Photon, Electron and Utlrasonic Emission from Nanocrystalline Porous Silicon Devices
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PHOTON, ELECTRON AND UTLRASONIC EMISSION FROM NANOCRYSTALLINE POROUS SILICON DEVICES N. KOSHIDA, B. GELLOZ, A. KOJIMA, T. MIGITA, Y. NAKAJIMA, T. KIHARA, T. ICHIHARA*, Y. WATABE*, and T. KOMODA* Dept of Electrical and Electronic Engineering, Tokyo University of A & T, Tokyo, Japan * Advanced Technology Research Laboratory, Matsushita Electric Works, Ltd., Osaka, Japan ABSTRACT For quantum-sized nanocrystalline silicon (nc-Si), various optical and electronic effects have been clarified in addition to a significant band gap widening. As typical examples of these induced effects, some emission properties of nanocrystalline porous silicon (PS) are described in this paper including the present status of application studies. The first one is electroluminescence (EL) of PS diodes. It is shown that following a drastic improvement in the external quantum and power efficiencies, stability has been significantly enhanced by the formation of covalent termination nc-Si surfaces. Next topic is the cold electron emission from PS diodes. When the nanostructure of the PS drift layer is appropriately controlled, injected electrons are accelerated ballistically toward the outer surface and emitted via tunneling through a thin-film top electrode perpendicular to the device surface as energetic electrons. As an efficient surface-emitting electron source, there are many advantages in this emitter over the conventional cold cathodes. The applicability of this emitter to either vacuum-type or solid-state flat-panel display is demonstrated. Finally, the usefulness of a PS device as a thermally induced ultrasonic emitter is presented on a basis of its fundamental characterizations. Technological potential of this emitter for functional acoustic devices is also discussed. INTRODUCTION Due to a strong quantum confinement in nanocrystalline silicon (nc-Si), various useful functions are induced in porous silicon (PS) in relation to the modified optical, electrical, thermal, and chemical properties [1]. A significant bandgap widening causes efficient visible photo-[2] and electro-luminescence [3]. To date, the external quantum and power efficiencies of PS-based electroluminescence (EL) have reached 1.1% and 0.4%, respectively, in the red-band [4]. The EL stability should be improved by the introduction of more complete surface passivation. The PS diode also operates as a planar cold cathode [5]. When the PS structure is appropriately controlled such that a tunneling transport becomes dominant, electrons can travel with a enlarged drift length via multiple tunneling through interfacial barriers between interconnected nc-Si, and emitted ballistically into vacuum [6]. This hypothesis has been confirmed by measurements of the energy distribution of emitted electrons [7,8] and by transient photocurrent analyses under a time-of-flight scheme [9]. A full-color 168×126 pixels flat panel display device has been developed using this surface-emitting cold cathode as an excitation source for a fluorescent screen [10,11]. Energetic electrons generate
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