Electrochemical Tailoring and Optical Investigation of Advanced Refractive Index Profiles in Porous Silicon Layers
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to fabricate a PS layer of desired thickness, porosity, pore size distribution and internal surface area. Good control over the etching process parameters is essential for fabrication of optimal and useful microstructures and morphologies in many applications. One important way of control is time modulation of the current density. As a result, complex structures such as Bragg reflectors [3], Fabry-Pdrot filters [4] and optical wave-guides [11 ] can be manufactured. These structures are basically fabricated by changing the current density in a periodic and wellcontrolled manner. As a result, periods with sublayers of different porosity and thereby indices of refraction can be fabricated. A computer-controlled continuous variation of the current density, demanding a more complex software as well as hardware, opens up new possibilities for tailoring more advanced structures with interesting application opportunities in sight. In this paper, we report on fabrication of non-conventional refractive index profiles in PS with, for example, exponential or sinusoidal shape. State of the art variable angle spectroscopic ellipsometry is employed to non-destructively characterize microstructural properties of the PS samples. This technique has previously been demonstrated to be an effective tool for both indepth investigations of PS morphologies and compositional depth variations in adsorption due to vapor and protein exposure [12,13]. Non-idealities in terms of deviations from expected mathematically defined patterns are resolved and discussed.
+ Corresponding author: [email protected] 195 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
EXPERIMENTAL AND THEORETICAL DETAILS PS samples were prepared from p-type 0.010-0.020 Q cm silicon wafers having (I11) crystal orientation from Okmetic OY (Finland). Anodization was performed in a mixture of HF, water and ethanol (volume ratio 1.6:2.4:6). The currents were predefined according to the desired porosity profile, and realized with a specially designed computer controlled current source. Prior to etching, the samples were placed in the etching solution for 1 minute to remove the native oxide. The counter electrode was a platinum wire positioned about 2 cm from the silicon wafer. After anodization, the samples were placed in ethanol for 10 minutes to remove remaining HF and finally blown dry with nitrogen gas. Optical measurements were done with a variable angle spectroscopic ellipsometer of the rotating analyzer type (J. A. Woollam Co., USA) [14]. Analysis of the optical data was done with the WVASE32 software package. The spectroscopic measurements were carried out at room temperature in air at six angles of incidence in the range 50'-75' (accuracy ± 0.0050) with an interval of 5' in the 400-1700 nm wavelength range. Variable angle spectroscopic ellipsometry is based on measurements of the change in the polarization state of a linear polarized light beam reflected at a sample surface at oblique incidence. The measured quantity in ellipsometry is the complex re
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