X.N. Xie

Oxygen-induced surface state on diamond (100)

Abstract The electronic structure of oxygenated diamond (100) surface is studied comparatively by experimental photoemission techniques and first principles calculations. Controlled oxygenation of the diamond (100) 2×1 surface at 300°C yields a smooth O:C (100) 1×1 surface with a distinctive emission state at ∼3 eV from the Fermi edge. Oxygenation of the hydrogenated surface at temperatures above 500°C, however, gives rise to extensive etching and roughening of the surface. The experimentally observed emission state at ∼3 eV following O adsorption is assigned to the O-induced surface state. When the oxygenated surface is annealed to 800°C to desorb chemisorbed O, the surface structure changes from 1×1 to 2×1 and another surface state emission at 2.5 eV associated with the clean surface reconstruction can be observed by UPS. This is attributed to the π-bond reconstruction of sub-surface carbon layers following the desorption of first layer CO from the surface. To understand the origin of the O-induced emission state, we calculated the density of states (DOS) of the oxygenated diamond using the first principles linear muffin-tin orbital (LMTO) method with atomic sphere approximation (ASA) based on density functional theory (DFT) and local density approximation (LDA).

Surface oxygenation studies on (100)-oriented diamond using an atom beam source and local anodic oxidation

Surface oxidation studies on pre-deuterated (1 0 0)-oriented single crystal diamond have been performed by oxidizing the diamond surfaces macroscopically using an oxygen atomic beam source as well as microscopically using local anodic oxidation by atomic force microscope (AFM). Oxygen-deuterium exchange on diamond (1 0 0) was investigated by X-ray photoelectron spectroscopy, elastic recoil detection and time-of-flight SIMS. Exchange of pre-adsorbed D by atomic O is thermally activated, with almost complete exchange of surface D by atomic O at 300 °C. At higher oxidation temperatures, oxidation states which are chemically shifted from the C 1s bulk peak by 3.2 eV was observed together with a disordering of the diamond surface. Micron-scale, localized oxygenation of the diamond surface at room temperature could be achieved with a biased AFM tip where we confirmed that the modified areas show a lower secondary electron yield and higher oxygen content. In addition, the electronic structure of the oxygenated diamond surface (on-top (OT) and bridging model) has been investigated by calculating the layered-resolved partial density of states using first principles plane wave ab initio pseudopotential method within the local density functional theory. For the oxygen OT model, sharp features due to occupied surface states in the valence band and unoccupied surface states in the gap exist. The increase in emission intensity near the valence band edge for oxygenated diamond (1 0 0) was verified by ultraviolet photoelectron spectroscopy study.

The evolution of 3×3,6×6,√3×√3R30° and 6√3×6√3R30° superstructures on 6H–SiC (0 0 0 1) surfaces studied by reflection high energy electron diffraction

Abstract The technique of reflection high energy electron diffraction (RHEED) has been applied to study the evolution of various superstructures on 6H–SiC (0xa00xa00xa01) as a function of annealing temperature. Between the evolution of the stable 3×3 and √3×√3 R 30° phases on a silicon-enriched 6H–SiC (0xa00xa00xa01), a mixed phase 3×3/2×2 reconstruction followed by a well-defined 6×6 reconstruction was observed by RHEED for the first time. The 6×6 reconstruction is distinct from the pseudo-periodic 6×6 structure suggested previously for graphite moire pattern on 6H–SiC (0xa00xa00xa01) [Surf. Sci. 48 (1975) 463; Surf. Sci. 256 (1991) 354]. The mechanisms for the formation of these superstructures in the sequence of 3×3,6×6,√3×√3 R 30° and 6√3×6√3 R 30° between 800°C to 1200°C were discussed. The 6×6 structure is proposed to evolve directly from the 3×3 following the missing of consecutive Si clusters in the twisted silicon adlayer model. Annealing the 6×6 reconstructed surface to 1000°C gives rise to a √3×√3 R 30° reconstruction. From here, the segregation of carbon domains occurs readily and these form an incommensurate 6√3×6√3 R 30 epilayer at 1200°C. At the early stages of the annealing, the 6√3×6√3 R 30 RHEED pattern consists of a series of cluster satellite streaks superimposed on 1×1 SiC. Further annealing results in the appearance of graphite streaks with its basis vectors rotated 30° to SiC. Prolonged annealing of the graphitized surface results in the growth of single crystalline graphite multilayers on the 6H–SiC substrate.

A spectroscopic study of the negative electron affinity of cesium oxide-coated diamond (111) and theoretical calculation of the surface density-of-states on oxygenated diamond (111)

Abstract The modification of the electron affinity of clean and oxygenated C(111) with cesium has been studied using ultra-violet photoelectron spectroscopy. Oxygenated C(111) shows strong valence emission features at 4.2 eV attributable to CO bonding orbital. Adsorption of cesium on the oxygenated diamond results in the formation of cesium oxide features at 4.2 eV and 8 eV and the condition of negative electron affinity (NEA). The cesium oxide adlayer is thermally stable to 500 °C and the NEA condition is not removed even at saturated O exposures, suggesting that cesium oxide-modified C(111) may act as a good photo-cathode. In order to evaluate the effect of oxygen atom adsorption on the surface states of C(111) 2×1 surface, the density of states of the O:C(111) surface adopting two different oxygen binding configurations have been calculated using the periodic density functional theory. The surface gap states on clean C(111) 2×1 surface are passivated by the adsorption of 1/2 monolayer oxygen in a bridging ‘epoxy’ fashion (C–O–C) across the carbon in the Pandey chain. At 1 monolayer oxygen coverage, the 2×1 reconstruction is lifted and an ‘on-top’ carbonyl (CO) binding mode results. Layered-resolved partial DOS calculation reveals that the surface band gap is closed with the adoption of the CO bonding configuration due to the introduction of quasi-continuous surface band states.

Oxidation of the 3×3 6H-SiC (0001) adatom cluster: A periodic density functional theory and dynamic rocking beam analysis

The atomic reconstruction and the adsorption of oxygen on 6H-SiC (0001) surfaces have been investigated by reflection high energy electron diffraction (RHEED) dynamic rocking beam analysis. The various possible chemisorption states on the surface following the adsorption of one, two, and three oxygen molecules have been studied using periodic density functional theory. RHEED rocking beam analysis provided insights into the atomic structure of 6H-SiC (0001) 3×3 in terms of vertical and lateral displacements, as well as the initial chemisorption state of oxygen on the reconstructed surface.

Ultrathin oxide interfaces on 6H–SiC formed by plasma hydrogenation: Ultra shallow depth profile study

Silicon oxide ultrathin films grown on silicon carbide (6H–SiC) by plasma hydrogenation have been studied using ultrashallow depth profiling with time–of–flight secondary ion mass spectrometry. Plasma hydrogenation gives rise to an epitaxial 3×3R30° silicate structure on 6H–SiC(0001) and 6H–SiC(0001). By selecting appropriate sputtering conditions, an ultrathin and atomically abrupt interface delineating the boundary between the silicate epilayer (SiO+,Si2O+, and SiO3−,SiO2−) and bulk silicon carbide (SiC+) was observed on both C(0001) and Si(0001) faces. Differences in the sputtering profile between the C and Si faces suggest an enrichment of the interface stoichiometry by Si and O on the Si face. Our results support the structural models of the silicate on the C and Si-face 6H–SiC(0001) proposed by Starke [Appl. Phys. Lett. 74, 1084 (1999); J. Vac. Sci. Technol. A 17, 688 (1999)].

Chemical etching study of probe-grown ultrathin nano-oxides by atomic force microscopy

We report the study of the etching characteristics of atomic force microscopy (AFM) probe-grown ultrathin oxides (AFM oxides, up to 5nm thick). In our method, an AFM localized depth analysis technique was employed to monitor the atomic layer-by-layer etching of AFM oxides. Insights into the growth mode and etching mechanism of AFM oxides were acquired on the basis of the etching results: it was found that AFM oxide growth is related to Si out-diffusion. For the formation of ultrathin oxides in ambient conditions, it is evident that oxidation-enhanced Si diffusion facilitates the layer-by-layer oxide growth in AFM anodic oxidation. The etching rate of ultrathin AFM oxides is dependent on the SiOH silanol reactive sites. Thermal annealing could reduce the content of silanol groups and enhance the chemical stability of AFM oxides against etching.

Reactive atom beam deposition of boron nitride ultrathin films and nanoparticles using borazine

Abstract The deposition of boron nitride nanomaterials and ultrathin films in high vacuum conditions (1×10 −6 –1×10 −3 Torr) using molecular borazine as well as pre-dissociated borazine has been investigated with the motivation of studying the nucleation efficiency of BN under high vacuum conditions. Molecular borazine displays negligible sticking probability on Si(1xa01xa01) 7×7 at room temperature. Pre-adsorbing the borazine on silicon at 140 K and subsequent annealing is effective in forming ultrathin hexagonal films on the sample. It is possible to nucleate BN nanomaterials on nickel-sputtered silicon with high efficiency if the borazine is first discharged in a radio-frequency plasma to form reactive radicals. High quality crystalline h-BN thin films, as well as BN nanotubes can be formed on nickel via this approach.