Physical properties of a transition metal oxide: optical and photoelectric properties of single crystal and thin film molybdenum trioxide

The optical absorption spectra of single crystals and thin films of MoO$_3$ have been measured in the temperature range 150 to 340 $^\circ$K with polarized light. At room temperature, the absorption spectrum of a single crystal consists of two peaks at $\lambda$ 4130 and $\lambda$ 3930 $\overset{\circ}{\mathrm A}$ with E || C and only one absorption peak a t 4250 $\overset{\circ}{\mathrm A}$ with E $\perp$ C followed by a rapid rise in absorption. The temperature and frequency dependence of the absorption coefficient in the edge over a range of absorption magnitudes 10$^2$-10$^5$ cm$^{-1}$ are described by expressions of the form $K(v.T)=K_0exp[-(\beta/kT) (E_0-hv)].$ The temperature dependence of the absorption edge was found to be linear from 340 to 150 $^\circ$K with a temperature coefficient of -6.2 x 10$^{-4}$ and -9.3 x 10$^{-4}$ eV/$^\circ$K for E || C and E $\perp$ C, respectively. The corresponding temperature coefficients in thin films are -2.7 x 10$^{-4}$ and -4.0 x 10$^{-4}$ eV/$^\circ$K for E || and $\perp$ to the film surface, respectively. Measurements have been made of the refractive indices of a single crystal and thin films. Ultraviolet irradiation of a thin film of MoO$_3$ produces a broad colour centre band having a maximum at 8700 $\overset{\circ}{\mathrm A}$. Several absorption peaks are resolvable in polarized light. On cooling down to 150 $^\circ$K, the peak position shifts to shorter wavelength by 400 $\overset{\circ}{\mathrm A}$ (4.67 x 10$^{-4}$ eV/$^\circ$K). An e.s.r. signal with g = 2.001 $\pm$ 0.005 has been observed in the colour film. The electrical conductivity has been measured on single crystals and polycrystalline samples in the temperature range 25 to 500 $^\circ$C, and the activation energies are found to be 1.83 $\pm$ 0.01 eV (intrinsic) and 0.29 to 0.70 eV (extrinsic). Photoconductivity has been measured in single crystals and thin films as a function of photon energy, temperature, and irradiation intensity. Trapping plays a significant role in the conduction phenomena. The thermal activation energies associated with different trapping levels were determined from the photoconductive decay curves and the electrical glow peaks measurements and were found to be in the range 0.16 to 0.64 eV.