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LUMINESCENCE ACTIVATION OF POROUS SILICON BY POST-ANODIZATION TREATMENT A. Kux, F. Miller, and F. Koch Physik-Department E16, Tech. Univ. Miinchen, 8046 Garching, Germany

ABSTRACT We prepare "nonluminescing" porous Si by electrochemical etching (50 mA/cm2 in 50% HF diluted 1:1 with ethanol) of 1 (i-cm (100) p-type wafers in the absence of light in order to study the subsequent luminescence activation by postprocessing. The treatments are : photochemical etching, ageing under ambient conditions, thermal oxidation. The study reveals remarkable inhomogeneities in the depth distribution of the luminescence and allows us to comment on the relative importance of particle size, spin density and chemical composition for the luminescence.

INTRODUCTION It is becoming increasingly clear in the electrochemical preparation of luminescing porous Si that precise details of how the process was carried out, as well as the drying and sample storage after the anodization, strongly influence the efficiency of photoluminescence. The conventionally cited sample doping, acid composition, current density and time are necessary but not sufficient parameters that enter into the preparation steps. A number of publications have pointed to the decisive influence of light on the electrochemical etching [1]. Others have noted chemical postanodization influences [2, 3]. The drying process and long-term storage has been mentioned by others as an essential factor in the sample preparation. There is a lot of speculation on the why and how of the peripheral treatment steps. In the literature one finds discussed whether a given treatment causes or reduces the dangling bond density, whether or not there are chemical composition changes, or if it is the particle size that is influenced by the post-anodization treatment. We want to use post-treatments as a tool by which to turn on the luminescence, in otherwise inactive electrochemically prepared porous Si. It is hoped from this perspective to learn more about the fundamental origin of the luminescence. Another motivation is to explore possible inhomogeneity effects that occur for the activation treatments.

SAMPLE PREPARATION PROCEDURES We start with (100) Si wafers of p-type, 1 Q-cm material. Samples are prepared from this one batch by electrochemical etching. The anodization solution is a 1:1 by volume mixture of 50% HF and ethanol. The electrolytic cell is made from Teflon and uses a platinum electrode. Everything is totally conventional, except for the fact that samples are etched in the absence of any illumination. The thickness is -,10/tm. The latter is large enough to be resolved in an optical microscope for edge-on observation. The etching time is of order 5min, short enough to avoid a purely chemical surface etching such as that described in ref. [4]. The resulting sample material after removal from the electrolyte and drying under nitrogen, shows only a very weak red luminescence under UV excitation. The luminescence is less than 1% of that for typical efficiently luminescing porous Si. Mat.