High performance gallium-zinc oxynitride thin film transistors for next-generation display applications Tae Sang Kim1, Hyun-Suk Kim1, Joon Seok Park1, Kyoung Seok Son1, Eok Su Kim1, Jong-Baek Seon1, Sunhee Lee1, Seok-Jun Seo1, Sun-Jae Kim1, Sungwoo Jun2, Kyung Min Lee2, Dong Jae Shin2, Jaewook Lee2, Chunhyung Jo2, Sung-Jin Choi2, Dong Myong Kim2, Dae Hwan Kim2, Myungkwan Ryu1, Seong-Ho Cho1, and Youngsoo Park1 1 Samsung Advanced Institute of Technology,
[email protected], 2Kookmin University,
[email protected] Introduction: High speed thin film transistors (TFTs) are in great need for next-generation TVs which will employ ultra high definition resolution (3840×2160) panels and possibly include multi-view autostereoscopic 3D technology which will negate the use of glasses for 3D viewing mode. In order to achieve high mobility devices, various types of metal oxide semiconductors have been extensively studied, including the most popular In-Ga-Zn-O, with typical field effect mobilities ranging between 10 to 30 cm2/Vs. Although these numbers are much higher than that of conventional amorphous silicon (0.5~1.0 cm2/Vs) TFTs, there is a strong demand for even higher mobility semiconductors which can exhibit excellent uniformity over a large area. Recently, new class of material based on zinc oxynitride (ZnON) has been introduced and it demonstrated superior characteristics compared to conventional amorphous metal oxide semiconductors [1,2]. In this work, we present results from both experiment and first principle calculation showing that by controlling the amount of nitrogen and oxygen within the ZnON film and adding suitable dopant such as Ga, the TFT performance can be greatly improved to be applicable for next-generation TV backplanes. Experimental: ZnON flims were deposited by reactive sputtering Zn metal target with N2 and O2 gas as reactants. Ga doping was realized by co-sputtering Ga2O3 target during ZnON film deposition. ZnON TFTs were fabricated on glass substrates (150mm×150mm) using standard semiconductor fabrication process (Fig. 1). The density-functional theory (DFT) calculations were performed with VASP code using GGA +U for the exchange correlation energy. 320-atom supercell for Zn3N2 (cubic antibixbyte structure) was used in the defect calculations. For extraction of the density of states (DOS) over a total subbandgap energy range (EV≤E≤EC), including the acceptor-like states near conduction band minimum EC (gA(E)) and the donor-like states near valence band maximum EV (gD(E)), we used a monochromatic photonic capacitance-voltage spectroscopy (MPCVS) by using a capacitance-voltage (C-V) characteristics under dark and photonic conditions with subbandgap optical source (λ≈1306 nm; hν=Eph=0.95 eV < Eg,ZnON=1.3 eV and the optical power Pop=0.2 mW) [3]. The energy band diagrams illustrating the concept of MPCVS are schematically shown in Fig. 2(a) and 2(b). Cross-sectional views with experimental setups are illustrated in Fig. 3 and the related model parameters and equations are summarized in Table 1. Results and Discussion: Hall effect measurements were taken for 50nm ZnON films on glass substrates. Fig. 4 shows the Hall effect mobility and carrier concentration of ZnON films depending on the oxygen content within the film. Both the Hall effect mobility and carrier concentration decreases with the increase in oxygen content of the grown films. Positive correlation between the Hall effect mobility and carrier concentration is a similar behavior seen in common
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amorphous oxide semiconductors such as IGZO, which is described by the percolation theory (Fig. 5). It is noted that the Hall effect mobility of ZnON is much higher than that of IGZO at similar carrier concentration regime which is explained by the lower electron effective mass of ZnON compared to that of oxide semiconductors [2]. To investigate the details of ZnON film, grazing incidence angle X-ray diffraction measurement was taken (Fig. 6). ZnON flims were nominally amorphous in nature, but for N-rich films, small peak around 53 degrees [Zn3N2 (440)] was seen. Nano beam diffraction pattern confirms the existence of cubic Zn3N2 nanocrystalline phase within the N-rich ZnON films. It is postulated that N-rich ZnON films have more of Zn3N2 nanocrystals within highly disordered phase. Through DFT calculation, Zn3N2 crystal was constructed (Fig. 7) and various types of point defects were studied. Table 2 shows the formation energy of each point defect within Zn3N2 and the results suggest that nitrogen vacancy is the most likely defect to be formed with the smallest formation energy. Fig. 8 shows the DOS of clean Zn3N2 and DOS of Zn3N2 with a nitrogen vacancy. For Zn3N2 with N-vacancy, EF is shifted well above the conduction band to produce free electron carrier to show n-type character and this supports the experimental results of N-rich ZnON having higher carrier concentration. Fig. 9 shows the characteristics of ZnON TFTs. Although Nrich ZnON demonstrates higher field effect mobility, high Ioff due to excessive carrier concentration makes it inadequate for display backplane applications. Increasing the O content will suppress the carrier concentration to lower Ioff but undesirable increase of subthreshold swing (S.S.) is observed. To find an alternative method to decrease the carrier concentration without degrading S.S., cation doping was considered. Through DFT calculation, Ga doping was found to effectively increase the formation energy of nitrogen vacancy from 1.97eV for pure Zn3N2 to 2.77 eV resulting in less carrier concentration. Fig. 10 shows the experimental Hall effect measurement of Ga-ZnON which describes the effect of Ga doping in ZnON films. As seen in Fig. 11, 3% Ga doping effectively lowers Ioff without greatly reducing mobility and moreover, S.S. is lowered to improve transfer characteristics. Fig. 12 shows the extracted DOS from MPCVS. The level of gA(E) is well matched with TFT characteristics where O-rich TFT showed inferior S.S. with the highest gA(E) and GaZnON exhibiting the lowest gA(E) level with improved S.S. Summary and conclusion: We have investigated high mobility ZnON material by both experiment and first principle calculation. N-vacancy was identified to be the origin of carriers within the material and Ga doping in ZnON was shown to be an effective method to optimize TFT performance. Fabricated TFTs exhibited great potential for applications in next generation display backplanes. References: [1] Y. Ye et al., J. Appl. Phys. 106, 074512 (2009). [2] Kim, H-S. et al. Sci Rep. 3 1459 (2013). [3] H. Bae et al. SID'13 Dig. Tech. Papers 1033 (2013).
27.1.1
IEDM13-660
Source/Drain
200 nm (AlNd)
ES
100 nm (CVD SiO2 )
Active layer
50 nm
Gate dielectric
350 nm / 50 nm (CVD SiNx/SiO2)
Gate
200 nm (Mo)
Fig. 1. Schematic cross-section of the TFT device structure. All patterning was done using photolithography and appropriate etching process.
Fig. 3. A cross sectional view and setup for MPCVS-based extraction of DOS in n-channel ZnON TFTs with an inverted staggered bottom gate and equivalent circuit model under dark and photonic conditions (a) in depletion region (VOFF
