The atomic layer deposition (ALD) of Al 2 O 3 using sequential exposures of Al(CH 3 ) 3 and O 3 was studied by in situ transmission Fourier transform infrared (FTIR) spectroscopy and quadrupole mass spectrometry (QMS). The FTIR spectroscopy investigations of the surface reactions occurring during Al 2 O 3 ALD were performed on ZrO 2 particles for temperatures from 363 to 650 K. The FTIR spectra after Al(CH 3 ) 3 and ozone exposures showed that the ozone exposure removes surface AlCH 3 * species. The AlCH 3 * species were converted to AlOCH 3 * (methoxy), Al(OCHO)* (formate), Al(OCOOH)* (carbonate), and AlOH* (hydroxyl) species. The TMA exposure then removes these species and reestablishes the AlCH 3 * species. Repeating the TMA and O 3 exposures in a sequential reaction sequence progressively deposited the Al 2 O 3 ALD film as monitored by the increase in absorbance for bulk Al 2 O 3 infrared features. The identification of formate species was confirmed by separate formaldehyde adsorption experiments. The formate species were temperature dependent and were nearly absent at temperatures g650 K. QMS analysis of the gas phase species revealed that the TMA reaction produced CH 4 . The ozone reaction produced mainly CH 4 with small amounts of C 2 H 4 (ethylene), CO, and CO 2 . Transmission electron microscopy (TEM) was also used to examine the Al 2 O 3 ALD films deposited on the ZrO 2 particles. These TEM images observed conformal Al 2 O 3 ALD films with thicknesses that were consistent with an Al 2 O 3 ALD growth rate of 1.1 Å/cycle. The surface species after the O 3 exposures and the mass spectrometry results lead to a very different mechanism for Al 2 O 3 ALD growth using TMA and O 3 compared with Al 2 O 3 ALD using TMA and H 2 O.

Metal atomic layer deposition (ALD) on oxides can display long nucleation periods and high growth temperatures that may be caused by surface poisoning by reaction products. Exposures of trimethylaluminum (TMA) during Pd ALD using Pd(hfac)2 and formalin on Al2O3 surfaces can shorten the nucleation period and reduce the growth temperatures. Fourier transform infrared spectroscopy studies indicate that TMA removes Al(hfac)∗ species that block surface sites. Pd ALD nucleates more readily and grows at lower temperatures because higher temperatures are not needed to desorb Al(hfac)∗ species. Transmission electron microscopy analysis shows the differences between Pd ALD films deposited with and without TMA.

In2S3 atomic layer deposition (ALD) with indium acetylacetonate (In(acac)3) and H2S was studied with quartz crystal microbalance (QCM), X-ray reflectivity (XRR), and Fourier transform infrared (FTIR) spectroscopy techniques. Subsequent In2S3 ALD on TiO2 nanotube arrays defined a model semiconductor sensitized solar cell. For In2S3 ALD on initial Al2O3 ALD surfaces, the In2S3 ALD displayed a nucleation period of ∼60−70 cycles followed by a linear growth region. These results were obtained under ALD conditions that were not completely self-limiting for the In(acac)3 exposure because of the low In(acac)3 vapor pressure. The growth per cycle decreased at higher temperature over the temperature range from 130 to 170 °C at these same reactant conditions. The growth per cycle was 0.30−0.35 Å per cycle at 150 °C as determined by QCM and XRR measurements at higher In(acac)3 exposures where the surface reactions were self-limiting chemistry versus In(acac)3 and H2S exposures. The FTIR examinations revealed that the nucleation period on Al2O3 ALD surfaces may be related to the formation of Al(acac)* species that act to poison the initial Al2O3 ALD surface. X-ray diffraction investigations revealed β-In2S3 ALD films and X-ray photoelectron measurements were consistent with In2S3 films. The In2S3 ALD was employed as a semiconductor sensitizer on TiO2 nanotube arrays for solar conversion. Scanning electron microscopy and energy dispersive X-ray analysis imaging revealed In2S3 over the full length of the TiO2 nanotube array after 175 cycles of In2S3 ALD at 150 °C at reactant exposure conditions that were self-limiting on flat substrates. The photoelectrochemical properties of these In2S3 ALD-sensitized TiO2 nanotube arrays with a Co2+/Co3+ electrolyte were then characterized by measuring the photocurrent density versus voltage and the external quantum efficiency versus photon energy. A small quantum efficiency of ∼10% was observed that can be attributed to charge recombination losses and charge injection/collection processes.

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