PDF Journal of Materials Chemistry C

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Cite this:J. Mater. Chem. C, 2020, 8, 5163

Oxygen incorporated solution-processed high-j La

₂

O

₃

dielectrics with large-area uniformity, low leakage and high breakdown field comparable with ALD deposited films†

Longsen Yan,‡âWaner He,‡âXiaoci Liang,^bChuan Liu,*^bXihong Lu, ^c Chunlai Luo,âAihua Zhang,âRuiqiang Tao,âZhen Fan, âMin Zeng,â Honglong Ning, ^dGuofu Zhou,êXubing Lu *âand Junming Liu âf

Low-power, form-free electronics require low-temperature and solution-processed high-k dielectric materials with extremely low leakage current and good flexibility, which is still very challenging. For example, the large band-gap material La2O3 has been hampered from low-power electronics applications due to the poor stability and inability to be solution-processed. Here, we develop oxygen- incorporated solution-deposition to obtain a high-kLa2O3dielectric film at a low temperature (1201C).

The thin film exhibits a uniform large area, a high-kvalue (412), a large band gap (46.3 eV), a high breakdown electric field (47 MV cm ¹), and a very low leakage current (10 ⁸A cm ²at 1 MV cm ¹) with excellent and stable insulating characteristics comparable with ALD deposited films. This method eﬃciently improves the chemical reaction and wettability of the precursor solution for densification and is also applicable for other high-kdielectric materials like HfO2and ZrO2. The film endures compressive strain at the limit of the PET substrate (B2.5%) and enables flexible CMOS circuits with stable organic thin-film transistors (OTFTs) that exhibit a high gain (490), a low operation voltage (2 V), and a low static power consumption (B0.5 nW). The excellent characteristics enable the presented film and method to generally advance low-power and high-performance electronics with printable and flexible properties.

Introduction

The gate dielectric is one of the critical components in the new generation of field-eﬀect transistors (FETs)^1,2 or thin film transistors (TFTs) for constructing form-free and low-power electronics including logic, memory, or sensing circuits and information displays.^3–7 Ideally, these TFTs should feature a low operational voltage, a low leakage current, and a fast switching speed,^8–10therefore demanding dielectric materials with a high dielectric constant (k) especially for organic flexible TFTs with relatively low carrier mobility values.^11,12 Although some oxide materials based on HfO2 and ZrO2 have been developed as solution-processed gate dielectrics,^13–17it is still a big challenge to obtain a dielectric film with an excellent combination of properties, including a high-k value, a wide band-gap, a low surface roughness, a nearly defect-free structure, a low leakage current, a high breakdown voltage, a high tolerance of mechanical strain, and, especially for form-free electronics, a low-processing temperature under ambient conditions.^18,19

The main problem to be solved is how to densify a metal oxide film from solution with an extremely low degree of

aInstitute for Advanced Material and Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, China.

E-mail: luxubing@m.scnu.edu.cn

bState Key Laboratory of Optoelectronic Materials and Technologies, School of Electronics and Information Technology, Sun Yat-Sen University, Guangzhou, 510274, China. E-mail: liuchuan5@mail.sysu.edu.cn

cMOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-Carbon Chem and Energy Conservation of Guangdong Province, KLGHEI of Environment and Energy Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, China

dState Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou, 510640, China

eInstitute of Electronic Paper Displays and Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, China

fLaboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, 210093, China

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc06210f

‡Longsen Yan and Waner He contributed equally.

Received 13th November 2019, Accepted 26th February 2020 DOI: 10.1039/c9tc06210f

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defects and perfect dielectric properties with a low deposition temperature. Compared with vacuum-based techniques (e.g., pulsed laser deposition,²⁰magnetron sputtering,²¹chemical vapor deposition,²² atomic layer deposition,^23,24 etc.), solution- processed deposition has much less restrictions on its applic- ability for low-cost and large-area flexible electronics.^25–31 However, until now, high-temperature treatment (often over 4001C and beyond the tolerance of most soft substrates) for film densification^18,32is still needed for many oxide materials to reduce various physical and chemical defects generated by pinhole, strain released during the heating and cooling process, organic residues,etc.^33–35

In particular, the dielectric material La2O3is one of the high-k dielectric materials that is most difficult to be low-temperature processed by solution and it has rarely been reported, although it has the unique advantage of a large bandgap and strong resistance to crystallization and it has been shown to be a good dielectric layer in MOSFET applications.^36,37Importantly, theoretical calcula- tion and experimental work have demonstrated that it has the electrical nature of an ultra-low leakage current when compared with other representative high-k materials such as Al₂O₃and HfO₂.^38,39 To overcome the difficulty of densification and sensitivity to moisture, we carefully design a coating solution and accurately control the defect density in the film. At a sufficiently low temperature of 1201C (without UV radiation), we fabricated a La2O3dielectric film that is verified to feature a high-k value, an ultra-low leakage current, and long-term stability in air. The mechanism for the remarkable improvement has been investigated and it is also applicable to improving densification and film properties of other high-k dielectrics including HfO2and ZrO2. Based on these films, low-voltage OTFTs and CMOS logic circuits with high gain and low power have been demonstrated for low-power, form-free electronics.

Results

Eﬀect of oxygen incorporation on the solution and film properties

For solution deposition of La2O3 films, we use acetylacetone lanthanum andN,N-dimethylformamide (DMF) as the precursor compound and solvent, respectively. At first, we found that a small amount of oxygen infusion can significantly change the properties of the precursor solution, enabling it to have a better ability for spreading out on the substrate surface. Then, we carried out a systematic study based on controlling the oxygen infusion into the precursor solution (as shown in Fig. 1a).

We compared solutions that received oxygen for 0 min, 1 min, 5 min, or 30 min, respectively. After a stirring process, the color of the solution varies from light to dark, i.e., from transparent to yellow, brown red, and crimson, as shown in Fig. 1b. The solution with O₂infusion for 30 min (simplified as

‘‘30 min-O₂’’ below) showed a much smaller contact angle than that of the solution without O2infusion when they were dipped onto the surface of Au film, as shown in Fig. S1 in the ESI.†The better wetting properties illustrate the enhanced polarity and

were related to the increased number of OH groups in the solutions (Fig. 1c), as revealed by Fourier transform infrared (FT-IR) spectroscopy, as shown in Fig. S2 in the ESI.†To further reveal the eﬀect of oxygen incorporation in the precursor solution, we also carried out¹H-NMR measurement. As shown in Fig. S3 (ESI†), the ¹H-NMR spectra of the four types of precursor solutions show nearly the same peaks, indicating that there should not exist significant chemical reactions. The same peak positions of FT-IR spectra shown in Fig. S2 (ESI†) also support the same conclusion as that of the¹H-NMR results.

The solutions were deposited into La2O3films, which were annealing at 1201C, as schematically shown in Fig. S4 in the ESI.†Compared with those made without oxygen infusion, the films made by using 30 min-O2 solution show significant improvement in the surface smoothness and film uniformity.

The improvement on wafer-scale substrates could be directly observed by the naked eye, as shown in Fig. 1b and Fig. S1 in the ESI.† Details of the morphology of the films can be observed in the AFM images (shown in Fig. S5a in the ESI†).

The films were confirmed to be amorphous as no crystalline peaks could be found in the XRD patterns (Fig. S5b in the ESI†).

Moreover, it is oxide but not hydroxide that has been formed, as the signals corresponding to the OH groups are not observable in the FT-IR spectra (Fig. S6c in the ESI†).

To characterize the electrical properties of the La2O3films, we fabricated Au/La2O3/Au(MIM) arrays on a flexible PET substrate, as shown in Fig. 1d. The high-resolution transmission electron microscopy (HRTEM) images of the La2O3films with no O2or 30 min O2infusion solution on Au/PET substrates are shown in Fig. 1e. Of the similar film thickness (70–79 nm) obtained after six-cycle spin-coating, the 30 min-solution- coated film shows a much better thickness uniformity than that of the film made without oxygen infusion. The highest temperature for all the processes including pre-annealing and densification of La₂O₃ films was 1201C, which was carefully determined by thermal gravity (TG) analysis (see Fig. 1f and Fig. S7 in the ESI†). In terms of electric properties, the La₂O₃ films exhibit decreased leakage current as the oxygen infusion time increases before coating, as shown in Fig. 1g. All these films show stable capacitance and maintain high permittivity at various frequencies (see Fig. 1h).

The statistical data of insulating properties for the La2O3

films are summarized in Fig. 1i and j and Table S1 in the ESI,†

showing their leakage currentJiat 1 MV cm ¹and breakdown fieldEb, respectively. Data of individual devices are shown in Fig. S8 in the ESI.†For the 18 devices with 30 min-oxygen La2O3

films, very lowJi(o10 ⁷A cm ² at 1 MV cm ¹) and high Eb

(around 7 MV cm ¹) values were observed. In comparison, for the 25 devices with non-oxygen La₂O₃films, we observed more than 4 orders of magnitude higher leakage current (from 10 ³ A cm ² to 100 A cm ² at 1 MV cm ¹) and much lower E_b (1 MV cm ¹to 3 MV cm ¹) values also with a broad distribution.

The insulating properties are comparable with the La₂O₃ film with a similar thickness grown by atomic layer deposition (ALD) in previous studies, where EBD is about 4 MV cm ¹.⁴⁰Moreover, a series of 30 min-O2 MIM devices annealed at diﬀerent

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temperatures from 1201C to 6001C was fabricated, and their Ji–Eiand permittivity frequency characteristics were measured, as shown in Fig. S9 in the ESI.† Improvement has also been found when using the same method to make HfO2and ZrO2thin films and the electrical properties of the MIM devices (20 for each) are shown in Fig. S8 in the ESI,†where those made with oxygen-incorporated deposition exhibit lower leakage current and higher breakdown fields.

Electrical and mechanical properties on flexible PET substrates The Au/La₂O₃/Au devices with diﬀerent La₂O₃film thicknesses (from 42 to 78 nm) were fabricated on flexible PET substrates by using 3 to 6 cycles of spin coating. TheJi–Eiand permittivity–

frequency characteristics of the devices are shown in Fig. 2a

and Fig. S10 in the ESI.†The leakage current for 78 nm La2O3is below 10 ⁷A cm ²at 1 MV cm ¹andB10 ⁶A cm ²at 3 MV cm ¹. Even with such a low processing temperature, the leakage current level of the solution-deposited La2O3films is already comparable with that of high-temperature processed (4400 1C) high-k dielectric films such as ZrO2, HfO2 and Al₂O₃,^14,19,20or vacuum process (iCVD) deposited polymers.²¹ The observed dielectric constant k is B12 for the four films with different thicknesses and the value is comparable with that of the solution-processed HfO₂ and ZrO₂. An important feature for permittivity is good frequency stability in a wide frequency range from 100 to 1 MHz. Also, due to the large bandgap, the La2O3dielectric film shows high transparency up to 94% in the visible light range (Fig. S11 in the ESI†) that is Fig. 1 Solution preparation and film characterization. (a) Experimental setup for oxygen infusion into precursor solution with a gas flowmeter (60 ml min ¹).

(b) Solution with various oxygen infusion times after 12 hour reaction and optical images of spin-coated films on silicon wafers (the diameter is 5 cm).

(c) Measurements of Fourier transform infrared spectroscopy (FT-IR) for the OH vibration. (d) A photo and a schematic diagram view of a MIM structure on a flexible PET substrate. (e) Cross-sectional HRTEM images of films (non-O₂or 30 min O₂) with the structure of PET/Au/La₂O₃/glue. (f) Thermogravimetric (TG) analysis of precursor solution to test weight loss processes (the initial weights of solution were 21.67 mg). (g) and (h) Insulating and dielectric properties for the films with diﬀerent times of O₂infusion, manifested as (g) the current density as a function of electric field (J_i–E_icurves) and (h) permittivity as a function of testing frequency (from 1 k to 2 MHz). (i) and (j) Leakage current and breakdown electrical field (E_b) based on the film (2 layers, annealed at 1201C, deposited on heavily doped Si substrate) made from solution with no O₂or 30 min O₂infusion.

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favorable for transparent electronics. A comparison of the main characteristic parameters for representative high-kfilms deposited by a solution process and thermal annealing is shown in Fig. 2b and Table S1 in the ESI.†The La₂O₃film in this study features the lowest processing temperatures and leakage current, while it has a permittivity comparable with that of the other reported materials.

Stability and reproducibility were then examined by taking the 26 nm La₂O₃ films as an example. The values of E_bwere measured for 18 devices on a 1.5 cm1.5 cm substrate and all were found to be over 7 MV cm ¹ (Fig. S8 in the ESI†). The durability of La2O3films to various strengths of electric fields was investigated (Fig. 2c) and the leakage current in each constant electric field cycle (100 s) remained stable until the Fig. 2 Performance of flexible La₂O₃MIM devices on PET substrates. (a) Insulating properties of flexible MIM devices with diﬀerent spin-coated cycles (three to six cycles, approximately 42 nm to 78 nm):J_i–E_icurves. (b) Comparison of leakage current at 1 MV cm ¹and permittivity as a function of annealing temperature for oxide dielectrics made by solution-process and thermal annealing. (c) Time-dependentJ_i–E_icurves at various electric field, increasing from 1 MV cm ¹to 6 MV cm ¹by 1 MV cm ¹and the stress duration for each step is 100 s. (d) Evolution ofJ_iwithE_stressof 1.5 MV cm ¹for approximately 45k s. (e) Schematic diagrams of flexible MIM devices. (f) Flexible MIM devices (thickness of La₂O₃layer is 78 nm) under tensile (down) and compressive (up) bending tests andJ_iis measured at 1.2 MV cm ¹. The strain (S) is calculated using the relation ofS=d_sub/2R, in whichd_subis the thickness of a substrate andRis bending radius (R). (g) Bending fatigue test under a tensile strain of 1.25% at 1.5 MV cm ¹. The La₂O₃dielectrics can sustain up to 6000 bending cycles.

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exerted electric field increased over 6 MV cm ¹, indicating excellent durability to high electric fields. The properties are already comparable to that of the ALD (atomic layer deposition)- grown Al₂O₃dielectrics (Fig. S12 in the ESI†). Furthermore, time- dependent leakage density was tested with a constant electric field of 1.5 MV cm ¹in air (Fig. 2d). The leakage only increases from 5.810 ⁷A cm ²to 3.3 10 ⁶ A cm ²after the retention of 4.5 10⁴ seconds, showing an excellent stability under a high electric field. Given the previous reports that La₂O₃films usually degrade quickly in air as they are sensitive to moisture,^41,42it is surprising that the solution-deposited La2O3 film exhibits such good air stability simply after covering the top metal electrode.

To investigate mechanical flexibility, compressive and tensile bending measurements were performed, as illustrated in Fig. 2e. The MIMs (dLa₂O₃ = 26 nm) fabricated on a PET substrate (250mm) exhibited excellent insulation with an ultra- small Ji (Fig. 2f) upon compressive stress with bending to a radius of curvature (R) of about 5 mm and upon tensile stress with aRof 7.2 mm. These values correspond to a compressive strain of 2.47% and a tensile strain of 1.80%, respectively. This curvature has already reached the ultimate deformation of 250mm-thick PET substrates, where plastic deformation takes place whenRreaches 5 mm (2.47% strain). For comparison, it has been reported that the MIM devices with the ALD-Al₂O₃ dielectric showed leakage-current failure between 0.99 and

1.33% applied strain.⁴³ In addition, under a tensile strain of 1.25% and a high electric field of 1.5 MV cm ¹, the MIM devices can sustain up to 6000 bending cycles with low leakage current (Fig. 2g). For comparison, the dielectric film can only sustain 500 bending cycles at a fixed tensile strain of 1.12% for iCVD- pC1D1 MIM devices.⁴⁴ The measured levels of insulating property and the mechanical flexibility are unprecedented for solution processed high-kdielectrics in recent years, and they are comparable to that of the high-k materials deposited by ALD.

Flexible organic transistors and complementary circuits based on the La2O3gate dielectric

Bottom-gate, top-contact OTFTs with the La2O3dielectric were fabricated on flexible PET substrates to explore applications in soft electronics. A schematic diagram of the OTFT used in this work is shown in Fig. S13 in the ESI.† A 10 nm thick poly- (a-methylstyrene) (PaMS) layer was spin-coated onto La2O3to modify the interface. Organic semiconductors of PTCDI-C8 and pentacene were deposited by vacuum evaporation to construct the n-type OTFT and p-type OTFT, respectively. As shown in Fig. 3a and b, well-behaved transfer (I_DS–V_GS) and output (I_DS–V_DS) characteristics were observed for both OTFTs. Small threshold voltages (V_ths) of 0.59 V (p-type) and 0.98 V (n-type) were obtained due to the high permittivity of the La2O3

Fig. 3 Flexible TFTs using 26 nm-thick La₂O₃as the dielectric with various semiconductors. (a) Transfer curves of PTCDI-C8 and pentacene OTFTs (the width and length of the channel are 750mm and 50mm, respectively). (b) Output curves of pentacene OTFTs. The organic semiconductors were deposited onto a modification layer (PaMS). (c) and (d) Positive bias stress (PBS) and negative bias stress (NBS) of PTCDI-C8 OTFTs. (e) Time dependent drain current and gate leakage current under a constant gate voltage of 6 V and a constant drain voltage of 6 V for the OTFT with semiconductor PTCDI-C8.

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dielectric layer. With only a smallVGSof 3 V, the ON/OFF ratio reaches 410⁴(n-type) and 310⁵(p-type), indicating its great potential for low-voltage soft electronics. Importantly, both OTFTs have very low gate leakage currents (maximumB10 ¹⁰A), which would guarantee the low-power and reliable operation. The V_th-shifts under positive bias stress (PBS) and negative bias stress (NBS) were measured for the n-type La₂O₃-OTFTs, as shown in Fig. 3c and d. After stressing at1 V for 13 200 s, theV_th-shifts under PBS and NBS are only 0.2 V and +0.1 V, respectively. The changes of IDSandIGSat constant bias are shown in Fig. 3e as a function of time for the retention of 46 000 s, which indicates that the high quality La2O3dielectric provides highly reliable operation of the OTFTs.

By connecting the p-channel pentacene and n-channel PTCDI-C8 OTFTs, we realized a low-voltage complementary inverter and a complementary two-input NAND gate on flexible PET substrates. The schematic diagram of the inverter layout and the photograph of the actual organic circuit are shown in Fig. 4a and b, respectively. The electrical characteristics of the inverter with 26 nm-La₂O₃gate dielectrics are shown in Fig. 4c

and d, indicating sharp switching behavior with an operation voltage down to 2 V and the signal gain as high as 90. Again, the electrical characteristics of the above inverters are comparable to that of the inverters made with ALD-grown Al₂O₃dielectric films (Fig. S14 in the ESI†). In addition, the static currents (whenV_in,B=VorV_dd=V_in,A) in our complementary gates are very small, usually below 200 pA. The static power dissipation is then less than 0.5 nW per logic gate.⁴⁵ Fig. 4e–g show the circuit, the schematic diagram, and truth table of a two-input NAND circuit. The transfer characteristics of the La2O3-NAND gate are shown in Fig. 4h, demonstrating successful operation of the Boole logic function with a small voltage down to 2 V.

Discussion

The factors and mechanisms to obtain excellent insulating properties in the above MIM, OTFTs, and inverters are discussed below. The first factor is that the oxygen infusion in the solution improves the wetting process during film deposition.

Fig. 4 Flexible complementary circuits based on La₂O₃-OTFT. (a) A schematic diagram of complementary CMOS inverter circuits. (b) Photograph of a fabricated flexible CMOS inverter by using the solution processed La₂O₃dielectric film (26 nm). (c) and (d) Output voltage and small-signal gain as a function of input voltage for supply voltages between 2 and 6 V. The inverter shows the rail-to-rail output switch and a small-signal gain as large as 90.

(e) and (f) Connected circuit and circuit schematic of 2-input NAND gate. (g) Truth table of the 2-input NAND gate. (h) Transfer characteristics with an operated voltage low to 2 V and device currentI_SS.

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The enriched OH groups in the precursor solution (Fig. 1c) effi- ciently improves the affinity between the liquid and the solid substrate, leading to reduced physical defects and improved film uniformity. This effect was quantitatively examined on wafer-scale films, as shown Fig. 5a and b, where the thickness of different spots was measured. The location of measurement spots on the 2 inch wafer-scale films are shown in Fig. S15 in the ESI.†Apparently, the 30 min-O₂solution leads to a much narrower distribution of film thickness,i.e., 1.65 to +1.35 nm in the 2 inch film.

The second factor is related to improved chemical reaction.

In experiments, the color of the solution with oxygen infusion

was gradually deepened when stirring and heating at 1201C, while the solution without oxygen infusion remained transparent even when heated to 1601C. This implies that the flowed oxygen may also serve as a ligand and facilitate the dissociation of lanthanum acetylacetone. The acetylacetone ions will probably combine with the protons of H₂O and produce OH ions. Then, the DMF molecule and OH ions may replace the original acetylacetonate ligand as the new ligands of lanthanum(III) ions.

Combining the above analysis, the deeper color may be related to the change of the ligand numbers, which show diﬀerent lumines- cence performance, as shown in Fig. S16 and S17 in the ESI.†^46,47

Fig. 5 Study of transport mechanism in La2O3films (30 min ODS-Fs, 1201C). (a) Statistic data of thickness measured in the wafer-scale La2O3films (2 layers, annealed at 1201C, deposited on 2 inch Si substrates) made from solution with no O2or 30 min O2infusion. (b) Contours of measured thickness for diﬀerent spots of the wafer-scale La₂O₃films. (c)J_i–E_icharacteristics of a MIM device with Au/La₂O₃/Si⁺(La₂O₃is 13 nm) measured at various device temperature (T_dev). The enlarged J_i–E_iplots showing that theJ_iplots for the temperature range from 78 to 273 K are almost the same. (d) The corresponding Arrhenius plots forJ_i. (e) Plots of ln|J_i/E_i2| as a function of 1/E_irevealing a F–N tunneling region. (f) Measurement results of XPS. Band-gap E_gwas fitted from the beginning of energy loss for 30 min-O₂film and non-O₂film. (g) The energy band diagram of La₂O₃obtained from valence electron spectroscopy and ideal band-alignment with organic semiconductors.

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As the reaction goes on, more and more La–OH bonds are formed under the O2 treatment, which easily become La2O3 under the calcination.^48,49Also, in post-treatment of the films, the adopted gradient heating process allows the organic components in the gel to volatilize gradually as compared with a direct and quick heating process and, also, the gradient cooling process releases the stress in the La₂O₃film and thus reduces the stress-induced physical defects.

Thirdly, a very low degree of defects is obtained in the film, as manifested in the electrical transport mechanisms discussed below. The Ji–Ei characteristics were measured at various temperatures. Over a wide temperature range from 78 K to 273 K, the measured Ji–Ei characteristics vary to a very small extent, indicating that charge transport through the La2O3layer is mainly governed by tunneling.^35,50Such temperature-independent carrier conduction indicates that thermally assisted conduction is significantly suppressed (Fig. 5c) and can be seen only with sufficiently low defect density.^51,52 Almost ideal tunneling-based conduction is confirmed by theJ_i–E_icurves at a relatively highE_i. A plot of ln|J_i/E_i²|versus1/E_ipresented in Fig. 5d, e clearly shows two distinct regions, indicating the transition from direct tunneling at a low field to Fowler–Nordheim (F–N) tunneling at a high field.

The transition occurs approximately at anE_iof 5 MV cm ¹. These observations strongly confirm that the solution-processed films, even though processed at a low temperature, could still reach the state with a very low density of defects and feature ultra-low leakage current.

Lastly, the prominent advantage of La2O3with a wide band-gap is further enhanced by oxygen-incorporated deposition. Using X-ray photoelectron spectroscopy (XPS) to test the dielectric layer,^53–55we extracted the band-gap of the La2O3films by fitting the O1s energy loss spectrum as 6.31 eV for the 30 min-O2film (Fig. 5f), which guarantees the ultralow leakage current described above. To further clarify the positions of the conductive band and the barrier height, valence band spectrum measurements were carried out.⁵⁶The 30 min-O₂films have a much larger band-gap than that of the non-O₂film. The energy band positions ofE_cand E_v and the corresponding ideal band alignments with organic semiconductors (e.g., pentacene and PTCDI-C8)^57,58are schematically shown in Fig. 5g. The large energy offset sufficiently suppresses the interfacial charge transfer in transistor operations.

Overall, the excellent insulating properties of La2O3 are attributed to the improved wetting and chemical reaction during deposition and densification as well as a very low degree of defects and a widened band-gap in solid films. The method has also been applied to improve HfO2and ZrO2films and may be applicable for other oxide dielectric films.

Conclusions

In conclusion, we demonstrate that high-k dielectric oxide films with excellent insulating properties could be obtained at a low temperature by an oxygen-incorporated solution process. The obtained La2O3 films exhibit a very low leakage current, high breakdown voltage, high permittivity, and large-area uniformity,

which is comparable with those of the ALD-deposited films. Also, the long-standing problem of the poor reliability of La2O3films under stress in air is also overcome as a result of the extremely low degree of defects. We also take advantage of the good flexibility of La₂O₃ films to the limit of the PET substrate and demonstrate flexible, low-power CMOS-like inverters with a static power consumption of 0.5 nW. In contrast with the existing eﬀorts to grow oxide dielectrics by solution, our results demonstrate a new concept in the solution system that could be generally applicable for other dielectrics. Further development of low-temperature, solution-processed La2O3 will aim at scalability and further inte- gration of common circuits towards their low-cost flexible wearable applications.

Experimental section

Preparation of precursor solution

The organometallic salt precursor solution was prepared inside a nitrogen glove box with 0.2181 g of La(C5H7O2)3xH₂O (Sigma- Aldrich, 98%) dissolved in 10 ml of DMF (Sigma-Aldrich, 99.8%) at a concentration of 0.05 mol L ¹. Oxygen flows into the solution at the rate of 60 ml min ¹for a controlled time.

After that, the sealed solution was stirred vigorously at 801C for 12 h and then placed in a drying cabinet for cooling to room temperature. Finally, the solution was centrifuged for 15 minutes and the supernatant was taken.

High-jlanthanum oxide thin films by solution processing The supernatant of the precursor solution was filtered through a 0.22mm PTFE filter membrane prior to spin coating. For the low temperature process, the La₂O₃ dielectric layer was spin- coated on a pre-cleaned substrate (PET or silicon) at 500 rpm for 5 s and 2000 rpm for 40 s, and then softly baked at 1201C for 10 min on a hotplate. Several spin-coating cycles were adopted to obtain the desired physical thickness, and finally a post-treatment process (120 1C for 1 h) was carried out to improve the film densification.

Characterization of precursor solution

All solutions were characterized using FT-IR spectroscopy (Nicolet 6700),¹H-NMR spectroscopy (3T MAGNETOM Trio Tim) and computer-controlled F-4600 fluorescence spectrophotometry (HITACHI, Japan). A contact-angle goniometer (DataPhysics OCA) developed by Stuttgart was used to examine the surface wettability with water as the sessile drop for the gold and annealed La₂O₃films.

Characterization of La2O3thin films

The surface roughness and morphology were observed using atomic force morphology (AFM, Asylum Research Ltd) in tapping mode. X-ray diﬀraction (XRD, Philips X’pert MRD meter) was used to measure the crystal form in each thin film. The electronic structure of the diﬀerent dielectric stack was investigated by using XPS (ULVAC-PHI, PHI 5000 Versa Probe) with an Al Ka X-ray source (1486.6 eV). The thickness and crystalline phase were

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characterized by obtaining high resolution transmission electron micrographs (HR-TEM, Tecnai G2 F20 S-Twin). The transparency of La₂O₃ onto PET was tested using ultra-violet and visible spectrophotometry (UV-Vis, UV-2600, SHIMADZU).

Metal–insulator–metal device fabrication

For the MIM structures, heavily-doped silicon or flexible PET served as the substrate. First, heavily-doped silicon with a photoresist as a covering layer was cleaned through ultrasonic cleaning with acetone, isopropanol (IPA) and DI water, respectively. Next, a mixed solution of hydrogen peroxide and sulfuric acid (1 : 4) was used to remove the residual organic contamina- tion and then hydrofluoric acid was used to remove the excrescent silicon oxide thin layer. Besides, flexible PET substrates were cleaned with acetone, IPA and DI water, respectively through ultrasonic cleaning. As an electrode, 20 nm-thick Au was fabricated as a bottom gate by thermal evaporation through shadow masks under a base pressure lower than 10 ⁴Pa at a deposition rate of about 0.2 Å s ¹. After UV–ozone treatment, all the substrates were on standby to follow the above treatments. Finally, a 40 nm-thick Au top electrode was evaporated under the same conditions to achieve a MIM device.

Electrical properties of devices

For all MIM and TFT devices, their electrical characteristics were tested in a vacuum environment using a semiconductor parameter analyzer (B1500A, Agilent). Capacitance versus voltage characteristics of MIM devices were measured using an Impedance Analyzer (E4990A, Keysight). TheJi–Ei characteristics of MIM devices were measured at various temperatures on the probe station connected with liquid N2and a thermal heater. To evaluate the electrical properties of La2O3 under bending, bending cycle tests under an ideal curvature radius andin situmeasurements were carried out.

TFT device fabrication

Bottom gate top contact (BGTC) TFT devices were fabricated on PET substrates, with three active layers including pentacene (Sigma, 98%, thermal evaporation) and PTCDI-C8 (Sigma, 98%, thermal evaporation). First, 16 nm-thick Au was thermally evaporated through a shadow mask on PET substrates. Then, a 26 nm-thick La2O3(two cycles of coating) insulating layer was prepared by a spin-coater. For the TFT devices of pentacene and PTCDI-C8, the deposition of the PaMS (B10 nm) layer as the modified layer was done by spin-coating and annealing at 1201C for 5 min in air, then the devices were completed by sequential thermal evaporation of 40 nm-thick active channel layers (B0.22 Å s ¹). Finally, source and drain electrodes (50 nm-thick Au) were thermally evaporated through a shadow mask. The channel length and width were 50mm and 750mm, respectively.

Flexible circuit fabrication

Based on PTCDI-C8 and pentacene-OTFTs, flexible PPMOS, CMOS and NAND circuits were successfully fabricated using 26 nm-thick La2O3as the insulating layers. Adhesive tape was

used to block the other half area when evaporating PTCDI-C8 or pentacene on PaMS layers. Finally, source/drain electrodes and interconnection electrodes were evaporated through a shadow mask. The channel length and width were 50mm and 750mm, respectively.

Author contributions

X. B. L. and C. L. suggested the designs, planned and super- vised the work. L. S. Y. came up with the experiments on oxygen- incorporated solution preparation. L. S. Y. and W. E. H. carried out the experiments on the use of oxygen-incorporated-solution processed high-k La₂O₃ dielectrics for insulators of related electronic devices and analysed the associated experimental results. X. C. L. also carried out some related experiments and characterization. X. H. L., L. S. Y. and W. E. H. analysed the factors of low defect density. L. S. Y., W. E. H., X. B. L. and C. L.

cowrote the manuscript. All authors read and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foun- dation of China (Contract No. 51872099, 61774174). X. B. L.

acknowledges the support of the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016). C. L. acknowledges the support of the Guangdong Natural Science Funds for Distinguished Young Scholars under Grant 2016A030306046. This work was also supported by Science and Technology Planning Project of Guangdong Province (No.

2016B090907001), the Guangdong Innovative Research Team Program (No. 2013C102), the Guangdong Provincial Key Labora- tory of Optical Information Materials and Technology (Grant No.

2017B030301007) and the 111 Project.

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