In₂Ga₂ZnO₇ oxide semiconductor based charge trap device for NAND flash memory

The programming characteristics of charge trap flash memory device adopting amorphous In₂Ga₂ZnO₇ (a-IGZO) oxide semiconductors as channel layer were evaluated. Metal-organic chemical vapor deposition (MOCVD) and RF-sputtering processes were used to grow a 45 nm thick a-IGZO layer on a 20 nm thick SiO₂ (blocking oxide)/p⁺⁺-Si (control gate) substrate, where 3 nm thick atomic layer deposited Al₂O₃ (tunneling oxide) and 5 nm thick low-pressure CVD Si₃N₄ (charge trap) layers were intervened between the a-IGZO and substrate. Despite the identical stoichiometry and other physicochemical properties of the MOCVD and sputtered a-IGZO, a much faster programming speed of MOCVD a-IGZO was observed. A comparable amount of oxygen vacancies was found in both MOCVD and sputtered a-IGZO, confirmed by x-ray photoelectron spectroscopy and bias-illumination-instability test measurements. Ultraviolet photoelectron spectroscopy analysis revealed a higher Fermi level (E _F) of the MOCVD a-IGZO (∼0.3 eV) film than that of the sputtered a-IGZO, which could be ascribed to the higher hydrogen concentration in the MOCVD a-IGZO film. Since the programming in a flash memory device is governed by the tunneling of electrons from the channel to charge trapping layer, the faster programming performance could be the result of a higher E _F of MOCVD a-IGZO.