Mn-doped ZnO Thin Films Research Articles

Influence of annealing temperature on structural, optical and magnetic properties of Mn-doped ZnO thin films prepared by sol–gel method

Thin films of Zn 1− x Mn x O ( x=0.01) diluted magnetic semiconductor were prepared on Si (1 0 0) substrates by the sol–gel method. The influence of annealing temperature on the structural, optical and magnetic properties was studied by X-ray diffraction (XRD), atom force microscopy (AFM), photoluminescence (PL) and SQUID magnetometer (MPMS, Quantum Design). The XRD spectrum shows that all the films are single crystalline with (0 0 2) preferential orientation along c-axis, indicating there are not any secondary phases. The atomic force microscopy images show the surfaces morphologies change greatly with an increase in annealing temperature. PL spectra reveal that the films marginally shift the near band-edge (NBE) position due to stress. The magnetic measurements of the films using SQUID clearly indicate the room temperature ferromagnetic behavior, and the Curie temperature of the samples is above room temperature. X-ray photoelectron spectroscopy (XPS) patterns suggest that Mn 2+ ions were successfully incorporated into the lattice position of Zn 2+ ions in ZnO host. It is also found that the post-annealing treatment can affect the ferromagnetic behavior of the films effectively.

Ferromagnetism above room temperature in Mn-doped ZnO thin films

Ferromagnetism has been investigated up to 500 K in Mn-doped ZnO thin films grown by pulsed laser deposition starting from targets of different Mn atomic concentrations (C Mn) (4–10 at.%) and varying the oxygen pressure and the substrate temperature. The films with the largest saturation magnetization ( M s ) (0.36–1 emu/g) and coercive field ( H c ) (80–200 Oe) were grown at O 2 pressure of 2×10 −3 mbar and at substrate temperature ranging from 480 ∘C to 600 ∘C, depending on the Mn content. At room temperature M s decreases with increasing the Mn content, whereas H c increases. Magnetic measurements, performed at 500 K ( M s ≈ 0.3 emu / g ) indicate a Curie temperature above 500 K. The observed high temperature ferromagnetism is attributed to the defects in the base materials, intrinsic or induced by the Mn dopant atoms.

Magnetic Properties in Co and Mn-Doped ZnO Powders and Thin Films

Zn0.86Co0.14O powder and thin films were prepared by standard solid-state reaction processes and radio-frequency (RF) magnetron sputtering. Magnetic measurements indicate that the powder is paramagnetic for temperatures above 3 K, while the thin films annealed in vacuum are ferromagnetic at room temperature. The saturated magnetization was found to be about 0.6 microB/Co, while the coercive force was found to be 200 Oe at room temperature. The very similar results were also obtained in Zn0.96Mn0.04O powder and thin films. Such different results for the powder and thin films indicate that growth conditions and defects play an important role in producing ferromagnetism.

On the defect induced ferromagnetic ordering above room-temperature in undoped and Mn doped ZnO thin films

AbstractEvidence for long range ferromagnetic order above room-temperature, RTFM, in pristine ZnO, In2O3, TiO2 nanoparticles and thin films, containing no nominal magnetic elements have been reported recently. This could question the origin of RTFM in doped dilute alloys if for example the ZnO matrix itself develops a defect induced magnetic order with a significant moment per unit cell. In this presentation we report a systematic study of the film thickness dependence of RTFM in pure ZnO deposited by DC Magnetron Sputtering. We observe a maximum in the saturation magnetization, MS, value of 0.62 emu/g (0,018 μB/unit cell), for a ˜480 nm film deposited in an oxygen ambience of appropriate pressure. Above a thickness of around 1 μm the films are diamagnetic as expected. We thus see a sequential transition from ferromagnetism to para- and eventual diamagnetism as a function of film thickness in ZnO. We also find that in such a ZnO matrix with a maximum intrinsic defect induced moment, on doping with Mn the maximum enhanced MS value of 0.78 emu/g is obtained for 1at.% Mn doping. With this approach of appropriate doping in a defect tailored matrix, we routinely obtain RTFM in both undoped and Mn- doped ZnO thin films.

Activation of room-temperature ferromagnetism in Mn doped ZnO thin films by N codoping

Mn doped ZnO films with and without N codoping have been fabricated by oxidative annealing of sputtered Zn:Mn and Zn2N3:Mn films on silicon substrates in flowing O2 ambient. It was found that the ZnO:Mn films show very weak ferromagnetic behavior, while for those with N codoping, significant ferromagnetism with a moderate saturation magnetization of about 0.23—0.61 μB per Mn2+ ion was observed at room temperature. It suggests that significant ferromagnetism in ZnO:Mn films could be activated by N codoping. The results indicate that holes are favorable for ferromagnetic ordering of the doped Mn2+ ions in ZnO, in agreement with the recent theoretical studies.

Suppression of conductivity in Mn-doped ZnO thin films

We studied the dopant concentration distribution and conductivity in ZnO:Mn films grown by metalorganic chemical vapor deposition. The ion beam, surface, and microstructural properties of undoped ZnO films were compared with Mn-doped ZnO films. Suppression of ZnO conductivity was observed for Mn doping up to ∼4.5 at. %. The presence of Mn2+, confirmed by x-ray photoelectron spectroscopy, is correlated with the reduction in conductivity. Variable-temperature Hall effect measurements yield an activation energy of 170 meV, consistent with deep donors in the bulk or at the interface. The results suggest that the incorporation of substitutional Mn suppresses the formation of native defects such as oxygen vacancies.

Effect of Doping Concentration on Zn1-XMnxO Thin Films Grown by RF Magnetron Sputtering

We have investigated the effect of doping concentration on structural, optical and magnetic properties of Mn-doped ZnO thin films deposited on Si (100) substrate by RF magnetron sputtering. The films have been characterized by X-ray diffraction (XRD), photoluminescence (PL) and superconducting quantum interference device (SQUID) magnetometry. It is observed from XRD, that the increase of Mn content increases the FWHM which indicates the degradation of crystalline quality. The photoluminescence spectrum reveals that the incorporation of Mn ions suppresses the deep level emissions considerably in comparison to those observed in pure ZnO. The near band edge (NBE) emission of Mn-doped films is shifted to the lower energy side (red shift) in comparison to pure ZnO film. The room temperature SQUID magnetometer results reveal that all the films show paramagnetic behaviour due to the lack of interactions among Mn moments.

STUDY OF THIN FILMS AND AQUEOUS CHLORINE SOLUTION USING X-RAY ABSORPTION FINE STRUCTURE SPECTROSCOPY

The capability of thin film measurement and performance at low energy using XAFS at the X-ray demonstration and development beamline (XDD) at SSLS has been explored. XDD is supplied with synchrotron radiation from one of the two superconducting dipoles of the Helios 2 storage ring. The photon energy is tunable from 2.5 keV to 10 keV using a channel-cut Si(111) monochromator and the photon flux is of the order of 109 photons/s at energies of several keV. The energy resolution, detector, and control system required by XAFS were optimized. As a demonstration, the XAFS spectra of Mn-doped ZnO thin films at Mn K-edge and chlorine aqueous solution at Cl K-edge were measured and discussed.

Investigation on Mn doped ZnO thin films grown by RF magnetron sputtering

In this paper, we have investigated the Zn 1 − x Mn x O ( x = 0.05, 0.10 and 0.15) thin films grown by RF magnetron sputtering. The grown films on sapphire [Al 2O 3(0001)] substrates have been characterized using X-ray Diffraction (XRD), Photoluminescence (PL) and Vibrating Sample Magnetometer (VSM) in order to investigate the structural, optical and magnetic properties of the films respectively. It is observed from XRD that all the films are single crystalline with (002) preferential orientation along c-axis. PL spectra reveal that the addition of Mn marginally shifts the Near Band Edge (NBE) position towards the higher energy side. The magnetic measurements of the films using VSM clearly indicate the ferromagnetic nature.

Manganese behaviour in ZnO layers deposited by magnetron sputtering for spintronic application

AbstractIn this work, we carry out structural analysis of ferromagnetic Mn‐doped ZnO thin films deposited by radio frequency magnetron sputtering, using transmission electron microscopy (TEM). On top of sapphire (0001) substrates, Mn rich precipitates and an interface reaction layer are observed follow‐ ing the deposition of Zn(Mn)O layers above 500 °C. The crystalline quality of ZnO layers deposited by magnetron sputtering is highly improved at 500 °C as well as the measured ferromagnetic response. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray absorption and magnetic circular dichroism characterizations of Mn doped ZnO

We report the near-edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments on well characterized Mn doped ZnO thin films that show ferromagnetism at room temperature. The NEXAFS measurements at O K edge clearly exhibit a preedge spectral feature which evolves with Mn doping, similar to one observed in hole-doped cuprates and manganites. The Mn L3,2 edge NEXAFS spectra exhibit divalent Mn apart from mixed valent Mn3+∕Mn4+ states. The spectral features of XMCD at Mn L3,2 edge demonstrate that ferromagnetism comes from Mn2+ ions and its dichroism shape is independent of Mn concentration.

Defect induced activation of Raman silent modes in rf co-sputtered Mn doped ZnO thin films

We study the influence of Mn doping on the vibrational properties of rf sputtered ZnO thin films. Raman spectra of the Mn doped ZnO samples reveal two additional vibrational modes, in addition to the host phonon modes, at 252 and 524 cm−1. The intensity of the additional modes increases continuously with Mn concentration in ZnO and can be used as an indication of Mn incorporation in ZnO. The modes are assigned to the activation of ZnO silent modes due to relaxation of Raman selection rules produced by the breakdown of the translational symmetry of the crystal lattice with the incorporation of Mn at the Zn site. Furthermore, the A1 (LO) mode is observed with very high intensity in the Raman spectra of undoped ZnO thin film and is attributed to the built-in electric field at the grain boundaries.

Magnetization dependence on carrier doping in epitaxial ZnO thin films co-doped with Mn and P

The magnetic and transport properties of Mn-doped ZnO thin films co-doped with P are examined. Superconducting quantum interference device magnetometry measurements indicate that the films are ferromagnetic with an inverse correlation between magnetization and electron density as controlled by P doping. In particular, under conditions where the acceptor dopants are activated leading to a decrease in free-electron density, magnetization is enhanced. The result is consistent with hole-mediated ferromagnetism in Mn-doped ZnO, in which bound acceptors mediate the ferromagnetic ordering. Increasing the electron density decreases the acceptor concentration, thus quenching the ferromagnetic exchange. This result is important in understanding ferromagnetism in transition metal doped semiconductors for spintronic devices.

Enhancement of thermopower of Mn doped ZnO thin film

We have studied the thermopower of Mn doped ZnO thin films experimentally for various Mn concentrations in the films and a theoretical attempt was taken to gain an insight into the origin of the enhancement of thermopower due to unbalancing of up spin and down spin electrons in the conduction band due to unfilled d-orbital of Mn. Also hopping contribution caused by partially filled d-orbital of Mn was taken into account.

Magnetic properties of Mn-doped ZnO powder and thin films

Zn 0.96Mn 0.04O powder and thin films were prepared by standard solid-state reaction processes and radio-frequency (RF) magnetron sputtering. Magnetic measurements indicate that the powder is paramagnetic for temperatures above 3 K, while the thin films annealed in vacuum are ferromagnetic at room temperature with a transition temperature of about 400 K. The largest saturated magnetization ( M s) was found to be about 1.05 μ B/Mn, while the coercive force was found to be 100 Oe at room temperature. The very different results for the powder and thin films indicate that growth conditions and defects play an important role in producing ferromagnetism.

Structure and magnetic properties of Mn-doped ZnO thin films

We report the structure and magnetic properties ofZn1−xMnxO thin films grown on Si(001) substrates by pulsed laser deposition. Structure and phaseevolution with Mn doping has been studied using x-ray diffraction, electron diffraction, andhigh-resolution electron microscopy. The undoped and 1% Mn-doped ZnO films arecompletely (001) oriented, and further Mn doping deteriorates the (001) orientation. ForMn concentrations below 3%, only the hexagonal ZnO phase exists in the filmswithout secondary phases. As the Mn concentration reaches 5%, secondary phaseMn2O3 was found aggregating at grain boundaries. All the Mn-doped films show ferromagneticproperties at room temperature, and the magnetic moment decreases as the Mnconcentration increases. Our results suggest that the ferromagnetism observed inZn1−xMnxO thin films is intrinsic rather than associated with secondary phases.

The Fine Structure of the Mn Distribution in ZnO Layers Deposited by Magnetron Sputtering for Spintronic Application

AbstractIn this work, we carry out structural analysis of ferromagnetic Mn-doped ZnO thin films deposited by radio frequency magnetron sputtering, using transmission electron microscopy (TEM) and high resolution x-ray diffraction. On top of sapphire (0001) substrates, Mn rich precipitates and an interface reaction layer are observed following the deposition of Zn(Mn)O layers above 500°C. The crystalline quality of ZnO layers deposited by magnetron sputtering is highly improved at 500°C as well as the measured ferromagnetic response.

Structural study of Mn-doped ZnO films by TEM

The structural study of diluted magnetic semiconductors is important for interpreting the ferromagnetic behavior associated with the materials. In the present work, a series of low concentration Mn-doped ZnO thin films synthesized by pulsed laser deposition was studied by electron microscopy. All films show the wurtzite structure with (001) preferred growth orientation on the Si substrate. Electron diffraction experiments indicate the deterioration of the growth orientation in some areas of the films with increasing Mn concentration, and the existence of a secondary phase, of Mn 2O 3-type, in the films with larger Mn concentrations. High-resolution electron microscopy images confirm the existence of the secondary phase in the grain boundary of the Mn-doped ZnO phase. The magnetic properties of Mn-doped ZnO are discussed in relation to the structures of the films.

TEM Study of Mn-Doped ZnO Thin Films Synthesized by PLD

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Dc and ac transport properties of Mn-doped ZnO thin films grown by pulsed laser ablation

Mn x Zn 1− x O ( x = 0.20) thin films were deposited on Pt coated Si substrates using pulsed laser ablation technique. The structural characteristics of the films were investigated by X-ray diffraction (XRD), while the dielectric response of the films was studied as a function of frequency and ambient temperature by employing impedance spectroscopy. It was found that all the films deposited on Pt coated Si substrates had c-axis preferred orientation perpendicular to the substrate, with full width at half maximum (FWHM) of the (0 0 2) X-ray reflection line being less than 0.5°. The dc and ac electrical conductivity of Mn-doped ZnO films were investigated as a function of temperature. The ac conductivity, σ ac( ω), varies as σ ac( ω) = Aω s with s in the range 0.4–0.9. The complex impedance plot showed data points lying on a single semicircle, implying the response originated from a single capacitive element corresponding to the bulk grains. The value of the activation energy computed from the Arrhenius plot of both dc and ac conductivities with 1000/ T were 0.2 eV suggesting hopping conduction mechanism. The optical properties of Zn 0.8Mn 0.2O thin films were studied in the wavelength range 300–900 nm. The data were analyzed in the light of the existing theories and reflected a Burstein–Moss shift in these films. The films show magnetic properties, which are best described by a Curie–Weiss type behavior.