Professors Ramdas and Rodriguez Symposium
Spectroscopy of Semiconductors: Science and Technology - October 21-22, 2000
To recognize the scientific contributions of Professor Anant K. Ramdas and Professor Sergio Rodriguez and to underscore Purdue University's role in the physics of semiconductors.
8:00-9:00Saturday, 21 October 2000
Continental Breakfast, Physics Building, Room 242
(Juice, coffee, and bagels will be served).
9:00-10:20
SESSION I, Physics Bldg., Room 223
Marcos Grimsditch, Session Chair
9:00-9:05
Welcome, Andrew S. Hirsch, Department of Physics, Purdue University
9:05-9:25
Elias Burstein, Department of Physics, University of Pennsylvania
Resonant Inelastic Light Scattering by Intersubband Electronic Excitations Revisited
In 1978, we suggested that it should be possible to observe resonant inelastic light scattering (ILS) by 2D plasmas via two-step and three-step processes at energy gaps where the interband transitions involve carrier occupied states. Shortly afterwards, Abstreiter et al and Pinczuk et al reported the observation of resonant ILS by intersubband spin-flip and non-spin-flip excitations in GaAs/AlGaAs heterostructures using photon energies in resonance with the E0 + ∆0 gaps. On the other hand, Ching et al subsequently reported the observation of ILS coupled non-spin-flip intersubband excitation-LO phonon modes using photon energies in resonance with the E1gap of InAs2. They did not observe ILS by spin-flip intersubband excitations. We now understand the reason for this. The macroscopic electric field, which is the origin of the coupling between the non-spin-flip intersubband excitations with LO phonons, can scatter the virtual electron-hole pairs in the intermediate state at the E1 gap, or at any gap in the Brillouin zone, via the direct Coulomb interaction (i.e., via Froehlich and Franz-Keldysh mechanisms which have large matrix elements). The exchange Coulomb interaction of the intermediate state electron-hole pairs with the spin-flip and non-spin-flip intersubband excitations, on the other hand, requires a spatial overlap of the wave functions of the interband electron-hole pair intermediate states at the optical gap with the intersubband excitations. In the case for resonanant ILS at an E1 gap of direct gap III-V semiconductors, the carriers involved in the intersubband excitations occupy states near the zone boundary. As a consequence, the contributions of the exchange Coulomb interaction to the scattering cross-section of the resonant ILS at the E1 gap by non-spin-flip and spin-flip intersubband excitations are quite small.
9:30-9:50
R.L. Aggarwal, MIT Lincoln Laboratories
Temperature Dependence of the Breakdown Voltage for Reverse-Biased GaN p-n-n+Photodiodes
B, due to impact ionization at high electric fields, for reverse-biased GaN P-n-n+ diodes has been measured at temperatures T between 98 and 248 K. The observed increase of VB with T is in excellent agreement with a simple model for the scattering of carriers by phonons with an effective energy of 42 meV. The impact ionization coefficeints for the electrons and holes are nearly equal, and their geometric mean has the value 4x104 cm-1 at the breakdown electric fields, in good agreement with recent theoretical results for 300 K.
9:55-10:15
Manuel Cardona, Max-Planck-Institut für Festkörperforschung
Anant, Sergio, and Me: A Tightly Bound Pair Weakly Coupled to a Singlet. A Paradigm in Scientific and Personal Interaction
For about forty years, I have interacted with the Anant-Sergio pair, in Princeton, Stuttgart, Providence, Santiago de Chile, at Purdue and probably elsewhere. The interaction was not only scientific but also at the personal and political level. Several examples of this interaction, ranging from the physics of semiconductors to cutthroat South-American science politics, will be discussed.
10:20-10:50 Coffee Break
10:50-12:00
SESSION II, Physics Building, Room 223
H.R. Chandrasekar, Session Chair
10:50-11:10
Earl W. Prohofsky, Department of Physics, Purdue University
Why My Friends Won't do Biological Physics
Condensed matter physics is often done on extremely well characterized samples. Good experimenters can obtain spectra with well defined sharp lines and theorist can compute observables which can be compared to experiment that often achieve excellent agreement. Things are not so simple in biological physics the way it is practiced and this can be a detriment to attracting good scientists to the field. Is this the fault of the way the field is practiced?
11:15-11:35
Stuart A. Solin, NEC, Princeton, NJ
Extraordinary Magnetoresistance in Narrow-Gap Semiconductors: Fundamental Physics and Practical Applications
Current interest in materials for magnetic sensors has been almost exclusively focused on MAGNETIC structures, such as spin valves, which exhibit giant magnetoresistance (GMR) and layered oxides which exhibit colossal MR (CMR). But basic research on nonlinear transport in semiconductor superlattices [1,2] led to the realization that narrow (zero)-gap NON-MAGNETIC semiconductors may be competitive/superior materials for a number of magnetic sensor applications. [3] Recently, we have measured, in narrow-gap simiconductor/non-magnetic metal composites, the largest MR yet reported for any system at room temperature. [4] We call this extraordinary MR or EMR. Room temperature EMR as high as 100%, 9,100% and ~750,000% at fields of 0.05, 0.25 and 4.0T, respectively have been obtained in non-biased structures Following a brief introduction to the physics of narrow-gap semiconductors, recently discovered novel phenomena such as EMR and self-biasing [5] will be described and explained. Finally, the potential impact of these new phenomena on magnetic sensor technology will be addressed.
[1] M. Lee, S.A. Solin and D.R. Hines, Phys. Rev., B48, 11,921 (1993)
[2] G.T. Seidler, S.A. Solin and A.C. Marley, Phys. Rev.Lett.76, 3049 (1996)
[3] Tineke Thio and S.A. Solin, Appl. Phys. Lett, 72, 3497 (1998)
[4] S.A. Solin, Tineke Thio, D.R. Hines and J.J. Heremans, Science, in press.
[5] S.A. Solin, Tineke Thio, J.W. Bennett, D.R. Hines, M. Kawano, N. Oda, and M. Sano Appl. Phys. Lett. 69, 4105(1996)
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11:40-12:00
Supriyo Datta, School of Electrical Engineering, Purdue University
Molecular Electronics: A new Frontier in "Semiconductor" Physics
Several experimental groups have recently reported measurements of the current-voltage (I-V) characteristics of individual or small groups of molecules. Particularly exciting is the demonstration of an abrupt voltage-induced switching from one current level to another in certain molecules. Such phenomena raise the possibiltiy that with improved understanding molecules can eventually be designed to exhibit desired I-V characteristics. In this talk I will review our current state of understanding emphasizing the similarities and differences with semiconductor physics.
12:00-1:30 Lunch, Purdue Memorial Union, La Posada del Sol, Ground Floor
1:30-3:20
SESSION III, Physics Building, Room 223
Hyunjung Kim, Session Chair
1:30-1:55
Franco Bassani, Scuola Normale Superiore, Pisa, Italy
Coherence and Interference in Optical Transitions
Interference effects in the optical transition amplitude are briefly recalled in linear and nonlinear optics. In particular, electromagnetically induced transparency in cuprous oxide (Cu2O) will be described. Implications for a modification of the Fresnel-Fizeau drag effect in a moving medium will be discussed.
2:00-2:30
George D. Watkins, Department of Physics, Lehigh University
Intrinsic Defects in Semiconductors: From Lark-Horowitz to Today
What we think we now know about vacancies and interstitials in semiconductors will be discussed, as will the long tortuous journey it took to get there. Remaining challenges will also be presented.
2:35-2:55
Harald Pascher, University of Bayreuth, Germany
Coherent Raman Spectroscopy of CdTe/MnTe Short Period Superlattices
A brief introduction in the basic principles of coherent Raman spectroscopy will be given. Results will be presented on the application of the method on short period (CdTe)m/(MnTe)n superlattices. As in spontaneous Raman scattering spin splittings of de-localized band carriers as well as of electrons localized to magnetic ions can be observed. The analysis of the data yields information on magnetic ordering, particularly at the heterointerfaces.
3:00-3:20
David G. Seiler, NIST, Washington, D.C.
Optical Spectroscopies for Semiconductor Manufacturing
To successfully construct semiconductor devices, the microelectronics industry must measure fundamental material parameters, especially when developing new materials; measure the quality of the material as it is grown; accurately determine the structural details of thin films, quantum wells, and other microstructures at the heart of devices; and measure properties of the devices themselves. Contactless, nondestructive optical methods and optical spectroscopies are used to characterize many of these critical properties of materials, processes, and devices in the semiconductor industry. We present the results of an industrial survey that determined the extent of use and relative importance of various optical methods/spectroscopies in industry. The survey also sought to identify both advantages and limitations of these methods as well as future requirements for optical characterization methods.
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3:25-3:50 Coffee Break
3:50-5:10
SESSION IV, Physics Building, Room 223
Gusty Feyder, Session Chair
3:50-4:10
Mark Lundstrom, School of Electrical Engineering, Purdue University
Simple Physics of Nanotransistors
4:15-4:35
Roberto Colella, Department of Physics, Purdue University
Multiple Bragg Scattering and the Phase Problem. Applications to Quasicrystals and to Resonant Scattering
Multiple Bragg scattering, a situation in which two or more Bragg reflections are excited at the same time, is a source of phase information. Applications to quasicrystals will be presented. The general problem of centrosymmetry in a quasicrystal, or lack of it, will be discussed, along with experimental results. Recently, this technique has been used to determine the phase of resonant scattering. An example will be shown for the case of germanium.
4:40-5:10
Giuseppe C. LaRocca, Scuola Normale Superiore, Pisa and University of Salerno, Italy
Microcavity Polariton Scattering
Semiconductor microcavities in which a light Fabry-Perot mode is resonant with a quantum well exciton transition have recently attracted much interest. In the strong coupling regime the eigenmodes of the system are cavity polariton, i.e., a linear combination of the light and the exciton modes. The linewidth of a cavity polariton is affected in a complex way by processes, which broaden either the Fabry-Perot mode or the quantum well exciton. The scattering of cavity polaritons on well width fluctuations will be considered in detail.
6:30-7:00 RECEPTION, University Inn and Conference Center, 3001 Northwestern Avenue, West Lafayette
7:00 DINNER, University Inn and Conference Center
8:00-9:00
SUNDAY, 22 OCTOBER 2000
Continental Breakfast, Physics Building, Room 242
(Juice, coffee, and bagels will be served.)
9:00-10:10
SESSION V, Physics Building, Room 223
S. Venugopalan, Session Chair
9:00-9:20
Eugene E. Haller, Lawrence Berkeley Laboratory, and UC, Berkeley
Physics with Isotopically Controlled Semiconductors
The end of the Cold War provided new possibilities for collaborations on isotopically controlled semiconductors between scientists of FSU and Western countries. Isotopically controlled structures of a significant number of semiconductors, including Si, Ge, GaAs, GaP and GaSb have been used in these studies. A few prominent results will be reviewed in this talk.
9:25-9:45
John J. Quinn, Department of Physics, University of Tennessee
Fractionally Charged Excitons and Luminescence in Quantum Hall Systems Containing a Valence Band Hole
The energy spectrum of a system containing N electrons confined to a plane z=0 interacting with a valence band hole confined to a plane z=d in the presence of a large magnetic field in the z-direction is studied numerically as a function of filling factor v and layer separation d. For d small compared to the magnetic length, the hole binds one or more electrons to form a neutral (X0) or charged (X-) exciton. The low lying states can be understood in terms of a generalized composite fermion picture, and the photoluminescence (PL) spectrum probes these excitonic states, not the correlations in the N electron system. For larger values of d the electron-hole interaction is too weak to bind an electron. Instead fractionally charged excitons (FCX), consisting of one or more Laughlin quasielectrons bound to the valence band hole, can occur. The PL spectra of these states do contain useful information about correlations in the N electron system.
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9:50-10:10
Chul Koo Kim, Department of Physics, Yonsei University, Seoul, Korea
A Variational Perturbation Theory for Strongly Correlated Electron Systems
A variational perturbation theory is formulated for strongly correlated electron systems. Using the variational action obtained through Jensen-Peierl's inequality, a perturbative expansion scheme for the thermodynamic potential of interacting electron systems is established. A modified Wick's theorem allows one to carry out systematic calculations of higher order terms without worrying about the double counting problem.
10:15-10:45 Coffee Break
10:45-12:30
SESSION VI, Physics Building, Room 223
K.K. Bajaj, Session Chair
10:45-11:05
David Nolte, Department of Physics, Purdue University
Time-Reversal in Holographic Adaptive Optics
Time is perhaps the most enigmatic of physical coordinates. It is the only coordinate that can have only one direction of motion--into the future. But time also enters symmetrically into all classical laws, allowing time-reversed motion to be indistinguishable (microscopically) from time-forward motion.
The equations of electrodynamics are no exception to time symmetry, and optics provides one of the best backdrops for dramatic demonstrations of the physics of classical time reversal. For instance, time-reversed light can be readily generated through nonlinear optics in a process called optical phase conjugation. In this process three coherent laser beams interact inside a nonlinear optical medium to generate a phase conjugate laser beam that has properties closely analogous to the time-reversed properties of one of the original beams.
We are testing the validity of the analogy between time reversal and nonlinear phase conjugation. By using magnetic fields to remove time-reversal symmetry in the solid state, we can identify which aspects of the time-reversal analogy hold deeply, and which are merely superficial.
Our exploration of time in the context of optics has also moved into the time-domain, in which we can directly manipulate time within picosecond optical pulses. In a nonlinear optical process called spectral holography, an optical pulse can be made to jump forward or jump backward in time by many times its own pulse duration. This surprising behavior shakes our intuitive expectation of immutable time, but can be explained through the simple imprinting of linear phase onto the spectrum of the optical pulse.
11:10-11:30
Miles Klein, Department of Physics, University of Illinois
Excitons Created by Scattering Photons in Insulators
Excitons are usually associated with allowed optical transitions across a band gap, wherein the interaction between the electron and hole gives a bound state below the gap. Here we ask and partially answer whether they can be detected by scattering photons at energy shifts in the vicinity of a band gap. The materials for which results are available represent the insulating parent compounds of the cuprate high temperature superconductors.
11:35-11:55
Sunil Sinha, Advanced Photon Source, Argonne National Laboratory
New Opportunities for Lattice Dynamics of Semiconductors Using Synchrotron Radiation
12:25-12:40 Closing Remarks, Andrew S. Hirsch
Photographs
Prof. Ramdas
Prof. Rodriguez
Professors Hirsch, Ramdas and Rodriguez
