In situ photovoltage measurements using femtosecond pump-probe photoelectron spectroscopy and its application to metal–HfO2–Si structures

Abstract

We report in situ photovoltage measurements of metal-oxide-semiconductor (MOS) structures using femtosecond pump-probe photoelectron spectroscopy. This technique, which employs a single femtosecond laser, is a noncontact noninvasive measurement method for extracting the magnitude and direction of the band bending in Si substrates covered with high-k dielectric stacks and thin metal layers. We studied MOS structures consisting of thin metal layers of both high and low work functions deposited atop HfO2 grown on Si (100) substrates during various phases of processing. Excitation of the sample by a pulse of laser light flattens the bands of the Si substrate, which can be monitored as a rigid shift in the observed photoelectron spectrum. Particular attention is given to the potential effects of electron-hole recombination and metallic screening on the magnitude of the shift in the photoelectron spectra. We find that while as-deposited metals follow the metal-induced gap state model, thermal annealing of these structures drives the interface silicon Fermi energy to midgap. Charged oxygen-vacancy related defects at or near the HfO2/metal interface contribute significantly to the Fermi energy shift.