Dr Veer Dhaka and Prof. Harri Lipsanen, Aalto University
Bilateral joint project NWIRES aims to understand the fundamental properties of photovoltaics materials at the nanoscale, and attempts to build next generation radial pn junction based semiconductor nanowire solar cells on cheap substrates using different growth approaches, namely metalorganic chemical vapour deposition (MOCVD) in Aalto University and electrodeposition & chemical bath deposition in IIT Delhi, India. The project is now in its 2nd year, and is now progressing at a good pace with increased research cooperation with the partners, India in the form of regular exchange visits and sample exchanges. From the Aalto point of view, the main research highlight since the beginning of 2015 are listed below:
- To use nanowires in solar cells, the two important key issues are (1) surface passivation and (2) long-term protection of the device/nanowires from oxidation and the environmental attack. In the past, we had successfully developed various passivation methods for III-V nanowires. Recently, we have identified specific low-temperature (<200oC) atomic layer deposition (ALD) grown ultrathin dielectric capping layers for surface passivation. In particular, 2nm thick Al2O3 and 0.2nm thick AlN not only successfully passivated the InP and GaAs nanowires, respectively, but they also serve as protective capping layers to prevent long-term oxidation/environmental attack. The results were published recently in AIP Advances 2016 .
Fig. 1: Schematic for fabrication process of dual-type NWs arrays on a single substrate .
- We have developed a new method to grow two different types of nanowires side-by-side into a single array on a single substrate using the MOCVD growth method (Fig.1). The technique uses selective-area-epitaxy (SAE) and vapor-liquid-solid (VLS) growth methods in two separate runs, in this way, GaAs & InP nanowires can simultaneously be grown on a single platform. This dual-type nanowire array shows a significant improvement in light absorption compared to a single material array, and offers the possibility for wider absorption tuning of the solar spectrum. In addition to immediate application in solar cells, the technique has also good potential in fabricating thermoelectric generators & LEDs. The results were published in Nano Letters 2015 .
- We have developed yet another versatile technique to grow positioned defined ordered arrays of GaAs nanowires with tunable periods and diameters on a large-area wafer size scale using laser interference lithography (LIL). Typically, electron beam lithography (EBL) is used to pattern position defined (selective-area) III-V nanowire array. However, EBL is slow and the exposure is practically limited only to small areas (~100µm x100µm). With this new technique, wafer scale nanowire array can be patterned quickly without the use of expensive cleanroom processing .
- Further, Aalto and KTH University, Sweden has succeeded in jointly fabricating a radial pn junction solar cell based on top-down fabricated InP nano pillars and the MOCVD grown overlayers at Aalto University. The solar cell shows a promising 5.2% light conversion efficiency. By improving the process parameters, we aim to fabricate similar NW based solar cell in the coming years. The results were published in IOP’s Nanotechnology journal .
 Dhaka, V., Perros, A., Naureen, S., Shahid, N., Jiang, H., Kakko, J.P., Haggren, T., Kauppinen, E., Srinivasan, A. and Lipsanen, H., 2016. Protective capping and surface passivation of III-V nanowires by atomic layer deposition. AIP Advances, 6(1), p.015016.
 Kakko, J.P.P., Haggrén, T., Dhaka, V., Huhtio, T., Peltonen, A., Jiang, H., Kauppinen, E. and Lipsanen, H., 2015. Fabrication of Dual-Type Nanowire Arrays on a Single Substrate. Nano Letters, 15(3), pp.1679-1683.
 Kauppinen, C., Haggren, T., Kravchenko, A., Jiang, H., Huhtio, T., Kauppinen, E., Dhaka, V., Suihkonen, S., Kaivola, M., Lipsanen, H. and Sopanen, M., 2016. A technique for large-area position-controlled growth of GaAs nanowire arrays. Nanotechnology, 27(13), p.135601.
 Sanatinia, R., Berrier, A., Dhaka, V., Perros, A.P., Huhtio, T., Lipsanen, H. and Anand, S., 2015. Wafer-scale self-organized InP nanopillars with controlled orientation for photovoltaic devices. Nanotechnology, 26(41), p.415304.
 Bautista, G., Mäkitalo, J., Chen, Y., Dhaka, V., Grasso, M., Karvonen, L., Jiang, H., Huttunen, M.J., Huhtio, T., Lipsanen, H. and Kauranen, M., 2015. Second-Harmonic Generation Imaging of Semiconductor Nanowires with Focused Vector Beams. Nano Letters, 15(3), pp.1564-1569.