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 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

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  m Three different substrates – Thermally-grown 200 nm SiO 2 /Si – 100 nm Si 3 N 4 /Si – 1  m ITO/quartz

7. Results – ZnS NWs on SiO 2 /Si ZnS NWs catalyzed by annealed sputtered Au/Pd thin film: Predominantly vertically aligned wrt substrate; 75% of NWs within a range of ± 10 o to surface normal Stand upright w/o falling over NWs are 10-15  m 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 ±10 o to surface normal Do not stand upright ZnS NWs catalyzed by sputtered Au/Pd thin film deposited into circular 80  m features: ZnS NWs only grow in circular 80  m features Maintain vertical alignment even at the edges of the features NWs are >15  m long 40  m 500 nm

8. Results – ZnS NWs on SiO 2 /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

9. 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 SiO 2 clearly are embedded with almost half their volume into SiO 2 layer to depth of 15-20 nm EDS shows that the Au/Pd clusters are interdiffused with the thermal SiO 2 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 SiO 2 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 SiO 2 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

10. 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 SiO 2 layer to a depth of 20 nm (yellow arrow) EDS shows that Zn and S interdiffused with the amorphous SiO 2 approximately 20 nm across the interface, which is consistent with the length scale of the interdiffused Au/Pd in SiO 2 (previous slide) Indicates that interdiffusion of ZnS into SiO 2 starts early on in the VLS process, facilitated by the interdiffusion of Au/Pd into SiO 2

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 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) CdS NW on SiO 2 /Si substrate CdS NW on Si 3 N 4 /Si substrate CdS NW on ITO/quartz substrate

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