D.C. Reynolds

Publications (88)117.45 Total impact

• Article: Some Optical Properties of Group II‐VI Semiconductors (I)

  D. C. Reynolds, C. W. Litton, T. C. Collins
  physica status solidi (b) 03/2006; 9(3):645 - 684. · 1.32 Impact Factor
• Article: Some Optical Properties of Group II–VI Semiconductors (II)

  D. C. Reynolds, C. W. Litton, T. C. Collins
  physica status solidi (b) 03/2006; 12(1):3 - 55. · 1.32 Impact Factor
• Article: Role of near-surface states in ohmic-Schottky conversion of Au contacts to ZnO

  H. L. Mosbacker, Y. M. Strzhemechny, B. D. White, P. E. Smith, D. C. Look, D. C. Reynolds, C. W. Litton, L. J. Brillson
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  ABSTRACT: A conversion from ohmic to rectifying behavior is observed for Au contacts on atomically ordered polar ZnO surfaces following remote, room-temperature oxygen plasma treatment. This transition is accompanied by reduction of the “green” deep level cathodoluminescence emission, suppression of the hydrogen donor-bound exciton photoluminescence and a ∼0.75 eV increase in n -type band bending observed via x-ray photoemission. These results demonstrate that the contact type conversion involves more than one mechanism, specifically, removal of the adsorbate-induced accumulation layer plus lowered tunneling due to reduction of near-surface donor density and defect-assisted hopping transport.
  Applied Physics Letters 08/2005; · 3.84 Impact Factor
• Article: Phonon scattering of excitons and biexcitons in ZnO

  K. Hazu, T. Sota, S. Adachi, Sf. Chichibu, G. Cantwell, D. C. Reynolds, C. W. Litton
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  ABSTRACT: The phase relaxation time of biexcitons T2bi and that of excitons T2ex in a bulk ZnO have been measured by the use of femtosecond four-wave mixing as functions of excitation wavelength, excitation power, and temperature. The biexciton-acoustic phonon interaction coefficient has been determined to be βacbi≈53 μeV/K as an average value. For A excitons, the interaction coefficients with acoustic and optical phonons have been determined to be βacex≈8.4 μeV/K and βoptex≈14 meV, respectively. © 2004 American Institute of Physics.
  Journal of Applied Physics 07/2004; 96(2):1270-1272. · 2.17 Impact Factor
• Article: Impact of the k-linear term on nonlinear optical response of the C-exciton manifold in ZnO

  K. Hazu, K. Torii, T. Sota, S. Adachi, SF. Chichibu, G. Cantwell, D. C. Reynolds, C. W. Litton
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  ABSTRACT: Time-integrated and spectrally resolved four-wave mixing (FWM) has been applied to study the impact of the k-linear term on nonlinear optical response of the C-exciton manifold in ZnO. From the excitation wavelength dependence of the FWM signal, we have found possible evidence that the mixing of the Γ1 and Γ5L states leads to a three-branch excitonic polariton dispersion but the oscillator strength transfer is small. The energy separation between the lower polariton branch and the extra branch due to the k-linear term has been found to be 3.5 meV. In addition, a binding energy of biexcitons consisting of two C excitons of 1.4 meV has been obtained. © 2004 American Institute of Physics.
  Journal of Applied Physics 05/2004; 95(10):5498-5501. · 2.17 Impact Factor
• Article: Emission and reflection spectra from AlxGa1−xN/GaN single heterostructures

  D. C. Reynolds, J. Hoelscher, C. W. Litton, T. C. Collins, R. Fitch, G. D. Via, J. Gillespie, A. Crespo, T. J. Jenkins, A. Saxler
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  ABSTRACT: Emission and reflection spectra from AlGaN/GaN single heterostructures grown on SiC substrates were investigated. Two-dimensional electron gas (2DEG) transitions were observed in both emission and reflection. The transitions are sharp, associated with the excited state of the 2DEG, reflect the conservation of the K-selection rule, and are excitonlike. The transitions are also associated with both the A- and B-valence bands. To verify the origin of the reflection and emission spectra, the top AlGaN layer was removed by reactive ion etching. After etching, only the excitonic reflection and emission spectra associated with GaN were observed. © 2003 American Institute of Physics.
  Journal of Applied Physics 09/2003; 94(7):4263-4266. · 2.17 Impact Factor
• Conference Proceeding: Growth and properties of n- and p-type ZnO

  C.W. Litton, D.C. Look, B.B. Claflin, D.C. Reynolds, G. Cantwell, D.B. Eason, R.D. Worley
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  ABSTRACT: We will focus our attention on growth and properties of low-resistivity, n-type ZnO single crystals (μ_e ∼ 230 cm²/V-s, n_e ∼ 1 × 10¹⁷/cm³, at RT), including study and identification of their characteristic shallow residual donors that are thought to control the high electrical conductivity observed in as-grown, n-type ZnO crystals. We will also direct some of our attention to study of the electrical and optical properties of recently demonstrated, nitrogen-doped, homoepitaxial p-type ZnO films grown by molecular beam epitaxy (MBE), either on low-resistivity (ρ) substrates, or on Li-diffused, semi-insulating ZnO substrates. Hall mobilities, carrier concentrations and resistivities of the p-type ZnO films ranged from μ_p = 1-2 cm²/V-s, n_p = 9 × 10¹⁶/cm³ to 1-5 × 10¹⁸/cm³, and ρ ∼ 10-40 ohm-cm, respectively. SIMS profiles have shown that the p-type films, with thickness typically in the range of 1-2 μm, contained large concentrations of N-atoms (as high as 1 × 10¹⁹/cm³ in the films) against a background of N in the substrates ∼ 1 × 10¹⁷/cm³, and temperature-dependent Hall measurements of activation energies have shown that the binding energy (BE) of N-acceptors in ZnO is -146 meV, comparable to the BE of shallow Mg acceptors in GaN (∼ 165 meV); conductivity measurements have shown that at least one of our p-type films is prone to instability and type conversion at cryogenic temperatures and under illumination with room light. MBE growth conditions necessary to achieve p-type ZnO:N films/ZnO are discussed together with the characteristic bandedge photoluminescence (PL) signatures of the p-type layers.
  Compound Semiconductors: Post-Conference Proceedings, 2003 International Symposium on; 09/2003
• Article: Temperature dependence of free excitons in GaN

  D. C. Reynolds, J. Hoelscher, C. W. Litton, T. C. Collins
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  ABSTRACT: The excitons involved in this study are the longitudinal and the Γ5 and Γ6 free excitons, as well as the donor bound exciton (D0,X). The temperature dependence of the energy positions of the Γ5, Γ6, and (D0,X) excitons are well accounted for by the Varshni equation (Y. P. Varshni, Physica (Amsterdam) 34, 149 (1967)). In the same temperature range, the energy positions of the longitudinal excitons depart from the predictions of the Varshni equation. The separation between the longitudinal- and transverse-mode free excitons has been previously reported. One component of this separation is the polarizability, which has a temperature dependence. The longitudinal exciton therefore has a band-gap temperature dependence, predicted by the Varshni equation, as well as an additional dependence due to polarizability. This temperature dependence has been accounted for by the Varshni equation, plus an additional linear and a quadratic temperature dependent term. © 2002 American Institute of Physics.
  Journal of Applied Physics 10/2002; 92(9):5596-5598. · 2.17 Impact Factor
• Article: Characterization of homoepitaxial p-type ZnO grown by molecular beam epitaxy

  D. C. Look, D. C. Reynolds, C. W. Litton, R. L. Jones, D. B. Eason, G. Cantwell
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  ABSTRACT: An N-doped, p-type ZnO layer has been grown by molecular beam epitaxy on an Li-diffused, bulk, semi-insulating ZnO substrate. Hall-effect and conductivity measurements on the layer give: resistivity = 4×101 Ω cm; hole mobility = 2 cm2/V s; and hole concentration = 9×1016 cm−3. Photoluminescence measurements in this N-doped layer show a much stronger peak near 3.32 eV (probably due to neutral acceptor bound excitons), than at 3.36 eV (neutral donor bound excitons), whereas the opposite is true in undoped ZnO. Calibrated, secondary-ion mass spectroscopy measurements show an N surface concentration of about 1019 cm−3 in the N-doped sample, but only about 1017 cm−3 in the undoped sample. © 2002 American Institute of Physics.
  Applied Physics Letters 09/2002; 81(10):1830-1832. · 3.84 Impact Factor
• Article: Polariton and free-exciton-like photoluminescence in ZnO

  D. C. Reynolds, D. C. Look, B. Jogai, T. C. Collins
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  ABSTRACT: An unusual photoluminescence line X has been observed in ZnO at an energy between that of the common donor-bound excitons (DBEs) and the free excitons (FEs). In the presence of a high carrier concentration, induced by a second below-band gap laser, the DBEs decrease in intensity, due to screening, and both the FEs and X increase. Thus, X has free-exciton, rather than bound-exciton, character. However, its electric-field vector lies in the plane perpendicular to the c axis, as is also found for the DBEs. The appearance of X is discussed in terms of the polariton picture. © 2001 American Institute of Physics.
  Applied Physics Letters 12/2001; 79(23):3794-3796. · 3.84 Impact Factor
• Article: Photoluminescence of GaN grown by molecular-beam epitaxy on a freestanding GaN template

  M. A. Reshchikov, D. Huang, F. Yun, L. He, H. Morkoç, D. C. Reynolds, S. S. Park, K. Y. Lee
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  ABSTRACT: Photoluminescence (PL) studies were performed on a 1.5-μm-thick GaN layer grown by molecular-beam epitaxy on a freestanding GaN template that in turn was grown by hydride vapor-phase epitaxy. PL spectra from both the epilayer and the substrate contain a plethora of sharp peaks related to excitonic transitions. We identified the main peaks in the PL spectrum. Taking advantage of the observation of donor bound exciton peaks and their associated two-electron satellites, we have determined the binding energies of two distinct shallow donors (28.8 and 32.6 meV), which are attributed to Si and O, respectively. © 2001 American Institute of Physics.
  Applied Physics Letters 12/2001; 79(23):3779-3781. · 3.84 Impact Factor
• Article: Fine structure on the green band in ZnO

  D. C. Reynolds, D. C. Look, B. Jogai
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  ABSTRACT: An emission band at 2.4 eV, called the green band, is observed in most ZnO samples, no matter what growth technique is used. Sometimes this band includes fine structure, which consists mainly of doublets, repeated with a longitudinal-optical-phonon-energy spacing (72 meV). We have developed a vibronic model for the green band, based on transitions from two separate shallow donors to a deep acceptor. The donors, at energies 30 and 60 meV from the conduction-band edge, respectively, are also found from Hall-effect measurements. © 2001 American Institute of Physics.
  Journal of Applied Physics 05/2001; 89(11):6189-6191. · 2.17 Impact Factor
• Article: Evidence for shallow acceptors in GaN

  D. C. Reynolds, D. C. Look, B. Jogai, R. J. Molnar
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  ABSTRACT: Two low-temperature photoluminescence lines in GaN, in the region of energies commonly interpreted as longitudinal optical-phonon replicas of free excitons, donor-bound excitons, or acceptor-bound excitons, are reinterpreted as acceptor-bound excitons (A0X’s) collapsing to n=2 and n=3 excited states, respectively, of the acceptors involved. Application of this model to two sets of A0X-related lines in hydride-vapor-phase-grown GaN gives acceptor energies of 85±1, and 115±1 meV, respectively. The existence of such shallow acceptor states, if confirmed, is of great technological importance. © 2001 American Institute of Physics.
  Journal of Applied Physics 05/2001; 89(11):6272-6274. · 2.17 Impact Factor
• Article: Short wavelength impurity exciton transitions in CdS at 1.2 K

  D C Reynolds, C W Litton, T C Collins
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  ABSTRACT: Emission lines on the high energy side of the ground state exciton have been observed in CdS crystals. These are very sharp lines characteristic of bound exciton transitions. Zeeman splittings of the lines confirm that excitons bound to both neutral and ionized centres are involved. The most logical interpretation is that the emission lines are the result of radiative dissociation of excited state exciton complexes whose holes derive from the Gamma 9 valence band.
  Journal of Physics C Solid State Physics 03/2001; 3(10):2092.
• Article: Combined effects of screening and band gap renormalization on the energy of optical transitions in ZnO and GaN

  D. C. Reynolds, D. C. Look, B. Jogai
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  ABSTRACT: The energy positions of the optical transitions in both GaN and ZnO were investigated when the samples were excited simultaneously with a HeCd laser and an Ar+ ion laser. The increased number of free electrons excited by the Ar+ ion laser will effectively screen both the free exciton and bound exciton transitions, resulting in a blueshift. The increased number of free electrons also produces many-body effects, which lead to a reduction of the band gap energy and thus a redshift. The resultant of screening and renormalization results in a redshift of the optical transitions in ZnO but a nearly vanishing shift in GaN. © 2000 American Institute of Physics.
  Journal of Applied Physics 11/2000; 88(10):5760-5763. · 2.17 Impact Factor
• Article: Identification of the Γ5 and Γ6 free excitons in GaN

  D. C. Reynolds, D. C. Look, B. Jogai, A. W. Saxler, S. S. Park, J. Y. Hahn
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  ABSTRACT: The Γ5 and Γ6 free excitons have been identified in GaN from emission measurements. Another emission peak is also observed which we believe to be the longitudinal free exciton. These measurements along with electrical measurements, which show the sample to have very high peak mobility, attest to the high quality of the sample. © 2000 American Institute of Physics.
  Applied Physics Letters 10/2000; 77(18):2879-2881. · 3.84 Impact Factor
• Article: Time-resolved photoluminescence lifetime measurements of the Γ5 and Γ6 free excitons in ZnO

  D. C. Reynolds, D. C. Look, B. Jogai, J. E. Hoelscher, R. E. Sherriff, M. T. Harris, M. J. Callahan
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  ABSTRACT: Time-resolved photoluminescence spectroscopy at 2 K was used to measure the radiative recombination lifetime of the allowed (Γ5) and forbidden (Γ6) free excitons in ZnO. The measurements were made on a sample containing internal strain, which altered the sample symmetry, and resulted in relaxed selection rules, allowing the Γ6 exciton to be observed. A radiative recombination lifetime of 259 ps was measured for the Γ5 exciton and 245 ps for the Γ6 exciton. The decay of the free excitons was of single-exponential form, and the decay times were obtained using a least-squares fit of the data. © 2000 American Institute of Physics.
  Journal of Applied Physics 08/2000; 88(4):2152-2153. · 2.17 Impact Factor
• Article: Strain variation with sample thickness in GaN grown by hydride vapor phase epitaxy

  D. C. Reynolds, D. C. Look, B. Jogai, J. E. Hoelscher, R. E. Sherriff, R. J. Molnar
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  ABSTRACT: High quality GaN crystals can be grown on sapphire by hydride vapor phase epitaxy. The thermal expansion mismatch between sapphire and GaN produces strain in the GaN crystal as it is cooled from the growth temperature to room temperature. The strain is evidenced by shifts in the photoluminescence and reflectance line positions. By analyzing the surface strain as the crystal thickness is increased, the thickness required to obtain zero surface strain can be estimated. This structure might provide a lattice matched and thermally matched substrate for further epitaxial growth of GaN. © 2000 American Institute of Physics.
  Journal of Applied Physics 07/2000; 88(3):1460-1463. · 2.17 Impact Factor
• Article: Dissociation of Al2O3(0001) substrates and the roles of silicon and oxygen in n-type GaN thin solid films grown by gas-source molecular beam epitaxy

  J. E. Van Nostrand, J. Solomon, A. Saxler, Q.-H. Xie, D. C. Reynolds, D. C. Look
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  ABSTRACT: Unintentionally doped and silicon doped GaN films prepared by molecular beam epitaxy using ammonia are investigated. Hall, secondary ion mass spectroscopy (SIMS), photoluminescence, and x-ray data are utilized for analysis of sources of autodoping of GaN epitaxial films in an effort to identify whether the n-type background electron concentration is of impurity origin or native defect origin. We identify and quantify an anomalous relationship between the Si doping concentration and free carrier concentration and mobility using temperature dependent Hall measurements on a series of 2.0-μm-thick GaN(0001) films grown on sapphire with various Si doping concentrations. SIMS is used to identify oxygen as the origin of the excess free carriers in lightly doped and undoped GaN films. Further, the source of the oxygen is positively identified to be dissociation of the sapphire substrate at the nitride-sapphire interface. Dissociation of SiC at the nitride-carbide interface is also observed. Finally, SIMS is again utilized to show how Si doping can be utilized to suppress the diffusion of the oxygen into the GaN layer from the sapphire substrate. The mechanism of suppression is believed to be formation of a Si–O bond and a greatly reduced diffusion coefficient of the subsequent Si–O complex in GaN. © 2000 American Institute of Physics.
  Journal of Applied Physics 07/2000; · 2.17 Impact Factor
• Article: Changes in electrical characteristics associated with degradation of InGaN blue light-emitting diodes

  Z. -Q. Fang, D. C. Reynolds, D. C. Look
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  ABSTRACT: Because of the high concentration of threading dislocations, the reverse current-voltage (I–V) characteristics for either homo- or heterojunctions made on GaN-based materials grown on sapphire often show a strong electric field dependence (called a soft breakdown characteristic), which can be described by a power law I=Vn, with n between 4 to 5. We find a significant increase of reverse currents associated with the early degradation of emission in InGaN blue single-quantum-well light-emitting diodes (LEDs) subjected to aging tests (injected current of 70 mA over a total time of about 300 h). The formation of dislocations might be due to the relaxation of strain in the thin InGaN active layer during the aging tests.
  Journal of Electronic Materials 03/2000; 29(4):448-451. · 1.47 Impact Factor