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F8.14.1

Influence of Different Atmospheres on the Life Time of Porous Silicon Light-Emitting Devices B.R. Jumayev #, H.L. Tam, K.W. Cheah and N.E. Korsunska a Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China a Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 03028 Kiev, Ukraine ABSTRACT In present report, we investigated the degradation processes in porous silicon light-emitting devices (LED) in different atmospheres (O2, N2, air and vacuum) by photoluminescence (PL), electroluminescence (EL), lifetime (LT) and I-V characteristic measurements as well as by Energy Dispersive X-ray Spectroscopy (EDS). The contacts were made by evaporation of Au and Au/Cu alloy. The LEDs emit in visible range at forward and reverse bias. As a rule, full width at half maximum of EL spectrum is wider than that of PL spectrum. The bias direction of applied voltage during degradation change EL, PL, I-V characteristics, and LT of the LEDs. At forward bias, LT degradation is less than that in reverse bias. The degradation of LEDs during forward bias did not produce any change in the spectral shape of EL and PL. At reverse bias, degradation led to red shift in the peak of EL and PL. The results show that the lifetime of LEDs with Au contact is longer than Au-Cu. Operating in different atmospheres, the LT in vacuum is longest and is more than 100 hours in reverse bias at room temperature. Possible mechanisms of degradation of LEDs are discussed. It is proposed that degradation is connected mainly with two processes: oxidation and metal diffusion. It is shown that the oxygen and metal in ionic state can diffuse quickly. Hence, in forward bias, the diffusion of metal would dominate, and in reverse bias, diffusion of oxygen dominates. INTRODUCTION The interest to porous silicon from point of view of its application in optoelectronic devices was stimulated by discovery of visible photoluminescence. A lot of works are devoted to investigations of this phenomena but the perspectives of its application in light emitting devices connected with electroluminescence (EL) that has not been studied in detail. The different EL spectra [1,2], a big dispersion of values of effectiveness [3,4], different diapasons of applied voltage [5,6] testify to complexity of investigated systems. In principle, electroluminescence devices based on porous silicon can be made in micron dimensions and combined with other silicon-based elements. But realisation of such device is hampered due to some problems: 1) Good electrical contact to porous layer providing injection of carriers is necessary. Translucent layers of metals, transparent conducting oxide of metals, polymers and liquid electrolytes are usually used as such contacts. The silicon substrate acts as the second electrode. Quantum efficiency in solid contact of EL structures is 10-4-10-2 %, appreciably large values of quantum efficiency ~0.3 % is obtained in systems with liquid contact, however their practical use is more difficult, than solid-state elem