Current switching by photo-assisted electron field-emission from diamond surfaces☆
Highlights
► Switching electron emission on and off by means of light illumination. ► FE increases monotonically by more than one order of magnitude upon illumination with visible light. ► May be used as a switching device.
Introduction
Electron emission from diamond has been extensively studied, following the discovery that hydrogen termination of diamond surfaces induces a negative electron affinity [1], facilitating all forms of electron emission from diamond [2]. This has lead to the development of various diamond-based electron sources [3]. Another important property of the diamond surface is that by exposing hydrogen terminated intrinsic diamond to H2O or to some other specific molecules (having high electro-negativity), it undergoes a process which induces surface (2D) p-type conductivity [4], [5]. This has been explained by the so called “transfer doping” mechanism by which electrons are transferred from the hydrogen terminated surface of the diamond to the adsorbed molecular layer, resulting in the deficiency of electrons in the diamond surface [6], [7] giving rise to a very thin, near surface, hole accumulation layer [8]. This layer has been shown to display excellent field emission (FE) properties. In particular, FE and Scanning Tunneling Spectroscopy (STS) measurements have confirmed that this accumulation layer forms a two dimensional quantum well in which discrete energy levels have been observed [9], [10]
The results of the present work show that illumination with visible light can increase the FE from transfer doped diamond by more than an order of magnitude, demonstrating that illumination can be used to modulate and externally control the electron emission. The photo-enhanced field emission reported here is found to be independent of photon energy for 532 nm, 637 nm and white light illumination. Hence not only can the electron emission from transfer doped diamond be controlled by the extracting electric field but its turn on and off as well as the extracted current intensity can be varied, at will, by illumination; thus opening up many possible applications.
Section snippets
Experiment and results
In the present work electron field emission (FE) was measured from one <100> and one <111> oriented natural type IIa diamond. Both samples were found, by AFM measurements, to be rather smooth, having an RMS roughness of about 1 nm. The sample surfaces were hydrogen terminated by exposure to a hydrogen micro-wave plasma at 700 °C for 1/2 h. Subsequent exposure to humid air induced the expected conductivity, consistent with the occurrence of transfer doping and the consequent formation of a 2D hole
Discussion
The important points of the present work are (i) The FE (I vs. V) curves for transfer doped diamond (lines a–h in Fig. 2) differ markedly from those expected for the emission from a continuum of electron emitting states (FN)) (line i). (ii) The FE levels off at a particular value of the applied voltage. (iii) The FE depends on illumination, increasing by about a factor of 10, as a result of increasing illumination power (by a factor of 1000), with no change in the general shape of the curves.
Conclusions
We have shown that electron field emission from undoped type IIa diamond whose surface has been rendered conductive by transfer doping (i.e. hydrogen termination followed by exposure to water molecules) can be much enhanced by visible light illumination at room temperature. The FE increases monotonically by more than one order of magnitude upon illumination at ever increasing intensity until it levels off at a particular photon flux. The process is found to be insensitive to the photon energy
Acknowledgements
Discussions with Dr. Cecile Saguy are much appreciated. LG and RK thank the school of physics of Melbourne University for the hospitality.
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Prime novelty: The effect of photon irradiation on the electron field emission from a transfer doped diamond surface is reported. No effect on light wave length is found. Some leveling off of the current with increasing extracting field is noticed. This may find application as an easy way to switch the emission current on and off by illumination.