Weakly dispersive band near the fermi level of GaMnAs due to Mn interstitials

The nature of the weakly dispersive electronic band near the Fermi level observed in photoemission experiments on the diluted magnetic semiconductor GaMnAs is investigated theoretically. The combination of experimental features appears puzzling. We show that the formation of the band is closely related to the presence of the Mn interstitial impurities. The states forming the band have predominantly minority-spin Mn-3d character. The low experimental Mn-3d intensity is explained by the low content of the interstitial Mn impurities. The features of the band are robust with respect to the calculational technique [local density approximation (LDA), LDA + U].     

Anion antisite defects in GaAs and GaP

The electronic properties of anion antisite defects and the related ideal cation vacancies are calculated based on tight-binding Hamiltonians and using a novel recursion method. For the antisite defects symmetric A₁ states are found in the upper part of the fundamental gaps, and for the ideal vacancies T₂ states are found in the lower part of the gaps. These results for antisite defects compare favorably with recent ESR experiments.     

Intrinsic point defects and light interstitials in metals

Clusters consisting of a radioactive probe atom and various point defects in metallic hosts were studied by means of NMR-ON and PAC. NMR resonance signals were observed for¹³¹IV₂ clusters in Fe and^114mInV₄ clusters in Ni. Decoration of¹¹¹InV₂ clusters by H atoms in W and Mo and by He atoms in W was monitored by PAC. The measurements yield the first reliable data on vacancy-hydrogen binding energies in these metals. The He-decoration results are in excellent agreement with the interpretation of He-desorption data and clearly show the existence of trap mutation.     

Atomic-scale visualization of antisite defects in LiFePO4

We visualize the antisite exchange defects in LiFePO4 crystals with an ordered olivine structure by using annular dark-field scanning transmission electron microscopy (STEM). A recognizable bright contrast is observed in some of the Li columns of STEM images in a sample annealed at a lower temperature, which directly demonstrates the disordered occupations by Fe atoms. Furthermore, such exchange defects appear to be locally aggregated rather than homogeneously dispersed in the lattice, although their overall concentration is fairly low. The present study emphasizes the significance of atomic-level observations for the defect distribution that cannot be predicted by macroscopic analytical methods.     

Role of surface antisite defects in the formation of heterojunctions

Fermi level pinning positions were measured for Si overlayers on cleaved, n-type GaAs and GaP substrates. The observed positions are 0.73 ± 0.1 eV and 0.98 ± 0.1 eV above the top of the substrate valence band. Theoretical calculations of the surface antisite acceptor levels predict the same chemical trend on going from GaAs to GaP. This agreement and other arguments suggest an important role for surface defects in the formation of semiconductor heterojunctions.     

Luminescence of excitons and antisite defects in the phosphors based on garnet compounds

Intrinsic emission (excitons, anti-site defects (AD)) of oxides with garnet structure has been analysed by means of investigation of the time resolved luminescence and decay kinetic spectra under excitation by the synchrotron and X-ray radiation at 10–300 K of the model objects: undoped single crystals and single crystalline films of Y₃Al₅O₁₂ and Lu₃Al₅O₁₂ garnets which are characterised by the significant differences in concentrations of AD Y_Al³⁺ and Lu_Al³⁺-types as analogs of cation isoelectronic impurities.     

Formation of antisite defects by gliding dislocations in sphalerite-structure crystals

Conditions are discussed under which an interstitial atom absorbed in the core of a gliding 60° dislocation belonging to the glide set can be transformed into an antisite defect. The mechanism considered may be responsible for an increase in the concentration of As_Ga defects observed in GaAs single crystals after their plastic deformation.     

Arsenic antisite defects as the main electron traps in plastically deformed GaAs

It is found from DLTS measurements that plastic deformation of GaAs single crystal creates a new kind of electron traps with an activation energy of 0.37 eV, and gives rise to an increase in the concentration of main electron traps with an energy of 0.80 eV. By comparing the concentrations of the main electron traps before and after deformation with analogous concentrations of As_Ga paramagnetic centers, found by EPR experiments, it is concluded that the centers observed in both cases are of the same origin. A nonstandard feature of the main traps is discovered: linear dependence of the DLTS-peak amplitude on the logarithm of the filling-pulse duration time. This feature can be explained in terms of the barrier-limited capture rate, assuming the traps are arranged in rows.     

Comparison of vacancy and antisite defects in GaAs and InGaAs through hybrid functionals

The formation energies and charge transition levels of vacancy and antisite defects in GaAs and In(0.5)Ga(0.5)As are calculated through hybrid density functionals. In As-rich conditions, the As antisite is the most stable defect in both GaAs and InGaAs, except for n-type GaAs for which the Ga vacancy is favored. The Ga antisite shows the lowest formation energy in Ga-rich conditions. The As antisite provides a consistent interpretation of the defect densities measured at mid-gap for both GaAs/oxide and InGaAs/oxide interfaces.     

The formation of arsenic antisite defects during plastic deformation of GaAs

The electron paramagnetic resonance (EPR) signal of arsenic antisite defects increases after plastic deformation of GaAs. This has been attributed in the preceeding paper to the formation of only compensating acceptors rather than additional antisites. A critical discussion of this alternative model is presented, taking into account new experimental results.     

Neutron spectroscopy study of the local modes of interstitials in transition metals

Astract Local modes of carbon, nitrogen, oxygen, and hydrogen have been measured in a number of transition-metal solid solutions by inelastic neutron scattering. The metal-impurity force constants have been calculated from experimental data and constructed as functions of the metal-impurity distance. The correlation of these functions with the data obtained in effective-medium theory, as well as the effect of interstitial atoms on the metal-metal bonding are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 23–27 (January 1997)     

Optically detected electron nuclear double resonance of anion antisite defects in GaP

Anion antisite defects in semi-insulating GaP single crystals (doped with group V atoms) are investigated with optically detected electron paramagnetic resonance and electron nuclear double resonance (ODEPR and ODENDOR) ODEPR and ODENDOR are measured as microwave- and microwave- and rf-induced changes of the magnetic circular dichroism of the optical absorption (MCDA), respectively. In GaP: V two P antisite defects are found in about equal concentration which within ENDOR precision have tetrahedral site symmetry with respect to the nearest P ligands. The two P_GaP₄ defects have only slightly differentg-factors and³¹P superhyperfine interactions, but differ by one order of magnitude in their spin-lattice relaxation times. Their optical properties are also different from those of the P_GaP₄ defects reported earlier in GaP: Cr. It is concluded that there is no experimental prove yet. that any of the P_GaP₄ defects hitherto observed in GaP in an isolated P_GaP₄ defect which is not associated with another defect to form an antisite related defect complex.     

Pre-edge features in X-ray absorption structure of Mn in GaMnN,GaMnAs and GeMn

_Ga1-xMnxN${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{N}$ samples with a wide range of concentrations, from x=0.003$x=0.003$ to 0.057, were grown by molecular beam epitaxy. X-ray diffraction and the simulation of the extended X-ray absorption fine structure (EXAFS) confirmed the wurtzite structure of the samples, without any secondary phase, and the location of Mn in the gallium sublattice of GaN. The valence state of Mn was studied using the X-ray absorption near-edge structure (XANES) at the K-edge of Mn. The shape of the measured XANES spectra does not depend on the Mn concentration: that implies the same valence state and local atomic structure around the Mn atom in all samples. A calculation of the electronic band structure of GaMnN, GaMnAs, GeMn and the XANES spectra of Mn in GaMnN was performed using the linearized augmented plane wave (LAPW) method. The calculated spectra fit well our experimental data. In particular, the comparison demonstrates that a peak in the pre-edge structure is directly related to the 3+$3+$ valence state of Mn. This was confirmed experimentally by measurements of the Mn K-edge in GaMn²⁺As${\mathrm{GaMn}}^{2+}\mathrm{As}$ and ZnMn²⁺Te${\mathrm{ZnMn}}^{2+}\mathrm{Te}$. An application of proposed interpretation for GeMn is discussed.     

Effect of annealing on the electric and magnetic properties of GaMnAs and Be-codoped GaMnAs

GaMnAs and Be-codoped GaMnAs films grown via molecular beam epitaxy (MBE) were heat treated and the stability of Mn in the matrix was investigated. MnAs had a stable phase at the low growth temperature, but MnGa was stable at the annealing temperature. Be-codoping did not prevent the precipitation processes, but Be itself was stable during the annealing process to maintain the GaAs matrix at the high conductivity.     

Determination of antisite defects in double perovskite Sr2FeMoO6 by diffraction method: Simulation studies

The crystal structure of double perovskite Sr₂FeMoO₆ synthesized via solid-state reaction at 1280 °C under a reduction atmosphere is refined by Rietveld technique based on X-ray powder diffraction (XRD) data in 2θ range of 15-140°. An antisite content (AS), i.e. Fe on the Mo sites (=Mo on the Fe sites), of 12.1(1)% is derived. In reference to the refinement results, a series of X-ray and neutron powder diffraction (NPD) data with different antisite contents, ranging from 0 (completely ordered) to 50% (completely disordered), are generated with a Poisonian noise added and subjected to Rietveld refinements with the same initial values for the refinable parameters. The AS is reproduced satisfactorily from the refinement of XRD data and the combined refinement of XRD and NPD data with a relative deviation smaller than 4%, whereas the relative deviation of AS derived from the refinement of NPD data can be as large as 50%. However, the atomic occupancies and isotropic temperature factors of all the atoms can be reasonably reproduced from the refinement of NPD data and the combined refinement of XRD and NPD data, whereas these data can be reproduced only for cations (Sr, Fe, Mo) from the refinement of XRD data. The present simulation studies shed light on understanding the controversial statements derived from XRD and NPD work regarding the antisite defects in Sr₂FeMoO₆, which in turn is indispensable for understanding the mechanism of large room temperature low-field magnetoresistance of the compound.     

Comparison of annealing effects on Zn-doped GaMnAs and undoped GaMnAs epilayers

To compare the annealing effects on GaMnAs-doped with Zn (GaMnAs:Zn) and undoped GaMnAs (u-GaMnAs) epilayers, we grew GaMnAs thin films at 200 °C by molecular beam epitaxy (MBE) on GaAs substrates, and they were annealed at temperatures ranging from 220 °C to 380 °C for 100 min in air. These epilayers were characterized by high-resolution X-ray diffraction (XRD), electrical, and magnetic measurements. A maximum resistivity at temperatures T_m close to the Curie temperatures T_c was observed from the measurement of the temperature-dependent resistivity ρ(T) for both the GaMnAs:Zn and the u-GaMnAs samples. We found, however, that the maximum temperature T_m observed for GaMnAs:Zn epilayers increased with increasing annealing temperature, which was different from the result with the u-GaMnAs epilayers. The formation of GaAs:Zn and MnAs or Mn-Zn-As complexes with increasing annealing temperature is most likely responsible for the differences in appearance.