Visible Luminescence from Laser-Induced Stain- and Dry Etched Silicon
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pulse width and frequency 10 Hz in atmosphere of air. The laser beam energy (LBE) and the IT were changed from 200 to 420 mJ per pulse and from 5 to 120 sec, respectively. The surface of DEPSL was not colored but after the laser treatment it becomes rough, which is visible to the eye. The study of the surface morphology of DEPSL by AFM shows that the surface has sphere-like objects characteristic for ECE and SE PS [7,8] with bigger differences in size 20-500 nm. This suggests smaller effective surface of porous Si than that obtained by ECE and SE where the crystallite size is between 2 and 15 nm [9]. As result the PL intensity of these samples is lower. The rough surface of DEPSL has made the IR transmission measurements impossible. The IR measurements were performed on a Bruker IFS66A Fourier Transform spectrophotometer equipped with a vacuum chamber. The PL measurements were carried out on a SPEX 1404 double spectrometer with GaAs photomultiplier tube in photon counting mode. The 413 nm line of a Krypton laser was used for PL excitation. The XPS studies were carried out in an Escalab MK II (VG Scientific) electron spectrometer and the details are described in [10]. All measurements were made at room temperature. RESULTS AND DISCUSSION Figure 1 illustrates characteristic PL spectra of two SEPSL (curves A and B) and one DEPSL (curve C) samples. The PL intensity of SEPSL is about one order of magnitude higher. The PL spectra of both types of samples can be fitted with two Gauss curves centered at about 2.6 eV (blue band-BB) and 1.9 eV (red band-RB). The variation of the PL of SEPSL with the preparation conditions is shown on Fig. 2. The data are the average of several sample sets. The scatter of the PL peak intensity and energy position are 20% and 5%, respectively. The PL peak intensity of the RB of SEPSL decreases with the Ar laser power. The peak intensity of the BB exhibits a maximum at about 240 mW/cm2 . For samples obtained after 5 min. treatment this dependence is more pronounced. It should be noted that the similar behaviour of the PL was observed for sets of samples prepared at the other LP
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Figure 1. Photolumninescence spectra of two SEPSL prepared at 120 mnW/cm 2 (curve A) and 240 mW/cm 2 (curve B) laser power and 5 mmn. irradiation time, and of a DEPSL prepared at 300 mJ laser beam energy and 20 sec. irradiation time (curve C). 504
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200 400 2 Laser power (mW/cm ) Figure 2. Dependence of the PL peak intensity of blue (2.6 eV) and red (1.9 eV) bands of SEPSL on laser power. The data for the blue band (10 min.) have been multiplied by 5 for clarity. and TI, not presented here. The PL intensity of both bands of DEPSL decreases with LBE from 300 to 420 mJ and goes through a maximum at about 15-20 sec treatment with Nd:YAG laser. In contrast with the majority of SEPSL the intensity of the blue band is higher than that of the red one as it has bee
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