Characterization of Stress Relaxation, Dislocations and Crystallographic Tilt Via X-ray Microdiffraction in GaN (0001) L
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Characterization of Stress Relaxation, Dislocations and Crystallographic Tilt Via Xray Microdiffraction in GaN (0001) Layers Grown by Maskless Pendeo-Epitaxy R.I Barabash1∗, G.E. Ice1, W. Liu1, S. Einfeldt2, D. Hommel2, A. M. Roskowski3, R. F. Davis3 1
Metals and Ceramics Div., Oak Ridge National Laboratory, Oak Ridge, USA Institute of Solid State Physics, University of Bremen, Germany 3 Materials Science and Engineering Department, North Carolina State University, Raleigh, USA 2
ABSTRACT Intrinsic stresses due to lattice mismatch and high densities of threading dislocations and extrinsic stresses resulting from the mismatch in the coefficients of thermal expansion are present in almost all III-Nitride heterostructures. Stress relaxation in the GaN layers occurs in conventional and in pendeo-epitaxial films via the formation of additional misfit dislocations, domain boundaries, elastic strain and wing tilt. Polychromatic X-ray microdiffraction, high resolution monochromatic X-ray diffraction and finite element simulations have been used to determine the distribution of strain, dislocations, subboundaries and crystallographic wing tilt in uncoalesced and coalesced GaN layers grown by maskless pendeo-epitaxy. An important parameter was the width-to-height ratio of the etched columns of GaN from which the lateral growth of the wings occurred. The strain and tilt across the stripes increased with the width-to-height ratio. Tilt boundaries formed in the uncoalesced GaN layers at the column/wing interfaces for samples with a large ratio. Sharper tilt boundaries were observed at the interfaces formed by the coalescence of two laterally growing wings. The wings tilted upward during cooling to room temperature for both the uncoalesced and the coalesced GaN layers. It was determined that finite element simulations that account for extrinsic stress relaxation can explain the experimental results for uncoalesced GaN layers. Relaxation of both extrinsic and intrinsic stress components in the coalesced GaN layers contribute to the observed wing tilt and the formation of sub-boundaries. INTRODUCTION Stress is present in almost all III-Nitride heterostructures. Lattice mismatch and high defect density produce intrinsic stress. The mismatch in the thermal expansion coefficients between the substrate and the nitride material structures produces an additional extrinsic stress component. Stress relaxation in the GaN layers causes formation of dislocations, boundaries and strain. Dislocations and tilt boundaries are known to impair the performance of GaN-based light emitting devices [1, 2]. Lateral epitaxial overgrowth (LEO) techniques such as pendeoepitaxy [3 – 5] have recently been shown to greatly reduce the density of threading dislocations. In the pendeoepitaxy ∗
Contact author: Rosa Barabash
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process, GaN wings are grown from the sides of GaN columns (Fig.1). The top of the column may or may not be covered with a mask (masked or maskless pendeoepitaxy) to inhibit GaN growth. When the LEO process is continued u
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