R. J. Nemanich

Arizona State University, Phoenix, Arizona, United States

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Publications (452)752.85 Total impact

  • Sean W. King, Robert J. Nemanich, Robert F. Davis
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    ABSTRACT: The Schottky barrier and interfacial chemistry for interfaces formed by evaporation of Sc onto 3C-SiC (111)-(1x1) surfaces at 600 °C has been investigated using in situ X-ray and ultra-violet photoelectron spectroscopy (XPS and UPS) and low energy electron diffraction (LEED). Sc was observed to grow in a two-dimensional manner and exhibit a (1x1) LEED pattern up to thicknesses of ∼2 nm beyond which diffraction patterns were no longer observable. XPS measurements of these same films showed a clear reaction of Sc with the 3C-SiC (111)-(1x1) surface to form a ScSix and ScCx interfacial layer in addition to the formation of a metallic Sc film. XPS measurements also showed the deposition of Sc induced ∼0.5 eV of upward band bending resulting in a Schottky barrier of 0.65 ± 0.15 eV.
    physica status solidi (b) 10/2014; · 1.49 Impact Factor
  • M. C. Zeman, R. J. Nemanich, A. Sunda-Meya
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    ABSTRACT: The growth and coarsening dynamics of dysprosium silicide nanostructures are observed in real-time using photoelectron emission microscopy. The annealing of a thin Dy film to temperatures in the range of 700–1050 °C results in the formation of epitaxial rectangular silicide islands and nanowires on Si(001) and triangular and hexagonal silicide islands on Si(111). During continuous annealing, individual islands are observed to coarsen via Ostwald ripening at different rates as a consequence of local variations in the size and relative location of the surrounding islands on the surface. A subsequent deposition of Dy onto the Si(001) surface at 1050 °C leads to the growth of the preexisting islands and to the formation of silicide nanowires at temperatures above where nanowire growth typically occurs. Immediately after the deposition is terminated, the nanowires begin to decay from the ends, apparently transferring atoms to the more stable rectangular islands. On Si(111), a low continuous flux of Dy at 1050 °C leads to the growth of kinked and jagged island structures, which ultimately form into nearly equilateral triangular shapes.
    Journal of Materials Science 02/2014; 49(4). · 2.31 Impact Factor
  • Sean W. King, Robert J. Nemanich, Robert F. Davis
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    ABSTRACT: In order to understand and predict the behavior of future scandium nitride (ScN) semiconductor heterostructure devices, we have utilized in situ x-ray and ultra-violet photoelectron spectroscopy to determine the valence band offset (VBO) present at ScN/3C-SiC (111) and 2H-GaN (0001)/ScN (111) interfaces formed by ammonia gas source molecular beam epitaxy. The ScN/3C-SiC (111) VBO was dependent on the ScN growth temperature and resistivity. VBOs of 0.4 ± 0.1 and 0.1 ± 0.1 eV were, respectively, determined for ScN grown at 925 °C (low resistivity) and 800 °C (high resistivity). Using the band-gaps of 1.6 ± 0.2 and 1.4 ± 0.2 eV previously determined by reflection electron energy loss spectroscopy for the 925 and 800 °C ScN films, the respective conduction band offsets (CBO) for these interfaces were 0.4 ± 0.2 and 0.9 ± 0.2 eV. For a GaN (0001) interface with 925 °C ScN (111), the VBO and CBO were similarly determined to be 0.9 ± 0.1 and 0.9 ± 0.2 eV, respectively.
    Applied Physics Letters 01/2014; 105(8):081606-081606-4. · 3.79 Impact Factor
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    ABSTRACT: Nitrogen-doped diamond has been under investigation for its low effective work function, which is due to the negative electron affinity (NEA) produced after surface hydrogen termination. Diamond films grown by chemical vapor deposition (CVD) have been reported to exhibit visible light induced electron emission and low temperature thermionic emission. The physical mechanism and material-related properties that enable this combination of electron emission are the focus of this research. In this work the electron emission spectra of nitrogen-doped, hydrogen-terminated diamond films are measured, at elevated temperatures, with wavelength selected illumination from 340 nm to 450 nm. Through analysis of the spectroscopy results, we argue that for nitrogen-doped diamond films on metallic substrates, photo-induced electron generation at visible wavelengths involves both the ultra-nanocrystalline diamond and the interface between the diamond film and metal substrate. Moreover, the results suggest that the quality of the metal-diamond interface can substantially impact the threshold of the sub-bandgap photo-induced emission.
    Diamond and Related Materials. 01/2014;
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    ABSTRACT: In this research, Al2O3 films were grown by remote plasma-enhanced atomic layer deposition using a nonpyrophoric precursor, dimethylaluminum isopropoxide (DMAI), and oxygen plasma. After optimization, the growth rate was determined to be ∼1.5 Å/cycle within a growth window of 25–220 °C; the higher growth rate than reported for thermal atomic layer deposition was ascribed to the higher reactivity of the plasma species compared with H2O and the adsorption of active oxygen at the surface, which was residual from the oxygen plasma exposure. Both effects enhance DMAI chemisorption and increase the saturation density. In addition, a longer oxygen plasma time was required at room temperature to complete the reaction and decrease the carbon contamination below the detection limit of x-ray photoemission spectroscopy. The properties of the subsequent Al2O3 films were measured for different temperatures. When deposited at 25 °C and 200 °C, the Al2O3 films demonstrated a single Al-O bonding state as measured by x-ray photoemission spectroscopy, a similar band gap of 6.8±0.2 eV as determined by energy loss spectroscopy, a similar index of refraction of 1.62±0.02 as determined by spectroscopic ellipsometry, and uniform growth with a similar surface roughness before and after growth as confirmed by atomic force microscopy. However, the room temperature deposited Al2O3 films had a lower mass density (2.7 g/cm3 compared with 3.0 g/cm3) and a higher atomic ratio of O to Al (2.1 compared with 1.6) as indicated by x-ray reflectivity and Rutherford backscattering spectroscopy, respectively.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2014; 32(2):021514-021514-9. · 1.43 Impact Factor
  • Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 01/2014; 32(1):011203-011203-6. · 1.36 Impact Factor
  • Diamond and Related Materials. 01/2014;
  • Jialing Yang, Brianna S. Eller, Robert J. Nemanich
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    ABSTRACT: The effects of surface pretreatment, dielectric growth, and post deposition annealing on interface electronic structure and polarization charge compensation of Ga- and N-face bulk GaN were investigated. The cleaning process consisted of an ex-situ wet chemical NH4OH treatment and an in-situ elevated temperature NH3 plasma process to remove carbon contamination, reduce oxygen coverage, and potentially passivate N-vacancy related defects. After the cleaning process, carbon contamination decreased below the x-ray photoemission spectroscopy detection limit, and the oxygen coverage stabilized at ∼1 monolayer on both Ga- and N-face GaN. In addition, Ga- and N-face GaN had an upward band bending of 0.8 ± 0.1 eV and 0.6 ± 0.1 eV, respectively, which suggested the net charge of the surface states and polarization bound charge was similar on Ga- and N-face GaN. Furthermore, three dielectrics (HfO2, Al2O3, and SiO2) were prepared by plasma-enhanced atomic layer deposition on Ga- or N-face GaN and annealed in N2 ambient to investigate the effect of the polarization charge on the interface electronic structure and band offsets. The respective valence band offsets of HfO2, Al2O3, and SiO2 with respect to Ga- and N-face GaN were 1.4 ± 0.1, 2.0 ± 0.1, and 3.2 ± 0.1 eV, regardless of dielectric thickness. The corresponding conduction band offsets were 1.0 ± 0.1, 1.3 ± 0.1, and 2.3 ± 0.1 eV, respectively. Experimental band offset results were consistent with theoretical calculations based on the charge neutrality level model. The trend of band offsets for dielectric/GaN interfaces was related to the band gap and/or the electronic part of the dielectric constant. The effect- of polarization charge on band offset was apparently screened by the dielectric-GaN interface states.
    Journal of Applied Physics 01/2014; 116(12):123702-123702-12. · 2.21 Impact Factor
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    ABSTRACT: Hydrogen terminated, nitrogen doped diamond thin films have been the focus of recent research for application in thermionic energy conversion devices and possibly in solar cells. Nitrogen doped diamond films can attain negative electron affinity (NEA) through treatment with hydrogen plasma, which also produces a very low work function surface. Photoemission and thermionic emission spectroscopy measurements confirm a work function of approximately 2 eV for such films. The research presented here includes results from imaging these thin films with photo-electron emission microscopy (PEEM) and thermionic electron emission microscopy (ThEEM), in addition to spectroscopic studies using ultraviolet photoelectron spectroscopy (UPS). From the images it can be concluded that the photo- and thermionic emission are spatially uniform and do not originate from different isolated emission sites. This observation holds true up to the highest resolution and for all temperatures investigated (300–800 K). While relatively uniform, the emission is found to be influenced by the surface morphology and film microstructure. The spatial intensity distributions of the PEEM and ThEEM images are very similar, as reflected by the structure present in both of these images. This observation indicates that both emission processes are enabled by the low work function of the film.
    Diamond and Related Materials 01/2013; 40:12–16. · 1.71 Impact Factor
  • Xin Liu, Sean W. King, Robert J. Nemanich
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    ABSTRACT: The thermal stability of 7 nm Ti, Pt, and Ru interfacial adhesion layers between Cu film (10 nm) and a Ta barrier layer (4 nm) has been investigated. The barrier properties and interfacial stability have been evaluated by Rutherford backscattering spectrometry (RBS). Atomic force microscopy was used to measure the surfaces before and after annealing, and all the surfaces are relatively smooth which excludes islanding or dewetting phenomena as a cause of the instability. The RBS showed no discernible diffusion across the adhesion layer/Ta and Ta/Si interfaces which provides a stable underlying layer. For a Ti interfacial layer, RBS indicates that during 400 °C annealing, Ti interdiffuses through the Cu film and accumulates at the surface. For the Pt/Cu system, Pt interdiffusion is detected which is less evident than Ti. Among the three adhesion layer candidates, Ru shows negligible diffusion into the Cu film indicating thermal stability at 400 °C.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2013; 31(2):2205-. · 1.36 Impact Factor
  • Brianna S. Eller, Jialing Yang, Robert J. Nemanich
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    ABSTRACT: GaN and AlGaN have shown great potential in next-generation high-power electronic devices; however, they are plagued by a high density of interface states that affect device reliability and performance, resulting in large leakage current and current collapse. In this review, the authors summarize the current understanding of the gate leakage current and current collapse mechanisms, where awareness of the surface defects is the key to controlling and improving device performance. With this in mind, they present the current research on surface states on GaN and AlGaN and interface states on GaN and AlGaN-based heterostructures. Since GaN and AlGaN are polar materials, both are characterized by a large bound polarization charge on the order of 1013 charges/cm2 that requires compensation. The key is therefore to control the compensation charge such that the electronic states do not serve as electron traps or affect device performance and reliability. Band alignment modeling and measurement can help to determine the electronic state configuration. In particular, band bending can determine how the polarization bound charge is compensated; however, the band bending is extremely sensitive to the specific processing steps such as cleaning, dielectric or metal deposition, postdeposition or postmetallization treatments, which affect oxygen coverage, carbon contamination, structural defects, bonding configurations, defect states, absorbates, and Fermi pinning states. In many cases, the specific effects of these treatments on the surface and interface states are not entirely clear as the nature of the electronic states has been obscured in complexity and subtlety. Consequently, a more systematic and methodical approach may be required.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2013; 31(5):050807-050807-29. · 1.43 Impact Factor
  • Franz A. M. Koeck, Robert J. Nemanich
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    ABSTRACT: Thermionic electron emission from low work function doped diamond films can be related to materials' properties, which include donor states, surface electron affinity, and substrate-diamond interface properties. The focus of this study is on how the properties of the substrate material affect the emission. Two aspects are considered, the substrate electrical resistance and the substrate Richardson constant, and the effects of tungsten, molybdenum and rhenium substrates are explored. Low work function diamond films were deposited on the substrates, and the thermionic emission was measured to ∼530 °C and described in terms of a fit to the Richardson-Dushman formalism. The results establish that all surfaces exhibit a similar work function but the Richardson constant and maximum emission current vary considerably. The rhenium based emitter displayed a low work function of 1.34 eV, a significant Richardson constant of 53.1 A/cm2 K2, and an emission current density of ∼44 mA/cm2 at a temperature of 530 °C. The results indicated that interface carbide formation could limit the emission presumably because of increased electrical resistance. For non-carbide forming substrates, an increased substrate Richardson constant corresponded to enhanced emission from the diamond based emitter.
    Journal of Applied Physics 12/2012; 112(11). · 2.21 Impact Factor
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    ABSTRACT: Vanadium oxide (VO2) is a narrow band gap material (Eg = 0.7 eV) with a thermally induced insulator-metal phase transition at ∼343 K and evidence of an electric field induced transition at T < 343 K. To explore the electronic properties of VO2, a sandwich structure was prepared with a 2 nm VO2 layer embedded between an oxidized Si(100) surface and a 2 nm hafnium oxide (HfO2) layer. The layer structure was confirmed with high resolution transmission electron microscopy. The electronic properties were characterized with x-ray and ultraviolet photoemission spectroscopy, and the band alignment was deduced on both n-type and p-type Si substrates. The valence band offset between VO2 and SiO2 is measured to be 4.0 eV. The valence band offset between HfO2 and VO2 is measured to be ∼3.4 eV. The band relation developed from these results demonstrates the potential for charge storage and switching for the embedded VO2 layer.
    Journal of Applied Physics 10/2012; 112(8). · 2.21 Impact Factor
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    ABSTRACT: Al{sub 2}O{sub 3} films, HfO{sub 2} films, and HfO{sub 2}/Al{sub 2}O{sub 3} stacked structures were deposited on n-type, Ga-face, GaN wafers using plasma-enhanced atomic layer deposition (PEALD). The wafers were first treated with a wet-chemical clean to remove organics and an in-situ combined H{sub 2}/N{sub 2} plasma at 650 Degree-Sign C to remove residual carbon contamination, resulting in a clean, oxygen-terminated surface. This cleaning process produced slightly upward band bending of 0.1 eV. Additional 650 Degree-Sign C annealing after plasma cleaning increased the upward band bending by 0.2 eV. After the initial clean, high-k oxide films were deposited using oxygen PEALD at 140 Degree-Sign C. The valence band and conduction band offsets (VBOs and CBOs) of the Al{sub 2}O{sub 3}/GaN and HfO{sub 2}/GaN structures were deduced from in-situ x-ray and ultraviolet photoemission spectroscopy (XPS and UPS). The valence band offsets were determined to be 1.8 and 1.4 eV, while the deduced conduction band offsets were 1.3 and 1.0 eV, respectively. These values are compared with the theoretical calculations based on the electron affinity model and charge neutrality level model. Moreover, subsequent annealing had little effect on these offsets; however, the GaN band bending did change depending on the annealing and processing. An Al{sub 2}O{sub 3} layer was investigated as an interfacial passivation layer (IPL), which, as results suggest, may lead to improved stability, performance, and reliability of HfO{sub 2}/IPL/GaN structures. The VBOs were {approx}0.1 and 1.3 eV, while the deduced CBOs were 0.6 and 1.1 eV for HfO{sub 2} with respect to Al{sub 2}O{sub 3} and GaN, respectively.
    Journal of Applied Physics 09/2012; 112(5). · 2.21 Impact Factor
  • Jialing Yang, Brianna Eller, Chiyu Zhu, Robert Nemanich
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    ABSTRACT: GaN-based transistors have shown immense promise because of their high saturation velocity and breakdown field, but their performance is limited by the high gate leakage. This limitation is mitigated with the use of metal/high-k oxide/III-N structures. This experiment investigates three promising high-k dielectrics deposited by plasma enhanced ALD: Al2O3, HfO2, and La2O3. The band gaps of these materials are 6.5eV, 5.8eV, and 4.3eV, while the dielectric constants are 9, 20, and 27, respectively. The large band gap associated with Al2O3 reduces the leakage current; however, the lower dielectric constant increases the equivalent oxide thickness. The band alignment of the high-k oxide/GaN interface plays a critical role in determining the confinement properties of semiconductor carriers and ultimately device performance. In situ photoemission gave valence band offsets for Al2O3, HfO2, and La2O3 with GaN as 1.8eV, 1.3eV, and 0.9eV. The results are described by the charge neutrality level and interface dipole models. We also investigated the use of Al2O3 as an interfacial passivation layer between HfO2 and GaN. This research is supported by the Office of Naval Research.
    02/2012;
  • Tianyin Sun, Franz Koeck, Robert Nemanich
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    ABSTRACT: Conversion of heat into electrical energy has been demonstrated using low effective work function diamond films achieved with n-type doping and surface hydrogen termination. Recently, visible light photo-electron emission has been demonstrated from the same diamond, and this work suggests that this effect could be utilized for a new approach to solar energy conversion namely combined photo and thermionic energy conversion. This work presents a spectroscopic study of photo- and thermionic electron emission from nitrogen doped diamond films on silicon substrates. In this experiment the diamond samples are heated from 100 C to 500 C, while being illuminated with light from 240 to 600 nm. The emission spectra show a significant increase of photo-emission intensity with elevated temperature and a lowering of the effective work function. The results are discussed in terms of the photo and thermal excitation, the carrier transport and the electron statistics. The results indicate the potential of diamond films in a combined photo and thermionic energy conversion solar cell. This research is supported through the Office of Naval Research.
    02/2012;
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    ABSTRACT: We report first results from our effort to couple a high resolution photoemission electron microscope (PEEM) to the OK-4 ultraviolet free electron laser at Duke University (OK-4/Duke UV FEL). The OK-4/Duke UV FEL is a high intensity source of tunable monochromatic photons in the 3–10 eV energy range. This tunability is unique and allows us to operate near the photoemission threshold of any samples and thus maximize sample contrast while keeping chromatic berrations in the PEEM minimal. We have recorded first images from a variety of samples using spontaneous radiation from the OK-4/ Duke UV FEL in the photon energy range of 4.0–6.5 eV. Due to different photothreshold emission from different sample areas, emission from these areas could be turned on (or off) selectively. We have also observed relative intensity reversal with changes in photon energy which are interpreted as density-of-state contrast. Usable image quality has been achieved, even though the output power of the FEL in spontaneous emission mode was several orders of magnitude lower than the anticipated full laser power. The PEEM has achieved a spatial resolution of 12 nm.
    Surface Review and Letters 01/2012; 05(06). · 0.28 Impact Factor
  • Chiyu Zhu, David J. Smith, Robert J. Nemanich
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    ABSTRACT: A gate stack structure with a thin ZnO layer between an oxidized Si(100) surface and an alloyed hafnium and lanthanum oxide (HfO2-La2O3) layer was prepared by plasma enhanced atomic layer deposition at ∼175 °C. High resolution electron microscopy indicated an amorphous structure of the deposited layers. The electronic properties were characterized with x-ray and ultraviolet photoemission spectroscopy. A significant amount of excess oxygen was observed in the as-deposited ZnO and (HfO2-La2O3) layers. A helium plasma postdeposition treatment can partially remove the excess oxygen in both layers. The band alignment of this structure was established for an n-type Si substrate. A valence band offset of 1.5 ± 0.1 eV was measured between a thin ZnO layer and a SiO2 layer. The valence band offset between HfO2-La2O3 (11% HfO2 and 89% La2O3) and ZnO was almost negligible. The band relationship developed from these results demonstrates confinement of electrons in the ZnO film as a channel layer for thin film transistors.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 01/2012; 30(5):051807-051807-8. · 1.36 Impact Factor
  • Fu Tang, Chiyu Zhu, David J. Smith, Robert J. Nemanich
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    ABSTRACT: In this work, we investigated the growth of Hf oxide, La oxide, and alloyed Hf–La oxide films using remote-plasma atomic layer deposition at temperatures ranging from ∼80 to ∼250 °C. The relative composition and atomic bonding structure of the film were determined by in situ x ray photoelectron spectroscopy (XPS). Atomic force microscopy and transmission electron microscopy were implemented to characterize the morphology and crystalline structure. The XPS results indicated that for low temperature Hf oxide growth, a significant amount of excess oxygen species was observed in the deposited film. This oxygen could lead to instabilities and adversely affect the function of thin film transistors. The authors established that a He plasma post deposition treatment can partially remove the excess oxygen. In addition, the pure Hf oxide films show a surface morphology with protruding islands over a smooth surface which reflects the crystallized nature of the Hf oxide domains. In order to suppress the crystallization of the Hf oxide and to obtain a smooth morphology, 1–3 cycles of La-oxide were employed between adjacent Hf-oxide cycles. The Hf–La oxide films showed reduced roughness compared with that of the pure Hf oxide film. Carbon residue in the alloyed film is also reduced compared with that of the La oxide film. Finally, the electrical properties of the deposited films were characterized by capacitance-voltage (C-V) and current-voltage (I-V) measurement. The I-V curves show that the alloyed Hf–La oxide films have a higher break down field than that of pure Hf oxide films.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2012; 30(1):01A147-01A147-6. · 1.43 Impact Factor
  • Xin Liu, Sandeep Gill, Fu Tang, Sean W. King, Robert J. Nemanich
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2012; 30(3):1212-. · 1.36 Impact Factor

Publication Stats

6k Citations
752.85 Total Impact Points

Institutions

  • 2007–2014
    • Arizona State University
      • Department of Physics
      Phoenix, Arizona, United States
    • Duke University Medical Center
      • Department of Biochemistry
      Durham, NC, United States
  • 1986–2014
    • North Carolina State University
      • • Department of Materials Science and Engineering
      • • Department of Physics
      Raleigh, North Carolina, United States
  • 2005–2009
    • Duke University
      • • Department of Chemistry
      • • Department of Physics
      Durham, NC, United States
  • 2004
    • University of Melbourne
      • School of Physics
      Melbourne, Victoria, Australia
  • 1995
    • Bergische Universität Wuppertal
      Wuppertal, North Rhine-Westphalia, Germany
  • 1989
    • Xerox Research Center Webster
      Webster, New York, United States
    • RWTH Aachen University
      • Institute of Semiconductor Electronics
      Aachen, North Rhine-Westphalia, Germany
  • 1978–1985
    • Palo Alto Research Center
      Palo Alto, California, United States
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 1984
    • Xerox Corporation
      Norwalk, Connecticut, United States
  • 1977
    • University of Illinois at Chicago
      • Department of Physics
      Chicago, IL, United States
  • 1976–1977
    • University of Chicago
      • James Franck Institute
      Chicago, IL, United States