Growth of Uncracked Al 0.80 Ga 0.20 N/GaN DBR on Si(111)
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Growth of Uncracked Al0.80Ga0.20N/GaN DBR on Si(111) M. B. Charles, M. J. Kappers and C. J .Humphreys Department of Materials Science and Metallurgy University of Cambridge Pembroke Street Cambridge CB2 3QZ, UK ABSTRACT The strain in Al0.80Ga0.20N/GaN distributed Bragg reflectors (DBRs) grown by metalorganic vapour phase epitaxy (MOVPE) on Si(111) was studied using high resolution X-ray diffraction (XRD). Previous studies have shown that a 50nm Al0.80Ga0.20N layer induced a compressive strain in Al0.12Ga0.88N capping layers and prevented crack formation. A seven period Al0.80Ga0.20N/GaN DBR was grown, but this was found to be cracked at room temperature, despite compression in the GaN layers. This problem was solved by growing an identical structure with the addition of a 650nm GaN cap, and due to the compression in this layer, the structure was crack-free. INTRODUCTION One of the challenges of growing GaN-based light emitting devices using Si(111) substrates is that silicon absorbs light in the visible range. This means that for high performance devices it would be advantageous if there was a reflector such as a distributed Bragg reflector (DBR) between the light emitting region and the silicon substrate. However, one of the other challenges in the use of silicon substrates is that GaN has a much larger thermal expansion coefficient than silicon [1], which can result in significant tensile strain in the GaN, and cracking as the structure is cooled from growth temperature of 1040°C to room temperature. This makes the growth of DBRs especially difficult, as even when grown upon sapphire, the strain can build up within the structure sufficiently to cause DBRs to crack [2]. In addition, in order to produce vertically contacted LED structures containing AlGaN/GaN DBRs, it would be necessary for the GaN and AlGaN layers of the DBR to be conducting; the silicon required for this further increases the tensile strain in the layers [3], and makes crack prevention even more difficult. EXPERIMENTAL DETAILS The structures were grown by MOVPE on a 6 x 2" Thomas Swan close-coupled showerhead reactor on Si(111) substrates (resistivity, ρ = 0.06-0.1 Ωcm). Using the precursors ammonia, trimethylaluminium, and trimethylgallium for N, Al, and Ga respectively, the samples were grown at pressures of 100 and 50 Torr for the GaN buffer layer and the DBR structures respectively. The layers were grown by thermally annealing the silicon substrates in-situ at 1120°C, then depositing a 30nm AlN layer at 715°C. A buffer layer of GaN was used before depositing the DBR. Both Al0.80Ga0.20N and GaN layers of the DBR were deposited at 1040°C,
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with a nominal silicon doping level of 1.5x1018 cm-3 and the thicknesses of the GaN and Al0.80Ga0.20N layers were approximately 54nm and 59nm respectively. The structures, illustrated schematically in figure 1, were grown with 7 and 10 periods, and were designed to produce a stop band around 520nm, with a structure as shown in figure 1. RESULTS AND DISCUSSION Previous work on AlN interlayers
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