Residual Water Vapor Research Articles

Water Dose influence to the ALD hafnium oxide process: Simulation and experiment

In this work, we report a two-dimensional gas-dynamic model for the formation of hafnium oxide from TDMAH and water during the atomic layer deposition (ALD) process. Application of the model is demonstrated by the example of water cycle, in which the monolayer of functional groups –OH is formed. The simulation results show that the duration of the water pulse affects the dynamics of surface coverage with the -OH groups. At short pulse durations, the resulting coverage of the film surface with -OH groups becomes incomplete and has heterogeneous distribution over the film surface, which is due to the design of the reaction chamber and regimes of gas supply. The amount of residual water vapor depends on the duration of the water pulse and pumping regimes. Experimental data show the influence of water pulse duration on the refractive index, which is related to the structure of the deposited layer (composition, morphology, etc.). Based on experimental data and modeling results, we explain the effect of water pulse duration on the refractive index by the change in reaction mechanisms depending on water amount in the chamber. With a lack of water, the incomplete monolayer of –OH groups is formed and a {HfO2-y} layer grows as a result of the monomolecular decomposition of the precursor. With an excess of water, we expect the implementation of multilayer adsorption of water, provoking the reaction of the precursor in the volume of the water film, leading to the formation of the product {HfO2+y}.

Operando study of HfO2 atomic layer deposition on partially hydroxylated Si(111)

The introduction of atomic layer deposition (ALD), to the microelectronics industry has introduced a large number of new possible materials able to be deposited in layers with atomic thickness control. One such material is the high-κ oxide HfO2; thermally stable and ultrathin HfO2 films deposited by ALD are a significant contender to replace SiO2 as the gate oxide in capacitor applications. We present a mechanistic study of the first deposition cycle of HfO2 on the Si(111) surface using tetrakis(dimethylamido) hafnium (TDMAHf) and water as precursors using operando ambient pressure x-ray photoelectron spectroscopy. Here, we show that the hydroxylation of the clean Si(111) surface by residual water vapor, resulting in a 0.3 monolayer coverage of hydroxyls, leads to instantaneous full surface coverage of TDMAHf. The change in the atomic ratio of Hf to C/N found during the first deposition half-cycle, however, does not match the assumed immediate ligand loss through reaction with surface hydroxyls. One would expect an immediate loss of ligands, indicated by a Hf:N ratio of approximately 1:3 as TDMAHf deposits onto the surface; however, a Hf:N ratio of 1:3.6 is observed. The partial hydroxylation on the Si(111) surface leads to binding through the TDMAHf ligand N atoms resulting in both N and CH3 being found remaining on the surface post water half-cycle. Although there is evidence of ligand exchange reactions occurring at Si–OH sites, it also seems that N binding can occur on bare Si, highlighting the complexity of the substrate/precursor reaction even when hydroxyls are present. Moreover, the initial low coverage of Si–OH/Si–H appears to severely limit the amount of Hf deposited, which we hypothesize is due to the specific geometry of the initial arrangement of Si–OH/Si–H on the rest- and adatoms.

Multi-stepwise pulse calorimetry for enthalpy and emissivity measurements on tantalum, considering gas–metal reactions at elevated temperatures

Refractory metals such as tantalum are widely used for high-temperature applications; however, the thermophysical properties of tantalum are not accurately determined at temperatures approaching the melting point. Combining dynamic and static calorimetric methods, the enthalpy and total hemispherical emissivity of tantalum were determined within the 1000–3230 K temperature range. The combined approach considers the influence of reactions between the metal, minor impurities, and residual gas within a high-vacuum environment. The tantalum sample consistently absorbs residual water vapor within the vacuum chamber. At temperatures near the tantalum melting point, the absorbed hydrogen and oxygen atoms are desorbed. The kinetic nature of the gas–metal reactions determines the appropriate conditions for enthalpy and emissivity measurements via dynamic and static calorimetry, respectively. The enthalpy measured under dynamic conditions favorably agrees with the literature data derived from levitation drop calorimetry, rather than those from classical pulse calorimetry, especially near the melting point.

Dynamic adsorption behavior of 1.1.1.2-tetrafluoroethane (R134a) on activated carbon beds under different humidity and moisture levels

1.1.1.2-Tetrafluoroethane (R134a) is developed as a tracer for mechanical leakage detection in military chemistry. Due to the weaker physical adsorption properties, water vapor competitive adsorption limits the use of tracer R134a. Thus, it is urgent to the analyze dynamic competitive adsorption behavior of R134a and water vapor on the activated carbon. In this study, R134a breakthrough curves on the fixed beds packed with activated carbons under different relative humidity and moisture content levels were systematically measured. Combining mass spectrometry, gas chromatography, and a novel fiber Bragg grating temperature sensor, the dynamic competitive adsorption processes and axial temperature of fixed beds were first reported. Interestingly, with increasing moisture content, R134a breakthrough time decreased, but dynamic adsorption capacity increased. This was attributed to the fact that the residual water vapor could increase the specific surface area and pore volume of activated carbon to enhance R134a dynamic adsorption capacity. Using the R134a as the standard, the dynamic competitive adsorption process was first revealed and could be divided into four stages including Co-adsorption, Co-breakthrough, Competitive Adsorption (“Rolling Up” and “Adsorption Platform”), and Dynamic Adsorption Equilibrium. “Rolling Up” and “Adsorption Platform” phenomena are mainly determined by the strength of intermolecular interactions, the concentration difference of adsorbate, and the thermal effect induced by water vapor adsorption/desorption.

An in-situ study on surface coloring mechanism of a 12Cr ferrite/martensite stainless steel during vacuum heat treatment

Color changes during oxidation of a 12Cr ferrite/martensite stainless steel were investigated in-situ by using a high vacuum (5 × 10−3 Pa) heating stage where the high vacuum is provided by an oil diffusion pump. The color of the oxide film changed from initial metallic silver to brown, then consecutively to purple, liberty blue, blue-gray and finally to azure blue during the 90 min heat treatment at 690 °C in high vacuum. In contrast to the above changes, during oxidation at 1130 °C, the color changes from metallic silver to brown, then to various blue and finally changed back to metallic silver. The color as a result of thin-film interference mainly depended on the oxide film thickness. The significant reduction of the thickness of the oxide film on the sample vacuum heat-treated at 1130 ℃ is owing to the volatilization of oxide, which is a result of the reaction of the already formed Cr2O3 with residual water vapor and oxygen to generate CrO2(OH)2 and CrO3 gas, which evaporated from the surface at 1130 °C. Ferrite grains have a much lower oxidation kinetics than that of martensite.

Flue gas carbonation curing of steel slag blocks: Effects of residual heat and water vapor

To make full use of residual heat (temperature) and water vapor (relative humidity, RH) retained in flue gas, the compound effects of temperature (60–140 °C) and RH (2–60 %) of 2-hour CO2 curing on the compressive strength and CO2 fixation of basic oxygen furnace slag (BOFS) blocks were systematically investigated. The results indicated that, in general, an increase of curing temperature increased the compressive strength of the blocks. The high early compressive strength gain (31.3 MPa) at 2-hour for carbonated BOFS blocks was directly related to the microstructure densification and pore refinement due to rich formation of calcite. It should be noted that the supply of sufficient water vapor (≥40 % RH) is particularly important at high temperature (e.g. ≥ 120 °C) to offset the water loss due to water evaporation. Furthermore, under an extremely dry environment (2 % RH), the continuous flow-through of CO2 gas at high temperature not only resulted in no carbonation reaction at the outer layer of the freshly pressed blocks, but may also deteriorate the integrity of the pore structure matrix.

Mechanisms of trace water vapor desublimation over airfoil in transonic cryogenic wind tunnels

Cryogenic wind tunnels can provide a larger operating Reynolds number compared to conventional ones. However, the internationally known “moisture contamination problem” resulting from the residual trace water vapor desublimation in the cryogenic wind tunnel may affect the accuracy of the aerodynamic data. Due to extreme difficulties in experiments, the detailed trace water desublimation characteristics remain unrevealed. An Euler–Euler two-phase flow model based on classical nucleation theory and a droplet growth model were established to predict the trace water vapor spontaneous desublimation in the nitrogen flow over airfoil in a transonic cryogenic wind tunnel. The proposed model was validated by experimental data obtained from the literature and showed good agreements. The verified model was applied to 0.152-m National Advisory Committee for Aeronautics 0012 airfoil in a Langley 0.3-m transonic cryogenic wind tunnel under a series of operating conditions. The simulated results reveal that the water vapor desublimation process in a transonic cryogenic wind tunnel can be divided into two patterns by a critical region. In pattern I, over the upper critical total temperature, the water vapor desublimates quickly during the rapid expansion of gas flow over the airfoil surface. In pattern II, below the lower critical total temperature, the water vapor is sufficiently supercooled and desublimates completely into small ice particles in the free-stream flow. For pattern I, the desublimation characteristics were analyzed in detail, and the influences of the desublimation process on the aerodynamic data were quantitatively evaluated, which can provide theoretical guidance for the practical operation of transonic cryogenic wind tunnels.

Plasma surface treatment of polystyrene in a low power low frequency argon glow discharge

A low power low frequency (50 Hz) argon plasma was used to treat the surface of polystyrene (PS) films. Plasma treatment for 3 min produced the best hydrophilic enhancement and it corresponded to doubling of the surface energy. At 45 Pa and flowrate of 44 sccm, a ‘saturation’ region in terms of lowest water contact angle (WCA) attained (WCA = 21°±3°), occurred at low discharge power of 0.15–0.25 W (applied power of 3–8 W). The induced hydrophilicity was attributed mainly to the oxygen-containing functional groups incorporated on the PS surface during the plasma treatment. Oxygen was speculated to be sourced from the dissociation of residual water vapour in the reactor chamber. Upon exposure to the laboratory environment, rapid hydrophobic recovery of the treated samples occurred but WCA stabilized after 72 h and remained at >30° below its value at pristine condition.

Study of a Mode Separation Due to Polarization Existing in a Cavity-Enhanced Absorption Spectroscopy.

A special phenomenon of resonance mode separation is observed during the study of a high sensitivity folded-cavity enhanced absorption spectroscopy for the measurement of trace gases. The phenomenon affects the measurement of gas absorption spectrum in the cavity. This resonant mode separation phenomenon of the resonant cavity is different from the resonant modes previously observed in linear-cavity enhanced absorption spectroscopy systems. To explore the mechanism of this phenomenon, a series of hypotheses are proposed. The most likely reason among these hypotheses is based on the different reflectance properties of the plane mirror at the fold of the cavity for S-polarized light and P-polarized light. Based on the matrix calculation method, the different reflectance and phase shift of the plane mirror for S-polarized light and P-polarized light are analyzed theoretically, and the results are in better agreement with the phenomena observed in the experiment. Finally, in order to eliminate the resonant mode separation phenomenon, line polarizers were added. By improving the system, the cavity enhanced absorption spectrum of residual water vapor in the cavity was successfully measured, and a minimum detectable absorption coefficient of αmin = 7.6 × 10−9 cm−1 can be obtained in a single laser scan of 10 s.

Evaporation of Methylammonium Iodide in Thermal Deposition of MAPbI3.

Thermal evaporation is an important technique for fabricating methylammonium lead iodide (MAPbI3), but the process is complicated by the need to co-evaporate methylammonium iodide (MAI) and PbI2. In this work, the effect of water vapor during the thermal deposition of MAPbI3 was investigated under high vacuum. The evaporation process was monitored with a residual gas analyzer (RGA), and the film quality was examined with X-ray photoelectron spectroscopy (XPS). The investigations showed that during evaporation, MAI decomposed while PbI2 evaporated as a whole compound. It was found that the residual water vapor reacted with one of the MAI-dissociated products. The higher iodine ratio suggests that the real MAI flux was higher than the reading from the QCM. The XPS analysis demonstrated that the residual water vapor may alter the elemental ratios of C, N, and I in thermally deposited MAPbI3. Morphologic properties were investigated with atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). It was observed that a sample grown with high water vapor pressure had a roughened surface and poor film quality. Therefore, an evaporation environment with water vapor pressure below 10−8 Torr is needed to fabricate high quality perovskite films.

Development of a high-power EUV irradiation tool in a hydrogen atmosphere

Extreme ultraviolet (EUV) lithography has recently been utilized as a high-volume manufacturing technology for advanced semiconductors. An EUV mirror can be easily contaminated in the existence of a residual hydrocarbon vapor gas inside an exposure chamber in a vacuum environment, which reduces the reflectance of the Mo/Si multilayer coating. To reduce this carbon contamination, hydrogen gas is introduced at a pressure of a few pascals in the EUV scanner. However, during this process, the multilayer may be damaged by hydrogen. In addition, the multilayer surface can become oxidized by residual water vapor in the vacuum chamber. Therefore, an EUV irradiation tool in hydrogen and water vapor atmospheres was developed and installed at BL-09 of the NewSUBARU synchrotron light facility to evaluate the cleaning effect and irradiation durability of the Mo/Si multilayer. The EUV irradiation intensity increased up to 6 W cm−2, and the hydrogen pressure reached 70 Pa.

Chemical kinetic analysis of in-cylinder ion current generation under direct water injection within internal combustion Rankine cycle engine

Direct water injection provides feasible solution for combustion optimization and efficiency enhancement within internal combustion Rankine cycle engine, while the feedback signal of close-loop direct water injection control is still absent. Ion current detection monitors in-cylinder electron variation which shows potential in revealing direct water injection process. For better understanding of unprecedented augment of ion current signal under direct water injection within internal combustion Rankine cycle engine, a chemical kinetic model is established to calculate the effect of intake oxygen fraction, fuel quantity, initial temperature, and residual water vapor on in-cylinder electron formation based on GRI Mech 3.0 and ion current skeleton mechanism. The simulation results indicate direct water injection process show significant impact on in-cylinder electron formation through chemical interactions between H2O and other intermediate species including HO2, O2, CH3, and H, these reactions provides additional OH radical for propane oxidation facilitation, which result in large portion of CH radical formation and therefore, lead to higher in-cylinder electron generation. The initial temperature plays a vital role in determining whether residual water vapor show positive or negative effect by in-cylinder temperature co-ordination of direct water injection. Results of this work can be used to explain phenomenon related to direct water injection and ion current signal variation under both internal combustion Rankine cycle or traditional petrol engine.

Impact of Residual Water Vapor on the Simultaneous Measurements of Trace CH4 and N2O in Air with Cavity Ring-Down Spectroscopy

Methane (CH4) and nitrous oxide (N2O) are among the most important atmospheric greenhouse gases. A gas sensor based on a tunable 7.6 μm continuous-wave external-cavity mode-hop-free (EC-MHF) quantum cascade laser (from 1290 to 1350 cm−1) cavity ring-down spectroscopy (CRDS) technique was developed for the simultaneous detection of CH4 and N2O in ambient air with water vapor (H2O) mostly removed via molecular sieve drying to minimize the impact of H2O on the simultaneous measurements. Still, due to the broad and strong absorption spectrum of H2O in the entire mid-infrared (mid-IR) spectral range, residual H2O in the dried ambient air due to incomplete drying and leakage, if not properly accounted for, could cause a significant influence on the measurement accuracy of the simultaneous CH4 and N2O detection. In this paper, the impact of residual H2O on the simultaneous CH4 and N2O measurements were analyzed by comparing the CH4 and N2O concentrations determined from the measured spectrum in the spectral range from 1311 to 1312.1 cm−1 via simultaneous CH4 and N2O measurements and that determined from the measured spectrum in the spectral range from 1311 to 1313 cm−1 via simultaneous CH4, N2O, and H2O measurements. The measured dependence of CH4 and N2O concentration errors on the simultaneously determined H2O concentration indicated that the residual H2O caused an under-estimation of CH4 concentration and over-estimation of N2O concentration. The H2O induced CH4 and N2O concentration errors were approximately linearly proportional to the residual H2O concentration. For the measurement of air flowing at 3 L per min, the residual H2O concentration was stabilized to approximately 14 ppmv, and the corresponding H2O induced errors were −1.3 ppbv for CH4 and 3.7 ppbv for N2O, respectively.

Nitrogen Beams with a National Electrostatics Corporation Alphatross Source and a 5SDH Accelerator

Over the past 15 years, Hope College has been producing hydrogen and helium ion beams with an Alphatross® ion source and 5SDH Pelletron® tandem Van de Graaff accelerator. The manufacturer stated the possibility of creating nitrogen ions from this source, but Hope College has not, up until now, attempted to do so. By mixing approximately 1% nitrogen into hydrogen source gas, imidogen (NH-) and amidogen (NH2-) ions are created and accelerated through the tandem accelerator. Oxygen and hydroxide beams are also present due to residual water vapor in the source after maintenance. Post acceleration, these ion beams were directed into a scattering chamber by a dipole bending magnet for identification. Alternate beams such as these open up new possibilities for future experiments such as nitrogen implantation.

Effect of residual water vapor on the performance of indium tin oxide film and silicon heterojunction solar cell

In this study, we investigate the effect of residual H2O pressure (PH2O) on the structural, electrical, and optical properties of indium tin oxide (ITO) film and the current-voltage (I-V) performance of silicon heterojunction (SHJ) solar cells. The ITO films and SHJ solar cell were prepared under different PH2O (3.75 × 10−7 Pa, 6 × 10−7 Pa, 10.25 × 10−7 Pa) by varying the pumping time before starting the fabrication process. The experimental results revealed the crystallinity of ITO films was suppressed with the increase in PH2O. Further, the sheet resistance of ITO films was reduced from 103 Ω/sq to 60 Ω/sq due to the enhanced carrier mobility, and the transmittance in near- infrared (NIR) region exhibited a slight increase, but the conversion efficiency of SHJ solar cells dropped dramatically from 23.3% to 22.79% due to the degradation of fill factor (FF). The analysis on n-a-Si:H/ITO and ITO/Ag contact properties indicated that such FF degradation could be attributed to the increased specific contact resistance for the two contacts, when PH2O was high. Finally, SHJ solar cell with conversion efficiency of 23.53% was achieved by restraining the residual H2O vapor during ITO deposition along with the optimization of transmittance in NIR region by adjusting the amount of O2 flow.

The Development of a New Ion Versus Log(Ig) Plot to Characterize Depletion-Mode High Electron Mobility Transistors with the Insertion of a Very Thin Evaporated Al Layer to AlGaN/GaN High Electron Mobility Transistors as an Example

Previously, we showed that the Id_on vs. log Id_off plot can be used to characterize silicon-based enhancement-mode MOS transistors for both p-channel and n-channel [1]. AlGaN/GaN high electron mobility transistors (HEMTs) have attracted world-wide interest for microwave power transistor applications. There are two kinds of AlGaN/GaN HEMTs: depletion-mode and enhancement-mode AlGaN/GaN HEMTs. In general, depletion-mode AlGaN/GaN HEMTs are more common. For depletion-mode devices with the source at 0 V, even though the gate voltage may be close to zero, the gate leakage current increases with the increase of the drain voltage and can be significant because of a large drain voltage. Thus the gate leakage current may limit the maximum drain voltage and thus the RF power output of the device. For depletion-mode high electron mobility transistors (HEMTs), it can be easily imagined that both the on current (Ion) and the gate leakage current (Ig) will be increased by using a low work function metal as gate metal. Thus the drain on current (Id_on) and the gate leakage current (Ig) alone will not be a good criterion to judge the quality of depletion-mode HEMTs. In this paper, we would like to propose the application of a new Id_on vs. log Ig plot to characterize depletion-mode HEMTs. In the literature, a lot of various approaches have been proposed to cut down the gate leakage current. Recently, Nanjo et al. proposed to use a thin Al layer inserted between AlGaN and the Schottky metal gate to reduce the gate leakage current [2]. The advantage of this approach was subsequently confirmed by Selvaraj and Egawa [3]; however, they emphasized on the increase of the drain current instead of the reduction in the gate leakage current. It is natural to wonder how Al insertion can ever increase the drain current. Hence, the physics behind this approach is not yet clear. In this paper, we will also propose our theory regarding the physics of Al gate for AlGaN/GaN HEMT’s. With the help of this new plot, the superiority of the sample with Al/Ni/Au gate compared to the control sample with Ni/Au gate can be easily seen because the on current is higher for the same gate leakage current.As shown in Fig. 1, it can be easily shown that the Al insertion significantly improves the device for the same gate leakage current. According to theory, the Schottky barrier height φb is given by φb = φm - χs, where φm and χs are the metal work function and the electron affinity of the semiconductor. It is known that the work function of Al is significantly lower than that of Ni. It can be easily predicted that AlGaN/GaN HEMT with Al gate should have much larger gate leakage current compared to that with Ni gate. In fact, this was experimentally observed by us if the Al was evaporated rapidly during sample deposition. Our explanation of the advantage of Al insertion was that there was residual water vapor present in the vacuum chamber during electron beam evaporation [4]. According to Kordos et al. [5], the electron mobility of the 2-D electron gas at the AlGaN/GaN interface can be improved by a thin layer of Al2O3 on top of AlGaN/GaN. This experimental observation was also reported by Liu et al. [6]. It can be easily imagined that the presence of a thick layer of Al2O3 on top of AlGaN/GaN will degrade the performance of AlGaN/GaN HEMT because the distance between the metal gate and the AlGaN/GaN interface becomes bigger. However, when the thickness of the Al2O3 layer is small enough, the improvement of the electron mobility will be more significant than the degradation due to the increase of the distance between the metal gate and the AlGaN/GaN interface, resulting in the improved performance of the AlGaN/GaN HEMT.

In Situ Preparation of Pr1-xCaxMnO3 and La1-xSrxMnO3 Catalysts Surface for High-Resolution Environmental Transmission Electron Microscopy

The study of changes in the atomic structure of a catalyst under chemical reaction conditions is extremely important for understanding the mechanism of their operation. For in situ environmental transmission electron microscopy (ETEM) studies, this requires preparation of electron transparent ultrathin TEM lamella without surface damage. Here, thin films of Pr1-xCaxMnO3 (PCMO, x = 0.1, 0.33) and La1-xSrxMnO3 (LSMO, x = 0.4) perovskites are used to demonstrate a cross-section specimen preparation method, comprised of two steps. The first step is based on optimized focused ion beam cutting procedures using a photoresist protection layer, finally being removed by plasma-etching. The second step is applicable for materials susceptible to surface amorphization, where in situ recrystallization back to perovskite structure is achieved by using electron beam driven chemistry in gases. This requires reduction of residual water vapor in a TEM column. Depending on the gas environment, long crystalline facets having different atomic terminations and Mn-valence state, can be prepared.

Global modeling of wall material migration following boronization in NSTX-U

Abstract Boronization is commonly utilized in tokamaks to suppress intrinsic impurities, most notably oxygen from residual water vapor. However, this is a temporary solution, as oxygen levels typically return to pre-boronization levels following repeated plasma exposure. The global impurity migration model WallDYN has been applied to the post-boronization surface impurity evolution in NSTX-U. A “Thin Film Model” has been incorporated into WallDYN to handle spatially inhomogeneous conditioning films of varying thicknesses, together with an empirical boron conditioning model for the NSTX-U glow discharge boronization process. The model qualitatively reproduces the spatial distribution of boron in the NSTX-U vessel, the spatially-resolved divertor emission pattern, and the increase in oxygen levels following boronization. The simulations suggest that oxygen is primarily sourced from wall locations without heavy plasma flux or significant boron deposition, namely the lower and upper passive plates and the lower private flux zone.

Correcting atmospheric CO<sub>2</sub> and CH<sub>4</sub> mole fractions obtained with Picarro analyzers for sensitivity of cavity pressure to water vapor

Abstract. Measurements of dry air mole fractions of atmospheric greenhouse gases are used in inverse models of atmospheric tracer transport to quantify their sources and sinks. The measurements have to be calibrated to a common scale to avoid bias in the inferred fluxes. For this purpose, the World Meteorological Organization (WMO) has set requirements for the interlaboratory compatibility of atmospheric greenhouse gas (GHG) measurements. A widely used series of devices for these measurements are the GHG analyzers manufactured by Picarro, Inc. These are often operated in humid air, and the effects of water vapor are corrected for in post-processing. Here, we report on rarely detected and previously unexplained biases of the water correction method for CO2 and CH4 in the literature. They are largest at water vapor mole fractions below 0.5 % H2O, which were undersampled in previous studies, and can therefore affect measurements obtained in humid air. Setups that dry sample air using Nafion membranes may be affected as well if there are differences in residual water vapor levels between sample and calibration air. The biases are caused by a sensitivity of the pressure in the measurement cavity to water vapor. We correct these biases by modifying the water correction method from the literature. Our method relies on experiments that maintain stable water vapor levels to allow equilibration of cavity pressure. In our experiments with the commonly used droplet method, this requirement was not fulfilled. Correcting CO2 measurements proved challenging, presumably because of our humidification method. Open questions pertain to differences among analyzers and variability over time. In our experiments, the biases amounted to considerable fractions of the WMO interlaboratory compatibility goals. Since measurements of dry air mole fractions of CO2 and CH4 are also subject to other uncertainties, correcting the cavity pressure-related biases helps keep the overall accuracy of measurements obtained with Picarro GHG analyzers in humid and potentially in Nafion-dried air within the WMO goals.

Mechanistic study of the atomic layer deposition of scandium oxide films using Sc(MeCp)2(Me2pz) and ozone

The atomic layer deposition (ALD) of scandium oxide (Sc2O3) thin films is investigated using Sc(MeCp)2(Me2pz) (1, MeCp = methylcyclopentadienyl, Me2pz = 3,5-dimethylpyrazolate) and ozone on hydroxyl-terminated oxidized Si(111) substrates at 225 and 275 °C. In situ Fourier transform infrared spectroscopy reveals that 1 not only reacts with surface hydroxyl groups at 275 °C, as expected but also with the SiO2 layer, as evidenced by losses in the SiO2 longitudinal optical and transverse optical phonon modes, resulting in the partial transformation of near-surface SiO2 to an ScSixOy interface layer. Ozone then combusts the MeCp groups of the O–Sc(MeCp)2 chemisorbed species, yielding surface carbonates, and oxidizes some of the underlying silicon, evidenced by gains in the SiO2 phonon modes. The Me2pz group from the next pulse of 1 reacts with these surface carbonates, leading to Sc–O–Sc bond formation (Sc2O3 deposition) and the restoration of an O–Sc(MeCp)2 surface. The reaction of the SiO2 substrate with 1 and the oxidation of silicon by ozone are temperature-dependent processes that occur during the initial cycles of film growth and directly impact the changes in the intensities of the SiO2 phonon modes. For instance, the intensity of the net gains in the phonon modes following ozone exposure is greater at 275 °C than at 225 °C. As the ALD cycle is repeated, the formation of an ScSixOy interface layer and deposition of an Sc2O3 film result in the gradual attenuation of the reaction of the SiO2 substrate with 1 and the oxidation of the underlying silicon by ozone. In addition to the ALD process, characterized by ligand exchange and self-limiting reactions, there are gas-phase reactions between 1 and residual water vapor near the substrate surface that lead to deposition of additional Sc2O3 and surface carbonates, the extent of which are also dependent on the temperature of the substrate. After 20 cycles of 1/ozone, the film thicknesses derived from ex situ X-ray photoelectron spectroscopy measurements are 2.18 nm (225 °C) and 3.88 nm (275 °C). This work constitutes the first mechanistic study of an Sc2O3 ALD process using ozone as the oxidant and emphasizes the significance of atypical reactions between the substrate and the reactants that influence the growth rate and near-surface stoichiometry during the initial cycles of film deposition.