Evidence for replacing zinc with cobalt and iron atoms in epitaxial ZnO:CoFe-diluted magnetic semiconductors

The structure and local environment of ZnO:CoFe were investigated. An epitaxial structure of ZnO thin film doped with CoFe on Al₂O₃ substrate was confirmed by XRD. By fitting the Zn EXAFS spectra, the bond distances and coordination numbers of Zn atoms in the ZnO:CoFe show that Co and Fe ions replace some positions of zinc atoms. The XANES study of Zn and O also show that CoFe cluster formation in this system can be excluded.     

Structural and Magnetic Properties of Codoped ZnO based Diluted Magnetic Semiconductors

Zn1-xCoxO （x = 0.01, 0.02, 0.05, 0.10 and 0.20） diluted magnetic semiconductors are prepared by the sol-gel method. The structural and magnetic properties of the samples are studied using x-ray diffraction （XRD）, extended x-ray absorption fine structure （EXAFS） and superconducting quantum interference device （SQUID）. The XRD patterns does not show any signal of precipitates that are different from wurtzite type ZnO when Co content is lower than x = 0.10. An EXAFS technique for the Co K-edge has been employed to probe the local structures around Co atoms doped in ZnO powders by fluorescence mode. The simulation results for the first shell EXAFS signals indicate that Zn sites can be substituted by Co atoms when Co content is lower than x = 0.05. The SQUID results show that the samples （x 〈 0.05） exhibit clear hysteresis loops at 300K, and magnetization versus temperature from 5 K to 350K at H = 100 Oe for the sample x = 0.02 shows that the samples have ferromagnetism above room temperature. A double-exchange mechanism is proposed to explain the ferromagnetic properties of the samples.     

Absence of room temperature ferromagnetism in transition metal doped ZnO nanocrystalline powders from PAC spectroscopy

Local structural and electronic environment around ¹¹¹In probe atoms in transition metal doped Zn_1???xT_xO (T=Mn, Co, V and Ni; x = 0.01, 0.02, 0.05) and Cu co-doped Zn_1???xCo_xCu_0.01O (x = 0.01–0.04) have been monitored on an atomic scale by Perturbed Angular Correlation (PAC) spectroscopy. Single phase nanocrystalline powders were synthesized at low annealing temperatures by sol-gel method. PAC measurements exhibited the well known quadrupole interaction frequency, $\upnu_{\rm Q} =$ 31 MHz, which have been attributed to the substitutional incorporation of ¹¹¹In in ZnO matrix. PAC results did not reveal any evidence of magnetic ordering down to 77 K in pure and doped ZnO, which is consistent with the recent observation of paramagnetic behavior in transition metal doped ZnO with synchrotron based studies.     

Direct evidence for As as a Zn-site impurity in ZnO

Arsenic has been reported in the literature as one of the few p-type dopants in the technologically promising II-VI semiconductor ZnO. However, there is an ongoing debate whether the p-type character is due to As simply replacing O atoms or to the formation of more complicated defect complexes, possibly involving As on Zn sites. We have determined the lattice location of implanted As in ZnO by means of conversion-electron emission channeling from radioactive (73)As. In contrast to what one might expect from its nature as a group V element, we find that As does not occupy substitutional O sites but in its large majority substitutional Zn sites. Arsenic in ZnO (and probably also in GaN) is thus an interesting example for an impurity in a semiconductor where the major impurity lattice site is determined by atomic size and electronegativity rather than its position in the periodic system.     

Indium-defect complexes in silicon studied by perturbed angular correlation spectroscopy

The formation of molecule-like complexes, consisting of a defect and a radioactive¹¹¹In atom, is studied using the perturbed angular correlation technique (PAC). The complexes are characterized by their defect specific electric field gradients which also contain information on the geometry of the formed complexes. Whereas the complex is formed with the¹¹¹In atom, its electric field gradient is measured after the decay of the radioactive¹¹¹In atom to¹¹¹Cd. Formation and dissolution of the molecule-like complexes is pursued for a variety of different conditions, such as sample temperature, dopant concentration and position of the Fermi level. In particular, the interaction of In atoms with the following defects in Si was investigated: Intrinsic defects, created by particle irradiation; substitutional donor atoms (P, As, Sb, Bi); and interstitial impurity atoms (Li, H, and an unidentified X defect); especially, the latter ones are known to passivate acceptor atoms in Si. Methodology and specific properties of the PAC technique will be illustrated with the help of these examples.     

EXAFS analysis of Er sites in Er–O and Er–F co-doped crystalline Si

We present extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) analyses of Er–O and Er–F co-doped Si. Samples were prepared by multiple implants at 77 K of Er and co-dopant (O or F) ions resulting in the formation of a2 μm thick amorphous layer uniformly doped with 1×10¹⁹ Er/cm³ and 3×10¹⁹ O/cm³, 1×10²⁰ O/cm³ or 1×10²⁰ F/cm³. EXAFS measurements show that the local environment of the Er sites in the amorphous layers consists of 6 Si first neighbors. After epitaxial regrowth at 620°C for 3 h, Er is fully coordinated with 8 F ions in the Er–F samples, while Si and O ions are concomitantly present in the first shell of O co-doped samples. Post regrowth thermal treatments at 900°C leave the coordination unchanged in the Er+F, while the Er+O (ratio 1 : 10) doped samples present Er sites with a fully O coordinated shell with an average of 5 O atoms and 4 O atoms after 30 s and 12 h, respectively. We have also found that the fine structure and intensity of the high-resolution PL spectra are strongly dependent on the Er-impurity ratio and on thermal process parameters in the Er–O co-doped samples, while this is not observed for the F-doped samples. The most intense PL response at 15 K was obtained for the 1 : 3 E : O ratio, suggesting that an incomplete O shell around Er is particularly suitable for optical excitation.     

Localized diffusion of helium atoms around indium impurities in aluminium observed by means of perturbed angular correlations

The behaviour of He atoms implanted in ¹¹¹In doped Al has been investigated by means of perturbed angular correlation (PAC) measurements. The onset of He mobility was found to be at about 250K, probably due to vacancy-assisted migration. Mobile helium is effectively trapped at In impurities in small vacancy clusters that accomodate at most 11 He atoms. The PAC spectra taken at temperatures from about 510 K to 670 K exhibit a relaxation effect that is ascribed to hopping of He atoms from one vacancy to another, the saddle-point energy for this localized diffusion being 0.67(5) eV. A tentative model of the cluster is given.     

Migration and agglomeration of heliumatoms in copper

The magration and agglomeration of Helium atoms in He irradiated Cu was studied between 10 K and 900 K by perturbed γγ angular correlation (PAC) measurements using the radioactive probe atom¹¹¹In. Trapping of interstitial and substitutional He atoms is observed already after irradiation at 10 K. Substitutional He (He-yacancy pair) is bound in the nearest neighbourhood to the¹¹¹In probe atom and seems to be stable up to 725 K (E _He1V1 ^b ≥2.1 eV). The onset of vacancy assisted He agglomeration in Cu is observed at 250 K.     

Ferromagnetism in phosphorus-doped ZnO: First-principles calculation

The electronic structure and magnetic properties of nonmagnetic phosphorus doped ZnO are investigated using first-principles calculation. Both generalized gradient approximation (GGA) and GGA + U calculations show that each substitutional P atom in ZnO induces a magnetic moment of about 1.0 μ_B, which come mainly from the partially filled p orbitals of the substitutional P and its 12 second neighboring O atoms. The magnetic coupling between the moments induced by P doping is ferromagnetic. The calculated electronic structures indicate that the ferromagnetic coupling can be explained in terms of the two band coupling model.     

Local structures of Mn in dilute magnetic semiconductor ZnMnO

The local structures of Mn atoms in (Zn, Mn)O have been investigated by extended x-ray absorption fine structure (EXAFS). For samples with lower Mn concentrations (<11%), EXAFS results show that the majority of Mn atoms are incorporated at Zn substitutional sites. Ferromagnetic behavior has been observed in those ZnMnO samples. It is the direct evidence to suppose that the ferromagnetism in (Zn, Mn)O is not due to Mn-related secondary phases but is intrinsic. When the Mn concentration is increased to 20% or higher, the EXAFS simulation shows the formation of MnO secondary phases and the films show a change from ferromagnetic (FM) to antiferromagnetic (AFM) state. We suggest that Mn-related secondary phases might be responsible for the transformation from FM to AFM state.     

Local structure and optical absorption characteristic investigation on Fe doped TiO_2 nanoparticles

The local structures and optical absorption characteristics of Fe doped Ti O2 nanoparticles synthesized by the sol-gel method were characterized by X-ray diffraction(XRD), X-ray absorption fine structure spectroscopy(XAFS) and ultraviolet-visible absorption spectroscopy(UV-Vis). XRD patterns show that all Fe-doped Ti O2 samples have the characteristic anatase structure. Accurate Fe and Ti K-edge EXAFS analysis further reveal that all Fe atoms replace Ti atoms in the anatase lattice. The analysis of UV-Vis data shows a red shift to the visible range. According to the above results, we claim that substitutional Fe atoms lead to the formation of structural defects and new intermediate energy levels appear, narrowing the band gap and extending the optical absorption edge towards the visible region.     

The lattice location and hyperfine field of implanted Yb in Fe as a function of annealing temperature

A combined study has been made of the lattice location and hyperfine field of Yb implanted into Fe single crystals. The location has been determined by ion channeling and back-scattering, the hyperfine field by perturbed angular correlation (PAC) measurements on¹⁶⁹Yb (decaying to excited states in¹⁶⁹Tm). The channeling experiments indicate that initially about 60% of the Yb atoms occupy substitutional sites in the Fe lattice, while the remaining atoms are not in any specific crystallographic site. On annealing, Yb migrates from substitutional to non-substitutional sites. No Yb atoms remain substitutional after a 600°C anneal. By making PAC measurements at two temperatures for two - cascades in¹⁶⁹Tm, it is found that the PAC pattern can be described using a combined static and time-dependent magnetic interaction. The PAC results show that the average hyperfine field and relaxation parameter decrease on annealing, and that the field disappears after a 600°C anneal. The correlation between the location and the hyperfine field is discussed. A comparison of the results with previous Mössbauer results for¹⁵¹Gd implanted in Fe, together with relaxation parameter measurements on a¹⁶⁹Yb₂O₃ source, suggests that the non-substitutional Yb is internally oxidized in the Fe host.Work partly carried out while at the Clarendon Laboratory, Oxford, England and Nuclear Physics Division, AERE, Harwell, England.     

Hyperfine interactions of111Cd in ion-implanted face centered cubic cobalt

The magnetic-dipole and electric-quadrupole hyperfine interactions of¹¹¹Cd probes in fcc cobalt after implantation of radioactive¹¹¹In⁺ ions have been investigated by PAC measurements with fast BaF₂ detectors. Six different sites of the probe atoms could be distinguished and characterized by their hyperfine parameters and annealing behaviour. Besides the substitutional site, three sites are assigned to In-vacancy complexes which are formed athermally in the implantation process or by thermally-activated trapping of lattice defects in annealing stage III. The remaining two sites are attributed to In located in stacking faults or hcp regions of the host. Systematic trends of impurity hyperfine fields in defect sites become evident from a comparison with other impurity-host combinations. The temperature dependence of the magnetic hyperfine fields has been measured between 20 K and 390 K. Large differences found for the various sites are discussed.     

Theoretical study of structural, optical and electrical properties of zirconium-doped zinc oxide

The structural, optical and electrical properties of zirconium-doped zinc oxide have been investigated by first principle calculations. Three possible structures including substitutional Zr for Zn (Zr_Zn), interstitial Zr (Zr_i) and substitutional Zr for O (Zr_O) are considered. The results show that the formation energy of Zr_Zn defect is the lowest, which indicates that Zr_Zn defect forms easier and its concentration may be the highest in the samples. It is also found that as the proportion of Zr increases, the lattice constants increase while the optical band gap first becomes larger and then smaller, which are consistent with our recently experimental results. The electronic structure calculations display that as Zr_Zn defect is introduced into ZnO, the Fermi-level shifts to the conduction band, and there are excess electrons in the conduction band, which may be a possible reason of the good conductivity of Zr doped ZnO film.     

PAC investigations of Au(110) 1×2-surfaces

Au(110) surfaces with (1×2)-reconstruction have been investigated using perturbed -angular correlation (PAC) spectroscopy. From the two observed electric-field-gradient tensors at¹¹¹In probe atoms, deposited at room temperature in a concentration of about 10^–4 ML, the occupation of substitutional sites in the densely-packed rows along [110]-directions and ninefold coordinated sites within the (111)-oriented microfacets, respectively, can be concluded. Annealing to about 600 K leads to bulk migration of the In atoms, detected by an increase of probe atoms with cubic surroundings. Due to this behaviour the order-disorder transitions of these surfaces occurring atT _c=649 K cannot be detected in our PAC experiments.     

Investigation of Li-induced structural disorder and phase transition in ZnO by Raman spectroscopy

Zn_0.8Li_0.2O ceramics with wurtzite structure have been fabricated by a solid reaction of ZnO and Li₂CO₃. The effects of substitutional Li atoms on the crystal structure and structural phase transition of ZnO are studied by Raman spectroscopy. The enhancement of E₁(LO) mode in Zn_0.8Li_0.2O ceramics reveals the occurrence of Li-induced structural disorder. Temperature dependent Raman spectra strongly indicate that a structural phase transition occurs at about 448 K in Zn_0.8Li_0.2O ceramics.     

Internal oxidation of 5sp impurities in noble metals

The internal oxidation of ion-implanted radioactive¹¹⁹In,¹¹⁹Cd, and¹¹⁹Sb in high-purity (5N) copper, silver, and gold single crystals has been investigated. The formation of the impurity-oxygen defect structures was monitored by Mössbauer emission spectroscopy on the 24 keV-radiation from the daughter¹¹⁹Sn after the-decay of the implanted radioactive isotopes. Molecule-like complexes are formed consisting of the implanted impurity and several oxygen atoms. The various Mössbauer parameters (isomer shift, quadrupole splitting, and Debye temperature) enable a distinction between different Sn-IV and Sn-II complexes, as well as between substitutional and vacancy-associated Sn atoms, which result from the ion implantations. The evolution of oxygen complexes with temperature during isochronal annealing was studied in the temperature range 300–1000 K. The formation (around 500 K) and the disintegration (around 900 K) of the complexes, in' silver in particular, are explained on the basis of diffusion properties of oxygen. A comparison of results from oxidations due to either thermal diffusion of oxygen or due to recoil implantations from a surface-oxide layer indicates that Sn-IV complexes are invariably formed in the first case, whereas in the latter case, a mixture of Sn-IV and Sn-II complexes is formed. Relations of the present results to similar results from PAC measurements on¹¹¹Cd in silver and from Mössbauer experiments on AgSn alloys are discussed.     

Local structure investigation of (Co,Cu) co-doped ZnO nanocrystals and its correlation with magnetic properties

Pure, Co doped and (Co, Cu) co-doped ZnO nanocrystals have been prepared by wet chemical route at room temperature to investigate the effect of Cu doping in Co doped ZnO nanocrystals . The nanocrystals have initially been characterized by X-ray diffraction, FTIR, Raman, optical absorption and EPR spectroscopy and the results were corroborated with DFT based electronic structure calculations. Magnetic properties of the samples have been investigated by studying their magnetic hysteresis behavior and temperature dependence of susceptibilities. Finally the local structure at the host and dopant sites of the nanocrystals have been investigated by Zn, Co and Cu K edges EXAFS measurements with synchrotron radiation to explain their experimentally observed magnetic properties.     

Defect states in Lu 3 Al 5 O 12 :Ce crystals

Wavelength-resolved TSL measurements after x-ray irradiation at room temperature were performed on undoped, Ce- and (Ce, Zr)-doped Lu 3 Al 5 O 12 crystals. Several glow peaks were observed in the 20-450 v C temperature region, whose intensities and emission spectra are influenced by doping. Namely, the TSL spectra are governed by defect and trace impurity-related emissions in the undoped crystal, while only the Ce 3+ emission is detected in the doped crystals. Moreover, EPR measurements performed at 12 v K revealed typical spectra of Ce 3+ (4f 1 , S =1/2) occupying Lu 3+ substitutional positions. In situ light irradiation (250-330 v nm) of the crystals results in a step-like decrease of the Ce 3+ EPR intensity. Such a behaviour can reflect a creation of Ce 4+ ions under UV irradiation or an increase of the Ce 3+ population in the excited (5d) state.     

Tetrahedral versus octahedral Mn site coordination in wurtzite and rocksalt Zn1−xMnxO investigated by means of XAS experiments under high pressure

We present the results of x-ray absorption measurements carried out in Zn_1−xMn_xO thin films under high pressure. The Mn environment remains essentially the same for nominal Mn concentrations given by x=0.05, 0.1, 0.15 and 0.25. Both the XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) indicate that Mn occupies the Zn site, being surrounded by four oxygen atoms at 2.02±0.01 Å. The substitutional hypothesis is reinforced by comparing the differences between the ambient (wurtzite) and high pressure (rocksalt) spectra, which correspond to tetrahedral and octahedral Mn environments.