Fabrication and Characterization of Nanowire Devices

Hardev Singh VirkProfessor Emeritus, Eternal

University, Baru Sahib (HP), India

Birth of Nanotechnology“There's Plenty of Room at the Bottom” • On December 29, 1959, Richard P. Feynman

gave the seminal talk at a meeting at Caltech of the American Physical Society. He presented a vision of the precise manipulation of atoms and molecules so as to achieve amazing advances in information technology, mechanical devices, medical devices, and other areas.

Changing Idea into Reality

Eric Drexler of MIT, the Chemist, established the modern field of nanotechnology, with a draft of his seminal Ph.D. thesis in the mid 1980s. His 1991 doctoral thesis at MIT was revised and published as the book "Nanosystems, Molecular Machinery Manufacturing and Computation" (1992), which received the Association of American Publishers award for Best Computer Science Book of 1992.

The Incredible Tininess of Nano

Billions of nanometersA two meter tall male istwo billion nanometers.

The pinhead sized dot is a million nm

Biological cells size isThousands of nm

DNA Molecules are about 2.5 nm in width

Hydrogen atom spans 0.1 nm2 Uranium atoms span 1 nm

• AFM Imaging of ATOMS of GOLD (Au 111)

Atomic Lattice Structure of HOPG in 3D Topography using Atomic Force Microscope

Introduction to Nanowires

Nanowires of metallic and semi-conducting materials have drawn a lot of research interest because of their potential applications in fields like nanoelectronics, opto-electronics and sensors. Nanowires exhibit unique electrical, magnetic, optical, thermoelectric and chemical properties compared to their bulk counterpart. Electronic conduction takes place both by bulk conduction and through tunneling mechanism.

Special Characteristics

• Nanowires exhibit high density of electronic state.

• They have diameter-dependant band gap.• They show enhanced surface scattering of

electrons and phonons.• They have increased excitation energy, high

surface to volume ratio and large aspect ratio.

Fundamental Features• One-dimensionality• – Wire diameter: several nm ~ hundreds of nm’s• – Aspect ratio: L:D >10:1• Material integrity• – Single crystalline nanostructure• Availability of numerous materials• – Superconductor, metal, semiconductor, insulator • Unique physical properties• – Large surface-to-volume ratio• – High transport/carrier mobility• – Quantum confinement/tunable band structure

Quantum Confinement Effects

– Quantum dots (0-D): confined states, and no freely moving ones

– Nanowires (1-D): particles travel only along the wire direction

– Quantum wells (2-D): confines particles within a thin layer

There is no confinement effect in Bulk materials.Refer to energy distribution.

Chemical Routes of Synthesis• Solution-Based Synthesis• – Solution-Liquid-Solid (SLS) Method• – Solvothermal Chemical Synthesis• – Template-Based Synthesis• Gas-Phase Synthesis• – Vapor-Liquid-Solid (VLS) Method• -- Laser abrasion• -- PVD• -- CVD (LPCVD, MOCVD)• – Vapor-Solid (VS) Method• – Oxide-Assisted Growth (OAG)

Growth of Semiconductor Nanowiresby VLS method

Laser ablation overcomes thermodynamic equilibrium constraints and enables liquid nanocluster formation.

GaN Nanowires Grown by VLS method

SEM image of GaN nanowires of diameters 10nm and lengths on the order of 10m (Huang et al., 2002).

ZnO Nanowires on Sapphire by VLS method

SEM images of ZnO nanowire arrays grown on a sapphire substrate, (a) shows patterned growth, (b) shows a higher resolution image of the parallel alignment of the nanowires, and (c) shows the hexagonal cross-section of the nanowires (Huang et al., 2001).

Nano-Lasers using ZnO Nanowires

ZnO nanowires grown by VLS method. Emission spectrum from ZnO nanowires.

Nanowire FabricationTemplate synthesis using polymer and

anodic alumina membranesElectrochemical deposition

Ensures fabrication of electrically continuous wires since only takes place on conductive surfaces

Applicable to a wide range of materialsHigh pressure injection

Limited to elements and heterogeneously-melting compounds with low melting points

Does not ensure continuous wires Does not work well for diameters < 30-40 nm

Chemical Vapor Deposition (CVD) or VLS technique Laser assisted techniques

Polymer Template Synthesis of Nanowires

Large Etched Ion TracksLarge Etched Ion Tracks

Anodization of aluminum Start with uniform layer of ~1m Al Al serves as the anode, Pt may serve as the

cathode, and 0.3M oxalic acid is the electrolytic solution

Low temperature process (2-50C) 40V is applied Anodization time is a function of sample size and

distance between anode and cathode Key Attributes of the process (per M. Sander)

Pore ordering increases with template thickness – pores are more ordered on bottom of template

Process always results in nearly uniform diameter pore, but not always ordered pore arrangement

Aspect ratios are reduced when process is performed when in contact with substrate

Anodic Alumina Template Preparation

Electrochemical mechanism• The overall reaction that takes place during

anodization is: 2Al + 3H2O => Al2O3 + 3H2

At the anode: 2Al + 3O2- => Al2O3 + 6e-

At the cathode:6H+ + 6e- ==> 3H2

• The Al is oxidized at the metal/oxide interface

• The oxide is etched away by the acid with the applied potential

• The pores are induced by the roughness of the top surface

TEM micrographs

(T. Sands/ HEMI group http://www.mse.berkeley.edu/groups/Sands/HEMI/nanoTE.html)

Anodic alumina (Al2O3) Template

100nmSi substrate

alumina template

(M. Sander)

Electrolytic CellElectrolytic Cell

Replica of Nanowires

Microtubule Fabrication

Electrochemical Synthesis

• Electrochemistry has been used to fabricate nanowires of Cu and heterojunctions of Cu-Se and Cd-S. The results of our investigations can be exploited for fabrication of nanodevices for application in opto-electronics and nano- electronics. During failure of our Experiments, exotic patterns ( nanoflowers, nanocrystals, nanobuds) were produced under nature’s self assembly.

Template Synthesis of Copper Template Synthesis of Copper NanowiresNanowires

The electro-deposition of metals is identical to The electro-deposition of metals is identical to an electroplating process. Polymer ITFs and anodic alumina and anodic alumina can be used as a template. The electrolyte used here is can be used as a template. The electrolyte used here is CuSO4.5H2O acidic solution. The rate of deposition of CuSO4.5H2O acidic solution. The rate of deposition of metallic film depends upon: current density, inter-metallic film depends upon: current density, inter-electrode distance, cell voltage, electrolyte electrode distance, cell voltage, electrolyte concentration, pH value and temperature etc. In our concentration, pH value and temperature etc. In our case, electrode distance was kept 0.5 cm and a current case, electrode distance was kept 0.5 cm and a current of 2mA was applied for 1 hour. The developed of 2mA was applied for 1 hour. The developed nanostructures were scanned under SEM for nanostructures were scanned under SEM for morphological and structural studies.morphological and structural studies.

Atomic Force Microscope(NT-MDT)

AFM image of hexagonal pores of AFM image of hexagonal pores of Anodic Alumina Membrane (AAM)Anodic Alumina Membrane (AAM)

SEM Images of Cu Nanowires using SEM Images of Cu Nanowires using Electrodeposition TechniqueElectrodeposition Technique

Copper Nanowire Bundles in AAMCopper Nanowire Bundles in AAM

Cu Nanowires under Constant CurrentCu Nanowires under Constant Current

Capping Effect of Current VariationCapping Effect of Current Variation

I-V Characteristics of Copper I-V Characteristics of Copper Nanowires grown in-situ in AAMNanowires grown in-situ in AAM

Copper Lillies grown due to over- Copper Lillies grown due to over- deposition of Copper in AAM deposition of Copper in AAM

A Garden of Copper NanoflowersA Garden of Copper NanoflowersA Garden of Copper NanoflowersA Garden of Copper Nanoflowers

Copper Nanoflowers grown in Polymer Template (100nm pores)

Copper Marigold Flower

SiC Crystalline Nanowire Flowers G. W. Ho (Nanotechnology, 2004)

Crystalline Nano-comb of ZnO NW H. Yan (JACS 2003)

SEM micrograph of Copper Buds

SEM micrograph of Nanocrystals of SEM micrograph of Nanocrystals of Polycrystalline CopperPolycrystalline Copper

XRD Spectrum of polycrystalline XRD Spectrum of polycrystalline Copper nanocrystalsCopper nanocrystals

Position [°2Theta] (Copper (Cu))

10 20 30 40 50 60 70

Counts

0

20000

40000

60000

36.6

37 [°

]38

.283

[°]

43.4

61 [°

]45

.448

[°]

48.9

20 [°

]50

.580

[°]

54.3

04 [°

]54

.956

[°]

64.8

09 [°

]

74.2

99 [°

]

KK1

XRD spectrum of Cu nanowiresXRD spectrum of Cu nanowires

Position [°2Theta] (Copper (Cu))

30 40 50 60 70 80 90

Counts

0

400

1600

Cu polycrystalline

SEM Image of CdS NanowiresSEM Image of CdS Nanowires

HRTEM image showing CdS Nanowire HRTEM image showing CdS Nanowire & Heterojunctions & Heterojunctions

I-V plot of CdS Nanowire arrays I-V plot of CdS Nanowire arrays showing RTD characteristics showing RTD characteristics

SEM image of Cu-Se NanowiresSEM image of Cu-Se Nanowires

Cu-Se nanowires exhibit p-n junction Cu-Se nanowires exhibit p-n junction diode characteristicsdiode characteristics

A Billion Dollar Question …

• What can nanowires offer for semiconductor nanoelectronics?

• Nonlithographic & extremely cost-effective• Reduced phonon scattering: High carrier

mobility but reduced thermal conductance(?) • Tunable electrical/optical properties• Large surface-to-volume ratio: Sensor

sensitivity & memory programming efficiency

Advantages of 1-D Nanowires• High-quality single-crystal wires with nearly

perfect surface• Scalable nanostructure with precisely

controlled critical dimensions• Best cross-section for surround-gate CMOS• Very cost-effective materials synthesis• High transport low-dimensionality structure• May use as both device and interconnect for

ultra-compact logic (e.g., SRAM)

Nanowire Field-Effect Transistor

• Ambipolar transport• Carrier mobility study• Quantum effect

A single device for numerous applications

Device physics study

Quantum-Wire Device M. Bjork (Nano Letters, Sept. 2004)

1. In-situ control of nanowire synthesis allows design of strongly confined quantum mechanical systems inside nanowires, possibly useful for SET .2. Next-generation nanoelectronic devices with extremely-low power, high performance, and radiation tolerance.

3-D Nanowire Logic Chip H. Ng (Nano Letters, July 2004)

Si NW Thermal Conductance D. Li (APL Oct. 2003)

Thermoelectric (TE) Conversion E.J. Menke (Nano Letters, Oct. 2004)

Bismuth Telluride (Bi2Te3) nanowires

Role of Nanowires for Next-Generation Electronics

• The chemical and physical characteristics of nanowires, including composition, size,

electronic and optical properties, can be rationally controlled during synthesis in a predictable manner, thus making these materials attractive building blocks for assembling electronic and optoelectronics nanosystems.

Some Observations & Remarks• Nanotechnology will be the driving force for

next technology revolution.• Nanowires open door to a wonderland where

the next generation electronics would emerge.• Scope for innovating new synthesis method

and complex functional nanostructures.• New device and interconnect concepts will

emerge from horizon, driven by materials synthesis.

Our Publications

• My website: http:// drhsvirk.weebly.com for list of our published research papers. Go to www.docstoc.com for purchase of

reprints. Free download of Review Paper on Nanowires: visit: ttp://www.intechopen.com/articles/show• Chapter 20 of Book “Nanowires - Implementations and

Applications”, InTech Open, Abbass Hashim (Ed).

AcknowledgementsAcknowledgements

• Reimer Spohr & Christina Trautman (GSI, Darmstadt)Reimer Spohr & Christina Trautman (GSI, Darmstadt)• Sanjit Amrita Kaur (GND University, Amritsar)Sanjit Amrita Kaur (GND University, Amritsar)• Vishal, Gurmit, Sehdev & KK (DAVIET, Jalandhar)Vishal, Gurmit, Sehdev & KK (DAVIET, Jalandhar)• Dr SK Mehta, Chemistry Deptt. (PU, Chandigarh) Dr SK Mehta, Chemistry Deptt. (PU, Chandigarh) • CSIO Chandigarh & IIT Roorkee for FESEM & TEM facility.CSIO Chandigarh & IIT Roorkee for FESEM & TEM facility.• SEM & TEM facility (SAIF, PU, Chandigarh)SEM & TEM facility (SAIF, PU, Chandigarh)• Rajeev Patnaik (Geology Deptt., PU, Chandigarh)Rajeev Patnaik (Geology Deptt., PU, Chandigarh)• DAV MC, New Delhi for Research Grants.DAV MC, New Delhi for Research Grants.• Dr. MS Atwal, VC, Eternal University, Baru Sahib.Dr. MS Atwal, VC, Eternal University, Baru Sahib.

Thank You !!!