Periodic Reporting for period 4 - ACrossWire (A Cross-Correlated Approach to Engineering Nitride Nanowires)
Periodo di rendicontazione: 2021-10-01 al 2023-11-30
Semiconductor nanowires exhibit outstanding potential for emerging applications in energy-efficient lighting, photonic integrated circuits, solar energy harvesting and terahertz-band photonics. Their geometry confers semiconductor nanowires with multiple unique properties that mitigate and overcome many of the challenges facing conventional planar semiconductor materials, and also adds extraordinary new functionality to these materials. However, the nanowire geometry also brings its own challenges. Progress towards III–V nanowire devices has been particularly hampered by the challenges in quantifying nanowire electrical properties using conventional contact-based measurements. This project overcame this problem through an unconventional approach - advanced contact-free electrical measurements – and by cross-correlating these measurements with crystal growth studies and device studies. 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 developed new methods to enable a 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 provided accurate, comprehensive and cross-correlated feedback to guide growth studies and expedited the targeted development of nanowires with specified functionality. This approach is highly effective for tailoring nanomaterials for diverse applications, including as photoelectrodes for solar photoelectrochemical water splitting.
ACrossWire delivered highly effective cross-correlated methodologies that enhance scientific understanding of nanomaterial properties. It then employed these methodologies to accelerate the development of nanowire-based optoelectronics. The project’s scientific and technological breakthroughs spanned the fields of crystal growth, advanced spectroscopy, materials characterisation and device engineering. Highlights are given here.
Tuning nanowire properties via targeted growth: The project provided unprecedented insight into the crystal growth mechanisms that govern nanowire electronic properties and ultimately dictate device performance. For silane-assisted GaN nanowire growth, the key nucleation conditions were identified. The project elucidated the role of dislocations in nanowire growth, and their relationship with nanowire morphology and optoelectronic properties. In the case of both III-nitride and III-arsenide material systems, cross-correlated optical, electrical and structural measurements revealed that radial heterostructures grown on the sidewalls were immune to defect-related problems that affected nanowire cores.
Contact-free measurements: We established a facility for broadband terahertz time-domain spectroscopy and optical pump-terahertz probe spectroscopy that achieved high signal-to-noise ratios and rapid data acquisition through a novel pulse acquisition scheme. Using this system, we ascertained device-relevant parameters such as charge carrier mobility, doping density and surface recombination velocity in nanowires. The findings guided device development. For example, we demonstrated highly effective surface passivation strategies for semiconductor nanowires using either atomic layer deposition of alumina or in-situ deposited ultra-thin epitaxial layers.
Cross-correlated measurements and automation: New high-throughput automated microscopy methodologies were developed for locating and characterising individual nanowires, and for automatically designing electrode patterns with high alignment accuracy. These methods combined machine learning with machine vision enabled by specialised alignment markers. We achieved an unprecedented yield of hundreds of bottom-, top- and side-gated nanowire field effect devices with reliable Ohmic source/drain contacts. These devices allow direct cross-correlation across different optical and electrical characterisation modalities with meaningful statistics.
Device architectures and integration: We developed new strategies for encapsulating, planarising and harvesting nanowire arrays that are suitable for the integration of nanowires into functional optoelectronic devices and systems. We demonstrated that these encapsulation strategies also successfully improved the stability of solar photoelectrochemical water splitting devices. We demonstrated novel device concepts for terahertz-band optical detection and photoresponsive memory systems that harness the exceptional optoelectronic properties of nanowires.
After the end of the project we will continue this work under two Proof-of-Concept grants that will translate the work from the laboratory to real-world applications.
Tuning nanowire properties via targeted growth: The project provided unprecedented insight into the crystal growth mechanisms that govern nanowire electronic properties and ultimately dictate device performance. For silane-assisted GaN nanowire growth, the key nucleation conditions were identified. The project elucidated the role of dislocations in nanowire growth, and their relationship with nanowire morphology and optoelectronic properties. In the case of both III-nitride and III-arsenide material systems, cross-correlated optical, electrical and structural measurements revealed that radial heterostructures grown on the sidewalls were immune to defect-related problems that affected nanowire cores.
Contact-free measurements: We established a facility for broadband terahertz time-domain spectroscopy and optical pump-terahertz probe spectroscopy that achieved high signal-to-noise ratios and rapid data acquisition through a novel pulse acquisition scheme. Using this system, we ascertained device-relevant parameters such as charge carrier mobility, doping density and surface recombination velocity in nanowires. The findings guided device development. For example, we demonstrated highly effective surface passivation strategies for semiconductor nanowires using either atomic layer deposition of alumina or in-situ deposited ultra-thin epitaxial layers.
Cross-correlated measurements and automation: New high-throughput automated microscopy methodologies were developed for locating and characterising individual nanowires, and for automatically designing electrode patterns with high alignment accuracy. These methods combined machine learning with machine vision enabled by specialised alignment markers. We achieved an unprecedented yield of hundreds of bottom-, top- and side-gated nanowire field effect devices with reliable Ohmic source/drain contacts. These devices allow direct cross-correlation across different optical and electrical characterisation modalities with meaningful statistics.
Device architectures and integration: We developed new strategies for encapsulating, planarising and harvesting nanowire arrays that are suitable for the integration of nanowires into functional optoelectronic devices and systems. We demonstrated that these encapsulation strategies also successfully improved the stability of solar photoelectrochemical water splitting devices. We demonstrated novel device concepts for terahertz-band optical detection and photoresponsive memory systems that harness the exceptional optoelectronic properties of nanowires.
After the end of the project we will continue this work under two Proof-of-Concept grants that will translate the work from the laboratory to real-world applications.
ACrossWire has harnessed the unique properties of semiconductor nanowires and exploited these properties to achieve not just superior optoelectronic device performances, but also truly novel functionalities. Photoresponsive memory devices, high-throughput device fabrication and cross-correlated characterisation methodologies are examples of novel capabilities achieved within the project.