Atomic Structure of Defects in GaN:Mg grown with Ga polarity
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Atomic Structure of Defects in GaN:Mg grown with Ga polarity Z. Liliental-Weber, T. Tomaszewicz, D. Zakharov, J. Jasinski, M.A. O’Keefe, S. Hautakangasa, A. Laaksoa, and K. Saarinena Lawrence Berkeley National Laboratory, Berkeley, CA 94720 m/s 62/203 a Helsinki University of Technology. Espoo, Finland ABSTRACT Electron microscope phase images, produced by direct reconstruction of the scattered electron wave from a focal series of high-resolution images, were used to determine the nature of defects formed in GaN:Mg crystals. We studied bulk crystals grown from dilute solutions of atomic nitrogen in liquid gallium at high pressure and thin films grown by the MOCVD method. All the crystals were grown with Ga-polarity. In both types of samples the majority of defects were three dimensional Mg-rich hexagonal pyramids with bases on the (0001) plane and six walls on {1123} planes seen in cross-section as triangulars. Some other defects appear in crosssection as trapezoidal (rectangular) defects as a result of presence of truncated pyramids. Both type of defects have hollow centers. They are decorated by Mg on all six side walls and a base. The GaN which grows inside on the defect walls shows polarity inversion. It is shown that change of polarity starts from the defect tip and propagates to the base, and that the stacking sequence changes from ab in the matrix to bc inside the defect. Exchange of the Ga sublattice with the N sublattice within the defect leads to 0.6±0.2Å displacement between Ga sublattices outside and inside the defects. It is proposed that lateral overgrowth of the cavities formed within the defect takes place to restore matrix polarity on the defect base. INTRODUCTION GaN can be easily grown with n-conductivity, at least up to a concentration of donors in the 10 cm-3 range. On the other hand, p-doping remains still difficult and not well understood. The only efficient p-type dopant is Mg, but the free hole concentration is limited to 2x1018cm-3 for Mg concentrations in the low 1019cm-3 range. This could limit further development of GaN based devices. Further increase of the Mg concentration, up to 1x1020cm-3 leads to a decrease of the free hole concentration. This is commonly interpreted as auto-compensation due to increased formation of N vacancies or vacancy complexes with Mg [1]. Another characteristic feature is the occurrence of the so-called “blue band” in the photoluminescence spectra of highly Mgdoped GaN grown by MOVPE [2]. Transmission electron microscopy (TEM) studies show the formation of different types of Mg–rich defects, both in bulk crystals grown from Ga excess under high N-pressure and in crystals grown by MOCVD [3-6]. The types of the defects strongly depend on crystal growth polarity (Fig. 1a). For bulk crystals grown with N-polarity, the planar defects (shown on Fig. 1a as lines) are distributed at equal distances (20 unit cells of GaN). These defects were described as inversion domains [6]. 19
For growth with Ga-polarity (for both bulk and MOCVD grown crystals) a different
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