Long-wavelength (1.3- to 1.6-µm) detectors for fiber-optical communications

A review of the current status of long-wavelength InGaAsP and AlGaAsSb quaternary alloy photodetectors for fiber-optical communications is presented. The material properties important for this application are discussed and compared for each alloy system. High-purity material necessary for low-capacitance p-n junction detectors can be routinely prepared in the InGaAsP alloy system, and techniques have been developed for the growth of compensated low-carrier-concentration AlGaAsSb alloys to satisfy this requirement. The dark current of photodiodes in both quaternary alloy systems is often limited by tunneling, and high-purity material can also be used to reduce this component of the dark current. There is some indication that tunneling current may be defect-assisted rather than band-to-band. Device structures designed to reduce the electric field in the low band-gap materials and thus reduce the tunneling dark current are also reviewed. The special enhancement of the ionization-coefficient ratio β/α in AlGaSb devices is described, and recent results of measurements of α and β in (lOO)-InP are presented. Finally, special structures which may permit artificial enhancement of the ionization-coefficient ratio and the design of low-noise high-gain long-wavelength avalanche photodiodes are reviewed.