Photoelectric emission from the negative electron affinity (100) diamond surface — exciton effects

Investigation of the photoelectric yield characteristics of the negative electron affinity NEA (100) diamond surface (produced by hydrogen plasma exposure) finds a striking similarity to the emission from NEA (111) diamond surfaces. Work on NEA diamond (111) surfaces has revealed the important role (sometimes dominant) of photoexcited excitons in the photoelectric yield. Comparison of the electron emission properties among surfaces with different crystallographic orientations can give further insight into electron emission mechanisms. We find that the kinetic energy distribution, the photon energy dependence of photoelectric yield, and the photon energy dependence of constant final state measurements of NEA (100) diamond are characteristically similar to the emission from NEA (111) diamond surfaces. Observed differences appear related to the change in band bending at the surface.     

STM images from diamond surfaces: steps towards comparisons of experiment and theory

Stable clean and hydrogenated diamond (100) and (111) surface reconstructions found by density functional molecular dynamics (DFMD) are characterized in their surface electronic properties. For sufficiently large surface slab supercells we have calculated the spatially resolved charge densities of the highest occupied surface states at constant height, which are compared with recently obtained images from scanning tunneling microscopy. The calculated charge-density distributions represent signatures of the considered surfaces which might be used to understand and to classify as-grown diamond surfaces.     

Phonon spectra of clean and Ni-terminated diamond (111) surfaces: An ab-initio study

The phonon densities of states (ph-DOSs) of clean and Ni-terminated C(111) surfaces with 1 × 1 and 2 × 1 surface structures were investigated using ab-initio density functional perturbation theory. The ph-DOSs showed vibrational spectra associated with the surface structures of C(111) and Ni/C(111). Further analyses of various surface phonon modes were performed to identify vibrational features involving the surface atoms of C(111) and Ni/C(111). These features provide important information for experimentally verifying the formation of a diamond bulk-like structure at Ni/C(111), as suggested in a previous study.     

Phase transitions on clean and nickel containing vicinal Si(111) surfaces

Structure and reversible structural phase transitions on clean vicinal Si surfaces inclined from the (111) plane towards [2 ] and [ 11] poles and the effect of nickel on the structure and the structural transitions on these surfaces have been studied by LEED. The structures of clean surfaces inclined towards [2 ] and [ 11] are different. Phase transitions take place at about 870°C and 800° C, respectively. Above the transition temperatures these two surfaces consist of regular steps with heights equal to one interplanar distance d₁₁₁ (3.135 Å). The terrace width between steps is determined by the angle of inclination to the (111) plane. Below the transition temperatures the surfaces inclined to [2 ] contain steps with height 3d₁₁₁, those inclined towards the [ 11] pole consist of combinations of (111) facets and some other, apparently (133). The effect of nickel leads to the formation of regular steps with height 2d₁₁₁ on the surfaces inclined towards [2 ] and with height 2d₁₁₁ or 1d₁₁₁, depending on the nickel concentration, on the inclined towards [ 11]. On nickel-containing surfaces there also take place reversible structural transitions with varying temperature.     

Surface diffusion of Pb on clean Si surfaces

Pb diffusion on clean Si(111), (100), and (110) surfaces was studied by Auger electron spectroscopy and low energy electron diffraction in the temperature range from 100 to 300°C. It is shown that lead transport along sillicon surfaces takes place via the mechanism of solid-phase spreading with a sharp moving boundary. The temperature dependence of the Pb diffusion coefficients on Si(111), (100) and (110) surfaces have been obtained. A Si(110)-4×2-Pb surface structure has been observed for the first time.     

氢、氧终端掺硼金刚石薄膜的电子结构

利用氢微波等离子体溅射和浓酸中沸煮方法分别制备了氢、氧终端掺硼金刚石薄膜.借助X射线光电子能谱及接触角检测对两种终端薄膜表面进行了分析，通过扫描探针显微镜研究了针尖和样品间的扫描隧道谱.结果表明，氢终端掺硼金刚石表面能带向上弯曲，在高于价带顶位置存在浅受主能级；氧终端表面能带向下弯曲，带隙较宽，带隙中不存在表面态.对两种终端金刚石薄膜的导电机理进行了讨论.     

氢、氧终端掺硼金刚石薄膜的电子结构

利用氢微波等离子体溅射和浓酸中沸煮方法分别制备了氢、氧终端掺硼金刚石薄膜.借助X射线光电子能谱及接触角检测对两种终端薄膜表面进行了分析,通过扫描探针显微镜研究了针尖和样品间的扫描隧道谱.结果表明,氢终端掺硼金刚石表面能带向上弯曲,在高于价带顶位置存在浅受主能级;氧终端表面能带向下弯曲,带隙较宽,带隙中不存在表面态.对两种终端金刚石薄膜的导电机理进行了讨论. 关键词： 氢终端 氧终端 掺硼金刚石薄膜 电子结构     

Double diffraction spots in RHEED patterns from clean Ge(111) and Si(001) surfaces

In RHEED patterns from clean Ge(111) and Si(001) surfaces, extra diffraction spots have been observed with superlattice reflection spots due to Ge(111) 2 × 8 and Si(001) 2 × 1 surface structures. The extra spots have not been found out in many previous LEED and RHEED patterns of clean Ge(111) and Si(001) surfaces. When the Ge(111) and Si(001) samples were rotated about an axis normal to the surfaces so as to vary the incident direction of the primary electron beam, the intensity of the extra spots showed a remarkable dependence upon the incident direction and they became invisible in some incident directions, in spite of the experimental condition that an Ewald sphere intersected reciprocal lattice rods of the extra spots. In this study, the extra spots are understood as forbidden reflection spots resulting from double diffraction of superlattice reflections of the surface structures, and the remarkable dependence of their intensity upon the incident direction is explained in terms of excitation of the surface wave of the superlattice reflections. These results suggest that the intensity of diffraction spots in RHEED patterns may be greatly influenced by the surface wave excitation of fundamental and superlattice reflections.     

The diamond surface: atomic and electronic structure

Experimental studies of the diamond surface with primary emphasis on the (111) surface are presented. Aspects of the diamond surface which are addressed include (1) the electronic structure, (2) the atomic structure, and (3) the effect of termination of the lattice by foreign atoms. Limited studies of graphite are discussed for comparison with the diamond results. Experimental results from valence band and core level photoemission spectroscopy (PES), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) are used to study and characterize both the clean and hydrogenated surface. In addition, the interaction of hydrogen with the diamond surface is examined using results from vibrational high resolution low energy electron loss spectroscopy and photon stimulated ion desorption (PSID) yield at photon energies just above the carbon k-edge. Both EELS and PSID verify that the mechanically polished 1 × 1 surface is hydrogen terminated and also that the reconstructed 2×2/2×1 surface is hydrogen free. We apply this basic knowledge of the clean diamond surface and of diamond-hydrogen systematics to understanding of the fundamental growth characteristics of diamond films.     

Vibrational EELS studies of CO chemisorption on clean and carbided (111), (100) and (110) nickel surfaces

Room temperature adsorption of CO on bare and carbided (111), (100) and (110) nickel surfaces has been studied by vibrational electron energy loss spectroscopy (EELS) and thermal desorption. On the clean (100) and (110) surfaces two configurations of CO adsorbed species, namely “terminal” and bridge bonded CO, are observed simultaneously. On Ni(111), only two-fold sites are involved. The presence of superficial carbon lowers markedly the bond strength of CO on Ni(111)C and Ni(110)C surfaces, while no adsorption has been detected on the Ni(100)C surface. Moreover, on the carbided Ni(110)C surface, the adsorption mode for adsorbed CO is changed with respect to the clean surface; only “terminal” CO is then observed.     

Observation of disorder-induced 2D mott-hubbard states of the alkali-earth metal (Mg,Ba)-adsorbed Si(111) surface

We report evidence of a disorder-driven Mott-Hubbard-type localization on the alkali-earth metal (AEM) (Mg,Ba)-adsorbed Si(111)-(7x7) surface. The clean metallic Si(111) surface is found to undergo a two-dimensional (2D) metal-insulator transition as randomly distributed AEM adsorbates cause disorder on the surface. A well-defined electron-energy-loss peak unique to the insulating phase is attributed to an interband excitation between the split Hubbard bands originated from a metallic surface band at Fermi energy. A quantitative analysis of the loss peak reveals that the AEM-induced insulating surfaces are of a Mott-Hubbard type driven essentially by disorder.     

First-principle study of Mg adsorption on Si（111） surfaces

We have carried out first-principle calculations of Mg adsorption on Si(111) surfaces. Different adsorption sites and coverage effects have been considered. We found that the threefold hollow adsorption is energy-favoured in each coverage considered, while for the clean Si(111) surface of metallic feature, we found that 0.25 and 0.5 ML Mg adsorption leads to a semiconducting surface. The results for the electronic behaviour suggest a polarized covalent bonding between the Mg adatom and Si(111) surface.     

不同晶面的氢终端单晶金刚石场效应晶体管特性

通过微波等离子体化学气相淀积技术生长单晶金刚石并切割得到(110)和(111)晶面金刚石片,以同批器件工艺制备两种晶面上栅长为6μm的氢终端单晶金刚石场效应管,从材料和器件特性两方面对两种晶面金刚石进行对比分析.(110)面和(111)面金刚石的表面形貌在氢终端处理后显著不同,光学性质则彼此相似.VGS=–4 V时,(111)金刚石器件获得的最大饱和电流为80.41 m A/mm,约为(110)金刚石器件的1.4倍;其导通电阻为48.51 W·mm,只有(110)金刚石器件导通电阻的67%.通过对器件电容-电压特性曲线的分析得到,(111)金刚石器件沟道中最大载流子密度与(110)金刚石器件差异不大.分析认为,(111)金刚石器件获得更高饱和电流和更低导通电阻,应归因于较低的方阻.     

On the correlation of geometrical structure and electronic properties at clean semiconductor surfaces

In their LEED patterns most clean semiconductor surfaces show extra spots in addition to the normal beams as expected for a continuation of the bulk lattice up to the topmost layer. This means, that these surfaces are reconstructed. The (111) surfaces of silicon and germanium crystals exhibit another interesting feature: the cleaved surfaces show a metastable superstructure which irreversibly converts to a new modification upon heat treatments above about $\text{1}\text{3}$ of the melting temperatures. The structural changes are correlated with changes in the electronic properties of the surfaces. This has been shown by investigations of surface conductivity, surface photoconductivity, surface photovoltage, internal reflection, electron energy-loss spectroscopy, photoemission, and work function. Recent theoretical studies have also revealed a distinct correlation between geometrical arrangements of the surface atoms and the corresponding band structures of the surface states. However, the atomic positions of the surface atoms used in these calculations are only a model since a detailed analysis of the LEED-intensity-versus-energy curves have not been carried out up to now.     

Electronic states of (2 × 1) and (1 × 1) (111) surfaces of Ge,Si, diamond,GaAs and Ge on Si

The “dangling-bond” surface state dispersion curves, E(k), have been calculated for the (2 × 1) and (1× 1) (111) surfaces of Ge, Si, and diamond, for (1 × 1) GaAs, and for (2 × 1) Ge on Si. The calculations employ the sp³s1 empirical tight-binding model of Vogl et al. and the atomic relaxation of Feder et al. The surface state band gaps are in good agreement with optical-absorption and electron-energy-loss measurements for Ge and Si. For the assumed epitaxial geometry, Ge on Si is predicted to shift the dangling-bond states downward by ≈0.1 to 0.4 eV.     

Characteristic energy band values and electron attenuation length of a chemical-vapor-deposition diamond (0 0 1)2 × 1 surface

The characteristic energy band values such as the Fermi-level position with respect to valence band top for a boron-doped p-type hydrogen-terminated chemical-vapor-deposition (CVD) diamond (0 0 1)2 × 1 surface and for a clean CVD diamond (0 0 1)2 × 1 surface have been determined by a new method with an accuracy of ±0.02 eV. The electron attenuation length for the clean diamond (0 0 1)2 × 1 surface for the electron kinetic energy of C 1s X-ray photoemission peak by Mg Kα excitation is experimentally determined to be 2.1-2.2 nm. These values are compared and discussed with the previously reported experimental and simulation values.     

Substrate dependent sublimation kinetics of mesoscopic ice films

We have measured the sublimation kinetics of 2–30 nm thick ice layers deposited in ultra high vacuum at 100 K, on different surfaces; Pt(111) and graphite (0001) surfaces with and without various pre-adsorbed monolayers. The results reveal (i) a much more complex sublimation kinetics than expected for a simple molecular solid, and (ii) a strong influence of the underlying substrate on the kinetics. These features, which are reproduced by computer simulations, are due to (i) a phase transition from amorphous to crystalline ice during sublimation, and (ii) substrate dependent nucleation and growth of the ice layers, respectively. The results are correlated with the hydrophobicity-hydrophilicity of the surfaces, and suggest a new method to characterize the wetting properties of solid surfaces. It can be easily performed with state-of-the-art surface cleanliness control, contrary to conventional wetting methods. Applied to the present results, and those of Smith et al. in the preceding paper [Surf. Sci. 367 (1996) L13], this gives, in order of decreasing wetting of water; clean Pt(111)>cleanRu(001)>Cs covered graphite>clean graphite>octane covered Pt(111)⩾clean Au(111).     

Atom force microscopy study on HTHP as-grown diamond single crystals

For understanding the mechanism of diamond growth at high temperature–high pressure (HTHP) from a metallic catalyst–graphite system, it is of great interest to perform atomic force microscopy (AFM) experiments, which provide a unique technique different from that of normal optical and electronic microscopy studies, to study the topography of HTHP as-grown diamond single crystals. In the present paper, we report first AFM results on diamond single crystals grown from a Fe-Ni-C system at HTHP to reveal the growth mechanism of diamond single crystals at HTHP. AFM images for as-grown diamond samples show dark etch pits on the (111) surface, indicating dislocations. Some fine particles about 100–300 nm in dimension were directly observed on the (100) diamond surface. These particles are believed to have been formed through transition of graphite to diamond under the effect of the catalyst and to have been transported to the growing diamond surface through a metallic thin film by diffusion. The roughness of the (100) diamond surface is found to be about several tens of nanometers through profile analysis. The diamond growth at HTHP, in a sense, could be considered as a process of unification of these fine diamond particles or of carbon-atom-cluster recombination on the growing diamond crystal surface. Successive growth interlayer steps on the (111) diamond surface were systemically examined. The heights of the growth interlayer steps were measured by sectional analysis. It was shown that the heights of the growth interlayer steps are quite different and range from about 10 to 25 nm. The source of the interlayer steps might be dislocations. The diamond-growth mechanism at HTHP could be indicated by the AFM topography of the fine diamond particles and the train-growth interlayer steps on the as-grown diamond surfaces. Received: 29 March 2001 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Interactions of CO molecules adsorbed on oxidised Cu(111) and Cu(110)

Reflection absorption infrared spectroscopy has been used in conjunction with LEED and surface potential measurements to study low temperature CO adsorption on the oxidised Cu surfaces Cu(111)O|$\text{3}\text{2}\text{?}\text{2}$|, Cu(110)O(2 × 1) and Cu(110)Oc(6 × 2). On all three surfaces adsorption at 80 K yields surface potential changes in excess of 0.6 V and does not lead to the formation of an ordered overlayer. At high coverages the adsorption enthalpy is lower than on the clean surfaces. Infrared spectra show the growth of a doublet band with components initially at 2100 and 2117 cm^?1 on the oxidised Cu(111) surface. Similar features seen on the oxidised Cu(110) surfaces are accompanied by a band at 2140 cm^?1: a very weak band at the same frequency on oxidised Cu(111) is attributed to defect sites. Studies of the temperature dependence of the spectrum from oxidised Cu(111) lead to the conclusion that two different binding sites are occupied. Spectra of ${}^{12}\text{CO}{}^{13}\text{CO}$ mixtures show that the molecules occupying these sites are in close proximity to each other, and that the spectrum is subject to large but opposing coverage-dependent frequency shifts.     

Passivation of Si (111) surfaces by ordered metal layers

The properties of √3 × √3 ordered gold and silver monolayers on a Si(111) substrate have been investigated by Auger, low energy electron diffraction and photo-emission analysis. It has been found that oxygen adsorption on these surfaces is considerably weaker than on clean Si surfaces. This new result clearly emphasizes the correlation between the oxidation properties of Si atoms and their local environment. A comparison is made with previous results concerning Au-Si amorphous metallic alloys where gold atoms act as a catalyst for the oxidation.