Field Effect Transistor with Electrodeposited ZnO Nanowire Channel
Graphical abstract
A template approach was employed for fabricating ZnO nanowires. These were further used as channels in field effect transistors with excellent properties.
Introduction
Fabrication of nanostructures with controlled morphology is an attractive field of research due mainly to the high application potential. Practically, precise shape control in the case of nano-objects enables one to further tailor materials functionality and to design devices which make use of consequent specific properties. An approach which enables one to obtain nanostructures with precise shape and dimensions is the so called template method, where the synthesis represents a replication of a chosen object [1], [2]. A wide range of templates were employed for this purpose, from nanoporous membranes such as anodic alumina [3], [4], polymer ion track membranes [5], [6] or titania nanotubes [7] to DNA [8] or proteins [9]. The replication methods are also diverse and a few examples are electrochemical [10], [11] or electroless deposition [12], chemical synthesis [13], atomic layer deposition [14].
Zinc oxide is one of the most intense studied material these days and the sought applications range from energy production [15] andphotocathalysis [16] to light emitting devices [17], sensors [18] or logic circuits [19]. A wide band gap (3.3 eV) semiconductor, with an excitonic binding energy of 60 meV, zinc oxide is attractive besides itsintrinsic physical properties to its high occurrence/low cost and non-toxicity. The material is also remarkable from the point of view of polymorphism of its nanostructures, from belts and rods to platelets and flowers, tunable as a function of preparationmethod and specific conditions. Different electronic devices were fabricated using zinc oxide, among them of utmost importance being field effect transistors [20], [21].
The group of methods which, at this moment, lead to the fabrication of zinc oxide nanowires with properties considered most appropriate for applications in nanoelectronicsare based onvapour/liquid/solid routes of preparation [22], [23]. However, these methods are rather costly, cheaper alternatives being sought.
Electrochemical deposition of semiconductors is a method which was proven as a relatively low budget, easily scalable techniquefor preparing semiconducting materials [24], [25]. When employed in conjunction with a template approach, electrochemical deposition allows the fabrication of semiconductor materials with tailored morphology, suited for certain functionality.
Zinc oxide electrodepositionwas extensively studied during the last two decades being a cheap and scalable alternative to more expensive preparation methods. By carefully choosing the deposition conditions one can tailor the properties of the desired material, including here its morphology, structure and its electrical and optical properties [26], [27], [28]. From this point of view electrochemical deposition it is one of the most flexible preparation methods.
In the present paper we describe our results regarding the preparation of field effect transistors using nanowires prepared by electrodeposition in ion track templates. Up to this moment such devices were fabricated exclusively using nanowires prepared by VLS approaches, more demanding in terms of infrastructure than electrodeposition. Besides the nanowire growth process, in the transistors fabrication algorithm, electron lithography, and subsequent annealing and surface passivation were employed.Our results show an excellent behaviour of the nanowire based devices, including here typical field effect transistor behaviour, with saturated source–drain characteristics and Ion to Ioff ratios of about 104. The transport properties show carrier mobility in excess of 100 cm2/Vs.
Weproved in this way that electrodeposited nanowires are an interesting alternative to those prepared by other approaches. Moreover, the template method allows a better control of the morphology of the nanostructure, enabling the fabrication of devices with precisely controlled characteristics.
Section snippets
Experimental
Polymer foils (30 micrometer thick polycarbonate) were irradiated at GSI's UNILAC with swift heavy ions with a specific energy of 11.4 MeV/nucleon. Each ion passing through the foil leaves a track which, in appropriate conditions, can be selectively removed, leading to the formation of nanopores with the shape and dimensions determined by the etching conditions [6].
Track etching was performed using 5 M NaOH and 10% vol. methanol solutions at 50 °C for 300 s, the etch rate for this experimental
Results and discussions
Zinc oxide electrodeposition is extensively studied since almost two decades. Several deposition routes were discovered, the one we chose for the present work being based on a nitrate bath [29]. In this case the possible electrode reactions are related to:
- (a)
water reductionO2 + 2H2O + 4e− → 4OH−H2O + 2e− → H2 + 2 OH−
- (b)
nitrate ions reduction:2e− +NO3− + H2O→NO2− + 2OH−
- (c)
(c) or zinc ions reduction:
The global reaction coresponding to zinc oxide formation is:Zn2+ + 2OH−→Zn(OH)2→ZnO↓ + H2O
In Fig. 1 the
Conclusions
Using a template approach ZnO nanowires were prepared by electrochemical deposition from a simple nitrate bath. A multi-step process starting with hydroxide formation by nitrate reduction at the electrode, zinc hydroxide precipitation and finally its decomposition and consequent formation of zinc oxide leads to nanowire growth. It was found that excellent quality nanowires with high aspect ratio are obtained for working electrode potential in the range -800 mV - -1200 mV. Structural
Acknowledgement
The authors acknowledge the financial contribution of UEFISCDI, contracts EUROC 5 and Idei 24/2013.
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