MONOENERGETIC POSITRON BEAM, POSITRON LIFETIME, AND HALL-EFFECT MEASUREMENTS IN III-V EPILAYERS GROWN AT LOW-TEMPERATURES BY MOLECULAR-BEAM EPITAXY

A monoenergetic positron beam has been utilized to depth profile defects in GaAs and in low temperature GaAs buffer layers (LTBL) grown by molecular beam epitaxy. Doppler broadening spectra were measured as a function of positron implantation depth and analyzed in terms of positron diffusion and annihilations of free and trapped positrons. Positron lifetime spectra were measured for the LTBL samples for temperatures in the range 10-295 K. These spectra show two temperature-dependent components resulting from the annihilations of free positrons and of positrons trapped in vacancy-type shallow defects. We have performed Hall-effect and low magnetic field temperature dependent resistivity measurements on the LTBL samples and have determined that the defect activation energy is = 0.78 eV. The magnetic field dependence of the Hall data for the LTBL samples shows several interesting new features including sharp stepwise increases for every 700 G between 3.5 and 6 kG.