Light Emission Versus Excitation from Porous Structures in Ion-Implanted Silicon
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653 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
The relationship between the microstructure and the growth conditions of PSL-s and also their optical behaviour has been studied by several authors using spectroscopic ellipsometry [6-8] Sagnes et al. [9] measured the optical transmission of free-standing porous films formed on p and p+ type substrates and showed that the absorption coefficient curves shift towards the higher energies with increasing porosity. The aim of this work was to analyse the effect of the mesoporous capping layer on mechanically stabilised buried microporous layers from the point of view of photoluminescence (PL) and electroluminescence (EL) under electrolyte contact. The reduced PL and strong EL exhibited by this sandwich structure was interpreted in terms of optical transmission deduced from spectroscopic ellipsometric (SE) measurements. EXPERIMENTAL Boron implantation of p-type silicon with 5 ohm-cm resistivity was performed by a Varian ion implanter with 40 keV accelerating voltage and doses which provides surface concentrations of 1018 and 1020 B/cm3 . The diffusion depth was 0.5 Am. Porous films were formed on ion implanted substrates in ethanol-containing aqueous electrolyte with an HF content between 12.5-25%. A 25 mA cm-2 constant current density was used for anodization. The porous layer thickness and porosity were measured by gravimetry. The etching time was defined for each electrolyte in order to get 2 ,.tm thickness for each sample of different double layers. The other single layer reference samples had different thicknesses and porosities which were determined from spectroscopic ellipsometry. Electroluminescence measurements were carried out during the anodic oxidation process performed with 0.1 N KNO 3 solution as electrolyte with an extended range PR-713/PC Spectrascan spectro-radiometer from Photo Research, USA. Spectral range is 390-1070 nm with 10 nm bandwidth. The measuring angle was 0.5 grad x 1.5 grad, the integration time was 6 sec. The photoluminescence was measured by a boxcar-based laser luminescence spectrometer. The excitation source was an N 2 laser (X=337 nm). The luminescence dispersed by a grating monochromator was detected by an RCA P28 photomultiplier. Spectroscopic ellipsometry was performed by a rotating analyser ellipsometer in the 270-750 nm wavelength range at 700 angle of incidence at Twente University. Other measurements were performed by a rotating polarizer ellipsometer in the 250-850 nm wavelength range at 750 angle of incidence with an ES4G device at SOPRA Co. The morphology variation with depth in this novel sandwich structure had been estimated by the oxygen incorporation in the porous structure after a thermal oxidation process at 800 0C for 2 minutes in a N 2 :0 2 mixture of volume ratio with 60:1. The oxygen profile determination were performed with a 950 keV deuterium beam using the backscattered D+ ions and the 2.5 MeV CCparticles emitted by 016 atoms. Using standard 15 keV energy resolution silicon detectors at 1
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