Takashi Harumoto

Controlled polarity of sputter-deposited aluminum nitride on metals observed by aberration corrected scanning transmission electron microscopy

The polarity determination process of sputter-deposited aluminum nitride (AlN) on metals has been analyzed using aberration corrected atomic resolution scanning transmission electron microscope. Direct growth of c-axis orientated AlN on face centered cubic metals (fcc) (111) with the local epitaxy has been observed, and the polarity was determined at the AlN/metal interface. We found that the AlN polarity can be controlled by the base metal layer: N-polarity AlN grows on Pt(111) while Al-polarity AlN forms on Al(111). Based on these results, the growth mechanism of AlN on metals is discussed.

In situ transmission infrared spectroscopy of hydroxylation of aluminum thin film and subsequent dehydration processes

A transmission Fourier-transform infrared spectroscopy apparatus for the in situ observation of chemical reaction in an aqueous solution has been developed. Reforming from an aluminum film to aluminum hydroxide (boehmite) during hydrothermal treatment and a subsequent dehydration process from boehmite to gamma alumina by heating in vacuum are quantitatively observed. According to the absorption peaks of the Al–O–H vibration of boehmite and the H–O–H vibration of liquid water, the hydrothermal reaction of the Al film occurs through two stages: the formation of boehmite and the penetration of excess water into the boehmite crystal.

Emergence of (001) preferred orientation in AlN film using ultrathin intermediate Pt layer

Abstract The preferred orientations of AlN films with an intermediate Pt layer have been examined using X-ray diffraction and transmission electron microscopy. It is found that the Pt layer encourages the AlN film into c axis preferred orientation deposition, even though the sputtering conditions would normally lead the AlN film to be deposited with other preferred orientations. The Pt layer also promotes the crystallisation of AlN films. The local epitaxial growth of AlN (001) on Pt (111) may elucidate the effect of the Pt layer on the preferred orientation control.

Room-temperature spin glass and near band edge properties of highly disorder (FeCo)0.03Zn0.97O and (FeCoNi)0.03Zn0.97O nanorods

This paper is intended to give a detailed study of (FeCo)0.03Zn0.97O and (FeCoNi)0.03Zn0.97O magnetic nanorods, which have been synthesized using a versatile low temperature chemical solution method. The detailed structural and microstructural investigations verify the single hexagonal wurtzite structure of ZnO and rule out the formation of any secondary phase. The Raman study further reveals that the codoped magnetic transition metals are successfully doped into host lattice, while the structure is highly defective and disordered due to random distribution of different codopants. The prepared nanorods exhibit room and low temperatures spin glass magnetic phase arising from the random arrangement of codopants magnetic moments and their exchange interactions at lattice sites of ZnO. The optical investigations of these codoped nanorods show a redshift in near band edge emission, which is possibly raised from the strong s-p and p-d exchange interactions of codopants.

Perpendicular coercivity enhancement of CoPt/TiN films by nitrogen incorporation during deposition

The effect of N incorporation on the structure and magnetic properties of CoPt thin films deposited on glass substrates with TiN seed layers has been investigated. During the deposition of CoPt, introducing 20% N2 into Ar atmosphere promotes the (001) texture and enhances the perpendicular coercivity of CoPt film compared with the film deposited in pure Ar and post-annealed under the same conditions. From the in situ x-ray diffraction results, it is confirmed that N incorporation expands the lattice parameter of CoPt, which favors the epitaxial growth of CoPt on TiN. During the post-annealing process, N releases from CoPt film and promotes the L10 ordering transformation of CoPt.

Rhombohedral distortion analysis of ultra-thin Pt(111) films deposited under Ar–N2 atmosphere

A rhombohedral analysis method for analysing the lattice distortion in a (111)-textured face-centred cubic film under rotationally symmetric stress is proposed. Because no material constants, such as diffraction elastic constants, are required, the expressions of the distortion, namely the angle and the lattice parameter, are universal and can be readily used to compare different films. Using this rhombohedral distortion analysis method, (111)-textured Pt films deposited under argon–nitrogen atmosphere are systematically investigated, and the thickness-dependent lattice deformation in as-deposited and annealed films is described by the two geometrical parameters of the rhombohedral cell.

Development of superlattice during thermal annealing in Pt/AlN multilayer films

The structures of Pt/AlN multilayer films were characterised by a combination of X-ray reflectivity (XRR) and X-ray diffraction (XRD). As-deposited film has accurate periodicity and abrupt interfaces, which make possible specular reflections up to 2q = 15°. The reflectivities of 1st Bragg reflection of the specimens annealed at temperatures below 600°C are almost constant about 65%. By annealing, intensity modulations overlap on the 111Pt reflection, indicating the formation of a kind of superlattice in the multilayer films. Annealing is considered to cause the release of sputtering gases, recovery and small orientation adjustment of Pt layers. On the other hand, AlN layers do not show changes during annealing at these temperatures. As a result, adjacent Pt layers with preferred orientation come to show a correlation in diffraction intensity although they are separated by AlN.

Epitaxial growth of BiFeO3 films on TiN under layers by sputtering deposition

BiFeO3/TiN/MgO (001) films have been prepared by magneton sputtering, where TiN serves as a conductive under layer. X-ray diffraction profiles and cross-sectional transmission electron microscopy images reveal that not only (001)-epitaxial BiFeO3 films are obtained, but also both tetragonal and rhombohedral phases co-exist in BiFeO3 films. Their crystallographic relationship is shown as following: tetragonal-BiFeO3 (001) [100]//TiN (001) [100]//MgO (001) [100] and rhombohedral-BiFeO3 (001) [100]//TiN (001) [100]//MgO (001) [100]. Besides, an oxidized TiN layer (∼ 20 nm) has also been detected between BiFeO3 and TiN layers and its formation may originate from oxygen inter-diffusion from BiFeO3 layer. Despite of the existence of the oxidized TiN layer, it does not affect the epitaxial growth of BiFeO3 films. On the other hand, the coercivity electric field obtained in ferroelectric loop of BiFeO3 is greatly enhanced to 49 MV/cm due to the existence of oxidized TiN layer.

Effect of substrate temperature on the structure and magnetic properties of CoPt/AlN multilayer films

The effect of substrate temperature on the structure and magnetic properties of CoPt/AlN multilayer films has been investigated. The crystallinity of CoPt has been improved with increasing substrate temperature from room temperature to 400 °C. After post-annealing process, L10 CoPt structure transformation has also been promoted. However, since the easy magnetic axis of L10 CoPt is in [001] orientation, the promotion of L10 CoPt transformation causes the change of easy magnetic axis in (111) textured CoPt layers, which impairs the perpendicular magnetic anisotropy. The optimum substrate temperature should be room temperature to obtain the strongest perpendicular magnetic anisotropy according to the results of the present work.

Disc-shaped nanocrystal model for simulating the diffraction peak profile from a one-dimensional superlattice and its application to Pt/AlN superlattice films

On the basis of the film structure of a Pt/AlN multilayer film which exhibits a superlattice peak after annealing, a disc-shaped nanocrystal model is proposed for simulating the diffraction line profile from a one-dimensional superlattice. The proposed model demonstrates that a superlattice peak can arise even from just two disc-shaped nanocrystals and even with a large misorientation (a few degrees), provided both the thickness and the in-plane crystal size of the nanocrystals are of the order of nanometres. Using the model, the superlattice peaks from Pt/AlN superlattices are analysed quantitatively and the effect of annealing on the film is discussed.