The temperature dependence of the EFG for 172 Lu( 172 Yb) in GaN

Wide band-gap semiconductors have become an important base material for applications in opto-electronics and in high power, high temperature electronics. After doping with various rare earths, electro-luminescence throughout the whole visible spectrum has been observed. This very interesting application could become much more versatile if doping by implantation would be possible. We use the perturbed angular correlation technique and the probe nucleus ¹⁷²Lu(¹⁷²Yb) to study its behaviour after implantation into GaN. An unusually large variation of the quadrupole interaction frequency between 25 K and 700 K is observed. The results can be described excellently with a simple model if one assumes that the RE’s partially filled 4f shell is responsible for this behaviour.     

Combination of emission channeling,photoluminescence and Mössbauer spectroscopy to identify rare earth defect complexes in semiconductors

Implanted radioactive ¹⁶⁷Tm / ¹⁶⁷Er and ¹⁶⁹Yb / ¹⁶⁹Tm impurities in Si and GaN were studied with emission channeling and photoluminescence spectroscopy. The effect of co-doping with oxygen on the rare earth (RE) lattice sites and their luminescence behavior was investigated. Tm and Yb occupy near-tetrahedral sites in Si and substitutional sites in GaN after room temperature implantation and annealing. O-RE complexes are formed upon co-doping with O resulting in modified luminescence signals. RE impurities remain substitutional in O-doped GaN, but are displaced from tetrahedral sites in O-doped Si. We discuss the feasibility of Mössbauer studies using ¹⁵¹Eu, ¹⁶⁹Tm and ¹⁶¹Dy to determine the RE valence state and to identify RE defect complexes.

     

Paramagnetic fluctuations in Yb3Ga5O12 investigated with 172Yb PAC probe

¹⁷²Yb PAC measurements were carried out on Yb₃Ga₅O₁₂ at temperatures ranging from 14 to 1100 K. The time dependent hyperfine interaction is clearly evident below room temperature. It is attributed to spin fluctuations of Yb ion in the ground state of the ²F_7/2 multiplet. Above 500 K, the temperature dependence of the electric field gradient is shown to result from a varying population of the crystal electric field levels. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Incorporation of the transition metal Hf into GaN

The perturbed angular correlation (PAC) technique was applied to study the incorporation of the transition metal Hf into GaN after implantation. To this end the PAC probe ¹⁸¹Hf(¹⁸¹Ta) was implanted into epitaxial Wurtzite GaN layers (1.3 μm on sapphire) with an energy of 160 keV and doses of 7× 10¹² at/cm². PAC spectra were recorded during an isochronal annealing programme, using rapid thermal annealing (RTA) and furnace annealing, in the 300–1000oC temperature range. After implantation the spectra show a damped oscillation corresponding to a quadrupole interaction frequency (QIF) of ν_Q= 340 MHz for 30% of the probe nuclei. Annealing up to 600oC reduces the damping of this frequency without an increase of the probe atom fraction f_s in these sites. Above 600oC f_s grows rapidly until after the 900oC RTA step more than 80% of the Hf probes experience a well defined QIF due to the incorporation of Hf on undisturbed sites of the hexagonal GaN wurtzite lattice. An interaction frequency of ν_Q= 340 MHz is derived. RTA and furnace annealing yield similar results for annealing up to 800oC, where the undisturbed fraction reaches about 60%. Then RTA at higher temperatures increases this fraction, while furnace annealing leads to a decrease down to 22% after annealing at 1000oC. To our knowledge this is the first time that a transition metal probe like Hf is incorporated to such a large extent into a semiconductor lattice. This revised version was published online in August 2006 with corrections to the Cover Date.     

Determination of the quadrupole ratio Q3+/Q2+ in172sYb

Statistically perturbed angular correlations of¹⁶⁰Dy and¹⁷²Yb have been time differentially investigated in liquid sources. As solvents we used ethylalcohol and concentrated hydrochloric acid at various temperatures. From our results we have calculated the ratio of the spectroscopic quadrupole moments of the 2⁺ rotational and the 3⁺ isomeric level in¹⁷²Yb asQ ³⁺/Q ²⁺=1.33±0.14.     

On the effective anisotropy of the 91–1,094 keV γγ-cascade in 172Yb

The suitability of the 91–1,094 keV γγ-cascade in ¹⁷²Lu(¹⁷²Yb) for perturbed angular correlation studies has been demonstrated several decades ago. In comparison with widely used probe nuclei like ¹¹¹In or ¹⁸¹Hf its quadrupole moment and anisotropy coefficient are quite large and the life time of the intermediate state is sufficiently long. However, the effective anisotropy observed in actual experiments is generally much lower than the literature value. If this isotope is to be used for any quantitative analysis, especially in solid state investigations, it is very important to understand the origin of this discrepancy since the ratio of these values is directly related to the fraction of probe atoms in any given lattice environment. To this end, the influence of Compton events from competing cascades in the 91 keV start energy window of the 91–1,094 keV γγ-cascade was studied in a numerical simulation. It is shown that this Compton background is the main reason for the strong reduction of the observable anisotropy. A relation between the width of the 91 keV energy window and the effective anisotropy is established.     

Study of highT c superconductivity in Y1Ba2Cu3O7−x by PAC

HighT _c superconductivity in the YBaCuO superconductor has been investigated through the quadrupole interaction of the probe nuclei⁹⁹Tc. The quadrupole interactions were measured by the TDPAC method from 77 to 296 K. The probe nuclei⁹⁹Tc were introduced into the YBaCuO superconducting specimen by diffusion. The derived electric quadrupole interaction parameters show that the probe nuclei are subject to a unique EFG interaction and occupy a substitutional lattice site in the YBaCuO superconductor. A strong EFG of 10¹⁹ V/cm² was observed. The temperature dependence of the EFG exhibits a linear decrease with temperature increase. Anomalies of both EFG and ν were found in the superconducting transition temperature region. The role of the oxygen vacancies in the Cu−O chains is discussed.     

The quadrupole interaction at181Ta in Zn

The quadrupole interaction (QI) in hexagonal close packed zinc lattice was measured using the 482 keV, 10.6 ns probe state of¹⁸¹Ta employing the time-differential perturbed angular correlation technique. The electric field gradient (EFG) at¹⁸¹Ta in Zn was derived from the measured quadrupole interaction frequency at room temperature asV _zz =12.202×10¹⁷ V/cm². The quadrupole interaction measured at various temperatures displayed normal temperature dependence similar to that seen by this probe in many non-cubic hosts.     

Electronic structure of the surface of rare Earth metals

A qualitative model describing the surface of rare earth metals with an uncompleted 4f electron shell (Ce-Yb) and the process of thermal evaporation from this surface is proposed. The model is based on a correlation established between the energies of the lowest-lying levels of the 4f^N?15d6s² configuration and the temperatures of equal saturated-vapor pressures of these metals.     

Search for ferromagnetic ordering in Pd doped wide band gap semiconductors GaN and ZnO

GaN and ZnO are possible candidates for dilute magnetic semiconductors with Curie temperatures above room temperature. Doping with transition metals like Co, Mn or Fe could be a simple way to create such systems. The perturbed angular correlation (PAC) probe ¹⁰⁰Pd/¹⁰⁰Rh is isoelectronic to cobalt and therefore a perfect tool to investigate the incorporation of transition metals into these compounds as well as the influence of other impurities on internal magnetic fields. The (0001) and (10 $\bar{1}$ 10) surfaces of ZnO single crystals, freestanding GaN films, and GaN thin films (6 μm) on sapphire substrates were recoil-implanted with the ¹⁰⁰Pd/¹⁰⁰Rh probe. The probe was produced using the fusion evaporation reaction ⁹²Zr(¹²C, 4n)¹⁰⁰Pd at a beam energy of 69 MeV. Subsequently, the incorporation of the probe was studied by PAC spectroscopy during an isochronal annealing program. First results without and with an applied external magnetic field are indicative of a strongly disturbed lattice vicinity of Pd impurities in both hosts. No signs of spontaneous ferromagnetic ordering were observed.     

Projected total energy surface description for axial shape asymmetry in 172W

The projected total energy surface(PTES)approach has been developed based on the triaxial projected shell model(TPSM)hybridized with the macroscopic–microscopic method.The total energy of an atomic nucleus is decomposed into macroscopic,microscopic and rotational terms.The macroscopic and microscopic components are described with the liquid drop model and Strutinsky method,respectively,and the rotational energy is given by the TPSM,the term beyond the mean field.To test theory,the PTES calculations have been carried out for the yrast states of the well deformed rare earth nucleus172W,and the theoretical results are in good agreement with the experimental data.By using the equilibrium quardrupole deformations(ε2andγ)determined by the PTES,the calculation of the transition quardrupole moment(Qt)in function of spin also reproduces the experimental data.A comparison between the PTES and TRS methods has been made for theoretical and application uses.     

Yb-Based Kondo Lattices: Insight from 170Yb Mössbauer, Positive Muon and γ–γ Perturbed Angular Correlations Spectroscopies

The usefulness of local hyperfine techniques in the investigation of the physical properties of Kondo lattices is illustrated through three examples. First, we show how Mössbauer spectroscopy on ¹⁷⁰Yb down to very low temperature (0.025 K) provides evidence for the existence of an incommensurate modulated magnetic structure close to T=0 in YbPtAl; a modulated structure at T=0 is in principle forbidden for Kramers ions, but it is made possible in YbPtAl due to the presence of the Kondo coupling. Second, we present a μSR study of the Kondo insulator YbB₁₂, and we evidence the persistence of fluctuations of very small correlated Yb moments close to T=0. Third, we report on a study by Perturbed Angular Correlation spectroscopy on the isotope ¹⁷²Yb of the Kondo lattice Yb₂Co₃Ga₉, which is characterised by a high Kondo temperature (T ₀?260 K). Our aim was to compare, on a large temperature range, the thermal variations of the 4f quadrupole moment and of the magnetic susceptibility, to check whether the scaling property predicted by theory could be observed.     

Mössbauer study of 57Fe in GaAs and GaP following 57Mn+ implantation

Ion implantation provides a precise method of incorporating dopant atoms in semiconductors, provided lattice damage due to the implantation process can be annealed and the dopant atoms located on regular lattice sites. We have undertaken ⁵⁷Fe emission Mössbauer spectroscopy measurements on GaAs and GaP single crystals following implantation of radioactive ⁵⁷Mn^?+? ions, to study the lattice sites of the implanted ions, the annealing of implantation induced damage and impurity–vacancy complexes formed. The Mössbauer spectra were analyzed with four spectral components: an asymmetric doublet (D1) attributed to Fe atoms in distorted environments due to implantation damage, two single lines, S1 assigned to Fe on substitutional Ga sites, and S2 to Fe on interstitial sites, and a low intensity symmetric doublet (D2) assigned to impurity–vacancy complexes. The variations in the extracted hyperfine parameters of D1 for both materials at high temperatures (T?> 400 K) suggests changes in the immediate environment of the Fe impurity atoms and different bonding mechanism to the Mössbauer probe atom. The results show that the annealing of the radiation induced damage is more prominent in GaAs compared to GaP.     

The internal electric fieldgradient in antiferroelectric PbZrO3 studied by perturbed angular correlations

The internal electric fieldgradient (EFG) at the Zr-site in antiferroelectric polycristalline PbZrO₃ has been investigated as a function of temperature by time differential angular correlation measurements. For the application of this technique PbZrO₃ was doped with small amounts of radioactive¹⁸¹Hf. In the antiferroelectric phase of PbZrO₃ the EFG decreases continously with increasing temperature and exhibits a sharp discontinuity at 230 °C, the Curie point of the compound. In the cubic phase a small remaining EFG is observed which is probably caused by lattice imperfections in the source. The antiferroelectric distortion of the lattice decreases slightly when the temperature approaches the Curie point, as shown by the temperature dependence of the asymmetry parameter of the EFG. The measured EFG at room temperature is compared with the result of a lattice sum calculation.     

Measurement of Quadrupole Interactions in LaMO3 (M = Cr,Fe, Co) Perovskites by TDPAC

The perturbed angular correlation (PAC) technique has been used to study the electric field gradient (EFG) in LaCoO₃ perovskite. The results are compared with those for LaCrO₃, LaFeO₃ measured earlier. The PAC probe, ¹¹¹In → ¹¹¹Cd, was introduced in the oxide lattice by means of chemical reaction during sample preparation. In the present work, the temperature dependence of the electric quadrupole interaction parameters, for LaCoO₃ was investigated. The resulting systematics of EFG at ¹¹¹Cd, in La(Cr,Fe,Co)O₃ perovskites, reveals a linear dependence with temperature. This revised version was published online in September 2006 with corrections to the Cover Date.     

Fe-Hydroxysulphates from Bacterial Fe2+ Oxidation

The perturbed angular correlation method (PAC) was applied to investigate the lattice location of implanted ¹¹¹In probe ions in Hf₂Ni and Zr₂Ni intermetallic compounds. It is concluded that the ¹¹¹In/¹¹¹Cd probe nuclei experiencing the highly asymmetric electric field gradient (EFG) occupy the unique hafnium or zirconium 8(h) sites in the investigated phases. Above room temperature, the EFGs decrease linearly with temperature. The results are compared with that of previous PAC measurements with ¹⁸¹Ta probes.     

Hyperfine fields on recoil implanted nuclei 62≦Z≦74 in a polarized Fe-, Co- and Ni-environment

Hyperfine fields acting on Sm, Gd, Dy, Er, Yb, Hf and W nuclei in a polarized ferromagnetic environment of Fe, Co and Ni have been investigated. The nuclei have been implanted into the host lattice by the recoil associated with Coulomb excitation with 11 MeV C¹²-ions from a 5.5 MV Van de Graaff accelerator. The precession of theγ-ray angular distribution has been measured and substantial perturbations were found. It is shown that the perturbation, at least for the case of the strongly perturbed rare earth nuclei, cannot be explained by a combined static electric quadrupole and magnetic dipole interaction.     

Temperature dependent energy relaxation time in AlGaN/AlN/GaN heterostructures

The two-dimensional (2D) electron energy relaxation in Al_0.25Ga_0.75N/AlN/GaN heterostructures was investigated experimentally by using two experimental techniques; Shubnikov-de Haas (SdH) effect and classical Hall Effect. The electron temperature (T_e) of hot electrons was obtained from the lattice temperature (T_L) and the applied electric field dependencies of the amplitude of SdH oscillations and Hall mobility. The experimental results for the electron temperature dependence of power loss are also compared with the current theoretical models for power loss in 2D semiconductors. The power loss that was determined from the SdH measurements indicates that the energy relaxation of electrons is due to acoustic phonon emission via unscreened piezoelectric interaction. In addition, the power loss from the electrons obtained from Hall mobility for electron temperatures in the range T_e > 100 K is associated with optical phonon emission. The temperature dependent energy relaxation time in Al_0.25Ga_0.75N/AlN/GaN heterostructures that was determined from the power loss data indicates that hot electrons relax spontaneously with MHz to THz emission with increasing temperatures.     

Characterisation of defects in rare earth implanted GaN by deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) was used to investigate the electrical properties of GaN implanted with the rare earth (RE) ions erbium and thulium. The GaN layers have been grown by metal-organic chemical vapor deposition (MOCVD) onto (0001) sapphire substrates. We used the channeled implantation geometry to implant a dose of 5×10¹⁴ RE cm⁻² with an energy of 150 keV. For each species, two different annealing procedures were used in a nitrogen atmosphere for 120 s. Indeed, the annealing temperature plays an important role in the lattice recovery, even if RE-related defects remain present. After annealing at 1000 C, the appearance of two new peaks, for both studied RE ions, is associated with the lattice damage induced by the implantation, such as the presence of nitrogen vacancies. After annealing at 1100 C, the recovery of the lattice is observed while a hole trap appears for both implanted RE ions with corresponding energy values E_v+0.61 eV and E_v+1.59 eV, in the case of Er and Tm, respectively.