Ultrahigh Vacuum Studies Research Articles

Characterization of the Si(111)-Ga interface using optical second-harmonic generation

Ultra high-vacuum studies of optical second-harmonic generation (SHG) from well-characterized Si(111)-Ga interfaces in the submonolayer coverage regime are reported. Both intensity and phase measurements of the SH signal have been made. The chi xxx and chi xxx susceptibility tensor components are shown to have resonant enhancement around a third of a monolayer, for 1064 nm excitation. The origin of this behaviour is discussed in terms of the structure and bonding at the interface.

Stepwise thermolysis of tris(allyl)rhodium to rhodium metal: elucidation of thermal process on bulk titania through ultrahigh vacuum studies on titania(001)

Stepwise thermolysis of tris(allyl)rhodium to rhodium metal: elucidation of thermal process on bulk titania through ultrahigh vacuum studies on titania(001)

Electrochemical hydration and reaction processes on metal surfaces studied by gas phase adsorption

The distinction between surface hydration and reaction processes in ultrahigh vacuum studies of electrochemical phenomena is illustrated by coadsorption of H 2O with: (i) Cs on Cu(110); at low caesium coverages, solvation of the adsorbed alkali is observed and above a critical coverage of θ Cs = 0.15 a surface reaction occurs forming hydroxide and hydrogen. Work function measurements have also been used to characterize the dielectric screening by interfacial water; (ii) hydrogen on Cu(110) and Pt(111): TDS and HREELS measurements using isotopic substitution of hydrogen by deuterium have been used to investigate the formation of (H 3O +) ad. Whereas on Cu(110) hydrogen is merely solvated by water, on Pt(111) there is evidence for the production of hydrated protons.

Solid state cesium ion guns for surface studies

Three cesium ion guns covering the energy range of 5–5000 V are described. These guns use a novel source of cesium ions that combine the advantages of porous metal ionizers with those of aluminosilicate emitters. Cesium ions are chemically stored in a solid electrolyte pellet and are thermionically emitted from a porous thin film of tungsten at the surface. Cesium supply to the emitting surface is controlled by applying a bias across the pellet. A total charge of 10.0 C can be extracted, corresponding to greater than 2000 h of lifetime with an extraction current of 1.0 μA. This source is compact, stable, and easy to use, and produces a beam with >99.5% purity. It requires none of the differential pumping or associated hardware necessary in designs using cesium vapor and porous tungsten ionizers. It has been used in ultrahigh-vacuum (UHV) experiments at pressures of <10−10 Torr with no significant gas load. Three different types of extraction optics are used depending on the energy range desired. For low-energy deposition, a simple space-charge-limited planar diode with a perveance of 1×10−7 A/V3/2 is used. Current densities of 10.0 μA/cm2 at the exit aperture for energies ≤20 V are typical. This type of source provides an alternative to vapor deposition with the advantage of precise flux calibration by integration of the ion current. For energies from 50 to 500 V and typical beam radii of 0.5 to 0.2 mm, a high perveance Pierce-type ion gun is used. This gun was designed with a perveance of 1×10−9 A/V3/2 and produces a beam with an effective temperature of 0.35 eV. For the energy range of 0.5 to 5 keV, the Pierce gun is used in conjunction with two Einzel lenses, enabling a large range of imaging ratios to be obtained. Beam radii of 60 to 300 μm are typical for beam currents of 50 nA to 1.0 μA. Results are presented and discussed for UHV studies of ion implantation, electronic surface changes induced by adsorbates, and negative secondary-ion mass spectrometry.

Solid state cesium ion guns for surface studies (abstract)

Three cesium ion guns covering the energy range of 5–5000 V are described. These guns use a novel source of cesium ions that combine the advantages of porous metal ionizers with those of aluminosilicate emitters. Cesium ions are chemically stored in a solid electrolyte pellet and are thermionically emitted from a porous thin film of tungsten at the surface. Cesium supply to the emitting surface is controlled by applying a bias across the pellet. A total charge of 10.0 C can be extracted, corresponding to greater than 2000 h of lifetime with an extraction current of 1.0 μA. This source is compact, stable, and easy to use, and produces a beam with >99.5% purity. It requires none of the differential pumping or associated hardware necessary in designs using cesium vapor and porous tungsten ionizers. It has been used in ultrahigh-vacuum (UHV) experiments at pressures of <10−10 Torr with no significant gas load. Three different types of extraction optics are used depending on the energy range desired. For low-energy deposition, a simple space-charge-limited planar diode with a perveance of 1×10−7 A/V3/2 is used. Current densities of 10.0 μA/cm2 at the exit aperture for energies ≤20 V are typical. This type of source provides an alternative to vapor deposition with the advantage of precise flux calibration by integration of the ion current. For energies from 50 to 500 V and typical beam radii of 0.5 to 0.2 mm, a high perveance Pierce-type ion gun is used. This gun was designed with a perveance of 1×10−9 A/V3/2 and produces a beam with an effective temperature of 0.35 eV. For the energy range of 0.5 to 5 keV, the Pierce gun is used in conjunction with two Einzel lenses, enabling a large range of imaging ratios to be obtained. Beam radii of 60 to 300 μm are typical for beam currents of 50 nA to 1.0 μA. Results are presented and discussed for UHV studies of ion implantation, electronic surface changes induced by adsorbates, and negative secondary-ion mass spectrometry.

Cryogenic sample manipulator for multipurpose sample analysis

A cryogenic sample manipulator for ultrahigh vacuum studies of surfaces by Raman spectroscopy of physisorbed gasses is described. The development of a cryogenic off−axis sample manipulator that can be cooled to 15 K and allows the sample to be placed at any distance desired from the rotational axis is reported in this paper. (AIP)

Detection of dry etching product species with i n s i t u Fourier transform infrared spectroscopy

I n situ Fourier transform infrared (FTIR) spectroscopy has been utilized to detect gas-phase product species resulting from the dry etching of aluminum and heavily doped n-type polycrystalline silicon (n+ poly-Si) in Cl2 . The products of aluminum etching in Cl2 were investigated in the presence and absence of a plasma. With the plasma off, Al2 Cl6 was the only infrared-active etch product detected at both low (50 °C) and high (>120 °C) sample temperatures. With the discharge on, the spectrum was more complex, indicating partial fragmentation of the dimer etch product in the plasma forming AlCl3 and, perhaps, AlCl. Silicon tetrachloride was the only infrared-absorbing product detected during the Cl2 plasma etching of n+ poly-Si. Unsaturated silicon chlorides (SiClx , x=1–3) were not present at detectable levels. These results are compared with those of ultrahigh vacuum (UHV) beam studies; in comparing UHV results with those obtained in a processing plasma, allowances must be made for the considerable difference in pressure, and therefore, in species’ fluxes in these two experimental environments. Unlike UHV studies, in situ FTIR measurements permit detection of etch products under typical processing conditions. When interpreting the results, however, the possibility that the products leaving the sample surface react further upon entering the plasma must be considered.

Ultrahigh vacuum and electrochemical CO-characterization studies of Cu ON Ru(0001)

Sub-monolayer to multilayer quantities of Cu on a Ru(0001) substrate was studied both in ultrahigh vacuum (UHV) and in an electrochemical cell (EC). The experimental apparatus consisted of an EC attached directly to a UHV chamber with facilities for Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS). The UHV and electrochemical methods were compared directly via a series of co-characterization experiments in which Cu was deposited in the UHV environment and characterized electrochemically. A comparison of the linear sweep stripping and TDS of vacuum deposited Cu showed that the difference in the heat of desorption between multilayer and monolayer Cu was ≈ 6 kcal/mol and was approximately the same for both methods.

Ultrahigh vacuum studies of Pd metal-insulator-semiconductor diode H2 sensors

Hydrogen sensitive Pd metal/insulator/semiconductor diodes provide an ambient temperature, low power electronic sensor for hydrogen as a result of hydrogen trapping at the Pd/insulator interface. Current kinetic models consider the rate limiting step to be adsorption at the Pd surface followed by rapid transport to the interface. We have obtained both steady-state and kinetic results for diodes with clean Pd surfaces over hydrogen pressures ranging from 10−10 to 10−1 Torr. The sensitivity limit (equivalent to <1011 total H2 impacts/cm2 at the Pd surface) is set by our vacuum capabilities, and is at least seven orders of magnitude greater than that obtained for devices with contaminated surfaces. These results clearly show that the kinetic and sensitivity limitations reported for such devices are a result of surface contamination. For diodes with a clean Pd surface, analysis of the steady-state results requires at least two binding states (9 and 6.8 kcal/mol of H relative to H2(g)) for H at the Pd/SiO2 interface. The kinetics of the diode response to H2 are inconsistent with direct transfer to the bulk exclusively through strong chemisorbed surface states, but suggest with a ‘‘precursor’’ mechanism in which the initial step is adsorption into a weakly bound precursor state from which branching steps populate both strongly bound chemisorbed surface states and bulk states. The surface states act in part as traps for hydrogen in competition with transfer to the bulk (and therefore to the electronically active interface).

Ultrahigh vacuum studies of Pd metal/insulator/semiconductor diode H2 sensors

Hydrogen sensitive Pd metal/insulator/semiconductor diodes provide an ambient temperature, low-power electronic sensor for hydrogen as a result of hydrogen trapping at the Pd/insulator interface. We have obtained both steady-state and kinetic results for diodes with clean Pd surfaces over hydrogen pressures ranging from 10−10 to 10−1 Torr. The sensitivity limit (equivalent to <1011 total H2 impacts/cm2 at the Pd surface) is at least 7 orders of magnitude greater than that obtained for devices with contaminated surfaces, revealing that the kinetic and sensitivity limitations normally reported for such devices are a result of surface contamination. Analysis of the steady-state results is consistent with two binding states (9 and 6.8 kcal/mol of H relative to H2(g)) for H at the Pd/SiO2 interface. The kinetics of the diode response to H2 are consistent with a ‘‘precursor’’ mechanism in which the initial step is adsorption into a weakly bound precursor state from which branching steps populate both strongly bound chemisorbed surface states and bulk states.

Mechanisms of laser interaction with metal carbonyls adsorbed on Si(111)7×7: Thermal vs photoelectronic effects

Ultrahigh vacuum studies of the interaction of 514 nm radiation from a cw Ar ion laser and its second harmonic at 257 nm with mono- and multilayer coverages of Mo(CO)6, W(CO)6, and Fe(CO)5 adsorbed on Si(111)7×7 at 90 K using thermal desorption spectroscopy (TDS), laser induced desorption spectroscopy, high resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy were performed. A model for the temperature rise of the sample due to cw laser heating is developed. By directly measuring the substrate temperature, these experiments were able to distinguish between photoelectronic and thermal effects active in the decomposition and desorption mechanisms of the adsorbed carbonyls. Results from TDS and HREELS show that Mo(CO)6 and W(CO)6 are molecularly adsorbed, while Fe(CO)5 partially dissociates upon adsorption. The decomposition of adsorbed Mo(CO)6 is caused by electronic excitation due to direct absorption of the 257 nm radiation. Irradiation with 514 nm radiation results in no photochemistry. The same mechanism is dominant for adsorbed W(CO)6 and Fe(CO)5; however, new excitation mechanisms are available to these molecules that lead to bonding changes in W(CO)6 and Fe(CO)5 with 514 nm irradiation. The photodecomposition products of the adsorbed carbonyls are found to be different from the gas-phase decomposition products. The surface stabilizes the adsorbed carbonyls, preventing complete removal of all the CO ligands. Desorption of CO due to photoelectronic excitation is found to occur via sequential single photon absorption and extraction of CO ligands. Evidence of clustering of carbonyl fragments was observed after 257 nm irradiation.

Calorimetric heat of adsorption measurements on palladium: II. Influence of crystallite size and support on CO adsorption

Integral heats of adsorption of CO, Q ad , have been measured at 300 K on unsupported Pd powder and on a family of Pd catalysts prepared from SiO 2, η-Al 2O 3, SiO 2Al 2O 3, and TiO 2 using a modified differential scanning calorimeter. For the samples studied, there was no evidence that Q ad was noticeably affected by the supports used to prepare the catalysts, and although high-temperature reduction of Pd TiO 2 catalysts sharply decreased the CO chemisorption, the Q ad values for these samples were comparable to those for the other catalysts. On the other hand, Pd crystallite size was found to have a significant effect on Q ad . For poorly dispersed samples, which contained average crystallite sizes up to 1000 nm, the Q ad values were usually close to 22 kcal mole −1; however, they increased sharply when the average Pd crystallite size dropped below 3 nm. The highest value, 35 kcal mole −1, was measured on the most highly dispersed samples. Although changes in either surface geometry or electronic properties as Pd crystallites sizes decrease below 5 nm could affect Q ad , the 13 kcal mole −1 increase on very small crystallites was larger than expected from ultrahigh-vacuum studies of CO chemisorption on Pd single-crystal surfaces. Consequently, changes in the electronic properties as Pd crystallite size decreases appear to be the major factor causing the stronger COPd bond strengths, in agreement with recent model calculations.

Mechanisms of carbon and oxygen incorporation into thin metal films grown by laser photolysis of carbonyls

We report the results of a series of experiments studying metal film growth by low-power UV-laser photolysis of metal carbonyls. Small-area Mo, W, and Cr films were grown on Si substrates by photolysis in a background pressure of 0.1–0.2 Torr of the carbonyl. The different metals exhibited vastly different deposit morphologies. Elemental depth profiling by Auger electron spectroscopy (AES) revealed the presence of large amounts of carbon and oxygen in the films. Ultrahigh vacuum studies of the interaction of 257- and 514-nm radiation with multilayer coverages of Mo(CO)6 and W(CO)6 adsorbed on Si(111)7×7 using high-resolution electron energy-loss spectroscopy, laser-induced desorption, and AES were performed to clarify the mechanisms of adsorbed metal carbonyl decomposition and impurity incorporation into the films. Decomposition of both the adsorbed and gas-phase carbonyls proceeds through direct electronic excitation of the molecule by absorption of the 257-nm radiation; however, the photodecomposition products of the adsorbed carbonyls are different from the gas-phase decomposition products. The evidence suggests that both the surface and the previously deposited photofragments stabilize the partially photolyzed carbonyls, preventing removal of all the CO ligands from the adsorbed species.

An X-ray photoelectron spectroscopy study of the influence of hydrogen on the oxygen–silver interaction

Hydrogen treatment of a pure silver catalyst before adsorption of oxygen influences the position and form of the O(1s) peak of the X-ray photoelectron spectra. It is possible that both the formation of sub-surface OH groups and an increase in the concentration of sub-surface oxygen, found to be of importance in earlier work, are responsible for this effect. Furthermore, when oxygen adsorption is carried out with extremely high exposures and at elevated temperature, pronounced differences occur in spectra obtained at temperatures between 298 and 723 K as compared with the results of ultrahigh vacuum studies reported in the literature. These differences are again most probably caused by the presence of the sub-surface oxygen in the present study.

Decomposition pressure of hydrogen in the α and α + β phase fields of the TiH system

Solid solutions of hydrogen in the α and α + β phase regions of the TiH system were investigated by measuring the equilibrium hydrogen pressure as a function of temperature (370–860 °C) and concentration (0–17 at.% H). The data required corrections to account for the residual hydrogen originally present in the metal because of incomplete degassing. Exclusion of such corrections in published data may have been responsible for the previous uncertainties in the Sieverts constants for this system. The P- C- T relations for the α phase can be expressed by P 1 2 = 1.130 × 10 3 r 0.2009−r exp( −5.061 × 10 4 −2.781 × 10 3r−2.991 × 10 5r 2 RT ) where P is the hydrogen pressure in kilopascals, r is the hydrogen-to-titanium atomic ratio and R = 8.3146 J K −1 mol −1 is the gas constant. The coefficients in this equation were constrained to reproduce the Sieverts constant reported in a recent ultrahigh vacuum study, i.e. >K s= lim N H→O P 1 2 N H =5.625×10 3exp(− 5.061×10 4 RT where N H = r (1 + r) is the atom fraction of hydrogen. The α + β region was investigated at two compositions: 13.5 and 17.0 at.% H. Below about 540 °C the “plateau” pressures of both alloy compositions produced a single linear van't Hoff plot, corresponding to P = 1.182 × 10 6 exp( −1.073 × 10 5 RT) . Combined with the existing “plateau” pressure data for the β + γ region, this equation yielded a new eutectoid temperature of 281 °C for the TiH system. Above about 540 °C the van't Hoff plot splits into two curvilinear branches owing to the composition changes in the α and β phases with temperature. The α−(α+β) boundary was found to conform to the accepted phase diagram.

Photothermal displacement spectroscopy: An optical probe for solids and surfaces

We present a sensitive technique for determining the optical and thermal properties of solids, surfaces and thin films. This technique, photothermal displacement Spectroscopy, is based on the detection of the thermal expansion of a sample upon absorption of electromagnetic radiation. The technique is well suited for in situ ultrahigh vacuum studies and for experiments where wide temperature ranges are required. We show that surface and bulk optical absorption can be distinguished and that surface absorptions of αL=10−6/W of incident power can be measured. The theoretical basis of the signal generation is given, and excellent experimental and theoretical agreement is demonstrated. The implications of our findings to imaging and microscopy are discussed.

Novel oxygen source for ultrahigh vacuum studies: C Y Yang and W E O'Grady, J Vac Sci Technol, 20 (4), 1982, 1056–1059

Novel oxygen source for ultrahigh vacuum studies: C Y Yang and W E O'Grady, J Vac Sci Technol, 20 (4), 1982, 1056–1059

Chemistry of cyclic olefins and polyenes on nickel and platinum surfaces

Results of ultrahigh vacuum studies of C/sub 4/-C/sub 8/ cyclic olefins and polyene chemisorption on Ni and Pt surfaces are presented. A mechanism is proposed for the novel surface mediated conversions of cycloheptatriene and norbornadiene to benzene. Studies of adsorption and thermal desorption of norbornadiene-7-d/sub 1/ on Pt(111) and Pt(100) indicated that there is no deuterium in the benzene produced on Pt(111), but that produced on Pt(100) contained approx. 15 to 20% C/sub 6/H/sub 5/D. An exchange process between C/sub 6/H/sub 6/ and D atoms on Pt(100) has been established but not for Pt(111). (BLM)

Pyridine coordination chemistry of nickel and platinum surfaces

Isotopic labelling studies have been used to provide fundamental understanding of the reactions of pyridine on various crystal planes of Ni and Pt. Ultrahigh vacuum studies indicated interaction via ..cap alpha..-pyridyl bonding for some of the crystal planes. Reactions for pyridine complexes of the metals with hydrogen and deuterium were established as an H-D exchange process. (BLM)

Novel oxygen source for ultrahigh vacuum studies

High-temperature solid oxide electrolyte (e.g., yttria-stabilized zirconia) cells are compatible with ultrahigh vacuum systems. Since the oxygen activity at the electrode/electrolyte interface is controlled precisely by the applied potential, the cells are ideal as oxygen sources for UHV studies. A three-electrode oxide electrolyte cell has been constructed and incorporated into a UHV surface analysis chamber. The control of the cell is extremely sensitive, and it can be used either to maintain a constant oxygen pressure or to pulse oxygen similar to a molecular beam source in the UHV chamber.