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 cm² V^-1 s^-1, although a channel/gate insulator interface was formed by an IGZO-111 (μ_FE = ~12 cm² 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.