Performances Of Porous Silicon Optical Waveguides
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Mat. Res. Soc. Symp. Proc. Vol. 486 ©1998 Materials Research Society
gtm. Following the formation of the active PS layer, a cladding PS layer with a relatively small refractive index is continuously formed by changing the anodization current density. The anodization current density and time for the cladding layer are 100 mA/cm2 and 90 s, respectively. The cladding PS layer thickness corresponding to these conditions is 6 gim. The anodization process of all samples is carried out in the dark. Immediately after processing, the sample was treated by thermal oxidization at 700 'C for 10 min in order to reduce the selfabsorption for a lightwave propagated through the active PS layer. Finally, the wafers are cut and cleaved. The device was excited from the direction perpendicular to the sample surface by an N2 pulse (337 nm) or a He-Cd (442 nm) laser, and output light emitted from a cleaved edge was observed by a microscopic detection system at low temperatures of about 10 K. The pulse width and repetition rate of the N2 pulse laser were 5 ns and 10 Hz, respectively. From the PL spectra, the internal loss under pumping due to an self-absorption and scattering are evaluated. The net internal loss of the prepared sample are measured by varying the length of excitation
beam [13], L, as illustrated in Fig. 1. When the device was excited by the sharp line beam with the length of L, the observed intensity from the cleaved edge PLare given by (1)
IL = I(,0 exp(-ax) dx =I0{1 - exp(-aL)} / a
(2)
where a and 1 are attenuation coefficient and luminescence intensity per unit length, respectively. With combined measurements for two different excitation lengths of L and 2L, the a value can be obtained by following equation. a = I / L.-ln(I'L / IL _ 1)
(3)
Thus the a values are independent on the original PL behavior. The attenuation coefficient under pumping can be evaluated without any probe-beam in this way.
eEdge-PL
Mca
/ Active PS
Cladding PS
. Si
L
[ Detection
ISystem
Fig. 1 Schematical structure of an edge-emitting device with stepindex PS waveguide. Experimental set-up is also shown.The emitted light from the cleaved edge is detected through a microscope.
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Buried-type PS Waveguide The fabrication process of the PS waveguides with three-dimensionally buried structures [14] is shown in Fig. 2. The design concept of this structure is based on the doping-modulation technique [10], different from the process mentioned above. Boron ions were implanted into a selective area such that a core region is formed in the cladding silicon substrate with a resistivity of 8-12 f(cm. The doping concentration and an accelerating voltage were 10'" cm 2 and 200 kV, respectively. To obtain the core with a sufficient depth to confine the lightwave, a thermal diffusion was carried out at a temperature of 1000 °C. Subsequently, the wafer were anodized in a mixture of HF and ethanol at a constant current density of 20 mA/cm2 for 4 min, and then the device was thermally oxidized in an dry-oxgen ambient in order to reduce a self-abs
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