New UV Light Emitter Based on AlGaN Heterostructures with Graded Electron and Hole Injectors
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New UV Light Emitter Based on AlGaN Heterostructures with Graded Electron and Hole Injectors M.A.L. Johnson, J.P Long1 and J.F. Schetzina2 Department of Material Science and Engineering NC State University Raleigh, NC 27695-7907 1 Department of Physics, NC State University 2 Department of Electrical and Computer Engineering, NC State University ABSTRACT New ultraviolet (UV) light emitting device structures address the problems of small carrier concentrations and large band-offsets in wide bandgap Aluminum Gallium Nitride (AlGaN) heterostructures through the use of graded epilayers for electron and hole injection. For light emission at 280-290 nm, a multiple-quantum-well separate confinement heterostructure (MQWSCH) employs a graded AlGaN structure for the injection of majority carriers from the metal-semiconductor contact layers into the spacecharge region of the pn-junction with a higher bandgap energy. Sample LED mesa devices were fabricated and have shown light emission of 289 nm under a forward bias of 12V (20mA). These results provide a ‘proof-of-concept’ for this new graded device structure which can be employed for the development of both UV-LEDs and laser diodes. INTRODUCTION Research in wide bandgap semiconductors for optoelectronic devices has yielded tremendous advances in the past decade including the commercial introduction of LED across the blue and green wavelength range (400-525 nm) as well as the demonstration of UV-LEDs near 370nm for fluorescence based white general illumination. [1,2,3] These devices have been based on indium gallium nitride (InGaN) and gallium nitride (GaN) heterostructures thereby limiting usage to wavelengths longer than 365 nm (3.41 eV). The demonstration of optoelectronic devices at shorter UV wavelengths have used AlGaN or quaternary AlGaInN based heterostructures to achieve direct bandgap transition energies higher than 3.41eV [4]. UV optoelectronic devices based on this approach have included UV light emitters and p-i-n photodiodes for both visible blind and solar blind UV detection and imaging [5,6]. A recent potential application to be identified for wide bandgap semiconductors is UV light emitters for optical sources to be used in the fluorescence detection of biomolecules including the tryptophan, tyrosine and phenylalanine. It is well known from biochemistry that the aromatic structure of these amino acids provides a resonance for UV fluorescence spectroscopy. With and optical excitation source for fluorescence spectroscopy of 265-280nm (4.42- 4.67eV), characteristic luminescence is emitted at longer wavelengths for each compound [7]. AlGaN based UV light emitters are expected to form the technological basis of compact fluorescence detectors of such biomolecules. Our work identifies two specific issues to be addressed in the development of AlGaN based light emitters. The first is the unavailability of a suitable p-type acceptor dopant for AlGaN at bandgap energies needed for UV emission. The second is the
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