Atomic structures and carrier dynamics of defects in a ZnGeP2 crystal

Abstract

ZnGeP 2 (ZGP) crystals have attracted tremendous attention for their applications as frequency conversion devices. Nevertheless, the existence of native point defects, including at the surface and in the bulk, lowers their laser-induced damage threshold by increasing their absorption and forming starting points of the damage, limiting their applications. Here, native point defects in a ZGP crystal are fully studied by the combination of high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and optical measurements. The atomic structures of the native point defects of the Zn vacancy, P vacancy, and Ge-Zn antisite were directly obtained through an HAADF-STEM, and proved by photoluminescence (PL) spectra at 77 K. The carrier dynamics of these defects are further studied by ultrafast pump-probe spectroscopy, and the decay lifetimes of 180.49, 346.73, and 322.82 ps are attributed to the donor V p + → valence band maximum (VBM) recombination, donor Ge Zn + → VBM recombination, and donor-acceptor pair recombination of V p + → V Zn − , respectively, which further confirms the assignment of the electron transitions. The diagrams for the energy bands and excited electron dynamics are established based on these ultrahigh spatial and temporal results. Our work is helpful for understanding the interaction mechanism between a ZGP crystal and ultrafast laser, doing good to the ZGP crystal growth and device fabrication.

Electrical Relaxation and Transport Properties of ZnGeP2 and 4H-SiC Crystals Measured with Terahertz Spectroscopy

“…It could be supposed that the defects with energy levels close to the midgap can act as recombination centers (probably P vacancies in different charge states in ZnGeP 2 [8,10]). On the contrary, the defects with energy levels close to the energy bands act as traps for the charge carriers (Ge and Zn vacancies, as well as the Ge Zn antisite defect [8,10]).…”

“…It could be supposed that the defects with energy levels close to the midgap can act as recombination centers (probably P vacancies in different charge states in ZnGeP 2 [8,10]). On the contrary, the defects with energy levels close to the energy bands act as traps for the charge carriers (Ge and Zn vacancies, as well as the Ge Zn antisite defect [8,10]). It is believed that electron irradiation generates point defects, first of all vacancies of P, Zn and Ge, while the interstitial atoms are easily annealed and clustered [8].…”

“…The complex conductivity spectra were successfully fitted using the Drude-Smith model, but the reported carrier concentration and the static conductivity are several orders of magnitude higher than could be expected for undoped high-resistivity ZnGeP 2 . Recently, ultrafast optical pump-optical probe spectroscopy (the transient reflection measurement) was applied to ZnGeP 2 crystals, and a charge carrier lifetime of 294 ps was deduced [10].…”

“…This absorption is associated with the optically active point defects [8]. Singly ionized zinc and phosphorus vacancies can act as such defects [8,10,13,14], as well as the Ge Zn antisite defect [10]. It is known that after thermal annealing or irradiation with high-energy electrons, the "anomalous" absorption in ZnGeP 2 crystals is reduced [14,15].…”