W. J. Schaff

Cornell University, Ithaca, New York, United States

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Publications (494)880.11 Total impact

  • K. T. Tsen, D. K. Ferry, H. Lu, W. J. Schaff
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    ABSTRACT: Gallium nitride (GaN), aluminum nitride, indium nitride (InN), and their alloys have long been considered as promising materials for device applications. Recently, growth of high quality InN as well as InxGa1−xN have been demonstrated. In particular, progress in the manufacturing of very high quality, single-crystal InN thin films has opened up a new challenging research avenue in the Ill-nitride semiconductors. In contrast to earlier beliefs, it has recently been found that InN has a relatively narrow bandgap, only ∼0.8 eV. Consequently, it is expected that InN has the smallest effective mass of the III-N semiconductors. As a result, very high electron mobility and a very large saturation velocity are expected. Recent single-particle Raman scattering, supported by ensemble Monte Carlo simulations suggest that steady velocities of the order of 5 × 107 cm/s can be found in high quality, single crystal wurtzite films of InN [1]. Here, we report on these calculations for the transport and properties of the non-equilibrium longitudinal optical phonons. We use a high quality, single-crystal wurtzite InN film grown on GaN and study the transport with picosecond/subpicosecond Raman spectroscopy. The built-in polarization and piezoelectric stress lead to an electric field of ∼80 kV/cm in the sample, which is oriented in the growth direction. From the Raman data [1], we can determine not only the average velocities (the drift velocity), but also the distribution function of the carriers along the field direction.
    12/2007: pages 143-146;
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    ABSTRACT: We report an experimental study of femtosecond near-infrared optically excited THz-emission from InN thin films grown by molecular beam epitaxy (MBE) on sapphire substrates. THz-emission was investigated as a function of structural as well as electronic properties such as types of buffer layer, film thickness, electron mobilities, electron concentrations and doping of the InN with Si and Mg atoms. The THz-emission mechanism in InN has been analyzed. Ultrafast transient currents are identified as dominant THz-emission mechanisms. Ultrafast carrier recombination is identified as a limiting factor of THz-emission from n-type InN: Si.
    Infrared and Millimeter Waves, 2007 and the 2007 15th International Conference on Terahertz Electronics. IRMMW-THz. Joint 32nd International Conference on; 10/2007
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    ABSTRACT: Raman studies of Mg-doped InN films with a Mg content from N<sub> Mg </sub>=3.3×10<sup>19</sup> to 5.5×10<sup>21</sup> cm <sup>-3</sup> are reported. Raman and secondary ion mass spectroscopy data on the Mg content have been found to correlate well. Lattice dynamics of hexagonal InN with substitutional impurities and vacancies has been investigated in the framework of the cluster approach. Energy positions of local vibrational modes in InN have been calculated and compared with experimental findings. It is concluded that Raman spectroscopy is a good tool for quantitative characterization of Mg-doped InN.
    Applied Physics Letters 10/2007; · 3.79 Impact Factor
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    ABSTRACT: Transport studies of as-grown and proton-irradiated n-InN have been performed aiming at verification of the nature of localized donor states resonant with the InN conduction band. These resonant donor states (RDS) show a clear contribution to the electrical conduction in low electron concentration InN epitaxial layers. We used proton irradiation to increase the number of incorporated native point defects of donor character in InN layers. Then, the performed studies of pressure dependence of the Hall electron concentration clearly show no increase in the number of RDS in samples exposed to irradiation in spite of the increase in the conducting electron concentration.
    Semiconductor Science and Technology 09/2007; 22(10):1161. · 1.92 Impact Factor
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    ABSTRACT: Electron accumulation is found to occur at the surface of wurtzite (110), (0001), and (000) and zinc-blende (001) InN using x-ray photoemission spectroscopy. The accumulation is shown to be a universal feature of InN surfaces. This is due to the low Γ-point conduction band minimum lying significantly below the charge neutrality level.
    Applied Physics Letters 08/2007; 91(9):092101-092101-3. · 3.79 Impact Factor
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    ABSTRACT: We present the experimental development and characterization of GaN ballistic diodes for THz operation. Fabricated devices have been described and gathered experimental data is discussed. The major problem addressed is the domination of the parasitic resistances which significantly reduce the accelerating electric field across the ballistic region (intrinsic layer).
  • X. Chen, W. J. Schaff, L. F. Eastman
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    ABSTRACT: A direct-write patterning of InGaN during molecular-beam epitaxy has been achieved by using in situ focused thermal beam. The surface of growing InGaN is exposed to a 50 μ m diameter pulse laser beam that is directed to controlled locations. Indium (In) mole fraction is reduced from 0.85 where it is adjacent to laser exposure, and to 0.75 where exposure takes place, whereas it is 0.81 away from exposed regions during a nominal 78 nm deposition on a thick InGaN buffer. The effect of local heating increases surface diffusion of In without evaporating the written materials. One additional feature of direct-write patterning is the enhancement of photoluminescence efficiency, which increases by a factor of 7 compared to nonwritten regions. Gray scale features with composition variations are also demonstrated by laser direct write.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/2007; · 1.36 Impact Factor
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    ABSTRACT: Subpicosecond time-resolved Raman spectroscopy has been used to measure the lifetime of the A1(LO) and E1(LO) phonon modes in InN at T = 10 K for photoexcited electron–hole pair density ranging from 5 × 1017 to 2 × 1019 cm−3. The lifetime has been found to decrease from 2.2 ps at the lowest density to 0.25 ps at the highest density. Our experimental findings demonstrate that the carrier-density dependence of LO phonon lifetime is a universal phenomenon in polar semiconductors.
    Journal of Physics Condensed Matter 05/2007; 19(23):236219. · 2.22 Impact Factor
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    ABSTRACT: The authors investigated a 1 μm thick molecular beam epitaxy–grown InN film by means of full hemispherical x-ray photoelectron diffraction and high resolution x-ray diffraction. While x-ray diffraction reveals that this nominally hexagonal InN layer contains roughly 1% of cubic phase InN, a comparison between measured and simulated x-ray photoelectron diffraction data allowed them to directly determine the polarity of the crystal. Furthermore, the data indicate that the InN surface consists of a mosaic of domains oriented at an azimuth of 180° to each other, where the azimuth corresponds to the rotation angle around the [0001] axis.
    Applied Physics Letters 05/2007; 90(19):191912-191912-3. · 3.79 Impact Factor
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    ABSTRACT: Longitudinal optical phonons in InN have been studied by time-resolved Raman spectroscopy on a subpicosecond time scale. The lifetimes of both the A<sub>1</sub>( LO ) and E<sub>1</sub>( LO ) phonons have been directly measured. From the temperature dependence of their lifetimes, the authors demonstrate that both phonons decay primarily into a large wave vector TO phonon and a large wave vector TA/LA phonon, consistent with the accepted phonon dispersion relationship for wurtzite InN.
    Applied Physics Letters 05/2007; · 3.79 Impact Factor
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    ABSTRACT: We use positron annihilation spectroscopy to study 2MeV He+4 -irradiated InN grown by molecular-beam epitaxy and GaN grown by metal-organic chemical-vapor deposition. In GaN, the Ga vacancies act as important compensating centers in the irradiated material, introduced at a rate of 3600cm-1 . The In vacancies are introduced at a significantly lower rate of 100cm-1 , making them negligible in the compensation of the irradiation-induced additional n -type conductivity in InN. On the other hand, negative non-open volume defects are introduced at a rate higher than 2000cm-1 . These defects are tentatively attributed to interstitial nitrogen and may ultimately limit the free-electron concentration at high irradiation fluences.
    Physical Review B 05/2007; · 3.66 Impact Factor
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    ABSTRACT: A room temperature ozone induced oxidation of thin InN films is proposed to improve the electric transport properties. The sheet carrier density is reduced upon oxidation by a value which is in the order of the electron concentration of an untreated InN surface. Thus, ozone effectively passivates the surface defect states on InN and might be an effective method to prepare InN films for electronic applications. A model for the improved electron transport properties is proposed taking into account the decreased surface band bending and the decreased influence of surface electrons on the net mobility of InN layers.
    Applied Physics Letters 05/2007; · 3.79 Impact Factor
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    ABSTRACT: Electron–longitudinal optical phonon scattering rate in InN has been directly measured by subpicosecond time-resolved Raman spectroscopy. The experimental results show that for a thick layer of InN grown on GaN, the average total electron–longitudinal optical phonon scattering rate is (5.1±1.0)×1013 s−1. This enormous electron–longitudinal optical phonon scattering rate, which is comparable to that observed in GaN, has been attributed to the extremely polar nature of InN.
    Applied Physics Letters 04/2007; 90(17):172108-172108-3. · 3.79 Impact Factor
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    ABSTRACT: Irradiation of InN films with 2 MeV He+ ions followed by thermal annealing below 500 °C creates films with high electron concentrations and mobilities, as well as strong photoluminescence. Calculations show that electron mobility in irradiated samples is limited by triply charged donor defects. Subsequent thermal annealing removes a fraction of the defects, decreasing the electron concentration. There is a large increase in electron mobility upon annealing; the mobilities approach those of the as-grown films, which have 10 to 100 times smaller electron concentrations. Spatial ordering of the triply charged defects is suggested to cause the unusual increase in electron mobility.
    Applied Physics Letters 04/2007; 90(16):162103-162103-3. · 3.79 Impact Factor
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    ABSTRACT: We report on an investigation of superconductivity in n-type InN. There is an optimum carrier density for the occurrence of the superconductivity. The lowest carrier density is limited by the Mott transition of ne~2×1017cm-3 and the highest density is limited by the superconductor to metal transition of ne~5×1020cm-3. As in the layered cuprate high Tc superconductors, the vortex solid of InN is melted by thermal fluctuation and by the external magnetic field. The field dependence of the critical current Jc exhibits an exponential decay over a wide range of magnetic field, which is ascribed to the current tunneling through micro Josephson-junctions. We propose a mechanism where the anisotropy of Jc is related to the presence of In-In chains of finite length in the ab plane.
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    ABSTRACT: InN films grown by molecular beam epitaxy have been subjected to 2 MeV He+ irradiation followed by thermal annealing. Theoretical analysis of the electron mobilities shows that thermal annealing removes triply charged donor defects, creating films with electron mobilities approaching those predicted for uncompensated, singly charged donors. Optimum thermal annealing of irradiated InN can be used to produce samples with electron mobilities higher than those of as grown films.
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    ABSTRACT: The valence band offset of wurtzite-InN/AlN (0001) heterojunctions is determined by x-ray photoelectron spectroscopy to be 1.52±0.17 eV. Together with the resulting conduction band offset of 4.0±0.2 eV, a type-I heterojunction forms between InN and AlN in the straddling arrangement.
    Applied Physics Letters 03/2007; 90(13):132105-132105-3. · 3.79 Impact Factor
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    ABSTRACT: Energetic particle irradiation followed by thermal annealing has been used to create InN films with both high electron concentration and high mobility. The mobility values are larger than have been reported for as-grown, undoped InN films with comparable electron concentrations (> 10^19 cm-3). The high mobility can be explained by a thermally-induced ordering of the native point defects produced by the irradiation. An analysis of the concentration dependence of the electron mobility shows that the defects are triply charged, and therefore the strong Coulomb interaction energy between them is minimized by the formation of a donor superlattice. Here we present evidence for this ordering, including experimental results and theoretical modeling.
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    ABSTRACT: We present details of the development of a GaN ballistic diode designed with THz applications in mind. Parallel theoretical and experimental analysis has shown that negative-differential-resistance (NDR) can only be achieved when hot electrons are injected into the drift region, contact and spreading resistances must be minimized in order to maximize the field in the drift layer. NDR remains achievable even in the presence of collision dominated transport or when transport is space-charge limited. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 02/2007; 4(2):528 - 530.
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    ABSTRACT: In this paper we carry out detailed study and numerical modelling of the operation of the GaN/AlGaN High Electron Mobility Transistor (HEMT) with Field Plate (FP) optimisation. The FP has the effect of smoothing the potential gradient along the HEMT channel near the gate edge on the drain side, resulting in the enhancement of the breakdown voltage. We show that the main effect of the FP is a substantial decrease in the longitudinal component of the field EL, along with little variation of the normal component ET. In addition to the main peak of the EL field at the gate edge, there is a secondary peak under the FP edge which grows rapidly with drain bias and eventually surpasses the first. The appearance of the second peak should be taken into account in optimising of the HEMT design. The I-V characteristics of the FP-gated device remain practically the same as those of a device without a FP. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2007; 4(2):651 - 654.

Publication Stats

8k Citations
880.11 Total Impact Points


  • 1970–2014
    • Cornell University
      • • Department of Electrical and Computer Engineering
      • • School of Applied and Engineering Physics
      Ithaca, New York, United States
  • 2011
    • Chalmers University of Technology
      Goeteborg, Västra Götaland, Sweden
  • 2009
    • Instytut Wysokich Ciśnień
      Warszawa, Masovian Voivodeship, Poland
    • Anadolu University
      • Department of Physics
      Eskişehir, Eskisehir, Turkey
  • 2005–2009
    • Lawrence Berkeley National Laboratory
      • Materials Sciences Division
      Berkeley, CA, United States
    • University of Essex
      • School of Computer Science and Electronic Engineering
      Colchester, ENG, United Kingdom
    • Petersburg Nuclear Physics Institute
      Krasnogwardeisk, Leningrad, Russia
  • 2007
    • Ioffe Physical Technical Institute
      Sankt-Peterburg, St.-Petersburg, Russia
  • 2006–2007
    • Technische Universität Ilmenau
      • • Institut für Mikro- und Nanotechnologien
      • • Institute of Physics
      Ilmenau, Thuringia, Germany
    • University of Leipzig
      • Institute of Experimental Physics
      Leipzig, Saxony, Germany
  • 2004–2007
    • Arizona State University
      • • Department of Physics
      • • School of Electrical, Computer and Energy Engineering
      Mesa, AZ, United States
    • Rensselaer Polytechnic Institute
      • Department of Physics, Applied Physics, and Astronomy
      Troy, NY, United States
    • National Taiwan University
      • Center for Condensed Matter Sciences
      Taipei, Taipei, Taiwan
  • 2004–2006
    • The Ohio State University
      • Department of Electrical and Computer Engineering
      Columbus, OH, United States
  • 2004–2005
    • The University of Warwick
      • Department of Physics
      Warwick, ENG, United Kingdom
  • 2003–2005
    • Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
      Berlín, Berlin, Germany
    • University of California, Berkeley
      • Department of Materials Science and Engineering
      Berkeley, MO, United States
  • 2002
    • CSU Mentor
      Long Beach, California, United States
  • 2000
    • Macquarie University
      Sydney, New South Wales, Australia
  • 1998
    • Georgia State University
      • Department of Physics and Astronomy
      Atlanta, GA, United States
    • McMaster University
      Hamilton, Ontario, Canada
  • 1993
    • Duke University
      • Department of Electrical and Computer Engineering (ECE)
      Durham, NC, United States
  • 1992
    • University of Michigan
      • Center for Ultrafast Optical Science
      Ann Arbor, MI, United States