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Under ultrahigh vacuum conditions, extremely small Ge nanodots embedded in SiO2, i.e., Ge–SiO2 quantum dot composites, have been formed by ion implantation of 74Ge+ isotope into (0001) Z-cut quartz at a low kinetic energy of 9 keV using varying implantation temperatures. Transmission electron microscopy (TEM) images and micro-Raman scattering show that amorphous Ge nanodots are formed at all temperatures. The formation of amorphous Ge nanodots is different from reported crystalline Ge nanodot formation by high energy ion implantation followed by a necessary high temperature annealing process. At room temperature, a confined spatial distribution of the amorphous Ge nanodots can be obtained. Ge inward diffusion was found to be significantly enhanced by a synergetic effect of high implantation temperature and preferential sputtering of surface oxygen, which induced a much wider and deeper Ge nanodot distribution at elevated implantation temperature. The bimodal size distribution that is often observed in high energy implantation was not observed in the present study. Cross-sectional TEM observation and the depth profile of Ge atoms in SiO2 obtained from x-ray photoelectron spectra revealed a critical Ge concentration for observable amorphous nanodot formation. The mechanism of formation of amorphous Ge nanodots and the change in spatial distribution with implantation temperature are discussed. [ABSTRACT FROM AUTHOR]

Organized extremely small Ti–Sn alloy nanodots have been formed in the subsurface of SiO2 by dynamic coimplantation of isotopic 48Ti+ and 120Sn+ at a low kinetic energy of 9 keV into (0001) Z-cut quartz at different substrate temperatures. Transmission electron microscopy images show that the Ti–Sn alloy nanodots are single crystal and have been formed uniformly at room temperature. They are distributed in a two-dimensional array with similar size of ∼4 nm and constant interdot spacing between each dot. The regions beyond and below the two-dimensional array are depleted of detectable nanodots. At high temperature, the distribution and crystallinity were destroyed with much smaller amorphous nanodots in a slightly deeper region. The implantation was carried out by dynamic coimplantation, rather than the commonly used sequential implantation. These results indicate that dynamic low energy coimplantation is capable of forming well-ordered two-dimensional array of alloy nanodots. [ABSTRACT FROM AUTHOR]

The kinetics of isothermal adsorption and migration of atomic hydrogen on a Si(100) surface has been investigated by the time-of-flight scattering and recoiling spectrometry technique. A continuous decrease in saturation coverage with temperature under constant atomic hydrogen exposure has been observed for temperatures in the range 325–750 K. This observation is in contrast with a widely accepted view of the Si(100)/H surface as having three fixed coverage states within certain temperature windows. For T[sub S]=500–650 K, the decrease is described by a kinetic model in which the surface concentration of physisorbed hydrogen atoms is depleted due to the increased rate of migration from precursor sites to primary monohydride sites. The model suggests a mechanism to explain the dependence of the saturation value on temperature in this range. The migration constant obeys an Arrhenius expression with an activation energy of 0.71 eV. A significant concentration of hydrogen atoms occupying precursor states acts as a reservoir, saturating the monohydride dangling bonds after the hydrogen source is shut off and discontinuation of Eley–Rideal abstraction. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The linear optical absorption properties of a titanium nanoparticle composite formed by implantation of low energy Ti[sup +] into single crystal SiO[sub 2] are reported. Evolution of a surface plasmon resonance (SPR) induced by the formation of a metal nanoparticle composite is studied as a function of ion dose. At an implantation temperature of 25°C, the threshold dose for the appearance of the SPR is ∼3.5 × 10[sup 16] ions/cm², indicating that spontaneous nucleation and clustering of titanium nanoparticles in SiO[sub 2] occurs at ∼3.2 × 10[sup 22] ions/cm³ peak concentration of implants. The average particle size and volume fraction are dependent on the ion dose. The correlation of the optical response, i.e., the intensity and frequency of the observed SPR, with the nucleation/clustering of the metal nanoparticles is discussed on the basis of Mie scattering theory and Maxwell Garnett theory. [ABSTRACT FROM AUTHOR]

The optical absorption spectra of 5 keV Ar[sup +] and Ti[sup +] irradiated LiNbO[sub 3] Z-cut single crystals at various temperatures have been investigated. After Ar[sup +] irradiation to a dose of 10[sup 17] ions/cm², an optical absorption band develops in the range 326-800 nm at elevated temperatures. This band is ascribed to the formation of bulk oxygen vacancy related polarons, its intensity increasing with temperature. By using various sequences of irradiation and annealing, a synergetic effect between ion irradiation and substrate annealing on the oxygen vacancy generation in LiNbO[sub 3] is revealed. Irradiation with Ti[sup +], whose ballistic behavior is similar to Ar, produced an absorption band at room temperature, while for Ar[sup +] no such absorption was observed. This band is ascribed to Ti[sup 3+] transitions. However, at 660 °C, the Ti[sup +] irradiation induced absorption band had a significantly lower intensity than that induced by Ar[sup +]. It is proposed that this "chemical effect" of subplanted Ti in the surface region is a result of Ti reaction with oxygen, thereby trapping some of the outdiffusing oxygen as TiO[sub x] and reducing the oxygen deficiency in the surface region. A bulk oxygen outdiffusion mechanism is developed based on these observations, which involves preferential sputtering of oxygen during irradiation, rendering an oxygen deficiency in the surface region. This deficiency of surface oxygen enhances the outdiffusion of highly mobile oxygen in the bulk towards the surface at elevated temperatures. [ABSTRACT FROM AUTHOR]

The non-steady-state radiation enhanced diffusion of Ti in MgO(100) prebombarded with 7 keV Ar[sup +] and Cl[sup +] has been studied and the data have been first interpreted quantitatively in terms of a diffusion-reaction mechanism. Secondary ion mass spectroscopy was used for depth profiling and mass spectral measurements. The results revealed an unrealistic stoichiometry for formation of a volatile TiCl[sub x] compound, indicating the failure of the hypothesis of volatile species formation during diffusion. A modified Kapinos-Platonov model [Rad. Eff. 103, 45 (1987)] was developed which provides a consistent interpretation of the data. The model purports that during the thermal relaxation stage of the energetic collision cascade, the initially identical defect structures induced by Ar[sup +] and Cl[sup +] implantation undergo different chemical reaction processes, leading to different Mg vacancy cluster distributions. Statistically, Mg vacancy clusters in the Cl[sup +] prebombarded MgO are larger in size than those in Ar[sup +] prebombarded MgO. The difference in these defect structures is the reason for the difference in the subsequent Ti diffusion in MgO as we observed previously [J. Chem. Phys. 115, 446 (2001)]. From this model it is derived that after diffusion for an extended time period, Ti diffusion in Cl[sup +] prebombarded MgO is greater than that in Ar[sup +] prebombarded MgO due to the complete release of monovacancies stored in the vacancy clusters. This trend is just opposite of the case of short-time diffusion as in the previous case. This prediction was conformed by using prolonged diffusion experiments. [ABSTRACT FROM AUTHOR]

We report the linear optical properties of a titanium nanoparticle composite formed by implantation of low energy Ti[sup +] into single crystalline SiO[sub 2]. A surface plasmon resonance (SPR) induced absorption band is found in the 650-850 nm wavelength range, i.e., in the visible (red) and near infrared regions, indicating the formation of Ti nanoparticles in the subsurface layer of the SiO[sub 2] matrix. The average size of the particles is ∼2.6 nm and the volume fraction is as low as 0.016. At the implantation energy of 9 keV and total dose of 3 × 10[sup 16] ions/cm², the SPR absorption band becomes distinguishable only at temperatures greater than 600 °C and reaches its maximum at ∼800°C (½T[sub m]). Enhanced nucleation with nearly constant particle size with increasing temperature contributes to the increased SPR absorption intensity at temperatures ≤800 °C. The SPR absorption intensity decreases significantly near 1000°C (2/3T[sub m]). A blueshift of the SPR absorption maximum with substrate temperature is also observed. The shape of the SPR absorption band and the difference between low and high energy implantations is discussed. [ABSTRACT FROM AUTHOR]

The effect of postbombardment annealing, or postannealing, has been investigated for Ti diffusion in ion prebombarded MgO(100). MgO postannealing was performed in ultrahigh vacuum after ion prebombardment of MgO and prior to Ti evaporation and diffusion. For postannealing temperatures of T= 800, 900, and 1000 °C, Ti diffusion in 7 keV Ar[sup +] prebombarded MgO exhibits the expected exponential decays with postannealing time and more rapid decays with higher temperatures. Cationic diffusion in the ion prebombarded sample proceeds in a nonsteady state condition of extra defects known as nonsteady state radiation enhanced diffusion (NSRED). Thermal dissociation of small vacancy clusters is responsible for NSRED. From the data of T ≤ 1000 °C, the derived dissociation energy of the vacancy clusters is only ∼10% of the normal value. For some specific postannealing times at T = 1100 °C, the subsequent Ti diffusion was more enhanced than that without postannealing, a phenomenon which is contrary to the usual understanding of annealing effects. This same effect was also observed for Ti diffusion in 7 keV C1[sup +] prebombarded MgO postannealed at 1100 °C. Previous work indicates that the small vacancy clusters for NSRED are mainly Mg divacancies. We propose a divacancy creation mechanism that is due to the thermal dissociation of larger vacancy clusters during postannealing. The dissociation energy ratio of the effective large vacancy clusters (LVC) to that of the divacancies (DIV) is E[sub LVC]/E[sub DIV] ∼ 1.4. The monovacancy coalescence was found to be unimportant for divacancy creation. A model based on this mechanism explains the unusual diffusion enhancement observed. [ABSTRACT FROM AUTHOR]

Scattering of molecular nitrogen ions in the 1.5–4.5 keV range from gold and graphite surfaces results in a small fraction of surviving molecules and molecular ions in addition to atoms and atomic ions resulting from dissociation. The kinetic energy (Ek ) distributions of scattered N+2 and N+ ions have been measured directly by means of an electrostatic sector analyzer (ESA) and the velocity distributions of the scattered N2 and N neutrals plus ions have been measured by time-of-flight (TOF) techniques. Scattered ion fractions were determined from the TOF measurements. The relative Ek distributions of the scattered atomic ions indicate that dissociation from excited repulsive electronic states which are populated during the collision dominate the mechanism, rather than purely vibrational or rotational excitation from the X 2Σ+g ground state of N+2 . The excited dissociative C 2Σ+u and D 2Πu states of N+2 are accessible by Franck–Condon transitions from the X2Σ+g state. The data are consistent with a mechanism in which these two dissociative states contribute their repulsive energy to the large relative Ek distributions of the scattered N+ ions. The electronic excitation occurs via electron promotion during the scattering collision through the Fano–Lichten mechanism. [ABSTRACT FROM AUTHOR]

The chemisorption of benzene and phenol on a clean Ni{110}–(1×1) surface and an oxygen predosed Ni{110}–(3×1)–O surface near room temperature has been investigated by time-of-flight scattering and recoiling spectrometry accompanied by shadow cone calculations. The Ne scattering and H, C, and O recoiling fluxes exhibited strong angular anisotropies as a function of beam incident (α) and crystal azimuthal (δ) angles. These anisotropies are due to C and O atoms shadowing their neighboring atoms within the benzene molecules and resulting phenoxide species, demonstrating that scattering and recoiling spectrometry is capable of providing structural information on polyatomic molecular systems. The results show that both benzene and phenoxide are chemisorbed as molecules which have very good short-range order despite the absence of long-range order observable by low energy electron diffraction. Both benzene and phenoxide are oriented nearly parallel to the surface, with a maximum inclination angle of 15°. The C atoms in the para positions of benzene and the C–O bond in phenoxide are oriented along the <001>= azimuth. The C–H bond is bent out of the plane of the hexagonal ring so that the H atoms are above the C atom plane. Chemisorption on the oxygen predosed surface results in a reaction in which a H atom is abstracted from both benzene and phenol with the formation of surface hydroxide groups; the molecules remain well ordered on this surface also. [ABSTRACT FROM AUTHOR]

The oxygen induced reconstructed phases of the Ni{110} surface have been studied by time-of-flight scattering and recoiling spectrometry (TOF–SARS). The substrate structures are determined from experimental measurements of azimuthal angle (δ) and polar incident angle (α) anisotropies in the scattered Ne intensities coupled with classical trajectory simulations for shadow cone analysis. By monitoring features in the TOF–SARS scans that are unique to specific phases, it is possible to follow the migration of the first-layer Ni atoms as a function of O2 exposure. The results show that upon increasing exposures of the clean Ni{110}–(1×1) surface to O2, a series of LEED patterns [initial p(3×1), p(2×1), and final p(3×1)] is produced corresponding to three surface phases which differ only in the density of the first-layer Ni <001> rows. These nascent ‘‘added rows’’ are stabilized by bonding to oxygen atoms which reside in the long-bridge positions along the <001> rows. Structural models for the three phases are confirmed from the TOF–SARS data and the inhibiting effects of carbon and sulfur impurities on the reconstruction is considered. [ABSTRACT FROM AUTHOR]

The oxygen adsorption site on the Ir{110}-c(2×2)-O surface has been studied by time-of-flight scattering and recoiling spectrometry (TOF-SARS) using 4 keV Ne+ for backscattering and Ar+ for recoiling. The oxygen site was analyzed from scans of (i) backscattering intensity versus incident angle, (ii) oxygen recoil intensity versus incident and azimuthal angle, and (iii) oxygen recoil energy versus azimuthal angle. Calibrated shadow cones and trajectory simulations were used to obtain the site coordinates. This TOF-SARS data is contrasted with that of Ni{110}-p(2×1)-O, in which it is well established that the adsorption site is in the long-bridge position along the <001> rows. Adsorption of oxygen in the short-bridge sites above the <110> Ir rows is the only model consistent with all of the experimental data and simulations. The O–Ir bond length is estimated to be ≊1.8 Å. [ABSTRACT FROM AUTHOR]

Time-of-flight (TOF) spectra of the scattered and recoiled particles resulting from 1–10 keV He+, Ne+, Ar+, Kr+, and Xe+ ions impingent on surfaces of LiF thin films have been obtained. Measurements of directly recoiled (DR) neutrals plus ions and neutrals alone are used to calculate positive and negative ion fractions Y+,- from DR events. The oppositely charged ion fractions have a distinctly different behavior as a function of kinetic energy. The Y+ values exhibit a threshold at low energy followed by a plateau region at higher energy while the Y- values are maximum in the low energy region followed by a decreasing yield as energy increases. The energy dependence of Y+,- is interpreted in terms of the recently developed model [J. Chem. Phys. 85, 3615 (1986)] for electronic charge exchange in keV ion/surface collisions which considers electron promotions in the close atomic encounter and resonant and Auger transitions along the outgoing trajectory. The ionization potential of the primary ion relative to the energy levels of the target atom is shown to have a large influence on charge exchange in the close encounter. The ratio of direct recoil to scattering particle flux increases by a factor of >102 from He to Xe; scattering and recoil cross sections are used to model this process. [ABSTRACT FROM AUTHOR]

Examines amorphous diamondlike carbon (DLC) films grown by low-energy mass-selected ion-beam deposition using electron-energy-loss spectroscopy. Information on plasmon resonance; Characteristics of amorphous carbon films with diamondlike properties; Methods for the study of DLC films; Forms of carbon used for comparison spectra.

Presents a study which determined the substrate temperature dependence of homoepitaxial growth of silicon using mass selected ion beam deposition. Function for the reflection of high-energy electron diffraction and Rutherford backscattering spectrometry; Transition to an amorphous phase at lower film thickness; Effects of substrate temperature, contamination and surface damage on the growth mechanism.