José Menéndez
Current research interests
Experimental work in my group is carried out at the ASU Laser Facility.This facility features state-of-the-art instrumentation for optical spectroscopy, including a unique micro-Raman system based on a 2 m Double Monochromator (SOPRA).
Atoms in a solid are never at rest. (Not even when the temperature approaches the absolute zero). They vibrate about their equilibrium positions, and these vibrations can be described theoretically from a knowledge of the interatomic potential energy. Since the displacements from equilibrium are small (even when the solid is about to melt they rarely exceed one tenth of the interatomic spacing), it makes sense to expand the potential in a Taylor series in these displacements. The constant term in the series is irrelevant for the dynamics (there are colorful New Age WWW sites describing the great benefits of having "positive energy", but you know from elementary physics that when it comes to motion a constant energy can always be added arbitrarily&emdash;and with no adverse health effects). The terms linear in the displacements vanish because the forces are zero at the equilibrium position. Therefore, the first non-trivial terms in the series are those quadratic in the displacements. In the so-called "harmonic approximation" the series is truncated at this point and all higher-order terms are neglected. All physical effects which require these higher-order terms for their explanation are said to be due to "anharmonicitiy".
The vibrational frequencies calculated within the harmonic approximation are in excellent agreement with experiment in all semiconductors. Moreover, the calculations can be performed from "first principles" (with no experimental input), by cleverly applying the rules of quantum mechanics to the lattice of atoms. The density functional theory underlying this type of calculations earned Walter Kohn the 1998 Nobel Prize in Chemistry. Even though the harmonic approximation is so good, in real life there are measurable deviations from its predictions. In a harmonic world there is no thermal expansion, the vibrational frequencies are independent of any applied stress, and any particular vibration, once started, would last forever. Deviations from these predictions, can be easily measured in the laboratory and experienced in real life.
Our interest in anharmonic effects is twofold: on the one hand, we would like to find out if theory can come up with predictions for the third- and higher order terms in the expansion of the interatomic potential which are as good as the calculated quadratic terms. On the other hand, we would like to exploit anharmonicity to characterize stress fields in semiconductors by mesauring the changes in vibrational modes induced by the anharmonic terms. This characterization is very important for hte semiconductor industry. Semiconductor devices are very complicated structures with layers of different materials which excert large stresses on each other. These stresses can significantly enhance or degrade the device peformance and must be characterized in detail.
The technique of choice for our studies of anharmonicity is laser Raman spectroscopy, the inelastic scattering of light by optical vibrations (phonons). We test the accuracy of the calculated anharmonic potential terms by comparing the measured and predicted lifetime of the Raman-active phonons. When anharmonicity is present, vibrations are damped and die out. According to elementary Fourier analysis, the inverse of the lifetime is equal to the measured linewidth of the Raman peak. Measuring linewidths, however, is not easy. Our experiments take advantage of the very high spectrsal resolution of the SOPRA instrument.
Our studies of stress in semiconductors take advantage of the known stress-dependence of Raman active modes to attempt a reconstruction of the stress field from the measured spectra. We are also interested in performing stress studies with very high spatial resolution. This line of work is described below in more detail.
Recent work
M. Canonico, C. D. Poweleit, J. Menéndez, A. Debernardi, S.R. Johnson, and J.-H. Zhang, "Anomalous LO phonon lifetime in AlAs," Phys. Rev. Lett. 88 215502 (2002).
All forms of spectroscopy using visible light have a great advantage and a significant disadvantage. On the positive side, wavelengths can be measured with extraordinary accuracy, so that the different optical spectroscopies are the techniques of choice for the determination of electronic and vibrational energy levels in condensed matter. On the other hand, elementary diffraction theory states that two objects cannot be "resolved" if their separation becomes much less than the wavelength of the light used for the observation. Since the wavelength of visible light is on the order of 500 nm [1 nm (nanometer) = a billionth of a meter] this means that it is impossible to measure how things change over scales much shorter than 500 nm. It might appear that this is small enough, but you might reconsider when you compare this distance with the typical atomic separation in solids, which is of the order of 0.1-0.2 nm. There is clearly no much hope of ever seeing individual atoms with visible light, but even more mundane objects such as microprocessors are rapidly becoming inaccessible to light spectroscopy. The fast Pentium or PowerPC your computer have feature sizes as small as 175 nm (and, according to the National Roadmap for Semiconductors, 50 nm by 2012). This means that today it is nearly impossible to use Raman spectroscopy for the characterization of stress felds in these critically important devices.
In collaboration with Dr. Christian Poweleit, manager of the ASU Laser facility, we are trying to improve the spatial resolution of Raman spectrsocopy by using solid immersion lenses. The goal is to study not only inhomogeneous stresses in silicon devices but also a number of important materials and structures such as polymers, quantum dots, and nitride semiconductors alloys.We have recently been awarded a major NSF instrumentation grant to develop a solid immersion lens microscope for spectroscopic imaging.
Recent work
L. Shi, C.D. Poweleit, F.A. Ponce, J. Menéndez, and W.W. Chow, "Anisotropic diffusion and drift of photogenerated carriers near coreless dislocations in InGaN quantum well," Appl. Phys. Lett. 79, 75 (2001).
C.D. Poweleit, A. Gunther, S. Goodnick, and J. Menéndez, "Raman imaging of patterned silicon using a solid immersion lens", Appl. Phys. Lett. 73, 2275 (1998).
For several years we have concentrated on understanding the vibrational structure of fullerenes. These are caged carbon molecules which owe their names to the dome-like structures characteristic of the architectural work of Buckminster-Fuller. The most beautiful fullerene is C60, which has the shape of a truncated icosahedron (soccer ball). It was discovered in 1985, and three of its discoverers (Kroto, Curl, and Smalley) received the Nobel Prize in Chemistry. Five years later Krätschmer and Huffman (the latter a Professor of Physics at an otherwise unremarkable university south of Phoenix) discovered a procedure to make large quantitites of C60 and C70, and this lead to an explosion of research on these materials.
The unique symmetry of the C60 molecule has profound spectroscopic implications, which we have studied in detail both experimentally (using Raman spectroscop) and theoretically, in collaboration with Prof. John B. Page. Our work has led to what we believe is the most detailed and accurate picture of the molecular vibrations in C60. You can see live movies showing these vibrations here. This detailed knowledge is extremely useful for the characterization of new fullerene materials. For example, Page and collaborators have recently shown that using the Raman spectrum of C60 as a starting point, they can predict the Raman spectrum of fullerene-polymers and discriminate among several proposed structures.
The electronic structure of materials can also be probed with Raman spectroscopy by using the Resonance Raman Scattering technique, which uses tunable lasers to determine Raman intensities as a function of the laser photon wavelength. We have recently demonstrated that this approach is ideally suited to study the electronic structure of carbon nanotubes, and we have an ongoing effort in this field.
Recent work
M. Canonico, G. B. Adams, C. Poweleit, J. Menendez, J. B. Page, G. Harris, H. P. van der Meulen, J. M. Calleja, and J. Rubio, "Characterization of carbon nanotubes using Raman excitation profiles," Phys. Rev. B 65, 201402(R) (2002).
J. Menéndez and J.B. Page, "Vibrational Spectroscopy of C60," ( 5MB) in "Light Scattering in Solids VIII: Fullerenes, Semiconductor Surfaces and Coherent Phonons" edited by M. Cardona and G.Güntherodt, Berlin, 2000, pp. 27-95.
J. Menéndez "Electron-phonon interaction and Resonance Raman scattering in one-dimensioaal systems: applications to carbon nannotubes" PowerPoint format (1.2 MB), Acrobat PDF format (740K)