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characteristics whereas the bonding structure at the surface is deeply dependent on the processes temperature and time parameters [5, 6]. Some of the previous results show that the intensity of the luminescent emission increases during the transition from the hydrogen covered surface of fresh porous silicon to the completely oxidized surface [7, 81. The process has discontinuities or transition points, marking changes in the emission mechanism. The saturation of the emission intensity or the quenching at a given temperature or after a processing time can be related to changes in the structure of the surface states. In this study we present a direct correlation of the cathodoluminescence emission with the evolution of the surface state of porous silicon under oxidizing and inert atmosphere. It is shown that the intensity of the emission band at 640 nm is related to the removal of interface defect centers, with characteristic optical absorption band at 883 cml, during low temperature treatments. EXPERIMENT The porous layers with thickness of 50 gim were prepared from p-type, 18-22 Qcm resistivity silicon wafers anodized in HF:ethanol solution, for 30 min, at 25 mA/cm2 current density.

209 Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society

Four sets of treated samples were prepared. First one was annealed in dry oxygen atmosphere at temperatures of 450'C for 30 min or 1000°C for 10 minutes. A second set of samples was subjected to a sequential treatment in dry oxygen, consisting of a first step at 450'C for 30 min followed by a second step at 1000'C for 10 min, both in dry oxygen atmosphere. For the third set of samples, a low temperature treatment at 450'C was performed in nitrogen and forming gas (10% H2+90% N2) atmospheres, for 60 min each. The last set of porous silicon samples were treated at temperatures in the range of 100-500'C. The isochronal multistep annealing treatments were done in air for 10 minutes at 50'C temperature. Cathodoluminescence emission was measured using a Hitachi S2500 SEM under 30 keV beam accelerating voltage. PYE UNICAM SPI 100 and FTIR Perkin Elmer and Pye Unicam spectrometers were used for optical absorption measurements in the range 400-4000 cm-l. EPR measurements were carried out by using a JEOL X- band EPR Spectrometer.

RESULTS Cathodoluminescence (CL) emission of porous silicon (PS) subjected to oxidation processes shows an emission band in the blue region showing a stable peak intensity, while the intensities of the bands in the red region present a strong dependence on the thermal treatments. Porous silicon oxidized at low temperature shows CL emission in the spectral range of 420-640 nm, with bands at 420-480 nm, 540 nm and 640 nm. The maximum intensity, as can be observed in the figure 1 is obtained for the band situated at 420-480 nm. The CL spectrum of the high temperature oxidized sample (Figure 2) shows an intense band centered at 400-460 nm while the components at 540 nm and 640 nm appear almost totally quenched. The two steps oxidation annealing, first