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.     

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.     

Oxidation of indium implanted aluminium studied by the Rutherford backscattering and perturbed angular correlation methods

Aluminium foils oversaturated with 5.10¹⁵/cm² implanted¹¹⁵In⁺ ions were oxidized in 200 mbar oxygen or vacuum annealed at 370–870 K. The samples were analyzed by the Rutherford backscattering (RBS) and perturbed angular correlation (PAC) techniques, using some 10¹¹/cm² implanted radioactive¹¹¹In tracers. Furthermore, the oxygen surface profiles were also scanned with high resolution by using the nuclear resonance technique (NRA). The formation of passivating Al₂O₃ surface layers, preventing deeper oxygen diffusion and the indium diffusion into these oxidized surface layers and into the bulk, were studied. Several quadrupole interaction frequencies previously attributed to strained cubic indium precipitates and indium (-vacancy) clusters were observed. When the samples were oxidized above 750 K, the formation of In-O complexes and of substitutional¹¹¹In in Al₂O₃ was observed.     

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.     

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     

Doping of sapphire single crystals with 111In and 111Cd detected by perturbed angular correlation

The electric hyperfine interaction of ion beam implanted ¹¹¹In and ¹¹¹Cd probe atoms in sapphire (Al₂O₃) single crystals has been investigated using perturbed angular correlation spectroscopy. For both probe atoms the same distinctive electric field gradient was found, indicating that nearly all the implanted probe atoms form a stable substitutional configuration in the temperature range between 77 K and 873 K on the aluminum sublattice. A comparative study between ¹¹¹In and ¹¹¹Cd-measurements points to a dynamic interaction initiated by the electron-capture of ¹¹¹In(EC)¹¹¹Cd similar to In₂O₃ and La₂O₃. Size and orientation of the EFG are discussed in comparison to experimental results in Cr₂O₃ single crystals. 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.     

Resistivity- and perturbed angular correlation-study of implanted nickel hydrides

Hyperfine Interactions -     

Hyperfine interactions of 111Cd in TiO2 (Rutile) studied by perturbed angular correlations

The time-differential perturbed angular correlation for ¹¹¹Cd nuclei has been measured after ¹¹¹In implantation into polycrystalline TiO₂. The observed perturbation functions are characterized by a well-defined electric field gradient with the quadrupole coupling constant ν_Q = 105(1) MHz and the asymmetry parameter η = 0.18(1). We attribute these hyperfine parameters to ¹¹¹Cd on the (distorted) substitutional cation site in rutile. The PAC results are compared with those in SnO₂ rutile as well as with X-ray diffraction, RBS channeling experiments and point charge model calculations including relaxation of the probe atom surrounding.     

Search for magnetic interaction in In doped AlN using perturbed angular correlation spectroscopy

The possible presence of a large magnetic field due to spin polarization of a Cd nucleus (decay product of ¹¹¹In) at an Al substitutional site in AlN is investigated with perturbed angular correlation (PAC) spectroscopy. The PAC spectra of ¹¹¹In/¹¹¹Cd in AlN show two probe environments: a weak quadrupole interaction (quadrupole interaction constant, $\nu _{\rm Q}^{\,\,\,\rm lattice} = 30$  MHz) due to ¹¹¹In probes at a defect free Al substitutional site and an unknown large interaction ( $\nu _{\rm Q}^{\,\,\,\rm complex} = 300$  MHz) tentatively attributed to a nearest neighbour pair between ¹¹¹In and a nitrogen vacancy (V_N) aligned along the c-axis. Surprisingly, in density functional theory (DFT) calculations, such a large electric field gradient (EFG) could not be reproduced. However, an inclusion of spin polarization in the calculations indicates a strong magnetic field at ~50 % of the ¹¹¹In/¹¹¹Cd site. An attempt to verify the presence of the strong magnetic field and to explain the origin of the strong interaction is made. Orientation measurements show, the large interaction is not characterised by a magnetic interaction and is predominantly due to the EFG. However, in the presence of an external magnetic field, the strong interaction probe environment becomes more uniform and the EFG increases by ~10 %. This definitely hints towards some sort of magnetic interaction at the strong interaction probe site.     

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.     

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.     

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.     

Investigation of laser irradiated poly- and monocrystalline aluminium by perturbed angular correlation measurements

Poly- and monocrystalline samples of aluminium implanted with ¹¹¹In to a dose of about 10¹³ at/cm² were irradiated at a temperature of 80 K with a Q-switched pulsed ruby laser. The energy density of the 25 ns pulses was in the range of 1–5 J/cm². Perturbed angular correlation measurements revealed the formation of several In-vacancy clusters, some of which give rise to electric field gradient tensors oriented along low-index crystallographic directions. This clearly shows that laser irradiation acts as a fast quench and that the monocrystalline structure is not destroyed, not even by the highest laser pulse intensity.