Infrared Photodetectors based on Nanowire Arrays with Embedded Quantum Heterostructures

Optical sensors operating in the infrared range of the electromagnetic spectrum are key components in a variety of
applications including optical communication, night vision, medical diagnosis, surveillance, and astronomy.
Semiconductor nanowires have great potential for realizing broadband infrared photodetectors with excellent
responsivity, low dark current and low noise, and a unique compatibility with commercial silicon-based electronics.
In this thesis work, comprising three published articles in Nano Letters, we synthesized, characterized and
modeled disruptive infrared photodetectors based on InP nanowires with axially embedded InAsP quantum discs.
In the first article, we made a combined study of design, growth, device processing and optoelectronic properties
of n+−i−n+ InP detector elements comprising 4 million periodically ordered nanowires in arrays, including either a
single or 20 InAsP quantum discs. Optimized Zn compensation of the residual non-intentional n-dopants in the isegment
suppressed the dark current at room-temperature to a few pA/NW. The detector elements exhibit a
strong broadband photoresponse with contributions from both the InP and InAsP segments with a threshold
wavelength of about 2.0 μm and a bias-tunable responsivity reaching 7 A/W@ 1.38 μm at 2 V bias.
In the second article, we performed an in-depth experimental and theoretical investigation of the responsivity of
optimized photodetectors under different illumination conditions. The photodetectors exhibit strongly bias and
power-dependent responsivities reaching record-high values of 250 A/W at 980 nm/20 nW and 990 A/W at 532
nm/60 nW, both at 3.5 V bias. Complementary real device modeling revealed a new photogating mechanism,
induced by the complex charge carrier dynamics involving optical excitation and recombination in the quantum
discs and interface traps, which reduces the electron transport barrier between the n+ segment and the i-segment
under illumination.
Finally, in the last article, we demonstrate the first intersubband photocurrent response in a nanowire
heterostructure array photodetector. The infrared response from 3 to 20 μm is enabled by intersubband transitions
in the low-bandgap InAsP quantum discs. The intriguing optical characteristics, including unexpected sensitivity to
normal incident radiation, are partly explained by excitation of the longitudinal component of optical modes in the
photonic crystal formed by the nanostructured portion of the detectors.
Our results show that properly designed arrays of axial nanowire heterostructures are promising candidates for
realization of commercially viable broadband photodetectors.