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A Cross-Correlated Approach to Engineering Nitride Nanowires

Periodic Reporting for period 2 - ACrossWire (A Cross-Correlated Approach to Engineering Nitride Nanowires)

Reporting period: 2018-10-01 to 2020-03-31

Semiconductor nanowires exhibit outstanding potential for emerging applications in energy-efficient lighting, optoelectronics and solar energy harvesting. When tailored at the nanoscale, nanowires should overcome many of the challenges facing conventional planar semiconductor materials, and also add extraordinary new functionality to these materials. However, progress towards III–nitride nanowire devices has been hampered by the challenges in quantifying nanowire electrical properties using conventional contact-based measurements. Without reliable electrical transport data, it is extremely difficult to optimise nanowire growth and device design. This project aims to overcome this problem through an unconventional approach: advanced contact-free electrical measurements. Contact-free measurements, growth studies, and device studies are being cross-correlated to provide unprecedented insight into the growth mechanisms that govern nanowire electronic properties and ultimately dictate device performance. A key contact-free technique at the heart of this project is ultrafast terahertz conductivity spectroscopy: an advanced technique ideal for probing nanowire electrical properties. We are developing new methods to enable the full suite of contact-free (including terahertz, photoluminescence and cathodoluminescence measurements) and contact-based measurements to be performed with high spatial resolution on the same nanowires. This is providing accurate, comprehensive and cross-correlated feedback to guide growth studies and expedite the targeted development of nanowires with specified functionality. We are now applying this powerful approach to tailor nanowires as photoelectrodes for solar photoelectrochemical water splitting. This is an application for which nitride nanowires have outstanding, yet unfulfilled, potential. This project will thus harness the true potential of nitride nanowires and bring them to the forefront of 21st century technology.
A number of technical and scientific outputs have resulted in the first half of this project. We have established a new facility for broadband terahertz time-domain spectroscopy and optical pump-terahertz probe spectroscopy. This facility employs a regenerative laser amplifier and a pulse-detection scheme to achieve high signal-to-noise ratios and rapid data acquisition. We have validated the system by measuring metal halide perovskite materials and III-V nanowires supplied by collaborators. We can now achieve nanowire Hall bar devices with lateral contacts separated by just 20 nm, and nanowire field effect devices with reliable Ohmic contacts. These devices are ideal for our cross-correlation experiments, allowing direct comparison between terahertz data and device data. We have developed new means of passivating nanowires post-growth using atomic layer deposition of alumina. We have fabricated and tested device prototypes for solar energy harvesting, including solar photoelectrochemical water splitting.
We have demonstrated a number of results and methodologies beyond the state of the art, including
- A new ex-situ passivation process that improves the electrical properties of nanowires and increases their resistance to corrosion in aqueous solutions.
- The demonstration of nanowire devices and arrays gated with ionic gels, that are suitable for both contact-based and contact-free measurements of nanowire properties.
- Device optimisation guided by cross-correlated measurements.
The next phase of the project will address aspects of nanowire growth, including growth on unconventional substrates, the optimisation of doping, and crystal phase control. The project will culminate with the demonstration of solar photoelectrochemical water splitting devices that have been designed and optimised based on our cross-correlated approach.