Al_xIn_1-xAs_ySb_1-y Digital Alloy Avalanche Photodiodes for Low-Noise Applications

Avalanche photodiodes (APDs) are used in a wide variety of light-detecting applications due to their internal gain mechanism. Compared to traditional photodiodes, APDs can achieve higher optical sensitivity by multiplying the photogenerated electrical carriers. APDs achieve this multiplication gain through impact ionization, wherein high energy carriers collide with surrounding electrons in the crystalline lattice and set them free. While advantageous for sensitivity, the stochasticity of the impact ionization process introduces additional electronic noise to the system. Reducing this noise, therefore, is paramount in high-performance APD design.

Together with the University of Texas at Austin, I further investigated the characteristics of APDs in the AlInAsSb materials system and developed several new APD architectures which take advantage of its qualities. My work began by characterizing the temperature stability of simple PIN AlInAsSb homojunction APDs. After demonstrating extraordinarily high temperature stability, I worked in characterizing wide-bandgap 80%-Al AlInAsSb APDs and further improving fabrication processing techniques for the AlInAsSb materials system.

My proposed thesis projects were twofold. First was designing and demonstrating a separate absorption, charge, and multiplication (SACM) APD for 2-µm applications. To create an effective design and layer structure for this project, I needed to preform numerous band structure and electric-field simulations. I created an AlInAsSb material macro in APSYS Crosslight which I was able to tune to the required bandgaps for this project. After successful simulation, the designed layer structure was grown at the University of Texas at Austin and fabricated and tested at the University of Virginia. I successfully demonstrated a low-noise, temperature-stable SACM APD for 2-µm applications with comparable dark current densities to state-of-the-art HgCdTe APDs while operating at temperatures 75 to 95 K higher.

My second project was demonstrating multiple-step staircase APDs. Although the first operational 1-step staircase APD had been previously demonstrated in the AlInAsSb materials system, increasing the number of steps and therefore the gain was necessary to prove the viability of the staircase APD. While the University of Texas at Austin led the design and crystal growth of these devices, I successfully fabricated and characterized both 2-step and 3-step staircase APDs at the University of Virginia. Furthermore, through investigation into the relative noise power of these devices, promising new noise characteristics were identified.