Understanding terahertz emission properties from a metal–insulator–semiconductor structure upon femtosecond laser illumination

Abstract

Metal–insulator–semiconductor (MIS) is an essential structure in semiconductor devices. Owing to the increasingly complex development of semiconductor devices, the local information of MIS, such as passivation, defects, and transient carrier dynamics, is becoming more challenging to characterize. Laser-excited terahertz (THz) emission spectroscopy and imaging are promising for investigating such local properties in a noncontact and nondestructive manner; however, the discussions are speculative and phenomenological. In this study, we formulate the mechanism of THz radiation based on a Si metal–oxide–semiconductor (MOS) structure to semi-quantitatively evaluate the MOS devices. A simplified model for both n- and p-type Si MOS structures is derived from Poisson's equation, which can effectively explain the dependence of THz emission amplitudes on external bias voltages considering external effects, including the flatband voltage, hot carrier diffusion, and quick recombination through interface states in addition to the photocurrent due to photocarrier field acceleration. The result reveals that one can estimate various “local” parameters such as the surface potential, semiconductor doping level, and dielectric constant of the passivation layer. The simplified model fills the gap between theory and observation, resulting in an advanced analytical tool for semiconductor research and development.