David T. Crouse

Self-ordered pore structure of anodized aluminum on silicon and pattern transfer

A practical approach of transferring a hexagonal array of nanosized pores produced in porous alumina into silicon and other substrates is discussed. The alumina pores have dimensions of 25–250 nm pore diameters and 50–300 nm pore spacings depending on the anodization conditions used. The characteristics of the alumina pores and the alumina–silicon interface are studied for different substrate materials and anodizing conditions. The unique structure of the barrier layer allows for the alumina to be directly used as an etch mask for pattern transfer into the silicon substrate.

Polarization independent enhanced optical transmission in one-dimensional gratings and device applications

A review and analysis is performed of various resonance effects associated with subwavelength one-dimensional (1-D) metal gratings for transverse electric (TE) and transverse magnetic (TM) polarized incident radiation. It is shown that by tuning the structural geometry (especially the groove width) and material composition of the 1-D gratings, polarization independent enhanced optical transmission (EOT) can be achieved. Three different cases of EOT have been studied for 1-D metal gratings: a) EOT for TM-polarized incident radiation b) EOT for TE-polarized incident radiation, and most importantly c) EOT for un-polarized incident light. Potential uses of these results in the design and improvement of various optoelectronic devices, such as polarizers, photodetectors and wavelength filters are discussed.

Role of optical and surface plasmon modes in enhanced transmission and applications

An analysis of several types of one-dimensional transmission gratings structures with different metal contact geometries is used to study the role of horizontally oriented surface plasmons, cavity modes and other optical modes in enhanced transmission. Several competing theories of enhanced transmission are presented and the analysis of the structures in this work clearly establishes that horizontal surface plasmons can enhance or inhibit transmission depending on whether the HSPs establish vortices of energy that circulate in a direction that enhances or inhibits the flow of energy through the center of the grooves. Also, we show that enhanced transmission can be achieved using a different mechanism than previously reported in the literature. This new mechanism is a Fabry-Perot resonance produced by small notches in the top metal surface, which concentrates the energy from the incident beam and steers it through the slit openings and into the substrate. Finally, applications of the different structures and their optical modes are discussed including chemical and biological sensors and high bandwidth, high responsivity InGaAs metal-semiconductor-metal photodetectors.

Numerical modeling and electromagnetic resonant modes in complex grating structures and optoelectronic device applications

An extended surface impedance boundary condition algorithm is developed that allows for the optical properties of a wide variety of complex one-dimensional periodic grating structures to be modeled. Wood-Rayleigh anomalies, diffraction, and electromagnetic resonance modes including horizontally oriented surface plasmons and vertical surface resonances are identified and described as well as analyzing their structural and geometrical dependencies. Methods to combine these modes to produce hybrid modes that channel and localize light are described. Application of these modes to metal-semiconductor-metal photodetectors (MSM-PD) is discussed and an example silicon-based MSM-PD with over 30 GHz bandwidth and 0.3 A/W responsivity is described.

Electrical properties of wafer-bonded GaAs/Si heterojunctions

This letter reports on the fabrication and electrical characterization of wafer-fused GaAs/Si heterojunctions. A detailed study of the effect of surface preparation on bonding GaAs to Si was performed. The current–voltage (I–V) characteristics of both n-GaAs/n-Si and p-GaAs/p-Si were measured from 77 K to room temperature. The forward I–V characteristics were analyzed using a numerical model that includes thermionic emission across the heterojunction. Specifically, a p-GaAs/p-Si heterointerface of high electrical quality was obtained by direct hydrophobic bonding.

A method for designing electromagnetic resonance enhanced silicon-on-insulator metal–semiconductor–metal photodetectors

Methods of using a combination of electromagnetic resonance modes including surface plasmons and Fabry–Perot cavity resonances and other optical modes including Wood–Rayleigh anomalies and diffraction to enhance the performance of metal–semiconductor–metal photodetectors (MSM-PDs) fabricated on silicon-on-insulator (SOI) substrates are described. The characteristics of these optical modes are briefly described and their proper use in SOI MSM-PDs is described in detail. Three algorithms that model the electromagnetic field produced by the incident beam, the quasi-static electric field produced by the applied bias, and the charge carrier motion, recombination and induced photocurrent are integrated together in a time-dependent way to accurately calculate the bandwidth and responsivity for a series of optical pulses. By using a combination of optical modes, a structure with theoretical values of 50 GHz bandwidth and 0.17 A W−1 responsivity is designed. Other structures using different combinations of optical modes are described.

Nanoporous Alumina Template with In Situ Barrier Oxide Removal, Synthesized from a Multilayer Thin Film Precursor

A nanoporous alumina template made from a multilayer metal film structure has been developed that allows for the in situ removal of the electrically insulating alumina barrier layer, exposing a Pt electrode at the pore bases. This barrier-free nanoporous system is of great use for dc electrodeposition of a wide variety of materials in the alumina pores. This work in particular describes the development of a multilayer thin film precursor consisting of a Si substrate with thin Pt and Ti and a thicker Al layer in that order. After the Al is anodized, producing the porous alumina, the resulting TiO 2 is selectively removed at the base of the alumina pores exposing the Pt electrode. The metals in the precursor perform different roles in the fabrication and allow the alumina template to be fabricated directly on the final substrate with no film transfer technique involved. This allows Si to be used as the substrate, which could then include electronic circuitry. Several techniques are used to analyze the resulting template.

Self-assembled nanostructures using anodized alumina thin films for optoelectronic applications

Anodized aluminum has recently attracted much attention because of its desirable porous structure. The pore structure is a self-ordered hexagonal array of cells with cylindrical pores in an alumina matrix of variable sizes with diameters of 25 to 300 nm with depths exceeding 100 /spl mu/m depending on the anodizing conditions used. These properties make anodized aluminum a desirable material for many optoelectronic devices including polarizers, photonic crystals, low threshold current lasers, and investigation of light-surface plasmon interactions in metals. The conventional approach to fabricate porous alumina is to use bulk or thin sheets of aluminum and then replicate this pattern into the desired substrate by one of several methods involving a tedious film transfer. However, in this work, a more convenient and practical method of fabricating the porous alumina structure using an evaporated film of aluminum on silicon and other substrates, subsequent pattern transfer, and its use in optoelectronic applications will be discussed.

Light localization, photon sorting, and enhanced absorption in subwavelength cavity arrays

A periodically patterned metal-dielectric composite material is designed, fabricated and characterized that spatially splits incoming microwave radiation into two spectral ranges, individually channeling the separate spectral bands to different cavities within each spatially repeating unit cell. Further, the target spectral bands are absorbed within each associated set of cavities. The photon sorting mechanism, the design methodology, and experimental methods used are all described in detail. A spectral splitting efficiency of 93-96% and absorption of 91-92% at the two spectral bands is obtained for the structure. This corresponds to an absorption enhancement over 600% as compared to the absorption in the same thickness of absorbing material. Methods to apply these concepts to other spectral bands are also described.

Tuning the polarization state of enhanced transmission in gratings

The polarization characteristics of enhanced transmission of lamellar gratings with structural dimensions on the subwavelength scale were studied and experimental results were compared to numerical models. The ability to tune the polarization state of the transmitted beam by varying the grating’s structural parameters is discussed. Gratings were fabricated and tested in the microwave spectral region, and the results were compared to theoretically modeled results. Enhanced transmission produced by cavity modes was experimentally verified for both s-polarized and p-polarized incident beams of light. Applications of these results to photonic devices in the visible, infrared, and microwave spectral regions are discussed.