Polymer-Assisted Deposition of Gallium Oxide for Thin-Film Transistor

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Functional Inorganic Materials and Devices

Polymer Assisted Deposition of Gallium Oxide for Thin-Film Transistors Applications Lin Chen, Wangying Xu, Wen Jun Liu, Shun Han, Pei Jiang Cao, Ming Fang, De Liang Zhu, and You Ming Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10888 • Publication Date (Web): 23 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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ACS Applied Materials & Interfaces

Polymer Assisted Deposition of Gallium Oxide for Thin-Film Transistors Applications

Lin Chen, Wangying Xu,* Wenjun Liu, Shun Han, Peijiang Cao, Ming Fang, Deliang Zhu,* Youming Lu College of Materials Science and Engineering, Shenzhen University, Guangdong Research Center for Interfacial Engineering of Functional Materials, Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, China *Email: [email protected]; [email protected]

Abstract We report the fabrication of gallium oxide (GaOx) thin film by novel polymer assisted deposition (PAD) method. The influence and mechanism of post annealing temperature (200-800 ℃) on the formation and properties of GaOx thin film is investigated by complementary characterization analyses. The results indicate the solution-deposited GaOx experiences the elimination of organic residuals as well as the transformation of amorphous GaOx to crystalline GaOx, with increasing annealing temperature. High quality GaOx could be achieved with smooth surface, wide bandgap, and decent dielectric performance. Moreover, the solution-processed In2O3 TFTs based on optimized GaOx dielectric demonstrate outstanding electrical performance, including a low operating voltage of 5 V, a mobility of 3.09 cm2V-1s-1, on/off current ratio of 1.8 × 105, and subthreshold swing of 0.18 V/decade, respectively. Our study suggests that GaOx achieved by PAD shows great potential for further low-cost and high-performance optoelectronic applications.

Keywords: polymer assisted deposition; GaOx; annealing temperature; dielectric; thin film transistors

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1.

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Introduction Gallium oxide (GaOx) has been extensively investigated in recent years due to its

outstanding material properties, such as superior thermal/chemical stability, high transparency and wide band gap (4.9 eV).1-3 It has been widely used in many semiconductor

fields

such

as

power

devices,4-5

gas

sensing,6-7

deep-UV

photodetectors,8-9 and solar cells.10-11 In addition, GaOx is also a promising high-K dielectric to replace conventional SiO2 due to its relatively high dielectric constant (~10), good breakdown voltage (8 MV/cm), and excellent stability.12-14 However, GaOx film is often fabricated by vacuum-processing methods, such as electron beam evaporation,15 sputter deposition,16 chemical vapor deposition (CVD),17 and atomic layer deposition (ALD).18 This usually causes high processing cost and time consumption. Chemical solution deposition has been emerging recently due to its simplicity, low cost, and high throughput large-scale production.14, 19-21 Unfortunately, there are only few reports on solution preparation of GaOx film and detailed investigations are still lacking.14, 19-20, 22-23 Besides, conventional sol-gel method has the difficulty in controlling the film thickness, roughness, stoichiometry, and chemical reactivity among the precursor solution. For example, to achieve the desired thickness of oxide film, it is necessary to repeating the coating and drying steps, which will undoubtedly increase the process complexity or increase defect states among different coating layer. Polymer assisted deposition (PAD) is an aqueous chemical solution route first described in 2004 and has been achieved great successes for the growth of oxide thin films since then.24-25 PAD method uses metal ions coordinated to polymers as precursor solution, enabling accurate control of thickness, homogeneousness, stoichiometry, and interface roughness.26-27 A large variety of oxide thin films is realized by PAD process with desired physical and structural properties.25,

28-30

However, GaOx, as one of the most promising oxide materials, has not been demonstrated by PAD method to the best of our knowledge. In this work, by utilizing PAD method, we fabricate high quality GaOx thin film for the first time by facile single-step spin-coated process. We systematically study the


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impact of annealing temperature on the structures and properties of GaOx thin films. High quality GaOx could be realized with smooth surface and good electrical performance. To certify the application of GaOx film as gate dielectric, In2O3 TFTs using GaOx dielectric are demonstrated, affording an optimized field effect mobility of 3.09 cm2V-1s-1, on/off current ratio of 1.8 × 105, threshold voltage of 0.83 V, and subthreshold swing of 0.18 V/decade, respectively. 2.

Experimental

2.1 Synthesis of precursor solutions The GaOx precursor solution (1 M) was synthesized by dissolving gallium nitrate hydrate (GaN3O9 · xH2O, 99.9%, Sigma-Aldrich) in deionized (DI) water. Then precursor solution was stirred vigorously in a magnetic stirrer for at least 5 h at 25 ℃. After stirring, the PEI (average MW ≈ 25000 by LS, average Mn ≈ 10000 by GPC, branched, Sigma-Aldrich) was added to the GaOx precursor solution to achieve gallium nitrate hydrate/PEI mass ratio of 3:1. The In2O3 precursor solution (0.1 M) was synthesized by dissolving indium nitrate hydrate (InN3O9 · xH2O, 99.9%, Sigma-Aldrich) into DI water. All precursor solutions were vigorously stirred at 25 ℃ for 6 h, and filtered through a 0.22-μm syringe filter prior to use. 2.2 Thin film fabrication and characterization The substrates of heavily doped p++-Si were ultrasonicated for 10 minutes with acetone, ethanol, and DI water, and then treated with O2 plasma. The GaOx precursor was spin-coated on the p++-Si substrates at 3500 rpm/s for 20 s, and following annealed on a hot plate with a specific temperature (200, 350, 500, 650 and 800 ℃) for 1 h under ambient atmosphere. The thicknesses of GaOx films were measured by ET-4000M (step meter, Japan). The thermal and chemical characteristics of the GaOx powder (dried for 18 h at 80 ℃) was measured by thermogravimetric analyzer (TGA-Q50) and Fourier transform infrared spectroscopy (FTIR, Nicolet 6700). The microstructures of the GaOx thin films were analyzed by using grazing incidence X-ray diffraction (GIXRD, SmartLab), and surface morphologies of GaOx films were measured by using an atomic force microscopy (AFM, Bruker Dimension ICON). The transmittance and chemical structure properties of the GaOx films were analyzed by



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ultraviolet visible spectroscopy (UV-vis, PerkinElmer Lambda 950) and X-ray photoelectron spectroscopy (XPS, Microlab 350), respectively. To evaluate the dielectric properties of the deposited GaOx film, the metal−insulator−metal (MIM) structure was fabricated by thermal evaporated an Al top electrode (100 nm). The capacitance-frequency characteristic of the GaOx film was measured using an impedance analyzer (Keysight E4980A) in the frequency range of 102 to 105 Hz. The dielectric leakage current (Jg) was measured by using a semiconductor parameter analyzer (Keithley 2614B). 2.3 TFT Device fabrication and measurement For the fabrication of In2O3 TFTs with conventional bottom-gate and top-contact structure, the In2O3 precursor was spin-coated on annealed GaOx films at 3500 rpm/s for 30 s, and then these samples were annealed at 250 °C for 1 h under ambient atmosphere. Then a source and drain (S/D) Al electrodes of 100 nm thick were fabricated by thermal evaporation using a metal shadow mask to form the channel with width (W) of 1500 μm and length (L) of 100 μm. The selected large W/L ratio and low frequency (100 Hz) here can efficiently limit the mobility of TFTs to be overestimated according to previous studies.31-32 The average electrical parameters of 15 TFTs devices were measured with a semiconductor parameter analyzer (Keithley 2614B) in the dark ambient atmosphere. Threshold voltage (Vth) was usually calculated from the saturation region of the transfer curve by plotting (IDS)1/2 vs. VGS and extrapolating to IDS = 0 plots. The mobility (μ) and subthreshold swing (S) were extracted from the following expressions:33 I DS =

S=

  μC i W 2L

 d(log I   dV 10

GS

DS

VGS -Vth 

)

  

2

-1

 ln10

(1) kT q

 qN   1+   C  t

(2)

i

Here Ci, W, and L, Nt are the capacitance per unit area of the gate insulator, width and length of channel, and trap states densities, respectively. 3.

Results and discussion To understand the formation process of GaOx, systematical characterization


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techniques were performed, with relevant results summarized in Table 1. Figure 1 displays the thermal characteristics of the GaOx precursor powder. The abrupt weight loss at 200 ℃ can be due to the evaporation of solvent and hydrolysis process, in line with previous reports.14,

34

The gradually weight loss in the range of 350-550 ℃ is

related to the removal of PEI polymers, as reported in literatures.35-36 It is noted that this result is consistent with the thermal characteristics of the PEI, as shown in Figure S1. The precursor films annealed at 200 and 350 ℃ contain large amount of PEI polymers, so it is better to call them GaOx:PEI films.37-38 FTIR spectroscopy can farther illustrate the evolution process of the GaOx (GaOx:PEI) films, as shown in Figure 2. The 200 ℃ annealed GaOx precursor powder clearly shows the characteristics of the amine modes at 1385 cm-1 (C-H bending), 1683 cm-1 (N-H bending), which are due to the amine groups of PEI.37 The peak intensity weakens with increasing annealing temperature, suggesting the gradually removed of PEI from the film. The peak within this scope of 3000-3700 cm-1 is usually related to hydroxyl (OH) group stretching vibrations, which weakens with annealing temperature.14 The 684 and 463 cm–1 peaks gradually increase with annealing temperature, which could be due to the Ga-O bond vibrations.39 The FTIR analysis suggests that GaOx undergoes the removed of PEI and the transformation of gallium hydroxide to form gallium oxide with rise of annealing temperature. Figure 3 displays the AFM images of the GaOx (GaOx:PEI) thin films under various annealing temperatures. The root-mean-square (RMS) values of the GaOx (GaOx:PEI) thin films annealed at 200, 350, 500, 650, and 800 ℃ are 0.473, 0.472, 0.326, 0.834, and 2.03 nm, respectively. The ultra-smooth surfaces of the GaOx (GaOx:PEI) thin films at annealed temperature ≤ 500 ℃ can be attributed to their amorphous nature and the application of a polymer-assisted precursor solution. For the gate dielectric used for bottom-gated TFTs, a smooth surfaces, as well as the amorphous nature, is critical to reducing leakage current caused by the surface roughness and achieving high device performance.40 The apparent enlarge RMS value of GaOx films annealed at the 650 and 800 ℃ could be due to high temperatures driving crystallization-induced agglomeration phenomenon.41



The thickness of GaOx (GaOx:PEI) film annealed at 200, 350, 500, 650, and 800 ℃ are 161, 70, 55, 48.3, and 47.6 nm, respectively. The dramatical reduction of the film thickness between 200 ℃ and 350 ℃ is due to the decomposition of PEI polymers, consistent with TGA and FTIR results. At 500 ℃, most of the PEI polymers are removed, resulting GaOx film with thickness of 55 nm. Further reduction of film thickness is related to complete removal of residual PEI polymers. The film thickness reported here is higher than previous studies, highlighting the advantage of PAD method to increase the viscosity of the aqueous solution.14, 26 Figure 4 shows the GIXRD patterns of GaOx (GaOx:PEI) films. No significant diffraction peaks could be observed for films annealed at temperature ≤ 500 ℃, suggesting the amorphous nature of GaOx (GaOx:PEI) films, in line with previous reports.2, 42 The GaOx thin film has begun to crystallize at 650 ℃ as the monoclinic polycrystalline phase, the crystallization gradually improves and forms β-GaOx with the increased temperature. Crystalline grain size is 10.7 and 14.4 nm for the 650 and 800 ℃-annealed samples, respectively, calculated by the Scherrer formula. For dielectric, polycrystalline film is usually undesirable because the grain boundaries could provide the pathways for impurity diffusion and current leakage.43 As shown in Figure 5, the GaOx (GaOx:PEI) thin films exhibit average transmittance over 90% in the visible range. Besides, the transmittance of the GaOx (GaOx:PEI) thin films increases with the annealing temperature increased from 200 to 650 ℃, which can be attribute to the decomposition of the organic impurities. By 800 ℃, the slightly decreased transmittance could be due to the photon scattering induced by higher-surface roughness.44 The optical bandgap (Eg) of the GaOx (GaOx:PEI) films are calculated via Tauc plot approach, as shown in the inset of Figure 5.45 The Eg of GaOx (GaOx:PEI) decreases from 5.24 to 4.62 eV with increasing annealing temperature from 200 to 800 ℃, which could be related to crystallization phenomenon in agreement with previous reports.46 The Eg values of GaOx (GaOx:PEI) in this work compare well with those reported in the literature. XPS was executed to analysis the chemical constitutions of the GaOx (GaOx:PEI) thin films at various annealing temperatures, as shown in Figure 6a-c. The wide


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spectra and C 1s peaks of the GaOx (GaOx:PEI) films are presented in Figure S2. It can be observed that the C 1s peak gradually weakens with the rise of annealing temperature, indicating that the PEI residual are gradually decomposed in the GaOx:PEI thin films. The O 1s peaks in Figure 6a can be deconvoluted into two kinds of peaks at 530.6 eV and 531.85 eV, corresponding to the oxide lattice (M-O-M) and bonded oxygen species (e.g. carbon-oxygen species, the hydroxyl species, or H2O absorbed on the surface of film), respectively.47 Here, OⅠ and OⅡ are defined as the areas of the oxide lattice and the bonded oxygen species, and Ototal is defined as the total area of O 1s peak. Figure 6b summarizes the ratios of OⅠ/Ototal and OⅡ/Ototal. The OⅠ/Ototal ratio increases from 32.9 % to 83.7 % as the annealing temperature increases from 200 to 800 ℃, indicating that the residual species are progressively decomposed and forms more M-O-M bonds in the GaOx:PEI thin film. For the gate dielectric layer, the bonded oxygen species should be kept small enough because they usually act as defect states, leading to the increasing of the leakage current and reduction of breakdown electric field.48 Figure 6c displays the Ga 2p core-levels for the GaOx (GaOx:PEI) films at various annealing temperatures. All the films exhibit a typical Ga 2p spectra with spin-orbital bimodal (2p 1/2 and 2p 3/2) spacing ~27 eV, confirming the formation of GaOx thin films.47,

49

The Ga 2p peak shifts towards

lower binding energies with increasing temperature, which is attributed to the strong binding strength of Ga–O. This result indicates the progressive oxidization of GaOx with the increased annealing temperature.50 Moreover, the O/Ga ratio of GaOx (GaOx:PEI) films annealed at 200, 350, 500, 650, and 800 ℃ are 1.93, 1.71, 1.49, 1.52, and 1.54, respectively. The higher O/Ga ratio for 200 and 350 ℃-annealed GaOx:PEI films may be attributed to the carbon-oxygen bonds, originating from the residue of PEI polymers. Stoichiometric GaOx films could be formed at annealing temperature higher than 500 ℃. The PL spectra of the GaOx (GaOx:PEI) films at various annealing temperatures are shown in Figure 7. The peak at around 545 nm could be observed, which can be related to O vacancies, Ga vacancies, or Ga-O vacancy pairs generated by partial incomplete oxidation and crystallization during the growth of GaOx.51-55 The apparent



reduction of PL peak at 350 ℃ can be attributed to the decomposition of PEI polymers, in line with FTIR and XPS analyses. The peak intensity decreases with increasing annealing temperature suggests the formation of GaOx metal-oxide framework. The dielectric and electrical characterizes of the GaOx (GaOx:PEI) films were measured by the MIM structure of Al/GaOx/p++-Si and summarized in Table 1. Figure 8a presents the areal capacitance (C) vs. frequency (f) characteristics of the GaOx (GaOx:PEI) films at various annealing temperatures. The GaOx (GaOx:PEI) films annealed at 200, 350, 500, 650, and 800 ℃ have areal capacitances of 220, 157, 178, 186, and 187 nF/cm2 (@100 Hz), corresponding to dielectric constants of 40.1, 12.6, 10.8, 10.1, and 10.2, respectively. It can be observed that the areal capacitance falls abruptly with the increase annealing temperature from 200 to 350 ℃, which can be attributed to the decomposition of PEI polymers, in line with previous reports.56 At temperatures above 350 ℃, the increased areal capacitance can be due to the densification process and the formation of metal-oxide framework.57 Figure 8b displays the leakage current density (Jleak) vs. electric field (E) characteristics for the GaOx (GaOx:PEI) films at various annealing temperatures. The 200 ℃ annealed GaOx:PEI film shows a large leakage current, which is mainly connected to a large amount of residual organic groups. With annealing temperature increases to 500 ℃, the GaOx film displays the lowest leakage current density of 5.6 × 10-6 A/cm2 (@1.5 MV/cm). This could be due to the reduction of defect concentrations since most of the PEI polymers are removed at this annealing temperature, supported by the above TGA, FTIR, XPS, and PL analyses. However, as the annealing temperature increases to 650 or 800 ℃, the leakage current further enhances, owing to a combination of the grain boundaries issues and the formation of semiconducting β-GaOx as reported in literatures.19, 43 In order to further understand the formation mechanism and properties of GaOx by PAD technique, we conduct further experimentation for GaOx film annealed at 580 ℃, as shown in Figure S3, with relevant parameters summarized in Table S1. The 580 ℃-annealed GaOx shows amorphous phase with increased surface roughness of 0.601 nm. At 580 ℃, PEI


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polymers are completely removed. However, the leakage current of 580 ℃-annealed GaOx is larger than the 500 ℃-annealed GaOx. This could be due to the increase of surface roughness, which favors both the Schottky emission and the Poole-Frenkel effect, contributing to the leakage current.43 To certify the applicability of the GaOx dielectric films as a gate insulator, we construct

TFTs

with

bottom-gate

and

top-contact

structure

using

aqueous-solution-based In2O3 as channel layer. The detailed properties of In2O3 could be found in our previous studies.56 Figure 9a shows the transfer characteristics of the In2O3 TFTs with GaOx (GaOx:PEI) dielectrics at various annealing temperatures, corresponding electrical parameters summarized in Table 2. With the increase annealing temperature from 200 to 500 ℃, the mobility of the device improves from 0.19 to 3.09 cm2V-1s-1, on/off current ratio from 6.7 × 102 to 1.8 × 105, and subthreshold swing (SS) from 0.73 to 0.18 V/decade, respectively. The promotion of mobility could be ascribed to the decomposition of residual organic groups and the formation of GaOx metal-oxide framework. These residuals could act as trapping sites, causing device mobility degradation. The fall of Ioff current with annealing temperature is in good agreement with the improvement of dielectric properties of GaOx. Generally, the subthreshold swing (SS) value of the devices can reflect the traps located in the interface of channel and dielectric.58 The interface trap states densities (NSmax) are calculated as 1.43 × 1013, 5.06 × 1012, and 2.19 × 1012 cm-2, corresponding to In2O3 TFTs based on GaOx annealed at 200, 350, and 500 ℃, respectively. The reduction of interface trap densities with annealing temperature is also due to the formation of GaOx framework, in line with mobility and on/off current ratio promotion. No field effect characteristics are observed in the In2O3 TFTs with 650 or 800 ℃-annealed GaOx, which can be due to poor surface roughness of crystallized dielectric. Note that carrier transport of TFT is limited to a narrow region in the interface of channel and dielectric, so the rough surface morphology of dielectric would exacerbate the scattering events, resulting in severe degradation of device mobility.59 In addition, we also demonstrate the In2O3 TFTs with 580 ℃-annealed GaOx dielectric, with the mobility of 0.97 ± 0.07 cm2V-1s-1, on/off



current ratio of 4.7 × 104, and subthreshold swing of 0.23 ± 0.02 V/decade, as shown in Figure S3(e)-(f) and Table S2. The device performance is poor than the 500 ℃-annealed GaOx-gated device, attributed to the increased GaOx surface roughness. The output characteristics of the In2O3 TFTs based on GaOx (GaOx:PEI) dielectrics annealed at 200, 350, and 500 ℃ are shown in Figure 9b-d. The devices show typical n-type transistor characteristics. No obvious current crowding is observed in the low drain voltage region, indicating excellent ohmic contact between the In2O3 channel layer and the Al electrodes. Besides, all the In2O3/GaOx TFTs show ultra-low operating voltage of 5 V compared to conventional SiO2-based TFTs, indicating the suitable in low-power electronics applications. Besides, we also tried using IGO (In:Ga=7:3) as TFTs channel material to extend the application of solution-processed GaOx as dielectric, as shown in Figure S4.60 The IGO TFTs shows good device performance, with a mobility of 1.98 ± 0.14 cm2V-1s-1, on/off current ratio of 9.0 × 104, subthreshold swing of 0.28 ± 0.02 V/decade, respectively. Large clockwise hysteresis is observed in the In2O3 TFTs with 200 ℃-annealed GaOx:PEI, which could be ascribed to the electron trapping at channel/dielectric interface.61 With the forward-scan of gate voltage, the transferred electrons are filled into the unoccupied surface states; when the back--scan of gate voltage, the trapped electrons remain filled until thermally released.61 With the increase annealing temperature from 200 to 500 ℃, the hysteresis of the In2O3 TFTs has clearly improved, which can be due to the decreased interface trap states. The interface trap states densities (NSmax) decreases from 1.43 × 1013 to 2.19 × 1012 cm-2 eV-1, with increasing the annealing temperature from 200 to 500 ℃ for GaOx (GaOx:PEI). This further supports that the improvement of hysteresis for GaOx-gated TFTs is due to the decrease of interface trap densities. To further evaluate the stability of the In2O3/GaOx TFTs, we carried out positive gate-bias-stress (PBS), negative-bias-stress (NBS), negative-bias-illumination-stress (NBIS), and environmental stability characterizations test, as shown in Figure 10 and Figure S5. Figure 10a exhibits the transfer curve evolution of the In2O3/GaOx TFTs under a gate bias voltage of 2 V for various intervals. In the PBS process, the device


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exhibits a positive VTH shift with negligible change of SS value. The negligible change of SS value indicates that no additional defect was created at In2O3/GaOx interface during PBS test.62 The positive VTH shift is usually ascribed to the electron trapping at the In2O3/GaOx interface. Besides, since our device is unpassivated, the VTH shift may be also related to the O2 adsorption at the channel surface layer and the depletion of the electron carriers.41 Figure 10b exhibits the transfer curve evolution of the In2O3/GaOx TFTs under a gate bias voltage of -2 V for various intervals. In the NBS process, the device exhibits a negative VTH shift, as shown in Figure 10b. Figure 10d summarized the VTH shift values versus stress time under different bias stress conditions. The negative VTH shift may be related to the accumulation of positive charges generated by the moisture adsorbed mechanism, expressed by H2O(g) + h+ ↔ H2O+(s).63-64 Under negative electrical field, the resultant buildup of water molecules with positive charge easily induces the delocalized electron carrier, resulting in a negative VTH shift. If the NBS test is done under the light illumination, H2O molecules may continue to combine with photogenerated holes, further exacerbating device degradation. This is the case for NBIS stability, as shown in Figure 10c. Figure S5 shows the environmental stability for In2O3/GaOx TFTs after exposure to a 75% humidity room temperature air environment for 7, 14 and 28 days, corresponding electrical parameters summarized in Table S3. The increased subthreshold swing, off current, and the negative shifting of the turn-on voltage could be observed after humidity air exposure. This could be attributed to the absorbed water at the In2O3 surface. According to the previous study, the adsorbed water can act as either electron donor or acceptorlike trap sites, leading to the increase of off current and degradation of subthreshold swing.65-66 Therefore, we need to introduce some techniques such as passivation and surface treatment to improve the environmental stability of In2O3/GaOx TFTs. 4.

Conclusions In summary, a novel approach based on polymer assisted deposition (PAD) has

been developed to fabricate the GaOx dielectric films. The formation mechanism of PAD-derived GaOx is carefully analyzed by complementary characterizations. The



500 ℃-annealed GaOx film using the simple single-step spin-coated process exhibits smooth surface, amorphous nature and excellent dielectric performance. The corresponding In2O3 TFTs with GaOx dielectric exhibits good electrical performance with a low operating voltage of 5 V, mobility of 3.09 cm2V-1s-1, on/off current ratio of 1.8 × 105, and SS value of 0.18 V/decade, respectively. We believe that the high quality GaOx prepared by PAD here could extend to other optoelectronic applications, such as power devices, deep-UV photodetectors, and solar cells. 5.

Supporting information TGA curves of the PEI. XPS survey spectra and the C 1s peaks of the GaOx thin

films annealed at various temperatures. Transfer characteristics of the IGO TFTs with 500 ℃-annealed GaOx dielectrics. Some characteristics and of the 580 ℃-annealed GaOx film and corresponding In2O3 TFTs. Environmental stability curves and electrical parameters of the In2O3 TFTs with 500 ℃-annealed GaOx dielectrics. Supporting Information is available free of charge from the Internet. 6.

Acknowledgements This work is supported by National Natural Science Foundation of China

(61704111, 51371120, 51872187, and 11774241), Natural Science Foundation of Guangdong Province (2017A030310524), the Science and Technology Foundation of Shenzhen (JCYJ20170817100611468 and JCYJ20170818143417082), and Natural Science Foundation of SZU (827000243 and 2017001).

References (1) Ma, H.-P.; Lu, H.-L.; Wang, T.; Yang, J.-G.; Li, X.; Chen, J.-X.; Tao, J.-J.; Zhu, J.-T.; Guo, Q.; Zhang, D. W. Precise Control of the Microstructural, Optical, and Electrical Properties of Ultrathin Ga2O3 Film through Nanomixing with Few Atom-Thick SiO2 Interlayer Via Plasma Enhanced Atomic Layer Deposition. Journal of Materials Chemistry C 2018, 6, 12518-12528. (2) Ramana, C. V.; Rubio, E. J.; Barraza, C. D.; Miranda Gallardo, A.; McPeak, S.; Kotru, S.; Grant, J. T. Chemical Bonding, Optical Constants, and Electrical Resistivity of Sputter-Deposited Gallium Oxide Thin Films. Journal of Applied


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(31) Xu, W.; Wang, H.; Xie, F.; Chen, J.; Cao, H.; Xu, J. B. Facile and Environmentally Friendly Solution-Processed Aluminum Oxide Dielectric for Low-Temperature, High-Performance Oxide Thin-Film Transistors. ACS Appl Mater Interfaces 2015, 7, 5803-5810. (32) Choi, H. H.; Cho, K.; Frisbie, C. D.; Sirringhaus, H.; Podzorov, V. Critical Assessment of Charge Mobility Extraction in FETs. Nat Mater 2017, 17, 2-7. (33) Fortunato, E.; Barquinha, P.; Martins, R. Oxide Semiconductor Thin-Film Transistors: A Review of Recent Advances. Adv Mater 2012, 24, 2945-2986. (34) Liu, A.; Guo, Z. D.; Liu, G. X.; Zhu, C. D.; Zhu, H. H.; Shin, B.; Fortunato, E.; Martins, R.; Shan, F. K. Redox Chloride Elimination Reaction: Facile Solution Route for Indium-Free, Low-Voltage, and High-Performance Transistors. Advanced Electronic Materials 2017, 3, 1600513. (35) Burrell, A. K.; Mark McCleskey, T.; Jia, Q. X. Polymer Assisted Deposition. Chem Commun (Camb) 2008, 1271-1277. (36) McCleskey, T. M.; Shi, P.; Bauer, E.; Highland, M. J.; Eastman, J. A.; Bi, Z. X.; Fuoss, P. H.; Baldo, P. M.; Ren, W.; Scott, B. L.; Burrell, A. K.; Jia, Q. X. Nucleation and Growth of Epitaxial Metal-Oxide Films Based on Polymer-Assisted Deposition. Chem Soc Rev 2014, 43, 2141-2146. (37) Huang, W.; Zeng, L.; Yu, X.; Guo, P.; Wang, B.; Ma, Q.; Chang, R. P. H.; Yu, J.; Bedzyk, M. J.; Marks, T. J.; Facchetti, A. Metal Oxide Transistors Via Polyethylenimine Doping of the Channel Layer: Interplay of Doping, Microstructure, and Charge Transport. Advanced Functional Materials 2016, 26, 6179-6187. (38) Huang, W.; Guo, P.; Zeng, L.; Li, R.; Wang, B.; Wang, G.; Zhang, X.; Chang, R. P. H.; Yu, J.; Bedzyk, M. J.; Marks, T. J.; Facchetti, A. Metal Composition and Polyethylenimine Doping Capacity Effects on Semiconducting Metal Oxide-Polymer Blend Charge Transport. J Am Chem Soc 2018, 140, 5457-5473. (39) Rambabu, U.; Munirathnam, N. R.; Prakash, T. L.; Vengalrao, B.; Buddhudu, S. Synthesis and Characterization of Morphologically Different High Purity Gallium Oxide Nanopowders. Journal of Materials Science 2007, 42, 9262-9266. (40) Avis, C.; Jang, J. High-Performance Solution Processed Oxide Tft with Aluminum Oxide Gate Dielectric Fabricated by a Sol–Gel Method. Journal of Materials Chemistry 2011, 21, 10649. (41) Zhu, L.; He, G.; Li, W.; Yang, B.; Fortunato, E.; Martins, R. Nontoxic, Eco-Friendly Fully Water-Induced Ternary Zr-Gd-O Dielectric for High-Performance Transistors and Unipolar Inverters. Advanced Electronic Materials 2018, 4, 1800100. (42) Chang, T. H.; Chiu, C. J.; Chang, S. J.; Tsai, T. Y.; Yang, T. H.; Huang, Z. D.; Weng, W. Y. Amorphous Ingazno Ultraviolet Phototransistors with Double-Stack Ga2O3/SiO2 Dielectric. Applied Physics Letters 2013, 102, 221104. (43) Xu, W.; Wang, H.; Ye, L.; Xu, J. The Role of Solution-Processed High-Κ Gate Dielectrics in Electrical Performance of Oxide Thin-Film Transistors. Journal of Materials Chemistry C 2014, 2, 5389-5396. (44) Liu, A.; Liu, G.; Zhu, H.; Song, H.; Shin, B.; Fortunato, E.; Martins, R.; Shan, F. Water-Induced Scandium Oxide Dielectric for Low-Operating Voltage N- and P-Type Metal-Oxide Thin-Film Transistors. Advanced Functional Materials 2015, 25,



7180-7188. (45) Yoo, D.; Kim, I.; Kim, S.; Hahn, C. H.; Lee, C.; Cho, S. Effects of Annealing Temperature and Method on Structural and Optical Properties of TiO2 Films Prepared by Rf Magnetron Sputtering at Room Temperature. Applied Surface Science 2007, 253, 3888-3892. (46) Kumar, S. S.; Rubio, E. J.; Noor-A-Alam, M.; Martinez, G.; Manandhar, S.; Shutthanandan, V.; Thevuthasan, S.; Ramana, C. V. Structure, Morphology, and Optical Properties of Amorphous and Nanocrystalline Gallium Oxide Thin Films. The Journal of Physical Chemistry C 2013, 117, 4194-4200. (47) Mi, W.; Ma, J.; Zhu, Z.; Luan, C.; Lv, Y.; Xiao, H. Epitaxial Growth of Ga2O3 Thin Films on MgO (110) Substrate by Metal–Organic Chemical Vapor Deposition. Journal of Crystal Growth 2012, 354, 93-97. (48) Yoo, Y. B.; Park, J. H.; Lee, K. H.; Lee, H. W.; Song, K. M.; Lee, S. J.; Baik, H. K. Solution-Processed High-K HfO2 Gate Dielectric Processed under Softening Temperature of Polymer Substrates. Journal of Materials Chemistry C 2013, 1, 1651-1658. (49) Girija, K.; Thirumalairajan, S.; Mangalaraj, D. Morphology Controllable Synthesis of Parallely Arranged Single-Crystalline β-Ga2O3 Nanorods for Photocatalytic and Antimicrobial Activities. Chemical Engineering Journal 2014, 236, 181-190. (50) Zhu, C.; Liu, A.; Liu, G.; Jiang, G.; Meng, Y.; Fortunato, E.; Martins, R.; Shan, F. Low-Temperature, Nontoxic Water-Induced High-K Zirconium Oxide Dielectrics for Low-Voltage, High-Performance Oxide Thin-Film Transistors. Journal of Materials Chemistry C 2016, 4, 10715-10721. (51) Harwig, T.; Kellendonk, F. Some Observations on the Photoluminescence of Doped β-Galliumsesquioxide. Journal of Solid State Chemistry 1978, 24, 255-263. (52) Liang, C. H.; Meng, G. W.; Wang, G. Z.; Wang, Y. W.; Zhang, L. D.; Zhang, S. Y. Catalytic Synthesis and Photoluminescence of β-Ga2O3 Nanowires. Applied Physics Letters 2001, 78, 3202-3204. (53) Song, Y. P.; Zhang, H. Z.; Lin, C.; Zhu, Y. W.; Li, G. H.; Yang, F. H.; Yu, D. P. Luminescence Emission Originating from Nitrogen Doping of β-Ga2O3 Nanowires. Physical Review B 2004, 69, 075304. (54) Yang, H.; Shi, R.; Yu, J.; Liu, R.; Zhang, R.; Zhao, H.; Zhang, L.; Zheng, H. Single-Crystalline β-Ga2O3 Hexagonal Nanodisks: Synthesis, Growth Mechanism, and Photocatalytic Activities. The Journal of Physical Chemistry C 2009, 113, 21548-21554. (55) Cheng, Y.; Yang, K.; Peng, Y.; Yin, Y.; Chen, J.; Jing, B.; Liang, H.; Du, G. Research on the Structural and Optical Stability of Ga2O3 Films Deposited by Electron Beam Evaporation. Journal of Materials Science: Materials in Electronics 2013, 24, 5122-5126. (56) Xu, W.; Long, M.; Zhang, T.; Liang, L.; Cao, H.; Zhu, D.; Xu, J.-B. Fully Solution-Processed Metal Oxide Thin-Film Transistors Via a Low-Temperature Aqueous Route. Ceramics International 2017, 43, 6130-6137. (57) Liu, A.; Liu, G.; Zhu, H.; Shin, B.; Fortunato, E.; Martins, R.; Shan, F.


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Eco-Friendly Water-Induced Aluminum Oxide Dielectrics and Their Application in a Hybrid Metal Oxide/Polymer Tft. RSC Advances 2015, 5, 86606-86613. (58) 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. Applied Physics Reviews 2016, 3, 021303. (59) Guo, Z.; Liu, A.; Meng, Y.; Fan, C.; Shin, B.; Liu, G.; Shan, F. Solution-Processed Ytterbium Oxide Dielectrics for Low-Voltage Thin-Film Transistors and Inverters. Ceramics International 2017, 43, 15194-15200. (60) Goncalves, G.; Barquinha, P.; Pereira, L.; Franco, N.; Alves, E.; Martins, R.; Fortunato, E. High Mobility a-IGO Films Produced at Room Temperature and Their Application in TFTs. Electrochemical and Solid State Letters 2010, 13, II20-II22. (61) Park, J. H.; Yoo, Y. B.; Lee, K. H.; Jang, W. S.; Oh, J. Y.; Chae, S. S.; Baik, H. K. Low-Temperature, High-Performance Solution-Processed Thin-Film Transistors with Peroxo-Zirconium Oxide Dielectric. ACS Appl Mater Interfaces 2013, 5, 410-417. (62) Liu, G. X.; Liu, A.; Shan, F. K.; Meng, Y.; Shin, B. C.; Fortunato, E.; Martins, R. High-Performance Fully Amorphous Bilayer Metal-Oxide Thin Film Transistors Using Ultra-Thin Solution-Processed ZrOx Dielectric. Applied Physics Letters 2014, 105, 113509. (63) Liu, P.-T.; Chou, Y.-T.; Teng, L.-F. Environment-Dependent Metastability of Passivation-Free Indium Zinc Oxide Thin Film Transistor after Gate Bias Stress. Applied Physics Letters 2009, 95, 233504. (64) Yang, S.; Cho, D.-H.; Ryu, M. K.; Park, S.-H. K.; Hwang, C.-S.; Jang, J.; Jeong, J. K. Improvement in the Photon-Induced Bias Stability of Al–Sn–Zn–In–O Thin Film Transistors by Adopting Alox Passivation Layer. Applied Physics Letters 2010, 96, 213511. (65) Park, J.-S.; Jeong, J. K.; Chung, H.-J.; Mo, Y.-G.; Kim, H. D. Electronic Transport Properties of Amorphous Indium-Gallium-Zinc Oxide Semiconductor upon Exposure to Water. Applied Physics Letters 2008, 92, 072104. (66) Nayak, P. K.; Jang, J.; Lee, C.; Hong, Y. Effects of Li Doping on the Performance and Environmental Stability of Solution Processed ZnO Thin Film Transistors. Applied Physics Letters 2009, 95, 193503.



100

Weight loss (%)

80 60 40 20 100 200 300 400 500 600 700 800 o

Temperature ( C) Figure 1. TGA curves of GaOx precursor powder from 25 to 800 ℃.

200 oC 650 oC

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

4000

3500

3000

350 oC 800 oC

2500

2000

500 oC

1500

Wavelength (nm)

1000

500

Figure 2. FTIR spectra of GaOx precursor powder annealed at various temperatures.


Page 19 of 26

(-712)

(-312)

(111) (-311)

(-401) (-202)

Figure 3. AFM images of the GaOx (GaOx:PEI) thin films with various annealing temperatures of (a) 200, (b) 350, (c) 500, (d) 650, and (e) 800 ℃.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

800 C o

650 C o

500 C o

350 C o

200 C

30

40

50

2 (degrees)

60

70

Figure 4. GIXRD patterns of the GaOx (GaOx:PEI) thin films annealed at various temperatures.



80

o

200 C o 350 C o 500 C o 650 C o 800 C

40 20 0 200

300

200 oC 350 oC 500 oC 650 oC 800 oC

2

60

(h) (a.u.)

Transmittance (%)

100

400

1

2

5.24eV 5.23eV 5.21eV 4.71eV 4.62eV

3

4

5

6

Photon energy (eV)

500

600

Wavelength (nm)

700

7

800

Figure 5. Optical transmittances of the GaOx (GaOx:PEI) thin films annealed at various temperatures. The inset in panel shows the optical bandgap of the GaOx dielectric films.

(a)

Content (%)

O 1s

200 oC

350 oC

OII

90 80 70 60 50 40 30 20 10

o

M-O-M M-OH/OR

200 300 400 500 600 700 800

Temperature (oC)

500 C

o

650 C

800 oC 536 534

(b)

OI

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

532

530

528

Binding Energy (eV)

(c) o

Ga 2p2/3

Ga 2p1/2

800 C 650 oC 500 oC 350 oC 200 oC

1150

1140

1130

1120

1110

Binding Energy (eV)

Figure 6. (a) XPS spectra of O 1s peaks of the GaOx (GaOx:PEI) thin films annealed at various temperatures. (b) The variation of oxygen components at various annealing temperatures. (c) The Ga 2p spectra of the GaOx thin films


Page 21 of 26

Intensity (a.u.)

200 oC 350 oC 500 oC 650 oC 800 oC

300

400

500

600

700

wavelength (nm) Figure 7. Room temperature PL of the GaOx (GaOx:PEI) thin films annealed at various temperatures.

)

(a) o

200 C 350 oC 500 oC 650 oC 800 oC

10

2

10

3

10

1

(b)

2

240 220 200 180 160 140 120 100 80

Current density (A/cm

Capacitance (nF/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

3

10

4

Frequency (Hz)

10

5

10

-1

10

-3

10

-5

10

-7

0.0

o

200 C o 350 C o 500 C o 650 C o 800 C

0.5

1.0

1.5

Electric field (MV/cm)

2.0

Figure 8. (a) Capacitance-frequency curves and (b) leakage current density-voltage characteristic of the GaOx (GaOx:PEI) thin films annealed at various temperatures.



(a)

VDS= 2V

-3 -2 -1 0

6.0x10-6 V = 0-5V@1V step 5.0x10-6 GS -6 4.0x10 3.0x10-6 2.0x10-6 1.0x10-6 0.0 -1.0x10-6 0.0 0.5 1.0 1.5

(b)

IDS (A)

o

200 C o 350 C o 500 C o 650 C o 800 C

1

2

VGS (V)

3.0x10-5 V = 0-5V@1V step 2.5x10-5 GS -5 2.0x10 1.5x10-5 1.0x10-5 5.0x10-6 0.0 -5.0x10-6 0.0 0.5 1.0 1.5

VDS (V)

3

4

5 (c)

VDS (V)

6x10-5 V = 0-5V@1V step 5x10-5 GS 4x10-5 3x10-5 2x10-5 1x10-5 0 -1x10-5 0.0 0.5 1.0 1.5

2.0 (d)

IDS (A)

IDS (A)

10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10

IDS (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

2.0

VDS (V)

2.0

Figure 9. (a) Transfer characteristics of the In2O3 TFTs with GaOx (GaOx:PEI) dielectrics annealed at various temperatures. (b-d) output characteristics of the In2O3 TFTs with GaOx (GaOx:PEI) dielectrics annealed at 200, 350, and 500 ℃, respectively.


10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10

0s 30s 90s 180s 360s 720s 1440s

-3 -2 -1 0s 30s 90s 180s 360s 720s 1440s

-3 -2 -1

(a)

VDS= 2V

VTH= 0.39V

0

1

2

VGS (V)

3

4

5

(c)

VDS= 2V

VTH= 0.83V

0

1

2

VGS (V)

3

4

10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

0s 30s 90s 180s 360s 720s 1440s

5

VTH= 0.50V

0

1

2

VGS (V)

PBS, dark NBS, dark NBS, light

0

30

(b)

VDS= 2V

-3 -2 -1

VTH (V)

IDS (A)

10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10

IDS (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


IDS (A)

Page 23 of 26

3

4

5

(d)

90 180 360 720 1440

Stress time (s)

Figure 10. Transfer characteristics of the In2O3 TFTs with GaOx dielectrics annealed at 500 ℃ under (a) PBS, (b) NBS, (c) NBIS test, and (d) VTH shift values versus bias stress time.


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Table 1.

Page 24 of 26

Microstructural, optical, XPS derived Oxygen 1s content and dielectric characteristics of solution-based GaOx (GaOx:PEI) thin films

at different annealing temperatures. Annealing

Areal capacitances Thickness

Roughness

Crystalline grain size

(nm)

(nm)

(nm)

temperature

leakage current density

Optical bandgap O/Ga ratio

M-O-M (%)

M-OH/OR (%)

at 100Hz

K

at 1.5 MV/cm

(eV) (nF/cm2)

(℃)

(A/cm2)

200

161

0.473

Amorphous

1.93

32.9

67.1

5.24

200.7

40.1

66.0

350

70

0.472

Amorphous

1.71

59.0

41.0

5.23

157.6

12.6

2.8×10-5

500

55

0.326

Amorphous

1.49

63.1

36.9

5.21

177.9

10.8

5.6×10-6

650

48.3

0.834

10.7

1.52

69.3

30.7

4.71

186.5

10.1

6.5×10-5

800

47.6

2.03

14.4

1.54

83.7

16.3

4.62

187.3

10.2

6.9×10-4


Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Electrical parameters of In2O3/GaOx TFTs under different annealing temperatures. Annealing

Mobility

Threshold

On/off

Subthreshold

temperature (℃)

(cm2 V-1 s-1)

voltage (V)

current ratio

Swing (V/decade)

Interface trap densities (cm-2·eV-1)

a

200

0.19±0.08

0.46±0.21

6.7×102

0.73±0.19

1.43×1013

350

1.97±0.13

0.86±0.18

1.0×104

0.36±0.01

5.06×1012

500

3.09±0.22

0.83±0.12

1.8×105

0.18±0.01

2.19×1012

650

No field effect

800

No field effect

Average values of 15 devices



TOC


Page 26 of 26

Polymer-Assisted Deposition of Gallium Oxide for Thin-Film Transistor

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