Static and dynamic responses of excitons in state-of-the-art bulk and epitaxial ZnO are reviewed to support the possible realization of polariton lasers, which are coherent and monochromatic light sources due to Bose condensation of exciton-polaritons in semiconductor microcavities (MCs). To grasp the current problems and to pave the way for obtaining ZnO epilayers of improved quality, the following four principal subjects are treated: (i) polarized optical reflectance (OR), photoreflectance (PR) and photoluminescence (PL) spectra of the bulk and epitaxial ZnO were recorded at 8 K. Energies of PR resonances corresponded to those of upper and lower exciton-polariton branches, where A-, B-and C-excitons couple simultaneously to an electromagnetic wave. PL peaks due to the corresponding polariton branches were observed. Longitudinal-transverse splittings (ω LT ) of the corresponding excitons were 1.5, 11.1 and 13.1 meV, respectively. The latter two values are more than two orders of magnitude greater than that of GaAs being 0.08 meV. (ii) Using these values and material parameters, corresponding vacuum-field Rabi splitting of exciton-polaritons coupled to a model MC mode was calculated to be 191 meV, which is the highest value ever reported for semiconductor MCs and satisfies the requirements to observe the strong exciton-light coupling regime necessary for polariton lasing above room temperature. (iii) Polarized OR and PR spectra of an out-plane nonpolar (1 1 2 0) ZnO epilayer grown by laser-assisted molecular beam epitaxy (L-MBE) were measured, since ZnO quantum wells (QWs) grown in nonpolar orientations are expected to show higher emission efficiencies due to the elimination of spontaneous and piezoelectric polarization fields normal to the QW plane. They exhibited in-plane anisotropic exciton resonances according to the

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Epitaxial ZnO thin films were grown by laser molecular-beam epitaxy on lattice-matched ScAlMgO 4 substrates following the deposition and annealing of suitable buffer layers. The samples were characterized by low-temperature photoluminescence ͑PL͒, absorption, and reflectivity measurements. PL from higher order (nϭ2) excitons ͑A exciton͒ was observed at temperatures lower than 40 K. The absorption spectrum contained lines and the reflection spectrum exhibited anomalies that were assigned to the excited-states (nϭ2,3) of A and B excitons. The optical quality could be improved dramatically by using annealed ZnO or MgZnO buffer layers.

We studied photoluminescence (PL) properties of eighteen samples of wurtzite ZnO/MgxZn1−xO multiple quantum wells (x=0.12 and 0.27) with various well widths (Lw) of 0.7–4.65 nm. Radiative recombination of the electron–hole pairs that are spatially separated due to the quantum-confined Stark (QCS) and Franz–Keldish (QCFK) effects was observed in two thicker samples at 5 K. This PL band is located ≈ 40 meV in energy below the emission band of the localized excitons and ≈ 60 meV below the absorption energy of the free exciton transition. One can not observe such kind of luminescence unless both of the following conditions are accomplished: (1) higher Mg concentration (x=0.27) and (2) Lw⩾4.23 nm. These experimental findings do not contradict the following two characteristic features for the QCS and QCFK effects; the magnitude of the electric field due to spontaneous and piezoelectric polarizations and the depth of the triangle-shaped potential wells are the monotonically increasing functions of Mg concentration and the Lw, respectively. The coupling strength with longitudinal-optical phonons, which is determined from the relative luminescence intensities of the phonon replicas, is significantly larger than that between the localized excitons and phonons. It is considered that the strong electric field increases the distance between electron and hole charge distributions from that determined by the Coulomb force and leads to the enhancement in the phonon interaction.

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Electron energy loss spectroscopy (EELS) provides direct information on the local electronic structure of a material. In ionic systems, EELS measurements of formal valences give insight and place remarkable constraints on the structure and stability of heterointerfaces and defects. This is of considerable importance to the field of oxide electronics and ferroelectrics, where been the growth of atomically-abrupt heterointerfaces has been a central goal. However, when the interfaces are between polar and nonpolar layers, electrical and atomic abruptness turn out to be mutually incompatible goals. This is the bulk analogy of the surface reconstructions found in polar systems where a material terminated along a bulk polar plane has a net charge and a divergent surface energy. Unlike conventional semiconductors, for multivalent oxides, physical roughness is not the only option -atomically abrupt interfaces are allowed if electrons can be redistributed at lower energy cost than it takes to redistribute ions. These extra electrons are detectable by EELS.Recent work on SrTiO 3 /LaAlO 3 [001] interfaces has found that Ti-La interfaces are conducting, but Sr-Al interfaces are insulating. Our EELS studies show that for the Ti-La terminated interface, excess electrons are found on the Ti sites, but not for the Sr-Al termination (where an extra 1/2 hole would be expected theoretically). Instead significant compensating oxygen vacancies are present and no free holes are found, probably explaining the electrical asymmetry. Controlling the interface termination lets us tune between insulator and conductor, trading chemical for electronic roughness [1].When both sides of the interface contain mixed-valence ions, the interface can be atomicallyabrupt, but electronically diffuse. Under these conditions, interface phases and charge-modulated structures can be constructed [2]. Structures that are thermodynamically unstable in the bulk can be stabilized by their interface energies in thin layers. As the layer thickness is increased, the structure can fault, in an attempt to recover its bulk valence (fig 1). For the LaTiO 3 /SrTiO 3 however, the stable oxidized La-phase is the pyrochlore La 2 Ti 2 O 7 which has a unit cell of ~4 La layers along the [001] direction [3]. In thinner structures a new fault, that does not exist in the bulk, develops along the (001) plane in which a single layer of La sites splits into a double layer of a ½ occupied Ruddleston-Popper-like fault. This oxidation reduces the electron count on the neighboring Ti sites (Fig 2). While thin layers are stable against oxidation, they also do not have the bulk electron count, so the stable electron count is limited to ~40% of the bulk value after thermal annealing (Fig 3). The unfaulted structure is stable up to ~400C [4].

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