Amorphous oxide semiconductors (AOSs) based on In₂O₃ are promising due mainly to high carrier mobility^1,2 and excellent optical transmittance in visible regime.  Therefore, AOS materials have been integrated as active layers² and electrodes² into a variety of electronic devices such as high performance thin film transistors² (TFTs) and fast response photo detectors³.  This class of materials has been gaining particular attention in next generation high-resolution displays due to their much higher TFT field effect mobility (>20 vs 1 cm²/Vsec) and low temperature (T) processability (RT-100 °C vs ~300 °C) compared to conventional amorphous Si (a-Si)-based TFTs.  Some AOS materials such as In-(Ga)-Zn-O is now being implemented in high performance and flexible active-matrix liquid crystal displays and active-matrix organic light emitting diodes technologies.  Additional advantages of AOSs besides high mobility, low T process and optical transparency include isotropic wet etch characteristics and compatibility with mass productions all of which make this material suitable for large area, flexible, and transparent devices on inexpensive polymer substrates.

In previous works, we have extensively contributed to the development of high performance and stable amorphous In-Zn-O (a-IZO) TFTs.  These efforts include^2,4-9 the first room-temperature fabrication of high mobility, compositionally homogeneous channel/metallization a-IZO TFTs; the first measurement of the specific contact resistance in a-IZO metallized thin a-IZO channel structure; the first identification of native defect doping in a-IZO using ultra-high pressure oxidation.  Amorphous/crystalline phase stability and thermal-stress-induced threshold voltage instability of a-IZO TFTs have also been reported.  Metallization strategies for In₂O₃-based TFTs were proposed, and the report on the relevance of metallization selection to the performance of a-IZO TFTs validated the usefulness of the proposed strategies.

One next challenge is to scale down AOS TFTs for the implementation in ultra-high definition (UHD) displays.  Since these next-generation technologies utilize much smaller pixel size in order to realize UHD resolution, AOS TFTs that are employed as pixel driving elements must be scaled down as well.  When the dimensions of TFT devices (e.g., channel length and width) are reduced, the important device characteristics such as the field effect mobility, device saturation behavior and threshold voltage are likely influenced by scaling as shown in conventional MOSFET devices.  However, studies on this scaling effect on the performance of AOS TFTs are currently quite limited in the literature.

In this presentation, we present a fundamental study of the scaling behavior of a-IZO-based TFTs.  The devices with various channel lengths (L) and widths (W) and aspect ratios were fabricated at room temperature and patterned using a photolithographical technique and a lift-off process.  We have found that the TFT field effect mobility decreases with decreasing channel length (L) from ~40 cm²/Vsec (L=50 µm) to ~17 cm²/Vsec (L<5 µm) in spite of the devices with the same aspect ratio.  In addition, the long channel devices present excellent drain current (I_D) saturation while the short channel a-IZO TFTs lose the I_D saturation behavior and show much higher off-sate current compared to long channel TFTs.    The electrical properties of channel conductance and carrier density were evaluated as a function of channel length and width.  The relevant device physics including transmission line model (TLM) analysis were used to fundamentally understand the behaviors of scaling a-IZO TFTs.  The findings in this presentation may be significant to In₂O₃-based AOS TFTs including a-IZO and a-IGZO for the next-generation UHD resolution display technologies.

¹               K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature 432, 488 (2004).

²               S. Lee, H. Park, and D. C. Paine, Journal of Applied Physics 109, 063702 (2011).

³               S. Cosentino, P. Liu, S. T. Le, S. Lee, D. Paine, A. Zaslavsky, D. Pacifici, S. Mirabella, M. Miritello, I. Crupi, and A. Terrasi, Applied Physics Letters 98 (2011).

⁴               S. Lee, B. Bierig, and D. C. Paine, Thin Solid Films 520, 3764 (2012).

⁵               S. Lee and D. C. Paine, Applied Physics Letters 98, 262108 (2011).

⁶               S. Lee and D. C. Paine, Applied Physics Letters 102, 052101 (2013).

⁷               S. Lee and D. C. Paine, Applied Physics Letters 104, 252103 (2014).

⁸               S. Lee, H. Park, and D. C. Paine, Thin Solid Films 520, 3769 (2012).

⁹               S. Lee, K. Park, and D. C. Paine, Journal of Materials Research 27, 2299 (2012).

Figure. Typical output characteristics of a-IZO TFTs as a function of channel length.