Compositional Ordering in In x Ga 1-x N and its influence on optical properties
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E11.19.1
Compositional Ordering in InxGa1-xN and its influence on optical properties Z. Liliental-Webera, D. N. Zakharova , K. M. Yua, J. Wua,b, S. X. Lia,b, J.W. Ager IIIa, W. Walukiewicza, E.E. Hallera,b ,H. Lu c, and W. J. Schaff c a Materials Sciences Division, Lawrence Berkeley National Lab, Berkeley, CA 94720 b Materials Science and Engineering, University of California, Berkeley, Berkeley, CA 94720 c Department of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853 ABSTRACT InxGa1-x N layers grown with compositions with the predicted miscibility gap have been studied using Transmission Electron Microscopy (TEM), x-ray diffraction and optical measurements (photoluminescence and absorption). The samples (0.34 < x < 0.8) were deposited by Molecular Beam Epitaxy at 800°C using 200 nm AlN buffer layer grown directly on sapphire substrates. Another sample with x = 0.34 was grown on a GaN buffer layer. Dislocation densities in the InGaN layers were typically in the mid-1010 cm-2 to1011 cm-2 range. Edge dislocations were the most prevalent. For In concentration x = 0.5 compositional ordering is observed leading to extra diffraction spots in electron and x-ray diffraction. The ordering was not observed for the sample with x=0.34 grown on GaN. Based on TEM measurements the estimated period of ordering ∆ was about ∆ = 45 Å for x = 0.5 and ∆ = 65Å for x = 0.78. The sample with x = 0.5 had the highest dislocation density. In addition to the presence of threading dislocations two types of domain boundaries on (0001) and (0110) planes were also observed in this sample. This sample has a broader photoluminescence (PL) that is redshifted compared to the absorption edge (“Stokes shift”). INTRODUCTION In-containing III-nitrides are used as the active layer in short-wavelength light emitting diodes and lasers [1,2]. These materials are also intense recent interest due to reevaluation material properties such as the band-gap, the lattice parameter, decomposition temperature, etc. [3-7]. However, the large size difference between Ga and In atoms makes growth of InxGa1-xN challenging. A miscibility gap for this material was predicted theoretically and phase separation been observed experimentally in films grown by Metalorganic Chemical Vapor Deposition (MOCVD) and Molecular Beam Epitaxy (MBE) for x = 0.25 [8-12]. Especially in alloys grown by MBE dependence on growth temperature was clearly demonstrated [11]. For the layers grown in the temperature range of 725-750°C with In concentration x > 0.35 two compositions have been reported: In0.97Ga0.03N and In0.37Ga0.63N. For such composition (x = 0.37) phase diagram predicts spinodal decomposition, resulting in In0.95Ga0.05N and In0.05Ga0.95N [13, 14] and also the presence of the In0.37Ga0.63N as due to kinetic limitation. For the lower growth temperatures (650-675°C) for x>0.35 diffused and split In0.37Ga0.63N indicating large inhomogeneities was observed without a strong separation observed in the samples grown with higher temperatures, similarly as was earlier
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