| AVS 63rd International Symposium & Exhibition | |
| Spectroscopic Ellipsometry Focus Topic | Friday Sessions |
| Session EL+AS+EM+MI+TF-FrM |
| Session: | Spectroscopic Ellipsometry: Novel Applications and Theoretical Approaches |
| Presenter: | Nuwanjula Samarasingha, New Mexico State University |
| Authors: | N. Samarasingha, New Mexico State University C. Rodriguez, New Mexico State University J.M. Moya, New Mexico State University N.S. Fernando, New Mexico State University S. Zollner, New Mexico State University P. Ponath, University of Texas at Austin K. Kormondy, University of Texas at Austin A. Demkov, University of Texas at Austin D. Pal, Indian Institute of Technology Indore, India A. Mathur, Indian Institute of Technology Indore, India A. Singh, Indian Institute of Technology Indore, India S. Dutta, Indian Institute of Technology Indore, India J. Singhal, Indian Institute of Technology Indore, India S. Chattopadhyay, Indian Institute of Technology Indore, India |
| Correspondent: | Click to Email |
The presence of excitonic features in the optical constants and ellipsometry spectra of bulk semiconductors and insulators has been known for many years. In Si, Ge, and GaAs, the E1 critical points are strongly enhanced by two-dimensional excitons, even at room temperature. Three-dimensional excitons have been seen in ellipsometry spectra for GaP and Ge. Excitons also influence the dielectric function of SrTiO3. An exciton is an electron-hole pair bound by the Coulomb interaction, with properties similar to a hydrogen atom. The influence of excitonic absorption on the dielectric function was described by Tanguy.
In a thin epitaxial layer (with a thickness below or near the Bohr radius) on a substrate with a different band gap, the wave functions of the electron and hole are strongly modified. In a thin type-I quantum well, consisting of a narrow-gap semiconductor grown on a large-gap substrate, both the electron and the hole are confined, which leads to an increase in the dipole overlap matrix element. Therefore, the dominant absorption peak at 4.2 eV is larger in a 20 nm thick SrTiO3 layer on a LaAlO3 substrate than in bulk SrTiO3. (The band gap of LaAlO3 is larger than that of SrTiO3.)
On the other hand, in a staggered type-II quantum well, either the electron is confined, or the hole, but not both. Therefore, the overlap dipole matrix element (and thus the excitonic absorption) is strongly reduced, because one quasiparticle resides in the quantum well and the other one in the substrate. If a SrTiO3 layer is grown on Si or Ge, the valence band maximum occurs in the substrate, while the conduction band offset is very small. Therefore, the exciton wave function is delocalized (deconfined), which reduces the dipole overlap matrix element. Therefore, the real and imaginary part of ε of thin SrTiO3 layers on Si or Ge are much smaller than in the bulk and decrease monotonically with decreasing thickness. A similar effect can be seen for thin ZnO layers on Si as a function of thickness.
The dielectric function of SrTiO3 is not only affected by layer thickness. A very thick polycrystalline SrTiO3 layer on Si has a much lower dielectric function than a single-crystalline SrTiO3 substrate. In this case, we speculate that the magnitude of the dielectric function is related to other Tanguy parameters, perhaps the excitonic binding energy or the exciton decay rate (broadening). To investigate this further, we will perform temperature-dependent ellipsometry measurements on bulk zinc blende GaP, which has a much simpler band structure than wurtzite ZnO or the correlated metal oxide SrTiO3, but shows similar excitonic effects.