Surface Peak Area Research Articles

Low-energy ion-beam induced effects in Al(1 0 0) surface studied using Rutherford backscattering and channeling

The results of low-energy ion spectroscopy can be compromised by the damage that the ions cause in surface layers of target single crystals, so it is important to characterize the impact of the probe ion on the target. In this investigation Al(100) single crystal surfaces have been irradiated at room temperature with 1keV He+ and Ar+ ions typical of low-energy ion scattering spectroscopy (LEIS) and sample sputter cleaning, respectively, and the effects have been studied as a function of ion dose and annealing. Rutherford backscattering and channeling measurements were used to probe the depth distribution and annealing characteristics of the dechanneling yield induced by low-energy ion bombardment. Analysis of the Al surface peak area for channeled MeV He ions, as well as the dechanneling yield from well below the surface, indicated an unusual increase in the depth distribution of the backscattered ion yield from Al with increasing low-energy He+ ion dose. Thermal annealing of the crystal following irradiation with the highest He+ dose of 4×1017 ions/cm2 restored the dechanneled ion yield to its lower starting value, with the relatively low activation energy of 0.05eV. The increase in backscattered ions is attributed to implanted He atoms located in the [100] channels. The Ar+ ion induced effects in the channeling spectrum are not as significant as those for He+ ions, even at the highest Ar+ dose of ∼1017 ions/cm2.

Growth of thin Ti films on Al single-crystal surfaces at room temperature

The growth of thin Ti films on Al(001), Al(110) and Al(111) surfaces at room temperature has been studied using high-energy Rutherford backscattering spectroscopy (RBS) and channeling, x-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). Our results show that Ti atoms form a thin, metastable, fcc overlayer on Al(110) and Al(001) surfaces. The primary evidence for this conclusion is the reduced backscattering that occurs as the Ti atoms shadow the Al atoms in the fcc structure of the Al substrate. For the Al(111) surface the Al surface peak area in ion channeling, measured as a function of Ti coverage, shows a small decrease for the first monolayer (ML) of Ti coverage but then increases gradually with coverage, a characteristic of alloy formation. However, XPS and low-energy ion scattering (LEIS) results are generally consistent with overlayer growth of Ti on the Al(111) surface, and the LEED pattern indicates an ordered overlayer for Ti coverages from 2 to 12 ML, at which point the Al surface was completely covered by Ti. The results suggest the growth of incommensurate, ordered islands of hep Ti on Al(111) in a Stranski-Krastanov growth mode, in remarkable contrast to the pseudomorphic fcc Ti overlayer growth observed for Ti films on Al(001) and Al(110).

Epitaxial growth of fcc Ti films on Al(001) surfaces

High-energy ion scattering (HEIS), x-ray photoelectron spectroscopy, and x-ray photoelectron diffraction (XPD) were used to study the growth of thin Ti films on Al(001) surfaces. The Al surface peak area in the backscattered ion spectrum of MeV He{sup +} ions, incident along the [00{bar 1}] direction, was used to monitor the atomic structure of the Ti films during growth. An initial decrease in the area was observed indicating epitaxial film growth. This decrease continued up to a critical film thickness of about 5.5 ML, after which point the structure of the film changed. Titanium films 3, 5, and 9 ML thick were characterized using XPD in the {ital same} chamber. Both the HEIS and XPD results show that the Ti films grow with an fcc structure on Al(001). A tetragonal distortion of 2.4{percent} in the fcc Ti film was measured using ions incident along the [10{bar 1}] direction. Although there is a general similarity of fcc Ti growth on both Al(001) and Al(110), the submonolayer growth regime does show differences for the two surfaces. {copyright} {ital 1997} {ital The American Physical Society}

Rutherford backscattering and channeling studies of a TiO 2(100) substrate, epitaxially grown pure and Nb-doped TiO 2 films

We have investigated the crystalline quality of a TiO 2(100) substrate, homoepitaxially grown TiO 2 film and Nb-doped TiO 2 films using Rutherford backscattering (RBS) and channeling experiments. The minimum yields obtained from the aligned and random spectra are 2.4 ± 0.2% for the TiO 2(100) substrate, and 4.0 ± 0.2% for a homoepitaxial TiO 2 film. The minimum yields for Ti and Nb are 1.6 ± 0.2% and 7.0 ± 1.0%, respectively, for a Nb-doped TiO 2 film. Also, about 95% of the Nb atoms occupy cation sites in the Nb-doped TiO 2 film. The angular yield curves for Ti and Nb from the Nb-doped film confirm the good crystalline quality of the film in which most Nb atoms occupy the cation sites. The calculated surface peak areas for Ti and Nb using a model which incorporates Nb surface segregation from the bulk, agree very well with the corresponding surface peak areas for Ti and Nb extracted from the experiment.

Room temperature growth of thin Pd films on Al(110) surfaces

High-energy ion backscattering spectroscopy (HEIS) and X-ray photoemission spectroscopy (XPS) have been used to study the growth of thin Pd layers on Al(110) surfaces at room temperature. The results are not consistent with the layered growth model presented previously for this system, but suggest instead that an AlPd alloy forms at the interface. Measured Al and Pd surface peak areas for MeV He + ions incident normal to the surface show that the Pd atoms intermix with and displace Al substrate atoms. The interface mixing was observed to continue up to about 10 ML of Pd coverage, at which point the intermixing stops. XPS measurements of the Pd 3d photopeaks show a chemical shift that is consistent with the formation of an AlPd-like compound at the interface and the growth of Pd metal thereafter.

Growth of ultrathin Pd films on Al(001) surfaces

High-energy ion backscattering spectroscopy (HEIS) and X-ray photoelectron spectroscopy (XPS) were used to determine the growth mode and the interface structure of ultrathin Pd films deposited on Al(001) surfaces at room temperature. Measured Al and Pd surface peak areas for MeV He + ions incident normal to the surface show that Pd atoms intermix with and displace Al substrate atoms. The mixing continues for Pd coverages from 0–5 monolayers, at which point a Pd metal film begins to grow on the alloy surface. XPS measurements of the Pd 3d photopeaks show a chemical shift that is consistent with the formation of an AlPd-like compound during the mixing phase, and Pd metal thereafter. HEIS results further reveal that the alloyed overlayer as well as the Pd metal film have some degree of axial alignment with respect to the Al substrate. The XPS intensity measurements are consistent with this two-stage growth model.

Spectra of secondary electrons induced by channeled and nonchanneled ions in Si and Al.

Energy spectra are measured in the (1--2)-keV range of secondary electrons induced by bombardment of Si(100) and Al(110) with a 1.5-MeV ${\mathrm{He}}^{+}$ beam. The ion beam is either aligned with a major crystallographic direction or incident along a random direction. The shape of the experimental secondary electron spectra are successfully compared with that of spectra calculated with an efficient Monte Carlo model for electron-transport simulation. In addition, the effective layer thickness L for secondary electron generation under channeling incidence conditions is determined. It is found that ${\mathit{L}}_{\mathit{A}}$ for KLL Auger electron generation is equal to the surface peak area in the spectra of the backscattered ions. This similarity is a consequence of the small values of the backscattering collision diameter and the adiabatic radius for K-shell ionization, as compared to the atomic vibration amplitude. In contrast, ${\mathit{L}}_{\mathit{B}}$ for the generation of electrons by direct Coulomb ionization is much larger than that for Auger emission. The large value for ${\mathit{L}}_{\mathit{B}}$---an indication of a reduced channeling effect---is attributed to the relatively large contribution from the moderately localized L shell to the measured spectra. \textcopyright{} 1996 The American Physical Society.

Observation of ultrathin metastable fcc Ti films on Al(110) surfaces.

High-energy ion-scattering spectroscopy, x-ray-photoemission spectroscopy, and low-energy electron diffraction (LEED) were used to study the growth of very thin Ti films on Al(110) surfaces at room temperature. The Al surface peak areas in the backscattering spectra of 0.96-MeV ${\mathrm{He}}^{+}$ ions, incident along the [1\ifmmode\bar\else\textasciimacron\fi{} 1\ifmmode\bar\else\textasciimacron\fi{}0] direction of Al(110), decreased sharply during the deposition of the first five monolayers of Ti. This suggests a growth model in which a pseudomorphic Ti film with a fcc structure forms, shadowing the Al surface atoms. This model is supported by LEED measurements where sharp diffraction spots persisted with the rectangular symmetry of the substrate lattice structure. Throughout this coverage regime, the attenuation of the Al photopeak intensities as a function of Ti coverage agrees with the attenuation rate calculated using a laminar growth mode. With additional Ti deposition the Al surface peak intensities increase while the Al photopeak intensities remain unchanged. A discussion of possible growth models for the high-coverage regime is presented.

Room temperature reaction of thin Ni films with Al(110) surfaces

We report the results of high-energy ion backscattering and channeling (HEIS) experiments together with x-ray photoemission spectroscopy (XPS) to determine the interface structure for thin Ni films deposited on Al(110) single crystal surfaces at room temperature. Measurements of the Al and Ni surface peak areas (SPA) show that the Ni atoms do not form a simple overlayer on the Al substrate, but react with and displace Al substrate atoms. The reaction continues with two different rates for a range of Ni coverage from 0 to 8 monolayers (ML) before Ni begins to cover the surface, which differs significantly from the thermally reacted Ni/Al systems. For the first 2.3 monolayers of deposited Ni, the NiAl phase is formed with additional displaced Al atoms at the interface, while for Ni depositions between 2.3 and 8.1 ML, a Ni3Al phase is formed. These phase identifications were made using XPS measurements of the Ni-2p3/2 peak chemical shifts and the shape and separation of the energy loss peak. The reaction at the interface is simulated using the embedded atom method to calculate the interaction between Al and Ni atoms. One ML of Ni atoms was initially distributed in small clusters on the Al surface, and a Monte Carlo approach was used to generate snapshots of the evolving interface. Computer simulations of the HEIS yield from these snapshots show an increase in the SPA of Al in good agreement with our experimental measurements.

Growth of thin Ti films on Al(110) surfaces

High-energy ion scattering, x-ray photoemission spectroscopy, and low-energy electron diffraction (LEED) were used to study the interfacial reaction of very thin Ti films on Al(110) surfaces. The Al surface peak areas in the backscattering spectra of 0.96 MeV He+ ions incident along the [1̄1̄0] direction decreased sharply during the deposition of the first four monolayers of Ti. This suggests a growth model in which an ordered Ti overlayer shadows the Al surface atoms. This model is supported by electron diffraction measurements where sharp diffraction spots persisted with the rectangular symmetry of the substrate. Throughout this coverage regime, photopeak intensities due to Al were attenuated strongly with Ti coverage. With additional Ti deposition the Al surface peak intensities increase while the Al photopeak intensities remain unchanged, indicating a reaction at the Ti/Al interface that exposes additional Al surface atoms to the incident ion beam. Ion scattering spectra taken with the He+ ion beam incident along a random direction with respect to the Al substrate exhibit increased Ti peak areas indicating Ti shadowing. A LEED pattern was observed throughout the experiment suggesting the formation of an ordered overlayer. We present a growth model to explain these observations.

Ion scattering studies of clean Nb(110) surfaces before and after exposure to hydrogen

The Nb(110) surface was studied using high-energy ion backscattering and channeling (HEIS) to determine the structure of the clean surface, and to measure changes in structure induced by H2 adsorption at the surface and absorption into the bulk material. Measurements with He+ ions channeling in the [1̄1̄0] and [1̄1̄1] directions were made as a function of ion energy (0.6 to 1.8 MeV) and H2 exposure. Computer simulations of the experiment for the clean surface show good agreement with the data using negligible enhancement (≤2%) of the surface-atom vibration amplitudes, and negligible surface relaxation. To obtain agreement with experiment does require that the simulations include reduced bulk atom vibration amplitudes, which simulate the effect of correlated vibrations. Our conclusions are in good agreement with calculations based on experimental phonon frequencies. As the surface is exposed to H2, the Nb surface peak area increases by ∼12% (1 MeV ion energy) in the chemisorption regime (0 to 20 L), followed by a more gradual increase as the H is absorbed into the bulk Nb. We discuss possible explanations for these increases in ion yield in the context of recent models for hydrogen uptake kinetics.