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−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−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−1 seem specific for the Sb, Ga, and Fe dopants, respectively. Li doped ZnO did not reveal additional modes.

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 155 cm2/V s at 300 K in a narrow carrier concentration range from 2 to 5×1016 cm−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.

Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−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.

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Infrared spectroscopic ellipsometry in the spectral range from ω=360cm−1toω=1500cm−1 and Raman scattering spectroscopy are applied to study the long-wavelength optical phonon modes and dielectric constants of MgxZn1−xO thin films in the composition range 0⩽x⩽1. The samples were grown by pulsed laser deposition on sapphire substrates. X-ray diffraction measurements of the thin film samples reveal the hexagonal wurtzite crystal structure for x⩽0.53 and the cubic rocksalt crystal structure for x⩾0.67. A systematic variation of the phonon mode frequencies with Mg-mole fraction x is found for both hexagonal and cubic MgxZn1−xO thin films. The modified random isodisplacement model matches the observed composition dependence of the phonon mode frequencies for the hexagonal structure thin films [J. Chen and W. Z. Shen, Appl. Phys. Lett. 83, 2154 (2003)], whereas a simple linear approximation scheme is sufficient for the cubic structure part. We observe a discontinuous behavior of the transverse optical phonon modes (decrease), and the static and high-frequency dielectric constants (increase) within the phase transition composition region from the wurtzite structure part to the rocksalt structure part. On the contrary, the longitudinal phonon mode parameters increase almost linearly, and upon phase transition the splitting between the transverse and longitudinal modes increases. We associate this discontinuous behavior with the change of the nearest-neighbor coordination number from fourfold (wurtzite structure) to sixfold (rocksalt structure) in our samples and the associated increase in bond ionicity from ZnO to MgO. Accordingly, we propose that the reduced exciton mass parameter should approximately double upon changing from wurtzite to rocksalt crystal structure.

Infrared dielectric function spectra and phonon modes with polarization parallel and perpendicular to the c axis of high quality, highly relaxed, and single crystalline wurtzite MgxZn1−xO films with 0⩽x⩽0.2 prepared by pulsed-laser deposition on c-plane sapphire substrates were obtained from infrared spectroscopic ellipsometry (380–1200 cm−1) and Raman scattering studies. A two-mode behavior is found for the modes with E1 symmetry, a lattice mode and an impurity-type mode are obtained for the A1 symmetry phonons. Model dielectric function spectra will become useful for future infrared ellipsometry analysis of complex MgxZn1−xO-based heterostructures.

Bulk-like GaN with high structural and optical quality has been attained by hydride vapor-phase epitaxy (HVPE). The as-grown 330 µm-thick GaN layer was separated from the sapphire substrate by a laser-induced lift-off process. The full width at half maximum values of the X-ray diffraction (XRD) ω-scans of the free-standing material are 96 and 129 arcsec for the (1 0 -1 4) and (0 0 0 2) reflection, respectively, which rank among the smallest values published so far for free-standing HVPE-GaN. The dislocation density determined by plan-view TEM images is 1–2×107 cm-2. Positron annihilation spectroscopy studies show that the concentration of Ga vacancy related defects is about 1.5×1016 cm-3. The high-resolution XRD, photoluminescence, µ-Raman, and infrared spectroscopic ellipsometry measurements consistently prove that the free-standing material is of high crystalline quality and virtually strain-free. Therefore it is suitable to serve as a substrate for stress-free growth of high-quality III–nitrides based device heterostructures.

There is an increasing demand for thin films with tailored properties, which requires the use and control of adequate deposition techniques. Ion beam sputter deposition (IBSD) is a physical vapor deposition (PVD) technique that is capable of fulfilling the technological challenges. In contrast to other PVD techniques, IBSD offers a unique opportunity to tailor the properties of the film-forming particles (sputtered target and scattered primary particles) and, hence, thin film properties. This is related to the fact that the generation and acceleration of the primary particles from the ion beam source, the generation of film-forming particles at the target, and thin film growth on the substrate are spatially separated. Thus, by changing ion beam parameters (ion species and ion energy) and geometrical parameters (ion incidence angle and emission angle), the energy distributions of the film-forming particles are modified. Even though in use for several decades, IBSD was not investigated systematically until lately. Utilizing the full potential of IBSD requires a comprehensive understanding of the physical processes. This tutorial describes the systematics of IBSD: The correlation between process parameters, properties of the film-forming particles, and thin film properties. The most important process parameters are the scattering geometry and the primary particle species. Depending on the material, different film properties can be influenced. Examples are adhesion, structural properties, composition, surface roughness, mass density, optical properties, stress, and electrical resistivity. In addition to the experimental results, fundamental physical aspects, experimental setups, and techniques for thin film deposition and particle characterization are described.