III-V Compound Semiconductor Nanowire Terahertz Detectors

Abstract

III-V semiconductor nanowires have emerged over the past decade as promising nano-components for future electronic and optoelectronic devices and systems, including field-effect transistors, light-emitting diodes, photodetectors, lasers and solar cells. Recently, III-V semiconductor nanowires have been considered as ideal candidates for photoconductive terahertz (THz) detection, as they possess many desirable properties, such as a direct and tunable band gap, good carrier mobility and short carrier lifetime (on the picosecond to nanosecond timescale). Due to the one-dimensional structure and nanoscale size, such III-V nanowire THz detectors are promising as building blocks for advanced THz systems with compact configuration and enhanced functionalities (i.e. sub-wavelength resolution and high polarisation sensitivity). This dissertation presents the first attempt to examine the suitability of III-V semiconductor nanowires for their applications as photoconductive THz detectors. At first, a series of GaAs/AlGaAs core-shell nanowires were grown (using Au-catalyst metalorganic vapour phase epitaxy technique), characterised and compared for selection to detect the THz signal in a THz time-domain spectroscopy (THz-TDS) system. The fabricated GaAs/AlGaAs single nanowire THz detectors exhibited a pA-level THz response with good signal-to-noise ratio and high polarisation sensitivity however a narrow detection bandwidth (in the range of 0.1-0.6 THz). The origin of the narrow bandwidth for single nanowire detectors was thoroughly investigated using finite-difference time-domain (FDTD) simulations, revealing that the limited bandwidth arose from the strong low frequency resonance caused by the specific device geometry design (rather than the nanowire itself). By adjusting and optimising the nanowire detector geometry, broadband (0.1-1.6 THz) GaAs/AlGaAs single nanowire THz detectors were demonstrated. Furthermore, due to the nanoscale active material fabricated on an insulating substrate (z-cut quartz), single nanowire photoconductive THz detectors showed a very low dark current and a resultant low-noise nature when compared with the traditional (bulk) photoconductive THz detectors. This relaxes the ultra-short carrier lifetime requirement for the (semiconductor) detection material for photoconductive THz detection, since in traditional photoconductive detector the detection material has to have a carrier lifetime of a few picoseconds to minimise the noise current. Therefore, nanowires with longer carrier lifetime can also be used for photoconductive THz detection. Based on above findings, the high-quality core-only InP nanowires (grown by selective-area metalorganic vapour phase epitaxy technique) were investigated for photoconductive THz detection. With previously optimised device geometry and superior optoelectronic properties of InP nanowires, InP single nanowire THz detectors were fabricated and found to exhibit a broadband response (0.1-2.0 THz) and excellent sensitivity, which were then used to measure the transmission spectra for real material characterisation with performance comparable to the traditional (bulk) detectors. A longer time-domain sampling window (compared to the traditional bulk detectors) and thus a higher spectral response resolution were obtained for the InP single nanowire THz detectors, which have been ascribed to the small active material volume and thick THz-transparent z-cut quartz substrate, enabling the single nanowire detector to have less Fabry-Pérot reflections in measured signal. Furthermore, it was found that the contact quality significantly affects THz detector performance and is particularly crucial for the performance and reliability of single nanowire detectors due to their large surface-to-volume ratio. In the final part of this work, an axial n+-i-n+ InP nanowire structure was designed and investigated for use in nanowire THz detectors. The improved contact quality (due to contact doping) has led to further improvement of the nanowire THz detector performance particularly in its signal-to-noise ratio. In summary, this thesis demonstrates a series of photoconductive THz detectors fabricated from different III-V semiconductor nanowire materials and structures, showing excellent bandwidth and sensitivity, approaching that of the conventional THz detectors. The nanowire device design, fabrication, characterisation and related optical simulations described in this work have provided deep insights into the characteristics of the single nanowire THz detectors, which may serve as a useful guidance for future development of nano-device based THz systems.