ZnO-based Dilute Magnetic Semiconductors Research Articles

Effects of coexistence of Mo and Zn vacancies with different valence states and interstitial H on the magneto-optical properties of ZnO: First-principles calculations

Research progress has been achieved on the influence of transition metal Mo doping and Zn vacancy coexistence on the magnetic and optical properties of ZnO. However, the interstitial H impurity in the Mo-doped ZnO system has been neglected. In this work, first principles were used to calculate the effects of the coexistence of different valence states of Mo and Zn vacancies and interstitial H on the magneto-optical properties of ZnO under the framework of density functional theory. Magnetic results showed that when Mo and Zn vacancies have +6 and 0 valences, respectively, the Zn22Mo6+O24(VZn0) system without interstitial H shows the largest magnetic moment and strongest magnetism. In the doped system, the local electrons of the spin-polarized O-2p orbital behave as acceptors near the valence band maximum, and the itinerant electrons behave as donors near the conduction band minimum. The itinerant electrons of the O-2p orbital adjacent to the Zn vacancy mainly contribute to the magnetic properties of the Zn22Mo6+O24(VZn0) system, and they are most beneficial to the design and preparation of new ZnO-based dilute magnetic semiconductors. Analysis of optical properties found that the Zn22Mo6+HiO24(VZn0) system with interstitial H have the easiest separation of electron–hole pairs, the highest mobility, the longest relaxation time, and the strongest built-in electric field when the Mo and Zn vacancies have +6 and 0 valences, respectively. Therefore, this system has the longest carrier lifetime and best activity. In addition, the absorption spectrum of the Zn22Mo6+HiO24(VZn0) system shows the most remarkable red shift in the visible and near-infrared regions. When pH = 0/7, the Zn22Mo6+HiO24(VZn0) system shows the strongest photocatalytic reduction ability. Given its carrier lifetime and activity, absorption spectrum, and redox ability, the Zn22Mo6+HiO24(VZn0) system is relatively the best for the design and preparation of new ZnO-based photocatalysts.

A first-principles study on magnetic properties of the intrinsic defects in wurtzite ZnO.

Since the origin of magnetism in ZnO-based diluted magnetic semiconductors (DMSs) is still controversial, in this work, we presented a detailed study on the magnetic, structural, and electronic properties of wurtzite ZnO-based DMS systems with point and complex intrinsic defects. Two outer electrons from neutral oxygen vacancy (VO) occupy the a1 orbital, making the inducted magnetic moment to be zero, while a cluster including three VOs leads to a magnetic moment of ∼1 μB. The magnetic moment of the system with a Zn vacancy (VZn) is 1.65 μB. When two neutral VZns in different relative distances were created in respective supercells, the systems showed different magnetic moments induced by the unequal level between the highest electron occupied orbital of the defect state introduced by different VZn sites and the valence band maximum. The system of a neutral O occupying an octahedral site gives rise to a magnetic moment of 2 μB, while zinc interstitial and antisite defects do not cause spin polarization. The system with a complex defect of VO and VZn is magnetic when those vacancies are adjacent but still do not cause the compensation effect. The oxygen interstitial defect is unstable, and VZn easily turns into the complex defect. We suggest that VO clusters and VZn complex defects could likely be the origin of ferromagnetism in undoped ZnO.

Thermal tuning of electrical and dielectric characteristics of Mn-doped Zn0.95Fe0.05O dilute magnetic semiconductors

Structural and hence the electrical properties of transition metal doped ZnO-based dilute magnetic semiconductors can effectively be tuned thermally for their potential utilization in high temperature modern day spintronic devices. In this work, we investigate the effect of Mn doping on structural and its consequent effects on the electrical characteristics of Zn0.95−xMnxFe0.05O (x = 0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) synthesized using sol–gel auto-combustion. X-ray diffraction analysis indicated that the synthesized samples had the pure wurtzite-type hexagonal crystal structure. Average crystallite size as determined using Scherrer’s formula was decreased with the increase of Mn doping while the porosity was increased. Temperature dependent dielectric measurements revealed that dielectric constant and dielectric loss was increased by increasing the temperature for Mn-doped samples. The conductivity was also enhanced with increasing temperature due to the enhanced evolution of charge carriers between the grains. Temperature dependent impedance spectroscopy confirmed that resistance and reactance were inverse function of the temperature revealing negative temperature coefficient of resistance. Nyquist plot shows semi-circular behavior attributed to the grain boundary effect.

The local magnetic moment and electron transfer of ZnO-based dilute magnetic semiconductors

The electronic structures and magnetic properties of ZnO semiconductors doped with Cu, Co, C, Al and S are studied by first-principles calculation. The electronic transfer among Zn, O and doped atoms, and the differences of the number of electron between spin-up and spin-down, Δs, Δp, Δd for s, p and d orbits of these atoms, are analyzed in detail. It is found that, the ferromagnetic ground state is stabilized by its half-metallic electronic structure, and the strong local magnetic moments in Zn1−xCoxO, Zn1−xCuxO and ZnO1-xCx (x = 5.55%) DMSs originate mainly from the strong hybridizations between Cu-3d and O-2p, Co-3d and O-2p, Zn-3d and C-2p electrons. It is considered that the requirements to give rise to the ferromagnetism in the DMSs are the strong local magnetic moment and the electron transfer. The magnetic coupling in Zn1-xCoxO, Zn1-xCuxO and ZnO1-xCx is also considered to be a RKKY interaction.

Effect of Co and Ni codoping on the structural, magnetic, electrical and optical properties of ZnO

Co and Ni codoped ZnO-based dilute magnetic semiconductors having a general formula of Zn0.9Co0.1−xNixO (x=0.0, 0.02, 0.04, 0.06, 0.08 and 0.10) have been prepared using a simple solid-state reaction method. A single-phase hexagonal wurtzite structure, confirmed by X-ray diffraction, has been developed for Zn0.9Co0.1−xNixO (x<0.05), while minor secondary peaks of NiO were detected in samples having x=0.06, 0.08 and 0.10. The study of surface morphology revealed grain sizes in the range of 1.41–0.71μm. Raman spectra in the 200–800cm−1 range have been studied. The direct band gap energy for all the samples was found to be less than that of pure ZnO. Fourier transform infrared spectroscopy and UV–vis spectroscopy confirmed the incorporation of the Co and Ni ions into a ZnO lattice. Magnetic characterisation performed by vibrating sample magnetometer exhibited room-temperature ferromagnetism. Temperature-dependent electrical resistivity measurements were performed using a four-point probe technique.

Carriers mediated magnetic and impedance spectroscopic analysis of sol–gel synthesized Zn0.95−x Mn x Fe0.05O (0 ≤ x ≤ 0.05) DMSs

Zn0.95−x Mn x Fe0.05O (0 ≤ x ≤ 0.05) dilute magnetic semiconductors were prepared using sol–gel auto-combustion method for potential spintronic applications. Effect of Mn + Fe doping on dielectric and magnetic behavior of ZnO-based dilute magnetic semiconductors has been probed systematically. Investigation of crystal structure reveals that co-doping of Mn and Fe into the host ZnO does not transform its wurtzite-type hexagonal lattice. This efficient substitution of the dopants at the host site resulted in a gradual decrease in the lattice parameters attributed to the local coordination. Well-dispersed nanometer-sized grains have been revealed by images obtained using a field emission scanning electron microscope. Energy-dispersive X-ray analysis confirms the exact nominal composition of the stoichiometric elements present in the samples. Dielectric measurements performed using an impedance analyzer in a frequency range of 20 Hz to 20 MHz follow Koop’s theory and Maxwell–Wagner two-layered model. Nyquist plot (Cole–Cole) shows that the data points are laying on single semicircle, which shows the dominant behavior of the grain boundary’s effect. Magnetic characteristics explored by a vibrating sample magnetometer revealed that with increased Mn doping, enhanced values of both saturation magnetization and coercivity were obtained.

Dominant ferromagnetic coupling over antiferromagnetic in Ni doped ZnO: First-principles calculations

The low magnetic moment (MM) in diluted magnetic semiconductors (DMS) at low impurity doping levels has triggered considerable research into condensed magnetic semiconductors (CMS).This work reports an ab-initio investigation of the electronic structures and magnetic properties of ZnO in a zinc-blende (ZB) structure doped with nickel ions. Ni-doped ZnO-based DMS and CMS exhibit a dominance of ferromagnetic coupling over antiferromagnetic. A robust increase in the magnetization has been observed as a function of Ni impurity levels. This material favors short-range magnetic interactions at the ground state, suggesting that the observed ferromagnetism is defined by the double exchange mechanism. The spin-polarized density of states (DOS) of Ni-doped ZnO characterizes it as half-metallic with a considerable energy gap for up-spin components and as metallic for-down spins. Half-metallic Ni:ZnO based magnetic semiconductors with high magnetization are expected to have potential applications in spintronics.

EXAFS and XANES analysis of oxides at the nanoscale.

Worldwide research activity at the nanoscale is triggering the appearance of new, and frequently surprising, materials properties in which the increasing importance of surface and interface effects plays a fundamental role. This opens further possibilities in the development of new multifunctional materials with tuned physical properties that do not arise together at the bulk scale. Unfortunately, the standard methods currently available for solving the atomic structure of bulk crystals fail for nanomaterials due to nanoscale effects (very small crystallite sizes, large surface-to-volume ratio, near-surface relaxation, local lattice distortions etc.). As a consequence, a critical reexamination of the available local-structure characterization methods is needed. This work discusses the real possibilities and limits of X-ray absorption spectroscopy (XAS) analysis at the nanoscale. To this end, the present state of the art for the interpretation of extended X-ray absorption fine structure (EXAFS) is described, including an advanced approach based on the use of classical molecular dynamics and its application to nickel oxide nanoparticles. The limits and possibilities of X-ray absorption near-edge spectroscopy (XANES) to determine several effects associated with the nanocrystalline nature of materials are discussed in connection with the development of ZnO-based dilute magnetic semiconductors (DMSs) and iron oxide nanoparticles.

Elucidation of the enhanced ferromagnetic origin in Mn-doped zno nanocrystals embedded into a SiO2 matrix

The origin of the enhanced room temperature ferromagnetism in Mn-doped ZnO (ZnO:Mn) nanocrystals is investigated. ZnO:Mn nanocrystals, which were fabricated by using a laser irradiation method with a 248-nm KrF excimer laser, exhibited two-times increase in the spontaneous magnetization (∼0.4 emu/cm3 at 300 K) compared to the ZnO:Mn thin film (∼0.2 emu/cm3 at 300 K). The increased exchange integral of J 1/k B = 51.6 K in ZnO:Mn nanocrystals, in comparison with the ZnO:Mn thin film (J 1/k B = 46.9 K), is indicative of the enhanced ferromagnetic exchange interaction. This is attributed to the large number of acceptor defects in the SiO2-capped ZnO:Mn nanocrystals. Namely, the holes bound to the acceptor defects form microscopic bound-magneticpolarons with Mn ions; hence, long-range ferromagnetic coupling is enhanced. The results suggest that ferromagnetism in ZnO-based dilute magnetic semiconductors can be controlled by modulating the density of native point defects, which can be chemically and thermodynamically modified during the material synthesis or preparation.

D0Ferromagnetism in Oxide Nanowires: Role of Intrinsic Defects

In this report, we have investigated the origin of defect-induced room-temperature d 0 ferromagnetism in pure SnO2 and ZnO nanowires (NWs) with average diameter ~ 50 nm, prepared by template assisted route. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopic measurements show the singly ionized oxygen vacancy is inducing ferromagnetism in pure SnO2 NWs whereas cation (Zn) vacancy is found to responsible for the ferromagnetic behaviour in pure ZnO NWs. Besides, it is found that the Zn vacancy-induced ferromagnetism in ZnO can be tuned by substituting few percentage of nonmagnetic alkali metal like potassium (K) at Zn site. Saturation moment as well as Curie temperature has found to increase significantly with K-doping up to 4 at.% but a decrease of ferromagnetic response is observed for higher K-doping. X-ray photoelectron spectra show that K+1 ions substitute at Zn site and thus introduce hole through which a ferromagnetic interaction between Zn vacancies can be mediated. The direct correlation between the Zn vacancy concentration and the corresponding saturation moment indicates that Zn vacancyinduced ferromagnetism in ZnO can be successfully tuned by K-doping that can an exciting approach to prepare ZnO-based dilute magnetic semiconductors for modern spintronic technology.

First-principles study on the magnetism of Mn and Co codoped ZnO

The ferromagnetic properties of Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O are investigated using total energy of plane wave and ultrasoft pseudopotentials method based on the density functional theory (DFT). The results show that Zn0.875Co0.125O and cobalt (Co) and manganese (Mn) codoped ZnO-based dilute magnetic semiconductors (DMSs) are of the ferromagnetic order, and the ferromagnetism mostly comes from interaction of spin-polarized oxygen atoms, zinc atoms and transition metals. Meanwhile, it is also found that the magnetic moments are mostly contributed by the spin-polarized oxygen atoms and transition metals. In addition, the electron sharing for the 3d states of transition metals (Co, Mn) and 2p states of O atoms is exacerbated as compared to pure ZnO and Co doped ZnO, helping to obtain p-type ZnO. These results indicate that Co and Mn codoped ZnO are promising magneto-electronic materials and they can be used for nanoscale spintronics device material.

The Transition Metal Doped ZnO: The Self-interaction Corrected LSDA Calculation

Electronic structures of ZnO-based dilute magnetic semiconductors (DMS) are investigated within the self-interaction-corrected local density approximation (SIC-LDA). These results are compared with those calculated within the standard LDA. The Korringa–Kohn–Rostoker method combined with coherent potential approximation has been used. We find the differences between LDA and SIC-LDA calculations, in the band-gap energy. The energetic position of the Zn 3d bands and the description of the transition-metal d bands, have been studied. In contrast to the standard LDA calculations, the results obtained within SIC-LDA are in good agreement with the experimental ones.

Featured issue: zinc oxide

In the first two issues of 2013 we are highlighting fields which are topics of particular current interest in our discipline, as we did in the first two issues of 2012. The February featured papers will be devoted to electronic materials related to Solar Energy Generation and Energy Storage, but in this (January) issue we again highlight the material featured in the February 2012 issue—Zinc Oxide. Reasons for the high interest in this material was outlined in my editorial in the February 2012 issues as follows. While this II–VI semiconductor has been known for many years, the modern appreciation of its potential began in the 1950s, utilising properties such as piezoelectric, luminescent, and UV absorption characteristics, as well as its semiconductor features. The growth of single crystals facilitated the development of devices such as piezoelectric transducers, strain gauges, and pressure sensors, in addition to earlier devices such as varistors. Further work on the growth of large single crystals was stimulated by the potential for lattice matched epitaxial growth of optoelectronic III–V nitrides, but more recently it has become apparent that ZnO might be a good optoelectronic material on its own account, owing to similarities with GaN. Having a similarly wide band-gap, and similar lattice dimensions, the key advantageous feature of ZnO is that it has a free exciton binding energy of 60 meV, more than twice that of GaN. This combination offers the possibility of UV laser action at room temperature, and has given rise to much research to address outstanding problems, including that on doping and thin film growth by several different growth techniques. Further work is also pursuing the potential of ZnO-based dilute magnetic semiconductors in spintronics’. Continued interest in this subject has led to this second Featured issue from papers submitted as regular submissions, and updates the position last year. We hope you will find this issue of interest, and that it will help to guide further work on this important topic.

First-principles study of single intrinsic vacancy formation and its effect on the electronic density states and magnetic moment of V-doped ZnO

We calculated the formation energy of single vacancy in V-doped ZnO in different conditions (oxygen or zinc rich) by first principles. Effect of an intrinsic vacancy on the electronic density of states and magnetic moment of V-doped ZnO (Zn 15VO 16) with and without single vacancy was also calculated. Our calculation was performed by the CASTEP program within spin-polarized GGA approximation implemented in materials studio software. The formation energy showed that oxygen vacancy inclined to stay far from vanadium (V) and zinc vacancy preferred to stay at a position near V. The calculated formation energy also showed that a zinc vacancy may automatically occur but an oxygen vacancy may not appear automatically. Vanadium doping introduced spin-polarization around Fermi level. For an energy favorable vacancy, an oxygen vacancy had little effect on the electronic density of states. A zinc vacancy made the spin-polarization peaks around Fermi level broaden and decreased their magnitude. For the magnetic moment in energy favorable configurations, an oxygen vacancy had little effect on the magnetic moment; a zinc vacancy significantly decreased the magnetic moment (as high as 63.7%). Changes in magnetic moments were consistent with electronic density of states. Our calculation may interpret various experimental magnetic moment values. Our work also provided a reference for preparing V-doped ZnO-based dilute magnetic semiconductors.

Structural and magnetic stability of dopants in ZnO-based dilute magnetic semiconductors

Structural and magnetic configurations of Co/Mn ions, the most widely studied transitionmetal dopants in ZnO-based dilute magnetic semiconductors, have been investigated usingfirst-principles density functional calculations. The study provides a fundamentaltheoretical understanding on the distribution of the magnetic ions in the ZnO host and itscorresponding magnetism. Results show that the substituent magnetic ions at the Zn sitestrongly tend to aggregate chain-like via oxygen on the ab plane with an antiferromagneticcoupling in contrast to paramagnetic isolated free Co/Mn. Substitutional Cucodoping is found theoretically to reduce the magnetic dopant’s tendency towardschain-like aggregation, in good agreement with recent experimental observations.

Room temperature ferromagnetism in nanostructured ZnO–Al system

The origin of the room-temperature ferromagnetism (RTF) in ZnO-based dilute magnetic semiconductors remains controversial. We experimentally demonstrate here that it is possible to induce RTF in ball milled (ZnO)1−x/Alx without any ferromagnetic dopant. Our work shows that RTF in (ZnO)1−x/Alx (x=0–0.5) can be realized simply by milling a mixture of high purity ZnO and Al fine powders for 8 h. The spontaneous magnetization of the milled powders is found to increase by annealing under a reduced pressure. The magnetization value depends highly on both the ratio of Al to ZnO and the annealing temperature. X-ray photoelectron spectroscopy results have revealed that the Zn2+ ions in (ZnO)1−x/Alx are partially reduced into a lower ionic state. As there are no magnetic impurities present in our samples, the origin of ferromagnetism is most likely to be due to the charge transfer between Zn and Al at the interfaces of the ball milled nanograins. Our results reinforce the significant role played by the alterations of the electronic structures in the RTF of ZnO.

Progress in Zno-based diluted magnetic semiconductors

ZnO-based diluted magnetic semiconductors (DMS) have attracted a great deal of research attention and controversy over the past decade. The initial attention was sparked by the prediction of above-room-temperature ferromagnetism in Mn-doped ZnO by T. Dietl. This was followed by a surge of reports of ferromagnetism in thin film transition metal (TM)-doped ZnO. However, reported values of magnetic moments and Curie temperatures were inconsistent, which led to controversy over the origin of the observed ferromagnetism. In this paper we review a number of TM-doped ZnO-based DMS in order to clarify which materials are likely ferromagnetic.

Recent Progress in the Magnetism Theory of ZnO-based Diluted Magnetic Semiconductors

Diluted magnetic semiconductors(DMSs) are new functional materials formed by doping a few percentages of magnetic ions in nonmagnetic semiconductors.Recently,the ferromagnetism origin and magnetism theory are the research hotspot in the field of DMSs.It is still unclear how the ferromagnetism is induced between the doped magnetic ions and the carriers.In this paper,the recent progress in magnetism theory was reviewed.The typical theories including RKKY,mean field theory,double exchange and bound magnetic polaron were elucidated in detail.The hotspot of experimental,theoretical research and existing problems were evaluated.

Crystallographically oriented Co and Ni nanocrystals inside ZnO formed by ion implantation and postannealing

In the last decade, transition-metal-doped ZnO has been intensively investigated as a route to room-temperature diluted magnetic semiconductors (DMSs). However, the origin for the reported ferromagnetism in ZnO-based DMS remains questionable. Possible options are diluted magnetic semiconductors, spinodal decomposition, or secondary phases. In order to clarify this question, we have performed a thorough characterization of the structural and magnetic properties of Co- and Ni-implanted ZnO single crystals. Our measurements reveal that Co or Ni nanocrystals (NCs) are the major contribution of the measured ferromagnetism. Already in the as-implanted samples, Co or Ni NCs have formed and they exhibit superparamagnetic properties. The Co or Ni NCs are crystallographically oriented with respect to the ZnO matrix. Their magnetic properties, e.g., the anisotropy and the superparamagnetic blocking temperature, can be tuned by annealing. We discuss the magnetic anisotropy of Ni NCs embedded in ZnO concerning the strain anisotropy.

Magneto-optical and transport studies of ZnO-based dilute magnetic semiconductors

Thin film samples of ZnO doped with V were grown on sapphire substrates by pulsed laser deposition (PLD). The magnetization was measured by SQUID magnetometry and the films were found to be ferromagnetic at room temperature. The transmission, Faraday rotation and magnetic circular dichroism were measured as a function of frequency at room temperature over an energy range of 1.5–4.0 eV and carrier concentrations were determined from Hall effect measurements. Clear magneto-optical signals that are ferromagnetic in origin were observed at the ZnO band edge and the optimal conditions for observing large ferromagnetic magneto-optic signals are discussed.