1112 IEEE ELECTRON DEVICE LETTERS, VOL. 40, NO. 7, JULY 2019 0.6V Threshold Voltage Thin Film Transistors With Solution Processable Indium Oxide (In 2 O 3 ) Channel and Anodized High-κ Al 2 O 3 Dielectric Sagar R. Bhalerao , Donald Lupo, Amirali Zangiabadi, Ioannis Kymissis , Jaakko Leppäniemi, Ari Alastalo , and Paul R. Berger, Fellow, IEEE Abstract — Low-voltage operation and low processing temperature of metal oxide transistors remain a challenge. Commonly metal oxide transistors are fabricated at very high processing temperatures (above 500 ◦ C) and their operating voltage is quite high (30–50 V). Here, thin- ﬁlm transistors (TFT) are reported based upon solution processable indium oxide (In 2 O 3 ) and room temperature processed anodized high-κ aluminum oxide (Al 2 O 3 ) for gate dielectrics. The In 2 O 3 TFTs operate well below the drain bias (V ds ) of 3.0 V, with on/off ratio 10 5 , subthresh- old swing (SS) 160 mV/dec, hysteresis 0.19 V, and low threshold voltage (V th ) 0.6 V. The electron mobility (μ) is as high as 3.53 cm 2 /V.s in the saturation regime and normalized transconductance (g m ) is 75 μS/mm. In addition, the detailed capacitance–voltage (C–V) analysis to deter- mine interface trap states density was also investigated. The interface trap density (D it ) in the oxide/semiconductor inter- face was quite low, i.e., 0.99 × 10 11 –2.98 × 10 11 eV -1 ·cm -2 , signifying acceptable compatibility of In 2 O 3 with anodic Al 2 O 3 . Index Terms— Metal oxide semiconductors, solution processing, indium oxide (In 2 O 3 ), low voltage, TFT, anodiza- tion, interface state density. I. I NTRODUCTION M ETAL oxide semiconductors have been extensively studied in the last few years for a wide range of devices and device applications such as thin ﬁlm transistors (TFT) Manuscript received April 27, 2019; revised May 16, 2019; accepted May 19, 2019. Date of publication May 22, 2019; date of current version June 26, 2019. This work has been funded by the Business Finland under Grant 40146/14 and the Academy of Finland under Grant 311458. The review of this letter was arranged by Editor Z. Ma. (Corresponding author: Sagar R. Bhalerao.) S. R. Bhalerao and D. Lupo are with the Electrical Engineering, FI Tampere University, 33720 Tampere, Finland (e-mail: sagar.bhalerao@ tuni.ﬁ). A. Zangiabadi and I. Kymissis are with the Department of Electrical Engineering, Columbia University, New York, NY 10027 USA. J. Leppäniemi and A. Alastalo are with VTT Technical Research Centre of Finland Ltd., FI-02044 Espoo, Finland. P. R. Berger is with the Electrical Engineering, Tampere University, FI 33720 Tampere, Finland, and also with the Department of Electri- cal and Computer Engineering, The Ohio State University, Columbus, OH 43210 USA. Color versions of one or more of the ﬁgures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/LED.2019.2918492 for transparent and ﬂexible electronics, active matrix and ﬂat panel displays, bio/medical sensors and radio frequency (RF) circuits [1]–[3]. Metal oxide semiconductors gained special attention due to their diverse spectrum of properties that distinguishes them from those of conventional silicon, such as wide band gap, wide optical transparency, high mobility and low temperature solution processable deposition [4]. They have paved the way for the next generation thin ﬁlm and printed electronics [5]. Amongst all the metal-oxide semicon- ductors, indium oxide (In 2 O 3 ) is the most favorable n-type semiconductors for thin ﬁlm transistors with a band gap 3.6 – 3.75 eV and high carrier mobility [6]. Even though, TFTs with solution processed In 2 O 3 semi- conductor and anodized Al 2 O 3 have been reported, most of the solution processed In 2 O 3 thin ﬁlm transistors reported in the literature, are either based on the high vacuum/ temperature [6], [7], or costly deposition techniques [8], espe- cially for the dielectric oxide deposition and/or also for the In 2 O 3 [8] with high operating voltages [9]. Here, In 2 O 3 TFTs are fabricated by combining the solution-processing route for the semiconductor and an anodization pathway for the high-κ gate dielectric (aluminum oxide - Al 2 O 3 ), enabling low volt- age operation voltage i.e. <3V. The anodization process was carried out as reported previously [10]. The present study on In 2 O 3 TFTs are not only focused on electrical characterization, but also provides insight into the interface between metal oxide and dielectric, bestowing densities of interface trap states. Furthermore, as per our knowledge of the current state of oxide TFTs, we did not ﬁnd any report combining these two, materials and processes nor any report on the In 2 O 3 /Al 2 O 3 interface study i.e. densities of trap states. Therefore, this is the ﬁrst time we are reporting the detailed analysis of the interface traps states for In 2 O 3 with Al 2 O 3 . Anodization empowers the room temperature deposition of dielectric compatible with ﬂexible and printed electronics devices, bypassing high tem- perature [6]–[8] high vacuum processes [6]–[8] with added advantages such as low cost, nanoscale deposition, high quality and denser oxide [11]. II. EXPERIMENTAL The bottom gate, top contact (BGTC) topology, used here for the device fabrication, is shown in Fig. 1. TFTs were 0741-3106 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.