Atomic Step Structure Research Articles

Characterization of atomic step structures on CaF2(111) by their electric potential

The structure and polarity of step edges on cleaved CaF2(111) are investigated by non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy. Ledges produced by cleaving the crystal appear with two distinctly different polarities denoted as type I and type II arising from the sectioning of ledges with steps having different polarities. With respect to the stoichiometric terrace, the surface potential is slightly reduced at ledges predominately composed of type I steps, while the potential of ledges predominantly composed of type II steps is significantly higher (typically 100 mV). We propose that the positive potential of type II steps stems from low coordinated Ca2+ ions inducing a dipole at step edges and confirm this by atomically resolved NC-AFM images revealing the Ca2+ ion sub-lattice with repulsive-mode imaging contrast.

Atomically Smooth Gallium Nitride Surfaces Prepared by Chemical Etching with Platinum Catalyst in Water

A method for preparing atomically smooth gallium nitride (GaN) surfaces that involves chemical etching with a platinum catalyst in water is presented. The flattened GaN(0001) surface exhibits periodic structures composed of straight-edged steps the height of a single bilayer. Atomic step and terrace structures containing an extremely small number of scratches and pits can be achieved on wafer scale (2 inch diameter). Photoluminescence analysis revealed that the intensity of band-edge luminescence increases significantly after flattening. Low energy electron diffraction indicated that the flattened GaN surface is crystallographically well-ordered. Current density between the wafer and Pt is observed when they are rotated, suggesting that the GaN surface is etched by an electrochemical reaction initiated by the tribo effect between Pt and the wafer surface.

Lipid Bilayer Membrane with Atomic Step Structure: Supported Bilayer on a Step-and-Terrace TiO2(100) Surface

The formation of a supported planar lipid bilayer (SPLB) and its morphology on step-and-terrace rutile TiO 2(100) surfaces were investigated by fluorescence microscopy and atomic force microscopy. The TiO 2(100) surfaces consisting of atomic steps and flat terraces were formed on a rutile TiO 2 single-crystal wafer by a wet treatment and annealing under a flow of oxygen. An intact vesicular layer formed on the TiO 2(100) surface when the surface was incubated in a sonicated vesicle suspension under the condition that a full-coverage SPLB forms on SiO 2, as reported in previous studies. However, a full-coverage, continuous, fluid SPLB was obtained on the step-and-terrace TiO 2(100) depending on the lipid concentration, incubation time, and vesicle size. The SPLB on the TiO 2(100) also has step-and-terrace morphology following the substrate structure precisely even though the SPLB is in the fluid phase and an approximately 1-nm-thick water layer exists between the SPLB and the substrate. This membrane distortion on the atomic scale affects the phase-separation structure of a binary bilayer of micrometer order. The interaction energy calculated including DLVO and non-DLVO factors shows that a lipid membrane on the TiO 2(100) gains 20 times more energy than on SiO 2. This specifically strong attraction on TiO 2 makes the fluid SPLB precisely follow the substrate structure of angstrom order.

Atomically controlled surfaces with step and terrace of β-Ga 2O 3 single crystal substrates for thin film growth

The surface of β-Ga 2O 3 (1 0 0) single crystal grown with floating zone method was treated by chemical–mechanical-polishing (CMP) for 30–120 min followed by annealing in oxygen atmosphere at temperature 600–1100 °C for 3–6 h. The evolution of the step arrangement was investigated with reflection high energy electron diffraction and atomic force microscopy. Atomically smooth surfaces with atomic step and terrace structure of β-Ga 2O 3 substrates were successfully obtained after just CMP treatment as well as CMP treatment and post annealing at 1100 °C for 3 h. The uniform step height was 0.57 nm, and smooth terrace width was 100 nm, where the misorientation angle was about 0.36°. The obtained atomically smooth surface provides a potential application for the high-quality epitaxial film growth.

Initial Growth Behavior of Ultra-Thin c-Axis-Oriented Epitaxial SrBi2Ta2O9 films on SrTuO3

Abstractc-axis-oriented epitaxial SrBi2Ta2O9 ultra-thin films were grown by pulse-gas-introduced metalorganic chemical vapor deposition (pulsed-MOCVD) on (100)SrTiO3 single crystal substrates with atomic scale step structure and their growth behavior was investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Minimum growth unit was found to be “ghalf-unit-cell” of SrBi2Ta2O9. Height of steps and width of terraces observed at SrBi2Ta2O9 film surface were in good agreement with those at SrTiO3 substrate surface. This shape transfer was induced by lattice displacement of SrBi2Ta2O9 along c-direction formed at atomic step on SrTiO3 substrate. In-plane growth of half-unit-cell SrBi2Ta2O9 2D-islands striding across the step walls was observed. It was considered to be a special phenomenon for c-axis-oriented films of layer-structured compounds due to their large crystal anisotropy and/or several times larger half-unit-cell height than single step one of SrTiO3.

Realization of ultrasmooth surface with atomic scale step structure on LiNbO3 and LiTaO3 substrates

Z-cut (C-plane), X-cut (M-plane), Y-cut (A-plane) LiNbO3 and LiTaO3 substrates were annealed at high temperature over 1000 degrees C in air. On all annealed substrates, atomic scale step structures with uniform height were observed, which demonstrates that the surfaces of the substrates become atomically smooth. The step height on Z-cut substrates was 0.25 +/-0.02 nm, which was well accordance with the distance between oxygen layers along the c-axis of the hexagonal unit cell of LiNbO3 and LiTaO3 crystals. The step heights on X-cut and on Y-cut substrates were 0.45 +/- 0.04 nm and 0.50 +/- 0.05 nm, respectively, which corresponded well with the distances between A-planes and M-planes in the unit cell.

Initial growth behaviour of LiNbO 3 films on α-Al 2O 3 substrates with atomic scale step structure

Evolving morphology of the LiNbO 3 films growth on C(001), A(110) and R(102) α-Al 2O 3 substrates with atomic scale step structure was investigated during the initial stage of the growth. On C(001) substrate, LiNbO 3 nucleated randomly irrespective of the step structure on the substrate, while this material showed preferential nucleation along the step edges on the surface of A(110) and R(102) substrates. This tendency of initial growth of LiNbO 3 on A(110) and R(102) substrates enhances the step-flow growth, allowing the fabrication of nanometric wire and smoother films.

Simultaneous observation of atomic step and domain wall structure of ultrathin Co films by magnetic force microscopy in ultrahigh vacuum

We have applied magnetic force microscopy in ultrahigh vacuum to study the correlation between the atomic step and magnetic domain wall structure of ultrathin Co films prepared in situ on Au(111) substrates. For the first time we were able to achieve high-resolution images showing simultaneously a clear domain wall contrast and the underlying atomic step structure. Although for in-plane magnetized Co films the domain walls were found to run preferentially in a direction perpendicular to the steps, no such correlation could be observed for out-of-plane magnetized Co films.

AlN epitaxial growth on atomically flat initially nitrided α-Al 2O 3 wafer

The (1 1 02) α-Al 2O 3 surface and the initial nitriding AlN layer are investigated using the atomic-force microscope. The initial nitriding method is to convert the (1 1 02) α-Al 2O 3 surface to a nanometer-thick AlN single-crystal buffer layer. The epitaxial AlN film is deposited by metalorganic chemical vapor deposition with and without the initial nitriding. Atomic step structure is found to be formed after the H 2 annealing. The atomic step structure is maintained during the initial nitriding time up to 5 min. Smooth AlN epitaxial films have been successfully deposited on the 2 inch diameter (1 1 02) α-Al 2O 3 wafer without any inverse-twin throughout the wafer by keeping the atomic step structure of the initial nitriding buffer layer.

Growth of high quality CdTe on Si substrates by molecular beam epitaxy

We have systematically studied the growth of CdTe (lll)B on Si(001)with different atomic step structures, defined uniquely by miscut tilt angle and direction. X-ray double crystal rocking curve (DCRC) analysis has been used to evaluate the crystalline quality and twin content of the films. High-resolution electron microscopy has been used to examine the CdTe(lll)B/Si(001) interface and to follow the microstructural evolution as a function of distance from the interface. Our results show that the formation of double domains and twins is very sensitive to the tilt parameters. When growth conditions are optimized, twins are not observed at distances greater than about 2.5 microns from the substrate surface. The best quality films exhibit a DCRC FWHM of 60 arc sec, for a film thickness of 17 µm, the lowest value ever reported for heteroepitaxial growth of CdTe on Si or GaAs. In efforts to improve the nucleation process, precursors such as Te and As have been used, and we have shown that they improve the stability of the heterointerface.

Characterization of GaAs/AlAs interfacial atomic step structures on a (411)A-oriented substrate by transmission electron microscope

We observed atomic structures of (411)A GaAs/AlAs hetero-interfaces using a transmission electron microscope, and studied the numerical computer simulations of the lattice images for the first time. The observed specimens were GaAs/AlAs multi-layer structures fabricated by molecular beam epitaxy, where the AlAs layers were very thin (0.75–2.1 monolayer). From the hetero-interfacial structures observed in cross-sectional view, it was found that the (411)A atomic step structures were based on Shimomura's model, which has already been proposed in a previous paper. We will discuss the concrete interfacial structure.

Characterization of GaAs/AlAs interfacial atomic step structures on a (411)A-oriented substrate by transmission electron microscope

Characterization of GaAs/AlAs interfacial atomic step structures on a (411)A-oriented substrate by transmission electron microscope

Atomic Step Structure on Vicinal H/Si(111) Surface Formed by Hot Water Immersion

We investigated the hydride structures at the step edges on Si(111) surfaces after removing surface oxide with HF solution followed by immersion in boiling water, using polarized infrared attenuated total-reflection spectroscopy. Vicinal (111) surfaces misoriented toward the [112̄] direction at angles of 0°, 2°, and 4° were used to elucidate the hydride structures at the steps. We assigned absorption peaks to monohydride chains at the step edges along the [1̄10] direction, vertical dihydride (whose three atoms form a surface vertical to [111]) at the kinks, and monohydride on the terraces. We determined that the silicon monohydride chain forms an atomically straight step edge along the [1̄10] direction with a small amount of vertical dihydride kinks after 5-min immersion in boiling water. Based on these assignments, we reexamined the water temperature and time dependence of these hydride structures and discussed the step formation mechanism.

Anisotropic etching versus interaction of atomic steps: Scanning tunneling microscopy observations on HF/NH4F-treated Si(111)

After ex situ etching with various solutions of hydrofluoric acid (HF) and ammonium fluoride (NH4F) Si(111) samples are transferred into ultrahigh vacuum with an ultrafast load-lock and characterized by scanning tunneling microscopy (STM): Concentrated HF selectively removes any surface oxide and, thus chemically prepares the initially burried, isotropically rough Si/SiO2 interface while highly buffered HF (i.e., NH4F) attacks bulk silicon anisotropically. After a rapid homogenization of the chemical surface termination (HF: various hydrides, fluorine, ...) towards a perfect, unreconstructed monohydride phase, Si(111)-(1×1):H, NH4F etching leads to a time-dependent transformation of isotropic roughness into a pattern of triangular etch defects with monohydride steps perpendicular to <2̄11≳ due to a preferential removal of lower-coordinated atomic defect sites. A predominant atomic step structure due to sample miscut (vicinal surfaces with azimuth ≠<2̄11≳) can oppose the anisotropic NH4F etching: At low step density (small polar angle of miscut) a meandering of atomic steps with straight monohydride portions is observed while at high step density strong step-step interaction counterbalances anisotropic removal and forces an etching by a homogeneous flow of (nonmonohydride) steps along the macroscopic misorientation. Local findings obtained with STM are compared to macroscopically averaged results from a simultaneous quantitative analysis of low-energy electron diffraction profiles.

Surface morphology study on CdZnTe single crystals by atomic force microscopy

The study of the crystal surface morphology of CdZnTe is important for the understanding of the fundamentals of crystal growth in order to improve the crystal quality which is essential in applications such as substrates for epitaxy or performance of devices, i.e., room temperature nuclear spectrometers. We present here a first atomic force microscopy study on CdZnTe. Cleaved (110) surfaces were imaged in the ambient and an atomic layer step structure was revealed. The effects of thermal annealing on the atomic steps together with Te precipitation along these steps are discussed in terms of deformation due to stress relief and the diffusion of tellurium precipitates.

Observation of atomic step morphology on silicon oxide surfaces

Atomic step structures on oxidized Si(111) surfaces are investigated by scanning electron microscopy with grazing incidence of the primary electron beam and by scanning force microscopy. Atomic step structures similar to those on clean Si(111) are preserved on a thermally oxidized surface at 900 °C in dry O2 as well as on a naturally oxidized surface in air. The atomic step structures are also discernible on the SiO2/Si interface when a 20-nm-thick oxide layer is removed by HF etching. These results indicate that once a clean surface with atomic steps and terraces is formed, the surface morphology is stable against oxidation, and that the initial surface morphology is preserved as oxidation proceeds.

Low-energy electron diffraction studies of atomic step transitions on a Ni(111) vicinal surface

A low-energy electron diffraction system with a two-dimensional position sensitive detector was used to investigate the step clustering phase transition on a Ni surface vicinal to (111), i.e., Ni (111) 5° [112̄]. The directions parallel and perpendicular to the step edges were [112̄] and [11̄0], respectively, and the surface was miscut by 5° about the [112̄] axis. Split beams at high temperature, indicative of a single atomic height step structure, coalesced to single beams as the temperature was lowered. The transition occurred around 560 K, and was reversible with some hysteresis. Scans along the [111] truncation rod were performed by changing the incident beam energy. Beam profile analysis was done to quantitatively analyze the change of step and terrace distributions on the surface. Kinematic calculations of intensity distributions were made for a two phase model, in which the surface was taken to consist of different areas, one type with large terraces and the other with small terraces, and compared with the experimental data. Two gamma functions were used for the terrace distribution and one exponential function for the step distribution. Fitting of experimental results to the calculation showed that step clustering and (111) faceting occurred as temperature decreased. The measured hysteresis in the heating and cooling cycles might be due to nucleation in a first order phase transition involving coexistence of regions with single atomic height steps and regions with double atomic height steps. The present result is consistent with our previous reported observation on a Ni vicinal surface tilted by 5° about the [11̄0] axis, i.e., Ni(111) 5° [11̄0], using glancing incidence x-ray diffraction.

Atomic step structures on cleaved α-alumina (012) surfaces

Cleaved α-alumina (012) surfaces are imaged with reflection electron microscopy (REM). A RHEED-THEED diffraction method is demonstrated for on-line determination of surface orientation. The observed defect structures on the (012) surface include vacancy-type of terraces islands, kinks and ledges of a few atoms high. Steps of height possibly less than about 0.8 Å may be seen in REM images. The formation of the surface steps of different heights is described based on the atomic structure of the bulk crystal. The electron beam damage to the surface domains initially terminated with oxygen and aluminium layers, respectively, is interpreted based on the electrostatic desorption model after Auger decay. This effect is responsible for the bright-dark band-type contrast observed in REM images, and can be employed to identify the surface termination. It has been found that the REM image sensitivity is significantly improved with a field emission gun (FEG).

Morphology of Monolayer Films Observed by Photoelectron and Low-Energy-Electron Microscopy

ABSTRACTMany of the fundamentals of epitaxial growth, long predicted by theories of crystal growth, have been seen during the molecular beam epitaxial growth of monolayer films. In general, high temperatures and low deposition flux favor nucleation at atomic steps. The opposite extreme of low temperature and high flux favors island nucleation in terraces, and steps play little role. For intermediate conditions a wide variety of growth morphology is seen. High temperatures and high flux have been seen to produce two-dimensional dendritic growth. Atomic steps on substrates have been observed to migrate during sublimation. Pinning of steps during migration gives rise to complex atomic step and terrace structures and a distribution of terrace widths. For terraces wider than the diffusion length of adsorbed atoms, nucleation of islands is favored in the terraces, whereas narrow terraces favor nucleation at the atomic steps. For a constant substrate temperature and deposition flux, morphology can vary profoundly depending upon local terrace width. Complex structures in monolayer films are produced during sublimation. These include lockeime or hole nuclei which are one atomic step deep. Steps are seen to oscillate during step flow apparently due to considerable mass transport between steps and terraces.

Atomic step and defect structure on surfaces of Si{100} and Si{111} observed by low-energy electron microscopy

Abstract The Si{100} and Si{111} surfaces are studied by low-energy electron microscopy. Atomic step and terrace structure formed by misorientation of Si wafers is investigated as well as various migration patterns of atomic steps formed during sublimation. Slip traces ending typically at screw dislocations form along ‘110’ directions in the surface and create additional atomic steps. Analysis of migration patterns near slip traces implies that both partial and perfect dislocations (or their equivalent, namely two partials) form on Si {100}. Perfect dislocations are seen on Si{111}. Regions with the (7×7) structure on Si{111}, which contain stacking faults, are imaged. Surface domain structure and domain boundaries are seen on Si{111}. On Si{100} the partial dislocations create two new types of surface domain displaced relative to the two pre-existing (2 × 1) and (1 × 2) domains. The potential for in situ studies of process-induced defects on Si wafers is demonstrated.