Effect of thermal stress and diameter on I-V characteristics of template synthesized Cu-Se heterostructures S.K.Chakarvarti and Meeru Chaudhri 2 Department of Applied Physics, National Institute of Technology (Deemed University) Kurukshetra-36 9, India 2 Department of Electronic Science, Kurukshetra University, Kurukshetra-369, India ABSTRACT One-dimensional nanostructures, such as nano-wires and nano-belts, 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, which exhibit bulk properties. Many methods for the fabrication of nano-wires, nano-rods and nano-tubes have been developed including lithographic and nonlithographic techniques. Among them Template synthesis (non-lithographic technique) is a versatile, flexible and simple approach to the fabrication of metallic nano-wires and nano-tubes. We have successfully fabricated arrays of Cu-Se 5µm, µm, nm, 4nm hetero structures using a non-lithographic technique of filling cylindrical pores of track-etch membranes with Cu and Se materials. These were then analyzed by scanning electron microscope. I-V curves of deposited Cu-Se hetero structures of varying diameters were also recorded which show increase of negative differential resistance with decrease in diameters of these heterostructures. The effect of thermal annealing at different temperatures causing thermal stress on these synthesized structures was also studied. Keywords: Template Synthesis, Cu-Se hetero-structures, I-V Characteristics, Thermal Annealing, Size Effect.Introduction One-dimensional nanostructures, such as nanowires and nanobelts, 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, which exhibit bulk properties.an important phenomenon of resonant tunneling is being studied for many years in semiconductor hetero structures that causes negative differential resistance in I-V curves of these hetero structures. The devices fabricated by these semiconductor hetero structures are thus named as resonant tunneling diodes. InAs/AlSb/GaSb [,2], SiGe/Si [3], CaF 2 /Si/CaF 2 [4] resonant tunneling diodes have already been reported. In the recent years nanostructures are gaining popularity due to their unbelievable properties which these structures possess due to their small dimensions. Nanostructures can be in the form of films, wires, powders, dots. Physical routs and chemical routs like thermal evaporation [5], Sol-Gel synthesis [6], Template synthesis [7] have been employed to fabricate these structures. However, nanowires have attracted much attention due to their interesting magnetic, electrical and optical properties. Metals and semiconductor nanowires fabricated by Template synthesis technique via Electrodeposition [8,9,], via pressure injection techniques [], via electroless deposition are reported. These nanowires have been reported to possess giant magneto resistance properties [2], increased band gap with decrease in diameter [3], quantization of energy levels [4]. Many methods for the fabrication of nanowires, nanorods and nanotubes have been developed including lithographic and nonlithographic techniques. Among them Template synthesis (nonlithographic technique) is a versatile, flexible and simple approach to the fabrication of metallic nanowires and Device and Process Technologies for Microelectronics, MEMS, Photonics, and Nanotechnology IV edited by Hark Hoe Tan, Jung-Chih Chiao, Lorenzo Faraone, Chennupati Jagadish, Jim Williams, Alan R. Wilson Proc. of SPIE Vol. 68, 68H, (28) 277-786X/8/$8 doi:.7/2.759582 28 SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol. 68 68H-
nanotubes. Arrays of nanowires are obtained by filling a porous template that contains a large number of straight cylindrical pores. Depending on the properties of the material and the chemistry of the pore wall, this nanocylinder may be solid (a nanofibril) or hollow (a nanotubule). Holes can also be filled with two different materials stacked in an alternating fashion to form multilayers. Fabrication of metal & semiconductor structures and metal-semiconductor heterostructures has been reported by many workers. We have fabricated metal-semiconductors binary materials like Cu-Se, Zn-Se, Cd-Se applying the same technique but are reporting here on Cu-Se(5). The I-V curves were also taken and behaviour similar to double barrier resonant tunneling diodes was observed.. As properties of nano structures is a function of its size, so effect of size of diameter of template synthesized Cu-Se binary structures were also observed and found that peak to valley current ratio increases with decrease in diameters. Annealing effects cause improved electrical properties. 2.Experimental Electrodeposition of Cu-Se heterostructures was carried out applying the technique of Template Synthesis and the electrochemical cell described by Chakarvarti and Vetter was used (9). Polycarbonate track-etch membranes having average pores diameters (5µm, µm, nm, 4nm) and copper tape with conducting adhesive (3M) were used. Conducting copper tape with track-etch membrane as an overlay was attached to the cathode of the electrochemical cell. CuSo 4.H 2 (2g/l)+H 2 So 4 (2g/l) in milli Q 8M water was used as electrolyte. The electrodeposition was carried out for minutes at 2V (current.9a to.25a) at room temperature ( 25 c) with anode as pure copper sheet. After the electro-deposition was half-way through, the electrolyte was drained out and a second electrolyte having a composition of SeO 2 (9-4 M) with.5 ml of dilute H 2 SO 4 was introduced in the cell. A current of.2a to.25a at a voltage of 3V was allowed to pass for 2 minutes at a temperature of 6 C. The same procedure was repeated for track etch membranes of all diameters. Subsequently, the sample was removed from the cell, carefully washed, air dried for approximately half an hour to reveal the deposited material and the track etch membrane foil was removed in the solvent dichloromethane (CH 2 Cl 2 ). High resolution images of Cu-Se heterostructures (nm) are shown in the Fig \S' I '- Fig..Template synthesized Cu-Se resonant tunneling diodes Effect of annealing on the I-V characteristics of these samples was studied at 5 C, 8 C and 9 C. For this the hetero structures were heated up to 5 C by placing them on the hot plate assembly. After acquiring the desired temperature Proc. of SPIE Vol. 68 68H-2
and keeping for minutes at that temperature, heater was switched off and the sample was allowed to cool. I-V curves of the sample was then measured using Keitheley Electrometer. Same procedure was repeated for the samples at 8 C and 9 C. 3.Results and discussion The I-V data of Cu-Se nanostructures examined at different annealing temperatures are shown in the Fig.2. Fig.3 shows the current-voltage curves at lower voltages. The peak current is due to resonant tunneling through eigen states and valley current is dominated by nonresonant tunneling and thermal currents.from these curves it is concluded that ) With increase in temperature of annealing the voltage at which resonance occurs shifts towards left. 2) With increase in temperatures of annealing peak to valley current ratio increases. 3) There is steep fall in current from peak to valley current at high annealing temperatures. 4) With increase in annealing temperature cut in voltage decreases. The graph between voltage at which resonance occurs and the temperature is shown in the Fig.3. The graph between temperature and peak to valley current ratio (PVR) is shown in the Fig.4. 4 3.5 3 Current(uA) 2.5 2.5 Current at 9 C Current at 8 C Current at 5 C Current at room temperature.5.5.5 2 2.5 Voltage(volts) Fig.2 Current- Voltage plots of Cu-Se binary structures at different annealing Proc. of SPIE Vol. 68 68H-3
Current(uA).9.8.7.6.5.4.3.2..5..5.2.25.3.35.4.45 Voltage(volts) Current at 9 C Current at 8 C Current at 5 C Current at 27C Fig.3Current-voltage curves of template synthesized Cu-Se nano binary temperatures. structures at lower voltages at different Proc. of SPIE Vol. 68 68H-4
6 5 4 PVR 3 2 2 4 6 8 Temperature( o C) Fig.4 Peak to valley current ratio and Temperature curve Vr(volts).8.6.4.2.8.6.4.2 2 4 6 8 Temperature( o C) Fig.5. Variation of resonance potential with annealing temperature Proc. of SPIE Vol. 68 68H-5
Transport measurements(i-v characteristics) were carried out on Cu-Se heterostructures of varying diameters still in the polycarbonate membrane. For this, an ohmic contact was made using a silver contact on the top of selenium side of track etch membrane. The other side was placed on a metallic plate and the whole assembly was then connected to Keithley Electrometer. The I-V curves of Cu-Se heterostructures of diameters 5µm, µm, nm; 4nm are shown in the fig.6,7. I-V curve (Cu-Se microstructures) 4 3 2 Current(mA) -.5 - -.5.5.5 2 2.5 3 Current(5um) Current(um) - -2-3 -4 Voltage (volts) Fig.6. I-V curves of Cu-Se microstructures Proc. of SPIE Vol. 68 68H-6
I-V curve (Cu-Se nanostructures) 8 6 Current(microamperes) 4 2-3 -2-2 3 4 Current(4nm) Current(nm) -2-4 Voltage (volts) Fig.6. I-V curves of Cu-Se nanostructures Increase of negative differential resistance has been found with decrease in diameter. This is from the prediction of quantum mechanics that electrons confined to a particular region of space may exit only in a certain set of allowed energy levels. In bulk materials energy level spacing becomes so small that the discreetness of the allowed energies is undetectable. The spacing between these levels increases as the size of confining space decreases. From the I-V curve of Cu-Se heterostructures of 5µm, it is observed that current increases with voltage (bulk effect). From the I-V curve of µm decrease in current with increase in voltage (negative differential resistance) has been found (quantum effect). Negative differential resistance is due to discreet energy levels. However peak to valley current ratio is small due to small discreetness in levels. I-V curves of Cu-Se heterostructures of nm and 4nm show increase in peak to valley ratio with diameters due to decrease in confining space as spacing between discreet energy levels increases with decrease in confining space. From I-V curves of Cu-Se heterostructures of different diameters it has also been observed that voltage at which resonance occurs increases with decrease in diameters. This is due to increase in band gap that increases as diameter of structures decreases. ACKNOWLEDGEMENTS SKC gratefully acknowledges the financial support extended under TEQIP programme and the consistent encouragement from Dr.M.N.Bandyopadhyay, the Ditector, National Institute of Technology(Deemed University),Kurukshetra, India Proc. of SPIE Vol. 68 68H-7
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