Optical Properties of Carbon Doped Cubic GaN Epilayers Grown on GaAs (001) Substrate by Molecular Beam Epitaxy
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Optical Properties of Carbon Doped Cubic GaN Epilayers Grown on GaAs (001) Substrate by Molecular Beam Epitaxy D. J. As, U. Köhler, and K. Lischka, Universität Paderborn, FB-6 Physik, Warburger Strasse 100, D-33095 Paderborn, Germany, [email protected]
ABSTRACT The optical properties of Carbon doped cubic GaN epilayers have been investigated by temperature and intensity dependent photoluminescence measurements. RF-plasma assisted molecular beam epitaxy equipped with an e-beam-evaporation source for carbon doping is used to grow the cubic GaN layers on GaAs (001) substrates. With increasing Carbon flux a new photoluminescence line at 3.08 eV appeared at 2K. This line is attributed to a donor acceptor transistion, which involves the shallow CN acceptor. From the spectral position the binding energy of the C acceptor is estimated to be about EC = 0.215 eV. Our experiments demonstrate that C indeed introduces a shallow acceptor in cubic GaN with an acceptor binding energy, which is about 15 meV lower than that observed for the Mg acceptor in cubic GaN. However, at high C fluxes a deep red luminescence band appeared at 2.1 eV, indicating compensation effects.
INTRODUCTION For advanced optoelectronic and electronic devices, like light emitting diodes (LEDs), or laser diodes (LDs) controlled p-type doping and high hole conductivity is crucial [1]. Up to now, Magnesium is used as standard dopant in Molecular Beam Epitaxy (MBE) as well as Metalorganic Vapor Phase Epitaxy (MOVPE). However, Magnesium has several disadvantages which limit the usually reported doping efficiency of Mg in GaN to maximum hole concentrations in the upper 1017cm-3. This is mainly ascribed to large acceptor ionization energy and compensation effects [2]. MBE-growth further showed that Mg is very volatile, requires low substrate temperatures and N-rich growth conditions [3,4]. These conditions are disadvantageous for high quality epilayers, especially for the cubic phase, where N-rich conditions deteriorate phase purity [5]. Among possible alternative acceptor dopants especially Carbon has received a considerable interest due to its similarity to nitrogen in atomic radius and electronegativity. In previous reports, Carbon doping of hexagonal Gallium Nitride (h–GaN) led to semi–insulating properties or to a reduction in background electron concentration [7-9]. In cubic GaN Abernathy et al. [10] reported on p-type doping by carbon. However, due to the use of CCl4 a pronounced reduction in growth rate prohibited the incorporation of higher C concentrations and the maximum hole concentration reached was 3 x 1017 cm-3. In this contribution we report on the optical properties of carbon doped cubic GaN epilayers. Carbon doping has been performed by rf-plasma assisted molecular beam epitaxy using an ebeam evaporation source. Our experiments clearly demonstrate that C indeed introduces a shallow acceptor in cubic GaN with an acceptor binding energy, which is about 15 meV lower than that observed for the Mg in c-GaN. I2.3.1
EXPERIMENTAL Cubic GaN (c-GaN)
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