Mg doping of high quality, metal organic chemical vapor deposition grown GaN films results in distinct traces in their photoluminescence and photoluminescence excitation spectra. We analyze GaN:Mg grown on sapphire substrates and identify two Mg related acceptor states, one additional acceptor state and three donor states which are involved in the donor acceptor pair band transitions situated at 3.26 eV -3.29 eV in GaN:Mg. The presented determination of the donor acceptor pair band excitation channels by photoluminescence excitation spectroscopy in conjunction with temperature dependent photoluminescence measurements results in a direct determination of the donor and acceptor binding, localization, and activation energies which is put into a broader context based on Haynes's rule. Furthermore, we analyze the biexponential decay dynamics of the photoluminescence signal of the acceptor and donor bound excitons. As all observed lifetimes scale with the localization energy of the donor and acceptor related bound excitons, defect and complex bound excitons can be excluded as their origin. Detailed analysis of the exciton transfer processes in the close energetic vicinity of the GaN bandedge reveals excitation via free and bound excitonic channels but also via an excited state as resolved for the deepest localized Mg related acceptor bound exciton. For the two Mg acceptor states we determine binding energies of 164±5 meV and 195±5 meV which is in good agreement with recent density functional theory results. This observation confirms and quantifies the general dual nature of acceptor states in GaN based on the presented analysis of the photoluminescence and photoluminescence excitation spectra.

A dramatic enhancement in the electromechanical response of barium titanate thin films is demonstrated by understanding and optimizing the relationship between organic removal, crystallization, and microstructure, which therefore results in pore elimination, larger grain sizes, and superior densification. The combination enables one to produce bulk-like dielectric properties in a thin film with a room temperature permittivity value above 3000. This advancement in complex oxide thin film processing science creates a new perspective from which to compare, parameterize, and better understand a collection of literature data concerning the manner in which the dielectric response of BaTiO3 depends upon physical dimensions. We are consequently able to apply a single physical model to bulk ceramic and thin film systems, and so demonstrate that the existence of parasitic interfacial layers are not needed to explain dielectric scaling. This work is instrumental in illustrating that extrinsic contributions to scaling are predominant, and that a fundamental understanding of material synthesis provides important opportunities to broaden the spectrum of nonlinear electromechanical properties that can be achieved in ferroelectric thin films.

The relationships linking temperature and voltage dependent dielectric response, grain size, and thermal budget during synthesis are illustrated. In doing so, it was found that maximizing thermal budgets within experimental bounds leads to electrical properties comparable to the best literature reports irrespective of the processing technique or microstructure. The optimal film properties include a bulk transition temperature, a room temperature permittivity of 1800, a voltage tuning ratio of 10:1 at 450 kV/cm, and a loss tangent less than 1.5% at 450 kV/cm. The sample set illustrates the well-known relationship between permittivity and crystal dimension, and the onset of a transition temperature shifts at very fine grain sizes. A brick wall model incorporating a high permittivity grain and a low permittivity grain boundary is used to interpret the dielectric data. However, the data show that high permittivity and tunability values can be achieved at grain sizes or film thicknesses that many reports associate with dramatic reductions in the dielectric response. These differences are discussed in terms of crystal quality and maximum processing temperature. The results collectively suggest that scaling effects in ferroelectric thin films are in many cases the result of low thermal budgets and the consequently high degree of structural imperfection and are not from the existence of low permittivity phases at the dielectric-electrode interface.

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There have been a number of recent reports of anomalously large permittivities (ε r ≈ 10 4 ) in the material CaCu 3 Ti 4 O 12 . The dielectric spectra is characterized by a large, relatively temperature independent permittivity near room temperature which exhibits a dielectric relaxation above 100 K. The crystal structure of CaCu 3 Ti 4 O 12 can be described as pseudo-perovskite with a cubic unit cell with a lattice constant of 7.391Å. The ubiquitous occurrence of this dielectric behavior in ceramics, single crystals, and thin films suggests that the polarization is not related to a simple conducting grain/insulating grain boundary-type system. While the precise origin of the dielectric response is not entirely clear, in this work it is shown that processing conditions have a significant influence on the room temperature dielectric properties. Specifically, the permittivity and loss exhibit a strong dependence on the oxygen partial pressure and sintering time. In fact, studies of the effects of sintering time and supporting evidence from capacitance-voltage measurements conclusively show that there is no direct relationship between the permittivity and grain size, as is the case in classical boundary layer systems. Lastly, with aliovalent doping the room temperature dielectric properties can be optimized to provide a high permittivity (ε r ∼ 8,000) dielectric with relatively low loss (tan δ < 0.05 at 1 kHz).