Enhanced Mobility and Stability of Amorphous IZO TFTs With Homojunction Formation and Back-Channel Engineering

High-performance oxide semiconductor thin-film transistors (TFTs) are fabricated by forming a homojunction-structured channel layer with double-layer In-doped ZnO (IZO) with different In contents. The a-I0.9ZO/a-I0.5ZO TFTs exhibit a field-effect mobility ( $\mu_{\mathrm{FE}}$ ) of $31.5 \mathrm{~cm}^2 / \mathrm{V} \cdot \mathrm{s}$ , an on-off current ratio ( $I_{\text {on }} / I_{\text {off }}$ ) of $2 \times 10^9$ , a subthreshold swing (SS) of 78 mV/decade, and a threshold voltage ( $V_{\text {th }}$ ) of 1.3 V. The $\mu_{\mathrm{FE}}$ is 2 times higher than that of the single-layer a-I0.5ZO TFT, which is attributed to the formation of the quasi two-dimensional electron gas (q-2DEG) due to the existence of the conduction band offset at the a-I0.9ZO/a-I0.5ZO homojunction interface, thus weakening the electron scattering. Moreover, the electrical properties of the bilayer-channel IZO TFTs were further enhanced by using CF4-plasma back-channel treatment and an Al2O3 thin film as back-channel passivation layer (BPL). The device exhibits a high $\mu_{\mathrm{FE}}$ of $50.4 \mathrm{~cm}^2 / \mathrm{V} \cdot \mathrm{s}$ , a high $\mathrm{I}_{\mathrm{on}} / \mathrm{I}_{\text {off }}$ of $6 \times 10^9$ , and a low SS of 65 mV/decade. The threshold voltage shifts ( $\Delta V_{\text {th }}$ ) were only -0.21 V and 0.29 V when the device was subjected to positive and negative gate-bias stresses for 10,000 s, respectively. The involving mechanism of the enhancement of device performance was elucidated in detail based on ultraviolet photoelectron spectroscopy (UPS), UV-visible spectroscopy, X-ray photoelectron spectroscopy (XPS), and capacitance-voltage (C-V) profiling technique analyses.