Experiment and Theoretical Calculation of the Surface Space Charge Region Effect on Photocurrent Generation of SnO₂ Bilayer Photodiode Devices

Abstract

to prolong the lifetime of the photocarriers of the photodetectors. [8][9][10][11] Das et al. reported the surface oxygen ion-adsorption on the oxygen-annealed SnO 2 surface would trap free electrons from the conduction band of SnO 2 , which results in an electrical potential built in the space charge region in the SnO 2 surface layer of SnO 2 -based UV-photodetectors. [3] As the photocarriers are generated by UV illumination on the SnO 2 surface layer of SnO 2 -based UV-photodetectors, the photocarriers would be separated by the electrical field built in the surface space charge region. As a result, the lifetime of the photocarriers can be enhanced. The abovementioned built-in electrical potential in the surface space charge region in the SnO 2 layer greatly depends on the oxygen vacancy concentration in the SnO 2 phase of the SnO 2 -based UV-photodetectors. Lee et al.'s study also stated that a critical oxygen vacancy concentration in the SnO 2 phase of the UV-photodetectors is required to create a sufficient built-in electrical potential in the surface space charge region to generate the detectable photocurrent. [12] So, they concluded that the critical electrical field required in the SnO 2 surface layer relies on the two criteria: 1) an adequate vacancy concentration in the SnO 2 layer, 2) oxygen ion-adsorption on the SnO 2 surface by annealing SnO 2 photodiode device in the O 2 ambient at elevated temperature. [12] However, for SnO 2 -based UV-photodiodes, the oxygen vacancies in the SnO 2 phase (causing low mobility) would trap the photocarriers and greatly reduce the lifetime of the generated photocarriers. Thus, the SnO 2 photocurrent generation layer with a high oxygen vacancy concentration would not be a good transportation path for the photocarriers.So, in this work, a SnO 2 bilayers photodiode device is studied, which consists of a thin SnO 2 photocurrent generation layer and a SnO 2 thick film on the glass substrate. The built-in electrical field in the thin SnO 2 photocurrent generation layer would drive the photocarrier down to and flow in the photocarrier transportation layer, i.e., the high-mobility SnO 2 thick film on the glass substrate. The designed SnO 2 bilayers photodiode device could enhance the photocurrent generation and the resolution for the SnO 2 -based UV-photodetectors.In this work, a SnO 2 bilayers photodiode device is designed, which consists of a thin SnO 2 photocurrent generation layer and a SnO 2 thick layer on the glass substrate. The SnO 2 photocurrent generation layer with high oxygen vacancy concentration is intentionally prepared. With annealing in oxygen ambient, an electrical potential would be built in the surface space charge region in the thin SnO 2 photocurrent generation layer, which would drive the photocarriers down to and flow in the photocarrier transportation layer, i.e., the high-mobility SnO 2 thick film on the glass substrate. The designed SnO 2 bilayers photodiode device could enhance photocurrent generation and resolution for the SnO 2 -based UV-photodete...