Nano Express
High-Performance a-InGaZnO Thin-Film Transistors with Extremely Low Thermal Budget by Using a Hydrogen-Rich Al2O3 Dielectric
Yan Shao1,
Xiaohan Wu1,
Mei-Na Zhang1,
Wen-Jun Liu1 &
Shi-Jin Ding ORCID: orcid.org/0000-0002-5766-089X1
Nanoscale Research Letters volume 14, Article number: 122 (2019) Cite this article
Electrical characteristics of amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) are compared by using O2 plasma-enhanced atomic layer deposition Al2O3 dielectrics at different temperatures. High-performance a-IGZO TFTs are demonstrated successfully with an Al2O3 dielectric deposited at room temperature, which exhibit a high field-effect mobility of 19.5 cm2 V− 1 s− 1, a small subthreshold swing of 160 mV/dec, a low threshold voltage of 0.1 V, a large on/off current ratio of 4.5 × 108, and superior negative and positive gate bias stabilities. This is attributed to the hydrogen-rich Al2O3 dielectric deposited at room temperature in comparison with higher deposition temperatures, thus efficiently passivating the interfacial states of a-IGZO/Al2O3 and the oxygen vacancies and improving conductivity of the a-IGZO channel by generating additional electrons because of enhanced hydrogen doping during sputtering of IGZO. Such an extremely low thermal budget for high-performance a-IGZO TFTs is very attractive for flexible electronic application.
Amorphous In-Ga-Zn-O (a-IGZO)-based thin film transistors (TFTs) have attracted much attention in the past decade due to their high mobility, good uniformity, high visible light transparency, and low process temperature [1,2,3]. These merits make it a promising candidate for the application of next-generation electronics, such as transparent display, flexible devices, or wearable electronics. In particular, for the applications of flexible electronics, TFTs are generally fabricated on low thermally stable polymer substrates. Thus, it is necessary to reduce the thermal budget of a-IGZO TFT fabrication. For this purpose, many researchers have focus on a-IGZO TFTs with room temperature fabricated gate insulators, such as sputtering [4,5,6], solution process [7,8,9], e-beam evaporation [10], and anodization [11]. However, these dielectric films often suffer from high density of traps and strong dielectric/a-IGZO interfacial scattering, thus resulting in limited field-effect mobility, a large subthreshold swing, and a small on/off current ratio [4,5,6,7,8,9,10,11].
On the other hand, atomic layer deposition (ALD) is a promising technique, which can provide high-quality films, precise control of film thickness, good uniformity over a large area, and low process temperature [12,13,14]. Zheng et al. [15] reported that the a-IGZO TFT with ALD SiO2 dielectric exhibited excellent electrical performance without the need of post-annealing. However, a high substrate temperature of 250 °C is required for the ALD of SiO2 films [15], which is higher than glass transition temperatures of most flexible plastic substrates. Interestingly, it is reported that ALD of Al2O3 films can be realized even at room temperature (RT) [16, 17]; meanwhile, the Al2O3 film deposited at RT contains a large amount of hydrogen (H) impurities [17]. However, to the best of our knowledge, the abovementioned H-rich Al2O3 film has never been utilized as a gate insulator in a-IGZO TFT. Therefore, it is desirable to explore the a-IGZO TFT with a RT ALD Al2O3 gate insulator.
In this letter, high-performance a-IGZO TFT was successfully fabricated with a room temperature deposited Al2O3 gate dielectric. By comparing the characteristics of the a-IGZO TFTs with various Al2O3 gate insulators deposited at different temperatures, the underlying mechanism was addressed.
Highly doped p-type silicon wafers (< 0.0015 Ω cm) were cleaned by standard RCA processes and served as gate electrodes. Forty-nanometer Al2O3 films were deposited in a commercial ALD system (Picsun Ltd.) using trimethylaluminum (TMA) and O2 plasma as a precursor and reactant, respectively. One growth cycle consisted of 0.1 s TMA pulse, 10 s N2 purge, 8 s O2 plasma pulse, and 10 s N2 purge. The TMA was maintained at 18 °C for a stable vapor pressure and dose, and the O2 gas flow rate was fixed at 150 sccm with a plasma generator power of 2500 W. Subsequently, 40-nm a-IGZO films were deposited by RF sputtering using an IGZO ceramic target with an atomic ratio of In:Ga:Zn:O = 1:1:1:4. During sputtering, working pressure and Ar and O2 gas flow rates were fixed at 0.88 Pa and 48 and 2 sccm, respectively. The active region was formed by photolithography and wet etching. After that, source/drain electrodes of 30-nm Ti/70-nm Au bilayers were prepared by electron beam evaporation and a lift-off method. No further annealing processes were applied on these devices.
The electrical properties of a-IGZO TFTs were characterized using a semiconductor device analyzer (Agilent Tech B1500A) in a dark box at room temperature. The device stabilities were measured under positive and negative gate bias stresses, respectively. The depth profiles of elements and chemical composition were measured by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS), respectively.
Figure 1a compares the dielectric constants of the Al2O3 films deposited at different temperatures as a function of frequency (i.e., from 10 Hz to 105 Hz). As the deposition temperature increases from 100 to 150 °C, the film shows a gradual decrease in dielectric constant. A similar trend was also reported in previous literatures for the deposition temperature changing from RT to 150 °C [18, 19]. This is because the RT Al2O3 film contains the highest concentration of hydrogen (H) in the form of OH groups. Thus, the corresponding dielectric constant is enhanced due to a rotation of more OH groups in an electric field [20]. In terms of the measurement frequency of 10 Hz, the extracted dielectric constants for the RT, 100 °C, and 150 °C Al2O3 films are equal to 8.6, 7.9, and 7.4, respectively, which are used for the extraction of the field-effect mobility (μFE) and interfacial trap density (Dit) of the fabricated TFT device. Figure 1b shows the leakage current characteristics of different Al2O3 films. It is found that the RT Al2O3 film exhibits a small leakage current density of 2.38 × 10− 8 A/cm2 at 2 MV/cm and a breakdown electric field of 5.3 MV/cm. In addition, the breakdown electric field increases gradually with increasing deposition temperature from 100 to 150 °C.
Electrical properties of Al2O3 films deposited at different temperatures. a Dielectric constant versus frequency. b Leakage current density versus electric field
Figure 2 shows the typical transfer curves of the a-IGZO TFTs with different Al2O3 gate insulators. The RT Al2O3 TFT exhibits the best performance, such as high μFE of 19.5 cm2 V− 1 s− 1, a small subthreshold swing (SS) of 160 mV/dec, a small threshold voltage (VT) of 0.1 V, and a large on/off current ratio (Ion/off) of 4.5 × 108. However, the a-IGZO TFTs with Al2O3 gate insulators deposited at both 100 and 150 °C show a much poorer performance, i.e., reduced on-currents (10− 7 and 3 × 10− 9 A) and degraded SS. The Dit at the interface of Al2O3/a-IGZO can be calculated based on the following equation [21]:
$$ {D}_{\mathrm{it}}=\left(\frac{\mathrm{SS}\times \lg e}{kT/q}-1\right)\frac{C_{ox}}{q^2} $$
where e, k, T, and q represent the Euler's number, Boltzmann constant, absolute temperature, and unit electron charge, respectively. Cox is the gate dielectric capacitance per unit area. For the RT Al2O3 TFT, the Dit is equal to 1.1 × 1012 eV− 1 cm− 2, which is over one or two times lower than those for the TFTs with the Al2O3 gate insulators deposited at 100 and 150 °C.
Transfer curves of the a-IGZO TFTs with ALD Al2O3 gate insulators deposited at different temperatures together with the extracted device parameters
The gate bias stabilities of the devices were further measured by applying negative and positive voltages. Figure 3 shows the VT shift as a function of bias stress time for different TFTs. In terms of negative gate bias stress (NGBS), the RT Al2O3 TFT exhibits a negligible VT shift of − 0.04 V after being stressed at − 10 V for 40 min. However, higher-temperature Al2O3 gate insulators generate larger VT shifts especially for 150 °C. Such a high NGBS stability for RT Al2O3 should be attributed to a low concentration of oxygen vacancies (VO) in the a-IGZO channel [22]. With respect to positive gate bias stress (PGBS), the RT Al2O3 TFT shows a VT shift of 1.47 V, which is much smaller than those (8.8 V and 12.1 V) for the 100 and 150 °C Al2O3 TFTs. Moreover, the influence of storage time on the device performance was investigated, as shown in Fig. 4. Although no passivation layer is covered on the back channel, the device still maintains an excellent performance after being kept in a cabinet (20% RH) for 60 days at 30 °C; meanwhile, no significant variations in μFE and SS are observed. This indicates the RT Al2O3 TFTs without any passivation layer have good storage-time-dependent stability in the current ambience.
VT shift as a function of bias stress time under NGBS = − 10 V and PGBS = 10 V for the TFTs with Al2O3 insulators deposited at different temperatures
Time-dependent stability of RT Al2O3 TFT after being kept in a cabinet (20% RH) at 30 °C. a Transfer curves. b Mobility and subthreshold swing
Table 1 compares the performance of our RT Al2O3 TFT with other reports. It is found that our device exhibits a zero-near VT, smaller SS, and larger Ion/off in the case of comparable mobility [4, 23]. Although using a Ta2O5 gate insulator can obtain higher mobility of 61.5 cm2 V− 1 s− 1, both SS and Ion/off deteriorate remarkably [10]. In a word, our RT Al2O3 TFT possesses a superior comprehensive performance in comparison with the 100 and 150 °C Al2O3 TFTs. Since all processing steps are identical except the deposition step of Al2O3, such significant differences in electrical performance should originate from the Al2O3 gate insulators.
Table 1 Comparison of the electrical parameters of our RT Al2O3 a-IGZO TFT and other a-IGZO TFTs fabricated at low temperatures (Tmax denotes the maximum process temperature)
To understand the underlying mechanism, the depth profiles of the elements in the a-IGZO/Al2O3 stacked films were analyzed by SMIS. Figure 5a shows the dependence of H concentration on depth in the stacks of IGZO/Al2O3, where the Al2O3 films were deposited at RT and 150 °C, respectively. For comparison, an IGZO film deposited on a bare Si substrate was also analyzed. The IGZO film deposited on bare Si contains an H concentration of ~ 3 × 1021 cm− 3, which originates from the residual gas in sputtering system and absorbed H2/H2O molecules on the Si surface. Both IGZO films deposited on the Al2O3 films contain higher H concentrations than that on the bare Si substrate. This indicates that the increased H concentrations should come from the release of H impurities in the underlying Al2O3 films during sputtering of IGZO. Moreover, it is observed that the H concentration in the IGZO film atop the RT Al2O3 film is higher than that on the 150 °C one in the interface-near region, which can provide more efficient passivation of interfacial states. This thus improves the SS and PGBS stability of the RT Al2O3 TFT by reducing interfacial carrier trapping. Additionally, the O 1s XPS spectra of the a-IGZO films near the interface of IGZO/Al2O3 were analyzed, as shown in Fig. 5b. The fitted peaks are located at 530.2 ± 0.1 eV, 530.9 ± 0.1 eV, and 531.6 ± 0.1 eV, corresponding to O2− ions bound with metal (O1), VO (O2), and OH groups (O3), respectively [13, 24]. The percentage of O2 is 26.3% in the a-IGZO layer atop the bare Si; however, it decreases to 12.3% and 6.8% for the 150 °C and RT Al2O3 underlying films, respectively. This indicates that more VO in the IGZO channel can be effectively passivated by additional H impurities originating from the underlying Al2O3 films, especially for the RT Al2O3 film with a higher H concentration. It is reported that when VO and H both are present in the a-IGZO film, they can combine to form a stable state in which H is trapped at VO (VOH), and the resulting VOH is a shallow-level donor [25,26,27]. Thus, enhanced H doping into the IGZO channel atop the RT Al2O3 improves the channel conductivity by providing additional electrons. Furthermore, the small VT shift under the NGBS for the RT Al2O3 TFT can also be attributed to the effective H passivation of VO [28]. As reported in literatures, the instability of TFT under NGBS originates from ionization of neutral VO (VO → VO2++2e−) [17, 29]. Moreover, the O3 percentage of the a-IGZO film on the RT Al2O3 is 6.9%, which is higher than those on the 150 °C Al2O3 (5.3%) and the bare Si (4.6%), respectively. The OH group could originate from the reaction O2− + H → OH− + e− during deposition of IGZO films [30]. Thus, the enhanced H doping into the IGZO channel atop the RT Al2O3 film generates more OH groups and also contributes to improve the channel conductivity.
a SIMS profiles of hydrogen concentration in Al2O3 deposited at RT and 150 °C. b High-resolution O1s XPS spectra of the IGZO channel deposited on RT Al2O3, 150 °C Al2O3, and bare Si
A high-performance a-IGZO TFT was fabricated successfully under the extremely low thermal budget of RT using an H-rich Al2O3 gate dielectric prepared by O2 plasma-enhanced ALD. This is ascribed to the fact that the Al2O3 dielectric deposited at RT contains more hydrogen impurities than those deposited at higher temperatures. Thus, the released H impurities during sputtering of IGZO generated more electrons, and efficiently passivated the interfacial states of a-IGZO/Al2O3 and the VO in the a-IGZO channel.
a-IGZO:
Amorphous In-Ga-Zn-O
ALD:
Atomic layer deposition
D it :
Interfacial trap density
I on/off :
On/off current ratio
NGBS:
Negative gate bias stress
PGBS:
Positive gate bias stress
RT:
SIMS:
Secondary ion mass spectrometry
Subthreshold swing
TFT:
Thin-film transistor
V O :
Oxygen vacancy
V OH:
Hydrogen trapped at oxygen vacancy
V T :
Threshold voltage
XPS:
X-ray photoelectron spectroscopy
μ FE :
Field-effect mobility
Nomura K, Ohta H, Takagi A et al (2004) Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 432:488–492
Park JS, Maeng WJ, Kim HS et al (2011) Review of recent developments in amorphous oxide semiconductor thin-film transistor devices. Thin Solid Films 520:1679–1692
Zan HW, Yeh CC, Meng HF et al (2012) Achieving high field-effect mobility in amorphous indium- gallium-zinc oxide by capping a strong reduction layer. Adv Mater 24:3509–3514
Zheng Z, Zeng Y, Yao R et al (2017) All-sputtered, flexible, bottom-gate IGZO/Al2O3 bi-layer thin film transistors on PEN fabricated by a fully room temperature process. J Mater Chem C 5:7043–7050
Huang XD, Ma Y, Song JQ, Lai PT (2016) High-performamce amorphous InGaZnO thin-film transistor with ZrLaO gate dielectric fabricated at room temperature. J Disp Technol 12:1522–1527
Nag M, Bhoolokam A, Steudel S et al (2015) Impact of the low temperature gate dielectrics on device performance and bias-stress stabilities of a-IGZO thin-film transistors. ECS J Solid State SC 4:N99–N102
Hsu CC, Chu MW, Sun JK, Chou HT (2016) Low temperature fabrication of an amorphous InGaZnO thin-film transistor with a sol-gel SiO2 gate dielectric. J Disp Technol 12:1043–1050
Seul HJ, Kim HG, Park MY, Jeong JK (2016) A solution-processed silicon oxide gate dielectric prepared at a low temperature via ultraviolet irradiation for metal oxide transistors. J Mater Chem C 4:10486–10493
Jo JW, Kim YH, Park J et al (2017) Ultralow-temperature solution-processed aluminum oxide dielectrics via local structure control of nanoclusters. ACS Appl Mater Interfaces 9:35114–35124
Chiu CJ, Chang SP, Chang SJ (2010) High-performance a-IGZO thin-film transistor using Ta2O5 gate dielectric. IEEE Electron Device Lett 31:1245–1247
Shao Y, Xiao X, He X et al (2015) Low voltage a-InGaZnO thin-film transistors with anodized thin HfO2 gate dielectric. IEEE Electron Device Lett 36:573–575
Ok KC, Park SHK, Hwang CS et al (2014) The effects of buffer layers on the performance and stability of flexible InGaZnO thin film transistors on polyimide substrates. Appl Phys Lett 104:063508
Sheng J, Han J, Choi W, Park J, Park JS (2017) Performance and stability enhancement of in−Sn−Zn−O TFTs using SiO2 gate dielectrics grown by low temperature atomic layer deposition. ACS Appl Inter Mater 49:42928–42934
Levy DH, Nelson SF (2011) Thin-film electronics by atomic layer deposition. J Vac Sci Technol A 30:018501
Zheng LL, Ma Q, Wang YH et al (2016) High-performance unannealed a-InGaZnO TFT with an atomic-layer-deposited SiO2 insulator. IEEE Electron Device Lett 37:743–746
Groner MD, Fabreguette FH, Elam JW, George SM (2003) Low-temperature Al2O3 atomic layer deposition. Chem Mater 16:639–645
Potts SE, Keuning W, Langereis E et al (2010) Low temperature plasma-enhanced atomic layer deposition of metal oxide thin films. J Electrochem Soc 157:P66–P74
Kim SK, Lee SW, Hwang CS et al (2005) Low temperature (<100°C) deposition of aluminum oxide thin films by ALD with O3 as oxidant. J Electrochem Soc 153:F69–F76
Niskanen A, Arstila K, Ritala M, Leskela M (2005) Low-temperature deposition of aluminum oxide by radical enhanced atomic layer deposition. J Electrochem Soc 152:F90–F93
Pethrick RA, Hayward D, Jeffry K, Affrossman S, Wilford P (1996) Investigation of the hydration and dehydration of aluminium oxide-hydroxide using high frequency dielectric measurements between 300 kHz-3 GHz. J Mater Sci 31:2623–2629
Schroder DK (2005) Semiconductor material and device characterization, 3rd edn. Wiley, Hoboken
Moon YK, Lee S, Kim DH et al (2009) Application of DC magnetron sputtering to deposition of InGaZnO films for thin film transistor devices. Jpn J Appl Phys 48:031301
Ning H, Zeng Y, Kuang Y et al (2017) Room-temperature fabrication of high-performance amorphous In−Ga−Zn−O/Al2O3 thin-film transistors on Ultrasmooth and clear nanopaper. ACS Appl Mater Interfaces 9:27792–27800
Tsao SW, Chang TC, Huang SY et al (2010) Hydrogen-induced improvements in electrical characteristics of a-IGZO thin-film transistors. Solid State Electron 54:1497–1499
Nakashima M, Oota M, Ishihara N et al (2014) Origin of major donor states in In-Ga-Zn oxide. J Appl Phys 116:213703
Xu L, Chen Q, Liao L et al (2016) Rational hydrogenation for enhanced mobility and high reliability on ZnO-based thin film transistors: from simulation to experiment. ACS Appl Mater Interfaces 8:5408–5415
Chen C, Cheng KC, Chagarov E, Kanicki J (2011) Crystalline In–Ga–Zn–O density of states and energy band structure calculation using density function theory. Jpn J Appl Phys 50:091102
Chen C, Yang BR, Li G et al (2019) Analysis of ultrahigh apparent mobility in oxide field-effect transistors. Adv Sci:1801189
Kim Y, Kim S, Kim W et al (2012) Amorphous InGaZnO thin film transistors—PART II: modeling and simulation of negative bias illumination stress-induced instability. IEEE Trans Electron Devices 59:2699–2706
Bang J, Matsuishi S, Hosono H (2017) Hydrogen anion and subgap states in amorphous In–Ga–Zn–O thin films for TFT applications. Appl Phys Lett 110:232105
Xiao X, Zhang L, Shao Y et al (2017) Room-temperature-processed flexible amorphous InGaZnO thin film transistor. ACS Appl Mater Interfaces 10:25850–25857
Lim W, Jang JH, Kim SH et al (2008) High performance indium gallium zinc oxide thin film transistors fabricated on polyethylene terephthalate substrates. Appl Phys Lett 93:082102
There is no acknowledgement.
This work was supported by the National Natural Science Foundation of China (61874029, 61474027, 51603151), and the National Key Technologies R&D Program of China (2015ZX02102-003).
All datasets are presented in the main paper and freely available to any scientist wishing to use them for non-commercial purposes, without breaching participant confidentiality.
School of Microelectronics, Fudan University, Shanghai, 200433, People's Republic of China
Yan Shao, Xiaohan Wu, Mei-Na Zhang, Wen-Jun Liu & Shi-Jin Ding
Yan Shao
Xiaohan Wu
Mei-Na Zhang
Wen-Jun Liu
Shi-Jin Ding
YS and MNZ carried out the main part of the fabrication and analytical works. YS, XW, and SJD participated in the sequence alignment and drafted the manuscript. SJD and WJL conceived the study and participated in its design. All authors read and approved the final manuscript.
Correspondence to Shi-Jin Ding.
Shao, Y., Wu, X., Zhang, MN. et al. High-Performance a-InGaZnO Thin-Film Transistors with Extremely Low Thermal Budget by Using a Hydrogen-Rich Al2O3 Dielectric. Nanoscale Res Lett 14, 122 (2019). https://doi.org/10.1186/s11671-019-2959-1
Received: 28 December 2018
Accepted: 26 March 2019
Hydrogen-rich Al2O3
Applications of Atomic Layer Deposition