K. S. Liu

Electrical characteristics of barium titanate films prepared by laser ablation

The BaTiO3 ferroelectric films of quality suitable for applying as dielectrics in 64 and 256 Mb dynamic random access memory have been successfully synthesized using the laser ablation technique. The dielectric constant and dielectric strength of the films are e=200 and 1 MV/cm, respectively. The switching characteristics are Qc=0.66 μc/cm2, ts=0.1 ps, and J1=1.57 μA/cm2 at 2.5 V, for charge storage density, writing time, and leakage current density, respectively. The quality of the BaTiO3 films is superior to the Ta2O5 dielectric films and is comparable to the lead‐zirconate‐titanate ferroelectric films.

Co-firing process using conventional and microwave sintering technologies for MnZn- and NiZn-ferrites

Abstract Effect of co-firing process on the characteristics of MnZn-ferrite materials was examined. The crystal structure and microstructure are not markedly modified, whereas the initial permeability-temperature ( μ i – T ) and power loss ( P L ) properties are pronouncedly altered due to co-firing process, no matter whether the materials are densified by conventional sintering (cs) or microwave sintering (ms) process. The degradation of the magnetic properties is attributed to the strain induced in the MnZn-ferrites. The decreases in initials permeability ( μ i ) and the increases in power loss ( P L ) is less extent for the materials densified by the microwave sintering process. Such a phenomenon is attributed to the smaller grain size of MnZn-ferrite when the materials are microwave sintered.

Electron transport in In-rich InxGa1−xN films

We have performed electrical transport measurements on metal-organic vapor phase epitaxy grown In-rich InxGa1−xN (x=1, 0.98, and 0.92) films. Within the experimental error, the electron density in InGaN films is temperature independent over a wide temperature range (4K⩽T⩽285K). Therefore, InxGa1−xN (0.92⩽x⩽1) films can be regarded as degenerate semiconductor systems. The experimental results demonstrate that electron transport in In-rich InxGa1−xN (x=1, 0.98, and 0.92) films is metalliclike. This is supported by the temperature dependence of the density, resistivity, and mobility which is similar to that of a metal. We suggest that over the whole measuring temperature range residue imperfection scattering limits the electron mobility in In-rich InxGa1−xN (x=1, 0.98, and 0.92) films.

AlGaN films grown on (0001) sapphire by a two-step method

A two-step growth method, commonly used for GaN on sapphire, was applied to grow high-quality Al0.2Ga0.8N on sapphire. Comparing to the one grown on a low-temperature grown AlN buffer layer, the decomposition, recrystallization, and islands coalescence processes of the two-step growth increased the surface flatness, the crystal quality, the electrical property, suppressed the phase separation, and released the biaxial tensile strain. A 2.0μm thick high-quality crack-free nearly GaN-free Al0.2Ga0.8N epilayer was obtained.

SrTiO3 buffering effect on Pb1-xLaxTiO3 thin films prepared by pulsed laser deposition

Thin films of phase‐pure perovskite PLT (Pb0.95La0.05Ti0.9875O3) were deposited in situ onto Si, Pt/Ti/SiO2/Si, and SrTiO3/Si substrates by pulsed laser deposition from stoichiometric targets. No Pb loss was observed in the near‐surface region. The blocking of the interdiffusion between inner Si substrate and the outer PLT films by SrTiO3 buffer layer was evidenced using x‐ray diffraction (XRD) and secondary ion mass spectrometry (SIMS) analyses. The formation of TiO2 second phase in PLT/Pt/Ti/SiO2/Si films was indicated by XRD and SIMS spectra to be the outward diffusion of Ti atoms from the underlying Ti layer through the Pt layer, reacting with ambient O2 at the PLT‐to‐Pt interface.

Electrical characteristics of (Sr0.2Ba0.8)TiO3 positive temperature coefficient of resistivity materials prepared by microwave sintering

The resistivity‐temperature (ρ‐T) properties of the (Sr0.2Ba0.8)TiO3 materials densified by the microwave sintering process were investigated. Neither the sintering temperature (1100–1180u2009°C) nor the soaking time (10–40 min) in this process exert a significant influence on the positive temperature coefficient of resistivity behavior of the as‐sintered samples. Contrarily, reducing the cooling rate from 154u2009°C/min to 4u2009°C/min or post‐annealing the samples at 1250u2009°C for 2 h would increase the maximum resistivity (ρmax) more than three orders of magnitude. Consequently, the resistivity jump with the maximum to minimum resistivity ratio ρmax/ρmin≊107 has been achieved. The increase in maximum resistivity by these processes is ascribed to the increase in cationic vacancies, which subsequently act as effective electronic traps. The trap level (Ese) of the cationic vacancies is estimated from the Arrhenius plots of maximum resistivity (ρmax) and temperature (Tmax) to be Ese≊1.5–2.0 eV. On the other hand, the ρ‐...

Modification of PTCR behavior of (Sr0·2Ba0·8)TiO3 materials by post-heat treatment after microwave sintering

Abstract (Sr0.2Ba0.8)TiO3 materials have been effectively densified by microwave sintering process. The grain size (~ 6 μm) and PTCR characteristics (Tc = 50 °C, ϱ max ϱ min ≈ 10 2 ) of the as-sintered samples vary insignificantly with sintering temperature (1100–1180 °C) and soaking time (10–40 min). However, lowering the cooling rate after sintering substantially increases the resistivity Jump ( ϱ max ϱ min ) from 102 to 105.3, without altering the microstructure. Subsequent annealing, on the other hand, substantially modifies the microstructure and PTCR characteristics. The resistivity Jump increases monotonously with heat treatment temperature (Tht) and reaches ( ϱ max ϱ min ) ≈ 10 7 for samples heat-treated at 1300 °C for 2 h. The grain size remains nearly unchanged for Tht ≤ 1200 °C and grows markedly for samples heat-treated at a higher temperature. The effective trap level is estimated to be Es = 1·46 eV. The activation energy for the densification rate is QMS = 8·2 kcal/mol for microwave sintering and Qcs = 62·5 kcal/mol for conventional sintering process.

DOUBLE CRITICAL TEMPERATURE CHARACTERISTICS OF SEMICONDUCTING (BA0.7PB0.3)TIO3 MATERIALS PREPARED BY MICROWAVE SINTERING

In this work, we obtain (Ba0.7Pb0.3)TiO3 materials possessing double critical temperature Tc in resistivity–temperature (ρ–T) behavior by microwave sintering at 1050–1080u2009°C for 5 min. The cooling‐rate control and postannealing processes modify the relative magnitudes of low‐ and high‐Tc resistivity jumps without altering the Tc values. The donor Ed and trap Es levels of those materials are estimated to be Ed≂0.05–0.07 eV and Es≂1.07–1.32 eV. According to experimental results the voltage sensitivity and transient responsivity of the current passing through double‐Tc materials are observed to be superior to those of single‐Tc materials prepared by the conventional furnace sintering method. Also, the double‐Tc characteristics are attributed to the dual phases with a core‐shell structure. Moreover, the change in the relative proportion of the two phases accounts for the influence of post‐heat treatment processes on the materials’ ρ–T behavior.

Modified transmission line model and its application to aluminum ohmic contacts with n-type GaN

A modified transmission line model (MTLM) of ohmic contact measurement is presented. This model preserves the simplicity of the circular transmission line model but eliminates the shortcoming of the possibility of obtaining misleading results. This model was applied to n-type GaN ohmic contacts and results similar to those obtained by Hall measurement were obtained. The ohmic contact pattern used in MTLM method occasionally exists during the fabrication of several devices. In such cases, the method can be used to determine the device processing quality without the need for any other test pattern.

CONVENTIONAL AND MICROWAVE SINTERING STUDIES OF SRTIO3

Using the nonconventional sintering technique, such as microwave sintering, it is observed to enhance the densification rate of SrTiO 3 materials as effectively as employing the highly active powders prepared by the chemical route. Although the chemically derived powders demonstrate better sinterability than the mixed oxide powders, the thermal analysis indicates that the segregation of Ti 4+ -containing clusters during decomposition of precursors in the direct pyrolysis (DP) process induces the occurrence of TiO 2 particles (anatase phase) prior to the formation of SrTiO 3 phase. These particles retard the necking process required to sinter the materials. The spray pyrolysis (SP) process can circumvent the preferential nucleation of TiO 2 phase and, therefore, produce powders exhibiting superior sintering behavior to the DP-derived powders. The microwave sintering technique, on the other hand, substantially enhances the rate of diffusion of the ions in the materials such that even the mixed oxide powders can be sintered at a temperature about 200 °C lower than that needed to achieve the same density in a conventional sintering process. Fine grain (∼4 μm) microstructure is obtained for the materials microwave sintered at 1220 °C for 10 min. The migration of grain boundaries requires higher temperature to initiate than the formation of neckings between the grains. The grain growth occurs only when the material was sintered at temperatures higher than 1250 °C.