Growth mechanism, ambient stability and charge trapping ability of Ti

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Growth mechanism, ambient stability and charge trapping ability of Ti-based maleic acid hybrid films by molecular layer deposition Yan-Qiang Cao, Wei Zhang, Lina Xu, Chang Liu, Lin Zhu, Lai-Guo Wang, Di Wu, Ai-Dong Li, and Guoyong Fang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04137 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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Growth mechanism, ambient stability and charge trapping ability of Tibased maleic acid hybrid films by molecular layer deposition Yan-Qiang Cao#,†, Wei Zhang#,†, Lina Xu#,‡, Chang Liu†, Lin Zhu†, Lai-Guo Wang†, Di Wu†, Ai-Dong Li*, †, Guoyong Fang*, ‡ †National

Laboratory of Solid State Microstructures, Jiangsu Key Laboratory of Artificial

Functional Materials, Materials Science and Engineering Department, College of Engineering and Applied Sciences, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, No.22 Hankou Road, Gulou District, Nanjing, Jiangsu 210093, P. R. China. ‡Key

Laboratory of Carbon Materials of Zhejiang Province, College of Chemistry and Materials

Engineering, Wenzhou University, No. 276 Xueyuanzhong Road, Wenzhou, Zhejiang 325035, P. R. China.

ABSTRACT Ti-based maleic acid (MA) hybrid films were successfully fabricated by molecular layer deposition (MLD) using organic precursor MA and inorganic precursor TiCl4. The effect of deposition temperature on the growth rate, composition and bonding mode of hybrid thin films has been investigated systematically. With increasing temperature from 140 oC to 280 oC, the growth rate decreases from 1.42 Å to 0.16 Å per MLD cycle with basically unchanged composition ratio of C : O : Ti in the films. Fourier transform infrared spectra indicate that all hybrid films show preference for bidentate bonding mode. Further analyses of X-ray photoelectron spectroscopy and in situ quartz crystal microbalance elucidate that 1

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as-deposited MLD Ti-MA hybrid films are consisted of inorganic Ti-O-Ti units and organic-inorganic Ti-MA units. In addition, density functional theory calculation was performed to investigate the possible reaction mechanism of TiCl4-MA MLD process, which is consistent well with experimental results. More importantly, by comparing with TiCl4-fumaric acid MLD system, it is demonstrated that the cis- and trans configurations of butenedioic acid influence the MLD growth, bonding mode, stability and charging ability of MLD hybrid films. Ti-MA hybrid films exhibit better stability and charging ability than Ti-FA hybrid films, benefiting from the inorganic Ti-O-Ti units in the hybrid films.

INTRODUCTION Atomic layer deposition (ALD) is a special chemical vapor deposition (CVD) technique based on alternating self-limiting gas-to-surface reactions, which enables material growth at the atomic level.1-4 With this nature, ALD has been widely used for synthesizing a myriad of inorganic materials,5 covering single elements,6 binary compounds (e.g., oxides,7 sulfides,8-9 nitrides,10 and fluorides11), ternary and even more complex compounds.12-13 With the development of ALD, this technique was firstly used for depositing a relatively new class of organic-inorganic hybrid films in 2006 by Nilsen and Fjellvag.14 Thereafter, various organic-inorganic hybrid films with many interesting properties have been developed by ALD.15-31 In addition, this technique is frequently called as molecular layer 2


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deposition (MLD) rather than ALD due to the molecular nature of the deposition process.32 Due to the differences in their bonding and elemental composition, inorganic and organic materials exhibit very distinct properties. In many cases, inorganic and organic matters are often complementary in properties. The organic constituents can provide structural flexibility, reduced density, photoconductivity, lower cost, etc., whereas the inorganic parts provide high carrier mobility, thermal and mechanical stability, etc. In addition, the organic-inorganic hybrid films usually exhibit properties that are intermediate between organic and inorganic materials. For example, the elastic modulus of inorganic ALD Al2O3 films is 198 GPa33 and organic polymers generally have elastic modulus in the range of 1-4 GPa. Lee et al. reported that the elastic modulus of MLD trimethyl aluminum(TMA)ethylene glycol(EG) organic-inorganic hybrid films is around 21 GPa.33 Hence organicinorganic hybrid films can offer the opportunity to develop new materials with tunable properties for applications in a variety of fields, such as micro-electronics,26, 34 energy,15-17, 35-36

catalysis,37-38 photoluminescence,39 etc.

To date, MLD processes have developed many types of organic-inorganic hybrid films. These hybrid films are usually called as “metalcones”, such as alucones,31 titanicones,29 zircones,40 zincones,34, 41-42 hafnicones,43 mangancones,44 and vanadicones,16 etc. However, MLD is still in its infancy to date compared to ALD.45 Compared to inorganic precursors, there are much more selections in the functional groups, chain backbone, chain length and molecular structure for organic precursors used in MLD. For example, to date there have 3


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been many organic precursors reported for MLD growing various alucones, where TMA is the predominant precursor as the Al source.45 These organic precursors usually have two or more functional groups, such as hydroxyl, amino, carboxyl, etc. They can be categorized into

two

classes:

homobifunctional

(i.e.,

EG,31

hydroquinone

(HQ),23

1,4-

benzenedicarboxylic (BDC)46) and heterobifunctional (i.e., ethanolamine (EA) and maleic anhydride,47 glycidol (GLY)30) reactants. There are different growth characteristics with the different MLD couples and different properties of the resultant alucones films. Homobifunctional organic reactants typically suffer from a symmetric “double-end” surface reaction. This parasitic reaction process decreases the density of reactive sites available for the subsequent half-reaction and results in slow growth rates and poor material stability.30, 41-42 To avoid the double reaction, heterobifunctional reactants and ring open reaction were proposed.32 To this end, Yoon et al. first reported a three-step ABC MLD reaction sequence using TMA, EA, and maleic anhydride as precursors, in which a heterobifunctional precursor EA and a ring-open reaction of maleic anhydride was demonstrated.47 In addition, through smartly selecting precursors with the same functional groups but different backbones, MLD enables tuning the properties of hybrid films. For example, EG can be substituted with the aromatic hydroquinone (HQ, HOC6H4OH) for MLD alucones, where using the aromatic backbone can provide structural stability and contribute largely to the electrical properties of the resultant hybrid films.23, 31 Moreover, there are a special group of organic molecules called isomers, which possess the same 4


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functional groups and backbones with different structures. For examples, benzene dicarboxylic acid (bdc) has there isomers (1,2-bdc, 1,3-bdc and 1,4-bdc).46 And butenedioic acid shows two isomers as cis-butenedioic acid (maleic acid, MA) and trans-butenedioic acid (fumaric acid, FA).48 Klepper et al. have reported MLD alucones using various carboxylic acids,28, 46, 48 demonstrating the cis- and trans configurations of butenedioic acid precursor can influence the growth rate and preference for bonding type of hybrid thin films.48 However, the research about effect of organic structure, especially the isomers, on MLD growth is still rare. Recently, our group prepared Ti based-FA hybrid films using TiCl4 as metal precursor and FA (trans-butenedioic acid) as organic precursor, and the growth characteristics, chemical and thermal stability, and charge trapping ability of hybrid films have been studied.21 In this work, MA (cis-butenedioic acid), which is the isomer of FA, and TiCl4 were used as precursors to fabricate Ti-MA hybrid films by MLD. The effect of deposition temperature from 140 oC to 280 oC on the growth rate, composition and bonding mode of the hybrid thin films has been investigated systematically. The ambient stability of the TiMA hybrid films against open air was explored. The charge trapping ability and stability of hybrid films as the charge trapping layer in a charge trapping memory capacitor was also evaluated. More importantly, the Ti-MA and Ti-FA hybrid films have been made systematical comparison so as to clarify the impact of cis- and trans- configurations of the organic precursors on the MLD growth, the stability and charge trapping ability of MLD 5


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hybrid films.

EXPERIMENTAL SECTION Ti-MA hybrid films deposition: The Ti-MA hybrid films were deposited in a commercial flow-type hot-wall ALD reactor (Picosun SUNALETM R-200B) using TiCl4 (99%, Nanjing Chemical Reagent CO. LTD) and cis-butenedioic acid (maleic acid, MA, COOH-CH=CH-COOH, 99%, J&K SCIENTIFIC LTD) as precursors. The inorganic precursor TiCl4 and organic precursor MA was evaporated at room temperature and 135 oC,

respectively. Pure N2 (99.999 %) was used as the carrier gas and purging gas. The

deposition was carried out under a 6-8 mbar pressure onto p-Si(100) using optimal pulse sequence of 0.3 s TiCl4/ 4 s N2/ 2 s MA/ 10 s N2. The deposition temperature was varied from 140 oC to 280 oC with 200 cycles. Additionally, the blocking and tunnel Al2O3 layer in memory capacitor were deposited at 200 oC using TMA as metal source and H2O as oxygen source with the pulse sequence of 0.1 s TMA/ 4 s N2 purging/ 0.1 s H2O/ 4 s N2 purging. Characterization: The evaporation characteristics of the MA precursor in flowing N2 were determined by the thermo gravimetric and differential scanning calorimetry (TG/DSC, Netzsch STA 409PC/PG) under atmosphere pressure. In situ quartz crystal microbalance (QCM) measurements were performed to investigate MLD process of Ti-MA hybrid films. These experiments used 6 MHz quartz sensors (INFICON) mounted in an INFICON ALD sensor head. The sensor head was modified to purge the back side of the crystal with N2 to 6


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ensure that ALD only occurred on the front side.49 The QCM thickness monitor (INFICON SQM-160) reported mass changes in units of ng/cm2. The thickness of Ti-MA hybrid films was determined by ex situ spectroscopic ellipsometry (SE, GES-5, Sopra). Tauc-Lorentz model was used for fitting the ellipsometry data. The topography and surface roughness of the hybrid films were analyzed by atomic force microscopy (AFM, Cypher, Asylum Research). The root-mean-square (RMS) roughness values were obtained from 3 μm × 3 μm images. The chemical composition of the hybrid films was investigated by ex situ Xray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha) with a standard Al Kα (1486.7 eV) X-ray source. The take-off angle of XPS is 90°, and the binding energy scale was calibrated using the energy position of the adventitious C 1s peak at 284.6 eV. In order to characterize the bonding mode and group information in the hybrid films, the Fourier transform infrared spectra (FTIR, Spectrum, PerkinElmer) in transmission mode of hybrid films deposited on double side polished Si were collected. A pristine Si substrate was used as the reference, and the Si background was subtracted. Density functional theory (DFT) calculation details: In order to investigate the reaction mechanism of the TiCl4-MA MLD process, the cluster models, Si10H15-TiCl3 and Si10H15-Ti(OH)3, were adopted. To model the surface, the bottom atoms, Si and H, of cluster model were fixed. The reactants include water and maleic anhydride. All stationary points in the Ti-MA MLD were fully optimized using the GD3 dispersion-corrected M062X functional (M06-2X-GD3) and 6-311G(d,p) basis set in the Gaussian 09 program.50-56 7


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The vibrational frequency and intrinsic reaction coordinate (IRC) calculations were further carried out to verify whether the stationary point is a minimum or transition state.57 Charge trapping properties test: A flash memory capacitor structure consist of Pt/Al2O3/Ti-MA hybrid films/Al2O3/Si was fabricated. 100-nm-thick Pt top electrodes were DC sputtered through a shadow mask with a diameter of 150 μm using the Q150T system. The electrical properties of the charge trapping capacitors, including memory window, endurance and retention characteristics, were measured by means of a semiconductor characterization system (Keithley 4200-SCS) with a probe platform (Cascade summit 12000B-M) at a high frequency (1 MHz).

RESULTS AND DISCUSSION Growth of Ti-MA hybrid films by MLD The TG/DSC result of MA from room temperature to 300 oC at a heating rate of 10 oC/min under flowing N is shown in Figure S1. It can be found that MA begins to volatilize 2

at about 135 oC and the volatilization rate turns very quick above 150 oC. A strong endothermic peak at ~154 oC is attributed to the sublimation/fusion of MA. MA source almost evaporates completely with only 0.51% residual when temperature reaches ~175 oC.

In the following MLD experiments, the temperature of MA source was set at 135 oC.

The effect of dosing time of the precursor TiCl4 and MA on the growth per cycle (GPC) for Ti-MA hybrid films deposited at 160 oC has been examined by in situ QCM, as illustrated in Figure 1. In Figure 1a, the dosing time of MA source changed from 0 s to 8 8


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s. While the TiCl4 dosing time was set at 2 s enough long to make sure that a surplus of TiCl4 was introduced during the TiCl4 pulse. It can be observed that the GPC first increases with the dosing time of MA extending to 2 s and then remains constant after dosing time longer than 2 s.

Figure 1. GPC values gained by in situ QCM for Ti-MA hybrid films deposited at 160 oC as a function of (a) MA and (b) TiCl4 dosing time.

Similarly, the dosing time of TiCl4 source varied from 0 s to 1 s with the MA dosing time of 2 s. The GPC value basically keeps unchanged when the TiCl4 dosing time reaches 0.3 s in Figure 1b. It reveals that MLD process of the Ti-MA hybrid films derived from TiCl4 9


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and MA shows self-limiting surface reaction at 160 oC. After the above experiments, the optimal MLD growth sequence for the Ti-MA hybrid films was fixed to be 0.3 s TiCl4/4 s N2 purging/2 s MA/10 s N2 purging. Besides, mass growth observed through in situ QCM measurements during Ti-MA hybrid films MLD at 160 oC is quite linear with MLD cycles, as shown in Figure 2.

Figure 2. In situ QCM mass gain profile for Ti-MA MLD process at 160 oC.

GPC value of Ti-MA hybrid films was also measured by ex situ spectroscopic ellipsometry on Si substrates. And GPC was studied as a function of deposition temperature and MLD cycles. Figure 3a shows the GPC dependence on deposition temperature from 140 oC to 280 oC. When the growth temperature increases from 140 oC to 180 oC, the GPC slowly decreases from 1.42 Å to 1.17 Å. With further raising the temperature to 200 oC, the GPC value of 0.50 Å can be observed with a sudden drop. While the temperature reaches to 280 oC, the GPC value becomes much smaller and is only 0.16 Å. Evidently, the growth rate is more temperature-dependent for the Ti-MA system than Ti-FA hybrid films.21 The GPC value of Ti-FA hybrid films declines from 1.10 Å to 0.52 Å with the 10


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deposition temperature from 180 °C to 350 °C. Usually, the reduced GPC can be ascribed to the increased thermal motion and desorption of molecules on growth surface.41-42, 58 Similar phenomenon has been observed in many MLD-derived organic-inorganic hybrid films such as alucones and zincones.31, 48, 59

Figure 3. (a) Growth rate for Ti-MA hybrid films deposited at different temperatures from 140 oC to 280 oC for 200 cycles, (b) The thickness of Ti-MA hybrid films versus number of MLD cycles at 160 oC.

In Figure 3b, the film thickness of Ti-MA versus the deposition cycles was measured at 160 oC. The film thickness is linearly related to the number of MLD cycles, which meets the typical growth feature of the ALD. There is excellent agreement between the linear Ti11


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MA hybrid films MLD growth determined by in situ QCM measurements in Figure 2 and ex situ SE analysis in Figure 3b. Characterizations and growth mechanism of Ti-MA hybrid films by MLD The refractive index of Ti-MA hybrid films grown at 160 oC was evaluated, as seen in Figure S2. The Ti-MA hybrid films exhibit the refractive index of 1.81 at 590 nm, which is nearly the same as the value of reported Ti-based hybrid films, such as Ti-FA21 and TiEG27 hybrid films. The refractive index of titanicones films is much lower than that of the ALD-derived TiO2 films (2.4 at 590 nm).27 Compared with the parent metal oxides, a lowering of the refractive index for the metalcones has also been observed in other metalcones.27-28, 31, 48 It can be ascribed to the fact that the density of the hybrid films is smaller than the parent metal oxides because of the organic species. AFM images of Ti-MA hybrid films deposited at different temperatures are recorded in Figure S3. All of the MLD Ti-MA hybrid films exhibit very smooth surfaces with small RMS values of ~0.2 nm except the sample deposited at 140 oC. The deposition temperature of 140 oC is very close to source temperature of MA (135 oC). Therefore, some precursor vapor might be involved into hybrid films during MLD process, resulting in a higher RMS value of ~0.5 nm for 140 oC sample. In addition, all Ti-MA hybrid thin films deposited at 140-280 °C exhibit amorphous nature based on the XRD scans, consistent with most reported literatures about titanicones, alucones and zincones films.27, 31, 59-60 The bonding mode between Ti ions and carboxyl group in Ti-MA hybrid films were 12


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measured by FTIR spectroscopy, as presented in Figure 4. Absorption bands corresponding to the asymmetric and symmetric stretch of carboxylate groups are visible in all spectra. The splitting between the asymmetric and symmetric carboxylate stretching bands (Δ) is also marked in Figure 4. For all our samples, the Δ is within the range 50-150 cm-1, suggesting the preference for bidentate bonding mode between Ti ion and carboxyl.61 TiFA hybrid films deposited at 200 oC shows the preference for bridging bonding mode with Δ of 158 cm-1.21 This is quite consistent with TMA-MA/FA systems reported by Klepper et al.,48 where Al-MA and Al-FA hybrid films show the preference for bidentate and bridging bonding mode, respectively. The cis- and trans configurations of butenedioic acid precursor can also influence the preference for bonding mode in TiCl4-MA/FA systems, the schematic of these two bonding modes are shown in Figure S4.

Figure 4. FTIR spectra of Ti-MA hybrid films deposited on double side polished silicon wafer at 160, 13


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200, 240 and 280 oC. The wavenumber splitting (cm-1) between the asymmetric and symmetric carboxylate bands (Δ) is given.

Figure 5. XPS spectra collected for (a) C 1s, (b) Ti 2p, (c) O 1s, and (d) Cl 2p of Ti-MA hybrid films deposited at various temperatures.

High resolution XPS were performed to characterize the chemical state and composition of Ti-MA hybrid films deposited at various temperature, as shown in Figure 5. The XPS spectra of C 1s, Ti 2p, O 1s, and Cl 2p of hybrid films are shown in panels (a), (b), (c) and (d), respectively. In the case of carbon, XPS spectra display two significant peaks centered at about 284.6 eV and 288.5 eV, corresponding to the C-C (backbone chain carbon) and O-C=O (carboxyl) bonds.62 The presence of C-C and O-C=O bonds indicates that organic molecules exist in the as-deposited hybrid films, shown in Figure 5a. As seen in Figure 5b, the Ti 2p XPS spectra of hybrid films can be fitted into two peaks centered at 464.5 eV and 14


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458.7 eV, which can be ascribed to Ti 2p1/2 and Ti 2p3/2 peaks of Ti-O bonds with the spin orbit splitting energy of 5.8 eV, in accordance with the literature value of TiO2.63 Two peaks are observed at 530.0 eV and 531.6 eV in Figure 5(c), corresponding to the O-Ti bonding and O-C bonding, respectively.40 The O-Ti and O-C bonding structures are shown in Figure S5. Oxygen of O-Ti is only bonded to Ti atom, while oxygen of O-C is from carboxyl. Along with raising the deposition temperature, it seems that both position and intensity of C 1s, Ti 2p, and O 1s peaks nearly remain constant. Figure 5(d) shows the Cl 2p XPS spectrum of hybrid films deposited at 160 oC. The doublet at 198.0 eV and 199.6 eV can be assigned to Cl 2p1/2 and Cl 2p3/2 peaks with the spin orbit splitting energy of 1.6 eV. It reveals that there is some remnant Cl embedded in the hybrid films. The atomic ratio of Cl/Ti is determined to be around 9%-17%. In comparison with literature value (Cl/Ti) of 15%-35% in Ti-4,4-oxydianiline hybrid films and 36%-44% in Ti-EG hybrid films,27, 29 the Cl impurity of 9%-17% is much lower in TiMA hybrid films. It can be ascribed to the unreacted Ti-Cl species with MA or absorbed HCl. However, the Cl impurity in Ti-MA hybrid films is a little higher than the reported value (Cl/Ti) of 3%-8% in Ti-FA hybrid films.21 The influence of growth temperature on the Ti-MA hybrid films component of C : O : Ti : Cl is summarized in Table 1. It can be found that, with increasing temperature from 140 oC to 280 oC, the atomic ratio of C : O : Ti shows little change. C : O : Ti ratio is stable at around 7.6(±1):5.5(±0.6):1, revealing that the effect of deposition temperature on the 15


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component of the Ti-MA hybrid films can be negligible. This is quite different from TiCl4FA MLD process, where C : O : Ti ratio changes from 8.35 : 7.49 : 1.00 of 180 oC to 4.66 : 4.80 : 1.00 of 350 oC.21 Table 1. Atomic ratio of Ti-MA hybrid films deposited at different temperature determined from XPS Spectra.

Deposition

Atomic ratio

Temperature (oC)

C

O

Ti

Cl

140

7.74

5.64

1.00

0.17

160

6.68

4.96

1.00

0.16

180

8.23

6.18

1.00

0.16

200

8.67

5.40

1.00

0.09

240

8.06

5.32

1.00

0.11

280

7.56

4.89

1.00

0.13

Further, the influence of deposition temperature on the atomic ratio of OO-Ti/OO-C and OO-Ti/Ti of Ti-MA hybrid films was also analyzed. The atomic ratio is calculated from the O 1s and Ti 2p XPS spectra, as shown in Figure 5. Besides, corresponding data of our previously reported Ti-FA hybrid films is adopted for comparison.21 The atomic ratio of OO-Ti/OO-C and OO-Ti/Ti of Ti-MA hybrid films also keeps nearly constant for various deposition temperature. In contrast, the atomic ratio of OO-Ti/OO-C and OO-Ti/Ti of Ti-FA hybrid films increases along with increasing growth temperature. It has been demonstrated that component change in Ti-FA system is arisen from the thermal decomposition of FA molecules at high temperature.21 After further carefully analysis, however, it can be found that OO-Ti/OO-C and OO-Ti/Ti of Ti-MA hybrid films is nearly same as the values of Ti-FA 16


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hybrid films at high temperate of ~300 oC. The atomic ratio of OO-Ti/OO-C and OO-Ti/Ti of Ti-MA hybrid films is around 0.26 and 1.4, respectively. Therefore, there are ~20.8% of O atoms are only bonded to Ti atoms (Ti-O-Ti). For TiCl4-FA MLD process, it has been demonstrated the produced by-product H2O from FA decomposition could react with TiCl to form Ti-O-Ti bonding at high temperatures.

Figure 6. The atomic ratio of (a) OO-Ti/OO-C and (b) OO-Ti/Ti obtained from XPS spectra of Ti-MA and Ti-FA hybrid films deposited at different temperatures.

Therefore, gas chromatography-mass spectrometry (GC-MS) measurements of MA dissolved in ethanol were performed in order to explore the thermal stability of MA. The GC-MS results show that only maleic anhydride can be detected at both low (160 oC) and high (280 oC) temperature, as shown in Figure S6. It indicates that all of MA molecules 17


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would decompose into maleic anhydride by splitting off H2O above 160 oC, as seen in eqn. (1). (1) This is quite different from FA, which only thermally decomposes at high deposition temperature.21 Therefore, the component of Ti-FA hybrid films exhibits the temperaturedependent behavior.

Figure 7. In situ QCM mass growth profiles of Ti-MA MLD process with (a) 0.1 s and (b) 2 s MA dosing time, the dosing time of TiCl4 was set as 0.3 s.

Although all of MA molecules are totally decomposed, the signal of organic species still can be detected in the MLD films (Figure 4 and Figure 5a). It can be ascribed to the fact 18


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that maleic anhydride can undergo a ring open reaction for MLD, which has been adopted for alucones MLD deposition.47 Therefore, the in situ QCM growth profiles were further deeply investigated to explore the mechanism of TiCl4-MA MLD system. Figure 7 illustrates the in situ QCM growth profiles with different MA dosing time (0.1 s or 2 s). It can be easily seen that mass change profiles are quite different with different MA dosing time. The TiCl4 dose leads to a large mass gain, and short MA dose (0.1 s) dose produces a sharp mass decrease (Figure 6a). When extending the MA dose time to 2 s, the mass decreases firstly and then increases slightly, as shown in Figure 6b. Due to the thermal decomposition, MA dose introduces the mixture of H2O and maleic anhydride. When reacting with the Ti-Cl surface, H2O can lead to the mass decrease while maleic anhydride can result in the mass gain. Therefore, it can be speculated that the by-product H2O from MA decomposition would firstly react with the Ti-Cl group to form Ti-OH, resulting in the mass decrease. This reaction is the fast reaction here. When extending the MA dosing time, maleic anhydride would undergo a ring open reaction with the Ti-OH. This ring open reaction here is the slow reaction, which leads to the mass gain. The corresponding possible reaction mechanism is illustrated in Figure 8. In addition, we can define the mass gain after TiCl4 dose as m1, and the mass gain for the whole MLD cycle as m2. Figure S7 plots the relationship between m2/m1 and MA dosing time. It can be seen that the value of m2/m1 increases along with extending the MA dosing time, arising from more MA species with high-molecular-weight incorporating into hybrid films. However, even extending the MA 19


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dosing to saturated dosing time of 2 s, the m2/m1 is still smaller than 1, indicating that not all the OH group can be reacted with maleic anhydride. It may be arisen from the steric hindrance from incorporated MA molecule with large size. Therefore, much O-Ti bonding can be remained in hybrid films, in consistent with XPS data.

Figure 8. Proposed possible reaction mechanism for Ti-MA MLD process.

Figure 9. C 1s XPS spectra of Ti-MA hybrid films with different MA dosing time.

Further, ex situ XPS was also adopted to measure the component of Ti-MA hybrid films with different MA dosing time. The amount of organic species in hybrid films can be determined by the intensity of C=O peak. As shown in Figure 9, the peak of C=O is very weak when the MA dosing time is very short of 0.1 s, indicating only a little MA existing in hybrid films. When extending the MA dosing time to 2 s, the C=O peak intensity increases dramatically, implying that much more MA are incorporated into the hybrid films. 20


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Above ex situ XPS data are well consistent with in situ QCM data, where it needs a long dosing time for maleic anhydride to incorporate into hybrid film.

Figure 10. (a) Possible hydrolysis reaction mechanism of Cl-terminated surface. (b) Possible addition reaction mechanism of maleic anhydride at Ti active site.

Moreover, DFT calculation was performed to investigate the possible reaction mechanism of TiCl4-MA MLD process. As shown in Figure 10a, H2O can coordinate with Ti active site on the surface. At the same time, H2O can also interact with maleic anhydride via strong hydrogen bonding. Due to the strong Lewis basicity, maleic anhydride can dramatically accelerate the hydrolysis reaction of Cl-terminated surface. The catalytic mechanism is similar to that of ammonia- and pyridine-catalyzed SiO2 ALD, which has been previously studied in detail.64-67 The energy barrier (Ea) of the catalytic hydrolysis from the intermediate Im1 to Im2 via the transition state (TS1) is about 8.5 kcal/mol. At the same time, as an weak Lewis base, H2O itself can assist the hydrolysis reaction of Clterminated surface.68 The barrier of the hydrolysis reaction of Cl-terminated surface can be further reduced. Then, maleic anhydride can be absorbed at Ti active site on the surface, 21


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which leads to the formation of five-coordination Ti intermediate (Im3) with maleic anhydride ligand (Figure 10). Subsequently, under the role of one water molecule, the hydroxyl ligand can attack adjacent maleic anhydride ligand, which can form new C-O bond and intermediate (Im4) via an addition transition state (TS2) with Ea of 11.4 kcal/mol. Lastly, the H atom of hydroxyl ligand can transfer to furan-like O atom and open furan ring of maleic anhydride. In fact, this proton transfer process from Im4 is difficult via a direct four-membered ring (4MR) transition state with a very high Ea of 35.2 kcal/mol. It may go through H2O-attended intermediate (Im5) and transition state (TS3) with the energy barrier of 11.7 kcal/mol to break old C-O bond of maleic anhydride and form the Ti-MA intermediate (Im6). According to above computational results, the energy barrier for the hydrolysis reaction of Cl-terminated surface is much lower than that for the addition reaction of maleic anhydride at Ti active site. Therefore, the addition and opening-ring reactions of maleic anhydride at Ti active site is slower than the hydrolysis reaction of Clterminated surface, in accordance well with experimental results. Moreover, it can be seen that bidentate bonding is formed based on above DFT calculation, which agrees with FTIR spectra. The following TiCl4 dose will react with the surface OH group, as shown in Figure S8, forming both inorganic Ti-O-Ti unit and organic-inorganic Ti-MA unit. Furthermore, TiCl4-H2O-maleic anhydride ABC-type deposition was also conducted, as shown in Figure S9(a). It can be seen that maleic anhydride can’t react with the Ti-OH surface. DFT calculation reveals that the direct ring-open reaction for Maleic anhydride on 22


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Ti-OH is very difficult with the energy barrier of 35.2 kcal/mol. However, with the help of one water molecule, the Ea of ring-open can be decreased to 11.4 kcal/mol. Therefore, we then performed the TiCl4-H2O-(H2O + maleic anhydride) ABC-type deposition, as shown in Figure S9(b). Both H2O and maleic anhydride are injected in the third step. It can be seen that maleic anhydride can be added into the film with the help of H2O. It is consistent well with DFT calculation. It can be seen that H2O is very important, therefore, there is also no deposition observed for TiCl4-maleic anhydride (Figure S10). On the basis of above analysis, although MA and FA possess the same functional groups and backbone, different molecular structure of organic precursor shows great influence on the MLD process. MA is the cis-isomer of butenedioic acid, whereas FA is the transisomer. MA is a less stable molecule than FA. FA only decomposes at high temperature, while MA is totally decomposed at whole MLD deposition temperature range. As a result, TiCl4-MA and TiCl4-FA MLD process exhibits quite different growth behavior. A temperature-dependent growth characteristic has been observed in the Ti-FA hybrid films. On increasing the temperature from 180 °C to 300 °C, the XPS composition ratio of C : O : Ti of the films changes from 8.35 : 7.49 : 1.00 to 4.66 : 4.80 : 1.00. For Ti-MA hybrid films, the composition nearly keeps unchanged when changing the deposition temperature with the C : O : Ti ratio of 7.6(±1) : 5.5(±0.6) : 1. However, there are lots of Ti-O-Ti units in as-deposited Ti-MA hybrid film. Therefore, the TiCl4-MA MLD as-deposited films are actually consisted of inorganic Ti-O-Ti units and organic-inorganic Ti-MA units. In this 23


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work, the as deposited films are still called as Ti-MA hybrid films. Stability of Ti-MA hybrid films Our previous study has demonstrated that Ti-FA hybrid films are not stable in air, it can react with H2O in air. Therefore, the stability of Ti-MA hybrid films in air was also explored for comparison by spectroscopic ellipsometry, XPS and FTIR, using the sample deposited at 160 oC of 200 cycles. The hybrid films were exposed to air atmosphere for one year. Firstly, the thickness change was measured by spectroscopic ellipsometry. The film thickness of the sample changes from 28.2 nm to 29.1 nm after one year air exposure. Obviously, there is only ~3.2% change in the thickness of the hybrid films.

Figure 11. The comparison of FTIR spectra of as-deposited Ti-MA hybrid films and the same sample aged in the open air for one year.

Figure 11 shows the FTIR spectra for Ti-MA hybrid films deposited 160 oC aged in open air for one year. In comparison with as-deposited Ti-MA hybrid films, a new weak broad absorption band appears after one year exposure in open air, which is visible in 3700-2800 cm-1 regions of the spectra resulting from the presence of OH groups. The wavenumber splitting between the asymmetric and symmetric carboxylate bands change from 91 to 110 24


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cm-1, still remaining the bidentate bonding mode. What we can see from the change is that the water in the open air can be absorbed by the Ti-MA hybrid films, which have not much influence on the bonding mode of the hybrid films.

Figure 12. (a) C 1s and (b) O 1s spectra of as-deposited Ti-MA hybrid films at 160 oC and the sample aged in open air for one year and immersed in deionized water for 1 h.

Figure 12 shows C 1s and O 1s XPS spectra of as-deposited Ti-MA hybrid films at 160 oC,

the sample aged in open air for one year and immersed in deionized water for 1 h.

Compared with the as-deposited hybrid films, the sample aged in open air for one year only exhibits a little change in O 1s spectra. Only a weak new peak centered at around 286.3 eV comes out, corresponding to the C-O (C related to the OH groups) from C 1s. The new C25


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O may come from the reaction between the Ti-MA hybrid films surface with water in air. Table 2 compares the stability of Ti-MA and Ti-FA hybrid films in air. After one year exposure in the open air, the atomic ratio of OO-Ti/OO-C and OO-Ti/Ti for Ti-MA hybrid films only changes 20.0% and 14.3%, respectively. The Cl/Ti atomic ratio of the hybrid films exposed in open air after one year is much lower than the as-deposited films, attributing to the unreacted Cl react with H2O or the evaporation of HCl. Compared to Ti-FA hybrid films, it can be easily seen that Ti-MA hybrid films are much more stable in air. There is little change for Ti-MA hybrid films, however, Ti-FA hybrid films would react with water in air to form more O-Ti bond with enlarged atomic ratio of OO-Ti/OO-C and OO-Ti/Ti. It can be speculated that the inorganic Ti-O-Ti component in Ti-MA hybrid films can improve its stability in air. However, the Ti-MA hybrid films are still not stable in water, as shown in Figure 12, it can be seen that the C=O intensity decreases and O-Ti intensity increases after immersed in deionized water for 1 h. Table 2. The stability comparison of Ti-MA and Ti-FA hybrid films in air for year.

Ti-MA hybrid films (160 oC)

Ti-FA hybrid films (200 oC)

As-dep.

Air-1 year

As-dep.

Air-1 year

thickness (nm)

28.1

29.2

18.6

17.0

OTi-O/OC-O

0.40

0.32

0.11

0.47

OTi-O/Ti

1.47

1.26

0.7

1.54

Cl/Ti

0.16

0.06

0.08

0.05

91

110

157

159

Δ ( cm-1)

26


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Bonding mode

Bidentate

Bidentate

Bridging

Bridging

Charge trapping ability of Ti-MA hybrid films

Figure 13. (a) High frequency (1 MHz) C-V curves of the samples with 7 nm Ti-MA hybrid films as CTL at different sweeping voltages; and (b) the dependence of memory window on sweeping gate voltage for Ti-MA and Ti-FA hybrid films based memory cells.

Furthermore, we also compared the charge trapping ability of Ti-MA and Ti-FA hybrid films. A flash memory capacitor structure consist of a top electrode, blocking oxide, charge trapping layer (CTL), tunnel oxide and p-Si substrate were fabricated.69 The schematic of the flash memory capacitor structure of Al2O3/Ti-MA hybrid films/Al2O3/Si by ALD is illustrated in Figure S11. The Ti-MA hybrid films deposited at 160 oC was used as a CTL and Pt was used as top electrode. What is a significant technological superiority of our 27


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memory capacitor in terms of fabrication is that all layers of tunnel oxide (6 nm Al2O3), CTL (7 nm Ti-MA hybrid films) and blocking oxide (15 nm Al2O3) can be continuously deposited on p-Si substrate without breaking the vacuum of ALD reactor chamber. Figure 13a plots the capacitance-voltage (C-V) curves of the sample with 7 nm Ti-MA hybrid films CTL at different sweeping voltages at 1 MHz. The capacitor shows the largest memory window of 11.04 eV at the sweeping gate voltage of ±16 V, indicating a good charge storage capability. Furthermore, with increasing the sweeping voltage, memory window of the capacitors becomes larger, as indicated in Figure 13b. In addition, Figure 13b also compares the performance of the memory cells using Ti-MA or Ti-FA hybrid films as CTL. It can be easily seen that Ti-MA hybrid films based memory cell exhibits much larger memory window than the Ti-FA hybrid films based memory cell. It can be ascribed to the fact that as-prepared Ti-MA hybrid films here are consisted of inorganic TiO-Ti units and organic-inorganic Ti-MA units, as demonstrated above. Compared with simple organic-inorganic Ti-FA hybrid films, the Ti-MA hybrid films may possess more media that can store charges. For example, there may be many defects or interfaces, which can enhance the charge trapping capability of the devices.70 Furthermore, the stability of the memory cell in air was also tested after 100 days’ exposure in the open air. Figure S12 shows the C-V curves with different sweeping voltage of the sample after air exposure, it can be easily seen that the memory cell still exhibits good charge trapping capability. The capacitor still shows the memory window of 9.65 V 28


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at the sweeping gate voltage of ±16 V. In comparison with data of as-prepared memory cell, the memory window of the air exposed capacitor only shows a little decrease, as shown in Figure 14a. At the sweeping gate voltage of ±16 V, the memory window decreases only 12.6 % compared to as prepared sample.

Figure 14. (a) The dependence of memory window on sweeping gate voltage, and (b) the endurance characteristics under 104 cycles of ±10 V, 10 ms program/erase pulse for Ti-MA hybrid films based memory cells before and after air exposure.

The endurance characteristic of the Ti-MA hybrid films based memory cell under a Program/Erase (P/E) voltage of ±10 V with pulse time of 10 ms is illustrated in Figure 14b. After 104 cycles of P/E pulse at room temperature, it exhibits very good endurance property. 29


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In addition, after 100 days’ air exposure, the endurance just shows 8.8% decrease. From what has mentioned above, it has been demonstrated the Ti-MA hybrid films exhibit excellent charge storage capability with good endurance property. Ti-MA hybrid films based memory cell exhibits much larger memory window than the Ti-FA hybrid films based memory cell. Furthermore, it can be found that the memory capacitor with Ti-MA hybrid films as CTL is relatively stable in air. The exposure in the open air for 100 days only takes the edge off the charge trapping ability of Ti-MA hybrid films slightly with 12.6% decrease in memory window and 8.8% decrease in endurance.

CONCLUSIONS Ti-MA hybrid films were successfully fabricated by MLD through sequential surface reactions of separately introduced MA and TiCl4 pulses. The effect of deposition temperature on the growth rate, composition and bonding mode of hybrid thin films has been investigated systematically. With increasing temperature from 140 oC to 280 oC, the growth rate dramatically deceases from 1.42 Å to 0.16 Å per MLD cycle. Moreover, the influence of the cis- and trans configurations of organic butenedioic acid precursor was explored. The cis- and trans configurations of organic precursors can influence the MLD growth behavior, the preference for bonding mode, the stability and charge ability of hybrid films. Ti-MA and Ti-FA hybrid films show the preference for bidentate and bridging bonding mode, respectively. The XPS composition ratio of C : O : Ti of Ti-MA films has little change with deposition temperature, whereas Ti-FA shows the temperature dependent 30


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composition. Both ex situ XPS and in situ QCM demonstrate that the as-deposited MLD Ti-MA hybrid films are consisted of inorganic Ti-O-Ti units and organic-inorganic Ti-MA units. In addition, DFT calculation reveals that the energy barrier for the hydrolysis reaction of Cl-terminated surface is much lower than that for the addition reaction of maleic anhydride at Ti active site during the MA pulse. Therefore, the addition and opening-ring reactions of maleic anhydride at Ti active site is slower than the hydrolysis reaction of Clterminated surface, in accordance well with experiment data. The inorganic Ti-O-Ti units in hybrid films can improve the stability and charge ability of Ti-MA hybrid films. As a result, Ti-MA hybrid films are very stable in open air for one year. Furthermore, Ti-MA hybrid films based memory cell exhibits much larger memory window than the Ti-FA hybrid films based memory cell. Moreover, the Ti-MA hybrid films based memory is relatively stable in air. The charge trapping ability of Ti-MA hybrid films only decreases slightly after 100 days’ exposure in the open air. Considering the flexibility of organic-inorganic thin films, the CTL based on MLD organic-inorganic thin films can be one of the promising candidates for the application in the emerging flexible electronic devices.

ASSOCIATED CONTENT Supporting Information Available: TG/DSC curves and GC-MS spectra of maleic acid. Refractive index and AFM images of Ti-MA hybrid films. Schematic of bonding type, O-C/O-Ti bonding, Ti-O-Ti/Ti-MA units. In situ QCM growth profile for TiCl4-H2O31


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Maleic anhydride and TiCl4-H2O-( H2O + Maleic anhydride) 3 step MLD. The absolute energies and Cartesian coordinates of all stationary points (Im1 to Im6, TS1 to TS3). Schematic and charging properties of Ti-MA hybrid film based flash memory capacitor structure. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected] Author Contributions #

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported in part by the Natural Science Foundation of China (51721001, 51571111, 51802150, 21873073), Jiangsu Province (BK2016230, and BK20170645) and Zhejiang Province (LY17B030003), and a grant the State Key Program for Basic Research of China (2015CB921203), China Postdoctoral Science Foundation (2017M611778) and the Fundamental Research Funds for the Central Universities (021314380117). We thank the National Supercomputer Center in Guangzhou and the High Performance Computing Center of Nanjing University for providing computing resources. We also thank the support from the open project of NLSSM (M30038, M30031). 32


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Growth mechanism, ambient stability and charge trapping ability of Ti

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