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1.
Engin?Arslan Mustafa?K.?Ozturk ?zgür?Duygulu Ali?Arslan?Kaya Suleyman?Ozcelik Ekmel?Ozbay 《Applied Physics A: Materials Science & Processing》2009,94(1):73-82
In the present paper, the effect of in-situ substrate nitridation time on crystalline quality of GaN films grown on Si (111)
substrates by metal organic chemical vapor deposition (MOCVD) were investigated. A thin buffer layer of silicon nitride (SiN
x
) with various thicknesses was achieved through the nitridation of substrate at different nitiridation times ranging from
0 to 660 s. The structural characteristics, such as dislocation densities, correlation lengths of columnar crystallites, the
tilt and twist of the mosaic structure, and the angles of rotational disorder, were all studied in detail by using a planar
and cross-sectional view of high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (HRXRD) performed
at different scattering geometries. It was found that the dislocation densities, lateral coherence lengths, vertical coherence
lengths, and the tilt and twist of mosaic blocks in GaN films monotonically varies with the nitridation time. The experimental
findings showed that the nitridation times had more influence on edge dislocation densities than the screw type. 相似文献
2.
《Surface science》1986,165(1):191-202
Several GeSi alloy films with different surface properties were prepared from a 500 Å thick Ge film that had previously been grown on a Si(111)-7×7 substrate by molecular beam epitaxy. The films were prepared by combinations of sputtering, annealing and Ge deposition from an evaporator. The surface properties were studied by Auger electron spectroscopy (AES) and by low energy electron diffraction (LEED). A novel LEED system employing position-sensitive detection was used. The Ge film surface gave a superposition of 7×7 and c(2×8) LEED patterns. A 7×7 → 1×1 phase transition was observed at 425±10°C. An irreversible 7×7 → c(2×8) transition was observed when the sample was heated above 500°C. The Ge film melted at 750±30°C and formed a GexSi1−x (x = 0.85±0.05) alloy whose surface gave a 7×7 LEED pattern. A 7×7 → 1×1 phase transition was observed at 600±0.15°C. Prolonged sputtering and annealing resulted in a GexSi1−x (x = 0.53±0.05) alloy whose surface gave a 5×5 LEED pattern. An apparent 5×5 → 1×1 phase transition was observed at 870±10°C but at that temperature the film was converted irreversibly to one with a much lower Ge atom fraction (x = 0.025±0.005) whose surface gave a 7×7 LEED pattern. A surface with a 5×5 pattern identical to that for the x = 0.53 alloy was prepared by deposition or Ge on Si. A similar 5×5 surface was prepared by deposition of Ge on a facetted GeSi alloy surface originally showing a superposition of 5×5 and 7×7 patterns. The intensity distributions in all of the 7×7 LEED pattern were found to be similar to those for Si(111)-7×7 at nearly the same electron energies. The characteristics of the 7×7 → 1×1 phase transitions were discussed in direct comparison with those of the Si(111)7×7 → 1×1 and Ge(111)-c(2×8) → 1×1 transitions observed with the same LEED system. 相似文献
3.
Thin films of Al–Ni–Co alloy with an average thickness of 15?nm were produced by means of conventional vacuum deposition technique on (0001) sapphire substrates heated at various test temperatures. The microstructures and textures of the films obtained were thoroughly investigated by atomic force microscopy, X-ray diffraction and transmission electron diffraction and imaging techniques. The diffraction measurements have evidenced that the vacuum deposition of Al72Ni15Co13 alloy on the substrates heated above 400°C allows a homogeneous poly-quasicrystalline film, consisting of the Ni-rich basic decagonal phase to grow. It has been further indicated by in-plane XRD analysis that the film deposited at 550°C contains a considerable amount of the decagonal grains epitaxially grown on the sapphire substrate. Possible epitaxial relations occurring between the deposit and the substrate will be detailed on the basis of results obtained from electron diffraction measurements. 相似文献
4.
Dynamic Monte Carlo simulations are performed to investigate the interface mixing of Co atoms deposited on a Cu (001) substrate. A tight-binding potential was used to determine the input parameters (jump probabilities and energy barriers) for the Dynamic Monte Carlo model. The results show that more Co adatoms penetrate into the substrate as the temperature rises and/or as the deposition rate decreases, and that the intermixing between the layers becomes concomitantly more pronounced. Cu atoms migrating into the Co layer via exchange processes during the growth of consecutive Co layers are proposed to be responsible for the intermixing. Furthermore, an initial Co clustering followed by a layer-by-layer growth mode was observed in the simulations, with the surface concentration of Cu atoms depending on the fraction of migrating Cu atoms and decaying into the Co film following a power law. The fraction of Cu atoms migrating into the Co layer can be adjusted by varying the deposition rate and the substrate temperature. 相似文献
5.
This paper reports on a study of the adsorption of CO molecules on the surface of ytterbium nanofilms of different thicknesses, which were sublimed on Si(111)7 × 7 at room temperature. Dependences of two types were investigated: the surface concentration of adsorbed molecules vs. CO dose expressed in langmuirs and the work function of films vs. CO dose. It was shown that the behavior of these dependences is mediated by size effects and Friedel oscillations generated by the ytterbium-silicon interface. Both effects exert an influence on the binding of CO molecules to the surface. At low molecule concentrations, this binding is effected through lone electron pairs localized at the carbon ends of the molecules. These electrons form a donor-acceptor bond to the vacant 5d level of the metal, with the level dropping below the Fermi level. At high CO molecule concentrations, the pattern becomes more complex; indeed, the enhanced Coulomb interaction gives rise to a partial transfer of electrons from the 5d level to the vacant 2π* orbital of CO molecules. 相似文献
6.
D. A. Muzychenko A. I. Oreshkin S. I. Oreshkin S. S. Ustavschikov A. V. Putilov A. Yu. Aladyshkin 《JETP Letters》2017,106(4):217-222
The initial stage of the adsorption of Ge on an Au(111) surface was investigated. The growth and stability of the structures formed at the surface were studied by ultrahigh-vacuum low-temperature scanning tunneling microscopy and analyzed using density functional theory. It was established that the adsorption of single Ge atoms at the Au(111) surface at room temperature leads to the substitution of Au atoms by Ge atoms in the first surface layer. An increasing of surface coverage up to 0.2–0.4 monolayers results in the growth of an amorphous binary layer composed of intermixed Au and Ge atoms. It was shown that the annealing of the binary layer at a temperature of T s ? 500 K, as well as the adsorption of Ge on the Au(111) surface heated to T s ? 500 K for coverages up to 1 monolayer lead to a structural transition and the formation of an Au–Ge alloy at least in the first two surface layers. Based on experimental and theoretical data, it was shown that the formation of single-layer germanene on the Au(111) surface for coverages ≤1 monolayer in the temperature range of T s = 297–500 K is impossible. 相似文献
7.
Michael A. Henderson 《Surface science》2010,604(19-20):1800-1807
The photochemical properties of the Cr-terminated α-Cr2O3(0001) surface were explored using methyl bromide (CH3Br) as a probe molecule. CH3Br adsorbed and desorbed molecularly from the Cr-terminated α-Cr2O3(0001) surface without detectable thermal decomposition. Temperature programmed desorption (TPD) revealed a CH3Br desorption state at 240 K for coverages up to 0.5 ML, followed by more weakly bound molecules desorbing at 175 K for coverages up to 1 ML. Multilayer exposures led to desorption at ~ 130 K. The CH3Br sticking coefficient was unity at 105 K for coverages up to monolayer saturation, but decreased as the multilayer formed. In contrast, pre-oxidation of the surface (using an oxygen plasma source) led to capping of surface Cr3+ sites and near complete removal of CH3Br TPD states above 150 K. The photochemistry of chemisorbed CH3Br was explored on the Cr-terminated surface using post-irradiation TPD and photon stimulated desorption (PSD). Irradiation of adsorbed CH3Br with broad band light from a Hg arc lamp resulted in both photodesorption and photodecomposition of the parent molecule at a combined cross section of ~ 10? 22 cm2. Photodissociation of the CH3–Br bond was evidenced by both CH3 detected in PSD and Br atoms left on the surface. Use of a 385 nm cut-off filter effectively shut down the photodissociation pathway but not the parent molecule photodesorption process. From these observations it is inferred that d-to-d transitions in α-Cr2O3, occurring at photon energies < 3 eV, do not significantly promote photodecomposition of adsorbed CH3Br. It is unclear to what extent band-to-band versus direct CH3Br photolysis play in CH3–Br bond dissociation initiated by more energetic photons. 相似文献
8.
Katawut Chuasiripattana Oliver Warschkow Bernard Delley Cathy Stampfl 《Surface science》2010,604(19-20):1742-1751
The polar Zn-ZnO(0001) surface is involved in the catalysis of methanol synthesis and the water–gas-shift reaction. We use density functional theory calculations to explore the favorable binding geometries and energies of adsorption of several molecular species relevant to these reactions, namely carbon monoxide (CO), carbon dioxide (CO2), water (H2O) and methanol (CH3OH). We also consider several proposed reaction intermediates, including hydroxymethyl (CH2OH), methoxyl (CH3), formaldehyde (CH2O), methyl (CH3), methylene (CH2), formic acid (HCOOH), formate (HCOO), formyl (HCO), hydroxyl (OH), oxygen (O) and hydrogen (H). For each, we identify the preferred binding geometry at a coverage of 1/4 monolayers (ML), and report calculated vibrational frequencies that could aid in the identification of these species in experiment. We further explore the effects on the binding energy when the adsorbate coverage is lowered to 1/9 and 1/16 ML. 相似文献
9.
In this paper, some samples of Al-Si alloy with various silicon content were treated by laser beam. The effects on structure, hardness and substructure of samples were investigated. The experimental results show that the primary crystal Al and eutectic silicon in the laser treated samples is got thinning obviously, the mosaic dimension is decreased and the dislocation density is increased. 相似文献
10.
R. A. Zhachuk S. A. Teys B. Z. Olshanetsky 《Journal of Experimental and Theoretical Physics》2011,113(6):972-982
The diffusion of strontium atoms on the Si(111) surface at room temperature has been investigated using scanning tunnel microscopy
and simulation carried out in terms of the density functional theory and the Monte Carlo method. It has been found that the
reconstruction of a clean silicon surface with a 7 × 7 structure has a profound effect on the diffusion process. The average
velocity of motion of a strontium atom in a unit cell of the 7 × 7 structure has been calculated. The main diffusion paths
of a strontium atom and the corresponding activation energies have been determined. It has been demonstrated that the formation
of scanning tunnel microscope images of the Si(111)-7 × 7 surface with adsorbed strontium atoms is significantly affected
by the shift of the electron density from the strontium atom to the nearest neighbor silicon adatoms in the 7 × 7 structure. 相似文献
11.
The interaction of methanethiol molecules CH3SH with the Au(111) surface is investigated, and it is found for the first time that the S-H bond remains intact when the methanethiol molecules are adsorbed on the regular Au(111) surface. However, it breaks if defects are present in the Au(111) surface. At low coverage, the fcc region is favored for S atom adsorption, but at saturated coverage the adsorption energies at various sites are almost isoenergetic. The presented calculations show that a methanethiol layer on the regular Au(111) surface does not dimerize. 相似文献
12.
《Surface science》1986,175(1):45-54
The effect of the sulfur induced reconstruction of the Pt(S)−[6(111) × (100)] surface on CO adsorption was investigated using thermal desorption spectroscopy (TDS). On the unreconstructed surface, desorption states are observed from the (111) terraces and from the high coordinate step edge site. Following the sulfur induced reconstruction of the surface, a new desorption state, ascribed to the upper (100) plane of the two atom high step is observed. Lateral repulsive interactions between S and CO are found to affect the desorption energy on the (111) terrace and the upper face of the (100) step edge. SuIfurCO or COCO interactions do not affect the CO desorption energy from the high coordinate step edge site. Suppression of each CO desorption state appears to be due to sulfur site blocking, as sulfur coverage is increased. 相似文献
13.
The phase composition, electronic structure, and magnetic properties of ultrathin cobalt films (no thicker than 20 ?) applied
on a Si(111)7 × 7 surface at room temperature are studied by high-resolution photoelectron spectroscopy using synchrotron
radiation and magnetic linear dichroism. It is shown that, as the cobalt thickness increases, first interface cobalt silicide
and then an island (discontinuous) film of silicon-in-cobalt solid solution form on the silicon surface. A metal cobalt film
starts growing after the deposition of a ∼7-?-thick Co layer. It is found that the ferromagnetic ordering of the system, which
is characterized by surface magnetization, sets in after the deposition of a ∼6-?-Co layer at the stage of Co-Si solid solution
formation. 相似文献
14.
《Physics letters. A》2020,384(30):126782
α-Sn is on the boundary of a couple of distinct topological phases. It will transform into a topological insulator under a suitable strain. However, a clear picture of its topological surface states (TSSs) is still lacking. Here we perform first-principles calculations on the electronic structure of α-Sn(111) surface to identify its TSSs and reveal their properties. The results show that the presence of valence band reorganizes the TSSs in the inverted sp gap into two Dirac cones. The lower one is in the valence band continuum; the upper one resides in the gap between the valence and conduction bands. We also demonstrate the transformation of the surface states by switching on or off of strain and/or spin-orbit coupling. Without spin-orbit coupling, only the TSSs associated with the lower Dirac cone survive, and they are spin unpolarized. The results are useful for understanding and engineering the topological properties of α-Sn. 相似文献
15.
《Surface science》1995,344(3):L1201-L1206
The dissociative chemisorption of phosphine, PH3, on the Si(111)-(7 × 7) surface has been examined employing supersonic molecular beam techniques. The initial probability of reaction, SR,0, has been found to be sensitive to substrate temperature, Ts, where SR,0 increases sharply by approximately a factor of 4–5 as Ts is increased above 800°C, which corresponds well with the (7 × 7) ↔ “(1 × 1)” phase transition. The reaction probability, SR, measured as a function of dose for PH3 reacting on Si(111)-(7 × 7) at Ts < 800°C, exhibits a dramatic increase as the surface is exposed to the PH3 molecular beam. This unique autocatalytic behavior is consistent with a mechanism in which submonolayer coverages of P(a) are capable of lifting the (7 × 7) reconstruction thus giving rise to a more reactive “(1 × 1)-like” phase. The reaction probability of Si2H6 on Si(111)-(7 × 7) is also observed to pass through a maximum with increasing P(a) coverages, and can be explained by considering similar changes in surface structure and reactivity. 相似文献
16.
The mechanism responsible for liquid crystal (LC) alignment on mechanically buffed or UV exposed polymer films is poorly understood. A comprehensive study of LC alignment on variously prepared substrates unequivocally shows that the anisotropy in the surface roughness of the substrate completely determines the direction of LC alignment. In all the cases studied, including those where an anchoring transition occurs with temperature, the LC director (re)aligns in the directions of low roughness. 相似文献
17.
B. Y. Man J. Wei C. Yang C. S. Chen M. Liu 《Applied Physics A: Materials Science & Processing》2009,96(4):827-831
ZnO buffer layers have been used to fabricate GaN thin films by using pulsed laser deposition on Si (111) substrates. c-axis GaN thin films were obtained by annealing in NH3 atmosphere at 950°C for 15 min. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and atomic force
microscopy (AFM) have been used for the characterizations of the crystalline quality, composition, and surface morphology
of the films. The annealing in ammonia (NH3) atmosphere markedly affects the preparation of GaN films and the least annealing time is 15 min under our experimental conditions.
The mechanism of the effects of the ZnO buffer layers was studied. In the beginning, Zn–O bonds are destroyed in the interface
of the films; a few O and Zn atoms depart from their positions, while N and Ga atoms fill in the empty positions and form
a hexagonal structure of a special component. Many bonds (such as Ga–O bonds, Zn–N bonds) existed then. The number of Zn–O
bonds decreases and the number of Ga–N bonds increases in the films with increasing of the annealing time. Many other bonds
(such as Ga–O bonds, Zn–N bonds) also decreased and more Ga–N bonds formed with annealing time increasing. After having been
annealed for 15 min under our experimental conditions, the quality of the hexagonal structure GaN films was markedly improved
by the destroying of the Zn–O bonds during high-temperature annealing. 相似文献
18.
《Journal of Electron Spectroscopy and Related Phenomena》1987,44(1):281-288
We report a bond-charge-model calculation for Si(111):H(1×1). This configuration may be assumed to represent the “ideal” (1×1) surface, as long as the saturation of the dangling bonds by H atoms produces a slight perturbation of the dynamical matrix. We display also the geometric folding of these dispersion curves into the (2×1) configuration and compare our results to recent He scattering data and “ab initio” calculations. Thus we are in the position to discuss which dynamical features of the 2×1 pattern arise from the mere folding of the (1×1) pattern and which are due to the physical changes produced by the specific reconstruction. 相似文献
19.
20.
We have determined the growth mode of graphene on SiC(0001) and SiC(0001ˉ) using ultrathin, isotopically labeled Si(13)C "marker layers" grown epitaxially on the Si(12)C surfaces. Few-layer graphene overlayers were formed via thermal decomposition at elevated temperature. For both surface terminations (Si face and C face), we find that the (13)C is located mainly in the outermost graphene layers, indicating that, during decomposition, new graphene layers form underneath existing ones. 相似文献