Nanowire Presentation

Alexandra Ford4/9/08

NSE 203/EE 235

Paper I Will Be Presenting Today:

Assembly of Vertical Nanowire (NW) Arrays

• Assembly of vertical NW arrays is key to taking full advantage of nanowire sublithographic dimensions for building high-density NW devices

• Large-area vertically aligned arrays of NWs on arbitrary substrates makes fabrication of transistors, light and field emission displays, and photovoltaics possible

• Vertically aligned NW arrays also provide direct charge-transport pathways for connecting top/bottom electrodes

How to Make Vertical NW Arrays

• Traditionally, vertical single-crystalline NW arrays have been fabricated by epitaxial growth on lattice-matched crystalline substrates

• This technique is expensive and limits the type of substrate and nanowire materials that can be used

• Prevents the use of amorphous substrates

A New Way to Make Vertical NW Arrays

• It is therefore desirable to find a way to make vertical single-crystalline NW arrays on arbitrary substrates

• This paper demonstrates a way to achieve this through use of an annealed plasma-sputtered Au/Pd thin film as a catalyst for vapor-liquid-solid (VLS) NW growth

• This paper compares the traditional method of using colloidal Au catalysts to a new method of using an annealed plasma-sputtered Au/Pd thin film catalyst on various substrates for VLS growth

• The paper specifically demonstrates how the two different catalyst deposition methods affect the ability to grow vertically-aligned NW arrays

Experimental

• II-VI nanowires grown (ZnS and CdS) by the VLS process• Two different catalysts are compared

– Colloidal Au (80 nm) catalysts– Plasma-sputtered Au/Pd thin film (10-15 nm thick) catalyst

• Annealed at 890 C for 10 min to produce 30-100 nm nanoclusters• Also deposited using a stainless steel hard mask with circular features

of 80 mm

• Three different substrates– Thermally-grown 200 nm SiO2/Si

– 100 nm Si3N4/Si– 1 mm ITO/quartz

Results – ZnS NWs on SiO2/SiZnS NWs catalyzed by annealed sputtered Au/Pd thin film:• Predominantly vertically aligned wrt substrate; 75% of NWs within a range of ± 10o to surface normal• Stand upright w/o falling over• NWs are 10-15 mm long, 50-80 nm in diameter

ZnS NWs catalyzed by colloidal Au• Are not vertically aligned wrt substrate; <10% of NWs within a range of ±10o to surface normal• Do not stand upright

ZnS NWs catalyzed by sputtered Au/Pd thin film deposited into circular 80 mm features:• ZnS NWs only grow in circular 80 mm features• Maintain vertical alignment even at the edges of the features• NWs are >15 mm long

40 mm

500 nm

Results – ZnS NWs on SiO2/Si

• Shorter wires grow completely vertically:

• All ZnS NWs were determined to be single-crystalline fcc sphalerite along [111] growth direction as determined by XRD and HRTEM

• Particles at tips of wires were composed of Au/Pd, confirming Au/Pd-catalyzed VLS growth

Completely vertical 160 nm long ZnS NW catalyzed by sputtered Au/Pd thin film

Results – Investigation of Catalyst Particles

BLUE = Au, GREEN = Pd, RED = Si profiles;YELLOW = EDS (Energy Dispersive Spectrometry) scan

Annular dark-field scanning TEM shows:• Annealed Au/Pd nanoclusters on SiO2

clearly are embedded with almost half their volume into SiO2 layer to depth of15-20 nm• EDS shows that the Au/Pd clusters areinterdiffused with the thermal SiO2 to a depth of 15 nm (yellow arrow), whileSi is distributed evenly over the entireAu/Pd nanocluster

• Annealed 80 nm colloidal Au on SiO2

Shows almost no embedded interfaciallayer (<3nm) in contrast to sputteredAu/Pd nanoclusters above• EDS shows that Au and Si are segregated,suggesting the absence of a reactiveinterface between the metal and SiO2

layer, again in contrast to the sputteredAu/Pd nanoclusters

Sputtering – Au/Pd atoms have highkinetic energies, allowing them to reactwith substrate surface

Results – How Does the Embedded Interfacial Layer Lead to Vertically Aligned Nanowire Growth?

BLUE = Zn, GREEN = S, RED = Si profiles;YELLOW = EDS (Energy Dispersive Spectrometry) scan

• (111) single crystal structure of the ZnS NWs was observed to extend into the amorphous SiO2 layer to a depth of 20 nm (yellow arrow)

• EDS shows that Zn and S interdiffused with the amorphous SiO2 approximately 20 nm across the interface, which is consistent with the length scale of the interdiffused Au/Pd in SiO2 (previous slide)• Indicates that interdiffusion of ZnS into SiO2 starts early on in the VLS process, facilitated by the interdiffusion of Au/Pd into SiO2

Results – “Rooting” of the NWSchematic showing the initial stage of NW growth on a rough amorphous substrate, showing how the interfacial state of the metal/substrate affects the NW growth direction:

Growth catalyzed by colloidal Au: • No vertically aligned NW growth• Even a small degree of surface roughnessaffects the metal/substrate interface byproviding random sites for NW nucleation

Growth catalyzed by sputtered Au/Pdthin film:• Vertically aligned NW growth• Surface roughness is compensated forby the embedded nanocluster; this provideslarger interfacial area for NW nucleation

IMPLICATION: Vertically aligned NWs of any material can be obtained on any substrate using the sputtered catalyst film approach

Results – CdS NW Growth on Three Different Substrates Using Sputtered Catalyst Approach

• Growth is close to vertically-aligned in all three cases• Room for improvement: use lower base pressures (here 300 Torr is used, which is relativelyhigh compared to MBE pressures ~ mTorr)

CdS NW on SiO2/Si substrate

CdS NW on Si3N4/Si substrate

CdS NW on ITO/quartz substrate

Conclusion• A method to grow single-crystalline, vertically

aligned NWs on arbitrary substrates using sputtered thin film catalyst deposition has been developed

• The key to achieving vertical alignment in this process is the embedding of the catalyst in the substrate which provides mechanical stability to the NWs

• This represents an important step toward achieving vertical nanowire arrays for a number of electronic, photonic, and photovoltaic applications