Effect of leakage current and dielectric constant on single and double layer oxides in MOS structure

MOS structure of Al/Al₂O₃/n-Si, Al/TiO₂/n-Si and Al/Al₂O₃/TiO₂/n-Si was obtained by deposition of Al₂O₃ and TiO₂ on silicon substrate by RF Magnetron Sputtering system. The total thickness of the oxide layer ~ 40 ± 5 nm in the MOS structure was kept constant. Samples were characterized by X-Ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), Impedance analyzer and Current-voltage (J-V) characteristics. The variations in the dielectric constant and tan δ of the MOS capacitor in the frequency range of 1000Hz-1MHz were measured by impedance analyzer. The variation in dielectric constant of the Al/Al₂O₃/TiO₂/n-Si multilayer compared to single layer of Al/Al₂O₃/n-Si and Al/TiO₂/n-Si is due to high probability of defects, lattice mismatch and interface interactions. The steep rise of Tan δ values in the Al/Al₂O₃/TiO₂/n-Si structure is due to the resonance effect of both Al₂O₃ and TiO₂ layers. The leakage current mechanisms of MOS structures were extracted from Schottky coefficient and Poole-Frenkel coefficient. Theoretical values of Schottky coefficients (β_SC) and Poole-Frenkel coefficients (β_PF) for each sample were estimated using the real part of the dielectric constant. The experimental values were calculated from J-V characteristics and compared with theoretical values. The appropriate model has been proposed. It was found that Schottky and Poole-Frenkel mechanisms are applicable at low and high field respectively for all MOS structures. The combination of Al/Al₂O₃/TiO₂/n-Si is found to be a promising structure with high dielectric constant and low leakage current suitable for MOS devices.