Impact ionization in AlAs0.56Sb0.44 photodiodes

The aim of this work is to characterize the impact ionization characteristics of AlAs0.56Sb0.44 towards its use as the avalanche medium in separate absorption and multiplication avalanche photodiodes (SAM-APDs) based on InP substrate for optical communication systems. The previous studies of the AlAs0.56Sb0.44 material were only undertaken on very thin p-i-n structures where we cannot accurately estimate the impact ionization coefficient and excess noise behavior due to “dead-space” effects. In this work, much thicker AlAs0.56Sb0.44 homojunction diodes were investigated systematically. The absorption coefficient was fitted by 1-D quantum efficiency model. Comprehensive multiplication and excess noise measurements based on AlAs0.56Sb0.44 homojunction diodes over a wide range of thickness were performed at room temperature. The bulk electron and hole ionization coefficients, α and β respectively, were found to be very disparate and ‘silicon like’ at low electric fields and α > β over the whole electric field range. The ionization coefficients were determined from 220-1250 kV/cm for α and from 360-1250 kV/cm for β. The β was found to rapidly drop at the low electrical field, but the α was similar to that of InP and InAlAs. Noise measurements carried out on the thickest p-i-n structure exhibits the best reported excess noise based on InP substrate, k = 0.005 at room temperature. The SAM-APDs using AlAsSb show potentially better performance than those using InP/InAlAs as the multiplication layer.