Surface-induced effects in GaN nanowires
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Toma Stoica Peter Grünberg Institut (PGI-9), Forschungszentrum Jülich, D-52425 Jülich, Germany; and Jülich-Aachen Research Alliance, D-52425 Jülich, Germany
Oliver Brandt and Lutz Geelhaara) Paul-Drude-Institut für Festkörperelektronik, 10117 Berlin, Germany (Received 10 June 2011; accepted 16 June 2011)
Semiconductor nanowires (NWs) are characterized by an extraordinarily large surface-to-volume ratio. Consequently, surface effects are expected to play a much larger role than in thin films. Here, we review a research focused on the impact of the surface on the electrical and optical properties of catalyst-free GaN NWs with growth direction ,0001.. Using a combination of complementary experimental techniques, it has been shown that the Fermi level is pinned at the NW sidewall surfaces, resulting in internal electric fields and in full depletion for NWs below a critical diameter. Deoxidation of the surfaces unpins the Fermi level, leading to enhanced radiative recombination of excitons. Prominent absorption below the bandgap is caused by the Franz-Keldysh effect. Close to the surface, the ionization energy of donors is reduced. The consideration of surfaceinduced effects is mandatory for an understanding of the physical properties of NWs as well as their application in devices. I. INTRODUCTION
Semiconductor nanowires (NWs) are considered as promising building blocks for nanoscale electronic and optoelectronic devices.1,2 The characteristic feature of NWs is their high aspect ratio which results in an extremely large surface-to-volume ratio. One beneficial consequence of the NW geometry is the efficient elastic relaxation of strain in lattice-mismatched heterostructures.3 Thus, dissimilar materials can be epitaxially combined while retaining an exceptionally high material quality.4 At the same time, devices based on NWs are very sensitive to surface effects, making them very attractive for sensing applications.5,6 However, surface effects may also have undesired consequences for NW device performance. In particular, the functionality of any semiconductor device depends on dopants, internal electric fields, and the resulting charge carrier distributions. Here, we review exemplary results related to the effects of the surface on electric fields and dopants for the specific case of catalyst-free GaN NWs. We expect that most of these insights hold for semiconductor NWs in general. GaN NWs were grown by plasma-assisted molecular beam epitaxy (MBE)7–12 or metal–organic chemical vapor deposition (MOCVD)13 without any external catalyst/collector. The advantage of this approach
II. FERMI LEVEL PINNING AND INTERNAL ELECTRIC FIELD
The electronic structure at semiconductor surfaces typically differs from that in the bulk and is then described by surface states. In the presence of surface states, the Fermi level EF is pinned at the surface. This
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.211 J. Mater. Res., Vol. 26, No. 17, Sep 14, 2011
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