Localized Nature of Photoluminescence from Anodically Oxidized Porous Silicon

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T.ITO, K.FURUTA, T.YONEDA, O.ARAKAKI, A.HATTA AND A.HIRAKI Osaka University, Department of Electrical Engineering, Suita, Osaka 565, Japan

ABSTRACT Photoluminescence (PL) properties from anodically oxidized porous silicon (PS) have been investigated. Large temperature coefficients (-0.5 meV/K) of PL peak energy are observed for relatively strongly oxidized PS specimens which show saturation of PL peak blue shift upon further anodic oxidation. Time-resolved PL data show that the PL decay is characteristic of thermal activation process with an energy of 9-27 meV at low temperatures between 80 and 180 K. Those results can be explained using a luminescence model which assumes several PL centers in the energy gap and considers thermally activated and tunneling processes.

INTRODUCTION Visible light emitting porous silicon (PS) has been widely investigated1 ) since Canham's report. 2) The mechanism of the light emission has, however, not completely been understood yet although there are several models which consider the quantum confinement effect of nm-size Si crystallites, localized luminescence centers and/or Si-dcrivcd materials.1-3) Since the formation process of ultra fine structure of PS cannot be controlled, a simple accumulation of various data taken for PS formed under different conditions may not necessarily lead to a universal conclusion. 4) Oxidation of PS can stabilize the fine structure, and therefore, oxidized PS is almost free from the natural oxidation during measurements and analysis of its properties, which gives further uncontrollability.5' 6) We have been studying anodically oxidized PS7 8' ) and have reported that oxidized PS shows blue shifts in photoluminescence (PL) peak energy with increasing oxidation times at the initial anodic oxidation stage and that the blue shifts are saturated for sufficiently oxidized specimens.8 ) In the present paper, we describe experimental results mainly on PL observed from anodically oxidized PS which shows a room-temperature PL peak at -650 nm. The PL mechanism is discussed on the basis of temperature dependences of deconvoluted PL spectra and PL decay times measured at low temperatures below room temperature (RT). Our results are compared with a previous report on anodically oxidized PS which was investigated at temperatures above RT.9)

EXPERIMENTAL PS films were formed on p-type Si (100) substrates of 14-20 Q'cm in an ethanoic electrolyte obtained by a mixture of equal parts of 50 wt% HF and ethanol. The anodic current of 20 mA/cm 2 flowed for 20 s, leading to a =300-nm PS layer. (Anodization times longer than 30 s 477

Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society

resulted in an obvious inhomogeneity of PS specimens in depth structure.,)) Electrochemical oxidation of PS thus obtained was performed with the same experimental setup but with a different electrolyte solution composed of HCI,H 20 and ethanol (7:53:40). The anodic current was kept constant during the oxidation and was 0.2 mA/cm 2 while the oxidation time ranged from 0 to 20

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