Electron spin resonance and some electrical and optical properties of GeO2/SiO x thin films

Electron spin resonance measurements show a reduction in the spin density associated with unpaired electron dangling bonds of a mixed complex of co-evaporated GeO₂/SiO _x films compared with that of SiO _x films investigated under similar conditions. Unlike the behaviour of BaO/GeO₂ complex films, these samples show a decrease in the optical gap as the thickness increases for films of fixed compositions. However, it increases as the SiO percentage increases in films of the same thickness. D.C. measurements at low applied fields below 10⁵ V cm^–1 agree well with the spin density results.     

A.c. conduction in evaporated MoO3/SiO amorphous thin films

The dielectric properties of vacuum-deposited MoO₃/SiO films of different compositions studied in the frequency range 10² to 10⁶ Hz at various temperatures (193 to 393 K) are reported. The properties of the film capacitor are found to be temperature and frequency dependent. The decrease in a.c. conductance with increasing concentration of SiO in MoO₃ may be attributed to the increasing number of trapping centres generated in MoO₃/SiO films during the evaporation process.     

Structural and electrical properties of evaporated ZnTe thin films

Zinc telluride thin films of various thicknesses are deposited by vacuum evaporation onto glass substrates at room temperature. The X-ray diffraction technique is used to determine the crystalline structure and grain size of the films, respectively. The structure was found to be cubic with preferential orientation along a (1 1 1) plane and crystallite size of about 50-80 nm. The degree of preferred orientation and crystallite size are increased as the film thickness increases. The current density-voltage (J-V) characteristics showed ohmic conduction in the lower voltage range and space-charge-limited conductivity in the higher voltage range. Capacitance measurements indicated that the films have a relative permittivity, ε_r, of approximately 8.19. Further evidence for this conduction process was provided by linear dependence of V_t on d². Analysis of the results yielded hole concentration , which is correlated with the structural properties.     

Electrical properties of vacuum evaporated PbSnS3 thin films

Electrical conduction of evaporated PbSnS₃ films of thickness ranging between 0.1 and 2.0 μm were studied by measuring the dc current in both parallel (planar) and transverse (cross plane) directions to the substrate surface. Conduction mechanisms relevant to various regions of the current-voltage characteristics are discussed. The obtained film conductivities were of the order 10⁻⁵ S cm⁻¹ at room temperature and increased exponentially with increasing temperature. No consistent modification of the conductivity values and nature were observed when the films are doped with CdCl₂, PbCl₂ and CuI impurities. While planar conductivity activation energies were constant with voltage and increased slightly with deposition temperature, the cross plane values were found to depend on both voltage and film deposition temperature.     

Some d.c. properties of evaporated thin aluminium fluoride films

The measurements of some d.c. properties of samples of evaporated aluminium fluoride thin films sandwiched between metal electrodes are reported. The technique of Rutherford scattering of megaelectronvolt alpha particles was used to study the stoichiometry and impurity content of the films. Measurements were made in the temperature range 77–393 K, and the thickness of the films ranged from 800 to 4000 Å. At high temperatures and high fields the conduction process can be explained on the basis of the Poole-Frenkel mechanism in amorphous materials, as described by Hill. At low temperatures the conductivity fits the Mott relation $\text{σ ∝ exp(-T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$. The breakdown electric field, measured as a function of film thickness d in the range 700–7500 Å, obeys the Forlani-Minnaja relation.     

Electron spin resonance and electrical properties of co-evaporated SiO/SnO2 thin films

Electron spin resonance (ESR) measurements are reported for various compositions of SiO/SnO₂ thin films and indicate a decrease in the value of spin density for increasing SnO₂ content in the SiO. Annealing of the device further reduces the value of the spin density. The electrical activation energy and resistivity have been found to increase after annealing of the device and the results are compared with the optical and ESR measurements.     

Structural, optical and electrical properties of thermally evaporated Ag2S thin films

Thin films of Ag₂S are prepared on glass and quartz substrates by a thermal evaporation method. The structural studies show that the films are well crystallized with an acanthite structure. The optical properties of the films are investigated using spectrophotometric measurements of transmittance and reflectance at normal incidence in the wavelength range 500-2200 nm. The refractive index, n, and the absorption index, k, of Ag₂S are determined from the absolute values of the measured transmittance and reflectance. The dispersion of refractive index in Ag₂S is analyzed using the concept of the single oscillator. Within this concept the oscillator energy, E₀, and the dispersion energy, E_d, can be determined as 5 and 32.5 eV, respectively. It is interesting to note that Ag₂S appears to fall into the ionic class. The values of the lattice dielectric constant and the ratio of the carrier concentration to the effective mass are also determined as 7.77 and 1.7×10⁴⁷ kg⁻¹ m⁻³, respectively. The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region reveals an indirect allowed transition with a band gap of 0.96 eV and associated phonons of 0.05 eV. Measurements of the dark electrical resistivity is studied as a function of film thickness and temperature. The dark electrical resistivity decreases with increasing film thickness. Graphical representation of log ρ as a function of reciprocal temperature yields two distinct linear parts indicating the existence of two activation energies ΔE₁ and ΔE₂ as 0.18 and 0.28 eV respectively. Discussion on the obtained results and their comparison with the previous published data is also given.     

Crystal structures and electrical properties of titanium films evaporated in high vacuum

Thin titanium films were deposited on glass substrates maintained at various elevated temperatures. X-ray studies showed that higher substrate temperatures caused growth of the $\text{{10}\text{1}\text{1}}$ orientation in preference to the {0002} orientation and also increased the grain size in the films. The dependences of the electrical resistivity and the Hall coefficient on the substrate temperature were investigated: the variation of the resistivity with substrate temperature can be explained qualitatively in terms of the grain sizes of the films, but the main feature of the Hall coefficient was found to be closely related to the differences in preferred orientation of the crystals.     

D.c. conduction studies on thermally evaporated neodymium oxide thin films

Aluminium-neodymium oxide-aluminium thin film capacitors have been prepared by thermal evaporation and the d.c. conduction properties of these films have been studied. The thicknesses of the films have been determined by a multiple beam interferometer. The current-voltage power-law dependence showed that the conduction in these films is space-charge limited. The linear dependence of the current density on the square root of the applied field confirmed the exponential trap distribution. The trap density has been found to be of the order of 10²⁶ m^–3. It has also been observed that the Schottky type of conduction is predominant in the high-field region and the height of the Schottky barrier has been determined. It is seen that the conduction mechanism is an activated process with the activation energy decreasing with increasing field.     

Composition dependence of electrical properties of simultaneously evaporated Sb-Se thin films

Amorphous thin films of Sb-Se are prepared using the three-temperature method. The films are prepared with atomic compositions from 5–90 at.% Sb. The electrical resistivity, Hall voltage and thermoelectric power of annealed samples have been measured in the temperature range 25 to 250°C. On heat treatment the sharp fall of resistance of the annealed films is attributed to radical structural transformation from amorphous to crystalline. Electrical resistivity, Hall constant and thermoelectric power are found to vary with thickness and composition of the film. Paper presented at the 7th International Conference on Thin Films, New Delhi, December 7–11, 1987     

Structural characterization of vacuum evaporated ZnSe thin films

Thermally evaporated ZnSe thin films deposited on glass substrates within substrate temperatures (T _s)at 303 K-623 K are of polycrystalline nature having f.c.c. zincblende structure. The most preferential orientation is along [111] direction for all deposited films together with other abundant planes [220] and [311]. The lattice parameter, grain size, average internal stress, microstrain, dislocation density and degree of preferred orientation in the film are calculated and correlated with T _s.     

Structural and optical properties of vacuum evaporated CdxZn1−xS thin films

II–VI semiconductors are of great importance due to their applications in various electro-optic devices. Sulphides of zinc and cadmium have been utilized effectively in various opto-electronic devices. We have prepared vacuumed CdZnS films by the vacuum evaporation method. Wide band gap binary films have wide application in solar cells. The structural and optical properties of these films have been studied. The band gap of these films is studied by absorption spectra in the wavelength range 400–650 nm. The films have a direct band gap, which varies from 3.50 eV for zinc sulphide to 2.44 eV for cadmium sulphide. The X-ray diffraction pattern of these films for structural analysis is also reported.     

Enhanced birefringence in vacuum evaporated silicon thin films

We report an experimental study of enhanced optical birefringence in silicon thin films on glass substrates. Form anisotropy is introduced as an atomic-scale morphological structure through dynamic control of growth geometry. The resulting birefringence is large compared with naturally anisotropic crystals and is comparable to two-dimensional photonic crystals. The films are fabricated with serial bideposition onto a substrate held at a fixed tilt angle relative to the impinging vapor. Films were analyzed by spectroscopic ellipsometry and scanning electron microscopy, the latter clearly revealing form anisotropy in a morphology of bunched columns perpendicular to the deposition plane with dimensions of hundreds of nanometers and smaller. The observed linear birefringence varies with wavelength and tilt angle, with a maximum of 0.4 at a 630-nm wavelength and 0.25 at 1500 nm.     

D.c. conduction in thin films of SiO/In2O3 before and after electroforming

Electrical measurements on thin co-evaporated SiO/In₂O₃ films before and after electroforming are reported. The high-field conduction (expressed in terms of circulating currentl _c and bias voltageV _b) in thin film sandwich structures of Al–SiO/In₂O₃-Al and Cu-SiO/In₂O₃-Cu is found to obey a relation of the form logI _c V _b ^1/2 . The electroformed samples show voltage-controlled negative resistance, voltage memory, thermal-voltage memory and pressure-voltage memory effects and the results are explained in terms of the filamentary model of electrical conduction.     

A.c. conduction mechanism and dielectric properties of co-evaporated SiO/B2O3 amorphous thin films

The a.c. electrical properties on Al-SiO/B₂O₃-Al sandwich devices with various compositions of SiO/B₂O₃ were studied over a frequency range of 2×10² to 1×10⁶ Hz and in the temperature range 158 to 463 K. The a.c. conductance G varies with frequency according to the relation G ^s, where the exponent s was found to be 0.92 (at 158 K) in the frequency range 8×10² to 2×10⁴ Hz and above 10⁵ Hz the conductance shows a square law dependence on frequency. These results suggest that the a.c. conduction mechanism in SiO/B₂O₃ thin films is due to an electronic hopping process. The numbers of localized sites were calculated using the conductivity relations given by Elliott and by Pollak and the results are compared. The effects of composition, temperature and annealing on the dielectric constant and loss factor were studied. The relative dielectric constant and loss factor were found to decrease with the increase of B₂O₃ content in SiO. Annealing of the samples reduces the values of the dielectric constant, loss factor tan , temperature coefficient of capacitance and a.c. conductivity. The variation in capacitance with the composition of SiO/B₂O₃ was investigated at room temperature, the results being normalized to 10 kHz. The dispersion was found to decrease with the addition of B₂O₃ into SiO.