Transition metal encapsulated hydrogenated silicon nanotubes: Silicon-based half-metal

One-dimensional hydrogenated silicon nanotubes (H-SiNTs) with transition metal atom encapsulated were systematically studied by using density functional theory. The band structures and magnetic properties of the H-SiNTs can be tailored by doping transition metal (TM) (TM = Cr, Mn, Fe, Co) atoms within the tube. The hydrogenated silicon nanotubes are semiconductors with wide band gaps. TM doping turns H-SiNTs to be metals or semiconductors with a very small gap, and TM atoms at the center of the tubes keep large magnetic moments. Robust half-metallicity is observed in Mn-doped H-SiNTs and it is free from Peierls distortion. Thus, H-SiNTs with encapsulated magnetic elements may find important applications in spintronic devices.     

单层SbAs和BiSb的表面修饰调控

基于密度泛函理论的第一性原理计算,研究了V族二维层状材料SbAs和BiSb在全氢化和全氟化后体系的晶体结构、稳定性和电子结构.计算结果表明,全氢化后SbAs和BiSb由buckled结构转变为准平面结构,而全氟化后则转变为low-buckled结构.同时,本征、全氢化和全氟化的SbAs和BiSb均具有很好的稳定性,具备实验合成的可能性.电子结构的分析表明,全氢化和全氟化后SbAs和BiSb均由宽带隙半导体转变为窄带隙的直隙半导体,且其能带结构仍具有很好的线性色散.通过对准平面和low-buckled结构SbAs和BiSb电子结构的进一步分析,揭示了全氢化和全氟化后体系能带变化的原因.在h-BN衬底上的计算结果显示,由于两者间的弱耦合作用,使得全氢化和全氟化SbAs的直隙半导体特征得以保留,表明其在未来光电子设备等领域中具有广泛的应用前景.     

Structural and electronic properties of monolayer hydrogenated honeycomb III–V sheets from first-principles

Using first-principles calculations, we investigate the structural and electronic properties of monolayer hydrogenated honeycomb III–V sheets. The lattice constants and cohesive energies of the hydrogenated III–V (XY H₂, X=B, Al, Ga, and Y =N, P, As) sheets depend on the III–V elements and follow the same trend as the atomic radii of the elements. We find that the short lattice constants correspond to the large cohesive energies of the hydrogenated III–V sheets. Similar to the graphane sheet, the hydrogenated BP and BAs sheets prefer the chair conformation. While for the hydrogenated BN, AlN, AlP, and GaN sheets, the boat conformation is favored. For the hydrogenated AlAs, GaP, and GaAs sheets, the chair and boat conformations are degenerate structures. We obtain that all the hydrogenated III–V sheets are wide-gap semiconductors. With GW corrections, the band gaps of hydrogenated III–V sheets follow the order of nitrogen > phosphorous > arsenic compounds for both the chair and boat conformations.     

Hydrogenated BN monolayers: A first principles study

In the present contribution we apply first principles calculations to investigate the electronic structures and stability of BN hydrogenated monolayers which include a substitutional carbon atom. For comparison, additional C hydrogenated structures are considered. The obtained results demonstrate that BN chair-like monolayers are more stable than boat-like configurations. It is found that the most stable structures present bond angles quite similar to the characteristic one observed for s p ³ hybridization. Moreover, a net magnetic moment arises from the introduction of a substitutional carbon impurity. In addition, the results indicate that carbon substitutionals can induce a remarkable reduction of the work function.     

氧化物稀磁半导体的研究进展

稀磁半导体是一种能同时利用电子的电荷和自旋属性,并兼具铁磁性能和半导体性能的自旋电子学材料。本文主要介绍ZnO、In2O3等氧化物稀磁半导体的研究进展,一是从实验角度介绍其制备、结构、磁性、电输运性质等特性;二是从理论角度对其磁交换能、电子结构、居里温度和磁性产生的机制进行阐述;三是在稀磁半导体的基础上进一步延伸,介绍其相关的异质结构的磁电阻效应,并在文章的最后对氧化物稀磁半导体的研究进行总结和展望。     

Electronic and structural properties of hydrogen on semiconductor surfaces

In this paper we will review the scientific literature which addresses the atomic geometry and electronic structure of clean and hydrogenated semiconductor surfaces. In particular, results related to vibrational studies will be presented. First, surfaces of elemental semiconductors (Ge, Si), Ge/Si-alloys, and III–V compound semiconductors chemisorb in a first stage atomic hydrogen by saturating surface atom dangling bonds. In a second step surface bonds are broken and a change of the geometrical structure results. Finally, higher hydrogen exposures are able to etch semiconductor surfaces. Best understood to date are surfaces of Si(1 0 0), Si(1 1 1), Ge_xSi_1−x(1 0 0), and III–V's after cleavage which have been modeled by dimerized and undimerized structures. (1 0 0) surfaces of III–V semiconductors, like GaAs and InP, tend to be dimerized, too.     

Nuclear magnetic resonance investigation of H, H2 and dopants in hydrogenated amorphous silicon and related materials

Nuclear magnetic resonance has been successfully applied to the study of the microstructure of hydrogenated amorphous silicon and related materials. It has been used to determine the local bonding and structural environment of the host atoms, the hydrogen, and the dopants. First, we review some of these NMR experimental results on the hydrogen microstructure in hydrogentaed amorphous semiconductors and compare the results on plasma deposited hydrogenated amorphous silicon (a-Si:H), remote hydrogen plasma deposited a-Si:H, thermally annealed a-Si:H, doped a-Si:H, microcrystalline Si and amorphous (Si, Ge):H alloys. A common feature is that these materials exhibit a heterogeneous distribution of hydrogen bonded to the semiconductor lattice in dilute and clustered phases. In addition, the lattice contains voids of varying number and size that contain non-bonded molecular hydrogen whose quantity is altered by deposition conditions and thermal treatment. Second, we review some aspects of the local bonding structure of dopants in a-Si:H. A significant fraction of the dopants are found to be in dopant-hydrogen clusters similar to those proposed to explain hydrogen passivation in crystalline silicon. Implications of the determined local structure on the doping efficiency are discussed.     

On the longevity of H-mediated ferromagnetism in Co doped TiO2: A study of electronic and magnetic interplay

Recent studies on dilute magnetic semiconductors have largely been focused on hydrogen induced giant magnetization. On the contrary, there are hardly any attempts to establish whether the H-induced magnetization is a permanent feature or likely to degrade with time. This aspect warrants a firm confirmation prior to planning their possible applications in spintronics and microelectronics. Motivated by this, we have systematically investigated this behaviour in Co doped TiO₂. In one study, we gave successive re-heat treatment to hydrogenated pellets and studied their magnetic, transport and electronic properties versus re-heating time. In another, we stored the hydrogenated sample in dry atmosphere for several months and studied the ageing effect on the induced ferromagnetism. The H-induced magnetization quickly vanishes upon re-heating the specimen; in addition, it also tends to degrade gradually with passage of time. The electronic and transport properties show a close parallelism, without any departure from structural properties. On the other hand, the Co-induced ferromagnetism, albeit a weak effect, does not show such degradation effects. As regards the mechanism of ferromagnetism, the oxygen vacancies are suggested to mediate the coupling.     

Ab initio investigation of local magnetic structures around substitutional 3d transition metal impurities at cation sites in III-V and II-VI semiconductors

The local magnetic structures around substitutional 3d transition metal impurities at cation sites in zinc blende structures of III-V (GaN, GaAs) and II-VI (ZnTe) semiconductors are investigated by using a spin-polarized density functional theory. We find that Cr-, Co-, Cu-doped GaN, Cr-, Mn-doped GaAs and Cr-, Fe-, Ni-doped ZnTe are half metallic with 100% spin polarization. The magnetic moments due to these 3d transition metal (TM) ions are delocalized quite significantly on the surrounding ions of host semiconductors. These doped TM ions have long range interactions mediated through the induced magnetic moments in anions and cations of host semiconductors. For low impurity concentrations Mn in GaAs also has zero magnetic moment state due to Jahn-Teller structural distortions. Based upon half metallic character and delocalization of magnetic moments in the anions and cations of host semiconductors these above mentioned 3d TM-doped GaN, GaAs and ZnTe seem to be good candidates for spintronic applications.     

Homogeneous and inhomogeneous magnetic oxide semiconductors

Magnetic oxide semiconductors are significant spintronics materials.In this article, we review recent advances for homogeneous and inhomogeneous magnetic oxide semiconductors.In the homogeneous magnetic oxide semiconductors,we focus on the various doping techniques including choosing different transition metals, codoping, non-magnetic doping,and even un-doping to realize homogeneous substitution and the clear magnetic origin.And the enhancement of the ferromagnetism is achieved by nanodot arrays engineering, which is accompanied by the tunable optical properties.In the inhomogeneous magnetic oxide semiconductors, we review some heterostructures and their magnetic and transport properties, especially magnetoresistance, which are dramatically modulated by electric field in the constructed devices.And the related mechanisms are discussed in details.Finally, we provide an overview and possible potential applications of magnetic oxide semiconductors.     

Photo-induced magnetic polarons in semiconductors

Exciton magnetic polarons observed in dilute magnetic semiconductors were investigated by steady-state and pico-second time-resolved photoluminescence measurements and have shown characteristic behavior of exciton localization processes in bulk-Cd_1-x Mn _x Te and also in the quantum structures composed of the dilute magnetic semiconductors. For the quantum structures spin-dependent coherent polarizations associated with excitons and biexcitons were studied by degenerate four-wave mixing experiment. Also investigated for different chalcogenide spinel ferromagnetic semiconductors was photo-induced enhancement of exchange interaction between magnetic ions by direct magnetic flux detection in the vicinity of the Curie temperatures.     

On the gate capacitance of MOS structures of kane-type semiconductors under magnetic quantization

An attempt is made to formulate the gate capacitance of MOS structures of Kane-type semiconductors under magnetic quantization, without any approximations of weak or strong electric field limits, on the basis of the fourth-order effective mass theory and taking into account the interactions of the conduction, light-hole, heavy-hole, and split-off bands. It is found, taking n-channel Hg_1–x Cd _x Te as an example, that the gate capacitance exhibits spiky oscillations with changing magnetic field, which is in qualitative agreement with experimental observations, reported elsewhere, in MOS structures of the same semiconductor. The corresponding results for n-channel inversion layers on parabolic semiconductors are also obtained from the expressions derived.     

Theory of excitons in cubic semiconductors in arbitrary magnetic fields: Application to GaAs

We have calculated the energies of six lowest optically allowed states of a direct exciton, in semiconductors with diamond and zinc-blended crystal structures, in the presence of an arbitrary magnetic field taking into account the effects of degeneracy and anisotropy of the valence bands, using a variational approach. Using the experimentally suggested values of the various Kohn-Luttinger parameters in GaAs, we have evaluated the energies of these levels. The values thus obtained are compared with those derived from magneto-reflection measurements in high-purity epitaxial layers of GaAs, and good agreement is found.     

第一原理计算稀磁半导体(In1-xMnx)As的晶格常数,磁性和电子结构

用紧束缚近似线性Muffin-tin轨道的方法计算了稀磁半导体(In1-xMnx)As(x=1/2,1/4和1/8)的晶格常数,磁性和电子结构.给出了Mn掺杂浓度的变化对(In1-xMnx)As的晶格常数,磁性和电子结构的影响.     

Magnetic properties of Mn-doped tellurite flakes like microstructure

In the present study, we report the magnetic properties of manganese-doped (Mn-doped) tellurite flakes, which are microstructure-like arrangements synthesized using the sol-gel method. These flakes exhibit ferromagnetism at room temperature. A temperature-dependent magnetic study (M-T) revealed the structures to exhibit ferromagnetism (FM) below 300 K. From the X-ray absorption and magnetic studies, it is evident that ferromagnetism originates due to Mn doping. Because ferromagnetism occurs at room temperature, Mn-doped tellurite flakes could be a candidate for spintronic devices as dilute magnetic semiconductors.     

Amorphous thin-film solar cells

This report gives an overview of the present status of thin-film solar cells made from hydrogenated amorphous semiconductors (a-Si:H, a-Ge:H) together with new results emphasizing the physics of amorphous materials and devices. Preparation techniques, quality and performances of a-Si:H and a-Ge:H films as well as solar cells with pin structures are reviewed. Dark and light current-voltage I(V) characteristics and spectral response measurements give information about photovoltaic diodes and allow further insights into the physics of these kinds of materials and solar cells. Simulation calculations and device modelling of such solar cells have increased our understanding of amorphous semiconductors and their devices. The introduction of pin/pin stacked and/or tandem structures has improved the long-term stability and conversion efficiency of amorphous solar cells.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Photoinduced effects and metastability in amorphous semiconductors and insulators

Amorphous semiconductors, being intrinsically metastable in nature, exhibit a wide variety of changes in their physical properties, particularly when photoinduced using bandgap illumination. This article reviews the photoinduced phenomena exhibited by amorphous semiconductors such as amorphous hydrogenated silicon (and other tetrahedrally coordinated materials) and chalcogenide glasses. Features exhibited in common by all types of amorphous semiconductors, whether in the experimentally observed photoinduced metastability or the theoretical models used to account for such behaviour, are stressed.     

Electrical Investigation of Armchair Graphene-Graphdiyne-Graphene Nanoribbons Heterojunctions

In this study,the structural and electronic properties of armchair graphdiyne nanoribbons,which have different widths are studied using the first principle calculation.The results indicate that all studied AGDYNRs show semiconducting behavior in which the band gap values decrease with the increase of nanoribbons width.The electronic and electrical properties of the graphdiyne sandwiched between two graphene nanoribbons are also investigated.The findings of our study indicate that among 4 investigated n-G-GDY-G-NR structures,the highest current is calculated for n = 3(3-G-GDY-G-NR),due to phase transition.     

Spin-polarized calculations of electronic structures in ferromagnetic and antiferromagnetic Zn0.75TM0.25Se (TM=Cr,Fe, Co and Ni)

In this work, we aim to examine the spin-polarized electronic band structures, the local densities of states as well as the magnetism of Zn_1−xTM_xSe (TM=Cr, Fe, Co and Ni) diluted magnetic semiconductors in the ferromagnetic (FM) and antiferromagnetic (AFM) phases, and with 25% of TM. The calculations are performed by the developed full-potential augmented plane wave plus local orbitals method within the spin density functional theory. As exchange-correlation potential we used the generalized gradient approximation (GGA) form. We treated the ferromagnetic and antiferromagnetic phases and we found that all compounds are stable in the ferromagnetic structure. Structural properties are computed after total energy minimization. Our results show that the cohesive energies of Zn_0.75TM_0.25Se are greater than that of zinc blende ZnSe. We discuss the electronic structures, total and partial densities of states, local moments and the p–d exchange splitting. Furthermore, we found that p–d hybridization reduces the local magnetic moment of TM and produces small local magnetic moments on the nonmagnetic Zn and Se sites. We found also that in the AFM phase the TM local magnetic moments are smaller than in the FM phase; this is due to the greater interaction of the TM d-up and d-down orbitals.     

Magnetic percolation in diluted magnetic semiconductors

We demonstrate that the magnetic properties of diluted magnetic semiconductors are dominated by short ranged interatomic exchange interactions that have a strong directional dependence. By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we reproduce the observed critical temperatures of a broad range of diluted magnetic semiconductors. We also show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned. This suggests that the ordering of diluted magnetic semiconductors is heavily influenced by magnetic percolation, and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations.