Self-Assembled Monolayers (SAMs)/Al2O3 Double Layer Passivated InSnZnO Thin-Film Transistor

We fabricated high-performance InSnZnO (ITZO) thin-film transistors (TFTs) with self-assembled monolayers (SAMs)/Al<sub>2</sub>O<sub>3</sub> double passivation layers (PVLs). The presented SAMs/Al<sub>2</sub>O<sub>3</sub> double passivation leads to high-performance ITZO TFTs with a steep subthreshold slope (~85 mV/dec), a low threshold voltage (~0.9 V), high mobility (~19.8 cm<inline-formula> <tex-math notation="LaTeX">$^{2}\text{V}^{-1}\text{s}^{-1}$ </tex-math></inline-formula>), and high on-off current ratio (<inline-formula> <tex-math notation="LaTeX">$\sim 8.7\times 10^{9}$ </tex-math></inline-formula>). Moreover, compared to devices with Al<sub>2</sub>O<sub>3</sub>PVL, ITZO TFTs with SAMs/Al<sub>2</sub>O<sub>3</sub> double PVLs show better stability in ambient air with a relative humidity of 60% under positive bias stress (PBS) and negative bias stress (NBS). This enhanced stability is attributed to the presence of a high-quality SAM/Al<sub>2</sub>O<sub>3</sub> dual PVL, which not only inhibits Vo-related trap sites and reduces overall trap density, but also protects the back-channel from environmental influences.


I. INTRODUCTION
In recent years, amorphous oxide semiconductor (AOS) transparent thin-film transistors (TFTs) have attracted widespread attention in driving AMLCDs in next-generation flat panel displays and AMOLED displays. Although AOS TFTs have superior performance compared to traditional silicon-based TFTs, their instability is still one of the key issues to be solved because they are sensitive to the environment, such as instability under gate bias.
In the commercial application of AOS TFTs, high-quality performance and high stability in a humid environment are essential [1]- [6]. In order to improve the stability of the AOS TFT, a passivation layer is necessary to protect the AOS active layer from the surrounding environment. As an alumina (Al 2 O 3 ) passivation layer (PVL) is a good barrier layer, it can effectively reduce atmosphere-induced instabilities in the backchannel of AOS film when it reaches a certain The associate editor coordinating the review of this manuscript and approving it for publication was Jenny Mahoney. thickness [7]- [9]. In addition, the Al 2 O 3 passivation layer can inhibit the oxygen atoms in the AOS film from escaping to form Vo during the annealing process, thereby significantly improving the stability of the AOS TFT, especially the negative bias stress (NBS) stability [9], [10]. Compared to Al 2 O 3 , vapor-phase self-assembled monolayers are an environmental barrier layer with better humidity resistance [11], [12]. Vapor-phase self-assembled monolayers (SAMs), made of organosilane compounds with different functions, can be used to modify the surface characteristics of oxides to form a hydrophobic layer, which helps improve the reliability of the device in humid environments [11]- [15]. For example, the present authors' research group has successfully applied different self-assembled monolayer films to the backchannel surface of metal oxide TFTs, leading to high positive bias stress (PBS) stability for TFT devices in humid environments [11]. However, since the self-assembled monolayer is very thin (2∼6 nm), its ability to inhibit the adsorption and diffusion of oxygen molecules is limited, and the improvement of the negative bias (NBS) stability of TFT devices is limited [11]. In this paper, combining the advantages of both SAMs and Al 2 O 3 PVLs, self-assembled monolayers (SAMs)/ Al 2 O 3 dual PVLs were designed to improve the reliability of ITZO TFTs. As a result, the electrical performance and device stability of ITZO TFTs have been significantly improved, including the stability of NBS and PBS in a humid environment.  Fig.1 shows the device structure of a bottom gate ITZO TFT with SAMs/Al 2 O 3 dual PVLs. A gate electrode was prepared by depositing a 300-nm-thick Al-Nd alloy layer (3 wt% Nd) on a glass substrate by DC sputtering and was patterned by photolithography and a wet etching process. An anodization process is then performed to form a 200-nm-thick AlO x :Nd layer as the gate dielectric layer. Subsequently, a 60-nm-thick ITZO film was deposited on the AlO x :Nd film by a ZnO target and ITO target (90 wt% In 2 O 3 and 10 wt% SnO 2 ) co-sputtering with an Ar/O 2 flow rate of 9/6 sccm. During the co-sputtering process, the sputtering power of ITO and ZnO was set to 105 W DC and 130 W RF, respectively. X-ray photoelectron spectroscopy (XPS) analysis revealed that the metal cation ratio of the deposited ITZO film was In: Sn: Zn = 52.9%: 4.2%: 42.9%. The source/drain (S/D) electrodes were prepared by sputtering 240 nm thick ITO in a pure Ar atmosphere. Shadow masks were used to pattern the ITZO channel and S/D electrodes, defining a channel width/length of 300 um/300 um. To fabricate the Al 2 O 3 passivation layer, 80-nm thick Al 2 O 3 was deposited by pulsed laser deposition (PLD) with an O 2 flow rate of 65.5 sccm and a laser power of 450 mJ. Subsequently, a post-annealing process of the device was performed on a hot plate of 300 • C in air for 2 hours. For the SAM treatment, n-octyltriethoxysilane (OTES) was used to react with the Al 2 O 3 surface of the device in the vapor phase by heat treatment. The device was inverted on a tungsten evaporation boat, and 0.05 ml of siloxane polymer droplets were dropped on the evaporation boat. This assembly was heated to 120 • C for 2 hours in a vacuum oven, and then the device surface was cooled to room temperature. The thickness of SAMs measured by the ellipsometer is ∼6nm.    Table 1. Herein, the field-effect mobility (µ FE ) can be obtained by transconductance (g m ) at a low drain voltage (V ds = 0.1V), which is given by [16]

III. RESULTS AND DISCUSSION
where L, W, and C ox are the channel length, width, and gate dielectric unit capacitance, respectively. The threshold swing (SS) is calculated by V th is defined as the corresponding gate voltage (V gs ) when the drain current (I ds ) reaches W/L×100nA at V ds = 10.1V. As shown in Fig. 2(a), the ITZO TFT without the passivation layer exhibits a large I off (∼10 −12 A) and a small I on (∼10 −5 A), corresponding small current ratio (I on /I off ), VOLUME 8, 2020 and a small field-effect mobility (µ FE ), indicating poor electrical performance. While the ITZO TFTs with Al 2 O 3 passivation exhibit dramatically modulated electrical performance, and have low I off (∼10 −14 A), low subthreshold swing (SS) (∼93 mV/dec), large I on /I off (∼10 9 ), and large µ FE (∼15.5 cm 2 / V· s). This result is consistent with previous research results [17], [18]. More importantly, compared with Al 2 O 3 passivation, the device performance is further enhanced after double passivation of SAMs/Al 2 O 3 , with a high field-effect mobility (µ FE ) of 19.8 cm 2 /V· s, a large I on /I off of 8.7 × 10 9 , and a low subthreshold swing (SS) of 85 mV/dec. The SS value of the oxide TFT devices is closely related to the total charge trap density (N T ), including the bulk channel layer (N bulk ) and the gate insulator/active layer interface traps (N it ). The relationship between SS and N T can be expressed as [7] where C ox is the gate capacitance per unit area (38 nF/cm 2 ), q is the elementary charge, and k B T is the thermal energy. Compared to unpassivated devices, the threshold voltage of ITZO TFT with an Al 2 O 3 PVL has a positive shift from 2.0 V to 2.9 V. Threshold voltage is expressed by the following formula [19]: where Q i , Q d , C ox , and φ ms is the effective MOS interface charge per unit area, depletion region charge per unit area, insulation capacitance per unit area, and metalsemiconductor work function potential difference, respectively, and E i and E F is the Fermi intrinsic level and the equilibrium Fermi level, respectively. All parameters mentioned above, except Q i /C ox , are affected by the charge carrier concentration of the channel layer, while Q d /C ox is affected by the total trap density consisting of the bulk trap density and interface trap density [19]. Therefore, the positive shift of V th and the reduction of SS and I off are all benefits from the reduction of oxygen vacancies (V O ) and electron concentration in the ITZO channel [17], [18], [20]. It is well known that V O (as donor-like states) is related to the electrons generated in the ITZO channel. In order to confirm the beneficial effects of the Al 2 O 3 passivation layer, XPS was used to examine the O 1s spectra of the backchannel of the ITZO TFT without a PVL (Fig. 3 (a)), with an Al 2 O 3 PVL (Fig. 3(b)), and with SAMs/Al 2 O 3 double PVLs (Fig. 3(c)). As shown in Fig. 3 (a)-(c), the XPS spectra of the O 1s peak can be divided into three peaks: O1, O2, and O3, and further fitted by the Gaussian-Lorentzian. The O1 peak centered at ∼530.4 eV corresponds to the oxygen bonded in the lattices, such as Zn-O, In-O, and Sn-O [21], [22]. The O2 peak centered at ∼531.2 eV corresponds to oxygen deficiencies in the lattices, such as V O . The O3 peak centered at ∼532.3 eV is attributed to chemisorbed oxygen, such as adsorbed O 2 , H 2 O, and the bonded oxygen in -OH groups [21], [22]. It is shown that the relative area of the O2 peak is reduced from 27.5% to 14.0% after Al 2 O 3 passivation. It is reported that Al 3+ as an oxygen binder can effectively reduce V O in AOS film [17], [18]. This could be due to the lower standard electrode potential (SEP) (−1.66 V) of Al than that of In (−0.342 V), Zn (−0.761 V), and Sn (−0.13 V), which could combine easily with the oxygen vacancies (V O ) [17], [19], [20]. This results in a reduction of oxygen vacancy and a decrease of electron concentration in the ITZO channel, and the V th of the device is positively shifted accordingly. Compared to the ITZO TFT with an Al 2 O 3 PVL, the device performance is further enhanced after double passivation of SAMs/Al 2 O 3 , and the device hysteresis has been significantly reduced, as shown in Fig. 2(a). The origin of hysteresis in oxide TFT is related to the shallow trap state produced by the water adsorbed on the semiconductor surface [6], [23]. Reducing the adsorption of water on the back-channel of the device is helpful to reduce the hysteresis of the device and improve the electrical performance of the device, such as the on current and the field-effect mobility [11], [13], [23]. This can be confirmed by XPS results. Fig. 3(d) shows the XPS depth profile for the back-channel of the ITZO TFT with SAMs/Al 2 O 3 dual PVLs. It shows that SAMs only exist with the Al 2 O 3 surface and no element diffusion from SAMs to Al 2 O 3 and ITZO layer. Comparing the XPS results of the ITZO TFT with Al 2 O 3 PVL and with SAMs/Al 2 O 3 dual PVLs, the relative area of the O3 peak is reduced from 32.1% to 18.9% after the surface passivation of SAMs, as shown in Fig. 3 (b) and (c). This indicates that the adsorption process of water or oxygen molecules on the back-channel of the ITZO TFT is effectively suppressed [23]. SAMs with silane groups are usually applied to oxide surfaces such as SiO 2 , ITZO, and Al 2 O 3 to adjust the interface trap density, charge carrier density, work function, surface energy, etc [11], [24], [25]. To investigate the beneficial effect of SAM treatment on the Al 2 O 3 PVL, specific analyses for oxide and hydroxide were done by deconvoluting the O 1s spectra of the Al 2 O 3 PVL and SAMs/Al 2 O 3 dual PVLs, as shown in Fig. 4 (a) and (b). The O 1s peak was deconvoluted into two peaks centered at 530.3 eV and 531.7 eV, which is corresponded to the binding energy of the oxide and the hydroxyl groups [20]. After SAM treatment, the number of hydroxyl groups has a decrease for the Al 2 O 3 PVL, which is from 10.7% to 6.8%. A small number of hydroxyl groups can ensure TFT reliability [20]. For the SAM treatment, the anchor groups of silane SAMs undergo dehydration, condensation reaction with the hydroxyl groups on the surface of the oxide to form a strong chemical bond, while the hydrophobic surface is formed by the hydrophobic functional group (-CH 3 ) of silane SAMs, as shown in Fig. 5 (d). Since H 2 O adsorbed on the backchannel surface of the TFT seriously affects the electrical characteristics of the device, suppressing the adsorption/desorption effect on the backchannel surface is beneficial to the improvement of the device performance [1], [2], [5], [6]. Fig. 5 (a)-(c) shows the water contact angle measurement image of ITZO film without a PVL, with an Al 2 O 3 PVL, and with a SAMs/Al 2 O 3 dual PVL, respectively. Generally, solid surfaces are divided into hydrophilic and hydrophobic surfaces according to the size of the water contact θ. When θ > 90 • , the solid surface appears hydrophobic, and when θ < 90 • , the solid surface appears hydrophilic. According to the results of the contact angle test, the surface of ITZO is changed from hydrophilic to hydrophobic after passivation by SAMs/Al 2 O 3 . It can be seen that compared with the lower hydrophobicity of the Al 2 O 3 passivation layer, the modification of the Al 2 O 3 passivation layer by SAMs further strengthens the hydrophobic property of the passivation layer and enhances the barrier ability of the passivation layer to water in the air, which is conducive to the improvement of the device performance, especially the stability of the device [11]- [14], [26].
Positive bias (PBS) and negative bias (NBS) tests were used to confirm the stability of TFTs with different passivation layers. For the positive/negative bias stress (PBS/NBS) test, V gs = +10 V / −10 V was applied to the device, and the relative humidity (RH) of ambient air is 60%. The V th shifts of the ITZO TFTs with different passivation layers under PBS/NBS are compared in Fig. 6. Without passivation, the device shows poor bias stability with a V th shift of 4.0/1.9 V under PBS/NBS for 3600 s, as shown in Fig. 6 (a) and (b). In contrast, ITZO TFTs with an Al 2 O 3 PVL have better bias stability, especially negative bias stability ( Fig.6 (d)), showing only a V th shift of 0.1 V for 3600 s. The Al 2 O 3 PVL significantly improves the stability of ITZO TFTs by suppressing V O -related trap sites formed in the ITZO film due to the escape of oxygen atoms [8], [18]. However, there is still a large positive bias drift (Fig.6 (c)), which indicates that Al 2 O 3 film with a thickness of 80 nm cannot effectively block the invasion of H 2 O molecules in the ambient air [9], [11], [27]. Compared to the ITZO TFT with an Al 2 O 3 PVL, the bias stability of the device is further enhanced after double passivation of SAMs/Al 2 O 3 , as shown in Fig.6 (e) and (f). This is mainly due to the modification of the Al 2 O 3 passivation layer by SAMs, which makes a thin hydrophobic layer formed on the surface of the Al 2 O 3 passivation layer. The hydrophobic layer enhances the barrier ability of the passivation layer to H 2 O molecules and improves the effect of the passivation layer [11], [12], [26].  The electrical stability of the ITZO TFTs with SAMs/ Al 2 O 3 dual PVLs under various humidity conditions was also investigated, as shown in Fig. 7 (a). All samples were subjected to different humidity conditions for 2 h. It can be seen that the transfer curves of the ITZO TFTs with SAMs/Al 2 O 3 dual PVLs nearly coincide under different relative humidities (RH). Fig. 7 (b) shows the change in V th , SS, and field-effect mobility (µ/µ 0 ) of the ITZO TFTs with SAMs/Al 2 O 3 dual PVLs under different relative humidities. Overall, the V th , SS, and field-effect mobility (µ/µ 0 ) did not change significantly under different humidity conditions and remained within acceptable ranges, indicating the device with SAMs/Al 2 O 3 dual PVLs has excellent humidity resistance [13], [23]. The -CH 3 group of the SAM is hydrophobic, and a dense layer of alkyl chains is capable to serve as a diffusion barrier, which can isolate the surface of the device from H 2 O molecules in the environment. Therefore, SAMs/Al 2 O 3 dual PVLs can provide stable transistor performance under different humidity environments [13].

IV. CONCLUSION
In summary, ITZO TFT passivated by a SAMs/Al 2 O 3 double-layer film is fabricated. The SAMs/Al 2 O 3 doublelayered passivation structure enhances the performance of ITZO TFTs through the film's intrinsic properties. The Al 2 O 3 passivation acts as an effective carrier suppressor by Al 3+ acting as an oxygen binder to reduce V O in the back-channel region. The dense alkyl chain layer of SAMs can act as a diffusion barrier, preventing reactive substances such as O 2 and H 2 O from reaching the surface. As a result, the ITZO TFT with SAMs/Al 2 O 3 dual PVLs has excellent device performance and good bias stability in humid environments. Overall, the SAMs /Al 2 O 3 double passivation layer structure combines the advantages of Al 2 O 3 passivation and SAMs passivation, while reducing the number of Vo-related trap sites, it can also have a good passivation effect on the device. This provides a new structure for improving the reliability of ZnO-based transparent metal oxide TFTs. His current research interests include novel compound thin-film transistors based on ZnO, IGZO, GaN, and their application in active matrix displays. VOLUME 8, 2020