1 Structural Changes in a Single GaN Nanowire under Applied Voltage 2 Bias 3 Sergey Lazarev, †,‡ Dmitry Dzhigaev, † Zhaoxia Bi, § Ali Nowzari, § Young Yong Kim, † Max Rose, † 4 Ivan A. Zaluzhnyy, †,∥ Oleg Yu. Gorobtsov, †,⊥ Alexey V. Zozulya, †,# Filip Lenrick, § Anders Gustafsson, § 5 Anders Mikkelsen, § Michael Sprung, † Lars Samuelson, § and Ivan A. Vartanyants* ,†,∥ 6 † Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, D-22607 Hamburg, Germany 7 ‡ National Research Tomsk Polytechnic University (TPU), Lenin Avenue 30, 634050 Tomsk, Russia 8 § NanoLund, Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden 9 ∥ National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe shosse 31, 115409 Moscow, 10 Russia 11 * S Supporting Information 12 ABSTRACT: GaN nanowires (NWs) are promising building 13 blocks for future optoelectronic devices and nanoelectronics. 14 They exhibit stronger piezoelectric properties than a bulk GaN. 15 This phenomena may be crucial for applications of NWs and 16 makes their study highly important. We report on an 17 investigation of the structure evolution of a single GaN NW 18 under an applied voltage bias along polar [0001] crystallographic 19 direction until its mechanical break. The structural changes were 20 investigated using coherent X-ray Bragg diffraction. The three-dimensional (3D) intensity distributions of the NWs without 21 metal contacts, with contacts, and under applied voltage bias in opposite polar directions were analyzed. Coherent X-ray Bragg 22 diffraction revealed the presence of significant bending of the NWs already after metal contacts deposition, which was increased 23 at applied voltage bias. Employing analytical simulations based on elasticity theory and a finite element method (FEM) 24 approach, we developed a 3D model of the NW bending under applied voltage. From this model and our experimental data, we 25 determined the piezoelectric constant of the GaN NW to be about 7.7 pm/V in [0001] crystallographic direction. The ultimate 26 tensile strength of the GaN NW was obtained to be about 1.22 GPa. Our work demonstrates the power of in operando X-ray 27 structural studies of single NWs for their effective design and implementation with desired functional properties. 28 KEYWORDS: GaN nanowires, coherent X-ray Bragg diffraction, piezoelectric effect, finite element method 29 S emiconductor nanowires (NWs) based on gallium nitride 30 (GaN), indium nitride (InN), and indium gallium nitride 31 (InGaN) have promising applications for light-emitting diodes, 32 low-cost solar cells, transistors, single photon sources, and 33 other devices. 1−6 The wurtzite (WZ) (hexagonal) crystal 34 structure of GaN NWs is noncentrosymmetric and has an 35 internal electric field along the [0001] crystallographic 36 direction. 7,8 Local deformation of the GaN unit cell leads to 37 a formation of an internal piezoelectric field and vice versa. 9 38 Nitride-based light-emitting diodes (LEDs) grown on c-plane 39 GaN substrate usually show the blue shifts with increased 40 injected current. This effect is usually attributed to the 41 screening of the piezoelectric field when the injected electron 42 density is high, which balances the band bending. It was also 43 demonstrated that a single GaN NW exhibits stronger 44 piezoelectricity 10 than a bulk GaN. 11 Integration of the NWs 45 into an electric circuit by metallic contacts may induce 46 additional strain and, therefore, may lead to additional 47 piezoelectric effects in the structure. This may dramatically 48 influence electron−hole pair recombination and alter the 49 efficiency of optoelectronic devices based on GaN NWs. 12 50 Moreover, better knowledge of the relation between piezo- 51 electric effect and strain field in single GaN NWs with the sizes 52 of hundreds of nanometers could potentially contribute to 53 understanding of inefficient recombination of electron−hole 54 pairs in quantum wells. 13−15 Therefore, investigation of the 55 influence of applied voltages on the structure of a single GaN 56 NW is of significant importance. 57 Different methods may be employed to reveal structural 58 changes in NWs under applied voltage such as scanning 59 electron microscopy (SEM) or transmission electron micros- 60 copy (TEM). 16 X-ray nanodiffraction, developed recently at 61 synchrotron sources, is an alternative approach that allows one 62 to determine structural properties of single NWs in a 63 nondestructive way. 17−20 Unfortunately, resolution of this 64 method is limited by the X-ray beam size. Newly developed X- 65 ray coherent scattering methods such as Bragg coherent X-ray 66 diffractive imaging (CXDI) and ptychography 21−24 allow to Received: May 3, 2018 Revised: July 17, 2018 Published: July 23, 2018 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.8b01802 Nano Lett. XXXX, XXX, XXX−XXX mac00 | ACSJCA | JCA11.1.4300/W Library-x64 | research.3f (R4.0.i9 HF05:4883 | 2.1) 2018/07/18 12:44:00 | PROD-WS-118 | rq_76143 | 7/24/2018 07:42:52 | 7 | JCA-DEFAULT