Mechanism of in-Situ Photoluminescence Decay in Porous Silicon and Its Application to Maskless Patterning
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MECHANISM OF IN-SITU PHOTOLUMINESCENCE DECAY IN POROUS SILICON AND ITS APPLICATION TO MASKLESS PATTERNING
Mikio Takai, Sanae Indou, and Hisanori Murase Faculty of Engineering Science and Research Center for Extreme Materials, Osaka University, Toyonaka, 560 Osaka, Japan
ABSTRACT PL decay in a time scale of ms for fresh porous Si surfaces was investigated for different atmospheres and excitation intensities to clarify the mechanism of the PL decay. The PL excitation intensity and atmosphere were found to affect the decay of PL, which was due to laser enhanced oxidation. Locally PL excited areas of porous Si were also found to be etched in a HF solution. Maskless patterning using this effect could be performed by a scan of a focused laser beam followed by HF etching.
INTRODUCTION Porous silicon (Si) has recently drawn much interest in optoelectronic fields because of its light emitting feature [1-6] and easiness in preparation [7]. However, mechanism of the light emission has not yet been clarified. Process technology for porous Si to fabricate light emitting devices such as etching and doping has not been established, since porous Si itself is not stable, e.g., the light emitting property changes in time, environment and thermal treatment [5, 6]. In this study, photoluminescence (PL) from porous Si at room temperature in the air was found to decay in a time scale of ms. The PL intensity, once decreased, was not recovered by re-excitation of PL. This PL decay was investigated for various sample atmospheres and excitation intensities to clarify the mechanism of PL decay. Locally PL excited areas of porous Si, i.e. laser-irradiated areas, were also found to be etched in a HF solution. Maskless etching was performed using this effect by a scan of focused laser beams over the porous Si sample.
EXPERIMENTAL PROCEDURES Porous Si was prepared by anodization of (100) oriented Si with a sheet resistivity of 10 20 f~cm (p-type). Anodization was performed in a 50 % HF:C 2 H5 OH solution for 30 s with a current density of 40 mA/cm 2 . Photoluminescence (PL) was excited at room temperature using a 488 nm line of an argon ion laser with a spot diameter of 1.2 gim (at l/e intensity) and detected by a photon counting system with a photomultiplier mounted on a monochromator as shown in Fig. 1.. PL signals are in-situ processed digitally using a computer. PL decay with time was measured on the fresh surface of porous Si, i.e. just after preparation. Auger spectra before and after laser irradiation were obtained to investigate stoichiometric change at irradiated surfaces of porous Si samples. Depth dependence of Auger signal intensity, i.e. composition change with depth was obtained by layer removal using argon ion sputtering. Maskless patterning was performed by a scan of the laser beam at 2 pm/s with a beam spot diameter of 1.2 gtm.
RESULTS AND DISCUSSION Figure 2 shows the typical PL spectrum for porous Si prepared by anodization for 30 s with a current density of 40 mA/cm 2 . The PL spectrum at room temperature has a broad pea
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