Defect Density-of-states in a-Si:H TFTs

Abstract

AbstractThe novel version of the field effect conductivity method for the direct density-of-states (DOS) determination has been developed. The combination of both, a new analytical solution, in the form of integral equation which relates the surface potential to the gate voltage, and new data-analysis algorithms allows one to solve the problem by using the results of measurements of TFT quasi-static transfer characteristics. This sensitive, and easy to use, method provides the detailed information on the defect density in the gap of hydrogenated Si. The ability of this technique to extract parameters of the TFT, including the flat band voltage, from the dependence of the surface potential on the gate voltage, and to resolve fine-scale features in the midgap DOS of a-Si:H has been demonstrated.We have shown that the dependence of the DOS on the energy in the range 0–0.15 eV from the conduction band is close to the exponent, while deeper in the gap, the DOS can be described as a series of localized defect bands with amplitude values in the range 1016 cm−3 eV−1-2×1018 cm−3 eV−1.The integrated charge density and the surface potential as functions of gate voltage have been investigated. The difference between the surface potential and activation energy from temperature dependence of drain current has been discussed. The short-time current relaxation and metastable defect transformations have been studied and analyzed.