Ultra thin transition metal oxide coatings on diamond for thermionic applications
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Ultra thin transition metal oxide coatings on diamond for thermionic applications
Amit K. Tiwari1 , Jonathan P. Goss1 , Patrick R. Briddon1 , Nick G. Wright1 , Alton B. Horsfall1 and Mark J. Rayson2 1 2
Electrical and Electronic Engineering, Newcastle University, NE1 7RU, UK Eng. Science and Mathematics, Lule˚ a University of Technology, Lule˚ a S–97187, Sweden
ABSTRACT We have investigated, using density functional simulations, the energetics and the electronic properties of oxides of selected transition metals, TMs, adsorbed onto a diamond (001) surface. We find that stoichiometric oxides of TMs, particularly Ti and Zn, influence the electron affinity of diamond strongly. The electron affinities of stoichiometric oxides of Ti and Zn are calculated to be around −3 eV, significantly higher than 1.9 eV of commonly used H–termination. The reactions of TMs with an oxygenated diamond are found to be highly exothermic. Based upon the energetics and the electronic properties, we propose that in the regime of ultra thin films, oxides of TMs are promising options for surface coating of diamond–based electron emitters, as these coatings are compatible with semiconductor device fabrication processes, while having the benefit of inducing a large negative electron affinity.
INTRODUCTION The electron affinity, χ, of a semiconductor is the difference in energy between the conduction–band minimum (Ec ) and the vacuum energy level (Evac ) [1–3]. Semiconductors for which χ is negative, i.e. Ec lies above Evac , hold great potential for various applications, including photocathode and advanced electron emitters [1–5]. This is because a negative electron affinity (NEA) can reduce the the energetic barrier to electron emission, so that conduction–band electrons can be released into the vacuum even at low temperatures. Due to the relative ease of tailoring χ by use of suitable surface treatments, electron emission from diamond surfaces has attracted much attention. In recent years, significant effort has been devoted to investigating the effect of surface termination on the electronic properties of diamond and it has been established that the terminations such as hydrogen and CsO reduce the work function by inducing an NEA [1, 6], while halogens and alkali–halides yield positive χ (PEA) and therefore have the opposite effect on the work function [2]. Although H and CsO terminations significantly reduce the work function of clean diamond (5.5–6 eV) [1, 6], the resulting value of the work function in the range 2–3.5 eV is still too high for low temperature thermionics, while due to the weak thermal stability, high temperature (above 600 K) operation results in an adsorbate free clean diamond surface. Lithium–oxide termination, which has recently been explored [7], can cause a significant downward shift of 4.50 eV in the work function. However, Li is is not
compatible with typical semiconductor device fabrication processes and hence, a low work function and thermally stable diamond surface for an efficient electron emitter remains to
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