Non-equilibrium thermal processing towards p-n junction formation in TCOs

Wide band gap semiconductors (> 3.0 eV) like ZnO and TiO2 are widely investigated in the field of optoelectronics for ultraviolet lasers, heterojunction solar cells, thin film transistors and light emitting diodes. Nowadays, the transparent conductive oxides (TCO) like Al- or F-doped ZnO (AZO or FZO) are basic materials for the front contact in thin-film photovoltaics. A highly doped n-type ZnO thin layer is an attractive candidate to replace the much more expensive indium-tin-oxide layer in the microelectronics industry. The optoelectronic properties of TCOs are determined by the type of doping and carrier concentration. The n-type conductivity of ZnO is easily achieved by substitution of Zn by group III elements (Al, Ga, In), or by doping with halogen elements (F, Cl or I) substituting oxygen in the lattice site. In the case of TiO2 the n-type material can be achieved by doping with Nb, Ta or F ions while p-type TiO2 can be realized by e.g. Cr doping. Here, we will present the utilization of highly non-equilibrium thermal processing of TCO/Si heterojunctions using millisecond (ms) range flash lamp annealing (FLA) techniques for the structural modification and dopant activation to form highly doped p- and n-type TCOs films on silicon substrate. The n- and p-type doping in ZnO was made by incorporation of Al and F or N and P into ZnO, respectively. While the conductivity of TiO2 films was controlled by efficient incorporation of Ta and Cr into the lattice side of titania. It will be presented that via millisecond range FLA treatment not only the optoelectronic properties but also the crystallographic orientation and phase formation of TCOs can be modified. The optical properties of fabricated TCOs were investigated using temperature dependent photoluminescence, Raman and transmission spectroscopy. X-Ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM) were utilized to study the microstructural properties while the electrical properties of the TCO layers and heterojunctions were measured using Hall Effect and current-voltage characterization, respectively. Moreover, it is shown that the annealing atmosphere, even during ms range annealing, can efficiently passivates the surface state and bulk defects in the TCOs significantly improving the near band gap emission. - See more at: http://www.european-mrs.com/2015-fall-symposium-g-european-materials-research-society#sthash.tu4VYSCO.dpuf