We report on III-nitride based blue vertical cavity surface emitting lasers using defect-free highly reflective AlInN/GaN distributed Bragg reflectors grown on c-plane free-standing GaN substrates. Lasing is demonstrated at room temperature under pulsed electrical injection. The high lasing threshold current density still prevents devices from continuous wave lasing because of large self-heating. The reasons for such a high threshold are discussed and we show that it mainly comes from large light absorption in the indium tin oxide current spreading layer. Properly tuning both its thickness and its position with respect to the electrical field could remarkably decrease the threshold.

International audienceWe demonstrate polariton lasing in a bulk ZnO planar microcavity under non-resonant optical pumping at a small negative detuning (delta~-1/6 the 130 meV vacuum Rabi splitting) and a temperature of 120 K. The strong coupling regime is maintained at lasing threshold since the coherent nonlinear emission from the lower polariton branch (LPB) occurs at zero in-plane wavevector well below the uncoupled cavity mode. The contribution of multiple localized polariton modes above threshold and the non-thermal polariton statistics show that the system is in a far-from-equilibrium regime, likely related to the moderate photon lifetime and in-plane photonic disorder in the cavity

We report on the epitaxial growth of high-quality core−shell nonpolar m-plane GaN/InAlN multiple quantum wells (MQWs) on the sidewall facets of c-oriented hexagonal GaN wires. Pseudomorphic growth without generation of threading dislocations has been established for planar GaN/InAlN (In = 15%) MQWs grown on m-GaN substrates, although m-plane InAlN epilayers cannot be grown perfectly lattice-matched to GaN along the two in-plane directions. Calculations based on elasticity theory indicate that the significant amount of strain oriented along the c-axis is the likely factor favoring the formation of cracks along this direction. For the core−shell wire geometry, such cracks are not observed, leading to high structural quality MQWs. A significant UV emission centered around 3.7 eV at room temperature with a strong polarization perpendicular to the wire axis is observed for those core−shell wires, which is consistent with k•p method calculations, proving the absence of quantum confined Stark effect on nonpolar m-plane surfaces. These excellent optical features reported in the UV spectral range are attributed to the defect-free nature of the GaN/InAlN MQWs, thereby opening promising opportunities for the realization of UV light emitters.

A detailed discussion of the optical properties of Al-rich Al 1−x In x N alloy films is presented. The (0001)-oriented layers with In contents between x = 0.143 and x = 0.242 were grown by metal-organic vapor phase epitaxy on thick GaN buffers. Sapphire or Si(111) served as the substrate. High-resolution X-ray diffraction revealed pseudomorphic growth of the nearly lattice-matched alloys; the data analysis yielded the composition as well as the in-plain strain. The complex dielectric function (DF) between 1 and 10 eV was determined from spectroscopic ellipsometry measurements. The sharp onset of the imaginary part of the DF defines the direct absorption edge, while clearly visible features in the high-photon energy range of the DF, attributed to critical points of the band structure, indicate promising crystalline quality of the AlInN layers. It is demonstrated that the experimental data can be well reproduced by an analytical DF model. The extracted characteristic transition energies are used to determine the bowing parameters for all critical points of the band structure. In particular, strain and the high exciton binding energies for the Al-rich alloys are taken into account in order to assess the splitting between the valence band with Γ v 9 symmetry and the Γ c 7 conduction band at the center of the Brillouin zone. Finally, the compositional dependence of the high-frequency dielectric constants is reported.

The crossover from an exciton gas to an electron-hole plasma is studied in a GaN/(Al,Ga)N single quantum well by means of combined time-resolved and continuous-wave photoluminescence measurements. The twodimensional Mott transition is found to be of continuous type and to be accompanied by a characteristic modification of the quantum well emission spectrum. Beyond the critical density, the latter is strongly influenced by band-gap renormalization and Fermi filling of continuum states. Owing to the large binding energy of excitons in III-nitride heterostructures, their injection-induced dissociation could be tracked over a wide range of temperatures, i.e., from 4 to 150 K. Various criteria defining the Mott transition are examined, which, however, do not lead to any clear trend with rising temperature: the critical carrier density remains invariant around 10 12 cm −2 . At sufficiently low temperature T and carrier density n, free electrons and holes in a semiconductor bind and form neutrally charged quasiparticles. These so-called excitons represent the fundamental electronic excitation of a semiconductor and obey Bose statistics in the low-density limit. However, when increasing T or n beyond a certain limit, the exciton complexes get ionized and the system switches from an insulating state to a conductive electron-hole plasma (EHP)-the Mott transition (MT) [1]. While exciton dissociation induced by a hot phonon bath represents a classical process and occurs when the thermal energy becomes of the order of the exciton binding energy E b X ≈ k B T , the breakup of excitons due to an increasing carrier population relies on much more complex mechanisms. In simplified terms, when n approaches the hard-sphere limit, that is, when the interparticle distance is reduced to the order of the exciton Bohr radius a B , the fermionic properties of the exciton constituents become dominant: Coulomb screening and phase-space filling cause a reduction of E b X and eventually the dissociation of the excitonic bound state around a certain critical density n crit [2]. Note that the latter is usually overestimated by the simple hard-sphere criterion [3].Whereas the MT was initially claimed to be a first-order phase transition [1,4], later experiments led to partially conflicting results. Especially in three dimensions (3D), studies exist that argue to evidence the first-order nature of the MT [5,6], while others point toward a second-order transition [7][8][9]. However, note that in optically-probed bulk systems, the exponential absorption profile leads to an emission signal that mixes inhomogeneously injected regions and may conceal certain characteristics of the MT. This drawback can be circumvented in 2D systems. Here, most of the studies suggest a rather smooth MT [10][11][12][13]. On the contrary, experimental studies concerning the T dependence of the MT are extremely scarce. In a simple framework, an increasing Debye-screening length gives reason to expect a rise in n crit with T . Even if such a behavior was claimed for bulk ...