Spatially Resolved Characterization of Microstructure, Defects and Tilts in GaN Layers Grown on Si(111) Substrates by Ma
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0934-I09-06
Spatially Resolved Characterization of Microstructure, Defects and Tilts in GaN Layers Grown on Si(111) Substrates by Maskless Cantilever Epitaxy R. I. Barabash1, C. Roder2, G. E. Ice1, S. Einfeldt2, J. D. Budai1, W. Liu3, O. M. Barabash1, S. Figge2, and D. Hommel2 1 Materials Science and Technology Division, Oak Ridge National Laboratory, One Bethel Valley Road, Oak Ridge, TN, 37831-6118 2 Institute of Solid State Physics, University of Bremen, Bremen, 28334, Germany 3 Advanced Photon Source, Argonne, IL, 60439 ABSTRACT The spatially resolved distribution of strain, misfit, threading dislocations, and crystallographic orientation was studied by white-beam Laue x-ray microdiffraction, scanning electron microscopy, and orientation imaging microscopy in uncoalesced GaN layers grown on Si (111) substrates by maskless cantilever epitaxy. Tilt boundaries formed at the column/wing interface with the misorientation strongly dependent on the growth conditions. A depth-dependent deviatoric strain gradient is found in the GaN. Types and density of misfit dislocations as well as their arrangement within different dislocation arrays was quantified. The results are discussed with respect to the miscut of the Si (111) surface and misfit dislocations formed at the interface. INTRODUCTION A high density of threading dislocations (typically 108-1010 cm-2) is usually found in GaN films grown on different substrates [1 - 4]. Various lateral epitaxial overgrowth (LEO) techniques have recently been developed which greatly reduce the density of threading dislocations. One of them is cantilever epitaxy (CE) with GaN wings overhanging over the trenches produced in Si substrates by photolithography methods [59]. Very often the laterally grown wings are found to be crystallographically tilted. This wing tilt is crucial as it can result in dislocations or cracks at the coalescence point of the wings. In this paper, our previous work on CE GaN grown on Si(111) substrates [9, 10] is continued with the objective to investigate the additional factors beyond thermal induced stress that influence wing tilt. Our previous analysis indicates that a strain gradient forms with depth in the GaN layer. In this study we address this issue explicitly. Comparison of the results obtained by different methods such as polychromatic X-ray microdiffraction (PXM) high resolution x-ray diffraction (HRXRD), scanning electron microscopy (SEM), and FE simulations allows us to obtain detailed information about the depthdependent local microstructural distribution of strain, lattice rotations and misfit dislocations density in GaN layers grown by CE on c-plane Si substrates. EXPERIMENTAL DETAILS The two samples under investigation (A and B) are identical to those presented in Refs. 9 and 10. The Si (111) substrate consists of about 4.5 µm wide ridges separated by about 5.0 µm wide trenches in the Si [112] direction. The substrate was overgrown with a 100 nm thick AlN nucleation layer followed by a thick GaN layer using metal-organic vapor phase epitaxy. The
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