Development of Silicon-Based UV-Photodetector Prototypes using Photoluminescent Nanocrystalline Silicon Overlayers
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DEVELOPMENT OF SILICON-BASED UV-PHOTODETECTOR PROTOTYPES USING PHOTOLUMINESCENT NANOCRYSTALLINE SILICON OVERLAYERS Carlos Navarro, Luis F. Fonseca, Guillermo Nery, O. Resto, and S. Z. Weisz Department of Physics, University of Puerto Rico, San Juan, PR. 00931-2243
ABSTRACT The maximum photoresponse of a normal silicon photodetector, that uses a p-n junction as the active zone, is obtained when the incident radiation wavelength is around 750nm. This response diminishes significantly when the incident radiation is near or in the UV region. Meanwhile, nanocrystalline silicon (nc-Si) films with high transparency above 650nm and high absorbance in the UV can be prepared. By quantum confinement effects, a fraction of this absorbed UV energy is re-emitted as visible photons that can be used by the junction. We study the enhancement of the UV-photoresponse of two silicon detector prototypes with a silicon p-n junction active zone and with a photoluminescent nc-Si overlayer. One prototype is made with a porous silicon/n-type silicon/p-type silicon/p++-silicon/metal configuration and the other with an Eu-doped Si-SiO2 overlayer instead of the porous silicon one. The comparison between both prototypes and the control is presented and discussed stressing on the enhancement effect introduced by the photoluminescent overlayers, stability and reproducibility. INTRODUCTION In the UV region, silicon-based photodetectors face the problem of surface states that strongly reduce the interband light absorption. In a typical commercial detector of this kind, the photosensitivity at 400nm is approximately 5 times smaller than at 800nm, where the detector approaches its maximum sensitivity. Various methods have been developed to overcome such difficulties and produce the so-called UV-enhanced silicon detectors. At present, a new possibility to enhance the UV-photoresponse of silicon-based detectors is the inclusion of photoluminescent layers in the configuration of the device. After the work of L. T. Canham [1], it is known that the photoluminescence (PL) quantum yield of nanocrystalline silicon (nc-Si) structures is orders of magnitude higher than in bulk silicon. The most popular of these structures is porous silicon (PSi). The PL spectrum of PSi shows a broad band in the visible region with its maximum changing with the characteristics of the silicon wafer and the preparation method but in the range between 800nm and 650nm for a typical freshly dried and exposed to air sample. Recently, efficient visible photoluminescence has been also obtained from other nc-Si structures such as nc-Si/SiO2 composites. These composites have been prepared by ion implantation, RF co-sputtering and other means. This composite material is more stable and mechanically more resistant than PSi because the Si particles are isolated and embedded in a solid SiO2 matrix instead of being exposed and under the influence of external conditions, as with PSi. The main idea in improving the sensitivity of a silicon-based UV detector using visibly photoluminescent nc-Si
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