A Study on Solution-Processed Y2O3 Films Modified by Atomic Layer

Article A Study on Solution-Processed Y2O3 Films Modiﬁed by Atomic Layer Deposition Al2O3 as Dielectrics in ZnO Thin Film Transistor

Haiyang Xu 1, Xingwei Ding 1,2,*, Jie Qi 3, Xuyong Yang 1,2 and Jianhua Zhang 1,2

1 Key Laboratory of Advanced Display and System Application, Ministry of Education, Shanghai University, Shanghai 200072, China; [email protected] (H.X.); [email protected] (X.Y.); [email protected] (J.Z.) 2 School of Mechatronics and Automation, Shanghai University, Shanghai 200072, China 3 Research and Development Department, Air Liquide Innovation Campus Shanghai, Shanghai 201108, China; [email protected] * Correspondence: [email protected]; Tel.: +86-21-5633-1976; Fax: +86-21-3998-8216

Abstract: In this work, Y2O3–Al2O3 dielectrics were prepared and used in ZnO thin ﬁlm transistor as gate insulators. The Y2O3 ﬁlm prepared by the sol–gel method has many surface defects, resulting in a high density of interface states with the active layer in TFT, which then leads to poor stability of

the devices. We modified it by atomic layer deposition (ALD) technology that deposited a thin Al2O3 film on the surface of a Y2O3 dielectric layer, and finally fabricated a TFT device with ZnO as the active layer by ALD. The electrical performance and bias stability of the ZnO TFT with a Y2O3–Al2O3 laminated dielectric layer were greatly improved, the subthreshold swing was reduced from 147 to 6 8 88 mV/decade, the on/off-state current ratio was increased from 4.24 × 10 to 4.16 × 10 , and the threshold voltage shift was reduced from 1.4 to 0.7 V after a 5-V gate was is applied for 800 s.

Citation: Xu, H.; Ding, X.; Qi, J.; Yang, X.; Zhang, J. A Study on Keywords: Y2O3 films; Y2O3–Al2O3 laminated dielectric; atomic layer deposition; thin film transistors

Solution-Processed Y2O3 Films Modiﬁed by Atomic Layer Deposition

Al2O3 as Dielectrics in ZnO Thin Film Transistor. Coatings 2021, 11, 969. 1. Introduction https://doi.org/10.3390/ In recent years, metal oxide thin ﬁlm transistors (TFT) have attracted a lot of attention coatings11080969 due to their high transmittance, high current switching ratio, insensitivity to visible light, and technical advantages, such as solution processing and low temperature deposition. Academic Editor: Angela De Bonis They are widely used in ﬂat panel displays and large-scale integrated circuits and, thus, show a huge application value [1,2]. Among the various metal oxide TFTs, transparent Received: 26 July 2021 ZnO-based TFTs have been extensively studied as a replacement for silicon-based TFTs in Accepted: 13 August 2021 Published: 15 August 2021 large area electronic displays [3–5]. In this regard, ZnO-based TFTs exhibit good electrical and optical properties such as high electron mobility, good uniformity, and excellent

Publisher’s Note: MDPI stays neutral transparency to visible light, making them a promising candidate for practical application with regard to jurisdictional claims in in next generation flat panel displays [6,7]. In this work, ZnO thin films in TFT were published maps and institutional affil- deposited by ALD technology. iations. As an important part of TFT, the gate dielectric layer plays an important role in the performance of TFT. With the reduction in the critical size of semiconductor devices, a traditional SiO2 dielectric layer, which has a low dielectric constant, can no longer meet the requirements of device preparation due to the secondary effect becoming prominent [8]. Therefore, a new high-performance dielectric layer is developed to replace it. Therefore, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. high-k materials, such as ZrO2 [9,10], HfO2 [11], and TiO2 [12], have received extensive This article is an open access article attention from researchers. However, high-k materials are difficult to apply on a large distributed under the terms and scale due to their high cost, easy crystallization, large leakage current, and high surface conditions of the Creative Commons roughness [13]. The interface modification method, i.e., the emergence of laminated Attribution (CC BY) license (https:// dielectric layers, has better solved the problems faced by high-k materials, and further creativecommons.org/licenses/by/ promoted the large-scale application of high-k dielectric layers. Waggoner et al. successfully 4.0/). prepared the ZrO2–Al2O3 laminated dielectric layer and realized the regulation of its

Coatings 2021, 11, 969. https://doi.org/10.3390/coatings11080969 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 969 2 of 7

dielectric properties [14]. Chang et al. successfully applied the gate dielectric layer of the Al2O3/HfO2/Al2O3 structure to the ZnO TFT. Compared with a single HfO2 dielectric layer, its hysteresis has been significantly improved, and it has higher electrical stability [15]. Ding et al. reported that, by inserting Al2O3 as a modified layer into the IGZO TFT with ZrO2 as the dielectric layer, the gate leakage current was obviously reduced and better transfer and output characteristics were obtained [12]. Y2O3 is a promising candidate for use as a gate insulator since it could present low leakage current, high breakdown voltage, and good high-temperature reliability due to both wide band gap (5–6 eV) and excellent thermal stability. Many research groups have studied IGZO transistors employing Y2O3 as gate insulator [16–18]. We prepared a Y2O3 dielectric layer by the sol–gel method and modified it by ALD Al2O3. The Y2O3 film prepared by the sol–gel method has many surface defects, resulting in a high density of interface states with the active layer in TFT, leading to poor stability of the devices. We modified it by atomic layer deposition (ALD) technology that deposited a thin Al2O3 film on the surface of the Y2O3 dielectric layer. This ALD technology has many advantages. For example, the prepared film has a uniform surface, high thickness controllability, excellent repeatability, and low deposition temperature [19]. At the meanwhile, an Al2O3 film is prepared by a direct spin-coating process to modify the Y2O3. This would make it clear that there is a distinct advantage of using an additional ALD layer rather than a direct spin- coating process. The performance of ZnO–Y2O3 (named ‘device A’) and ZnO–Y2O3–ALD and Al2O3 (named ‘device B’) TFT were examined to confirm the expected performance of Y2O3 and the effect of ALD Al2O3.

2. Experiments

A 0.2-M Y2O3 and 0.4-M Al2O3 precursor solution were synthesized by dissolving a certain amount of Y(CH3COO)3·xH2O and Al(NO3)3·9H2O in ethylene glycol methyl ether. After the precursor solution was placed in a magnetic stirrer and stirred for 10 min to completely dissolve the solute, ethanolamine was added as a stabilizer to avoid turbidity and precipitation of the precursor solution. The preparation process of precursor solution was carried out in a glove box filled with nitrogen to isolate water and oxygen in the air. The prepared precursor solution was stirred in a water bath heating pot with a magnetic stirrer at 60 ◦C for 2 h, and then aged at room temperature for 12 h to obtain homogeneous hydrolysis and the best viscosity. Before depositing the film, the p-type Si substrate was ultrasonically cleaned in acetone, alcohol, and deionized water for 10 min to remove stains and grease. Then, the Si substrate was treated in an ozone and ultraviolet environment for 10 min to improve the hydrophobicity of the substrate surface and enhance the uniformity of film growth. Next, the Y2O3 precursor solution was dropped onto the Si substrate through a syringe, first spin-coated at a speed of 500 r/min for 5 s, and then at a speed of 3000 r/min for 30 s. This process was repeated three times to obtain a 60-nm-thick films. The Al2O3 precursor solution was spin-coated on it at a speed of 2000 r/min for 20 s to obtain a 10-nm-thick film. Then, the film was placed on a hot plate at 120 ◦C to cure for 10 min. The film was ◦ then placed in a muffle furnace and annealed at 400 C for 2 h. Subsequently, an Al2O3 film of approximately 10 nm was deposited on another solution-processed 60-nm-thick ◦ Y2O3 film by ALD (TFS-200, Beneq) at 200 C using trimethylaluminum and deionized water. Then, high-purity diethyl zinc (DEZ) and deionized water were used to deposit a 20-nm ZnO active layer at 150 ◦C on the dielectric layer, with a purge time of 5 s. Finally, Al films deposited by thermal evaporation were used as source/drain electrode of TFTs with channel width (W) = 1000 µm and channel length (L) = 200 µm, respectively. The schematic structure of the ZnO TFT with Y2O3–Al2O3 (ALD) as gate insulator are shown in Figure1a. The surface morphology of the films was characterized by atomic force microscope (AFM, nanonaviSPA-400 SPM, SII Nano Technology Inc. Chiba City, Japan). The AFM measurement mode used was a tapping mode. The parameters of the AFM tip (Tap150AL- G) were of a resonant freq. of 150 KHz and a force constant of 5 N/m. The measurement Coatings 2021, 11, x FOR PEER REVIEW 3 of 8

The surface morphology of the films was characterized by atomic force microscope (AFM, nanonaviSPA-400 SPM, SII Nano Technology Inc. Chiba City, Japan). The AFM measurement mode used was a tapping mode. The parameters of the AFM tip (Tap150AL-G) were of a resonant freq. of 150 KHz and a force constant of 5N/m. The measurement geometry was rectangle and the acquisition time was 4 min. The structure of Y2O3 film was analyzed by X-ray diffraction (XRD). The transfer characteristics were measured at room temperature by a semiconductor parameter analyzer (Keithley, 4200, Tektronix Inc, Beaverton, OR, USA).

3. Results and Discussion In order to study the thermal decomposition characteristics of the precursor solution, we tested the thermogravimetric curve of the Y2O3 precursor solution by thermogravimetric analysis (TGA), as shown in Figure 1b. The test conditions are that the temperature rises from room temperature to 800 °C in an air environment, and the heating rate is 10 °C/min. It can be seen from the thermogravimetric curve that the precursor solution has a greater weight loss between room temperature and 120 °C, which is mainly caused by the Coatings 2021, 11, 969 3 of 7 decomposition and hydrolysis reaction of the precursor [20]. There is a small weight loss between 120 and 400 °C. The weight loss in this range is mainly caused by the conversion of related hydroxides into corresponding oxides through a dehydroxylation reaction [21]. Whengeometry the temperature was rectangle is higher and than the acquisition 400 °C, the time weight was of 4 the min. precursor The structure solution of Yalmost2O3 ﬁlm remainswas analyzedstable, indicating by X-ray that diffraction the precursor (XRD). solution The transfer has completely characteristics formed were a dense measured oxide at film.room According temperature to the analysis by a semiconductor result of the thermogravimetric parameter analyzer curve, (Keithley, we choose 4200, 400 Tektronix °C as the Inc, annealingBeaverton, temperature OR, USA). of the film.

FigureFigure 1. (a 1.) The(a) Theschematic schematic structure structure of the of theZnO ZnO TFT TFT with with Y2O Y32–AlO32–AlO3 2(ALD)O3 (ALD) as gate as gate insulator. insulator. (b)Thermogravimetric(b)Thermogravimetric curve curve of precursor of precursor solution. solution.

3.Figure Results 2a and shows Discussion the XRD pattern of Y2O3 film. No obvious crystallization peak is observedIn in order the tospectrum, study the which thermal indicates decomposition that the characteristicsY2O3 film prepared of the precursorby the sol–gel solution, methodwe tested has an the amorphous thermogravimetric structure. curve Amorph of theous Y2O structure3 precursor plays solution an important by thermogravimet- role in high-performanceric analysis (TGA), TFT as[22]. shown Amorphous in Figure struct1b. Theure testhelps conditions to form area film that with the temperaturehigh uni- formityrises and from smooth room temperaturesurface, provides to 800 a fast◦C intransport an air environment,path for carriers, and and the is heating beneficial rate is ◦ to large-size10 C/min. oxide It can TFTs. be seen In order from theto test thermogravimetric the transmittance curve of the that Y2 theO3 film, precursor we deposited solutionhas ◦ a Y2aO greater3 film on weight quartz loss glass. between As shown room in temperature Figure 2b, the and light 120 transmittanceC, which is mainly of the caused Y2O3 film by the exceedsdecomposition 90% in the andvisible hydrolysis light range. reaction The ofresults the precursor show that [20 the]. ThereY2O3 film is a smallis suitable weight for loss the betweenpreparation 120 of and transparent 400 ◦C. The electronic weight lossdevice ins, this and range the film is mainly with high caused transmittance by the conversion has potentialof related in preparation hydroxides of into flexible corresponding and transparent oxides devices. through a dehydroxylation reaction [21]. When the temperature is higher than 400 ◦C, the weight of the precursor solution almost remains stable, indicating that the precursor solution has completely formed a dense oxide film. According to the analysis result of the thermogravimetric curve, we choose 400 ◦C as the annealing temperature of the film. Figure2a shows the XRD pattern of Y 2O3 film. No obvious crystallization peak is observed in the spectrum, which indicates that the Y2O3 film prepared by the sol–gel method has an amorphous structure. Amorphous structure plays an important role in high- performance TFT [22]. Amorphous structure helps to form a film with high uniformity and smooth surface, provides a fast transport path for carriers, and is beneficial to large-size

oxide TFTs. In order to test the transmittance of the Y2O3 film, we deposited a Y2O3 film on quartz glass. As shown in Figure2b, the light transmittance of the Y 2O3 film exceeds 90% in the visible light range. The results show that the Y2O3 film is suitable for the preparation of transparent electronic devices, and the film with high transmittance has potential in preparation of flexible and transparent devices. As shown in Figure3, the surface morphology of the Y 2O3 dielectric layer and Y2O3– Al2O3 laminated dielectric layer was analyzed by AFM. Based on the AFM results, the root mean square roughness of the Y2O3,Y2O3–Al2O3 (ALD), and (c) Y2O3–Al2O3 (solution processed) laminated a dielectric layer are 1.08, 0.66, and 0.86 nm, respectively. The lower surface roughness value of the Y2O3–Al2O3 (ALD) laminated dielectric layer indicates that the surface morphology is the smoothest, which is critical to the formation of good interface contact between the dielectric layer and the active layer. Therefore, we choose Y2O3–Al2O3 (ALD) as a dielectric layer in the following part. Y2O3–Al2O3 (solution processed) will not be discussed. Coatings 2021, 11, x FOR PEER REVIEW 4 of 8

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Figure 2. (a) XRD pattern of Y2O3 film. (b) Transmission rate of Y2O3 film.

As shown in Figure 3, the surface morphology of the Y2O3 dielectric layer and Y2O3– Al2O3 laminated dielectric layer was analyzed by AFM. Based on the AFM results, the root mean square roughness of the Y2O3, Y2O3–Al2O3 (ALD), and (c) Y2O3–Al2O3 (solution processed) laminated a dielectric layer are 1.08, 0.66, and 0.86 nm, respectively. The lower surface roughness value of the Y2O3–Al2O3 (ALD) laminated dielectric layer indicates that the surface morphology is the smoothest, which is critical to the formation of good interface contact between the dielectric layer and the active layer. Therefore, we choose Y2O3– Al2O3 (ALD) as a dielectric layer in the following part. Y2O3–Al2O3 (solution processed)

FigureFigurewill not 2. 2. ( abe()a XRD) discussed. XRD pattern pattern of of Y Y2O2O3 film.3 ﬁlm. (b ()b Transmission) Transmission rate rate of of Y2 YO23O film.3 ﬁlm.

By preparing capacitors based on Al/Y O /Si and Al/Al O -Y O /Si structures, the By preparing capacitors based on Al/Y22O3/Si3 and Al/Al2O32-Y23O3/Si2 3structures, the ca- capacitance–frequencypacitance–frequency characteristic characteristic analyzer analyzer was wasused used to test to testthe relationship the relationship between between the the unit capacitance value and frequency of Y O film and Y O –Al O film, as shown unit capacitance value and frequency of Y2O3 film2 and3 Y2O3–Al2O23 film,3 as2 shown3 in Figure in Figure4a. The capacitance per unit area of the Y O and Y O –Al O dielectric layers 4a. The capacitance per unit area of the Y2O3 and Y2O23–Al3 2O3 dielectric2 3 2layers3 measured at 2 measuredlow frequency at low (20 frequency Hz) are 162.9 (20 Hz)and are131.3 162.9 nF/cm and2, 131.3respectively. nF/cm The, respectively. capacitance The per capac- unit 2 itancearea measured per unit at area high measured frequency at (100 high kHz) frequency is 152.2 and (100 121.0 kHz) nF/cm is 152.22. The and capacitance 121.0 nF/cm per . The capacitance per unit area of Y O and Y O –Al O dielectric layer films varies little unit area of Y2O3 and Y2O3–Al2O3 dielectric2 3 layer2 3 films2 varies3 little with frequency (~8%). with frequency (~8%). Figure4b tests the leakage characteristics of the Y O film and Figure 4b tests the leakage characteristics of the Y2O3 film and the Y2O3–Al2O2 3 3film. The the Y O –Al O film. The leakage currents of Y O and Y O –Al O dielectric layers are leakage2 3currents2 3 of Y2O3 and Y2O3–Al2O3 dielectric2 layers3 are2 1.13 × 102 −73 and 4.5 × 10−8 A/cm2 Figure 3. AFM−7 image of (a) Y−2O8 3, (b) Y2O3–Al2O3 (ALD) and (c) Y2O3–Al2O3 (solution processed). 1.1at a× field10 strengthand 4.5 of× 210 MV/cm,A/cm respectively,at a field and strength the breakdown of 2 MV/cm, electric respectively, field is 5 and and 5.7 the breakdown electric field is 5 and 5.7 MV/cm, indicating that the Y O –Al O laminated MV/cm, indicating that the Y2O3–Al2O3 laminated dielectric layer 2has3 better2 3insulation dielectricBy preparing layer has capacitors better insulation based on properties Al/Y2O3/Si than and the Al/Al Y O2O3dielectric-Y2O3/Si structures, layer. the ca- properties than the Y2O3 dielectric layer. 2 3 pacitance–frequencyThe method of characteristic extracting field-effect analyzer wa mobilitys used to (µ )test et the al. relationship of device parameters between the are unitdescribed capacitance in our value previous and frequency reports [of23 Y].2O Table3 film1 andsummarizes Y2O3–Al2O the3 film, electrical as shown performance in Figure 4a.parameters The capacitance of device per A,unit B area and comparesof the Y2O3 them and Y with2O3–Al other2O3 TFTs.dielectric It can layers be seenmeasured from theat 2 lowtable frequency that the (20 Al2 Hz)O3 intermediate are 162.9 and layer 131.3 greatly nF/cm , improves respectively. the performanceThe capacitance of ZnOper unit TFT. 2 2 areaSpecifically, measured the atµ highincreased frequency from (100 2.65 kHz) to 7.12 is cm152.2/Vs, and the 121.0 on/off-state nF/cm . The current capacitance ratio (Ion per/Ioff ) unitincreased area of from Y2O3 4.24and ×Y210O36–Alto 4.162O3 dielectric× 108, and layer the films subthreshold varies little swing with (SS frequency) decreased (~8%). from Figure0.147 to4b 0.088tests the V/decade. leakage Positivecharacteristics bias voltage of the Y stability2O3 film (PBS) and the is veryY2O3 important–Al2O3 film. for The the leakagepractical currents application of Y2O of3 and oxide Y2O TFT3–Al devices.2O3 dielectric Figure layers5a,b showare 1.1 the × 10 positive−7 and 4.5 bias × 10 voltage−8 A/cm of2 atdevice a field A strength and B, respectively.of 2 MV/cm, Therespectively, test condition and the of PBSbreakdown is a forward electric voltage field ofis 5 and V under 5.7 MV/cm,room temperature indicating andthat darkthe Y environment,2O3–Al2O3 laminated and the duration dielectric is 800layer s. Bothhas better of the twoinsulation devices

propertiesshift towards than athe positive Y2O3 dielectric direction layer. with increasing PBS time, which is attributed to the numerous vacancies at the interface between the dielectric and the channel layer or the full bulk region [10]. The electron trapping occurred at the channel–insulator interface.

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Figure 4. (a) Capacitance–frequency curve per unit area of Y2O3 film and Y2O3–Al2O3 film (b) Leak- age characteristic curve of Y2O3 film and Y2O3–Al2O3 film.

The method of extracting field-effect mobility (μ) et al. of device parameters are described in our previous reports [23]. Table 1 summarizes the electrical performance parameters of device A, B and compares them with other TFTs. It can be seen from the table that the Al2O3 intermediate layer greatly improves the performance of ZnO TFT. Specifi- cally, the μ increased from 2.65 to 7.12 cm2/Vs, the on/off-state current ratio (Ion/Ioff) increased from 4.24 × 106 to 4.16 × 108, and the subthreshold swing (SS) decreased from 0.147 to 0.088 V/decade. Positive bias voltage stability (PBS) is very important for the practical application of oxide TFT devices. Figure 5a,b show the positive bias voltage of device A and B, respectively. The test condition of PBS is a forward voltage of 5 V under room temperature and dark environment, and the duration is 800 s. Both of the two devices shift towards a positive direction with increasing PBS time, which is attributed to the numerous vacancies at the interface between the dielectric and the channel layer or the full bulk re- gionFigure [10]. 4. 4. (a (Thea) )Capacitance–frequency Capacitance–frequency electron trapping occurred curve curve per per at unit the unit area channel–insulator area of Y of2O Y32 filmO3 film and andYinterface.2O3 Y–Al2O23O–Al 3 film2O 3(bfilm) Leak- (b) ageLeakage characteristic characteristic curve curve of Y2O of3 Yfilm2O3 andfilm Y and2O3–Al Y2O2O3–Al3 film.2O 3 film. Table 1. Electrical properties of device A, B and compare with other TFTs. Table 1. ElectricalThe method properties of extracting of device field-effect A, B and compare mobility with (μ other) et al. TFTs. of device parameters are de- Dielectric Layer Methodscribed in our Active previous Layer reports Vth (V)[23]. Tableμ (cm 12 /summarizesVs) Ion/Ioff the electricalSS (V/dec.) performance Ref. pa- 2 Dielectric Layer Method Active Layer Vth (V) µ (cm /Vs) Ion/Ioff SS (V/dec.) Ref. Y2O3–HfO2 RF-ALDrameters of deviceIGZO A, B and compares1.1 them3.3 with other 10TFTs.7 It can be0.180 seen from the[17] table Y O –HfO RF-ALD IGZO 1.1 3.3 107 0.180 [17] 2 Y3 2O3 2 thatALD the Al2O3 intermediateIGZO layer3.9 greatly improves7.6 the performance- 0.350of ZnO TFT. Specifi-[18] Y2O3 ALD IGZO 3.9 7.6 - 0.350 [18] Y2O3–Al2O3 Spraycally, pyrolysis the μ increased ZnO from 2.65 - to 7.12 cm 342/Vs, the on/off-state 105 current- ratio (Ion/I[24]off) in- Spray 5 Y2O3–Al2O3 ZnO6 -8 34 10 -[24] SiO2–Al2O3 pyrolysiscreasedALD from 4.24IGZO × 10 to 4.16 ×8.67 10 , and the6.03 subthreshold 10 swing9 (SS) 0.230decreased from[25] 0.147 9 SiO2–Al2O3 ALD IGZO 8.67 6.03 10 7 0.230 [25] HfO2 to RF 0.088 V/decade. ZnO Positive bias2.55 voltage stability12.2 (PBS)7 10is very important2.550 for the practical[26] HfO2 RF ZnO 2.55 12.2 10 2.550 [26] application of oxide TFT devices. Figure 5a,b show the7 positive7 bias voltage of device A ZrO2 2 ALDALD ZnOZnO 0.10.1 36.8 36.8 10 10 0.0690.069 [27[27]] 6 Y O (Device A) solutionand B, respectively. ZnO The 3.64 test± 0.5 condition 2.65 ±of1 PBS is~4.24 a forward× 10 6voltage0.147 ± of0.01 5 V under room Y22O33 (Device A) solution ZnO 3.64 ± 0.5 2.65 ± 1 ~4.24 × 10 0.147 ± 0.01 This This work Y O –Al O (Device B) solution-ALD ZnO 2.84 ± 0.85 7.12 ± 1 ~4.16 × 108 0.088 ± 0.01 2 3 2 3 temperature and dark environment, and the duration is 800 s. 8Both of the two devices shift Y2O3–Al2O3 (Device B) solution-ALD ZnO 2.84 ± 0.85 7.12 ± 1 ~4.16 × 10 0.088 ± 0.01 work towards a positive direction with increasing PBS time, which is attributed to the numerous vacancies at the interface between the dielectric and the channel layer or the full bulk region [10]. The electron trapping occurred at the channel–insulator interface.

Table 1. Electrical properties of device A, B and compare with other TFTs.

Dielectric Layer Method Active Layer Vth (V) μ (cm2/Vs) Ion/Ioff SS (V/dec.) Ref. Y2O3–HfO2 RF-ALD IGZO 1.1 3.3 107 0.180 [17] Y2O3 ALD IGZO 3.9 7.6 - 0.350 [18] Y2O3–Al2O3 Spray pyrolysis ZnO - 34 105 - [24] SiO2–Al2O3 ALD IGZO 8.67 6.03 109 0.230 [25] HfO2 RF ZnO 2.55 12.2 107 2.550 [26] ZrO2 ALD ZnO 0.1 36.8 107 0.069 [27]

Y2O3 (Device A) solution ZnO 3.64 ± 0.5 2.65 ± 1 ~4.24 × 106 0.147 ± 0.01 This Figure 5. ((aa)) Positive Positive bias bias stress stress of of device A and ( b) Positive bias stress of device B. Y2O3–Al2O3 (Device B) solution-ALD ZnO 2.84 ± 0.85 7.12 ± 1 ~4.16 × 108 0.088 ± 0.01 work

Figure6 shows the threshold voltage shift ( ∆Vth) of device A and B under different stress times. The values of ∆Vth for the device A and B are 1.4 and 0.7 V, respectively. The smaller ∆Vth indicates that the Y2O3–Al2O3 laminated dielectric layer has a lower surface defect state density. The ∆Vth dependence of time agrees with a stretched-exponential equation: ( " #) t β ∆V = (V − V ) 1 − exp − (1) th GS th τ

where Vth is the initial threshold voltage, τ is the carrier trapping time, and β is the stretch index. The τ values of the Y2O3 dielectric layer and the Y2O3–Al2O3 laminated dielectric layer of the ZnO TFT extracted from the equation are 1.60 × 105 and 7.47 × 105 s, and the β value is 0.21 and 0.41, respectively. An oxide TFT device with a high τ value has better stability, so the failure time of a ZnO TFT device with an Y2O3–Al2O3 laminated dielectric layer is prolonged. Figure 5. (a) Positive bias stress of device A and (b) Positive bias stress of device B.

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Figure 6 shows the threshold voltage shift (ΔVth) of device A and B under different stress times. The values of ΔVth for the device A and B are 1.4 and 0.7 V, respectively. The smaller ΔVth indicates that the Y2O3–Al2O3 laminated dielectric layer has a lower surface defect state density. The ∆Vth dependence of time agrees with a stretched-exponential equation:

β t  Δ=VVV() −1exp − − th GS th  (1) τ  

where Vth is the initial threshold voltage, τ is the carrier trapping time, and β is the stretch index. The τ values of the Y2O3 dielectric layer and the Y2O3–Al2O3 laminated dielectric layer of the ZnO TFT extracted from the equation are 1.60 × 105 and 7.47 × 105 s, and the β Coatings 2021, 11, 969 value is 0.21 and 0.41, respectively. An oxide TFT device with a high τ value has better6 of 7 stability, so the failure time of a ZnO TFT device with an Y2O3–Al2O3 laminated dielectric layer is prolonged.

Figure 6. (a) The dependence of threshold voltage shift on stress time of device A and B. (b) and (c) are the ﬁttedfitted curves described by EquationEquation (1)(1) ofof devicedevice AA andand B,B, respectively.respectively.

4. Conclusions

In this work, an Y2O3 dielectricdielectric layer layer was was prepared prepared by the sol–gel method, and a ZnO TFT device with with an an Y Y2O2O3 dielectric3 dielectric layer layer and and an anY2O Y32–AlO32–AlO3 laminated2O3 laminated dielectric dielectric layer layerwere wereprepared prepared by combining by combining with ALD with ALDtechnology. technology. It is found It is found that, that,compared compared with withthe ZnO the ZnO TFT device with the Y O dielectric layer, the electrical performance and bias stability TFT device with the Y2O3 dielectric2 3 layer, the electrical performance and bias stability of of ZnO TFT with the Y O –Al O laminated dielectric layer have been greatly improved. ZnO TFT with the Y2O32–Al3 2O3 2laminated3 dielectric layer have been greatly improved. For For example, the SS dropped from 147 mV/decade to 88 mV/decade, the I /I ratio example, the SS dropped from 147 mV/decade to 88 mV/decade, the Ion/Ioff ratioon increasedoff increased from 4.24 × 106 to 4.16 × 108, and the ∆V was reduced from 1.4 to 0.7 V after a from 4.24 × 106 to 4.16 × 108, and the ΔVth was reducedth from 1.4 to 0.7 V after a 5 V gate voltage5 V gate applied voltage for applied 800 s. for The 800 improvement s. The improvement of the electrical of the electrical performance performance and bias andstability bias stability of ZnO TFT is attributed to the lower interface trap state density of the Y2O3–Al2O3 of ZnO TFT is attributed to the lower interface trap state density of the Y2O3–Al2O3 lami- laminated dielectric layer. nated dielectric layer.

Author Contributions: Conceived the original ideas: X.D.; analyzed the data and created the figure Author Contributions: conceived the original ideas: X.D.; analyzed the data and created the figure plots: H.X. and X.D.; participated in part of the data collection: J.Q. and X.Y.; participated in data plots: H.X. and X.D.; participated in part of the data collection: J.Q. and X.Y.; participated in data analysis: X.D. and H.X.; participated in writing the paper: X.D. and H.X.; supervised the project: X.D. analysis: X.D. and H.X.; participated in writing the paper: X.D. and H.X.; supervised the project: and J.Z. All authors have read and agreed to the published version of the manuscript. X.D. and J.Z. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported in part by the Natural Science Foundation of China (61804093), the Funding: This work is supported in part by the Natural Science Foundation of China (61804093), National Science Foundation for Distinguished Young Scholars of China (51725505), and in part by the National Science Foundation for Distinguished Young Scholars of China (51725505), and in part byScience Science and and Technology Technology Commission Commission of Shanghai of Shanghai Municipality Municipality Program Program (19DZ2281000). (19DZ2281000). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. ConflictsConflicts ofof Interest:Interest: The authors declaredeclare nono conflictconflict ofof interest.interest. References 1. Nguyen, C.P.T.; Raja, J.; Kim, S.; Jang, K.; Le, A.H.T.; Lee, Y.-J.; Yi, J. Enhanced electrical properties of oxide semiconductor thin-film transistors with high conductivity thin layer insertion for the channel region. Appl. Surf. Sci. 2017, 396, 1472–1477. [CrossRef] 2. Hosono, H. Recent progress in transparent oxide semiconductors: Materials and device application. Thin Solid Film. 2007, 515, 6000–6014. [CrossRef] 3. Yu, X.; Marks, T.J.; Facchetti, A. Metal oxides for optoelectronic applications. Nat. Mater. 2016, 15, 383–396. [CrossRef] 4. Petti, L.; Münzenrieder, N.; Vogt, C.; Faber, H.; Büthe, L.; Cantarella, G.; Bottacchi, F.; Anthopoulos, T.D.; Tröster, G. Metal oxide semiconductor thin-film transistors for flexible electronics. Appl. Phys. Rev. 2016, 3, 021303. [CrossRef] 5. Kamiya, T.; Hosono, H. Material characteristics and applications of transparent amorphous oxide semiconductors. NPG Asia Mater. 2010, 2, 15–22. [CrossRef] 6. Fortunato, E.M.C.; Barquinha, P.M.C.; Pimentel, A.C.M.B.G.; Gonçalves, A.M.F.; Marques, A.J.S.; Martins, R.F.P.; Pereira, L.M.N. Wide-bandgap high-mobility ZnO thin-film transistors produced at room temperature. Appl. Phys. Lett. 2004, 85, 2541–2543. [CrossRef] Coatings 2021, 11, 969 7 of 7

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