S. Toyoda

Chemistry and band offsets of HfO2 thin films for gate insulators

Interfacial chemistry and band offsets of HfO2 films grown on Si(100) substrates are investigated using high-resolution angle-resolved photoelectron spectroscopy and are correlated with interfacial structures revealed by transmission electron microscope. Hf 4f and O 1s spectra show similar chemical shifts indicating the existence of a double layer structure consisting of a HfO2, upper layer and a SiO2-rich Hf1−xSixO2 lower layer. Two types of valence band offsets are clearly determined by a double subtraction method to be 3.0 and 3.8 eV that can be attributed to ΔEv1 for the upper layer HfO2/Si and ΔEv2 for the lower layer Hf1−xSixO2/Si, respectively.

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...

Scanning photoelectron microscope for nanoscale three-dimensional spatial-resolved electron spectroscopy for chemical analysis.

In order to achieve nondestructive observation of the three-dimensional spatially resolved electronic structure of solids, we have developed a scanning photoelectron microscope system with the capability of depth profiling in electron spectroscopy for chemical analysis (ESCA). We call this system 3D nano-ESCA. For focusing the x-ray, a Fresnel zone plate with a diameter of 200 μm and an outermost zone width of 35 nm is used. In order to obtain the angular dependence of the photoelectron spectra for the depth-profile analysis without rotating the sample, we adopted a modified VG Scienta R3000 analyzer with an acceptance angle of 60° as a high-resolution angle-resolved electron spectrometer. The system has been installed at the University-of-Tokyo Materials Science Outstation beamline, BL07LSU, at SPring-8. From the results of the line-scan profiles of the poly-Si/high-k gate patterns, we achieved a total spatial resolution better than 70 nm. The capability of our system for pinpoint depth-profile analysis and high-resolution chemical state analysis is demonstrated.

Nitrogen doping and thermal stability in HfSiOxNy studied by photoemission and x-ray absorption spectroscopy

We have investigated nitrogen-doping effects into HfSiO{sub x} films on Si and their thermal stability using synchrotron-radiation photoemission and x-ray absorption spectroscopy. N 1s core-level photoemission and N K-edge absorption spectra have revealed that chemical-bonding states of N-Si{sub 3-x}O{sub x} and interstitial N{sub 2}-gas-like features are clearly observed in as-grown HfSiO{sub x}N{sub y} film and they decrease upon ultrahigh vacuum (UHV) annealing due to a thermal instability, which can be related to the device performance. Annealing-temperature dependence in Hf 4f and Si 2p photoemission spectra suggests that the Hf-silicidation temperature is effectively increased by nitrogen doping into the HfSiO{sub x} although the interfacial SiO{sub 2} layer is selectively reduced. No change in valence-band spectra upon UHV annealing suggests that crystallization of the HfSiO{sub x}N{sub y} films is also hindered by nitrogen doping into the HfSiO{sub x}.

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.

Significant increase in conduction band discontinuity due to solid phase epitaxy of Al2O3 gate insulator films on GaN semiconductor

We have investigated band discontinuities and chemical structures of Al2O3 gate insulator films on n-type GaN semiconductor by photoemission and x-ray absorption spectroscopy. It is found that the solid phase epitaxy at the GaN crystal during annealing procedures at 800 °C leads to phase transformation of Al2O3 films from amorphous to crystalline. Changes in crystallographic structures closely correlate with the significant increase in conduction band discontinuity at the Al2O3/GaN interface, which suggests that epitaxial Al2O3 films on GaN semiconductor, free from grain boundaries of Al2O3 polycrystalline, hold the potential for high insulation performance.

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.