Electrical Properties of GaN/InGaN MQW Heterojunction Diodes as Affected by Various Plasma Treatments
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ELECTRICAL PROPERTIES OF GaN/InGaN MQW HETEROJUNCTION DIODES AS AFFECTED BY VARIOUS PLASMA TREATMENTS J. Kim, B. Luo, R. Mehandru, F. Ren, Department of Chemical Engineering, University of Florida, Gainesville FL 32611, USA, K. P. Lee, S.J. Pearton, Department of Materials Science and Engineering, University of Florida, Gainesville, FL; A.Y. Polyakov, N.B. Smirnov, A.V. Govorkov, Institute of Rare Metals, Moscow, 109017, B. Tolmachevsky, 5, Russia; A.V. Osinsky, P.E. Norris, Corning Applied Technologies, Woburn, MA 01801
ABSTRACT Effects of UV/O3 or deuterium plasma treatment, of annealing in air at 550 oC, of annealing in N2 at 500 o C and various combinations of these treatments on leakage current and resistance in the forward direction of GaN/InGaN multiquantum-well light emitting diodes MQW LEDs were studied. It was shown that the best results are achieved with 5 minutes long UV/O3 treatment. LED structures thus prepared showed effects of strong tunneling in their I-V characteristics. The space charge region was shown to be located in the GaN/InGaN superlattice SL. Passing of moderately high forward current through the structure for several hours enhanced the overall tunneling through the structure and facilitated faster tunneling between the layers in the GaN/InGaN SL.
INTRODUCTION Light emitting diodes (LEDs) based on AlGaN/InGaN multiquantum well MQW structures have now become commercially available from several manufacturers. Depending on design these LEDs emit light in the blue, green or white spectral range and show excellent quantum efficiency and brightness (see e.g. [1]). However, quite a few technological and scientific issues still need to be resolved to fully capitalize on the superior materials properties of the AlGaN/InGaN system. For example, at present the diode structures are commonly defined by dry etching and excessive surface leakage caused by radiation damage in such a process needs to be suppressed to improve the quantum efficiency. The exact location of the space charge region in the diodes is often not known which makes optimization more difficult. Also, one rather often observes short-term or long-term changes in current-voltage and in electroluminescence characteristics of the diodes [2,3] and the nature of these processes needs to be better understood. We address some of these issues in this paper.
EXPERIMENTAL The LED structures studied in this paper were fabricated from LED wafers grown by metalorganic chemical vapor deposition on sapphire. According to the wafers manufacturer the structure consisted of a 4.5 µm of n+-GaN, a ten period superlattice of GaN/InxGa1-xN (9nm/9nm, x=0.15-0.20), a 0.1 µm pAlxGa1-xN(Mg) (x=0.12) layer and a 0.45 µm p-GaN(Mg) contact layer. The structures were processed into 300 µm in diameter mesa diodes by reactive ion etching in Cl2/Ar [4 ]. The ohmic contacts to the top pGaN layer, 100 µm in diameter, were prepared by sputtering Ti/Au, photolithography and lift-off, with subsequent rapid thermal annealing at 950 oC. Each mesa was surrounded by a ring o
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