Physics of Voltage-Profile Modifications in P-N Photodetectors

Abstract

The photovoltaic signal is a crucial characteristic of photodetectors, including those based on p-nor p-i-n photodiodes, among others. When the detector is excited by ultrafast (femto or nanosecond) pulses in an open-circuit configuration, a photovoltaic signal is generated, which exhibits a slow decay on the microsecond time scale. If the dominant physics in the detector is the recombination of photo-excited charge carriers, one would expect the signal to decay without changing its sign. However, certain experiments using short-pulse excitation have observed that the photovoltaic signals can undergo a sign change over time following the excitation. Initially, positive signals are detected immediately after the excitation, but they transition to negative signals several microseconds later. In this study, we investigate various physical effects such as density, temperature, electrostriction, pressure, photostriction, and band gap renormalization to determine their impact. We specifically examine their role in the following scenario: after ultrafast excitation, if the carrier density and temperature increase, and during the relaxation process, the system approaches a state close to quasi-thermal equilibrium where the carrier density remains elevated but lower than the intrinsic thermal equilibrium density at the elevated temperature, then the photovoltaic signal can become negative.