Properties of the shallow O-related acceptor level in ZnSe

Zinc selenide layers grown by molecular beam epitaxy (MBE) and doped with ZnO have been characterized using low temperature photoluminescence (PL) measurements as a function of excitation level, temperature, and laser energy (i.e., selectively excited donor-acceptor pair luminescence or SPL), as well as reflectance measurements. An O-related donor-to-acceptor (D⁰-A ⁰) pair band is clearly observed in all of the ZnO-doped layers, whose position varies from 2.7196 to 2.7304 eV, depending on the excitation level. The same peak occurs in a number of undoped, As-doped, and Ga-doped MBE samples, showing that O can occur as a residual impurity. Temperature-dependent measurements reveal the existence of a corresponding conduction band-to-acceptor (e-A⁰) peak at 2.7372 eV (39.8 K), confirming the existence of the acceptor level. The binding energy of this acceptor is about 84±2 meV, which is 27 meV shallower than that of N. The SPL measurements reveal four excited states of the shallow acceptor level, separated from the 1s _3/2 ground state by 48.2 (2p_3/2), 57.1 (2s_3/2), 64.3 (2p_5/2:Γ₇), and 67.7 meV (3p _3/2:Γ₈), respectively (all values ±1 meV). These energies fit well to conventional effective mass theory, which demonstrates that this O-related acceptor level is effective-mass-like. However, luminescence and secondary ion mass spectrometry show that the ZnO doping technique introduces shallow donor impurities into the material in addition to O acceptors, specifically high levels of chemical contaminants (mainly B and Ga) originating from the doping source. This effect may account for the lack of reproducibility in obtaining p-type conduction with ZnO doping, and suggests that more effective O incorporation methods should be devised.