AlxGa1−xN Ultraviolet Avalanche Photodiodes With Avalanche Gain Greater Than $10^{5}$

Abstract

Ultraviolet (UV) avalanche photodiodes (APDs) based on Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N layers (x = 0 and 0.02) were introduced instead of a thick n-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.05</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.95</sub> N: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 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> were achieved at reverse biases of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> > 90 V, indicating the device approaches to avalanche multiplication.