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The $p\text{\ensuremath{-}}T$ conditions of the solid-solid phase transition from the wurtzite to rocksalt structure in GaN are determined both experimentally and by ab initio calculations. Experimental evaluation was based on the x-ray absorption measurements in a laser-heated diamond anvil cell. At 300 K, the transition was observed near 47 GPa. At lower pressures, the wurtzite to rocksalt transition has been induced by high temperature: 1420 K at 42 GPa and about 2100 K at 37 GPa. Thus the slope of the wurtzite-rocksalt borderline could be evaluated as negative and nonlinear. On the part of the theory, the $p\text{\ensuremath{-}}T$ borderline was determined from a comparative analysis of the temperature dependences of the Gibbs potential of the wurtzite and rocksalt structures for different pressure. The Gibbs potentials were calculated within the quasiharmonic approximation and the self-consistent phonon approach. The results obtained with the self-consistent phonon approach show that the inclusion of the anharmonic phonon effects is indispensable to obtain a very good agreement with the experimental data. Possible consequences of the observed anharmonicity for the still unknown melting behavior of GaN are discussed. In particular, it is suggested that the melting temperature of the rocksalt-GaN, at pressure around 37 GPa, is not much higher than 2100 K.

High-pressure measurements were employed to study mechanisms of (i) the 3.27 eV emission band (and phonon replicas) and (ii) 2.9--3.1 eV blue photoluminescence in different types of GaN:Mg samples. GaN layers with low Mg doping, grown on bulk GaN crystals and on sapphire substrates, were used to study the 3.27 eV uv band. This band results from donor-acceptor pair (DAP) recombination (with both impurities of shallow character). For high Mg doping resulting in blue light emission at about 3 eV, a bulk GaN crystal and a layer grown on sapphire were employed. For the all samples the pressure coefficient of the corresponding luminescence bands was measured at 77 K by means of the diamond anvil technique. The energy shift of the 3.27 eV band under hydrostatic pressure is used to determine the linear pressure coefficient characteristic for (shallow) DAP recombination. It has a value of about 35 meV/GPa, close to the pressure shift of the GaN band gap (about 40 meV/GPa). The 2.9--3.1 eV blue luminescence band exhibits much lower pressure coefficients of about 23 meV/GPa. It shows that the initial state of the blue emission has localized character in both types of GaN material and supplies a strong argument against models of radiative recombination in highly-Mg-doped/compensated GaN material that postulate an involvement of delocalized (shallow) electronic states as the initial states.

Magnetoreflectivity measurements on GaN are used to resolve the symmetry of three excitonic resonances, arising from the crystal-field and spin-orbit effects on the valence band. The symmetry characteristic for each valence subband is directly seen in the observed Zeeman pattern. The linear magnetic-field effect for all exciton components is well accounted for by introducing only two coupling constants: the electronic g factor for the conduction band and the Luttinger $\ensuremath{\kappa}$ parameter for the valence band. The strength of the electron-hole exchange interaction is determined.

The pressure dependence of the ${A}_{1}(\mathrm{TO})$ and ${A}_{1}(\mathrm{LO})\mathrm{}$ phonon modes of bulk, single crystals of wurtzite GaN has been measured by Raman scattering in a diamond-anvil cell up to 40 GPa. The measured phonon frequencies have been used to determine the LO and TO phonon-mode Gr\"uneisen parameters and also the value of effective transverse charge ${e}_{T}^{*}=2.6$ and its pressure derivative ${\mathrm{de}}_{T}^{*}/dP=\ensuremath{-}2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3} {\mathrm{GPa}}^{\ensuremath{-}1}$. The experimentally obtained values are compared with results of calculations by means of tight-binding formalism combined with ab initio linear muffin-tin orbital calculations.

We present a comprehensive study of the anisotropic optical properties of nonpolar GaN/AlGaN multiple quantum wells intentionally designed to act as an active region of a planar microcavity operating in the strong-coupling regime. The strain induced by the underlying AlGaN-based Bragg reflector leads to a redistribution of exciton oscillator strength as revealed by photoluminescence and reflectivity measurements. Complementary k . p calculations show an excellent agreement with experiments and emphasize the opportunity to tune the nature of the light-matter coupling in a microcavity by means of strain engineering. Finally, the validity of the developed model is proven by angle-resolved photoluminescence studies carried out on the complete microcavity structure. The recorded eigenmode spectra reveal the coexistence of the weak- and the strong-coupling regime along the two orthogonal polarization planes.

International audience; Both weakly and strongly confined excitons are studied by time-resolved photoluminescence in a nonpolar nitride-based heterostructure grown by molecular beam epitaxy on the a-facet of a bulk GaN crystal, with an ultralow dislocation density of 2 × 105 cm−2. Strong confinement is obtained in a 4 nm thick Al0.06Ga0.94N/GaN quantum well (QW), whereas weakly confined exciton-polaritons are observed in a 200 nm thick GaN epilayer. Thanks to the low dislocation density, the effective lifetime of strongly confined excitons increases between 10 and 150 K, proving the domination of radiative recombination processes. Above 150 K theQWemission lifetime diminishes, whereas the decay time of excitons in the barriers increases, until both barrier and QW exciton populations become fully thermalized at 300 K. We conclude that the radiative efficiency of our GaN QW at 300 K is limited by nonradiative recombinations in the barriers. The increase of exciton-polariton coherence lengths caused by low dislocation densities allows us to observe and model the quantized emission modes in the 200 nm nonpolar GaN layer. Finally, the low-temperature phonon-assisted relaxation mechanisms of such center-of-mass quantized exciton-polaritons are described.

We investigate the properties of doping-induced metal-insulator transition in GaN:Si by means of electron spin resonance and the Hall effect. While increasing the doping concentration, Si-related bands are formed below the bottom of the GaN conduction band. The ${D}^{0}$ band of single-occupied Si donor sites is centered 27 meV below the bottom of the GaN conduction band, and the ${D}^{\ensuremath{-}}$ band of double-occupied Si states is centered at 2.7 meV below the bottom of the GaN conduction band. Strong damping of the magnetic moment occurs due to filling of the ${D}^{\ensuremath{-}}$ states at Si concentrations approaching the metal-insulator transition. Simultaneously, shortening of electron spin relaxation time due to limited electron lifetime in the single-occupied ${D}^{0}$ band is observed. The metal-insulator transition occurs at the critical concentration of uncompensated donors equal to about 1.6 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{18}$ cm${}^{\ensuremath{-}3}$. Electronic states in metallic samples beyond the metal-insulator transition demonstrate the nonmagnetic character of double-occupied states.

Gallium nitride was studied by Raman scattering and x-ray-absorption spectroscopy up to 60 GPa. A high-pressure structural phase transition was observed in gallium nitride at 47 GPa by means of Raman scattering and x-ray-absorption spectroscopy. We also report the direct determination of the bulk modulus ${\mathit{B}}_{0}$ of this compound (245 GPa). Gr\"uneisen parameters of the four observed phonon modes were established. The transition pressure is compared with existing calculations.

We report on highly resolved photoluminescence (PL) and reflectance (RF) spectra of a homoepitaxial GaN layer grown by metal-organic vapor phase epitaxy. This sample exhibits narrow linewidths of several PL emission peaks, down to $\ensuremath{\approx}100\ensuremath{\mu}\mathrm{eV}$ full width at half maximum for some bound exciton transitions. As a consequence, we have detected new PL features as, e.g., a fivefold fine structure of the donor-bound exciton line at $\ensuremath{\approx}3.471\mathrm{eV},$ and other known PL transitions could be determined with high precision. In RF measurements, the extraordinary quality of the epitaxial layer allowed observation of weakly damped excitonic ground-state transitions and of narrow excited exciton transitions with high signal-to-noise ratio.

Magnetization measurements of the wurtzite highly resistive bulk crystals of GaN:Mn, codoped with Mg, are presented. Strong anisotropy of magnetization at low temperatures s 2‐1 0 Kd was observed. The data were analyzed assuming Mn ions in d 4 configuration. The crystal field model taking into account cubic field of tetrahedral symmetry, trigonal field along the c-axis simulating hexagonal structure, tetragonal static JahnTeller distortion, and the spin-orbit interaction provides good description of the experimental magnetization data.

We combine transmission electron microscopy, high-resolution x-ray diffraction, cathodoluminescence, and photoluminescence experiments with first-principles calculations to study the formation, thermodynamic stability, structural, and optical properties of chemically ordered ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys $(0lxl1)$. Our results reveal that group-III-nitride surfaces exhibit chemically highly sensitive adsorption sites at step edges and that these sites can be used to kinetically engineer chemically ordered ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys. The ordered alloys have unique properties: (i) the band gap is redshifted up to $110\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ with respect to the disordered alloy of the same composition and (ii) the band gap reduction is caused by localization of the band edge wave functions in the GaN layer. Ordered ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ thus can be seen as a natural quantum well structure where electrons and holes are localized and confined in monolayer GaN quantum wells.

We present magneto-optical studies of intra-center absorption band related to neutral Mn acceptor in GaN bulk crystals. The band is built of a zero-phonon line at $1.4166\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ followed by GaN phonon spectrum. Magneto-optical experiments performed in magnetic fields up to $22\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in both Faraday and Voigt configurations revealed very small influence of magnetic field on the spectrum. In particular no splitting of the zero-phonon line was observed. The only characteristic feature recorded in high magnetic fields was a step-like behavior of the spectral position of zero-phonon line measured in Faraday configuration. The step appeared at the magnetic field of about $7\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, with a shift in energy of about $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. We interpret the behavior of the zero-phonon line in magnetic field based on crystal-field model for ${\mathrm{Mn}}^{3+}({d}^{4})$ ion in trigonal crystal field, undergoing Jahn-Teller distortion and spin-orbit coupling. Applied model explains reasonably experimental results, supporting strongly localized character of Mn neutral acceptor in GaN.

Stimulated emission under quasi-resonant photoexcitation was studied in high-quality homoepitaxial GaN layers. Emission escaping perpendicular to the excited surface as well as propagating along the surface was analyzed as a function of the excitation power density in the temperature range from 8 to $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Contributions of stimulated emission due to inelastic exciton\char21{}exciton/carrier interaction and recombination in electron-hole plasma (EHP) were revealed and simultaneously observed from the sample edge. The concurrent action of two mechanisms of stimulated emission was interpreted to be caused mainly by spatially inhomogeneous Mott transition due to separation of EHP located at the very surface of the layer and dense exciton gas located deeper in the layer. The separation is facilitated by high carrier diffusion length in homoepitaxial GaN. Stimulated emission due to inelastic exciton\char21{}exciton/carrier interaction was unambiguously traced up to the temperature of $440\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

We have applied positron annihilation spectroscopy to show that 2-MeV electron irradiation at 300 K creates primary Ga vacancies in GaN with an introduction rate of 1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The Ga vacancies recover in long-range migration processes at 500--600 K with an estimated migration energy of 1.5 (2) eV. Since the native Ga vacancies in as-grown GaN survive up to much higher temperatures (1300--1500 K), we conclude that they are stabilized by forming complexes with oxygen impurities. The estimated binding energy of 2.2 (4) eV of such complexes is in good agreement with the results of theoretical calculations.

We have applied a low-energy positron beam and secondary ion mass spectrometry to study defects in homoepitaxial and heteroepitaxial GaN layers. Positron experiments reveal high concentrations of Ga vacancies in nominally undoped n-type GaN, where the conductivity is due to unintentional oxygen incorporation. Ga vacancies are observed in both homoepitaxial and heteroepitaxial layers, indicating that their formation is independent of the dislocation density. No Ga vacancies are detected in p-type or semi-insulating samples doped with Mg, as predicted by the theoretical formation energies. In samples where n-type conductivity is due to Si doping and the incorporation of oxygen impurities is suppressed, the concentration of Ga vacancies is much lower than in n-type samples containing oxygen. This indicates that the presence of oxygen donor in GaN promotes the formation of Ga vacancy. We suggest that this effect is due to the creation of ${\mathrm{V}}_{\mathrm{Ga}}{\ensuremath{-}\mathrm{O}}_{\mathrm{N}}$ complexes during the epitaxial growth.

The pronounced optical in-plane anisotropy of a suitably designed nonpolar GaN-based microcavity, mainly inherited from the valence-band complexity of wurtzite semiconductors, allows the coexistence of two different light-matter coupling regimes along orthogonal polarization planes. When increasing the excitation power density, a transition to a nonlinear coherent emission is observed under nonresonant optical pumping for both polarization directions at carrier densities remaining well below the exciton-saturation level. Their differing nature is experimentally revealed by means of polarization-resolved Fourier space and near-field imaging. While one polarization direction exhibits all specificities of the strong light-matter coupling regime and features polariton lasing at high injection, the other one lacks a straightforward interpretation due to its complex coupling regime. Possible mechanisms are addressed and evaluated.