SIO x Related Photoluminescence Excitation in Porous Silicon
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ABSTRACT Using PL, PLE, secondary ion mass spectroscopy (SIMS) and EPR investigations we show in this paper that two luminescence bands with different PL excitation spectra exist on the surface of silicon wires in PS. INTRODUCTION A large number of investigations have been carried out on the photoluminescence (PL) and photoluminescence excitation (PLE) of porous silicon (PS). However, the origin of visible light emitted of PS and the mechanisms of PL excitation are still under investigation. The current points of view on PL are follows: two-dimensional quantum confinement effects in the silicon quantum wires [1]; light emission of silicon clusters [2]; luminescence of polysilans [3], siloxene [4], silicon oxide [5,6] or amorphous silicon [7]. Using PL, PLE, secondary ion mass spectroscopy (SIMS) and EPR investigations we show in this paper that PL of PS at room temperature is caused by the two luminescence objects on the silicon wires surface with the different PL excitation spectra.
EXPERIMENTAL RESULTS AND DISCUSSION The PS samples were prepared by electrochemical etching in hydrofluoric acid (HF:H 2 0:C2 H5 OH=1:1:2) of n-type and p-type (4-5 Q - cm) silicon wafers of (100) ani (111) orientations. The etching current density (Ie) was in the range 20-200 mA /cm and the etching time (te) was in the range 2-15 min. The PS layer thicknesses were near 10 pm. PL were measured at room temperature under excitation by the 337 nm line of a N2 laser or by xenon lamp light. In the latter case a monochromator MDR-23 was used. SIMS measurements have been performed by means of a spectrometer LAS 2000 (RIBER) in the range of masses 1-100 m.un. using step to step etching by an Ar beam of 4 keV energy. EPR measurements have been performed also. The PL spectra of the as-prepared samples excited by the UV (337 nm) and visible (430 nm) light are shown in Fig.1. The shape, the intensity and the peak position of the luminescence bands depend on the preparation regimes (Ie and te) and excitation light wavelength. The luminescence band intensity increases and peak position shifts the lower photon energy when etching time te and 159
Mat. Res. Soc. Symp. Proc. Vol. 405 @1996 Materials Research Society
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Fig.1 PL spectra of the as-prepared samples excited by the 337nm (a) and 430nm (b) light. Preparation regimes: Ie = 25 (1,2,3) and 100 (1',21,3') mA/cm 2; te = 2 (1,1'), 8 (2,2') and 15 (3,3') min.
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150 2 200 100 50 0 15 10 le , mA/cm te , min Fig.2. Dependence of luminescence band intensity and peak position from the etching time te (a) and current density Ie (b). Excitation light: 337nm (1',2',3') and 430nT (1,2,3). Preparation regimes: Fig.2a - Ie = 25 (1,1*), 50 (2,2*) and 100 (3,3*) mA/cm Fig.2b - te = 2 (1,1*), 8 (2,2*) and 15 (3,3*) min.
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excitation light wavelength increase (Fig.2a). The peak shifts to larger photon energies
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