Lisa Porter - US grants

The PI will establish a new research program investigating the metallisation schemes which offer potential for implementation if wide band-gap (GaN, SiC) semiconductor devices. Her research will combine knowledge of interfacial reactions and materials properties with an understanding of the physics of Schottky barriers and the associated interfacial current transport mechanisms.

9802917 Porter This award provides support for the acquisition of an ultra-high vacuum system with the capability of multiple techniques for the controlled deposition of many types of thin conducting films. The deposition system will have a rare combination of techniques and attributes that will allow for the growth of a variety of conducting films, ranging from ultra-pure elements to refractory compounds, in a highly controlled environment. This system will permit leading research involving studies of the physics and electrical, chemical, and microstructural properties of materials at conducting-to-semiconducting interfaces. Some of the existing and planned research programs include an NSF-supported study of thermally stable contacts to SiC and GaN for high temperature electronic applications and the development of contacts for both SiC-based solar probes and SiC-, GaN-, or BN-based radio isotope batteries. Additional programs that will benefit include "Surface Phase Transitions in Alloys", "Stability of Fine Microstructures in Thin Films", and the "Mesoscale Interface Mapping Project". %%% While all of these research programs involve the fabrication of conducting contacts on semiconducting or insulating substrates, the nature of conducting films varies in terms of the techniques which are most appropriate for depositing the films. The system addresses these differences in its inclusion of two sources for dc sputtering, a multi-pocket electron-beam source for evaporation, and the future capability for the addition of an ion source for ion-beam assisted deposition. The sputtering and evaporation techniques will allow for easier deposition of most types of the films which are being studied, while the ion- assisted process will be used in the future to introduce more control over the parameters for depositing compound materials. ***

9875186
Porter

This proposal describes research on novel structures and metallizations for wide bandgap semiconductor devices, the development of integrated laboratory-classroom modules associated with semiconductors and thin film science, and the extension of coordinated advising and outreach activities.

Due to its large bandgap (3.4 eV), high thermal conductivity, and high saturation drift velocity, gallium nitride is being intensively pursued for a variety of optoelectronic and high temperature, high frequency, and high power electronic devices. Recent advances in the quality of GaN epitaxial films have prompted efforts to develop structures which can be used in making efficient devices. In this project novel structures comprising conducting layers and semiconducting GaN films will be investigated. This work will employ the selective growth of GaN and its subsequent overgrowth to produce embedded conducting layers (ECL) within the GaN epitaxial films. These structures will have potential applications in devices such as ultra-violet photodetectors and permeable base transistors.

A closely related challenge for the development of both GaN and SiC devices involves the fabrication of p-type ohmic contacts with low specific contact resistivities (SCRs). Because of the large bandgaps and large workfunctions of these semiconductors, metal contacts on p-type material yield large Schottky barrier heights, or energy barriers for electron transport across the metal-semiconductor interface. These large barrier heights increase SCRs even on relatively highly doped material. To reduce the Schottky barrier heights InxGa1-xN will be investigated in the form of thin (~50-100 A) interlayers between selected metal contacts and the p-type GaN or SiC semiconductor substrates. Indium nitride and gallium nitride are completely soluble in one another, such that the composition of InxGa1-xN may be varied over the range from pure GaN to pure InN. By varying the composition, and thus the bandgap, of the lnxGal-XN interlayer, it is believed that the barrier heights can be significantly reduced.

The long-term outlook for the advancement of wide bandgap semiconductor technology will depend on the availability of suitably educated graduates. Recent research indicates that students learn best when laboratory courses are integrated with classroom experiences. For this reason the Materials Science & Engineering Department at CMU plans to implement a new undergraduate curriculum with extensive integration of laboratory and classroom courses. The development of instructive experiments will be critically important to the successful implementation of this program. This proposal discusses plans for developing selected laboratory modules and expanding student advising and outreach programs.
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This project is a joint collaboration between Prof. Lisa Porter/Carnegie Mellon University (CMU) and Prof. Anita Lloyd Spetz from S-SENCE (Center for Sensor Technology) at Linkoping University, Linkoping, Sweden. The aim is through the combined collaborative expertise to push forward materials science understanding and subsequent development of high-temperature sensors, and to understand mechanisms that limit their performance. Examples of applications for these sensors include monitoring of selective catalytic reduction in automobile combustion engines and of flue gases from power plants. The approach is to develop and characterize ohmic contacts and insulators for SiC based chemical sensors that demonstrate substantial improvements in long-term stability at high temperatures (e.g., 300-800 C) required for optimum detection of certain gas species (e.g., hydrocarbons or NH3). Specifically, test devices (capacitors and Schottky diodes) will be fabricated based on contact and insulator materials, as well as associated processing conditions, developed by the CMU group. The devices will then be tested at S-SENCE for their gas sensor response at high temperatures. Initial research at Carnegie Mellon will consist of materials selection for ohmic contacts (e.g., TaC and PtSi), gate metals (e.g., Pt3Si) and gate insulator (e.g., SiO2 or AlN) followed by device fabrication. The devices will consist of TLM patterns for contact resistance measurements, MIS capacitors and Schottky diodes. The stability of these devices will then be measured using current-voltage (I-V) and/or capacitance-voltage (C-V) measurements as a function of both annealing temperature and measurement temperature. Selected samples will be sent, or brought, to S-SENCE for measurements of their gas response. This will be accomplished by mounting each sample onto a ceramic heater, which is attached to a 16-pin holder. After introducing specified gases into the assembly, the sensor response will be measured as the voltage at a constant current of 0.1 or 1 mA. Promising materials structures will be incorporated in MISiCFET devices. An important part of this research will include investigations of the morphology and interfacial chemistry and their relationship to the electrical properties of the sensors. The morphologies of the contact films will be characterized using scanning electron microscopy. The interfacial chemistry of the metal-insulator, insulator-semiconductor and metal-semiconductor will be characterized by Auger electron spectroscopy (AES), TEM, XRD and SIMS.
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The project addresses fundamental research issues associated with electronic materials having technological relevance. An important feature of the project is the integration of research and education, and an international collaboration providing both scientific and educational benefits. Broader impacts associated with the project are exemplified by the education of undergraduate and graduate students in a unique technical, cultural and professional context. The approach includes: 1) graduate student exchange through visits to and from Sweden, 2) supervision of an undergraduate research project pertaining to chemical sensors, and 3) introduction of a lunch/speaker series designed to inspire and retain women graduate students and post-docs in materials science. This NSF project is a Cooperative Activity in Materials Research between the NSF and Europe (NSF 02-135).

The objective of this research is to understand the physics and chemistry of ohmic contact formation to semiconducting polythiophene films and to optimize the contacts for electronic devices, such as organic field-effect transistors, light emitting diodes, and photovoltaics. The approach emphasizes a consideration of both the chemistry and the energy level alignment at the contact-polythiophene interfaces. Specifically, chemical bonding at the interfaces will be enhanced via regiochemical positioning of side chains. Selected metals will be deposited using both metal-on-organic and organic-on-metal configurations. Thin-film transistor structures will be fabricated and measured to yield key electrical properties, such as the carrier mobility and the contact resistance.

The broader impacts include the technology necessary for the development of highly versatile and inexpensive electronic devices, such as electronic displays on flexible substrates and large-area solar cells. The latter application is particularly exciting, because the low cost in combination with theoretical efficiencies of organic solar cells makes wide-scale production of renewable energy a realistic goal. This research program emphasizes an interdisciplinary, cross-college collaboration, an approach that is becoming more valuable with the increasing complexity of current and future devices. In addition, the project emphasizes the education and training of graduate students in essential future technology and includes minority students from a Historically Black University. The supervision of one or two undergraduate research projects will also be conducted to inspire undergraduate students to continue their education and training in science and engineering.

Title: Formative Research Program on Gallium Oxide Semiconductor Materials and Interfaces to Hasten the Development of Ultra-High Efficiency Electronics

Non-Technical:
The global energy crisis presents an urgency to develop ultra-high efficiency electronics for energy conversion and transport. Electricity accounts for more than a third of primary energy consumption in the U.S., and a large portion of that electricity is handled by power electronics. For example, power electronics are required for efficient electrical switching within the electrical grid, as well as for power supplies in hybrid electric vehicles and for power converters in Naval ship platforms and to integrate renewable energy systems into the electrical grid. In each of these applications, the power semiconductor device is the critical component that determines the energy conversion efficiency, in addition to the size and cost of the system. Because of its recent material (crystal) availability and its extreme properties, gallium oxide (Ga2O3) is a novel semiconductor material that is expected to yield superior devices for ultra-high efficiency electronics. Although there is strong interest in the U.S., to date, the vast majority of research and development (R&D) on gallium oxide has been conducted in Japan. This research project is designed to hasten the development of ultra-high efficiency electronics (especially power devices) based on gallium-oxide materials and interfaces and to establish a foundation for R&D activities on gallium-oxide within the U.S. Ultimately, advances in power electronics will account for substantial gains in energy savings and an associated reduction in carbon emissions. As part of this program graduate and undergraduate students will receive training and develop professional skills needed to work in the semiconductor industry. Additional outcomes will include scientific outreach to middle and high school students and teachers, and conference sessions to inform and to help build R&D activities in this emerging field.

Technical:
Specifically, in this project researchers at Carnegie Mellon University will conduct a formative study focusing on the physics, chemistry, and processing conditions that determine the behavior of electrical contacts to doped Ga2O3 single crystal substrates and epitaxial layers. A number of devices based on Ga2O3, including Schottky diodes, MESFETs, and MOSFETs, have been demonstrated. However, as research on Ga2O3 as a wide bandgap semiconductor is in its very early stages, there is little understanding of how to control device relevant interfaces to this material. Specifically, this research will focus on (1) forming low resistivity ohmic contacts, informed by thermodynamic calculations and consideration of other physical and chemical properties, and (2) characterizing Schottky diodes for a wide range of metals on different crystal orientations of Ga2O3 and comparing the Schottky barrier heights with predicted values. Knowledge of the effects of metal workfunctions, crystal orientation, interfacial reactions, and processing conditions on the contact properties will serve as a platform to enable informed designs that device engineers can use to achieve optimized devices based on Ga2O3, and, potentially, on alloys and heterostructures of (Al,Ga,In)2O3 materials in the future.