The perovskite manganites with generic formula RE1?xAExMnO3 (RE = rare earth, AE = Ca, Sr, Ba and Pb) have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). The most fundamental property of these materials is strong correlation between structure, transport and magnetic properties. They exhibit extraordinary large magnetoresistance named CMR in the vicinity of the insulator?metal/paramagnetic?ferromagnetic transition at relatively large applied magnetic fields. However, for applied aspects, occurrence of significant CMR at low applied magnetic fields would be required. This review consists of two sections: in the first section we have extensively reviewed the salient features, e.g.?structure, phase diagram, double-exchange mechanism, Jahn?Teller effect, different types of ordering and phase separation of CMR manganites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for various doped manganites having natural grain boundaries such as polycrystalline, nanocrystalline bulk and films, manganite-based composites and intrinsically layered manganites, and artificial grain boundaries such as bicrystal, step-edge and laser-patterned junctions. Some other potential magnetoresistive materials, e.g.?pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc, are also briefly dealt with. The review concludes with an overview of grain-boundary-induced low field magnetotransport behavior and prospects for possible applications.

https://iopscience.iop.org/article/10.1088/0953-8984/20/27/273201/pdf

Low field magnetotransport in manganites

The perovskite manganites of general formula RE_1-xAe_xMnO_3 (RE= rare earth,AE=Ca, Sr, Ba and Pb)have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). They exhibit extraordinary large magnetoresistance pronounced as CMR in the vicinity of insulator-metal/paramagnetic-ferromagnetic transition at a relatively large applied magnetic fields. However, for applied aspectes, occurence of significant CMR at low applied magnetic fields would be required. This review consists of of two sections: In the first section we have extensively reviewed the salient features e.g. structure, phase diagram, double exchange mechansim, Jahn Teller effect, different types of ordering and phase separation of CMR mangnaites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for doped manganites such as polycrystalline La_0.67Ca_0.33MnO_3 films, Ag admixed La_0.67Ca_0.33MnO_3 films, polycrystalline (La_0.7Ca_0.2Ba_0.1MnO_3)and epitaxial (La_0.67Ca_0.33MnO_3) films on different substrates, nanophasic La_0.7Ca_0.3MnO_3, mangnaite-polymer composites (La_0.7Ba_0.2Sr_0.1MnO_3-PMMA and La_0.67Ca_0.33MnO_3-PMMA)and double layered polycrystalline (La_1.4Ca_1.6-xBa_xMn_2O_7) and films (La_1.4Ca_1.6Mn_2O_7). Some other potential magnetoresistive materials e.g. pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc have aslo been briefly dealt with. The review concludes with the summary of results for low field magnetotransport behaviour and prospectes for applications.

Low Field Magnetotransport in Manganites

Thin-film transistors made of solution-processed metal oxide semiconductors hold great promise for application in the emerging sector of large-area electronics. However, further advancement of the technology is hindered by limitations associated with the extrinsic electron transport properties of the often defect-prone oxides. We overcome this limitation by replacing the single-layer semiconductor channel with a low-dimensional, solution-grown In2O3/ZnO heterojunction. We find that In2O3/ZnO transistors exhibit band-like electron transport, with mobility values significantly higher than single-layer In2O3 and ZnO devices by a factor of 2 to 100. This marked improvement is shown to originate from the presence of free electrons confined on the plane of the atomically sharp heterointerface induced by the large conduction band offset between In2O3 and ZnO. Our finding underscores engineering of solution-grown metal oxide heterointerfaces as an alternative strategy to thin-film transistor development and has the potential for widespread technological applications.

https://advances.sciencemag.org/content/advances/3/3/e1602640.full.pdf

Heterojunction oxide thin-film transistors with unprecedented electron mobility grown from solution

This study analyzes the current spreading effect and light extraction efficiency (LEE) of lateral and vertical light-emitting diodes (LEDs). Specifically, this study uses a fully 2-D model that solves drift-diffusion and Poisson equations to investigate current flow paths and radiative recombination regions. The ray-tracing technique was used to calculate the LEE of the top surface. First, this study discusses the current spreading effect of the lateral and conventional vertical LED and determines the efficiency droop even with a transparent conducting layer. Different electrode configurations in the vertical LED were tested to optimize the current spreading effect, which, in turn, suppresses the carrier leakage and mitigates the efficiency droop under high injection conditions. This study also discusses the wall-plug efficiency in overall cases to identify the design rules for higher power conversion efficiency.

Study on the Current Spreading Effect and Light Extraction Enhancement of Vertical GaN/InGaN LEDs

Multilayer MgZnO/ZnO thin films for UV photodetectors

This paper reports the formation of two-dimensional electron gas (2DEG) in rf-sputtered defective polycrystalline MgZnO/ZnO heterostructure via the screening of grain boundary potential by polarization-induced charges. As the MgZnO thickness increases, the sheet resistance reduces rapidly and then saturates. The enhancement of the interfacial polarization effect becomes stronger, corresponding to a larger amount of resistance reduction, when the Mg content in the cap layer increases. Monte Carlo method by including grain boundary scattering effect as well as 2D finite-element-method Poisson and drift-diffusion solver is applied to analyze the polycrystalline heterostructure. The experimental and Monte Carlo simulation results show good agreement. From low temperature Hall measurement, the carrier density and mobility are both independent of temperature, indicating the formation of 2DEG with roughness scattering at the MgZnO/ZnO interface.

Two dimensional electron gases in polycrystalline MgZnO/ZnO heterostructures grown by rf-sputtering process

ZnO has shown great promise for application in optoelectronic devices, in which the modulation of conductivity is crucial to device performance. In this paper, we have applied the Monte Carlo method to analyze the mobility of single-crystalline and polycrystalline MgZnO/ZnO heterostructure thin film layers. The effects of grain boundary scattering and ionized impurity scattering, as well as phonon scattering, are considered. Our studies show that, with careful design of modulation doping that considers the effects of spontaneous and piezoelectric polarization, the grain boundary potential can be suppressed to improve the mobility of the ZnO layer by at least one order of magnitude. Simulation results are also confirmed by our experimental work, which shows that the polarization effect does play an important role in attracting carriers and increasing the mobility.

Mobility Enhancement of Polycrystalline MgZnO/ZnO Thin Film Layers With Modulation Doping and Polarization Effects

The effects of structural, electrical and magnetic properties with the isovalent chemical substitution of Ca2+ into the Sr2+ sites in La12(Sr18−xCax)Mn2O7 (x=0–08) are investigated The highest magnetoresistance ratio [ρ(0)/ρ(H)] of 208% (H=15 T) at a temperature of 102 K was observed for the x=04 sample The Curie temperatures decreased from 135 to 102 K for x=0–04, respectively Moreover, the Weiss constants θ were varied from the positive to negative value with increasing Ca concentration The antiferromagnetic behavior with Neel temperature around 30 K was found in the x=08 sample The magnetization measurements show that the hysteresis phenomenon appeared at the temperatures below the Curie or Neel temperatures

Structural, electrical and magnetic properties of two-dimensional La1.2(Sr1.8−xCax)Mn2O7 manganites

Neutron diffraction and ac magnetic susceptibility measurements have been performed to study the magnetic ordering of the Mn spins in polycrystalline Nd0.45Ca0.55MnO3. Three peaks around 260, 90, and 40 K were observed in the temperature dependence of the ac susceptibility. Structural analysis using high-resolution neutron diffraction patterns shows that the peak at 260 K is associated with the charge ordering of the Mn ions. Neutron magnetic diffraction shows that the peak at 90 K originates from the ordering of the Mn spins. Both ferromagnetic and antiferromagnetic couplings between the Mn spins were found. The spin structure in the basal plane consists of tilted ferromagnetic chains (along the [110] direction) aligned antiferromagnetically, whereas the interlayer coupling is ferromagnetic. A spin reorientation occurs below 15 K, which results in a change in the title angle. The Mn spins order at TN≈120 K, and the moment saturated at 30 K at 〈μZ〉=2.29(4)μB.

Magnetic order and spin reorientation in Nd0.45Ca0.55MnO3

The study of transparent conducting oxide (TCO) has become an important area due to the applications of lighting and display technology. Therefore, high mobility and conductivity TCO materials would be a key issue to the industry. In this paper, we have applied the Monte Carlo method to analyze the mobility of single and poly-crystalline MgZnO/ZnO thin film layer. The effects of grain boundary scattering, ionized impurity scattering as well as phonon scattering have been included in our program. The grain boundary potential size and carrier screening effect has been analyzed with our developed 2D Poisson and drift-diffusion solver. With a careful design of modulation doping and including the effect of spontaneous and piezoelectric polarization, the grain boundary potential can be suppressed and thus the mobility of the ZnO layer can be improved.

Chih-I Huang

Papers

Two dimensional electron gases in polycrystalline MgZnO/ZnO heterostructures grown by rf-sputtering process

Mobility Enhancement of Polycrystalline MgZnO/ZnO Thin Film Layers With Modulation Doping and Polarization Effects

Structural, electrical and magnetic properties of two-dimensional La1.2(Sr1.8−xCax)Mn2O7 manganites

Magnetic order and spin reorientation in Nd0.45Ca0.55MnO3

Mobility Study of Polycrystalline MgZnO/ZnO Thin Film Layers with Monte Carlo Method