Role of thickness on the optical and electrical properties of physically evaporated TiO2 thin films and silicon-based n-TiO2/p-Si heterojunction (HJ) configurations

TiO2 thin films of about 50, 100, and 150 nm thicknesses and their n-TiO2/p-Si heterojunctions (HJs) were prepared by physical evaporation of TiO2 powders. The role of thickness on the electrical properties of the HJ diodes and the optical properties of the films were compared. The morphological roughness and granularity of the films were determined by the SEM imaging. UV-vis spectroscopy of the films showed that the optical transmittance and bandgap values decreased with increasing thickness, whilst the refractive index, extinction coefficient, and Urbach energy increased. The 50 nm thick TiO2 thin film has a relatively high FOM value of 7.99 x 10(-5) Omega(-1). Electrical characteristics of the HJs were studied by current-voltage (I-V), and capacitance/conductance-voltage (C/G-V) measurements with the thickness variable. Important diode parameters of the HJs such as ideality factor (n), barrier height (Phi(b)), series resistance (Rs), interface state density (Nss), built-in potential (Vbi), and acceptor density (NA) were calculated. The n and Phi(b) of the HJs were determined by the I-V method to be 2.27, 2.48, 2.11 and 0.61, 0.55, 0.58, respectively. The Phi(b) values obtained from the C-V properties were 0.73, 0.62 and 0.64, respectively, which were higher than those obtained from the I-V method and decreased with thickness. The Nss of the HJs, calculated by the I-V approach are 2 x 10(17), 3.33 x 10(17) and 1.09 x 10(18) eV(-1)cm(-2), respectively. The capacitance-voltage/frequency (C-V/F), conductance-voltage/frequency (G-V/F), and impedance-frequency (Z-F) characteristics of the HJs were investigated as a function of TiO2 layer thickness. All measurements showed that the optical and electrical properties of the films and HJs are affected by the thickness of the TiO2 layer, and that the thinner one (similar to 50 nm) may be suitable for optoelectronic device designs.