Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

SnO2: A comprehensive review on structures and gas sensors

Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

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Structural properties of self-organized semiconductor nanostructures

Metal oxides possess exceptional potential as base materials in emerging technologies. In recent times, significant amount of research works is carried out on these materials to assess new areas of applications, including optical, electronic, optoelectronic and biological domains. In such applications, the response and performance of the devices depend crucially, among other factors, on the size, shape and surface of the active oxide materials. For instance, the electronic and optical properties of oxides depend strongly on the spatial dimensions and composition [1] . The large number of atoms on the surface, and the effective van der Waals, Coulombic and interatomic coupling significantly modify the physical and chemical properties of the low dimensional oxide materials vis-a-vis its bulk counterparts. As a result, low dimensional oxide materials, such as nanoparticles, nanospheres, nanorods, nanowires, nanoribbon/nanobelts, nanotubes, nanodisks, nanosheets evoke vast and diverse interests. Thermal and physical deposition, hydro/solvothermal process, spray-pyrolysis, assisted self-assembly, oil-in-water microemulsion and template-assisted synthesis are regularly employed to synthesis one-, two- and three-dimensional nanostructures, which have become the focus of intensive research in mesoscopic physics and nanoscale devices. It not only provides good scopes to study the optical, electrical and thermal properties in quantum-confinement, but also offers important insights for understanding the functional units in fabricating electronic, optoelectronic, and magnetic devices of nanoscale dimension. Tin oxide (SnO 2 ) is one such very important n-type oxide and wide band gap (3.6 eV) semiconductor. Its good quality electrical, optical, and electrochemical properties are exploited in solar cells, as catalytic support materials, as solid-state chemical sensors and as high-capacity lithium-storage. Previously, Chopra et al. [2] reviewed different aspects of transparent conducting SnO 2 thin films. Wang et al. [3] discussed device applications of nanowires and nanobelts of semiconductor oxides, including SnO 2 . Batzill et al. [4] discussed about the surface of single crystalline bulk SnO 2 . However, it is understood that neither there is any comprehensive review on various crystallographic phases, polymorphs, bulk modulus, lattice parameters and electronic states of SnO 2 , nor there is any updated compilation on the recent progress and scope on SnO 2 nanostructures. Therefore, the proposed review covers the past and recent progress on the said topics and is summarized in the following manner. The available theoretical and experimental works on crystal structures, bulk modulus, lattice parameters are reviewed in details. The electronic states and the band structures of these phases are discussed next. Active crystal surfaces of SnO 2 play vital roles in its many interesting properties, including sensing and catalytic applications. So, a short review is written on its different surfaces, its electronic structures and density of states. The discussion on the importance of morphological variations on the properties of SnO 2 is followed by a review on different methods for obtaining such structures. A detail survey on the existing literature on techniques and mechanisms for the growth of nanostructures are included. SnO 2 is efficiently employed in gas sensing applications. A review on such applications is compiled based on the role of morphology and performance. The future course of SnO 2 as an important material in the contemporary research is also discussed.

SnO2: A Comprehensive Review on Structures and Gas Sensors

By antimony doping tin oxide, SnO2:Sb (ATO), below 1.0% Sb concentration, controllable n-type doping was realized. Plasma-assisted molecular beam epitaxy has been used to grow high quality single crystalline epitaxial thin films of unintentionally doped (UID) and Sb-doped SnO2 on r-plane sapphire substrates. A UID thickness series showed an electron concentration of 7.9×1018cm−3 for a 26nm film, which decreased to 2.7×1017cm−3 for a 1570nm film, whereas the mobility increased from 15to103cm2∕Vs, respectively. This series illustrated the importance of a buffer layer to separate unintentional heterointerface effects from the effect of low Sb doping. Unambiguous bulk electron doping was established by keeping the Sb concentration constant but changing the Sb-doped layer thickness. A separate doping series correlated Sb concentration and bulk electron doping. Films containing between 9.8×1017 and 2.8×1020 Sb atoms/cm3 generated an electron concentration of 1.1×1018–2.6×1020cm−3. As the atomic Sb concentration...

Electron transport properties of antimony doped SnO2 single crystalline thin films grown by plasma-assisted molecular beam epitaxy

The surface roughness of In2O3(001) films is a roadblock to potential semiconductor applications of this material. Using plasma-assisted molecular beam epitaxy we found that In2O3(001) films grow rough by the formation of {111} facets and In2O3(111) films grow smooth without facetting due to the conventionally used (oxygen-rich) conditions. This behavior indicates that the (111) surface is thermodynamically prefered over the (001) surface. We demonstrate that under indium-rich growth conditions these thermodynamics are changed allowing In2O3(001) films to grow smoothly without facetting. Surface indium plays a key role by acting as an auto surfactant that lowers the surface free energy difference between the (001) and the (111) surface.

Plasma-assisted molecular beam epitaxy of high quality In2O3(001) thin films on Y-stabilized ZrO2(001) using In as an auto surfactant

Plasma-assisted molecular beam epitaxy of SnO2 on TiO2

Using a spin light emitting diode (spin-LED) we study the injection of spin-polarized holes and electrons from a (Ga,Mn)As epitaxial film into self-assembled InAs quantum dots (QDs). The electroluminescence polarization, integrated over the QD ensemble, is ∼1% for both carrier types, consistent with quantum well (QW) spin-LEDs. However, spectrally resolved measurements reveal a monotonic decrease in polarization with increasing energy for hole injection, while no spectral dependence is observed for electron injection. This is in contrast to previous measurements of QW based structures.

Spin injection from (Ga,Mn)As into InAs quantum dots

We report on the epitaxy of Fe thin films on GaAs(001) using molecular-beam epitaxy with two different growth methods aimed at suppressing Fe and GaAs interdiffusion. These methods make use of low-temperature deposition at −150 °C and/or of an ultrathin Al interlayer, respectively. Good-quality single-crystal Fe films were obtained. The magnetic properties of the Fe films show square hysteresis loops and clear in-plane magnetic anisotropy with well-defined easy hard axes. The photoluminescence of an Al0.3Ga0.7As/GaAs quantum well in close proximity to the Fe film is measured in order to examine the quality of the Fe/GaAs interface.

Properties of a Fe/GaAs(001) hybrid structure grown by molecular-beam epitaxy

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