State-of-the-art MBE-STM Facility:
First in the Nation

During the last year, the physics department successfully brought on-line a state-of-the-art semiconductor device fabrication and characterization facility. Our new facility was officially opened by Chancellor John White during a ribbon cutting ceremony in April. The ceremony was a fantastic success. Over one hundred people attended, and several articles were written in the local city and campus papers. In addition, the ribbon cutting and several interviews were shown on television.

Our new facility is a multi-chamber stainless steel system measuring approximately twenty feet by ten feet. Within the walls of the chambers is the best vacuum one can produce on Earth, that is, an ultra-high vacuum (10-14 atmospheres of pressure). Within an ordinary room, air molecules can only travel one-billionth of a meter before colliding into another air molecule. Within the ultra-high vacuum facility an air molecule would travel several hundred miles before hitting another. To obtain such ultra-low pressures, we consume over 1000 gallons of liquid air (i.e., liquid nitrogen) per week to cool several major sections of the facility to a temperature of minus 200 degrees Celsius. Operating this complex facility requires five full-sized racks filled with sophisticated electronics, three personnel computers, and two workstations. In addition, the room was specially designed to provide a dust-free and vibration-free working environment.

There are two broad classes of semiconductors. Everyone has heard of silicon-based semiconductor devices. Silicon electronics make up the chips inside our computers and represents a trillion dollar a year industry. The primary manner in which silicon devices are manufactured is by using a particle accelerator to implant ions into a crystal of silicon. With the aid of magnetic steering fields the ions can be positioned at any three dimensional point inside the silicon. This technique allows millions of transistors to be "written" into a single chip.

Our new semiconductor fabrication facility focuses on another class of semiconductors called III-V compound semiconductors (examples include GaAs, InP, GaN). These semiconductor devices have very different properties when compared to silicon. Their development has resulted in new technologies that would be impossible with silicon. For example, every compact-disc (CD) player uses a III-V laser to read the information off the disc. Fiber optic communications uses lasers and detectors made from the III-V semiconductors. Cellular phones, direct satellite TV, and global positioning systems all are possible because of III-V semiconductors.

Unlike silicon-based devices, which are produced primarily by ion implantation techniques, the III-V device structures must be formed by depositing one plane of atoms after another onto a crystal surface until the entire device structure is "grown." This fabrication technique is a form of vapor phase growth called molecular beam epitaxy (MBE), and it is precise enough to allow the deposition of material to be controlled down to less than a single plane of atoms. We can now fabricate any arrangement of atoms we desire, literally allowing us to artificially produce crystals that would never occur in nature. For example, we can produce layers so thin that the properties of the electrons trapped inside these layers are governed by quantum mechanics. Since, the quantum behavior of electrons is so unusual, there is a tremendous opportunity to produce a new class of electronic devices which have been termed multi-functional devices. That is, the electronic characteristics of a single multi-functional device duplicate the properties of over one hundred conventional electronic devices. This necessarily leads to faster, lower-power consumption electronics.

Because compound semiconductor device fabrication occurs solely at a semiconductor surface, the better one can control and manipulate the motion of atoms on surfaces, the more sophisticated the device structures one can achieve. Consequently, there is a critical need for atomic-scale characterization of the device structure during the fabrication process. To address this need, the Physics Department at the University of Arkansas has combined, for the first time, the state-of-the-art in III-V device fabrication (gallium arsenide, for example, is a III-V material) using MBE with the powerful atomic-scale surface characterization of scanning tunneling microscopy (STM). This is a relatively new characterization technique that utilizes quantum mechanical tunneling to produce the world's most powerful microscope, one that can image individual atoms. Since atoms are the smallest building blocks of semiconductor devices, the ability to image them represents the ultimate and final step in structural characterization. Our department is the first in the nation to combine the state-of-the-art in III-V device fabrication in the important area of telecommunications with the state-of-the-art in atomic-scale characterization.

Paul Thibado, a new physics faculty member (see his Faculty Profile in Reflections for Spring 1997 edition), played an important role in bringing the unique facility on-line. His research interests span from the basic physics of atoms moving on surfaces to making the next generation of high speed transistors. During the last year he won several major research grants, awarded by the National Science Foundation, Office of Naval Research, Research Corporation, and the Arkansas Science and Technology Authority which total almost 1.5 million dollars. Thibado also won the prestigious National Science Foundation-Career award. This is a very competitive program designed to identify the nation's most promising young scientists. The Career grant focuses on developing our understanding of how individual atoms move on a material surface. This is one of the most basic bits of information required to fully understand the device fabrication process.

The significance of this facility for our students, faculty, and community cannot be overstated. By fabricating devices in a manner identical to companies, our school is a training ground for the future scientists working in this industry. In addition, faculty can now directly interact with industry on research issues that will impact the companies' productivity. Lucent technologies (formerly AT&T Bell Labs) has agreed to form a partnership with our research effort. We have also formed a research partnership with the Army Research Laboratory (ARL), where we will manufacture a new device that can allow ultra-fast optical switching. The possibility of this interaction leading to a manufacturing business located in the Fayetteville area is very high. Never before has the basic physics of atoms moving on surfaces been so closely tied to industry as it is with next generation of devices.*