Evidence of palladium-defect pairing in intrinsic germanium

Time Differential Perturbed Angular Correlation measurements were performed in intrinsic germanium with the ¹⁰⁰Pd/¹⁰⁰Rh probe. The probe was produced via ⁹²Zr(¹²C, 4n)¹⁰⁰Pd and recoil-implanted into samples. Parallel measurements with the ¹¹¹In/¹¹¹Cd probe confirmed the quality of the germanium studied. The measurements with the ¹⁰⁰Pd/¹⁰⁰Rh probe show a modulation pattern in the ratio function with a quadrupole interaction frequency of 8.3(2) Mrad/s. The pattern depends on sample orientation and it is most pronounced after annealing at 500°C. After annealing at 700°C the effect vanishes due to strong damping of the ratio function. The pattern may be caused, similar to what has been observed for highly doped n-type silicon, by the pairing of the Pd-atom with a vacancy located in the < 111 > direction. The disappearance of the pattern would indicate the dissociation of this pair. Pair formation and dissociation may be relevant to palladium-induced-crystallization processing of germanium.     

Hot́ hole anisotropic effect in silicon and germanium

Anisotropic effect of the hole drift velocity in silicon and germanium has been investigated with the time of flight technique by applying the electric field parallel to the <100 and <111 crystallographic axis. The measurements were performed for electric fields ranging from 10 to 3 × 10⁴V/cm and temperatures from 40 to 200°K. The results indicate that the anisotropic effect v_d<100/v_d<111 increases with decreasing temperature and increasing electric field, and reaches a saturation value at high electric fields (? 10⁴V/cm). The maximum anisotropic effect for Ge is 1.25 at 40°K and for Si is 1.2 at 45°K. A qualitative analysis of the experimental data indicates that the anisotropic effect is due to the warped heavy-valence-band shape.     

Structure and energy level of native defects in as-grown and electron-irradiated zinc germanium diphosphide studied by EPR and photo-EPR

The properties of defects in as-grown p-type zinc germanium disphosphide (ZnGeP₂) and the influence of electron irradiation and annealing on the defect behavior were studied by means of electron paramagnetic resonance (EPR) and photo-EPR. Besides the well-known three native defects (V_Zn, V_P, Ge_Zn), an S=1/2 EPR spectrum with an isotropic g=2.0123 and resolved hyperfine splitting from four equivalent I=1/2 neighbors is observed in electron-irradiated ZnGeP₂. This spectrum is tentatively assigned to the isolated Ge vacancy. Photo-EPR and annealing treatments show that the high-energy electron irradiation-induced changes in the EPR intensities of the zinc and phosphorus vacancies are caused by the Fermi level shift towards the conduction band. Annealing of the electron-irradiated samples induces a shift of the Fermi level back to its original position, accompanied by an increase of the EPR signal associated with the V_Zn⁻ and a proportional increase of the EPR signal assigned to the V_P⁰ under illumination (λ<1 eV) as well as generation of a new defect. The results indicate that the EPR spectra originally assigned to the isolated V_Zn⁻ and V_P⁰ are in fact associated defects and the new defect is probably the isolated phosphorus vacancy V_Pi.     

Intrinsic V- and Vo centers in single crystal SrO

The intrinsic V^- center (a positive hole trapped at a cation vacancy) has been observed in single crystals of SrO following x-irradiation at 77 K. The ESR spectrum corresponds to a spin-$\text{1}\text{2}$ defect having <100> axial symmetry with g₆ = 2.0012(3) and g_⊥ = 2.0703(3). The concentration of V^- centers was enhanced significantly by heating crystals in oxygen at 1000°C and rapidly quenching. V^o centers (two holes trapped at a cation vacancy) were also observed after x-irradiation at low temperature, with g₆ = 2.0011(3), g_⊥ =2.0751(3), and D = 380.4(5) MHz.     

Mössbauer Study of Lanthanum–Strontium Ferromanganite Oxides

TDPAC has been employed to study the local structure of implanted palladium in silicon utilizing 87–75 keV γ–γ cascade of probe nucleus ¹⁰⁰Pd. The observed hyperfine parameters revealed the presence of Pd–V defect pair only in highly doped n-type silicon. A dumbbell structure with substitutional palladium and silicon vacancy as nearest neigbor is suggested for this defect.     

Status of the Germanium Detector Array (GERDA) in the search of neutrinoless ββ decays of 76Ge at LNGS

The Germanium Detector Array (GERDA) in the search for neutrinoless ββ decays of ⁷⁶Ge at LNGS will operate bare germanium diodes enriched in ⁷⁶Ge in an (optional active) cryogenic fluid shield to investigate neutrinoless ββ decay with a sensitivity of T _1/2 > 2 × 10²⁶ yr after an exposure of 100 kg yr. Recent progress includes the installation of the first underground infrastructures at Gran Sasso, the completion of the enrichment of 37.5 kg of germanium material for detector construction, prototyping of low-mass detector support and contacts, and front-end and DAQ electronics, as well as the preparation for construction of the cryogenic vessel and water tank. GERDA Collaboration     

TDPAC and first-principles study of electronic and structural properties of a Pd-vacancy complex in undoped germanium

Pd is one of the metals suitable for inducing low-temperature crystallization in Ge. However, it is not clear how residual Pd atoms are integrated into the Ge lattice. Therefore, time-differential γ–γ perturbed angular correlations (TDPAC) technique using the ¹⁰⁰Pd(→¹⁰⁰Rh) nuclear probe produced by recoil implantation has been applied to study the hyperfine interactions of this probe in single-crystalline undoped Ge. A Pd-vacancy complex aligned along the <111> crystallographic direction with a unique interaction frequency of 8.4(5) Mrad/s has been identified. This complex was measured to have a maximum relative fraction of about 76(4)% following annealing at 350 ^°C. Further annealing at higher temperatures reduced this fraction, possibly via dissociation of the complex. Calculations suggest dissociation energy of 1.94(5) eV for the complex. DFT calculations performed in this work are in reasonable good agreement with the experimental values for the electric-field gradient of the defect complex in Ge and Si for comparison. The calculations predict a split-vacancy configuration with the Pd on a bond-centred interstitial site having a nearest-neighbour semi-vacancy on both sides (V-Pd_BI-V) in Ge and Si.     

Silicon-germanium nanostructures with high germanium concentration

The impact of nucleation conditions on the quality of epitaxial layers of germanium and GeSi alloys containing a high Ge mole fraction grown on (100) silicon substrates using electron-beam epitaxy is considered. The Ge_xS_{1 -x}/Ge superlattices are grown on a Ge_ySi_{1 -y} (x > y) relaxed buffer layer. X-ray diffractometry, atomic force microscopy and Auger spectroscopy are the main techniques used to study the properties of the grown structures.     

Second upper-bound on the three-phonon resistivity in Ge

Contributions of different allowed three-phonon resistances in Ge have been presented in the second upper-bound W^>₁. It is found that the deviation of W^>₁ from W^>₀, the first upper-bound, is markable. As a matter of fact W^>₁ is shown to be less than W^>₀ by about 14% at 100°K and about 5% at 300°K.     

First principles study of electronic and structural properties of the Ge/GeO2 interface

The electronic and structural properties of the Ge/GeO₂ interface are addressed through a density functional simulation scheme which includes the use of hybrid functionals for achieving accurate band gaps, band offsets and defect levels. The present work discusses the germanium dangling bond levels, the thermodynamics of GeO_x, the stability of the oxygen vacancy across Ge/HfO₂ interfaces, the atomic structure of GeO_x, electron and hole trapping in GeO_x, and the band alignment at the Ge/GeO₂ interface.     

Limits of Majorana neutrino mass from combined analysis of data from <Superscript>76</Superscript>Ge and <Superscript>136</Superscript>Xe neutrinoless double beta decay experiments

We present effective Majorana neutrino mass limits <m _ββ> obtained from the joint analysis of the recently published results of ⁷⁶Ge and ¹³⁶Xe neutrinoless double beta decay (0νββ) experiments, which was carried out by using the Bayesian calculations. Nuclear matrix elements (NMEs) used for the analysis are taken from the works, in which NMEs of ⁷⁶Ge and ¹³⁶Xe were simultaneously calculated. This reduced systematic errors connected with NME calculation techniques. The new effective Majorana neutrino mass limits <m _ββ> less than [85.4–197.0] meV are much closer to the inverse neutrino mass hierarchy region.     

Ab initio study of the formation of vacancy and hydrogen-vacancy complexes in palladium and its hydride

We report on the results of ab initio calculations of vacancy and hydrogen-vacancy complexes in palladium and palladium hydride. Comparative analysis of the energies of the formation of defect complexes in palladium and its hydride has revealed that the formation of vacancy clusters is easier in the palladium hydride structure. Investigation of hydrogen-vacancy complexes in bulk crystalline palladium has shown that a hydrogen atom and a vacancy interact to form a stable hydrogen-vacancy (H-Vac) defect complex with a binding energy of E _b = ?0.21 eV. To investigate the initial stage in the formation of hydrogen-vacancy complexes (H _n -Vac _m ), we consider the clusterization of defects into clusters containing H-Vac and H₂-Vac complexes as a structural unit. It is found that hydrogen-vacancy complexes form 2D defect structures in palladium in the (100)-type planes.     

Spin relaxation of conduction electrons in highly-doped n-type germanium at low temperature

We present a theory of the spin relaxation time of the conduction electrons in highly-doped n-type germanium at liquid helium temperature. The theory is compared with some of our measurements and the experimental data available in the literature on As-doped germanium. The observed linewidth at T = 10 K is accounted for in the whole metallic concentration range (N_D > 3 × 10¹⁷cm^?3).In the lower concentration range (3 × 10¹⁷ < N_D < 10¹⁸cm^?3), the relevant mechanism is the random jumping of the g factor upon intervalley scattering. The agreement with experiment is good without any adjustable parameter.In the higher concentration range (N_D > 10¹⁸cm^?3), the dominant process for the linewidth is the spin-flip scattering by ionized donors (Elliott process); the usual theory is shown to be insufficient and the greater effectiveness of scattering by the localized part of the donor potentials is pointed out. The calculated linewidth is related to the intervalley scattering time T_iv. The agreement with experiment is good and predictions are given for the linewidth in the case of other shallow donors in the same concentration range.     

Search forββ decay of76Ge to the excited states in76Se

Lower limits on the half-life of theββ(2v+0v) decay of⁷⁶Ge to the excited states in⁷⁶Se have been obtained using the results of low-background measurements with a HPGe detector surrounded by passive germanium shielding: T_1/2(0⁺ → 2 ₁ ⁺ ) >1.1 · 10²¹ y, T_1/2(0⁺ → 0 ₁ ⁺ ) > 1.7 · 10²¹ y, T_1/2(0⁺ → 2^2/+) > 1.4 ·10²¹ y.     

EPR of the lithium ion-dangling bond complex in germanium containing dislocations

We have observed a new EPR spectrum in a lithium-diffused germanium crystal N_D-N_A ≈ 10¹³ cm^-3) containing dislocations. The principal g values are 1.917 ± 0.002, 1.896 ± 0.002, and 0.855 ± 0.010 along the axes <1$\text{1}$0>, <11$\text{2}$>, and <111>, plus equivalent sets. As this new spectrum is seen in lieu of the dislocation dangling bond spectrum, we attribute it to a lithium ion- dislocation dangling bond complex.     

Point defects in NiAl near the equiatomic composition

Perturbed γ-γ angular correlation spectroscopy was applied to study the structure and properties of point defects near¹¹¹In probes in quenched and annealed NiAl samples with five compositions in the range c_Ni=48–52%. Quadrupole interaction signals are analysed in terms of near-neighbor Ni-vacancy and nextnear-neighbor Ni antisite atom defects. Large mono- and divacancy concentrations were observed in all samples after quenching. For c_Ni<50%, the vacancy defects could not be annealed out (structural defects). For C_Ni>50%, large quenched-in vacancy concentrations annealed out near 500°C. For c_Ni=50%, most vacancies annealed out near 500°C, but about 35% of the¹¹¹In probes retained a trapped monovacancy. This is attributed to a very low value of the formation enthalpy of a vacancy next to the In probe, estimated to be 0.16(2) eV. A lesser annelaing stage detected at 300°C is tentatively attributed to diffusion of Al-vacancies.     

Correlated pairs in the attractive Hubbard model

The attractive Hubbard Model is considered in the strong coupling limits (U?t) by treating the hopping integral, t, as a perturbation. A phase diagram emerges with two critical temperatures: itk_BitT_ct²/U and itk_BT_UU/4. For T<T_c, there is regime of strongly correlated pairs reminescent of superconductivity. For T_c<T<T_U, there is a domain of uncorrelated pairs. For T>T_U, one has a normal metal.     

Kaonic atoms measurements at the DAΦNE accelerator

Radioactive ⁵⁷Mn⁺(T _1/2?= 1.5 min) ions have been implanted at the ISOLDE facility at CERN with 60 keV energy to fluences <10¹²/cm² into p-type Si_1???x Ge _x (x < 0.1) single crystals held at 300–600 K. The implantation and annealing processes result in the majority of the implanted Mn ions occupying substitutional lattice sites. In the subsequent ⁵⁷Mn nuclear β ^???-decay to the 14.4 keV Mössbauer state of ⁵⁷Fe (T _1/2?= 100 ns), an average recoil energy of 40 eV is imparted to the ⁵⁷Fe daughter atoms which results in a large fraction being expelled into tetrahedral interstitial sites and the creation of a vacancy. The remainder occupies substitutional sites. This technique of recoil production of ^57m Fe_I thus allows for the study of the diffusion characteristics of interstitial Fe. From the temperature dependent line broadening, the activation energies have been determined and decrease with increasing Ge concentration which contributes significantly to the increase of the jump frequency. A similar result has been obtained in n-type SiGe but there the values for the activation energies were much higher.     

IR absorption and Raman spectra of single crystals of stable germanium isotopes

The Raman and IR absorption spectra of single crystals of germanium isotopes ⁷²Ge, ⁷³Ge, ⁷⁴Ge, and ⁷⁶Ge in the region of phonon absorption and interband electronic transitions are studied at room temperature. The dependence of the Raman peak position on the atomic mass has the form ν ~ M^–1/2. The shifts of the phonon absorption peaks of individual isotopes with respect to germanium of natural isotopic composition ^natGe are determined. With increasing average atomic mass of germanium, these peaks shift to longer wavelengths. In the region of interband electronic transitions, the intrinsic absorption edge of ⁷⁶Ge is observed to shift by 1 meV to higher energies with respect to Ge of natural isotopic composition. For isotopes with atomic masses close to that of natural germanium (⁷²Ge,⁷³Ge, ⁷⁴Ge), we found no significant difference in the band gap width at room temperature.     

Anomalous electrical conductivity of the potassium deficient linear chain platinum compound: K1.75Pt(CN)4·1.5H2O

Four probe single crystal dc conductivity, σ, has been measured for K_1.75Pt (CN)₄·1.5H₂O for temperature 360K > T > 30K on crystals prepared via electrochemical growth, with σ (295K)

25–50 Ω^?1cm^?1. An anomalous behavior in σ (T) is reported with σ (T) nearly constant for T > 300K and 160K > T > 80K, and rapidly decreasing σ (T) observed for 300K > T > 160K and T < 80K. The data for 200K < T < 308K are analyzed in terms of a three-dimensional second order phase transition occurring at T_c = 308K. These results contrast sharply with those of the well studied K₂Pt(CN)₄X_0.3·3H₂0 (X=Cl,Br).