Enhanced Phase Stability and Carrier Suppression of Ternary Cation-Based Amorphous Oxide Semiconductor Thin Film Transistors
Authors: Liu Mingyuan , Song Han Wook , Lee Sunghwan 
Published: July 2021

Transparent optoelectronic materials have gained significant attention for application in high performance devices such as next generation displays1-2, photovoltaics3, electrochromic devices4, and sensors5-6. Metal oxide-based semiconductors, specifically, are a promising group of transparent optoelectronic materials, now considered to be the vital building blocks for future device applications due to the unique combined properties of excellent optical transparency (from the visible to near-infrared regime), and high electrical conductivities7-8. Metal oxides have already seen wide implementation in various optoelectronic devices, depending on their electrical conductivity, as transparent electrodes by degenerately-doped oxides such as indium tin oxides (ITO) and doped zinc oxides (ZnO), as semiconducting active layers based on In2O3, ZnO or SnO2, and as insulators (e.g., SiO2, Al2O3, HfO2) for dielectrics or encapsulations.

The key pixel driving switches in display technologies, thin film transistors (TFTs), require a vast array of demanding material properties such as high carrier mobilities (usually defined as the “TFT field effect mobility”), low thermal budgets during processing, resilient phase stability, and reliable device performance under high thermal and bias stress conditions. Amorphous oxide semiconductors (AOSs), specifically those based on indium oxides, are receiving unique attention for TFT implementation due to their promisingly high carrier mobilities (> 5-20 cm2/Vs) compared to conventional amorphous Si (<~1 cm2/Vs), low processing temperature requirements (ambient to 200 °C), superior mechanical flexibility to their crystalline counterparts, and large area process-ability. Indium oxide-based binary and ternary cation material systems show tremendous promise for use in next generation displays as these materials exceed the aforementioned material property and fabrication requirements. Unfortunately, undoped In2O3 experiences a rapid onset of microstructural crystallization at very low homologous temperatures (T/Tm<0.19) at 150 °C and struggles to maintain its amorphous phase structure. The inclusion of Zn in In2O3 has revealed a viable and promising binary cation material which specifically addresses the structural instability of undoped In2O3, Indium Zinc Oxide (IZO), as the addition of Zn into In2O3 proves to stabilize the temperature-sensitive amorphous phase of indium oxide. Furthermore, the reported carrier mobility of IZO has shown to be as high as 20-40 cm2/Vs for both Hall and 15-30 cm2/Vs for TFT field effect mobilities. Studies have dug deeper to further unveil the effects of doping of In2O3-basd materials, and the ternary cation system of indium gallium zinc oxides (IGZO) has proven to be even more popular than binary systems as the addition of Ga in IGZO allows for the controllable suppression of channel carrier density during TFT applications (preferred for TFT devices where a low device off-state current is desired). In addition to Ga, more third cation species have been investigated as suitable material candidates such as Hf, Si, and Zr, but the carrier mobilities (~3-10 cm2/Vs) are around 3-10 times lower than that of In2O3 or IZO.

Therefore, securing strategies to develop a material system which maintains both high carrier mobility (e.g., >~20 cm2/Vs) and suppresses carrier generation for TFT channel application is of significant importance, and is necessary to expedite the realization of next-generation transparent displays which possess reliable performance, fast switching speed, and, consequently, ultra-high definition resolution. In this study, the ternary cation oxide system of indium aluminum zinc oxide (IAZO) is investigated with varying Al concentration in IAZO thin films. The IAZO thin films were deposited using magnetron co-sputtering at room temperature. The structural, optical, and electrical properties were systematically characterized as a function of Al concentration and compared to baseline IZO samples. The carrier transport characteristics, as well as the dominant mechanisms for carrier density and resistivity and their relation to Al concentration, are discussed. Furthermore, amorphous IAZO-based TFTs were developed to objectively compare and validate device performance and parameters against IZO-based TFTs.

The authors gratefully acknowledge the financial supports of the U.S. NSF Award No. ECCS-1931088; the Purdue Research Foundation (Grant No. 60000029); and the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 20011028) by KRISS.

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Figure 1