Growth of vertically aligned Si wire arrays over large areas (>1cm2) with Au and Cu catalysts

Abstract

Arrays of vertically oriented Si wires with diameters of 1.5 m and lengths of up to 75 m were grown over areas Ͼ1 cm 2 by photolithographically patterning an oxide buffer layer, followed by vapor-liquid-solid growth with either Au or Cu as the growth catalyst. The pattern fidelity depended critically on the presence of the oxide layer, which prevented migration of the catalyst on the surface during annealing and in the early stages of wire growth. These arrays can be used as the absorber material in novel photovoltaic architectures and potentially in photonic crystals in which large areas are needed. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2779236͔Photovoltaic devices designed to achieve high cell efficiency with low-quality materials must have optically thick absorber layers, yet must simultaneously allow efficient collection of low diffusion length charge carriers. An attractive approach involves an array of vertically aligned semiconducting wires to enable carrier collection in the wires' radial direction, a distance that is short relative to their optical thickness ͑i.e. length͒.1 Well-defined wire arrays have been produced using lithographic patterning followed by anisotropic etching, 2,3 but such methods require large areas of high-quality substrate materials. In contrast, wires of various materials 4 have also been grown 'bottom up' by the vaporliquid-solid ͑VLS͒ process.5 Control of the size and position of VLS-grown wires has been demonstrated, 6,7 particularly in the case of Si by patterning of a surface oxide.8-10 Wire array growth, however, has only been achieved over relatively small areas, unless a template is used.11 We demonstrate herein the VLS growth of arrays of Si wires having diameters of 1.5 m and lengths of Ͼ70 m, with very low defect densities, over areas Ͼ1 cm 2 , without the use of a template.Attempts to grow Si wire arrays did not yield high pattern fidelity when the catalyst was not confined. Wires were grown by photolithographically patterning S1813 photoresist ͑Microchem͒ on a clean Si͑111͒ wafer, then exposing it for 5 s to buffered HF͑aqueous͒ ͑Transene, Inc., 9% HF, 32% NH 4 F͒, followed by evaporation of 500 nm of Au and lift-off of the resist. This produced a square array of 3 m diameter Au islands with a center-to-center pitch of 7 m. Samples were then annealed in a tube furnace at 900-1000°C for 20 min under 1 atm of H 2 at a flow rate of 1000 SCCM ͑SCCM denotes cubic centimeters per minute at STP͒, followed by wire growth under 1 atm of H 2 and SiCl 4 , at flow rates of 1000 and 20 SCCM, respectively. This produced arrays of low fidelity, with no control over the wire diameter or wire position ͑not shown͒. Examination of the samples after a 20 min H 2 anneal only revealed that this behavior was due to substantial agglomeration of the catalyst ͑Fig. 1͒.The successful production of large-area Si wire arrays involved the use of an oxide buffer layer to confine the VLS catalyst to the desired areas in the pattern. To implement this approach, a 300 nm oxide was ther...

Photoelectrochemical Hydrogen Evolution Using Si Microwire Arrays

“…Si microwire arrays were grown using a process similar to that described previously. 1,2 Boron-doped p + -Si (111) wafers, having a resistivity of < 0.001 ·cm (Silicon Quest International), were used as growth substrates. The wafers were coated with 450 nm of thermal oxide that had been patterned with 4-μm-diameter circular holes arranged on a square lattice with a 7 μm pitch.…”

Evaluation of Pt, Ni, and Ni–Mo electrocatalysts for hydrogen evolution on crystalline Si electrodes

“…Si microwire arrays were grown on p + -Si substrates by an atmospheric pressure chemical vapor deposition (CVD) technique that used Cu as a vapor-liquid-solid (VLS) growth catalyst. 9 The hexagonal wire pattern was determined by the pattern of the Cu, which was defined by evaporation and lift-off of Cu into lithographically etched holes in a SiO 2 buffer layer on the Si substrate. For wire growth, SiCl 4 (g) was the Si source and BCl 3 (g) was the boron source, resulting in p-type doping of the crystalline Si microwires.…”

“…The microwire array geometry orthogonalizes the directions of light absorption and charge-carrier collection, thereby faciliusing high growth rate methods such as the vapor-liquid-solid growth technique. 9,10 Exploitation of the orthogonalization approach has to date yielded Si microwire array-based radial n + -p junction photovoltaic devices that, while only absorbing 60% of the light, have resulted in 7% solar energy conversion efficiencies under simulated Air Mass (AM) 1.5 illumination conditions. 11 Radial n + -p junction Si microwire array photocathodes that have been decorated with islands of Pt as an electrocatalyst have yielded, with only $50% light absorption, >5% thermodynamically based energy conversion efficiencies for the production of H 2 (g) from 0.5 M H 2 SO 4 (aq) under simulated AM1.5 illumination.…”

Fundamental limits in the external quantum efficiency of single nanowire solar cells

“…7,8,11,12 Recently nanowire based solar cells with both axial and radial p-n junctions have been demonstrated in various material systems. [13][14][15][16] Even though the ultimate device should consist in a forest of vertical nanowires, to date many experiments are carried out on single nanowire devices that are placed on a substrate with the light incident perpendicular to the substrate surface. 13,15,16 This geometry offers more flexibility for basic material characterization and is also promising for powering nanoelectronic devices.…”