Surface Thermolytic Behavior of Nickel Amidinate and Its Implication

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Surface Thermolytic Behavior of Nickel Amidinate and Its Implication on the Atomic Layer Deposition of Nickel Compounds Ran Zhao,† Shuang Xiao,‡,§ Shihe Yang,‡,§ and Xinwei Wang*,† †

School of Advanced Materials, Shenzhen Graduate School, and ‡Guangdong Key Laboratory of Nano−Micro Material Research, School of Chemical Biology and Biotechnology, Shenzhen Graduate School, Peking University, Shenzhen 518055, China § Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

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S Supporting Information *

ABSTRACT: Atomic layer deposition (ALD) is a highly important technology to fabricate nickel and nickel compound thin films. The quality of the ALD films relies much on the surface chemistry reactions involved in the ALD process. Aiming to achieve high-quality ALD films, a careful surface chemistry study is carried out in this work to investigate the surface thermolysis behavior of an amidinate-type nickel precursor, bis(N,N′-di-tertbutylacetamidinato)nickel(II) (Ni(amd)2). Using the in situ technique of X-ray photoelectron spectroscopy, this work reveals a number of implications which are important for the engineering of the ALD processes. The Ni(amd)2 precursor is shown to be reactive to the SiOx surface even at room temperature, which suggests a good suitability for low-temperature ALD. The surface amidinate moiety is found to decompose at 250 °C, which suggests the limitation of Ni(amd)2 for high-temperature ALD. On the other hand, the byproduct of the surface reaction, amidine, can be adsorbed on the surface at low temperature, which might be trapped in the deposited film, inducing carbon and nitrogen impurities. Therefore, if the goal is to achieve a low impurity, a relatively higher deposition temperature (e.g., at 150− 200 °C) would be favored. To further demonstrate the important value of the above implications from the thermolysis study, an example of the ALD of NiO is investigated experimentally. Indeed, the NiO films can be successfully deposited at an ever-low temperature of 90 °C, and increasing the deposition temperature to 200 °C can eliminate the accumulation of carbon and nitrogen impurities at the film−substrate interface, whereas the deposition at 250 °C leads to drastic increase in impurities, likely owing to the thermal decomposition of the surface amidinate moiety. All these experimental findings are well consistent with the implications from the surface thermolysis study, which highlights the important value of this surface thermolysis approach for rational ALD process engineering. Given the similarity and broad use of the amidinate-type metal precursors, the results reported herein should also be of significant value for many other ALD processes.

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INTRODUCTION Thin films of nickel and its compounds (e.g., NiOx, NiNx, NiCx, NiSix, and NiSx) have a wide variety of important applications in cutting-edge microelectronics,1−4 catalysis,5−10 and energy technologies.11−15 Atomic layer deposition (ALD), which relies on alternate self-limiting surface reactions, is one of the most important technologies to fabricate these thin-film materials, especially when the films are required to be conformally coated on complex topographies with atomicprecise control in film thickness.16−18 For instance, a metallic Ni film could be deposited into Si hole structures by ALD in order to prepare NiSi deep-hole contacts for microelectronic engineering;2,3 NiO thin film, with its thickness precisely controlled by ALD, has been demonstrated as an efficient holetransport layer in solar cells;13,14 and ALD-prepared NiSx and NiCx ultrathin films have been demonstrated of high catalytic activities for the electrochemical oxygen and hydrogen evolution reactions from water, respectively,9,10 and therefore © XXXX American Chemical Society

they are of great promise for use in renewable energy conversion and storage technologies. Ideally, an ALD process can offer many unique merits for film fabrication such as extraordinary control over film thickness, uniformity, conformality, and reproducibility,16 but in reality, many ALD processes suffer from nonideal factors, such as undesired side surface reactions, which may result in the films incorporating unwanted impurities.19−22 More than often, the film quality is of essential importance for its applications, and the impurity incorporation could be seriously detrimental in this regard.23 For instance, the impurities in the NiO films could form defect sites, which could significantly impede the hole transport, thereby limiting the efficiency of the corresponding solar cells.24 Received: March 30, 2019 Revised: June 18, 2019 Published: June 19, 2019 A

DOI: 10.1021/acs.chemmater.9b01267 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials To achieve high-quality ALD film growth with minimal impurity incorporation, one often needs to carefully design (or choose) the ALD precursors and optimize the process conditions (e.g., temperature) in order to enable the facile detaching of the ligands from the precursor complex upon its adsorption on the surface while retaining the molecular integrity to avoid the side reactions that may cause the impurity incorporation.19 To this end, surface thermolysis investigation on the precursor compounds is of critical importance to reveal the necessary information in order to assess the precursor reactivity for the ALD purposes and also to uncover the possible side reaction routes with their onset temperatures, thereby shedding light on the strategies for ALD process engineering. In this work, we investigate the surface thermolysis of a representative amidinate-type Ni precursor, that is, bis(N,N′di-tert-butylacetamidinato)nickel(II) [Ni(amd)2], by using in situ X-ray photoelectron spectroscopy (XPS), and reveal a series of implications that are highly important for better engineering of the ALD processes for nickel compounds. The chosen Ni amidinate precursor is a volatile compound, and it has been employed to deposit a large variety of nickel compounds including NiO,13,14,25,26 NiNx,27 NiSx,9,28,29 and NiCx30 as well as the Ni metal itself.31 Despite the numerous applications, the ALD surface mechanisms involving this precursor are not yet well understood. Previously, a couple of detailed in situ surface chemistry studies from our group revealed some special and intriguing attributes for the ALD of NiS at 200 °C (e.g., initial agglomeration32 and nonvolatile acid−base complex formation33), yet the ALD processes for other nickel compounds or at other temperatures were not studied on that detailed mechanistic level, therefore leaving many important questions unanswered such as whether lowtemperature ALD can be achieved with this precursor and how to minimize the impurity level in the ALD films. The former question is critical for the film deposition on thermally sensitive substrates such as polymers and plastics, aiming for flexible electronics applications, and the latter is important for applications in (opto)electronic devices that require highquality films.34−36 In the following, we show that these questions can be well answered by the surface thermolysis study, and the answers are later verified experimentally. The results reported herein not only provide direct implications to the ALD process engineering for the nickel compounds but can also potentially extend the applications of the ALD nickel compounds to an even boarder scope.

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increase its vapor pressure (∼140 mTorr) for being dosed into the ALD chamber. A flat SiOx/Si wafer sample was used as the substrate to investigate the surface thermolytic behavior of Ni(amd)2. The SiOx/Si substrate was a p-type Si(100) wafer with a thin layer of SiOx (∼2 nm) on the surface, and it was sequentially cleaned in acetone, methanol, isopropanol, and deionized water and then treated with ultraviolet/ozone for 30 min before loading into the ALD chamber. The substrate was then heated under vacuum at 450 °C in the ALD chamber for 4 h to completely remove any organic surface residue. Subsequently, the substrate was cooled down to room temperature (25 °C) inside the chamber, and then the Ni(amd)2 vapor was dosed onto the SiOx/Si surface with an exposure of ∼0.1 Torr s. This exposure number was particularly chosen to resemble the conditions used in practical ALD processes.9,31,38 Then, the Ni(amd)2-dosed SiOx/Si sample was transferred back and forth between the analysis chamber for measuring the XPS spectra and the ALD chamber for the heat treatments at various temperatures from 100 to 400 °C. ALD of NiO thin films was carried out in a home-built tubular ALD reactor.9 The deposition employed Ni(amd)2 as the Ni precursor and H2O as the coreactant. During the deposition, Ni(amd)2 was mildly heated to 70 °C while H2O was kept at room temperature. The Ni precursor vapor was delivered into the deposition chamber with the assistance of purified N2 gas (through a GateKeeper inert gas purifier). As for the H2O vapor, it was first delivered into a ∼5 mL gas trap and then delivered into the deposition chamber. In each ALD cycle, the exposures of the Ni(amd)2 and H2O vapors were approximately 0.1 and 0.3 Torr s, respectively. Purified N2 gas was also used as the purge gas. A long purge time of 60 s was used after each Ni(amd)2 dose and after each H2O dose. The purity of the deposited NiO films was examined by both XPS and time-of-flight secondary-ion mass spectrometry (TOF SIMS). To obtain the elemental depth profiles by XPS, 1 keV Ar+ ions were used for sputtering. To obtain the elemental depth profiles by SIMS (ION-TOF GmbH, TOF SIMS V), 3 keV Cs+ primary ions (27 nA, 500 × 500 μm2) were used, and the negatively charged secondary ions were collected for the analysis.

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RESULTS AND DISCUSSION Figure 1 shows the acquired high-resolution core-level XPS spectra of Ni 2p and the Auger electron spectra of Ni L3VV. As shown in Figure 1a, the core-level XPS spectrum of the initially

EXPERIMENTAL SECTION

All in situ experiments were carried out on a modified XPS system (Thermo Scientific, Escalab 250Xi), which was described in detail in our previous publications.33,37 In brief, the system consisted of an XPS analysis chamber, a hot-wall ALD chamber, and a connecting intermediate chamber, which allowed for transferring samples without vacuum break. A monochromatic Al Kα X-ray source was used for XPS, and the spectrum binding energy (BE) was referenced to Au 4f7/2 (83.96 eV). High-resolution XPS spectra were acquired using pass energies of 20 eV for O 1s and 50 eV for N 1s, C 1s, and Ni 2p because the intensities for the latter three were relatively low. During the XPS measurement, the spectrum was always taken on a new spot for each measurement so as to avoid potential damage from the X-ray. Bis(N,N′-di-tert-butylacetamidinato)nickel(II) [Ni(amd)2] was used as the representative nickel amidinate compound as it has been employed as the Ni precursor in many reported ALD processes.9,14,25,27−29,32,33,38 Ni(amd)2 was mildly heated to 70 °C to

Figure 1. (a) Ni 2p core-level XPS and (b) Ni L3VV Auger spectra acquired after the dose of Ni(amd)2 on a SiOx/Si substrate at 25 °C and after the heat treatments at various temperatures. Peak fitting was performed for the Ni 2p3/2 bands, and the blue and green curves show the fitted peak components for Ni−O and metallic Ni, respectively. B

DOI: 10.1021/acs.chemmater.9b01267 Chem. Mater. XXXX, XXX, XXX−XXX

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position of the Auger emission also changed, and the most prominent change was again found when elevating the temperature from 200 to 250 °C, which is in good consistence with the previous core-level results. Also, the Auger peak position was found to eventually shift to a higher KE of 843 eV at a temperature ≥350 °C, and this KE value is a good indicator of the formation of metallic Ni.45 Therefore, the Auger results shown in Figure 1b corroborate well the previous core-level observations. High-resolution XPS spectra of O 1s, N 1s, and C 1s were also measured. As shown in Figure 2a, the O 1s spectrum of

bare SiOx/Si substrate does not contain any Ni 2p signals; but, after the dose of Ni(amd)2 at room temperature (25 °C), the spectrum exhibits pronounced Ni 2p signals with two spin− orbit-split 2p1/2 and 2p3/2 bands. The Ni 2p3/2 band consists of a main peak at 856.5 eV and a broad satellite band at 860−868 eV in BE. The BE of the main Ni 2p3/2 peak was appreciably higher than that of the molecular Ni(amd)2 (855.7 eV), as measured in a control experiment on physisorbed Ni(amd)2 (Figure S1). This apparent difference in the BE suggests that Ni(amd)2 was not physisorbed on the SiOx surface but probably formed Ni−O bonds with the surface oxygen atoms, as oxygen has stronger electronegativity than nitrogen. The position and line shape of the Ni 2p3/2 satellite band resemble the features typically for nickel hydroxide/oxide,39,40 which is also consistent with the formation of the Ni−O bonds. The oxygen atoms to form the Ni−O bonds were probably from the surface hydroxyl moieties, which are known to be present on the SiOx surface32,41 and could possibly react with Ni(amd)2 to afford the Ni−O bonds while hydrogenating the amidinate ligand to afford N,N′-di-tert-butylacetamidine (Hamd) as the byproduct. Previously, similar surface reactions and bond formations were found at an elevated temperature (for instance 18542 or 200 °C32). Interestingly, we herein found that the surface reaction could also proceed, without apparent kinetic difficulty, even at a temperature as low as room temperature (25 °C). This behavior suggests that Ni(amd)2 is a highly reactive compound, and therefore it is a highly promising Ni precursor for low-temperature ALD. After the heat treatments of the Ni(amd)2-dosed sample at various temperatures, considerable changes were observed in the Ni 2p spectra as shown in Figure 1a. The most drastic change of the spectrum line shape was found when the heating temperature was elevated from 200 to 250 °C, as a new prominent peak (853.7 eV) appeared on the low BE side of the main Ni 2p3/2 peak. The intensity of the new 853.7 eV peak continued to rise as the heating temperature was further elevated, and eventually, this peak dominated the Ni 2p3/2 spectrum at temperatures ≥350 °C. The relatively low BE of 853.7 eV suggests that metallic Ni was likely formed at a high temperature.43,44 To further analyze the variation of the Ni 2p spectra, careful peak fitting was carried out for the Ni 2p3/2 spectra shown in Figure 1a. The 25 °C spectrum was first fitted by using three peak components to capture the features of the Ni−O bonds.32 Of the three peak components, one major component (856.5 eV) was used to describe the main 2p3/2 peak, and the other two minor components (861.4 and 865.1 eV) were together used to account for the satellite band. These three components were then grouped together by fixing their relative BEs and intensities and used as a whole to fit for the Ni−O bonds in the other spectra. Similarly, another group of three peak components was used to describe the spectrum for the metallic Ni, and the associated relative BEs and intensities were adopted from the spectrum fitting for the bulk Ni metal (Figure S2). Using the above two groups of peak components, all Ni 2p3/2 spectra could be well fitted (Figure 1a), and the fitted parameters are shown in Table S1. Figure 1b displays the Ni L3VV Auger electron spectra collected along with the Ni 2p core-level spectra. After the dose of Ni(amd)2 at room temperature (25 °C), a broad Ni Auger peak was observed at the kinetic energy (KE) of 841 eV. This KE matches well with the value for nickel oxide,45 which again suggests the formation of Ni−O bonds at room temperature. Upon heat treatments, the line shape and peak

Figure 2. XPS spectra of O 1s core-level emission acquired after the dose of Ni(amd)2 on a SiOx/Si substrate at 25 °C and after the heat treatments at various temperatures. Panel (b) shows the blowup of the shoulder region of the O 1s spectra (a).

the initially bare SiOx/Si surface displayed a strong peak at 532.8 eV. Upon the dose of Ni(amd)2 at room temperature (25 °C), a small shoulder peak at the lower BE of 530.9 eV appeared in the O 1s spectrum. This BE value falls in the range for bulk nickel hydroxide/oxide (529.4−531.1 eV), again suggesting the formation of Ni−O bonds on the surface.39,40 After the heat treatments, the intensity of this shoulder peak gradually reduced, suggesting that some surface Ni−O bonds disappeared at high temperature. Peak fitting was also performed for O 1s (Table S2), and the results will be discussed later. Figure 3a shows the high-resolution XPS spectra of N 1s. After the dose of Ni(amd)2, the spectrum (25 °C) shows a pronounced N 1s peak at 399.0 eV in BE. Notably, the line shape of this peak is slightly left-skewed, indicating that there is another minor peak component at the high BE side. Accordingly, we performed the peak fitting using two peak components, and the result showed that the minor peak was at 400.9 eV in BE (Table S3). According to previous studies,32,33 the main peak at 399.0 eV could be assigned to the nickelbonded N in the surface amidinate moiety (−Ni−amd). As for the minor peak, because it has a higher BE and judging from the atom electronegativity, a possible assignment could be the nitrogen in molecular Hamd, which was a byproduct of the reaction between Ni(amd)2 and the surface hydroxyl. The electronegativity of H is greater than that of Ni, so the expectation on the relative size of the BE was in line with the experimental results. Also, Hamd is a liquid at room C

DOI: 10.1021/acs.chemmater.9b01267 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 3. XPS spectra of (a) N 1s and (b) C 1s core-level emissions acquired after the dose of Ni(amd)2 on a SiOx/Si substrate at 25 °C and after the heat treatments at various temperatures.

temperature, so it is fairly reasonable that Hamd could be adsorbed on the SiOx surface at low temperature. To further corroborate this assignment, we additionally conducted a control experiment by directly dosing the molecular Hamd vapor on a clean SiOx/Si substrate and measuring the XPS spectra. As shown in Figure 4, there was only one single peak in the N 1s spectrum, and the BE of this peak was indeed at 400.9 eV, which well supports our assignment of the 400.9 eV peak. Figure 3a also shows the N 1s spectra of the sample after various heat treatments. Although the spectrum line shape remained roughly the same, the overall N 1s intensity considerably decreased as the heating temperature increased, and eventually the N 1s intensity reduced below the noise level for the temperature ≥350 °C. Peak fitting was also carried out, and the results are provided in Table S3. Note that as the main 399.0 eV peak was found to slightly shift to 399.2−399.3 eV upon the heat treatments, we use 399.2 eV to denote this peak component in the later discussion. Figure 3b shows the high-resolution XPS spectra of C 1s. After the dose of Ni(amd)2, the spectrum (25 °C) shows pronounced C 1s core-level emission signals. The line shape of this spectrum is considerably left-skewed because the amidinate moiety of Ni(amd)2 contains three types of carbons, namely, alkyl, amido, and amidine carbons with the numbers of 7, 2, and 1 atoms in each amidinate ligand, respectively. Previous studies showed that the amidinate ligand remained as a whole structure at room temperature,32,42,46 and therefore, three peak components with a fixed intensity ratio of 7:2:1 were used to fit the 25 °C spectrum. The fitting gave the BE values of 285.5, 286.7, and 287.5 eV for the alkyl, amido, and amidine carbons, respectively, and these numbers match well with previous studies on this compound32 as well as other studies on a similar-structure copper amidinate compound.47 After the heat treatments, the overall C 1s intensity considerably declined as the heating temperature increased. Careful peak-fitting analysis was carried out for all these C 1s spectra, and the detailed results are provided in Table S3. We found that for the heating temperature ≤200 °C, the spectrum line shape could be well fitted using the above group of the three carbon peak components, suggesting that the amidinate

Figure 4. XPS spectrum of N 1s core-level emission acquired after the dose of Hamd on a SiOx/Si substrate at 25 °C.

moiety remained as a whole structure at ≤200 °C. However, when the heating temperature was elevated above ≥250 °C, an additional peak at ∼285.0 eV had to be included in the fitting. This additional peak continued to develop as the heating temperature further increased and became dominant when the temperature was ≥350 °C. According to its BE (∼285.0 eV), this peak is likely associated with alkyl or alkenyl carbon,48,49 which could be the thermal decomposition product of the amidinate moiety.47 It is worth noting that no carbide carbon was observed during the entire heating process. The BE of the carbide carbon for NiCx was reported to be 283.4 eV30 (see also Figure S3), which is in stark contrast to the observed value as above. This observation is quite interesting because Ni(amd)2 was previously used with the H2 plasma for the successful ALD of NiCx,30 but the thermolysis herein suggests that heating itself is not sufficient to completely break up the precursor alkyl moiety nor to drive the formation of NiCx, at least at the initial ALD stage, which therefore points out that it is of great necessity to use the energy-enhanced species to drive the surface reactions toward affording metal carbides. Quantitative XPS analysis was performed by converting the XPS peak areas to relative atomic contents with the use of sensitivity factors. Given that this work deals with essentially a monolayer surface reaction, “surface” sensitivity factors (rather than the conventional bulk sensitivity factors) should be used for the quantification.50 The surface sensitivity factors were D

DOI: 10.1021/acs.chemmater.9b01267 Chem. Mater. XXXX, XXX, XXX−XXX

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suggest that both the surface-adsorbed Hamd and the nickelbonded amidinate moiety were liberated from the surface. Interestingly, the decline of the 399.2 eV N content started at a lower temperature (150 °C) than the onset of the metallic Ni formation [also true for the C(amd) content as shown in Figure 5c], which implies that the liberation of the amidinate on Ni could occur with a lower thermal budget than the agglomeration of the Ni atoms. A possible surface reaction to account for the amidinate liberation at low temperature could be that the moiety (−O−Ni−amd), when liberating the amidinate, affords a bridging O−Ni−O moiety with a neighboring oxygen on the surface. In fact, we found a gradually increasing trend in the BE of the Ni 856.5 eV peak, starting from 150 °C, to 857.1 eV at 400 °C (Table S1), and this trend could be explained by the replacement of the bonded amidinate with a second oxygen since the latter has a stronger electronegativity. It is worth noting that the bridging O−Ni−O moiety could also be formed at the first place when two neighboring surface hydroxyls react with one Ni(amd)2 molecule, and indeed, the experimental atomic ratio of N (399.2 eV):Ni was appreciably lower than that in the stoichiometry of −Ni−amd (∼1.6 vs 2) even at 25 °C, which suggests that the bridging O−Ni−O moiety was also formed at 25 °C (see Figure S5 for more discussion). On the other hand, the content of N 400.9 eV dropped quickly at 150 °C and became almost negligible at 200 °C, which suggests that the desorption of Hamd was almost complete at 200 °C. We also found that for the temperature ≤200 °C, the experimental atomic ratios of N(total)/C(total) were always roughly 0.2, and this number is consistent with the elemental composition of the amidinate moiety, which again suggests that the amidinate moiety remained as a whole structure below 200 °C. However, at 250 °C, the surface amidinate moiety started to decompose as the additional C content appeared in the plot at this temperature (Figure 5c). The decomposition of the amidinate moiety was suggested to involve the scission of the C−N bonds, which could form the surface alkyl or alkenyl groups.47 The additional C content continued to increase as the temperature was further elevated, and eventually, it became the only residue at ≥350 °C, without any observable amidinate carbon or nitrogen on the surface. On the basis of the above results, Scheme 1 summarizes the observed surface thermolytic behavior of Ni(amd)2 on SiOx. The Ni(amd)2 vapor could react with the SiOx surface at room temperature to afford both O−Ni−amd and O−Ni−O moieties on the surface, and the reaction could also afford the byproduct Hamd which could be adsorbed on the surface. The desorption of Hamd started at about 150 °C and mostly completed at 200 °C. The structure of the amidinate moiety could retain as a whole to at least 200 °C, but it started to

derived from the bulk counterparts by taking into account the difference in the photoelectron mean free paths50 (Table S4). The results of the quantification are shown in Figure 5, where

Figure 5. Variation of the atomic contents with respect to the heat treatment temperature. Data were extracted from the XPS (a) Ni 2p3/2 856.7 and 853.7 eV and O 1s 530.9 eV, (b) N 1s total and N 1s 399.2 and 400.9 eV, and (c) C 1s total, C 1s amidinate (amd), and additional C (285.0 eV) peaks.

all atomic contents were normalized to that of Ni in the 25 °C spectrum. The 856.7 and 853.7 eV Ni contents shown in Figure 5a correspond to the Ni−O bonds and metallic Ni, respectively. At temperature below 200 °C, the 853.7 eV (metallic) Ni content was essentially negligible, and almost all Ni formed the Ni−O bonds on the SiOx surface; but when the heating temperature was elevated beyond 200 °C, the 856.7 eV Ni content considerably decreased, while the 853.7 eV Ni content considerably increased, showing a crossover at 250 °C in the plot, which corresponds to the conversion of the Obonded Ni to metallic Ni at high temperature. This conversion is also accompanied by the concurrent reduction of the 530.9 eV O content (Figure 5a), which corresponds to the oxygen in the Ni−O bonds. The atomic contents of N and C also showed drastic reduction upon heating (Figure 5b,c), which suggests that the formation of metallic Ni was accompanied by liberating the organic species on the surface. Possibly, the surface amidinate ligand was liberated (at least partially) by heating, so the Ni atoms became mobile on the surface and therefore agglomerated to form clusters of metallic Ni. Indeed, the surface morphology images taken by ex situ atomic force microscopy (AFM) (Figure S4) showed that nanoparticles were formed after the heat treatment at 350 °C. Similar agglomeration processes have also been suggested in some other ALD studies (e.g., Pt,51 Cu,42 and NiS32). As for the N contents shown in Figure 5b, we noticed that both the 400.9 and 399.2 eV N contents dropped with temperature, which Scheme 1. Surface Thermolysis of Ni(amd)2 on SiOx

E

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Chemistry of Materials decompose at 250 °C, and the metallic Ni was also formed at that temperature. The thermolytic behavior of Ni(amd)2 has many important implications to practical ALD, especially considering that the Ni(amd)2 compound has recently been used in an increasing number of ALD processes for the nickel compounds (e.g., NiO,14,25 NiSx,9,28,29 NiCx,30 and Ni metal31). First, the thermolytic behavior suggests that Ni(amd)2 is suitable for a fairly wide temperature range of deposition because it exhibited very good surface reactivity at room temperature as well as good thermal stability up to at least 200 °C. The suitability for low-temperature ALD is particularly interesting, as it would be highly useful for the depositions on plastic or polymer substrates toward the applications in flexible electronics. Prior work has reported the successful ALD of Ni-containing films at temperature as low as 80 °C (Ni3Cx30), but the room-temperature ALD has not yet been assessed. The thermolytic behavior revealed here indicates that at least the surface reaction in the Ni(amd)2 half-cycle should be of no thermodynamic or kinetic barriers at room temperature, although a successful low-temperature ALD also requires good reactivity of the accompanying coreactant in the other half-cycle.52,53 Additionally, caution should also be paid to the possible condensation of Ni(amd)2 at low temperature, and perhaps long-time purge is needed after the Ni(amd)2 dose. Nevertheless, Ni(amd)2 is a highly promising precursor for low-temperature ALD. On the other hand, the high reactivity at low temperature also implies a potential issue for the use and storage of Ni(amd)2 because the Ni(amd)2 vapor could possibly react with the walls of the vapor-delivering tubes and the storage containers. This issue is often overlooked, partly because the surface reactions are still self-limiting, and the consumption of the precursor vapor is usually small. However, if the vapordelivering tubes contain a large overall surface area and the surface is not presaturated by the precursor, a significant amount of the precursor vapor could be consumed during delivery, thereby causing an apparent deficiency of the precursor vapor for the ALD film growth. This phenomenon has been observed in our ALD experiments, and therefore particular caution should be taken in this regard. At a high temperature above 250 °C, the surface amidinate moiety decomposed, which suggests that the incorporation of carbon impurities could be an issue for the ALD films. In fact, surface carbon species such as graphitic carbon are known to be stable on Ni,54,55 and they are often difficult to be completely removed. The decomposition of the surface amidinate moiety could also lead to nonideal CVD-like film growth, and once it occurs, the conformality of the deposited films may be compromised. On the other hand, the formation of the metallic Ni at ≥250 °C is also interesting because it may be useful for synthesizing nanoscale Ni clusters for catalysis applications,5,8,56 although possible side effects from the precursor ligand breakdown should be cautioned. For the temperature ranging from 25 to 200 °C, Ni(amd)2 is apparently a good ALD precursor. However, caution should be taken for the possible adsorption of Hamd on the surface because the adsorbed Hamd may be trapped as the impurity inside the ALD film and/or at the film−substrate interface. Therefore, if the goal is to achieve a low impurity, a relatively higher deposition temperature (e.g., at 150−200 °C) would be favored.

To demonstrate the important value of the above implications, we further investigated an example of using Ni(amd)2 for ALD, that is, to use Ni(amd)2 and H2O for the ALD of NiO. This ALD NiO process has been reported in several papers,13,14,25,26 but all those papers focused on the applications of the deposited NiO films; no details on the ALD process itself have been carefully reported so far, and in particular, the impurity of the deposited NiO has not been carefully assessed. Also, all reported deposition temperatures were above 150 °C, and thus whether lower-temperature ALD is feasible still remains a question. To this end, we first explored the ALD of NiO at a low temperature of 90 °C. Indeed, we found that the NiO film could be deposited at 90 °C with a growth rate of 0.8 Å/cycle, which demonstrates the feasibility of Ni(amd)2 for low-temperature ALD. Then, we studied the impurity of the ALD NiO films deposited at low and high temperatures. As per the previous discussion, two representative deposition temperatures of 90 and 200 °C were chosen for comparison, and the deposited NiO films (150 cycles) were carefully examined by both XPS and SIMS for their elemental depth profiles. Figure 6 shows the XPS depth

Figure 6. XPS elemental depth profiles for the ALD NiO films (150 cycles) deposited at (a) 90 and (b) 200 °C on SiOx/Si. Gray color zone denotes the interface between NiO and SiOx. To highlight the small numbers, the atomic ratios of C and N were enlarged by 5 times in the plots.

profiles for the ALD NiO films deposited on SiOx/Si at 90 and 200 °C. Notably, there is a significant difference in the impurity levels of C and N at the interface between NiO and SiOx. For the low deposition temperature of 90 °C, the C and N impurities tended to accumulate at the interface and reached the maxima of 3 and 1 at. %, respectively, whereas for the deposition at 200 °C, there was no such impurity accumulation at the interface, and the C level was only ∼0.8 at. % and the N level was below the detection limit (