Ultraviolet Photodetector Based on GaN/AlN Quantum Disks in a Single Nanowire L. Rigutti,* ,† M. Tchernycheva, † A. De Luna Bugallo, † G. Jacopin, † F. H. Julien, † L. F. Zagonel, ‡ K. March, ‡ O. Stephan, ‡ M. Kociak, ‡ and R. Songmuang § † Institut d’Electronique Fondamentale, University of Paris Sud XI, UMR 8622 CNRS, 91405 Orsay, France, ‡ Laboratoire de Physique des Solides, University of Paris Sud XI, UMR 8502 CNRS, 91405 Orsay, France, and § CEA-CNRS group “Nanophysique et Semiconducteurs”, Institut Ne ´el, 25 Rue des Martyrs 38042, Grenoble Cedex 9, France ABSTRACT We report the demonstration of single-nanowire photodetectors relying on carrier generation in GaN/AlN QDiscs. Two nanowire samples containing QDiscs of different thicknesses are analyzed and compared to a reference binary n-i-n GaN nanowire sample. The responsivity of a single wire QDisc detector is as high as 2 × 10 3 A/W at λ ) 300 nm at room temperature. We show that the insertion of an axial heterostructure drastically reduces the dark current with respect to the binary nanowires and enhances the photosensitivity factor (i.e., the ratio between the photocurrent and the dark current) up to 5 × 10 2 for an incoming light intensity of 5 mW/cm 2 . Photocurrent spectroscopy allows identifcation of the spectral contribution related to carriers generated within large QDiscs, which lies below the GaN band gap due to the quantum confined Stark effect. KEYWORDS Nanowires, quantum structures, photodetector, nitrides T he recent progress in the controlled synthesis of wide- band-gap nanowires has enabled the development of nanoscale photonic devices in the visible-UV spectral range. Single-wire optically pumped lasers, 1 nanolight emit- ting diodes covering the whole visible spectrum, 2 and pho- tovoltaic devices 3 have been demonstrated using core-shell InGaN/GaN multiquantum well nanowires grown by catalyst- assisted MOCVD. UV photodetectors based on ZnO or GaN nanowires have been also demonstrated. 4-7 The small nanowire size and the high photoconductive gain demon- strated in nanowire photodetectors 4,6 are very promising for the fabrication of focal plane arrays with diffraction-limited spatial resolution and high responsivity. Numerous studies have been devoted to the photocon- duction properties of GaN nanowires. It has been pointed out that the nanowire surface has a strong impact on the wire photoconduction. 8 Due to the Fermi level pinning, the bands bend close to the lateral surface and create a region depleted of electrons, which changes the nanowire conduc- tion and also induces a Franz-Keldysh effect responsible for sub-band-gap photocurrent. 9 All these studies focus on binary GaN nanowire; no nanowire photodetectors based on QDisc III-N heterostructures have been reported so far. In the present work we report the demonstration of an ultraviolet photodetector based on a single GaN nanowire containing GaN/AlN QDiscs. The doped extremities of the nanowires provide an electrical access allowing probing of the carrier photogeneration in the QDisc region. Two het- erostructured nanowire samples containing QDiscs of dif- ferent thickness are analyzed and compared to a reference binary n-i-n GaN nanowire sample. The responsivity of a single wire QDisc detector has been measured to be as high as 2 × 10 3 A/W at λ ) 300 nm at room temperature. We show that the insertion of an axial heterostructure drastically reduces the dark current with respect to the binary nanow- ires and enhances the photosensitivity factor (i.e., the ratio between the photocurrent and the dark current) up to 5 × 10 2 at 5 mW/cm 2 illumination power density. Photocurrent spectroscopy allows identifying the spectral contribution related to carriers generated within large QDiscs, which lies below the GaN band-gap due to quantum confined Stark effect (QCSE). The study of photocurrent at different tem- peratures shows that photogenerated carriers escape from QDiscs mainly by thermal activation. Catalyst-free GaN nanowires were grown by radio fre- quency plasma-assisted molecular beam epitaxy (PA-MBE) on Si(111) substrates under N-rich atmosphere at 790 °C, according to the technique described in ref 10. The nanowire length is approximately 1.2 μm and the diameter is 25-80 nm. For nanowire heterostructures, a stack of 20 AlN/GaN QDiscs was inserted in the middle of Si-doped GaN nanow- ires, as schematically shown in Figure 1a. Quantum disks have been formed by switching from Ga to Al flux without any growth interruption. A reference sample containing n-i-n GaN binary nanowires was grown using similar conditions. The length of the nominally undoped segment is 100 nm. The STEM analyses (Figure 1b,c) show that the thickness of AlN barriers is 2-3 nm for both samples and the QDisc thickness is 3-5 nm (1-3 nm), for sample 1 (sample 2). As seen from STEM images, the QDisc thickness increases progressively toward the nanowire top. An AlN shell of about * Corresponding author, lorenzo.rigutti@ief.u-psud.fr. Received for review: 03/29/2010 Published on Web: 07/09/2010 pubs.acs.org/NanoLett © 2010 American Chemical Society 2939 DOI: 10.1021/nl1010977 | Nano Lett. 2010, 10, 2939–2943