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INTENSE VISIBLE LUMINESCENCE FROM THERMALLY-OXIDIZED POROUS SILICON S. MIYAZAKI, K. SHIBA, K. SAKAMOTO and M. HIROSE Department of Electrical Engineering, Hiroshima University Higashi-Hiroshima 724, Japan

ABSTRACT Photoluminescence from porous silicon oxidized at 800 or 900'C in an N2+0 2 gas mixture has been investigated. The ideal passivation of the porous Si surface with thermally grown oxide results in stable, intense visible-light emission. The steady-state and time-resolved luminescence measured as functions of temperature and excitation power have indicated that a possible pathway for the light emission is the radiative recombination through localized states.

INTRODUCTION The recent discoveries of the visible luminescence from porous Si(PS)[1] and the blueshift in the optical absorption spectrum with respect to the bulk Si[21 have drawn much interest in the physics of silicon nanostructure. The importance of quantum confinement effects in the Si nanocrystal has often been discussed to explain the optical properties of PS[1, 2]. Alternative explanations have also been made on the basis of the formation of luminescent chemical compounds such as (Si 60 3+-nH Im) on an internal PS surface[3] or the existence of 6 the high surface stress in the PS[4]. It has been shown that hydrogen termination of the PS surface plays an important role on the efficient light emission as revealed by the hydrogen desorption studies[5-71. In order to understand a correlation between bonded hydrogen and the visible light emission, PS has been thermally oxidized[8-11]. It has been demonstrated that the surface passivation with good quality oxide grown by rapid-thermal process[10J, low pressure oxidation[9] or a dilute 02 oxidation[11] leads to the efficient light emission. In this paper, we report the steady state luminescence for thermally oxidized PS, whose surface is passivated with ultra-thin SiO 2, as functions of temperature and excitation intensity. The temporal decay of the luminescence is also measured. A possible luminescence mechanism is discussed.

EXPERIMENTAL P-type Si(100) wafers with a resistivity of 2-~0Qcm were anodized in an HF based electrolyte consisting of C 2H OH : H 2 0 : HF = 1 : 5 : 5 with a current density of 25mA/cm 2 . Under these conditions, -10•m thick PS layers with a mirrorlike top surface were formed after 10min anodization. Subsequent oxidation was carried out at a temperature of 800'C or 900'C in a furnace with a constant gas flow of a mixture of oxygen and nitrogen for 1-60min. The gas molar fraction of 02 to N 2 was maintained at 10%. The temperature and excitation intensity dependences of luminescence were measured by using a lock-in detection system, where a 488nm light from an Ar+ ion laser was chopped at a frequency of 34Hz. The specimens were placed in vacuum on a copper susceptor connected with a closed-cycle Mat. Res. Soc. Symp. Proc. Vol. 283. 01993 Materials Research Society

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refrigerator and cooled in the temperature range from 18K to 296K. The 488nm laser intensity was changed i