Effects of Stress-relieving AlN Interlayers in GaN-on-Si Grown by Plasma-assisted Molecular Beam Epitaxy
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1068-C06-04
Effects of Stress-relieving AlN Interlayers in GaN-on-Si Grown by Plasma-assisted Molecular Beam Epitaxy Adam Adikimenakis1,2, Suman-Lata Sahonta3, George Dimitrakopulos3, Jaroslav Domagala4, Philomela Komninou3, and Alexander Georgakilas1,2 1 MRG - IESL, FORTH, Vassilika Vouton, Heraklion, 71110, Greece 2 Physics department, University of Crete, Heraklion, 71203, Greece 3 Physics department, University of Thessaloniki, Thessaloniki, 54124, Greece 4 Institute of Physics, Polish academy of science, Warsaw, Poland ABSTRACT The insertion of an AlN interlayer for tensile strain relief in GaN thin films grown on Si (111) on-axis and vicinal substrates by nitrogen rf plasma source molecular beam epitaxy has been investigated. The 15 nm AlN interlayer was inserted between the bottom 0.5 micron GaN layer and the top 1.0 micron GaN layer. The interlayer was very effective to reduce the tensile stress in the overall 1.5 micron GaN/Si film to the level required for complete avoidance of microcracks, which were present in high densities in GaN/Si heterostructures grown without an AlN interlayer. The strain of the AlN interlayer, as well as the strain in all the layers of the entire GaN/Si heterostructure was analyzed by x-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Reciprocal space map in XRD indicated that the 15 nm AlN interlayer was coherently strained with the GaN films. However TEM observations revealed that the AlN interlayer was partially relaxed in local regions. The AlN interlayer was also observed to interfere with the GaN growth process. In particular, above morphological features such as Vdefects, GaN was overgrown with a large density of threading dislocations and inversion domain boundaries. INTRODUCTION The heteroepitaxial growth of III-V semiconductors on Si has attracted significant research efforts for more than two decades [1-11]. The most ambitious target of this work concerns the monolithic integration of III-V optoelectronic and high frequency electronic devices with Si digital electronic circuits. However, the replacement of conventional substrates (used for III-V growth) with the low cost, large diameter and high quality Si substrates is also a strong motivation. For these reasons, significant research efforts have been recently devoted to the GaN growth on (111) Si [2-10] and much attention has been paid on the realization of good quality, crack-free GaN films on Si (111) layers. The epitaxial orientation relationship between GaN and Si is (0001)GaN/(111)Si, < 1 1 20 >GaN/< 1 1 0 >Si, for which the large lattice mismatch (16.9% with respect to GaN) induces strain, leading to misfit and threading dislocations in the epitaxial films. Moreover, the difference in the thermal expansion coefficients between the two materials leads to thermally induced tensile stress, causing microcracks in the GaN films [4]. Microcracks can be avoided by utilising one or more intermediate AlN layers [2,3] in order to introduce compressive residual strain in the overgrown GaN lay
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