Quantum Confinement Effect in Amorphous In–Ga–Zn–O Heterojunction Channels for Thin-Film Transistors

Electrical and carrier transport properties in In–Ga–Zn–O thin-film transistors (IGZO TFTs) with a heterojunction channel were investigated. For the heterojunction IGZO channel, a high-In composition IGZO layer (IGZO-high-In) was deposited on a typical compositions IGZO layer (IGZO-111). From the optical properties and photoelectron yield spectroscopy measurements, the heterojunction channel was expected to have the type–II energy band diagram which possesses a conduction band offset (ΔE(c)) of ~0.4 eV. A depth profile of background charge density indicated that a steep ΔE(c) is formed even in the amorphous IGZO heterojunction interface deposited by sputtering. A field effect mobility (μ(FE)) of bottom gate structured IGZO TFTs with the heterojunction channel (hetero-IGZO TFTs) improved to ~20 cm(2) V(−1) s(−1), although a channel/gate insulator interface was formed by an IGZO−111 (μ(FE) = ~12 cm(2) V(−1) s(−1)). Device simulation analysis revealed that the improvement of μ(FE) in the hetero-IGZO TFTs was originated by a quantum confinement effect for electrons at the heterojunction interface owing to a formation of steep ΔE(c). Thus, we believe that heterojunction IGZO channel is an effective method to improve electrical properties of the TFTs.