Coaxial Impact Collision Ion Research Articles

Critical Role of Terminating Layer in Formation of 2DEG State at the LaInO3/BaSnO3 Interface

AbstractBased on the interface polarization model, the 2D electron gas (2DEG) at LaInO3(LIO)/BaSnO3(BSO) interfaces is understood to originate from a polarization discontinuity at the interface and the conduction band offset between LIO and BSO. In this scenario, the direction of polarization at the interface is determined by whether the first atomic LIO layer at the interface is LaO+ or InO2−. The role of the terminating layer is investigated at the LIO/BSO interface in creating the 2DEG. Based on conductance measurements of the in situ grown LIO/BSO heterostructures, it has been reported in this work that the 2DEG only forms when the BSO surface is terminated mainly with a SnO2 layer. The terminating layer is controlled by additional SnO2 deposition on the BSO surface. It has been shown that the as‐grown BSO surface has a mixed terminating layer of BaO and SnO2 while the BSO surfaces prepared with additional SnO2 deposition are terminated mainly with the SnO2 layer. The terminating layer is confirmed by coaxial impact collision ion scattering spectroscopy. The finding is consistent with the interface polarization model for 2DEG formation at LIO/BSO interfaces, in which the direction of the interfacial polarization in LIO is determined by the terminating layer of the BSO surface.

Oxygen adsorption on wet-chemically cleaned GaN(0001) surface studied by coaxial impact collision ion scattering spectroscopy

Oxygen adsorption on wet-chemically cleaned GaN(0001) surface has been studied by coaxial impact collision ion scattering spectroscopy (CAICISS). The intensity variation of time-of-flight spectra obtained by CAICISS was compared with simulation results. The CAICISS analysis showed that oxygen is adsorbed at the top of Ga atoms at the surface. The Ga–O bond distance was estimated to be between 1.80 and 1.85 Å. The estimated bond distance is consistent with the Ga–O tetrahedral bond distance in β-Ga2O3.

Kagome-like structure of germanene on Al(111)

The structure of a 3 \ifmmode\times\else\texttimes\fi{} 3 reconstruction formed by Ge deposition onto Al(111) [Al(111)3 \ifmmode\times\else\texttimes\fi{} 3-Ge] was investigated by coaxial impact collision ion scattering spectroscopy, which is one of the most surface-sensitive structural analysis techniques. It has been revealed that Al(111)3 \ifmmode\times\else\texttimes\fi{} 3-Ge forms a structure with 10 Ge atoms in a unit cell, similar to the recently reported ``kagome-like'' silicene on Al(111) rather than the previously reported honeycomb lattice. A surface band structure of the Al(111)3 \ifmmode\times\else\texttimes\fi{} 3-Ge has also been investigated by angle-resolved photoemission spectroscopy (ARPES). The calculated band structure of our proposed model based on a density functional theory well reproduces ARPES spectra, which also indicates that Al(111)3 \ifmmode\times\else\texttimes\fi{} 3-Ge forms a kagome-like structure.

Unpredicted surface termination of α-Fe2O3(0001) film grown by mist chemical vapor deposition

We analyze the surface structure of an α-Fe2O3(0001) film grown on a c-plane sapphire substrate by mist chemical vapor deposition (CVD), which has been recently developed as a simple, safe, and cost-effective film growth method. Using coaxial impact-collision ion scattering spectroscopy, we found that the atomic-layer sequence of the surface termination of an α-Fe2O3(0001) film grown by mist CVD was Fe–O3–Fe– from the top layer. This surface termination is predicted to form in an oxygen-poor environment by density functional theory combined with a thermodynamical approach despite that the mist CVD process is performed with atmospheric-pressure air. The surface structure markedly changes after annealing above 600°C in ultrahigh vacuum. We found that only a couple of layers from the top layer transform into Fe3O4(111) after 650°C annealing, which would be so-called biphase reconstruction. Complete transformation into a Fe3O4(111) film occurs at 700°C, whose atomic-layer sequence is determined to be Fe–O4–Fe3– from the top layer.

Surface termination structure of α-Ga2O3 film grown by mist chemical vapor deposition

The surface structure of α-Ga2O3(0001) grown on an α-Al2O3(0001) substrate by mist chemical vapor deposition was studied by coaxial impact-collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). The minimum step height observed in the AFM image was 0.21 ± 0.01 nm, coinciding with the height of three atomic layers of α-Ga2O3(0001). It was revealed by CAICISS analysis that the surface of α-Ga2O3(0001) is terminated by a Ga layer followed by an O layer, which is consistent with the surface termination of α-Al2O3(0001). A structural model taking surface relaxation into account was also constructed by fitting the simulated curve for the azimuth angle dependence of the Ga intensity to the experimental dependence. The resultant structural model is similar to the model of an α-Al2O3(0001) surface, which indicates analogous behavior in corundum crystals.

Surface structure analysis of BaSi2(100) epitaxial film grown on Si(111) using CAICISS

Geometry and surface structure of a BaSi 2 (100) film on Si(111) formed by reactive deposition epitaxy (RDE) have been investigated using coaxial impact-collision ion scattering spectroscopy and atomic force microscopy. BaSi 2 (100) film can be grown only when the Ba deposition rate is sufficiently fast. It is revealed that a BaSi 2 (100) film grown at 600 °C has better crystallinity than a film grown at 750 °C owing to the mixture of planes other than (100) in the RDE process at higher temperatures. The azimuth angle dependence of the scattering intensity from Ba shows sixfold symmetry, indicating that the minimum height of surface steps on BaSi 2 (100) is half of the length of unit cell. By comparing the simulated azimuth angle dependences for more than ten surface models with experimental one, it is strongly indicated that the surface of a BaSi 2 (100) film grown on Si(111) is terminated by Si tetrahedra. • Surface structure of a BaSi 2 (100) is analyzed using ion scattering spectroscopy. • BaSi 2 (100) film can be grown only when the Ba deposition rate is sufficiently fast. • RDE at 600 °C is the best condition for growth of highly crystalline BaSi 2 (100) film. • We propose that the BaSi 2 (100) film grown on Si(111) is terminated by Si tetrahedra.

Surface structure of GaP(110): Ion scattering and density functional theory study

The structure of the GaP(110) surface has been investigated using coaxial impact collision ion scattering spectroscopy (CAICISS) and density functional theory (DFT). CAICISS simulations based on structural parameter values of the well known buckled dimer model obtained in quantitative low energy electron diffraction studies in the 1980s were found to fit well with experimental data measured in the $[\overline{1}10]$ azimuth, but offered a relatively poor fit in all other incident geometries. A new surface structure derived from DFT calculations, involving small changes to bond angles and interlayer spacings, was optimized during the analysis of CAICISS data, until good fits of the data were obtained for all three azimuths ($[\overline{1}10]$, $[001]$, and $[\overline{1}11]$). The key feature of the new structure found to be required for this improved agreement with experiment is the inclusion of relaxations both parallel and perpendicular to the surface between the second and third layers.

Polarity replication across m-plane GaN/ZnO interfaces

We have investigated the crystal plane alignment at m-plane GaN/ZnO heterointerfaces prepared by a room temperature epitaxial growth technique. Coaxial impact-collision ion scattering spectroscopy was used to show that the +c directions for GaN and ZnO are aligned at the GaN/ZnO (11¯00) hetero interface, which makes a striking contrast to polar c-plane GaN (0001)/ZnO (0001¯) interfaces, where polarity-flipping always occurs. Theoretical calculations revealed that an atomic alignment at the m-plane GaN/ZnO interface that maintains the +c direction across the interface is energetically favorable, although there could be an in-plane shift in the positions of the anions and cations at the interface.

Structural analysis of the Cu(100)–p(3[formula omitted])R45°–Sn surface using low and medium energy ion scattering spectroscopies

The atomic structure of the Cu(100)–p(32×2)R45°–Sn surface has been studied using medium energy ion scattering (MEIS), co-axial impact collision ion scattering spectroscopy (CAICISS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The p(32×2)R45° reconstruction was observed with LEED following the deposition of 0.42ML of Sn from a Knudsen cell on to the clean Cu(100) surface. The existence of sub-surface Sn atoms was immediately ruled out by the lack of blocking dips in the MEIS Sn scattered yield. Previously reported Sn overlayer and Cu–Sn surface layer alloy models were tested against the medium and low energy ion scattering data, with the best fit realised following structural optimization of the surface alloy model proposed by Pussi [K. Pussi et al., Surf. Sci. 549 (2004) 24]. Different out-of-plane shifts were observed for Sn atoms depending on their proximity to the missing Cu row.

Analysis of the interactions between He + ions and transition metal surfaces using co-axial impact collision ion scattering spectroscopy

The interactions between low energy He + ions and a series of transition metal surfaces have been studied using co-axial impact collision ion scattering spectroscopy (CAICISS). Experimental data were collected from the Ni(110), Cu(100), Pd(111), Pt(111) and Au(111) surfaces using ion beams with primary energies between 1.5 keV and 4.0 keV. The shadow cone radii deduced from the experimental surface peak positions were found to closely match theoretical predictions. Data analysis was performed using both the FAN and Kalypso simulation codes, revealing a consistent requirement for a reduction of 0.252 in the screening length correction in the Molière approximation within the Thomas–Fermi (TFM) interaction potential. The adjustments of the screening length in the TFM potential, predicted by O'Connor, and the uncorrected Ziegler–Biersack–Littmark (ZBL) potential both yielded inaccurate results for all of the surfaces and incident energies studied. We also provide evidence that, despite their different computational methodologies, the FAN and Kalypso simulation codes generate similar results given identical input parameters for the analysis of 180° backscattering spectra.

Reply to “Comment on `Computer Simulation of Coaxial Impact-Collision Ion Scattering Spectroscopy Spectra of Clean Pt(111) Surface and Pt(111)–p(2×2)-O Surface' ”

Reply to “Comment on `Computer Simulation of Coaxial Impact-Collision Ion Scattering Spectroscopy Spectra of Clean Pt(111) Surface and Pt(111)–p(2×2)-O Surface' ”

Comment on “Computer Simulation of Coaxial Impact-Collision Ion Scattering Spectroscopy Spectra of Clean Pt(111) Surface and Pt(111)–p(2×2)-O Surface”

The clean Pt(111) and Pt(111)-p(2×2)-O surfaces have been investigated using the computer simulation of coaxial impact-collision ion scattering spectroscopy (CAICISS). The computer simulations employing the atomic collisions in a crystalline target (ACOCT) program code, which treats the atomic collisions three-dimensionally and is based on the binary collision approximation (BCA), were carried out for the case of 3 keV He+ ions incident along the [211] azimuth of the clean Pt(111) and Pt(111)-p(2×2)-O surfaces. The comparisons between ACOCT results and experimental CAICISS data show that the experimental results on the clean Pt(111) surface are relatively well reproduced by the ACOCT simulations for no surface relaxation, and that the ACOCT simulations for the Pt(111)-p(2×2)-O surface appear the oxygen occupancy in the fcc-hollow sites over the hcp-hollow sites. The ACOCT results for the clean Pt(111) and Pt(111)-p(2×2)-O surfaces are consistent with the analyzed results of low-energy electron diffraction (LEED).

Computer simulation of CAICISS spectra of clean Cu(1 0 0) surface and Cu(1 0 0) surface by Pt deposition

The clean Cu(100) surface and Pt/Cu(100) surface by Pt deposition at room temperature have been investigated using the computer simulation of coaxial impact-collision ion scattering spectroscopy (CAICISS). The computer simulations employing the ACOCT program code, which treats the atomic collisions three-dimensionally and is based on the binary collision approximation (BCA), were carried out for the case of 3keV He+ ions incident along the 〈100〉 and 〈110〉 azimuths of the clean Cu(100) and Pt/Cu(100) surfaces. The comparisons between ACOCT results and experimental CAICISS data show that the experimental results on the clean Cu(100) surface are relatively well reproduced by the ACOCT simulations including the inward relaxation of 1.2% in the first interlayer spacing and the outward relaxation of 1.6% in the second interlayer spacing, and that the ACOCT simulations for the Pt deposition with coverages of 2.35ML and 2.75ML on the Cu(100) surface appear the concentrations of 0.24ML of Pt sitting 2.3Å and 0.25ML of Pt sitting 2.5Å above the outermost atomic layer, respectively.

Interface structure and polarity of GaN/ZnO heterostructure

Gallium nitride (GaN) films were grown with and without lattice-matched (In,Ga)N buffer layers on the c(+) and c(-) faces of ZnO single-crystal substrates using molecular beam epitaxy, and their interface structures, including the relationship between the crystallinity of the GaN film and the polarity of the ZnO substrate, were investigated. Growth at a high temperature (e.g., 800°C) was made possible by using an (In,Ga)N buffer layer, which improved the crystallinity of the GaN films compared with that of GaN films grown directly on a ZnO substrate. Ion scattering, i.e., coaxial impact-collision ion scattering spectroscopy, and electron beam channeling, i.e., convergent beam electron diffraction, profiles revealed that the GaN films grown on c(+)- and c(-)-ZnO substrates with an (In,Ga)N buffer layer had a (0001) surface (i.e., c(+) polarity). That is, polarity inversion occurs when GaN film is grown with an (In,Ga)N buffer layer on a c(-)-ZnO substrate.

Change in polarity of zinc oxide films grown on sapphire substrates without insertion of any buffer layer

We have investigated the polarity of zinc oxide (ZnO) and Al-doped ZnO films grown on (11¯20) and (0001) sapphire substrates, using coaxial impact collision ion scattering spectroscopy. The films grown by pulsed laser deposition with a nominally undoped ZnO ceramic target had a (000¯1) surface, whereas the films prepared with a 1 mol% Al-doped ZnO ceramic target had a (0001) surface. The usage of Al-doped and undoped targets caused no difference in the in-plane lattice orientation. Electron microscope observations revealed that polarity change due to doping occurred without the formation of any interfacial phase between ZnO and sapphire.

Growth process and surface structure of MnSi on Si(1 1 1)

The solid-phase epitaxial growth process and surface structure of MnSi on Si(111) were investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). For the Si(111) sample deposited with 30 monolayers (ML) of Mn at room temperature, the intermixing of Mn and Si gradually started at 100°C and reached equilibrium at approximately 400°C. At this equilibrium state, the Mn atoms were transformed into crystalline MnSi film. Further annealing caused the desorption of Mn atoms. We identified the structure of MnSi as cubic B20 and the crystallographic orientation relationships as Si(111)//MnSi(111) and Si[1¯21¯]//MnSi[1¯10]. The MnSi(111) surface was found to have a dense Si terminating layer on its topmost surface. On the other hand, 3ML of Mn deposited on Si(111) reacted with Si even at room temperature and formed a pseudomorphic structure. This structure was transformed into MnSi after annealing. A filmlike morphology with protrusions was observed for the sample with 30ML of Mn, while island growth occurred for the sample with 3ML of Mn.

Coverage Analysis of Molecularly Thin Lubricant Films Using Molecular Dynamics Simulations and CAICISS Measurements

In order to examine the coverage of molecularly thin lubricant films on magnetic recording media, a method using coaxial impact collision ion scattering spectroscopy (CAICISS) is proposed. In this method, it is presumed that if a lubricant film of perfluoropolyether completely covers the surface, the fluorine-to-carbon atomic ratio (F/C ratio) of the surface should become 2. In this paper, we reproduce molecularly thin lubricant films on a carbon overcoat by full-atom molecular dynamics simulations, and calculate their coverage and F/C ratios. assuming the CAICISS measurement, the F/C ratios were computed considering the shadow cones formed behind the surface atoms. By comparing the F/C ratios with the coverage, we conclude that the coverage measurement method using CAICISS can accurately evaluate the coverage of molecularly thin lubricant films.

Surface Characterization of GaN(0001) Grown by Liquid Phase Epitaxy Using Coaxial Impact-Collision Ion Scattering Spectroscopy

We characterized the surface cleanliness, structure, and composition of GaN(0001) grown by liquid phase epitaxy (LPE) using coaxial impact-collision ion scattering spectroscopy (CAICISS). Contamination on the as-received LPE-GaN(0001) was removed by annealing at 900 °C, but annealing was unable to remove the O atoms from the as-grown surface, indicating that the as-grown sample incorporated O atoms. Although the as-grown sample contained O atoms, its structural perfection was maintained. These facts were explained by localization of O atoms near dislocations. The etched surface exhibited high crystalline quality, which was comparable to that of the sample grown by vapor phase epitaxy (VPE).

Computer Simulation of Coaxial Impact-Collision Ion Scattering Spectroscopy Spectra of Clean Pt(111) Surface and Pt(111)-p(2×2)-O Surface

The clean Pt(111) and Pt(111)-p(2×2)-O surfaces have been investigated using the computer simulation of coaxial impact-collision ion scattering spectroscopy (CAICISS). The computer simulations employing the atomic collisions in a crystalline target (ACOCT) program code, which treats the atomic collisions three-dimensionally and is based on the binary collision approximation (BCA), were carried out for the case of 3 keV He+ ions incident along the [211] azimuth of the clean Pt(111) and Pt(111)-p(2×2)-O surfaces. The comparisons between ACOCT results and experimental CAICISS data show that the experimental results on the clean Pt(111) surface are relatively well reproduced by the ACOCT simulations for no surface relaxation, and that the ACOCT simulations for the Pt(111)-p(2×2)-O surface appear the oxygen occupancy in the fcc-hollow sites over the hcp-hollow sites. The ACOCT results for the clean Pt(111) and Pt(111)-p(2×2)-O surfaces are consistent with the analyzed results of low-energy electron diffraction (LEED).

CAICISS and STM study of [formula omitted] and [formula omitted] tin phases on Si(1 0 0)

Tin (Sn) adsorption on Si(100) surface has been studied by scanning tunneling microscopy (STM) and co-axial impact collision ion scattering spectroscopy (CAICISS). The c(8×4) and (5×1) phases were analyzed to reveal their atomic structure. It has been confirmed that the c(8×4) structure consists of buckled Sn dimers. The (5×1) structure has been found to be partially irregular along (5×1) rows and to consist of double Sn layer. The Sn atoms in the first layer are undimerised while those in second layer form asymmetric dimers.