Photoelectron Emission Mechanism From Hydrogen Terminated Nano-Crystalline Diamond
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0956-J11-05
Photoelectron Emission Mechanism From Hydrogen Terminated Nano-Crystalline Diamond Daisuke Takeuchi1, Kazuhiko Saeki2, Christoph Erwin Nebel3, Satoshi Yamasaki1, and Oliver Aneurin Williams4 1 Nanotechnology Research Institute (NRI), National Institute of Advanced Industrial Science and Technology (AIST), AIST TC2-13, 1-1-1 Umezono, Tsukuba, 305-8568, Japan 2 Industiral Technology Center of Tochigi Prefecture, Utunomiya, 321-3224, Japan 3 Diamond Research Center, AIST, Tsukuba, 305-8568, Japan 4 Institute for Materials Research (IMO), University of Hasselt, Diepenbeek, B-3590, Belgium
ABSTRACT We investigated the electron affinity on hydrogen-terminated nano- (NCD) and ultranano-crystalline diamond (UNCD) films to reveal influences of defects, sp2 and grain boundaries on photoemission properties. To compare properties, single crystalline IIa diamond, optical grade poly-, nano-, ultranano-crystalline diamond films, a diamond-like carbon film, a highly oriented pyrolytic graphite have been characterized. All hydrogen terminated SCD, PCD and NCD samples show continuous sub-band yield, arising at about 4.4 eV, regardless of their crystal and/or grain sizes, while UNCD, DLC, and HOPG do not show the specific photoemission in the regime hν < 6 eV. Note that the UNCD film grown with some amount of hydrogen in the gas phase (UNCD-H) shows a comparable yield spectrum as hydrogenterminated SCD. According to the normalized spectra, obviously, the electron emission mechanism on hydrogen-terminated NCD films is the same as that of SCD with a NEA of -1.1 eV. The sub-band yield from NCD is even larger than that of SCD, which is attributed to surface enlargement. INTRODUCTION Nano-crystalline diamond (NCD) films grown by chemical vapor deposition (CVD) attracts a lot of attention in fields such as electron emission devices and bio electronics as they can be grown on large areas, are cheap and still exhibit the superior properties of diamond. [1-3] Electron emission from hydrogen-terminated diamond surfaces with negative electron affinity (NEA) is one of the key mechanisms required to be optimized in such devices. [4-6] Nanodiamond films with a dense matrix of grain boundaries show up to 5% sp2-bonded carbon. [1,7] Their effects on electron emission properties are not yet understood. [8,9] Up-to-now, the NEA on nano-diamond and ultranano-diamond films have not been investigated in detail using photoelectron emission experiments. [10] We have reported firstly a photoexcitation mechanism of hydrogen-terminated diamond surfaces below the band-gap energy, which corresponds to a direct photoemission on NEA from the valence band to the vacuum through unoccupied surface states. [11,12] According to this direct measurement of NEA, we investigated the electron affinity on hydrogen-terminated nano- and ultranano-crystalline diamond films. A hydrogenated single crystalline diamond, an optical-grade polycrystalline diamond, and a diamond-like carbon
Table 1 Growth conditions of samples used in this paper. Sample
Substrate
NCD
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