G. L. Liu

Effects of interlayer and annealing on chemical states of HfO2 gate insulators studied by photoemission spectroscopy

We have performed photoemission spectroscopy of high-k gate insulators HfO2/HfSiON/Si to investigate the interlayer formation by Hf metal predeposition and the annealing effect systematically. Comparing the line shapes of core-level photoemission spectra for two systems with and without Hf-metal predeposition, we found that Hf-metal predeposition effectively reduces the growth of interface layer. Hf 4f core-level spectra revealed that the annealing at 1000 °C for both samples causes the formation of the metallic Hf and Hf-silicide clusters. Surface morphology was also observed by atomic force microscopy.

Crystallization in HfO2 gate insulators with in situ annealing studied by valence-band photoemission and x-ray absorption spectroscopy

We have investigated the valence-band and conduction-band electronic structures of HfO2 gate insulators on Si substrates and their dependence on the annealing temperature in ultrahigh vacuum and the Hf-metal predeposition at the interface by photoemission spectroscopy and x-ray absorption spectroscopy. In the case with the Hf-metal predeposition before the HfO2 deposition, the valence-band spectra were split into double-peak structures and the line shapes of O K-edge x-ray absorption spectra became sharp due to the annealing at 800 and 900 °C. On the other hand, without the Hf-metal predeposition, annealing-temperature dependence in these spectra was not observed. Cross-sectional transmission electron microscopy images reveal that the changes in both valence-band and O K-edge absorption spectra are related to the crystallization of the HfO2 layer, although it is difficult to distinguish the crystallization in Hf 4f core-level spectra. It suggests that the valence-band photoemission and x-ray absorption sp...

Chemical reaction and metallic cluster formation by annealing-temperature control in ZrO2 gate dielectrics on Si

Thermal stability of the ZrO2∕Zr–silicate∕Si structure and the Zr–silicide formation were investigated by photoemission spectroscopy depending on the annealing temperature in ultrahigh vacuum. By the annealing below 860°C, the interfacial layer thickness of the Zr–silicate decreased although the ZrO2 top layer was not affected. The annealing at 860°C caused the interfacial Zr–silicate layer to disappear. By the annealing above 860°C, the metallic Zr components appeared and the metallic clusters were formed. High-resolution photoemission spectra have revealed that the clusters consist of a ZrSi2 layer. Valence-band spectra depending on the annealing temperature provide us with the information about the crystallization in the ZrO2 layer.

Precise determination of band offsets and chemical states in SiN∕Si studied by photoemission spectroscopy and x-ray absorption spectroscopy

We have investigated chemical states and band offsets in SiN∕Si by photoemission spectroscopy and x-ray absorption spectroscopy. N1s photoemission spectra in SiN for three kinds of layer-thickness films are fitted by a single component, suggesting that a nitrogen atom is surrounded by three silicon and nine nitrogen atoms for the first and the second nearest neighbor, respectively. Valence-band offsets between SiN and the Si substrates are determined to be 1.6 eV using valence-band spectra by subtracting the contribution from Si substrates. Band gap of SiN is estimated to be 5.6–5.7 eV from valence-band, N1s core level, and NK-edge-absorption spectra. Furthermore, time-dependent measurements of N1s photoemission spectra reveal that the x-ray irradiation time is a significant factor to determine the precise valence-band offsets excluding the differential charging effects.

Control of oxidation and reduction reactions at HfSiO∕Si interfaces through N exposure or incorporation

Using synchrotron-radiation photoemission spectroscopy, we have investigated oxidation and reduction reactions of HfSiO(N)∕Si gate stack structures annealed in a N2 or O2 atmosphere. It is found that both oxidation and reduction reactions can be suppressed by using nitrogen-incorporated HfSiO films in the annealing process at proper partial pressure of N2 gas (PN2∼100Torr). The detailed analysis of “SiO2 equivalent thicknesses” for annealed HfSiO and HfSiON films reveals that ambient N2 gas suppresses only the reduction reaction, while nitrogen atoms incorporated in dielectrics suppress both oxidation and reduction reactions.

Mechanism of Hf-silicide formation at interface between poly-Si electrode and HfO2∕Si gate stacks studied by photoemission and x-ray absorption spectroscopy

We have investigated the mechanism for silicidation by chemical reactions at polycrystalline-Si (poly-Si)∕HfO2∕Si gate stacks by annealing in ultrahigh vacuum using photoemission spectroscopy and x-ray absorption spectroscopy. Si 2p, Hf 4f, and O 1s high-resolution photoemission spectra have revealed that a Hf-silicide formation starts at as low temperature as 700°C and that a Hf silicate is also formed at the interface between poly-Si electrodes and HfO2. The metallic Hf silicide is formed at the interface between HfO2 and Si substrates, which changes the band offsets on Si substrates. We have found that poly-Si electrodes promote the interfacial reaction between HfO2 and Si substrates, while the crystallization in a HfO2 layer is independent of the silicide formation. The silicidation mechanism based on photoemission spectra is also confirmed from the thermodynamical analysis considering the Gibbs’ free energy.

Chemical reaction at the interface between polycrystalline Si electrodes and HfO2∕Si gate dielectrics by annealing in ultrahigh vacuum

We have investigated the chemical reaction at the interface between polycrystalline-Si (poly-Si) electrodes and HfO2∕Si gate dielectrics by photoemission spectroscopy and x-ray absorption spectroscopy depending on the annealing temperature in an ultrahigh vacuum. From Si2p and Hf4f high-resolution core-level photoemission spectra, we revealed that the Hf-silicide formation starts at as low temperature as 700°C and that the Hf-silicate layer is also formed at the interface between poly-Si electrodes and HfO2. Crystallization of the amorphous HfO2 layer even at 700°C was suggested from valence-band and OK-edge absorption spectra. By the annealing at 800°C, the HfO2 layer disappeared completely and the Hf-silicide clusters were formed on the Si substrate. Direct contact between poly-Si electrodes and HfO2 promotes the interfacial reaction compared to the case without poly-Si electrodes.

Annealing-time dependence in interfacial reaction between poly-Si electrode and HfO2∕Si gate stack studied by synchrotron radiation photoemission and x-ray absorption spectroscopy

We have investigated annealing-time dependence in interfacial reactions between polycrystalline-Si (poly-Si) electrodes and HfO2∕Si gate stacks using synchrotron radiation photoemission and x-ray absorption spectroscopy. From photoemission core-level spectra, we found that silicidation started at the upper interface between poly-Si electrodes and HfO2 gate dielectrics under the conditions of 700°C and 3min in ultrahigh vacuum. Before silicidation, we observed nonstoichiometric silicon oxide at the upper interface, suggesting the formation of oxygen vacancies which may cause Fermi level pinning. The interface layer between HfO2 gate dielectrics and Si substrates was changed into silicide by further annealing. In addition, from valence-band photoemission and O K-edge absorption spectra, we have found that the crystallization of HfO2 gate dielectrics is independent of silicidation.

Photoinduced charge-trapping phenomena in metal/high-k gate stack structures studied by synchrotron radiation photoemission spectroscopy

We have demonstrated photoinduced charge-trapping phenomena in metal/high-k gate stack structures using time-dependent photoemission spectroscopy with synchrotron radiation. Pt metal gate electrode with a large work function releases trapped negative charges near the surface of the HfSiON film while TiN metal gate electrode with a lower work function keeps negative charges in the HfSiON film. The release of negative trapped charges reveals a possibility of positive charge trapping at the interface in the HfSiON film. The location of energy level for negative charges is concluded to be between Pt and TiN Fermi-level in the band gap of the HfSiON film.

Thermal stability in a-Si/HfSiO(N)/Si gate stack structures studied by photoemission spectroscopy using synchrotron radiation

We have investigated thermal stability in amorphous-Si/HfSiO(N) gate stack structures using synchrotron-radiation photoemission spectroscopy. Core-level photoemission spectra depending on annealing temperature have revealed the mechanism of metallization reaction at the upper interface between a-Si cap layer and HfSiO(N) films under ultrahigh vacuum annealing. Silicidation reaction starts by annealing at 700 °C for both HfSiO and HfSiON films. By annealing at 800 °C, metallization reaction is rapidly promoted for the HfSiO film, while the Hf-silicide component changes into the Hf-nitride component due to its thermal stability and metallization reaction mildly proceeds for the HfSiON films.