Effect of Rapid Thermal Oxidation on Blue and Red Luminescence Bands of Porous Silicon
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LAGOWSKI*
"Center for Microelectronics Research and *Department of Physics, University of South Florida, Tampa, FL 33620 ABSTRACT
Photoluminescence (PL) of porous silicon after rapid thermal oxidation was studied using excitation energies of 5.0 and 6.4 eV. The emission spectra in both cases are dominated by a broad blue band centered at 430-450 nm and a much weaker red band positioned at 680-720 nm. Using atomic force microscopy (AFM), we have found a correlation between blue PL intensity and increase in feature size caused by progressive oxidation. The blue luminescence, found in porous Si only after oxidation, is identical, with respect to spectrum and fast decay, to that of thermally grown oxide on crystalline Si. Based on these findings, we conclude that the blue luminescence originates from Si0 2 rather than from Si nanocrystals embedded in the oxide matrix. The red luminescence, on the contrary, has a different origin. The intensity of red band is a function of oxidation temperature and at RTP above 1050 0C, the red band is completely eliminated. The red luminescence characteristics are discussed in relation to the possible mechanism. INTRODUCTION
Porous silicon material has generated intense research owing to its strong room temperature photoluminescence in red and blue spectral regions [1-3]. The FL characteristics of freshly prepared porous Si on p- and n-type wafers are a strong function of substrate and anodization parameters as well as the excitation energy. Under Ar laser (2.41 eV) excitation, the steady state photoluminescence from asprepared porous Si peaks in the red at around 750-870 nm. This luminescence is prone to laser induced degradation. Rapid Thermal Processing (RTP) in oxygen enhances and stabilizes the luminescence intensity [4-5]. Following oxidation, the red PL peak shows blue and red shifts for p- type and n- type Si substrates, respectively [6]. The final peak position being very similar in both cases. Recently blue luminescence (435-440 nm) in RTP oxidized porous Si have been observed under ultraviolet (UV) excitation [2,7]. In our previous study we reported a quantitative correlation between the blue PL intensity and the increase in feature size caused by progressive oxidation [3]. In this paper we compare the PL characteristics of RTP oxidized porous Si and thermal oxides grown on crystalline Si wafers. Appropriate comparison will be made between results obtained with UV and visible (2.41 eV) excitations. The PL results are correlated with AFM and FTIR (Fourier-transform infrared spectroscopy) measurements. Based on these results, the behavior of blue and red 369 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
PL bands, in RTP oxidized porous Si is discussed. EXPERIMENTAL DETAILS
Porous Si films were prepared by anodic etching in 49 % HF : ethanol (1:1) mixture on both p- and n-type (100) silicon wafers with resistivities between 3-10 Qcm. The anodization current density and time were typically 20 mA/cm2 and 20 min, For n-type wafers, anodization was perf
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