Self-limiting deposition of anatase TiO2 was accomplished by plasma-enhanced atomic layer deposition using TiCl4 and O2. Film growth occurred at a rate of ∼1.35 Å/cycle, independent of temperature. The refractive index increased monotonically with temperature, and the presence of the anatase phase was confirmed by FTIR and XRD for T ≥ 110 °C. Films were free of impurities with respect to FTIR and XPS, and conformal coverage was confirmed by cross-section microscopy. These results show that OH groups, the most common surface termination in ALD, is not required for this process.

The self-limiting deposition of anatase TiO 2 was accomplished by pulsed plasma-enhanced chemical vapor deposition ͑PECVD͒ using a simultaneous delivery of TiCl 4 and O 2 . The amorphous to anatase phase transition was examined as a function of temperature, plasma power, and film thickness. Films deposited at T = 120°C, with the plasma power set at 100 W were amorphous, containing residual amounts of chlorine. At 200 W, the films displayed an anatase structure, and no chlorine was detected by X-ray photoelectron spectroscopy. Spectroscopic ellipsometry, Fourier transform infrared, and X-ray diffraction measurements concur that a minimum film thickness of ϳ25 nm is required for the formation of the anatase phase.Titanium dioxide ͑TiO 2 ͒ thin films are of major technological interest due to their versatile physical and chemical attributes. Titania's high refractive index makes it a common component in optical filters and coatings. 1,2 TiO 2 is a leading photocatalyst 3-5 and serves as a critical component in emerging photovoltaic technology. 6,7 With its extraordinarily high dielectric constant, TiO 2 thin films are a leading candidate for both memory and thin-film transistor applications. 8,9 A variety of synthesis techniques have been used, including chemical vapor deposition ͑CVD͒, 8,10 sputtering, 1,2 plasma-enhanced CVD ͑PECVD͒, 11,12 and atomic layer deposition ͑ALD͒. 4,5,13-18 Emerging applications demand improved quality and control. ALD imparts digital control over film thickness and composition, as well as exceptional film quality. The main drawback of ALD is its low rate, which precludes its use for the commercial synthesis of mesoscale structures ͑100-1000 nm͒ for single-wafer processing such as optical components. ALD typically requires temperatures Ͼ200°C to produce the anatase phase, which is desired for photocatalyst applications. In this article, we describe the selflimiting production of anatase films by pulsed PECVD at temperatures well below 200°C.Our group has established pulsed PECVD as an alternative to ALD for the self-limiting growth of metal oxide thin films, including Ta 2 O 5 , 19,20 Al 2 O 3 , 21,22 and ZnO. 23 The process has been described in detail previously 24,25 and is briefly reviewed here. In pulsed PECVD, O 2 and the metal precursor are mixed and delivered simultaneously. To be self-limiting, the precursor must be unreactive with O 2 so that no deposition occurs with the plasma off. TiCl 4 is unreactive with O 2 below 400°C, 10 but other precursors such as TiI 4 that are reactive with O 2 26 would not produce self-limiting growth by this technique. Purge steps are eliminated, and growth occurs discretely by modulating the plasma power at low frequency ͑ap-proximately in hertz͒. The nature of the self-limiting growth is fundamentally different from ALD. Instead of relying on surface chemistry, growth terminates during each plasma step due to the consumption of the precursor. The process is self-limiting in the sense that no deposition occurs with the plasma off or with the plas...

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Effect of wall conditions on the self-limiting deposition of metal oxides by pulsed plasma-enhanced chemical vapor deposition

Anatase TiO 2 photoanodes were deposited by self-limiting growth techniques at low temperature. The optical bandgap and flatband voltage of the as-deposited films agree with the values obtained from single crystal anatase. The donor density could be increased by both UV illumination and cathodic polarization in acidic solutions. Improvements in photocurrent scaled closely with changes in carrier concentration, with over 20-fold enhancements observed over the as-deposited films. The threshold potential for hydrogen intercalation was Ϫ0.6 V vs Ag/AgCl. At this level the carrier concentration could be manipulated with no change in optical transmission. At lower potentials irreversible changes are observed which are attributed to the reduction of the underlying indium tin oxide contact. In contrast, no changes were observed when fluorinated tin oxide was used as the contact layer.TiO 2 films have been widely studied as photocatalysts for water splitting and degradation of organic contaminants since the early 1970s. 1,2 Various techniques have been used to synthesize TiO 2 thin films including oxidation of titanium, 3 chemical vapor deposition ͑CVD͒, 4 plasma-enhanced chemical vapor deposition ͑PECVD͒, 5 and sputtering. 6 More recently, self-limiting growth techniques such as atomic layer deposition have been introduced to provide digital control. 7,8 Our group has recently developed both plasma-enhanced atomic layer deposition ͑PEALD͒ 9 and pulsed PECVD 10 processes for the synthesis of titania at low temperature ͑Յ200°C͒ using TiCl 4 and O 2 . In PEALD, perfect conformality is achieved, allowing uniform deposition on complex topographies. The resulting films produced by both techniques displayed a polycrystalline anatase morphology with no detectable chlorine impurities. 9,10 In this work we focus on the photoelectrochemical properties of these films and compare their performance with that of TiO 2 films deposited by other techniques. Photoanodes were formed by depositing TiO 2 on glass coated with transparent conducting oxides ͑TCOs͒. In particular, we systematically explore electrochemical doping using cathodic polarization as a means to control the carrier concentration and enhance the photoresponse. In addition, the role of the underlying TCO contact was evaluated by comparing photoanodes produced with indium tin oxide ͑ITO͒ and fluorinated tin oxide ͑FTO͒. ExperimentalTiO 2 thin films were deposited on TCO-coated glass by both PEALD and pulsed PECVD. Details on the two deposition processes are found in the literature. 9,10 In all cases, the deposition temperature was T = 200°C. Thin ͑70 nm͒ and thick ͑255 nm͒ sets of films were produced to explore the effect of film thickness. Film thickness was determined by spectroscopic ellipsometry ͑J. A. Woollam͒. Most of the results presented below were achieved with pulsed PECVD films, but nominally identical results were obtained with PEALD films. The two TCOs employed were commercial FTO ͑Libby-Owens-Ford, R s ϳ 10 ⍀/ᮀ͒ or 200 nm thick ITO layers that were sputtered in...