Abstract
Positron annihilation spectroscopy was employed to elucidate the nature and thermal behavior of defects induced by Cu in freestanding GaN crystals. Cu atoms were intentionally introduced in GaN lattice through thermally activated diffusion from an ultrathin Cu capping layer. During isochronal annealing of the obtained Cu-doped GaN in the temperature range of 450–850 K, vacancy clusters were found to form, grow and finally vanish. Doppler broadening measurements demonstrate the presence of vacancy-like defects across the 600 nm-thick layer below the surface corresponding to the Cu-diffused layer as evidenced by secondary ion mass spectrometry. A more qualitative characterization of these defects was accomplished by positron lifetime measurements. We found that annealing at 450 K triggers the formation of divacancies, whereas further increase of the annealing temperature up to 550 K leads to the formation of large clusters of about 60 vacancies. Our observations suggest that the formation of these vacancy-like defects in bulk GaN is related to the out-diffusion of Cu.
Highlights
Positron annihilation spectroscopy was employed to elucidate the nature and thermal behavior of defects induced by Cu in freestanding GaN crystals
The lifetime positron annihilation spectroscopy (PAS) probe of Cu-diffused GaN crystals shows that annealing is associated with the appearance of a relatively long-lifetime component attributed to vacancy clusters, whereas a small change in the average lifetime is detected
At a fixed positron energy E, the S parameter is described as S(E) = ηs(E)Ss + ηb(E)Sb + i ηdi (E)Sdi, where Ss and Sb are the characteristic values of S parameter for positron annihilation at the surface and in defectfree bulk GaN lattice, respectively. η stands for the fraction of positron annihilations at each state
Summary
Positron annihilation spectroscopy was employed to elucidate the nature and thermal behavior of defects induced by Cu in freestanding GaN crystals. The annealing at 550 K of the Cu-diffused GaN sample induces an increase in the average positron lifetime by 4 ps. Open-volume defects introduced in the sample presumably during the out-diffusion of Cu. Note that this increase is comparable to earlier observations of 2 and 5 ps increase in the average lifetime in GaN after electron irradiation at fluences of 3 × 1017 and 1 × 1018 cm−2, respectively [29].
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