Mesoporous Gallium Nitride
Tongtong Zhu1, Yingjun Liu1, Peter Griffin1, Helen Springbett1, Fabien Massabuau1, and Rachel A. Oliver1
1Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS
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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