Basics: Direct and Inverse Photoemission

Photoemission and inverse photoemission are two highly complementary techniques that provide a detailed understanding of the electronic structure above and below the Fermi level. Photoemission is based on the familiar photoelectric effect. In simplest terms, one measures the energy and angular distributions of electrons emitted from a sample that is excited by monochromatic light. Operationally, inverse photoemission is the time reversed process of photoemission, where monoenergetic electrons impinge on a sample and the emitted photons are detected. Experimental implementation is illustrated schematically in the figure above.

       Sample Results: Direct and Inverse Photoemission

Metallic quantum well states in the Cu/fccFe/Cu(100) system:  The figure on the left is a series of normal emission photoemission spectra obtained from a Cu wedge grown on a 5 ML fccFe/Cu(100) structure. Spectra from different positions on the wedge (which correspond to different thicknesses of the Cu overlayer) are displayed as an intensity plot.  Dark colors indicate low intensity, bright yellow indicates high intensity. The bright bands moving towards the Fermi level with increasing Cu thickness are the metallic quantum well (MQW) states in the Cu overlayer.    The figure at the As suggested by the discussion above, as the thickness of a metallic quantum well changes, MQW state energies change. The figure above on the left shows a series of inverse photoemission spectra obtained from the Cu/fccFe/Cu(100) system as a function of increasing film thickness. The tick marks show that MQW states move upward with increasing thickness. The figure at the right shows normal incidence IPS spectra from planar Cu/fccFe/Cu(100) structures with increasing Cu thickness.  The occupied MQW states highlighted by each color in the photoemission data is seen above the Fermi level in the IPS data. 

HOMO and LUMO level alignment for the N3 dye molecule on the TiO2(110) surface: The most efficient dye sensitized solar cells (DSSCs) to date are fabricated with anatase titania nano-particles sensitized with N3 dye.  The alignment of the N3 HOMO and LUMO levels with the conduction band minimum (CBM) and valence band maximum (VBM) of the underlying titania semiconductor is critical for solar cell performance.  Using our unique apparatus that houses both photoemission in the same experimental chamber, we can study the level alignment from the model N3/rutile-TiO2(110) system.

Band alignment in high-K dielectric/semiconductor systems: As scaling in the semiconductor industry moves towards critical dimensions of ~ 45 nm, the effective oxide thickness needed for metal semiconductor oxide (MOS) devices approaches 1 nm.  At these thicknesses, the leakage current through an SiO2 oxide is unacceptably large, and high-K dielectrics such as HfO2 are now being employed.  Band alignment between the oxide and the semiconductor, as well as between a candidate metal gate and the oxide, can be measured.  The figure at the left shows combined photoemission and inverse photoemission spectra that give the band gap for HfO2, HfSiOx, and SiO2.   

Read more: Unoccupied electronic structure of Ru(0001) W.-K. Siu and R.A. Bartynski, Phys. Rev. B, 75, 235427 (2007)

       Recent Publications                                                                       (back to top)