Influence of buffer layer and processing on the dark current of 2.5 μm-wavelength 2%-mismatched InGaAs photodetectors

An extensive study is presented in which different buffer layers for the growth of 2.5 μm wavelength mismatched InGaAs photodetectors are compared. The dark current of the photodetectors is measured to judge the quality of the buffer layers. These differ in material composition (InGaAs or InAsP), grading mechanism (linear or stepwise), total buffer-layer thickness and number of steps. It is shown that the detectors with a thick InAsP buffer, grown on a 2°-off-oriented substrate, lead to the lowest dark currents. Different processing schemes are compared and it is shown experimentally that the dark current of a mesa-type detector consists of a part proportional to the circumference and one proportional to the detector area. The latter part is shown to be equal to the dark current of a planar Zn-diffused detector with the same dimensions