Langmuir Exposure Research Articles

Effects of gases on the field emission performance of silicon gated field emitter array

Effects of gases on field emission performance were measured using silicon-gated field emitter arrays. Gas was injected into a vacuum chamber with a 1000 × 1000 tip array, which was driven by a DC gate and collector voltages. The collector voltage was fixed at 200 V while the gate voltage was swept to 40 V. For the gas exposure study, N2, He, and Ar were used. The sets of partial pressures, 5 × 10−6, 5 × 10−5, and 5 × 10−4 Torr, were used for the experiment. It was observed that N2 had the least effect and Ar had the worst effect on emission current performance. The degradation of collector current at 5 × 10−4 Torr pressure for Ar was ≈65% where for the N2, at the same level of pressure, the degradation was ≈41%. However, further experiments with high purity Ar gas showed that it was the water vapor present in the gas itself that was the primary cause of reduction in emission current and not the gas itself. The results expressed in reduction in emission current versus Langmuir exposure versus the current clearly showed the effect of water vapor. After the vacuum was recovered, the work function again restored partially to its original value. After ultraviolet light cleaning, the emission current was restored completely to the original state.

Self-Limiting Film Growth of Transparent Conducting In2O3 by Atomic Layer Deposition using Trimethylindium and Water Vapor

Conformal and self-limiting film growth of transparent conducting In2O3 is demonstrated with atomic layer deposition (ALD) using trimethylindium (TMIn) and water vapor. Various parameters, including pulsing times of TMIn and H2O and deposition temperature, are varied to confirm the ALD processing window of In2O3. It is found that an extremely large Langmuir exposure of H2O (∼2 Torr s) is required to fully react the surface-adsorbed In-(CH3)* groups with H2O. Self-limiting film growth is obtainable at the deposition temperatures between 200 and 251 °C where the film growth is controlled by surface adsorption of TMIn molecules. Within the ALD window, ALD-In2O3 exhibits a film growth rate of ∼0.039 nm/cycle at 217 °C. In addition, the film exhibits an excellent step coverage of ∼97% on a high-aspect-ratio (∼11) nanostructure. The resistivity of the ALD-In2O3 film deposited at 200 °C is about 2.8 × 10–3 Ω cm, and this is attributed to the high Hall mobility of 84 cm2/(V s). Finally, the reduction of the Hall mobility of the films with respect to increases in the deposition temperature is correlated to the evolution of microstructures.

The Reactive Chemisorption of Alkyl Iodides at Cu(110) and Ag(111) Surfaces: A Combined STM and XPS Study

The chemisorption of methyl and phenyl iodide has been studied at Cu(110) and Ag(111) surfaces at 290 K with STM and XPS. At both surfaces dissociative adsorption of both molecules leads to chemisorbed iodine, with the STM showing c(2 x 2) and (square root 3 x square root 3)R30 structures at the Cu(110) and Ag(111) surfaces, respectively. At the Cu(110) surface a comparison of coexisting c(2 x 2) I(a) and p(2 x 1) O(a) domains shows the iodine adatoms to be chemisorbed in hollow sites with evidence at low coverage for diffusion in the (110) direction. In the case of methyl iodide no carbon adsorption is observed at either the silver or the copper surfaces, but chemisorbed phenyl groups are imaged at the Cu(110) surface after exposure to phenyl iodide. The STM images show the phenyl groups as bright features approximately 0.7 nm in diameter and 0.11 nm above the iodine adlayer, reaching a maximum surface concentration after approximately 6 Langmuir exposure. However, the phenyl coverage decreases with subsequent exposures to PhI and is negligible by approximately 1000 L exposure, consistent with the formation and desorption of biphenyl. The adsorbed phenyls are located above hollow sites in the substrate, they are stabilized at the top and bottom of step edges and in paired chains (1.1 nm apart) on the terraces with a regular interphenyl spacing within the chains of 1.0 nm in the (110) direction. The interphenyl ring spacing and diffusion of individual phenyls from within the chains shows that the chains do not consist of biphenyl species but may be a precursor to their formation. Although the XPS data shows carbon present at the Ag(111) surface after exposure to PhI, no features attributable to phenyl groups were observed by STM.

Friction and Adhesion Properties of Clean and Oxidized Al–Ni–Co Decagonal Quasicrystals: A UHV Atomic Force Microscopy/Scanning Tunneling Microscopy Study

The tribological properties of adhesion and friction between 10-fold Al–Ni–Co decagonal quasicrystals and conductive W2C and TiN coated tips were studied in ultrahigh vacuum (UHV) with an atomic force microscope. Contacts between the tip and clean quasicrystals are dominated by strong adhesion forces, which result in irreversible deformations and material transfer. The irreversible adhesion was suppressed following an oxygen exposure of 10 Langmuir, which also reduced the adhesion force by a factor of two. An additional 2/3 reduction in adhesion force occurred after several 100 Langmuir exposure. A much larger decrease (by a factor 10) occurred by air-oxidation. The friction force decreased also with oxygen exposure although not as dramatically. A linear decrease by a factor two, was observed between the clean surface and the surface exposed to 200 Langmuir of oxygen. After that the friction force remained constant. Air-oxidation reduced friction by roughly another factor of two. In contrast with the clean

Preadsorbed Oxygen Atoms Affect the Product Distribution and Kinetics of Acetylene Cyclization to Benzene on Pd(111): A Laser-Induced Thermal Desorption/Fourier Transform Mass Spectrometry Study

The study presented here focuses on determining the role of oxygen as a modifier on Pd(111) and its effects on the cyclization of acetylene to benzene. Laser-induced thermal desorption/Fourier transform mass spectrometry (LITD/FTMS) is used as a sensitive tool for measuring the in situ kinetics of benzene formation from acetylene on O/Pd(111). Low exposure of acetylene on O/Pd(111) leads to the anticipated formation of benzene and 1,3-butadiene. Though there is no evidence of furan formation on the surface, oxidation products, such as CO and H2O, are observed. An enhancement in the yield of benzene has been observed with increasing oxygen preexposure. Our evidence suggests that this enhancement is caused by oxygen-island compression of acetylene molecules into bare patches of Pd, which effectively increases the local coverage of acetylene in those regions. Isothermal kinetic studies of 1.1 langmuirs of acetylene on a 50% saturated layer of O on Pd(111) (from a 0.25 langmuir exposure of O2 at 250 K) yield an Ea of 37.8 ± 3 kJ/mol using initial rates (and 36.2 ± 3 kJ/mol using a pseudo-first-order model). Both the activation energy and preexponential factor from a 50% saturation coverage of oxygen on Pd(111) correspond to the values expected for twice the acetylene exposure on a clean surface. The apparent contradiction between increased benzene yields and activation barrier for the O/Pd system can be rationalized by the compensation effect, where a more tightly bound reactant can lead to a greater entropy of activation.

The adsorption of Fe on W(110) and of O and H on Fe-covered W(110)

The adsorption of Fe monolayers on W(110) and adsorption of oxygen and hydrogen on the Fe layers have been examined via work-function, XPS, Auger, LEED, and thermal desorption measurements. Two distinct forms of Fe 1 W(110) exist, although both show sharp p(1 × 1) LEED patterns. For the monolayer formed at 90 K, Δф = −836 meV , while it is −530 meV for the layer formed or annealed at 300–600 K. Heating the 90 K layer converts it irreversibly to the high-temperature form. Adsorption of H or O on these layers gives different ф and Δф values. Initial sticking coefficients for H 2 at 90 K are s 0 = 0.36 for the 90 K Fe layer and 0.3 for the 300/600 K layer. The amounts adsorbed at 90 K after 40 Langmuir exposure (where s<0.05) are (in units of H W , i.e. unity corresponding to 1.42 × 10 15 atoms cm −2) 0.69 for the 90 K and 0.63 for the 600 K layer. H 2 thermal desorption spectra for H/Fe 1/W(110) and for Fe 1/H 1/W(110) are similar in shape and peak at 320 K, much lower than on W(110) or on Fe(110). O is adsorbed to the same extent and with virtually identical sticking coefficients on both Fe layers, with s 0 ≈ 0.88 at 90 K. Fe 2p peak-shifts indicate that O uptake leads to chemisorption for O W<0.5 , followed by oxide formation. The maximum O uptake corresponds to O W ≈ 1.35 , and suggests that Fe 3O 4 is formed at saturation, as also indicated by the Fe 2p peak shift. Work function increments are different for the two layers, being 713 meV for the 90 K layer and 773 meV for the 600 K layer for chemisorption, i.e. for O W = 0.42 . On heating saturated layers to T> 1200 K, Fe, WO 2 and traces of FeO are desorbed with peak temperatures at 1340 K. For O 0.5/Fe 1/W(110) or Fe 1/O 0.6/W(110), oxidation of Fe does not occur; in the former case O becomes chemisorbed on the W surface on heating to 450 K. In both cases this is followed by segregation on heating to 800 K, as indicated by decreases in Fe Auger intensities and formation of a p(1 × 1) LEED pattern, corresponding to close-packed oxygen islands, as observed previously with other metal-oxygen coadsorption systems on W(110). A brief discussion of the similarities and differences of the present results with those for Cu, Ag, Hg, Ni, and Pd on W(110) is given.

Observation of surface symmetry for Si/XeF2 and Si/Cl2 system using second-harmonic generation

Observation of surface symmetry change with etching reaction for the Si(111)/XeF2 and Si(111)/Cl2 systems has been demonstrated using second-harmonic generation (SHG) is reported. SHG output decays quickly with spontaneous XeF2 etching because of the formation of a SiFx reaction layer. Surface symmetry is broken by 1–2 langmuir exposure to XeF2 and recovered by ohmic heating to 800 °C. SHG output also decays quickly with exposure to Cl2. However, surface symmetry seems to remain after 20 langmuir exposure to Cl2 and SHG output is recovered by ohmic heating to 1000 °C.

Reactions of Primary Organosilanes on Transition Metal Surfaces. Identification of the First Surface-Bound Silylynes

Transition metal silicides have numerous applications in the microelectronics industry. Primary organosilanes (RSiH[sub 3]) are attractive alternative silicon sources for the synthesis of metal silicides, since many of these compounds are stable in air. A Pt(111) surface in an ultrahigh vacuum chamber (10[sup [minus]10] Torr) was treated with a 7 langmuir exposure of t-BuSiH[sub 3] at 105 K, and subsequent reactions of the silane were studied by temperature-programmed reaction spectroscopy (TPRS). Confirmation of the formation of surface silylyne species was obtained by reflection-absorption infrared (RAIR) spectroscopy. Methylsilane also dehydrogenates on Pt(111) surfaces to give a stable organosilylyne product. The present data reveal that the S-H bonds of primary organosilanes readily cleave on Pt surfaces. The number of Si-H bonds cleaved is dependent on the temperature and coverage, but by 250 K all three are cleaved to yield organosilylyne units. Like alkylidyne units, with which they are isoelectronic, the organosilylyne species are probably bound to 3-fold hollow sites. 18 refs., 2 figs.

Optical interference in second-harmonic generation from a metal surface with molecular adlayers

A strong oscillation pattern was observed in optical second-harmonic generation (SHG) at a Ag(111) surface under pyridine adlayers with variable thickness. This oscillation appears only when s-, but not p-, polarized fundamental light is used. This effect is attributed to the modulation of the radiation efficiency of the fundamental light penetrating into the Ag bulk and of the second-harmonic light radiating out to the vacuum by pyridine overlayers through optical interference. A standard optical interference model has been developed to quantitatively analyze the oscillation patterns and determine the adlayer thickness. It was found that at 90 K the pyridine overlayer growth rate is 1.4 Å per Langmuir exposure.

X-Ray Photoelectron Study of the Methane Interaction with LaCoO3

Abstract The interaction of CH4 with LaCoO3 has been studied at room temperature with the use of an X-ray photoelectron spectroscopy (XPS) analyzer. CH4 dissociated on the surface of LaCoO3, and new carbonaceous products were detected at 104 langmuir exposure on the spectra for C1s. Since the intensities of the adsorbed oxygen on the spectra for O1s gradually decreased in proportion to the increase in the amount of CH4 exposure, methane reacted with adsorbed oxygen of LaCoO3 at room temperature in an XPS analyzer.

Morphological effect of a clean Si surface on NH 3 dissociative adsorption

The interaction of NH 3 in the 10 −2–10 2 langmuir exposure range with different Si surfaces, namely (111)2 × 1 and 7 × 7 reconstructed as well as (100) 2 × 1 either stepped or not, is compared. Upon saturation, characterized by a removal of the surface dangling bonds (DB) and a net decrease of the ionization energy, Si(111) 7 × 7 differs from other surfaces in saturating at one NH 3 molecule per 2 DB against 4 for the others. The various results are explained within a common picture where the NH 3 molecule dissociates into one NH 2 radical and one hydrogen atom, each species being able to remove 2 DB, if they are close enough.

Step effects in the thermal decomposition of methanol on Pt(111)

Methanol decomposition to adsorbed CO and hydrogen on Pt(111) was observed using time-resolved high resolution electron energy loss spectroscopy. No reaction was seen at temperatures ≲ 180 K, the molecular methanol desorption temperature. Between 180 and 200 K the decomposition reaction is controlled by the small amount of defects present on our carefully prepared sample. This was principally demonstrated by the fact that only 1–2% of a monolayer of CO was formed regardless of the initial methanol coverage. A steady state reaction run between 200 and 240 K showed only a low rate of CO formation (only a few percent of a monolayer even after many langmuir exposure of methanol). At a 300 K surface temperature, decomposition proceeds much faster, with about 10% of a monolayer of CO being formed. The origin of this temperature dependence is attributed to the ability of the product H and CO to diffuse away from sites where they were initially formed. Using a He seeded methanol beam, there was no apparent incident energy dependence for the methanol decomposition reaction.

Electronic properties of CoO(100) surfaces: Defects and chemisorption.

Ultraviolet photoemission spectroscopy, Auger-electron spectroscopy, and low-energy electron diffraction have been used to study the electronic structure of CoO(100) surfaces obtained by cleaving single crystals in ultrahigh vacuum. Two distinct types of surface defect states that can be produced by ${\mathrm{Ar}}^{+}$-ion bombardment are identified. Both types of defects result from oxygen vacancies near the surface. The first type of defect consists primarily of surface oxygen vacancies; it forms after any degree of reduction of the surface and probably exists in small concentration even on cleaved surfaces. Its electronic structure is interpreted in terms of the occupation of exchange- and ligand-field--split d orbitals of Co ions at defect sites. The second type of defect appears only after heavy reduction of the surface and presumably consists of interacting oxygen vacancies or clusters of Co atoms. The interaction of ${\mathrm{O}}_{2}$, CO, and ${\mathrm{H}}_{2}$O with the nearly perfect surface and with surfaces containing one or both types of defects at room temperature has also been investigated. The nearly perfect surface is relatively inert to all three molecules studied, exhibiting either no reaction or minimal chemisorption only at residual defect sites below at least ${10}^{4}$ langmuir exposure. ${\mathrm{O}}_{2}$ initially adsorbs dissociatively at both types of defect site; high ${\mathrm{O}}_{2}$ exposures result in a molecularly adsorbed species. Exposure to CO further reduces surfaces that contain predominantly only the first type of defect, but not the more heavily reduced surfaces that contain both types of defects. ${\mathrm{H}}_{2}$O adsorbs dissociatively on the heavily reduced surface, resulting in adsorbed ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ ions. The results for CoO(100) are compared and contrasted with those obtained previously for NiO(100) and MnO(100).

Chemisorption of sulfur on Ge(100)

The adsorption of elementary sulfur on Ge(100) has been studied with AES, LEED and various surface photoemission spectroscopy techniques. The adsorption of sulfur reaches saturation after a few langmuir exposure and yields an ordered (1×1) structure with one sulfur atom per surface unit cell bonded on a bulk-like bridge site. Sulfur deposition exceeding one monolayer is possible, probably due to defects. During room temperature submonolayer adsorption no sulfur islands are formed. The adsorbed single atoms are bonded in well-defined bridge sites as in the case of full monolayer coverage. The valence band structure of the Ge surface is changed under sulfur adsorption: The surface states of clean Ge(100)(2×1) disappear, for S/Ge(100)(1×1) one strongly and two weakly dispersing sulfur induced states with E B=2−4 eV, E B θ5 eV and E B θ7.5 eV, respectively, are identified. The resulting two-dimensional band structure confirms the idea of covalent SGe bonds wiht (1×1) periodicity. The system S/Ge(100)(1×1) can be regarded as an ideally terminated surface.

Poisoning and reactivation processes in oxide-type cathodes: Part II. Single crystal BaO films

The work described here is a continuation of the studies reported in part I of shelf-life poisoning and reactivation observed with mixed Ba/Sr oxide-type cathodes after exposure in a vacuum tube ambient. Emphasis in part II is placed on the identification and determination of the basic mechanisms involved in the individual processes that make up the overall effects observed in part I. In order to achieve this, single crystal BaO films deposited on an inert Ir substrate were studied with respect to poisoning and reactivation by the various individual gas components found in typical vacuum ambients. The electronic work function changes were separated into their components of surface dipole and bulk-type Fermi level changes using the analysis of shifts in the low energy electron reflection (LEER) patterns obtained with an incident electron beam. Corresponding compositional changes were correlated with the electronic changes using multiple Auger energy analysis and also by noting changes in the characteristic shapes of the LEER patterns. The results showed that poisoning during shelf-life takes place chiefly by O 2, CO 2 and H 2O. O 2 poisoning occurs mainly by neutralizing O-vacancy donors and is more of a bulk-type effect altering the Fermi level. CO 2 and H 2O, on the other hand, form partial monolayers of surface carbonates and hydroxides which alter the surface dipole, i.e. electron affinity. These electronic changes in band structure are verified by the compositional measurements indicating the presence of an adsorbed surface layer with CO 2 and H 2O but not with O 2. The surface layers have also been identified as the carbonates and hydroxides from characteristic LEER shapes as well as from comparison to dissociation temperatures and gas desorption results of part I. The poisoning rate for a 10 Langmuir exposure is about 1 eV change in Fermi level for O 2, about 1 eV change in electron affinity for CO 2 and about 1 3 eV change in electron affinity for H 2O. The actually observed vacuum ambient poisoning rates are in very good agreement with that expected from the above mentioned gas poisoning rates when combined in the ratios of the partial pressures of these gases found in typical vacuum ambients. Reactivation is largely a desorption of the gas adsorbed during shelf-life and this is usually completed for oxide-type cathodes when the reactivation temperature reaches ∼1200 K, a typical operating temperature. Reactivation can also be enhanced by choosing substrate materials which will aid in the generation of O-vacancy donors or in the formation of a Ba surface layer. Care must be taken though to avoid interaction effects such as the formation of an oxide residue at the interface which may impede the donor generation process.