Energy level alignment in metal/oxide/semiconductor and organic dye/oxide systems (2008)

The alignment between the energy levels of the constituent materials of metal-oxide-semiconductor field effect transistors (MOSFET's) and dye sensitized solar cell (DSSC's) is a key property that is critical to the functions of these devices. We have measured the energy level alignment (band offsets) for metal/oxide/semiconductor (MOS) systems with high-k gate oxides and metal gates, and for organic dye/oxide systems. The combination of UV photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS) in the same vacuum system was used to measure both the occupied and unoccupied density of states (DOS), respectively, of these materials systems. Additional soft X-ray photoemission spectroscopy (SXPS) measurements were made of both the valence bands and core levels of the high-k systems. The combination of the UPS, IPS and SXPS measurements were used to determine the band offsets between the high-k oxides and the Si substrates of thin film oxide/Si samples. To find the metal-oxide band offsets, thin metal layers were sequentially deposited on the oxide surfaces, followed by spectroscopic measurements. These measurements, combined with the measurements from the clean oxide surfaces, were used to find the metal-oxide band offsets. Metal-oxide band offset values were also calculated by the Interface Gap State (IGS) model. We compared the experimental metal-oxide conduction band offset (CBO) values with those calculated using the IGS model, and found that they tended to agree well for Ru/oxide and Ti/oxide systems, but not as well for Al/oxide systems. Through core level spectroscopy, we correlated observations of the composition of the metallic layers with the trends in agreement between the experimental and IGS CBO values, which led to the conclusion that the IGS model gives accurate values for the CBO for systems with chemically abrupt interfaces. Core level spectroscopy of the MOS systems also showed that Al and Ti overlayers reduced the interfacial SiO2 layers of HfO2/SiO2/Si and Hf0:7Si0:3O2/SiO2/Si systems, while leaving the composition of high-k layers essentially unchanged. We also measured the energy level alignment for 3 organic dye/oxide systems, N3 dye on rutile TiO2(110), N3 dye on anatase TiO2 nanoparticle, and N3 dye on epitaxial ZnO(1120) film substrates. For the N3/TiO2(110) system, we found the the N3 highest occupied molecular orbital (HOMO) was 0.9 eV above the TiO2 valence band maximum (VBM) and the N3 lowest unoccupied molecular orbital (LUMO) was 0.5 eV above the TiO2 conduction band minimum (CBM). The energy level alignment for the N3/TiO2 nanoparticle system was similar to that for N3/TiO2(110). The alignment between the N3 HOMO and oxide VBM for the N3/ZnO systems was found to be similar to those of the N3/TiO2 systems, whereas the alignment between the N3 LUMO and oxide CBM alignment was found to differ markedly between the N3/ZnO and the respective N3/TiO2 systems. The difference in the LUMO-CBM alignments is attributed to the different interactions between the N3 LUMO and the ZnO and TiO2 conduction bands. In addition, we measure the energy level alignment for a prototype dye molecule, isonicotinic acid (INA), on TiO2(110) and ZnO substrates. These measurements showed that the LUMO of INA is similar to that of N3, and that the HOMO of INA is much different than that of N3, in keeping with expectations based on the compositions and theoretical electronic structures of these molecules.