Annihilation of Positrons in Electron‐Irradiated Silicon Crystals

Abstract

In honour of Prof. Dr. J. STUKE'S Goth birthday Lifetime and Doppler-broadening measurements in positron annihilation are used to investigate defects in silicon crystals irradiated with electrons of 1 MeV energy. The lifetime in undoped defect-free Si is found to be (218 8) ps. Isochronal annealing studies in the temperature range 12 K < T, < 600 K show that in undoped Si a t low temperature positrons annihilate a t neutral monovacancies and double vacancies with lifetimes of (268 & 10) and (318 & 15) ps, respectively.In boron-doped crystals before and after irradiation no special defect reveals in the spectra up to 400 K.

Defects in electron-irradiated Si studied by positron-lifetime spectroscopy

“…The calculated positron lifetime of the divacancy is 306 ps, 18 and the experimentally determined values are in the range from 318 to 327 ps. [20][21][22] The annealing range of the investigated sample ͑see Fig. 1͒ agrees with the annealing range of the divacancy between 450 and 623 K which was found by EPR measurements.…”

“…This value is in good agreement with calculated, and also earlier measured positron lifetimes in Si divacancies. 26,20 Below a measurement temperature of 100 K, the positron lifetime at first decreases due to the effectiveness of shallow positron traps. After the disappearance of the detrapping from these defects below 60 K ͑see Fig.…”

Stability of large vacancy clusters in silicon

“…Recent results on positron annihilation in neutron irradiated silicon 7,20 and on high-dose electron irradiated silicon 1,21 give a quite consistent picture on stable vacancy clusters formed during thermal treatment by mobile primary defects: After neutron irradiation, larger vacancy clusters are formed during thermal treatment around 870 K with defectrelated positron lifetimes of def ϭ420Ϯ20 ps ͑Ref. 7͒ ͓430Ϯ30 ps ͑Ref.…”

“…Vacancy clusters in silicon are detected by electron paramagnetic resonance ͑EPR͒, positron annihilation spectroscopy ͑PAS͒, and other methods not only after damage ͑electron [1][2][3][4] and neutron irradiation [5][6][7] or plastic deformation 8 ͒, but also in as-grown samples. 9 Due to a lack of computational data on structure and stability of vacancy clusters detected in silicon in the 1970's and 1980's, their size has been under discussion for a long time.…”

Lifetimes of positrons trapped at Si vacancies

“…The increase in the lifetime correlates positively with the vacant space inherent to each multivacancy. Positron annihilation experiments in the past indeed observed several lifetimes of the positron, depending on irradiation and annealing conditions: 270 ps, 5 295-325 ps, 8 and longer lifetimes ranging from 320 to 500 ps, 9,10 all of which are longer than 218 ps ͑Ref. 5͒ observed in vacancy-free samples.…”