Most phosphate precursors used in atomic layer deposition are either thermally unstable or require activation. Therefore, our aim is to identify additional phosphate precursors that have not yet been widely used by the scientific community. This paper describes the deposition of titanium phosphate coatings via atomic layer deposition using in various pulse sequences titanium tetraisopropoxide (TTIP), tris(trimethylsilyl) phosphate (TTMSP), and water as the precursors. We performed the deposition predominantly onto carbon fibers. For x-ray photoelectron spectroscopy (XPS), in a limited number of cases, we coated flat silicon wafers, bearing 100 nm oxide layers. Film growth without a water pulse (pulse sequence TTMSP/TTIP) did not yield reliable results. Therefore, the pulse sequences TTMSP/H2O/TTIP, TTMSP/TTIP/H2O, and TTMSP/H2O/TTIP/H2O were used. All these sequences exhibited self-limiting growth behavior at 200 °C. The growth per cycle (GPC) was 0.21–0.24 nm/cycle for TTMSP/H2O/TTIP and TTMSP/H2O/TTIP/H2O, while TTMSP/TTIP/H2O yielded a lower GPC of 0.11 nm/cycle. Chemical analyses of the deposited coatings via induction coupled plasma-optical emission spectrometry (ICP-OES) and XPS revealed a molar ratio P/Ti in the range of 0.31–0.91. These values are lower than the value of 1.33, which is expected for Ti3/4PO4. Therefore, these coatings have compositions between titanium phosphate and titanium oxide. The coatings with the TTMSP/TTIP/H2O pulse sequence comprised the highest phosphorus content: XPS sum formula = TiP0.53O3.1C0.2, ICP-OES P/Ti = 0.52–0.91. The sequence TTMSP/H2O/TTIP yielded XPS sum formula = TiP0.38O2.8C0.3 and ICP-OES P/Ti = 0.32–0.45. The sequence TTMSP/H2O/TTIP/H2O yielded a similar composition with XPS sum formula = TiP0.39O3.1C0.1 and ICP-OES P/Ti = 0.35–0.53. We increased the P/Ti ratio using multiple subcycles of TTMSP/H2O up to a maximum P/Ti value of 1.47. All coated fibers were subjected to thermogravimetric analysis in air to test their ability to increase the oxidation resistance of the fibers. As a criterion for the oxidation resistance, we selected the temperature at which a mass loss of 3% with respect to the mass at 400 °C occurs. For uncoated fibers, this 3% mass loss occurs at a temperature3% of 648 °C. All coated fibers showed a moderate upshift of this temperature3%. The maximum temperature3% that we achieved was 711 °C with coatings deposited via the pulse sequence TTMSP/H2O/TTIP/H2O.
Klapper et al. (Mon,) studied this question.