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Determination of carrier density of ZnO nanowires by electrochemical techniques

Iván Mora-Seró, Francisco Fabregat-Santiago, Benjamin Denier, Juan Bisquert, Ramón Tena-Zaera et al. Citation: Appl. Phys. Lett. 89, 203117 (2006); doi: 10.1063/1.2390667

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Determination of carrier density of ZnO nanowires by electrochemical techniques

Iván Mora-Seró,a ͒Francisco Fabregat-Santiago,Benjamin Denier,and Juan Bisquert

Departament de Ciències Experimentals,Universitat Jaume I,12071Castelló,Spain

Ramón Tena-Zaera,Jamil Elias,and Claude Lévy-Clément

LCMTR,Institut des Sciences Chimiques Seine Amont,CNRS,2/8rue Henri Dunant,94320Thiais,France

͑Received 4September 2006;accepted 4October 2006;published online 16November 2006͒The carrier density of ZnO nanowires has been determined by means of electrochemical impedance spectroscopy.A model taking into account the geometry of ZnO nanowires has been developed and the differences with the standard flat model,as curved Mott-Schottky plots,are discussed.The as-grown electrodeposited samples present a high donor density of 6.2ϫ1019cm −3,dramatically reduced by two orders of magnitude after an annealing in air at 450°C during 1h.The results show that the surface of the ZnO nanowires is active;therefore this system appears as a useful structure to support a functionalized nanostructured devices.©2006American Institute of Physics .͓DOI:10.1063/1.2390667͔

Single-crystal ZnO nanowire arrays have attracted a lot of research interest in recent years because they may be used as building blocks for a new generation of devices in differ-ent technological domains such as optoelectronics,1,2solar cells,3,4gas sensing,5field emission,6,7and piezoelectrics.8Until now,most of the work has been focused on deposition methods,mainly on those from vapor phase techniques.9Nevertheless,a solution technique such as electrodeposition not only appears as a low temperature alternative,but it is also well suited for cheap large-scale production.10

There have been some reports on the electrical charac-teristics of single nanowires,11–13but the electrical properties of nanowires arrays,which may be crucial for attaining a good device performance,have not been explored in detail.The particular morphology of nanowire arrays makes their electrical characterization difficult by using standard solid-state techniques.In contrast,by establishing a semiconductor/liquid junction it is possible to investigate the electrical properties of nanowire arrays using standard elec-trochemical methods such as impedance spectroscopy.Here we show that this technique gives information on the elec-tronic properties of the entire nanowire surface,that is essen-tial in nanostructured devices.

ZnO nanowires arrays were electrodeposited on com-mercial transparent conducting SnO 2:F/glass substrates ͑10⍀/sq ͒,that were previously coated with a thin continu-ous sprayed ZnO buffer layer.The electrolyte was an aque-ous solution of ZnCl 2͑5ϫ10−5M ͒and KCl ͑0.1M ͒,satu-rated with bubbling O 2.More details are given elsewhere.14The morphology of a representative sample is shown in Fig.1.The nanowire density is 2.8ϫ109cm −2and the mean size of each nanowire is 93nm in diameter and 830nm in length ͑Fig.1͒.Electrochemical impedance spectroscopy ͑EIS ͒measurements were performed in a three-electrode cell using a Pt wire as counter electrode and a standard Ag/AgCl in 3M KCl as reference electrode.A carbonate propylene elec-trolyte ͑0.1M LiClO 4͒was used to avoid ZnO decomposi-tion.EIS data were obtained using a FRA equipped

PGSTAT-30from Autolab.Each measurement was done ap-plying a 20mV ac sinusoidal signal over the constant ap-plied bias with the frequency ranging between 500kHz and 5mHz.After the electrochemical measurement,the as-grown samples were annealed in air at 450°C for 1h at atmospheric pressure,followed by cooling in ambient air.

Mott-Schottky ͑MS ͒analysis is a standard technique,commonly used to determine both dopant density and flat-band potential at semiconductor/liquid contacts.15In flat electrodes the capacitance per unit area of surface is.16,17

1

C s

2=

2

q ␧N D

͑V sc +V 0͒,͑1͒

where V sc is the potential difference across the semiconduc-tor space-charge region,and V 0takes into account the con-tribution of the Helmholtz layer and the flatband potential.In the nanowire semiconductor structure,a circular depletion layer will grow from the surface towards the center of the wire at changing bias potential,and the geometry may intro-duce significant changes with respect to Eq.͑1͒that we con-sider in the following.Each nanowire is described as a cyl-inder of radius R with axial symmetry and donor density N D .The Poisson equation

1r ץץr ͩr ץV ץr ͪ

=−q

␧

N D

͑2͒

is solved in the depletion approximation for the voltage V .In Eq.͑2͒,␧=␧r ␧0,␧r =10is the dielectric constant of the ZnO

a ͒

Author to whom correspondence should be addressed;electronic mail:

sero@exp.uji.es FIG.1.SEM micrographs of electrodeposited ZnO nanowire arrays.͑a ͒Plan view and ͑b ͒cross section.

APPLIED PHYSICS LETTERS 89,203117͑2006͒

0003-6951/2006/89͑20͒/203117/3/$23.00©2006American Institute of Physics

89,203117-1