J Mater Sci: Mater Electron (2006) 17:993–997 DOI 10.1007/s10854-006-9000-z
Effect of diameter on I–V characteristics of template synthesized Cu–Se nano/micro structures Meeru Chaudhri Æ A. Vohra Æ S. K. Chakarvarti
Received: 13 April 2006 / Accepted: 6 July 2006 / Published online: 31 August 2006 Springer Science+Business Media, LLC 2006
Abstract We have successfully fabricated arrays of Cu–Se nano/micro structures of varying diameters (40 nm, 100 nm, 1 lm, 2 lm) using a non-lithographic technique involving filling of pores of track-etch membranes (TEMs) with Cu and Se materials. The I–V curves of synthesized Cu–Se nano/micro structures of varying diameters were also recorded which show increase of negative differential resistance with decrease in diameters of these structures. A suitable explanation to this behaviour has also been presented.
1 Introduction One-dimensional nanostructures, such as nanowires offer a high degree of interest for furthering the current state of nanotechnology research and development. Higher aspect ratio, diameter dependent band-gap, and increased surface scattering for electrons are some of the more significant features in which nano-wires differ from their normal counterparts. A variety of methods for the fabrication of nanowires, nanorods and nanotubes have been developed including lithographic and nonlithographic techniques [1]. Among them Template synthesis (TS) M. Chaudhri (&) Æ A. Vohra Department of Electronic Science, Kurukshetra University, Kurukshetra 136119, India e-mail:
[email protected] S. K. Chakarvarti Department of Applied Physics, National Institute of Technology (Deemed University), Kurukshetra 136119, India
is a versatile, nonlithographic, flexible and simple approach to the fabrication of metallic nanowires and nanotubes [2]. Arrays of nanowires are obtained by filling a porous template (TEM) that contains a large number of straight cylindrical pores of requisite low dimensions. Depending on the properties of the material and the chemistry of the pore wall, these nanocylinders may be solid (nanofibrils) or hollow (nanotubules). Pores can also be filled with two different materials stacked in an alternating fashion to form bilayers or junctions. Fabrication of metal and semiconductor structures and metal-semiconductor binary materials have been reported by many workers [3–5] Characterization and explanations of electronic properties of nanowires are extremely important due to their potential applications in electronic devices. In the present work, an attempt has been made to fabricate Cu–Se structures of varying diameters, besides their corresponding I–V curves are also recorded. Cu–Se binary system has been studied as it is an important material system for CuInSe2 solar cells, for photovoltaic cells or for Shottky diodes[6]. In the present work this material system was taken for fabrication of high packing density low dimensional resonant tunneling diodes (RTD).
2 Experimental Electrodeposition of Cu–Se binary structures was carried out applying the technique of TS and the electrochemical cell described by Chakarvarti and Vetter was used [7]. The schematic diagram of the fabrication process of binary structures is shown in Fig. 1.
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Fig. 1 Schematic diagram of fabrication process of Cu–S enano/micro binary structures
Membranes used were polycarbonate track etch membranes (PC TEMs). TEMs are fabricated by exposing appropriate track forming insulating materials (in our case polycarbonate polymers) in a nuclear reactor or heavy ion accelerator and subsequently chemically etching with a suitable solvent to enlarge the radiation damage produced by heavy ions in order to produce the ‘through hole’. Polycarbonate (PC) TEMs having average pores diameters 40 nm, 100 nm, 1 lm, 2 lm and copper tape with conducting adhesive (3 M) as substrate were used. Conducting copper tape with track-etch membrane as an overlay was attached to the cathode of the electrochemical cell. CuSO4 Æ 5H20 (200 g/L)+H2SO4 (20 g/L) in milli Q 18 M water was used as an electrolyte. The electrodeposition was carried out for 10 min at 2 V (current 0.0190–0.0250 A) at room temperature (25 degrC) with anode as pure copper rod. After the electro-deposition was half-way through, the electrolyte was drained out and a second electrolyte having a composition of SeO2 (9 · 10–4 M) with 0.5 mL of dilute H2SO4 was introduced in the cell. A current of 0.020–0.025 A at a voltage of 3 V was allowed to pass for 12 min at a temperature of 60C. The same procedure was repeated for track etch membranes of all diameters. From our earlier experience, time of electrodeposition process was found reduced for TEMs having smaller pore diameters. Subsequently, the samples were removed from the cell, carefully washed, air dried for approximately half an hour to reveal the deposited material and the TEM foils with insitu Cu–Se nano/micro structures were used for finding I–V characteristics. However for SEM characteristics, membranes were dissolved in the solvent dichloromethane (CH2Cl2), leaving behind the structures. 3 Results and discussions High resolution SEM images of Cu–Se nanostructures (100 nm and 40 nm) are shown Figs. 2 and 3.
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Fig. 2 (a, b) SEM micrographs of Cu–Se nanostructures (100 nm)
D.C. transport measurements were carried out on in situ Cu–Se binary structures of varying diameters embedded in the PC TEMs. For this, an ohmic contact was made using a silver contact on the top of selenium side of the TEM template. The other side was placed on a metallic plate and the whole assembly was then connected to a Keithley Electrometer (model 617). The setup for I–V measurment is shown in Fig. 4.
J Mater Sci: Mater Electron (2006) 17:993–997
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The I–V curves of Cu–Se binary structures of diameters 2 lm, 1 lm, 100 nm; 40 nm are shown in Figs. 5 and 6. A prominent feature as observed from the I–V curves is the negative differential resistance (i.e. decrease in current with increase in the voltage) instead of schottky type behaviour. The I–V behaviour of Cu–Se binary structures can be explained on the basis of quantum size effects that dominate at small dimensions. In the bulk material, conduction electrons are restricted to the macroscopic dimensions of the solid, and the energy-level spacing becomes so small that the discreteness of the allowed energies is undetectable. As
Fig. 6 I–V curves of Cu–Se nanostructures
the size of the device is reduced and approaches nanometer range, the quantum size effects become pronounced. In such mesoscopic solids, however, the energy level spacing in the conduction band is large enough that the discreteness of the energy levels become apparent [8]. This discreteness of energy levels also affects the electronic properties of such solids and devices [8, 9]. From the I–V properties, it is evident that the fabricated structures behave as RTDs. The resonant tunneling effect in the fabricated structures can be
Fig. 4 Schematic diagram of I–V apparatus
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attributed to single barrier structures [10]. As the dimensions are reduced (in our case the diameter of structures) the energy levels in conduction band of Se become discrete due to quantum size effects [8]. With an increase in the potential across the device, the electrons now do not have continous energy states to conduct and instead remain confined to one energy level till the potential across the Cu–Se device becomes efficient to send them to the next energy level. This appears as a drop in current from peak to valley in I–V curves of Cu–Se (Figs. 5 and 6) structures and is responsible for resonance in single barrier based RTDs structures of Cu–Se. From Figs. 5 and 6 the negative differential resistence has been found to increase with decrease in diameter. From the I–V curve of the Cu–Se structures of 2 lm, it is observed that the current increases with the voltage which represents bulk effects. However, it is evident from the I–V curves of 1 lm diameter structures, there is decrease in the current as the applied voltage is increased i.e. the negative differential resistance appears due to quantum size effects at small dimensions. However PVR (peak to valley current ratio) is small due to less discreteness in levels. I–V curves of Cu–Se structures of 1 lm, 100 nm and 40 nm show increase in PVR with decrease in diameter, e.g., PVR is 1.02 when diameter is 1 lm, PVR is 2 when diameter is 100 nm and PVR is 2.5 when diameter is 40 nm. This can be attributed to the increase in spacing between discrete energy levels as the dimensions become smaller. From I–V curves of Cu–Se binary structures, it is also observed that the voltage at which resonance occurs increases with decrease in diameters. A plot between voltage at which resonance occurs (Vr) and the structures diameter is shown in Fig. 7. This can be attributed to shifting of quantized energy levels upwards in energy. Fig. 8 (a and b) show the I–V curves for different diameters for voltages between 0 V and 1 V. Increase in cut in voltage with decrease in
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Fig. 8 (a, b) I–V curves of Cu–Se binary structures at low voltages
diameter is observed. This can be attributed to an increase in bandgap with decrease in diameter of device. Such an increase in bandgap with decrease in diameters of structures for Bi nanowires fabricated by TS have been reported in [9]. Acknowledgments Thanks are due to Dr. Henri HENOT of Universite´ catholique de Louvain, Belgium for providing PC track etch membranes of 40 nm and GSI, Darmstadt, Germany for providing irradiation facilities at UNILAC for producing other TEMs.
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Fig. 7 Voltage Vr- structures diameter curve
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