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Optical Properties of Germanium Doped Cubic GaN Donat J. As1, Michael Deppe1, Jürgen Gerlach2 and Dirk Reuter1 1 Universität Paderborn, Department Physik, Warburger Strasse 100, 33098 Paderborn, Germany 2 Leibniz-Institut für Oberflächenmodifizierung (IOM) e.V., Permoserstr. 15, 04318 Leipzig, Germany ABSTRACT We report on recent doping experiments of cubic GaN epilayers by Ge and investigate in detail the optical properties by photoluminescence spectroscopy. Plasma-assisted molecular beam epitaxy was used to deposit Ge-doped cubic GaN layers with nominal thicknesses of 600 nm on 3C-SiC(001)/Si(001) substrates. The Ge doping level could be varied by around six orders of magnitude by changing the Ge effusion cell temperature. A maximum free carrier concentration of 3.7×1020 cm-3 was measured in the GaN layers via Hall-effect at room temperature. Low temperature photoluminescence (PL) showed a clear shift of the donoracceptor emission to higher energies with increasing Ge-doping. Above a Ge concentration of ~ 2x1018cm-3 the near band edge lines merge to one broad band. From temperature dependent measurements of the observed excitonic and donor-acceptor transitions a donor-energy of ~ 36 meV could be estimated for Ge. INTRODUCTION Germanium was recently introduced as a highly favorable n-type dopant in hexagonal GaN (h-GaN) [1]. In comparison to the standard Si donor, the superiority of Ge doped h-GaN is demonstrated by very high free carrier concentrations above 1020cm-3 with smooth surfaces and reduced tensile strain [2]. However, due to symmetry reasons h-GaN, if grown in c-plane, exhibits strong spontaneous and piezoelectric polarization fields at interfaces and surfaces, which limit the recombination efficiency in e.g. double heterostructures or quantum wells. To overcome these harmful effects non-polar or semi-polar h-GaN may be grown or as an additional alternative way the metastable cubic phase of GaN (c-GaN) may be used, where these fields are absent. Therefore, for device applications it will be very attractive to investigate the behavior of Ge as an alternative n-type dopant in cubic group III-nitrides. EXPERIMENT Cubic GaN (c-GaN) epilayers were grown by plasma-assisted molecular beam epitaxy (MBE) in a Riber 32 system. 10 μm thick 3C-SiC (001) layers, which were deposited on 500 μm Si (001) were used as substrates [3]. Ga, Ge, and Si molecular beams were provided by effusion cells and an Oxford Applied Research HD25 radio frequency plasma source was used to generate activated nitrogen atoms. All samples have been grown at a substrate temperature around 720 °C employing a Ga flux of 3.4 × 1014 cm-2 s-1. A series of samples each consisting of a nominally 600 nm thick Ge-doped c-GaN layer was grown. The Ge effusion cell temperature was varied between 600 °C and 1000 °C in steps of 100 °C to achieve doping levels varying over several orders of magnitude. Time of flight secondary ion mass spectroscopy (ToF-SIMS) demonstrates the incorporation of Ge into c-GaN

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