B. Chao

OPTICAL PROPERTIES OF GE:SB:TE TERNARY ALLOYS

The complex dielectric function of the three stoichiometric compositions GeSb4Te7, GeSb2Te4, and Ge2Sb2Te5 has been measured by spectroscopic ellipsometry in the energy range of 1.4–5.1 eV. Each composition was measured in the amorphous and in the two crystalline states, the metastable face-centered-cubic and the stable hexagonal phases. The complex dielectric function shows strong differences between the amorphous and the crystalline samples, these differences are responsible for the drastic contrast in the reflectivity spectra. The ellipsometric spectra are fitted with models which fulfilling the Kramers–Kronig relations. Thus the Forouhi–Bloomer model was used for the amorphous samples and the Lorentz harmonic oscillator model for the crystalline samples. The temperatures at which the phase transformations are achieved were determined by measuring the ac electrical resistance at 100 Hz while the samples were heated. The temperature derivative of the electrical conductivity shows two well defined peaks ...

CD SELF-DOPING OF CDTE POLYCRYSTALLINE FILMS BY CO-SPUTTERING OF CDTE-CD TARGETS

Cadmium self-doped CdTe polycrystalline films were grown on Corning glass substrates at room temperature by cosputtering from a CdTe–Cd target. The electrical, structural, and optical properties of the films were analyzed as a function of the Cd concentration. Films with a stoichiometric composition, and slightly below and above it, were prepared. In films where the Te exceeds 50 at. %, it is found segregation of Te and its electrical resistivity is about 107 Ω cm. In those with an excess of Cd, the electrical resistivity drops several orders of magnitude, the carrier concentration increases, and the resistivity activation energy drops. From these results, we concluded that using this deposition method, n-type Cd self-doped CdTe polycrystalline films can be produced.

The growth kinetics of rf sputtered Ba2Si2TiO8 thin films

Radio‐frequency sputtered barium titanium silicate (BST), Ba2Si2TiO8, thin films were grown on crystalline Si(100) substrates at substrate temperatures ranging from 750 to 955 °C and were characterized using x‐ray diffraction, optical microscopy, and scanning electron microscopy. The result of x‐ray diffraction analysis indicates that the BST films deposited at an optimum substrate temperature of 845 °C were strongly c‐axis oriented. The corresponding film growth rate in the direction normal to the film surface and lateral grain growth rate were 1.95 nm/min and 0.77 μm/min, respectively, at the initial stage of deposition. The former decreased with sputtering time and the latter increased with grain size. The fast lateral grain growth rate indicates a strong interaction between the overgrown BST film and the Si substrate. The increase in lateral grain growth rate suggests a surface diffusion controlled nucleation and growth mechanism in the initial stage of the deposition, and a coalescence mechanism domi...

Low temperature crystallization of sputtered carbon films

The crystallization of amorphous carbon films, under inert atmospheres, occurs at annealing temperatures above 800 °C. In this work we have found that when the annealing of carbon films is performed under atmospheric conditions, crystallization occurs at temperatures as low as 200 °C. The catalytic effect of oxygen in the crystallization process is understood in terms of the generation of a porous structure in the carbon film due to the vaporization of carbon oxides.

Optical and structural properties of sol-gel SiO2 layers containing cobalt

SiO2 coatings containing cobalt were prepared using the sol-gel method and the Si to Co nominal atomic ratio in the coatings was varied from 1.3 to 7.1. The structure and optical properties of the coatings, heat-treated in air at 300 and 500 °C, were characterized using optical, x-ray diffraction, and Auger depth profile measurements. The optical transmission data in the UV-visible range, in samples with low cobalt concentrations, show only the absorption bands associated with the tetragonal Co2+, regardless of the heat treatment temperature. Coatings with a larger amount of Co treated at 300 °C show absorption bands associated with both tetragonal and octahedral Co2+. When these coatings are heat treated at 500 °C, most of the cobalt migrates to the free surface of the coatings, where it is oxidized by the atmospheric oxygen and forms a top layer of Co3O4. The thickness of the cobalt oxide layer depends on the heat treatment temperature and on the Co concentration. To describe the absorption bands of bot...

Crystallization of SbTe Phase Change Optical Films

The aim of this work was to investigate the crystallization properties and crystallization temperature in SbTe thin films, with different content of Sb, using DSC, optical reflection at 650 nm and XRD measurements. DSC studies showed that crystallization temperature of the films depend on the Sb content. XRD measurements have shown that films with low Sb contents (less that 73 at. %) directly crystallized in orthorhombic Sb2nTe3 and films with higher Sb contents first crystallized in rhombohedral Sb phase and at more high temperature appear Sb2nTe3 stable crystalline phase. Optical measurements showed a gradual increase of reflectivity with the temperature depending on the Sb content. The results of this research show that it is possible to obtain mixtures with different crystallization temperatures, changing the Sb content in the SbTe alloys.

CdTe-Cd : a new semiconductor with polytype structure

Abstract Cd-doped CdTe thin films were prepared by sputtering on unheated glass substrates. Targets of stoichiometric CdTe with inclusions of elemental Cd were used to vary the Cd concentration of the films in the approximate range 47–55at.%. The amount of Cd in the films is directly related to the area of the Cd pieces on the CdTe target. The electrical and structural properties of the films were analysed using Hall measurements and X-ray diffraction. It is found that both properties strongly depend on the Cd concentration. For Cd concentrations below or equal to the stoichiometric value, the crystalline structure corresponds to the regular zincblende, and the electrical transport is dominated by the extended states. For Cd concentrations above the stoichiometric value, the X-ray data show an unusual 6H polytype structure and the electrical conduction takes place by hopping along localized states. It is proposed that this latter behaviour in the conductivity occurs because the excess of Cd atoms enter the CdTe lattice in interstitial sites, creating energy levels in the forbidden bandgap.

Effect of substrate temperature on the orientations of radio frequency sputtered Ba2Si2TiO8 thin films

The effect of substrate temperature on orientations of rf sputtered Ba2Si2TiO8 (BST) thin films grown on Si (100) and (111) substrates was investigated using x‐ray diffraction, optical microscopy, and scanning electron microscopy. The texture of the films deposited on Si (100) substrates varies from an amorphous structure at 750 °C, to a randomly oriented polycrystalline structure, then to (001), (111), (320), and (410) orientations as the substrate temperature increases to 950 °C. The sequence of textures coincides with the decrease in the planar atomic density of the films and the increase in the BST film/substrate interfacial energy for the corresponding orientations. The films deposited on Si(111) substrates show a similar temperature dependency of textures to that observed on films deposited on Si (100) substrates, but the optimum temperature for a particular orientation shifts to higher temperature than that for deposition on Si (100) substrate. The capillarity theory of nucleation is used to explai...