Structural Study of V-like Columnar Inversion Domains in AlN Grown on Sapphire
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Structural Study of V-like Columnar Inversion Domains in AlN Grown on Sapphire J. Jasinski, T. Tomaszewicz, Z. Liliental-Weber, Q. S. Paduano1, D. W. Weyburne1 Lawrence Berkeley National Laboratory, Materials Science Division, Berkeley, CA 94720 1 Air Force Research Laboratory, Sensors Directorate, Hanscom AFB, MA 01731 ABSTRACT V-like columnar inversion domains with a divergence angle of about 4.5o ± 1o grown in AlN films with N-polarity were studied using transmission electron microscopy (TEM) and atomic force microscopy. Such domains emerge at the surface forming a small islands in form of hexagonal, truncated pyramids. A model of such pyramid was proposed. TEM studies indicate a displacement of c/2 along the [0001] direction at the inversion domain boundary. A boundary itself is composed of long segments located on the {1100} planes, which are alternated by short segments on some inclined planes. INTRODUCTION C-plane sapphire often remains the substrate of choice for epitaxial growth of III-nitrides films. However, due to the large lattice and thermal expansion coefficient mismatch between the substrate and the layer, high densities of structural defects are formed inside such films. Different types of structural defects (threading dislocations, dislocation loops, basal stacking faults, nanopipes, and inversion domains) in GaN grown on sapphire have been addressed by numerous studies. On the other hand, much less has been done in case of AlN films grown on sapphire despite the growing interest in their potential applications in high power and high temperature devices [1,2]. In this work, we investigated AlN films containing specific type of defects, referred latter as V-like columnar inversion domains. Their structure and model of the inversion domain boundary will be discussed. EXPERIMENTAL DETAILS AlN films were grown on c-plane sapphire by metalorganic chemical vapor deposition (MOCVD). Substrates were first heated to 1000oC and kept in hydrogen ambient. This predeposition heat treatment was followed by the nitridation at 930oC after which the actual growth was performed at 1150oC. A detailed growth description can be found elsewhere [3]. Atomic force microscopy (AFM) was used to study surface morphology of these AlN films. On the other hand, their microstructure was investigated using transmission electron microscopy (TEM) in both, cross-sectional and plan-view configurations. TEM specimens were prepared using standard method of mechanical thinning followed by Ar ion milling. RESULTS AND DISCUSSION Bright-field TEM images of two, ~0.5 µm- and ~1.25 µm-thick, AlN layers observed in cross-section along the [1100] direction, are shown in Figs. 1a,b. One can see that both layers have high density of V-shape columnar defects in addition to threading dislocations. These defects nucleate at the interface with sapphire and propagate through the entire layer. At their
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AlN
0.2 µm
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AlN
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sapphire
[1120]
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Figure 1. Bright-field
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