Preparation and Chemical Investigation of Porous Silicon
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PREPARATION AND CHEMICAL INVESTIGATION OF POROUS SILICON S.POULIN*, Y.DIAWARA*, J.F.CURRIE*, A.YELON*, S.C.GUJRATHI** AND V.PETROVA-KOCH***. *Ecole Polytechnique, Ddpartement de genie physique, Groupe des Couches Minces, C.P.6079, Succ.A, Montreal, Oc, Canada, H3C 3A7. **Universit6 de Montreal, Ddpartement de physique, Groupe des Couches Minces, C.P. 6128, Succ.A, Montrdal, Qc, Canada, H3C 3J7. ****Physik- Department E 16, Tech. Univ. Munchen, 8046 Garching, Germany. ABSTRACT
We have begun a systematic study of the chemistry of light emitting porous Si (LEPOS) using X-ray photoemission spectroscopy and time-of-flight elastic recoil nuclear scattering. The relation of chemical composition to preparation parameters and its evolution with rapid thermal annealing in N2:H2 and in vacuum were studied. The relation between chemical and photoluminescence properties and stability was investigated. The composition of LEPOS is extremely sensitive to preparation conditions and to subsequent treatment. The concentration of C and F in addition to Si, H and 0 vary with thermal treatment conditions and with time after preparation. However, preparation under the cleanest possible conditions increase both reproducibility and stability. INTRODUCTION Since the first reports of visible photoluminescence (PL) (1) and of a gap in the visible (2) in porous silicon at room temperature, many studies have been undertaken to understand these phenomena and other properties of this material (3-5). One of the difficulties involved in determining the intrinsic properties of porous silicon is its sensitivity to oxidation. For example the intensity and the lifetime of the luminescence have been found to change substantially (1,6) and rapidly as soon as the sample surface is exposed to air or to certain solvents(7). It is clear that the chemistry of the sample preparation and subsequent handling are determining factors in material characterisation and ultimately in the manufacture of useful longlived optoelectronic devices. In this paper we report a systematic study of the chemistry of light emitting porous silicon (LEPOS) using X-ray Photoemission Spectroscopy (XPS) and Time-Of-Flight Elastic Recoil Detection (TOF-ERD), whose objective is to explore the effect of sample preparation conditions. The relation between chemical treatment and PL properties and stability was also investigated. We show that samples prepared carefully using microelectronic procedures and annealed under high vacuum are highly stable, as shown by the stability of PL characteristics of samples exposed to air for months after preparation. EXPERIMENTAL Three series of porous silicon samples were prepared from 2 and 5 fl-cm borondoped (100) silicon wafers by anodic etching in HF/C 2H5OH (1:1), as described elsewhere (8,9). This was followed by three different conditioning steps. Samples were either stored in water (series A) or in ethanol (series B) , or immediatly treated by Rapid Thermal Oxidation (RTO) in a pure 02 ambient (series C). The exact temperature versus time program
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