Growth of thick and flat high quality GaN using nano-column compliant layers

Lead Research Organisation: University of Bristol
Department Name: Physics

Abstract

This research will primarily develop a novel technique to grow a high volume of high quality gallium nitride for use in optoelectronic, microelectronic and biomedical devices. It is currently very difficult to produce large single crystal gallium nitride due to the high pressures and temperatures required, and this is inhibiting the wide uptake of this disruptive material technology. Instead, the technique involved in this proposal uses a high density of minature columns with dimensions on the nano-scale to initiate the chemical growth of a large crystal from a crystal of another material that has a different crystal structure. The columns reduce the problems associated with the different crystal structures and has the potential to produce high quality gallium nitride at relatively low cost.The research will determine the optimum column size and the best conditions for the crystal growth by theoretical modelling and experimentation. Three different complementary growth techniques at the universities of Nottingham and Bath will be used, and the advanced characterisation techniques and expertise at Bristol University will provide essential feedback and understanding of the nanostructures to the growth personnel.

Publications


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Cherns D (2010) GaN devices based on nanorods in Journal of Physics: Conference Series
 
Description Solar cells based on InGaN nanostructures, EPSRC grant EP/1035501/1
Amount £450,000 (GBP)
Funding ID EP/1035501/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom of Great Britain & Northern Ireland (UK)
Start 09/2011 
End 08/2014
 
Title Solar cell for forming solar panel, solar concentrator and power plant for producing electricity comprises at least one sub-cell containing several specific indium gallium nitride nanocolumns 
Description In solar cell comprising sub-cell(s) with several InxGa1-xN nanocolumns/nanorods, each nanocolumn/nanorod is substantially defect free; as nanocolumns/nanorods vary in composition along their length, and have adjacent portions separated by tunnel junctions formed by doping nanocolumn, such that tunnel junctions are defect free. Further, under lateral growth conditions, continuous layer is grown on top of nanocolumns/nanorods. The precursor layer and overlayer are doped. Continuous InxGa1-xN layers formed by lateral growth of nanocolumns/nanorods have much lower defect densities than continuous epilayers e.g. 107 to 108 cm-2, as opposed to 109 to 1010 cm-2. The nanorod/nanocolumn devices have several key advantages over devices based on continuous epilayers. The nanorods are perfect single crystals, free of threading defects (less than 108 cm-2), and remain free of defects as they grow laterally until coalescence. The nanorods are compliant structures and have right geometry such that misfit stresses can be eliminated by lateral relaxation, in content to be grown pseudomorphically on GaN base, where composition change is graded. Misfit dislocations and layer stresses which lead to phase separation can thus be avoided. This is not possible with continuous epilayers as there is 7% mismatch between GaN and InN. There is also good evidence that high crystal quality InGaN nanorods can be grown by molecular beam epitaxy (MBE) for all compositions up to pure InN. If threading dislocations are generated e.g. in misfit dislocation source, they are eliminated on nanorod sides; where driving force is relaxation of strain energy. Further, threading defects can be generated in nanorods and are propagated, where they provide top surface growth step. However, for solar cell probability of such defect generation is low (less than 5-10%). Stress relaxation in nanorods also means that composition can be reversed to give InxGa1-xN overlayers with low x. Thus, enables p-type layers (needed in two-junction device) having low x value. This provides crucial flexibility in doping needed to integrate sub-cells into multi-junction device. The solar cell offers advantage of using optimum combination of band gaps for improved overall efficiency, combined with lower epitaxy and processing costs, and without use of toxic materials. The manufacturing is maskless and does not require use of etching chemicals or only in relatively small amounts. InxGa1-xN also has intrinsic properties which are advantageous for solar cells, including high optical absorption of 2x 105 cm-1, giving greater than 90% absorption in 200 nm, compared with greater than or equal to 10 microns for Si, and high carrier mobility and drift velocity which can reduce carrier recombination rates. Moreover, high piezoelectric and spontaneous electric fields present in GaN heterostructures can be used to enhance tunneling through reverse biased junctions. For x greater than 0.43, nanorod/nanocolumn surfaces show electron accumulation, although depth of electron accumulation layer should be 2-3 nm. This suggests that electrons and holes may become spatially separated, and reduce probability of electron-hole recombination 
IP Reference WO2012076901-A1 ; EP2649650-A1 ; US2013269763-A1 
Protection Patent granted
Year Protection Granted 2011
Licensed No
Impact The structures envisaged in the patent have been demonstrated