S. L. Guo

PbZr0.5Ti0.5O3/La0.5Sr0.5CoO3 heterostructures prepared by chemical solution routes on silicon with no fatigue polarization

PbZr0.5Ti0.5O3/La0.5Sr0.5CoO3 (PZT/LSCO) heterostructures have been grown directly on (100)Si by chemical solution routes. Field-emission scanning electron microscopy and x-ray diffraction analysis show that PZT and LSCO thin films are polycrystalline and entirely perovskite phase. PZT thin films grown on LSCO thin films showed no preferential orientation and excellent ferroelectricity. The remnant polarization Pr is about 22.8 μC/cm2 and the coercive field Ec is about 95 kV/cm at an applied electric field of 400 kV/cm. The ferroelectric capacitor has been fabricated and showed no polarization fatigue after 3×109 fatigue cycles. The Pt/PZT/LSCO capacitor is suitable for nonvolatile random access memory application.

Properties of highly (100) oriented Ba0.9Sr0.1TiO3/LaNiO3 heterostructures prepared by chemical solution routes

Highly (100) oriented Ba0.9Sr0.1TiO3/LaNiO3 heterostructures have been grown on Si(100) using chemical solution routes. X-ray diffraction analysis shows that Ba0.9Sr0.1TiO3 thin films are high (100) orientation (α100=0.92). Atomic force microscopy investigation shows that they have large grains about 80–200 nm. A Pt/Ba0.9Sr0.1TiO3/LaNiO3 capacitor has been fabricated and showed excellent ferroelectricity, the remnant polarization and coercive field are 10.8 μC/cm2 and 96 kV/cm, respectively. The electric field dependence of capacitance measurement shows that the capacitor has large capacitance tuning ([Cmax−Cmin]/Cmax×100%) of 63%. The Ba0.9Sr0.1TiO3 thin films have high dielectric constant (e) of 200 at 1 MHz.

Ferromagnetism and superparamagnetism of ZnCoO:H nanocrystals

ZnCoO:H nanocrystals are weak ferromagnetic at room temperature with a small coercivity and a small remanence, whereas ZnCoO nanocrystals are paramagnetic. The thermal irreversibility of zero-field cooling and field cooling magnetizations of ZnCoO:H nanocrystals corresponds to the existence of superparamagnetism due to the nanosize effect. X-ray photoelectron spectra show the incorporation of Co(2+) ions inside the ZnO lattice without changing the wurtzite structure. Our data suggest that hydrogen can induce ferromagnetism in ZnCoO and that ferromagnetic ZnCoO:H crystals with small particle size can show the superparamagnetism

Temperature dependence of ferroelectric and dielectric properties of PbZr0.5Ti0.5O3 thin film based capacitors

The temperature dependence of the ferroelectric and dielectric properties of PbZr0.5Ti0.5O3 thin films deposited on LaNiO3-coated SrTiO3 substrate was investigated. The results showed that both the saturation polarization and remanent polarization increased with decreasing temperature from 300 to ∼50 K, and decreased as the temperature continued to decrease below 40 K. The capacitance of the PbZr0.5Ti0.5O3 ferroelectric thin film capacitor as a function of small ac field and temperature was measured, and the data were processed using Rayleigh law. It was demonstrated that both the reversible and irreversible contributions to the dielectric constant decreased with decreasing temperature; however, they showed an increase when the temperature dropped below 50 K. The anomalous behavior of the temperature dependence may be attributed to a phase transition in the PbZr0.5Ti0.5O3 thin film in the vicinity of 50 K.

Subband electron properties of modulation-doped AlxGa1−xN/GaN heterostructures with different barrier thicknesses

Magnetotransport properties of modulation-doped Al0.22Ga0.78N/GaN heterostructures with different barrier thicknesses of 25–100 nm have been investigated in magnetic fields up to 9 T at 1.4 K. Fast Fourier transform has been applied to obtain the subband density and mobility of the two-dimensional electron gas in these heterostructures. High electron density of 1.18×1013 cm−2 and quantum mobility of ∼8200 cm2 V−1 s−1 are obtained when the barrier thickness is 75 nm, which indicates that there exists a critical barrier thickness between 50 and 100 nm in the modulation-doped Al0.22Ga0.78N/GaN heterostructures. We also find that the elastic strain relaxation of the barrier does not significantly enhance the quantum mobilities of the ground subbands, however, it has strong effect on the mobilities of the excited states. The experimental values obtained in this work are useful for the design and optimization AlxGa1−xN/GaN device.

Giant reversible magnetocaloric effect in cobalt hydroxide nanoparticles

The magnetocaloric effect associated with magnetic phase transitions in -CoOH 2 nanoparticles has been investigated. A sign change in the magnetocaloric effect is induced by a magnetic field, which is related to a field-induced transition from the antiferromagnetic to the ferromagnetic state below the Neel temperature. The large reversible magnetic-entropy changeS m 20.9 J /kg K at 15 K for a field change of 7 T indicates that -CoOH 2 is a potential candidate for application in magnetic refrigeration in the low-temperature range.

Multisubband transport of the two-dimensional electron gas in AlxGa1−xN/GaN heterostructures

Multisubband transport of the two-dimensional electron gas (2DEG) in modulation-doped Al0.22Ga0.78N/GaN heterostructures has been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. It is found that the mobility of the 2DEG in the first subband in a triangular quantum well at the heterointerface decreases significantly, while the mobility of the 2DEG in the second subband increases, when the Al0.22Ga0.78N barrier is partially relaxed. Such behavior of the 2DEG mobility is explained by the nonuniformity of the piezoelectric polarization field at the heterointerface induced by the Al0.22Ga0.78N relaxation and strong interface scattering. Meanwhile, it is concluded that the scattering from the remote ionized donors is the main mechanism contributing to the quantum scattering time and responsible for the intersubband scattering in the quantum well at the heterointerface.Multisubband transport of the two-dimensional electron gas (2DEG) in modulation-doped Al0.22Ga0.78N/GaN heterostructures has been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. It is found that the mobility of the 2DEG in the first subband in a triangular quantum well at the heterointerface decreases significantly, while the mobility of the 2DEG in the second subband increases, when the Al0.22Ga0.78N barrier is partially relaxed. Such behavior of the 2DEG mobility is explained by the nonuniformity of the piezoelectric polarization field at the heterointerface induced by the Al0.22Ga0.78N relaxation and strong interface scattering. Meanwhile, it is concluded that the scattering from the remote ionized donors is the main mechanism contributing to the quantum scattering time and responsible for the intersubband scattering in the quantum well at the heterointerface.

Changes in the interface capacitance for fatigued lead–zirconate–titanate capacitors

From the capacitance–voltage dependence of Pt/PbZr0.5Ti0.5O3/Pt capacitors, we mathematically separated the capacitance into two parts corresponding to two regions of the samples, the uniform electric-field region and the nonuniform region. They are correlated with the bulk ferroelectric film region and the Pt/PbZr0.5Ti0.5O3 Schottky barrier interface region, respectively. The calculations based on the in-series capacitor model show a slight decrease of dielectric permitivity for the fatigued bulk films. By assuming a much smaller dielectric permitivity of the interface region than that of bulk films, it was found that the interface capacitance decreased remarkably compared with that of the bulk ferroelectric film after fatigue. This decrease was attributed to the lowering of ferroelectricity in the interface layer, which suggests that the fatigue is mainly an interface state controlled process. The asymmetricity in the interface capacitance–voltage curve is attributed to the different defect concentratio...

Evolution of Rayleigh constant in fatigued lead zirconate titanate capacitors

The capacitance of Pt/PbZr0.5Ti0.5O3/Pt capacitors versus the strength of applied ac electric field E has been measured for samples that fatigued at different square pulse switching cycles. Based on the in-series capacitor model, the variation of interface capacitance and bulk ferroelectric capacitance at different stages of fatigue were treated separately. A simplified method was derived to calculate the initial dielectric constant and Rayleigh constant for the bulk ferroelectric film from the ac electric field dependence of the sample capacitance. It was found that the initial dielectric constant and Rayleigh constant for the bulk ferroelectric film changed a little when the number of cycles is less than 3×106, whereas it decreased remarkably when the number of cycles is greater than 3×106. The suppression of polarization was attributed to pinning of the domain walls at the electrode/ferroelectric interface.

Weak antilocalization and beating pattern in high electron mobility AlxGa1−xN/GaN two-dimensional electron gas with strong Rashba spin-orbit coupling

The weak antilocalization (WAL) effects of the two-dimensional electron gas (2DEG) in high mobility AlxGa1−xN/GaN heterostructure as well as beating patterns in the Shubnikov–de Haas (SdH) oscillatory magnetoresistance have been investigated by means of magnetotransport measurements before and after illumination. The zero-field spin splitting mainly arising from the Rashba spin-orbit coupling effect is studied using the weak antilocalization and beating patterns analysis, respectively. The Rashba spin-orbit coupling constant α deduced using the weak antilocalization analysis showed a good agreement with that estimated from the analysis of the beating patterns for the sample before and after illumination. For our sample, the electron motion in the high mobility system is in the ballistic regime, the experimental WAL curves were fitted by a simulated quantum conductance correction according to a model proposed by [Golub [Phys. Rev. B 71, 235310 (2005)].