Perturbed Γγ Angular Correlation Spectroscopy Research Articles

Preparation of In-doped Y2O3 ceramics through a sol-gel process: Effects on the structural and electronic properties

The Pechini-type sol-gel (PSG) process has been used for the preparation of doped oxides due to its capability to overcome most of the difficulties that frequently occur by using other producing methods. In this work we analyze the case of samples of pure and In-doped yttria (Y2O3) prepared by the PSG process. We experimentally characterize the synthesized samples by x-ray diffraction, micro-Raman spectroscopy, electrochemical impedance spectroscopy (EIS), and time-differential perturbed γ-γ angular correlation (PAC) spectroscopy, and we compare these results with those obtained starting from commercial oxide powders. We found that the PSG process can be used to successfully produce doped yttria in the cubic phase, with the impurities substitutionally located at the cationic sites of the structure. By the proposed PSG route, the inclusion of impurities does not affect the particle size nor the resistivity. However, when we compare the PSG samples with other samples produced from commercial powder, we found that the first have lower resistivities at grain interiors. On the other hand, PAC spectroscopy in 111In(→111Cd)-doped yttria allows the study of the dynamic hyperfine interactions observed by the radioactive 111Cd impurity-probe, which can be used to “sense” the host electron availability near the impurities after the electron-capture decay of 111In. Differences between PAC spectra for PSG samples and the commercial powder suggest that the PSG process introduces additional donor defects into the yttria electronic structure, which is consistent with the lower resistivity observed in the PSG samples by EIS spectroscopy.

Electric field gradient in nanostructured SnO2 studied by means of PAC spectroscopy using 111Cd or 181Ta as probe nuclei

Electric quadrupole interactions were studied in pure and Mn-doped powder samples and thin films of SnO2 using perturbed γγ angular correlation spectroscopy (PAC). The powder samples were prepared by Sol gel method and the thin film were prepared on the Si (100) substrate by sputtering technique using Sn in the oxygen atmosphere. The samples were characterized by x-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The thickness of the film was 100 nm. The average particle size of the SnO2 powder samples was determined to be smaller than 60 nm. The radioactive 111In and 181Hf tracers were introduced in the powder samples during the sol gel chemical process. Radioactive 111In was implanted on the SnO2 thin films using the University of Bonn ion implanter (BONIS). PAC measurements were carried out in a four BaF2 detector spectrometer in the temperature range of 77–973 K for samples annealed at different temperatures. The PAC results for both nuclear probes show the presence of two electric quadrupole interactions. The major fractions in both cases correspond to the substitutional sites in the rutile phase of SnO2. The results are compared with previous PAC measurements.

Structural properties of the donor indium in nanocrystalline ZnO

The structural properties of the nanocrystalline semiconductor ZnO (nano-ZnO) doped with the donor Indium were investigated by perturbed γγ angular correlation spectroscopy (PAC) and extended X-ray absorption fine structure measurements (EXAFS). Up to an average concentration of one In atom per nanocrystallite, PAC measurements show that about 12% of the 111In atoms are incorporated on substitutional Zn sites. At higher In concentrations, new In defect complexes are visible in the PAC spectra, which dominate the spectra if the average In concentration exceeds one In atoms per nanocrystallite. In addition, the local environment of Zn and In atoms in In doped nano-ZnO was investigated by EXAFS. The measurements at the K edge of Zn show that the crystal structure of nano-ZnO corresponds to bulk ZnO. In heavily In doped nano-ZnO the EXAFS experiments at the K edge of In exhibit an expansion of the first O shell about the In site. Since about four O atoms are detected in this first shell a substitutional incorporation of the In atoms in the ZnO lattice is suggested. The second shell to be occupied by Zn atoms as well as higher shells are almost invisible, which might have the same microscopic origin as the occurrence of defect complexes observed by PAC.

Investigation of mechanical alloying of Ti–Al compounds using perturbed γγ-angular correlation spectroscopy, x-ray diffraction, and differential scanning calorimetry

Ti0.50Al0.50 and Ti0.75Al0.25 compounds were mechanically alloyed by ball milling of elemental Ti and Al powders. Radioactive 111In atoms incorporated into these compounds were used to investigate the different locally ordered crystalline structures by perturbed γγ-angular correlation spectroscopy (PAC). The formation of the intermetallic compounds γ–TiAl and α2–Ti3Al was observed on an atomic scale and occurred as a consequence of the heat treatment of mechanically alloyed Ti0.50Al0.50 and Ti0.75Al0.25, respectively. Due to the sensitivity of PAC to local order on an atomic scale, information about formation conditions and thermal stability of a new metastable phase with an ordered tetragonal crystal structure is presented for Ti0.50Al0.50 samples. In addition, the formation of the ordered phase Ti2AlN was observed, indicating the incorporation of N during the milling process. The PAC investigations were complemented by x-ray diffraction and differential scanning calorimetry measurements.

Formation of 117In–H pairs after annealing of GaN implanted with 117Cd( 117In)

We report on the annealing behaviour of GaN grown on sapphire after implantation of 117Cd( 117In). Isochronic annealing has been performed in order to remove the implantation-induced lattice damage in evacuated quartz ampoules with additional Al. The extent of the lattice damage is investigated with perturbed γγ-angular correlation spectroscopy (PAC). The implantation damage is annealed at temperatures above 1073 K to a large extent. 82(5)% of the probes are found on the Ga-site characterized by a quadrupole coupling constant of 20.9(1) MHz. After H loading, an additional interaction appears which is attributed to 117Cd( 117In)–H pairs.

Magnetic Properties of Grain Boundaries of Nanocrystalline Ni and of Ni Precipitates in Nanocrystalline NiCu Alloys

Perturbed γγ-angular correlation spectroscopy (PAC) was used to investigate nanocrystalline Ni and NiCu alloys, which are prepared by pulsed electrodeposition (PED). Using diffusion for doping nanocrystalline Ni with 111In four different ordered grain boundary structures are observed, which are characterized by unique electric field gradients. The incorporation of 111In on substitutional bulk sites of Ni is caused by moving grain boundaries below 1000 K and by volume diffusion above 1000 K. The nanocrystalline NiCu alloys prepared by PED are microscopically inhomogeneous as observed by PAC. In contrast, this inhomogeneity cannot be detected by X-ray diffraction. The influence of the temperature of the electrolyte, the current density during deposition, and the optional addition of saccharin to the electrolyte on the homogeneity of nanocrystalline NiCu alloys was investigated.

Ion implanted dopants in GaN and AlN: Lattice sites, annealing behavior, and defect recovery

The recovery of structural defects in gallium nitride (GaN) and aluminum nitride (AlN) after implantation of In+111 and Sr+89 in the dose range (0.1–3) 1013 cm−2 and ion energies of 60–400 keV has been investigated as a function of annealing temperature with emission channeling (EC) and perturbed γγ angular correlation spectroscopy. The implanted In and Sr atoms occupied substitutional sites in heavily perturbed surroundings of point defects after room temperature implantation. No amorphization of the lattice structure was observed. The point defects could be partly removed after annealing to 1473 K for 10–30 min. Lattice site occupation of implanted light alkalis, Na+24 in GaN and AlN as well as Li+8 in AlN, were also determined by EC as a function of implantation and annealing temperature. These atoms occupied mainly interstitial sites at room temperature. Lithium diffusion and the occupation of substitutional sites was observed in GaN and AlN at implantation temperatures above 700 K. A lattice site change was also observed for sodium in AlN, but not in GaN after annealing to 1073 K for 10 min.

Response of the Electric Field Gradient in Ion implanted BaTiO3 to an External Electric Field

AbstractSingle crystalline, ferroelectric BaTiO3 as material with the highest piezoelectric constants among the perovskites with ordered sublattices was implanted with 111In(111Cd). The electric field gradient at the Ti position was measured with perturbed γγ-angular correlation spectroscopy (PAC) while the crystal was exposed to an external electric field. A quadratic dependence could be observed: νQ(E) = (34.8(1) + 0.16(4) E/kV/mm + 0.080(2) E2/kV2/mm2) MHz. Point charge model calculations reproduce the linear change of Vzz, but not the quadratic term. The polarizability of the host ions of BaTiO3 is known to be nonlinear with respect to an electric field. The resulting quadratic shift of the electron density is reflected in the strength of the EFG.

Annealing of ion-implanted GaN

111mCd and 112Cd ions have been implanted into GaN. With photoluminescence spectroscopy and perturbed γγ angular correlation spectroscopy (PAC) the reduction of implantation damage and the optical activation of the implants have been observed as a function of annealing temperature using different annealing methods. The use of N2 or NH3 atmosphere during annealing allows temperatures up to 1323 and 1373K, respectively, but above 1200K a strong loss of Cd from the GaN has been observed. Annealing GaN together with elementary Al forms a protective layer on the GaN surface allowing annealing temperatures up to 1570K for 10min.

Oxygen dynamics in epitaxial YBa 2 Cu 3 O 7-δ thin films

We have investigated epitaxial YBCO films by Perturbed γγ-Angular Correlation Spectroscopy (PAC) using 111In probe atoms. The probes substitute on the yttrium lattice site and hyperfine parameters have been determined unambiguously. Excellent agreement is achieved with full potential electric field gradient (efg) calculations. Good reproducibility has been achieved by our preparation method allowing us to study the influence of oxygen motion on the structural orthorhombic to tetragonal phase transition.

Local magnetism of ultrathin fcc-Co films on Cu(100) investigated by 111In probes

Structural aspects and magnetic properties of ultrathin fcc-Co films epitaxially grown on Cu(100) have been investigated. Insight into the local structure and magnetism has been gained through perturbed γγ-angular correlation spectroscopy (PAC) using radioactive 111In probes. In 2-monolayer-thick Co films, a strongly reduced magnetic hyperfine field of B hf = 5 T compared with the bulk value ( B hf(bulk) = 23.6 T) is observed, which is indicative for an outward shift of the 111In from the substitutional surface site. Diffusion of Cu atoms from the Cu substrate on to the Co film occurs at elevated temperatures and leads to the formation of mixed CoCu environments at the 111In probes.

Radioactive probe atoms in semiconductors

Progress in semiconductor technology is accompanied by progress in knowledge and control of intrinsic and extrinsic defects in these materials, i.e. of vacancies or self-interstitials and dopant or impurity atoms, respectively. In spite of the large number of different experimental techniques, in particular electrical and optical ones, employed for the characterisation of these defects, there is a shortage of techniques that are able to identify the chemical nature, local structure, and dynamic properties of defects on an atomic scale. The missing sensitivity towards low defect concentrations in the range of 1016 cm−3 excludes many techniques which are known to fulfil these requirements. There is a growing number of experimental techniques, however, having in common the application of radioactive isotopes, so-called probe atoms, that deliver the required information also at low defect concentrations. Among the nuclear techniques, which will be discussed in context of their applicability to semiconductors, are the perturbed γγ angular correlation spectroscopy, Mossbauer spectroscopy, andβ-nuclear magnetic resonance. But also non-nuclear techniques, such as the classical electrical and optical methods, being highly sensitive to low defect concentrations, will be shown to gain the missing sensitivity to the nature of defects if radioactive atoms are employed. For elemental semiconductors as well as for III–V, II–VI, and IV-IV compounds, it will be illustrated to what extent radioactive probe atoms contribute to delivering the needed knowledge and control of defects.

Atomic configurations of group V acceptors in ZnSe, ZnTe, and CdTe

The formation of donor–acceptor pairs was detected by perturbed γγ angular correlation spectroscopy. In ZnSe, ZnTe, and CdTe crystals doped with the donor 111In and one of the acceptors N, P, As, or Sb the strength of the electric field gradient shows a systematic correlation of the bond length between the respective acceptor and the neighboring cation. For N acceptors, an inward relaxation of the neighboring cations is concluded to occur.

First Microscopic Observation of Cadmium-Hydrogen Pairs in GaN

ABSTRACTThe formation and properties of acceptor-hydrogen pairs in GaN have been studied using radioactive 111mCd acceptors and the perturbed γγ angular correlation spectroscopy (PAC). After H-loading by low energy implantation (100 eV) at temperatures between 295 K and 473 K, the formation of two Cd-H complexes involving about 30% of the Cd-acceptors is observed. The complexes have been identified as single hydrogen atoms bound to the Cd acceptor in two different configurations. The dissociation enthalpies of these configurations have been determined as 1.1(1) eV and 1.8(1) eV, respectively.

Microscopic investigations of the nanocomposite WGa

The nanocomposite WGa was investigated by perturbed γγ-angular correlation spectroscopy (PAC) using the probe atom 111In. The WGa samples were prepared by inert gas condensation; the Ga content ranged between 10 at.% and 50 at.%. The radioactive 111In atoms were diffused into the WGa samples at temperatures between 600 K and 740 K. Two distinct locations of the 111In probe atoms were observed. One was identified as a substitutional site within precipitated α-Ga, for which a depression of the melting temperature was observed. The other was due to specific properties of the composite WGa and is stable up to temperatures of at least 500 K.

Destabilization of egg lecithin liposomes on the skin after topical application measured by perturbed γγ angular correlation spectroscopy (PAC) with 111In

Unilamellar and oligolamellar liposomes were topically applied to the skin of porcine ears. Perturbed angular correlation spectroscopy of 111In-DTPA, initially entrapped into the liposomes, was used to study the release kinetics of the hydrophilic probe. After drying of the liposomal suspensions, a decrease of the time-integrated perturbation factor 〈 G 22(∞)〉 within 30 min indicated destabilization of the liposomes and total marker release in the upper epidermal layers.

Feasibilities of nuclear techniques for the study of molecular defects in metals and semiconductors

Nuclear techniques, like Mossbauer effect and perturbed γγ angular correlation spectroscopy (PAC), have proven themselves to be sensitive tools for labelling and identifying probe atom-defect complexes. In these experiments, the “molecular defect” is investigated via the nuclear hyperfine interaction, which is measured at the site of the radioactive probe atom. Here, we shall put the emphasis on the PAC spectroscopy, which often uses111In/111Cd as radioactive probe atom. In metals, based on the identification of simple probe atom-defect pairs, the agglomeration of defects after cold-working and the interaction of vacancies with He atoms will be discussed. In semiconductors, it will be focussed on the interaction between dopant atoms, which strongly determines the electrical properties of these materials.