J. Okabayashi

Fabrication, magnetic properties, and electronic structures of nanoscale zinc-blende MnAs dots (invited)

Ferromagnetic nanoscale zinc-blende MnAs dots were successfully fabricated on a sulfur-passivated GaAs (001) surface by molecular-beam epitaxy. Transmission electron microscopy and selected area electron diffraction showed that the crystalline structure was not the same as that of bulk MnAs with NiAs-type hexagonal crystalline structure, but of zinc-blende type. In in situ photoemission spectroscopy of the zinc-blende MnAs dots, the Fermi edge was not clearly observed and the Mn 3d partial density of states was similar to that of the diluted ferromagnetic semiconductor Ga1−xMnxAs, which also supports the fabrication of zinc-blende MnAs in the nanoscale.

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.

Magnetic properties of Fe and Co codoped SnO2 prepared by sol-gel method

SnO2 codoped with Fe and Co was prepared using the sol-gel method. Compared with SnO2 doped with only Fe or Co, the ferromagnetic saturation magnetization of the codoped SnO2 was enhanced because of the lattice distortion induced by codoping. Mossbauer spectra consist of two paramagnetic doublets and one broad sextet of high-spin Fe(III) species. The Mossbauer spectra revealed that the magnetic sextet with broad peaks is drastically modulated by doping with 0.5%–3% Co and 1% Fe codoped SnO2. Our experimental results suggest correlation between the enhancement of the saturation magnetization and the appearance of sextet peaks.

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...

Novel Mn-doped chalcopyrites

Abstract Heavily Mn-doped II–VI–V 2 semiconductors, such as CdGeP 2 and ZnGeP 2 have been prepared by depositing Mn on single crystalline substrate at nearly 400 °C in an ultra high vacuum chamber. Well-defined ferromagnetic hysteresis with a saturation behavior appears in the magnetization curve up to above room temperature. The chemical states of the ZnGeP 2 :Mn interface has been clarified by a careful in situ photoemission spectroscopy. The as-prepared surface consists of Ge-rich, metallic Mn compound. In and below the sub-surface region, dilute divalent Mn species as precursors of the DMS phase exist. No MnP phase was observed at any stage of the depth profile. Theoretical band-calculation suggests that the system with vacancies (Cd, V c , Mn)GeP 2 or a non-stoichiometric (Cd, Ge, Mn)GeP 2 are ferromagnetic and energetically stable although ferromagnetism is not stable in a stoichiometric compound (Cd, Mn)GeP 2 .

X-ray absorption spectroscopy of transition-metal doped diluted magnetic semiconductors Zn1-xMxO

We have investigated the electronic structure of Zn1−xMxO (M: 3d transition metal) by x-ray absorption spectroscopy. Using configuration–interaction cluster-model analyses, electronic structure parameters have been deduced and their chemical trend is discussed. Results show that the p–d exchange constant Nβ is negative and large in cases of Mn, Fe, and Co, which is consistent with the enhancement of magnetic circular dichroism.

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}.

Perpendicular magnetic anisotropy at the interface between ultrathin Fe film and MgO studied by angular-dependent x-ray magnetic circular dichroism

Interface perpendicular magnetic anisotropy (PMA) in ultrathin Fe/MgO (001) has been investigated using angular-dependent x-ray magnetic circular dichroism (XMCD). We found that anisotropic orbital magnetic moments deduced from the analysis of XMCD contribute to the large PMA energies, whose values depend on the annealing temperature. The large PMA energies determined from magnetization measurements are related to those estimated from the XMCD and the anisotropic orbital magnetic moments through the spin-orbit interaction. The enhancement of anisotropic orbital magnetic moments can be explained mainly by the hybridization between the Fe 3dz2 and O 2pz states.

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.