Stabilization of Porous Silicon Electroluminescence by Surface Capping With Silicon Dioxide Films
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STABILIZATION OF POROUS SILICON ELECTROLUMINESCENCE BY SURFACE CAPPING WITH SILICON DIOXIDE FILMS N. KOSHIDA, J. KADOKURA, M. TAKAHASHI*, and K. IMAI* Dept of Electrical and Electronic Engineering, Tokyo University of A&T, Tokyo, Japan *NTT Telecommunications Energy Laboratories, Atsugi, Kanagawa, Japan ABSTRACT To prolong the operation life of electroluminescence (EL) from porous silicon (PS) diodes without affect on the efficiency, the surface passivation technique is applied to the active PS layer. By combining a postanodization electrochemical oxidation (ECO) technique for the PS layer with a surface capping by SiO2 films sputtered using electron cyclotron resonance (ECR) method, the degradation of the EL efficiency is effectively suppressed for a long time over several hours under a cw operation. It is shown that from thermal desorption spectra analyses, the capability of capped films as a barrier against penetration of water molecules is a key factor for stabilizing the EL operation due to preventing luminescent PS layers from current-induced oxidation. INTRODUCTION For practical applications of electroluminescence (EL) from nanocrystalline porous silicon (PS) [1], the external power efficiency (EPE) should be at least 1%, the operating voltage below 10 V and the stability longer than 10,000 h. We recently developed a PS-EL device which offers the red band emission performance with an EPE of 0.4% [2], an external quantum efficiency (EQE) exceeding 1% [3], and an operating voltage of about 5 V. Our EL device is composed of an indium tin oxide (ITO) top contact, a PS layer treated by electrochemical oxidation (ECO), an n+-type Si wafer, and an ohmic back contact. An attained remarkable improvement in the efficiency is owing to the introduction of post-anodization ECO technique [4]. Further attempts should be sought to improve the stability to a practical level. On a basis of the previous results, an approach to stabilize EL of PS device is proposed in this paper. Its effects on the EL characteristics are reported, including the key factor for stabilizing EL operation. POSSIBLE WAYS FOR STABILIZING EL By controlling PS structures and device configurations, the EQE of PS based EL devices has been significantly improved from the values in the early stage. Recently a significant improvement was attained by employing the post-anodization ECO treatment of PS in a H2SO4 solution [4]. It promotes preferential oxidation of nonconfined residual silicon surfaces, while hydride surfaces of depleted nanocrystalline silicon remain unchanged. Thus, both an increase in the spatial density of luminescent nanocrystallites and a decrease in the leakage current are selfregulatedly obtained by an one-step processing. This causes a dramatic improvement in the EQE and EPE up to 1.1% and 0.4%, respectively [2,3]. Regarding the stability of EL, there is still a problem of limited life time possibly due to gradual surface oxidation of luminescent silicon nanocrystallites in PS during long operation and upon storage in ambient. It deter
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