Elsevier

Surface Science

Volume 505, May 2002, Pages 93-114
Surface Science

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

https://doi.org/10.1016/S0039-6028(02)01103-2Get rights and content

Abstract

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.

Introduction

The chemistry of oxygen chemisorption on diamond surfaces and the electronic properties of oxygenated diamond present a rich area for fundamental surface science studies [1], [2], [3], [4], [5], [6], [7]. Controlled oxygenation of single crystal diamond to produce a well-defined, smooth oxygenated surface is interesting from both fundamental and technological perspectives. In contrast to silicon, the oxygenated diamond interface is limited to a monolayer (ML) even at high-pressure or high-temperature oxidation conditions. The ultrathin, oxygenated interface imparts distinctive electronic and physical properties to the diamond. The oxygenated diamond surface is hydrophilic, has a positive electron affinity and a low surface conductivity. In contrast, the hydrogenated diamond surface is hydrophobic and exhibits negative electron affinity (NEA) and low resistivity [8]. By creating spatially resolved oxygenated and hydrogenated domains on the diamond surface, a special type of lateral transistor which utilizes the property of surface conductivity at the gate for controlling the tunneling barrier height has been demonstrated recently [9], [10]. On the other hand, desirable surface properties of the hydrogenated diamond such as NEA and surface conductivity are easily corrupted by oxygen and limit the application of diamond as photocathodes [11] and electrodes. Introducing oxygen into the chemical vapor deposition (CVD) gas feed has been known to promote deposition of high quality crystalline diamond films at lower temperature, prompting questions about the role of oxygen during diamond CVD [12].

The controlled uptake of oxygen on the diamond surface, or procedures for the effective exchange of surface chemisorbed oxygen with hydrogen, or vice versa, has not been established clearly. Molecular oxygen shows no appreciable sticking probability on diamond in vacuum, but moisture may cause a slow oxidation of the surface under ambient conditions. Oxidation can be effected by subjecting the diamond to high-temperature treatment in an oxygen flow tube at high pressure [13]. The difficulty in preparing a well-characterized oxygenated surface stems from the very facile etching and roughening of the diamond surfaces at high temperature. It is desired to search for a less vigorous route for the oxygenation of diamond surface so that the surface structure of the diamond can be maintained for surface science investigation. Pehrsson et al. had examined in detail the oxidation chemistry of hydrogenated C(1 0 0) by activating molecular oxygen over an iridium filament and observed peroxy, carbonyl, ether and hydroxyl groups on the surface, pointing to the complexity of the co-adsorbed hydrogen/oxygen C(1 0 0) system [1], [2]. However the filament activation method produces thermally excited O2 rather than atomic O, and caution has to be exercised to control the filament temperature to prevent the evaporation of metal oxides on the surface. Recently we reported that atomic oxygen beam treatment of the surface using a radio-frequency atom beam source in high vacuum constitutes an efficient route to generate an oxidized diamond surface without destroying the surface structure [14]. In this work, we directly probe for the efficiency of O–D exchange on pre-deuterated diamond by performing ERDA, time-of flight SIMS (TOF-SIMS) and high resolution XPS. Besides macroscopic oxidation with the atom beam source, we have also researched on microscopic oxidation of the diamond surface to investigate whether low temperature, controlled oxidation of the diamond could be performed under ambient conditions.

Theoretical studies of clean and hydrogenated diamond (1 0 0) surfaces have been performed using various empirical and semiempirical techniques with the level of sophistication ranging from slab-MINDO [15], empirical tight binding methods [16], [17] to non-self-consistent local density functional (LDF) calculations [18], [19]. However the electronic structure of oxygenated diamond has received little attention. Previously we have calculated the density of states (DOS) on diamond (1 0 0) previously using first principles linear muffin-tin orbital (LMTO) method [14] and showed that distinct oxygenated states exist in the valence band for oxygenated diamond (1 0 0). In this study, the more sophisticated ab initio calculations were performed to consider the detailed layered projection DOS.

Section snippets

Experimental

The experiments were carried out in a UHV chamber equipped with surface analysis facilities such as UPS, XPS and RHEED. A 13.56 MHz RF plasma atom beam source was installed in the same chamber such that the surface modification of the diamond samples could be studied in situ using these analysis techniques. The sample used was a semiconducting diamond single crystal (4×4×0.5 mm3) grown homoepitaxially on synthetic diamond (1 0 0) face by microwave enhanced CVD. The diamond crystal was mounted on

Oxidation studies on diamond (100)

A clear 2×1/1×2 reconstruction could be observed on the deuterium-terminated diamond surface prepared by post-treatment of the as-grown diamond by RF-deuterium plasma beam. The RF-deuterium plasma beam was applied at a glancing incidence to facilitate the surface polishing. Preliminary experiments looked at the transformation in the RHEED pattern on the smooth 2×1/1×2 deuterium-terminated diamond after being irradiated with atomic O beam source for different lengths of time. The 2×1/1×2 face as

Discussion

The experiments show that the oxygenation of diamond must proceed under controlled conditions to prevent etching and roughening of the diamond surfaces. At low oxidation temperature (below 300 °C), only partial coverage of the hydrogenated surface with oxygen could be achieved, as verified by both ERDA and TOF-SIMS. This observation is supported by the HREELS and Auger studies of Pehrsson and Mercer [1], [2] which reported that oxidation proceeded in two stages on heated diamond sample. The

Conclusion

Both macroscopic and microscopic oxidation of the diamond (1 0 0) surface has been investigated. Atomic O beam treatment of C(1 0 0) at 300 °C is effective for the surface exchange of chemisorbed D by O. High-temperature atomic O treatment leads to the rapid roughening of the diamond surface and the production of high oxidation states on the surface. The desorption of O from the diamond surface at 800 °C results in the appearance a 2×1 surface state due to the clean surface π-bond reconstruction.

Acknowledgements

This project is funded by NUS academic research grant number R-143-000-061-112 entitled “Growth and etching of wide band gap semiconductors”.

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