Characterization of Optical Lifetime in Silicon-on-Insulator Wafers by Photoluminescence Decay Method
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Characterization of Optical Lifetime in Silicon-on-Insulator Wafers by Photoluminescence Decay Method Shigeo Ibuka, Michio Tajima and Atsushi Ogura1 Institute of Space and Astronautical Science, Sagamihara, 229-8510, Japan. 1 System Devices and Fundamental Research, NEC Corporation, Tsukuba, 305-8501, Japan. ABSTRACT We report observation of temporal decay of luminescence due to electron-hole condensation in silicon-on-insulator (SOI) wafers. The condensate luminescence was observable in SOI wafers under ultraviolet light excitation, because of shallow penetration depth of the light and confinement of photo-excited carriers in the top-Si layer. We found that the temporal decay of the luminescence depended on the surface/interface condition and fabrication method. These findings can be explained by the difference in the recombination process via surface, interface and defect states in the top-Si layer. We propose that the decay measurement of the condensate luminescence has great potential for characterization of SOI wafers.
INTRODUCTION The silicon-on-insulator (SOI) wafer is one of the most attractive materials for fabrication of next generation devices with superior properties of low-power, high-speed and radiation-hardness. With increasing application of these wafers, there is strong and growing requirement for more sensitive and non-destructive characterization of the top-Si layer with a thickness of less than 200 nm. We previously reported that photoluminescence (PL) using an ultraviolet (UV) laser light as an excitation source has been an effective tool for characterization of SOI wafers[1,2]. The UV light excitation enables us to detect the luminescence from the top-Si layer, because of its shallow penetration depth and presence of a buried oxide (BOX) layer acting as a diffusion barrier of photo-excited carriers. As a result, information about type and distribution of defects in the top-Si layer is obtained from the PL spectrum and mapping[2]. Furthermore, luminescence due to the condensed phase of high-dense carriers, known as electron-hole droplets (EHD), is detectable under the UV light excitation. The detection can be explained by confinement of photo-excited carrier in the top-Si layer[3,4]. In this paper, we studied temporal decay of the EHD luminescence in a wide variety of commercial SOI wafers[5]. The observation of the decay was realized employing a pulsed UV light as an excitation source. The decay of SOI wafers with a surface oxide was slower than that of the wafers without the oxide for all wafers. The decay of a bonded SOI wafer with a bonding interface between the top-Si and BOX was faster than that of the wafer with the interface between the BOX and substrate. These results indicate that the recombination via surface and interface states is effective for the decay process of the EHD luminescence. There was dependence of the temporal decay on the fabrication techniques, although there were negligible differences in the luminescence spectral shape among the wafers. We will demonstrate that the
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