Positron annihilation studies of defect properties in semi-insulating GaAs

The positron lifetime spectra have been measured in three semi-insulating (SI) GaAs samples, one n-type GaAs doped with Te, and one p-GaAs doped with Zn. The average lifetimes in all SI-GaAs samples were 229 ps, and were larger than the bulk lifetime found in p-GaAs. Moreover, the annihilation rate distribution (ARD) in SI-GaAs was much wider than that in p-type GaAs. This showed that gallium vacancies existed in SI-GaAs. By measuring its Doppler broadening spectra as a function of temperature between 10 and 300 K, we observed that the S-parameter decreased with temperature increasing from 10 to 120 K. Thus the charge state of VGa was determined to be negative. Another defect VAs in SI-GaAs has also been detected under photoexcitation at 77 K by the increase of the S-parameter with the driving current in GaAs LED which was not observed in the dark.