Spectral sensitivity of graded composition AlGaAs/GaAs nanowire photodetectors

A photodetection model for AlGaAs/GaAs nanowire photodetectors with graded composition is developed based on the numerical solution of coupled Poisson and continuity equations. Photocurrents and spectral sensitivities of nanowire photodetectors have been simulated as a function of Al content range, the wavelength of the incident light, the length of the nanowire, and contact types. The results demonstrate Ohmic–Schottky devices have the highest sensitivities among the four types of nanowire devices, which can be attributed to the strong built-in electric field induced by the graded composition and the dominant unidirectional built-in electric field due to the Schottky barrier. Schottky–Schottky devices show the lowest sensitivities due to the additional blocking effect of the Schottky barrier. The optimum nanowire length in which an Ohmic–Schottky structured nanowire photodetector can achieve maximum sensitivity is 7.4 μm for a nanowire with a linearly graded Al content range of 0–0.4 (x Al = 0.4). The optimum nanowire lengths for Ohmic–Ohmic (x Al = 0.3), Schottky–Schottky (x Al = 0.1), and Schottky–Ohmic (x Al = 0.0) nanowire devices are 6.5, 3.5, and 11 μm, respectively.