James E. Downes

Comparison between experiment and calculated band structures for DyN and SmN

Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079, USA(Dated: March 20, 2008)We investigate the electronic band structure of two of the rare-earth nitrides, DyN and SmN.Resistivity measurements imply that both materials have a semiconducting ground state, and bothshow resistivity anomalies coinciding with the magnetic transition, despite the different magneticstates in DyN and SmN. X-ray absorption and emission measurements are in excellent agreementwith densities of states obtained from LSDA+U calculations, although for SmN the calculationspredict a zero band gap.

Surface electronic structure of p-type GaN(0001̄)

The surface electronic structure of p-type wurtzite GaN(0001) 1×1 grown by molecular beam epitaxy has been investigated using synchrotron radiation excited angle resolved photoemission spectroscopy. Four discrete surface states were clearly identified and characterized. All the states were removed by exposure of the surface to atomic hydrogen. Three of the states were of pz orbital character, while the fourth is derived from s orbitals. Three of the surface states lie below the bulk valence band maximum throughout the surface Brillouin zone, and one surface state was observed to disperse into the bulk optical band gap. Comparison with theory suggests that this surface state is Ga-derived, consistent with a model of Ga-terminated, but N polar, GaN. The other three surface states compare well with states observed earlier from n-type GaN.

Electronic structure of thin film silicon oxynitrides measured using soft x-ray emission and absorption

The elementally resolved electronic structure of a thin film silicon oxynitride gate dielectric used in commercial device fabrication has been measured using soft x-ray emission and absorption spectroscopies. The SiOxNy was grown by annealing SiO2 in NH3. Soft x-ray emission and soft x-ray absorption were used to measure the valence and conduction band partial density of states in the interfacial region of both the nitrogen and oxygen states. The elementally specific band gap for the O 2p states was measured to be 8.8 eV in the interfacial region, similar to that of pure SiO2. The elementally specific band gap for the N 2p states in the interfacial region was measured to be approximately 5 eV.

Resonant soft X-ray emission spectroscopy of V2O3, VO2 and NaV2O5

Resonant soft X-ray emission (RSXE) spectra of V2O3, VO2 and NaV2O5 were recorded for a series of excitation energies at resonances of the V L- and O K-absorption band. The V L- and O K-emission in these vanadium oxide bands possess considerable overlap. By resonant excitation we can tune the energy to the absorption thresholds, thereby eliminating this overlap. Hereby we obtain the V 3d and O 2p projected density-of-states of the valence band. Resonant inelastic X-ray scattering (RIXS) is found to be weak in V2O3, which we explain as being due to its metallic character at room temperature. Vanadium dioxide (VO2), semiconducting at room temperature, shows considerable RIXS features only at the O K-emission band. Distinct RIXS structures are visible in the RSXE spectra of the insulator NaV2O5. In the emission spectra excited at the V L-thresholds of this ternary vanadium oxide, dd-excitations of the V dxy subband at an energy loss of -1.7 eV are observed. Our observation, that RIXS is stronger for insulators than for metals, should be taken advantage of for studying insulator-to-metal transitions in vanadium compounds in the future.

Electronic band structure information of GdN extracted from x-ray absorption and emission spectroscopy

The electronic structure of GdN films grown by pulsed laser deposition has been investigated by soft x-ray absorption (XAS) and x-ray emission spectroscopy (XES) at the N K-edge. Density functional calculations within the local spin density approximation with Hubbard-U corrections of the N p weighted bands and density of states are used to extract band information from the spectra. Gd M4,5 XAS and XES spectra are also presented. The XES-XAS separation is shown to give information on the f-band spin splitting and the XAS line shapes are shown to reflect atomic multiplet effects.

Electronic structure of the organic semiconductor Alq3 (aluminum tris-8-hydroxyquinoline) from soft x-ray spectroscopies and density functional theory calculations

The element-specific electronic structure of the organic semiconductor aluminum tris-8-hydroxyquinoline (Alq(3)) has been studied using a combination of resonant x-ray emission spectroscopy, x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, and density functional theory (DFT) calculations. Resonant and nonresonant x-ray emission spectroscopy were used to measure directly the carbon, nitrogen and oxygen 2p partial densities of states in Alq(3), and good agreement was found with the results of DFT calculations. Furthermore, resonant x-ray emission at the carbon K-edge is shown to be able to measure the partial density of states associated with individual C sites. Finally, comparison of previous x-ray emission studies and the present data reveal the presence of clear photon-induced damage in the former.

Observation of multiple Zhang-Rice excitations in a correlated solid: Resonant inelastic X-ray scattering study of Li2CuO2

Multiple Zhang-Rice type spectral features have been observed in resonant inelastic X-ray scattering (RIXS) from the quasi–one-dimensional cuprate charge transfer insulator Li2CuO2. The first feature appears at constant emission energy, and is associated with a Zhang-Rice singlet final state. The second is an interplaquette charge transfer excitation that results in a novel triplet Zhang-Rice–type final state. It is accompanied by the presence of a O 2p nonbonding to upper Hubbard band excitation at an energy close to that of a calculated triplet charge transfer Zhang-Rice–type excitation. The site selectivity and polarization rules associated with RIXS allows these two excitations to be distinguished.

Soft x-ray emission studies of the bulk electronic structure of AlN, GaN, and Al0.5Ga0.5N

The electronic structure of wurtzite GaN, Al0.5Ga0.5N, and AlN has been studied using synchrotron radiation excited soft x-ray emission spectroscopy. In particular, the elementally resolved partial densities of states has been measured and found to agree well with calculations. The shift in energy of the valence band maximum as x varies from 0 to 1 in AlxGa1−xN was measured by recording N K-emission spectra, and found to be linear. Furthermore, N K-emission spectra revealed resonantlike hybridization of N 2p and Ga 3d states at 19 eV below the GaN valence band maximum. The spectral intensity of this feature is proportional to Ga content.

Low activation energy for the removal of excess nitrogen in nitrogen rich indium nitride

For some InN films large amounts of excess nitrogen are seen at low growth temperatures. Recent studies have revised downward the defect formation energies for several forms of nitrogen rich point-defects in InN. Here we calculate an activation energy of 0.4 ± 0.1 eV for the thermally activated removal of much of the excess nitrogen, believed to be interstitial nitrogen. This low energy barrier is shown to support the case for a low defect formation energy of the same native defect, although it is pointed out that non-equilibrium plasma based conditions are required to reach these lower defect formation energies.

Electronic properties of (Ga,Mn)N thin films with high Mn content

The authors gratefully acknowledge financial support from the New Zealand Foundation for Research Science and Technology through its New Economy Research Fund and through a postdoctoral fellowship of one of the authors B.J.R.. The work of the MacDiarmid Institute is supported by a New Zealand Centre of Research Excellence award. Another author S.G. wishes to thank Education New Zealand for financial support of the XANES measurements.