Perturbed Γγ Angular Correlation 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.

TDPAC measurements in pure and Fe-doped In 2 O 3

Measurements of the electric quadrupole interactions were used to characterize pure and Fe-doped In2O3 samples using perturbed γ-γ angular correlation (PAC) technique with 111In-111Cd radioactive probe. The samples of pure as well as 1 % and 5 % Fe-doped In2O3 were prepared by sol–gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDS). The PAC measurements were carried out with a conventional fast-slow coincidence set-up using four BaF2 detectors as a function of temperature from 295 K to 1073 K. The powder XRD spectra analyzed with Rietveld method as well as SEM and EDS results showed that Fe-doped samples are homogeneous without any secondary iron oxide phases. The PAC spectra of pure and 1 % Fe-doped In2O3 show well-known characteristic quadrupole frequencies for the two non-equivalent sites in the bixbyte structure. The hyperfine parameters in these cases change little with temperature. For the 5 % Fe-doped sample however the PAC spectra changed significantly and a third frequency with large η appears.

Hyperfine interaction study of CeRh2Si2 with perturbed γ-γ angular correlation spectroscopy using C111d and C140e probes

The intermetallic compound CeRh2Si2 has been investigated by means of perturbed γ-γ angular correlation (PAC) spectroscopy using I111n(C111d) and L140a(C140e) probe nuclei. Measurements with C111d were carried out in the temperature range of 8–295 K. The results showed a highly distributed and almost temperature independent quadrupole interaction. No magnetic interaction was observed with this probe down to the lowest measured temperature. Measurements with L140a(C140e) probe were carried out in the temperature range of 8–36 K to determine the magnetic hyperfine field at Ce sites. A well defined magnetic interaction was observed at around 36 K. The PAC spectra below 25 K showed an additional magnetic interaction which was assigned to C140e probe substituting Ce positions with a different magnetic structure.

Investigation of Nanocrystalline Materials Using Radioactive Isotopes

The potential of detecting the hyperfine interaction at the nuclear site of radioactive isotopes for the investigation of nanocrystalline (nc) materials is discussed. The subject is illustrated with help of different examples for characterizing nc samples. Using the perturbed γ γ angular correlation technique (PAC) for detecting the hyperfine interaction at the site of the radioactive probe 111In/111Cd experiments regarding the doping of nc semiconductors ZnO and InP, the shift of the Néel temperature in nc antiferromagnetic NiO, the structure of grain boundaries in Ni, and the homogeneity of nc Ni-Cu alloys are presented. The resulting information is compared with that obtained by Mössbauer spectroscopy and by standard techniques used in materials science.

Doping and Characterisation of Nanocrystalline Materials

The synthesis behaviour and characterisation of nanocrystalline materials is presented. The materials synthesised are ZnO and InP doped with shallow donors and acceptors, respectively. Characterisation was performed with radioactive isotopes using the perturbed γγ angular correlation technique (PAC), thereby yielding local information on an atomic scale. The characterisation was supplemented by X-ray diffraction, transmission electron microscopy, UV/VIS absorption spectroscopy, photoluminescence spectroscopy, and extended X-ray absorption fine structure spectroscopy. It was shown that the successful incorporation of dopants in nanocrystalline ZnO and InP requires annealing at temperatures at which the growth of the nanocrystals in the sample becomes a significant process.

Effects of external stress on defect complexes in semiconductors

Crystal field gradients that exist at lattice sites in solids depend on the symmetry of chargedistribution around atomic sites. The charge symmetry could be broken either by thepresence of impurity complexes in the host matrix or by external stress on the samples,which leads to an observable magnitude of electric field gradients (EFGs). The perturbedγ–γ angular correlation (PAC) method is employed here to investigate the effect of uniaxial stress on111Cd sites in crystalline doped semiconductors.

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.

Grain Size Effect on the Temperature Dependence of the Electric Field Gradient in Nanocrystalline In

Nanocrystalline indium (nano-In) was prepared with different particle sizes by electrochemical deposition. The temperature dependence of the local electric field gradient (EFG) of nano-In was investigated in a temperature range of 20–300 K using the probe 111In for perturbed γγ angular correlation (PAC) experiments. The temperature dependence of the EFG of nano-In can be described by a (1 − BT 3/2) dependence as in bulk-In. It is shown that the low temperature value of the EFG and the proportionality constant B vary systematically with particle size.

Doping of the nanocrystalline semiconductor zinc oxide with the donor indium

Doping of the nanocrystalline semiconductor ZnO with the donor In111 was achieved by the incorporation of In111 atoms during the growth process followed by a hydrothermal treatment at 473 K. The incorporation of In111 on substitutional Zn sites was shown by the perturbed γγ angular correlation technique. The structural quality of nanocrystalline ZnO with a mean grain size of 11 nm is significantly improved by annealing at 473 K, as revealed by x-ray diffraction, transmission electron microscopy, optical absorption measurements, and photoluminescence spectroscopy. It is shown that the incorporation of In111 on undisturbed Zn sites in nanocrystalline ZnO seems to be supported by the onset of crystal growth and by the removal of intrinsic defects.

Incorporation of the Donor Indium in Nanocrystalline ZnO

Nanocrystalline zinc oxide (n-ZnO) doped with 111 In has been prepared by the method of electrochemical deposition under oxidizing conditions (EDOC). After growth, a crystal size of 5 nm was determined. Investigations of the incorporation of 111 In in the host lattice are performed by the method of perturbed γγ-angular correlation (PAC). After growth, the PAC experiments show that 111 In is incorporated in a not yet identified complex local structure. Subsequent annealing at about 573 K incorporates 111 In on substitutional Zn site in n-ZnO, which is a prerequisite for the incorporation of In as a shallow donor.

Studies of semiconductors

The recent progress in semiconductor technology calls for an excellent control of defects, both intrinsic defects (vacancies, interstitials, antisites) and extrinsic (dopants, impurity atoms). This requires techniques that are able to detect defects at constantly decreasing concentrations. The ability of identifying very low defect concentrations is the strength of spectroscopic techniques using radioactive dopants, like Mössbauer effect, perturbed γγ angular correlation, or tracer diffusion. Similarly, electrical and optical spectroscopic techniques profit from the use of radioactive dopants. The potential of these techniques will be illustrated by presenting results obtained for various intrinsic and extrinsic defects in Si, Ge, III–V, and II–VI semiconductors.

Identification of defects in semiconductors via their electric field gradients

Recent theoretical calculations show for defect complexes in semiconductors, characterized by electric field gradients (EFG), that their chemical compositions, electronic charge states and the induced lattice relaxations can be obtained by comparing the experimental and calculated EFG. The experimental data are obtained by perturbed γγ angular correlation experiments and the calculations are performed using ab initio full potential methods in the framework of density functional theory.

Defect complexes formed with Ag atoms in CdTe, ZnTe, and ZnSe

Using the radioactive acceptor 111Ag for perturbed γγ-angular correlation (PAC) spectroscopy for the first time, defect complexes formed with Ag are investigated in the II–VI semiconductors CdTe, ZnTe, and ZnSe. The donors In, Br and the Te-vacancy were found to passivate Ag acceptors in CdTe via pair formation, which was also observed in In-doped ZnTe. In undoped or Sb-doped CdTe and in undoped ZnSe, the PAC experiments indicate the compensation of Ag acceptors by the formation of double broken bond centres, which are characterised by an electric field gradient with an asymmetry parameter close to η=1. Additionally, a very large electric field gradient was observed in CdTe, which is possibly connected with residual impurities.

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.

Compensating defects and electrical activation of donors in CdS

Electrical deactivation of donors in CdS was studied by using Perturbed γγ Angular correlation (PAC) spectroscopy and temperature dependence of Hall mobility, resistivity, and free-carrier concentration. PAC spectra and electrical properties were monitored as a function of thermal treatment either under S or Cd pressure in a temperature range from RT to 1073 K. For samples annealed above 800 K under S pressure, Hall effect showed increased electrical compensation whereas PAC detected spontaneous creation of cadmium vacancies, VCd, via the formation of (InCd–VCd) pairs. The increase of the concentration of compensating acceptors, determined from electrical measurements, precisely correlates with the increase of the concentration of (InCd–VCd) pairs found by means of PAC. In contrast to that, thermal treatment under Cd pressure up to 1073 K does not provoke the formation of compensating native defects up to [In] >1019/cm3.

Defect complexes induced by diffusion of group I acceptors into CdTe

The formation of defects following the diffusion of the group I acceptors Cu, Ag, or Au into CdTe was investigated by perturbed γγ-angular correlation (PAC) using the radioactive donor 111InCd. The formation of A-centres (InCd–VCd pairs) with the donor 111InCd was observed, indicating that the incorporation of the group I acceptors Cu, Ag, or Au is accompanied by the generation of cation vacancies. In addition, the formation of InCd–AgCd or InCd–AuCd pairs was observed in CdTe. The concentration of cation vacancies formed after diffusion of Cu, Ag, or Au was found to depend only weakly on the respective group I element. In contrast, the migration energy of the cation vacancy was observed to be different in CdTe crystals doped with Ag or Au, yielding 0.76 (3)eV and 0.84 (3)eV, respectively. In Ag-doped CdTe the binding energies of the InCd–VCd pair and the InCd–AgCd pair were determined to 0.18 (2)eV and 0.19 (2)eV, respectively.

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.

Doping of CdTe with radioactive [formula omitted] during MOCVD growth

CdTe epilayers grown on GaAs by MOCVD were doped in situ with the donor In by using a mixture of radioactive trimethylindium (TM 111In) and stable TMIn. By measuring the γ-radiation of the radioactive dopant 111 In the amount of In atoms incorporated into the CdTe layers during growth and the deposition of In within the MOCVD setup was determined. The presence of compensating defects in the grown layers was detected at the site of the donor 111 In by the perturbed γγ-angular correlation (PAC) technique. The PAC experiments, complemented by photoluminescence spectroscopy, show the presence of A-centres, consisting of a Cd vacancy and an In donor.

Pairing between Radioactive Solutes and Point‐Defects in Semiconductors

AbstractThe study of solute point‐defect interactions in crystals is performed on an atomic scale by employing radioactive solutes as local observers. Thereby, the presence of a point‐defect next to the solute atom becomes observable via the altered electronic charge distribution about the solute, giving rise to a defect‐specific electric field gradient. Using the perturbed γγ angular correlation technique, this electric field gradient is detected via the hyperfine interaction occurring at the site of the nucleus of the radioactive atom. The sensitivity to solute point‐defect pairs is typically in the order of 1016cm–3. Several properties of these pairs, such as their chemical nature and thermodynamical behaviour are discussed. The point‐defects addressed in this paper are intrinsic, such as lattice vacancies, and extrinsic, such as substitutional and interstitial impurity atoms. Results for the elemental semiconductor Si and different II‐VI compound semiconductors will be discussed.