Ga 3d States Research Articles

An insight into the electronic, optical and magnetic properties of CeO2-X (X=Al, Ga) materials for optoelectronic applications

We investigate electronic, optical and magnetic properties of Al and Ga doped CeO2 using first principle calculations. In order to approximate exchange and correlation potential, Perdew-Burke-Ernzerhof (PBE) based generalized gradient approximation is used in Wien2k code. Spin-polarized calculations manifest non-magnetic nature of CeO2 while Al and Ga doped CeO2 points magnetic character with net magnetic moment of 3.09 μB and 2.03 μB, respectively. p-d hybridization with enormous influence of Ce 4f-, Al 3p- and Ga 3d-states is noted which tune electronic properties of selected materials. Optical absorption spectrum of pure CeO2 shows blueshift upon incorporation of impurities. Upon addition of impurities, significant reduction in band gap of CeO2 is noted. Improved absorption and conductivity along with decrease in reflectivity in the UV region embellish uses of these materials in the field of optoelectronics, photonics, solar, photocatalysis and spintronics.

Effect of Vacancy Defects on the Electronic Structure and Optical Properties of GaN

The effect of gallium vacancy (VGa) and nitrogen vacancy (VN) defects on the electronic structure and optical properties of GaN using the generalized gradient approximation method within the density functional theory were investigated. The results show that the band gap increases in GaN with vacancy defects. Crystal parameters decrease in GaN with nitrogen vacancy (GaN:VN) and increase in GaN with gallium vacancy (GaN:VGa). The Ga vacancy introduces defect levels at the top of the valence band, and the defect levels are contributed by N2p electron states. In addition, the energy band shifts to lower energy in GaN:VNand moves to higher energy in GaN:VGa. The level splitting is observed in the N2p states of GaN:VNand Ga3d states of GaN:VGa. New peaks appear in lower energy region of imaginary dielectric function in GaN:VNand GaN:VGa. The main peak moves to higher energy slightly and the intensity decreases.

Electronic properties on GaN(0001) surface – ab initio investigation

Density Functional Theory (DFT) calculations were used to determine physical properties of bulk GaN and GaN(0001), i.e. N-terminated surface. The bulk data are in agreement with the theoretical and experimental data reported by Magnuson et al. [M. Magnuson et al., Phys. Rev. B 81 (2010) 085125], demonstrating that bonding in GaN is of a mixed nature in which Ga 4s, Ga 4p, Ga 3d, and N 2s, N 2p orbitals are involved. Ga 4s and Ga 4p states are sp3 hybridized while the N states are involved independently in bonding. Ga sp3 states overlap with the N 2s states creating the upper subband extending down to 6 eV below the valence band maximum (VBM). The lower valence band (VB) subband is composed of Ga 3d states overlapping with N 2s states whose energy levels range from 13 to 17 eV below VBM. GaN bulk bonding enforces creation of the two surface states at clean GaN(0001) surface: the lower energy state, located 15 eV below VBM is composed of N 2s states. The upper energy surface state, located below VBM, is composed of N 2pz nonsaturated state, i.e. it is of the broken bond type. The Fermi energy is pinned by this dispersionless acceptor state. Ga adsorption leads to emergence of donor state, which pins the Fermi level at 1.1 eV above VBM which is compatible with the Fermi level pinned at the surface for p-type GaN and is inconsistent with n-type bulk. The total energy of slab has the energy lower by about 2 eV for the adsorbed than for the separate Ga atom. The inconsistent data obtained from thermodynamic stability and band diagram analyses suggest further investigations to explain inconsistencies between these two pictures.

Optical Study of Filled Tetrahedral Compounds Li3AlN2 and Li3GaN2

A detailed analysis of the optical properties of filled tetrahedral semiconductors Li3AlN2 and Li3GaN2 has been performed, using the full potential linearized augmented plane wave method within the density functional theory. The real and imaginary parts of the dielectric function e(ω), the optical absorption coefficient I(ω), the reflectivity R(ω), and the electron energy loss function are calculated within the random phase approximation. The interband transitions responsible for the structures in the spectra are specified. Looking at optical matrix element, we note that the major peaks are dominated by transition from metal s, N 2p states to N 2p, Ga 3d states. The theoretical calculated optical properties and electron energy loss spectrum yield a static dielectric constant of 5.34 and a plasmon energy of 19.47 eV for Li3GaN2. In the Li3AlN2 compound, the static dielectric constant decreases to 4.75 and yields a plasmon energy of 18.5 eV. The effect of spin–orbit coupling on the optical properties is also investigated and found to be quite small, especially in the low-energy region. In order to check the reliability of our calculations, analogous results obtained for Be3N2 in the same structure [space group Ia3(206)] are included in this work.

Electronic structure of LiGaS 2

X-ray photoelectron spectroscopy (XPS) measurement has been performed to determine the valence band structure of LiGaS 2 crystals. The experimental measurement is compared with the electronic structure obtained from the density functional calculations. It is found that the Ga 3d states in the XPS spectrum are much higher than the calculated results. In order to eliminate this discrepancy, the LDA+ U method is employed and reasonable agreement is achieved. Further calculations show that the difference of the linear and nonlinear optical coefficients between LDA and LDA+ U calculations is negligibly small, indicating that the Ga 3d states are actually independent of the excited properties of LiGaS 2 crystals since they are located at a very deep position in the valence bands.

Electronic Structure of a Nitrogen Vacancy in Cubic Gallium Nitride

The electronic structure of a nitrogen vacancy in zinc-blende GaN has been calculated using two different supercells with the plane-wave pseudopotential (PWPP) and the tight-binding linear muffin-tin orbitals (TB-LMTO) methods. The Ga 3d states are included in the valence states. Relaxation near a nitrogen vacancy site was examined with the PWPP method using a 31-atom unit cell. The nearest neighbor Ga atoms were found to move toward the vacancy site by 0.04 A resulting in a relaxation energy of only 0.04 eV. Given the absence of large relaxations, TB-LMTO calculations were then performed for 31- and 63-atom unit cells using an ideal (unrelaxed) geometry. Densities of states and charge density maps show that a nitrogen vacancy can induce a partially filled band, which overlaps with the conduction states, resulting in n-type conductivity. The energy shift of this band under pressure was investigated for volume compressions up to 12% and compared with results from recent high-pressure measurements for GaN.

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.

Surface and bulk electronic structure of thin-film wurtzite GaN

The bulk and surface valence-band electronic structure of thin-wurtzite GaN has been studied using angle-resolved photoemission spectroscopy. The bulk band dispersion along the {Gamma}{Delta}A, {Gamma}{Sigma}M, and {Gamma}TK directions of the bulk Brillouin zone was measured. Our results indicate the local-density approximation band-structure calculations using partial-core corrections for the Ga 3d states predict the relative dispersion of many of the observed bands with a high degree of accuracy. Furthermore, a nondispersive feature was identified near the valence-band maximum in a region of k space devoid of bulk states. This feature is identified as emission from a surface state on GaN(0001)-(1{times}1). The symmetry of this surface state is even with respect to the mirror planes of the surface and polarization measurements indicate that it is of sp{sub z} character, consistent with a dangling-bond state. {copyright} {ital 1997} {ital The American Physical Society}

Bulk and Surface Electronic Structure of GaN Measured Using Angle-Resolved Photoemission, Soft X-ray Emission and Soft X-ray Absorption

ABSTRACTThe electronic structure of thin film wurtzite GaN has been studied using a combination of angle resolved photoemission, soft x-ray absorption and soft x-ray emission spectroscopies. We have measured the bulk valence and conduction band partial density of states by recording Ga L- and N K- x-ray emission and absorption spectra. We compare the x-ray spectra to a recent ab initio calculation and find good overall agreement. The x-ray emission spectra reveal that the top of the valence band is dominated by N 2p states, while the x-ray absorption spectra show the bottom of the conduction band as a mixture of Ga 4s and N 2p states, again in good agreement with theory. However, due to strong dipole selection rules we can also identify weak hybridization between Ga 4s- and N 2p-states in the valence band. Furthermore, a component to the N K-emission appears at approximately 19.5 eV below the valence band maximum and can be identified as due to hybridization between N 2p and Ga 3d states. We report preliminary results of a study of the full dispersion of the bulk valence band states along high symmetry directions of the bulk Brillouin zone as measured using angle resolved photoemission. Finally, we tentatively identify a non-dispersive state at the top of the valence band in parts of the Brillouin zone as a surface state.

X-ray photoelectron spectroscopy and theory of the valence band and semicore Ga 3d states in GaN.

The effects of the Ga 3d semicore levels on the electronic structure of GaN are discussed. While band-structure theory using the local-density approximation predicts these states to overlap with the N 2s band and to have important effects on the total energy, x-ray photoelectron spectroscopy (XPS) shows that they occur \ensuremath{\sim}3 eV below the N 2s band. This apparent discrepancy is resolved by means of a so-called \ensuremath{\Delta} SCF or difference of self-consistent-fields calculation, in which the binding energy is calculated as a total-energy difference including solid state screening effects by means of the excited-atom model. The calculated valence-band densities of states are found to be in good agreement with the XPS spectrum. The differences between zinc blende and wurtzite GaN are discussed.

Structural and electronic properties of bulk GaAs, bulk AlAs, and the (GaAs)1(AlAs)1 superlattice.

Ground-state electronic and cohesive properties of the pure compounds GaAs and AlAs and of the (GaAs${)}_{1}$(AlAs${)}_{1}$ (001) superlattice are investigated using a highly precise local-density all-electron total-energy band-structure approach---the self-consistent full-potential linearized augmented-plane-wave (FLAPW) band method---to obtain the energy bands, density of states, and total energies. The effects of Ga 3d states, spin-orbit interactions, and pressure on the energy gap are analyzed quantitatively. The energy gap of the (1\ifmmode\times\else\texttimes\fi{}1) superlattice is found to be direct. The instability of the (1\ifmmode\times\else\texttimes\fi{}1) superlattice relative to the constituent pure compounds at T=0 is determined from total-energy differences to be 13.5 meV.