退火温度对N+注入ZnO:Mn薄膜结构及室温铁磁性的影响

采用射频磁控溅射法在石英玻璃衬底上制备了ZnO:Mn薄膜, 结合N⁺ 注入获得Mn-N共掺ZnO薄膜, 进而研究了退火温度对其结构及室温铁磁性的影响. 结果表明, 退火后ZnO:(Mn, N) 薄膜中Mn²⁺和N^3-均处于ZnO晶格位, 没有杂质相生成. 退火温度的升高 有助于修复N⁺注入引起的晶格损伤, 同时也会让N逸出薄膜, 导致受主(N_O)浓度降低. 室温铁磁性存在于ZnO:(Mn, N)薄膜中, 其强弱受N_O浓度的影响, 铁磁性起源可采用束缚磁极化子模型进行解释.     

Magnetic anisotropy of Co2+ as signature of intrinsic ferromagnetism in ZnO:Co

We report on the magnetic properties of thoroughly characterized Zn(1-x)Co(x)O epitaxial thin films, with low Co concentration, x = 0.003-0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.     

Mn和N共掺ZnO稀磁半导体薄膜的研究

使用对Zn₂N₃:Mn薄膜热氧化的方法成功制备了高含N量的Mn和N共掺ZnO的稀磁半导体薄膜.在没有N离子共掺的情况下,ZnO:Mn薄膜的铁磁性非常微弱;如果进行N离子的共掺杂,就会发现ZnO:Mn薄膜在室温下表现出非常明显的铁磁性,饱和离子磁矩为0.23 μ_B—0.61 μ_B.这说明N的共掺激发了ZnO:Mn薄膜中的室温铁磁性,也就是受主的共掺引起的空穴有利于ZnO中二价Mn离子的铁磁性耦合,这和最近的相关理论研究符合很好. 关键词： 磁性半导体 受主掺杂 空穴媒介的铁磁性     

Structure and room-temperature ferromagnetism of Co-doped ZnO DMS films

Co-doped ZnO diluted magnetic semiconductor films were prepared on Si(100) substrates by magnetron sputtering system and the Co content varies from 0.01 to 0.15. The X-ray diffraction results showed ZnO of the wurtzite structure. The ferromagnetism was observed at room temperature. The X-ray near-edge absorption spectroscopy revealed that Co substitutes for Zn²⁺ ions in the valence of +2 state in the Co-doped ZnO films.     

Room-temperature ferromagnetism of Cu ion-implanted Ga-doped ZnO

1 MeV Cu²⁺ ions have been implanted into un-doped ZnO and Ga-doped ZnO films with a dose of 1 × 10¹⁷ ions/cm² at room-temperature. Cu ion-implanted Ga-doped ZnO had ferromagnetism at room-temperature and the saturation magnetization of this sample was estimated to be 0.12 μ_B per Cu, while the Cu ion-implanted un-doped ZnO did not show ferromagnetic behavior. Near-edge X-ray fine structure (NEXAFS) spectroscopy revealed that a partial amount of implanted Cu ions existed as Cu²⁺ (d⁹) state in Ga-doped ZnO film. On the other hand, almost Cu atoms existed as Cu¹⁺ (d¹⁰) state in un-doped ZnO film. However, the subsequent annealing at temperature above 800 °C on this ferromagnetic sample induced the annihilation of ferromagnetism due to the formation of non-ferromagnetic Cu₂O phase.     

Ferromagnetism induced by oxygen-vacancy complex in (Mn,in) codoped ZnO

Mn doped Zinc oxide (ZnO) thin films were prepared by metal organic chemical vapor deposition (MOCVD) technique. Structural characterizations by X-ray diffraction technique (XRD) and photoluminescence (PL) indicate the crystal quality of ZnO films. PL and Raman show a large fraction of oxygen vacancies (V_O²⁺) are generated by vacuum annealed the film. The enhancement of ferromagnetism in post-annealed (Mn, In) codoped ZnO could result from V_O²⁺ incorporation. The effect of V_O²⁺ on the magnetic properties of (Mn, In) codoped ZnO has been studied by first-principles calculations. It is found that only In donor cannot induce ferromagnetism (FM) in Mn-doped ZnO. Besides, the presence of V_O²⁺ makes the Mn empty 3d-t_2g minority state broadened, and a t_2g-V_O²⁺ hybrid level at the conduction band minimum forms. The presence of V_O²⁺ can lead to strong ferromagnetic coupling with the nearest neighboring Mn cation by BMP model based on defects reveal that the ferromagnetic exchange is mediated by the donor impurity state, which mainly consists of Mn 3d electrons trapped in oxygen vacancies.     

Epitaxial Properties of Co-Doped ZnO Thin Films Grown by Plasma Assisted Molecular Beam Epitaxy

High quality Co-doped ZnO thin films are grown on single crystalline Al2O3（0001） and ZnO（0001） substrates by oxygen plasma assisted molecular beam epitaxy at a relatively lower substrate temperature of 450℃. The epitaxial conditions are examined with in-situ reflection high energy electron diffraction （RHEED） and ex-situ high resolution x-ray diffraction （HRXRD）. The epitaxial thin films are single crystal at film thickness smaller than 500nm and nominal concentration of Co dopant up to 20%. It is indicated that the Co cation is incorporated into the ZnO matrix as Co^2＋ substituting Zn^2＋ ions. Atomic force microscopy shows smooth surfaces with rms roughness of 1.9 nm. Room-temperature magnetization measurements reveal that the Co-doped ZnO thin films are ferromagnetic with Curie temperatures Tc above room temperature.     

H掺杂对ZnCoO稀磁半导体薄膜结构及磁性能的影响

利用磁控溅射法,采用亚分子分层掺杂技术交替溅射Co靶和ZnO靶,在Si衬底上制备了不同氢氩流量比的H:ZCO薄膜样品,研究了氢氩流量比对薄膜结构特性和磁学性能的影响。所制备的薄膜样品具有c轴择优取向。由于H对表面和界面处悬挂键的钝化作用,随H₂流量比的增加,薄膜的择优取向变差。磁性测量结果显示,薄膜样品的铁磁性随着氢氩流量比的增大而增强。XPS结果表明,随着H含量的增大,金属态Co团簇的相对含量逐渐增加,而氧化态Co离子的相对含量逐渐减小。H:ZCO样品中的铁磁性可能来源于Co金属团簇,H的掺入促使ZnO中的Co离子还原成Co金属团簇,从而增强了薄膜样品的室温铁磁性。     

Magnetic and Raman scattering studies of Co-doped ZnO thin films grown by pulsed laser deposition

Phase pure Zn_1?x Co _x O thin films grown by pulsed laser deposition have transmittance greater than 75 % in the visible region. Raman studies confirm the crystalline nature of Zn_1?x Co _x O thin films. Zn_0.95Co_0.05O thin films show room temperature ferromagnetism with saturation magnetization of 0.4μ _B /Co atom. The possible origin of paramagnetism at higher Co doping concentrations can be attributed to the increased nearest-neighbor antiferromagnetic interactions between Co²⁺ ions in ZnO matrix. XPS confirms the substitution of Co²⁺ ions into the ZnO host lattice.     

Electrical and optical spectroscopy on ZnO:Co thin films

Magnetic oxide semiconductors, for example the highly transparent and intrinsically n-type conducting zinc oxide doped with the 3d transition metal Co (ZnO:Co), are promising for the emerging field of spintronics [1]. We investigated n-conducting ZnO:Co thin films with a Co content of nominal 0.02, 0.20, or 2.00 at. %. The substitution of Co cations in the tetrahedral sites of wurtzite ZnO with Zn was confirmed at low temperature by the 1.877 eV photoluminescence between crystal field split d-levels of Co²⁺ (d⁷) ions. Based on theoretical studies, it is predicted that the formation of electron levels with zinc interstitials (I_Zn) or hole levels with zinc vacancies (V_Zn) is necessary to induce ferromagnetism, whereas the formation of electron levels with oxygen vacancies (V_O) is detrimental for ferromagnetism in ZnO:Co [2]. Cobalt generates a hole level in ZnO [3]. We investigated the generation of electron levels in n-conducting ZnO:Co in dependence on the Co content by means of deep level transient spectroscopy (DLTS). However, because of the ambiguous categorization of deep defects in n-conducting ZnO (V_O, I_Zn), an optimization of defect-related ferromagnetism in ZnO:Co is not possible at the moment. PACS 78.30.Fs; 91.60.Ed; 91.60.Mk     

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.     

Study of Sm-doped ZnO samples sintered in a nitrogen atmosphere and deposited on n-Si(1 0 0) by evaporation technique

The samarium doping zinc oxide (Zn_1-xSm_xO) with (x=0.0, 0.04, 0.05 and 0.17) polycrystalline thin films have been deposited on n-Si(1 0 0) substrate using thermal evaporation technique. Ceramic targets for deposition were prepared by the standard solid-state reaction method and sintered in nitrogen atmospheres. X-ray diffraction and scanning electron microscopy analyses show that the bulk and films features reveal wurtzite crystal structure with a preferential (1 0 1) crystallographic orientation and grows as hexagonal shape grains. According to the results of the Hall effect measurements, all the films show p-type conductivity, possibly a result of nitrogen incorporation into the Sm-doped ZnO samples. Magnetic measurements show that ferromagnetic behavior depends on the Sm³⁺ concentration. For a film with lower Sm₂O₃ contents (x=0.04), a phenomenon of paramagnetism has been observed. While, with further increase of Sm³⁺ contents (x=0.05) the ferromagnetic behavior has been observed at room temperature. However, at higher doping content of Sm³⁺, the ferromagnetic behavior was suppressed. The decrease of ferromagnetism with increasing doping concentration demonstrates that ferromagnetism observed at room temperature is an intrinsic property of Zn_1-xSm_xO films.     

Defect induced ferromagnetism in carbon-doped ZnO thin films

We report on room temperature ferromagnetism in C-doped ZnO thin films prepared by electron beam evaporation. Magnetization, Hall effect, X-ray photoemission spectroscopy (XPS) and X-ray diffraction studies have been conducted to investigate the source and nature of ferromagnetism in C-doped ZnO. The samples were observed to have n-type conduction with the carrier concentration increasing with C doping. XPS does not give any evidence for C substituted at the O site, and is more consistent with the formation of C-O bonds and with the presence of C primarily in the +4 state. It is suggested that the ferromagnetism originates in the development of Zn vacancies that are stabilized due to the incorporation of C in a high valence state (C⁴⁺).     

Two magnetic regimes in doped ZnO corresponding to a dilute magnetic semiconductor and a dilute magnetic insulator

Films of ZnO doped with magnetic ions Mn and Co and, in some cases, with Al have been fabricated with a very wide range of carrier densities. Ferromagnetic behavior is observed in both insulating and metallic films, but not when the carrier density is intermediate. Insulating films exhibit variable range hopping at low temperatures and are ferromagnetic at room temperature due to the interaction of the localized spins with static localized states. The magnetism is quenched when carriers in the localized states become mobile. In the metallic (degenerate semiconductor) range, robust ferromagnetism reappears together with very strong magneto-optic signals and room temperature anomalous Hall data. This demonstrates the polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.     

Room temperature ferromagnetic pure ZnO

Pure ZnO films were prepared by pulsed laser deposition on oxidized Si substrates under different oxygen pressure and substrate temperature. Clear room temperature ferromagnetism has been observed in the ZnO film prepared under high vacuum and room temperature. The observation of anomalous Hall effect confirms the intrinsic nature of the ferromagnetism. The photoluminescence and X-ray photoelectron spectroscopy spectra show the high concentration of oxygen vacancies in the ferromagnetic ZnO film. Our results clearly demonstrate the ferromagnetic contribution of the oxygen vacancies mediated by the spin polarized electrons hopping between discrete states in pure ZnO.     

Room-Temperature Ferromagnetism in Zn1-xMnxO Thin Films Deposited by Pulsed Laser Deposition

Zn1-xMnxO （x = O.Olq3.1） thin films with a Curie temperature above 300K are deposited on Al2O3 （0001） substrates by pulsed laser deposition. X-ray diffraction （XRD）, ultraviolet （UV）-visible transmission and Raman spectroscopy are employed to characterize the microstructural properties of these films. Room temperature ferromagnetism is observed by superconducting quantum interference device （SQUID）. The results indicate that Mn doping introduces the incorporation of Mn^2＋ ions into the ZnO host matrix and the insertion of Mn^2＋ ions increases the lattice defects, which is correlated with the ferromagnetism of the obtained films. The doping concentration is also proven to be a crucial factor for obtaining highly ferromagnetic Zn1-xMnxO films.     

Ho3+掺杂对ZnO薄膜的微观结构和磁性的影响

研究Ho³⁺掺杂对氧化锌半导体材料的微结构和磁学性质影响. 利用热蒸发技术制备了一系列沉积在Si(100)衬底的Zn_1-xHo_xO(x=0.0、0.04、0.05）薄膜. X射线光谱、表面形貌以及磁性的实验结果表明,Ho³⁺掺杂对ZnO薄膜材料的性能影响很大. X射线衍射图显示峰位出现高角度转变并且趋向于(101)取向,在ZnO晶格显示Ho³⁺置换. 扫描电子显微镜和能谱仪对薄膜的表面形貌以及化学     

Nitrogen doping-mediated room-temperature ferromagnetism in insulating Co-doped SnO2 films

Co-doped SnO₂ films codoped with nitrogen (N) have been prepared by magnetron sputtering to investigate the effect of p-type defects on magnetic properties. The incorporation of N modifies the preferential growth orientation of the films. Multiple characterization techniques reveal that the incorporated Co²⁺ and N³⁻ ions substitute for Sn⁴⁺ and O²⁻ sites in SnO₂ lattice, respectively. As N concentration increases, the band gap of the films decreases because of the formation of Sn-N bond. Room-temperature ferromagnetism is observed in (Co, N)-codoped SnO₂ films, and the saturated magnetic moment is sensitive to the incorporated N concentration. The variations in the magnetic properties as a function of N concentration are discussed on the basis of bound magnetic polaron model.     

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.     

Ferromagnetism in Mn-doped ZnO due to impurity bands

We have prepared quasi-homoepitaxial Zn_0.95Mn_0.05O-films on ZnO substrates by pulsed laser deposition. The thin films were characterized by in situ RHEED during growth, X-ray diffraction and reflectometry, as well as transmission electron microscopy (TEM). The magnetic properties have been analyzed by SQUID magnetometry and by measuring the anomalous Hall effect. By varying the growth conditions (substrate temperature: 200–600 ^∘C, deposition atmospheres: Ar, O₂) a different concentration of impurities has been generated. Our results show that the films are electron doped and exhibit a ferromagnetic coupling that depends on the impurity concentration.