Intersecting Basal Plane and Prismatic Stacking Fault Structures in GaN/AlN epilayers on on-axis and off-cut 6H-SiC subs
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0892-FF22-02.1
Intersecting Basal Plane and Prismatic Plane Stacking Fault Structures in GaN/AlN epilayers on on-axis and off-cut 6H-SiC substrates J. Bai, X. Huang, M. Dudley Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275, U.S.A.
ABSTRACT Comparative TEM studies have been carried out on GaN/AlN epifilms grown on both onaxis and off-cut 6H-SiC substrates to study the defects formed in the GaN/AlN films and the state of strain relaxation at the interface. Prismatic Stacking Faults (PSFs) are observed to form at I1 type substrate steps in both the on-axis and vicinal samples. In the vicinal samples, the PSFs expand into GaN/AlN film forming intersecting stacking fault configurations comprising faults that fold back and forth from the basal plane (I1 Basal-Plane Stacking Faults; BSFs) to the prismatic plane (PSFs). On the other hand, in the on-axis sample the PSFs are observed to mostly annihilate each other to form enclosed domains confined to the near-interface region. In addition, HRTEM studies suggest the existence of Geometric Partial Misfit Dislocations (GPMDs) at the SiC/AlN interface of the vicinal sample, which form at I2 type substrate steps. These GPMDs simultaneously accommodate the lattice mismatch and stacking sequence mismatch present at the SiC/AlN interface. This provides explanation of the improved strain relaxation observed in the vicinal versus the on-axis sample. INTRODUCTION In spite of significant research effort, the improvement of the quality of GaN films grown on the currently available non-native substrates (SiC and sapphire) remains a significant obstacle to the extensive development of GaN-based devices. The use of vicinal surface epitaxy (VSE) has been shown to be a promising strategy for this purpose. In previous studies, GaN films grown on vicinal substrates have been demonstrated to exhibit improved crystalline quality and optical performance [1-3]. This effect has been widely attributed to the reduction of threading dislocation density, but the detailed mechanism is far from understood. Here, we have conducted detailed studies of epilayers grown on both on-axis and vicinal SiC substrates. For the vicinal samples, we report the observation of intersecting stacking fault configurations and their formation mechanisms. In addition, strain relaxation of GaN/AlN films grown on off-cut SiC substrates is studied and a GPMD-driven strain relaxation mechanism is suggested. EXPERIMENTAL Transmission electron microscopy (TEM) specimens were prepared from two sets of samples: samples #1 and #2 are GaN films (1µm thick) grown with an AlN buffer (0.1µm thick) on on-axis 6H-SiC substrate and 3.5° off-cut 6H-SiC substrate (towards [1 2 10] direction), respectively. TEM studies were carried out on a CM12 and a JEOL JEM 3000F system. High resolution X-ray diffraction (HRXRD) was performed on a Bede D1 system. RESULTS AND DISCUSSION
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Figure 1 shows a series of TEM images recorded from specimens cut from samp
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