Congshun Wang

Enhanced tunability due to interfacial polarization in La0.7Sr0.3MnO3∕BaTiO3 multilayers

BaTiO3 single layer and La0.7Sr0.3MnO3∕BaTiO3 multilayer films were fabricated by laser molecular-beam epitaxy. The voltage tunability of these films was investigated systematically in the frequency ranging from 10kHzto1MHz. The results suggest that the sizable tunability arises from the interfacial polarization which can be strongly suppressed by applied dc biases. In multilayer films, remarkable enhancement in voltage tunability was observed, because the interfacial polarization was greatly enhanced by an interfacial polarization associated possibly with the Maxwell-Wagner relaxation. The authors’ results indicate that the voltage tunability in low frequency (⩽1MHz) has a dominating contribution from the interfacial polarization.

Low-frequency negative capacitance in La0.8Sr0.2MnO3/Nb-doped SrTiO3 heterojunction

Low-frequency (100Hz⩽f⩽1MHz) dielectric properties of La0.8Sr0.2MnO3∕Nb-doped SrTiO3 heterojunctions were investigated in detail at room temperature. Negative capacitance was observed at low frequencies under positive dc biases. This phenomenon was found to result from the combinational contributions from the Maxwell–Wagner interfacial relaxation and the dipolar relaxation related to detrapped carriers which give rise to inductive effect under an applied electric field.

Electronic transport characteristics through individual ZnSnO3 nanowires

Composite ZnSnO3 nanowires are synthesized via a one-step thermal evaporation method. The nanowires are of core-shell structures with the presence of grain boundary and out-of-phase boundaries. Transport through individual nanowires shows nonlinear current-voltage (I-V) characteristics in the range of the voltage from −3to3V. Such a behavior can be attributed to the presence of the barrier at the grain boundary, and the effective barrier height is estimated to be about 0.22eV by analyzing the I-V curves at various temperatures. The current at −3V jumps from 0.12to6.0μA within 30s at 300K as exposed to UV illumination. Such jump can be well explained in terms of effective barrier height and depletion width.

TEMPERATURE-DEPENDENT DIELECTRIC STRENGTH OF A MAXWELL–WAGNER TYPE RELAXATION

A relationship between the temperature and the dielectric strength of a Maxwell–Wagner (MW) type relaxation was deduced based on the conventional two-layer model for inhomogeneous systems and under the usual assumptions that the conductivity of each layer obeys the thermally activated law and the dielectric constant of each layer is relatively temperature independent. The relation shows that the relaxation peak height in the imaginary part of the complex permittivity for a MW-type relaxation increases with decreasing temperature and saturates at low enough temperature. This behavior was well-proved by both numeral and experimental results and therefore could be regarded as a fingerprint of a MW-type relaxation in most practical cases.

Visible-blind, ultraviolet-sensitive photodetector based on SrTiO3 single crystal.

High-sensitivity and visible-blind ultraviolet (UV) photoconductive detectors based on SrTiO(3) single crystal with interdigitated electrodes are reported. The responsivities of photovoltage and photocurrent can reach 2.13x10(5) V/W and 213 mA/W, respectively, at 330 nm at ambient temperature, and the corresponding quantum efficiency eta reaches 80.2%. The dark current is lower than 50 pA at 10 V bias, and the UV/visible contrast ratio is about four orders of magnitude with a sharp cutoff at 390 nm. The experimental results demonstrate that SrTiO(3) single crystal has potentially wide applications in UV detection.

Modulation of the two-photon absorption by electric fields in HgCdTe photodiode

We demonstrate the tunability of the two-photon absorption (TPA) coefficient by adjusting the electric field in a HgCdTe (MCT) photodiode with cutoff wavelength of 5.2μm. The TPA coefficient was measured by using a picosecond pulsed laser with wavelength of 7.92μm. An enhancement of the TPA coefficient occurs in the space charge region of the MCT pn junction, which can be attributed to the Franz–Keldysh effect induced by the built-in electric field. By applying a reverse bias to intensify the built-in field, the TPA coefficient is found to be further enhanced by a factor of 18.9. This electric field dependence of the TPA coefficient has been fairly interpreted by the pn junction model with the Franz–Keldysh effect included.

Experimental determination of interfacial-layer thickness from polarization-voltage hysteresis loops in Pb(Zr0.4Ti0.6)O3 thin films

Interfacial layers near top and bottom electrodes with low resistivity in Pb(Zr0.4Ti0.6)O3 (PZT) thin film are identified and modeled through frequency-dependent polarization-voltage (P-V) hysteresis loops at frequencies below 20kHz. Actual voltage drops, as well as built-in imprint voltage across the intrinsic ferroelectric layer, are found to be frequency dependent, as shown from the linear voltage shift of P-V hysteresis loops against applied external voltage at different frequencies with respect to one referenced hysteresis loop. Calculated interfacial-layer thickness is about 32±2nm for an Ir∕IrO2∕PZT∕Pt∕SiO2∕Si capacitor with a PZT film thickness of 100nm, in good agreement with the resistive measurements by Chu et al. [Appl. Phys. Lett. 81, 5204 (2002)].

In2O3 nanowires grown from Au/In film on glass

Two kinds of In2O3 nanowires (NWs), i.e., straight and tapered ones, are grown from 20-nm-thick Au∕300-nm-thick In and 20-nm-thick Au∕1-μm-thick In films on glass at 400°C by a one-step annealing method, respectively. All the NWs are single crystalline. The growth of the NWs is initiated by Au catalyst particles via vapor-liquid-solid growth mechanism, and an additional side growth might be responsible for the nonuniform diameters of the tapered NWs. It is revealed that a certain content of oxygen in the In film facilitates the large-scale growth of the In2O3 NWs. The field-emission studies show that the In2O3 NWs on glass have a low turn-on electric field of about 4.3V∕μm. Our growth method has potential applications in the in situ fabrication and integration of the In2O3 NWs-based devices, especially with glass as substrates.

Extended mode in blocked impurity band detectors for terahertz radiation detection

We demonstrate the existence of an interfacial barrier in blocked impurity band (BIB) detectors using temperature-dependent dark current and corresponding theoretical calculations. Considering the effects of the interfacial barrier, the calculated photoresponse is in good agreement with the experimental results. A dual-excitation model, including the direct excitation over the full barrier and excitation to the band minimum with subsequent tunneling into the blocking layer, is proposed to quantitatively explain the observed photoresponse extension. A concept of extended-mode detection is developed to suggest the option for some selective photoresponse in the terahertz region and open the possibility of extending BIB photoresponse to lower frequency.

Model to explain the anisotropic phenomenon of effective mobility of organic field-effect transistors

Recent research has revealed that the overall effective mobility of organic field-effect transistors depends on the anisotropy film. To account for this behavior, a modified model is proposed based on the previous grain boundary trap model. In this model, the degeneration factor, which is related to the maximal misorientation angle of grains, is introduced to describe the difference between aligned and nonaligned organic films. More explicit definitions of effective grain length and grain boundary are given concerning this anisotropic phenomenon. The simulation results are consistent with experimental data for aligned pentacene and copper phthalocyanine (CuPc).