A. Chack

Structural evolution of SnO2TiO2 nanocrystalline films for gas sensors

Thin films (50‐200 nm) of SnO2TiO2 were deposited on SiO2:(001)Si substrates by RF-sputtering and by molecular beam before they were annealed in vacuum at 200‐900°C. In-situ TEM, XRD, SEM, Raman and IR-spectroscopy were used to analyze the structure transformations in the SnO2TiO2 films. In the as-deposited state, the films are amorphous. They crystallize at higher temperatures (starting at about 500°C) forming nanosized grains. The problem of the spinodal decomposition in the SnO2TiO2 system observed earlier at high temperatures is discussed also for low-temperature processing. The stoichiometry of the films of both groups (reactive ion sputtered and high-vacuum e-gun sputtered) is being compared.

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.

ORDERING DEPENDENCE OF PYROELECTRICITY IN GAXIN1-XP

The pyroelectric coefficient of GaxIn1−xP thin films epitaxially grown on GaAs was measured on samples with x close to 0.5 and order parameter, η, varying between 0.02 and 0.34. It was found that the pyroelectric coefficient varies with the square of the order parameter in accordance with theory. This work also shows that the perfectly random alloy, η=0, is pyroelectric as well.

Spontaneous Ordering in Thin Polycrystalline Cd1–xZnxTe Films during Annealing

Polycrystalline films of Cd 1-x Zn x Te (0 < x < 1) of 0.1 μm thickness on glass substrates were grown by Metal Organic Chemical Vapor Deposition at 480°C. X-ray diffraction and transmission electron microscopy (TEM) methods were used for the film characterization. TEM in-situ heating the Cd 1-x Zn x Te films in the range of 200 to 300°C caused the following structure transformations: strong recrystallization, twinning, and partial decomposition. The self-diffusion coefficient, as estimated by TEM in-situ recrystallization monitoring at 300 °C was found to be on the order of 10 -8 cm 2 /s. A CuPt-type superstructure grew spontaneously in the films during the film annealing.

Effects of doping on the kinetics of laser-induced low-temperature crystallization of amorphous silicon

Substantial effects of boron and phosphorus doping on the kinetics of laser-induced crystallization (LIC) in hydrogenated amorphous silicon (a-Si:H) are reported. A kinetic nanoscopic electron-related LIC model that suggests predictions and explanations of observed effects of B and P doping on the LIC temperatures and crystallite size in a-Si:H is presented. The LIC is considered to be the integral effect of a huge number of nanoscale picosecond material reconstructions, each of which is generated by a nanoscopic short-lived (picosecond) large-energy fluctuation. The LIC in doped a-Si:H occurs at temperatures substantially lower than those found in the crystallization in a furnace. Crystallite size in B-doped a-Si:H is half of that in P-doped and undoped material.

Cross-section of Si:H solar cells prepared by PECVD at the edge of crystallization

Abstract The cross-section of p–i–n Si:H solar cells deposited by PECVD at 13.56 MHz on ZnO-covered glass substrates was studied by TEM and Raman spectroscopy. Two cells prepared under different conditions were compared. Independently of the deposition parameters, the i-layer of the SiH films was found to consist mostly of nanocrystallites agglomerated in filaments oriented along the growth direction, surrounded by amorphous material. No incubation layer for the crystallites was seen, and it was shown that the crystallization is not influenced by the Ag contact. No drastic difference in morphology was observed by TEM between the relatively high (6.1%) and low efficiency (2.9%) single-junction solar cells studied. However, Raman spectroscopy showed the high efficiency cell to be more uniform and have less amorphous material.

Structure of Si:H Films Fabricated by Plasma-Enhanced Cvd using Hydrogen Diluted Plasma

The structure of undoped Si:H films deposited at a high rate of 6-9 A/s in an RF (13.56 MHz) plasma from hydrogen-silane gas mixtures at various substrate temperatures was studied using TEM (with in-situ annealing), XRD, Raman spectroscopy, optical absorption and hydrogen effusion. It is found that under our conditions the amorphous to crystalline transition occurs in a relatively narrow range of parameters, influenced mainly by hydrogen dilution and to a lesser degree by the substrate temperature. In the crystalline range the material is found to be nanocrystalline (average grain size 20 nm) and the crystals are essentially stable up to 800 o C annealing. The crystal structure of a mixed amorphousnanocrystalline phase of samples deposited near the edge of crystallinity is also found to be rather stable. Nanocrystalline Si films deposited under these latter deposition conditions reveal in hydrogen effusion a relatively compact material and show high solar cell efficiencies (6-8%) when incorporated as i-layers in pin solar cells.

Influence of ferroelectricity on the electrical properties of Cd1 − xZnxTe solid solutions

Abstract Measurements of the electric conductivity and the Hall effect near the ferroelectric-paraelectric transition temperature of Cd1 − xZnxTe are presented. We observed anomalies in the temperature variation of the carrier density, as well as of the mobility. We relate this behavior to the ferroelectric properties. In particular we show that ( ∂E ∂T ) p , where E is the position of the acceptor level, is proportional to the specific heat. We demonstrate the simultaneous appearance of a change in the acceptor energy level and a change in mobility of carriers at the phase transition of a ferroelectric semiconductor.

New phonon in cadmium zinc telluride

The discovery of an unreported phonon in Cd/sub 1-x/Zn/sub x/Te is presented. This phonon is only present in thin films, and only when 0<x<1. The phonon is seen at 195 cm/sup -1/ and the Raman line is a resonant at an excitation in the vicinity if 600 nm. We believe this phonon is caused by microcrystalline clusters of ZnTe embedded in the Cd/sub 1-x/Zn/sub x/Te film, however, this assignation is not final, as further experiments are needed to rule out other possibilities such as a phonon from the ordered phase.