Illumination effects on the characteristics of nanocrystalline silicon ballistic emitter as an active electrode in solutions

Abstract

As reported previously [1], the nanocrystalline silicon (nc-Si) ballistic electron emitter shows some specific features: emission of high energy electrons with several eV in average [2], surface emission [3], small angle dispersion [4], operation in various medium, including vacuum, atmospheric-pressure gases [5], and solutions [6,7]. When the emitter is driven in aqueous solutions without using any counter electrode, hydrogen is generated at the device surface. The present paper reports the effects of illumination on the activity of the nc-Si ballistic emitters in solutions. The experimental results of the operation in D 2 O solutions are also presented.The nc-Si emitter was composed of a thin Au film (10 nm), nc-Si layer (1ȝm), polycrystalline silicon, and n + -Si substrate. The nc-Si layer was formed by photo-anodization technique and oxidized rapidly to enhance the electric field effect. Then, some treatments (high pressure water vapor annealing, annealing in forming gas, surface chemical modification; replacement of residual metastable Si-H bonds by stable covalent Si-C bonds) were performed to decrease oxide film defects and to passivate nc-Si layer.The device was packaged such that only emitter surface area was exposed to the solution. A Halogen Lamp was used to illuminate the emitting surface in some experiments. As shown in Fig. 1, cyclic voltammograms in the three electrode configuration were measured under the dark or illumination. The species of gas evolved in solutions during the operation with no counter electrode were also analyzed by gas chromatography. The change in the properties of solutions was characteristics by pH value and dissolved hydrogen content.Figure 2 shows the voltammograms characteristics of the nc-Si ballistic electron emitter as an active electrode in H 2 SO 4 solution (0.1 mol/l, pH=1) at V PS =30 V. The characteristics under Pt-Ptconfiguration is also shown for reference. The redox potential of H 2 SO 4 solution is also indicated by the arrows. When the device surface is illuminated, both the diode current and the H 2 generation rate are increased in comparison to those observed in the dark. This is presumably due to a photo-induced effect leading to an enhancement in the energetic electron injection into solutions. The assistance of illumination at appropriate wavelength possibly improves the electrochemical activity.As in every aqueous solution, in which the emitter operation generates hydrogen with no by-product, only deuterium is generated in D 2 O as well. The pH value also exhibited a shift toward the alkaline side during the device operation with no counter electrodes.It has been confirmed that this device operates as an active electrode which supplies strongly reducing electrons into solutions. The hydrogen generation rate depends not only on H + ions concentration at the interface but also the difference between the energy of emitted electrons and redox potential of solutions. The nc-Si ballistic emitter is useful in solutions not only for hydrogen generation but also ...