Electronic and structural properties of the InP(1 0 0)(2 × 4) surface studied by core-level photoemission and scanning tunneling microscopy

The (2 × 4)-reconstructed InP(1 0 0) surfaces have been investigated by scanning tunneling microscopy (STM) and synchrotron-radiation core-level photoelectron spectroscopy. STM observations show that the α2 model describes the atomic structure of the InP(1 0 0)(2 × 4) surface in a limited range of the surface-preparation conditions, as predicted theoretically but not previously observed. STM results also support the accuracy of the previously found mixed-dimer structure for the InP(1 0 0)(2 × 4) surface under less P-rich conditions. A study of P 2p core-level photoelectron spectra, measured with different surface-sensitivity conditions, demonstrates that P 2p photoemission from the mixed-dimer InP(1 0 0)(2 × 4) surface consists of at least two surface-core-level-shift (SCLS) components which have kinetic energies approximately 0.4 eV higher and 0.3 eV lower than the bulk emission. On the basis of the surface-sensitivity difference between these SCLSs, they are related to the third-layer and top-layer P sites in the mixed-dimer structure, respectively.     

Core-level shifts of InP(1 0 0)(2 × 4) surface: Theory and experiment

Surface core-level shifts (SCLSs) of the (2 × 4)-reconstructed InP(1 0 0) surface with the established mixed In–P dimer structure have been investigated by first-principles calculations and photoelectron spectroscopy. Theoretical values were calculated using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy functional. The obtained theoretical values are quite similar within both approximations. The found differences originate in the tiny structural differences. It is concluded that the expansion or contraction of the crystal lattice has smaller effect on the SCLSs than the geometrical details of the reconstruction, which suggests that the Madelung potential has the dominant effect on the SCLSs. The results support the presence of a P 2p peak at higher binding energy (BE) compared to bulk peak, as proposed with recent measurements [P. Laukkanen, J. Pakarinen, M. Ahola-Tuomi, M. Kuzmin, R. E. Perälä, I. J. Väyrynen, A. Tukiainen, V. Rimpiläinen, M. Pessa, M. Adell, J. Sadowski, Surf. Sci. 600 (2006) 3022], and reveal several hitherto not reported SCLSs. The calculated SCLSs reproduce the measured spectra within reasonable accuracy. Furthermore, the atomic origins of the InP(1 0 0)(2 × 4) SCLSs are solved. In particular, it is shown that the lowest SCLS of P 2p of the InP(1 0 0)(2 × 4) arises from the topmost In–P dimers.     

Boron and indium substitution in GaAs(0 0 1) surfaces: Density-functional supercell calculations of the surface stability

Using the density-functional method and surface supercells the surface formation energies are calculated for the most stable GaAs(0 0 1) surface reconstructions without and with up to four indium or/and boron substitutions. Optimal conditions for the growth of the alloys are derived from calculated surface stability diagrams. The incorporation of indium into GaAs without phase separation is possible under strong As-rich conditions and medium to In-rich conditions. Less As-rich conditions can lead to the formation of an InAs phase. Ga-rich conditions give an InGa phase. Isovalent boron incorporation into GaAs without phase separation is possible under strong arsenic and reduced boron exposure. A BAs phase can be formed under more B-rich conditions. More Ga-rich conditions lead to the boron substitution in arsenic positions. The formed boron dimers can be a starting point for the formation of a boron phase. A true antisite boron substitution is less probable. Using the suitable growth conditions obtained for the ternary alloys it is energetically more favourable to incorporate both indium and boron (formation of BInGaAs) than to incorporate only one of the two elements (In or B). The antisite boron incorporation is not favoured in combination with isovalent boron or indium.     

GaAs(0 0 1) (2 × 4) to c(4 × 4) transformation observed in situ by STM during As flux irradiation

Atomic resolution scanning tunnelling microscopy (STM) has been used to study in situ the As-terminated reconstructions formed on GaAs(0 0 1) surfaces in the presence of an As₄ flux. The relationship between the As-rich (2 × 4) and c(4 × 4) surfaces is observed throughout the gradual evolution of the reconstruction transformation. The results suggest that during the initial stage of the transformation, Ga-rich As-terminated variations of the c(4 × 4) form in order to accommodate excess mobile Ga produced by pit formation. These transient structures later planarize, as excess Ga is incorporated at step/island edges. Successive imaging of the same sample area during As₄ irradiation allows point-by-point adatom binding to be analysed in a way inaccessible to MBE–STM systems relying on sample quenching and transfer.     

Adsorption and NiO(1 0 0) formation by atomic and molecular oxygen on Ni(1 1 0)

The structure of the ordered p(2 × 1) and p(3 × 1) phases of adsorbed oxygen as well as the formation of ultrathin NiO(1 0 0) layers on a Ni(1 1 0) single crystal are investigated by grazing scattering of fast hydrogen atoms. Via ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of oxygen adsorbates and ultrathin nickel oxide layers. For oxidation using atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude. We compare the experimental results with computer simulations based on classical projectile trajectories for grazing scattering of fast hydrogen atoms and test structure models for oxygen adsorbed on Ni(1 1 0) and NiO(1 0 0).     

Density-functional study of the CO chemisorption on bimetallic Pd–Sn(1 1 0) surfaces

We study the ordered PdSn c(2 × 2), (2 × 1), and PdSn₂ (3 × 1) overlayers deposited on Pd(1 1 0) by using first-principles density-functional calculations. It appears that the two PdSn structures are almost degenerate in the energy. Pd–Sn surfaces we consider do not display the marked buckling with Sn atoms displaced towards vacuum that is common for Pt–Sn surfaces. Low-coverage CO chemisorption at these overlayers and on analogous surface structures on Pd₃Sn is considered. It is shown that inclusion of an empirical correction to the CO adsorption energy changes the stable adsorption site from the long-bridge to the top one in most cases. The adsorption energy decreases with the number of Sn atoms in the vicinity of the adsorption site, and this property correlates well with the position of the centre of gravity of the local Pd d-electron band, and also with the variation of the local density of d-electron states at the Fermi level. The centre-of-gravity value is used to assess the core-level shifts for Pd atoms in various geometries. Most of the calculated data compare rather well with the recent measurements on Pd–Sn overlayers at Pd(1 1 0) as well as with other data on related bimetallic systems.     

Formation of a √3 × √3 surface on Si/Ge(1 1 1) studied by STM and LEED

We have performed a detailed study of the formation and the atomic structure of a √3 × √3 surface on Si/Ge(1 1 1) using both scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Both experimental methods confirm the presence of a √3 × √3 periodicity but unlike the Sn/Ge(1 1 1) and the Sn/Si(1 1 1) surfaces, the Si/Ge(1 1 1) surface is not well ordered. There is no long range order on the surface and the √3 × √3 reconstruction is made up of double rows of silicon atoms separated by disordered areas composed of germanium atoms.     

Observation of charge transfer states of F4-TCNQ on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface

We have investigated the valence electronic states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface using valence photoelectron spectroscopy. Since the electron affinity of condensed F₄-TCNQ is 5.24 eV and the energy from the valence band maximum of the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface to the vacuum level is 4.1 eV, spontaneous charge transfer would be expected in the present system. At sub-monolayer coverage of F₄-TCNQ, characteristic peaks are observed at 1.1 and 2.5 eV below Fermi energy. The former peak is assigned to a singly occupied affinity level, and the latter is ascribed to a relaxed highest occupied molecular orbital of adsorbed F₄-TCNQ. The work function change is increased up to +1.3 eV as a function of F₄-TCNQ coverage. These results support the occurrence of charge transfer into F₄-TCNQ on the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface.     

Atomic and electronic structure of group-IV adsorbates on the GaAs(0 0 1)-(1 × 2) surface

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the atomic and electronic structure of the group-IV adsorbates (C, Si, Ge, Sn, and Pb) on the GaAs(0 0 1)-(1 × 2) surface considered in two different models: (i) non-segregated Ga-IV-capped structure and (ii) segregated structure in which the group-IV atoms occupying the second layer while the As atom floats to the surface. The non-segregated structure is energetically more favorable than the segregated structure for Sn and Pb, whereas it is the other way around for C, Si, and Ge.     

Growth of M-plane GaN on γ-LiAlO2(1 0 0): the role of Ga adsorption/desorption

We study the adsorption and desorption kinetics of Ga on using in situ reflection high-energy electron diffraction. Two stable surface phases are identified, which manifest themselves at low temperature by a (1×2) reconstruction at bilayer coverage and a (4×4) reconstruction at trilayer coverage. At growth temperature, Ga adsorbs layer-by-layer up to bilayer coverage after which Ga cluster and eventually droplet formation occurs. The bilayer desorption is delayed by “feeding” from this excess Ga. The optimum growth conditions with regard to surface morphology are those giving rise to trilayer coverage. This finding is in contrast to the case of GaN(0 0 0 1) where the optimum growth conditions are related to the formation of a Ga bilayer at the growth front.     

Structural properties of Cu clusters on Si(1 1 1):Cu2Si magic family

Basing on the results of the scanning tunneling microscopy (STM) observations and density functional theory (DFT) calculations, the structural model for the Cu magic clusters formed on Si(1 1 1)7 × 7 surface has been proposed. Using STM, composition of the Cu magic clusters has been evaluated from the quantitative analysis of the Cu and Si mass transport occurring during magic cluster converting into the Si(1 1 1)‘5.5 × 5.5’-Cu reconstruction upon annealing. Evaluation yields that Cu magic cluster accommodates 20 Cu atoms with 20 Si atoms being expelled from the corresponding 7 × 7 half unit cell (HUC). In order to fit these values, it has been suggested that the Cu magic clusters resemble fragments of the Cu₂Si-silicide monolayer incorporated into the rest-atom layer of the Si(1 1 1)7 × 7 HUCs. Using DFT calculations, stability of the nineteen models has been tested of which five models appeared to have formation energies lower than that of the original Si(1 1 1)7 × 7 surface. The three of five models having the lowest formation energies have been concluded to be the most plausible ones. They resemble well the evaluated composition and their counterparts are found in the experimental STM images.     

GaP低阻欧姆接触层的AES和SIMS分析

本文用AES和SIMS分析讨论了p-GaP与三层金属膜Pd/Zn/Pd形成良好欧姆接触层的性质.     

吸附于纳米磷化镓粉体表面碱性品红的拉曼光谱(英文)

本文中,对吸附于纳米磷化镓(GaP)粉体表面的碱性品红拉曼光谱进行了研究。通过将吸附碱性品红与纯碱性品红晶体样品的拉曼光谱进行对比、分析可知,碱性品红在纳米GaP粉体表面发生了化学吸附。在吸附碱性品红样品的拉曼光谱中,位于1200~1320cm-1范围内的光谱特征表明可能有新的化学键(P-O-C+或Ga-O-C+)形成。碱性品红分子的中央碳正离子(C+)与GaP表面具有孤对电子的氧原子形成配位键。红外光谱结果表明,氧原子与纳米GaP粉体表面的磷原子或镓原子键合,以P-O,Ga-O或P-O-Ga形式存在于GaP表面。碱性品红分子的呼吸振动,N-苯环伸缩振动,以及苯环C-C伸缩振动散射强度与纯碱性品红晶体样品相比皆有所增强。N-苯环伸缩振动散射强度增加意味着N原子是除C+离子以外的另一个可以与GaP表面发生化学作用的活性中心,这种化学作用是由N原子与GaP表面存在共轭效应造成的。     

Atomic and electronic properties of furan on the Si(0 0 1)-(2 × 2) surface

The atomic and electronic properties of the adsorption of furan (C₄H₄O) molecule on the Si(1 0 0)-(2 × 2) surface have been studied using ab initio calculations based on pseudopotential and density functional theory. We have considered two possible chemisorption mechanisms: (i) [4 + 2] and (ii) [2 + 2] cycloaddition reactions. We have found that the [4 + 2] interaction mechanism was energetically more favorable than the [2 + 2] mechanism, by about 0.2 eV/molecule. The average angle between the CC double bond and Si(1 0 0) surface normal was found to be 22°, which is somewhat smaller than the experimental value of 28°, but somewhat bigger than other theoretical value of 19°. The electronic band structure, chemical bonds, and theoretical scanning tunneling microscopy images have also been calculated. We have determined a total of six surface states (one unoccupied and five occupied) in the fundamental band gap. Our results are seen to be in good agreement with the recent near edge X-ray absorption fine structure and high resolution photoemission spectroscopy data.     

Tetra-σ bonding adsorption of pyridine on Si(1 0 0)-2 × 1

We studied adsorption of pyridine on Si(1 0 0) at room temperature using high resolution photoemission spectroscopy (PES) and near edge X-ray adsorption fine structure (NEXAFS) in the partial electron yield (PEY) mode. The Si 2p, C 1s, N 1s spectra of pyridine on Si(1 0 0) showed that pyridine is chemisorbed on Si(1 0 0)-2 × 1 through the formation of the tetra-σ-bonded structure with the N atom and three C atoms. NEXAFS was conducted to characterize the adsorption geometry of pyridine on Si(1 0 0). The π* orbital of CC bond showed a good angle dependence in C K-edge NEXAFS spectra, and we were able to estimate the adsorption angle between chemisorbed pyridine of CC bond and the Si(1 0 0) surface using an analytical solution of NEXAFS intensity. We find the coexistence of two different tight bridges with the adsorption angles 42 ± 2° and 45 ± 2° with almost equal abundance.     

A comparative study of clean and Bi-stabilized InP(1 0 0)(2 × 4) surfaces by the core-level photoelectron spectroscopy

The bismuth-stabilized (2 × 4)-reconstructed InP(1 0 0) surface [Bi/InP(1 0 0)(2 × 4)] has been studied by synchrotron-radiation core-level photoelectron spectroscopy. The spectra are compared with previous core-level data obtained on a clean InP(1 0 0)(2 × 4) surface. The findings support that the P 2p surface-core-level shift (SCLS) of the clean InP(1 0 0)(2 × 4), which has higher kinetic energy than the bulk emission, arises from the third-layer P atoms and that the second P 2p SCLS, which has lower kinetic energy than the bulk, arises from the top-layer P atoms. Similar In 4d SCLSs are found on the clean and Bi-stabilized InP(1 0 0)(2 × 4) surfaces, indicating that these shifts contain contributions of the In atoms that lie in the second and/or fourth layers. In addition to this, the results improve our understanding of the atomic structure of the Bi/InP(1 0 0)(2 × 4) surface and lead to refined surface models which include Bi-Bi and Bi-P dimers.     

Reduction of surface defects on the GaP window layer of 630 nm AlGaInP LED using post-Zn diffusion process

We have investigated the characteristics of the surface of the GaP window layer of 630 nm AlGaInP LED, which was improved by post-Zn diffusion process. The measured resistance and the amount of hole concentration of the post-Zn-diffused GaP window layer have remarkably decreased and increased, respectively. Moreover, the ECV system showed that the amount of doping concentrations on the surface of the GaP window layer was significantly increased because of the diffusion of Zn atoms. The amount of surface defects observed on the post-Zn-diffused GaP window layer was also reduced. Furthermore, it was found out that the efficiency of 630 nm AlGaInP LED chip was increased due to the surface improvement of the GaP window layer. At an injection current of 40 mA, the LED chip with a Zn diffusion layer obtained a higher output power of 11 mW compared to the 7.5 mW output of the conventional LED chip.     

Interaction of hydrogen with InN thin films elaborated on InP(1 0 0)

III-V semiconductor compound structures are widely applied in technology of advanced microelectronics, optoelectronics, and gas sensors. In this paper, we report on the use of XPS to characterize in situ the interaction of thermally activated hydrogen atoms and hydrogen molecules with InP(1 0 0) surfaces covered by thin InN overlayers. XPS spectra were taken with an ESCALAB-210 spectrometer after repeated hydrogenation cycles at temperatures up to 350 °C. The evolution of the In 3d, In 4d, P 2p, N 1s, O 1s and C 1s photoelectron spectra was carefully monitored. The XPS spectra of the hydrogen exposed surface revealed significant differences compared to those from the non-hydrogenated surface. InN films were found to be weakly reactive to hydrogen under experimental conditions explored. The behavior of P atoms at the hydrogenated surface was dependent on the parameters characterizing each hydrogenation (exposure, hydrogen species used, annealing temperature). Moreover, the heavily hydrogenated surface exhibited a phosphorus enrichment.     

Adsorption of an organic zwitterion on a Si(1 1 1)-7 × 7 surface at room temperature

The couple sulfonato/Si(1 1 1)-7 × 7 leads to remarkable 2D chiral molecular assembly with a stability improved at room temperature. The voltage-dependency of the STM images has been experimentally investigated and the correlation between STM images and PDOS has been studied. The proposed empirical model of the adsorption of molecules on Si(1 1 1)-7 × 7 has been justified by the experimental and theoretical data.     

First-principles study of small palladium clusters on NiAl(1 1 0) alloy surface

Theoretical calculations focused on the geometry, stability, electronic and magnetic properties of small palladium clusters Pd_n (n=1–5) adsorbed on the NiAl(1 1 0) alloy surface were carried out within the framework of density functional theory (DFT). In agreement with the experimental observations, both Ni-bridge and Al-bridge sites are preferential for the adsorption of single palladium atom, with an adsorption energy difference of 0.04 eV. Among the possible structures considered for Pd_n (n=1–5) clusters adsorbed on NiAl(1 1 0) surface, Pd atoms tend to form one-dimensional (1D) chain structure at low coverage (from Pd₁ to Pd₃) and two-dimensional (2D) structures are more stable than three-dimensional (3D) structures for Pd₄ and Pd₅. Furthermore, metal-substrate bonding prevails over metal–metal bonding for Pd cluster adsorbed on NiAl(1 1 0) surface. The density of states for Pd atoms of Pd/NiAl(1 1 0) system are strongly affected by their chemical environment. The magnetic feature emerged upon the adsorption of Pd clusters on NiAl(1 1 0) surface was due to the charge transfer between Pd atoms and the substrate. These findings may shade light on the understanding of the growth of Pd metal clusters on alloy surface and the construction of nanoscale devices.