Elaboration and Light Emission Properties of Low Doped p-Type Porous Silicon Microcavities
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function of the current density, Fig. a) and b), respectively. The values for n have been obtained by a fit of the experimental reflectivity spectrum obtained for single layer. The values of the
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=1120 nm
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Current densityj (rnA/cm )
Fig 1: Calibrationcurves of the refractive index (a) and the formation velocity (b) as a function of the anodisation currentdensity.
air
PS
Bulk
refractive indices are given at a wavelength of 1120 nm. The same layer was later dissolved to perform thickness measurements in order to determine the formation velocity. The values of j (1.66 to 333 mA) were chosen so that well defined porosities and thicknesses could be obtained. As it is well known the porosity increases with j. After optimization, the minimum and maximum current densities enable us to obtain 56.5% and 72%, respectively, which leads to a variation of the refractive index by 0.47. On figure lb) we first observed a linear dependence of the formation velocity as a function of the current density. Then, a saturation begins to appear for high value of j. The inset illustrate this effect showing the dependence of the ratio v/j, (i.e. the valence of the anodisation) as a function of j. These calibration curves were used to control the vertical modulation of the porosity during silicon anodisation. Figure 2 shows a SEM photograph of a multilayer structure. A periodical change of the contrast is clearly observed
Fig. 2: SEM photograph of a PS multilayer. The structure was obtained repeating30 times the current sequence of 150 mA/cm 2 - 1.25 s and 15 mA/cm2 - 10.25 s. The periodicity and the total layer thickness are 0.39 and 11.7 pm, respectively. The overexposure near the top is due to surface charge effect. The arrow indicates the dissolution (i.e. formation) direction. 712
throughout the entire thickness of the PS film. Due to the low conductivity of the porous silicon, surface charge effects can not be avoided in the microscope and give rise to the overexposure observed at the top of the layer. Since this level of brightness is much higher than the bulk one (bottom part of the photograph) it can not be attributed to a porosity gradient in the depth. The periodicity of this structure (bright and dark stripes) is 0.39 pm and remains unchanged in the depth, attesting for the control of the layer thickness during the formation. However, the photograph also shows irregular undulations at the interfaces. This is more pronounced near the bulk (bottom) interface. This effect is due to fluctuations in the formation velocity [5]. A direct consequence of this on the light propagation is the increase of the losses in the structure due to scattering [6]. However, comparing two successive interfaces one see that these fluctuations are correlated which explains why the optical interferences are
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