Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P11: Dopants and Defects in Semiconductors - OxidesFocus
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Sponsoring Units: DMP DCOMP FIAP Chair: Joel Varley, Lawrence Livermore National Laboratory Room: LACC 303A |
Wednesday, March 7, 2018 2:30PM - 3:06PM |
P11.00001: Doping and Compensation in Wide-Band-Gap Oxides Invited Speaker: David Look Wide-band-gap oxides are extensively used as active or passive elements in various electronic and optoelectronic devices. Advantages include transparency, high breakdown voltages, and a wide range of conductivities, from semi-insulating to highly conducting. The conductivities are controlled by donors (ND) and acceptors (NA) so it is important to know their identities (impurities or defects), concentrations, formation energies, and electronic energies (ED and EA). In this talk, we will concentrate on three important oxides, Ga2O3, ZnO, and InGaZnO. The values of ND, NA, and ED, are determined by theoretical fits of temperature-dependent Hall-effect measurements and the donor and acceptor identities are determined by comparison with SIMS and positron annihilation measurements, as well as density functional theory (DFT). For Ga2O3 and ZnO, it is easy to achieve ND > 5 x 1020 cm-3 by doping with Si or Ga, respectively, and indeed we find that ND ≈ [Si] in Ga2O3:Si, and ND ≈ [Ga] in ZnO:Ga. Moreover, the resulting high Fermi levels can drive the formation of native acceptors, Ga and Zn vacancies, respectively, so that, e.g., NA ≈ [VZn] in ZnO:Ga, rather than consisting of impurity-type acceptors. Optical measurements are also affected by the high carrier concentrations; e.g, bulk plasmons can produce a strong reflectance near hν ≈ 1 eV, for n ≈ 1021 cm-3. Finally, we can observe interesting quantum corrections to the resistivity, some following expected theoretical dependences on temperature and magnetic-field strength, and some not. |
Wednesday, March 7, 2018 3:06PM - 3:18PM |
P11.00002: Hidden Hydrogen in β-Ga2O3 W Fowler, Ying Qin, Philip Weiser, Michael Stavola Hydrogen impurities may play a crucial role in the electrical conductivity of β-Ga2O3 by acting as shallow donors and by passivating cation-vacancy acceptor complexes [1,2]. IR spectroscopy of β-Ga2O3 single crystals treated in an H2 (D2) ambient yields a strong, polarized vibrational line at 3437 (2546) cm-1 originating from a defect containing two equivalent O-H(D) partners. Theoretical calculations using the CRYSTAL06 code [3] with hybridized DFT Hamiltonian yield a plausible model for this defect in which the two H(D) are associated with a relaxed Ga(1) vacancy[4]. The observed production of this defect under a two-step annealing process[5] signifies the existence of “hidden” hydrogen that acts as a source of H not readily seen in vibrational spectroscopy. Candidates for this species, including H2 at interstitial or vacancy sites, are analyzed theoretically. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P11.00003: Intrinsic Point Defects, Charge Neutrality, and Self-Doping in β-Ga2O3 Elan Weiss, Christian Oberdorfer, Travis Withrow, Kevin Krymowski, Yaxian Wang, Wolfgang Windl We present a holistic method to determine intrinsic doping and identify the charge neutrality point of semiconductors using density functional theory. By relying strictly on the physical meaning of charge neutrality point we are able to more accurately determine the Fermi level of semiconductors doped with native point defects at temperature, especially when coupled to chemical potentials for the defect-energy calculations that are coupled to the specific stoichiometry of the crystal. This method is applied to β-Ga2O3, a wide bandgap semiconductor known for being a deep UV transparent conducting oxide. Using hybrid-functional based DFT, we examine the tunability of the Fermi level and intrinsic point defects with respect to composition for the two extremes of crystal growth: instantaneous quenching and adiabatic cooling. In the case of stoichiometric β-Ga2O3 we’ve found the traditional method to computationally determine charge neutrality points to not properly describe the intrinsic Fermi level, whereas our method does. Our results suggest an explanation for the n-type nature of the grown crystals based on the grown stoichiometry and the imperfections present. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P11.00004: Surveying acceptor dopants in beta-Ga2O3 John Lyons With a wide band gap, high critical breakdown voltage, and low-cost substrates, Ga2O3 is a promising material for power electronics. As with many wide-band-gap semiconductors, obtaining better control over its electrical conductivity is critically important. Since both theory and experiment have indicated holes self-trap in Ga2O3, efficient p-type doping is not expected. However, a better understanding of acceptor dopants will be necessary for the full development of this material. In this work, the properties of group-II and group-V acceptor impurities in beta-Ga2O3 are explored using first-principles calculations based on hybrid density functional theory. Acceptor ionization energies and formation energies of these potential dopants are compared, and optical transitions are also determined for comparison with experiment. |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P11.00005: The Production of Hydrogen Defects in Ga2O3 Ying Qin, Philip Weiser, Michael Stavola, W Fowler, Stephen Pearton We have recently discovered an O-H vibrational line at 3427 cm-1 in beta-Ga2O3. This line was found to be strongly polarized along the [010] direction of a (-201)-oriented wafer and has been assigned to a VGa-2H complex [1]. Surprisingly, this defect has been found to be most effectively produced by a two-step annealing process. A first anneal in an H2 ambient at 900°C produces a weak line at 3437 cm-1. A second anneal in flowing N2 at 450°C greatly increases the intensity of the 3437 cm-1 line. These results show that a “hidden” form of H can be produced in Ga2O3 that can be converted into the VGa-2H complex by an addition thermal annealing treatment in an inert ambient. Similarly, the implantation of protons into Ga2O3 at room temperature produces a weak line at 3427 cm-1 that is made roughly 4 times more intense by a subsequent anneal at 450°C. The identity of this “hidden” form of H in Ga2O3 remains as a puzzle. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P11.00006: Hydrogen is the prime suspect for persistent photoconductivity in SrTiO3 Violet Poole, Jesse Huso, Matthew McCluskey When annealed at 1200°C, strontium titanate (SrTiO3) exhibits large persistent photoconductivity (PPC) at room temperature. Upon exposure to sub-band-gap light, the resistance drops by 3 orders of magnitude, a change that persists “forever” after the light is turned off. Recently PPC was exploited to write a conductive path on a crystal, using a 405 nm laser. The defect responsible for this remarkable effect has not been identified. In this talk, the results of varying the annealing gas will be discussed. The experiments provide evidence that light causes hydrogen to leave the substitutional oxygen site and form O-H bonds. This liberates electrons, causing PPC. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P11.00007: Computational search for useful hole-electron dopants in SnO and SnO2 Migle Grauzinyte, Jose Flores Livas, Stefan A Goedecker
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Wednesday, March 7, 2018 4:18PM - 4:30PM |
P11.00008: Strain modulated oxygen vacancy and polaron formation in rutile TiO2 Yi-feng Zheng, Xingao Gong Strain is routinely used as a powerful method to achieve certain functional materials and the formation of oxygen vacancies (OVs) largely determines the material properties in many oxides like rutile TiO2. In this work by hybrid functional (HSE06) calculations we find that strain can effectively modulate the formation of OVs in rutile . Biaxial tensile strain significantly lowers the formation energies of OVs and the concentrations of conduction electrons and OVs increase by several magnitudes, in consist with the semiconductor-to-metal transition observed in experiments. Biaxial compressive strain, however, increases the formation energies of OVs, in contrast with the continuum elastic model. The failure of the model is due to that strain changes the polaronic configurations as in the spin charge densities. The formation of OVs in rutile TiO2 is intimately related with the formation of polarons, while the latter is favored under tensile strain but less favored under compressive strain, which may be a reasonable explanation for the discrepancy of OV formation energies under different strain types. This provides a new insight into the defect and polaron formations and their interplay with strain in rutile TiO2 and possibly other similar materials. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P11.00009: Ab initio calculation of lattice constant variations with stoichiometry: Application to UO2+x Fabien Bruneval, Jean-Paul Crocombette, Michel Freyss Non stoichiometric uranium dioxide experiences a shrinkage of its lattice constant with increasing oxygen content, for both the hypo- and the hyper-stoichiometric regimes. Based on ab initio calculations within the DFT+U approximation, we have developed a point defect model that accounts for the volume of relaxation of the most significant intrinsic defects of UO2. Our model takes a special care about the treatment of the charged defects in the equilibration [1] and in the determination of reliable defect volumes of formation [2]. |
Wednesday, March 7, 2018 4:42PM - 4:54PM |
P11.00010: Properties of point defects in nano-porous SiC insulators Joseph Noonan, Blair Tuttle Point defect assisted leakage throughout nanoporous SiC insulators is important for integrated circuits. Using atomic models of nano-porous SiC, we explore the properties of the point defects using density functional calculations. We calculate H passivation energies, gap levels, and hyperfine parameters. The present results are combined with experimental and theroetical efforts to understand leakage in nano-porous SiC insulators. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P11.00011: Al2O3 Dielectric Layers on H-terminated Diamond: Controlling Surface Conductivity Yu Yang, Franz Koeck, Maitreya Dutta, Xingye Wang, Srabanti Chowdhury, Robert Nemanich Dielectric layers on H-terminated diamond have enabled recent breakthroughs in high voltage and high temperature FET operation. This study investigates how the surface conductivity of H-terminated diamond can be preserved and stabilized by using a dielectric layer with an in situ post-deposition treatment. Thin Al2O3 layers were grown by PEALD on H-terminated undoped diamond (100) surfaces. The changes of the hole accumulation layer were monitored by correlating the binding energy of the diamond C 1s core level with electrical measurements. The initial Al2O3 PEALD resulted in an increase of the C 1s core level binding energy consistent with a reduction of surface conductivity. A hydrogen plasma step restored the C 1s binding energy and the diamond surface resistance within the range for surface transfer doping. Further PEALD growth did not appear to degrade the surface conductive layer according to the position of the C 1s core level and electrical measurements. This work provides insight into approaches to establish and control the two-dimensional hole-accumulation layer of H-terminated diamond and improve the stability and performance of H-terminated diamond electronic devices. |
Wednesday, March 7, 2018 5:06PM - 5:18PM |
P11.00012: Electrical Impedance Studies of Iron-Doped ZnO nanostructures Ganga Neupane, Parameswar Hari Undoped and iron-doped (5%, 10%, and 15%) zinc oxide (Fe-ZnO) nanostructures were synthesized via a hydrothermal precipitation method. Structural properties of Fe-ZnO were studied by means of X-ray diffraction spectroscopy (XRD), Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). XRD studies showed the existence of wurtzite ZnO structure in all Fe-ZnO samples. SEM and TEM studies of Fe-ZnO revealed changes in from nano particles to rod shape when the temperature of the precipitation bath was increased from 70 C to 90 C. Electrical transport properties of Fe-ZnO were analyzed by Cole-Cole plot using impedance spectroscopy from 70C to 90 C and were modeled by equivalent RC circuit. The cole-cole spectrum reveals that the resistance of doped sample decreased 70 C to 80C as the morphology changes from nanoparticle to nanorods. From 80 C – 90 C, the electrical resistance of Fe-ZnO nanorod structure increased from 4500 Ω to 5500 Ω. We will discuss the implications of these results based on a transport model. |
Wednesday, March 7, 2018 5:18PM - 5:30PM |
P11.00013: Phosphorus doping of insulating metal-oxide surfaces Muhammed Acikgoz, Michele Pavanello
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