Electrical and Optical Properties of Carbon-Doped GaN Grown by MBE on MOCVD GaN Templates Using a CCl 4 Dopant Source
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Electrical and Optical Properties of Carbon-Doped GaN Grown by MBE on MOCVD GaN Templates Using a CCl4 Dopant Source Rob Armitage, Qing Yang, Henning Feick*, Yeonjoon Park, and Eicke R. Weber Department of Materials Science and Engineering, University of California, and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
ABSTRACT Carbon-doped GaN was grown by plasma-assisted molecular-beam epitaxy using carbon tetrachloride vapor as the dopant source. For moderate doping mainly acceptors were formed, yielding semi-insulating GaN. However at higher concentrations p-type conductivity was not observed, and heavily doped films (>5×1020 cm-3) were actually ntype rather than semi-insulating. Photoluminescence measurements showed two broad luminescence bands centered at 2.2 and 2.9 eV. The intensity of both bands increased with carbon content, but the 2.2 eV band dominated in n-type samples. Intense, narrow (~6 meV) donor-bound exciton peaks were observed in the semi-insulating samples. INTRODUCTION Carbon is a major residual impurity in gallium nitride grown by metal-organic chemical vapor deposition (MOCVD) [1], and has found practical use as an intentional dopant for semi-insulating base layers in transistor structures [2]. Despite this, the defects formed by carbon and their influence on GaN properties remain poorly understood. Among possible defects, theorists considered CN, CGa, carbon pairs, and CGa in a DX-configuration [3,4,5]. However there is disagreement in calculated values for the defect formation energies and (in some cases) their levels in the GaN band gap. The problem is complicated since the energetically favored type of defect may depend on the growth conditions (Ga- or N-rich limit) and the Fermi level position. Previous experiments on carbon-doped GaN yielded inconsistent results. Several groups pursued carbon as an alternative to magnesium for p-type doping, but only two succeeded [6,7]. One of these studies [7] involved zincblende-structure GaN and both used GaAs substrates. Other groups found that carbon compensated background donors to yield semi-insulating GaN [2,8,9], but could not obtain p-type conductivity. Still other studies found that carbon produced predominantly donors, or showed mixed acceptor/donor behavior depending on the dopant concentration [10,11]. A feature observed in all luminescence studies of heavily carbon-doped GaN was a deep-level yellow emission band [12]. Many earlier studies were complicated by factors such as the use of unconventional substrates, carbon dopant sources that introduced additional impurities, and relatively poor-quality GaN reference material. The present investigation avoids such *
On leave from center for advanced european studies and research (Caesar), Bonn, Germany
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