InP/InAsP Quantum Discs-in-Nanowire Array Photodetectors: Design, Fabrication and Optical Performance

This thesis focuses on design, processing and electro-optical investigations of two- and three-terminal photodetectors based on large arrays
of around three million n+-i-n+ InP nanowires with embedded InAsP quantum heterostructures for broadband detection. The first part of the
thesis work dealt with a general investigation of the room-temperature optoelectronic behavior of two-terminal photodetectors under
broadband visible to near-infrared illumination, and in particular the response under selective 980 nm excitation of the 20 axially-embedded
InAsP quantum discs in each of the nanowires. The photodetectors show a non-linear optical response, which we attribute to a novel
photogating mechanism resulting from electrostatic feedback from trapped interface charges between the nanowire and SiOx cap layer,
similar to the gate action in a field-effect transistor. From detailed analyses of the complex charge carrier dynamics involving these traps
in dark and under illumination was concluded that electrons are trapped in two interface acceptor states, located at 140 and 190 meV below
the conduction band edge. The non-linear optical response was investigated at length by photocurrent measurements recorded over a wide
power range. From these measurements were extracted responsivities of 250 A/W (gain 320) @ 20 nW and 0.20 A/W (gain 0.2) @ 20 mW
with a detector bias of 3.5 V, in excellent agreement with the proposed two-trap model. Finally, a small signal optical AC analysis was
made both experimentally and theoretically to investigate the influence of the interface traps on the detector bandwidth. While the traps
limit the cut-off frequency to around 10 kHz, the maximum operating frequency of the detectors stretches into the MHz region.
In the second part of the thesis, we report on the detection of long-wavelength infrared radiation originating from intersubband transitions
in the embedded quantum discs at low temperature. For this purpose, we developed a technique for depositing ultra-thin ITO top contact
layers which not only improved the photon flux reaching the quantum discs, but also maintained electrical characteristics similar to those
of the previously used thick ITO layers. Theoretical calculations of the optical transition matrix elements using an 8-band k·p simulation
model along with solving drift-diffusion equations shed light on both interband and intersubband transitions. The calculations showed a
possible intersubband transition around 135 meV (9.2 μm) between the ground state and first excited state in the conduction band of the
discs in very good agreement with the observed experimental data. Conventionally, an out-of-plane electric field component is required for
intersubband resonances in a planar quantum well. Interestingly, the intersubband signal in our photodetector was collected under normal
incidence conditions which nominally only generates an in-plane electric field component. We attribute this unexpected intersubband signal
to a scattering in the nanowire array which effectively creates an electric field component along the nanowires.
In the last part of the thesis, we focus on device processing and optoelectronic characterization of the first reported three-terminal
phototransistors based on similar InP/InAsP nanowire/quantum discs heterostructures, now with a buried global gate-all-around contact
around the i-segments of the nanowires comprising the quantum discs. Furthermore, an elaborate theoretical model of the phototransistors
was developed in excellent agreement with the experimental results. In particular, we highlight a unique possibility to electrically tune the
spectral sensitivity and bandwidth of the detector. The transparent ITO gate-all-around contact facilitates a radial control of the carrier
concentration by more than two orders of magnitude in the nanowires and quantum discs. The transfer characteristics reveal two different
transport regimes. In the subthreshold region, the photodetector operates in a diffusion mode with a distinct onset at the bandgap of InP. At
larger gate biases, the phototransistor switches to a drift mode with a strong contribution from the InAsP quantum discs. Besides the
unexpected spectral tunability, the detector exhibits a state-of-the art non-linear responsivity reaching 100 A/W (638 nm/20 μW) @
VGS=1.0V/VDS=0.5V, and a response time of the order of μs, limited by the experimental setup, in excellent agreement with the developed
comprehensive real-device model.