Coaxial Impact Collision Ion Scattering Spectroscopy Spectra Research Articles

Analysis of polar direction of AlN grown on (0001) sapphire and 6H‐SiC substrates by high‐temperature metal‐organic vapor phase epitaxy using coaxial impact collision ion scattering spectroscopy

AbstractHigh‐crystalline quality AlN layers with an atomically flat surface are grown on (0001) sapphire and 6H‐SiC substrates by metal‐organic vapor phase epitaxy at 1400 °C without low‐temperature buffer layer. The polar direction of AlN layers is investigated by coaxial impact‐collision ion scattering spectroscopy (CAICISS). For AlN growth on sapphire substrate, trimethylaluminum (TMAl) is supplied for 10 sec before introducing ammonia (NH3). For AlN growth on SiC substrate, TMAl and NH3 are supplied at the same time. The CAICISS spectrum of AlN layers is analyzed by measuring the dependence of the Al atoms signal intensity on the angle of ion beam incidence considering the shadowing and focusing effects for determining the polar direction of AlN layers. The CAICISS spectra clearly indicate the polar direction of AlN layers and both AlN layers are found to be had the Al‐polarity. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Atomic arrangement of Al-induced clusters on Si(0 0 1) surface at high temperature

The atomic structure of the Al-induced clusters on Si(0 0 1) surface formed by the annealing of 0.5 ML Al/Si(0 0 1) at 500 °C has been studied using coaxial impact collision ion scattering spectroscopy (CAICISS). CAICISS results proposed that the Al atoms occupy the cave site (T4 site) and off-centered T4 site. To determine the structure of the Al-induced clusters definitely, classical ion-scattering trajectory simulations using scattering and recoiling imaging code (SARIC) have been performed for the recently proposed most possible four different cluster models (Bunk, Zotov, Kotlyar, and Zavodinsky model). Our CAICISS spectra and simulation results show that the Bunk model is the best plausible one among the models. As the results of the simulations, it is found that Al–Si dimers has been oriented on the topmost layer of the Si(0 0 1) surface with a bonding length (Δ z) of 1.00 ± 0.05 Å.

In situ Monitoring of Surface Processes in Plasma by Coaxial Impact-Collision Ion Scattering Spectroscopy

We demonstrated in situ monitoring of surface processes in plasma, using coaxial impact-collision ion scattering spectroscopy (CAICISS) operable in a gas atmosphere, taking the plasma nitridation processes of Si surfaces with (111) and (001) faces as examples. From the results of real-time monitoring of CAICISS spectra, it was found that (1) the top layer of the Si(111) surface is readily nitrided even at room temperature, while the Si(001) surface requires thermal activation for its nitridation and that (2) at a high temperature, the nitridation of Si(001) proceeds to layers deeper than in the case of Si(111).

Initial adsorption structure of ethylene on Si(0 0 1) surface at room temperature

Ethylene (C 2H 4) on the Si(0 0 1) surface has been a subject of numerous investigations over a decade. Despite a wealth of experimental and theoretical studies, the adsorption structures are not clearly understood yet. In order to elucidate this controversial issues clearly, C 2H 4 molecules have been non-dissociatively chemisorbed on Si(0 0 1)-(2 × 1) surface at room temperature with an exposure of 100 L, and C 2H 4 on Si(0 0 1) surface structure has been investigated by coaxial impact collision ion scattering spectroscopy (CAICISS). To determine the adsorption structure of the C 2H 4 molecules definitely, the computer simulation with the two-dimensional trajectory counting method has been performed for the recently proposed most possible two single molecular adsorption configurations (di-σ on-top and di-σ end-bridge). The CAICISS spectra show a better agreement with the simulation results for the di-σ on-top structure than that for the di-σ end-bridge structure. The bond length of Si dimer separation on the Si(0 0 1) surface with the adsorption of 100 L C 2H 4 is found to be 2.35 ± 0.05 Å. This implies that the bonding of Si dimers stays intact. It is also found that the bonding length of C–Si and C–C is 1.81 ± 0.05 Å and 1.61 ± 0.05 Å respectively.

Ｓｉ（００１）ｃ（４×４）‐Ｓｎの走査トンネル顕微鏡―同軸型直衝突イオン散乱分光法による解析

The c (4×4) super-lattice structure of Sn/Si (001) system has been studied using scanning tunneling microscopy (STM) and coaxial impact collision ion scattering spectroscopy (CAICISS). After heating at 1200°C, the clean Si (001) 2×1 surface is confirmed by using low-energy electron diffraction (LEED) and STM. After Sn deposition of 0.5 monolayer (ML), successively annealed it at 600°C, a c (4×4) super-lattice structure is formed. We obtain three kinds of protrusions in high-resolution STM images of Si (001) c (4×4) -Sn structure. One of them is very similar to that of recently reported c (4×4) -Si reconstruction. Others are new protrusions in c (4×4) -Sn, so it seems that these are ascribed to Sn dimer. We analyzed CAICISS spectra scattered from Sn atoms, compared with simulated results. As a result, Sn dimer is found to be buckled. Thus, we proposed the mixed ad-dimer model for c (4×4) -Sn structure.

Control of crystal polarity in a wurtzite crystal: ZnO films grown by plasma-assisted molecular-beam epitaxy on GaN

ZnO/GaN heterointerfaces are engineered to control the polarity of ZnO films grown by plasma-assisted molecular beam epitaxy on Ga-polar GaN templates. The polarity of ZnO films is determined both by coaxial impact collision ion scattering spectroscopy (CAICISS) and by convergent beam electron diffraction (CBED). Polarity inversion can be achieved by inserting an interface layer with a center of symmetry, because the polarity comes from a lack of the center of symmetry. An O-polar (anion-polar) ZnO film can be grown on Ga-polar (cation-polar) GaN by inserting a ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ layer at the interface, while Zn-polar ZnO is grown on GaN without forming an interface layer. A single-crystalline monoclinic ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ layer, which has a center of symmetry, is formed by O-plasma preexposure on the Ga-polar GaN surface prior to ZnO growth, while the ZnO/GaN interface without any extra layer is formed by Zn preexposure. The orientation relationship between ZnO, ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3},$ and GaN is determined as $[2\ensuremath{-}1\ensuremath{-}10{]}_{\mathrm{ZnO}}\ensuremath{\parallel}[010{]}_{{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}}\ensuremath{\parallel}[2\ensuremath{-}1\ensuremath{-}10{]}_{\mathrm{GaN}}$ and ${(0001)}_{\mathrm{ZnO}}\ensuremath{\parallel}{(001)}_{{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}}\ensuremath{\parallel}{(0001)}_{\mathrm{GaN}}.$ The CAICISS results reveal the growth of an O-polar ZnO film on O-plasma-preexposed GaN, while a Zn-polar ZnO film on Zn-preexposed GaN. The origin of the observed features in polar-angle-dependent CAICISS spectra can be analyzed by considering the shadow cones of Zn and O atoms formed by incident ions and shadowing and focusing effects of scattered ions. Azimuthal-angle-dependent CAICISS spectra reveal the surfaces of both Zn- and O-polar ZnO films as mixture of c and $c/2$ planes with a ratio of about 50:50. The ZnO film with a ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ interface layer shows a degradation in the crystal quality as evidenced by a broadening of the x-ray rocking curves. The CBED results for the O-plasma-preexposed samples reveal Ga-polar GaN and O-polar ZnO for the O-plasma-preexposed samples, which directly confirms polarity inversion from cation to anion polar. On the other hand, Zn-polar ZnO CBED patterns are obtained from ZnO films grown on Zn-preexposed Ga-polar GaN, which indicates the same cation polarity for a ZnO/GaN interface without the formation of an interface layer. It is noted that no planar or faceted inversion domain boundaries are formed to invert the polarity (from Ga polar to O polar). This indicates that we can control the polarity by engineering interfaces.

In situ analysis of the room-temperature epitaxial growth of CeO2 ultrathin films on Si (111) by coaxial impact-collision ion scattering spectroscopy

The room-temperature epitaxial growth of CeO2 films on Si(111) substrates was examined in situ by combined use of a coaxial impact-collision ion scattering spectroscopy (CAICISS) and the laser molecular beam epitaxy (laser MBE). It was found that the crystal quality of CeO2 ultrathin films (∼3 nm thick) as-grown in UHV (∼10−9 Torr) could be improved remarkably by a few minutes of O2 gas exposure (∼10−5 Torr) at room temperature. A three-fold symmetry in the Ce signal intensity of azimuth rotational CAICISS spectra, which exhibited the type-B epitaxial growth ([1̄10]CeO2‖[11̄0]Si), was observed for the films thicker than about 1 nm.

Study of the topmost surface structure of a y-cut LiNbO 3 single crystal with coaxial impact collision ion scattering spectroscopy (CAICISS)

The topmost surface structures of a y-cut LiNbO 3 single crystal were studied with coaxial impact collision ion scattering spectroscopy (CAICISS). The observed CAICISS spectra of the azimuthal intensity variation of Nb atoms of that had mirror plane symmetry which was unexpected from the space group of LiNbO 3 ( R3 c, C 3 v 6) itself. By the simulation of CAICISS, it was strongly suggested that Nb atoms at the topmost few layers of y-cut LiNbO 3 single crystal were rearranged to give a mirror plane of symmetry in their three-dimensional atomic configuration rather than to form other related oxides.

A study on initial oxidation of Si(100)-2×1 surfaces by coaxial impact collision ion scattering spectroscopy

By using coaxial impact collision ion scattering spectroscopy (CAICISS), we have studied the structural changes extending over the first several layers from oxidized Si(100)-2×1 surfaces. A simulation of ion scattering was used for the analysis of CAICISS spectra. At an oxide thickness of 0.87 monolayer (ML), a peak in the CAICISS spectrum, which originates from the first-layer Si atoms, disappears. This finding indicates that the first-layer Si atoms move due to the structural relaxation accompanying the adsorption of oxygen atoms onto SiSi dimer sites and back-bond sites. In the CAICISS spectrum for 1.1-ML-thick oxide, some peaks appear due to the oxygen adsorption onto SiSi bonds between the second- and the third-layer Si atoms. From this result, we can deduce that the inward oxidation occurs before the lateral oxidation finishes.

Surface Structure of ZnO Single Crystals Analysed by Ion Scattering Spectroscopy

A ZnO crystal of wurtzite-type structure has polar surfaces of (0001) and (0001), which are terminated by Zn and O ions, respectively. Observation conducted by atomic force microscopy showed that step/terrace structures were formed on the (0001) and (0001) surfaces, whose step height was predominantly half of lattice parameter, c0. The ionic arrangement of the surfaces was determined by coaxial impact-collision ion scattering spectroscopy (CAICISS); the CAICISS spectra had a periodicity of 60° for azimuth angle dependence. From the analyses of CAICISS spectra it was shown that the (0001) and (0001) surfaces had a single dangling bond, and no ionic distortion occurred.

Identification of the terminating structure of 6H–SiC(0001) by coaxial impact collision ion scattering spectroscopy

The terminating structure of 6H–SiC(0001) substrates fabricated by the Acheson method was directly identified by means of coaxial impact collision ion scattering spectroscopy (CAICISS). The CAICISS spectra showed that the topmost surfaces of the samples were Si-terminated planes for both the front and rear faces. It was also shown that the (0001)Si face was composed of Si-terminated flat terraces and steps, the height of which corresponded to one-half the unit cell length along the 6H–SiC c axis.

Monte Carlo simulation of time-of-flight spectra of coaxial impact collision ion scattering spectroscopy applied to MoS 2(0001) and SrTiO 3(001)

We have synthesized a Monte Carlo program to simulate the time-of-flight (TOF) spectra of coaxial impact collision ion scattering spectroscopy (CAICISS). This program is based on the binary collision model considering inelastic energy loss and follows the incoming and outgoing trajectories for each incident ion from a top surface up to 300 monolayers (MLs) below a surface. The present simulation reproduced well the observed overall TOF spectra from MoS 2(0001), whose surface structure was already known. It was also found that the ions backscattered from the atoms located in the depths from top surface down to 150th atom layer contributed to the TOF spectrum. In order to extract quantitative information on surface structures from CAICISS spectra, it is essential to estimate accurately the background originated from the backscattering at the depths. The CAICISS measurement followed by the Monte Carlo simulation was applied to structure analysis of a TiO 2-terminated SrTiO 3(001) surface prepared by a chemical treatment and confirmed that the topmost layer comprised a TiO 2 layer and the SrO fraction was estimated to be less than 5%.

Identification of the Topmost Atomic Layer of 6H-SiC(0001) Surface by CAICISS.

The terminating structure of 6H-SiC (0001) fabricated by the Acheson method was directly identified by means of Coaxial Impact Collision Ion Scattering Spectroscopy (CAICISS). The CAICISS spectra proved that the topmost surfaces of samples were Si-terminated planes for both the front and the rear faces. Furthermore, according to the azimuthal dependence of Si peak in TOF spectra, it was also proved that the (0001) Si face was composed of the Si-terminated flat terraces and steps whose height is 7.56 A corresponding to a half unit cell length of the c-axis of 6H-SiC.

Topmost surface analysis of SrTiO3 (001) by coaxial impact-collision ion scattering spectroscopy

The terminating atomic plane of SrTiO3 (001) surface was investigated by means of coaxial impact-collision ion scattering spectroscopy (CAICISS). CAICISS spectra proved that SrTiO3 (001) surfaces of as-supplied substrates as well as of O2-annealed substrates were predominantly terminated with TiO2 atomic plane, while the SrO atomic plane came at the topmost surface of SrTiO3 (001) homoepitaxial film. This indicates the structural conversion of the topmost atomic layer from TiO2 to SrO occurred during the SrTiO3 homoepitaxial growth. The azimuth rotational CAICISS spectra exhibited a fourfold symmetry in the surface atom alignments, showing the square lattice structure of a terminating plane.