Doped ZnO

The phonon mode frequencies of wurtzite- and rocksalt-structure Mg Zni- O thin films vs. x, as obtained by combination of Raman scattering and IRSE, are plotted for 0 x 1 in Fig. 3.13. 

For the wurtzite-structure Mg Zni- O (x 0.53) thin films, an one-mode behavior with a further weak mode between the TO- and LO-mode for the phonons with E - and Ai-symmetry was found. The Ai(TO)-, Ai(LO)-, and the upper branch of the Ai(LO)-modes of the wurtzite-structure thin films show an almost linear behavior, whereas the lower branch of the Eq(LO)-modes and the two Ai(TO)-branches exhibit a nonlinear behavior. In [132] the modified random element isodisplacement (MREI) model was suggested 

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In addition to the processing technique, the properties of the oxides also changed by preparing them in a composite way. Arefian et al. [18] have synthesized SnO/ ZnO nanocomposite using the sonochemical method and studied the effects of temperature and power on the morphologies generated. Recently Mg doped ZnO... 

Kittilstved KR, Gamelin DR (2005) Activation of high-Tc ferromagnetism in Mn+2-doped ZnO using amines. J Am Chem Soc 127(15) 5292-5293... 

Anandan, S., Vinu, A., Mori, T., Gokulakrishnan, N., Srinivasu, P., Murugesan, V. and Ariga, K. (2007) Photocatalytic degradation of 2,4,6-trichlorophenol using lanthanum doped ZnO in aqueous suspension. Catalysis Communications, 8, 1377-1382. 

The most detailed NMR study of impurity band formation in a semiconductor in the intermediate regime involved 31P and 29 Si 7). line width and shift measurements at 8 T from 100-500 K for Si samples doped with P at levels between 4 x 1018 cm 3 and 8 x 1019 cm 3 [189], and an alternate simplified interpretation of these results in terms of an extended Korringa relation [185]. While the results and interpretation are too involved to discuss here, the important conclusion was that the conventional picture of P-doped Si at 300 K consisting of fully-ionized donors and carriers confined to extended conduction band states is inadequate. Instead, a complex of impurity bands survives in some form to doping levels as high as 1019 cm 3. A related example of an impurity NMR study of impurity bands is discussed in Sect. 3.8 for Ga-doped ZnO. 

The photovoltaic devices were then completed with a 50-nm layer of chemically deposited CdS, 50 nm of radio frequency (RF) sputtered intrinsic ZnO, and 350 nm of Al-doped ZnO and bilayer Ni/Al top contacts deposited by e-beam. Finally, a 100-nm layer of MgF2 is deposited by e-beam to minimize... 

A magnetic semiconductor thin him is made by doping ZnO with the 3d3 7 ion Co2+. The crystal hied splitting of the d orbitals in a tetrahedral site is opposite to that in an octahedral site, with the lower pair of levels labeled e and three upper orbitals labeled t2- (a) What is the spin state of the Co2+ ion in ZnO (b) What is the expected magnetic moment on the Co2+ ions (c) The spectrum has an absorption peak at 660 nm. What is the crystal held splitting of the Co2+ ion in the tetrahedral crystal held of ZnO ... 

H. Kim, J.S. Horwitz, W.H. Kim, S.B. Qadri, and Z.H. Kafafi, Anode material based on Zr-doped ZnO thin films for organic light-emitting diodes, Appl. Phys. Lett., 83 3809-3811, 2003. 

Antimony-doped SnOi films were deposited by adding SbCls to the deposition solution. Sb is a well-known n-type dopant used to increase the conductivity of SnOi films. The Sb concentration in the films increased hnearly with that in the deposition solution and was somewhat less than the solution concentration (e.g., 6% Sb in solution gave ca. 4% in the film). The Sb doping increased both the visible/near-lR transmission and mid-lR reflectance of the films, compared to the undoped films. These spectra are similar to those for doped ZnO (Fig. 7.3), and the effect of doping can be explained in the same way. The bandgap increased to 4.1 eV, compared to 3.56 eV for the undoped film, explained through band filhng by free electrons. 

Al-doped ZnO films were also deposited by adding AICI3 to the deposition solution. The amount of A1 in the films (given as at.% with respect to the Zn concentration) was somewhat smaller than that in the deposition solution but was proportional to the concentration in solution (up to the maximum measured concentration in the films of 5.5%). 

Fig. 21. Photocurrent during light pulse as in Fig. 19 at highly doped ZnO electrode (jVd cm-3) (1) no hydrochinon (2) 10 3 M hydrochinon...

V doped ZnO (V composition 0.05-0.15) with higher conductivity shows ferromagnetic behavior at room temperature, whereas that with lower conductivity is nonmagnetic (Saeki etal. 2001). 

Figure 6. ZnO band-gap (a) intensities and (b) energies collected during synthesis of Co2+-doped ZnO nanocrystals, plotted versus initial dopant concentration. [Adapted from (52).]...

Such polarized EXAFS measurements were carried out to study the structure of undoped and indium doped ZnO thin films prepared by the pyrosol process [11, 20]. Such films are promising materials as gas sensors, ultrasonic oscillators, piezoelectric transducers, and transparent electrodes in solar cells. 

Figure 4. a) FT of the polarized Zn K-edge EXAFS k3y(k) spectra for undoped and indium doped ZnO films at normal (full line) and grazing (dashed line) incident angles, h) Polarized FeFF6 simulations with the X-ray polarization vector parallel to the [101] direction (full line) and to the [010] direction (dashed line). 

Eu3+ Recently, Zhang et al. (2005) observed energy transfer from the host to Eu3+ in Eu3+-doped ZnO nanociystals synthesized by high temperature calcination method. The excitation spectrum consists in the strong band of the ZnO host (385 nm) and the characteristic excitation line spectrum of Eu3+ when the Eu3+ luminescence at 616 or 708 nm is monitored. Note that the latter wavelength was used to avoid the overlap with the host emis-... 

Ishizumi and Kanemitsu (2005) have studied PL properties of Eu3+ doped ZnO nanorods fabricated by a microemulsion method. The PL of bound exciton recombination and ZnO defects was observed near 370 and 650 nm under 325-nm light excitation, but no emission of Eu3+ occurred. On the other hand, the sharp PL peaks due to the intra-4f transitions of Eu3+ ions appeared under nonresonant excitation below the band-gap energy of ZnO (454 and 457.9 nm) in addition to direct excitation to 5D2 (465.8 nm). Therefore the authors concluded that the energy transfer occurs from the ZnO nanorods to Eu3+ ions through ZnO-defect states. This energy transfer mechanism seems very different from the previous one and more spectroscopic evidence is required to confirm it. 

Oxidation reactions have been the focus of several researchers in recent years. Several pathways are available for the activation of methane with two predominant ones being partial oxidation via oxidative coupling and the other being formation of oxygenates. Sojka, Herman and Klier have recently reported that formaldehyde selectivity can be enhanced by using a doubly promoted (Cu-Fe) doped ZnO with yields of 76 g HCHO (kg cat.) 1h . 31 These reactions were carried out at 750°C at 2.5% conversion. Singly doped Cu-ZnO catalysts yielded CO2 and H2O via deep oxidation whereas singly doped Fe-ZnO primarily yielded HCHO. Doubly doped Cu-Fe-ZnO minimized the formation of C2 products and therefore, the Cu-Fe-ZnO catalyst decreased C2 products and enhanced HCHO formation. 

Fig. 1.15. Electron concentration (dashed line) of Sn-doped indium oxide and Al-doped ZnO in dependence on oxygen partial pressure for a dopant concentration of 1 % [117]. With increasing oxygen partial pressure the donors become compensated by oxygen interstitials (In20s) or by zinc vacancies (ZnO). Reprinted with permission from [117]. Copyright (2007) by the American Physical Society...

Fig. 2.8. (a) Hall mobility as a function of the temperature for an undoped epitaxial ZnO layer and (b) Hall mobility of Ga-doped ZnO layers as a function of the carrier concentration. The ZnO films were grown epitaxially on lattice-matched ScAlMg04 (SCAM) by Makino et al. [64], In (a) the calculated mobilities for acoustical, polar-optical, piezoelectric, and ionized impurity scattering are shown, together with the total theoretical mobility. In (b) the solid curve is the fit curve (2.24) from Fig. 2.6, while the dashed line is the theoretical curve, calculated by Makino et al. [64]. The dotted line was calculated for transport across depletion regions at grain barriers (see Sect. 2.2.3), also present in epitaxial films [106]... 

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