Low temperature, infrared, and visible-ultraviolet absorption spectra of yttrium aluminum garnet (YAG) bulk crystals and epitaxial layers doped with Ce are presented. In the region of intra-configurational 4f–4f transitions, the spectra of the bulk YAG crystals exhibit existence of at least two different Ce3+ related centers, a major one associated with Ce in regular positions substituting yttrium and also additional center, due to so called antisite positions in the garnet host, i.e., ions in the Al positions. Crystal field analysis based on exchange charge model exhibit excellent agreement with the experimental data for major Ce3+ center.

Low temperature, infrared absorption spectra of gadolinium gallium garnet crystals doped with Ce are presented. In the region of intraconfigurational 4$f$-4$f$ transitions the spectra exhibit existence of at least two different, major Ce${}^{3+}$ related centers in the GGG crystals and also some other centers at lower concentration. The spectrum of 4$f$-4$f$ intrashell transitions of Ce${}^{3+}$ ions extends up to about 3700 cm${}^{-1}$ due to the large splitting of the ${}^2$ $F_{7/2}$ excited state. In the visible region the absorption spectrum shows influence of symmetry-related selection rules. The absorption coefficient changes in the region of 4$f^1$-5$d^1$ transitions due to thermal population of the second level, belonging to the ${}^2$ $F_{5/2}$ ground state. This suggests that the symmetry of the site occupied by Ce${}^{3+}$ ions, which substitute Gd${}^{3+}$, is higher than $D_2$ expected for garnet hosts. DOI: http://dx.doi.org/10.1103/PhysRevB.87.045114 ©2013 American Physical Society

Trends in the behavior of band gaps in short-period superlattices (SLs) composed of CdO and MgO layers were analyzed experimentally and theoretically for several thicknesses of CdO sublayers. The optical properties of the SLs were investigated by means of transmittance measurements at room temperature in the wavelength range 200–700 nm. The direct band gap of {CdO/MgO} SLs were tuned from 2.6 to 6 eV by varying the thickness of CdO from 1 to 12 monolayers while maintaining the same MgO layer thickness of 4 monolayers. Obtained values of direct and indirect band gaps are higher than those theoretically calculated by an ab initio method, but follow the same trend. X-ray measurements confirmed the presence of a rock salt structure in the SLs. Two oriented structures (111 and 100) grown on c- and r-oriented sapphire substrates were obtained. The measured lattice parameters increase with CdO layer thickness, and the experimental data are in agreement with the calculated results. This new kind of SL structure may be suitable for use in visible, UV and deep UV optoelectronics, especially because the energy gap can be precisely controlled over a wide range by modulating the sublayer thickness in the superlattices.

Understanding the origin of the strong difference of electrical parameters between as grown and annealed undoped ZnO films prepared at a temperature range of 100–200 °C by thermal atomic layer deposition is essential for their future applications. In this paper, we show that the conductivity drop by up to 4 orders of magnitude as effect of post-growth annealing is accompanied by multiple simultaneous effects like a two orders of magnitude decrease of hydrogen impurity content, a strong width reduction of the luminescence peaks, and an increase of crystallite sizes influencing the carrier scattering. We show that a level of structural and optical improvement as well as the final electrical parameters of annealed films strongly depend on the previously employed growth temperature, which is related to transition from oxygen- to zinc-rich conditions influencing a type and concentration of native point defects. The growth temperature does not only influence the bandgap energy but also the binding energies of existing donors and the relative ratio between the number of donors and acceptors; hence, it determines the final electrical characteristics of the films. This means that electrical properties of undoped ZnO-atomic layer deposition films can be tuned by native defects engineering.