Effect of shell coating technique on carrier collection properties of core/shell nanostructures

Abstract

The core/shell nanowire (nanorod) arrays are promising candidates of being building blocks in many nanoscale device applications such as solar cells and photodetectors. Fast transportation of charge carriers, which requires high quality interface of core and shell materials, is crucial in this kind of applications. In this study, we performed a theoretical and experimental investigation on different physical vapor deposition (PVD) techniques to discover the most efficient deposition technique for a conformal shell coating around nanowires. We conducted Monte Carlo (MC) simulations and fabricated indium sulfide (In2S3) nanorods/silver (Ag) core/shell devices for photocurrent measurements. Glancing angle deposition (GLAD) technique was used to grow In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanorods. Our MC simulation results show that wider angular distribution of atomic flux (e.g. sputtering at high working gas pressures) can provide a conformal shell around nanowire arrays compared to a directional incident flux. The core/shell devices with Ag shell deposited by sputtering at a relatively high working gas pressure (3.4*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> mbar) show the highest photocurrent compared to other devices that involved more directional incident flux types of Ag sputter deposition. The shortened transit times calculated from photocurrent profiles can also indicate a conformal shell obtained with high pressure sputtering. On the other hand, our MC simulation results reveal that a small angle (~30°) of incoming flux w.r.t. the substrate surface normal might be required for PVD techniques with a highly directional incoming flux (e.g. thermal or e-beam evaporation) for a sufficient conformal shell. Overall, we demonstrate that already well-known and well-established industrial PVD techniques can be utilized simply with small modifications in order to fabricate high quality core/shell structures for high efficiency, low-cost photovoltaic and photoconductive devices.