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InZnO nanorods by liquid indium seeded vapour phase deposition |
| Andrea Zappettini , Sathish Chander Dhanabalan , Marco Villani , Laura Lazzarini , Davide Calestani |
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IMEM- CNR (IMEM), Parma, Italy |
| Abstract |
| Metal oxide nanowires have received a continuously growing attention in the last years. Among them, in particular, zinc oxide nanowires (or nanorods) have been proposed for a large number of applications such as photovoltaics, UV emitters, piezo energy harvesting, gas sensing, and transparent electronics. Undoped zinc oxide shows typically n-type conductivity, generally ascribed to stoichiometric defects (e.g. oxygen vacancies or zinc interstitials) or due to unavoidable contamination of hydrogen that acts as interstitial donor. Most of the applications actually require a good control of electrical conductivity that can be achieved by doping or alloying ZnO. In particular, high n-type conductivity can be achieved by alloying zinc oxide with group III elements (such as Al, In or Ga) in ternary or even quaternary oxide compounds. Indeed, numerical calculations recently showed that 1-3% concentration of an element such as Al in a ZnO matrix, pushes Fermi level to penetrate into the conduction band, thus giving rise to a metallic behaviour [1]. Similar results were obtained in the case of doping with Ga or In. This is, for example, very important for the production of transparent conductors as in the case of TCOs in solar cells. Recently, a vapour phase technique to grow vertically self-aligned ZnO nanorods by a vapour-phase technique over a ZnO or Al:ZnO (AZO) film [2] has been reported. The main result of that growth technique is that, even if Zn vapours are used for the growth, the required temperature is low enough (450-480 °C) to allow the use of low-cost glass substrates, as the ones typically adopted for solar cell technology. By growing such nanostructures starting from a metallic Zn source, without the use of any catalyst or metal-organic precursor, contaminations from reaction environment (often intrinsic and noteworthy in wet chemical methods) has been drastically reduced allowing to grow ZnO nanorods with high crystalline quality and controlled physical properties. It is important to note that in the described technique a layer of liquid metal droplets is formed on the substrate during the first growth stage and that only afterwards, when oxygen is introduced, a corrugated oxide wetting layer is formed and nanostructures start to grow. This two-step process was successfully used in the past by our group also for growing SnO2, In2O3, and ZnO nanowires and mainly differs from standard VLS (vapour-liquid-solid) growth techniques because in this case the liquid phase is not used as an inert solvent through which the solid phase of different materials precipitate but to induce a localized preferential fast nucleation point that also provides high metal vapour supersaturation for a self-catalyzed vapour-solid growth. In general, the growth of ternary oxides by a similar vapour deposition technique, starting directly from the co-evaporation of two different metals, is much more difficult because vapour pressure of the two metals may differ by several orders of magnitude. This is the case, for example, of zinc and the mentioned group III elements, whose boiling point are generally over 2000°C and, hence, have insufficient vapour pressure around 500°C. For this reason, growth of InZnO nanowires was achieved up to now at temperatures well above 500°C (typically on the range 700-1400 °C), and thus not compatible with the use of low-cost glass substrates. In this work, we show that it is possible to grow indium zinc oxide nanorods by evaporating Zn and using liquid indium as a seed on the substrate at a temperature lower than 500°C. Microanalysis performed by TEM on single nanowires show an indium concentration larger than 1% (that is the concentration required to get metallic behaviour). The same technique has been also successfully used to grow at low temperature GaZnO and SnZnO nanowires. The possibility to use the same technique to grow nanowires of a wider family of ternary oxides is discussed.
[1] Mirco Bazzani, Andrea Neroni, Arrigo Calzolari, and Alessandra Catellani, , “Optoelectronic properties of Al:ZnO: Critical dosage for an optimal transparent conductive oxide”, Applied Physics Letters, 98 121907 (2011) [2] D. Calestani, M. Zha, L. Zanotti, M. Villani, A. Zappettini, “Low temperature thermal evaporation growth of aligned ZnO nanorods on ZnO film: a growth mechanism promoted by Zn nanoclusters on polar surfaces”, CrystEngComm, Vol. 13 (2011) pp. 1707-1712 |
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Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 8, by Andrea ZappettiniSee On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17 Submitted: 2013-04-15 17:49 Revised: 2013-07-29 23:11 |