PL and FTIR Absorption Study on Porous Silicon in situ during Etching, in Oxygen Ambient, and after Chemical Oxidation
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PL AND FTIR ABSORPTION STUDY ON POROUS SILICON IN SITU DURING ETCHING, IN OXYGEN AMBIENT, AND AFTER CHEMICAL OXIDATION G. Mauckner, T. Walter, T. Baier, K. Thonke and R. Sauer Abteilung Halbleiterphysik, Universitit Ulm 7900 Ulm, GERMANY
ABSTRACT Steady state and time-resolved photoluminescence (PL) and Fourier-transform infrared (FTIR) spectroscopy have been performed in situ during etching, on "as prepared" porous Si in air under laser exposure and on chemically oxidized porous Si. We suppose that PL degradation of "as prepared" porous Si is caused by creating non-radiative defect centers during photooxidation. Chemically oxidized porous Si shows increased PL intensity and longer recombination lifetimes as compared to non-oxidized samples. We conclude, that an oxide layer with low defect density on the inner surface of chemically oxidized porous Si reduces the non-radiative recombination rate. INTRODUCTION Efficient visible light emission and a significant blue shift of the absorption edge [1] of porous silicon have stimulated great interest in the origin of the PL. However, porous Si suffers from degradation by aging, ascribed to oxidation processes, which take place especially under illumination in oxygen ambient [2,3]. This process is accompanied by a decrease of the PL intensity. Determination of the origin of the PL and the control of the parameters governing the PL efficiency is obviously interesting for basic research as well as for potential device applications. We present an optical study of porous silicon in situ during preparation, freshly etched "as prepared" in oxygen ambient and after chemical oxidation. The investigation involves FTIR absorption, PL and time resolved PL spectroscopy. EXPERIMENTAL Boron-doped p-Si (100) samples (10 Ohmcm) are electrochemically anodized as described in Ref. [2]. The anodization is done under dark conditions with current densities between 10 and 40 mA/cm2 in 2:1 or 1:1 40% HF : Ethanol for I - 30 min. To prepare chemically oxidized porous silicon, we dip freshly etched samples into 20% aqueous HNO3 for 30 sec to 60 min. After etching or chemical oxidation the samples are rinsed in ethanol and dried in a gas stream of N2 . FTIR absorption spectroscopy was used to determine the chemical surface composition. The transmitted mid-IR was analyzed by a Bomem DA8 Fourier transform spectrometer equipped with a Globar (SiC) IR light source and a cooled mercury cadmium telluride photodetector. PL spectra are recorded using the blue 457.9 nm line of an argon ion laser for excitation and the Fourier spectrometer with a silicon photodiode or a 1/4 m monochromator with a multi-alkali photomultiplier tube for detection. Time resolved PL was obtained using excitation by an argon ion laser modulated with a Kerr cell. A time resolution of about 1 microsecond of the PL transients was achieved.
Mat. Res. Soc. Symp. Proc. Vol. 283. @1993 Materials Research Society
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IN SITU FTIR ABSORPTION First we studied FTIR absorption during anodic etching. Since ethanoic HF itself strongly abso
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