S. Dannefaer

Positron annihilation in diamond, silicon and silicon carbide

Recent positron lifetime and doppler broadening results on silicon, diamond and silicon carbide are presented in this contribution. In as-grown Czochralski Si ingols vacancies are found to be retained after growth at concentrations typically around 3×1016/cm3. 10 MeV eleciron irradiation of variously doped Si wafers shows that only high doping concentrations well in excess of the interstitial oxygen concentration causes an increase in the amount of monovacancies retained.In porous silicon very long-lived positronium lifetimes in the range 40–90 ns are found. Polycrystalline diamond films contain various types of vacancy agglomerates but these are found to be inhomogeneously distributed from crystallite to crystallite. Electron irradiation of silicon carbide results in two vacancy-related lifetimes which are interpreted as resulting from carbon and silicon vacancies.

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.

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.

Vacancies in As-Grown and Electron-Irradiated 4H-SiC Epilayers Investigated by Positron Annihilation

Epilayers of 4H-SiC were investigated by positron annihilation spectroscopies: four epilayers and their substrates were investigated. The epilayers (47 to 220 mum thick) contained significantly low ...

The Role of Nitrogen in the Annealing of Vacancies in 4H-SiC

Annealing of 2.2 MeV electron irradiated 4H-SiC epilayer and bulk 4H-SiC was investigated by means of positron annihilation spectroscopy. The concentration of N was 5×10 16 cm -3 in the epilayer and ~10 18 cm -3 in the bulk. Annealing was found to be independent of N concentration up to the temperature of 1450 o C, at which temperature all radiation-produced defects detectable by positrons were annealed. Introduction Former positron investigations [1] have shown that 2.2 MeV electron irradiation at room temperature creates silicon vacancies (VSi) and also enhances the positron response from grownin vacancy clusters; these clusters have sizes in the range of 6 to 12 divacancies. Carbon vacancies (VC) could not be observed directly due to the experimental difficulty of separating the positron lifetimes from VSi and VC when these vacancies coexist in a sample. Nitrogen in the concentration range of 5×10 16 to ~10 18 cm -3 was concluded to have no effect on the concentration/charge of the radiation induced vacancies. In this work is investigated if N plays a role during annealing of radiation-produced vacancies. Experimental The sample consisted of a 220 μm thick 4H-SiC epilayer deposited on a 300 μm thick bulkgrown wafer, and was irradiated by 2.2 MeV electrons to a dose of 2×10 17 cm -2 ; the sample is identical to one of the formerly investigated layers [1]. Isochronal annealing was done in steps of 25 o C up to 1450 o C (1⁄2 hr. per step), and high purity N2 was used as a protective gas. Positron lifetime spectra and Doppler broadening of the annihilation gamma quanta (511 KeV) were measured at room temperature after each annealing step. Lifetime spectra required analyses [2] using two vacancy-associated lifetimes in order to obtain a chi-square value of 1.00 ± 0.06. Based on this decomposition, the trapping rate for each of the vacancy-associated lifetimes can be calculated from the trapping model [3]. The trapping rate is important since it is proportional to the vacancy concentration, and is also very sensitive to defect charge. Doppler spectra cannot be resolved into individual components as in the case of lifetime spectra. Doppler data are, therefore, analyzed using the so called S parameter defined as the number of counts within a narrow (±0.7 KeV) region centered at 511 KeV divided by the number of counts in the whole of the energy spectrum. In the case of a sample that contains no positron traps the value obtained for S is denoted SB and the lifetime spectrum contains only one lifetime component denoted τB, where the subscript B stands for bulk. When trapping by vacancies occurs the value of S normally increases due to a higher value, SV specific for the vacancy, i.e. S = (1-F) SB + FSV. (1) The parameter F in Eq. (1) cannot be determined from Doppler data, but can from lifetime data according to Materials Science Forum Online: 2005-05-15 ISSN: 1662-9752, Vols. 483-485, pp 481-484 doi:10.4028/www.scientific.net/MSF.483-485.481

Vacancies In Electron Irradiated 6H-SiC

Annealing of electron irradiated bulk n-type 6H-SiC has shown that neutral carbon vacancies and neutral silicon vacancies undergo a major reduction in concentration in the 20–200 °C temperature interval after which only slight changes occur up to 1200 °C. The experiments suggest that the positively charged carbon vacancy, detected by electron paramagnetic resonance, constitutes only a small fraction of all carbon vacancies.