Electrical properties of TeAsGe films

The effect of temperature T, electrode separation d and substrate temperature T_s on the I–V characteristics of amorphous films 3500 Å thick, vapour deposited from the alloy TeAsGe (53:36:11 at. %) onto a glass substrate at room temperature, were investigated. The material displayed the behaviour of a negative resistance device with a memory. The behaviour for T=constant is described by the relation V = CIexp (?αI), where C and α are constants for a specimen at constant temperature. The threshold voltage at which the off state transforms to the negative resistance state decreased with T according to the relation V_th = V₀exp (E_v/2 kT), where E_v = 0.21 eV. V_th increased with d and decreased with T_s and was related to changes in resistance and structure. Microscope examination showed the formation of filaments containing recrystalized structure arising from Joule heating.     

Electrical transport properties of amorphous Se78−x Te22Bi x films

D.C. Conductivity measurements on the thin films of a-Se_78–x Te₂₂Bi _x system (where x = 0, 0.5, 2 and 4) are reported in the temperature range 213–390 K and the density of states (DOS) near the Fermi level is calculated using dc conductivity data. It is found that the conduction in all the samples takes place in the tails of localized states. The conduction in the high temperature region 296–390 K is due to thermally assisted tunneling of electrons in the localized states at the conduction band edge. In the low temperature region 213–296 K conduction takes place through variable range hopping in the localized states near the Fermi level.     

Electrical properties of polyimide Langmuir-Blodgett films sandwiched between superconducting electrodes: detection of microwaves

We have examined the electrical properties of ultrathin insulating layers of cadmium arachidate (C₂₀), polyamic acid long alkyl amine salts (PAA) and polyimide (PI) by using tunnel junctions with structures of (Pb-Bi)/PAA (or C₂₀)/(Pb-Bi), Au/PI/(Pb-Bi) and Au/PI/Au. Typical current-voltage (I-V) characteristics of weakly coupled superconductors were obtained for (Pb-Bi)/PAA (or C₂₀)/(Pb-Bi) junctions with one, two or three deposited layers at a temperature below the critical temperature of a Pb-Bi alloy. On the contrary, typical I-V characteristics of tunnel junctions were obtained for Au/PI/(Pb-Bi) and Au/PI/Au junctions, when the number of deposited layers is greater than about 30. Finally, we concluded that the efficiency of detecting microwaves is excellent in (Pb-Bi)/PAA (or C₂₀)/(Pb-Bi) junctions, although bridging filaments exist in the PAA (or C₂₀) Langmuir-Blodgett (LB) layers, and that the PI LB layers become good electrical insulating spacers when the thickness of deposited layers is more than about 11 nm.     

5. VCNR type characteristic in thin film amorphous TeGeAsSi films

In the hope of further understanding the memory and switching effects of amorphous evaporated semi-conductor films, the current-voltage curve of the quarternary system TeGeAsSi was experimentally examined and our findings are discussed below.     

Transient photoconductivity in amorphous thin films of Ge22Se78

Transient photoconductivity measurements on amorphous thin films of Ge₂₂Se₇₈ are reported in the present paper. The measurements were made in red light (660 nm) in the temperature range 302–352 K at various levels of illumination. The transient photocurrent was also studied for various illumination times. At all temperatures and intensities and for all illumination times the decay in the photocurrent was found to be non-exponential. The decay was found to be independent of temperature in the temperature range 302–322 K. However, it became faster at higher temperatures (above 322 K). The decay became slower as the intensity or illumination time was increased. The results are explained in terms of recombination within localized states in the mobility gap of this material.     

Electrical breakdown in amorphous As2S3 films

We investigated the electrical breakdown under d.c. electric fields of amorphous As₂S₃ films about 100–1000 Å thick sandwiched between two aluminium electrodes. It was found that the breakdown field decreased with increases in either the temperature or the thickness of the sample. Attempts were made to fit the observed electrical breakdown voltages with several previously proposed theories of thermal and electrical breakdown. It was found that a satisfactory fit could only be obtained by a modification of Klein's theory provided that the current was observed to depend on the voltage in an exponential manner in accordance with the Poole mechanism in which the carriers are released from high density localized states within the forbidden energy region. According to this modification, the breakdown field should depend inversely on the temperature and this agrees well with the experiment. The formation of localized conducting channels within the sample because of breakdown was verified by taking a scanning electron micrograph of the junction area of the broken film.     

Preparation and investigation on glasses in the Te46As32Ge10Si12 and Te41As37Ge10Si12 systems

Thin films of Te_46–xAs_32+xGe₁₀Si₁₂ (x=0,5) of different thicknesses are deposited on glass substrate by vacuum evaporation. X-ray diffraction revealed the formation of amorphous films. The value of the optical band gap, E _g, is found to increase with the thickness of the films and with increasing As content. The films are heat treated at different elevated temperatures from 298 to 423 K. The values of E _g are found to decrease with increasing temperature of heat treatment. The band tail, E _e, obey Urbach's empirical relation.     

Thickness dependence of optical parameters for Ge–Se–Te thin films

The present work deals with thickness dependent study of the thin films of Ge₁₀Se_90 − xTe_x (x = 0, 10) chalcogenide glasses. Bulk samples of Ge₁₀Se₉₀ and Ge₁₀Se₈₀Te₁₀ have been prepared by melt quenching technique. Thin films (thickness d = 800 nm and 1100 nm) of the prepared samples have been deposited on glass substrate using vacuum evaporation technique. The optical parameters i.e. optical band gap (E_g^opt), absorption coefficient (α), refractive index (n) and extinction coefficient (k) are calculated from the transmission spectrum in the range 400–1500 nm. The optical band gap decreases with the increase of thickness from 1.87 ± 0.01 eV (d = 800 nm) to 1.80 ± 0.01 eV (d = 1100 nm) for Ge₁₀Se₉₀ and from 1.62 ± 0.01 eV (d = 800 nm) to 1.48 ± 0.01 eV (d = 1100 nm) for Ge₁₀Se₈₀Te₁₀ thin films.     

Electrical transport in amorphous Se1−xSbx thin films

Amorphous Se_1–x Sb _x (x=0.1, 0.15 and 0.2) films of different thicknesses were prepared by the electron gun evaporation technique. Their structure was studied using reflection or transmission electron diffraction patterns. The d.c. electrical conductivity, thermoelectric power and magnetoresistance measurements, were made in the temperature range 80–300 K. These measurements confirm intrinsic conduction in the entire range of investigation, and were found to be independent of the film thickness. The effect of antimony impurity on electrical transport properties of selenium is understood in terms of the reduction of the Se₈ ring population.     

2.3 Investigations of metal contacts to amorphous evaporated Ge films

Amorphous Ge films are used as non-injecting ohmic contacts to high-resistivity n-silicon radiation detectors, but the function of this contact is not yet fully explored. One part problem is the role of the metal films used as external contacts to the amorphous film. In this paper we investigate the function of different contacting metals, such as Au, Al, Cr by measuring the I-V-characteristics of sandwich structures with two metals on both sides of the amorphous evaporated Ge film (of typical thickness 1 μm). It was found that while the symmetric CrGeCr structure (also AuGeAu) had low resistance (leading to resistivity values of ?4 × 10⁴ωcm for the Ge film), the unsymmetric structures e.g. AlGeCr and AlGeAu, showed higher resistance. This was again found for the AlGeAl structure, which also showed some polarity dependence. Rutherford backscattering was used to investigate diffusion effects between the layers; also the microstructure of the Ge films was investigated by electron microscopy.     

Chalcogenide coatings of Ge15Sb20S65 and Te20As30Se50

Chalcogenide coatings are investigated to obtain either optical components for spectral applications or optochemical sensors in the mid-infrared. The deposition of Ge(15)Sb(20)S(65) and Te(20)As(30)Se(50) chalcogenide glasses is performed by two physical techniques: electron-beam and pulsed-laser deposition. The quality of the film is analyzed by scanning electron microscopy, atomic force microscopy, and energy dispersive spectroscopy to characterize the morphology, topography, and chemical composition. The optical properties and optical constants are also determined. A CF(4) dry etching is performed on these films to obtain a channeled optical waveguide. For a passband filter made by electron-beam deposition, cryolite as a low-refractive-index material and chalcogenide glasses as high-refractive-index materials are used to favor a large refractive-index contrast. A shift of a centered wavelength of a photosensitive passband filter is controlled by illumination time.     

Ellipsometry and optical measurements on amorphous thin films of As2Se3

Thin films of amorphous AsSe_3/2 have been prepared by thermal evaporation of the material under a vacuum of 1.33×10^?3 Pa. Reflectivity, transmission and ellipsometric measurements of the films have been carried out. The optical energy gap and the absorption coefficient as a function of wavelength were obtained. Two absorption bands were observed and interpreted in terms of defects in the AsSe_3/2 system (homopolar bonds). Analysis of reflection and transmission spectra shows that the electron density at band tails of both conduction and valency bands follows N(E)?E^1/2 (Taue plots). No considerable variations were observed on changing the film thickness.     

A structural model of the amorphous alloy As0.40Se0.30Te0.30 by random technique

An analysis is performed of the atomic radial distribution function of the amorphous alloy As_0.40Se_0.30Te_0.30, obtained from the quenching of the molten mixture of the elements. A spherical-shaped model of this alloy has been studied by means of the random Monte Carlo method. The model describes the experimental radial distribution function quite well.     

Electrical and optical properties of evaporated Ag10As30Te60 thin films

The temperature dependence of the electrical resistance and the spectral dependence of the optical absorbance of evaporated Ag₁₀As₃₀Te₆₀ thin films were studied. It was found experimentally that the heat treatments cause significant changes in the electrical resistance and the optical absorbance of the chalcogenide film. The results obtained were correlated with the X-ray analyses.