Silicon nanowires (SiNWs) have emerged as a promising material for high-sensitivity photodetection in the UV, visible
and near-infrared spectral ranges. In this work, we demonstrate novel planar SiNW phototransistors on silicon-oninsulator
(SOI) substrate using CMOS-compatible processes. The device consists of a bipolar transistor structure with an
optically-injected base region. The electronic and optical properties of the SiNW phototransistors are investigated.
Preliminary simulation and experimental results show that nanowire geometry, doping densities and surface states have
considerable effects on the device performance, and that a device with optimized parameters can potentially outperform
conventional Si photodetectors.
We report on the growth, fabrication and characterization of GaInAsSb <i>p-i-n</i> photodiodes with a high sensitivity in the short-wave infrared. Uniquely these photodiodes are grown on a GaAs substrate using the interfacial misfit array technique, which accommodates the lattice mismatch at an abrupt GaAs/GaSb interface. Top illuminated mesa
photodiodes with varying area were fabricated and characterized at room temperature. A zero bias resistance area
product of 260 Ωcm<sup>2</sup> is measured, together with a responsivity of up to 0.8 AW<sup>-1</sup> without an anti-reflection coating. The D* at zero bias is estimated to be 4.5x10<sup>10</sup> Hz<sup>1/2</sup>W<sup>-1</sup> which is approaching the best results reported for GaInAsSb photodiodes. Hence this work presents a promising alternative to GaInAsSb detectors grown lattice matched on GaSb substrates, strained InGaAs detectors grown on InP substrates or HgCdTe detectors. Making use of cheaper GaAs substrates, available in larger diameters and dry etch chemistry, the GaInAsSb photodiode technology reported here is particularly well placed to support future lower cost, larger area focal plane arrays approaching gigapixel resolution.
The dilute nitride GaInNAs(Sb) alloy system is challenging to grow and defects can cause short diffusion lengths and
high background doping densities. Despite these difficulties, extremely high cell efficiencies have recently been
achieved in multi-junction solar cells utilising 1 eV GaInNAs absorber layers. This study aims to highlight the tradeoffs
between the electrical and optical characteristics related to the performance of GaInNAs(Sb) diode structures
grown by molecular beam epitaxy , with band gaps ranging from 0.90 to 1.04 eV. Post-growth annealing was necessary
in some instances to reduce the background doping and dark current densities. The incorporation of Sb into GaInNAs
has enabled the possibility of producing a dilute nitride cell with a band gap lower than 0.80 eV, although with an
increased dark current.
Two of the key challenges in the realisation of focal plane arrays based on type-II InAs/GaSb superlattices (T2SL) are
the difficulty in achieving a good sidewall profile and the increased dominance of surface leakage current as the device
dimensions shrink. We report the electrical and morphological results of test pixels for mid-wave infrared T2SL
photodiodes etched using a Cl<sub>2</sub>/Ar based inductively coupled plasma reactive ion etching (ICP-RIE) process and
passivated using SU-8 epoxy photoresist. The etch rate and sidewall surface morphology of GaSb, InAs, and InAs/GaSb
T2SL materials are compared after dry etching under the same conditions, leading to the determination of an optimal
etch rate. The effect of surface treatment using selected wet chemical etchants before passivation on the surface leakage
current is presented. Limitations of the dry etching recipe and further improvement of the sidewall verticality and
smoothness are also discussed. Good sidewall profiles and
bulk-limited dark currents are demonstrated for T2SL
photodiodes etched to depths between 1.5 and 3.5 μm with a pitch size down to 12 μm.
InGaAsN is a promising material system to enable low-cost GaAs-based detectors to operate in the telecommunication
spectrum, despite the problems posed by the low growth temperature required for nitrogen incorporation. We
demonstrate that InGaAsN p<sup>+</sup>-i-n<sup>+</sup> structures with nominal In and N fraction of 10% and 3.8%, grown by molecular
beam epitaxy (MBE) under non-optimal growth conditions, can be optimized by post growth thermal annealing to match
the performance of optimally grown structures. We report the findings of an annealing study by comparing the
photoluminescence spectra, dark current and background concentration of the as-grown and annealed samples. The dark
current of the optimally annealed sample is approximately 2 μA/cm<sup>2</sup> at an electric field of 100 kV/cm, and is the lowest
reported to date for InGaAsN photodetectors with a cut-off wavelength of 1.3 μm. Evidence of lower unintentional
background concentration after annealing at a sufficiently high temperature, is also presented.