  Avalanche photodetectors (APDs) are key components in optical communication
systems due to their increased photocurrent gain and short response time as
compared to conventional photodetectors. A detector design where the
multiplication region is implemented in a large bandgap material is desired to
avoid detrimental Zener tunneling leakage currents, a concern otherwise in
smaller bandgap materials required for absorption at 1.3/1.55 um.
Self-assembled III-V semiconductor nanowires offer key advantages such as
enhanced absorption due to optical resonance effects, strain-relaxed
heterostructures and compatibility with main-stream silicon technology. Here,
we present electrical and optical characteristics of single InP and InP/InAsP
nanowire APD structures. Temperature-dependent breakdown characteristics of
p+-n-n+ InP nanowire devices were investigated first. A clear trap-induced
shift in breakdown voltage was inferred from I-V measurements. An improved
contact formation to the p+-InP segment was observed upon annealing, and its
effect on breakdown characteristics was investigated. The bandgap in the
absorption region was subsequently varied from pure InP to InAsP to realize
spatially separate absorption and multiplication APDs in heterostructure
nanowires. In contrast to the homojunction APDs, no trap-induced shifts were
observed for the heterostructure APDs. A gain of 12 was demonstrated for
selective optical excitation of the InAsP segment. Additional electron
beam-induced current measurements were carried out to investigate the effect of
local excitation along the nanowire on the I-V characteristics. Our results
provide important insight for optimization of avalanche photodetector devices
based on III-V nanowires.
