Bruce D. McCombe

Professor of Physics

Office : 138 A Fronczak Hall & 237 Fronczak Hall ; Lab : 142 Fronczak Hall.
Phone : (716)645-2389 or (716)645-2017
Fax : (716)645-2507
email : mccombe@acsu.buffalo.edu
See also: McCombe Lab Home Page

BRIEF BIOGRAPHY

Professor McCombe was born (longer ago than Professor Gonsalves) in a small town (Sanford) in the Grand State of Maine. He attended undergraduate school at Bowdoin College (A.B. 1960) and graduate school at Brown University, obtaining the PhD degree officially in 1966. Continuing his quest for warmer weather he accepted an NAS/NRC postdoctoral position at the Naval Research Laboratory, Washington, D.C. in December 1965, and after wending his way through various positions of increasing responsibility, finally "floated up" to the position of Superintendent of the Electronics Technology Division in 1979. Two years of this position, combined with a surfeit of both hot and humid weather and the federal bureaucracy, led him to accept a position as Professor of Physics at what was then SUNY at Buffalo (and is now University at Buffalo, the State University of New York) in 1982. At that time he was blessedly unfamiliar with the bureaucracy in the state of New York, but in the intervening years has unfortunately become progressively more intimate with this entity. He is a Fellow of the American Physical Society. For reasons that are presently difficult to recall, he took the position of Chairman of the Department of Physics in 1987 (it was at that time called the Department of Physics and Astronomy), and he remained in that position till 1996. He is presently the Director of the Center for Advanced Photonic and Electronic Materials (CAPEM) and is the Associate Dean for Research and Sponsored Programs in the College of Arts and Sciences (another baffling decision). During the past year he became a SUNY Distinguished Professor. Even at his advanced age, Professor McCombe occasionally indulges in one of his favorite physical pastimes, playing basketball.

RESEARCH INTERESTS

Professor McCombe is primarily interested in the basic physics and applications of semiconductor nanostructures and spin effects in semiconductors.

A semiconductor nanostructure is usually some combination of semiconductors in layered form created by fancy (and expensive) growth methods (See Professor H. Luo), which may be further patterned in one or both of the lateral dimensions either lithographically or by growth techniques. The characteristic dimensions of such structures lie in the range of 1 to 100 nanometers (thus the name). Since these dimensions are comparable to or less than the characteristic wavelength (the deBroglie wavelength) of charge carriers in semiconductors, the resulting structures confine electrons or holes quantum mechanically in one or more directions. This confinement (sounds vaguely criminal, doesn't it?) leads to electronic behavior that is characterized as quasi-two- dimensional, quasi-one-dimensional or quasi-zero-dimensional, and the confining structures are often called quantum wells, quantum wires and quantum dots, respectively. The lowered dimensionality leads to some very interesting behavior, some of which is well-understood and has led to applications such as quantum-well lasers and detectors, quantum cascade lasers, and high electron mobility transistors. Other areas, including electron-electron interactions and spin effects (see below) are not as well understood. We're interested in contributing to understanding the latter and in devising new applications of these structures.

Understanding spin effects in semiconductors is important in the context of the rapidly developing field of Spintronics. In very simple terms conventional electronics is concerned with manipulating the charge of the electron via (usually) electric fields to perform various useful functions (e.g., logic, memory, amplification, etc.). Generally speaking, Spintronics can be described as activities directed at manipulating another intrinsic property of the electron, its spin, to perform improved, or entirely new functions. Producing, characterizing and understanding the basic physics of ferromagnetic semiconductors (particularly materials like GaMnAs, GaMnSb and GaMnAs), as well as fabricating and studying device building blocks, are all areas of interest in Professor McCombe's laboratory (in collaboration with Professor H. Luo and others in the Department). This work is presently part of the research activities of a large consortium funded by a DARPA/ONR SpinS grant; UB is the lead institution.

Within these rather broad and rapidly expanding areas his specific interests are: electron-electron and electron-hole interactions, particulary for magneto-excitons and charged magneto-excitons in quantum wells and quantum dots; how reduced dimensionality affects the electron-optical phonon interaction; vibrational modes in nanoparticles; optical, infrared and far infrared properties of Mn in III-V semiconductors; and magnetic and magneto-transport properties of Ferromagnetic semiconductors.

Professor McCombe employs visible, near infrared and far infrared spectroscopic techniques and electrical transport techniques at low temperatures and high magnetic fields to obtain information relevant to the above interests.

PRESENT RESEARCH ACTIVITY

  1. We are studying the coupling of the single particle energy states across the heterointerface and the (Coulomb) interactions between spatially separated electrons and holes in the InAs/AlxGa1-xSb heterostructure system. This is a unique semiconductor heterostructure system in which the valence band maximum of AlxGa1-xSb alloys can lie higher than the conduction band minimum of InAs for a certain range of alloy compositions. As a function of x this system makes a transition from semiconducting behavior (x > 0.3) in which the valence band of AlxGa1-xSb lies below the conduction band in InAs to a semimetallic behavior (x < 0.3) in which the bands overlap. In either case holes are confined in the AlGaSb barriers, spatially separated from the electrons, which are confined in the InAs wells. We have recently discovered a new absorption line in far infrared magnetooptical studies of single InAs quantum wells in the InAs/AlxGa1-xSb (for x < 0.3) system, which we believe provides evidence an exciton-like state in the presence of many electrons. This state involves the binding together of electrons (in the InAs) and holes (in the AlGaSb) by their mutual Coulomb attraction in a lower energy state than that obtained if the electrons and holes are presumed to remain unbound in their free carrier, single particle states. We continue to study this fascinating system.
    We are also studying the effects of coupling of the conduction band states in InAs to the valence band states in GaSb across the heterointerface in InAs/GaSb single quantum-well structures. We are employing cyclotrons resonance to probe these states in a magnetic field, and have recent results that show strong differences between interfaces containing In-Sb bonds relative to interfaces containing Ga-As bonds.


  2. Optically Detected Resonances (ODR) in semiconductor nanostructures (in collaboration with Professor A. Petrou). We have recently developed a technique that combines far infrared and near infrared/visible spectroscopies. This is based on earlier work in the microwave region in which resonant absorption at the cyclotron frequency was shown to modulate the photoluminescence (PL) of semiconductors in the frequency region near that corresponding to the fundamental energy gap. This modulation was sufficiently sensitive that cyclotron resonance could be detected as changes in the PL intensity. Others have also extended this technique to the far infrared through the use of far infrared lasers, but we have recently shown that in GaAs/AlGaAs quantum-well structures this technique can also be used to detect sensitively the resonances of shallow impurities. This technique has a very wide range of applications, from simply determining the effective masses of charge carriers in samples that are not intentionally doped with impurities to unraveling some of the complex recombination mechanisms of photo-induced electrons and holes. We have been focusing recently on studying internal transitions of neutral and charged magneto-excitons in GaAs/AlGaAs quantum wells and the effects of excess electrons on these transitions. The recent results include the first observation of internal transitions of negatively charged excitons and the blue shift of the bands that result from the introduction of excess electrons. We will be extending these studies to internal transitions of excitons in single lateral-fluctuation quantum dots in the GaAs system.


  3. One of our efforts in spin effects in semiconductors is focusing on studying the magnetotransport properties of so-called digital alloys of GaAs/Mn, GaSb/Mn InAs/Mn and InSb/Mn in magnetic fields up to 33 T (at the National High Magnetic Field Laboratory at Florida State University) and at temperatures between 1.5 K and 400 K. Measurements up to 17 T are carried out in Buffalo. Digital alloys are formed by a combination molecular beam epitaxy and atomic layer epitaxy. Sub-monolayers of Mn are inserted between layers of As or Sb and separated by multiple layers of GaAs, GaSb, InAs, or InSb in a superlattice configuration. The materials are grown by Professor Luo and his students here and collaborators at Notre Dame University. These materials are particularly interesting because recently Professor Luo and our group in collaboration with the UND group and a group at the Pennsylvania State university have shown that they are ferromagnetic at room temperature. We have developed a model of this behavior and are pursuing various structural, magnetic and electrical transport studies to determine the basic physics underlying these results and how they can be used in potential spin polarizers operating at room temperature. Our own work is focusing on understanding the interesting magnetotransport properties, the anomalous Hall effect and the magnetoresistance and what can be learned from such measurements about the underlying spin states.


  4. We are also studying the optical properties of these materials in the hopes of determining how Mn is incorporated into the structure as well as what the frequency dependent conductivity can tell us about the role of the carriers in these materials. These measurements are carried out from the far infrared to the visible through the use of our Fourier transform spectrometer and optically pumped far infrared laser systems. Studies are underway of epitaxial layers of GaAs:Mn (grown at high temperatures, where Mn is known to predominantly occupy the Ga sites substitutionally) and also ferromagnetic random alloys (grown at low temperatures) for comparison. We are also investigating the digital alloys and comparing the dc conductivity results with the ac conductivity measured in the far infrared and also determined by pulsed THz spectroscopy in Professor Markelz's laboratory. Finally, we are employing far infrared laser cyclotron resonance to probe the sign of carriers in the digital alloy materials as a characterization tool.


  5. We have recently begun studies of nanoparticles of GaP produced by novel chemical means in Professor Prasad's laboratory. We are probing the lattice vibrational properties of these nanoparticles, and intend to correlate these properties, particularly shifts in the optical modes and activation of other modes due to the confinement, with other measurements. Eventually we hope to be able to probe the electronic atom-like states of the nanoparticles and to investigate the effects of incorporation of transition metal ions. This work is part of a large AFOSR DURINT grant (Professor Prasad is the PI).


EXPERIMENTAL FACILITIES

Professor McCombe's laboratory is equipped to carry out a wide range of spectroscopic and electrical transport studies. Major equipment includes: