Arthur C. Gossard - US grants

This proposal describes basic research on low-dimensional heterostructures and quantum structures, hereafter termed ultrastructures. The ultrastructures will have radically new properties introduced by their new degrees of quantization. The research will involve an integrated investigation of formation, fundamental characterization and device implementation of these extremely small structures. Ultrastructure microfabrication will be achieved by in situ epitaxial crystal growth and high- resolution focused ion beam methods. Characterization of the structures will be carried out using sensitive, spatially resolved structural and spectroscopic methods. The novel physical properties originating from carrier confinement will be investigated and incorporated in exploratory electronic and photonic devices.

Instrumentation will be acquired for the metallo-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) synthesis of III-V nitrides using funds from the Academic Research Infrastructure Program. The major items include a RF heated III-V MOCVD nitride system and a nitrogen plasma source for MBE growth. The aluminum, gallium and indium nitride materials are the most promising materials for optoelectronic devices to operate in the ultraviolet to blue wavelength regime. The research activities will focus on: 1) development of novel epitaxial growth technologies for III-V nitride semiconductors, 2) fundamental studies of the epitaxial growth mechanism, and 3) the fabrication of UV/blue laser diodes, high temperature electronics, and vacuum electronics devices. This research involves substantial support and interest from industrial collaborators. Studies of the epitaxial growth of III-V nitride semiconductors will be conducted with the ultimate objective of producing high quality laser diodes for use in the ultraviolet and blue wavelength regimes. The III-V nitride semiconductors are useful materials for high temperature electronics and also for vacumn electronics devices. The III-V nitrides are technologically important materials for devices, particularly at shorter wavelengths and at higher temperatures.

9724436 Sherwin A tunable, far-infrared, picosecond source will be developed based on the free electron laser (FEL) facility at the University of California-Santa Barbara (UCSB). The pulsed, far- infrared source will support research in several areas of condensed matter physics, mostly nonlinear dynamics of semiconductor heterostructures and of superconductors at terahertz frequencies. %%% The light source to be developed will fulfill a current need for a short-pulse, far-infrared user facility in the country. The new FEL capability has potential applications in other areas besides condensed matter physics, such as molecular spectroscopy, molecular chemistry, and biophysics. ***

This project aims to combine the facilities and personnel of the University of California at Santa Barbara and a group at the University of Erlangen, headed by Gottfried Dohler. Research focuses on the growth and characterization of non-stoichiometric compound semiconductors and graded semiconductor quantum structures, and will combine molecular beam epitaxy of samples and electronic characterization at Santa Barbara with optoelectronic characterization at Erlangen. There will be an exchange of researchers with the goals of transferring knowledge and allowing students to gain hands-on knowledge of each groups experimental methods.

With this award from Instrumentation for Materials Research Program the University of California, Santa Barbara, will develop a Laser Driven Terahertz System which will provide new research instrumentation for high-resolution linear terahertz spectroscopy of materials, material structures, and devices. The instrument will deliver microwatts of tunable radiation up to ~1 terahertz (THz). Above ~1 terahertz the power will be substantially less but more than adequate for linear, high-resolution spectroscopy of materials and material structures. Up to 1 terahertz, with suitable methods of excitation and sampling, the system will also be used to test the performance of state-of-the-art electronics at frequencies that exceed the capability of existing network analyzers. At the same time, the system will provide a test bed to explore new non-linear materials and devices for optical difference frequency generation of terahertz radiation by photoconductivity and non-linear susceptibility. The research and development of this new instrument will provide interdisciplinary education and training for postdoctoral researcher and to graduate and undergraduate students in physics, materials science and solid state electronics.
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With this award from Instrumentation for Materials Research program the University of California Santa Barbara will develop a Laser Driven Terahertz System which will provide new research instrumentation for high-resolution linear terahertz spectroscopy of materials, material structures, and devices. The terahertz part of electromagnetic spectrum, from 100 GHz to 10 THz, is science rich but relatively technology poor. The Laser Driven Terahertz System will provide new research instrumentation for high-resolution linear terahertz spectroscopy of materials, material structures, and devices. The research and development of this new instrument will provide interdisciplinary education and training for the post-doctoral researcher committed to the project and to graduate student and undergraduate researchers who will use the instrument in physics, materials science and solid state electronics. The instrument will deliver microwatts of tunable radiation up to ~1 terahertz (THz). Above ~1 terahertz the power will be substantially less but more than adequate for linear, high-resolution spectroscopy of materials and material structures.

Technical Abstract

The University of California Santa Barbara will acquire a new molecular beam epitaxy (MBE) system for the growth of high-performance oxide thin films. Numerous research programs at UCSB require the growth of insulating or semiconducting oxide thin films. These include tunable dielectrics for microwave devices, oxide thin films for optoelectronics and sensing, gate dielectrics for the development of CMOS devices employing high-mobility semiconductor channels and for high-electron mobility transistors with reduced gate leakage and high charge densities. These applications require the deposition of structurally perfect oxide thin films with low impurity and point defect concentrations, control over interface atomic structures and compatibility with underlying active device layers. Oxide thin films grown by MBE will allow for the understanding of the basic physics and materials science of oxides that currently lags far behind that of other electronic materials. Experimental testing and realization of the theoretical predictions requires high-quality, pure materials and the atomic layer control afforded by MBE. We anticipate that the high-purity, structurally perfect oxide films synthesized by MBE will lead to new scientific insights that generate new device applications. Graduate students and post-doctoral researchers are the primary 'hands-on' users of MBE at UCSB. The proposed MBE system will be operated as a shared facility, impacting the education and training of a large number of students in a wide range of interdisciplinary research activities at UCSB and collaborating academic institutions - we will build on the strong culture for MBE of compound semiconductors at UCSB and house the tool in the same large shared facility. Formal training in MBE is offered in graduate courses and weekly MBE seminars in the Materials Department while hands-on-training is provided by two development engineers. The oxide MBE system will significantly extend the opportunities that have previously been offered to student and teacher research interns and education programs aimed at underrepresented groups.


Lay Abstract

Molecular beam epitaxy is unique among the techniques used for making new electronic materials that enable modern electronic and optical devices, such as transistors and lasers. The performance of these devices depends largely on the degree of materials perfection. In molecular beam epitaxy, layers that are a few atoms thick can be stacked and materials with different electronic properties can be combined. Molecular beam epitaxy allows for unprecedented purity of these layers - the impurity levels can be as low as a few ten parts per billion. The new molecular beam epitaxy system at the University of California Santa Barbara will be utilize these unique capabilities to develop new classes of electronic thin film materials, based on metal oxides. We anticipate that the high-purity, structurally perfect oxide films synthesized by molecular beam epitaxy will lead to new technologies, such as transistors with higher operating speeds and capacitors that enable new wireless communication devices. The oxide molecular beam epitaxy system will contribute greatly to the education and training of students at the University of California Santa Barbara, who will be the primary hands-on-users of the new system. The oxide MBE system will also significantly extend the opportunities that have previously been offered to student and science teacher interns and education programs aimed at underrepresented groups.

Intellectual merit: This project is awarded under the Nanoelectronics for 2020 and Beyond competition, with support by multiple Directorates and Divisions at the National Science Foundation as well as by the Nanoelectronics Research Initiative of the Semiconductor Research Corporation. Progress in transistor and integrated circuit (IC) scaling has slowed, in part because of physical limits of transistor operation at small dimensions, but primarily because power consumption and power density are becoming excessive as complexity and density are further increased. IC power density results from opposing constraints in transistor and circuit design; the electron thermal distribution sets a minimum transistor control voltage for low off-state dissipation, while the dissipated energy on interconnects increases as the square of voltage. Addressing these limitations, radical changes in transistor design are proposed. To increase the on-current, designs are proposed that will overcome the so-called density of states (DOS) bottleneck in III-V semiconductors, adding additional valleys to those used in transport, therefore increasing the amount of charge that can be transported through the device at a high velocity. To increase drive current in N-channel field effect transistors (FETs) and the IC speed at reduced voltages III-V transistors will be develop using for the first time transport in the L (satellite) valleys, i.e. L-valley electronics. These will use the light electron part of their dispersion in the transport direction for fast carriers and will use the heavy electron characteristics to pack multiple bands into the ?same? energy space. Similar density of states engineering will be applied to P-channel FETs, achieved using light- and heavy-hole states mixed by strain and quantum confinement. To reduce supply voltages, steep transistors will be developed, having I-V characteristics varying much more rapidly than a thermal distribution. In addition to established tunnel injection devices having only moderate on-current, high-current steep-FETs will be developed. These use transport in energy bands of tightly constrained energy range, produced using 1-D semiconductor superlattices. Combining these two classes transisto rs, state-density-engineered transistors designed for high drive currents at low voltage, and steep transistors designed for very low off-state leakage, the program will explore new logic gate designs providing low power and high speed.

Broader Impacts: The proposed work seeks to increase the speed and complexity, and reduce the power consumption of ICs. The industry is of enormous worldwide value. The participants interact regularly with the VLSI industry, communicating ongoing work and seeking guidance, and will continue with this model in the NSF program. Development of high-speed yet low-power logic devices will circumvent present power-consumption limits now constraining VLSI speed and complexity. This program will enable further large increases in the speed and power-limited computational performance of ICs, benefiting applications in industry, commerce, and personal use. Ph.D. students will be trained in semiconductor materials, device physics, and IC design. Their training will emphasize the interaction of system and circuit design with device design. Simulation tools will be developed and distributed by nanoHub to a worldwide user community. The program will operate a summer internship program, affiliated with that of the NNIN, providing laboratory experience exposure to a research environment for 8 undergraduate students.

Microprocessors containing billions of transistor switches are at the heart of PCs, cell phones, computer servers answering our internet searches, and supercomputers modeling the weather and designing new drugs and aircraft. Modern machinery is controlled by microprocessors; a typical car uses 50. After 50 years of rapid improvement, since 2000 progress has stalled, primarily because the transistors consume too much energy when they switch. As transistors are made smaller, more fit on a chip, and the energy consumed increases. The battery is drained quickly and chip becomes hot. Slowing the switching reduces heating, but then the software runs slowly. In this program, a new transistor design will be investigated. If successful, these transistors will consume 100 times less switching energy, allowing faster, more powerful chips.

The challenge is the power supply voltage; reducing the voltage by 2:1 reduces the switching energy 4:1. Between ~1990-2005, voltages were reduced from 5 to 1 Volt. Unfortunately, with normal (MOS) transistor switches, below ~0.7 Volts the transistor's switching becomes imperfect, with the transistor not turning completely off. This finite off-state leakage current increases energy consumption, hence it has not been possible to supplies much below 0.7 Volts. Ten years ago, tunnel transistors were proposed, as these can turn off nearly completely even at supplies as low as 0.3 Volts. Unfortunately, tunnel transistors do not turn on well, and microprocessors using them will therefore operate slowly. This limitation becomes much worse if the supply is dropped to 0.1 Volts. This program will research a new design, the triple-heterojunction tunnel transistor. This has added semiconductor junction layers which increased the on-current by as much as 100:1. If successful, rapidly-switching microprocessors will be feasible with even a 0.07V supply, and would consume as little as 1% of the energy of today's technology.