Dislocations around precipitates in AlGaN epilayers
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Shin Tsunekawa Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Atsuo Kasuya Center for Interdisciplinary Research, Tohoku University, Aoba-ku, Sendai 980-8578, Japan (Received 9 April 2002; accepted 15 May 2002)
Dislocations around precipitates in undoped AlGaN were investigated by transmission electron microscopy. The dislocation images were taken under different diffraction conditions. The dislocations are classified into two types, a pure edge dislocation loop and a close-coiled helical dislocation. Both types of dislocations were found to depend on the shape and size of the precipitate sources. It is suggested that the pure edge dislocation loop results from homogeneous shear stress and the close-coiled helical dislocation is caused by spherically symmetrical stress concentration at round ends of the precipitates and chemical force due to defect concentration change.
I. INTRODUCTION
II. SAMPLES AND EXPERIMENTAL DETAILS
GaN and its alloys have recently attracted great attention because they are excellent candidates for applications in solar blind ultraviolet detectors, high-voltage, high-power, and high-temperature devices. The coalescences in the materials are believed to bring about the formations of threading dislocations,1 boundaries,2 nanopipes,3–5 precipitates,6 and cracks.7 In our previous works, only the defects caused by the coalescence were focused.4 –8 Along with the rapid development of the epitaxial growth technique,9–13 the coalescence quality has drastically improved. The reduction of defects in perfect coalescence epilayers is a key issue for the performance improvement of nitride semiconductor devices. However, little information has been gained on the dislocations caused by precipitation in perfect coalescence AlGaN epilayers. An understanding of the dislocations around the precipitates is of increasing importance. In this work, undoped GaN-based layers and AlGaN epilayers were grown by metalorganic vapor phase epitaxy (MOVPE). The precipitates were identified by atomic force microscopy (AFM) and scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS). The structures of dislocations around the precipitates were observed by transmission electron microscopy (TEM). The formation mechanisms of the dislocations are discussed.
Samples used in this study were 0.4-m-thick AlGaN epilayers grown on top of GaN layer at 1080 °C by MOVPE. The 0.6-m-thick GaN layer was grown on the (0001) surface of an Al2O3 substrate at 1071 °C. X-ray microanalysis showed that the Al mole fraction was about 0.22. None of the epilayers was intentionally doped. As-grown epilayers were transparent with specular surfaces. The surface morphology of the epilayers was imaged by an AFM (NanoScope IIIa, Digital Instruments). A typical plane-view image is shown in Fig. 1(a). A number of bright contrast regions with densities of about 1.5 × 109 cm−2 appear in the AFM images. Almost all of the bright regions show a more or less oval shape in the size range from several tens t
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