| AVS 60th International Symposium and Exhibition | |
| Spectroscopic Ellipsometry Focus Topic | Thursday Sessions |
| Session EL+AS+EN+PS+SS+TF-ThM |
| Session: | Spectroscopic Ellipsometry for Photovoltaics and Instrument Development |
| Presenter: | T. Willett-Gies, New Mexico State University |
| Authors: | T. Willett-Gies, New Mexico State University C.J. Zollner, Cornell University E. DeLong, New Mexico State University S. Zollner, New Mexico State University |
| Correspondent: | Click to Email |
Using FTIR ellipsometry, we have determined the dielectric function of twinned single-crystalline lanthanum aluminate (LaAlO3) and spinel (MgAl2O4) wafers which are often used as substrate materials for oxide epitaxy. Measurements were taken at 300 K in the region of lattice vibrations between 250 and 1000 cm-1. LaAlO3 is a rhombohedrally distorted perovskite with two formula units per unit cell, leading to eight IR-active phonon modes [1]. Two of these eight are below our spectral range, one is very weak, and two are nearly degenerate [1]. We thus expect four TO peaks in the imaginary part of the dielectric function. The polar character of LaAlO3 also causes strong LO-TO splittings. Unlike previously published FTIR reflectance studies (which require a Kramers-Kronig analysis to determine the TO/LO phonon peaks), our FTIR ellipsometry measurements allow the direct determination of TO and LO phonon energies as peaks in the dielectric function ε and the loss function 1/ε, respectively.
Magnesium aluminate spinel (MgAl2O4) belongs to the cubic Oh7 space group and has two formula units per primitive cell. Of its 39 optic modes, factor group analysis [2] shows that there are only four IR-active modes with T1u symmetry. The lattice dynamics of spinel has long been controversial and differences have been found between natural crystals (which are believed to be fully ordered) and synthetic crystals (which often contain a small amount of Mg/Al disorder).
A good description of the dielectric functions of these materials can be found using a sum of Lorentz oscillators (for the TO phonons in our spectral range) and two poles for electronic and low-energy phonon absorption outside of our spectral range. A factorized model [3] with LO/TO phonon pairs and a UV pole yields even better agreement with the data. The classical Lorentz model assumes a frictional force proportional to the velocity of the atoms resulting in a single broadening parameter, while the Lowndes model [3] takes into account the anharmonic phonon decay and assigns independent broadening parameters to the LO and TO phonons. Our FTIR ellipsometry measurements yield LO and TO parameters (energies, broadenings, and oscillator strengths) with unprecedented accuracy, far exceeding those from previous FTIR reflectance results. We will compare our experimental phonon energies with those obtained from ab initio density-functional theory for both LaAlO3 and MgAl2O4.
[1] P. Delugas, V. Fiorentini, and A. Filipetti, Phys. Rev. B71, 134302 (2005).
[2] A. Chopelas and A.M. Hofmeister, Phys. Chem. Minerals18, 279 (1991).
[3] R. P. Lowndes, Phys. Rev. B1, 2754 (1970).