Effect of Laser Illumination on Oxidization of Porous Silicon
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ON
EFFECT OF LASER ILLUMINATION SILICON OXIDIZATION OF POROUS
L.Z. ZHANG*, J.C. MAO*, B.R. ZHANG*, W.X. ZHU'-, Y.L. He -, H.Z. SONG*, J.Q. DUAN* AND G.G. QIN* * Department of Physics, Peking University, Beijing 100871, China. ** General Research Institute of Non-ferrous Metals, Beijing 100083, China. + Amorphous Physics Research Lab, BUAA Beijing 100083, China. ABSTRACT We have studied the effect of laser illumination ( argon laser line of 488 nm) on the oxidization process of the inner surfaces of porous silicon (PS) by measuring the photoluminescence (PL), Fourier-transform infrared (FTIR) absorption and x-ray photoelectron spectroscope and contrasted the variations of PL and FTIR spectra of the PS treated in the following four ways: 1. In vacuum with laser illumination (LI) with power density of 12 mW / mm 2. 2. In oxygen with LI. 3. In oxygen without LI. The times for all the above three treatments were 1 h. 4. Storage in air for 2 months without LI. The PL peak of PS showed serious degradation and a blue shift in case 2 but only a moderate degradation and no shift in case 1. The results of FTIR absorption show that the LI in an atmosphere of oxygen enhanced greatly the increase of oxygen-related absorption bands and the decrease of various silicon-hydrogen vibrational mode absorption bands. INTRODUCTION Research on porous silicon (PS) has already a history of more than thirty years 1.2., and great progress has been made in experimental research since 1990. Canham 3 observed strong visible photoluminescence (PL) of PS. At the same time, Lehmann and Gosele 4 measured its light absorption and observed a shift of the absorption edge to 1.76eV, which is much larger than the forbidden gap of crystalline Si at room temperature. They regarded PS as a quantum-wire array, the diameters of the wires being several nanometers, and explained their results using the quantum confinement effect of carriers. Recently, visible electroluminescence has also been observed in PS 5. All these experimental results imply the possible optoelectronic applications for silicon, and thus greatly stimulate related research. One focus of research in PS Is the mechanism of visible luminescence (VL). Besides the point of view which considers the quantum confinement effect as the origin of strong VL in PS, other views are Mat. Res. Soc. Symp. Proc. Vol. 283. @1993 Materials Research Society
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that VL is caused by an Increment of the energy gap of amorphous SI containing hydrogen or oxygen 6, or by the siloxene produced on surfaces of PS during anodization 7, or by SiH 2 8 or by the other molecules 9 sticking to surfaces of PS. For application, the stability of VL of PS is extremely important. Using secondary Ion mass spectrum, spreading resistance measurments etc., Canham et al. 10 studied the variations of chemical composition, optical properties and resistivity of PS during its storage in air. Tischler et al.11 pointed out that illumination in the presence of oxygen causes substantial decrease in the PL efficiency of PS. We have studied
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