Surface Recombination and Vacuum/GaN/AlGaN Surface Quantum Wells
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Surface Recombination and Vacuum/GaN/AlGaN Surface Quantum Wells Xiyao Zhang, I. P. Wellenius, A. L. Cai, J. F. Muth Electrical and Computer Engineering, North Carolina State University, Raleigh NC 27695-7911 John Roberts, Pradeep Rajagopal, Jim Cook, Eddie Piner, and Kevin Linthicum Nitronex Corporation, 628 Hutton Street, Suite 106, Raleigh, North Carolina 27606
Abstract Surface quantum wells of gallium nitride have been grown by Metal Organic Vapor Phase Epitaxy on top of AlGaN/GaN heterostructures. One boundary of the quantum well is vacuum (or air)/GaN interface, the other is GaN/AlGaN interface, and the width of the quantum well is the thickness of gallium nitride cap, and quantum confinement is demonstrate by the energy shift in photoluminescence, and cathodoluminescence as the GaN cap thickness is varied. The efficiency of the quantum well emission is sensitive to the surface environment and resulting surface recombination velocity. In this study the surface is altered by surface preparation treatments and resulting in changes in the luminescence. The changes in the efficiency of quantum well luminescence with surface treatments are attributed to changes in surface recombination velocity and surface electric fields.
Introduction In III-Nitride devices, thin Gallium Nitride capping layers have been used in a variety of ways, for example to improve gate leakage1, or to modify the band structure shape for devices that depend on piezoelectric charge2. With the correct material parameters it is also possible to form a surface quantum well with the vacuum/GaN interface forming one boundary, GaN cap forming the quantum well, and AlGaN/GaN interface forming the other boundary. We have observed efficient emission photoluminescence and cathodoluminescence from these structures without any special surface passivation.3 In this study we examine changes in photoluminescence and cathodoluminescence that result from simple surface chemical treatments. There are several motivations for investigating surface quantum wells. With the intense interest in forming and using nanostructures for device applications, the effects of the vacuum potential and surface states become increasingly important. Also as nanostructures are widely used, the surface area to volume ratio of the resulting devices substantially increases and one can expect environmental influences, and degradation processes such as oxidation to become increasingly important. The emission of surface quantum wells can provide a sensitive tool with which to monitor these processes. The observation of surface quantum wells has been observed in other material systems and was first demonstrated on the GaInP and GaInAs material systems.4,5 For example Yablonovitch et al. monitored the photoluminescence shifts as InGaP quantum wells were etched, and investigated the surface recombination velocities when the surfaces were passivated with sodium sulfide and other solutions. More recently it was observed that similar passivation schemes could be used to increa
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