Native vacancies in nitrogen-doped and undoped ZnSe layers studied by positron annihilation

We have used a low-energy positron beam to investigate the defects in N-doped and undoped ZnSe layers grown on GaAs:Si or ZnSe substrates. In N-doped ZnSe we observe positron trapping at vacancies, identified as V_SeN_Se complexes, and at negative ions. Based on positron data, we give estimates for the concentrations of vacancies and negative ions. The results suggest that in addition to forming isolated N_Se acceptors or V_SeN_Se pairs, incorporated nitrogen also forms donor-type defects leading to compensation of holes. In undoped ZnSe layers, the results show presence of Zn vacancies in concentrations 10¹⁶-10¹⁷ cm^-3. The Zn vacancy concentration correlates with the dislocation density at the ZnSe/GaAs interface.