Non-lithographic Fabrication of Ni-se Heterojunction Nanowires and their Electrical Characterization

In this paper, heterojunction nanowires of Ni-Se are fabricated via template-assisted electrodeposition process. Nanowires are characterized by scanning electron microscopy (SEM), Energy dispersive X-Ray Spectroscopy (EDS), X-ray diffractometry and electrical transport studies. SEM photographs reveal the uniform and dense growth of Ni-Se nanowires. X-ray diffraction pattern shows the crystalline nature of Ni-Se nanowires. EDS spectrum shows the much higher percentage of Ni as compared to Se. A collective current-voltage characteristic of heterojunction nanowires shows the resonant tunneling diodes (RTDs) like behavior with peak to valley current ratio 1.37 at room temperature.


INTRODUCTION
The recent development in the technology of heterojunction has brought tremendous applications in our day to day life. The application of heterojunction based electronics devices include double heterostructure laser (DH) in telecommunication system, light emitting diodes (LEDs), high electron mobility transistor, satellite TV system, and resonant tunneling diodes (RTDs) [1-3 and references therein]. The double heterostructure laser (DH) found in virtually every home as part of compact disc (CD) player. In microelectronics industry, current manufacturing processes make use of lithographic techniques to imprint circuits on semiconductor materials, which is approaching to its physical limits. To continue the size reduction of semiconductors, new non-lithographic techniques that can manipulate objects at nano-or atomic scale should be developed. Materials at nanoscale find step toward miniaturization with a qualitatively new scale. Nanowires have been investigated for their intrinsic ability to make smaller devices by scientists and engineers. A variety of techniques have been used such as thermal evaporation, chemical vapor deposition (CVD), chemical self assembly, sol-gel process, and template synthesis for the fabrication of nanowires [5 and references therein]. Template synthesis is an elegant approach for the fabrication of 1D structure such as nanowires and nanotubes [4][5][6][7][8]. This technique makes use of porous membrane such as track-etch membrane, anodic alumina oxide (AAO), and mica etc. The desire material can be deposited into the pores of membrane via sol-gel, chemical vapor deposition (CVD), electrochemical deposition, chemical deposition, and electroless deposition. The size and shape of the Nanowires is true replica of pores geometry. In this paper we describe here the fabrication and characterization of metal-semiconductor heterojunction nanowires of Nickel-Selenium (Ni-Se) via template-assisted electrodeposition technique.

EXPERIMENTAL DETAIL
All chemicals purchased from Merck are of analytical grade with high purity. Anodic alumina oxide (AAO) templates of diameter 100nm, thickness 60 micron and pore density 10 9 pore/cm 2 are purchased from what man, USA. Ag/AgCl is used as Reference electrode and a Gamry Potentiostat Ref 600 is used as dc source for electrode position at room temperature (28 degree Celsius). Platinum (Pt) electrode is used as counter electrode (anode).
A thin film of gold is sputtered onto one side of AAO template using sputterer coater. This gold layer along with conductive adhesive copper tape acts as working electrode and provides a stable substrate (cathode) for the growth of Nanowires. The electrodeposition is carried out using bath conditions NiSO 4 .6H 2 O (250 g/l), NiCl 2. 6H 2 O (40 g/l) H 3 BO 3 (40 g/l) for deposition of Nickel and Selenium dioxide {SeO 2 (8×10 -4 M)} with 0.5 ml of 35% dilute H 2 SO 4 for deposition of Selenium, the inter-electrode distance was kept 0.5 cm. The amount of metal deposited and time required to fill the pores is calculated using Faraday's Laws. The deposition of Ni and Se is done for the calculated time (i.e. 1300 sec for Ni and 200 sec for Se) in order to get heterojunction nanowires of Ni & Se. The quality of electrode position depends upon several parameters such as inter-electrode distance, pH value, agitation, current density, temperature etc. After the deposition was over, the membrane was washed with distilled water for several times and the sample was dried in room temperature.

SEM Characterization
To study the morphology of nanowires, the nanowires are freed from template by dissolving it with 1M NaOH solution followed by rinsing with distilled water. The SEM photograph as shown in Fig. 1 reveals the good quality deposition with large scale and uniform formation of heterojunction nanowires. The elemental composition was confirmed by Energy dispersive X-ray spectroscopy (EDS) as shown in Fig. 2

Electrical Characterization
Electrical characterization of Ni-Se heterojunction Nanowires embedded in AAO template is done at room temperature by using "Keithley Digital Source Meter" 2400. The collective Current-Voltage (I-V) measurement reveals the resonant tunneling diode like behavior at low sweep voltage 0 V to 1 V as shown in Fig. 4. I-V characteristics show RTD behavior with peak to valley current ratio (PVCR) 1.37. Lower value of PVCR may be due to nonhomogeneous hetero-interface. Similar results have been reported in literature for Cu-Se RTDs by Biswas et al and Chaudhri et al. [9][10].

CONCLUSION
Metal-semiconductor heterojunction nanowires of Ni-Se fabricated via Template synthesis shows the resonant tunneling diode (RTD) like behavior with peak to valley ratio 1.37 at room temperature. This method is suitable to synthesize mono dispersive nanowires of desired diameter and length. Peak current is observed at low voltage which is an important feature and offers the possibility of fabricating low-power dissipation electronic devices.