Quenching and Recovery of the Photoluminescence in Porous Si After Pulse IR Irradiation
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POROUS Si AFTER PULSE IR IRRADIATION J. Diener, S. Ganichev*, M.Ben-Chorin, D. Kovalev*, V.Petrova-Koch and F. Koch Technische Universitdt Minchen,Physik-DepartmentE16, D-85747 Garching,Germany * A.F. Ioffe PhysicotechnicalInstitute, RussianAcademy of the Sciences, St. Petersburg, 194021, Russia
ABSTRACT A pulsed, high-power TEA CO2 laser with lines in the region from 9.2 to 10.6 pm has been used to irradiate luminescent porous Si samples. The visible luminescence quenches and then recovers to its initial value on a time scale of one hour. It is found that the quenching is efficient when the IR wavelength is within the Si-O absorption band. We suggest that the resonant excitation of the Si-O bonds results in a metastable reconfiguration of the oxygen together with the creation of dangling bonds. These non-radiative centers are responsible for the PL quenching. INTRODUCTION The origin of photoluminescence (PL) inporous Si (PS) is a subject of intense research. It is now generally accepted that quantum confinement changes the band-gap of the Si nano-particles, and their optical properties. However, the states responsible for the light emission are still not identified. Calcott et al. [1] propose that the luminescence results from recombination of excitons in the volume of the crystallites. Koch et al. [2] argue that the radiative recombination process occurs predominantly via surface states. Surface defect states are known to play an important role in the non-radiative processes. Rapid thermal oxidation and hydrogen effusion were used to show that the PL properties are strongly related to the density of dangling bonds [3]. There is an anticorrelation between the dangling bond concentration and the intensity of the red luminescence band [4]. The exposure of the PS internal surfaces to organic solvents results in reversible changes in the luminescence intensity [5]. All these results indicate that the chemical and structural conditions of the surface should be considered in the luminescence studies. In this work we show that CO 2 laser radiation can influence the PL properties, due to the creation of metastable states on the surface of the Si crystallites. The radiation can be tuned to be resonant with the Si-O stretching mode. Such Si-O bonds become strongly excited and can even dissociate. The process results in a decrease of the luminescence intensity which then recovers in a time scale of hours. Here, we focus our attention on the reversible quenching effect. Irreversible quenching of the PL can also be obtained. SAMPLES AND EXPERIMENTAL SETUP The samples used in this work are prepared from (100) p-type B-doped, 1lcm Si wafers. Before anodization an ohmic back contact is provided by B implantation. Anodization is carried out in a 1:1 vol. mixture of ethanol:HF (49% in water). Etching current density is 30mA/cm 2 . The typical sample thickness is in the range of 2 pm. Following anodization samples are removed from the cell, washed with propanol and left to dry under ambient conditions. The freshly prepared samples
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