Surface characterisation of contact materials for thin film CdTe solar cells

The deposition of tellurium onto the low index faces of copper single crystals induces a range of structures such as the (2√3×2√3)R30° surface substitutional alloy on the Cu(111) surface. These structures have been studied using a combination of scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), X-ray photoemission (XPS) and ultraviolet photoemission (UPS). The deposition of tellurium on to the Cu(111) surface produced several structures including (2√3×2√3)R30° phase for a coverage of 0.17ML, For a coverage of 0.33ML a (√3×√3)R30° surface alloy was found. For coverages greater than 0.66ML a bulk alloy was formed which is consistent with the Cu3Te2 phase. The deposition of tellurium onto the Cu(110) surface produced a c(2×2) structure for a coverage of 0.5ML. Further deposition causes uniaxial compression of the over-layer as indicated by the LEED patterns. STM of this surface revealed two hexagonal domains rotated 30° with respect to each other. The deposition of tellurium on to the Cu(100) surface produced a p(2×2) over−layer for a coverage of 0.25ML. Further deposition lead to a series of coincidence lattices showing co-existing structures including split c(2×2) spots in the LEED pattern. A heavily streaked split c(2×2) LEED pattern was observed for tellurium coverages over 1ML dosage where STM images revealed a heavily striped surface with two domains perpendicular to each other. The larger structures observed suggest 3D growth of these stripes. On all 3 surfaces the work function was found to increase upon tellurium deposition. The electronic structure of indium oxide thin films was investigated after different treatments in ultra high vacuum using XPS and UPS. The (111) surface was investigated using a combination of hard and soft X-rays showing the lower portion of the valence band to be dominated by oxygen orbitals while the higher portion of the valence band indium orbitals. The valence band was then probed and the work function measured after different annealing treatments. For all 3 samples, (111) (110) and (100) the work function was found to increase when annealing in oxygen when compared to annealing in vacuum. This is attributed to the movement of the Fermi level as the carrier concentration at the surface is altered. The lowering of observed gap states when annealing in oxygen also suggests they may originate from oxygen vacancies within the surface