Improved NPIN Junctions for 100 GHz InP Mach-Zehnder Modulators

High-bandwidth (>100 GHz) and compact indium phosphide Mach-Zehnder modulators with npin junctions, designed for low contact resistance, can experience parasitic photocurrent self-amplification, leading to unpredictable performance. This work introduces a design modification to address self-amplification by inserting a new n-doped material at the np-junction. This material has a lower conduction-bandenergy-minimum than the neighboring layers, thereby increasing the barrier to electron flow across the npin junction. Mach-Zehnder modulators fabricated with this modification demonstrate a 20% reduction in self-amplification compared to unmodified devices. Moreover, the modification improves performance consistency, decreasing the standard deviation of measured self-amplification across devices from 14% to 5%. Using a calibrated model, we identify the electron-hole recombination lifetime due to defects within the p-doped layer as a key factor influencing self-amplification. Our model, which includes perimeter-induced short lifetimes of 7 ps in the p-doped layer reproduces the experimental results. Furthermore, the model predicts that devices with well passivated sidewalls resulting in lifetimes of 10 ns will have a self-amplification exceeding 20 unless the modification is incorporated, in which case self-amplification is 1.5. The contact resistance of npin structures is verified to be two orders of magnitude lower than that of corresponding pin structures.