Ultrafast thin film GaAs photoconductive detectors

Abstract

Recent progress in the development of GaAs thin film photoconductive detectors for future optical communication systems at near-IR wavelengths is reported. The devices consist of a thin high purity epitaxial layer of GaAs grown on top of a chromium-doped semi-insulating substrate, two metal contacts and a contact layer on top of the “active” layer. Intrinsic detector response times of the order of 28 ps are found after excitation with fast laser pulses. The initial fast photocurrent decay is followed, if and only if the contacts are ohmic, by a second decay which is orders of magnitude slower. The amplitude of the slow tail decreases and finally disappears at higher excitation intensities or after the superposition of extremely faint continuous wave light. It is suggested that a small number of deep centres which are present in the active layer or close to the interface with the substrate (probably chromium) and which trap and re-emit charge carriers are responsible for the long tail. These centres are saturable. The results of studies of current gain and noise equivalent power of GaAs and a comparison with In 0.53Ga 0.47As/InP detectors are given and possible means of future improvements are indicated.