Subwavelength structures (SWS), a form of diffractive optic, are well-known for their ability to function as polarizers and anti-reflection coatings. They can also be used to create narrowband optical filters whose surface reflectance spectra exhibit resonant peaks that are highly wavelength dependent, especially as the surface index is modified by the deposition or adsorption of biomaterials such as molecules or cells. In this study, we report on the design and fabrication of SWS structures in silicon that are suitable for use as biosensors in sensitive molecular detection. The structures combine a two-dimensional dielectric grating and Si/SiO2 optical waveguide to create a surface that can function as a narrowband optical filter. The SWS structures were fabricated using a combination of three-beam interference lithography and reactive-ion etching in a CBrF3 plasma. This produced a two-dimensional periodic nanostructure grating array, having a period of ~450 nm and air pores of ~265 nm, within a 300 nm thick silicon layer that serves as the core waveguide region of the filter. The ability to achieve sensitive molecular detection (< nm) is expected by virtue of working with high-index silicon-based structures, but may be practically limited by the need to detect the reflectance at near-infrared, rather than visible, wavelengths.
The results of experimental studies that investigate the reactive ion etching characteristics of Si nanostructures using CBrF3 plasmas is reported. Etch rates and anisotropies were studied as functions of RF and ICP power (0 - 200 W), gas pressure (10 - 190 mT), gas flow, and O2 gas mixtures in a commercial ICP etching system. Both isotropic and anisotropic etching regimes were identified, and used to create < 600 nm feature periodic motheye and photonic crystal nanostructures that possess pyramidal and near- anisotropic profiles, respectively. The ability of CBrF3 to effectively control sidewall profile and etch depth when used with different resist mask profiles makes it an attractive etchant for the fabrication of periodic optical nanostructures that have stringent sidewall and nano-tolerance requirements.
HgCdTe epitaxial layers were grown on (111)B CdTe substrates in
a commercially purchased and modified vertical MOCVD reactor. DIPTe,
DMCd and elemental Hg were used as reactants and growth takes place
at 360°C. Run-to-run reproducibility of composition and electrical
properties was studied over a hundred growth runs. Hall measurements
show that n-type material is consistently obtained with carrier concentrations
in the mid 10'4/cm3 and mobilities as high as good bulk
material. LWIR photoconductive devices were fabricated with this low
carrier concentration material and tested. The responsivity and the
lifetime measurements indicated that these devices perform as well as
those fabricated with purchased LPE material. Excellent device performance
was obtained on 80-element photodiode arrays fabricated with
x''0.3 p-type HgCdTe samples by formin9 flf/p junctions by ion implantation.
R0A values obtained are ..1x1Ob Q-cm2 at 77K for Xco 5.7 1m
and are highly uniform within an 80-element array. We also report on
an all MOCVD grown p-on-n double-layer heterojunction (DLHJ)
photodiode with Xco 8.1 um.
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