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 $\text{644\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},$ 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 $\text{277\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ 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 $\text{720\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ seem specific for the Sb, Ga, and Fe dopants, respectively. Li doped ZnO did not reveal additional modes.

Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 ${\mathrm{cm}}^{\mathrm{-1}})$ spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry analysis and compared with previously measured near-band-gap index-of-refraction data using the Lyddane–Sachs–Teller relation. The ZnO model dielectric function spectra will become useful for future infrared ellipsometry analysis of free-carrier parameters in complex ZnO-based heterostructures.

A multistep pulsed-laser deposition (PLD) process is presented for epitaxial, nominally undoped ZnO thin films of total thickness of 1 to 2 μm on $c$ -plane sapphire substrates. We obtain reproducibly high electron mobilities from 115 up to $\text{155\hspace{0.17em}}{\mathrm{cm}}^2/\mathrm{V s}$ at 300 K in a narrow carrier concentration range from 2 to ${\text{5×10}}^{16}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}.$ The key issue of the multistep PLD process is the insertion of 30-nm-thin ZnO relaxation layers deposited at reduced substrate temperature. The high-mobility samples show atomically flat surface structure with grain size of about 0.5–1 μm, whereas the surfaces of low-mobility films consist of clearly resolved hexagonally faceted columnar grains of only 200-nm size, as shown by atomic force microscopy. Structurally optimized PLD ZnO thin films show narrow high-resolution x-ray diffraction peak widths of the ZnO(0002) ω- and 2Θ-scans as low as 151 and 43 arcsec, respectively, and narrow photoluminescence linewidths of donor-bound excitons of 1.7 meV at 2 K.

Zinc oxide (ZnO), with its excellent luminescent properties and the ease of growth of its nanostructures, holds promise for the development of photonic devices. The recent advances in growth of ZnO nanorods are discussed. Results from both low temperature and high temperature growth approaches are presented. The techniques which are presented include metal-organic chemical vapour deposition (MOCVD), vapour phase epitaxy (VPE), pulse laser deposition (PLD), vapour-liquid-solid (VLS), aqueous chemical growth (ACG) and finally the electrodeposition technique as an example of a selective growth approach. Results from structural as well as optical properties of a variety of ZnO nanorods are shown and analysed using different techniques, including high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), photoluminescence (PL) and cathodoluminescence (CL), for both room temperature and for low temperature performance. These results indicate that the grown ZnO nanorods possess reproducible and interesting optical properties. Results on obtaining p-type doping in ZnO micro- and nanorods are also demonstrated using PLD. Three independent indications were found for p-type conducting, phosphorus-doped ZnO nanorods: first, acceptor-related CL peaks, second, opposite transfer characteristics of back-gate field effect transistors using undoped and phosphorus doped wire channels, and finally, rectifying I-V characteristics of ZnO:P nanowire/ZnO:Ga p-n junctions. Then light emitting diodes (LEDs) based on n-ZnO nanorods combined with different technologies (hybrid technologies) are suggested and the recent electrical, as well as electro-optical, characteristics of these LEDs are shown and discussed. The hybrid LEDs reviewed and discussed here are mainly presented for two groups: those based on n-ZnO nanorods and p-type crystalline substrates, and those based on n-ZnO nanorods and p-type amorphous substrates. Promising electroluminescence characteristics aimed at the development of white LEDs are demonstrated. Although some of the presented LEDs show visible emission for applied biases in excess of 10 V, optimized structures are expected to provide the same emission at much lower voltage. Finally, lasing from ZnO nanorods is briefly reviewed. An example of a recent whispering gallery mode (WGM) lasing from ZnO is demonstrated as a way to enhance the stimulated emission from small size structures.

ZnO films were prepared by pulsed laser deposition on $a$ -plane sapphire substrates under ${\mathrm{N}}_2$ atmosphere. Ferromagnetic loops were obtained with the superconducting quantum interference device at room temperature, which indicate a Curie temperature much above room temperature. No clear ferromagnetism was observed in intentionally Cu-doped ZnO films. This excludes that Cu doping into ZnO plays a key role in tuning the ferromagnetism in ZnO. 8.8% negative magnetoresistance probed at $5\mathrm{K}$ at $60\mathrm{kOe}$ on ferromagnetic ZnO proves the lack of $s\text{-}d$ exchange interaction. Anomalous Hall effect (AHE) was observed in ferromagnetic ZnO as well as in nonferromagnetic Cu-doped ZnO films, indicating that AHE does not uniquely prove ferromagnetism. The observed ferromagnetism in ZnO is attributed to intrinsic defects.