J. W. Allen

Theory of mixed valence: Metals or small gap insulators (invited)

Mixed valence systems exhibit an interesting variety of low temperature electronic phenomena, in particular, a remarkable division of the known mixed valence compounds into metallic and insulating groups, exemplified respectively by CePd3 and SmB6. We show that for Sm compounds it is appropriate to use a two‐band Hubbard Hamiltonian comprised of a narrow f band with a large Coulomb interaction U, a wide d band with negligible interactions, and f‐d hybridization, Coulomb, and exchange interactions. Using the Green’s function decoupling methods of Hubbard, we show that intersite correlations profoundly affect states near the Fermi energy and may lead to a small insulating gap if the two bands contain an even integral number of electrons per rare earth ion, as in stoichiometric SmB6. Magnetic phase transitions are discussed in terms of exchange vs. hybridization energies, and a first order transition from a paramagnetic insulator to a ferro‐ or ferrimagnetic metal is predicted within the mixed valence regime.

Electronic structure of silicon nitride and amorphous silicon/silicon nitride band offsets by electron spectroscopy

The film thickness, chemical state, and polarization screening for a‐SiN1.4 :H films deposited by glow discharge over hydrogenated amorphous silicon (a‐Si:H) were determined by x‐ray photoelectron spectroscopy (XPS) and Auger spectroscopy. The nitride films were observed to be single phase and the escape depth for 1400‐eV electrons in the a‐SiN1.4 :H film was determined to be 30 A. The band offsets for the a‐Si:H/a‐SiN1.4 :H interface were determined by XPS and Bremsstrahlung isochromat spectroscopy (BIS) to be 1.2 eV for the valence band and 2.2 eV for the conduction band, while the band gap for a‐SiN1.4 :H was found to be 5.3 eV in accordance with the optical gap. By combining optical absorption measurements with the valence‐band density of states and conduction‐band density of states determined by electron spectroscopy, a semiquantitative estimate of the band tailing within the nitride gap was obtained. Correlation of the defect absorption with the electron spin resonance measurements suggest that the ...

Electron spectroscopy study of the Si–O bonding and the polarization screening near the Si‐SiO2 interface

Electron spectroscopy studies of thermally grown SiO2 on Si, α‐quartz, α‐cristobalite, and fused silica show that the variation in the Si–O–Si bond angle in SiO2 does not cause significant change in the charge transfer in the Si–O bond. The relative core level shifts are sensitive to etching and to variations in the oxidation process. Measurements of the Auger parameter for Si show no difference in polarization screening between the Si‐SiO2 interface and the bulk SiO2 and that the nearest neighbors of Si in the SiO4 tetrahedron contribute predominantly to the polarization screening.

Mixed valent SmB6 and gold‐SmS: Metals or insulators?

We review the dc transport properties of mixed valent compounds SmB6 and gold‐SmS and find the character of the ground state, metallic or insulating, to be an open question. The room temperature optical properties of SmB6 and gold‐SmS display ordinary metallic behavior at high energy, but are very nonmetallic at low energy in that E1 becomes positive like an insulator from about 0.1 eV down to the lowest energy measured. We show that these room temperature optical properties are consistent with a ’’bad‐metal’’ model having either a sharp 4f resonance at the Fermi level, or a collective low frequency density fluctuation, and extract from the data a value ∼.08 eV for the 4f‐5d mixing matrix element. But we point out that the possibility of an insulating ground state remains.

γ-α Transition in Ce from the large N Anderson model

The equation of state and 4f occupation of Ce are calculated from the degenerate Anderson model with volume-dependent interactions, using the 1/N expansion to first order and the exact entropy for the N = 6 Kondo model. Maximum instability occurs at nf ∼ 0.95, within the Kondo regime, and the α phase saturates at nf ∼ 2/3.

Electronic structure of CePd3 from resonant photoemission and optical reflectivity spectra

Optical reflectivity and resonant photoemission measurements on CePd3 yield a 4f binding energy Ef between 0.7 and 1.0 eV and set an upper limit of 0.3 eV on a surface shift of Ef. The 4f halfwidth at half maximum δ is between 0.5 and 0.8 eV. The spin fluctuation energy Γ = 20 meV is successfully related to Ef and δ by the Kondo impurity Ansatz.

The conduction band of hydrogenated amorphous silicon

The valence and conduction band density of states of hydrogenated amorphous silicon (a‐Si:H) are determined using x‐ray photoemission (XPS) and bremstrahlung isochromat spectroscopy (BIS), respectively. Evidence for the Si‐H antibonding orbital of polyhydrides is found near the conduction band edge. Implications of these results for the transport and recombination are discussed. In samples doped with 1% boron, unoccupied gap states are found. Finally, corrections used in photoemission are applied to BIS spetra to determine a more accurate density of states.

Energy Dependence of the Optical Matrix Element for Hydrogenated Amorphous Silicon

The valence band and conduction band density of states of hydrogenated amorphous silicon are determined using x-ray photoemission and bremstrahlung isochromat spectroscopy, respectively. From these densities of states and measurements of the imaginary part of the dielectric function using ellipsometry, the energy dependence of the average transition matrix element squared has been determined. The matrix element exhibits a maximum near the direct edge of crystalline silicon (~3.35 eV) and is very similar to the energy dependence calculated for a complex crystalline form of silicon with short range disorder and long range order.