Optical evaluation of pretreated InGaN quantum well structures
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Optical evaluation of pretreated InGaN quantum well structures T. Böttcher, F. Bertram 1, P. Bergman 2, A. Ueta, J. Christen 1 and D. Hommel Universität Bremen, PO Box 330440, 28334 Bremen, Germany 1 Universität Magdeburg, PO Box 4120, 39016 Magdeburg, Germany 2 Linköping Universitet, SE-581 83 Linköping, Sweden
ABSTRACT In order to optimize the quantum efficiency of InGaN quantum wells, different MOVPE growth sequences are compared using photo- and electroluminescence. In one study, the surface was pretreated with trimethylindium (TMIn) prior to the well deposition. In another study, growth interruptions were performed after the quantum well deposition to desorb segregated indium. In both cases, the room-temperature photoluminescence (PL) intensity is strongly enhanced. For the samples grown with TMIn preflow the wavelength distribution in low-temperature cathodoluminescence (CL) wavelength mappings is narrowed, which can be attributed to more homogeneous quantum wells. Furthermore, the decay times of the radiative recombination increase both at RT and 2K. A reason for this could be an improved indium profile along the growth direction or a more homogeneous In wetting layer due to the pre-wetted surface. INTRODUCTION GaN-based light emitters designed for the blue-violet spectral region have been studied intensively over the last years due to their high commercial potential. All those devices are based on InGaN quantum wells forming the active region in combination with an AlGaN electron blocking layer, which separates the two doping regimes. Thus, the quantum efficiency of the quantum well is one of the most crucial factors in the performance of these devices. In this work, the modification of the InGaN quantum wells by means of the indium supply was studied. Since indium tends to segregate on the semiconductor surface, it will affect the indium distribution both in the lateral as well as in the growth direction. To address this issue, two growth sequences were compared. In the first experiment, indium was supplied to the growth surface prior to the quantum well deposition [1]. This additional indium supply is supposed to pre-wet the surface, such that the indium profile is improved. In the second experiment, a growth interruption was inserted after the well deposition, during which a small flow of hydrogen was supplied [2]. The hydrogen should remove excessive indium segregated to the surface and thereby improve the upper interface of the quantum well. EXPERIMENTAL To gain a better understanding of the quantum well luminescence in these devices, different quantum well structures were compared in PL and CL at room and at low temperature. Based on these quantum wells, LEDs were then fabricated and evaluated in view of their device
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performance. In particular, integral and time-resolved PL as well as spatially resolved CL were measured. The LED performance was compared on the basis of L-I curves. All structures were grown by metal organic vapor phase epitaxy (MOVPE) on sapphire substrates, using a
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