PHOTOCONDUCTING TIO2 PREPARED BY SPRAY PYROLYSIS USING TICL4

TiO2 was prepared by the oxidation of TiCl4 at relatively low deposition temperatures using spray pyrolysis. For a fixed carrier gas (N2) flow rate, the substrate temperature is the critical parameter determining the film properties. Films made near 595 K contain only the anatase phase, adhere well to the glass substrate, and have a relative optical transmission of about 75% (between 450 and 850 nm), the value one calculates for the non-absorbing anatase phase in this region. These films have an absorption edge characteristic of a direct gap semiconductor with allowed transitions and optical gaps of 3.5-3.7 eV, depending on the porosity. At higher substrate temperatures (above 635 K), the resulting films have a cloudy appearance and they contain both the rutile and the anatase modifications, confirmed by X-ray diffraction. Lower temperature substrates reduce the deposition rate and yield more porous films. For the pure anatase films, the dark conductivities are about one order of magnitude larger, and the photoconductivity is about one order of magnitude smaller when measured with the sample in air, than those observed when the sample is in a vacuum. When the samples are in a vacuum, the conductivities of the clear films increase by about two orders of magnitude (from about 10(-10) to about 10(-8) S cm-1) when illuminated with light from an electrically programmable read-only memory eraser (mercury light), which gives an intensity of about 30-mu-w cm-2 at the sample. The photoresponse has two parts; the fast portion rises in less than 1 s to 99% of its final value, followed by a small slow rise portion. The slow response, which is more pronounced in the decay process, is interpreted as being due to the presence of surface trapping states.