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.
KeywordsAmorphous InGaZnO Thin-film transistor Er2O3 Er2TiO5
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.
Results and discussion
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.
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