P. Mascher

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.

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.

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 studies of vacancies in proton irradiated silicon

Annealing of vacancies produced by heavy proton irradiation of float‐zone (Fz) and P‐doped Czochralski‐grown (Cz) silicon has been investigated by positron lifetime spectroscopy. In Fz‐Si divacancies are retained after irradiation, and these defects are completely annealed out at 700u2009°C. In Cz P‐doped silicon, impurities are found to enhance both the amount of retained vacancies as well as the tendency for vacancy clustering. Two annealing stages appear at 100u2009°C and close to 450u2009°C which seem to be a result of interstitial migration. Vacancy migration takes place in a wide temperature range between 100 and 1000u2009°C.

Comment on “Amorphous hydrogenated silicon studied by positron lifetime spectroscopy”

H.-E. Schaefer, R. Wurschum, R. Schwarz, D. Slobodin, and S. Wagner [Appl. Phys. A40, 145 (1986)] have recently assigned positron lifetimes to various vacancy sizes. In this comment we will show that their discussions are ill founded.

Heat‐treatment‐induced defects in low‐resistivity silicon

Czochralski‐grown silicon has been investigated with doping levels up to 5×1018 cm−3 of boron or phosphorus by positron annihilation. For the highly boron‐doped samples vacancies could be created upon heat treatment in the 700–1000u2009°C range up to a concentration of 1017 vacanciesu2009cm−3. Reducing either the boron concentration or the interstitial oxygen concentration (by prolonged heat treatment at 1200u2009°C) reduced the amount of created vacancies. No vacancies were detected in highly phosphorus‐doped materials. Combined positron‐lifetime and Doppler‐broadening measurements indicate that the vacancies are essentially of monovacancy nature and have impurities close by which contribute with high‐momentum electrons. Isothermal heat treatments at 700, 750, 900, and 950u2009°C indicate a reaction of the form X+Y⇄Z, where Z contains a vacancy. The forward reaction has an activation enthalpy of 2.7 eV, while the back reaction has the value 3.0 eV. It is suggested that X corresponds to substitutional boron and Y to inte...

Vacancy complexes in Cr-doped GaAs

Cr-doped semi-insulating GaAs has been investigated by means of position lifetime spectroscopy. In as-grown GaAs the dominant positron trap is a CrGa·VAs complex. Upon annealing the concentration of this complex increases around 260°C and then decreases at temperatures higher than 500°C. No vacancy agglomeration took place. In low temperature (130 K) e--irradiated (28 MeV) Cr-GaAs, di- and trivacancies were observed together with the gallium antisite, GaAs.