The Structure of Dislocations in GaN Grown by MBE as a Function of the Gallium to Nitrogen Ratio
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The Structure of Dislocations in GaN Grown by MBE as a Function of the Gallium to Nitrogen Ratio Marcus Q. Baines1, David Cherns1, Sergei V. Novikov2, Michael J. Manfra3 and C. Thomas Foxon2 1 HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, U.K. 2 Department of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. 3 Bell Laboratories, Lucent Technologies, 700 Mountain Avenue, Murray Hill, NJ 07974, U.S.A.
ABSTRACT Transmission electron microscopy (TEM) and atomic force microscopy (AFM) have been used to analyse the core structure of dislocations in GaN grown by molecular beam epitaxy (MBE) as a function of the gallium to nitrogen ratio. Ga-rich samples had a much smoother morphology; TEM observations showed that amorphous deposits decorated some dislocations and occasional surface pits, but weak beam and end-on imaging suggested that, away from the growth surface, dislocations of all types had closed core structures, in contrast to previous observations (Hsu et al, Appl. Phys. Lett. 78, 3980 (2001), Baines et al, Mat.Res.Soc.Symp.Proc. 743, L2.5 (2003)). Ga-poor samples were found to have much rougher surfaces; dislocations were often at the centers of deep surface pits but were observed to be undecorated and to have closed core structures. It is concluded that in growth under Ga-rich conditions, decoration of dislocation cores depends on the accumulation of Ga at surface pits, rather than being a fundamental property of dislocation formation. INTRODUCTION There is widespread evidence that dislocations in GaN can have open core structures, depending on both growth and on doping conditions. In undoped and Si-doped GaN films grown by metalorganic chemical vapour deposition (MOVCD), dislocations with Burgers vectors of ctype (screw dislocations) can have open core structures [1,2]. However, in MOCVD films grown with high concentrations of Mg, we have shown that a (edge) and c + a (mixed) type dislocations can have open core structures [3]. In recent work, Hsu et al [4] and Baines et al [5] have shown that dislocations with screw or mixed character in GaN films grown by molecular beam epitaxy (MBE) under Ga-rich conditions can have open cores decorated by a deposit believed to be Garich. In this paper, we examine in more detail the core structure of dislocations in films grown by MBE, as a function of the gallium to nitrogen ratio. Studies were carried out by transmission electron (TEM), and by atomic force microscopy (AFM). TEM studies, carried out on plan-view samples, showed that some, but not all, dislocations in the Ga-rich films were decorated by an amorphous deposit. Apart from the immediate surface region, these dislocations were also found to have closed core structures. Studies of large areas suggested that amorphous deposits were associated with surface pitting rather than due to dislocations per se. It is argued that, unlike the
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case for Mg-doped material there is no driving force for segregation of Ga to dislocation
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