Defect study on electron irradiated GaAs by means of positron annihilation

Measurements of the positron lifetime and Doppler-broadened annihilation-radiation have been performed in electron-irradiated GaAs. The positron lifetime at the irradiation induced defects was 0.250 ns at 300 K. The defect clustering stage was found to occur at around 520–620 K, and the coarsening and annealing stage is believed to be above 620 K. Similar annealing stages were also observed in GaAs lightly doped with Si (0.2×10¹⁸ cm^–3). Both the lifetime and the S-parameter in the irradiated GaAs were found to decrease with temperature from 300 K to 100 K, suggesting the coexistence of shallow traps in electron irradiated GaAs.     

Positron annihilation in ultrafine particles of Fe

Measurements of two-photon annihilation in ultrafine particles of Fe (～ 100 A in diameter) show that a considerable fraction of the positrons annihilate in the state of positronium. The emission of slow positrons from a metal surface can account for these results.     

Microstructural evolution of ZnS during sintering monitored by optical and positron annihilation techniques

Positron lifetime and optical absorption techniques were employed to track the microstructural evolution of polycrystalline ZnS grown by Chemical Vapor Deposition (CVD). As grown material and material treated with Hot Isostatic Pressure (HIP) was sintered at temperatures ranging from 400 to 1000°C for 2–18 h. A 290 ps defect lifetime could be resolved in all samples, while an additional longer lifetime (=430 ps) was found only in samples annealed at low temperatures. This component gradually disappeared during annealing at 800°C. Associated with the disappearance of the long-lived component, the apparent bulk lifetime of the material changed from 235 to 215 ps. A 215±2 ps bulk parameter was also found for HIP-treated material annealed at temperatures greater than 400°C and hence is taken to represent the delocalized state of the positrons in ZnS. Optical absorption measurements showed that annealing at 800°C also caused the absorption profiles of the CVD and HIP samples to converge. The rate of the bulk lifetime transition correlates with the absorption changes. The observed sharpening of the absorption profile is attributed to a decrease in scattering from grain boundaries and voids, and a decrease in absorption from point defects. The 430 ps lifetime is believed to be due to trapping at voids and grain boundaries, while the 290 ps lifetime likely is due to a monovacancy stabilized as a small complex.Paper presented at the 132nd WE-Heraeus-Seminr on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

A positron annihilation study in some amorphous scintillators

Two organic scintillators — NE104 and a liquid solution of 2,5 diphenyloxazole (PPO) — were investigated through positron annihilation spectroscopy to study the influence of the structural order on the magnetic quenching effect on positron bound systems in solids.Preliminary note in [11]     

Evaluation of the formation entropy of a single vacancy by means of positron annihilation

Analyzing positron annihilation data it was found that the formation energy of a single vacancy is proportional to the characteristic temperature T_c in f.c.c. metals. Assuming that the equilibrium fractional concentrations of single vacancies at T_c are constant, one finds that the formation entropy of a single vacancy is (1.6 ± 0.1)k for all f.c.c. metals. For zinc and cadmium, the formation entropies are 2.5k and 3.3k respectively.     

Review of defect investigations by means of positron annihilation in II-VI compound semiconductors

1-x Cd_xTe after post-growth annealing. The experiments on defect generation and annihilation after low-temperature electron irradiation of II-VI compounds are also reviewed. The characteristic positron lifetimes are given for cation and anion vacancies. Received: 19 November 1997/Accepted: 20 November 1997     

The study of defects in crystals by positron annihilation

The study of defects in crystals through their interaction with thermalized positrons is usually based on the trapping model. The theoretical foundation of the trapping model, based on the theory of diffusion-limited reactions, is reviewed with particular emphasis on short-range interactions between positrons and trapping defects. For this case the analytical properties of the trapping model are developed in some detail, including the effects of the initial pair distribution on the trapping rate, the effects of detrapping, and the effects associated with more than one type of trap. The assumptions that are usually made in determinations of monovacancy formation enthalpies from positron annihilation experiments are critically discussed. The need for additional experiments capable of providing information on the temperature dependence of the various parameters involved is, stressed. As an illustration we treat in detail monovacancies with finite detrapping rate plus divacancies, both in thermal equilibrium. In the final section a few remarks are added on cases with long-range interactions between positrons and traps, such as electrically charged point-defects in insulators or semiconductors, and dislocations.     

A study on the influence of impurities on the recovery and recrystallisation behaviour of cold rolled Ni above room temperature by means of positron annihilation

The recovery and recrystallisation behaviour of cold rolled Ni of different purity such as Ni doped with Sb, Ti and B between room temperature and 550°C was studied by measurement of the peak counting rate of 2γ-angular correlation during annealing with constant heating rate. The results indicate a strong dependence of the annealing behaviour on type and concentration of impurity atoms. For Ni of purity 99.8% and especially for NiSb the peak counting rate shows a large increment in the temperature region 200–400°C which is explained to be caused by the formation of vacancy clusters.     

Monovacancy formation energy in copper,silver, and gold by positron annihilation

Monovacancy formation energies in copper, silver, and gold have been deduced from the temperature variation of the peak counting rate in the angular correlation curve of positron annihilation radiation from these metals. The counting rate was temperature dependent over the entire temperature range, including temperatures so low that no trapping of positrons at vacancies is effective. At these temperatures the increase in counting rate results from thermal expansion of the lattice. By separating this thermal expansion effect from the vacancy trapping effect at higher temperatures, we obtained values for the monovacancy formation energyE _1v for copper, silver, and gold to 1.29±0.02 eV, 1.16±0.02 eV, and 0.97±0.01 eV, respectively.     

Characterization of diamond single crystals by means of double-crystal X-ray diffraction and positron annihilation

The crystallinity of synthesized and natural crystals of diamond was characterized by double-crystal X-ray diffraction and positron annihilation. The two-dimensional angular correlation of annihilation radiation and positron lifetime measurements revealed that in natural crystals positroniums are formed in a high fraction. The synthesized crystal Ib showed both an extremely small width for the diffraction and a positron lifetime spectrum with a single component of the lifetime of 115 ps. In contrast, the natural diamonds contain a long-lived component of lifetime longer than 2 ns. The diffusion length of positrons was also measured by a variable-energy positron beam. In the synthesized crystal IIa, a diffusion length of about 100.8 nm was observed.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

A study of mono and divacancies in Cu and Au by positron annihilation

The peak counting rate of the angular correlation curve was measured for Cu and Au in the temperature range between ?196°C and 1200°C. The experimental data were analysed by means of the trapping model under consideration of positron trapping by mono-and divacancies and detrapping of positrons from monovacancies. Such an analysis leads to acceptable results for both monovacancy and divacancy parameters.     

A study of porous silicon prepared under different HF concentrations by positron annihilation

Two-dimensional angular correlation of positron-electron annihilation radiation and positron-lifetime experiments have been performed on lightly doped porous silicons prepared under different HF concentrations. Positronium formation in the etched pores and positron trapped in voids are observed in both experiments. A surprising result is found that both positron lifetime and momentum spectra show a reduction in the size of the etched pores with decreasing HF concentration in the etching solution. This trend is different from the intuitive expectation that the pore size increases with increasing porosity. Our result can be explained in terms of the formation mechanism of porous film in lightly doped silicon.     

A positron annihilation study of carbon black and carbon-black-filled polybutadiene

Studies have been made on positron lifetimes in carbon blacks and carbon black filled polybutadiene. The results for the carbon blacks can be interpreted with the aid of the theoretical results of Brandt and Paulin: Phys. Rev. B5, 2430 (1972) and show that while positron annihilation occurs principally from a bound state, diffusion in the carbon black is extremely small. A numerical upper limit on the diffusion coefficient has been obtained. The data from the rubber is discussed with the aid of a specially developed extension of the previous theory. Here one finds that whereas the variations in the long-component intensity with carbon black loading can be very satisfactorily explained, there is again no evidence for any diffusion effects in the rubber, and an upper limit can be placed on the diffusion coefficient here also. Indeed the variation in the long-component intensity with particle size goes the opposite way to what one would expect if diffusion were dominant. This effect is attributed to the increase in the amount of bound rubber as the particle size is reduced, and the consequent increase in the number of annihilation sites.     

Two-dimensional electron momentum distribution in graphite revealed by means of angular correlation of positron annihilation

Two-dimensional angular correlation measurements with positron annihilation radiation (2D-ACAR) on a single crystal of Kish graphite reveal that the motion of the annihilating electrons is nearly free along the graphite layers and restricted within some distance shorter than the lattice constant in the direction perpendicular to the layers. The hollows appearing on the 2D-ACAR surfaces are explained in terms of preferential annihilation of positrons with π-electrons in graphite.     

A note on nuclear excitation by positron annihilation with K-shell electrons

Experimental data for the nuclear excitation of¹¹⁵In by positron annihilation with K-shell electrons have been examined, taking account of the effective thickness of an In target used, and then the cross section _res for resonance excitation to the 1078 keV energy level by positron annihilation has been reevaluated. The cross section for the process is induced by the measured effective cross section _eff of nuclear excitation of the isomeric level^115mIn. In this work the effective thicknesses of the In target for positrons with kinetic energies 83.9 and 470 keV have been estimated. The effective thickness r has been determined from a relation r=E/ ¦dE/dr¦, where E is the positron energy width at 83.9 or 470 keV, r is the mean distance traversed by them, and ¦dE/dr¦ the stopping power of indium for them. In the present case, _K, the atomic-level width of the K-level of indium, is used as E. Neglecting a contribution from the 1464 keV level being much smaller than that from the 1078 keV level, a reevaluated value of _res has been obtained as 1.7×10^–25 cm².     

Polydimethylsiloxane rubber gamma radiation effect studies by positron annihilation lifetime spectroscopy

Polydimethylsiloxane rubber was irradiated at various radiation doses up to 800 kGy in air. The lifetime and intensity of the long component τ₃ were obtained by positron annihilation lifetime spectroscopy (PALS). The crystallization property and the molecular flexibility were estimated using the differential scanning calorimetry and dynamic thermomechanic analysis. Thermal volatile property was determined by thermogravimetry analysis. It was proved that the cross-linking reaction made the chain flexibility of the rubber to reduce gradually during gamma radiation, which resulted in the reduction of the o-Ps intensity observed by PALS. Although the degradation effect existed during gamma radiation, it was still less significant than the cross-linking effect when the radiation dose was up to 800 kGy.     

The study of native defects in as-grown GaAs by positron annihilation

A mono-energetic positron beam was applied for a study of native defects in GaAs. From measurements of Doppler broadening profiles and lifetime spectra of positrons, it was found that Ga-vacancies were introduced by Si-doping. On the other hand, for p-type GaAs, interstitial clusters were found to be introduced by Zn-doping. The observed species of defects are in agreement with those expected from the Fermi level.     

Theoretical study on the Rocksalt structured positron annihilation in magnesium oxide

Based on the atomic superposition approximation （ATSUP） and first-principles pseudopotential plane-wave methods, the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Borofiski-Nieminen positron-annihilation-rate interpolation formula respectively. The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results. The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.     

A study of Fermi surfaces of the α-phase Cu-Ge and Cu-Si alloys by positron annihilation

Angular correlation measurements on the fcc solid solutions of Cu?Ge and Cu?Si alloys have been carried out on single crystals with four crystallographic orientations by a crossed-slit geometry. The results on both the alloys are nearly the same. The 〈111〉-neck radius and the 〈100〉-radius of the Fermi surfaces increase almost linearly with increasing electron concentration in accord with previous results on Cu?Al and Cu?Zn alloys. The Fermi surface does not touch the square faces of the Brillouin zone at the solubility limit. The results agree fairly well with a calculation based on the sinking-conduction band model.     

Theoretical study on the positron annihilation in Rocksalt structured magnesium oxide

Based on the atomic superposition approximation(ATSUP) and first-principles pseudopotential plane-wave methods,the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Boron’ski-Nieminen positron-annihilation-rate interpolation formula respectively.The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results.The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.