K.H. Wong

High tunability in compositionally graded epitaxial barium strontium titanate thin films by pulsed-laser deposition

Compositionally graded barium strontium titanate [(BaxSr1−x)TiO3—BST, x=0.75, 0.8, 0.9, and 1.0] thin films are fabricated by pulsed-laser deposition on a LaAlO3 substrate with (La0.7Sr0.3)MnO3 as the bottom electrode. A high dielectric permittivity and temperature characteristic without Curie–Weiss law are obtained. A tunability of over 70% is obtained at frequency of 1 MHz, which is higher than that of single BST layer with the same compositions. All the results indicate that the graded thin films have better electrical properties than a single-layer film.

Evidence of the influence of phonon density on Tm3+ upconversion luminescence in tellurite and germanate glasses

Spectral properties of blue upconversion luminescences in Tm3+ doped tellurite (PWT, PbF2-WO3-TeO2) glasses and germanate (PWG, PbF2-WO3-GeO2) glasses pumped by a tunable dye laser were studied at room temperature. Two emission bands centered at 453 and 477 nm, corresponding to the 1D2→3H4 and 1G4→3H6 transitions of Tm3+ ions respectively, were observed. The two-photon absorption mechanism responsible for the 477 nm luminescence was confirmed by a quadratic dependence of luminescent intensities on the excitation power. Tellurite glasses showed a weaker upconversion luminescence than germanate glasses. This observation was inconsistent with the prediction from the phonon sideband measurement. In this article, Raman spectroscopy and transmittance measurement were employed to investigate the origin of the difference in upconversion luminescences in the two glasses. Compared with phonon sideband spectroscopy, Raman spectroscopy extracts more information, including both phonon energy and phonon density, and th...

Top-interface-controlled fatigue of epitaxial Pb(Zr0.52Ti0.48)O3 ferroelectric thin films on La0.7Sr0.3MnO3 electrodes

Epitaxial Pb(Zr0.52Ti0.48)O3/La0.7Sr0.3MnO3(PZT/LSMO) and LSMO/PZT/LSMO heterostructures have been grown on LaAlO3(001) substrates by pulsed-laser deposition. Three types of ferroelectric capacitors, i.e., Pt/PZT/LSMO (A), LSMO/PZT/LSMO (B), and Pt/PZT/LSMO (C) have been fabricated, where the Pt electrode was sputter deposited onto as-grown (capacitor A) and in situ annealed (capacitor C) PZT/LSMO films, respectively. It is found that the LSMO/PZT/LSMO capacitor shows a low coercive field and good fatigue endurance up to 1010 switching cycles. Similar properties are also obtained for capacitor A. However, the capacitor C, with the PZT film in situ annealed at reduced oxygen pressures, exhibits higher switching voltages and starts to fatigue rapidly at about 106 bipolar switching cycles. Lead deficiency at the surface of the annealed PZT films has been observed. Our results demonstrate that the fatigue performance of PZT/LSMO films, although affected greatly by the electrode configurations, is intrinsicall...

Epitaxial growth and planar dielectric properties of compositionally graded (Ba1−xSrx)TiO3 thin films prepared by pulsed-laser deposition

We have heteroepitaxially deposited compositionally graded (Ba1−xSrx)TiO3 (BST) thin films with increasing x from 0.0 to 0.25 on (100)-oriented MgO substrates using pulsed-laser deposition. The compositional gradients along the depth in the graded films were characterized by Rutherford backscattering spectroscopy. By using surface interdigital electrodes, the planar dielectric response of epitaxial graded BST films was measured as a function of frequency, temperature, and dc applied voltage. At room temperature, the dielectric constant of the graded BST film was about 450 with a dielectric loss, tan δ of 0.007 at 100 kHz. Measurements varying the dc bias voltage showed hysteresis of the dielectric response and a tunability of 25% at an applied electric field of 80 kV/cm. The graded BST films undergo a diffuse phase transition with a broad and flat profile of the capacitance versus temperature. Such behavior of the dielectric response in graded BST films is attributed to the presence of the compositional a...

Field-induced crossover from cluster-glass to ferromagnetic state in La0.7Sr0.3Mn0.7Co0.3O3

The thermodynamics and kinetics of the magnetic states of the semiconductor-like compound La0.7Sr0.3Mn0.7Co0.3O3 were studied by detailed magnetic measurements. The spontaneous magnetic state was found to be a cluster-glass phase which can cross over to a ferromagnetic state under an external magnetic field H. The defreezing temperature Tf for the cluster-glass phase and the merging temperature Tm for zero field cooled and field cooled magnetizations are related to the field H by Hj=H0j(1−Tj/Tc)nj, where H0j and nj are constants and j=f,m. Codoping tends to destroy the double exchange in Mn4+–O–Mn3+ and broadens the coexistence region of the cluster-glass and ferromagnetic states. The magnetic relaxation was found to deviate from the usual logarithmic time dependence and follow a power law.

Intrinsic resputtering in pulsed‐laser deposition of lead‐zirconate‐titanate thin films

Pulsed‐laser deposition (PLD) of lead‐zirconate‐titanate [Pb(Zrx,Ti1−x)O3] (PZT) thin films under low ambient pressure has been investigated by studying the angular deposition distributions of the constituent elements of the films. Nonstoichiometric profiles are observed and a dip occurs near the target surface normal of the deposition profile of lead. Experimental results show that intrinsic resputtering of the film is important in the PLD process and is responsible for the anomalous distribution of lead.Pulsed‐laser deposition (PLD) of lead‐zirconate‐titanate [Pb(Zrx,Ti1−x)O3] (PZT) thin films under low ambient pressure has been investigated by studying the angular deposition distributions of the constituent elements of the films. Nonstoichiometric profiles are observed and a dip occurs near the target surface normal of the deposition profile of lead. Experimental results show that intrinsic resputtering of the film is important in the PLD process and is responsible for the anomalous distribution of lead.

Growth and characterization of Hf-aluminate high-k gate dielectric ultrathin films with equivalent oxide thickness less than 10 Å

Ultrathin amorphous Hf–aluminate (Hf–Al–O) films have been deposited on p-type (100) Si substrates by pulsed-laser deposition using a composite target containing HfO2 and Al2O3 plates. Transmission electron microscopy observation of Hf–Al–O films showed that the amorphous structure of Hf–Al–O films was stable under rapid thermal annealing at temperatures up to at least 1000 °C. Capacitance–voltage measurement of a 38 A Hf–Al–O film revealed that the relative permittivity of the film was about 16. Such a film showed very low leakage current density of 4.6×10−3 A/cm2 at 1 V gate bias. The Hf–Al–O film under optimized condition did not show any significant interfacial layer at the interface and an equivalent oxide thickness of less than 10 A has been achieved. The formation of Hf–O and Al–O bonds in the film was revealed by x-ray photoelectron spectroscopy.

Study of interfacial reaction and its impact on electric properties of Hf-Al-O high-k gate dielectric thin films grown on Si

Amorphous thin films of Hf–Al–O (with atomic ratio of Al/Hf of about 1.4) were deposited on (100) p-Si substrates by pulsed-laser deposition using a HfO2 and Al2O3 composite target. Transmission electron microscopy was employed for a detailed study of the interfacial reaction between the Hf–Al–O films and the Si substrates. Islands of Hf silicide formed from interfacial reaction were observed on the surface of the Si substrate. The formation of Hf silicide is attributed to the presence of Al oxide in the films that triggers the reaction between Hf atoms in the amorphous Hf–Al–O films and Si under an oxygen deficient condition. The impact of silicide formation on the electrical properties was revealed by high-frequency capacitance–voltage (C–V) measurements on metal–oxide–semiconductor capacitors. The observed abnormal C–V curve due to interfacial reaction was discussed.

Epitaxial growth of yttrium-stabilized HfO2 high-k gate dielectric thin films on Si

Epitaxial yttrium-stabilized HfO2 thin films were deposited on p-type (100) Si substrates by pulsed laser deposition at a relatively lower substrate temperature of 550 °C. Transmission electron microscopy observation revealed a fixed orientation relationship between the epitaxial film and Si; that is, (100)Si//(100)HfO2 and [001]Si//[001]HfO2. The film/Si interface is not atomically flat, suggesting possible interfacial reaction and diffusion. X-ray photoelectron spectrum analysis also revealed the interfacial reaction and diffusion evidenced by Hf silicate and Hf–Si bond formation at the interface. The epitaxial growth of the yttrium stabilized HfO2 thin film on bare Si is via a direct growth mechanism without involving the reaction between Hf atoms and SiO2 layer. High-frequency capacitance–voltage measurement on an as-grown 40-A yttrium-stabilized HfO2 epitaxial film yielded an effective dielectric constant of about 14 and equivalent oxide thickness to SiO2 of 12 A. The leakage current density is 7.0×1...

Formation of core/shell structured cobalt/carbon nanoparticles by pulsed laser ablation in toluene

Magnetic cobalt nanoparticles encapsulated in shells of layered structure have been produced by the technique of pulsed laser ablation in toluene. The morphology, microstructure, and magnetic properties of the prepared nanoparticles were characterized by electron microscopy, micro-Raman spectroscopy, and superconducting quantum interference device magnetometry, respectively. The results indicated that the cobalt nanoparticles fabricated are noncrystalline but coated with the graphitic carbon layers. It is believed that the formation of these carbon layers well-protect the cobalt nanoparticles to be oxidized thus maintaining the superparamagnetic property. This is an important feature that makes the cobalt nanoparticles a useful material for medical and many other magnetic based applications.