Surface Science LettersNEA peak of the differently terminated and oriented diamond surfaces
Introduction
Electrons in the conduction band are generally prevented from escaping into the vacuum by the electron affinity barrier χ. However, the diamond (100), (110) and (111) surfaces are known to exhibit a negative electron affinity (NEA) when terminated with hydrogen [1], [2], [3], [4], [5], [6]. NEA is defined if the vacuum level Evac lies below the conduction band minimum (CBM) at the surface and, hence, electrons excited into the conduction band can easily escape into the vacuum. The attractive NEA property of diamond surfaces has gained a lot of interest in recent years as a result of its potential application as a low-voltage field emission device [7], [8], [9], [10], or as a surface channel field-effect transistor [11], [12]. The diamond-based devices show even better characteristics owing to a hydrogen termination, inducing a NEA behavior [13], [14], [15]. Field-emission measurements [8], [9], [10] on nitrogen-doped diamond show threshold fields less than 0.5 V μm−1. The mechanism of emission and the corresponding link to NEA are still not well understood. Very recently, Geis et al. [16] proposed a new surface electron-emission mechanism in diamond cathodes based on the enhancement of electric fields at metal–diamond–vacuum triple junctions. They explained the mechanism as electrons tunneling from a metal into diamond surface states where they are accelerated to energies sufficient to be ejected into vacuum. For a better understanding of the emission process and its optimization for the synthesis of thin diamond films, the exact determination of the NEA peak as well as the NEA value as a function of surface termination and surface orientation are of prime importance.
In this work we present an investigation of the NEA peak as well as quantitative NEA values of the different terminated and oriented diamond surfaces. We discuss the presence of the NEA peak combined with the k||-conservation in the photoemission process. We demonstrate that for differently oriented diamond surfaces, the NEA peak is present when the k||-conservation in the photoemission process is satisfied. We will further show that NEA is not only limited to hydrogen termination, but also a hydrogen oxide at the surface can even result in a stronger NEA in agreement with calculations.
Section snippets
Experimental methods
The hydrogen plasma cleaned crystals [6], [17] are mounted on a heatable (up to 1200°C) sample holder. They are transferred to a VG ESCALAB Mk II spectrometer with a base pressure of 2×10−11 mbar, equipped with a MgKα (hν=1253.6 eV) anode and a helium discharge lamp (He I, hν=21.2 eV). The energy resolution is at its best 0.9 eV for X-ray photoelectron spectroscopy (XPS) and 35 meV for ultraviolet photoelectron spectroscopy (UPS) with He I radiation. The diamond substrates used in this study are
Results and discussion
In Fig. 1 we present the low kinetic energy part of the He I normal emission spectra of the hydrogen-terminated diamond (100), (110) and (111) surfaces. The numbers in the graph present the cutoff energy position with extrapolation to zero intensity estimating an error of 0.1 eV. This error value is estimated from the error for the determination of the energy position and by the reproduction of the results (surface preparation) [6]. At low kinetic energies the hydrogen-terminated (100), (110)
Conclusions
In summary, the NEA peak of differently terminated and oriented diamond surfaces was investigated by means of ultraviolet photoelectron spectroscopy . Electron emission from energy levels below the CBM up to the vacuum level Evac allowed the quantitative calculation of the upper limit of the NEA value. The inelastic scattering at the surface to the vacuum interface and the emission of electrons from the unoccupied surface states, situated in the band gap, are the mechanisms responsible for
Acknowledgements
The authors gratefully acknowledge P. Reinke for supplying the (110) diamond crystal and F. Bourqui, C. Neururer, E. Mooser and O. Raetzo for the skillful technical assistance. This work was supported by the Swiss National Science Foundation.
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