Preparation and Characterization of the Active Layer for an Led Based on Oxidized Porous Silicon
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Department of Physics & Astronomy, University of Rochester, Rochester NY 14627
*** Also Laboratory for Laser Energetics, Department of Physics & Astronomy and The Institute of Optics, University of Rochester, Rochester NY 14627 **** DRA Malvern, St Andrews Rd, Malvern, Worcs WR14 3PS, UK Abstract We have studied the photoluminescence (PL) in oxidized porous silicon (PSi), prepared from anodized crystalline Si followed by annealing at temperatures ranging from 700 to 1000 0 C. It has been found that two PL bands with spectral peaks at 1.6 eV (near-IR band) and near 2 eV (red band) exist with a strong dependence on preparation (annealing) conditions. Recent experimental results show a correlation between the intensity of the near-IR band and the level of leakage current in the diode-like structure. The suppression of the near-IR emission results in improved carrier transport, and the enhancement of the red band emission maximizes the electroluminescence (EL) efficiency. The PL study of thermally oxidized PSi indicates different recombination mechanisms. The red PL band is associated with a mechanism similar to bandtail-recombination within the quasi-bandgap of Si nanograins, whereas the near infra-red PL is associated with recombination via defect centers. These mechanisms will be discussed.
Introduction Crystalline silicon (c-Si) is the dominant semiconductor material for microelectronics, and there has been an effort over several decades to find a place for silicon in optoelectronics. Major approaches are band-structure engineering in SiGe superlattices [1] and doping by isoelectronic impurities [2] or rare-earth metals [3,]; however, none have been found to provide efficient luminescence at room temperature. In contrast, electrochemically prepared porous Si (PSi) exhibits strong visible room temperature photoluminescence (PL) with > 1 % external quantum efficiency (EQE) [4]. However, the PL in PSi is not stable and can degrade after several hours of storage at the room temperature in a laboratory environment. The degradation of the PL and the electroluminescence (EL) has been found and studied intensively [5, 6, 7]. The proposed mechanism is the effusion of hydrogen from the as-anodized PSi surface [7]. The rate of hydrogen effusion increases drastically at high temperatures (T > 4000C). The irreversible thermal quenching of the PL and EL makes it impossible to use standard microelectronic thermal processes (T - 10000 C) in the fabrication of PSi-based devices. The EL in PSi was reported shortly after the discovery of PL [8]. A typical PSi-based LED consists of a transparent or semitransparent contact (Au, ITO or conducting polymers) and a PSi layer (thickness between 1-10 1im) on a crystalline silicon (c-Si) substrate (p- or n-type) [8-10]. The characteristics of these structures, which can be modeled as a type of Schottky
687 Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997 Materials Research Society
junction, are not promising due to the lack of an ohmic contact to the PSi layer and a large number of defects at P
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