AVS2009 Session TF2-TuA: ALD/CVD: Oxides and Barriers

Titanium dioxide thin films were deposited using pulsed plasma-enhanced chemical vapor deposition at low temperature (T_s <200 ºC). Self-limiting deposition (~1 Å/cycle) was accomplished via simultaneous delivery of TiCl₄ and O₂. TiCl₄ is shown to be inert with molecular oxygen at process conditions, making it a suitable precursor for pulsed PECVD. The process was examined as a function of TiCl₄ exposure, plasma power, and substrate temperature. Crystalline anatase formation was observed at temperatures as low as 120^oC. Depositions at high power also had a significantly greater refractive index. For process conditions, digital control over film thickness is demonstrated. Film uniformity is exceptional, with thickness variations less than 1% across 100 mm silicon wafers. Photocatalytic activity has been examined using methylene blue decomposition experiments, UV-VIS spectroscopy, and electrochemical analysis. Mott-Schottky plots show that the band edge position of these thin films is in agreement with measurements from anatase single crystals. The photocatalytic activity of these films for both hydrogen production and organic remediation is assessed. We also plan to present new results on the production of the titania-vanadia alloys with enhanced light response in the visible regime.

In this work, TiO₂ ALD is carried out in an integrated ALD reactor/UHV chamber that allows for X-ray photoelectron spectroscopy (XPS) analysis after each precursor pulse without vacuum break. Titanium tetrachloride (TiCl₄) and water (H₂O) are selected as precursors due to their molecular simplicity and broad operating temperature range that result in several achievable TiO₂ phases. The initial growth at 100^oC on two substrates − chemical oxide on silicon prepared by piranha treatment, and hydrogen-terminated silicon prepared by HF etch − is compared. The intensities and binding energies of characteristic peaks from the XPS spectra are used to analyze the elemental compositions and chemical state of each species as the deposition progresses. TiO₂ growth on both SiO₂ and H-Si surfaces exhibits linear behavior, as normally achieved by ALD, but the TiO₂ growth rate is lower on hydrogen-terminated surface than on silicon dioxide surface. Interestingly, no incubation period is observed on either surface.

The chemical shifts of the Si 2p, O 1s and Ti 2p XPS peaks after TiO₂ deposition on the SiO₂ substrate suggest bond formation between titanium and silicon-bound oxygen at the interface. The data also suggest that some chlorine is trapped at the SiO₂/TiO₂ interface and that the titanium oxide right at the interface is sub-stoichiometric. The results on the hydrogen-terminated Si surface show different interfacial properties. There is no detectable amount of oxidized silicon species on hydrogen-terminated silicon after deposition under vacuum. Together with the results of ex situ studies, it can be concluded that interfacial silicon dioxide grows after air exposure, not during ALD reactions. The absence of silicon oxide and a shift in the Si 2p binding energy in the as-grown samples suggest the possibility of an ALD mechanism which involves direct bonding between titanium and silicon on the surface. The differences between the two substrates will be discussed.

In this presentation, the authors will discuss the surface reaction mechanism during the plasma-assisted atomic layer deposition (ALD) of TiO₂ using titanium tetraisopropoxide and an O₂-Ar plasma at substrate temperatures ≤150 °C. In situ attenuated total reflection Fourier-transform infrared (IR) spectroscopy was used to detect surface species generated or consumed during each half-reaction cycle with a sensitivity down to a fraction of a monolayer. Our IR data showed that the reactive species on the TiO₂ surface for TTIP chemisorption were both surface carbonates and –OH groups, identified in the 1450-1700 and 3400-3800 cm^-1 regions, respectively. Based on this observation, we conclude that plasma-assisted ALD of TiO₂ involves a combination of the mechanisms previously reported for O₃- and H₂O-based ALD using TTIP as the metal precursor: we had recently reported surface carbonates as the reactive sites using O₃ as the oxidizing agent and –OH groups have previously been reported with H₂O as the oxidant. We hypothesize the following reaction mechanism. Combustion products such as CO₂, CO, and H₂O are formed during the O₂-plasma cycle due to the plasma-assisted combustion of the isopropoxy ligands on the surface. A fraction of this CO₂ reacts with the surface to generate the carbonates, similar to the O₃-based ALD mechanism. We explain the simultaneous presence of the surface –OH groups in addition to the carbonates due to the plasma activation of H₂O, also generated during the O₂ plasma cycle, and the subsequent reaction of these activated species with the surface. The latter step does not occur when O₃ is used as the oxidant. In fact, the ratio of the carbonates and the surface –OH groups could be varied by controlling the residence time of the reaction products in the plasma. A growth per cycle of ~0.8 Å was obtained at 150 °C, which was significantly higher compared to H₂O- and O₃-based ALD of TiO₂ at similar temperatures. In situ and ex situ IR measurements showed no significant carbon contamination in the films. Ex situ IR data showed the Ti-O-Ti transverse optic mode at 440 cm^-1, a characteristic of anatase. The ex situ x-ray diffraction measurements further confirmed anatase as the dominant crystal phase. The crystallinity of the films may be the reason for the higher growth per cycle compared to that observed for amorphous films deposited from the same metal precursor.

The realization of compact fiber optic networks is limited by the control of the rare earth (RE) dopant ion’s identity and its distribution. Each RE ion contains spectral energy levels, whose transitions can be promoted by controlling the ion-ion proximity. In this work, a radical enhanced atomic layer deposition (RE-ALD) process, utilizing highly reactive radicals to activate surface reactions, is developed to design complex metal oxides with multiple dopants, whose concentration variation and spatial distribution control enable the synthesis of a wide range of multifunctional materials with tunable properties including magnetic, spectral, and electronic. Specifically, the control of dopant proximity and concentration is used to enhance desirable spectral transitions related to amplification at 1.54 micron for compact planar optical amplifier applications. The spatial distributions between Er³⁺ and various rare earth sensitizers (e.g. Yb and Eu) are investigated. The ability to control the spacing of each sensitizer allows for a unique study of the correlation between energy transfer and dopant proximity. Thin films of approximately 10 nm are synthesized by sequential radical-enhanced ALD of Y₂O₃, Er₂O₃, Yb₂O₃, and Eu₂O₃ at about 350°C. The composition, microstructure, cation distribution, local chemical bonding and optical properties of the as-synthesized thin films were determined by x-ray photoelectron spectroscopy, electron microscopy and photoluminescence measurements. To optimize the effect of the sensitizer and minimize the concentration quenching, the concentration and spatial distance between Yb³⁺/Eu³⁺ sensitizers and Er³⁺ were controlled by changing the global deposition cycle sequence. Extended x-ray absorption fine structure analysis verified the spatial control of dopants in the Y₂O₃ host. It was found that multi-dopant spatial control allows for incorporation of minimal active sensitization sites, allowing for the possibility of maximizing the active Er concentration while maintaining efficient energy transfer. This additional degree of control allows for a class of complex materials that can potentially outperform their conventional counterparts by many-folds in luminescence.

La₂O₃ was incorporated into hafnium dioxide grown by atomic layer deposition (ALD) to stabilize the amorphous phase during high temperature annealing. The incorporation was achieved by depositing HfO₂ and La₂O₃ alternatively in different ALD cycles. X-ray photoelectron spectroscopy compositional analysis shows that the Hf and La atomic percentage ratio can be controlled by varying the number of separate Hf and La ALD cycles. Microstructure was determined with X-ray diffraction and cross-sectional transmission electron microscopy. The introduction of La increases the film crystallization temperature from 5 00 °C for a HfO₂ film to 800 °C, 900 °C and 950 °C for 10 nm films containing 13% La (metal basis), 25% La and 43% La, respectively. Due to the ALD growth mechanism, the film is a HfO₂-HfLa_xO_y periodic structure in which La just interacts with a limited thickness of HfO₂, and La-free layers mainly composed of HfO₂. The presence of periodic HfO₂ thin interval layers adds an extra advantage in amorphous stabilization during high temperature annealing over that found for homogeneous HfLa_xO_y mixtures and less overall La is required. Therefore ALD incorporating La is a potential method to grow amorphous HfO₂-La₂O₃ high-к dielectric thin films.