AlxGa1−xN Ultraviolet Avalanche Photodiodes With Avalanche Gain Greater Than

Ultraviolet (UV) avalanche photodiodes (APDs) based on Al_xGa_1-xN wide-bandgap semiconductor alloys (x = 0.05) are reported. The epitaxial structure was grown by metalorganic chemical vapor deposition on a GaN substrate having a low dislocation density. Step graded n-type Si-doped Al_xGa_1-xN layers (x = 0 and 0.02) were introduced instead of a thick n-Al_0.05Ga_0.95N:Si layer to minimize strain-induced defects and crack formation, resulting in reduced leakage current densities of the devices with various circular mesa diameters. Under UV illumination at λ = 280 nm, high avalanche gains greater than 1.5 × 10⁵ were achieved at reverse biases of V_R 94 V for the APDs with mesa diameters of 30-70 μm. In addition, significantly increased spectral responsivities of devices having a 70-μm mesa diameter was observed at reverse biases of V_R > 90 V, indicating the device approaches to avalanche multiplication.