Medium-energy Ion-scattering Study Research Articles

Medium-energy ion-scattering study of strained holmium silicide nanoislands grown on silicon (100)

We have used medium-energy ion scattering (MEIS) to quantitatively analyze the structure of holmium silicide islands grown on the Si(100) surface. Structure fitting to the experimental data unambiguously shows that the tetragonal silicide phase is present and not the hexagonal phase, which is associated with the growth of nanowires at submonolayer coverages. Islands formed with a lower holmium coverage of 3 ML are also shown to be tetragonal, which suggests that the hexagonal structure is not a low coverage precursor to the growth of the tetragonal phase. MEIS simulations of large nanoislands, which include the effects of lateral strain relief, have been performed and these compare well with the experimental data.

MEIS Study of Sb Implantation in O or N pre-implanted Si(001) and SIMOX

Medium energy ion scattering (MEIS) was used to study distribution of ion-implanted Sb dopant in Si with excess vacancies and SIMOX substrates and the effects of thermal treatments. Extra vacancies in Si were generated by N or O pre-implantations at high temperature. Effects related to the different chemical nature of the pre-implanted species are expected under these conditions. Different Sb annealing behavior and distribution were observed for O and N pre-implanted Si. The oxygen containing samples (SIMOX and O pre-implanted Si) presented higher substitutionality after long annealing times. The nitrogen pre- implanted Si presented the lowest amount of segregated Sb and more uniform dopant distribution. Both pre-implanted samples, N and O, had a large dopant loss to the atmosphere during annealing.

Adsorbate-Induced Segregation in the Ni{111}/Au/(R,R)-Tartaric Acid System

The adsorption of the catalytically important chiral modifier, (R,R)-tartaric acid, was investigated on bimetallic Ni/Au surfaces created by annealing thin Au films on Ni{111}. Reflection adsorption infrared spectroscopy measurements revealed that the adsorption mode of (R,R)-tartaric acid depends very strongly on the surface composition. Deprotonation to produce an adsorbed bitartrate species was achieved only when small Ni clusters became available in the bimetallic surface. Medium-energy ion-scattering studies showed that, once the bitartrate species was able to be produced, significant adsorbate-induced Ni segregation was observed. Scanning tunneling microscopy (STM) investigations suggest that segregation is particularly facile in the proximity of step edges. In addition, STM revealed that the ordered (9 × 9) structure previously reported by Jacobsen et al.1 is stable even with relatively high concentrations of Ni in the surface layer.

Medium energy ion scattering for the characterisation of damage profiles of ultra shallow B implants in Si

High depth resolution medium energy ion scattering (MEIS) in the double alignment mode has been used to determine the pre- and post-annealing damage distributions following 0.1–2.5 keV B + implantation into Si(1 0 0) at different substrate temperatures. Samples were irradiated to doses ranging from 1×10 14 to 2×10 16 cm −2 at substrate temperatures of −150°C, 25°C and 300°C. Rapid thermal processing (RTP) was carried out to temperatures ranging from 400°C to 1000°C for 10 s, to monitor the annealing of damage caused by the B + implant. For the room temperature (RT) implants, two distinct damage distributions were observed. The first was a narrow, near-surface damage peak which grows out from the virgin Si surface peak to a maximum depth of 3 nm, much shallower than the TRIM predicted mean projected range of e.g. 1 keV B + ions ( R p≈5.3 nm). The width of this damage layer showed only a weak dependence on the B + ion energy and strong dependence on the dose. The number of displaced atoms in this layer for dilute damage conditions is in good agreement with modified Kinchin Pease predictions. For 1 keV B +, a second, deeper damage peak appeared only after a B dose of 1×10 15 cm −2 , having a maximum at a depth of ≈7.5 nm, well beyond the R p of 5.3 nm. MEIS showed that this post-implant damage structure which develops for irradiations performed at 25°C and 300°C, is the result of dynamic annealing processes that are highly effective in the region in between the two peaks, in which Frenkel defects have their maximum production rates. The observed growth of the surface damage layer with implant dose is ascribed to the migration of point defects, created along the bombardment cascade, to the Si/SiO 2 interface. For 500 eV B + implants, due to proximity of this surface sink, the residual damage is greater even at 300°C. Implantations at −120°C resulted in a single, heavily damaged layer stretching from the surface to the position of the deep damage. These damage profiles show a direct correlation between the displaced Si and the implanted B distributions. MEIS yields approached random level, showing near or total amorphisation of the Si lattice; epitaxial regrowth, even after 30 s RTP at 600°, was however only partial, apparently arrested at B containing I clusters formed near R p of the B distribution. RTP at 400°C and 500°C of the samples implanted at room temperature leads to substantial reduction in the Si damage, especially in the width of the near-surface peak. It suggests a substantial rearrangement of Si atoms in the lattice that occurs without the release of Si interstitials, in view of the absence of TED at these temperatures and may involve a degree of realignment of the damage structure with the channelling direction. The annealing behaviour measured by MEIS at higher temperatures is consistent with XTEM observations, showing the formation and growth in size of extended interstitial defects and their ultimate dissolution at high temperature. As well as moving into the bulk where they cause TED, a fraction of the released interstitials migrate to the surface and increase the width of the surface damage region. MEIS studies also indicates the occurrence of reverse annealing for high temperature implant conditions.

Molecular-beam-deposited yttrium-oxide dielectrics in aluminum-gated metal–oxide–semiconductor field-effect transistors: Effective electron mobility

We report on high effective mobilities in yttrium-oxide-based n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) with aluminum gates. The yttrium oxide was grown in ultrahigh vacuum using a reactive atomic-beam-deposition system. Medium-energy ion-scattering studies indicate an oxide with an approximate composition of Y2O3 on top of a thin layer of interfacial SiO2. The thickness of this interfacial oxide as well as the effective mobility are found to be dependent on the postgrowth anneal conditions. Optimum conditions result in mobilities approaching that of SiO2-based MOSFETs at higher fields with peak mobilities at approximately 210 cm2/V s.

CN-induced surface layer expansion of Ni(1 1 0): a MEIS study

Medium energy ion scattering (MEIS) using 100 keV H + incident ions has been used to determine the outermost substrate layer spacings of clean Ni(1 1 0) and Ni(1 1 0)c(2×2)-CN. Experimental blocking curves were obtained using [ 1 0 0], [ 1 1 2 ] and [ 1 0 1 ] ion incident directions and these have been interpreted with the aid of theoretical simulations for different model structures. For the Ni(1 1 0)c(2×2)-CN surface phase the ion scattering data indicate a substrate outermost layer expansion of 21.2±1.6%, providing independent confirmation of the large CN-induced surface layer relaxation obtained in an earlier scanned-energy mode photoelectron diffraction study of this surface.

Erratum: Investigation of a two-dimensional dysprosium germanide on Ge(111): A medium-energy ion-scattering study including direct observation of a reversal of top-layer buckling upon adsorption of hydrogen [Phys. Rev. B62, 5016 (2000)

Received 29 September 2000DOI:https://doi.org/10.1103/PhysRevB.63.049901©2001 American Physical Society

Investigation of a two-dimensional dysprosium germanide on Ge(111): A medium-energy ion-scattering study including direct observation of a reversal of top-layer buckling upon adsorption of hydrogen

A two-dimensional dysprosium germanide of stoichiometry ${\mathrm{DyGe}}_{2}$ on the Ge(111) surface, with a nominal Dy coverage of one monolayer, has been investigated using medium-energy ion scattering. A quantitative structural analysis has revealed that the Dy atoms reside beneath a well-ordered Ge(111) bilayer, rotated by $180\ifmmode^\circ\else\textdegree\fi{}$ with respect to the bulk. Upon adsorption of hydrogen, the buckling of the top layer was seen to reverse with a corresponding expansion of the Dy-Ge bond length. Possible generalizations for other rare earth/semiconductor systems are discussed.

Medium-energy ion-scattering study of the temperature dependence of the structure of Cu(111)

We have studied the surface structure and vibrations of Cu(111) at temperatures between 300 and 1180 K using medium-energy ion scattering. At 300 K, first and second interlayer spacing contractions of -1.0% and -0.2%, respectively, relative to the bulk spacing are found. Above 1040 K, the first interlayer spacing expands significantly more rapidly than the bulk spacing, so that at 1180 K a +4.3% expansion is found. This expansion is smaller than the one previously determined for Ag(111), in agreement with predictions. Monte Carlo simulation of spectra show also that the vibration amplitudes of the Cu(111) surface atoms are enhanced by \ensuremath{\sim}30% below 910 K, and \ensuremath{\sim}100% at 1180 K, with respect to their bulk values. \textcopyright{} 1996 The American Physical Society.

Quantitative structural analysis of fcc Fe(2 x 1) on Cu(001): A medium-energy ion-scattering study.

The detailed structure of a 6-ML Fe film grown epitaxially at 310 K on a Cu(001) substrate was determined by high-resolution medium-energy ion scattering through comparisons with Monte Carlo simulations of the ion-scattering experiment. The observation of excess ion scattering from the Cu substrate atoms indicates that the lattice parameter of the Fe film deviates from the Cu lattice constant, i.e., the Fe film grows nonpseudomorphically. The Fe film is ordered fcc and has a reconstructed surface layer in the form of atom row glide along [110] directions with an expansion of the surface atom layer. Improved agreement between the ion-scattering simulation and the experimental data is obtained when the Fe lattice is rotated by about 1.2\ifmmode^\circ\else\textdegree\fi{} relative to the Cu substrate.

Composition, structure, and morphology for Ag deposition on Cu(110): A medium-energy ion-scattering study.

Caracterisation de la composition, de la structure, et de la morphologie interfaciales, resultantes du depot d'Ag sur Cu(110) a 300 K, par diffusion d'ions (d'energie intermediaire) a haute resolution. Observation d'un mecanisme de croissance d'un agregat de couche prononce pour lequel il n'y a aucun melange mesurable entre l'Ag et le substrat Cu Succedant la formation d'un revetement epais d'une monocouche uniforme (1.30×10 15 atomes/cm 2 ), les agregats d'Ag sont rapidement nuclees, donnant une epaisseur moyenne superieure a 20 A pour un depot de moins de deux monocouches. Les clusters croissent comme une extension du type-(110) de la cristallographie du substrat, et ne sont pas une evolution continue de la monocouche hexagonale d'Ag formee durant le depot. Les clusters formes a 300 K sont instables aux temperatures elevees, et se decomposent pour donner des agglomerats profonds d'epaisseur ∼300 A apres recuit a 575 K

Medium-energy ion-scattering analysis of the c(2 x 2) structure induced by K on Au(110).

The Au(110) surface is reconstructed and shows a (1\ifmmode\times\else\texttimes\fi{}2) low-energy electron-diffraction (LEED) pattern, which is known to correspond to a missing-row reconstruction. At a coverage of half a monolayer of potassium, a c(2\ifmmode\times\else\texttimes\fi{}2) LEED pattern is seen, which is not easily rationalized in terms of the faceted structures observed at lower alkali-metal coverages. Two different models have been proposed for the atomic arrangements in this structure. We have performed a medium-energy ion-scattering study, using channeling and blocking, of this surface. We find very good agreement with a model first proposed by Ho et al.: The top layer of atoms consists of intersecting c(2\ifmmode\times\else\texttimes\fi{}2) arrays of Au and K atoms. The top layer of Au atoms is shifted toward the bulk, and the K atoms are displaced outward relative to this plane. The third layer of Au atoms is also buckled, all in quantitative agreement with the calculation.

Missing-row surface reconstruction of Ag(110) induced by potassium adsorption.

We report a medium-energy ion-scattering study of the (1\ifmmode\times\else\texttimes\fi{}2) reconstruction of the Ag(110) surface induced by submonolayer amounts of K. A qualitative comparison between ion-scattering data from the clean and K-covered surfaces shows that the (1\ifmmode\times\else\texttimes\fi{}2) periodicity is caused by a missing-row reconstruction of the Ag substrate. Computer simulations support this conclusion and provide detailed information on the atomic positions. Our observations support a recent model by Heine and Marks explaining surface relaxation and reconstruction in terms of the occupation of s-p and d electronic levels.

Determination of the surface geometry of gaAs(110) by the total energy and force methods

The equilibrium surface geometry of GaAs(110) is determined by using the total energy and force methods, within the pseudopotential and local-density schemes. The results, which are in agreement with recent LEED and medium-energy ion-scattering studies, suggest that the surface reconstructs with a top layer tilt angle of 31° ± 1° and lateral shifts of up to 0.53 and 0.33 Å for Ga and As, respectively. Calculated surface state energies for the equilibrium geometry are in agreement with photoemission data.