CHARACTERIZATION OF NATIVE VACANCIES IN EPITAXIAL GaN AND ZnSe SEMI- CONDUCTOR LAYERS BY POSITRON ANNIHILATION SPECTROSCOPY

Point defects in epitaxial GaN and ZnSe semiconductor layers have been studied using a low-energy positron beam. Ga vacancies are found to be present in n-type GaN grown by metal organic chemical vapor deposition, where the conductivity is due to residual oxygen. When n-type silicon impurity doping is done, clearly less vacancies are observed. In Mgdoped p-type and semi-insulating materials the vacancies are not observed. In GaN layers grown by molecular-beam epitaxy also bigger vacancy clusters are detected. The formation of Ga vacancies is found to depend strongly on the stoichiometry during the growth, and much less on the structural quality of the layers. In GaN layers positrons are trapped also at edge-type dislocations, which are shown to be negatively charged but not to contain open-volume defects. Undoped Znse layers are found to contain negative Zn vacancies. In nitrogen doped ZnSe and ZnS0.06Se0.94 layers Se vacancies are detected. These are most likely part of a defect complex with N impurity. Positron trapping at negative N acceptors is also observed. By combining the results of positron annihilation, secondary ion mass spectrometry, and capacitance-voltage measurements, a detailed picture of the deactivation of N impurities in ZnSe is obtained.