R. J. Nemanich

Arizona State University, Phoenix, Arizona, United States

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Publications (387)590.72 Total impact

  • 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). · 2.16 Impact Factor
  • F.A.M. Koeck, R.J. Nemanich, J. Sharp
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    ABSTRACT: Thermionic energy conversion is a process that allows direct conversion of heat into electrical energy without mechanically moving components. In a thermionic converter electrons from the emitter traverse a small gap, are collected by a counter-electrode, the collector, and a self generated voltage develops across the gap. We have prepared prepared an ultra-nanocrystalline diamond (UNCD) based thermionic electron emitter that exhibits a low effective work function of typically 1.4 eV. This was attributed in part to reduced band bending and to the negative electron affinity (NEA) surface. A thermionic energy converter comprised of 2 diamond electrodes were positioned to establish a 25 micron gap and the emitter which was operated at temperatures up to 700 Celsius with a self generated open circuit voltage of 0.35 V. The reduced power output of the device was in part attributed to space charge effects and diamond film resistivity. Utilizing surface ionization effects at the emitter by introducing atomic hydrogen into the converter gap resulted in significant power output increase. With atomic hydrogen in the gap, the converter was operated up to 750 Celsius indicative of efficient surface ionization for charge transfer as well as a stable NEA diamond surface.
    Vacuum Nanoelectronics Conference (IVNC), 2013 26th International; 01/2013
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    ABSTRACT: Results are presented on the photo-induced electron emission from nitrogen doped diamond films prepared on doped silicon substrates. In contrast to results for films on metal substrates, a significant increase of emission intensity was observed at elevated temperatures. The results suggest a contribution from photon enhanced thermionic emission.
    Vacuum Nanoelectronics Conference (IVNC), 2013 26th International; 01/2013
  • 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-.
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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
  • 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
  • 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: The first part of this study examined oxide stability and cleaning of Ru surfaces. The surface reactions during H2 plasma exposure of Ru polycrystalline films were studied using x-ray photoelectron spectroscopy (XPS). The ∼2 monolayer native Ru oxide was reduced after H-plasma processing. However, absorbed oxygen, presumably in the grain boundaries, remains after processing. A vacuum thermal anneal at 150 °C substantially removes both surface oxide and absorbed oxygen which is attributed to a reduction by carbon contamination. The second part of the study examined the thermal stability of Cu on a Ru layer. The thermal stability or islanding of the Cu film on the Ru substrate was characterized by in situ XPS. After plasma cleaning of the Ru adhesion layer, the deposited Cu exhibited full coverage. In contrast, for Cu deposition on the Ru native oxide substrate, Cu islanding was detected and was described in terms of grain boundary grooving and surface and interface energies. The oxygen in the grain boundary has a negligible contribution to the surface energy and does not contribute to Cu islanding.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2012; 30(5):2203-.
  • 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-.
  • Tianyin Sun, Franz A. M. Koeck, Chiyu Zhu, Robert J. Nemanich
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    ABSTRACT: This study reports a photoemission threshold of ∼1.5 eV from nitrogen-doped nanocrystalline diamond, which ranks among the lowest photo-threshold of any non-cesiated material. Diamond films on molybdenum substrates have been illuminated with light from 340 to 550 nm, and the electron emission spectrum has been recorded from ambient to ∼320 °C. The results display combined thermionic and photo-electron emission limited by the same low work function and indicate that the two emission processes are spatially separated. These results indicate the potential for a solar energy conversion structure that takes advantage of both photoemission and thermionic emission.
    Applied Physics Letters 11/2011; 99(20):202101-202101-3. · 3.79 Impact Factor
  • Anderson Sunda-Meya, David J. Smith, Robert J. Nemanich
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    ABSTRACT: This study establishes that under conditions of epitaxial Si deposition, embedded Ti-silicide nanostructures undergo shape transitions and migrate upward to the surface during capping with a thin epitaxial Si layer. Many of these structures display a near-hemispherical shape which is attributed to minimization of their surface and interface energies. The density and size of the nanostructures are observed to be temperature-dependent. The buried islands induce inhomogeneous stress profiles on the capping layer surface. Atomic-force micrographs of the islands show square holes at the surface aligned along [110] directions which suggests that the sloping surfaces of the pits approximate to (111) surfaces, and the silicide interface is also facetted to match Si (111) planes. Cross-sectional electron micrographs reveal that many islands display faceting. The observed structural changes can be rationalized in terms of the interplay between thermodynamics and kinetics, solid-state capillarity, and the roughening transition.
    Journal of Applied Physics 11/2011; 110(9). · 2.21 Impact Factor
  • Yang Sun, Brianna S. Eller, Robert J. Nemanich
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    ABSTRACT: This research focuses on the formation of Ag nanopatterns on periodically poled lithium niobate (PPLN). The photo-induced process employs UV-light exposure while the PPLN is immersed in a AgNO3 solution. The Ag deposition was consistent with previous results, showing preferential deposition along the domain boundary as well as an increased density of particles on the positive domain surface in comparison to the negative domain. By tuning the chemical solution concentration and the UV-light intensity, the Ag+ ion flux and the electron flux are varied and the deposition pattern could be controlled to either enhance the nanowire-like structures along the domain boundary or create a more uniform deposition pattern over the positive and negative domains. To understand the deposition process, we investigated the relationship between the Ag+ ion flux because of diffusion and the electron flux initiated by the UV exposure of the ferroelectric surface. The subsequent results suggest that this relationship is responsible for the different deposition patterns. The observed variation of boundary-enhanced or boundary-depressed deposition is explained by consideration of the electric field distribution and the ratio of the Ag+ ion and photon flux. The results establish that the ratio can be controlled by varying the solution concentration and/or UV-light intensity to generate enhanced nanowire-like structures along the domain boundary or a more uniform deposition pattern over the positive and negative surface. Moreover, the specific value of the Ag+/photon flux ratio where the pattern changes is dependent on other factors including the nucleation limited growth mechanism and the Stern layer on the lithium niobate.
    Journal of Applied Physics 10/2011; 110(8):084303-084303-7. · 2.21 Impact Factor
  • Yang Sun, Robert J. Nemanich
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    ABSTRACT: This research addresses the wavelength dependence of the fabrication of Ag nanostructures through photoinduced deposition using single crystal ferroelectric lithium niobate as a template. The photoinduced deposition involves ultraviolent light illumination of polarity patterned lithium niobate while immersed in a AgNO3 solution. The results focus on the differences of the Ag nanostructure formation process on the positive and negative domains and domain boundaries. The results indicate that for below-band-gap excitation, a very low density of nanostructures is observed. However, for all above-gap-excitation wavelengths, deposition occurs on both polarity surfaces and at the domain boundaries. The density is greatest at the domain boundaries and reduced densities of smaller nanostructures are observed to form on both the positive and negative domains. The deposition on the domain surfaces is greatest for the shortest wavelengths, whereas the domain selectivity is increased for wavelengths just above the band gap. The external screening and weak band bending of single crystal lithium niobate introduces an enhanced electric field at the domain boundary. The enhanced electric field leads to migration of electrons to the domain boundary and consequently enhanced formation of Ag nanoparticles along the boundary. The variation in the reduction rate versus illumination wavelength is attributed to the light absorption depth and the competition between the photochemical and photoelectric deposition processes. To explore the transition from surface to bulk screening of the polarization charge, oxygen implanted PPLN surfaces were prepared and used for the Ag photoinduced deposition. Consistent with the transition to internal (bulk) screening, the Ag nanoparticle formation on the oxygen implanted PPLN surfaces showed suppressed boundary nanowire formation.
    Journal of Applied Physics 05/2011; 109(10):104302-104302-7. · 2.21 Impact Factor
  • Tianyin Sun, Franz A. Koeck, Robert J. Nemanich
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    ABSTRACT: Energy conversion cells utilize either direct photon illumination or indirect thermal energy for electron excitation. Nitrogen-doped, hydrogen terminated nanocrystalline diamond films display a negative electron affinity and have shown low temperature thermionic emission which can be employed for energy conversion in a vacuum thermionic emission cell. However, the low work function of such films suggests that the current could be enhanced through visible light illumination to induce photoelectron emission. We present measurements of the spectrum of emitted electrons from N-doped diamond films for light illumination between 600 and 340nm, while the film is heated from ambient to 500C. Features due to thermionic and photo-emission are identified, and a complex interaction is observed between the two processes at various temperatures and illumination wavelengths. The results indicate the potential application of diamond emitters as combined thermal and photon energy converters, and we present a new approach to enhance the performance of diamond-film energy converters.
    03/2011;
  • Franz A. M. Koeck, Jeff Sharp, Robert J. Nemanich
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    ABSTRACT: Vacuum thermionic energy conversion is based on electron transfer from a hot emitter across a vacuum gap to the collector. Our approach for an efficient emitter utilizes nanostructured, negative electron affinity doped diamond films. With a low effective work function of less than 1.3 eV thermionic emission commences at 260 C and observes the law of Richardson -- Dushman with a significant emission current > 5 mA at 500 C. Pairing this emitter with a similar collector results in a potential across the gap and introduction of an ohmic load establishes a current indicative of energy conversion. Utilizing ionization processes of gaseous species at the emitter surface can enhance inter-electrode charge transfer and increase output power. In the ionization process an electron is trapped in an occupied molecular orbital establishing a negative ion state. The electron affinity and negative ion binding energy determines stability of the transient negative species, and we present results for H2 and CH4. As these species are introduced in the inter-electrode gap an increase in output power is observed with a concurrent shift of maximum output power to lower load resistance.
    03/2011;
  • Xin Liu, Sandeep Gill, Fu Tang, Sean King, R. J. Nemanich
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    ABSTRACT: Low-k inter-layer dielectrics (ILD) with copper interconnects display advantages for reducing energy consumption in silicon technology. However, the processing induced degradation of the ILD low-k properties has become a challenge. In this work, we have employed remote N2/H2 plasma processes to simultaneously clean both low-k ILD (k=2.5) and chemical-mechanical polished (CMP) Cu surfaces. FTIR and C-V results indicate that N2 plasma cleaning processes show low carbon abstraction as well as a relatively small increase in the dielectric constant (k=2.6). A carboxamide layer is formed which apparently inhibits further etching. In contrast, the k value increases to 3.5 after an H2 plasma treatment. For the CMP-Cu surfaces, an N2/H2 plasma process at 380C effectively removes the oxide and carbon contamination. In addition, the affects of plasma-induced UV light has been studied, and the results indicate enhanced carbon depletion in the ILD. Degradation of the low-k properties is attributed to carbon abstraction which is enhanced by the plasma induced UV and hydrophilic character. The results establish a range of N2/H2 plasma processes for simultaneous cleaning of CMP Cu and low-k ILD surfaces.
    03/2011;
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    ABSTRACT: Thermionic electron emitters are a crucial component in applications ranging from high power telecommunication, electron guns, space thrusters and direct thermal to electrical energy converters. One key characteristic of diamond based electron sources is the negative electron affinity (NEA) properties of hydrogen terminated surfaces which can significantly reduce the emission barrier. Nitrogen and phosphorus doped diamond films have been prepared by plasma assisted chemical vapor deposition on metallic substrates for thermionic emitter application. Electron emission current versus temperature was measured and analyzed with respect to the Richardson–Dushman relation, with work function and Richardson constant deduced from the results. Initial emission measurements up to 500°C in vacuum were followed by emitter characterization while the sample was exposed to methane. Vacuum measurements indicated a work function of 1.18eV and 1.44eV for phosphorus and nitrogen doped diamond films, respectively. Introduction of methane resulted in a significant increase of the emission current which was ascribed to contribution from ionization processes which increase charge transfer from the emitter surface. This phenomenon was utilized in a thermionic energy conversion structure by introduction of methane in the inter electrode gap where a two-fold increase in output power was observed upon introduction of the gaseous species.
    Diamond and Related Materials 01/2011; 20(8):1229-1233. · 1.71 Impact Factor
  • Franz Koeck, Robert Nemanich
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    ABSTRACT: Vacuum thermionic energy conversion utilizes thermionic emission to release electrons from an emitter into vacuum and collection at a counter-electrode. In our approach for an efficient thermionic emitter a multi-layer diamond thin film structure was synthesized by plasma-assisted CVD on a metallic substrate with controlled surface roughness including a nanodiamond pretreatment step. Introduction of nitrogen during ultra-nanocrystalline diamond (UNCD) film growth resulted in a low resistivity interstitial layer significantly enhancing emission current density which can be related to the Richardson constant. The top layer of polycrystalline nitrogen doped diamond was exposed to a hydrogen plasma inducing negative electron affinity characteristic presenting a low effective emitter work function 2 A/cm^2 K^2 was extracted and at a temperature of 500^oC a thermionic emission current > 5 mA was measured. This may well be the highest current density reported from a thermionic emitter operating at the moderate temperature of 500^oC.
    03/2010;

Publication Stats

2k Citations
590.72 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
  • 1988–2010
    • North Carolina State University
      • • Department of Physics
      • • Department of Materials Science and Engineering
      Raleigh, NC, 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
  • 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