Interface and interlayer barrier effects on photo-induced electron emission from low work function diamond films

doi:10.1016/j.diamond.2014.02.008

Diamond and Related Materials

Volume 44, April 2014, Pages 123-128

https://doi.org/10.1016/j.diamond.2014.02.008 Get rights and content

Highlights

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Spectra of photo-induced electron emission from N-doped diamond is reported.
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Separate energy thresholds are observed for photo-induced and thermionic emission.
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Thresholds are attributed to variance in the samples’ electronic structure.
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Photoelectron generation involves both the UNCD and the metal substrate.

Abstract

Nitrogen-doped diamond has been under investigation for its low effective work function, which is due to the negative electron affinity (NEA) produced after surface hydrogen termination. Diamond films grown by chemical vapor deposition (CVD) have been reported to exhibit visible light induced electron emission and low temperature thermionic emission. The physical mechanism and material-related properties that enable this combination of electron emission are the focus of this research. In this work the electron emission spectra of nitrogen-doped, hydrogen-terminated diamond films are measured, at elevated temperatures, with wavelength selected illumination from 340 nm to 450 nm. Through analysis of the spectroscopy results, we argue that for nitrogen-doped diamond films on metallic substrates, photo-induced electron generation at visible wavelengths involves both the ultra-nanocrystalline diamond and the interface between the diamond film and metal substrate. Moreover, the results suggest that the quality of the metal–diamond interface can substantially impact the threshold of the sub-bandgap photo-induced emission.

Introduction

Diamond is unusual for its property of obtaining a negative electron affinity (NEA) surface after hydrogen passivation [1], [2], [3]. With an NEA and n-type doping, a low effective work function can be achieved, which enables thermionic electron emission from the diamond surface at relatively low temperatures (< 500 °C). Current state-of-the-art techniques for preparing nitrogen-doped diamond thermionic electron emitters involve introducing sufficient sp² bonds at the grain boundaries to reduce the upward band bending, and an effective work function of 1.3 eV has been reported [4]. Low energy photons have also produced electron emission from N-doped diamond films. This visible light photo-induced emission from N-doped diamond was found to share the same low threshold energy as the thermionic emission [5]. Combining these emission mechanisms may enable applications in thermionic energy conversion [6], [7], and use as a photocathode [8].

A recent study suggested that photon-enhanced thermionic emission (PETE) [9] could be an advantage for combining photo-induced and thermionic emission processes in a novel device structure. Application in a concentrated solar cell was suggested. The proposed cell is composed of two parallel plates serving as the electron emitter and collector, and a vacuum gap that separates the two plates. Solar light illuminates the emitter to induce PETE. According to Ref. [9], in this structure the electron affinity of the semiconductor emitter provides a significant contribution to its PETE efficiency. Based on this effect we proposed that, by coating a semiconductor with a low work function diamond film, high efficiency solar energy conversion could be achieved due to the reduced emission threshold [5]. Efficient transport of electrons through the interface between the substrate and the diamond film thus becomes a key objective in engineering the related materials. Understanding the effect of film structure on the photo-induced emission from the diamond emitters will be crucial in the further development of such multilayer structures. In this work we report a spectroscopic study of photo-induced and thermionic electron emission from N-doped diamond films on metal substrates with different interface and interlayer conditions.

Section snippets

Experiment

In this research, microwave plasma enhanced chemical vapor deposition (MPCVD) was employed to prepare nitrogen-doped diamond emitters on 25 mm diameter molybdenum substrates. Four variations were prepared for comparison of different interface structures. They include: 1) a combination of mirror polished Mo substrate/nitrogen-incorporated ultra nanocrystalline diamond ((N)UNCD) inter-layer/N-doped polycrystalline diamond (N-diamond) surface layer; 2) bead-blasted Mo substrate/(N)UNCD/N-diamond;

Results

In order to compare the effects of different layers, optical absorbance measurements were taken from an efficient emitter diamond sample and one without the top N-diamond layer, both on similarly polished fused silica substrates. Data are shown in Fig. 1, which span from 200 to 900 nm. Comparison between the two curves suggests that a significant portion of the light was absorbed in the (N)UNCD layer, and the absorption was stronger in the UV regime. When the wavelength was below 300 nm, the

Discussion

The spectroscopic results show several intriguing features of the low energy photo-induced emission. From all samples tested, the emitted electrons present a low energy cut-off that can be observed from the thermionic emission. Yet, unlike the thermionic emission spectra, the photo-induced emission spectra extend to a maximum kinetic energy that approximately corresponds to the respective photon energy. Assuming Spicer's three-step model for photoelectron emission [16], photo excited electrons

Conclusion

Combined photo-induced and thermionic electron emission of nitrogen-doped, hydrogen-terminated diamond samples was examined with different interface and interlayer conditions between the metal substrate and the top N-doped diamond film. Multiple photo-induced emission thresholds were observed with sub-bandgap photons. PEEM imaging shows that these thresholds were either due to variation of the electronic structure of the different sample sets, or from separated emissive domains, the size of

Prime novelty statement

Hydrogen terminated and nitrogen-doped diamond films exhibit a low work function. This research investigated photo-induced and thermionic electron emission from a range of nitrogen-doped diamond films deposited on molybdenum substrates. The results show that the interface properties of the films can substantially affect the characteristics of the electron emission spectrum.

Acknowledgments

This research was supported through the Office of Naval Research under contract number N00014-10-1-0540. The authors thank Gary G. Hembree for technical support for PEEM imaging. We gratefully acknowledge the LeRoy Eyring Center for Solid State Science at Arizona State University and Diana Convey for the use of the UV–vis spectrometer.

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View full text

Interface and interlayer barrier effects on photo-induced electron emission from low work function diamond films

Highlights

Abstract

Introduction

Section snippets

Experiment

Results

Discussion

Conclusion

Prime novelty statement

Acknowledgments

Surf. Sci.

Diam. Relat. Mater.

Diam. Relat. Mater.

Diam. Relat. Mater.

Diam. Relat. Mater.

Diam. Relat. Mater.

Diam. Relat. Mater.

Diam. Relat. Mater.

Solid State Electron.

Quantum Photoyield of Diamond (111) - A Stable Negative-Affinity Emitter

Phys. Rev. B

Negative-Electron-Affinity Effects on Diamond (100) Surface

Phys. Rev. B

Combined Visible Light Photo-Emission and Low Temperature Thermionic Emission from Nitrogen Doped Diamond Films

Appl. Phys. Lett.