Spin-flip Raman-scattering studies of compensating donor centers in nitrogen-doped zinc selenide grown by molecular-beam epitaxy

When ZnSe is doped p type by the incorporation of nitrogen, a compensation process sets in for effective acceptor concentrations between 10¹⁷ and 10¹⁸ cm^-3. It is generally agreed that this is due to the creation of compensating donors which have been reported to lie at a depth of about 45 meV. It has previously been shown from optically detected magnetic resonance and spin-flip Raman-scattering experiments that these donors have a gyromagnetic ratio (g value) of about 1.4, compared with the g value of 1.1 for the more usual shallow donors (which lie at about 26 meV). We report here a systematic study of a series of ZnSe layers (grown by molecular-beam epitaxy) in which the nitrogen concentration is gradually increased, and we confirm the correlation between the onset of the compensation (at an effective acceptor concentration of about 1.7×10¹⁷ cm^-3) and the appearance of the 45-meV donor. The spin-flip Raman scattering is a resonance process that requires the laser energy to coincide with the excitonic transition associated with the donor, and we have exploited this effect to determine the difference in the localization energies of excitons bound at the two types of donor. The results show that Haynes's rule is obeyed for donor depths E_D extending at least to 45 meV, the exciton localization energy being 0.20E_D.