Surface Reconstruction Patterns Research Articles

High-Yield Production of Quantum Corrals in a Surface Reconstruction Pattern

The power of surface chemistry to create atomically precise nanoarchitectures offers intriguing opportunities to advance the field of quantum technology. Strategies for building artificial electronic lattices by individually positioning atoms or molecules result in precisely tailored structures but lack structural robustness. Here, taking the advantage of strong bonding of Br atoms on noble metal surfaces, we report the production of stable quantum corrals by dehalogenation of hexabromobenzene molecules on a preheated Au(111) surface. The byproducts, Br adatoms, are confined within a new surface reconstruction pattern and aggregate into nanopores with an average size of 3.7 ± 0.1 nm, which create atomic orbital-like quantum resonance states inside each corral due to the interference of scattered electron waves. Remarkably, the atomic orbitals can be hybridized into molecular-like orbitals with distinct bonding and antibonding states. Our study opens up an avenue to fabricate quantum structures with high yield and superior robustness.

Surface Reconstruction Pattern Recognition Technology Based on Scattered Point Cloud Data.

Surface reconstruction technology based on cloud data has broad prospects in the fields of reverse engineering, cultural heritage protection, and smart city construction. This article studies the surface reconstruction pattern recognition technology based on scattered point cloud data. The candidate feature points are extracted according to the surface variation, and the precise method of point cloud is used to fit the clustering plane, and the feature points are selected from the candidate feature points. Use the area increase method to construct the initial grid of the specific three-dimensional point group data. In the construction process, the normal vector of the point group data does not need to be separated, but defines the angle of the normal vector of the adjacent triangular grids, thereby separating relatively flat areas. Using the projection parameterization method, the scattering points in the domain are projected onto the curved surface, and the parameter values of the projection points are counted as the parameter values of the scattering points. All sampling points on the common boundary have tangent vectors along the two directions of the boundary. The direction of the bisector of the angle between the two tangent vectors is calculated as the direction of the connection vector outside the boundary of the sampling point. It can be seen from the experimental data that the search radius of the normal vector and feature descriptor when calculating the feature description operator is 0.01 and 0.02 m, instead of 0.005 and 0.006 m of the bunny data. Using the local feature size to refine the point cloud data can reduce the number of point clouds, remove redundant data in the point cloud, and realize dynamic adjustment and adaptive reconstruction of nonuniform point clouds.

Interface Studies of III-V Semiconductors and Oxides with an I n-Situ Spectroscopic Ultra-High Vacuum (UHV) System Capable of Atomic Layer Deposition (ALD) and Atomic Layer Etch (ALEt)

Preparation of gate dielectrics and high-quality interfaces by atomic layer deposition onto III-V and other sensitive substrates, which typically have multicomponent native oxides after air exposure, is a challenging problem that cannot be completely understood by ex-situ surface analysis techniques. Surface oxides can be detrimental or, in specific cases, even beneficial to device performance depending on how the interface affects the Fermi-level of the semiconductor.1,2 One such system is GaSb, which is a case study here for a high mobility p-type material. In this work, surfaces of GaSb <001> have been examined, with methods of oxide removal including H-atoms, Ar+ ions, and both, simultaneously tested as a function of temperature for determining the threshold of surface damage. The re-oxidation of the interface is examined after oxygen and water exposure, and after oxide films growth. Description of the unique system designed and built at NRL for these type of studies will be presented.3 It combines a surface science UHV chamber; with X-ray photoelectron and Auger electron spectroscopies (XPS & AES), transmission or reflection-absorption thin-film FTIR spectroscopy, low-energy electron diffraction (LEED), reflection electron energy loss spectroscopy (REELS), and surface preparation, with a chamber-within-a-chamber design for performing ALD and ALEt at pressures from mTorr to Torr. Without exposure to ambient, the ALD system is pumped out and opened to the UHV environment for spectroscopy. A quantified, micro-dosing gas reagent delivery system is also implemented, so quantitative ALD reactant exposure may be correlated with actual surface reactions.4 Spectroscopic results and surface reconstruction patterns for GaSb interfaces acquired in above system will be presented. 1L.B. Ruppalt et al., Appl. Phys. Lett. 101, 231601 (2012). 2V.M. Bermudez, J. Appl. Phys. 114, 024903 (2013). 3V.M. Bermudez, Rev. Sci. Instrum. 85, 114101 (2014). 4T.J. Larrabee, T.E. Mallouk, D.L. Allara, Rev. Sci. Instrum. 84, 014102 (2013).

Energetic, electronic, and magnetic properties of Mn pairs on reconstructed (001) GaAs surfaces

We study energetic, magnetic, and electronic properties of diluted substitutional Mn-pairs on the reconstructed $(001)$ GaAs surfaces. The studies are based on first-principles calculations in the framework of the density functional theory. We demonstrate that the stability of the systems strongly depends on the position, orientation, and the distance between the Mn-atoms constituting the pair. Independently of the considered surface reconstruction pattern, the Mn-pairs with Mn-atoms being the nearest neighbors (NN) on cationic sublattice turn out to be energetically more favorable than the pairs with the larger distance between the Mn-atoms. However, the preferential build-up orientation of the Mn-NN-pair depends on the surface reconstruction and is parallel either to $[110]$ or $[1\bar{1}0]$ crystallographic direction. We reveal also the mechanisms of the magnetic ordering of Mn-NN-pairs. The Mn-NN-pairs along the $[110]$ crystallographic direction exhibit always ferromagnetic alignment of Mn spins, whereas the spins in the Mn-NN-pairs along $[1\bar{1}0]$ direction are mostly anti-ferromagnetically aligned. In the electronic structure of the systems containing Mn-pairs with ferromagnetically aligned spins, we observe the valence band hole states in the neighborhood of Fermi energy. This indicates that the surface ferromagnetism in this prototype of dilute magnetic semiconductors can be explained in terms of the $p$-$d$ Zener model.

Perpendicular magnetic properties of ultrathin L10-Mn1.67Ga films grown by molecular-beam epitaxy

Materials with large perpendicular magnetic anisotropies (PMAs) have drawn great attention because of their potential applications in advanced spintronic devices such as spin-transfer-torque magnetic random access memory (STT-MRAM) and ultrahigh-density perpendicular magnetic recording. To date, a large variety of PMA materials have been investigated, such as L10-ordered FePt, CoPt granular films, Co/(Pt,Pd,Ni) multilayers, ultra-thin CoFeB alloys and perpendicularly magnetized Co2FeAl films. Among the various kinds of materials with PMA, MnGa film with L10-structure has received the most attention because it has large PMA (Ku～107 erg/cm3), ultralow Gilbert damping constant (0.008) and theoretically predicted high spin polarization (more than 70%). All these properties make L10-ordered MnGa a good candidate for spintronic devices such as STT-MRAM and spin-torque oscillators. Meanwhile, from the viewpoint of STT related spintronic device, it is necessary to fabricate ultrathin perpendicularly magnetized L10-MnxGa films to lower the critical current for magnetization reversal. However, up to now, in the main researches the ultrathin L10-MnxGa films have been grown on MgO substrates, which makes it difficult to integrate the MnGa-based magnetic tunnel junctions into the semiconductor manufacturing process.In this work, ultrathin L10-Mn1.67Ga films with different thickness values (1-5 nm) are grown on traditional GaAa (001) substrates by a molecule-beam epitaxy system. During the deposition, in situ streaky surface reconstruction patterns are observed from reflection high-energy electron diffraction, which implies high crystalline quality of the L10-Mn1.67Ga film. Only MnGa superlattice (001) and MnGa fundamental (002) peaks can be observed in the X-ray diffraction patterns in a range from 20 to 70, which means that the L10-Mn1.67Ga film is a good single-crystalline with c-axis along the normal direction. The magnetic properties of these films are measured by superconductor quantum interference device magnetometer in a field range of 5 T. The perpendicular M-H curves are almost square, while the in-plane curves are nearly hysteresis-free, each with a remnant magnetization (Mr) of around zero, which clearly evidences the PMA of the ultrathin L10-Mn1.67Ga film. Moreover, as the thickness of L10-Mn1.67Ga film decreases from 5 nm to 1 nm, the ratio of Mr/Ms also decreases from 1 to 0.72, which indicates that the PMA loses as thickness decreases. We also estimate the perpendicular anisotropy constant (Ku) from the relation Ku=Keff+2 Ms2, and the maximum Ku of 14.7 Merg/cm3 is obtained for the 5 nm MnGa film. Although the Ku decreases with thickness decreasing, a Ku value of 8.58 Merg/cm3 is observed in a 2 nm thick film. The obtained results are important for developing the L10-MnGa-based spin-transfer torque Gbit class magnetic random access memory.

Phase separated reconstruction patterns on strained FCC (111) metal surfaces

In this paper, we use the newly developed shear-induced reconstruction computer simulation method to explore various phase separated reconstruction patterns on strained Au(111), Pt(111), Ni(111) and alloy Ag/Pt(111) systems via molecular dynamic computer simulations. We find that specific surface reconstruction patterns are not confined to specific metallic systems; instead the results show that almost any reconstruction pattern could arise in almost any of the model systems dependent on the substrate to reconstruction layer lattice constant mismatch, and the substrate to reconstruction layer interaction strength.

Surface reconstruction transition of metals induced by molecular adsorption

Two surface reconstruction patterns, strikingly different from the Au(111) intrinsic herringbone reconstruction, have been observed at the interfaces between Au(111) and the self-assembled monolayer of perylene or iron phthalocyanine by scanning tunneling microscopy (STM). These interfacial structures, elucidated by using the two-dimensional Frenkel-Kontorova model and density-functional theory (DFT) calculations, are attributed to the anisotropic surface stress induced by interfacial charge transfer. The length scales of the induced reconstruction periodicities are theoretically estimated, in quantitative agreement with experiments. Our findings afford the possibility of tailoring the contact structure of molecular electronic devices by molecular and surface engineering.

Surface reconstruction on stishovite SiO2, HfO2 and rutile TiO2 (001)

This paper systematically investigates the surface reconstruction processes and patterns on stishovite SiO2, HfO2 and rutile TiO2 (001) by using classical molecular dynamics. It is found that these three surfaces relax instead of reconstruction at 0 K, and have little possibility to reconstruct below 40 K. Above 40 K, surface reconstructions take place as collective atomic motion which can be speeded by higher temperature or compressed strain. Several reconstruction patterns with approximate surface energies are found, and electrostatic potentials on them are also provided in comparison with possible microscopic results.

Optimization of GaMnAs growth in low temperature molecular beam epitaxy

We present a detailed growth optimization procedure and experimental results for the growth of GaMnAs magnetic semiconductors in low-temperature molecular beam epitaxy. They were explored by using in-situ monitoring of the surface reconstruction patterns, double crystal/high-resolution x-ray diffraction, conductivity measurement, and superconducting quantum interference device measurements. The results showed strong correlations among the measurements. The room temperature conductivity measurement, in particular, was found to be a useful tool in forecasting the ferromagnetic transition temperature of the films. High quality GaMnAs films could contain Mn up to ≈5% without MnAs segregation at substrate temperatures of 215–275°C. The highest transition temperature of 80 K, however, was measured from the sample with 3.7% Mn grown at the substrate temperature of 250°C and As4 pressure of 1.4×10−6 torr for a growth rate of 0.25 μm/hr.

Surface reconstruction patterns of AlN grown by molecular beam epitaxy on sapphire

We present here a study of the growth of AlN by molecular beam epitaxy on sapphire (0 0 0 1) substrates using reflection high energy electron diffraction and atomic force microscopy. The surface morphology and reconstruction have been monitored as a function of temperature using initially nitrided sapphire substrates. We obtained a (2×2) reconstruction on AlN after growth using two different procedures; either by cooling or by remaining at high temperature for an extended period of time. During the subsequent growth of AlN we can maintain this (2×2) reconstruction for some time, before it reverts to a (1×1) pattern. We have also studied the influence upon the reconstruction of post-growth deposition of Al at high temperature onto the AlN surface after growth. By desorbing the excess Al, the (2×2) surface reconstruction recovery time was found to be directly proportional to the Al deposition time. This strongly suggests, therefore, that the (2×2) reconstruction may be intrinsic to AlN.

Lateral OMVPE growth of GaAs on patterned substrates

GaAs was grown on patterned 〈1 0 0〉 on- and off-axis GaAs substrates by organometallic vapor-phase epitaxy (OMVPE). Patterned mesas were observed to change shape because lateral growth rates varied by more than an order of magnitude in different crystallographic directions. For this study, misoriented GaAs (1 0 0) wafers were polished 3° toward the nearest [1 1 0] or [1 1 1] family of directions, and 320 nm high cross-shaped mesas were fabricated. OMVPE growth was performed between 550°C and 650°C for 1 h at a vertical growth rate of approximately 1.3 μm/h. Atomic force microscopy showed that three effects have a powerful influence on lateral growth initiated at mesa sidewalls. First, the symmetry of the dominant surface reconstruction has a major effect on the diffusion of Ga adatoms. Rapid Ga diffusion occurs along the 〈0 1 1〉–〈0−1−1〉 axis in OMVPE, or the perpendicular 〈0−1 1〉–〈0 1−1〉 axis in molecular beam epitaxy, and appears to be a result of the different surface reconstructions which exist in the two growth ambients. Second, misorientation of the wafer causes a growth asymmetry as Ga adatoms move preferentially from high-to-low terraces. When terrace steps descend toward a mesa wall, rapid lateral growth away from the wall is always observed. When terrace steps descend away from a mesa wall, little lateral growth occurs and even reduced vertical growth may be observed. When the misorientation and reconstruction symmetries align, the surface acts like an atomic diode and the rapid lateral growth can exceed the vertical growth rate by more than an order of magnitude. Third, on misoriented substrates, step bunching increases with increasing temperature, and this can lead to significant changes in the original shape of a mesa. A growth model is presented which relates the lateral growth rate in different crystallographic directions to the substrate misorientation, the growth temperature, and the partial pressure of As during growth. It is also shown that different surface reconstruction patterns are related to chemical species with continuously varying concentrations rather than thermodynamically distinct phases.

Surface reconstruction patterns of GaN grown by molecular beam epitaxy on GaN bulk crystals

An investigation of the conditions giving rise to surface reconstruction for GaN grown by molecular beam epitaxy on (0 0 0 1̄) bulk GaN substrates has been carried out. The results of different surface reconstructions were investigated by supplying Ga and active nitrogen separately to GaN surfaces. A (2×2) reconstruction on (0 0 0 1̄)GaN substrates and for GaN layers grown on such substrates was observed. The (2×2) surface reconstruction is stable in the presence of an active nitrogen flux at high temperature, but it disappears on cooling the GaN sample below ∼400°C. The (2×2) reconstruction is considered to be due to an additional amount of relatively tightly bound Ga on this (1×1) surface. Adding further Ga to the (2×2) reconstructed surface resulted in a return to a (1×1) reconstruction.

Surface sensitivity of impurity incorporation: Mg at GaN (0001) surfaces

The interplay of surface termination, reconstruction patterns, and availability of species involved determines the incorporation of impurities during growth. We study ab initio this interplay for Mg at the GaN (0001) surfaces and find that optimal incorporation conditions strongly depend upon surface orientation and cannot be predicted using bulk stoichiometric arguments. With reasonable assumptions on the kinetics, high densities of Mg can be achieved in the absence of hydrogen and the Ga surface displays superior incorporation characteristics to that of the N surface.

Gallium-induced surface reconstruction patterns of GaN grown by molecular beam epitaxy

We report on an investigation of the mechanisms giving rise to surface reconstruction for GaN grown by molecular beam epitaxy (MBE) on a range of different substrates. We have studied the effects of surface contamination by oxygen or arsenic and demonstrate that both can influence the surface reconstruction. We show that surface reconstruction measured by reflection high-energy electron diffraction (RHEED) is associated with excess Ga on the surface, which undergoes an order–disorder transition at about 400–500°C. For MBE, growth on hydride vapour phase epitaxy (HVPE) GaN/SiC composite substrates having the Ga polarity the 2×2 reconstruction is intrinsic. This intrinsic reconstruction can be destroyed by heating to a high temperature or by oxidation. The intrinsic 2×2 reconstruction can also be destroyed by adding an additional monolayer of more weakly bound Ga, which can be removed by desorption at high temperature. For growth by MBE on sapphire with nitridation, a 3×3 reconstruction is observed on cooling the sample to <400°C, but the reconstruction can be changed to 4×4 by contamination with oxygen. For growth of GaN with a N polarity by MBE on GaAs(111)B, a 4×4 reconstruction is observed on cooling to <400°C, which may be caused by arsenic.

Arsenic-free GaAs substrate preparation and direct growth of GaAs/AlGaAs multiple quantum well without buffer layer

High-temperature treatment of GaAs substrate without As flux in a preparation chamber was investigated as a substrate surface cleaning method for molecular beam epitaxial (MBE) growth. Oxide gases such as CO and CO 2 were almost completely desorbed at a temperature above which Ga and As started to evaporate from the substrate. During the cleaning at a temperature as high as 575°C for 30 min, about 100 nm thick GaAs was evaporated from the substrate, but its surface maintained mirror-like smoothness and showed streak pattern with surface reconstruction pattern in the reflection high energy electron diffraction (RHEED) observation. Direct growth of GaAs/Al GaAs quantum well (QW) structures was tried on such surfaces without introducing any buffer layers. The QW structure showed photoluminescence with both intensity and full width at half maximum comparable with those for the QW grown on the substrate cleaned by the conventional method with introducing a GaAs buffer layer.

Arsenic-free GaAs substrate preparation and direct growth of GaAs/AlGaAs multiple quantum well without buffer layer

High-temperature treatment of GaAs substrate without As flux in a preparation chamber was investigated as a substrate surface cleaning method for molecular beam epitaxial (MBE) growth. Oxide gases such as CO and CO 2 were almost completely desorbed at a temperature above which Ga and As started to evaporate from the substrate. During the cleaning at a temperature as high as 575°C for 30 min, about 100 nm thick GaAs was evaporated from the substrate, but its surface maintained mirror-like smoothness and showed streak pattern with surface reconstruction pattern in the reflection high energy electron diffraction (RHEED) observation. Direct growth of GaAs/Al GaAs quantum well (QW) structures was tried on such surfaces without introducing any buffer layers. The QW structure showed photoluminescence with both intensity and full width at half maximum comparable with those for the QW grown on the substrate cleaned by the conventional method with introducing a GaAs buffer layer.

Clean and damage-free GaAs surfaces prepared by ultrasonic running deionized water treatment

GaAs(001) surfaces treated with ultrasonic running deionized water (URDIW), after being etched by H 2SO 4 or NH 4OH were examined by using X-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED), XPS analysis revealed that the arsenic and gallium oxides on chemically etched surfaces were completely removed by the URDIW treatment. RHEED observation indicated that the H 2SO 4-etched GaAs surface shows a streaky (2×1) surface reconstruction pattern at 360°C and that the NH 4OH-etched surfaces show a (2×4) surface reconstruction pattern at 310°C. These experimental results indicate that chemically clean GaAs surfaces with little damage can be produced by URDIW treatment and that the stoichiometry of a URDIW treated surface can be controlled by varying the etching solutions.

Reflection high-energy electron diffraction observation of GaAs surface-prepared ultrasonic running de-ionized water treatment

GaAs (001) surface ultrasonically cleaned under running de-ionized water (U-RDIW) is investigated by reflection high-energy electron diffraction (RHEED). The RHEED observations of U-RDIW-treated surfaces show a spotty (1×1) pattern at room temperature and a (2×1) streaky surface reconstruction pattern at 360 °C. The experimental results indicate that chemically clean and damage-free GaAs surfaces can be produced by U-RDIW treatment. We discuss surface structures before/after heating using a hydrogen- terminated model.

Reflection high-energy electron diffraction studies of the growth of strained InxGa1−xAs on GaAs substrate by migration-enhanced epitaxy

Reflection high-energy electron diffraction oscillations have been studied during the growth of strained InxGa1−xAs on GaAs by molecular beam epitaxy and migration-enhanced epitaxy. The oscillations decay rapidly for x≳0.2 during molecular beam epitaxy, while they persist for a long while during migration-enhanced epitaxy. We believe that the altered surface reconstruction pattern in the latter case changes the growth mode from three-dimensional to a near perfect two-dimensional mode for high strain values. Using migration-enhanced epitaxy, we demonstrate improved channel mobility and performance of GaAs-based modulation-doped field-effect transistors and narrower linewidths in the low-temperature excitonic photoluminescence of In0.1Ga0.9As/Al0.3Ga0.7As quantum wells.

Investigation of SimGen strained monolayer superlattices by Rheed, Raman, and X-ray techniques

Ultrathin Si m Ge n strained monolayer superlattices (SMS) have been grown by molecular beam epitaxy. The critical thickness of the total superlattice is determined in situ by direct observation of the changes of surface reconstruction patterns using reflection high energy electron diffraction. Results are in close agreement with that of a single layer with the same thickness and the average SMS composition. Raman scattering and X-ray diffraction spectroscopy are used to study the thermal stability and surface abruptness. Subsequent annealing of these SMSs reveals that the transition from pure silicon and/or germanium layers to Si 1− x Ge x alloys becomes more pronounced as the annealing time and temperature are increased. The interdiffusion coefficients obtained from the X-ray spectrum are found to obey an Arrhenius relation with an activation energy of 3.1 ± 0.2 eV.