Abstract
Poly-crystalline anatase TiO2 layer fabricated by LPCVD using titanium-tetra-iso-propoxide and NbF5 in H2-ambient was treated in conc.-HCl solution after thin layer of IIIb-group metal was deposited on the TiO2 layer. Resistivity of the as-deposited layer about 1 × 10-1 Ω·cm was drastically reduced to 3 × 10-3 Ω·cm by the wet-treatment using indium. Temperature dependence of the resistivity increased with temperature above 100 K for the wet-treated layer was quite different from that decreased above 100 K for the as-deposited layer, whereas the resistivity was saturated at lower temperatures. The resistivity at room-temperature was decreased with the thickness before the wet-treatment but independent on the thickness above 100 nm for the wet-treated layer. Indium was more effective for the resistivity reduction than gallium but aluminum was not useful for the treatment. As the results that the wet-treatment using indium was examined for the TiO2 layers deposited by various conditions, the optimum deposition condition to reduce the resistivity of the layer after the wet-treatment was clearly different from that for the as-deposited layer.
Highlights
Anatase-TiO2 with wide energy band gap about 3.2 eV has been candidate of transparent conductive oxide (TCO) layer instead of indium-tin-oxide (ITO) as demonstrated by laser-ablation and reactive-sputtering [1] [2]
We demonstrated low-pressure chemical vapor deposition (LPCVD) of anatase-TiO2 layer by using titanium-tetra-iso-propoxide (TTIP: Ti(O-i-C3H7)4) as the metalorganic precursor and NbF5 to reduce the resistivity in O2- and H2-ambient [5]-[7]
Some case can be considered for the drastic reduction of the resistivity: 1) if thin metal-layer is formed on the TiO2 layer, the observed resistivity is significantly decreased due to the metal-layer, 2) if the surface Fermi-level is pinned in the conduction band, accumulation layer with high conductivity is formed at the surface, 3) if the interface defects in grain-boundaries are passivated and non-activated by ions, decrease of the Schottky-barrier height resultantly decrease the resistivity
Summary
Anatase-TiO2 with wide energy band gap about 3.2 eV has been candidate of transparent conductive oxide (TCO) layer instead of indium-tin-oxide (ITO) as demonstrated by laser-ablation and reactive-sputtering [1] [2]. We demonstrated low-pressure chemical vapor deposition (LPCVD) of anatase-TiO2 layer by using titanium-tetra-iso-propoxide (TTIP: Ti(O-i-C3H7)4) as the metalorganic precursor and NbF5 to reduce the resistivity in O2- and H2-ambient [5]-[7] It was found in the results of Nb and F co-doped layer that F substituted to the O-site (FO) contributes to the reduction of resistivity without VO [5], in which the FO acts to form Ti3+ [4], and the transparency in UV-Vis region is resultantly improved due to reduction of the absorption originated from VO-related centers [7]. Post surface wet-treatment in HCl solution by using metal species of IIIb-group is applied to poly-crystalline LPCVD-TiO2 layers deposited by various conditions to reduce the resistivity with the discussions for sufficient metal-species and carrier transport across the grain-boundaries
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