Electrical and optical properties of cobalt oxide thin films, prepared by ion-beam sputtering

Optical and electrical properties of cobalt oxide thin films obtained by ion-beam sputtering in argon atmosphere and argon with the addition of oxygen ($P_{\mathrm{Ar}}$ = 1.1$\cdot$10$^{-5}$ Torr) has been investigated in the work. Optical properties investigations showed that, for films of cobalt oxide obtained in argon atmosphere, the optical band gap is independent of the film thickness and is 3.24 eV, which is in the range of given in the literature for the CoO phase values. For cobalt oxide films obtained in mixed atmosphere of argon with the addition of oxygen, two direct optical transitions with energies of 1.45 and 2.1 eV were detected. The presence of two direct allowed optical transitions is associated with the variable valence of cobalt in the Co$_3$O$_4$ compound and the presence of two valence states Co$^{2+}$ and Co$^{3+}$. The dependences of specific electrical conductivity of the synthesized films on the magnitude of the electric field were studied. It was found that for all investigated samples, the electrical conductivity does not depend on the electric field strength up to the value of $E$ = 10$^6$ V/m. The nonlinearity of the dependence of specific electrical conductivity of the synthesized films on electric field strength at $E >$ 10$^6$ V/m is discussed in terms of the hopping conductivity model and the trap ionization model described by the Poole–Frenkel effect.