• band alignment;
  • energy barriers;
  • interfaces;
  • internal photoemission;
  • work function

Precise evaluation of the effective work function and the built-in voltage in metal–oxide–metal (MOS) or metal–insulator–metal (MIM) stacks is crucial for attaining the functionality of various oxide-based electron devices. We studied the impact of a low work function (WF) metal interlayer (IL), i.e. Ti, on the electron barrier height between the metal Fermi level and the conduction band of the oxide insulator. The experiments were performed on structures comprised of a layer of thermal SiO2 or an atomic layer deposited Al2O3 on heavily doped p- or n-Si substrates. In order to evaluate the impact of the Ti IL, 3, 6 or 10 nm thick Ti films have been deposited prior to TiNx sputtering. Spectral curves of electron photoemission from the metal layer into the oxide were analyzed in order to determine the energy barrier for electrons between the Fermi level of the metal and the conduction band of the insulator (Φe). As expected, the insertion of the low WF metal shifts the barrier to lower energies, depending on the IL thickness. However, while a 3-nm thick Ti IL is sufficient to lower the barrier by 0.5 eV in the SiO2-based stack, it has no measurable impact on the barrier in the Al2O3/Ti/TiNx stack. For the latter structure, both Ti and the Al2O3 thicknesses influence the Φe lowering trend, suggesting that oxygen scavenging from Al2O3 by Ti and nitridation of the Ti IL by subsequent TiN deposition may affect the band alignment between the metal and the oxide.