Characteristics of thin InAlAs digital alloy avalanche photodiodes

InP-based avalanche photodiodes (APDs) are widely used in short-wave infrared (SWIR) communications. In this work, a thin (200 nm nominal) InAlAs digital alloy layer consisting of two monolayer (ML) InAs and two ML AlAs was grown on InP substrate and investigated in detail. APDs with p-i-n and n-i-p structures were fabricated and characterized. The current-voltage, capacitance-voltage characteristics, and excess noise were measured at room temperature with different laser wavelengths, and the measured effective k value (ratio of impact ionization coefficients) is about 0.15 with the multiplication gain up to 12. The randomly-generated path length (RPL) model was carried out to analyze the dead space effect. Our thin digital alloy successfully reduced excess noise compared with conventional In_0.52Al_0.48As random alloy without introducing new elements and shows the potential for high-speed, low noise APD applications.