Contact properties of a low-resistance aluminum-based electrode with metal capping layers in vertical oxide thin-film transistors

Abstract

Thin-film transistors (TFTs) with a small pitch size are necessary to realize high-resolution displays for virtual reality and augmented reality applications. Particularly, electrodes require low-resistance metals to reduce the resistance–capacitance delay caused by the increased pixel density. However, low-resistance Al can easily oxidize in bottom-contact structures of vertical TFTs owing to the oxidative deposition environment. This study quantitatively analyzed the contact properties of an Al-based metal with Mo and Ti capping layers. The Mo/Al/Mo and Ti/Al/Ti were adopted as the source/drain (S/D) electrodes, and their contact properties were compared. The top-gate bottom-contact device with Mo/Al/Mo S/D exhibited better contact properties, with a 0.02 V turn-on voltage (V_on), 3.5 × 10⁷ ON/OFF ratio, and 5.7 kΩ contact resistance (R_SD). By contrast, the device with Ti/Al/Ti S/D exhibited degraded characteristics, with a −0.3 V V_on, 0.9 × 10⁷ ON/OFF ratio, and 17 kΩ R_SD owing to metal oxidation. The contact properties were further examined through ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. Vertical TFTs were fabricated using Mo/Al/Mo and Ti/Al/Ti electrodes, and their electrical properties were investigated. The vertical TFT with Mo/Al/Mo electrodes exhibited reasonable performance, with a field-effect mobility of 3.3 cm² V⁻¹ s⁻¹ and R_SD of 15 kΩ. Conversely, the device with Ti/Al/Ti electrodes yielded degraded transfer characteristics, with a mobility of 0.05 cm² V⁻¹ s⁻¹ and R_SD of 984 kΩ. The analysis indicates that electrode materials significantly influence the electrical performance of vertical TFTs. Therefore, electrode materials must be carefully selected and structured to realize high-end vertical TFT arrays.