# Presentation: 487694

## Electrically-Active Interfaces for Chemical SensorsLisa M. Porter, DMR-0354939

**Conventional technology:**

**Contacts developed at CMU:**

- shorting

- The intrinsic properties of silicon carbide (SiC), a wide bandgap semiconductor, allow it to be used in electronics at temperatures where silicon (Si) cannot be used. However, while the properties of SiC allow for higher operating temperatures, interfaces between dissimilar materials (e.g., metal-semiconductor) typically react at relatively low temperatures, which yields unstable devices.  In this study single-phase contacts with thermodynamic stability with SiC were demonstrated.

- Scanning electron microscopy images

- X-ray diffraction spectra

- Transmission electron microscopy images

- Smooth, uniform morphology

- No reaction with SiC

- Single PtSi phase

**Notes:**

There is strong interest in developing SiC-based devices for high-temperature electronics such as gas sensors that can be used at high temperatures and in aggressive environments. However, while the intrinsic properties of this wide band-gap semiconductor allow for high operating temperatures, interfaces between dissimilar materials (e.g., metal-semiconductor) often interact at relatively low temperatures. These interfaces (e.g., contacts) are necessary in all devices. 
We compare the conventional contact technology (Al-Ti) for p-type SiC to single-phase PtSi contacts that we developed and studied at CMU. The traditional approach to forming ohmic contacts to p-type SiC is to anneal an Al-based contact (e.g., TiAl) on p+ SiC at temperatures around 1000°C. However, as shown in the SEM image (upper left image) of a contact test pattern, Al-based contacts have morphological problems resulting from the high processing and/or operating temperatures.  These problems are associated with the high tendency for oxidation, phase separation with low melting point phases, etc. In contrast, the PtSi contacts showed a smooth morphology (lower left image) and no reaction with the SiC (TEM image with FFT ‘diffraction’ pattern, lower right). 
The fabrication process (represented in the diagram above) consists of sequential sputter deposition of the metal layers followed by annealing at ~1000 °C.  The thicknesses of the Si and Pt layers resulted in a controlled reaction with the formation of the PtSi phase, which is predicted to be thermodynamically stable with SiC (an important point for high-temperature stability!). Our results show that under the right conditions the kinetics of the reaction between the Si and Pt layers is sufficiently fast to prevent an undesirable reaction with the SiC.  If insufficient Si is present, the SiC reacts with the metal because of its low heat of reaction (not shown here, but we have studied using TEM).
Our studies are resulting in an understanding of the reactions to yield single-phase contacts on SiC. We are now focusing on characterizing the long-term stability of the contacts and developing a multi-layer contact structure, based on PtSi, that is both thermodynamically/electrically stable and oxidation resistant.  (An invention disclosure is planned.)

## Electrically-Active Interfaces for Chemical SensorsLisa M. Porter, DMR-0354939

- The broader impacts include: 1) promoting graduate student exchange via visits to Sweden, 2) supervising undergraduate research projects, 3) founding a lunch/speaker series designed to inspire and retain women graduate students and post-docs in materials science, and 4) running the MSE Days in the Engineering Your Future workshop for middle-school and high-school girls.
- The PI also participated as a presenter at the Space Day events in her hometown of Brewer/Bangor, Maine.  A total of more than 300 students participate in nine sessions.

- Engineering Your Future Workshop

*Middle school students measuring the low-temperature properties of superconductors.*

*High school students oxidiz-ing titanium and using it to make jewelry or art pieces.*

- Space Day

*Students experimenting with the amazing properties of superconductors (magnetic levitation) and marshmallows (thermal properties) after submersing them in liquid nitrogen.*

**Notes:**
To foster a strong collaboration with our European partner, annual meetings in both the U.S. and Europe are occurring.  The graduate students brought their samples with them to perform complementary characterizations. 
We have been supervising an undergraduate research project via a Research Experience for Undergraduates. Our European partner has supplied sensor structures for the undergraduate student to characterize via current-voltage measurements, SEM, etc. 
The PI has been hosting a series of lunches designed to inspire women graduate students in materials science to continue along a profession in science and engineering. The idea is for women graduate students and post-docs to come together as a group and share their ideas and similar experiences and to encourage them to consider various options for the future.  To inspire discussion, a speaker is  invited to give a short informal talk on a selected topic, such as how to balance personal and professional responsibilities and professional aspirations: academia vs. industry.  The series has been so popular that the MSE Dep’t has decided to sponsor even more lunches.
For the fifth year, the PI helped run the Engineering Your Future Workshop (sponsored by the Society of Women Engineers) for middle school and high school girls.  The middle-school session comprised hands-on experiments of several classes of engineering materials, including semiconductors, superconductors, metals, ceramics, and polymers.
This year the PI was invited to her hometown to participate as a presenter at Space Day.  In the course of two days, she presented materials science in nine sessions with a total of more than 300 students.