Institute Silicon Photovoltaics

Near Ultraviolet Photoelectron Spectroscopy

Basics

Photoelectron spectroscopy (PES) makes use of the external photoeffect to investigate the structure and electronic properties of materials. The basic set-up is sketched in Fig. 1 (left): The monochromatic photons that are emitted from the light source (e.g. an X-ray tube / gas discharge lamp / Xe high pressure lamp + Monochromator / …) have a photon energy E_phot = hν. They impinge onto the sample under investigation and excite electrons from occupied states below the Fermi level E_F into unoccupied states above E_F.

If the excited electrons reach the sample surface and their impulse is sufficient to overcome the work function barrier, they can exit the sample into the vacuum. Using an energy-selective detector, their kinetic energy E_kin is measured. From this, the initial energy of the photoelectron in the sample can be calculated. Usually, it is given in terms of a binding energy E_bind, i.e. the energy relative to the Fermi level in the sample.

Fig.1: (left) Basic set-up of a photoelectron spectroscopy (PES) experiment; (right) sketch of the energy scales ¹ relevant for the PES and comparison of the conventional UPS mode with the Constant Final State Yield spectroscopy (CFSYS).

¹ convention: states below E_F have negative binding energy.

Measurement setup

Our photoelectron spectroscopy set-up is equipped for XPS (Al and Mg anodes) and UPS (usually He gas discharge). In addition, it features a light source for low energy excitation with variable photon energy up to 7eV that is especially useful for the spectroscopy of the valence band and band gap states of amorphous silicon (a‑Si:H) layers.