A Novel Solid-State Light-Emitting Device Based on Ballistic Electron Excitation
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A NOVEL SOLID-STATE LIGHT-EMITTING DEVICE BASED ON BALLISTIC ELECTRON EXCITATION Y. NAKAJIMA, A. KOJIMA and N. KOSHIDA Department of Electrical and Electronic Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan ABSTRACT The concept of a novel solid-state light-emitting device is proposed on a basis of our previous report that the nanocrystalline porous silicon (PS) diode with a well-controlled structure operate as an efficient ballistic electron emitter. This device is composed of a semitransparent thin Au film, a fluorescent thin film, a PS layer, and n-type Si substrate. When a positive bias voltage is applied to the Au electrode, visible luminescence is emitted of which band corresponds to that of the deposited fluorescent material. The optoelectronic characteristics suggest that the light emission is based on direct excitation of fluorescent film by ballistic electrons generated in the PS layer. This result indicates another possibility of nanocrystalline silicon for photonic applications. INTRODUCTION Nanocrystalline porous silicon (PS) diodes operate as not only photoluminescent (PL) [1] and electroluminescent (EL) [2] devices but also surface-emitting cold cathodes in vacuum [3]. The PS emitter is simply composed of a thin Au film, a PS layer and a Si substrate. This PS device has many advantageous features as an electron source: simple structure, relatively low operation voltage, less sensitivity to vacuum pressure [4-6]. In addition, the PS diode with a well-controlled structure made by the porosity-modulation technique emits electrons under the ballistic mode [7,8]. Similar phenomenon has also been observed in the diode based on anodized polycrystalline silicon films [9]. The mechanism can be explained by generation of ballistic electrons via multiple-tunneling effect in interconnected silicon nanocrystallites under a high electric field. This function is very attractive for possible use of PS for novel ballistic devices, including large-area flat panel display, owing to the controllability of electron energy and little scattering losses. In this work, it is pursued to use ballistic electrons within the PS diode as excitation source of a light-emitting device without emitting into vacuum. The device configuration is presented including the light emission characteristics. EXPERIMENTAL PS layers were formed by anodizing heavily doped (0.005-0.018 cm) single-crystalline, n-type Si wafers with an ohmic contact in a solution of HF (55 wt%):ethanol=1:1. During the anodization, a sample was illuminated by a 500 W tungsten lamp from a distance of 20 cm. The PS layer structure was controlled by periodically modulating the anodization current. Based on the previously reported porosity-multilayer technique, PS layers were modified such that a graded-band multilayer (G-M) structure is made as shown schematically in Fig. 1. To make the
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effect of structural control clear, the normal PS devices prepared under a constant anodization current was also
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