G. A. Khan

Infrared absorption spectra of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

The optical absorption spectra of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films have been studied. For higher photon energies, the absorption is found to be due to a direct forbidden electronic transition process from the oxygen 2p band to the vanadium 3d band in a similar way to that observed in crystalline V2O5. The exponential behaviour of absorption edge for lower photon energies is attributed to the electronic transitions between the tailed-off d-d states corresponding to V4+ ions. For co-evaporated V2O5/B2O3 films the optical energy gap is observed to increase with the increase in V2O5 content of the composite films.

The behaviour of SiOx/SnO thin dielectric films in an alternating electric field

The a.c. electrical conduction through SiOx/SnO thin films prepared by the co-evaporation technique has been investigated. The conductivity shows ωs-type behaviour. The values ofs lie between 0.75 and 0.41 depending upon the temperature. The conduction may be explained by a single polaron hopping process as proposed by Elliott. The dependences of capacitance, tan δ and loss factor on the frequency of applied field and the temperature are also discussed. It is suggested that the centres taking part in both d.c. and a.c. conduction processes are the same. Under similar conditions, the co-evaporated SiOx/SnO films are found to be more disordered than those of SiOx/SnO2.

A.c. conduction through MIM sandwich samples of evaporated thin films of V2O5 and V2O5/B2O3

The a.c. electrical properties of evaporated V2O5 and co-evaporated V2O5/B2O3 films have been investigated. The conductivity in both types of film follows the theory of Elliott for single polaron hopping. The behaviour of capacitance, tangent of loss angle and dielectric loss agrees with the model proposed by Goswami and Goswami. The co-evaporated films of V2O5/B2O3 show better dielectric properties than simple V2O5 with increasing content of B2O3. Because of their hygroscopic character, the films with B2O3 content larger than 40% are observed to be unstable in the atmosphere.

Electrical conduction through MIM structures of evaporated V2O5 and V2O5/B2O3 amorphous thin films

The electrical conduction through vacuum-evaporated thin films of V2O5 and V2O5/B2O3 in MIM structures has been investigated. The high-field behaviour of both types of film is in accordance with the Poole-Frenkel type of mechanism. The increase in B2O3 content in co-evaporated V2O5/B203 films results in a decrease in the conductivity of the composite films. This is attributed to the expansion of the resultant film structure due to the network-forming effect of B2O3. The covaporated thin films of V2O5/B2O3 with a molar content of B2O3 larger than 40% are observed to be unstable because of their hygroscopic nature.

The effect of composition and substrate temperature on the optical energy gap of SiOx/SnO amorphous thin films

The fundamental absorption edge of co-evaporated SiOx/SnO thin films of various compositions has been investigated. The results have been analysed by assuming optical absorption by non-direct electronic transitions. The optical energy gap is seen to decrease with increase in the molar content of SnO which increases the level of disorder of the complex system. The effect of substrate temperature during deposition on the optical absorption is also studied. The optical energy gap increases with the substrate temperature. This is attributed to the annealing of some of the dangling bonds and hence to an increase in the order of the system. The Urbach tail is related to the broadening of the absorption edge as a result of charged defects present in the films and related to the Franz-Keldysh effect.

Infrared spectra of some thin amorphous films of MoO3-In2O3 deposited by vacuum evaporation

The infrared absorption spectra of vacuum-evaporated MoO3-In2O3 thin films are reported for a series of films of varying compositions within the spectral range 4000 to 400 cm−1 and interpreted in relation to the spectrum of the pure MoO3 films. The affects of changes in composition, film thickness, substrate deposition temperature and annealing on the absorption bands are observed. In some cases a shift in band frequency is also observed. The gradual increase in ordering and decrease in valency state with increase of annealing temperature is observed in the infrared curves. Some new peaks appear which reveal the presence of a lower valency state in MoO3. The shift in band frequency may be attributed to the incorporation of Mo(VI) ions in the indium interstitial positions.

X-ray photoelectron spectroscopic studies of thin amorphous films of MoO3/In2O3 (deposited by co-evaporation)

Abstract The Mo and In 3 d 3/2 and 3 d 5/2 XPS core level peaks are compared with those of MoO 3 and In 2 O 3 in mixed oxide thin amorphous films of MoO 3 /In 2 O 3 deposited by co-evaporation. The effects of changes in composition, film thickness, substrate deposition temperature and annealing on the binding energies of Mo(3 d ) and In (3 d ) in MoO 3 /In 2 O 3 are presented. Slight changes in the Mo(3 d ) and In(3 d ) peak positions are observed with the addition of In 2 O 3 to MoO 3 . These changes in binding energy may be attributed to the presence of a In 3+ ions in an MoO 3 lattice. With the increase of sample thickness and substrate deposition temperature, no change in the electron spectra is observed. The decrease in the binding energy of the Mo(3 d ) and In(3 d ) doublet with increased annealing temperature may be due either to incorporation of oxygen vacancies and indium interstitials or to the formation of Mo and In species of lower oxidation state.

Electron spin resonance studies of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

Electron spin resonance studies of evaporated V2O5 and co-evaporated V2O5/B2O3 amorphous thin films have been made. For lower molar contents of B2O3, the co-evaporated V2O5/B2O3 films show poorly resolved hyperfine structure, whereas for higher content of B2O3 the hyperfine spectra are well resolved. This behaviour of films is attributed to the increase in the lifetime of a particular V4+ ion due to Anderson localization of charge, as the degree of disorder increases with increase in the molar content of B2O3. The unpaired electron at a given time is localized on a single 51V nucleus. The low intensity of ESR signal for higher concentration of V2O5 in the co-evaporated V2O5/B2O3 films has been related to the less effective concentration of V4+ ions due to antiferromagnetic coupling of the V4+ ions.

Electrical conduction through co-evaporated SiOx-SnO thin films

D.c. electrical properties of SiOx-SnO (300 nm) thin films forming part of a metal-insulatormetal (MIM) structure are reported. The conduction process is related to the field-assisted thermal excitation of electrons from the trapping centres in the forbidden band gap to the conduction band (Poole-Frenkel effect). The energy gap between the trapping level and the bottom of the conduction band in 80% SiOx-20% SnO in thin films (300 nm) is found to be equal to 0.50 eV. The level of conduction in the composite films is observed to increase with an increase in the content of SnO.

X-ray photoelectron spectroscopic (XPS) investigations of etching and annealing effects on thin films of MoO3

Abstract XPS core level measurements are used to observe the surface changes of MoO 3 films resulting from Ar + ion bombardment. It is observed that the molybdenum on the surface of the MoO 3 films changes under the ion bombardment to a lower oxidation state (Mo 4+ ). Surface damage following Ar + ion bombardment for a longer time is also observed by scanning electron microscopy. Some samples are annealed in vacuum and the colour of the samples is changed to blue as a consequence of the formation of Mo 5+ ions in the Mo 6+ oxide lattice. The new oxidation states formed due to annealing and etching the samples is attributed to the internal electron transfer from oxygen 2 p to the molybdenum 4 d level.