T. Bretagnon

Vacancy‐type defects in crystalline and amorphous SiO2

Positron lifetime spectroscopy and two‐dimensional angular correlation of annihilation radiation have been used to investigate grown‐in vacancy structures in synthetic crystalline α‐SiO2, synthetic fused quartz, and in a 60‐μm‐thick chemical‐vapor‐deposited amorphous SiO2 film. For α‐SiO2 a ∼300 ps lifetime component suggests trapping by either silicon monovacancies or by oxygen divacancies (or both). The vacancies are neutral and present at a concentration level of 1017/cm3. The positron bulk lifetime for α‐SiO2 is estimated to be ∼238 ps in good agreement with semiempirical predictions. In the fused quartz significant positronium formation is found (80%) and the remaining positrons annihilate in voids yielding a lifetime of ∼500 ps. The amorphous SiO2 film contains a mixture of small vacancy clusters and voids and ∼30% of the positrons form positronium. Heat treatment above 950 °C results in a substantial reduction in defect concentration, but up to 1100 °C a small vacancy cluster contribution persists....

Indium vacancy in as‐grown InP: A positron annihilation study

The positron lifetime technique has been used to investigate grown‐in defects in various types of indium phosphide. A neutral monovacancy‐type defect has been detected independently of the nature (Zn,Fe,Si,S) and concentration of the dopants. The defect is stable at least up to 800 °C, and is suggested to be a trapped indium vacancy.

Positron annihilation investigation of vacancies in as-grown and electron-irradiated diamonds

Abstract Vacancy-type defects in the four main types of diamond (Ia, Ib, IIa and IIb) were investigated using positron lifetime, Doppler broadening and optical absorption spectroscopies. In unirradiated samples vacancy clusters were found in all types, synthetic as well as natural. These clusters are situated in highly defected regions, rather than homogeneously distributed, and their concentration varies significantly from sample to sample. For synthetic Ib diamonds vacancy clusters were investigated as a function of nitrogen content. The bulk lifetime for diamond is calculated to be 98±2 ps and the bulk Doppler S parameter is estimated to be 25% lower than that for silicon. Electron irradiation (2.3 MeV) produced neutral monovacancies in IIa diamond and the positron data correlated well, as a function of dose, with the GR1 optical zero-phonon line; the introduction rate was estimated to be 0.5±0.2 cm −1 . In Ib diamond, monovacancies were found to be negatively charged. The positron lifetime for monovacancies was (40±6)% larger than the bulk lifetime and the Doppler S parameter increased by (8±1)%. At-temperature Doppler measurements between 30 and 770 K indicated that irradiation-produced neutral monovacancies can convert to the negatively charged state above 400 K but this was dependent on diamond type. Isochronal annealing of irradiated Ib diamonds showed that the complex of a substitutional nitrogen and a vacancy, formed upon annealing close to 600°C, undergoes two detectable modifications between 600 and 870°C reaching a configuration stable to 1170°C. Key conclusions based on positron and optical data are in mutual accord.

Positron lifetime investigations of diamond films

Abstract Positron lifetime investigations on B-doped and undoped chemical vapor deposition diamond films have shown that B-doping at 500 ppm almost completely removes vacancies from the film. The 1.68 eV photo luminescence (PL) line is also removed. For undoped films, large concentrations (≈1018 cm−3) of vacancy clusters (approximately 6 vacancies) and mono- or divacancies are observed. Small observable effects arising from annealing up to 1100°C could be found in both B-doped or undoped films. There is no correlation between the width of the 1.68 eV PL line and the vacancy concentration.

Positron annihilation investigations of vacancies in InP produced by electron irradiation at room temperature

Positron lifetime investigations were done on a series of InP samples irradiated to various doses with 2.5 MeV electrons. In n-type materials, positron lifetimes of 265±5 and 338±15 ps are attributed to indium vacancy–interstitial complexes and divacancy–interstitial complexes, respectively. In p-type materials these defects were not observed. Thermal annealing took place up to 200 °C for both defect types. Introduction rates were estimated to be 0.1 cm−1 for VIn⋅InI and ∼0.05 cm−1 for the divacancies. The divacancies showed a temperature dependence of the trapping rate, which suggests a thermally activated process. No evidence for VP vacancies could be found in neither p-type nor n-type materials.

A positron lifetime investigation of InP electron irradiated at 100 K

Positron lifetime investigations have been made on variously doped InP samples irradiated at 100 K with 2.5 MeV electrons. Three irradiation‐produced positron lifetimes were found: 240±10, 265±10, and 330±20 ps which are, respectively, ascribed to VP, VIn, and VP⋅VIn vacancies in close association with interstitials. Total introduction rates of these defects were in the range of 0.6–1.2 cm−1. Observation of the defects depends on the position of the Fermi level. In n‐type materials no evidence could be found for VP⋅PI, while in p‐type material VIn⋅InI was not observed. Annealing studies up to 300 K show that VP⋅PI anneals slightly below 300 K, while VIn⋅InI anneals in part around 125 K, but a sizable fraction remains at 300 K. Divacancies begin annealing at 125 K, but some can persist to 300 K.

Characterization of vacancies in as-grown and electron irradiated α-quartz by means of positron annihilation

Synthetic α-quartz is shown to contain a significant concentration (several ppm) of vacancies. The major concentration of vacancies is suggested to be in the form of divacancies, giving rise to a positron lifetime of 285±5 ps, but in addition, there is a much smaller concentration of large vacancy clusters that are observable only after electron irradiation, whereupon they give rise to a positron lifetime close to 425 ps. Annealing between 900 and 1000 °C causes disappearance of divacancies and formation of vacancy clusters giving rise to a positron lifetime close to 300 ps. Above ∼950 °C positronium is formed with an exceptionally long lifetime (3–5 ns) ascribable to the formation of an amorphous phase connected with the thermal instability of β-quartz. Electron irradiation (2.3 MeV at 8 °C) gave rise to a 250±5 ps lifetime component interpreted to signify formation of neutral monovacancies, V0 and/or VSi. Their introduction rate is nonlinear, decreasing abruptly by a factor of ∼5 above a dose of 1×1017 ...

Character and distribution of vacancies in Czochralski‐grown silicon ingots

Positron lifetime investigations of vacancy distributions in ingots of silicon have shown that vacancies are retained after growth at nearly constant concentrations close to 3×1016 cm−3. The vacancies are generally monovacancies and are suggested to be trapped by oxygen clusters. Trapped divacancies can also be formed but they are unstable upon heat treatment at 1000 °C for 16 h. This observation is invoked to explain anomalous oxygen precipitation. This heat treatment has little effect on the distributions of monovacancies in the ingots investigated, so the complexes between vacancies and oxygen clusters are suggested to be formed at temperatures above 1000 °C during the growth.