W. J. Schaff

Cornell University, Ithaca, New York, United States

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Publications (498)857.29 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: InxGa1-xN and InxAl1-xN alloys are grown via MBE on sapphire substrates. A hole concentration of 7.7×1017 cm−3 is achieved on Mg-doped In0.04Ga0.96N. When x≫0.11, the Hall samples exhibit strong n-type polarity, whereas p-type polarity is confirmed by hot probe measurement for all Mg-doped InGaN and InAlN samples. Single p-i-n junction solar cells made of the same InxGa1-xN alloy composition are developed. Upon illumination by a 325 nm laser, Voc is measured at 2.5 V with a fill factor of 61% for all-GaN cell. Clear photo-responses are also observed in InGaN cells with 0.2 and 0.3 Indium content when illuminated by focused outdoor sunlight.
    Conference Record of the IEEE Photovoltaic Specialists Conference 01/2008;
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    ABSTRACT: Eu-doped GaN powder is produced by a high-yield and low-cost method. The effect of growth temperature between 950 and 1030 °C on Eu effective incorporation and luminescence was investigated by photoluminescence, X-ray diffraction (XRD) and Raman spectroscopy. The effective Eu concentration was extracted non-destructively by strain analysis of the correlated Raman and XRD data. A clear correlation between Eu incorporation and luminescence intensity was observed. The optimum Eu incorporation of 0.5 at% was obtained at 1000 °C. Samples grown at this temperature also displayed the best crystallinity.
    Journal of Crystal Growth 01/2008; 310(2):452-456. · 1.55 Impact Factor
  • Journal of Applied Physics 01/2008; 104(3):039906-039906-1. · 2.21 Impact Factor
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    ABSTRACT: Transmission electron microscopy was applied to study InN nanorods grown on the a-, c- and r-plane of Al2O3, and (111) Si substrates by non-catalytic, template-free hydride metal-organic vapor phase epitaxy (H-MOVPE). Single crystal nanorod growth was obtained on all substrates. However, the shape of the nanorods varied depending on the substrate used. For example, nanorods grown on r-plane sapphire and (111) Si have sharp tips. In contrast, growth on a- and c- planes of Al2O3 results in flat tips with clear facets on their sides. The structural quality of these nanorods and their growth polarity are compared to crystalline quality, surface roughness, defects and growth polarity of InN layers grown by MBE on the same planes of Al2O3.
    12/2007: pages 37-40;
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    ABSTRACT: The prospect of developing electronic and optoelectronic devices, including solar cells, that utilize the wide range of energy gaps of InGaN has led to a considerable research interest in the electronic and optical properties of InN and In-rich nitride alloys. Recently, significant progress has been achieved in the growth and doping of InGaN over the entire composition range. In this paper we present structural, optical, and electrical characterization results from InGaN films grown on Si (111) wafers. The films were grown over a large composition range by both molecular beam epitaxy (MBE) and the newly developed “energetic neutral atomic-beam lithography & epitaxy” (ENABLE) techniques. ENABLE utilizes a collimated beam of ∼2 eV nitrogen atoms as the active species which are reacted with thermally evaporated Ga and In metals. The technique provides a larger N atom flux compared to MBE and reduces the need for high substrate temperatures, making isothermal growth over the entire InGaN alloy composition range possible. Electrical characteristics of the junctions between n- and p-type InGaN films and n- and p-type Si substrates were measured and compared with theoretical predictions based on the band edge alignment between those two materials. The predicted existence of a low resistance tunnel junction between p-type Si and n-type InGaN was experimentally confirmed.
    MRS Proceedings. 12/2007; 1068.
  • 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; · 3.52 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. · 3.52 Impact Factor
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    ABSTRACT: We present an experimental and theoretical study of the role of band filling effects in the hydrostatic pressure dependence of photoluminescence PL from InN. The PL peak pressure coefficient dE PL / dp is shown to decrease from 27.3± 1.1 meV/ GPa to 20.8± 0.8 meV/ GPa when the electron concentration increases from 3.6 10 17 cm −3 to 1.1 10 19 cm −3 . We argue that this decrease is caused by the pressure sensitivity of the Fermi level in InN, which induces a lowering of dE PL / dp with respect to the band gap pressure coefficient dE G / dp. dE PL / dp is shown to depend on the electron concentration in accordance with predictions based on ab initio calculations, taking into account the influence of conduction-band nonparabolicity.
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    ABSTRACT: Surface states are known to pin the Fermi level in InN and In1−xGaxN, strongly affecting charge distribution and transport on the surface and at interfaces. By solving Poisson’s equation over a range of bias voltages for an electrolyte-based capacitance-voltage measurement configuration, we have calculated the band bending and space charge distribution in this system and developed an electronic model generally applicable to both p- and n-type group-III-nitride thin films. Both conduction band nonparabolicity and band renormalization effects due to the high surface electron concentration were included. The calculated space charge distributions, using the majority dopant concentration as a fitting parameter, are in excellent agreement with experimental data. The model quantitatively describes increasingly strong n-type electrical characteristics on the surface due to electron accumulation in p-type In1−xGaxN for decreasing values of x. This also provides a general understanding of the effect of mobile carriers on capacitance-voltage measurements.
    Physical review. B, Condensed matter 07/2007; 76(4). · 3.66 Impact Factor
  • 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: Pressure-dependent transport and photoluminescence (PL) measurements were performed on InN. In the transport measurements on the sample with a low electron concentration (n e lower than about 10 18 cm –3) a Localized Donor State (LDS) was seen to cross the Fermi level at elevated pressure magnitude. It was accompanied by the transfer of electrons from the Conduction Band to the LDS and a significant increase in electron mobility. In a sample with a higher concentration of electrons no transport anomaly was ob-served. Application of pressure helps to confirm that the disputed photoluminescence mechanism in InN consists in band-to-band recombination. No evidence of the participation of this LDS in radiative recom-bination processes was found in the pressure-dependent PL measurements. On the contrary, the depend-ence of the PL intensity on excitation power suggested the involvement of the LDS in nonradiative recombination processes.
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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).
    06/2007;
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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. · 3.52 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; · 3.52 Impact Factor
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    ABSTRACT: Spectroscopic ellipsometry is applied in order to determine the complex dielectric function DF for In-rich In x Ga 1−x N alloys with N-face polarity from near-infrared into the vacuum ultraviolet spectral region. The results are compared to corresponding data for metal-face films. The optical properties of both types of hexagonal films agree in the essential features which emphasizes that the extracted DFs do not depend on the polarity but represent therefore bulk characteristics. Besides the band gap, five critical points of the band structure are clearly resolved within the composition range of 1 x 0.67. Their transition energies are deter-mined by a fit of the third derivative of the DF. With increasing Ga content, all transitions undergo a continuous shift to higher energies characterized by small bowing parameters. Model calculations of the imaginary part of the DF close to the band gap that take the influence of band filling and conduction-band nonparabolicity into account are presented. A comparison to the experimental data yields the position of the Fermi energy. With the calculated values for the carrier-induced band-gap renormalization and the Burstein-Moss shift, the zero-density values for the fundamental band gaps are obtained. Their dependence on the alloy composition is described by a bowing parameter of b = 1.72 eV.
    Physical Review B 05/2007; · 3.66 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

Publication Stats

8k Citations
857.29 Total Impact Points

Institutions

  • 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