Structure and Electrophysical Properties of the Transparent Conducting Zinc and Indium Oxides Films V.A. Kulbachinskii V.G. Kytin, O.V. Reukova, D.S. Glebov, D.D. Melnik, A.R. Kaul, L.I. Burova M.V. Lomonosov Moscow State University, Moscow, Russia Yu.M. Galperin, A.G. Ulyashin University of Oslo, Oslo, Norway

1) Transparent conducting oxides Outline 1) Transparent conducting oxides 2) Zinc oxide: - crystal structure; - electronic structure; - defects and dopants MOCVD grown undoped Ga doped ZnO films Co doped ZnO films 3) Indium oxide (In2O3): - crystal structure - electronic structure - properties of Sn doped In2O3 4) Summary 2

Transparent conducting oxides and their applications 3

Crystal structure of ZnO Basic structures of ZnO* Zinc blende O Wurtzite Rocksalt Zn a=3.250 Å c=5.206 Å 4

Band structure of ZnO wurzite Band structure of ZnO calculated by non-local (solid lines) and local (dashed lines) empirical pseudo potential method* Brillouin zone of wurtzite ZnO Electron effective mass: mII≈m┴≈(0.25-0.3)m0* 5

Defects in ZnO Calculated formation energies of native defects in ZnO* Native donor defects: - Oxygen vacancies VO - Zinc interstitials Zni Native acceptor defects: - Zn vacancies ZnV - O interstitials Oi Calculated formation energies of native defects in ZnO* O rich conditions Zn rich conditions 6

Epitaxial films with orientation determined by substrate Structure of undoped ZnO films grown on R-Al2O3 and ZrO2(Y2O3)(111) substrates by oxygen assisted MOCVD XRD data of ZnO films grown on R-Al2O3 and ZrO2(Y2O3) (111) substrates: a) and b) on R-Al2O3 θ-scan and φ-scan; c) and d) on ZrO2(Y2O3)(111) θ-scan and φ-scan Epitaxial films with orientation determined by substrate 7

Structure of undoped ZnO films grown on MgAl2O4 (111) substrates by oxygen assisted MOCVD XRD data of ZnO films grown on MgAl2O4(111) substrates: a) θ-scan; b) φ-scan Epitaxial films with 2 in-plain orientations of ZnO with respect to substrate 8

Structure of undoped ZnO films grown by water assisted MOCVD XRD θ-scans of ZnO films grown on r-Al2O3 and ZrO2(Y2O3) (111) substrates at 300 0C by water assisted MOSVD No visible structure from ZnO in φ-scans Polycrystalline films with chaotic orientation of crystallites 9

Surface morphology of undoped ZnO films grown by oxygen and water assisted MOCVD AFM images of the surface of ZnO films grown on ZrO2(Y2O3) (111) substrates at 600 0C O2 assisted MOCVD H2O assisted MOCVD rms 4.68 nm rms 40.78 nm Surface of ZnO films grown by water assisted MOCVD is smoother than by oxygen assisted MOCVD 10

SEM image ZnO film grown on R-Al2O3 at 300 0C Surface morphology of undoped ZnO films grown by water assisted MOCVD at different temperatures SEM image ZnO film grown on R-Al2O3 at 300 0C SEM image ZnO film grown on R-Al2O3 at 500 0C 11

Magnetic properties of undoped ZnO films grown by water assisted MOCVD at different temperatures M(H) at room temperature for the films deposited by water-assisted CVD on r-sapphire substrates at 300 °C (R_W_300) and at 500 °C (R_W_500). 12

Resistivity of undoped ZnO films grown by oxygen assisted MOCVD Lowest resistivity have the most ordered films grown on R-Al2O3 and ZrO2(Y2O3)(111) substrate Highest resistivity have the films with 2 different orientation of crystallites grown on MgAl2O4(111) substrates 13

Hopping conductivity in undoped ZnO films grown by oxygen assisted MOCVD Mott's law: 14

Resistivity of undoped ZnO films grown by water assisted MOCVD Variable range hopping conductivity in a wide temperature range 15

Magnetoresistance of undoped ZnO films grown by oxygen assisted MOCVD 16

ZnO:Ga films X-ray data of ZnO:Ga films deposited at 600 0C on ZrO2(Y2O3)(111) substrate by oxygen assisted MOCVD: a) 1.7 at. % Ga; b) 3.6 at. % Ga. Shift of ZnO peak positions correspond to increase of lattice constant with increase of Ga content 17

Resistivity of ZnO:Ga films grown by oxygen assisted MOCVD ZnO:Ga on R-Al2O3 substrate ZnO:Ga on ZrO2(Y2O3)(111) substrate Resistivity decreases first with an increase of Ga content 18

Magnetoresistance of ZnO:Ga films grown by oxygen assisted MOCVD 19

Resistivity of ZnO:Ga films grown by water assisted MOCVD Variable range hopping conductivity in investigated temperature range 20

Magnetoresistance of ZnO:Ga films grown by water assisted MOCVD Estimate of r0 and g(EF) from positive magnetoresistance and ρ(T) Substrate Ga content, at. % r0, nm g(EF), 1019 cm-3eV-1 R-Al2O3 25 7 13 ZrO2(Y2O3)(111) 8 3.8 21 21

ZnO:Co films deposited by oxygen assisted MOCVD Epitaxial films with oriention determined by substrate 22

Structure of ZnO:Co films deposited by water assisted MOCVD No peaks in φ-scans. Polycrystalline structure with chaotic orientation of crystallites. 23

Cobalt substitutes Zn up to 33 at. % content EXAFS (extended X ray absorption fine structure ) spectra and Co state in ZnO:Co films EXAFS-спектроскопия — новый метод исследования вещества, позволяющий определять структурные параметры ближнего окружения атомов с выбранным Z, спектры которых изучаются. Среди этих параметров — межатомные расстояния, координационные числа, амплитуды тепловых колебаний. Существование дальнего порядка в исследуемых образцах не требуется. В зависимости от применяемой методики получения спектров можно анализировать ближнее окружение атомов, расположенных либо в объеме образца, либо на его поверхности. Cobalt substitutes Zn up to 33 at. % content 24

Magnetic properties of ZnO:Co film 25

Magnetoresistance of ZnO:Co films Value of positive magnetoresistance increases with an increase of Co content Value of positive magnetoresistance is larger for the films grown by oxygen assisted MOCVD Possible origin of positive magnetoresistance: reduction of the density of states at Fermi energy in magnetic field 26

Two inequivalent positions of In cations In2O3 Кубическая структура типа биксбита пространственной группы . Two inequivalent positions of In cations 27

Band structure of In2O3 Brillouin zone of In2O3 28

Investigated In2O3:Sn films Valence band XPS spectra of In2O3:Sn films Deposition method: magnetron sputtering Targets: 1) Oxide target (baked In2O3 and SnO2 9:1); 2) metal target In-Sn alloy Substrate: glass Target Deposition temperature, 0C Film thickness oxide RT 80 230 75 Valence band XPS spectra of In2O3:Sn films Bandgap states in films deposited from metal target 29

X-ray phoemission data about O state in In2O3:Sn films High resolution O 1s spectra acquired after slight sputtering using low energy (500 eV) Ar+ 30

XPS spectra of In and Sn states in In2O3:Sn films High resolution In 3d XPS spectra of ITO films deposited at different conditions High resolution Sn 3d XPS spectra of ITO films deposited at different conditions Estimate of the film composition determined from XPS data Target Deposition temperature In content, at. % Sn content, at. % O content, at. % oxide RT 47.41 3.65 0.96 230 0C 47.66 3.50 0.95 metal 47.93 4.04 0.92 31

Resistivity of In2O3:Sn films deposited from oxide target The conductivity of the films deposited at 230 0C is higher than the conductivity of the films deposited at room temperature. This correlates with the better crystallinity of the film deposited at 230 0C Treatment in H plasma leads to the increase of conductivity. The Effect is larger for 5 min treatment than for 30 min treatment 32

Magnetoresisitance of In2O3:Sn films deposited from oxide target Negative magnetoresistance is explained by weak localization theory 33

Resistivity and magnetoresistance of In2O3:Sn films deposited from metal target in oxygen deficit conditions 2D variable range hopping conductivity. Large localization length r0>35 nm. Negative magnetoresistance could be caused by increase of localization length in magnetic field 34

Summary - Electron mobility in ZnO and In2O3:Sn films correlates with degree of crystallinity: the better is the crystallinity the larger is the electron mobility. - Electron transport in highly crystalline ZnO, ZnO:Ga and films is bandlike. - Electron transport in polycrystalline ZnO, ZnO:Ga films and oxygen deficient In2O3:Sn films is hopping. - Electron concentration in ZnO:Ga films is essentially smaller than concentration of Ga atoms. - Electron transport in ZnO:Co films is hopping at low temperatures. Increase of Co content in ZnO films leads to increase of paramagnetic susceptibility and large positive magnetoresistance at low temperatures. This magnetoresistance could be explained by Zeemann splitting of electronic energy levels in magnetic filed. - Conductivity of In2O3:Sn films deposited from oxide target is larger than conductivity of ZnO:Ga films grown by oxygen assisted MOCVD due to larger electron concentration. The increase of substrate temperature from RT to 230 °C leads to essential increase of the electron mobility and film conductivity. 35