Akiharu Morimoto

Improved leakage and ferroelectric properties of Mn and Ti codoped BiFeO3 thin films

Polycrystalline BiFeO3 (BFO), Ti-doped BFO, Mn-doped BFO, and (Mn, Ti)-codoped BFO (BFMT) thin films were fabricated on Pt/SrTiO3 (100) substrate by pulsed laser deposition. Observed leakage current behavior in those ion-doped BFO films indicated the dominance of space-charge-limited current in the high electric field region. The leakage current of the BFMT film was much reduced in relation to the other films due to the formation of deep traps. In the BFMT film, well saturated P-E hysteresis curves were observed. Remanent polarization and coercive field for maximum electric field of 2100 kV/cm were 75 μC/cm2 and 310 kV/cm, respectively.

Defects in hydrogenated amorphous silicon‐carbon alloy films prepared by glow discharge decomposition and sputtering

Properties of hydrogenated amorphous silicon‐carbon alloy (a‐Si1−xCx :H) films prepared by radio frequency (rf) glow discharge decomposition and rf sputtering have been investigated by means of electron spin resonance (ESR), infrared absorption, optical absorption, and photoconductivity measurements. Although the number of C‐H per C atom [C‐H]/[C] is larger than that of Si‐H per Si atom [Si‐H]/[Si], the ESR spin density increases greatly with the C content. The increase in the density of dangling bonds may be related to the fact that the number of H atoms in gathered phase increases with an increase in x. ESR measurements also give useful information about the preferential formation of C or Si dangling bonds and the atomic distribution of Si and C through a compositional dependence of the g value. A remarkable feature for a‐Si1−xCx :H film is that the presence of C atoms in the amorphous network makes the Si‐H bond in a‐Si1−xCx :H more stable than that in a‐Si:H.

Ferroelectric properties of lead‐zirconate‐titanate films prepared by laser ablation

Ferroelectric lead‐zirconate‐titanate (PZT) thin films have been deposited by excimer laser ablation on sapphire substrates with and without an electrode. In preparation for the films, O2 gas pressure has greatly influenced the film structure and morphology. For the first time, we have confirmed the ferroelectric properties of PZT films prepared by laser ablation without post‐annealing. It appears to be possible to use these films for nonvolatile random access memories with some additional improvements in the film properties.

Properties of Hydrogenated Amorphous Si–N Prepared by Various Methods

The results of optical absorption, infrared absorption, electron spin resonance and photoluminescence measurements are presented for hydrogenated amorphous Si-N films prepared by three different methods; magnetron sputtering of an Si target in an Ar+N2+H2 gas mixture, glow-discharge decomposition of SiH4+N2, and glow-discharge decomposition of SiH4+NH3. The improvement in the properties of films prepared by the third method, e.g., the sharp band tail and low dangling-bond density, is explained mainly in terms of the structual softening due to the existence of N-H bonds. The fatigue of the photoluminescence due to illumination is found to be more prominent and the recovery due to annealing less prominent in N-incorporated film than in a-Si:H.

ESR and IR Studies on a-Si1-xGex:H Prepared by Glow Discharge Decomposition

This paper presents results on ESR measurement and IR absorption in a-Si1-xGex:H films prepared by glow discharge decomposition. In as-deposited films, the number of Ge dangling bonds per Ge atom [Ge↑]/[Ge] is found to be larger than that of Si dangling bonds per Si atom [Si↑]/[Si] by a factor of 4–15 because of preferential attachment of H to Si. [Ge↑]/[Ge] is reduced by an increase in Si content because H bonded to an Si atom works not only as a terminator for Si dangling bonds but also as a structural softener in these films. The differences in [Ge↑]/[Ge] among samples with various Si contents become smaller with increasing annealing temperature.

Nature of Localized States in Hydrogenated Si–Based Amorphous Semiconductor Films Elucidated from LESR and CPM

Measurements of low energy absorption by the constant photocurrent method, ESR and light-induced ESR were carried out for a-Si:H, a-Si1-xCx:H, a-Si1-xOx:H and a-Si1-xNx:H films as a function of the film thickness. As a result, both the surface density of dangling bonds in the disordered surface layer and the density of dangling bonds in the bulk region were obtained by discriminating between neutral and charged dangling bonds. It is found that many charged defects are present even in high-purity a-Si:H films and their density increases with the addition of O and N. We attribute these charged defects to Si3-+N4+ in a-Si1-xNx:H, in contrast to the model of Robertson and Powell, and to Si3-+O3- in a-Si1-xOx:H.

NMR and IR Studies on Hydrogenated Amorphous Si1-xCx Films

NMR and IR studies have been carried out on glow-discharge-deposited a-Si1-xCx:H films. The hydrogen content increases with an increase in x up to 0.4 mainly in gathered form contributing to the broad component of the NMR line. By combining the NMR and IR results, the proportionality between the number of H bonded to C and the intensity of IR absorption due to the C?H stretching mode vibration is ascertained. The proportionality constant is found to be AstrC?H=1.7?1021 cm-2.

Annealing and crystallization processes in tetrahedrally bonded binary amorphous semiconductors

Abstract Results of Raman scattering, infrared absorption, X-ray diffraction and electron spin resonance measurements as a function of annealing temperature are reported for Si-C, S-N, Si-Ge and Ge-C alloy films prepared by r.f. sputtering. Amorphous Si-C and Si-N Films tend to segregate into stoichiometric clusters (SiC and Si3N4) and excess element clusters with annealing, and the crystallization temperature for these films increases with increasing C or N content. Amorphous Si-Ge films are crystallized by annealing without segregation, and the crystallization temperature for these films decreases monotonically with increasing Ge content. Amorphous Ge-C films are crystallized by annealing with segregation into Ge- and C-like clusters, and the crystallization temperature has a maximum value around Ge0·42C0·58.

Preparation of Pb(Zn0.52Ti0.48)O3 Films by Laser Ablation

Pb(Zr0.52Ti0.48)O3 films were prepared on an r-plane sapphire substrate by laser ablation using the ArF excimer laser. This is the first report on the preparation of Pb(Zr0.52Ti0.48)O3 films by laser ablation. The composition of the deposited films was found to be fairly close to the composition of the target material for a wide range of the substrate temperature, 400–750°C. The substrate temperature greatly influences the crystal structure; a low substrate temperature produces a pyrochlore phase and a high substrate temperature (750°C) a perovskite phase.

Composition Dependence in BiFeO3 Film Capacitor with Suppressed Leakage Current by Nd and Mn Cosubstitution and Their Ferroelectric Properties

(Nd,Mn)-cosubstituted BiFeO3 (BFO) films with various Nd and Mn compositions were fabricated on a Pt/SrTiO3(100) substrate by pulsed laser deposition. X-ray diffraction patterns and atomic force microscopy images indicated that the suppression of impurity phases and a smooth surface morphology were realized by Nd substitution in the BFO films. Furthermore, by combining with Nd substitution, a small amount of Mn substitution in BFO films is effective for reducing the leakage current density by three orders of magnitude compared with that of a BFO film capacitor. The polarization vs electric field (P–E) curves showed a strong dependence of measurement frequency in the range of 0.1–2 kHz, and well-saturated P–E hysteresis loops were observed at 20 kHz at room temperature. The remanent polarization and coercive field at a maximum electric field of 1.9 MV/cm were approximately 70 µC/cm2 and 0.32 MV/cm, respectively.