Morphology transitions in ZnO nanorods grown by MOCVD D.N. Montenegro a,n , A. Souissi b , C. Martı ´nez-Toma ´s a , V. Mun ˜ oz-Sanjose ´ a , V. Sallet b a Departamento de Fı ´sica Aplicada y Electromagnetismo, Universitat de Valencia, Dr. Moliner 50, 46100 Burjassot, Spain b Groupe d’Etude de la Mati  ere Condense´e (GEMAC), CNRS-Universite´ de Versailles St Quentin, 45 avenue des Etats-Unis, 78035 Versailles Cedex, France article info Article history: Received 17 April 2012 Received in revised form 9 August 2012 Accepted 23 August 2012 Communicated by R. Fornari Available online 31 August 2012 Keywords: A1. Nanostructures A1. Growth models A1. X-ray diffraction A3. Metalorganic chemical vapor deposition B1. Zinc compounds B2. Semiconducting materials abstract Morphology transitions (nanorods–nanowalls and nanorods–nanotubes-layer) were induced in the growth of ZnO nanostructures by metal organic chemical vapor deposition (MOCVD) on c-sapphire, using helium as carrier gas, and dimethylzinc–triethylamine and nitrous oxide as zinc and oxygen sources, respectively. A systematic study of the influence of the VI/II ratio and precursor flow-rates on the morphology of ZnO nanorod arrays has been carried out, taking advantage of the ability of MOCVD to individually control the precursor partial pressures. Growth mechanisms are discussed to under- stand the evolution of the nanostructures morphology for different growth conditions. In particular, the influence of the gas phase supersaturation on the vertical alignment and aspect ratio of the nanorods is emphasized. & 2012 Elsevier B.V. All rights reserved. 1. Introduction In the recent years, the synthesis of vertically aligned quasi- one-dimensional (1D) nanostructures like nanowires and nanor- ods has attracted intensive research interest, due to the potential and, in some cases, demonstrated applications of these nanos- tructures for the fabrication of assembly operating nanodevices [1,2]. Special attention is being paid to nanorods based on ZnO and related alloys, due to the attractive physical properties of ZnO such as a direct band-gap of 3.37 eV, a high exciton binding energy of 60 meV at room temperature, piezoelectricity, and a surface chemistry sensitive to the environment [3]. All these properties make ZnO nanostructures good candidates for applica- tions in various devices like sensors, detectors, solar cells, nano- generators and light-emitting diodes [4–6]. Several methods have been established to synthesize ZnO nanorod arrays such as physical vapor transport [7–9], chemical vapor deposition [10,11], electrodeposition [12,13], pulsed laser deposition [14,15] and metal organic chemical vapor deposition (MOCVD) [16–18]. The latter technique, further to the scalable advantages due to its industrial character, has been demonstrated to be well adapted to the synthesis of ZnO nanowires and nanorods arrays with well controllable shape, good quality and reproducibility [19]. In most of the contributions related to ZnO MOCVD growth, low reactor-pressures, dimethylzinc (DMZn) or diethylzinc (DEZn) as zinc precursor, and nitrogen or argon as carrier gas [20–22], have been used. In previous studies, we have reported on the growth of vertically well aligned ZnO nanorod arrays by means of an atmospheric-MOCVD process using dimethylzinc–triethylamine (DMZn–TEN) and N 2 O as zinc and oxygen precursors, respectively, and helium as carrier gas [23,24]. The relative simplicity and the lower costs of atmospheric-MOCVD processes are well known. On the other hand, the use of DMZn–TEN has been demonstrated to prevent the premature gas-reactions as well as to improve the c-oriented ZnO growth [25]. From the hydrodynamic point of view, the low density of helium favors the laminar gas-regime, and so prevents local turbulences during the growth process. In this work, by carrying out a systematic study, we have focused on the influence of the oxygen/zinc partial pressure ratio (R VI/II ) and precursor flow rate on the morphological and struc- tural properties of ZnO nanorods. Zinc and oxygen flow rates were individually controlled and varied in order to analyze a wide range of R VI/II (80–510), i.e. attempting to modulate oxygen-rich conditions by changing either oxygen or zinc partial pressure. Growth rate effects have been also investigated by simulta- neously increasing the precursor flows at constant R VI/II ratio. The study has allowed to deepen the understanding of the growth mechanisms, and consequently, to get a better control of the properties of ZnO nanorods grown by this technique. 2. Experimental details Catalyst-free ZnO nanorods were synthesized on epiready c-sapphire substrates using an atmospheric MOCVD system Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.08.038 n Corresponding author. Tel.: þ34 9635 44908; fax: þ34 9635 43146. E-mail address: Diana.Montenegro@uv.es (D.N. Montenegro). Journal of Crystal Growth 359 (2012) 122–128