ABSTRACT: Porous semiconducting nitrides are effectively a new class of semiconducting material, with properties distinct from the monolithic nitride layers from which devices from light emitting diodes to high electron mobility transistors (HEMTs) are increasingly made. The introduction of porosity provides new opportunities to engineer a range of properties including refractive index , thermal and electrical conductivity , stiffness and piezoelectricity . Quantum structures may be created within porous architectures  and novel composites may be created via the infiltration of other materials into porous nitride frameworks .
Sub-surface multilayers of porous gallium nitride (GaN) are based on epitaxial structures consisting of alternating doped and undoped layers. Conventionally, trenches are formed using a dry-etching process, penetrating through the multilayer, and thereafter an electrochemical etching process is used in a lateral etching step, preferentially creating porosity in the doped material . We have developed a novel alternative etching process, which removes the requirement for the dry-etched trenches, with etching proceeding vertically from the top surface via defect-related channels . We make use of the ubiquitous threading dislocations found in GaN as etching pathways, resulting in an etch process which leaves an undoped top surface layer almost unaltered and suitable for further epitaxy.
This new defect-based etching process provides great flexibility for the creation of a variety of sub-surface porous architectures on top of which a range of devices may be grown. We have demonstrated the application of distributed Bragg reflectors (DBRs) based on porous GaN in improving the extraction efficiency and performance of both light emitting diodes and single photon sources . In this talk, I will also discuss other future applications of these novel materials, the techniques my group is developing to facilitate their characterization and their detailed three-dimensional structure.
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BIOGRAPHY: Professor Rachel Oliver is Director of the Cambridge Centre for Gallium Nitride. The focus of her research is understanding how the small scale structure of nitride materials effects the performance and properties of devices. She uses expertise in microscopy and materials growth to develop new nanoscale nitride structures which will provide new functionality to the devices of the future. Her work on porous nitrides led to the founding of startup company Poro Technologies Ltd, of which she is Chief Scientific Officer. She is passionate about communicating science to the general public and about widening participation in science by under-represented groups. Her combined achievements in her technical field and in campaigning for equality, diversity and inclusion in science led to her being named as one of the Women in Engineering Society's Top 50 Women in Engineering 2020.