Controllability And Homogeneity Of Optical Properties Of Thin Porous Silicon Films
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ABSTRACT Reflection spectroscopy was applied to evaluate the optical homogeneity and the Variation in PS layer optical thickness refractive index of thin porous silicon (PS) layer. was evaluated by measuring and mapping the reflectance over the surface area. For the circular anodization area of 17 mm diameter, the measured variation was less than 5 % on an area of 11 mm diameter. The anodization electrode position was found to have little influence on the homogeneity. A method to derive refractive index from a reflection spectrum is studied. The wavelength dependence of PS index measured by the method In showed fair coincidence with the calculation based on effective medium approximation. the two layer PS formation, optical thickness was found to change whether high porosity or low porosity layer is formed first. INTRODUCTION The refractive index of PS layer is controlled by the current density during anodization and thus desired refractive index profiles can be easily formed in the PS layer. This is attracting attention as a promising technique to realize silicon based optical devices integrable with Si LSI circuits. Index superlattice devices, including Brag reflectors [1-3], optical filters and Fabry-Perot micro cavities [4,5], have been reported hitherto and some studies on optical waveguides [6] are now being made. A number of technical requirements should be fulfilled in making these devices practical. Controllability and homogeneity of optical characteristics are among the most important. Because, controllability of the index is directly reflected on the device performance and homogeneity determines the yield of the device. In this paper we report the study on the evaluation of homogeneity and refractive index of PS layer. To the author's knowledge, little reports have been made on the surface area homogeneity. We evaluated the homogeneity in terms of optical thickness by scanning the PS layer surface by a focused He-Ne laser and mapping the reflectance. The reflectance spectrum provides a convenient way to measure the optical thickness of thin films. The most widely used method utilizing an equation comprising adjacent reflectance peak and valley wavelengths [4] tends to have increased errors in the dispersive wavelength region. In this paper we report the result of a study on a simple and accurate method which is applicable to thin optical thickness films. In fabricating superlattice devices, successive PS formation process is inevitable and data on only mono-layer is sometimes insufficient for device designing We studied the optical thickness controllability 119
Mat. Res. Soc. Symp. Proc. Vol. 486 01998 Materials Research Society
of two-layer PS structure. SAMPLE PREPARATION We used (100) p-Si wafers with 5-8 Qcm resistivity. The back unpolished side was Al deposited and sintered to form ohmic contact. The anodization was carried out in the dark using a mixture of 50% HF and C2H5 OH (1:1 or 2:1 volume ratio). The maximum anodization current density we tested was 25 mA/cm 2, because at higher current de
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