Microcrystallites in Oxidized Porous Silicon
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MICROCRYSTALLITES IN OXIDIZED POROUS SILICON V. LEHMANN*, H. CERVA*, B. JOBST*, V. PETROVA-KOCH**, A. KUX** AND T. MUSCHIK** *Siemens, Dept. ZFE BT , 8000 Munich 83, Otto-Hahn-Ring 6, Germany **Tech. Univ. Munich, 8046 Garching, Germany ABSTRACT Rapid thermal oxidation of porous silicon leads to desorbtion of hydrogen from the inner surface and formation of a thin oxide layer. Despite this dramatic change in the chemical composition oxidized microporous silicon (miPS) shows photoluminescence (PL) in the visible region. This is contradictory to the idea that the observed PL originates from chemical compounds like siloxene or polysilane, which would require a certain stoichiometry. If silicon microcrystallites are still present in the oxidized miPS, the observed luminescence can be explained in terms of a quantum confinement effect. It is the purpose of this work to prove the existence of microcrystallites in oxidized miPS using electron microscopy and X-ray diffraction. INTRODUCTION A key question in the discussion of the unusual optical properties (1,2) of micro porous silicon (miPS) is whether quantum confinement or chemistry (siliconhydride (3), siloxene (4)) is responsible for the observed visible luminescence. Recently it was found (5) that at low annealing temperatures (RT - 6000 C) the photoluminescence (PL) is suppressed, whereas the PL intensity recovers at higher oxidation temperatures (7000 C - 900 0 C). This is a strong argument for the quantum size approach, because rapid thermal oxidation (RTO) will change the stoichiometry of miPS from SiHx to SiOy by oxidizing the inner surface of the porous skeleton, without destroying the crystallite core. EXPERIMENTAL MiPS was formed on polished silicon substrates (p-type, 1-2Qcm, (100)) using ethanoic HF (1:1, Ethanol : 50wt% HF) as an electrolyte. Stripe shaped samples were immersed into the electrolyte and anodized in the dark for one minute using highly doped p-type silicon as a counter electrode. This method produces a current density distribution (0-300 mA/cm 2 ) over the length of the immersed stripe and consequently a variable miPS layer thickness (0-10pm). RTO was performed at different temperatures in dry oxygen (for 30s) using a Heatpulse 610. Oxide layers up to 14nm (1200 0C, 30s) on polished Si substrates can be produced by RTO (6). The PL spectra were recorded using an excitation wavelength of 488nm. X-ray diffraction measurements were performed in ambient air using oxidized miPS powder samples (50 mA/cm 2 , 20-50 pm) produced by carefully scraping the porous layer off the substrate.
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Wavelength (nm)m. Fig. 1 Optical transmission spectra of 12pm thick miPS layers oxidized at different temperatures 0 (normalized to a miPS sample oxidized at 1200 C).
Mat. Res. Soc. Symp. Proc. Vol. 283. 01993 Materials Research Society
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Fig.2 Peak wavelength and intensity of photoluminescence together with the ESR signal as a function of the oxidation temperature.
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