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Core-shell ZnO/ZnMnO nanowires on a-[Al.sub.2][O.sub.3] and GaN (buffer layer)/Si (111) substrates are prepared by pulsed laser deposition using a Au catalyst. Resonant Raman measurements have shown an increase in the multiphoton scattering caused by disorder in ZnO upon doping by Mn.

The synthesis of the ZnO nanowires on 10 x 10 m[m.sup.2] a-plane sapphire substrates by high-pressure pulsed laser deposition is described. A scanning electron microscopy and electron paramagnetic resonance study in the X-band is done and the observations are presented.

A consistent set of epitaxial, n-type conducting ZnO thin films, nominally undoped, doped with Ga or Al, or alloyed with Mg or Cd, was grown by pulsed laser deposition (PLD) on single-crystalline c-plane sapphire (0 0 0 1) substrates, and characterized by Hall measurement, and UV/VIS optical transmission spectroscopy.The optical band gap of undoped ZnO films at nearly 3.28 eV was shifted by alloying with Mg up to 4.5 eV and by alloying with Cd down to 3.18 eV, dependent on the alloy composition. In addition, highly doped ZnO:Al films show a blue-shifted optical absorption edge due to filling of electronic states in the conduction band.The Hall transport data of the PLD (Mg,Zn,Cd)O:(Ga,Al) thin films span a carrier concentration range of six orders of magnitude from 3 × 1014 to 3 × 1020 cm−3, which corresponds to a resistivity from 5 × 10−4 to 3 × 103 Ω cm. Structurally optimized, nominally undoped ZnO films grown with ZnO nucleation and top layer reached an electron mobility of 155 cm2/V s (300 K), which is among the largest values reported for heteroepitaxial ZnO thin films so far.Finally, we succeeded in combining the low resistivity of ZnO:Ga and the band gap shift of MgZnO in MgZnO:Ga thin films. This results demonstrate the unique tunability of the optical and electrical properties of the ZnO-based wide-band gap material for future electronic devices. [Copyright &y& Elsevier]

The relaxation dynamics of charge carriers in an epitaxial La[sub 0.7]Ca[sub 0.3]MnO[sub 3] film was studied by the light reflection method with a femtosecond time resolution in a wide temperature range. The relaxation of time-resolved photoinduced reflections was shown to be two-component in the whole temperature range including the paramagnetic dielectric–ferromagnetic metal transition at T ≈ 150–160 K. The fast relaxation component had a maximum lifetime (τ ∼ 500 ps) in the transition region, and its contribution to relaxation increased as temperature decreased. The lifetime of the slow component was minimum (τ ∼ 15 fs) in the transition region. In addition, fast oscillations assigned to coherent phonons appeared in photoinduced responses at T < 200 K. The dephasing time of these oscillations increased as temperature decreased, whereas their frequency changed insignificantly. © 2003 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

Polarized micro-Raman measurements were performed to study the phonon modes of Fe, Sb, Al, Ga, and Li doped ZnO thin films, grown by pulsed-laser deposition on c-plane sapphire substrates. Additional modes at about 277, 511, 583, and 644 cm[sup -1], recently assigned to N incorporation [A. Kaschner et al., Appl. Phys. Lett. 80, 1909 (2002)], were observed for Fe, Sb, and Al doped films, intentionally grown without N. The mode at 277 cm[sup -1] occurs also for Ga doped films. These modes thus cannot be related directly to N incorporation. Instead, we suggest host lattice defects as their origin. Further additional modes at 531, 631, and 720 cm[sup -1] seem specific for the Sb, Ga, and Fe dopants, respectively. Li doped ZnO did not reveal additional modes. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: ZnO/ZnO:Mn core–shell nanowires were studied by means of X-ray absorption spectroscopy of the Mn K- and L2,3-edges and electron energy loss spectroscopy of the O K-edge. The combination of conventional X-ray and nanofocused electron spectroscopies together with advanced theoretical analysis turned out to be fruitful for the clear identification of the Mn phase in the volume of the core–shell structures. Theoretical simulations of spectra, performed using the full-potential linear augmented plane wave approach, confirm that the shell of the nanowires, grown by the pulsed laser deposition method, is a real dilute magnetic semiconductor with Mn2+ atoms at the Zn sites, while the core is pure ZnO. [Copyright &y& Elsevier]

The ordinary optical dielectric functions of pulsed-laser-deposition grown wurtzite c-plane MgxZn1−xO (0≤x≤0.53) thin films have been determined by using spectroscopic ellipsometry in the photon energy range from 4.5 to 9.5 eV. The dielectric functions reveal features which resemble those previously detected in uniaxial AlGaN and identified as E1- and E2-type band-to-band transitions with no remarkable dependence of the transition energy on Mg content x. The E1- and E2-type transitions for ZnO are compared with pseudopotential band-structure calculations. [Copyright &y& Elsevier]

Generalized infrared spectroscopic ellipsometry for wave numbers from 370 to 1200 cm−1 was applied to study the anisotropic dielectric response of wurtzite-type ZnO thin films grown on r-plane sapphire. Combined wavelength-by-wavelength analysis of data measured at multiple sample in-plane orientations allows for simultaneous extraction of the dielectric function (DF) tensor, the film thickness and the respective ZnO c-axis orientation, specifically in relation to the sapphire substrate. For anisotropic materials and use of generalized ellipsometry, the correlation problem between thickness and DF does not exist. Subsequent line shape analysis of ordinary and extraordinary DFs provides complete A1- and E1-symmetry longitudinal and transverse optical phonon-mode and broadening parameters, together with the static dielectric constants. The epitaxial relations from the ellipsometry data analysis are (1 1 −2 0)ZnO∣∣(0 1 −1 2)sapphire and [0 0 0 1]ZnO∣∣[0 −1 1 1]sapphire, which was confirmed by XRD measurements. [Copyright &y& Elsevier]