Nanowire Presentation

1. Nanowire Presentation Alexandra Ford 4/9/08 NSE 203/EE 235

2. Paper I Will Be Presenting Today:

3. Assembly of Vertical Nanowire (NW) Arrays • Assembly of vertical NW arrays is key to taking full advantage of nanowiresublithographic 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

4. 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

5. 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

6. 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

7. Results – ZnS NWs on SiO2/Si • ZnS 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

8. Results – ZnS NWs on SiO2/Si • Shorter wires grow completely vertically: • All ZnSNWs were determined to be single-crystalline fccsphalerite 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

9. Results – Investigation of Catalyst Particles • 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 of • 15-20 nm • EDS shows that the Au/Pd clusters are • interdiffused with the thermal SiO2 to • a depth of 15 nm (yellow arrow), while • Si is distributed evenly over the entire • Au/Pd nanocluster • Annealed 80 nm colloidal Au on SiO2 • Shows almost no embedded interfacial • layer (<3nm) in contrast to sputtered • Au/Pd nanoclusters above • EDS shows that Au and Si are segregated, • suggesting the absence of a reactive • interface between the metal and SiO2 • layer, again in contrast to the sputtered • Au/Pd nanoclusters • Sputtering – Au/Pd atoms have high • kinetic energies, allowing them to react • with substrate surface BLUE = Au, GREEN = Pd, RED = Si profiles; YELLOW= EDS (Energy Dispersive Spectrometry) scan

10. Results – How Does the Embedded Interfacial Layer Lead to Vertically Aligned Nanowire Growth? • (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 BLUE = Zn, GREEN = S, RED = Si profiles; YELLOW= EDS (Energy Dispersive Spectrometry) scan

11. Results – “Rooting” of the NW Schematic 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 roughness • affects the metal/substrate interface by • providing random sites for NW nucleation • Growth catalyzed by sputtered Au/Pd • thin film: • Vertically aligned NW growth • Surface roughness is compensated for • by the embedded nanocluster; this provides • larger interfacial area for NW nucleation IMPLICATION: Vertically aligned NWs of any material can be obtained on any substrate using the sputtered catalyst film approach

12. Results – CdS NW Growth on Three Different Substrates Using Sputtered Catalyst Approach CdS NW on SiO2/Si substrate CdS NW on Si3N4/Si substrate CdS NW on ITO/quartz substrate • Growth is close to vertically-aligned in all three cases • Room for improvement: use lower base pressures (here 300 Torr is used, which is relatively • high compared to MBE pressures ~ mTorr)

13. 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