Metastability of Luminescent Porous Silicon
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METASTABILITY OF LUMINESCENT POROUS SILICON S. MIYAZAKI, K. SAKAMOTO, K. SHIBA and M. HIROSE Department of Electrical Engineering, Hiroshima University Higashi-Hiroshima 724, Japan
ABSTRACT Photoluminescence from 1-3rm thick porous Si layers prepared by anodization of p-type c-Si wafers and subsequent chemical etching exhibits an anomalous temperature dependence and light-induced degradation. The luminescence intensity is almost quenched at temperatures below 30K and recovered by laser irradiation at 48K. This quenching phenomenon is not observed for PS thicker than 10ýtm. The luminescence fatigue is partially recovered by annealing at 150*C for 5min during which no further oxidation takes place. These observations are interpreted in terms of the structural metastability of hydrogen-terminated porous Si.
INTRODUCTION Visible-light emission from porous Si (PS)[1] and a significant blue shift of the optical absorption edge with respect to the bulk silicon[2l have greatly stimulated our interest in its application to optoelectronic devices. It has frequently been speculated that the quantum confinement effect in the Si nanostructure is responsible for the unique optical properties. However, there is no direct evidence for this. Also, the formation of luminescent chemical compounds such as polysilanes[3] and siloxene derivatives[4] in the PS layer is suggested. It is reported that the photoluminescence (PL) intensity increases with decreasing temperature, reaching the maximum at a temperature of about 100-200K and it starts to decrease at the lower temperatures[5]. This suggests that the phonon participation is important in the radiative recombination process in PS. For the purpose of better understanding of the radiative recombination process in PS, we have investigated the temperature dependence and light-induced degradation of PL from the viewpoint of the structural metastability.
EXPERIMENTAL 1-31im thick-PS layers were formed by anodizing p-type Si(100) wafers with a resistivity of 2-10Qcm in 50% aqueous HF under the short circuit condition or in an ethanoic HF solution consisting of C-2 H5 OH(99.5wt.%) : HF(50wt.%) = 1 : 10 with external bias. The current density was maintained at 10mA/cm 2 under the biased condition, while in the short circuit condition the average current density was 50[tA/cm 2 . Under the short circuit condition, -1.5j.m-thick PS with an uniform and mirrorlike surface was prepared after 13hrs anodization. Samples exhibiting the efficient visible PL at room temperature were obtained by chemical etching in a solution of HF: C-H 5 OH : H 20 = 1 :3: 8 for 2-10min after the anodization. The temperature dependence of PL was measured by using a lock-in detection system, in which a 488nm light from an Ar+ ion laser was chopped at a frequency of 34Hz. The specimen was placed in vacuum on the copper susceptor connected with a closed-cycle refrigerator and cooled Mat. Res. Soc. Symp. Proc. Vol. 283. ©1993 Materials Research Society
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over the temperature range from 21 to 296K. Luminescence fatigu
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