Dilute Magnetic Semiconductor Thin Films Research Articles

Dual manipulation of ferromagnetism in co-doped ZnO thin films by surfactant and n-type carriers

We present a conceptually-new approach “dual manipulation effect” using the surfactant passivation and the electron carrier doping for mediating intrinsic ferromagnetism in Co-doped ZnO dilute magnetic semiconductor (DMS) thin films. The first-principles calculations show that the surface passivation by hydrogen serves as a magnetism switch for the Co-O-Co magnetic coupling at the surface of the thin film, and thus can control the spin polarization of the doped Co atoms. Meanwhile, the electron carrier doping can further function as an effective layerlike ferromagnetism mediator for the underneath layer. The dual manipulation effect sheds light on the essential magnetism origin of n-type Co:ZnO DMS thin films, and may be used as an alternative strategy for enhancing the ferromagnetism in other n-type DMS oxides thin films.

Co-doped ZnO dilute magnetic semiconductor thin films by pulsed laser deposition: Excellent transmittance, low resistivity and high mobility

The 120-nm-thick Co-doped ZnO (CZO) dilute magnetic semiconductor films were grown by pulsed laser deposition at various substrate temperatures (Ts) of 100–700 °C. During the film's growth, Ar or O2 or Ar/O2-mixed gas atmosphere was used. Hall measurements indicate the CZO films prepared in Ar atmosphere possess the lower resistivity. Then, the microstructural, optical, electrical and magnetic properties of the CZO films deposited in Ar atmosphere were investigated in detail. For the deposition in Ar atmosphere, the lowest resistivity of 4.30 × 10−2 Ω-cm appeared in the 400 °C-grown CZO film since it had more oxygen vacancies with 0 charge state (VO0). As the Ts was raised to 100–300 °C, the CZO films had low mobilities of 5.1–10.7 cm2/V. This is attributed to the formation of oxygen vacancies with 2+ charge state (VO2+), resulting from the appearance of lattice distortions in these films. A significant relaxation of lattice distortion occurred in the 400 °C-grown CZO film, causing the efficient reduction in the amount of VO2+. Therefore, the 400 °C-grown film had a relatively high mobility of 21.0 cm2/V. With increasing the Ts to 400–700 °C, the transmittances (@450 nm) of 84.1%–95.8% were obtained in the CZO films. Furthermore, the saturation magnetization values of 100, 400 and 700 °C-grown CZO films were 2.74 × 10−5, 3.37 × 10−5 and 5.33 × 10−5 emu, respectively. These results suggest the CZO films are highly potential for spintronic and optoelectronic applications, especially for the 400 °C-grown film.

Ultrahigh Tunability of Room Temperature Electronic Transport and Ferromagnetism in Dilute Magnetic Semiconductor and PMN‐PT Single‐Crystal‐Based Field Effect Transistors via Electric Charge Mediation

Multiferroic heterostructures composed of complex oxide thin films and ferroelectric single crystals have aroused considerable interest due to the electrically switchable strain and charge elements of oxide films by the polarization reversal of ferroelectrics. Previous studies have demonstrated that the electric‐field‐control of physical properties of such heterostructures is exclusively due to the ferroelectric domain switching‐induced lattice strain effects. Here, the first successful integration of the hexagonal ZnO:Mn dilute magnetic semiconductor thin films with high performance (111)‐oriented perovskite Pb(Mg1/3Nb2/3)O3‐PbTiO3 (PMN‐PT) single crystals is reported, and unprecedented charge‐mediated electric‐field control of both electronic transport and ferromagnetism at room temperature for PMN‐PT single crystal‐based oxide heterostructures is realized. A significant carrier concentration‐tunability of resistance and magnetization by ≈400% and ≈257% is achieved at room temperature. The electric‐field controlled bistable resistance and ferromagnetism switching at room temperature via interfacial electric charge presents a potential strategy for designing prototype devices for information storage. The results also disclose that the relative importance of the strain effect and interfacial charge effect in oxide film/ferroelectric crystal heterostructures can be tuned by appropriately adjusting the charge carrier density of oxide films.

Effect of electron beam rapid thermal annealing on crystallographic, structural and magnetic properties of Zn 1−x Sm x O thin films

Abstract Trivalent rare earth ions (Sm 3+ ) doped ZnO dilute magnetic semiconductor thin films (Zn 1− x Sm x O, where x =0.02, 0.04 and 0.06) of different thickness are grown on silicon (100) substrates using radio frequency magnetron sputtering and post annealed in high vacuum by electron beam rapid thermal annealing (ERTA) technique. X-ray diffraction analysis indicates that the thin films have a ZnO's hexagonal wurtzite structure. The unit cell constants a, c and the cell volume increases in the as-deposited sample as ‘ x ’ increases, whereas ERTA has a reverse effect on them. Topographic analysis by atomic force microscopy on as-deposited thin films shows nonlinear change in grain size as a function of Sm concentration, whereas annealed thin films show linear change. Magnetization studies by vibrating sample magnetometer on as-deposited and annealed Zn 1− x Sm x O thin films show ferromagnetic response, due to the oxygen vacancies introduced by Sm doping. The as-deposited 100 nm Zn 0.94 Sm 0.06 O thin film is weakly ferromagnetic (24 µemu), which after annealing becomes comparatively stronger (60 µemu). This indicates that apart from higher doping concentration of Sm, ERTA plays an important role in inducing oxygen vacancies.

Synthesis and properties of Zn(Cu–Mn)O dilute magnetic semiconductor thin films by chemical spray pyrolysis technique

The Mn–Cu co-doped ZnO thin films with Zn1–2xCuxMnxO (x=0.05) composition were synthesized by precursor solution spray pyrolysis at ≤300°C in the highly crystalline c-axis oriented wurtzite structural form. X-ray diffraction studies show absence of binary oxide phases and substitutional doping of Mn and Cu in ZnO. The chemical environment of the dopant ions probed by the XPS analysis of the binding energy shifts and structure of the Cu and Mn XPS peaks show that Mn2+ and Cu2+ occupy Zn2+ sites in the ZnO lattice with Mn ions having bonding with lattice and interstitial O2− ions. The lowering of the optical band gap energy from undoped ZnO films at ∼3.22eV to 3.16eV shows doping by Mn and Cu in ZnO gives rise to band gap bowing in Zn0.90Cu0.05Mn0.05O thin films. The magnetization studies of the Zn0.90Cu0.05Mn0.05O films analyzed at 2K and 300K using squid magnetometer show ferromagnetic behavior with magnetization value of 5×10−5emu and saturation magnetization at ∼15kOe at 300K. The observance of the ferromagnetism at room temperature is attributed to the mixed valence state of Mn in ZnO akin to the Mn3O4 spinel phase and the interstitial oxygen defects.

Defects in paramagnetic Co-doped ZnO films studied by transmission electron microscopy

We study planar defects in epitaxial Co:ZnO dilute magnetic semiconductor thin films deposited on c-plane sapphire (Al2O3), as well as the Co:ZnO/Al2O3 interface, using aberration-corrected transmission electron microscopy and electron energy-loss spectroscopy. Co:ZnO samples that were deposited using pulsed laser deposition and reactive magnetron sputtering are both found to contain extrinsic stacking faults, incoherent interface structures, and compositional variations within the first 3–4 Co:ZnO layers next to the Al2O3 substrate. The stacking fault density is in the range of 1017 cm−3. We also measure the local lattice distortions around the stacking faults. It is shown that despite the relatively high density of planar defects, lattice distortions, and small compositional variation, the Co:ZnO films retain paramagnetic properties.

Role of metallic cobalt in room temperature dilute ferromagnetic semiconductor Zn0.95Co0.05O1−δ

Using two suitably designed thin film systems, the impact of metallic cobalt nanodots on the mechanism of ferromagnetism (FM) in ZnO thin films was studied. At a relatively higher oxygen partial pressure, Co nanodots embedded ZnO thin films no longer show FM, as compared to Co-doped ZnO dilute magnetic semiconductor (DMS). The structural and magnetic properties of these two systems support the hypothesis that (1) the FM of DMS thin films is due to bound magnetic polarons instead of cobalt nanoclusters and (2) the critical defect concentration is the key parameter which controls the FM properties in DMS thin films.

Structural and magnetic properties of Ni1-xFex O thin films synthesized by pulsed laser deposition

Pulsed laser deposition (PLD) method was used to fabricate Ni1-xFexO (x=0.02，0.05) dilute magnetic semiconductor thin films with room temperature ferromagnetism. For the Ni0.98Fe0.03O sample with a lower Fe content，the crystalline structures of the Ni1-xFexO films are of the NaCl type as determined by X-ray diffraction. For the Ni0.95Fe0.05O sample with a higher Fe content，some additional weak diffraction peaks corresponding to α-Fe2O3 phase can be observed. From the X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS)， it is shown that the doped Fe impurities in Ni0.98Fe0.02O are substitutionally incorporated into the NiO host and no secondary phases with room temperature ferromagnetism are found. These results show the intrinsic ferromagnetism of the Ni1-xFexO samples.

Structure and magnetic properties of Zn1-xCoxO thin film

Zn1-xCoxO (x=0.01, 0.02) dilute magnetic semiconductor thin films deposited on Si (001) substrates at 650℃ by pulsed laser deposition method were studied by X-ray absorption fine structure, X-ray diffraction and magnetic measurement. The typical ferromagnetic hysteresis curves were obtained by superconducting quantum interference device magnetometry at room temperature. The X-ray diffraction results showed that Zn1-xCoxO films were of the wurtzite structure. The X-ray absorption fine structure results revealed that the Co atoms were incorporated into the ZnO lattice and located at the substitutional Zn sites, and a homogeneous phase of Zn1-xCoxO was formed. Comparing the experimental curves with the theoretical calculation results, the additional peak C was assigned to the oxygen vacancies, which indicated that the ferromagnetism of Zn1-xCoxO films was strongly correlated with the existence of oxygen vacancies.

Structure of MnxGe1－x dilute magnetic semiconductor films

The structure of MnxGe1-x dilute magnetic semiconductor thin films prepared by magnetron co-sputtering has been studied by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) techniques. The XRD results show that in the MnxGe1-x thin film with low Mn doping concentration (x=0.070), only diffraction peaks attributed to crystalline Ge can be observed. In samples with high Mn doping concentrations (x=0.250, 0.360), the secondary phase Ge3Mn5 appears, and its content enhances with Mn doping concentration. The XAFS results indicate that for the Mn0.07Ge0.93 thin film, Mn atoms are mainly incorporated into the Ge lattice and located at the substitutional sites of Ge atoms with the ratio of 75%, while for the Mn0.25Ge0.75 and Mn0.36Ge0.64 samples, most of the Mn atoms are aggregated to form Ge3Mn5.

EXAFS Analysis of Dilute Magnetic Semiconductor Thin Films Synthesized by the Ion Beam Technique

We have synthesized dilute magnetic semiconductor (DMS) thin films of CdMnTe and ZnMnSe using the ion beam technique. High doses of Mn ions (∼2–5×1016/cm 2) were implanted into single crystal CdTe and into ZnSe epilayers on GaAs, forming subsurface layers of Cdl.xMnxTe and Znl-xMnxSe alloys, respectively, with x∼0.15–0.22. Fluorescence extended x-ray absorption fine structure (EXAFS) measurements on these materials reveal that the Mn atoms in the CdMnTe and ZnMnSe layers, both as-implanted and annealed, have local environments similar to their corresponding bulk-grown DMS alloys. While the anion-cation distances (Ra-c) in the annealed samples are equivalent to those in the corresponding bulk-grown DMS, the Ra-c in the asimplanted samples are slightly larger (∼0.01Å) than those in the bulk-grown DMS. This is most likely due to the implantation damage in the as-implanted materials. Our results on the Ra-c of the ion beam synthesized layers deviate significantly from Vegard's law, but are consistent with the bimodal distribution model. The EXAFS results are corroborated with results from ion beam analysis and Raman spectroscopy.