Inﬂuence of surface and polarization potentials on the electronic and optical properties of In x Ga 1-x N/GaN axial nanowire heterostructures Oliver Marquardt, Christian Hauswald, Martin W¨ olz, Lutz Geelhaar, and Oliver Brandt Paul-Drude-Institut f¨ ur Festk¨ orperelektronik Hausvogteiplatz 5–7, 10117 Berlin, Germany Email: marquardt@pdi-berlin.de Abstract—We study the inﬂuence of surface and polarization potentials on the electronic properties of axial InxGa1-xN/GaN nanowire heterostructures. Our simulations indicate nontrivial, competing inﬂuences of both these potentials on the spatial separation of electrons and holes, which are well suited to explain previous experimental observations. I. I NTRODUCTION In x Ga 1-x N is considered to be an excellent, environ- mentally friendly canditate for novel light emitting devices spanning the whole visible spectrum [1], [2]. The growth of planar In x Ga 1-x N/GaN nanostructures, however, is limited by the large lattice mismatch between GaN and InN and the tendency of phase separation. Hence, the crystal quality required for light emitting devices cannot be achieved through- out the whole composition range. This drawback of planar In x Ga 1-x N/GaN heterostructures can in principle be avoided in In x Ga 1-x N/GaN axial nanowire heterostructures. Here, an In x Ga 1-x N disk serves as the active region for light emission in a GaN nanowire. The nanowire geometry facilitates elastic relaxation of the strained disk due to the free surfaces even for large In contents. In fact, many studies report green, amber, and red light emission from axial In x Ga 1-x N/GaN het- erostructures [3], [4], [5]. However, it was recently observed that the photoluminescence intensity monotonically decreases with decreasing In content, thus making light emission in the blue spectral range difﬁcult to achieve [6]. In this paper, we report on a systematic study of the inﬂuence of the In content and the thickness of the active layer on the electronic properties of axial In x Ga 1-x N/GaN nanowire heterostructures. The simulation is based on continuum elasticity theory and an eight-band k · p model implemented within a plane-wave framework [7], [8], and takes strain, piezoelectricity, and surface potentials into account. II. THE ROLE OF PIEZOELECTRIC POLARIZATION AND SURFACE POTENTIALS Free surfaces allow for elastic relaxation of nanowires, thus reducing the strain, but also the piezoelectric polarization in and around the active region. Nevertheless, strain and piezoelectricity cannot be fully eliminated in nanowires and still inﬂuence the electronic properties of the system. In particular, polarization potentials V P will occur that induce Fig. 1. Schematic line scan of the valence band edge at the bottom interface of an InxGa 1-x N insertion in a GaN nanowire along the wire’s diameter, including surface potentials (eV S ) and polarization potentials (eV P ). The dashed line indicates the bulk valence band edge. The shaded area depicts the wire diameter. a spatial separation of electrons and holes along the growth direction, as is well-known and understood for planar III- nitride heterostructures. Due to the existence of free surfaces, nanowires furthermore exhibit surface potentials V S resulting from Fermi level pinning and unintentional doping. These additional potentials represent an attractive potential for either hole or electron states and can therefore induce an in-plane spatial separation of the charge carriers. The inﬂuence of surface and polarization potentials on the valence band energy E vb is depicted schematically in Fig. 1. Correspondingly, the charge carrier conﬁnement and the resulting recombination rate will depend on the interplay between polarization poten- tials and surface potentials. As a ﬁrst step towards understanding this interplay, we have considered a surface potential which decays quadratically to- wards the central axis of the nanowire. Assuming a Fermi level pinning of 0.6 eV above the conduction band edge [9] and a di- ameter of the nanowire of 80 nm, a doping level of 10 17 cm -3 corresponds to a surface potential drop of eV S = 80 meV. In order to achieve a qualitatively meaningful description of the inﬂuence of surface and polarization potentials on the charge carrier conﬁnement, we have limited our study to the electron and hole ground states and used the spatial charge carrier overlap O as an indicator: O =  r1  r2  r3  e (r 1 ,r 2 ,r 3 ) h (r 1 ,r 2 ,r 3 ) (1) where r i denotes the spatial discretization of the super cell and  e,h represents the electron and hole charge densities. 978-1-4673-6310-5/13/$31.00 ©2013 IEEE NUSOD 2013 137