- Nano Express
- Open Access
Structural and electrical characteristics of high-κ Er2O3 and Er2TiO5 gate dielectrics for a-IGZO thin-film transistors
© Chen et al; licensee Springer. 2013
- Received: 31 October 2012
- Accepted: 5 December 2012
- Published: 8 January 2013
In this letter, we investigated the structural and electrical characteristics of high-κ Er2O3 and Er2TiO5 gate dielectrics on the amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) devices. Compared with the Er2O3 dielectric, the a-IGZO TFT device incorporating an Er2TiO5 gate dielectric exhibited a low threshold voltage of 0.39 V, a high field-effect mobility of 8.8 cm2/Vs, a small subthreshold swing of 143 mV/decade, and a high Ion/Ioff current ratio of 4.23 × 107, presumably because of the reduction in the oxygen vacancies and the formation of the smooth surface roughness as a result of the incorporation of Ti into the Er2TiO5 film. Furthermore, the reliability of voltage stress can be improved using an Er2TiO5 gate dielectric.
- Amorphous InGaZnO
- Thin-film transistor
Amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) are being extensively explored as a replacement for amorphous and polycrystalline silicon TFTs in large-area display technologies, such as active-matrix liquid crystal display devices and active-matrix organic light-emitting displays. This is due to their high field-effect mobility, low leakage current, excellent optoelectronic characteristics, good uniformity and stability, and low temperature fabrication.
To achieve a high drive current at a low gate voltage, we can either employ high-κ materials or thinner gate dielectrics. However, the decrease in the thickness of gate dielectric is limited due to the occurrence of electron tunneling. Consequently, high-κ gate dielectric materials, including Al2O3, ZrO2, Y2O3, and HfO2, have been studied to reduce the electron tunneling and maintain the large capacitance. However, HfO2 dielectric film has a critical disadvantage of high charge trap density between the gate electrode and gate dielectric, as well as the gate dielectric and channel layer. Recently, rare earth (RE) oxide films have been extensively investigated due to their probable thermal, physical, and electrical performances. To date, the application of RE oxide materials as gate dielectrics in a-IGZO TFTs has not been reported. Among the RE oxide films, an erbium oxide (Er2O3) film can be considered as a gate oxide because of its large dielectric constant (approximately 14), wide bandgap energy (>5 eV), and high transparency in the visible range[8, 9]. The main problem when using RE films is moisture absorption, which degrades their permittivity due to the formation of low-permittivity hydroxides. The moisture absorption of RE oxide films may be attributed to the oxygen vacancies in the films. To solve this problem, the addition of Ti or TiO x (κ = 50 to approximately 110) into the RE dielectric films can result in improved physical and electrical properties. In this study, we compared the structural and electrical properties of Er2O3 and Er2TiO5 gate dielectrics on the a-IGZO TFT devices.
The Er2O3 and Er2TiO5 a-IGZO TFT devices were fabricated on the insulated SiO2/Si substrate. A 50-nm TaN film was deposited on the SiO2 as a bottom gate through a reactive sputtering system. Next, an approximately 45-nm Er2O3 was deposited by sputtering from an Er target, while an Er2TiO5 thin film (approximately 45 nm) was deposited through cosputtering using both Er and Ti targets at room temperature. Then, postdeposition annealing was performed using furnace in O2 ambient for 10 min at 400°C. The a-IGZO channel material (approximately 20 nm) was deposited at room temperature by sputtering from a ceramic IGZO target (In2O3/Ga2O3/ZnO = 1:1:1). Top Al (50 nm) source/drain electrodes were formed by a thermal evaporation system. The channel width/length of examined device was 1,000/200 μm. The film structure and composition of the dielectric films were analyzed using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The surface morphology of the films was investigated by atomic force microscopy (AFM). The capacitance-voltage (C-V) curves of the Al/Er2O3/TaN and Al/Er2TiO5/TaN devices were measured using a HP4284 LCR meter. The electrical characteristics of the a-IGZO TFT device were performed at room temperature using a semiconductor parameter Hewlett-Packard (HP) 4156C (Palo Alto, CA, USA). The threshold voltage (VTH) was determined by linearly fitting the square root of the drain current versus the gate voltage curve. Field-effect mobility (μFE) is derived from the maximum transconductance.
The physical model to be presented is based on the structure of the Er2O3 and Er2TiO5 surfaces, as schematically depicted in Figure 5b,c, respectively. Briefly speaking, during dc stress, hydroxyl ions (OH–) are released from the erbium hydroxide (Er-OH) by breaking the Er-OH bonds. The electrons in the oxide have gained enough energy from the applied gate and drain voltages. They collide with strained Er-O-Er or Er-O-Ti bonds to generate trapped charges in bulk oxide, causing a threshold voltage shift. On the other hand, a-IGZO TFT with the Er2O3 dielectric has a larger drive current degradation than that with the Er2TiO5 one. The hygroscopic nature of RE oxide films forming hydroxide produces oxygen vacancies in the gate dielectric, leading to a larger flat-band voltage shift and higher leakage current. The incorporation of Ti into the Er2O3 dielectric film can effectively reduce the oxygen vacancies in the film.
In conclusion, we have fabricated a-IGZO TFT devices using the Er2O3 and Er2TiO5 films as a gate dielectric. The a-IGZO TFT incorporating a high-κ Er2TiO5 dielectric exhibited a lower VTH of 0.39 V, a larger μFE of 8.8 cm2/Vs, a higher Ion/Ioff ratio of 4.23 × 107, and a smaller subthreshold swing of 143 mV/dec than that of Er2O3 dielectric. These results are attributed to the addition of Ti into the Er2O3 film passivating the oxygen vacancies in the film and forming a smooth surface. Furthermore, the use of Er2TiO5 dielectric film could improve the stressing reliability. The Er2TiO5 thin film is a promising gate dielectric material for the fabrication of a-IGZO TFTs.
This work was supported by the National Science Council (NSC) of Taiwan under contract no. NSC-101–2221-E-182–059.
- Su LY, Lin HY, Lin HK, Wang SL, Peng LH, Huang JJ: Characterizations of amorphous IGZO thin-film transistors with low subthreshold swing. IEEE Electron Device Lett 2011, 32: 1245–1247.View ArticleGoogle Scholar
- Nomura K, Ohta H, Takagi A, Kamiya T, Hirano M, Hosono H: Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 2004, 432: 488–492. 10.1038/nature03090View ArticleGoogle Scholar
- Lee JS, Chang S, Koo SM, Lee SY: High-performance a-IGZO TFT with ZrO2 gate dielectric fabricated at room temperature. IEEE Electron Device Lett 2010, 31: 225–227.View ArticleGoogle Scholar
- Yang S, Hwang CS, Lee JI, Yoon SM, Ryu MK, Cho KI, Park SHK, Kim SH, Park CE, Jang J: Water-related abnormal instability of transparent oxide/organic hybrid thin film transistors. Appl Phys Lett 2011, 98: 103515. 10.1063/1.3551536View ArticleGoogle Scholar
- Yabuta H, Sano M, Abe K, Aiba T, Den T, Kumomi H, Nomura K, Kamiya T, Hosono H: High-mobility thin-film transistor with amorphous InGaZnO4 channel fabricated by room temperature rf-magnetron sputtering. Appl Phys Lett 2006, 89: 112123. 10.1063/1.2353811View ArticleGoogle Scholar
- Yuan L, Zou X, Fang G, Wan J, Zhou H, Zhao X: High-performance amorphous indium gallium zinc oxide thin-film transistors with HfOxNy/HfO2/HfOxNy tristack gate dielectrics. IEEE Electron Device Lett 2011, 32: 42–44.View ArticleGoogle Scholar
- Huff HR, Gilmer DC: High Dielectric Constant Materials: VLSI MOSFET Applications. Berlin: Springer; 2005.View ArticleGoogle Scholar
- Fanciulli M, Scarel G: Rare Earth Oxide Thin Film: Growth, Characterization, and Applications. Berlin: Springer; 2007.Google Scholar
- Giangregorio MM, Losurdo M, Sacchetti A, Capezzuto P, Bruno G: Metalorganic chemical vapor deposition of Er2O3 thin films: correlation between growth process and film properties. Thin Solid Films 2009, 517: 2606–2610. 10.1016/j.tsf.2008.10.041View ArticleGoogle Scholar
- Zhao Y, Toyama M, Kita K, Kyuno K, Toriumi A: Moisture-absorption-induced permittivity deterioration and surface roughness enhancement of lanthanum oxide films on silicon. Appl Phys Lett 2006, 88: 072904. 10.1063/1.2174840View ArticleGoogle Scholar
- Zhao Y, Kita K, Kyuno K, Toriumi A: Effects of europium content on the microstructural and ferroelectric properties of Bi4−xEuxTi3O12 thin films. Appl Phys Lett 2006, 89: 252908. 10.1063/1.2423242View ArticleGoogle Scholar
- van Dover RB: Amorphous lanthanide-doped TiOx dielectric films. Appl Phys Lett 1999, 74: 3041–3043. 10.1063/1.124058View ArticleGoogle Scholar
- Losurdo M, Giangregorio MM, Bruno G, Yang D, Irene EA, Suvorova AA, Saunders M: Er2O3 as a high-k dielectric candidate. Appl Phys Lett 2007, 91: 091914. 10.1063/1.2775084View ArticleGoogle Scholar
- Pan TM, Lin CW, Hsu BK: Postdeposition anneal on structural and sensing characteristics of high-κ Er2TiO5 electrolyte–insulator–semiconductor pH sensors. IEEE Electron Device Lett 2012, 33: 116–118.View ArticleGoogle Scholar
- Su NC, Wang SJ, Chin A: High-performance InGaZnO thin-film transistors using HfLaO gate dielectric. IEEE Electron Device Lett 2009, 30: 1317–1319.View ArticleGoogle Scholar
- Wang SD, Lo WH, Lei TF: CF4 plasma treatment for fabricating high-performance and reliable solid-phase-crystallized poly-Si TFTs. J Electrochem Soc 2005, 152: G703-G706. 10.1149/1.1955166View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.