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Schottky and Ohmic Contacts for High Power Electronic Applications
This research work is currently supported by NSF grant # ECS 0622086 under the project titled: “RUI: Improved Thermal Stability of Metal Contacts and Diffusion Barriers to SiC and AlxGa1-xN using Refractory Metal Borides”, PI: T. N. Oder, 11/01/2006 – 10/31/2009 with a no-cost extension to 10/2010. Funds = $150,000.
In order to fully utilize the excellent qualities of wide band gap semiconductors for high power and high temperature applications, high quality ohmic and Schottky metal contacts are required. A large number of electronics essential to military and commercial operations such as high power radars, power amplifiers for wireless communication, remote actuators, distributed high power control systems etc, require functionality at temperatures above 300oC. Both types of metal contacts are critical components in these devices and their thermal stability dictates the overall limit of the application of the devices, which can be as high as 600oC (automobile/industrial engines)., While the primary application of these contacts targets high temperature and high power electronic devices, further applications of relevance to the AFOSR Optoelectronics Program exist in the optoelectronic devices such as photodetectors, sensors, LEDs and LDs when direct bandgap materials are considered. Selection of ohmic or Schottky contact metal is generally guided by the reaction chemistry at the metal/semiconductor interface and by the Schottky-Mott theory, which predicts the energy barrier Φ0 (barrier height) to the flow of electrons:
For p-type materials:
q Φ 0 = EG + ΧS – φm (1)
For n-type materials:
qΦ0 = φm - ΧS (2)
where EG is the band gap of the semiconductor,ΧS is the electron affinity of the semiconductor and φm is the work function of the metal. For p-type materials, ohmic behavior results if a metal with high enough work function is chosen, because this will lead to low barrier to the flow of electrons, while the opposite is the case for n-type materials. Thus, metals with high work functions such as Pt, Ni, and Au (with φm = 5.65, 4.84, 4.58 eV, respectively) are considered for ohmic contact on p-type materials. However, after thermal stressing at 500oC, significant increases in specific contact resistance in some of these contacts have been noted.
Schottky (rectifying) contacts on the other hand, play key roles in devices such as photodetectors, sensors and heterojunction field effect transistors (HFETs). In microwave devices, the thermal stability of the Schottky contacts becomes a very important factor at high input power level. Due to their good high frequency behavior, Schottky barrier diodes are also being extensively used in different applications such as mixers and detectors and are actually the most progressed SiC power devices already commercially available. Improvements are being sought for Schottky diodes for operation in the THz frequency region., These applications call for high quality Schottky contacts with large barrier heights, low reverse leakage current and excellent thermal stability. To address the need for development of contacts to SiC and III-nitrides with improved thermal stability, the PI has conducted investigations of several selected transition metal borides: zirconium boride (ZrB2), chromium boride (CrB2), titanium boride (TiB2), hafnium boride (HfB2), Vanadium boride (VB2) and tungsten borides (WB, W2B and W2B5). These borides were chosen because of their promising qualities such as very narrow composition ranges with SiC and AlxGa1-xN, high hardness, very high melting points, and excellent chemical resistance and yet very low electrical resistance.
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Wide Bandgap Semiconductors
A Wide Band Gap (WBG) semiconductor in general terms can be defined as a semiconductor with an energy band gap (EG) above 2 eV. Included in this group are the group III-nitrides, silicon carbide (SiC), zinc selenide (ZnSe) etc. The focus of our group is on the group III-nitrides, silicon carbide and zinc oxide.
Go to WBG site
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This material is based upon work supported by the National Science Foundation under Grant No. DMR #0423914.
 Magneto-optics
Goal: To investigate the use of multilayer structures for Faraday and surface magneto-optic Kerr effect rotation and, in particular, to investigate the application of surface and band edge enhancements effects for optical modulation and optical isolation.
Background:Magneto-Optical effects such as the Faraday effect and the Surface Magneto Optic Kerr Effect (SMOKE) describe the rotation of the polarization of light in the presence of an applied magnetic field. Unlike other rotatory effects such as optical activity and electro-optic effects, the sense of rotation relative to the magnetic field direction is, for a given structure, independent of the direction of propagation of the light. Thus, the amount of rotation increases with repeated reflection back and forth through the same material. When a Faraday rotator is structured so that the effective net rotation angle during traversal is 45 degrees, the rotator, when combined with simple linear polarizers, enables the elimination of back reflections. The property that the rotation direction is independent of the propagation direction thus effectively allows one to build an optical isolator, a device which only conducts light in one direction. This elimination of potentially deleterious back reflections is very useful in, for example, stabilizing lasers, amplifiers and non-linear optical elements.
Such large angle rotations, however, require either large applied magnetic fields or large interaction lengths, the latter effectively preventing the miniaturization of Faraday rotation utilizing bulk materials. Multilayer interference, on the other hand, involves dramatic increases in interaction lengths for light propagating near the band edge of a multilayer photonic crystal. The use of multilayer materials may enable the miniaturization of optical isolators if materials and structures combining both large rotatory dispersion and large interaction lengths can be fabricated.
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Sample Spectral Filter
Spectral Filters
The design and production of polymer optical interference filters continues to be the subject of intense research and innovation due to the significant manufacturing advantages of polymers, such as cost, flexibility, and low weight. A fundamental problem for a variety of lighting and optical component applications is the reproduction of desired irregular spectral transmittance and reflectance curves near normal incidence. Applications include specialty filters for the lighting of artwork that are designed to eliminate regions of the visible spectrum that are not necessary for viewing a particular work. The use of extruded multilayer films for specialized spectral films offers unique opportunities as compared to more traditional sputtering, evaporation, or spin coating deposition techniques. Notably, the multilayer extrusion process enables rapid, low-cost, production of large-area free-standing films having a large number of layers. Custom multi-bandgap structures can be created through uneven splits during the layer multiplication process and through the post-processing coherent and incoherent layering of films. The layer multiplying process, however, also imposes unique design challenges requiring different optimization techniques. Though amenable to creating a large number of layers, the layer multiplication process limits the variety of different index materials that can be layered in a single process and is best suited to designs with layer profile symmetries compatible with the layer doubling steps of the multilayer extrusion process.
The work is being done in collaboration with Prof. Carl Dirk of University of Texas, El Paso and includes the design and testing of prototype spectral filters for custom optical filters and specialty lighting applications.
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<p><strong><span style="font-size: medium;">Polymer Microlens</span></strong></p>
</span><span style="font-family: tahoma, verdana, arial; line-height: 18px; text-align: left;"> </span><br /><span style="font-size: small; line-height: 18px;">Gap microlithography with negative photoresist will be used to pattern dome-shaped resist on the polymer surface prior to dry etching to achieve leyer-terminated plano-convex microlens on the CLiPS polymer materials. The resulting graded </span><span style="font-size: small; line-height: 18px;">index microlens has numerous applications including solar concentrators and micro-GRIN lens arrays which promise super resolution imaging and increased lighting efficiency</span><strong>.</strong></td>
Polymer Microlens
Gap microlithography with negative photoresist will be used to pattern dome-shaped resist on the polymer surface prior to dry etching to achieve leyer-terminated plano-convex microlens on the CLiPS polymer materials. The resulting graded index microlens has numerous applications including solar concentrators and micro-GRIN lens arrays which promise super resolution imaging and increased lighting efficiency.
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3-D Photonic crystals on Multilayer Polymeric Systems
Fabricate 3-D photonic crystal nanostructures (arrays of holes or pillars) on layered polymers. The planar arrays of holes or pillars provide a 2-D photonic crystal structure while the multilayer films in the polymer system will provide the third dimension in the 3-D structures. Arguably, this third dimension would be small with an expected refractive index difference between the layers of about 0.2 – 0.5, and would only be regarded as complementing the larger air/polymer refractive index difference. However, this contribution could yield a significant increase in the 2-D nature of the photonic crystal on LEDs for enhanced light extraction.
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