Surface Accumulation Induced Negative Schottky Barrier and Ultralow Contact Resistance in Atomic-Layer-Deposited In $_{\text{2}}$ O $_{\text{3}}$ Thin-Film Transistors

As transistor dimensions reach the 3-nm node, interface and surface engineering emerges as critical considerations. Challenges introduced by reduced conductivity and mobility due to surface depletion significantly impact thickness scaling and contact performance. In this work, we report the surface accumulation in atomic layer deposition (ALD) grown In $_{\text{2}}$ O $_{\text{3}}$ thin-film transistors (TFTs). The negative Schottky barrier enables an ultralow metal-to-semiconductor contact resistance of $\textit{R}_{\textit{c}} =\text{23.4}~\Omega\, \mu \text{m}$ at electron charge density $\textit{n}_{\text{2D}}=\text{5.0}$ $\times$ ${\text{10}}^{\text{13}}~\text{cm}^{-\text{2}}$ in a nanometer ultrathin In $_{\text{2}}$ O $_{\text{3}}$ channel. The effect of film thickness and annealing on contact resistance is investigated. Ultralow contact resistivity $\rho _{\textit{c}}\approx \text{1.3}$ $\times$ $ \text{10}^{-\text{9}}~\Omega\, \text{cm}^{\text{2}}$ and current transfer length $\textit{L}_{\textit{T}} \approx\text{2}~ \text{nm}$ are achieved in a 1-nm-thick film. The superior ohmic contact is made possible by the charge neutrality level (CNL) deeply aligned inside the conduction band for In $_{\text{2}}$ O $_{\text{3}}$ . Together with theoretical calculations, the Landauer quantum resistance limit of In $_{\text{2}}$ O $_{\text{3}}$ is discussed. Furthermore, the ALD-grown In $_{\text{2}}$ O $_{\text{3}}$ transistor is back-end-of-line (BEOL) compatible with a low thermal budget of 400 $^{\circ}$ C (even considering the optional O $_{\text{2}}$ annealing). Our work demonstrates that In $_{\text{2}}$ O $_{\text{3}}$ is also an up-and-coming candidate for ultra-scaled, high-performance BEOL transistors from even the contact engineering point of view.