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.     

Impurity induced EFG at111Cd in the fcc silver lattice

Electric field gradients (EFG) due to Cu, Zn and Ga impurities in silver were studied by the perturbed angular correlation technique using the probe atom¹¹¹In/¹¹¹Cd. A large deviation from the axial symmetry was observed for the EFG at a nearest neighbour site to Zn and Ga impurities. The molecular cluster method calculations can reproduce the magnitude of the EFG, however, does not account for the asymmetry parameter.     

Hyperfine interactions in 111Cd-doped lutetium sesquioxide

We report here first Perturbed Angular Correlation (PAC) results of the electric field gradient (EFG) characterisation at ¹¹¹Cd impurities located at both non-equivalent cation sites of the bixbyite structure of Lutetium sesquioxide, between room temperature (RT) and 1273 K. The comparison with results coming from a systematic ¹¹¹Cd PAC study in bixbyites and with point-charge model (PCM) predictions shows the presence of a trapped defect at RT in the neighbourhood of the asymmetric cation site, which is completely removed at T > 623 K. The anomalous EFG temperature dependence in Lu₂O₃ can be described in the frame of a “two-state” model with fluctuating interactions, which enables the experimental determination of the acceptor energy level introduced by the Cd impurity in the band-gap of the semiconductor and the estimation of the oxygen vacancy density in the sample. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

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.     

Electric field gradient in bixbyite rare-earth oxides R2O3 (R=Tl,Eu, Lu,Tm) measured by perturbed angular correlation

The electric field gradient (EFG) at ¹¹¹Cd nuclei dilutely substituting the cation site in bixbyite rare-earth oxides Tl₂O₃, Eu₂O₃, Lu₂O₃ and Tm₂O₃ has been measured using perturbed angular correlation technique. The ¹¹¹In(EC) ¹¹¹Cd probe nuclei were introduced into the samples by thermal diffusion. The experimental EFG values are compared with those calculated using the point charge model (PCM). The results are discussed in terms of a correlation between the electric field gradient and cation–oxygen bond length in metal oxides. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electric field gradients of 111Cd in the copper oxides CuO and Cu2O

Perturbed angular correlation measurements were performed after ¹¹¹In implantation into CuO and Cu₂O powder samples and 1 μm thick Cu₂O surface layers. The quadrupole hyperfine interaction of ¹¹¹Cd was studied in isochronal annealing cycles at 370–1170 K covering the CuO→Cu₂O phase transition. The electric field gradients obtained for ¹¹¹Cd on substitutional Cu lattice sites were associated with the repective oxygen coordinations. Annealing of Cu₂O surface layers on copper foils resulted in a texture with the efg pointing preferentially out of the surface plane.     

Dynamic hyperfine interactions in In(Cd)-doped ZnO semiconductor: PAC results supported by ab initio calculations

In this work, we present results of Time-Differential γ–γ Perturbed-Angular-Correlations (PAC) experiments performed in ¹¹¹Cd-doped ZnO semiconductor. The PAC technique has been applied in order to characterize the electric-field-gradient (EFG) tensor at (¹¹¹In (EC)→) ¹¹¹Cd nuclei located, as was later demonstrated, at defect-free cation sites of the ZnO host structure. The PAC experiments were performed in the temperature range of 77–1075 K. At first glance, the unexpected presence of low-intensity dynamic hyperfine interactions was observed, which were analyzed with a perturbation factor based on the Bäverstam and Othaz model. The experimental EFG results were compared with ab initio calculations performed with the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) method, in the framework of the Density Functional Theory (DFT), using the Wien2K code. The presence of the dynamic hyperfine interactions has been analyzed enlightened by the FP-APW+lo calculations of the EFG performed as a function of the charge state of the cell. We could correlate the large strength of the dynamic hyperfine interaction with the strong variation of the EFG due to changes in the electronic charge distribution in the Cd vicinity during the time-window of the PAC measurement. It was also revealed that the Cd impurity decays to a final stable neutral charge state (Cd²⁺) fast enough (in few ns) to produce the nearly undamped observed PAC spectra.     

Anomalous Temperature Dependence of the EFG in AlN Measured with the PAC-Probes 181Hf and 111In

¹⁸¹Hf and ¹¹¹In ions were implanted into AlN-layers in order to investigate their immediate lattice site environment and its temperature dependence by means of the Perturbed Angular Correlation (PAC) technique. After rapid thermal annealing at 1273 K up to 50% of the probe atoms were incorporated on undisturbed lattice sites defined by an electric field gradient (EFG) of 33 MHz for In and 572 MHz for Hf for measurement at room temperature. PAC-spectra taken at temperatures between 25 and 1200 K show that the EFG measured at the site of the undisturbed probes changes with temperature. While for Hf it decreases by 3%, for In it increases by 25% within the measured temperature range. Thus, the change cannot be due only to the thermal lattice expansion. In the case of In the fraction of probe atoms on substitutional sites increases with temperature until it reaches nearly 100% at 973 K. These effects are fully reversible. For the Hf probe, an additional EFG was detected at temperatures above 300 K.     

Damage studies on the system^{111} \mathop {In}\limits^ \to \underline {Ni}

Lattice damage after implantation of¹¹¹In in Ni has been studied applying the DPAC technique to the 171–245 keV -ray cascade in the daughter nucleus¹¹¹Cd. Implantations were carried out at 10 K and at 300 K. The low temperature implantation yields a higher regular substitutional fraction (80%) than the room temperature implantation (40%). The annealing behaviour of both implants above RT is the same. In addition, two distinct defect-associated sites were observed in isochronal annealing sequences. A microscopic model for these defects is presented, which takes into account magnetic and electric hyperfine interaction strengths, binding energies and site populations as a functions of temperature.     

Characteristic interactions of 111Cd probes with nonradioactive In impurities doped in ZnO

Local structures produced in 0.5 at.% In-doped ZnO were investigated by means of the time-differential perturbed angular correlation method using the ¹¹¹Cd probe nuclei generated in the disintegration of different parents, ¹¹¹In and ^111mCd. From distinct perturbation patterns obtained with the probes descended from the different parents, it was ascertained that the doped In atoms locally form a unique structure dispersed in ZnO matrix without forming macroscopic agglomerates of their own.     

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.     

Quadrupole interaction at111Cd in RRh3B2 compounds (R=Ce to Gd) and absence of transferred hyperfine field at111Cd in GdRh3B2

The quadrupole interaction at¹¹¹Cd in RRh₃B₂ compounds studied by the TDPAC technique supports the prediction that Ce in CeRh₃B₂ is not trivalent. The highly anisotropic EFG indicates that¹¹¹Cd in GdRh₃B₂ cannot occupy the rare earth site. The transferred magnetic hyperfine field at¹¹¹Cd is nearly zero which supports the conjecture that the impurity probe nuclei are at the 3g(Rh) site.     

Non-ionic contributions to the electric-field gradient at 111Cd/181Ta impurities in bixbyites

The electric-field-gradient (EFG) tensor at both cation sites of the bixbyite structure in ¹⁸¹Hf-implanted Lu- and Sm-sesquioxides was determined by the PAC technique. The cumulated EFG data at Ta-impurity sites in binary oxides enable us to discuss the “universal” character of the empirical correlation between local and ionic contributions to the EFG in these systems. An EFG factorization in terms of the electronic characteristics of the probe and the geometry of the cation coordination is proposed, which explains the experimental EFG results at Ta/Cd impurity sites in bixbyites and agrees with a simplified decomposition of the EFG valence contribution coming from ab-initio calculations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature dependence studies of the EFG at111Cd probes located in some intermetallic compounds

The temperature dependences of the electric field gradient (EFG) at¹¹¹Cd probes in nickel-indium and indium-tellurium intermetallic compounds have been studied by means of the time differential perturbed angular correlation of gamma rays (TDPAC). The room temperature interaction frequencies (e ² Qq/h) in each case were determined as In₂Te₅ (136 MHz), Ni₃In (13.8 MHz), NiIn (252 and 337 MHz) (two sites), and Ni₂In₃ (162 and 266 MHz) (two sites). The TDPAC pattern of¹¹¹Cd in NiIn did not agree with the previous assignment of the crystal structure. A general discussion of the results related to other temperature dependence studies of the EFG is presented and a general trend is discerned.Supported in part by the US Energy and Research Development Administration.     

Hyperfine Fields at the Cd Site in La0.67Cd0.25MnO3 CMR Manganites

Although Cd and Ca ions have the same valence and cation size, their incorporation into vacancy-doped La manganites induce different properties. While the incorporation of Ca leads to high Tc up to 250 K and induces a metallic-like behaviour, the incorporation of Cd severely reduces Tc and promotes insulator-like behaviour. In this work, the Cd hyperfine fields have been measured with the Perturbed Angular Correlations (PAC) technique after implantation and annealing of ^111m Cd in La–Cd–MnO₃ samples. The PAC results are compared with measurements of the resistivity and magnetization performed on the same samples. The mixed La and Mn site Cd occupancy is suggested as a possibility to explain the properties of the La–Cd–MnO₃ system.     

Electric field gradients of acceptor–donor pairs in semiconductors

The interaction between substitutional and interstitial donors and single or double acceptors in Si, GaAs, InP, and InAs has been studied by perturbed angular correlation spectroscopy (PAC). For the case of Si, complex formation between substitutional donors (As, P) and different radioactive acceptors (¹¹¹In, ¹¹¹Cd, ¹¹⁷Cd) has been observed. The formation of Cd–hydrogen pairs using either ¹¹¹Cd or ¹¹⁷Cd is discussed for GaAs, InP, and InAs. This revised version was published online in August 2006 with corrections to the Cover Date.     

An In-defect complex as a possible explanation for high luminous efficacy of InGaN and AlInN based devices

The role of indium in GaN and AlN films is investigated with the method of the perturbed angular correlation (PAC). Using the PAC probe ¹¹¹In in addition to indium on substitutional cation sites a large fraction of probes is found in a distinctly different microscopic environment which was attributed to the formation of an indium nitrogen-vacancy (V_N) complex. The influence of an electron capture induced after effect is ruled out by additional measurements with the PAC probes ^111mCd and ¹¹⁷Cd and using GaN with different dopants. It is shown that the V_N is not bound to substitutional Cd impurities suggesting that the In-V_N complex formation is a particularity of In in GaN and AlN. Finally, a preliminary model is presented to explain the temperature behavior of the electric field gradient, observed in the In-V_N complex measured with ¹¹¹In.     

Cavities in silicon investigated with the PAC-probe 111In

The perturbed angular correlation technique (PAC) was used to study the creation and development of He-induced cavities. In order to investigate the interaction of Indium atoms with cavities in Silicon the Bonn isotope separator was used to implant overlapping profiles of He (10 keV) and radioactive ¹¹¹In (160 keV) into undoped FZ-silicon. To get insight into the cavity formation mechanism samples were prepared with various He-doses (0.6, 2 and 6× 10¹⁶ ions/cm²). The samples were measured directly after implantation and after different annealing steps (t_hold= 10 min, T = 500–1100oC). Further, different implantation and annealing sequences were used. At higher He doses (2 and 6× 10¹⁶ ions/cm²) we find a large fraction of ¹¹¹In probe atoms subjected to an electric field gradient (EFG) corresponding to a quadrupole interaction frequency (QIF) of ν_Q= 411(6) MHz with η= 0.25(4). The corresponding defect configuration is formed most effectively after He implantation into annealed, ¹¹¹In doped Si. This and the affinity of In to vacancies leads us to the assumption that, similarly to the situation in metals, the Indium atoms act as nucleation centres for vacancy clusters (cavities) and are situated on the inner walls of the cavities. This revised version was published online in August 2006 with corrections to the Cover Date.     

Pressure and temperature dependence of the electric field gradient (EFG) in CdS

Using the time differential perturbed angular correlation technique (TDPAC), the electric hyperfine interaction of¹¹¹Cd in the II-VI-semiconductor CdS was investigated. The results of the temperature and pressure dependence of the electric field gradient (EFG) are discussed. The binding energyE _b for¹¹¹In at a Cd lattice site and a Cd vacancy (¹¹¹In_Cd–V_Cd–pair) could be estimated to 340 meV±80 meV by means of an Arrhenius-Plot. The disappearance of the 79 MHz and 73 MHz frequencies under a pressure of about 30 kbar could be due to their vacancy character or to the beginning of the phase transition to rocksalt structure.