The Influence of Local Ambient Atmosphere on the Electroluminescent Stability of Porous Silicon Diodes
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Libing Zhang and Jeffery L. Coffer* Departmentof Chemistry, Texas ChristianUniversity, Fort Worth, TX, 76129 Bruce E. Gnade CentralResearch Laboratories,Texas Instruments Inc., Dallas, TX, 75265 DaXue Xu and Russell F. Pinizzotto Centerfor MaterialsCharacterization,University ofNorth Texas, Denton, TX, 76203
Abstract In this work, the influence of surrounding ambient atmosphere on the stability of electroluminescent (EL) porous Si (PS) diodes fabricated from anodic oxidation of epitaxiallygrown p-type layers on n-type Si substrates is investigated. These structures are characterized using photoluminescence (PL), electroluminescence (EL), and infrared (IR) spectrosopies, as well as scanning electron microscopy (SEM). Such structures yield orange emission with maxima near 620 nm upon the application of moderate applied voltages (3-7 V). In strong oxidizing environments, EL intensity degrades completely within 30 minutes; in contrast, the integrated intensity remains essentially unchanged in the same timeframe in the presence of a vigorous flow of inert gases such as nitrogen and argon. Infrared spectroscopic studies strongly suggest that electroluminescence degradation is related to porous silicon surface oxidation. Introduction Inherent pessimism in the development of silicon-based optoelectronic materials was altered significantly in 1990 by the discovery that electrochemical fabrication of a thin porous layer on top of crystalline silicon under selected conditions creates a strongly luminescent material. 1 Of critical relevance to device implementation are recent accounts detecting efficient electroluminescence (EL) in porous silicon. 2 -7 These approaches range from simple metal/as7 56 24 prepared porous Si junctions " to conductive polymer , or transparent electrode (ITO) covered porous Si structures. A recurring problem for many of these structures is a deterioration of EL performance over time. The conductive polymer and ITO-covered approaches have shown some promise in improving the efficiency and stability of hole injection, but this type of derivatization thwarts the chemical sensitivity of the native porous Si surface. To further understand the physico-chemical nature of the processes causing this degradation of the electroluminescent response, we have carried out studies probing the effects of changing the gaseous ambient environment surrounding the electroluminescent porous Si from an oxidizing environment to an inert one. These effects are analyzed using photoluminescence 671 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
(PL), electroluminescence (EL), and infrared (IR) spectrosopies. This combined approach permits a more synergistic analysis of the structural changes (or lack thereof) which can accompany such degradation. Experimental Procedures Silicon p-n junctions were prepared using molecular beam epitaxy and were comprised of 2 ttm thick layers of p-type material on n-type substrates; boron concentration in the p-type layer was 1 x 1019 atoms / cm3 . Porous silicon (PS) l
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