Growth and Characterization of Silicon Nanowires for Solar Cell Applications

Si-nanowires are being introduced as an attempt to decrease the high recombination rate present in silicon based thin-film solar cells by employing radial pn-junctions instead of conventional planar pn-junctions. Previous publications have also shown an additional increase in the amount of absorbed light when covering a silicon-substrate in silicon nanowires which may result in a further increase in the total efficiency of a thin-film solar cell. Successful growth of Si-nanowires has earlier been performed by Chemical Vapour Deposition (CVD), employing gold (Au) as catalytic material. Au is a very stable catalytic material for nanowire growth but Au-residues are unwanted in solar cell applications, and the current experiment has therefore investigated aluminium (Al) as an alternative catalyst material. However, stable Al-catalysed growth has been proven to be difficult and is assumed to be mainly due to rapid oxidation of Al to Al2O3. Most of the nanowires were short, tapered and consisted of worm-like structures. Several unsuccessful in-situ NH3-based cleaning (CVD) processes were attempted. Tin (Sn) was also attempted as a protective coating for the Al-film in order to protect Al from exposure to air during sample transport, without any luck. As solar cells require both p-doped and n-doped sections in order to form pn-junctions, initial investigations were performed on the effect from the addition of dopant gases (B2H6 and PH3) on nanowire morphology. The addition of B2H6 to the gas flow seemed to have much larger effects than PH3 on the nanowire morphology compared to intrinsic nanowires. Both gases resulted in a continuous reduction in the average nanowire length with increasing dopant⁄SiH4 ratios, ultimately leading to a complete inhibition of nanowire growth. The highest usable dopant⁄SiH4 ratios before complete growth-inhibition were found to ~10^-3 for B2H6 and ~10^-1 for PH3. An undesirable tapering effect was also found when adding B2H6 to the gas-flow, resulting in radial growth of amorphous silicon on the nanowire walls already at the lowest dopant ratio (~10^-5). This may complicate the use of B2H6 as a dopant gas for p-type nanowires. Ignoring the fact that the addition of PH3 to the gas-flow reduces the nanowire growth rate PH3 may be assumed to be a good alternative for n-type doping of nanowires as no further effects on the nanowire morphology is observed. The actual implementation of dopant atoms into the nanowire structure may be determined by measuring the electrical resistivity in the nanowire, and a possible four-contact structure has been designed and partly optimized for this purpose. The contact structure has been designed in three layers where two of them are produced by photolithography while the smallest layer by electron-beam-lithography. Note that the structure has not been finalized because of time limitations. Some optimization of the four nanowire contacts remains as some final lift-off problems appeared, and is assumed to be related to either an incomplete development of the smallest features or an observed resist-bubbling because of high Titanium (Ti) deposition temperature. However, a robust three-point alignment procedure has been investigated and found useful for producing accurate contacts to single nanowires and leads to the conclusion of a promising structure.