Catalyst formation and growth of Sn- and In-catalyzed silicon nanowires
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Catalyst formation and growth of Sn- and In-catalyzed silicon nanowires Irène NGO 1, Benedict O’DONNELL 2, José ALVAREZ 1, Marie-Estelle GUEUNIER-FARRET 1 , Jean-Paul KLEIDER 1, Linwei YU 2, Pere ROCA i CABARROCAS 2 1 Laboratoire de Génie Electrique de Paris, 11 rue Joliot-Curie, Plateau de Moulon, 91192 Gifsur-Yvette Cedex, France 2 Laboratoire de Physique des Interfaces et des Couches Minces , Ecole Polytechnique, CNRS, 91128 Palaiseau, France ABSTRACT Silicon nanowires (Si NWs) were grown directly on transparent conductive oxide layers using a single pump down process in a plasma enhanced chemical vapour deposition (PECVD) system. Layers of ITO and SnO2 on glass substrates were exposed to a hydrogen plasma leading to the reduction of the oxide and to the agglomeration of the metal into catalyst droplets of a few tens of nanometers diameter. The diameter and the density of the nanowires depend on the catalysts droplets size and density, we studied step by step the evolution of the surface prior to and at the initial stage of the nanowire growth. The catalyst droplets size and distribution were essentially investigated through Scanning Electron Microscopy (SEM). INTRODUCTION Nanostructures offer promising building blocks for future microelectronics and optoelectronics devices. In the field of photovoltaics, the anisotropic vapour-liquid-solid (VLS) growth of crystalline silicon nanowire (SiNWs) offers an innovative solar cell geometry. Radial p-n junction enables the decoupling of the light trapping in the axial direction and the minority carrier collection in the radial direction. Constraints on the amount and the purity of the Si required for a cell are thereby reduced, while maintaining a comparable efficiency to that obtained in Si thin film technology. The conversion efficiency of that type of cell is predicted to achieve over 17% due to improved light absorption and minority carrier collection. These cells will also use less than one hundredth of the silicon required in a traditional wafer based cell [1] and thus lower the overall cost of the device. A well-known self organized growth mechanism for creating NWs is the VLS process, where the wires grow as silicon precipitates from a supersaturated catalyst droplet [2]. Gold has been the most popular catalyst for Si wire growth since the first publications in this field. This noble metal shows many attractive features as a VLS catalyst: it is stable and non toxic, it offers a low temperature eutectic point with silicon (363°C) and it dissociates silane with low activation energy (0.53 eV). Yet it should be avoided because of parasitic diffusion of gold in the crystalline Si wire which creates deep recombination centres at 0.54 eV below the conduction band. Many alternative catalysts are available for VLS growth, however the conditions for wire growth and the quality of the wires produced differ strongly according to the catalyst [3]. Here, we investigated silicon wire growth from indium (In) and tin (Sn). According to Nebol’sin et al.[4], growth fr
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