Solution-Processed Rare-Earth Oxide Thin Films for Alternative Gate

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Solution-Processed Rare-Earth Oxide Thin Films for Alternative Gate Dielectric Application Jiaqing Zhuang,† Qi-Jun Sun,† Ye Zhou,‡ Su-Ting Han,† Li Zhou,† Yan Yan,† Haiyan Peng,†,§ Shishir Venkatesh,† Wei Wu,† Robert K. Y. Li,† and V. A. L. Roy*,† †

State Key Laboratory of Millimeter Waves and Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China ‡ Institute for Advanced Study, Shenzhen University, Shenzhen, Guangdong 508060, PR China § Key Laboratory for Material Chemistry of Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China S Supporting Information *

ABSTRACT: Previous investigations on rare-earth oxides (REOs) reveal their high possibility as dielectric films in electronic devices, while complicated physical methods impede their developments and applications. Herein, we report a facile route to fabricate 16 REOs thin insulating films through a general solution process and their applications in low-voltage thin-film transistors as dielectrics. The formation and properties of REOs thin films are analyzed by atomic force microscopy (AFM), X-ray diffraction (XRD), spectroscopic ellipsometry, water contact angle measurement, X-ray photoemission spectroscopy (XPS), and electrical characterizations, respectively. Ultrasmooth, amorphous, and hydrophilic REO films with thickness around 10 nm have been obtained through a combined spin-coating and postannealing method. The compositional analysis results reveal the formation of RE hydrocarbonates on the surface and silicates at the interface of REOs films annealed on Si substrate. The dielectric properties of REO films are investigated by characterizing capacitors with a Si/Ln2O3/Au (Ln = La, Gd, and Er) structure. The observed low leakage current densities and large areal capacitances indicate these REO films can be employed as alternative gate dielectrics in transistors. Thus, we have successfully fabricated a series of low-voltage organic thin-film transistors based on such sol−gel derived REO films to demonstrate their application in electronics. The optimization of REOs dielectrics in transistors through further surface modification has also been studied. The current study provides a simple solution process approach to fabricate varieties of REOs insulating films, and the results reveal their promising applications as alternative gate dielectrics in thin-film transistors. KEYWORDS: sol−gel, rare-earth oxide, dielectric, low voltage, thin-film transistor

1. INTRODUCTION Rare-earth (RE) elements, regarded as jewels for functional materials, have attracted tremendous interest in past decades due to their numerous applications in luminescent materials, catalysts, magnets, batteries, and so on.1−6 They are a family of elements including all lanthanides from lanthanum (La) to lutetium (Lu) plus scandium (Sc) and yttrium (Y) with unique energy level configurations.1 Among diverse RE materials, the most stable compounds are RE oxides (REOs), in which most of the RE elements hold typically a trivalent state with the general formula of sesquioxide, Ln2O3 (e.g., Sc2O3, Y2O3, La2O3, Sm2O3, and Er2O3), while for Ce, Pr, and Tb, more stable oxides exist as CeO2, Pr6O11, and Tb4O7, respectively.7 The applications of REOs have now been broadened to luminescent, optical, and dielectric materials.7−14 Previous studies on REOs revealed that they possess remarkable electrical properties, such as high relative dielectric constant, © 2016 American Chemical Society

large areal capacitance, superior electrical breakdown strength, high transparency, and superior thermal stability which fulfill the requirements of dielectrics in electronics, especially thinfilm transistors (TFTs).8,15−18 Except for Pm2O3 due to its radioactive property, all the other 16 REOs films have been studied and employed as gate dielectrics in recent years, and the results revealed that the REOs are promising candidates as alternative gate dielectrics to traditional SiO2.18−31 Therefore, REOs, together with other potential oxides (e.g., Al2O3, ZrO2, and HfO2), have also been considered as solutions to the problems of large leakage current, high standby power consumption, and inferior gate dielectric reliability for continued downscaling of electronics.32−35 Usually, these Received: August 3, 2016 Accepted: October 20, 2016 Published: October 20, 2016 31128

DOI: 10.1021/acsami.6b09670 ACS Appl. Mater. Interfaces 2016, 8, 31128−31135

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ACS Applied Materials & Interfaces

Figure 1. Preparation procedures of solution-processed REO thin-film and device fabrication process: (a) spin-coating of REO precursor solutions, (b) postannealing of REO dielectric films, (c) schematic of compositions in REO films, (d) surface modification of REO film, (e) deposition of pentacene semiconductor film, and (f) Au source and drain electrodes. purchased from Tractus. All chemicals and solvents were used as received without purification. 2.2. Formation of REOs Thin Films on Si. RE acetates are chosen in this study as ultralow impurities would exist after a postannealing procedure compared with the RE nitrate, sulfate, or chloride sources. A quantity of dried Ln(CHCOO)3 powders (0.1 mol/L for Ln = Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu and 0.2 mol/L for Ln = Ce, Pr, and Tb; experiment on precursor preparation for Ln = Pm is unable to be conducted due to its radioactive property) was first added into 2-methoxyethanol, and all the acetates were insoluble in the solvent until DETA was added dropwise into the mixture under vigorous stirring.41,42 The molar ratio of Ln(CHCOO)3 to DETA was fixed at 1:3 for Ln = Y, Nd, Sm, Eu, Dy, Ho, Er, Tm, Yb, and Lu and 1:6 for Ln = Sc, La, Ce, Pr, and Gd. After stirring for 2 h, all the acetates were well-dissolved. Basically, all RE acetates have limited solubility in alcohols as the hydroxyl group of alcohols cannot provide coordination with Ln3+ to break down their monodentate, bidentate, and polymeric structures.43 Based on the Lewis acid−base theory, the acetates can be homogeneously dissolved into 2-methyoxyethanol after adding a small amount of DETA.42 All the precursor solutions are found to be quite stable even after 2 months, as shown in Figure S1. Then, the solutions were filtered through polytetrafluoroethylene (PTFE) filter with a pore size of 0.2 μm. Finally, 0.1 mL of deionized water was added into 1 mL of precursor and kept in an ultrasonic bath for 10 min to promote hydrolysis before use. For the preparation of REO thin films, the hydrolyzed solution was deposited on Si wafer by spin-coating at 5000 rpm for 60 s and heated at 120 °C for 10 min to remove residual solvents, followed by a postannealing process at 500 °C for 0.5 h in air, as shown in Figure 1. 2.3. Fabrication of Capacitors and Pentacene Transistors. Capacitors and pentacene-based TFTs are constructed based on Si− Ln2O3−Au and bottom-gate top-contact structures, respectively, on heavily doped Si wafers, as can be seen in Figure 1. The 16 capacitors were fabricated by depositing top Au electrodes at a rate of 0.2 Å/s in a vacuum evaporator at a base pressure of 1 × 106 Torr on the REOs films through a shadow mask with a dimension of 1 × 1 mm2. The fabrication of transistors was realized by depositing a 40 nm pentacene layer at a rate of 0.1 Å/s in the same evaporator under a base pressure of 1 × 106 Torr; subsequently, Au source and drain electrodes were deposited with channel length of 50 μm and width of 1000 μm. To improve the device performance, surface modification methods have been attempted through a thin polymer layer or two different SAMs on Er2O3 film, respectively. In detail, PS- and HMDS-modified Er2O3 films were prepared by spin-coating PS solution (2 mg/mL in toluene) and HMDS at 5000 rpm for 60 s and baking at 130 and 150 °C for 0.5

oxides were prepared by some traditional costly vacuum-based techniques, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and atomic layer deposition (ALD), which are expensive and time-consuming.19,20,36−40 In this regard, solution-processable dielectrics are more favorable in electronics as they are cost-effective and can be manufactured in large amounts.32−34 Sol−gel method is a well-known and popular technique as it offers the possibility of tuning properties of resulting products by adjusting the precursor solutions easily. It has been empolyed in numerous fields including the preparation of high performance dielectrics. However, the preparation and application of solution-processed REOs, especially for the formation of REOs on silicon, and the electrical properties of entire series of sol−gel derived REOs have not been studied yet. In this study, we report the formation and properties of 16 REO dielectric thin films with thicknesses around 10 nm through a facile solution process. The as-prepared thin films are amorphous with extremely low roughness (less than 0.1 nm) and exhibit hydrophilic surfaces. The formation of RE hydrocarbonates and silicates during postannealing process is verified through XPS analyses on the chemical states of RE and oxygen. Low leakage current densities are recorded for these REOs films (e.g., Ln2O3, Ln = La, Gd, and Er), and they can be further reduced through a simple surface modification. A large areal capacitance of 522−585 nF/cm2 at 1 kHz is obtained for Ln2O3 (Ln = La, Gd, and Er) films. The utilization of entire series of REOs films as alternative gate dielectrics for lowvoltage pentacene transistors is also demonstrated. A highest saturated mobility (μ) of 0.88 cm2 V−1 s−1, on−off current ratio (Ion/Ioff) of 1.61 × 106, and low subthreshold swing (SS) of 94 mV/dec have been obtained for transistors with ODPAmodified Er2O3 dielectric layer.

2. EXPERIMENTAL SECTION 2.1. Materials. All rare-earth acetates Ln(CHCOO)3 (Ln = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), diethylenetriamine (DETA), polystyrene (PS) (average molecular weight M = 280 000), hexamethyldisilazane (HMDS), and 2-propanol were purchased from Sigma-Aldrich. 2-Methoxyethanol was purchased from Acros Organics, and n-octadecylphosphonic acid (ODPA) was 31129


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Figure 2. Ultrasmooth REOs films: AFM images of solution-processed REOs (Ln2O3, (a) Ln = La, (b) Gd, and (c) Er) thin films; inset images present the corresponding water contact angle measurement results. h, respectively. ODPA-modified Er2O3 film was prepared by immersing the substrates with postannealed Er2O3 into 0.5 mmol/L ODPA/2-propanol solution for 12 h, followed by rinsing in 2-propanol 3 times and drying by nitrogen gas. 2.4. Characterizations. The crystallinity of thermal annealed REOs (Ln2O3, Ln = Y, La, Ce, Pr, Nd, Sm, Yb, and Er) films were verified by X-ray diffractometer (XRD, Bruker, D8 Phaser) using Cu Kα radiation (λ = 1.5418 Å). The surface morphologies of bare REO and pentacene films were observed by an atomic force microscopy (AFM, VEECO, Multimode V) through tapping mode. The surface hydrophilic−lipophilic properties of all REO films and surfacemodified Er2O3 films were obtained using a contact angle goniometer (ramé-hart, Model 200), by dropping 2 μL of deionized water or diiodomethane onto the REOs surfaces at a contact time of 5 s. The surface energy of interface is calculated based on the following equations: γS = γSd + γSp

(1)

γL(1 + cos θ) = 2 γSdγLd + 2 γSpγLp

(2)

⎡ d(lg IDS) ⎤−1 SS = ⎢ ⎥ ⎣ dVGS ⎦

where Ci is the capacitance of the gate dielectric per unit area, A is the contact area of Si/Ln2O3/Au capacitor, κ is the dielectric constant, d is the thickness of Ln2O3 films, W is the channel width, L is the channel length, IDS is the drain current, VGS is the gate voltage, and Vth is the threshold voltage.32,45

3. RESULTS AND DISCUSSION 3.1. Characterization of REO Films. Ellipsometry technique was used to characterize the thicknesses for all REO films, and the results are summarized in Table S1. The thicknesses of REO films are in a range of 8.4−10.6 nm for Ln2O3 (0.1 M, Ln = Sc, Y, La, Ce, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu) and 15.8 and 14.7 nm for Pr2O3 (0.2 M) and Tb2O3 (0.2 M), respectively. The variation of thickness is proposed as a result of different hydrolysis rates of the precursors as well as diverse forms of film formations at a same annealing temperature. Figure S2 presents the XRD patterns of REOs (Ln2O3, Ln = Y, La, Ce, Pr, Sm, Er, and Yb) films with single coating layer and (Ln2O3, Ln = La, Gd, and Er) films with five repeated coatings. The REOs exhibit amorphous behavior as poor crystallinity is observed for the selected REOs thin films with single layer (thickness around 10 nm) and even for thicker films (thickness around 40 nm). In addition, Figure S3 depicts the result of TGA-DSC analysis, and a very small weight loss (less than 5%) was recorded for all three (Ln2O3, Ln = La, Gd, and Er) powder samples, revealing their good thermal stabilities below 600 °C. The surface roughness of insulating films is critical to their potential application in corresponding capacitors and transistors. To characterize the morphologies of the sol−gel derived REO films, tapping-mode AFM is employed, and the results of three REO (Ln2O3, Ln = La, Gd, and Er) films are illustrated in Figure 2 as examples. As depicted in Figure 2a−c, ultrasmooth surfaces can be observed for Ln2O3 (Ln = La, Gd, and Er) films after postannealing treatment. Specifically, the root-mean-square (rms) roughnesses are 0.09, 0.10, and 0.08 nm for Ln = La, Gd, and Er, respectively, which are comparable to those of commercially available thermally grown SiO2 dielectric layer.45 In addition, the pristine ultrasmooth surface provides a good contact with top electrodes in capacitors or the semiconductor layers in transistors. To investigate the hydrophilic−lipophilic properties of REO films, we used water contact angle measurement, and the contact angles are in a range of 41.6−71.6°, indicating hydrophilic surfaces exist on the REO films.8,11,22,46 In general, a hydrophobic surface would exist on the REO films due to their unique electronic structures, which have also been found

where γS, γSd, and γSp are the total surface energy, dispersion component, and polar component of surfaces, θ is the measured contact angle, and γL, γLd, and γLp are the counterparts of the wetting liquids, respectively.44 The thicknesses of bare REO films were recorded by an ellipsometer (Rudolph Research, Auto/EL). Thermogravimetric analysis−differential scanning calorimetry (TGADSC) was employed to characterize the thermal stability of the solution process REOs. Surface compositional analysis of REO films and chemicals states of RE element (Ln = La, Gd, and Er) and oxygen were investigated using an X-ray photoelectron spectrometer (XPS, Physical Electronics, PHI-5802) with a monochromatic Al Kα X-ray source (1486.6 eV) under ultrahigh vacuum with a base pressure of 10−10 Torr, and the reference C 1s peak at 284.8 eV was used for calibration. The areal capacitance of all REO films and surfacemodified Er2O3 films were obtained by measuring Si/Ln2O3/Au capacitors (1 mm2) on an Agilent 4284A Precision LCR Meter in air. The leakage current of bare Ln2O3 (Ln = La, Ce, Gd, and Er) and surface-modified Er2O3 films, transfer, and output characteristics of all transistors based on bare or surface-modified REO dielectrics were measured using a Keithley 2612 source meter in an Mbraun glovebox filled with nitrogen. The threshold voltage (Vth) was extracted from the plot of square root of drain voltage (|IDS|1/2) versus gate voltage (VGS) and determined as the intercept at IDS = 0. The dielectric constant (κ) of REO films, estimated thicknesses of PS, HMDS, and ODPA layers on Si, saturated mobility (μ), and subthreshold swing (SS) of OTFTs are calculated using the following equations:

⎛ε × κ ⎞ ⎟ Ci = A⎜ 0 ⎝ d ⎠ IDS =

W μ Ci(VGS − Vth)2 2L sat

(5)

(3)

(4) 31130


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Figure 3. XPS spectra and peak deconvolution of C 1s for initial surfaces of solution-processed REOs (Ln2O3, Ln = (a) La, (b) Gd, and (c) Er) films.

Figure 4. Compositional analyses of REO films: XPS spectra and peak deconvolution of O 1s of initial surfaces and interiors after 5 nm sputtering of solution processed REOs (Ln2O3, Ln = (a) La, (b) Gd, and (c) Er) films.

in bulk RE ceramics.47 However, hydrophilic surfaces are expected considering the large ionic radius and low electronegativity of RE atoms and adhesions of hydroxyl groups. Additionally, water contact angles images were recorded, and their corresponding contact angles (55.6, 56.4, and 55.2° for La2O3, Gd2O3, and Er2O3 films) are given in the insets of Figure 2. As discussed above, surface modification is an effective way to engineer the surfaces and then obtain better device performance. Therefore, a polymer layer and two SAMs have been tried to modify the surfaces of REOs films and the contact angles increase to 75.6, 88.6, and 105.3° for hydrophobic HMDS-, PS-, and ODPA-modified Er2O3 films, respectively, as shown in Figure S4. 3.2. Compositional Analyses of REOs Films. To investigate the chemical composition of solution-processed REOs films, XPS technique was employed to analyze the formation of pristine REOs films. Except Si from the substrates and carbon contamination, dominant signals from Ln and O are observed in the spectra for Ln2O3 (Ln = La, Gd, and Er) films surface and interiors, as shown in Figure S5, indicating that no impurities or contaminations exist in these solution-processed REOs films. The accurate surface composition is analyzed by investigating the chemical states of relevant elements (i.e., C, O and Si) on the top surface and interiors at the depth of 5 nm.

The C 1s, O 1s, and Si 2s/Si 2p spectra are recorded for initial surfaces of three REOs (Ln2O3, Ln = La, Gd, and Er) films, and their fitting results are shown in Figures 3, 4 and S6 and summarized in Tables S2−S4. As shown in Figure 3, the C 1s spectra can be deconvoluted into four peaks, among them, three peaks with lower binding energies indicate the existence of adsorbed carbon contaminants.48 In addition, it is apparent that the intensity and area ratio of the deconvoluted peak with the highest binding energy, corresponding to the C from carbonates groups in RE hydrocarbonates, decrease for REOs (Ln2O3, Ln = La, Gd, and Er) films with increasing atomic number of lanthanide element, as can be seen in Figure 3 and Table S2. The formation of hydrocarbonates is proposed as a result of reaction between REOs with atmospheric carbon dioxide and water during the air-cooling process. Furthermore, the carbonation process is limited to the outer layers of the REOs films as no carbon signals can be observed after sputtering the films to a depth of 5 nm by argon ions. The observed phenomenon of reduction in RE hydrocarbonates is regarded as a result of lower reaction rate during the annealing of films due to the lanthanide contraction effect.7,49 In general, these inevitable surface contaminants and the formation of reacted carbonates during the annealing and cooling processes would deteriorate the performance of capacitors and transistors. 31131


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Figure 5. Electrical performance of REOs films. (a) Leakage current density versus gate bias voltage and (b) areal capacitance frequency versus of solution-processed REOs (Ln2O3, Ln = La, Gd, and Er) films as well as HMDS-, PS-, and ODPA-modified Er2O3 films.

5. As a dielectric material, the leakage current of a film under bias should be minimized to achieve efficient switching property and low power consumption during device operation. The existence of RE hydrocarbonates, observed by XPS analysis, will deteriorate the insulating performance. Therefore, we employed interface engineering through a facile modification by SAMs or polymer. The Er2O3 demonstrates an enhanced electrical performance as capacitor and TFT because Er2O3 shows least carbonates formation; consequently, the influences from pristine surface and subsequent modification processes can be minimized. Figure 5a shows the leakage current densities as a function of bias gate voltage for bare REOs (Ln2O3, Ln = La, Gd, and Er) films and Er2O3 films modified by HMDS, PS, and ODPA, respectively. A low leakage current density of 10−10−10−9 A/cm2 at zero bias is recorded for all three REOs (Ln2O3, Ln = La, Gd, and Er) films. At a gate bias of −3 V (electrical breakdown strength of ∼3 MV/cm), the leakage current density increases to 5.6 × 10−6, 2.0 × 10−7, and 8.5 × 10−7 A/cm2 for bare REOs (Ln2O3, Ln = La, Gd, and Er) films, respectively. The increase in leakage current density is due to the inferior crystallinity and the existence of adsorbed hydroxyl groups and RE hydrocarbonates as predicted, which are confirmed by XRD and XPS measurements. The difference among REOs (Ln2O3, Ln = La, Gd, and Er) films is ascribed to their variation of film densities due to different film formation. As mentioned above, the method of passivation on pristine films would improve their insulating performance. Therefore, the leakage current density of Er2O3 film, as expected, is reduced through surface modification either by HMDS, PS, or ODPA. The ODPAmodified Er2O3 film exhibits the best insulating behavior, as shown in Figure 5a. To further verify the function of surface modification, La2O3 film (with highest leakage current) was modified using the same methods. Similar results were obtained and the leakage current densities were reduced significantly, as shown in Figure S7. Furthermore, high areal capacitances are recorded for all REO films, as given in Figures 5b and S8. At a frequency of 1 kHz, 585, 534, and 522 nF/cm2 are found for Ln2O3 (Ln = La, Gd, and Er) films annealed at 500 °C, and the rest exhibits capacitances from 311 to 572 nF/cm2, respectively. It is worth mentioning that the concentrations for Pr2O3 and Tb2O3 are doubled to form thicker dielectric films as large leakage current is found for the films prepared using the

Therefore, a passivation method via surface modification is proposed to eliminate such adverse effects for practical applications. Figure 4 shows the O 1s spectra and deconvoluted results of initial surface and interiors after 5 nm of argon ions sputtering of REOs (Ln2O3, Ln = La, Gd, and Er) films. The spectra of O 1s monitored on the surfaces of REOs (Ln2O3, Ln = La, Gd, and Er) films can be deconvoluted into six peaks, corresponding to the oxygen in lattice Ln2O3 (Ln−OI) and nonlattice Ln 2 O 3 (Ln−O II ), carbon contaminants and lanthanide carbonates (C−O, CO, and O−CO), lanthanide silicates (Ln−O−Si), adsorbed hydroxyl groups (−OH), and silicon suboxide from the substrates (SiOx), respectively. The existence of O in lattice and nonlattice Ln2O3 films results from poor crystallinities due to relatively low annealing temperature and is further verified by analyzing the interface at the depth of 5 nm, as given in Figure 4 and Table S3.50 Furthermore, the ratios of Ln−OII/Ln−OI at the depth of 5 nm are larger than those on top surfaces for three Ln2O3 (Ln = La, Gd, and Er) films, indicating the solid evidence of poor crystallinities. The formation of RE silicates, which has been observed in previous reports, is also confirmed in our study by characterizing the binding energies of Si 2s or Si 2p for top surfaces and interiors of Ln2O3 films, as shown in Figure S6 and Table S4.51−53 The photoemission electrons of Si can be observed in the spectra monitored both on the top surfaces and interiors after 5 nm of argon ions sputtering, indicating that the migration of Si atom from substrate into the films occurs while annealing. On one hand, the formation of RE silicates is an advantage for the application of REOs films as dielectrics due to their low leakage current density, while on the other hand, the relative low dielectric constant deteriorates their potential application as capacitor. Moreover, we propose a schematic diagram (Figure 1c) of these solution-processed REOs films according to the results of compositional analyses of REOs films. In these films, surface carbon contaminants, hydrocarbonates, REOs, RE silicates, and native silicon oxide exist continuously from top surfaces to the interfaces on Si substrates, as shown in Figure 1c. 3.3. Electrical Properties of REOs Films. The insulating properties of solution-processed REOs films are characterized by measuring their leakage current densities versus bias voltage and areal capacitance versus frequency, as presented in Figure 31132


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REOs (Ln2O3, Ln = La, Gd, and Er) dielectric films. The transfer curves of remaining transistors with REOs other than La, Gd, and Er are also provided in Figure S11, and a typical ptype behavior can be found for all the transistors as expected. Table S5 presents the detailed results of Vth, SS, and Ion/Ioff as well as μ for all the REOs dielectrics based TFTs. As illustrated in Table S5, the Vth varies from −0.73 to − 0.61 and −0.65 V for transistors with bare Ln2O3 dielectrics for Ln = La, Gd, and Er, respectively. The SS, Ion/Ioff, and μ of all pentacene TFTs are 105−199 mV/dec, 1.0−9.1 × 104, and 0.09−0.25 cm2 V−1 s−1, respectively. To further enhance the performance of REOs as gate dielectrics in transistors, modification on REOs surfaces has been carried out. Figures 6b and S10 show the transfer and output characteristics of transistors based on Er2O3 films modified by HMDS, PS, and ODPA and typical p-type behavior is observed. In addition, the SS (94 mV/dec), Ion/Ioff (1.6 × 106), and μ (0.88 cm2 V−1 s−1) are recorded for ODPAmodified Er2O3 based transistor. As mentioned above, the interface between the dielectric and semiconductor affects the device performance significantly.54−57 Therefore, the analysis of surface energy of Er2O3 dielectric with the modification layer was conducted to investigate the performance enhancement. As shown in Figure S4, images of contact angle using water and diiodomethane were recorded, and the values of surface energy of pristine Er2O3 film and Er2O3 films modified by HMDS, PS, and ODPA were calculated according to eqs 1 and 2. All the water contact angles of pristine Er2O3 films increase after the introduction of PS or SAMs, indicating the formation of hydrophobic surface on Er2O3 films. The values of surface energy are 24.07, 40.77, and 46.70 mJ/m2 for Er2O3 modified by ODPA, HMDS, and PS, which agrees well with the trend of increasing mobility of corresponding transistors, indicating lower surface energy yields larger semiconductor mobility, consistent with reported results both in pentacene and polymer semiconductor based transistors.44,54,58,59 Furthermore, bias stress effects have been investigated to characterize the influence of surface modification on device stability. As shown in Figure S12, transistors based on ODPA-, HMDS-, and PS-modified Er2O3 dielectrics maintain 58.6, 52.5, and 90.4% of their initial drain currents after a bias of VGS = VDS = −3 V for 104 s. Superior stability is found in the transistor with PS-modified Er2O3 dielectric compared with that of the other two, suggesting that best passivation on the defects of pristine Er2O3 is formed owing to its densified polymeric structure.

precursor with a concentration of 0.1 M, resulting from the insufficient oxide formation as their oxidation states are usually higher than three. For CeO2 film preparation, a high leakage current is recorded for a thick film using 0.2 M precursor (see Figure S9), which may result from insufficient oxygen supply while annealing. In general, REO films have high dielectric constants. However, in our study, due to the existence of RE hydrocarbonates and silicates, the nominal dielectric constant is calculated and ranges from 5.3 to 6.1 for all REOs films, as summarized in Table S1. Moreover, as shown in Figure 5b, the areal capacitance of Er2O3 films reduces significantly to 251, 147, and 121 nF/cm2 after the introduction of low-permittivity HMDS, PS, or ODPA layer, respectively. 3.4. REOs Films as Gate Dielectrics. To demonstrate the function of the present solution-processed REOs films as alternative gate dielectrics in TFTs, we fabricated a series of pentacene transistors based on 15 kinds of REOs films. A systematic study on electrical performance such as threshold voltage (Vth), subthreshold swing (SS), on−off current ratio (Ion/Ioff), and saturated mobility (μ) of transistors has been conducted. To enhance the performance of REOs-based TFTs, we employed the method of surface modification on Er2O3 film as a demonstration. Figures 6a and S10 show the transfer and output characteristics of pentacene transistors based on bare

4. CONCLUSIONS A series of REO dielectric films were prepared through a facile sol−gel method. The composition, surface morphology, and electrical properties of these REO films have been intensively studied using various techniques. These pristine amorphous REOs films with a thickness around 10 nm exhibit ultrasmooth and hydrophilic surfaces. RE hydrocarbonates and silicates formed during the postannealing process are observed and verified through XPS analysis results. High areal capacitances are recorded in all the REO films, and low leakage current densities are found in REOs (Ln2O3, Ln = La, Gd, and Er) films. They can be further reduced through surface modification. Finally, 15 pentacene TFTs have been fabricated based on the REOs thin films to demonstrate their application as alternative gate dielectrics. A carrier mobility of 0.88 cm2 V−1 s−1, on−off current ratio of 1.61 × 106, and lowest subthreshold swing of 94 mV/dec have been achieved in a transistor based

Figure 6. Transfer characteristics of pentacene TFTs based on (a) Ln2O3 (Ln = La, Gd, and Er) dielectrics and (b) HMDS-, PS-, and ODPA-modified Er2O3 dielectric films; inset images present the AFM image of pentacene grown on corresponding dielectrics. 31133


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on ODPA-modified Er2O3 dielectrics. Our study proves that the present solution-processed REOs films are promising candidates as dielectrics in electronics.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09670. Photographs of precursor solutions, TGA-DSC curves of REO powders prepared at 500 °C, XRD patterns, summary of water contact angles, thicknesses, and calculated dielectric constants of sol−gel derived REO films, leakage current density versus bias voltage of CeO2 film, leakage current density versus bias voltage of bare and surface-modified La2O3 films, contact angle images (water and di-iodomethane) of Er2O3 films with modification, summary of threshold voltages, carrier mobilities, subthreshold swings, on−off current ratios of pentacene transistors based on sol−gel-derived REO dielectrics, summary of C 1s, O 1s, and Si 2s/Si 2p fitting results, XPS survey scan spectra of Ln2O3 (Ln = La, Gd, and Er) and fine scan spectra of Si 2s/Si 2p on initial surfaces and interiors after 5 nm sputtering, transfer and output characteristics of pentacene transistors based on bare surface-modified REO dielectrics, capacitance of REO dielectrics other than Ln2O3 (Ln = La, Gd, and Er) and CeO2 films, bias stress test results of transistors with bare Er2O3 and surface-modified Er2O3 dielectrics (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the City University of Hong Kong’s Applied Research Grant Project No. 7004378.

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REFERENCES

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