Vacancy-type Complexes Research Articles

Positron lifetime spectroscopy of lithium tetraborate Li2B4O7 glass

Free-volume structure of glassy lithium tetraborate Li2B4O7 is studied as compared with isocompositional crystal using positron annihilation lifetime spectroscopy in mixed positron-trapping and Ps-decaying modes exploring three-term decomposed positron lifetime spectra. The longest-lived lifetime component in Li2B4O7 glass is explained due to low-electron density spaces between neighbouring tetraborate groups, while second component is attributed to positron trapping in lithium vacancy-type complexes. Crystal-to-glass transition in free-volume structure of Li2B4O7 is examined with x3-x2-coupling decomposition algorithm. Assuming tight interconnection between atomic-deficient void structure of crystalline and glassy lithium tetraborates, the vitrification from crystalline state is described as expansion of positron-trapping complexes involving singly-ionized lithium vacancies in more spatially-extended positron-trapping sites, this being accompanied by large number of Ps-traps appeared preferentially in boron-oxide network of tetraborate structural units.

Effects of annealing temperature on buried oxide precipitates in He and O co-implanted Si

The effects of the processing conditions on the formation of buried oxide precipitates in He and O co-implanted Si were investigated by the combination of Fourier transform infrared (FTIR) absorption spectroscopy, depth-resolved positron annihilation Doppler spectroscopy, and transmission electron microscopy (TEM). Silicon wafers were implanted with 50 keV He ions at a fluence of 2 × 1016 cm−2 and subsequent 150 keV O ions at a fluence of 2 × 1017 cm−2. For comparison, reference Si wafers were only implanted with 150 keV O ions. The Si–O–Si stretching frequency increases while the peak width of the Si–O–Si stretching absorption band decreases with an increase in annealing temperature. After the same annealing, the peak width of the Si–O–Si stretching absorption band in the He and O co-implanted sample is significantly larger than that in the reference sample. Two kinds of vacancy-type defects are observed by positrons, i.e., vacancy-type defects and vacancy-oxygen complexes. The characteristic S values of vacancy-type defects and vacancy-type complexes in the He and O co-implanted sample are smaller than those of the reference sample. In addition, the thickness of the buried oxide layer in the He and O co-implanted sample is smaller than that in the reference sample. After annealing at 1473 K, the O content is larger in the He and O co-implanted sample compared to that in the reference sample.

Vacancy generation during Cu diffusion in GaAs

Positron lifetime and Doppler broadening spectroscopy were applied for a study of defect properties of semi-insulating GaAs after diffusion of copper. A 30 nm layer of Cu was deposited by evaporation to the undoped GaAs samples. The diffusion of Cu was performed during an annealing step at 1100 °C at different arsenic vapor pressures. The samples were quenched into room temperature water. The initial semi-insulating (SI) undoped GaAs sample shows no positron traps. After annealing, a vacancy-type complex and a shallow positron trap were observed to be efficient positron traps. Due to the Cu contamination during the annealing process, the shallow trap is believed to be the CuGa double acceptor. The nature of the vacancy-type defects could not be determined unambiguously. The concentration of these vacancies shows inverse relationship to the As vapor pressure that refers to the arsenic vacancy as a part of this complex. Moreover, Doppler-coincidence spectroscopy shows clearly that Cu atoms are not bound in the direct vicinity to the observed vacancies.

Investigation of vacancy-type complexes in GaN and AlN using positron annihilation

The momentum distribution of electron-positron pairs in GaN and AlN has been investigated for the first time by measuring the one-dimensional angular correlation of the annihilation radiation (1D-ACAR). The characteristic parameter of the electron density r s ′ (the radius of the sphere occupied by an electron) estimated from the experimental data differs noticeably from r s calculated in terms of a standard model of noninteracting particles: r s ′ (AlN)≈1.28r s, and r s ′ (GaN)≈1.66r s, where r s(AlN)≈1.6091 au, and r s(GaN)≈1.6568 au. The chemical nature of atoms in the environment of the annihilating positron in AlN and GaN was identified from the electron-positron ionic radius of Al3+, Ga3+, and N5+ cores, which were determined from the characteristics of the high-momentum component of 1D-ACAR curves. The analysis was based on a comparison of the electron-positron ionic radii with those considered standard for Al and Ga metals and the related compounds GaP, GaAs, and GaSb. A conclusion is made that positron annihilation dominates in the regions of vacancy-type “nitrogen antisite”-“vacancy” complexes, [N Ga + V Ga] and [N Al + V Al], in GaN and AlN, respectively.

Structure-Dependent Vibrational Lifetimes of Hydrogen in Silicon

The lifetimes of the Si-H vibrational stretch modes of the H(*)(2) ( 2062 cm(-1)) and HV.VH((110)) ( 2072.5 cm(-1)) defects in crystalline Si are measured directly by transient bleaching spectroscopy from 10 K to room temperature. The interstitial-type defect H(*)(2) has a lifetime of 4.2 ps at 10 K, whereas the lifetime of the vacancy-type complex HV.VH((110)) is 2 orders of magnitude longer, 295 ps. The temperature dependence of the lifetime of H(*)(2) is governed by TA phonons, while HV.VH((110)) is governed by LA phonons. This behavior is attributed to the distinctly different local structure of these defects and the accompanying local vibrational modes.

Defects agglomeration in the vicinity of hydrogen-related vacancy-type complexes in proton-implanted silicon

The results of IR studies of H interactions with radiation-induced defects in crystalline Si implanted with protons (Si : H) are presented. Wide doublets at ∼1980, 2060 and ∼720, 620 cm −1 with relatively low thermal stability (the doublets are fully annealed at 550 K) appear in IR absorption spectra of Si : H when implantation dose is increased. It is shown that the doublets may be assigned to nuclei of two hydrogenated amorphous phases with different densities.The Si : H stretching spectra of as-implanted samples are greatly simplified with the increasing implantation dose. Four groups of IR bands predominate at high implantation doses: 2222 cm −1—VH 4; 2072 cm −1—V 2H 2; the doublet at 2166, 2191 cm −1—V 2H 6; 1967 cm −1—an interstitial-type complex and the doublet at 2107, 2122 cm −1—a vacancy-type complex. It is shown that the doublet at 2107, 2122 cm −1 may be tentatively assigned to V 6H 12 which can serve as nuclei or precursors of the {1 1 1} platelets.

Defects in undoped and Mg-doped GaN and Al xGa 1− xN

We report on a quantitative identification of the shallow oxygen donor in GaN, compare the defects which are detected by optically detected magnetic resonance in the yellow and red emission bands in nominally undoped GaN, and demonstrate that independent of the doping level the microscopic structure of the Mg acceptor remains the same. There is a minor influence on the magnetic resonance parameters of Mg, which comes from a modification of the strain and changes by Mg incorporation. With increasing Al content in the Al x Ga 1− x N alloy the g-values become isotropic. The compensating centers are most likely vacancy-type complexes.

The impact ofin situphotoexcitation on the formation of vacancy-type complexes in silicon implanted at 85 and 295 K

Photoexcitation of silicon during low-fluence implantation with MeV Si and Ge ions is observed to suppress vacancy-type point-defect formation, as determined by in situ deep-level transient spectroscopy. The A-center formation after low-temperature implantation is extended over a wide temperature interval indicating that electrically inactive clusters, which emit vacancies during annealing, are formed in the end-of-range region during implantation at 85 K. The number of vacancies stored in these clusters is influenced by low-temperature in situ photoexcitation.

Room-temperature diffusivity of self-interstitials and vacancies in ion-implanted Si probed by in situ measurements

We have determined the room-temperature diffusivity of self-interstitials and vacancies in Si. Silicon p+−n junctions were realized in n-type epitaxial Si wafers, having an O and C content ⩽1015/cm3, and implanted at room temperature with 2.5 MeV He ions to fluences in the range 1×109–1×1012/cm2. The junctions were reverse biased at −30 V, in order to embody the entire damage profile of the ion in the depletion layer, and in situ leakage measurements were performed during and just after implantation. It is found that the leakage current increases monotonically during implantation while, at the beam turn off, it decreases by about a factor of 2 for times as long as 1 day. Ex situ deep level transient spectroscopy measurements show that the main contribution to leakage current is due to the deep levels introduced in the band gap by phosphorous–vacancy and divacancy complexes. This allowed us to associate the leakage current reduction at the beam turn off to the recombination of vacancy-type complexes by residual free interstitials. When, for a fixed fluence, the ion flux is increased, an initial faster transient, lasting up to ∼1000 s, is observed and has been attributed to the annihilation of residual free vacancies. Diffusivity values of 1.5×10−15 and 3.0×10−13 cm2/s for interstitials and vacancies, respectively, have been achieved from the analysis of these data.

Influence of growth conditions on the formation of deep photoluminescence bands in MBE-grown Si layers and SiGe/Si quantum structures

Influence of the growth parameters, i.e. growth temperature and ion bombardment, on the quality of SiGe/Si quantum structures and Si thin films, grown by molecular beam epitaxy (MBE), is studied using photoluminescence (PL) spectroscopy, optically detected cyclotron resonance and high-resolution X-ray diffraction measurements. The bombardment of even a low flux of Si + ions, as present in the MBE growth with biased substrates, is shown to degrade essentially the quality of epitaxial structures due to creation of both point-like and extended defects. The low temperature MBE growth primarily results in a quenching of SiGe-related emissions relative to the substrate-related PL due to formation of competing non-radiative channels associated, presumably, with vacancy-type complex defects.

Hyperfine interactions of impurities in defect sites in ion-implanted metals

The lattice location of ion-implanted radioactive isotopes in metals and their interactions with defects in their own radiation damage cascade are of importance for studies of the hyperfine interactions of such probe atoms by nuclear methods. Recent results from Mössbauer and PAC experiments in particular are reviewed. Emphasis is put on lattice site identifications, which can be inferred from measured lattice-dynamical and hyperfine interaction parameters of probe atoms. Some general conclusions on the hyperfine interactions in interstitial- and vacancy-type complexes are drawn.