Swift heavy ion induced modifications in cobalt doped ZnO thin films: Structural and optical studies

Modifications in the structural and optical properties of 100 MeV Ni⁷⁺ ions irradiated cobalt doped ZnO thin films (Zn_1−xCo_xO, x = 0.05) prepared by sol-gel route were studied. The films irradiated with a fluence of 1 × 10¹³ ions/cm² were single phase and show improved crystalline structure with preferred C-axis orientation as revealed from XRD analysis. Effects of irradiation on bond structure of thin films were studied by FTIR spectroscopy. The spectrum shows no change in bonding structure of Zn-O after irradiation. Improved quality of films is further supported by FTIR studies. Optical properties of the pristine and irradiated samples have been determined by using UV-vis spectroscopic technique. Optical absorption spectra show an appreciable red shift in the band gap of irradiated Zn_1−xCo_xO thin film due to sp-d interaction between Co²⁺ ions and ZnO band electrons. Transmission spectra show absorption band edges at 1.8 eV, 2.05 eV and 2.18 eV corresponding to d-d transition of Co²⁺ ions in tetrahedral field of ZnO. The AFM study shows a slight increase in grain size and surface roughness of the thin films after irradiation.     

Structure and ferromagnetic properties of Co-doped ZnO powders

Single-phase Zn_1−xCo_xO (x=0.02, 0.04) powders were synthesized by a simple co-precipitation technique. X-ray diffraction analysis reveals that the Co-doped ZnO crystallizes in a wurtzite structure. The lattice constants of Co-doped ZnO powders decrease slightly when Co is doped into ZnO. Optical absorption spectra show a decrease in the bandgap with increasing Co content and also give an evidence of the presence of Co²⁺ ions in tetrahedral sites. Raman spectra indicate that Co doping increased the lattice defects and induced another Raman vibration mode around at 538 cm⁻¹, which is an indicator for the incorporation of Co²⁺ ions into the ZnO host matrix. Magnetic measurement reveals that the Zn_1−xCo_xO (x=0.02, 0.04) powders clearly exhibit room-temperature ferromagnetic behavior, which makes them potentially useful as building components for spintronics.     

Co掺杂对ZnO薄膜结构和性能的影响

采用PVA溶胶-凝胶方法，在玻璃衬底上制备了Zn_1-xCo_xO薄膜，利用X射线衍射仪(XRD)研究了不同Co含量对其微结构的影响.采用振动样品磁强计(VSM)测量了Zn_0.88Co_0.12O样品室温下的磁性.采用荧光光谱仪研究了Zn_1-xCo_xO样品室温下的发光特性，分析掺杂含量对其发光性能的影响，发现随着掺杂含量的增加，蓝光发光峰有一定的红移现象. 关键词： PVA方法 ZnO 掺杂     

Optical properties of bulk Zn1−xCoxO magnetic semiconductors

Polycrystalline Zn_1−xCo_xO (x=0, 0.02, 0.05, 0.10 and 0.15) oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in open air. X-ray diffraction analyses using Rietveld refinement indicate that a stoichiometric single phase with a wurtzite-like structure was found in Zn_1−xCo_xO samples with x up to 0.10. The elemental mapping using energy dispersive X-ray spectroscopic analyses presents a uniform distribution of Co. Optical transmittance measurements show that several extra absorption bands appear in the Co-doped ZnO, which is due to the transitions between the crystal-field-split 3d levels of tetrahedral Co²⁺ substituting Zn²⁺ ions. Raman measurements show that limited host lattice defects are induced by Co doping. Magnetization measurements reveal that the Co-doped ZnO samples are paramagnetic due to the absence of free carriers and in low temperature the dominant magnetic interaction is nearest-neighbor antiferromagnetic.     

Giant temperature coefficient of resistance in Co-doped ZnO thin films

A novel high-performance thermistor material based on Co-doped ZnO thin films is presented. The films were deposited by the pulsed laser deposition technique on Si (111) single-crystal substrates. The structural and electronic transport properties were correlated as a function of parameters such as substrate temperature and Co-doped content for Zn_1?x Co _x O (x=0.005,0.05,0.10 and 0.15) to prepare these films. The Zn_1?x Co _x O films were deposited at various substrate temperatures between 20 and 280 °C. A value of 20 %/K for the negative temperature coefficient of resistance (TCR) with a wide range near room temperature was obtained. It was found that both TCR vs. temperature behavior and TCR value were strongly affected by cobalt doping level and substrate temperature. In addition, a maximal TCR value of over 20 %?K^?1 having a resistivity value of 3.6 Ω?cm was observed in a Zn_0.9Co_0.1O film near 260 °C, which was deposited at 120 °C and shown to be amorphous by X-ray diffraction. The result proved that the optimal Co concentration could help us to achieve giant TCR in Co-doped ZnO films. Meanwhile, the resistivities of the films ranged from 0.4 to 270 Ω?cm. A Co-doped ZnO/Si film is a strong candidate of thermometric materials for non-cooling and high-performance bolometric applications.     

Effect of indium incorporation in Zn1−xCoxO thin films

In the present paper, the preliminary investigations of a series of ZnO thin films co-doped with indium and cobalt with an objective to elucidate the correlation, if any, between the carrier concentration and the induced room temperature ferromagnetism (RTFM), are presented. The single-phasic (Zn_99.5In_0.5)_1−xCo_xO thin films are deposited by spray pyrolysis. The substitution of Zn²⁺ by Co²⁺ has been established by optical transmission analysis of these films. The films are ferromagnetic at room temperature; and the magnetization has higher value for indium and cobalt co-doped thin film as compared with Zn0₉₀Co_0.1O thin film (having no indium).     

Electronic structure and ferromagnetism of polycrystalline Zn1−xCoxO (0≤x≤0.15)

The electronic structure of polycrystalline ferromagnetic Zn_1−xCo_xO (0.05≤x≤0.15) and the oxidation state of Co in it, have been investigated. The Co-doped polycrystalline samples are synthesized by a combustion method and are ferromagnetic at room temperature. XPS and optical absorption studies show evidence for Co²⁺ ions in the tetrahedral symmetry, indicating substitution of Co²⁺ in the ZnO lattice. However, powder XRD and electron diffraction data show the presence of Co metal in the samples. This give evidence to the fact that some Co²⁺ ion are incorporated in the ZnO lattice which gives changes in the electronic structure whereas ferromagnetism comes from the Co metal impurities present in the samples.     

Influence of Co doping content on its valence state in Zn1−xCoxO (0 ≤ x ≤ 0.15) thin films

Zn_1−xCo_xO (0 ≤ x ≤ 0.15) thin films grown on Si (1 0 0) substrates were prepared by a sol-gel technique. The effects of Co doped on the structural, optical properties and surface chemical valence states of the Zn_1−xCo_xO (0 ≤ x ≤ 0.15) films were investigated by X-ray diffraction (XRD), ultraviolet-visible spectrometer and X-ray photoelectron spectroscopy (XPS). XRD results show that the Zn_1−xCo_xO films retained a hexagonal crystal structure of ZnO with better c-axis preferred orientation compared to the undoped ZnO films. The optical absorption spectra suggest that the optical band-gap of the Zn_1−xCo_xO thin films varied from 3.26 to 2.79 eV with increasing Co content from x = 0 to x = 0.15. XPS studies show the possible oxidation states of Co in Zn_1−xCo_xO (0 ≤ x ≤ 0.05), Zn_0.90Co_0.10O and Zn_0.85Co_0.15O are CoO, Co₃O₄ and Co₂O₃, with an increase of Co content, respectively.     

Structure and magnetism of Zn0.9Co0.1O DMS films prepared by chemical solution deposition method

The Zn_0.9Co_0.1O films are fabricated by chemical solution deposition method. All the films have the ZnO wurtzite structure with a preferential orientation along the c-axis. The analysis of X-ray near-edge absorption spectroscopy and X-ray photoelectron spectroscopy indicates that the valence of Co is +2, and there are oxygen vacancies in Zn_0.9Co_0.1O films annealed in Ar atmosphere. Extended X-ray absorption fine structure results reveal that Co²⁺ ions have dissolved into ZnO and substituted for Zn²⁺ ions. Magnetization measurements show that the film annealed in Ar exhibits ferromagnetism which can be explained by the formation of bound magnetic polarons.     

Structural and optical properties of cobalt doped ZnO nanocrystals

Zn_1−xCo_xO nanocrystals with nominal Co doping concentrations of x = 0–0.1 were synthesized through a simple solution route followed by a calcining process. The doping effects on the structural, morphological and optical properties were investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman, absorption and luminescence spectroscopy. The results indicated that a small amount of Co ions were incorporated into ZnO lattice structure, whereas the secondary phase of Co₃O₄ was segregated and precipitated at high Co doping concentrations, the solid solubility of Co ions in ZnO nanocrystals could be lower than 0.05. The spectra related to transitions within the tetrahedral Co²⁺ ions in the ZnO host crystal were observed in absorption and luminescence spectra.     

Microstructures, surface bonding states and room temperature ferromagnetisms of Zn0.95Co0.05O thin films doped with copper

Zn_0.95−xCo_0.05Cu_xO (ZCCO, where x = 0, 0.005, 0.01 and 0.015) thin films were deposited on Si (1 0 0) substrates by pulsed laser deposition technique. Crystal structures, surface morphologies, chemical compositions, bonding states and chemical valences of the corresponding elements for ZCCO films were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and X-ray photoelectron spectroscopy (XPS). XRD and FESEM results indicate that crystallite sizes of the highly (0 0 2)-oriented ZCCO films slightly decrease with increasing Cu content. When the Cu content increases from 0 to 0.015, Zn 2p_3/2, Co 2p, Cu 2p_3/2 and O 1s peaks of the ZCCO film shift towards higher or lower binding energy regions, and the reasons for these chemical shifts are investigated by fitting the corresponding XPS narrow-scan spectra. Both in-plane and out-of-plane magnetization-magnetic field hysteresis loops of the ZCCO films reveal that all the films have room temperature ferromagnetisms (RTFMs). The conceivable origin of the RTFM is ascribed to the combined effects of the local structural disorder resulted from (Co²⁺, Cu²⁺, Cu¹⁺)-cations which substitute Zn²⁺ ions in the ZnO matrices, ferromagnetic coupling between coupled dopant atoms caused by Co²⁺ (3d⁷4s⁰) and Cu²⁺ (3d⁹4s⁰) spin states, and exchange interactions between the unpaired electron spins originating from lattice defects induced by Cu doping in the Zn_0.95Co_0.05O matrices.     

Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films

We report on the analysis of optical transmittance spectra and the resulting ferromagnetic characteristics of sputtered Zn_1−xCo_xO films. Zn_1−xCo_xO films were prepared on (0001)-oriented Al₂O₃ substrates by the radio-frequency (rf) magnetron co-sputtering method. The XRD results showed that the crystallinity of films was properly maintained up to x=0.30 and no second phase peaks were detected up to x=0.40. The transmittance spectra showed both the increase of the absorption band intensity and the red shift of the absorption peak as well as the band edge with increasing x. We have proved experimentally that these changes depend on Co concentration. These optical properties suggest that sp-d exchange interactions and typical d-d transitions become activated with increasing x, which leads to the enhancement of ferromagnetic properties in Zn_1−xCo_xO films as shown in the AGM results. Therefore, it is concluded that the ferromagnetism derives from the substitution of Co²⁺ for Zn²⁺ without changing the wurtzite structure.     

Raman scattering investigations on Co-doped ZnO epitaxial films: Local vibration modes and defect associated ferromagnetism

Raman scattering spectroscopy has been performed on high quality Co-doped ZnO epitaxial films, which were grown on Al₂O₃ (0001) by oxygen-plasma assisted molecular beam epitaxy. Raman measurements revealed two local vibration modes (LVMs) at 723 and 699 cm⁻¹ due to the substitution of Co²⁺ in wurtzite ZnO lattice. The LVM at 723 cm⁻¹ is found to be an elemental sensitive vibration mode for Co substitution. The LVM at 699 cm⁻¹ can be attributed to enrichment of Co²⁺ bound with oxygen vacancy, the cobalt–oxygen vacancy–cobalt complexes, in Zn_1−xCo_xO films associated with ferromagnetism. The intensity of LVM at 699 cm⁻¹, as well as saturated magnetization, enhanced after the vacuum annealing and depressed after oxygen annealing.     

Room-temperature anomalous Hall effect and magnetroresistance in (Ga, Co)-codoped ZnO diluted magnetic semiconductor films

This paper reports that the(Ga,Co)-codoped ZnO thin films have been grown by inductively coupled plasma enhanced physical vapour deposition.Room-temperature ferromagnetism is observed for the as-grown thin films.The x-ray absorption fine structure characterization reveals that Co 2+ and Ga 3+ ions substitute for Zn 2+ ions in the ZnO lattice and exclude the possibility of extrinsic ferromagnetism origin.The ferromagnetic(Ga,Co)-codoped ZnO thin films exhibit carrier concentration dependent anomalous Hall effect and positive magnetoresistance at room temperature.The mechanism of anomalous Hall effect and magneto-transport in ferromagnetic ZnO-based diluted magnetic semiconductors is discussed.     

Study of Zn1−xCoxO (0.02≤x≤0.08) dilute magnetic semiconductor prepared by mechanosynthesis route

Single-phase Zn_1−xCo_xO (0.02≤x≤0.08) dilute magnetic semiconductor is prepared by mechanical milling process. The shift of XRD peaks towards the higher angle and a redshift in the band gap compared to the undoped ZnO ensure the incorporation of Co²⁺ ions in the semiconductor host lattice. Pure Zn_xCo_1−xO phases show the paramagnetic behavior in the temperature range 80 K≤T≤300 K. The room temperature volume magnetic susceptibility (χ_v) estimated in case of Zn_0.96Co_0.04O is ∼10⁻⁵ emu/Oe cm³. The temperature dependence of susceptibility χ_v can be fitted well with Curie law confirming the paramagnetic interaction. The observed crystal-field splitting of 3d levels of Co²⁺ ions inside Zn_1−xCo_xO has been successfully interpreted using Curie law.     

固相反应法制备Co掺杂ZnO的磁性和光学性能研究

研究了以固相反应法制备Co掺杂ZnO粉体的磁性和光学性能，测试结果表明对于均匀掺杂的Zn_0.95Co_0.05O粉体，Co²⁺随机取代Zn²⁺的位置进入ZnO晶格.Co²⁺之间的3d自旋电子耦合交换作用使得近邻的Co²⁺自旋反平行，Zn_0.95Co_0.05O粉体在3—300K表现为顺磁性，而非铁磁性. 关键词： ZnO 固相反应 稀磁半导体 顺磁性     

Co掺杂ZnO薄膜的局域结构和电荷转移特性研究

采用磁束缚电感耦合等离子体溅射沉积法在不同的氧气分压下制备了Zn_0.95Co_0.05O和Zn_0.94Co_0.05Al_0.01O薄膜.利用X射线吸收精细结构技术对薄膜O-K,Co-K和Co-L边进行了局域结构研究,结果表明：Co²⁺取代了四配位晶体场中的Zn²⁺而未改变ZnO的六方纤锌矿结构,高真空条件下制备的薄膜 关键词： Co掺杂ZnO 稀磁半导体 X射线吸收精细结构 共振非弹性X射线散射     

Zn1-xCoxO稀磁半导体薄膜的结构及其磁性研究

利用X射线吸收精细结构、X射线衍射和磁性测量等技术研究脉冲激光气相沉积法制备的Zn_1-xCo_xO (x＝0.01,0.02)稀磁半导体薄膜的结构和磁性.磁性测量结果表明Zn₁-xCo_xO样品都具有室温铁磁性.X射线衍射结果显示其薄膜样品具有结晶良好的纤锌矿结构.荧光X射线吸收精细结构测试结果表明,脉冲激光气相沉积法制备的样品中的Co离子全部进入ZnO晶格中替代了部分Zn的格点位置,生成单一相的Zn_1-xCo_xO 稀磁半导体.通过对X射线吸收近边结构谱的分析,确定Zn_1-xCo_xO薄膜中存在O空位,表明Co离子与O空位的相互作用是诱导Zn_1-xCo_xO产生室温铁磁性的主要原因. 关键词： 1-xCo_xO稀磁半导体')" href="#">Zn_1-xCo_xO稀磁半导体 X射线吸收精细结构谱 脉冲激光气相沉积法     

Fe,Co共掺杂ZnO薄膜结构及发光特性研究

采用溶胶凝胶法在玻璃衬底上制备了Fe,Co共掺Zn0.9FexCo0.1-xO(x=0,0.03,0.05,0.07)系列薄膜.通过扫描电镜(SEM)、X射线衍射(XRD)、X射线光电子谱(XPS)和光致发光(PL)谱对薄膜样品的表面形貌、晶体结构、成分和光学性能进行了研究.XRD结果表明所有ZnO薄膜样品都呈六方纤锌矿结构,在样品中没有观察到与Fe和Co相关的团簇,氧化物及其他杂相的衍射峰,表明共掺杂改善了Fe或Co在ZnO的分散性.XPS测试结果揭示样品中Co离子的价态为+2价;Fe离子的价态为+2价和+3价共存,但Fe相对浓度的增大导致Fe3+含量增加.所有样品的室温光致发光谱(PL)均观察到紫外发光峰和蓝光双峰,其中Fe,Co共掺ZnO薄膜的紫外发光峰较本征ZnO出现蓝移,蓝光双峰峰位没有变化,但发光强度有所减弱;而掺杂ZnO薄膜的绿光发光峰几乎消失.最后,结合微结构和成分分析对薄膜样品的发光机理进行了讨论.     

Optical and magnetic properties of laser-deposited Co-doped ZnO thin films

We report the optical and magnetic properties of laser-deposited Zn_1−xCo_xO (x=0.06-0.3) thin films with no intentional electrical carrier doping. The analysis of the high-temperature magnetization data provides an unambiguous evidence that antiferromagnetic superexchange interaction is the dominant mechanism of the exchange coupling between Co ions in Zn_1−xCo_xO alloy, yielding the value of the effective exchange integral J₁/k_B to be about −27 K. The low-temperature magnetization data reveals a spin glass transition in Zn_1−xCo_xO alloy for the Co content x>0.15, giving the value of the spin freezing temperature T_f to be ∼8 and ∼12 K for x=0.2 and 0.25, respectively. Optical spectra analysis shows a linear increase of the band gap E_g with the increase of the Co content following E_g=3.231+1.144x eV.