M. Anwar

An XPS study of amorphous MoO3/SiO films deposited by co-evaporation

X-ray photoelectron spectroscopic (XPS) core-level spectra of MoO3/SiO thin films are presented. The effects of changes in composition, substrate temperature during deposition and annealing on the binding energy of Mo(3d) and Si(2p) core lines in mixed films are compared with those of MoO3 and SiO. Appreciable changes in Mo(3d) peak positions and slight changes in Si(2p) peak positions are observed. The change in binding energy of the Mo(3d) doublet may be attributed to the effective incorporation of silicon ions in an MoO3 lattice which may cause the molybdenum orbital to be a little less tightly bound. This helps in the internal electron transfer from the oxygen (2p) to the molybdenum (4d) level as a result of which the molybdenum is readily changed to lower oxidation states during heat treatment. XPS spectra show that the position of the Si(2p) core state shifts monotonically with increasing oxygen concentration from the value of 101.8 to 102.6 eV.

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.

The correlation of various properties of thin films of MoO3 and of the mixed oxide systems MoO3-In2O3 and MoO3-SiO

A discussion of the general properties of three systems of dielectric films i.e. MoO3 and the mixed oxide systems MoO3-In2O3 and MoO3-SiO is presented. Composition, film thickness, substrate deposition temperature and annealing all have a substantial effect on the structure and various properties of the films. The general properties of these three systems of dielectric films include analysis by X-ray photoelectron spectroscopy, UV-visible and infrared spectroscopy including the Fourier transform technique, electrical properties both d.c. and a.c. at both low and high fields and electron spin resonance. An attempt is made to show how the variations of properties depend on many disposable parameters. In particular the use of X-ray photoelectron spectroscopy shows how the local atomic and molecular bonding changes as a result of varying preparation parameters and this feature is a recent facility in the field of thin dielectric-semiconducting films. It is expected that the general discussion in this paper may help in the interpretation of results on other thin dielectric and semiconducting films.

The optical absorption edge in amorphous thin films of MoO3-In2O3

A study of the effects of changes in composition, film thickness, substrate deposition temperature and annealing on the optical properties of MoO3-In2O3 is presented. The results are found to be compatible with the reduction in the value of optical energy gap of these materials as the molar fraction of In2O3 in the MoO3 thin film increases. This decrease of optical gap may be attributed to the incorporation of In(III) ions in an MoO3 lattice. The decrease in optical band gap with increasing thickness may be interpreted in terms of the incorporation of oxygen vacancies which are also believed to be the source of conduction electrons in the MoO3-In2O3 complex. The decrease of band gap with increasing substrate temperature may be attributed to the enhanced ordering of the samples and the decrease of band gap with annealing may be attributed to a reduction in the concentration of lattice imperfections.

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.

An infrared spectroscopic study of vacuum evaporated amorphous thin films of MoO3-SiO

A series of amorphous MoO3-SiO thin films of varying compositions were investigated by infrared spectroscopy. In some cases a shift in band frequency was observed. Some new peaks appeared when SiO was mixed in MoO3. These new peaks are due to bond-rocking vibrations of the bridging oxygen atoms (430 to 440 cm−1), bending motion of the same oxygen atom (800 to 810 cm−1), stretching vibration of non-bridging oxygen atoms (876 cm−1) and bond-stretching vibrations of oxygen atoms (1040 to 1060 cm−1) in SiO. The band of 940 cm−1 is attributed to the absorption of oxygen atoms in its two-fold-coordinated bridging Si-O-Si bond at an isolated site indicating that there is no oxygen atom as a second-nearest neighbour to any other oxygen atom.

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.

Electronic conduction in thin amorphous MoO3/SiO films deposited by co-evaporation

A study of the effects of changes in composition and temperature on the electrical properties of MoO3/SiO thin amorphous films is presented. The high-field conduction is probably due to the Poole-Frenkel effect as it is in simple SiO. At low temperature and low field the electron hopping process is dominant but conduction at higher temperatures is a contact-limited process. The decrease in conductivity with increasing concentration of SiO in MoO3 may be attributed to the increasing number of trapping centres introduced in MoO3/SiO films during the evaporation process. The increase in conductivity in MoO3/SiO films with increasing temperature is attributed to the increasing concentration and higher mobility of charge carriers.