Low-Noise AlInAsSb Avalanche Photodiodes

Avalanche photodiodes (APDs) have been used for a wide range of commercial, military, and research applications in realms ranging from optical communications to imaging to single photon detection. Their primary advantage relative to p-i-n photodiodes is that the APD’s gain can provide higher sensitivity, at the cost of additional noise. This excess noise motivates the study of low noise materials, as well as structures with inherently lower noise.

My studies of the AlInAsSb materials system began with an investigation into an impact ionization engineering device called a staircase photodiode, in collaboration with students at the University of Texas at Austin, who recently developed a process for growing the materials system AlInAsSb through molecular beam epitaxy (MBE). This staircase structure was designed alongside a control structure composed of a simple, homojunction photodiode made from the Al0.7In0.3As0.3Sb0.7 material. With successful, low-noise results from the staircase photodiode, I was motivated to investigate the noise properties of the control structure. When the control showed low noise as well, we expanded our studies into homojunction APDs composed of varying aluminum concentrations of AlInAsSb. The results of these homojunction APDs gave low noise and low dark current for some devices, while other devices exhibited absorption at longer wavelengths with higher dark current. We used the information from the homojunction studies to design and fabricate an SACM APD, which uses one material layer for absorption and another for multiplication. This helped us achieve a low-noise, low dark current APD that can absorb at telecommunication wavelengths (1.3 µm to 1.6 µm). Other studies branched off these, including investigations into multi-step staircase structures and characteristics of digital alloy material APDs versus random alloy material APDs.