Thin film transistors based on ultra-wide bandgap spinel ZnGa₂O₄

We report on thin film transistors (TFTs) based on spinel ZnGa₂O₄ (ZGO) that was recently spotlighted as an ultra-wide bandgap oxide semiconductor. The ZGO layers were grown in a spinel structure by pulsed laser deposition on the cubic spinel MgAl₂O₄ (MAO) as well as on cubic MgO substrates while changing the Zn/Ga ratio. The compressive strained epitaxial growth of ZGO on MgAl₂O₄ (100) and the tensile strained epitaxial growth of ZGO on MgO (100) without any misfit or threading dislocations were confirmed by the reciprocal space map and cross-sectional transmission electron microscopy. The electrical transport properties were demonstrated through TFTs based on ZGO as the channel layer, Al₂O₃ as the gate oxide, and Sn-doped In₂O₃ as the source, drain, and gate electrodes. When the Zn/Ga ratio is slightly lower than the ideal value of 0.5 on MgO substrates, the ZGO TFT showed the highest mobility of 5.4 cm²/V s. The I_ON/I_OFF ratio and subthreshold swing (S) value are 4.5 × 10⁸ and 0.19 V/dec, respectively.