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The ferroelectric properties of hafnium oxide and zirconium oxide based thin films are promising for applications in low power electronics, such as ultra-thin ferroelectric tunneling devices. However, the amount of ferroelectric phase in the film depends on their polycrystalline morphology, which changes with film thickness. Therefore, controlling the film thickness without changing the ferroelectric properties has remained challenging. Here, we propose the use of thermal atomic layer etching to decouple the ferroelectric phase stabilization from the film thickness. First, the ferroelectric phase fraction is maximized by crystallizing the film at an optimized film thickness. Subsequently, the ferroelectric film thickness is reduced to the desired range by atomic layer etching. We demonstrate the feasibility of this approach for a ferroelectric hafnium zirconium oxide film of 10 nm initial thickness, which we integrate into a double-layer ferroelectric tunnel junction. The atomic layer etch rate of ferroelectric hafnium zirconium oxide using HF and dimethylaluminum chloride is found to be ∼0.2 Å/cycle. Although the ferroelectric phase persists after atomic layer etching, the etching increases the surface roughness. For applications in ferroelectric tunnel junctions, we show that atomic layer etching of ferroelectric hafnium zirconium oxide can improve the read current by more than a factor of 200, while at the same time reducing the read voltage by 43%. The resulting tunneling electroresistance of about 2500 is the highest reported so far for polycrystalline hafnium zirconium oxide-based materials. [ABSTRACT FROM AUTHOR]

The nucleation and growth of Pt atomic layer deposition (ALD) on Al2O3 substrates was studied using (methylcyclopentadienyl)-trimethyl platinum (MeCpPtMe3) and O2 plasma as the reactants. The nucleation of Pt ALD was examined on Al2O3 ALD substrates at 300 °C using a variety of techniques including spectroscopic ellipsometry, x-ray reflectivity, x-ray photoelectron spectroscopy, and scanning electron microscopy. These techniques revealed that Pt ALD does not nucleate and grow immediately on the Al2O3 ALD substrates. There was negligible Pt ALD during the first 38 ALD cycles. The Pt ALD growth rate then increased substantially during the next 12 ALD cycles. Subsequently, the Pt ALD growth rate reached a steady state linear growth regime for >50 ALD cycles. These measurements suggest that the Pt ALD first forms a number of nanoclusters that grow slowly during the first 38 ALD cycles. These islands then merge during the next 12 cycles and yield a steady state Pt ALD growth rate of ∼0.05 nm/cycle for >50 ALD cycles. The Pt ALD film at the onset of the steady state linear growth regime was approximately 2-3 nm in thickness. However, the SEM images of these Pt ALD films appeared corrugated and wormlike. These films also had a density that was only 50-70% of bulk Pt. Film densities that were consistent with bulk Pt were not observed until after >100 ALD cycles when the Pt ALD films appeared much smoother and were 4-5 nm in thickness. The Pt ALD nucleation rate could be enhanced somewhat using different O2 plasma parameters. [ABSTRACT FROM AUTHOR]

Analysis of steady-state and transient photoconductivity measurements at room temperature performed on c-axis oriented GaN nanowires yielded estimates of free carrier concentration, drift mobility, surface band bending, and surface capture coefficient for electrons. Samples grown (unintentionally n-type) by nitrogen-plasma-assisted molecular beam epitaxy primarily from two separate growth runs were examined. The results revealed carrier concentration in the range of (3–6)×1016 cm-3 for one growth run, roughly 5×1014–1×1015 cm-3 for the second, and drift mobility in the range of 500–700 cm2/(V s) for both. Nanowires were dispersed onto insulating substrates and contacted forming single-wire, two-terminal structures with typical electrode gaps of ≈3–5 μm. When biased at 1 V bias and illuminated at 360 nm (3.6 mW/cm2) the thinner (≈100 nm diameter) nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA (noise level) to 0.1–1 μA. Under the same conditions, thicker (151–320 nm) nanowires showed roughly ten times more photocurrent, with dark currents ranging from 2 nA to 1 μA. With the light blocked, the dark current was restored in a few minutes for the thinner samples and an hour or more for the thicker ones. The samples with lower carrier concentration showed similar trends. Excitation in the 360–550 nm range produced substantially weaker photocurrent with comparable decay rates. Nanowire photoconductivity arises from a reduction in the depletion layer via photogenerated holes drifting to the surface and compensating ionized surface acceptors. Simulations yielded (dark) surface band bending in the vicinity of 0.2–0.3 V and capture coefficient in the range of 10-23–10-19 cm2. Atomic layer deposition (ALD) was used to conformally deposit ≈10 nm of Al2O3 on several devices. Photoconductivity, persistent photoconductivity, and subgap photoconductivity of the coated nanowires were increased in all cases. TaN ALD coatings showed a reduced effect compared to the Al2O3 coated samples. [ABSTRACT FROM AUTHOR]

Thin films grown by Al2O3 atomic layer deposition (ALD) and SiN plasma-enhanced chemical vapor deposition (PECVD) have been tested as gas diffusion barriers either individually or as bilayers on polymer substrates. Single films of Al2O3 ALD with thicknesses of ≥10 nm had a water vapor transmission rate (WVTR) of ≤5×10-5 g/m2 day at 38 °C/85% relative humidity (RH), as measured by the Ca test. This WVTR value was limited by H2O permeability through the epoxy seal, as determined by the Ca test for the glass lid control. In comparison, SiN PECVD films with a thickness of 100 nm had a WVTR of ∼7×10-3 g/m2 day at 38 °C/85% RH. Significant improvements resulted when the SiN PECVD film was coated with an Al2O3 ALD film. An Al2O3 ALD film with a thickness of only 5 nm on a SiN PECVD film with a thickness of 100 nm reduced the WVTR from ∼7×10-3 to ≤5×10-5 g/m2 day at 38 °C/85% RH. The reduction in the permeability for Al2O3 ALD on the SiN PECVD films was attributed to either Al2O3 ALD sealing defects in the SiN PECVD film or improved nucleation of Al2O3 ALD on SiN. [ABSTRACT FROM AUTHOR]

Nucleation phenomena are critical for the fabrication of W/Al2O3 nanolaminates using atomic layer deposition (ALD) techniques. The nucleation and growth of W ALD on hydroxylated Al2O3 ALD surfaces and Al2O3 ALD on fluorinated W ALD surfaces was studied using in situ quartz crystal microbalance (QCM) and ex situ atomic force microscope (AFM) techniques. The QCM investigations revealed that Al2O3 ALD readily nucleated on the fluorinated W surface and displayed “substrate-enhanced growth.” In contrast, W ALD required 4–10 ALD cycles to nucleate on the hydroxylated Al2O3 surface and displayed “substrate-inhibited growth.” The W ALD nucleation period was shorter for higher Si2H6 and WF6 reactant exposures. The most rapid nucleation of W ALD on the Al2O3 surface occurred with much larger Si2H6 and WF6 exposures on the initial ALD cycle with the WF6 exposure prior to the Si2H6 exposure. By analyzing the individual Si2H6 and WF6 mass gain per cycle (MGPC), three main regions were identified in the W ALD nucleation and growth: initial deposition on Al2O3, W island growth and coalescence, and steady state growth. The root mean square (rms) roughness of the resulting W ALD film was dependent on the Si2H6 exposures and the number of ALD cycles required to nucleate the W ALD. A linear dependence was observed between the rms roughness and the number of ALD cycles required to reach one-half the maximum W MGPC. The W ALD also displayed very periodic oscillations in the W MGPC that were consistent with island nucleation and growth. Four local minima and three local maxima were observed in the W MGPC versus the number of ALD cycles. Comparing the results for W ALD on Al2O3 surfaces with recent simulations of ALD nucleation helps to establish the relationship between the nucleation period and surface roughness with island growth during nucleation. [ABSTRACT FROM AUTHOR]

The adsorption kinetics for ethylsilane (ES), diethylsilane (DES), and diethylgermane (DEG) on Si(111) 7×7 were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The initial reactive sticking coefficients were determined as a function of surface temperature using LITD measurements. In these experiments, the ethyl coverage vs adsorption time was monitored using CH2=CH2 (ethylene) LITD signals that were produced by the β-hydride elimination of the surface ethyl groups, e.g. Si–CH2CH3(ad)→Si–H(ad)+CH2=CH2(g). The initial reactive sticking coefficients were S0≊2×10-3, 4×10-3, and 5×10-2 for DES, ES, and DEG, respectively, at 200 K. As expected from a precursor-mediated adsorption model, the initial reactive sticking coefficients were observed to decrease with increasing surface temperature. Experiments with preadsorbed hydrogen also demonstrated that the initial reactive sticking coefficients of DES and DEG were reduced as a function of hydrogen coverage. This behavior indicated that alkylsilane and alkylgermane adsorption on Si(111) 7×7 requires free dangling bond sites. LITD experiments revealed that the ethyl surface coverage saturated after large exposures. The saturation coverage corresponded to a deposited Si or Ge coverage of Θ=0.13 monolayer for DES and DEG and was independent of surface temperature between 200–400 K. DEG adsorption cycles were used to deposit increasing amounts of Ge on Si(111) 7×7. The deposited coverage was examined using H2 TPD studies which indicated that the germanium may be forming islands. LITD experiments were also used to monitor ethyl surface diffusion on Si(111) 7×7 after DES adsorption. No evidence of significant ethyl surface mobility (D≤1.0×10-10 cm2/s) was found for surface temperatures as high as 600 K. [ABSTRACT FROM AUTHOR]

The surface diffusion of n-butane on a stepped ruthenium {Ru(S)-[15(001)×2(100)]} surface was studied using laser-induced thermal desorption (LITD) techniques. Large aspect ratio laser desorption areas were used to measure the anisotropy of n-butane diffusion on the stepped Ru(001) surface. Surface diffusion coefficients at Θ=Θsat for diffusion parallel (D||) or perpendicular (D⊥) to the steps were measured by orienting these desorption areas either perpendicular or parallel to the step edges. At 115 K, the surface diffusion coefficient was much larger in the direction parallel to the step edges and D||/D⊥≊26. Surface diffusion coefficients for diffusion on a terrace (Dt) or over a step (Ds) were deconvoluted from the measured D|| and D⊥ values. The temperature-dependent D|| and D⊥ values yielded an activation barrier for n-butane diffusion on the Ru(001) terrace at Θ=Θsat of Et=2.8±0.4 kcal/mol and a diffusion preexponential of D0,t=1.6×10-2±0.2 cm2/s. For diffusion over a step at Θ=Θsat, the surface diffusion barrier was Es=4.8±0.5 kcal/mol and the diffusion preexponential was D0,s=6.1×10-1±0.3 cm2/s. The measured corrugation ratio for n-butane on the Ru(S)-[15(001)×2(100)] surface was Ω≡Edif/Edes=0.24 for diffusion on the terrace and Ω=0.41 for diffusion over a step. The surface corrugation ratio on the terrace of Ru(S)-[15(001)×2(100)] was similar to Ω=0.29 measured previously for n-butane on Ru(001). This similarity indicated that step defects did not dominate previous LITD measurements of alkanes on Ru(001). [ABSTRACT FROM AUTHOR]

Laser-induced thermal desorption (LITD) techniques were used to study the adsorption, desorption, and surface diffusion kinetics of NH3 on MgO(100) single-crystal surfaces. Isothermal LITD adsorption measurements revealed that the sticking coefficient of NH3 on MgO(100) was constant vs coverage and decreased with increasing surface temperature. The adsorption kinetics were consistent with a mobile precursor intermediate. In addition, the saturation NH3 coverage on MgO(100) was strongly dependent on temperature, and decreased by a factor of 5 as temperature increased from 130 to 165 K. Isothermal and linear temperature ramp LITD experiments indicated that the desorption kinetics of NH3 from MgO(100) could be modeled using a first-order rate law with a coverage-dependent desorption activation energy. The coverage dependence of the desorption activation energy was consistent with repulsive lateral interactions between NH3 adsorbates on the MgO(100) surface. The surface diffusion of NH3 on MgO(100) was also examined using LITD techniques. In contrast to recent predictions of NH3 surface diffusion on MgO(100), no evidence of NH3 surface mobility (D≤10-9 cm2/s) was observed for temperatures as high as 165 K. [ABSTRACT FROM AUTHOR]

The surface diffusion and desorption kinetics for perfluoro-n-butane on Ru(001) were examined using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The surface diffusion displayed Arrhenius behavior and was coverage independent. The surface diffusion parameters for perfluoro-n-butane on Ru(001) were Edif=2.9±0.3 kcal/mol and D0=5.9×10-2±0.2 cm2/s. The desorption parameters for perfluoro-n-butane on Ru(001) were Edes=13.8±0.6 kcal/mol and νdes=2.8×1021±0.1 s-1. In comparison, the surface diffusion parameters for n-butane on Ru(001) were Edif=3.5±0.2 kcal/mol and D0=1.4×10-1±0.2 cm2/s. The desorption parameters for n-butane on Ru(001) were Edes=11.9±0.5 kcal/mol and νdes=3.6×1015±0.1 s-1. The corrugation ratio, defined as Ω≡Edif/Edes, was determined to be Ω=0.21 for perfluoro-n-butane on Ru(001). This corrugation ratio was substantially different than the corrugation ratio of Ω≊0.30 measured for n-butane and various other n-alkanes, cycloalkanes and branched alkanes on Ru(001). The comparison between perfluoro-n-butane and the other alkanes indicates that fluorination lowers the surface corrugation ratio on Ru(001). Likewise, fluorination significantly increases the preexponential for desorption from Ru(001). This study illustrates the magnitude of substituent effects on surface diffusion and desorption kinetics for a physisorbed molecule on a single-crystal metal surface. [ABSTRACT FROM AUTHOR]

The adsorption and desorption kinetics for SiCl4 on Si(111)7×7 were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. TPD experiments monitored SiCl2 as the desorption product at approximately 950 K using a heating rate of β=9 K/s. SiCl2 was also observed as the desorption product in the LITD yield at all surface coverages. LITD measurements determined the initial reactive sticking coefficient (S0) of SiCl4 on Si(111)7×7 versus surface temperature. The sticking coefficient was observed to decrease with surface temperature from S0≊0.18 at 160 K to S0≊0.03 at 600 K. TPD experiments revealed that the chlorine surface coverage saturated after large SiCl4 exposures and the saturation coverage was independent of surface temperature. Isothermal LITD studies enabled the surface chlorine coverage to be monitored as a function of time during SiCl2 desorption. These studies revealed second-order desorption kinetics for SiCl2 with a desorption activation barrier of Ed=67±5 kcal/mol and preexponential of νd=3.2×100±0.1 cm2/s. Experiments with preadsorbed hydrogen demonstrated that the initial sticking coefficient of SiCl4 was reduced as a function of increasing hydrogen coverage. This behavior indicated that the adsorption of SiCl4 on Si(111)7×7 requires free surface dangling bonds. LITD techniques were also used to study the surface diffusion of the chlorine on Si(111)7×7 after SiCl4 adsorption. No evidence of significant surface chlorine mobility (D≤2×10-11 cm2/s) was found for surface temperatures as high as 825 K. [ABSTRACT FROM AUTHOR]

The fluorescence from disordered phenanthrene adlayers on Al2O3(0001) was examined in ultrahigh vacuum at 20 K using laser-induced fluorescence techniques. The fluorescence spectra was consistent with emission from a phenanthrene excimer. The fluorescence lifetime of the phenanthrene excimer was studied versus phenanthrene coverage and versus distance from the Al2O3(0001) surface using xenon spacers. As a function of coverage on Al2O3(0001), the fluorescence lifetime decreased from τ=34±0.5 ns at phenanthrene coverages of Θ≥20 ML to τ=7±0.5 ns at Θ=1 ML. As a function of xenon spacer distance from the Al2O3(0001) surface, the fluorescence lifetime also decreased from τ=34±0.5 ns at distances of d>100 Å to τ=7±0.5 ns at d=7 Å. Fluorescence measurements versus phenanthrene coverage on xenon, butane, acetonitrile and methanol multilayers revealed that the phenanthrene excimer fluorescence lifetime was constant at τ≊35 ns on the molecular multilayer surfaces. These results indicated that the reduction of the fluorescence lifetime was particular to the Al2O3(0001) surface.The fluorescence lifetimes versus phenanthrene coverage and xenon spacer distance on Al2O3(0001) were equivalent when the phenanthrene coverage was converted to total adlayer thickness. This correspondence suggested that the excited electronic energy in disordered phenanthrene adlayers transfers rapidly to phenanthrene excimers at the phenanthrene–vacuum interface. Subsequently, competition occurs between fluorescence quenching by the Al2O3(0001) surface and phenanthrene excimer fluorescence. In addition, fluorescence lifetime measurements vs phenanthrene coverage on CaF2 thin films displayed similar fluorescence quenching. Fluorescence lifetimes versus phenanthrene coverage on O2 molecular multilayers also revealed fluorescence quenching that was attributed to a charge-transfer mechanism. The observed... [ABSTRACT FROM AUTHOR]

The surface diffusion of n-alkanes on Ru(001) was measured using laser-induced thermal desorption (LITD) techniques. The surface diffusion coefficients for propane, n-butane, n-pentane, and n-hexane all displayed Arrhenius behavior. The surface diffusion activation energies increased linearly with carbon chain length from Edif =3.0±0.1 kcal/mol for propane to Edif =4.8±0.2 kcal/mol for n-hexane. In contrast, the surface diffusion preexponentials remained nearly constant at D0 [bar_over_tilde:_approx._equal_to]0.15 cm2 /s. Measurements performed at different coverages also revealed that the surface diffusion coefficients were coverage-independent for all the n-alkanes on Ru(001). The surface corrugation ratio Ω was defined as the ratio of the diffusion activation energy to the desorption activation energy, Ω=Edif /Edes . The surface corrugation ratio was observed to be remarkably constant at Ω[bar_over_tilde:_approx._equal_to]0.3 for all the n-alkanes. This constant corrugation ratio indicated a linear scaling between the diffusion activation energy and the desorption activation energy. This behavior also suggested that the n-alkanes move with a rigid configuration parallel to the Ru(001) surface. [ABSTRACT FROM AUTHOR]

The desorption of CO from clean and sulfur-covered Ru(001) surfaces was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. CO was observed to desorb from clean Ru(001) with coverage-dependent kinetics. The isothermal desorption of CO was monitored with LITD measurements. The rates for CO desorption were determined using a simple Padé approximant method to evaluate coverage-dependent kinetic parameters. On the clean Ru(001) surface, the desorption activation energy and preexponential dropped sharply from Ed=34 kcal/mol and νd=5×1015s-1 for ΘCO<0.33 ML to Ed=23 kcal/mol and νd=2×1012s-1, for ΘCO>0.33 ML. The clean Ru(001) surface results agreed very well with earlier studies of CO desorption. The presence of surface sulfur shifted the TPD peaks for CO on Ru(001) to lower temperatures. Likewise, isothermal LITD measurements revealed that the CO desorption parameters at ΘCO=0.06 ML decreased from Ed=36 kcal/mol and νd=1×1016s-1 to Ed=22 kcal/mol and νd=5×1010s-1 as the sulfur coverage increased from ΘS=0.00 to 0.24 ML. The kinetic parameters derived from the isothermal LITD experiments were able to predict accurately the CO TPD spectra as a function of CO coverage, sulfur coverage and heating rate. The dramatic effect of sulfur on the CO desorption kinetics at low sulfur coverages argued for a long-ranged electronic effect for sulfur interacting with CO on Ru(001). [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence (LIF) and electronic absorption spectroscopy (EAS) were used to examine pyrene adsorbed on Al2O3(1120) as a function of surface coverage, temperature, and adlayer order. Temperature-programmed LIF and EAS techniques were employed to measure the desorption kinetics of pyrene from Al2O3(1120). An activation energy of Ed=18±1 kcal/mol was obtained for pyrene desorption in the submonolayer regime assuming a first-order desorption pre-exponential of ν1=1×1013 s-1. Multilayer desorption studies revealed zero-order desorption kinetics with an activation energy of Ed =20±2 kcal/mol and a pre-exponential of ν0=3×1026±1.5 molecules/cm2 s. The formation of pyrene excimers and the structure of the pyrene adlayer were also investigated using LIF and EAS techniques. The LIF spectra for pyrene adlayers on Al2O3(1120) at 21 K exhibited both monomer and excimer emission. The relative amount of excimer emission at 21 K increased vs pyrene coverage from Θ=0.30 monolayers (ML) to Θ=4.5 ML. In addition, the excimer emission increased and the monomer emission decreased vs surface temperature between 21 and 150 K at all measured coverages. The temperature and coverage dependence of the LIF spectra suggested that excimer formation was dependent on pyrene mobility. In support of a mobility mechanism for excimer formation, LIF studies demonstrated that the excimer emission at Θ=0.25 ML was quenched by xenon adsorption on the pyrene adlayer. LIF and EAS measurements also indicated that the pyrene adlayer crystallized when the adlayer was annealed above 230 K. The LIF spectra of the crystalline pyrene adlayers on the Al2O3(1120) surface displayed only excimer emission. [ABSTRACT FROM AUTHOR]

The resonant desorption of butane from Al2O3(1120) was studied using tunable infrared radiation from an infrared free electron laser. Resonant desorption was induced by exciting C–H stretching vibrations in butane adlayers at coverages of θ=1–7 ML of Al2O3(1120) at 90 K. The resonant desorption spectrum did not duplicate the infrared absorption spectrum of solid butane. In contrast, the asymmetric C–H stretches produced a greater desorption yield than expected in comparison to the symmetric C–H stretches. The greater desorption efficiency of the asymmetric C–H stretches was attributed to the orientation of butane molecules in an ordered adlayer on Al2O3(1120). The proposed orientation in this adlayer is butane in an all-trans configuration with the carbon backbone perpendicular to the surface. The resonant desorption yield increased approximately linearly with laser energy and surface coverage. Isotope experiments were also performed with equal amounts of butane-h10 and butane-d10 on the surface. Equivalent amounts of protonated and deuterated species were observed to desorb after excitation of butane-h10. The results were all consistent with a thermal mechanism for the resonant desorption of butane from Al2O3(1120). In this thermal mechanism, vibrational energy deposited in the C–H stretching vibrations is thermalized quickly and leads to resonant heating of the butane adlayer and subsequent thermal desorption. [ABSTRACT FROM AUTHOR]

The effects of coadsorbed carbon monoxide on the surface diffusion of hydrogen on Ru(001) were studied using laser-induced thermal desorption techniques. The surface mobility of hydrogen was measured as a function of CO surface coverage at 260 and 280 K. At both temperatures, the surface diffusion of hydrogen displayed an abrupt reduction at a coadsorbed CO coverage of ΘCO =0.12 ML. LEED studies revealed that a CO coverage of ΘCO =0.12 ML corresponded to the onset of the formation of ordered [Square_Root]3×[Square_Root]3 CO islands at 260 and 280 K. Temperature programmed desorption results demonstrated that a lateral repulsive interaction exists between hydrogen and carbon monoxide on Ru(001). Assuming that the lateral repulsive interaction leads to a hydrogen exclusion area around each CO admolecule, a hydrogen exclusion radius of rCO =2.2–2.7 Å was determined. The lateral repulsive interaction would also result in hydrogen exclusion from the interior of ordered [Square_Root]3×[Square_Root]3 CO islands and provide an explanation for the hydrogen surface diffusion results. According to this interpretation, the rigid [Square_Root]3×[Square_Root]3 CO islands obstruct the hydrogen surface diffusion pathways and thereby reduce the hydrogen surface mobility on Ru(001). [ABSTRACT FROM AUTHOR]

The desorption of hydrogen and deuterium from Si(111) 7×7 was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) mass spectrometry. Isothermal LITD measurements enabled the surface coverage of hydrogen and deuterium to be monitored as a function of time. These isothermal results were used to obtain accurate desorption kinetics of hydrogen and deuterium from the high-temperature β1 state on Si(111) 7×7. The desorption of hydrogen displayed second-order kinetics with an activation barrier of 61±4 kcal/mol and a preexponential factor of 1.2×101±1.3 cm2/s. Likewise, the desorption kinetics of deuterium displayed second-order kinetics with an activation barrier of 59±3 kcal/mol and a preexponential factor of 2.8×100±1.0 cm2/s. These desorption activation barriers yield upper limits of 82.6 and 81.6 kcal/mol for the Si–H and Si–D chemical bond energies, respectively, on Si(111) 7×7. TPD results obtained as a function of hydrogen coverage were consistent with second-order desorption kinetics. The TPD experiments were also used to measure hydrogen coverages and to calibrate the LITD signals. In addition, LITD techniques were used to study the surface diffusion of hydrogen on Si(111) 7×7. No evidence of significant hydrogen surface mobility (D≤10-9 cm2/s) was found for surface temperatures as high as 740 K. [ABSTRACT FROM AUTHOR]

A kinetic lattice gas model is applied to study collective surface diffusion of adsorbates on inhomogeneous surfaces. The diffusion coefficient is extracted from Monte Carlo (MC) simulations by observing the decay of the autocorrelation functions for density fluctuations. Calculations are presented for diffusion on a surface with various different coverages of randomly placed blocks and to diffusion on a surface with various distributions of traps of different binding energies. In the cases where analytical expressions for the collective diffusion coefficient can be derived, the MC results show excellent agreement with analytical predictions. [ABSTRACT FROM AUTHOR]

The decomposition reaction of methanol on Ru(001) was studied using laser induced thermal desorption (LITD). The LITD studies, combined with temperature programmed desorption and Auger electron spectroscopy measurements, allowed absolute product yields for the three competing surface pathways to be determined over the entire range of chemisorbed methanol coverages at a heating rate of β=2.6 K/s. At the lowest methanol coverages of θ≤0.07θs, where θs is the surface coverage of a saturated chemisorbed layer, all the methanol reacted between 220–280 K. This methanol decomposition reaction yielded desorption-limited H2 and CO as reaction products. At higher coverages, molecular desorption and the second methanol decomposition reaction involving C–O bond breakage became increasingly important. At θ=θs, 50% of the initial methanol coverage desorbed, 24% produced H2 and CO and 26% left C on the surface. Isothermal LITD kinetic measurements were carried out at low methanol coverages of θ≤0.07θs at various temperatures from 180 to 220 K. The initial decomposition rates obtained from the isothermal LITD studies displayed first order kinetics. The decomposition kinetics at later times could not be fit by first order kinetics and suggested a self-poisoned reaction. Subsequent LITD studies revealed that CO inhibited the decomposition reaction. The product CO inhibition was modeled by first order kinetics with a CO-coverage dependent activation barrier. The observed first order reaction kinetics at low methanol coverage could be expressed by the pre-exponential ν=106 s-1 and the coverage-dependent activation barrier E=7 kcal/mol+αθCO/θCO,s, where α=20 kcal/mol and θCO/θCO,s is the dimensionless CO coverage normalized to the CO saturation coverage θCO,s. Isotopic LITD studies revealed that the decomposition kinetics of CH3OH, CD3OH, and CH3OD were identical. This... [ABSTRACT FROM AUTHOR]

The effects of surface carbon on the surface diffusion of hydrogen on Ru(001) were studied using laser-induced thermal desorption techniques. The surface mobility of hydrogen decreased by approximately a factor of 60 as a function of increasing surface carbon coverage from θC=0 to θC=0.42 monolayer at T=300 K. The observed reduction of hydrogen surface mobility vs surface carbon coverage was consistent with the trapping of hydrogen atoms by carbide species on the Ru(001) surface. A simple trapping model suggests that the potential energy wells of the carbon trap sites are ΔE>2.4 kcal/mol deeper than regular hydrogen adsorption sites. This estimate is also consistent with the results of Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

The surface diffusion coefficient for hydrogen on Ru(001) at low coverage was measured using laser-induced thermal desorption techniques. In the temperature range between 260 and 330 K, the diffusion coefficients displayed Arrhenius behavior with an activation barrier Ediff=4.0±0.5 kcal and a preexponential factor D0=6.3×10-4 cm2/s. Agreement between the experimental and theoretical parameters suggests that hydrogen diffuses on the surface by moving from a threefold site to a neighboring threefold site via a twofold site. Surface contaminants such as carbon and oxygen were observed to produce dramatic effects on the hydrogen surface diffusion rate. [ABSTRACT FROM AUTHOR]

The surface mobilities of both CO and K coadsorbed on Ru(001) were studied using laser-induced thermal desorption (LITD) techniques. The LITD measurements revealed that CO was essentially immobile on the potassium-promoted Ru(001) surface when the CO surface coverage, ΘCO, was less than the potassium coverage, ΘK. The CO surface mobility increased dramatically when ΘCO>=ΘK. At ΘK=0.10 ML and 315 K, the CO diffusion coefficient was DCO<4×10-10 cm2/s for ΘCO<0.10 ML and increased to DCO=2.5×10-7 cm2/s for ΘCO>=0.10 ML. At ΘK=0.25 ML and 315 K, the CO surface diffusion coefficient was DCO<4×10-10 cm2/s for ΘCO<0.25 ML and increased to DCO=5×10-9 cm2/s for ΘCO>=0.25 ML. The potassium surface mobility also decreased dramatically as a function of CO coverage at both ΘK=0.10 ML and ΘK=0.25 ML. These diffusion results are consistent with a mutually stabilizing and trapping interaction between coadsorbed CO and K with a 1:1 CO:K stoichiometry. The thermal desorption spectra of both CO and K were also studied using temperature programmed desorption (TPD) techniques. At ΘK=0.10 ML and CO coverages ranging from ΘCO=0.06 ML to ΘCO=0.42 ML, CO TPD peak temperatures were consistently higher than CO peak temperatures obtained on clean Ru(001). These TPD results indicate that CO is stabilized by K on Ru(001). In addition, the potassium TPD spectra at various CO coverages with ΘK=0.10 ML and ΘK=0.25 ML revealed that K was stabilized by coadsorbed CO. The experimental results for CO and K coadsorbed on Ru(001) argue for a mutually stabilizing, short-ranged trapping CO–K interaction with a 1:1 CO:K stoichiometry. Monte Carlo simulations based on this model for the CO:K interaction were also consistent with the coadsorbate surface diffusion results. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigations of energy transfer between adlayers on single-crystal surfaces provide a unique opportunity to explore electronic energy transfer in restricted geometries. In this study, laser induced fluorescence techniques and donor quantum yield measurements were used to examine the distance dependence of electronic energy transfer between donor and acceptor adlayers on Al2O3(0001). The donor adlayer was p-terphenyl, the acceptor adlayer was 9,10-diphenylanthracene, and n-butane was the variable spacer adlayer. The electronic energy transfer rates vs spacer thickness were determined at both 30 and 85 K in ultra high vacuum. The butane spacer experiments showed that the donor energy transfer rate decreased with a 1/d3 dependence, where d is the thickness of the spacer adlayer. Given a Förster quantum mechanical or a Kuhn classical energy transfer mechanism with randomly oriented dipoles, a 1/d3 distance dependence is consistent with resonance electronic energy transfer from a two-dimensional donor adlayer to a three-dimensional array of acceptors. The spacer measurements yielded a critical transfer distance of d0=44 ±4 Å at 30 K and d0=33 ±6 Å at 85 K. The differences in the critical transfer distance at 30 and 85 K could be explained by the redshift in the p-terphenyl fluorescence spectrum at 85 K that reduces the overlap between the donor fluorescence and acceptor absorption spectra. Values of d0=44 Å at 30 K and d0=35 Å at 85 K were calculated theoretically from a 1/d3 analysis and were in excellent agreement with the experimental measurements. The rate of donor–donor intralayer energy migration was also determined by measuring the electronic energy transfer rate versus donor coverage on the acceptor adlayer. The donor quantum yield measurements versus donor adlayer coverage were consistent with the spacer results and indicated that electronic energy migration does not occur within the p-terphenyl adlayer. These... [ABSTRACT FROM AUTHOR]

Continuous and ultrathin platinum (Pt) films were deposited on tungsten (W) adhesion layers using atomic layer deposition (ALD) techniques. Pt ALD films were deposited at 120 °C using MeCpPtMe3 and H2 plasma as the reactants. X-ray reflectivity studies observed the rapid nucleation of the Pt film. X-ray photoelectron results were consistent with layer-by-layer growth suggesting a continuous Pt film at thicknesses ≥1.5 nm. The high surface energy of the W ALD adhesion layer enables the growth of continuous and ultrathin films of lower surface energy metals and should facilitate the use of precious metals for a variety of applications. [ABSTRACT FROM AUTHOR]

The nucleation and growth of tantalum nitride atomic layer deposition (ALD) was investigated on Al2O3 ALD surfaces on silicon substrates using tertbutylimino trisdiethylamino tantalum (TBTDET) and hydrogen radicals as the reactants. The hydrogen radicals were generated using a hot tungsten filament. Auger electron spectroscopy (AES) was utilized to monitor the atomic composition of the surface versus the TBTDET/hydrogen radical reaction cycles. These studies were conducted in a vacuum apparatus with a sample introduction port, two ALD reactors, and a connecting surface analysis chamber. Initial results with the silicon substrates at 250 °C revealed tantalum nitride ALD growth and the oscillation of the N AES signal during the TBTDET/hydrogen radical reaction cycles. However, TBTDET pyrolysis was a problem on the BN heater. The TBTDET pyrolysis threshold of ∼250 °C on the BN heater limited the maximum substrate temperature to 170 °C. The AES results revealed that the composition of the tantalum nitride ALD films grown at 170 °C was TaNx where x∼1. In addition, there were carbon and oxygen impurities in the TaNx film. Aluminum and tantalum AES signals were employed to determine the tantalum nitride ALD thickness on the underlying Al2O3 ALD surface during TBTDET/hydrogen radical reaction cycles at 170 °C. The Cumpson method of using the ratio of the Al and Ta AES signals to determine the TaNx ALD film thickness avoided the problem of normalization between different AES spectra. The TaNx ALD nucleated readily on the Al2O3 ALD surface. The Al AES signal approached zero after 20 reaction cycles. The corresponding tantalum nitride film thickness was determined to be 11.6 Å after 20 reaction cycles. The film thickness at 5, 10, 15, and 20 reaction cycles yielded an initial ALD growth rate of 0.5 Å/cycle. The ALD growth rate increased to 0.7 Å/cycle for >10 reaction cycles. The efficient initial nucleation and reasonable growth rates for tantalum nitride ALD are desirable for the formation of copper diffusion barriers on interconnection vias. [ABSTRACT FROM AUTHOR]

Tungsten (W) atomic layer deposition (ALD) was performed on cobalt (Co) nanoparticles using WF6 and Si2H6 as reactants. A variety of techniques were then applied to analyze both the Co nanoparticles and flat Co substrates after W ALD. Analysis of the W ALD-coated Co nanoparticles is complicated because a CoO layer may exist on the Co nanoparticles and a WO3 layer may be present on the W ALD coating. LECO measurements quantified the oxygen weight percent in the W ALD-coated Co nanoparticles. The oxygen weight percent decreased with increasing number of W ALD AB cycles. To determine the location of this oxygen, x-ray reflectivity (XRR) investigations measured the WO3 film thickness on flat W ALD films. The XRR measurements yielded a WO3 film thickness on flat W ALD films of ∼20 Å. X-ray photoelectron spectroscopy (XPS) studies also quantified the relative oxygen abundance at the W/Co interface for W ALD on flat Co films. The XPS measurements revealed that nearly all the oxygen was in the WO3 layer on the W ALD film. Only an immeasurably small amount of oxygen was bonded as CoO at the W/Co interface. To determine the thickness of W ALD film on the Co nanoparticle, surface profilometry of W ALD on flat Co substrates measured a W ALD growth rate of 3.9 Å per AB cycle. A geometric model was then constructed to incorporate the information from all the measurements on Co nanoparticles and flat Co substrates. Excellent agreement between the geometrical model and the oxygen weight percent versus the number of W ALD cycles was obtained when the CoO thickness was negligible and the WO3 thickness on the W ALD layer on the Co nanoparticles was 28.5 Å. This agreement indicates that the details of ALD on nanoparticles can be unraveled by a concert of techniques even when interfacial layers can form due to the high reactivity of nanoparticles. The W ALD-coated Co nanoparticles may be useful in fabricating WC–Co hardmetals with enhanced mechanical properties. [ABSTRACT FROM AUTHOR]

Challenges are encountered during atomic layer deposition (ALD) on large quantities of nanoparticles. The particles must be agitated or fluidized to perform the ALD surface reactions in reasonable times and to prevent the particles from being agglomerated by the ALD film. The high surface area of nanoparticles also demands efficient reactant usage because large quantities of reactant are required for the surface reactions to reach completion. The residence time of the reactant in a fluidized particle bed reactor may be too short for high efficiency if the ALD surface reactions have low reactive sticking coefficients. To address these challenges, a novel rotary reactor was developed to achieve constant particle agitation during static ALD reactant exposures. In the design of this new reactor, a cylindrical drum with porous metal walls was positioned inside a vacuum chamber. The porous cylindrical drum was rotated by a magnetically coupled rotary feedthrough. By rotating the cylindrical drum to obtain a centrifugal force of less than one gravitational force, the particles were agitated by a continuous “avalanche” of particles. In addition, an inert N2 gas pulse helped to dislodge the particles from the porous walls and provided an efficient method to purge reactants and products from the particle bed. The effectiveness of this rotary reactor was demonstrated by Al2O3 ALD on ZrO2 particles. A number of techniques including transmission electron microscopy, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy confirmed that the Al2O3 ALD film conformally coats the ZrO2 particles. Combining static reactant exposures with a very high surface area sample in the rotary reactor also provides unique opportunities for studying the surface chemistry during ALD. [ABSTRACT FROM AUTHOR]

Hydrogen radical concentrations traveling under flowing gas conditions through tubes of different materials were measured using a dual thermocouple probe. The source of the hydrogen radicals was a toroidal radio frequency plasma source operating at 2.0 and 3.3 kW for H2 pressures of 250 and 500 mTorr, respectively. The dual thermocouple probe was comprised of exposed and covered Pt/Pt13%Rh thermocouples. Hydrogen radicals recombined efficiently on the exposed thermocouple and the energy of formation of H2 heated the thermocouple. The second thermocouple was covered by glass and was heated primarily by the ambient gas. The dual thermocouple probe was translated and measured temperatures at different distances from the hydrogen radical source. These temperature measurements were conducted at H2 flow rates of 35 and 75 SCCM (SCCM denotes cubic centimeter per minute at STP) inside cylindrical tubes made of stainless steel, aluminum, quartz, and Pyrex. The hydrogen radical concentrations were obtained from the temperatures of the exposed and covered thermocouples. The hydrogen concentration decreased versus distance from the plasma source. After correcting for the H2 gas flow using a reference frame transformation, the hydrogen radical concentration profiles yielded the atomic hydrogen recombination coefficient, γ, for the four materials. The methodology of measuring the hydrogen radical concentrations, the analysis of the results under flowing gas conditions, and the determination of the atomic hydrogen recombination coefficients for various materials will help facilitate the use of hydrogen radicals for thin film growth processes. [ABSTRACT FROM AUTHOR]

A chemical reactor was constructed for growing thin films using atomic layer deposition (ALD) techniques. This reactor utilizes a viscous flow of inert carrier gas to transport the reactants to the sample substrates and to sweep the unused reactants and reaction products out of the reaction zone. A gas pulse switching method is employed for introducing the reactants. An in situ quartz crystal microbalance (QCM) in the reaction zone is used for monitoring the ALD film growth. By modifying a commercially available QCM housing and using polished QCM sensors, quantitative thickness measurements of the thin films grown by ALD are obtained in real time. The QCM is employed to characterize the performance of the viscous flow reactor during A1[sub 2]O[sub 3] ALD. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Quantitative Ca tests were used to determine the water vapor transmission rate (WVTR) through 25 nm thick Al2O3 gas diffusion barriers grown on plastic by atomic layer deposition (ALD). The measured WVTRs were 1.7×10-5 g/m2 day at 38 °C and 6.5×10-5 g/m2 day at 60 °C. Based on the apparent activation energy, the WVTR at 23 °C is estimated to be only 6×10-6 g/m2 day. The WVTR values for the Al2O3 ALD film are very similar to the WVTR value for the glass control. These Ca tests indicate that Al2O3 ALD gas diffusion barriers should enable display and lighting applications of highly moisture sensitive organic light-emitting diodes on plastic. [ABSTRACT FROM AUTHOR]

Thin films of Al2O3 grown by atomic layer deposition (ALD) were investigated as gas diffusion barriers on flexible polyethylene naphthalate and Kapton® polyimide substrates. Al2O3 ALD films with thicknesses of 1–26 nm were grown at 100–175 °C. For Al2O3 ALD films with thicknesses >=5 nm, oxygen transmission rates were below the MOCON instrument test limit of ∼5×10-3 cc/m2/day. Applying a more sensitive radioactive tracer method, H2O-vapor transmission rates of ∼1×10-3 g/m2/day were measured for single-sided Al2O3 ALD films with thicknesses of 26 nm on the polymers. Ultrathin gas diffusion barriers grown by Al2O3 ALD may enable organic displays and electronics on permeable, flexible polymer substrates. [ABSTRACT FROM AUTHOR]