Temperature dependence of the band gap shrinkage due to electron-phonon interaction in undoped n-type GaN

The photoluminescence spectra of band-edge transitions in GaN is studied as a function of temperature. The parameters that describe the temperature dependence red-shift of the band-edge transition energy and the broadening of emission line are evaluated using different models. We find that the semi-empirical relation based on phonon-dispersion related spectral function leads to excellent fit to the experimental data. The exciton-phonon coupling constants are determined from the analysis of linewidth broadening

Free carrier effects in gallium nitride epilayers: the valence band dispersion

The dispersion of the A-valence-band in GaN has been deduced from the observation of high-index magneto-excitonic states in polarised interband magneto-reflectivity and is found to be strongly non-parabolic with a mass in the range 1.2-1.8 m_{e}. It matches the theory of Kim et al. [Phys. Rev. B 56, 7363 (1997)] extremely well, which also gives a strong k-dependent A-valence-band mass. A strong phonon coupling leads to quenching of the observed transitions at an LO-phonon energy above the band gap and a strong non-parabolicity. The valence band was deduced from subtracting from the reduced dispersion the electron contribution with a model that includes a full treatment of the electron-phonon interaction.Comment: Revtex, 4 pages, 5 figure

Filtering of Defects in Semipolar (11−22) GaN Using 2-Steps Lateral Epitaxial Overgrowth

Good-quality (11−22) semipolar GaN sample was obtained using epitaxial lateral overgrowth. The growth conditions were chosen to enhance the growth rate along the [0001] inclined direction. Thus, the coalescence boundaries stop the propagation of basal stacking faults. The faults filtering and the improvement of the crystalline quality were attested by transmission electron microscopy and low temperature photoluminescence. The temperature dependence of the luminescence polarization under normal incidence was also studied

Strain-free bulk-like GaN grown by hydride-vapor-phase-epitaxy on two-step epitaxial lateral overgrown GaN template

Crack-free bulk-like GaN with high crystalline quality has been obtained by hydride-vapor-phase-epitaxy (HVPE)growth on a two-step epitaxial lateral overgrown GaN template on sapphire. During the cooling down stage, the as-grown 270-μm-thick GaN layer was self-separated from the sapphire substrate. Plan-view transmission electron microscopyimages show the dislocation density of the free-standing HVPE-GaN to be ∼2.5×10 exp 7  cm exp −2 on the Ga-polar face. A low Ga vacancy related defect concentration of about 8×10 exp 15 cm exp−3 is extracted from positron annihilation spectroscopy data. The residual stress and the crystalline quality of the material are studied by two complementary techniques. Low-temperature photoluminescence spectra show the main neutral donor bound exciton line to be composed of a doublet structure at 3.4715 (3.4712) eV and 3.4721 (3.4718) eV for the Ga- (N-) polar face with the higher-energy component dominating. These line positions suggest virtually strain-free material on both surfaces with high crystalline quality as indicated by the small full width at half maximum values of the donor bound exciton lines. The E1(TO) phonon mode position measured at 558.52 cm exp −1 (Ga face) by infrared spectroscopic ellipsometry confirms the small residual stress in the material, which is hence well suited to act as a lattice-constant and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality III-nitride device applications.Peer reviewe

Direct observation of threading dislocations in GaN by high-resolution Z-contrast imaging

Wide gap nitride semiconductors have attracted significant attention recently due to their promising performance as short-wavelength light emitting diodes (LEDs) and blue lasers. One interesting issue concerning GaN is that the material is relatively insensitive to the presence of a density of dislocations which is six orders of magnitude higher than that for III-V arsenide and phosphide based LEDs. Although it is well known that these dislocations originate at the film-substrate interface during film growth, thread through the whole epilayer with line direction along and are perfect dislocations with Burgers vectors of a, c, or c+a, the reason why they have such a small effect on the properties of GaN is unclear. To develop a fundamental understanding of the properties of these dislocations, the core structures are studied here by high resolution Z-contrast imaging in a 300kV VG HB603 scanning transmission electron microscope (STEM) with a resolution of 0.13 nm. As the Z-contrast image is a convolution between the probe intensity profile and the specimen object function, it is possible to obtain more detailed information on the specimen object function, i.e. the structure, through maximum entropy analysis (the maximum entropy technique produces the ``most likely`` object function which is consistent with the image)

DISTRIBUTION OF STRAINS AT Fe2+A SITE IN THE THIOSPINEL FeSc2S4

Le thiospinelle FeSc2S4 a été étudié par effet Mössbauer jusqu'à 1,5 K. Les spectres paramagnétiques ont été analysés par une distribution de doublets dont le nombre, l'intensité et la scission quadrupolaire ε varient avec T. – Une première interprétation considère l'influence sur les niveaux nucléaires du 57Fe d'un effet tunnel cohérent entre 3 puits de potentiel correspondant à 3 distorsions Jahn Teller équivalentes (généralisation du modèle de Hartmann Boutron). L'effet des contraintes et du couplage spin orbite sur le doublet fondamental 5Eg du Fe2+ en site A sont négligés. Les calculs ont été conduit dans le cas d'un effet Jahn Teller linéaire fort et comportent un terme d'anisotropie. Un système de 3 doublets est obtenu à basse T dans l'hypothèse de relaxation lente : les intensités et les positions des pics varient avec le rapport 3 Γ/ε où 3 Γ est l'énergie tunnel entre le doublet vibronique fondamental et le premier singulet excité. Mais les résultats expérimentaux ne correspondent pas à ceux du modèle. – Une seconde interprétation est proposée et qui tient compte de l'effet sur le doublet vibronique fondamental du couplage spin-orbite au 2e ordre et d'une distribution aléatoire de contraintes : ces dernières peuvent causer des levées de dégénérescence de l'ordre du cm-1 sur les niveaux d'énergie des ions présentant une dégénérescence orbitale. Le calcul conduit à un gradient de champ électrique axial à toute température : la variation avec T de la composante principale Vzz(< 0) de ce gradient est obtenue pour différentes valeurs de contraintes. Un accord qualitatif avec les résultats expérimentaux est observé pour des scissions énergétiques dues aux contraintes comprises entre 1 et 10 cm-1.The paramagnetic thiospinel FeSc2S4 was studied by Mössbauer effect down to 1.5 K. Spectra have been analyzed by a distribution of doublets whose number, intensity and hyperfine quadrupole splitting ε vary with temperature. – A first tentativ interpretation used a generalised Hartmann Boutron's model : the influence on the 57Fe nuclear levels of the coherent tunneling between three equivalent potential wells (corresponding to three Jahn Teller distorsions) is considered. Strains and spin orbit coupling are not taken into account on the 5Eg ground state of A site Fe2+. Calculation were done in the case of strong linear Jahn Teller coupling and include warping terms. At low temperatures in the case of slow relaxation, a system of three doublets is obtain : the intensities and peak's positions of them Vary with the ratio 3 Γ/ε, where 3 Γ is the tunneling energy between the vibronic doublet ground state and the first excited singlet. But experimental data cannot be fitted by such a model. – A second interpretation is proposed, taking into account the effect on the vibronic doublet ground state of second order spin-orbit coupling and of random strains, which are known to induce splitting of the order of 1 cm-1 in the orbitally degenerate state of ions. The variation with temperature of the principal component Vzz(< 0) of the electric field gradient - which is axially symetric - is obtained for different values of strains. Strains splitting of 1 to 10 cm-1 is deduced in qualitative agreement with experimental results

MÖSSBAUER STUDIES ON GRANULAR Fe-SiO2

Les composés granulaires Fe-SiO2 ont été préparés par copulvérisation chathodique en utilisant des cibles segmentées Fe-SiO2, un plasma d'argon (ou argon + H2) à 10-3 torr. Leurs spectres Mössbauer mettent en évidence la transition superparamagnétisme-ferromagnétisme. Cette température de transition dépend de la taille des grains de fer dans la matrice de SiO2. Une analyse fine des spectres Mössbauer montre qu'il existe plusieurs sites superparamagnétiques. Ceci s'interprète par des effets d'interface. Il existe aussi plusieurs sites magnétiques, ceci est dû à une distribution des tailles de grains à l'intérieur du composé. Les résultats de la spectrométrie Mössbauer donnent des détails sur la nature des composés granulaires qui n'avaient jamais été mentionnés auparavant.Granular compounds Fe-SiO2 were prepared by cosputtering using Fe and SiO2 targets, Argon (or Argon + H2) plasma under 10-3 torr. Mössbauer spectra from 4 to 300 K clearly show the transition superparamagnetism-ferromagnetism. The transition temperature depends on the size of the iron grain in the SiO2 matrix. A carefull analysis of the Mössbauer spectra show that several superparamagnetic sites exist. This was interpretated by interface effects. Several magnetic peaks exist too, which are due to a distribution of grains size among the sample. Mössbauer data gives details on the nature of granular compounds which were never seen before

SUPERPARAMAGNETISM IN GRANULAR COMPOUNDS AS SEEN BY MÖSSBAUER SPECTROSCOPY

Les films granulaires /1/ sont des matériaux composites constitués de particules de métal (15 à 200 Å) dispersés dans une matrice amorphe et isolante. Les composés étudiés Fe-SiO2 /2/, Fe-Al2O3 et Fe-ZrO2 ont été préparés par copulvérisation de Fe et de SiO2 (ou Al2O3 ou ZrO2). La microstructure et le diamètre des grains de fer dépendent de la proportion relative de fer et de matrice. L'effet Mössbauer sur le 57Fe permet d'étudier la transition ferro superparamagnétisme et de préciser des propriétés de l'interface métal-isolant en corrélation avec la microscopie électronique. Différentes compositions ont été étudiées de 4,2K 1 300K. Pour les matériaux peu riches en fer, les grains sont de petit diamètre et les composés sont presque amorphes. Pour les composés contenant de 50 à 60 % de fer, les diamètres des grains varient entre 50 et 100 Å et plusieurs sites de fer sont mis en évidence : deux sites, d'intensités faibles, correspondent d'une part à du fer paramagnétique dilué dans la matrice, d'autre part à des composés non définis Fe-O matrice (H = 200 à 250 kOe à 4,2K) ; un site principal correspond au fer massif métallique ; deux sites simulant une répartition de champs hyperfins (370 ≤ H ≤ 420 kOe) correspondent à des atomes de fer à l'interface /3/. Les intensités de ces sites varient beaucoup avec le diamètre des grains et la nature de la matrice.Granular films are composite materials made of small metals grains (15-200 Å) in an amorphous insulating matrix /1/. The granular compounds studied here were Fe-SiO2 /2/, Fe Al2O3 and Fe-ZrO2 with different ratio Fe/insulating matrix. These samples were prepared by cosputtering of Fe and SiO2 (or Al2O3 or ZrO2). The microstructure was studied by electron microscopy. Mössbauer effect was used to study the transition superparamagnetism ↔ ferromagnetism as a function of T and to get information about the interface Fe-matrix /2/. In 0.35 Fe, 0.65 ZrO2, the spectrum shows a large distribution of Hhpf around the value of metallic iron, this sample is amorphous and relaxation effects are present at 4.2K. In 0.53 Fe, 0.45 Al2O3 the distribution of Hhpf is narrow, this sample is almost amorphous. In other samples with higher concentration of iron, a magnetically ordered well defined spectra appear at 4.2K. The following features are shown : a paramagnetic site (low intensity) was found which corresponds to very tiny Fe clusters. A magnetic site (low intensity) with Hhpf = 200 to 250 kOe corresponds to some iron-oxygen-matrix compound. The magnetic site with Hhpf ≈ 340 kOe corresponds to bulk metallic iron. Other sites with Hhpf ≈ 370 kOe and 410 kOe correspond to iron atom at the surface of the grains /3/. Spectra at different temperature on samples 0.6 Fe, 0.4 Al2O3 and 0.5 Fe, 0.5 ZrO2 are typical of relaxation behaviour with transition temperature around 300 and 500K respectively. The nature of the matrix is of significant importance on the behaviour of the magnetic grains. For a given grain diameter the number of sites with Hhpf > 340 kOe depends strongly of the nature of the matrix. Amorphous material appears when the concentration of iron is very low. The size diameter measured from electron diffraction photograph is consistent with the size deduced from Mössbauer experiments. Fe-Al2O3 gives the narrowest linewidth, this means that the granular compound is well defined : interface effect are the lowest, concentrations of paramagnetic Fe and Fe compound are very low.Furthermore in Fe-Al2O3 there is a narrow range of grains diameter. Fe-Al2O3 appears to be the best defined granular compound, i.e. : magnetic metalgrains in an insulating matrix with a sharp distribution of diameters