D. Kerr

Oxygen in silicon: a positron annihilation investigation

Positron lifetime and Doppler broadening investigations of oxygen in silicon (Cz silicon) have been performed. It was found that positrons may be trapped by defects yielding a positron lifetime of only 100 ps and a momentum distribution of the annihilation quanta which is wider than that for defect‐free silicon. This unequivocally shows that clusters of oxygen constitute positron traps. The trapping process was found to be thermally activated in the 25–300 K range. Isochronal and isothermal annealing showed that the clusters alternate during growth between having an interstitial character and a vacancy character. At 450 and 500u2009°C, growth of oxygen clusters takes place with an activation energy of 1.5 eV, while at 650 and 714u2009°C, these clusters disappear with an activation energy of 1.1 eV. At 714 and 790u2009°C, growth of new clusters takes place with an activation energy of 2.6 eV.

Vacancy interactions in GaAs

Isochronal annealing of zinc‐ or silicon‐doped GaAs as well as undoped semi‐insulating or low‐resistivity materials has been investigated by positron lifetime measurements. For impurity concentrations larger than 4.5×1017 cm−3, only monovacancy complexes such as ZnGaVAs, u2009SiAsVGa, or AsGaVGa are observed and they yield a positron lifetime of 265±5 ps. For impurity concentrations less than 1×1017 cm−3, divacancies dominate and yield a lifetime of 295±5 ps. The concentrations of grown‐in vacancies, either VAs or VGa, are both estimated to be in the range of (1–4)×1017 cm−3.

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 950u2009°C results in a substantial reduction in defect concentration, but up to 1100u2009°C a small vacancy cluster contribution persists....

A study of defects in amorphous silicon films

Positron lifetime measurements have been performed in an investigation of amorphous silicon films deposited on single‐crystal silicon wafers. The amorphous films were produced by evaporation in vacuum or by sputtering in an atmosphere of either pure argon or a mixture of argon and fluorine. All films gave rise to a new lifetime component τ2 having values in the range of 400 to 500 ps. As many as 70% of the positrons annihilating in the film were calculated to be trapped by defects in the film. During isochronal annealing, the τ2 component disappeared around 700u2009°C for the evaporated films, while it persisted at least up to 1300u2009°C for the sputtered films. Another lifetime with a value of 2 ns was also observed. This lifetime had a constant intensity of 0.2 to 0.3% for both evaporated silicon and sputtered silicon using argon gas. For the sputtered film using both argon and fluorine, the intensity of this component was increased by a factor of 10 after annealing at 1100–1300u2009°C.

Defect characterization in diamonds by means of positron annihilation

Abstract Positron lifetime measurements on type Ia diamonds have shown that divacancies and larger vacancy clusters (>10 vacancies) have been retained in these stones. Upon irradiation with 3.5 MeV electrons, monovacancies are formed in their neutral state of charge. Some divacancies are also formed. The monovacancies are introduced at a rate of 0.3 cm−1 and no charge transfer involving monovacancies was found at thermal equilibrium up to temperatures of 800 K.

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 800u2009°C, and is suggested to be a trapped indium vacancy.

On the character of defects in GaAs

Positron lifetime measurements on GaAs are presented and discussed and former measurements are reviewed. The limitations and appropriate criteria for adequate spectrum analyses are considered in detail. It is shown that exceptionally good statistical accuracy is necessary for a reliable and consequential decomposition, and that source corrections are very important. Results for liquid phase electro-epitaxially grown GaAs conclusively show that the bulk lifetime for GaAs is 220+or-1 ps. All other GaAs samples (grown by either the liquid encapsulated Czochralski method or by the horizontal Bridgman method) show variable bulk lifetimes up to 232 ps. Shallow traps are concluded to be the cause for this. The binding energy of positrons in such traps is estimated to be about 23 meV, and the trapping rate into deep traps from such shallow traps is found to be approximately five times smaller than from the bulk state. The shallow trap is likely to be substitutional boron or nitrogen. The shallow traps anneal mainly around 700 K. Deep traps are found in all samples, yielding lifetimes tau DI approximately=260 ps and 290 ps<or= tau DII<or=315 ps. These lifetimes occur in n-type, p-type and semi-insulating materials. The results support the authors earlier contention that the 290-315 ps components are due to divacancies and the 260 ps lifetime is due to monovacancies, in complex form with some impurity. The divacancy anneals around 620 K while the mono-vacancy-impurity complexes generally anneal around 800 K.

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.

Deformation‐induced defects in GaAs

Semi‐insulating undoped GaAs was plastically deformed and then investigated by positron lifetime spectroscopy. Strains between 0% and 40% and temperatures of deformation of 450, 500, and 600u2009°C were investigated, with detailed investigations carried out for the lowest temperature of deformation. Between 0% and 4% strain a reduction of the grown‐in vacancy response takes place simultaneously with a slight increase in vacancy cluster size to 2 or 3 vacancies. Between 4% and 6% strain a very substantial increase in vacancy production occurs but nearly all of these vacancies are clustered into voids with a radius of about 50 A and density of the order of 1013–1014 cm−3. The total concentration of vacancies necessary to produce these voids is 1017–1018 cm−3. This clearly shows that vacancies are formed upon deformation and that they are mobile at 450u2009°C. The small vacancy clusters (2 or 3 vacancies) are present at a concentration of about 5×1016 cm−3, the same as for the 4% strained samples. Upon further defor...

Annealing of monovacancies in electron and γ-irradiated diamonds

Abstract Diamonds were irradiated by either 2.3xa0MeV electrons or by 60 Co γ-rays. In the case of electron irradiation, positron annihilation and optical absorption showed that 30–35% of the monovacancies (positron lifetime of 145xa0ps) were removed by interstitials between 700 and 1020xa0K. Above 1020xa0K, divacancies were formed giving rise to TH5 optical absorption and a positron lifetime of 185xa0ps. Above 1120xa0K, TH5 absorption was replaced by absorption at 507/517xa0nm, but no change in the positron lifetime was observed. Annealing of the γ-irradiated type Ib diamonds differed substantially from that for the electron irradiated samples; 60% of the monovacancies where removed between 350 and 525xa0K.