Investigation of Contact Resistance in IGZO Thin- Film Transistors through the Insertion of an ITO Interlayer = 인듐-주석 산화물 계면층 삽입을 통한 인듐-갈륨-아연 산화물 박막트랜지스터의 접촉저항 연구

다국어 초록 (Multilingual Abstract)

Indium–gallium–zinc–oxide (IGZO) thin-film transistors (TFTs) have attracted considerable attention for next-generation memory applications owing to the excellent properties of IGZO as a channel material, including ultra-low off state current, high electron mobility, and BEOL- compatible low temperature processing. However, as device dimensions continue to scale down, contact resistance increasingly dominates the total device resistance, severely limiting current drivability. Unlike Si-based MOSFETs, conventional contact engineering approaches, such as silicide formation, are not applicable to oxide semiconductor TFTs, necessitating alternative strategies for contact resistance reduction. In this study, the insertion of an indium tin oxide (ITO) interlayer at the metal/IGZO interface was investigated to improve contact properties. Considering that contact resistance in IGZO TFTs mainly originates from the Schottky barrier and interfacial oxide layers, an ITO interlayer was introduced to mitigate these factors. In direct Mo/IGZO contacts, the strong oxygen reactivity of the metal results in pronounced interfacial oxide layers, leading to increased contact resistance. In contrast, as a conductive metal oxide, ITO does not introduce additional interfacial oxide layers at the ITO/IGZO interface, thereby avoiding the formation of an extra tunneling barrier. Furthermore, the ITO interlayer does not induce direct oxygen vacancy formation in the IGZO channel, which is beneficial for channel reliability. Transmission line method (TLM) analysis revealed that the specific contact resistivity was reduced from 8.8×10⁻³ Ω·cm² for devices without an interlayer to 3.2×10⁻⁵ Ω·cm² when a 2 nm- thick ITO interlayer was optimally inserted, corresponding to an improvement of more than two orders of magnitude. As a result of the improved contact properties, the drain current increased from 3.8 × 10⁻³ μA/μm for devices without an interlayer to 6.9 × 10⁻³ μA/μm at a gate overdrive voltage of 1V, confirming a substantial enhancement in on-state performance. X-ray photoelectron spectroscopy (XPS) analysis further indicated that the insertion of the ITO interlayer did not induce a significant change in oxygen vacancy concentration at the IGZO interface, supporting the absence of direct channel degradation. In addition to TLM analysis, the extracted contact resistivity was cross-verified using a 1/VOV method. These results demonstrate an effective contact engineering strategy for IGZO TFTs and highlight their potential applicability to next-generation memory devices, including vertical channel transistor DRAM (VCT DRAM) and two transistor zero capacitor (2T-0C) DRAM architectures.

목차 (Table of Contents)

• I. Introduction 1
• 1.1 Advantages of a-IGZO TFTs 1
• 1.2 Necessity of a-IGZO TFTs for next-generation memory applications 3
• 1.3 Contact resistance engineering in Silicon-based MOSFETs 5
• 1.4 Challenges of contact resistance in scaled AOS TFTs 7
• 1.5 Components affecting contact properties in a-IGZO TFTs 8
• II. Experimental Section. 10
• 2.1 Concept of improving contact properties via ITO interlayer insertion 10
• 2.2 Fabrication of a-IGZO TFT and TLM device 14
• 2.3 Extraction of contact resistivity using the TLM method 16
• 2.4 Cross verification of the TLM method using the 1/VOV method 18
• III. Results and Discussion 19
• 3.1 Electrical characteristics 19
• 3.1.1 Contact properties and contact resistivity extraction 19
• 3.1.2 Transfer and output characteristics 22
• 3.2 Film characteristics 24
• 3.2.1 Structural characteristics and surface roughness 24
• 3.2.2 Carrier concentration and resistivity 25
• 3.3 Mechanistic analysis of contact properties with ITO interlayer insertion 26
• 3.3.1 Analysis of interface phenomena at the Mo/IGZO contact 26
• 3.3.2 Analysis of ITO interlayer insertion effects 28
• 3.3.3 Effect of ITO interlayer thickness 30
• 3.3.4 Transfer length analysis based on contact properties 33
• IV. Conclusion 36
• Summary 38
• References 40
• Acknowledgements 42
• Curriculum Vitae 44