Photo-and-dark-current-voltage characteristics of normal-incidence GaAs photodetectors with two types of electrode configurations

The characteristics of photo-current-voltage and dark-current-voltage for two-photon-response semi-insulating GaAs photodetectors responding to near-infrared wavelengths of 1.31 μm and 1.55μm are investigated. The semi-insulating GaAs photodetectors were fabricated into hemisphere on whose bottom two types of electrodes were deposited. In experiments, the incident laser was adjusted to travel normally to the photodetector and focus at the center of the bottom so as to improve the nonlinear photo-responsivity markedly. It is observed that the photocurrent dependent on bias exhibits quadratic nonlinearity for both lasers and both electrode configurations, which reflects frequency-doubled absorption responsible for the physical mechanisms of the photodetectors; and the reasonable analysis demonstrates the important role of the electric-field-induced frequency-doubled absorption in two-photon response. Furthermore, it is found that the photocurrent is quite more greater when the electrode positioned at the bottom center of the photodetectors (central electrode for short) is negatively charged than that in the case of it positively charged under the conditions of the identical bias voltage and the same incident optical power; while the dark-current varies in exactly the opposite mode compared to the photocurrent. The aforementioned disparate variations of the photocurrent and the dark-current are well interpreted by the theory of surface band-bending of semi-insulating GaAs, and such variations result in a large ratio of photo-current to dark-current in the case of the central electrode negatively charged. The investigated results also indicate that the optimization of electrode structure is essential to improve the photo-responsivity of the photodetector.