Photoluminescence and Electroluminescence in Partially Oxidized Porous Silicon
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at the Laboratory for Laser Energetics and the Institute of Optics, University of Rochester. 683 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
stable electroluminescence, which up to now we observe only in partially oxidized material. The stability of the luminescence is due to the presence of bonded oxygen and makes possible the successful operation of these devices for more than 100 hours of continuous operation at a current density up to 1 A/cm 2 . EXPERIMENTAL The partially oxidized porous silicon samples were formed by anodization of p-type 10 f2 cm Si wafers in HF-methanol-water (1:2:1) solution at the constant current density of 1 mA/cm 2 under illumination by 300 W halogen lamp. The thickness of the porous silicon films was approximately 1 gIm. Fourier-Transform Infra Red (FTIR) spectra taken immediately after anodization show a strong absorption related to oxygen in the bridging configuration at 1080 cm- 1 and back-bonded configuration at 2250 cm- and 870 cm- 1 [8]. The morphology of surface was observed by Atomic Force Microscopy: it was flat down to 5 nm. The semitransparent (T-40%) gold contacts were sputtered in vacuum directly on the porous silicon layer without any special surface preparation. The typical evolution of the time-integrated PL spectra in partially oxidized porous silicon is shown in Figure 1 as a function of temperature. The maximum of the PL is at 600 nm at room temperature and it shifts to 570 nm with decreasing temperature. The shape of the spectrum indicates the presence of a blue PL band. The blue PL band which peaks usually at 460-480 nm is expected in samples with a detectable amount of bonded oxygen [9]. We observe a decrease of intensity of the orange PL band with decreasing temperature whereas the blue PL band is nearly temperature independent. As a result, at temperature T < 50 K the blue PL band dominates (Fig. 1). The integrated intensity of the blue band correlates with the degree of oxidation [9] and for strongly oxidized samples at T < 50 K the main PL peak shifted to 460-480 nm. In Figure 2, we show the temperature dependence of the PL intensity (I PL) monitored at 570 nm. For all measured samples in the region T > 50K (temperature region 1) this curve can be fitted by an activation law IPL = Io exp (-Ea/kT) with Ea=10 meV. As the temperature is decreased further (temperature region 2) the PL becomes independent of temperature.
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Wavelength, nm Figure 1. Temperature dependence of the time-integrated PL spectra in POPS. The arrows show two significant PL bands at 570 rim and at 480 tim.
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Figure 2. Temperature dependence of the PL intensity in POPS monitored at 570 nm. We distinguish two regions. In region 1 (300K < T < 50K), the PL intensity is thermally activated. In region 2 (T < 50K), the PL intensity is te
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