Defects in CdS: In detected by perturbed angular correlation spectroscopy (PAC)

The local lattice environment of the donor In in CdS is investigated measuring the electric-field gradient at the site of the radioactive probe atom ¹¹¹In by the perturbed γγ angular correlation technique. It is shown that implantation of In into CdS with subsequent annealing drives 100% of the In atoms to Cd lattice sites. Diffusion of In into CdS under S overpressure results in the formation of In_Cd-V_Cd pairs which seem to be responsible for the self-compensation of In donors in CdS.     

Calculated perturbed angular correlations for111In doped cubic single crystals

The perturbed angular correlation functions pertaining to the quadrupolar hyperfine interaction of¹¹¹In activity in cubic single crystals were calculated for various low-index crystallographic orientations of the principle axes of the electric field gradient (efg). The asymmetry of the efg was varied. The capacity and limitations of the method are discussed, especially for the case of non-axial efg.     

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.     

Ionic and electronic transport in In2S3 studied via perturbed angular correlation spectroscopy

We report on Perturbed Angular Correlation measurements in polycrystalline In₂S₃ samples in the temperature range from 8 K to 1000 K where two different crystallographic phases β and α occur. As probes, implanted ¹¹¹In nuclei have been used. The three observed EFGs are attributed to probes residing substitutionally in the different sulfur-octahedra and -tetrahedra of β-In₂S₃. A strong damping between 150 K and 300 K has been attributed to EFG fluctuations following the ¹¹¹In(EC)¹¹¹Cd decay. The α-phase (above 680 K) is characterized by a different dynamical damping of the perturbation functions, caused by mobile In atoms. Therefore, the semiconductor In₂S₃ shows, in two different temperature ranges, dynamical PAC-spectra which correspond to different types of mobile charge carriers. Since ¹¹¹In is a self atom in In₂S₃, this compound is an ideal substance to study the charge transport phenomena by the PAC technique. This revised version was published online in August 2006 with corrections to the Cover Date.     

Monitoring positional changes in bixbyite oxides with perturbed angular correlation spectroscopy

The refinement of atomic positions with perturbed angular correlation spectroscopy for several isostructural sesquioxides M₂O₃ (M=Sc, In, Y, Yb, Dy, Ho, Gd, Sm) is presented. The experimental electric field gradients of¹¹¹Cd residing at the two substitutional sites were reproduced by the simple point charge model with an accuracy of less than 1% by varying the coordinates (available from X-ray and neutron diffraction experiments) within their experimental uncertainty. In this way, for each oxide coordinates were obtained having an accuracy of 0.3 pm, which is typically five times better than that of available X-ray and neutron diffraction results.     

Comments on time-differential perturbed angular correlation functions for static axial electric quadrupole interaction

A technique of computer aided derivation of analytical formulas has been applied to perturbed- angular correlation functions in case of the static axial electric quadrupole interaction in a single crystal. An arbitrary orientation of the EFG symmetry axis with respect to the detectors was considered. Explicit formulas for correlation functions in commonly used detector configurations= and=/2 have been obtained. Small corrections to the previously published formulas have been suggested.     

Intrinsic and extrinsic defects in semiconductors studied by perturbed γγ angular correlation spectroscopy

The application of the perturbed γγ angular correlation technique as an analytical tool for the characterisation of atomic defect configurations is discussed. Using the radioactive probe atom¹¹¹In/¹¹¹Cd, recent results on the compensation of acceptor and donor atoms in different II–VI semiconductors will be discussed, in particular the role of the cation vacancy defect.     

In situ studies of solid state reactions by perturbed angular correlation

We report on three examples of in situ studies of solid state reactions by time differential perturbed angular correlation of¹⁸¹Ta: (i) the oxidation of hafnium metal and the doping of ZrS₂ with Hf during iodine vapour transport crystal growth; (ii) the observation of sublattice melting during polymorphic phase transitions in TaS₂; (iii) the electrointercalation of 2H-TaS₂ with silver.     

Data reduction methodology for perturbed angular correlation experiments

Data reduction schemes currently used for time differential perturbed angular correlation measurements are evaluated in terms of (i) their relative effectiveness in eliminating irrelevant variables, including single counter efficiencies and spectrum time shifts; and (ii) their effectiveness in putting data in a form that can easily be fitted by theoretical correlation functions. It is pointed out that erroneous conclusions may be derived from improperly reduced data, but that properly analyzed experiments performed at four angles allow a good determination of both the time-dependent and time-independent parts of the correlation function. Correction of data for accidentals, source self-absorption and spectrum livetime differences are discussed.     

Determination of the crystal structures of In70-Ni30 and In70-Pd30 using perturbed angular correlation

According to phase diagrams based on x-ray measurements, In70-Pt30 has the cubic Sn₇Ir₃ crystal structure (D8_f, cI40) but the alloys In70-Ni30 and In70-Pd30 have been variously reported to have either a cubic gamma-brass (D8_1???3, cI52) or the Sn₇Ir₃ structures. In this study, hyperfine interaction measurements are applied as an alternate method to identify phases. Perturbed angular correlation (PAC) measurements were made of characteristic nuclear quadrupole interactions of ¹¹¹In/Cd probe atoms, and demonstrated a common, characteristic “signature” of the Sn₇Ir₃ structure in all three alloys. The Sn₇Ir₃ structure has two inequivalent Sn-sites with a 3:4 ratio of atoms and point symmetries indicate that the electric-field gradients at both sites should be axially symmetric. Measured perturbation functions for all three alloys exhibited two axially symmetric quadrupole interaction signals having the expected 3:4 ratio of amplitudes, as expected for the structure. Furthermore, ratios of the two quadrupole interaction frequencies in each alloy were characteristically large, with frequencies for probe atoms on In(3) sites roughly five times greater than on In(4) sites. Taken together, these observations confirm that all three phases have the Sn₇Ir₃ structure. Quadrupole interaction frequencies are also reported for isostructural alloys of gallium with Pt, Pd and Ni. Negligible inhomogeneous broadening was observed in measurements near room temperature in all six phases, indicating excellent atomic ordering at the stoichiometric 70:30 compositions.     

A perturbed angular correlation spectrometer for material science studies

A four-detector perturbed angular correlation (PAC) spectrometer has been developed with ultra-fast BaF₂ detectors to acquire four coincidence spectra simultaneously, two at 180° and two at 90°. This spectrometer has double efficiency compared to that of a three-detector set-up. Higher efficiency is desirable for PAC studies in solid state physics where large number of coincidences are required to obtain the PAC spectra with good statistics and is particularly useful when the half-lives of the parent probe nuclei used for PAC measurements are ∼2–3 days or less as in ¹¹¹In (2.8 d), ⁹⁹Mo (2.7 d) and ¹⁴⁰La (1.7 d). The performance of the spectrometer has been tested for the HfO₂ monoclinic crystal in the temperature range from 77 to 873 K and for the HfF₄·3H₂O crystal at room temperature. The polycrystalline HfO₂ has been synthesized from Hf metal by heating in air. The hydrated hafnium fluoride has been crystallized by dissolving Hf metal in 40% HF and drying slowly at room temperature.      

Molybdate polymerisation studied by perturbed angular correlation

We determined the average nuclear quadrupole interaction (NQI) at⁹⁹tc in frozen aqueous molybdate solutions at 77K as a function of pH. The polymerisation shows up clearly in the titration curve at pH 6.5 and leads to a dramatic increase of the NQI.on leave from Tanta University, Egypt     

Hyperfine-field measurements inCr Rh andCr Re alloys using perturbed angular correlation technique

The hyperfine fields on¹¹¹Cd probe nuclei in Cr (1 at% Rh), Cr (0.3 at% Rh) and Cr (0.5 at% Re) matrices are measured using time-differential perturbed (–) angular correlation of 173–247 keV cascade in the decay of¹¹¹In to the levels in¹¹¹Cd. The sources prepared by ion implantation of¹¹¹In (2.81 d) activity followed by appropriate annealing procedure were found to be of good quality. The addition of impurities, located to the right of chromium in the periodic table such as Rh and Re, to the chromium matrix increases the electron to atom ratio, resulting in an increase in the magnetic moment compared to pure chromium. This in turn is expected to result in an increase in the hyperfine fields at probe nuclei in these alloys. The measured hyperfine fields are in qualitative agreement with the expected changes in these alloys.On leave from Kanpur Institute of Technology.     

On the perturbed angular correlation attenuation factor for relaxing systems

The attenuation factor of gamma-gamma perturbed angular correlation in the isotropic random-phase approximation is deduced for an axially symmetric quadrupole interaction and spins 1, 3/2, 2 and 5/2. It is shown that modifications to the usual expression are significant. Useful expressions for fitting procedures are obtained.Work supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Financiadora de estudos e Projetos (FINEP).     

Time-dependent perturbed angular correlation study of181Ta in Mg

The quadrupole interaction of¹⁸¹Ta in Mg metal has been investigated at room temperature by time differential perturbed angular correlation method yieldingv _Q=127±7 MHz. The observed EFG is calculated to be 20.8×10¹⁶ V/cm².     

A special feature of the integral perturbed angular correlation method for the study of the electric quadrupole interaction

A method is discussed to detect the orientation and strength of the quarupole interaction for implanted samples, using an integral perturbed angular correlation. The formulation is applied to the case of¹²⁹Xe implanted in diamond.I.I.K.W. Navorser.     

Time differential perturbed angular correlation studies of hafnium hydrides

The dependence of the electric field gradients (EFG) in Hafnium-Hydride systems as a function of the composition H/Hf in the cubic δ and tetragonal ε phases were investigated using the time-dependent perturbed angular correlation (TDPAC) technique, with¹⁸¹Ta as the nuclear probe. Two EFGs were found in the ε phase, indicating the existence of two major symmetries surrounding the Ta probe. The results indicate that the trend of the EFGs, in the ε phase, are due to the changes in the lattice parametersa _o,c _o as hydrogen is added to the Hf-H system. In the δ phase, only one major symmetry was found. Both phases are characterized by broad frequency distributions and large anisotropies.