W. J. Schaff

Cornell University, Ithaca, New York, United States

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Publications (533)936.74 Total impact

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    ABSTRACT: Tandem solar cells using InxGa1-xN heterojunctions with silicon as the active junction were fabricated using gas-source molecular beam epitaxy (MBE) and by a novel deposition method incorporating an energetic nitrogen atom source. N-type InGaN layers were grown on p-Si(111) to evaluate predicted low-resistance tunnel junction properties. Ohmic behavior was observed, showing that these junctions can be used to connect the two pn subcells of an InGaN/Si tandem without the requirement of the heavily doped layers used in current multijunction cells. Undoped and Mg-doped films were grown by MBE on n-Si(111) using a AlN buffer layer. Depletion is observed on the Si side of the junction and efficiencies approaching 5% were measured for this “hybrid” cell design. Conditions for achieving depletion on the p-InGaN and producing a “single-junction” tandem cell are discussed.
    Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE; 06/2008
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    ABSTRACT: X-ray photoemission spectroscopy is utilised to determine the variation in surface Fermi level pinning across the composition range of n- and p-type c -plane Inx Ga1–xN alloys. The pinning relative to the charge neutrality level is used to explain a change in band bending direction causing a transition from surface electron accumulation (In-rich) to depletion (Ga-rich) at x ≈ 0.43 for n-type alloys and a transition from surface inversion to hole depletion at x ≈ 0.59 for p-type alloys where downward band bending occurs across the composition range. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (b) 05/2008; 245(5):881 - 883. DOI:10.1002/pssb.200778452 · 1.49 Impact Factor
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    J. W. Ager · N. Miller · R. E. Jones · K. M. Yu · J. Wu · W. J. Schaff · W. Walukiewicz ·
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    ABSTRACT: The bandgap range of InGaN extends from the near-IR (InN, 0.65 eV) to the ultraviolet. To exploit this wide tuning range in light generation and conversion applications, pn junctions are required. The large electron affinity of InN (5.8 eV) leads to preferential formation of native donor defects, resulting in excess electron concentration in the bulk and at surfaces and interfaces. This creates difficulties for p-type doping and/or measuring of the bulk p-type activity. Capacitance–voltage measurements, which deplete the n-type surface inversion layer, have been used to show that Mg is an active acceptor in InN and Inx Ga1–xN for 0.2 < x < 1.0, i.e. over the entire composition range. Mg acceptors can be compensated by irradiation-induced native donors. Thermopower measurements were used to provide definitive evidence that Mg-doped InN has mobile holes between 200 K and 300 K. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (b) 05/2008; 245(5):873-877. DOI:10.1002/pssb.200778731 · 1.49 Impact Factor
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    ABSTRACT: The band gap energies of the In1−xAlxN alloys are continuously tunable across the solar spectrum, making them good candidates for high efficiency solar cells. In particular, multijunction solar cells could be fabricated entirely from different compositions of this one alloy system. From modeling experimental measurements of the optical absorption coefficient in alloys with 0 ≤ × ≤ 0.6, a band gap bowing parameter of 4.8 ± 0.5 eV is found for the alloy system. With this number, the alloy compositions for two and three junction tandem cells with maximum theoretical power conversion efficiencies can be determined. Two junction InAlN / Si solar cells are also promising structures for high efficiency solar cells, due to the properties of the interface between n-type InAlN and p-type Si, as well as the band gaps of the respective materials. The theoretical efficiency (detailed balance) for the In0.60Al0.40N / Si tandem cell is 41%.
    Conference Record of the IEEE Photovoltaic Specialists Conference 05/2008; DOI:10.1109/PVSC.2008.4922884
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    ABSTRACT: Eu doped GaN powders have been produced by a low-cost and high-yield method. The effect of temperature on the optical properties and structure was investigated by photoluminescence, X-ray diffraction and Raman spectroscopy. The effective Eu incorporation was extracted by a non-destructive strain analysis correlating the Raman with the XRD data. Luminescence intensity was closely related to the effective Eu incorporation. The maximum Eu incorporation was determined to be 0.5 at% for the sample grown at 1000 °C, which showed the strongest red photoluminescence with the highest crystallinity. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 05/2008; 5(6):1495 - 1498. DOI:10.1002/pssc.200778413
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    ABSTRACT: Raman and SIMS studies of Mg-doped InN films with a Mg content from NMg = 3.3 × 1019 to 5.5 × 1021 cm–3 are reported. Lattice dynamics of hexagonal InN with substitutional impurities and vacancies has been investigated theoretically and calculated Raman spectra were compared with experimental ones. It is concluded that Raman spectroscopy is a good tool for quantitative characterization of Mg-doped InN. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 05/2008; 5(6):1648-1651. DOI:10.1002/pssc.200778563
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    ABSTRACT: The electrical properties of InGaN that is either undoped, or Mg doped, are compared to learn about the nature of p-type conductivity. For In alloy fraction beyond 5% Hall measurements do not indicate p-type polarity, even when Mg doping is employed. In contrast, hot probe measurements show that p-polarity can be measured for the entire range of Mg-doped In mole fractions. The conflicting polarity indications are primarily the result of surface electron accumulation. Parasitic surface electron conductivity can further be seen in p–n homojunctions at In fractions including 20% and 30%. This has an impact on structures such as solar cells. Temperature variable conductivity and PL from different layer structures provides further understanding of the nature of InGaN:Mg. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (b) 05/2008; 245(5):868 - 872. DOI:10.1002/pssb.200778710 · 1.49 Impact Factor
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    ABSTRACT: In this work, the authors report step-flow growth mode of InN on [0001] oriented GaN templates, using a production-style molecular beam epitaxy system, Veeco GEN200 registered , equipped with a plasma source. Using adaptive growth conditions, they have obtained a surface morphology that exhibits the step-flow features. The root mean squared roughness over an area of 5x5 m² is 1.4 nm with monolayer height terrace steps (0.281 nm), based on atomic force microscopy. It has been found that the presence of In droplets leads to defective surface morphology. From x-ray diffraction, they estimate edge and screw dislocation densities. The former is dominant over the latter. Micro-Raman spectra reveal narrow E² phonon lines consistent with excellent crystalline quality of the epitaxial layers. The Hall mobility of 1 m thick InN layers, grown in step-flow mode, is slightly higher than 1400 cm²/V s, while for other growth conditions yielding a smooth surface with no well-defined steps, mobility as high as 1904 cm²/V s at room temperature has been measured. The samples exhibit high intensity photoluminescence (PL) with a corresponding band edge that shifts with free carrier concentration. For the lowest carrier concentration of 5.6x10¹ cm³, they observe PL emission at 0.64 eV.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 05/2008; 26(3):399-405. DOI:10.1116/1.2899412 · 2.32 Impact Factor
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    ABSTRACT: p s s current topics in solid state physics c status solidi 1 Introduction With the increased availability of high-quality InN and In-rich InGaN samples, much pro-gress has been made in the understanding of the basic properties of these materials. The InGaN ternary alloy is of special interest since it spans a spectral range from the ul-traviolet to the infrared [1], taking into account the GaN band gap of 3.4 eV and the relatively recent discovery of a low band gap value of about 0.7 eV for InN [2]. The high quality of InN and In-rich InGaN samples that have recently become available enables precise inspec-tion of theoretical models describing the physical proper-ties of these materials. In the present work, we will be in-terested in the influence of hydrostatic pressure on the op-tical properties of InN and In-rich InGaN. The hydrostatic pressure coefficient dE G /dp of the optical band gap is a ba-sic property of a semiconductor material. For nitride mate-rials, values of dE G /dp are are ~50 meV/GPa [3], ~40 meV/GPa [4], and ~25 meV/GPa [5] for AlN, GaN, and
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    ABSTRACT: We have demonstrated step-flow growth mode of InN, with monolayer height terrace steps (0.281 nm), using a production-style PA-MBE system, GEN200®. The surface morphology exhibits the step-flow features on relatively large areas and the RMS roughness over an area of 5 x 5 μm2 is 1.4 nm. We also investigated the consequences of In droplets formation during the growth and we have found that the vapor-liquid-solid growth mechanism generates defective layer areas underneath droplets that have been formed early in the growth process. The Hall mobility of 1μm thick InN layers, grown in such step-flow mode is slightly higher than 1400 cm2/Vs while for other growth conditions we have obtained mobility as high as 1904 cm2/Vs at room temperature. The samples exhibit high intensity photoluminescence spectra with a band edge that shifts with free-carrier concentration. For the lowest carrier concentration of 5.6×1017 cm-3 we observe PL emission at ~0.64 eV.
    physica status solidi (c) 05/2008; 5(6):1642-1644. DOI:10.1002/pssc.200778561
  • K. D. Matthews · X. Chen · D. Hao · W. J. Schaff · L. F. Eastman ·
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    ABSTRACT: We report on the growth of high quality Mg-doped InGaN and InAlN alloys on (0001) sapphire substrates by molecular beam epitaxy. Phase separation does not occur in InxGa1-xN films with Indium content up to 0.88. Hall measurement shows that a hole concentration of 7.7x1017 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 of Mg-doped InGaN. Ni/Au film is deposited on Mg-doped InxGa1-xN as the p contact met al. A contact resistivity as high as 9.95 Ωcm2 is obtained when x = 0.25 while it is only 6.81x10–5 Ωcm2 when x = 0.85. High quality crack-free InAlN films are also grown on sapphire substrates with AlN/InN quantum-well buffer layers. When Mg is incorporated near the solid solubility limit, donor defects form and In0.62Al0.38N films show n-type polarity by Hall measurement, however lower Mg flux yields p-type polarity. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 05/2008; 5(6):1863 - 1865. DOI:10.1002/pssc.200778707
  • X. Chen · K. D. Matthews · D. Hao · W. J. Schaff · L. F. Eastman ·
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    ABSTRACT: The InGaN alloy system offers a unique opportunity to develop high efficiency multi-junction solar cells. In this study, single junction solar cells made of Inx Ga1–xN are successfully developed, with x = 0, 0.2, and 0.3. The materials are grown on sapphire substrates by MBE, consisting of a Si-doped InGaN layer, an intrinsic layer and an Mg-doped InGaN layer on the top. The I –V curves indicate that the cell made of all-GaN has low series resistance (0.12 Ω cm2) and insignificant parasitic leakage. Contact resistances of p and n contacts are 2.9 × 10–2 Ω cm2 and 2.0 × 10–3 Ω cm2, respectively. Upon illumination by a 200 mW/cm2, 325 nm laser, Voc is measured at 2.5 V with a fill factor of 61%. Clear photo-responses are also observed in both InGaN cells with 0.2 and 0.3 Indium content when illuminated by outdoor sunlight. But it is difficult to determine the solar performance due to the large leakage current, which may be caused by the material defects. A thicker buffer layer or GaN template can be applied to the future growth process to reduce the defect density of InGaN films. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Physica Status Solidi (A) Applications and Materials 05/2008; 205(5):1103 - 1105. DOI:10.1002/pssa.200778695 · 1.62 Impact Factor
  • Takashi Inushima · Kazutoshi Fukui · Hai Lu · William J. Schaff ·
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    ABSTRACT: Temperature dependence of the reflectivity of InN with a carrier concentration of (3.5–4.7)×1017 cm3 is investigated from 50 to 750 cm−1 using infrared synchrotron radiation. E1 phonon is separately observed from plasma oscillation, and in the energy range below E1(TO), phonon polariton is observed up to 104 cm−1. The lifetime of the E1(TO) phonon is directly determined by the reflectivity measurements. From the temperature dependence of the lifetime, the E1(TO) phonon primarily decays into phonons with a renormalized frequency of 177 cm−1. From the plasma edge position the electron effective mass is estimated to be me⊥ = 0.076m0 for the intrinsic InN.
    Applied Physics Letters 04/2008; 92(17):171905-171905-3. DOI:10.1063/1.2918848 · 3.30 Impact Factor
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    ABSTRACT: The variation in surface electronic properties of undoped c-plane InxAl1-xN alloys has been investigated across the composition range using a combination of high-resolution x-ray photoemission spectroscopy and single-field Hall effect measurements. For the In-rich alloys, electron accumulation layers, accompanied by a downward band bending, are present at the surface, with a decrease to approximately flatband conditions with increasing Al composition. However, for the Al-rich alloys, the undoped samples were found to be insulating with approximate midgap pinning of the surface Fermi level observed.
    Applied Physics Letters 04/2008; 92(17):172105-172105-3. DOI:10.1063/1.2913765 · 3.30 Impact Factor
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    ABSTRACT: The valence band density of states VB-DOS of zinc-blende InN001 is investigated using a combination of high-resolution x-ray photoemission spectroscopy and quasiparticle corrected density functional theory. The zinc-blende VB-DOS can be characterized by three main regions: a plateau region after the initial rise in the DOS, followed by a shoulder on this region and a second narrow but intense peak, similar to other III-V and II-VI semiconductor compounds. Good general agreement was observed between the experimental and theo-retical results. Tentative evidence for an s-d coupling due to the interaction between valence-like N 2s states and semicore-like In 4d states is also identified. Measurements and calculations for wurtzite InN112 ¯ 0 are shown to yield a VB-DOS similar to that of zinc-blende InN, although the nonzero crystal field and different Brillouin zone shape in this case lead to a more complicated band structure which modifies the DOS. In adlayers terminating the InN112 ¯ 0 surface are also evident in the experimental VB-DOS, and these are discussed.
    Physical Review B 03/2008; 77(11). DOI:10.1103/PhysRevB.77.115213 · 3.74 Impact Factor
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    ABSTRACT: The complex dielectric function DF of wurtzite InN and GaN as well as zinc blende GaN was measured by spectroscopic ellipsometry between 14 and 32 eV with synchrotron radiation. In this spectral region, the DF of InN and GaN originates from Ga 3d and In 4d core level transitions to unoccupied conduction-band states. The Ga 3d and In 4d electronic states are highly localized and show almost no dispersion. We use these core states as a reference in order to probe the conduction bands. For this purpose, the imaginary part is compared to the density of empty p-orbital-like electronic states located around the cation atomic site, as calculated by density-functional theory in the local density approximation. The constant splitting of absorption features in the DF is attributed to the spin-orbit splitting of the d states. d 5/2−3/2 is found to be 0.82 eV for the In 4d and 0.41 eV for the Ga 3d level, respectively. On wurtzite samples with the c axis in the surface plane, ellipsometry measurements give access to both independent dielectric tensor components and , respectively. The observed anisotropy is induced by a directional dependence of empty p states yielding a p -DOS density states different to the p -DOS.
    Physical review. B, Condensed matter 03/2008; 7720(11). DOI:10.1103/PhysRevB.77.115120 · 3.66 Impact Factor
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    ABSTRACT: InN has the largest electron affinity, 5.8 eV, of any known semiconductor. At its surface, the Fermi level is pinned ca. 0.9 eV above the conduction band edge, leading to an electron accumulation layer in n-type material and an inversion layer in p-type material. Recently, we have used capacitance-voltage measurements with an electrolyte contact to deplete the surface inversion layer in Mg-doped InN and observe space charge due to ionized acceptors [1]. However, these measurements do not give information about the proportion of acceptors that are ionized, or about hole transport. Here, thermopower measurements are used to deduce the majority carrier type under the surface inversion layer in Mg-doped InN. Observation of a positive Seebeck coefficient provides direct and definitive evidence of mobile holes in InN:Mg. Temperature-dependent measurements from 200-300 K are consistent with degenerate conduction in the highly doped films. Modeling of the experimental data to determine the carrier effective masses and scattering mechanisms will be presented. [1] R. E. Jones et al., Phys. Rev. Lett. 96, 125505 (2006).
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    ABSTRACT: The core-level lineshape in photoemission spectra of InN is studied by high-resolution X-ray photoemission spectroscopy. The In 3d and N 1s core-levels are asymmetric, displaying a high binding energy tail which is attributed to inelastic losses to and/or screening by conduction band plasmons in the accumulation layer present at InN surfaces. The extent of the asymmetric tail decreases with decreasing surface Fermi level position associated with a lower density of electrons in the accumulation layer.
    Surface Science 02/2008; 602(4):871-875. DOI:10.1016/j.susc.2007.12.026 · 1.93 Impact Factor
  • K. T. Tsen · Juliann G. Kiang · D. K. Ferry · Hai Lu · William J. Schaff · Hon-Way Lin · Shangjr Gwo ·
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    ABSTRACT: Time-resolved Raman spectroscopy on a subpicosecond time scale has been used to study the phonon dynamics of both the A1(LO) and the E1(LO) phonons in InN. From the temperature-dependence of their lifetimes, we demonstrate that both phonons decay primarily into a large wavevector TO phonon and a large wavevector TA/LA phonon consistent with the accepted phonon dispersion relationship for wurtzite InN. Their lifetimes have been found to decrease from 2.2 ps, at the low electron-hole pair density of 5×1017cm-3 to 0.25 ps, at the highest density of 2×1019cm-3. Our experimental findings demonstrate that carrier-density dependence of LO phonon lifetime is a universal phenomenon in polar semiconductors.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2008; 6892:2-. DOI:10.1117/12.762319 · 0.20 Impact Factor
  • B. Liu · R. Zhang · Z. L. Xie · H. Lu · Q. J. Liu · Z. Zhang · Y. Li · X. Q. Xiu · P. Chen · P. Han · S. L. Gu · Y. Shi · Y. D. Zheng · W. J. Schaff ·
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    ABSTRACT: This article reports on the study of microstructure and dislocation of InN films using high resolution x-ray diffraction grown on sapphire (0001) both by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). The mosaic tilt, twist, and correlation lengths of InN films are determined by using symmetrical and asymmetrical reflections as well as reciprocal space mapping. Deducing from these results, MBE-grown InN film exhibits the edge-type dislocations of 4.0×109 cm−2, which is about ten times higher than the density of screw-type dislocations. In MOCVD-grown InN sample, the edge-type dislocations density is as high as 2.1×1010 cm−2, and the screw-type dislocations density is 1.3×109 cm−2. They indicate that edge type is the predominant dislocation type in the InN films. By comparing the reported transmission electron microscopy results, the accuracy of evaluation for the dislocation density using the mosaic model is proved.
    Journal of Applied Physics 01/2008; 103(2):023504-023504-4. DOI:10.1063/1.2832753 · 2.18 Impact Factor

Publication Stats

14k Citations
936.74 Total Impact Points


  • 1970-2014
    • Cornell University
      • • Department of Electrical and Computer Engineering
      • • School of Applied and Engineering Physics
      Ithaca, New York, United States
  • 2007
    • Rensselaer Polytechnic Institute
      • Department of Physics, Applied Physics, and Astronomy
      Троя, New York, United States
  • 2004
    • Lawrence Berkeley National Laboratory
      Berkeley, California, United States
  • 2003
    • The Ohio State University
      Columbus, Ohio, United States
  • 2002
    • Wright-Patterson Air Force Base
      Dayton, Ohio, United States
  • 2000
    • Macquarie University
      Sydney, New South Wales, Australia
  • 1998
    • University at Buffalo, The State University of New York
      • Department of Physics
      Buffalo, NY, United States
    • McMaster University
      Hamilton, Ontario, Canada
  • 1993
    • University of Essex
      Colchester, England, United Kingdom
  • 1992
    • University of Cambridge
      Cambridge, England, United Kingdom