Lina S. Abdallah

Dielectric function of LaAlO3 from 0.8 to 6 eV between 77 and 700 K

The authors used spectroscopic ellipsometry to determine the dielectric function and the refractive index of LaAlO3 as a function of photon energy from 0.8 to 6 eV between 77 and 700 K. The ellipsometric angles were acquired over a broad range of incidence angles with a computer-controlled Berek waveplate compensator and with zone-averaging of the adjustable polarizer. The data were corrected for surface effects, such as surface roughness or adsorbed overlayers. The authors report Tauc–Lorentz model parameters for LaAlO3 at 300 K. After annealing in UHV for 20 h, the surface layer thickness decreased from 15 A to less than 1 A. They speculate that the anneal causes surface modifications, such as the evaporation of adsorbed molecular layers (hydrocarbons or water) or surface vacancies, and that surface diffusion leads to a reduction of surface roughness. They have also measured the temperature-dependence of the refractive index at 1.96 eV between 77 and 700 K and given a theoretical explanation of its origin.

Optical constants and band structure of trigonal NiO

Using spectroscopic ellipsometry and transmission measurements, the authors determined the optical constants (absorption coefficient, complex refractive index, and dielectric function) of bulk trigonal NiO from 0.08 to 6.5 eV. By careful discussion of the data, elastic scattering by oxygen bubbles was ruled out and the effects of surface roughness were removed numerically to obtain an accurate dielectric function of NiO. A direct band gap of 0.85 eV was found from transmission and assigned to direct interband transitions from the Ni-O hybrid valence band states to the Ni 4s conduction band at the center of the Brillouin zone. At 4 eV, the authors find the well-known charge transfer gap from the lower to the upper Hubbard band. Several intermediate sharp peaks were also found. The temperature dependence of the NiO charge transfer gap is similar to the E1 gap of Si between 100 and 700 K. At higher temperatures, heating NiO in vacuum leads to sublimation, which has drastic irreversible consequences for the p...

Two-phonon absorption in LiF and NiO from infrared ellipsometry

Using Fourier-transform infrared spectroscopic ellipsometry, the infrared lattice absorption of LiF and NiO was studied in the reststrahlen region. The transverse optical (TO) and longitudinal optical phonon energies, broadenings, and amplitudes were determined. Both materials also show a weak two-phonon absorption, which modifies the shape of the reststrahlen bands. The authors did not find any evidence of a splitting of the TO phonon in NiO due to antiferromagnetic ordering and place an upper limit of 17 cm−1 on this splitting.

Optical conductivity of Ni1 − xPtx alloys (0<x<0.25) from 0.76 to 6.6 eV

Using spectroscopic ellipsometry and Drude-Lorentz oscillator fitting, we determined the dielectric function and optical conductivity versus photon energy from 0.76 to 6.6 eV of 10 nm thick Ni1 − xPtx alloy (0<x<0.25) films deposited on thick thermal oxides. We find absorption peaks near 1.6 and 5.0 eV due to interband optical transitions. There is a significant broadening of the UV peak with increasing Pt content, since the bandwidth of the 3d electrons in Ni is smaller than that of the 5d bands in Pt. Our experimental observation is consistent with ab initio calculations of the density of states for Ni, Pt, and the Ni3Pt compound. Annealing the metals at 500°C for 30 s increases the optical conductivity.

Optical conductivity of Ni1−xPtxSi monosilicides (0 < x < 0.3) from spectroscopic ellipsometry

The optical constants of 22 nm thick Ni1−xPtxSi (0 < x < 0.3) monosilicide films were measured using spectroscopic ellipsometry, in the spectral range from 0.6 to 6.6 eV at room temperature. Ni1−xPtx films sputtered on clean Si were annealed at 500 °C for 30 s to form nickel platinum monosilicides. The correct silicide thickness was found by minimizing Si substrate artifacts in the optical constants of Ni1−xPtxSi determined from ellipsometric data analysis. Two interband transitions at 1.8 and 4.5 eV were observed (rather than three peaks in PtSi). The absorption peak at 4.5 eV broadens with increasing Pt content in the monosilicide.