculated using the correct value for the density of this compound. The corrected elastic constants are C11 = 8.769,c12 = 4.341, and C44 = 4.076 in units of 1011 dyn/cm2.

Structure of PECVD Si:H films for solar cell applications

Abstract The structure of undoped SiːH films and solar cells deposited under different hydrogen concentration and substrate temperatures were studied. The characterization techniques used were XRD, Raman spectroscopy, TEM, optical absorption, and hydrogen effusion. The high concentration films were amorphous in the as-deposited state but crystallized upon annealing at 700°C. Middle and low concentration films were nanocrystalline (nc) and remained nc up to 800°C annealing. A theoretical explanation is given for the stability of these films. Such films, on glass substrates, had optical absorption spectra close to those of amorphous material. The solar cell samples, showed some nc morphology in all-concentration states.