This project exploits a recently discovered electrochemical process to create new manufacturing techniques for GaN that have not been possible. The project represents a bold integration of epitaxial optoelectronics with scientific interdisciplines including electrochemical processes, solid-phase mass transport, and applied fracture mechanics. At the same time the investigators seek to develop concrete, manufacturable processes that will impact the energy-efficient lighting and power distribution industries, projected to capture markets of hundreds of billion dollars by 2020. Three concrete objectives are (1) manufacture-friendly thin-film LEDs for general illumination, (2) the regenerative usage of heteroepitaxial substrates to offset the substrate cost in LED production, and (3) hybrid integration of GaN membrane devices with flexible hosts. The combination of a nanoscale porosification process with MOCVD epitaxy enables a flexible embedding of nano-voids and nano-cavity layer underneath LED device layers. The presence and coalescence of these nano-cavities facilitate wafer splitting, large-area layer transfer and, simultaneously, substrate recycling. Furthermore, the transferred, free-standing thin-film devices provide an ideal configuration as hybrid inorganic-organic flexible devices in both optoelectronics and electronics. The investigated etching-epitaxy-splitting process will open up untapped opportunities in substrate utilization, growth design, and device fabrication.
GaN is a ubiquitous semiconductor that is finding applications in everyday life. This wide bandgap material is capable of producing efficient ultraviolet, blue, and green light for display and lighting. It is an ideal replacement for silicon as compact transistors for high-power electricity transmission and energy conversion. However, GaN is chemically inert and mechanically robust, thus imposing rigid constraints to fabrication techniques for mass production. In spite of the demonstrated superiority in optoelectronic and electronic performance, GaN LEDs and transistors still face a stiff bottleneck in the cost of manufacturing. This projects exploits a recently discovered electrochemical process to create new manufacturing techniques for GaN that have not been possible. It aims to propel the manufacture technology for solid state lighting into a smarter, greener, and more sustainable state based on nanoscale scientific insights. Using a simple anodization process to create a foam-like GaN, the investigators plan to demonstrate the possibility of micromachining GaN, including slicing and shaping thin layers, for manufacture-friendly and cost effectively fabrication of GaN devices for energy-efficient lighting and power distribution, an industry that is projected to capture markets of hundreds of billion dollars by 2020.
1. Project overview The goals of the project include (1) the development of a manufacturable smart-cutTM procedure to slice or split GaN LED layers off arbitrary epitaxial substrates, (2) the regenerative usage of heteroepitaxial substrates (sapphire, Si, and SiC) to offset the substrate cost in LED production, (3) the producing of low-dislocation vertical LEDs and concurrent reclaiming of homoepitaxial bulk GaN to increase the life cycles and reduce the average cost of bulk GaN wafers, and (4) the demonstration of hybrid organic-inorganic flexible GaN devices. 2. Major activities Major activities of the project include: Transformation of nanoporous GaN Overgrowth of GaN on nanoporous GaN Control of the porosity of NP GaN for liftoff Determination of the mechanical properties (hardness and moduli) of the NP GaN layer Correlating the porosity with the separation study Study of bonding materials and interface adhesion Large-area separation of GaN layers from sapphire using the vertical EC etching Preliminary results from an vertical LED device using nanoporous liftoff procedure Characterizations of GaN membrane-based distributed Bragg reflector (DBR) Preparation and transfer of large-area GaN nanomembranes (NMs) Confirmation of low-dislocation and high mobility of GaN NM NM InGaN/GaN NM based light emitting diodes (LEDs) Enhancement-mode GaN NM MOS field-effect transistor 3. Intellectual merit III-Nitride semiconductors are one of the most important electronic materials of our age. The proposed work represents a bold integration of epitaxial optoelectronics with scientific interdisciplinary including electrochemistry, solid-phase mass transport, and applied fracture mechanics. This project seeks to prepare for the first time III-N nanoporous GaN and nanomembranes in large area manner. This proposal is based on our recent discovery of a special electrochemical process in nano-drilling, porosifying, and splitting GaN epilayers that we believe will impact at multiple fronts the design paradigm and production of LEDs. Also, the encasing of such an active inorganic NM into flexible and versatile electronic hosts explore new frontiers in interface chemistry, band structure engineering, and electronic transport. Being a strongly piezoelectric semiconductor, GaN in NM form will be a perfect platform for the study of strain-induced polarization and its impact on device performance, and to exploit the opportunity of active tuning of the polarization electrostatics. Sharing and dissemination of the research products – wide bandgap nanoporous semiconductors, nanomembranaes and device fabrication – with other researchers was an essential component of this project. Because our nanoporous and nanomembrane manufacturing technique does not require complicated mechanical and chemical etching process, duplication of our processing steps by other researchers will be straightforward. The goals of this project were geared toward realizing such widespread dissemination by optimizing the processing techniques and demonstrating promising proof of- concept device fabrication 4. Societal impacts The successful demonstration of nanoporous GaN, nanomembrane and their applications will impact both the science and technology community. Special electrochemical process in nano-drilling, porosifying, and splitting GaN epilayers will impact at multiple fronts the design paradigm and production of LEDs. With a much reduced flexural rigidity and radius of bending, III-N NMs will provide a unique opportunity for active tuning of polarization electrostatics, enabling a new class of piezo-microelectronic and piezo-optoelectronic devices. Demonstration of NM based stretchable devices would greatly enhance the application bases of ubiquitous computing and smart living, such as wearable and mobile devices, human-machine interfaces, bionic devices, point-of-care diagnostics, embedded systems, energy harvesting, and management.