Atomic Layer Deposition of Cobalt Carbide Thin Films from Cobalt

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Atomic Layer Deposition of Cobalt Carbide Thin Films from Cobalt Amidinate and Hydrogen Plasma Qipeng Fan, Zheng Guo, Zhuangzhi Li, Zhengduo Wang, Lizhen Yang, Qiang Chen, Zhongwei Liu, and Xinwei Wang ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00006 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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ACS Applied Electronic Materials

Atomic Layer Deposition of Cobalt Carbide Thin Films from Cobalt Amidinate and Hydrogen Plasma Qipeng Fan,1,a Zheng Guo,2,a Zhuangzhi Li,3 Zhengduo Wang,1 Lizhen Yang,1 Qiang Chen,1 Zhongwei Liu,1,* and Xinwei Wang2,*

1

Laboratory of Plasma Physics and Materials, Beijing Institute of Graphic Communication,

Beijing 102600, China

2

School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen

518055, China

3

College of Physics and Information Engineering, Hebei Normal University, Shijiazhuang

050024, China

* E-mail: [email protected] (Z.L.); [email protected] (X.W.)

a

These authors contributed equally.

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ABSTRACT Atomic layer deposition (ALD) of cobalt carbide thin films is reported by using bis(N,N-diisopropylacetamidinato)cobalt(II) (Co(amd)2) and H2 plasma. The process shows a good self-limiting ALD film growth behavior for a fairly wide temperature range from 70−160 C, and the growth rate is 0.066 nm/cycle for the deposition within the temperature range. The deposited cobalt carbide thin films are generally smooth and pure, and the film composition is approximately Co3C0.7 for the deposition at 80−200 C. Notably, all the carbon in the as-deposited films forms cobalt carbide, and no carbon-carbon bonds are detected by X-ray photoelectron spectroscopy. Raman spectroscopy also confirms the absence of graphite or amorphous carbon in the as-deposited films. The films are nanopolycrystalline as deposited, and the crystal structure is the hexagonal Co3C structure. The films can decompose into hcp-Co metal and amorphous carbon upon the thermal annealing in N2 at 400 C. The resistivity and magnetization of the as-deposited films are also characterized. It is further shown that, using this plasma-assisted ALD process, highly conformal cobalt carbide films can be deposited into the trench structures with a high aspect ratio of 20:1. In the last, the ALD growth chemistry is studied by using in-situ quartz crystal microbalance (QCM) technique, and the QCM results suggest that the structure of the amidinate ligand in the Co(amd)2 precursor largely falls apart upon its reaction with the surface during the ALD.

Keywords: atomic layer deposition, cobalt carbide, cobalt amidinate precursor, hydrogen plasma, conformal film coating

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INTRODUCTION Transition-metal carbides (TMCs) have recently aroused great attention for their intriguing electronic structures1,2 and broad applications in energy storage,3 (electro)catalysis,4 biosensing,5 and biomedicine.6 Among the TMCs, cobalt carbides (e.g., Co2C, Co3C) are of particular interest, as they have shown many promising applications in various areas. For instance, in catalysis, cobalt carbide has been identified as the crucial catalytic site for ethanol dehydrogenation,7 and the nanoprisms of cobalt carbide could even enable the direct production of lower olefins from syngas under mild conditions.8 In microelectronics, cobalt-carbon films have been applied as the intermediate layers to fabricate CoSi2 contacts.9 Also, cobalt carbide is generally ferromagnetic, and the mixed phase of Co2C and Co3C has recently been shown with a fairly high coercivity at room temperature,10 which is highly promising for the use as an alternative to rare-earth magnets.11 Very recently, nanoparticular cobalt carbide was also reported as a good electrocatalyst for hydrogen evolution reaction.12 Traditionally, cobalt carbide was synthesized by wet-chemistry methods,12 mechanical alloying,13 reactive magnetron sputtering,14,15 and chemical vapor deposition.16 Recently, atomic layer deposition (ALD) has aroused great interest for the synthesis of nanomaterials.17-19 ALD utilizes alternate self-limiting surface reactions to grow the target material one layer at a time, and therefore it can enable uniform conformal film coatings on the structures of almost any geometries.20 ALD is also featured for its atomic-precise control of

film

thickness,

remarkable

large-scale

uniformity,

and

extraordinary

process

reproducibility. With these merits, ALD has been employed in numerous designs of novel nanomaterials and applications in cutting-edge technologies.17-19, 21-28

3



Over the years, ALD has grown very fast with over one thousand ALD processes developed to date for numerous types of materials.29 However, the reported ALD processes specifically targeting for metal carbides (Ni3Cx,30-32 Co2C,33 TiAlC,34 ErC2,35 WCx,36,37 and MoCx38) are still considerably few comparing to metal oxides or metals,29 despite that carbon is a common impurity in the ALD metal films. Therefore, there is certainly a great need to carefully study the carbon in the ALD metal films and develop new ALD processes for metal carbides. As for the ALD of cobalt carbide, a process using cobalt acetylacetonate (Co(acac)2) and propanol was recently reported.33 However, the process required a high deposition temperature of >300 ºC, and perhaps due to the partial thermal decomposition of Co(acac)2, the resultant films were rough and mixed of Co2C and Co and also possibly contained nanoclusters of carbon.33 On the other hand, in some ALD reports targeting for pure Co metal,39-41 appreciable carbon content was observed in film as impurity, but detailed characterizations on the carbon (or carbide) itself were not carried out. Nevertheless, these reports provided useful insights toward the development of new ALD processes for cobalt carbide.

Herein, we report a new plasma-assisted ALD process of cobalt carbide, by using a volatile amidinate-type cobalt precursor, i.e., bis(N,N-diisopropylacetamidinato)cobalt(II) (Co(amd)2), along with H2 plasma. The Co(amd)2 compound does not deposit film with molecular H2 at low temperature (≤260 C).42 We show that by using the highly reactive H2 plasma, the deposition can be processed at a low temperature (70 − 160 C) with good ALD film growth behavior. Careful film characterizations were carried out, and notably, we found that all the carbon in the as-deposited films formed cobalt carbide, indicating that it is a good 4


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ALD process for depositing cobalt carbide.

EXPERIMENTAL SECTION

Plasma-assisted ALD of Cobalt Carbide. The deposition of cobalt carbide was conducted in a home-built tubular ALD reactor, using Co(amd)2 and H2 plasma. The details of the ALD reactor setup were described in our previous publication.43 Briefly, the deposition chamber consisted of a 60 cm long quartz tube (40 mm in diameter). Inside the tube was placed a flat sample holder in the center zone. Copper coil was wrapped on the upstream part of the tube. Through the coil, pulsed radiofrequency (RF) power (13.56 MHz) of 40–80 W was supplied to generate the H2 plasma inside the upstream zone of the quartz tube. The Co(amd)2 precursor was placed in a glass container, and it was heated to 50 C to afford sufficient precursor vapor during the deposition. The Co(amd)2 vapor was delivered into the deposition chamber with the assist of high-purity N2 (99.999%) carrier gas at a constant flow rate of 50 sccm. High-purity H2 gas (99.999%) with a flow rate of 50 sccm was used as the discharge gas to generate the H2 plasma. It was previously reported that, without using plasma, Co(amd)2 and molecular H2 did not deposit any film at temperature below 260 C,42 and therefore molecular H2 gas was used as the purge gas during the plasma-assisted ALD process. This arrangement allows for convenient switch-on of the plasma, because it does not need the extra step to stabilize the gas flow as required for switching the gas types. The plasma RF power and the lengths of the precursor pulse, purge, and plasma pulse in each ALD cycle were systematically varied to investigate the film growth behavior. The deposition substrates were flat Si wafers and glass slides, and there was no appreciable difference for the films 5



deposited on these substrates. Prior to ALD, all the substrate samples were sequentially cleaned by acetone, methanol, and isopropanol, and then treated with H2 plasma for 1 min.

Characterizations of the ALD Films. The thickness of the ALD films was measured by first scratching the deposited films and then measuring the step profiles (Veeco, Dektak 150) across the scratched lines. For thick films, the obtained thicknesses were also verified by cross-sectional scanning electron microscopy (SEM) (Hitachi, SU8020). SEM and atomic force microscopy (AFM) (Veeco, diInnova) were employed to examine the film morphology. The film composition was analyzed by X-ray photoelectron spectroscopy (XPS) (Thermo Scientific, Escalab 250Xi). Raman spectra were taken with 532 nm excitation (Titan, RTS-2). The film crystallinity was analyzed by X-ray diffraction (XRD) (Rigaku, D/max-2200 PC). Transmission electron microscopy (TEM) (JEOL, JEM-3200FS) was used to examine the film microstructure. The film sheet resistance was measured by the four-point probes method (RTS-8). The measurement of magnetization was conducted on a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS-3). The magnetic hysteresis loop was measured with in-plane configuration, and the magnetic contribution from the Si substrate was subtracted. In-situ film growth behavior was investigated by depositing the film on a quartz crystal microbalance (QCM) with an Inficon SQC-310 controller to monitor the change of the oscillation frequency.

RESULTS AND DISCUSSION

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FIGURE 1. Growth rate of cobalt carbide as the functions of (a) the Co(amd)2 pulse length, (b) H2 plasma pulse length, (c) purge length following the Co(amd)2 pulse, and (d) plasma input power. Panel (a) inset displays the molecular structure of Co(amd)2. (e) Plot of film thickness with respect to the total ALD cycles. The linear fitting is shown as a dash line. (f) Growth rate of cobalt carbide as a function of deposition temperature. To study the ALD process of cobalt carbide, we first chose a representative ALD recipe as the following: each ALD cycle consisted of, sequentially, 5 s Co(amd)2 pulse, 15 s purge, 10 s H2 plasma pulse, and 10 s purge, and the plasma input power and the deposition temperature were respectively 60 W and 80 °C. Then, we individually varied these parameters while keeping fixed all the others to investigate the cobalt carbide film growth. Figure 1 presents the film growth behavior, where the reported film growth rates were calculated based on the thickness values of the 300-cycle films. The effect of the Co(amd)2 pulse length is displayed in Figure 1(a), where the film growth rate first increased from 0.015 to 0.066 nm/cycle as the pulse length increased from 1 to 5 s, and then the growth rate flattened out as the pulse length was greater than 5 s. Figure 1(b) shows a similar trend of the film growth rate with respect to the H2 plasma pulse length. As the plasma pulse length increased, the growth rate also initially increased and then flattened out at 0.066 nm/cycle when the plasma pulse 7



length was greater than 7 s. These results show that the reactions involved in this process were self-limiting, and the film growth in each ALD cycle reached saturation when the Co(amd)2 and H2 plasma pulse lengths exceeded 5 and 7 s, respectively. Figure 1(c) displays the effect of the purge time following the Co(amd)2 pulse, and the film growth rate reached constant as the purge length was greater than 5 s, which suggests that 5 s purge was sufficient for the complete removal of the reaction byproducts and unreacted excess precursors. The dependence of the plasma input RF power is shown in Figure 1(d). A slight increase in growth rate was observed when the power was initially increased from 40 to 50 W; when the power was further increased beyond 50 W, the growth rate remained almost the same. The plasma input power is positively correlated to the electron temperature and density in the plasma, and thus higher input power should be able to afford more hydrogen radicals. Therefore, the increase of the power would give out a similar effect to the increase of the plasma pulse length. The above results have demonstrated that the herein reported deposition process followed a typical ALD behavior, and the growth of cobalt carbide in each cycle was indeed self-limiting. Also, the deposition recipe we initially used was in the saturated film growth regime, and therefore, unless otherwise specified, this recipe was adopted in the following depositions.

Linear film growth behavior was investigated by varying the total ALD cycles from 100 to 600 cycles. Figure 1(e) plots the deposited film thickness with respect to the total ALD cycles, which appears a good linear dependence, well manifesting the characteristic layer-to-layer growth behavior for an ALD process. The linear fit of the data is also shown in Figure 1(e) as a dash line, and from the fitted slope, the film growth rate was 0.066 nm/cycle, 8


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which was identical as obtained previously from single-point thickness data. Also, the fitted intercept was approximately zero, which implies no nucleation delay for this process. We further investigated the temperature effect as shown in Figure 1(f). The growth rate was found to be almost constant at 0.066 nm/cycle for the deposition temperature ranging from 70 to 160 C. But, when the deposition temperature was increased to 180 C, the growth rate appeared a slight increase to 0.073 nm/cycle, and, when the temperature was further elevated to 200 C, the growth rate was found to considerably increase to 0.14 nm/cycle, possibly owing to the thermal decomposition of Co(amd)2.44 Nevertheless, the process exhibited an ALD temperature window of at least 70 − 160 C, and, in the following, we focused our characterizations on the films deposited within this temperature window.

FIGURE 2. (a,c,e) SEM and (b,d,f) AFM images for ~20 nm cobalt carbide films deposited at (a,b) 80 °C, (c,d) 120 °C, and (e,f) 160 °C by 300 ALD cycles. 9



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Figure 2 shows the SEM and AFM images of ~20 nm thick films deposited at 80 − 160 °C by 300 ALD cycles. The films were generally smooth, and the rms roughness values were fairly small as 0.509, 0.708, and 0.798 nm for the films deposited at 80, 120, and 160 °C, respectively. Some granular features were also observed in the SEM and AFM images, and these features suggest that the deposited films were polycrystalline.

FIGURE 3. XPS spectra for the ALD cobalt carbide films (~60 nm) deposited at 80 − 200 °C. (a) Survey spectra and high-resolution (b) Co 2p, (c) C 1s, (d) O 1s, and (e) N 1s spectra. The H2 plasma pulse length was 10 s. TABLE 1. Elemental composition of the cobalt carbide films deposited at 80 − 200 °C, with 10 s plasma pulse length. Data were extracted from the XPS results shown in Figure 3. N and O contents were both below 0.5 at.% (based on XPS detection limit) and therefore not included. There is about ±10% error in determining x. Deposition temperature (C)

Co (at.%)

C (at.%)

x in Co3Cx

80 120 160 200

80.7 80.3 80.8 80.5

18.5 19.1 18.5 18.6

0.69 0.71 0.69 0.69

10


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FIGURE 4. XPS spectra for the ALD cobalt carbide films (~60 nm) deposited at 80 C with various pulse lengths of the H2 plasma. (a) Survey spectra and high-resolution (b) Co 2p, (c) C 1s, (d) O 1s, and (e) N 1s spectra. TABLE 2. Elemental composition of the cobalt carbide films deposited at 80 C with various pulse lengths of the H2 plasma. Data were extracted from the XPS results shown in Figure 4. N and O contents were both below 0.5 at.% (based on XPS detection limit) and therefore not included. There is about ±10% error in determining x. Pulse length of H2 plasma (s)

Co (at.%)

C (at.%)

x in Co3Cx

5 10 15 20

80.6 80.7 84.6 85.9

18.8 18.5 14.5 13.3

0.70 0.69 0.51 0.46

XPS was used to analyze the chemical composition of the ALD cobalt carbide films. The spectra were collected after 60 s of 2 kV Ar+ sputtering to remove the adventitious carbon and oxide on the sample surface. Figure 3 displays the obtained XPS spectra for the ALD cobalt carbide films (~60 nm) deposited at various temperatures. As shown in Figure 3(a), all the survey spectra contained only the peaks associated with Co and C, suggesting that the films were quite pure. High-resolution spectra were further acquired for the core-level emissions of Co 2p, C 1s, N 1s, and O 1s. As shown in Figure 3(b) (also Figure S1a), the Co 2p core-level spectra display pairs of spin-orbit split peaks at the binding energies of 778.1 eV (Co 2p3/2) 11



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and 793.1 eV (Co 2p1/2), and these numbers are consistent with those for cobalt carbide33, 45. The Co 2p3/2 peak also has a weak broad satellite band at ~781.5 eV, which is due to the plasmon loss of the photoelectron.46, 47 It should be noted that the Co 2p binding energies for cobalt carbide and pure cobalt were too close to be unambiguously distinguished,33,

35

but

nevertheless we did not observe any other-type signals such as cobalt oxides.48 The C 1s core-level spectra are shown in Figure 3(c) (also Figure S1b). All the C 1s spectra display single peaks at 283.2 eV, and this number corresponds to the carbon in cobalt carbide.49, 50 Notably, no peaks corresponding to carbon-carbon bonds were observed (near 284.6 eV),49, 50 which indicates that all the carbon in the films formed cobalt carbide. As for the N 1s and O 1s spectra (Figure 3(d,e)), all the core-level emission signals were barely observable, and accordingly the N and O impurity levels were both below 0.5 at.% (taking into account the detection limit of XPS). We further calculated the atomic ratios of Co and C based on the Co 2p and C 1s peak areas, and the results are shown in Table 1 in the form of x in Co3Cx (vide infra). Provided a fixed H2 plasma pulse length of 10 s during the deposition, the x in Co3Cx was found to be approximately 0.70 for all the deposition temperatures (80−200 °C). We further investigated the plasma effect on the film composition by varying the H2 plasma pulse length during the deposition (at 80 °C). Figure 4 shows the corresponding XPS results (also Figure S2), and the calculated x are listed in Table 2. We found that x remained roughly constant at 0.70 for the plasma pulse length ≤10 s but started to decrease when the pulse length was ≥15 s. Although further mechanistic investigation is needed, this result might be related to the selective etching effect of the prolonged plasma exposure. Nevertheless, all the deposited films were highly pure, and no carbon-carbon bonds were detected from XPS. 12


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FIGURE 5. Raman spectra for the ALD cobalt carbide films (~80 nm) deposited at 80 C with various pulse lengths of the H2 plasma from 5 to 20 s. Spectra for a bare glass-slide substrate and an annealed cobalt carbide film (10 s plasma pulse length) are included for comparison. The Raman band at 1100 cm-1 was from the substrate. Raman spectroscopy was further used to examine the form of carbon in the deposited films. As reported previously,30,

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if carbon was present as nanocrystalline graphite or

amorphous carbon in a metal carbide matrix, the Raman spectrum should show the characteristic D and G bands of the carbon materials. Figure 5 shows the Raman spectra for the ALD cobalt carbide films (~80 nm) deposited at 80 C with various pulse lengths of the H2 plasm. All these spectra display only the Raman signals from the glass-slide substrate but no signals for the D and G bands. Therefore, the carbon atoms in the as-deposited films were likely isolated with each other just as in the Co3C crystal lattice, and this observation was also in good consistence with the XPS results. In addition, we also took the Raman measurement on a 10-s cobalt carbide film after annealing it at 400 C in N2 for 1 h. As also shown in Figure 5, the annealed sample displayed pronounced Raman signals of the D and G bands, which suggests that after the annealing the ALD cobalt carbide film decomposed into Co and amorphous carbon. 13



FIGURE 6. XRD results for the ALD cobalt carbide films (~80 nm) deposited at 80 C with various pulse lengths of the H2 plasma from 5 to 20 s. Patterns for a bare glass-slide substrate and an annealed cobalt carbide film (10 s plasma pulse length) are included for comparison. Figure 6 shows the XRD patterns of the ALD cobalt carbide films (~80 nm) deposited at 80 C with various pulse lengths of the H2 plasma, and for comparison purpose, the patterns for a bare substrate and the 400 C annealed cobalt carbide film are also plotted. For the as-deposited films, all the XRD patterns show a very broad diffraction peak at around 44. Although it was not possible to determine the crystal structure from this single XRD peak, we could use the Scherrer equation to estimate the crystallite size to be around ~4 nm. As will be shown later, this crystallite size was in good consistence with the direct observation of the crystallites under TEM. As for the annealed sample, the XRD pattern exhibits several peaks, which could be indexed to the (100), (002), (101), and (110) lattice planes of hcp-Co (PDF#05-0727). By using the Scherrer equation, the crystallite size of the annealed sample was estimated to be around 16 − 22 nm.

TEM was further employed to examine the ALD films. Figure 7(a) shows a representative TEM image for a cobalt carbide film (~26 nm) deposited at 80 C. The 14


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as-deposited film was found to be polycrystalline, and the average crystallite size was ~5 nm. Figure 7(b) displays the corresponding electron diffraction pattern, and the prominent diffraction rings indicate again that the as-deposited cobalt carbide film was polycrystalline. By carefully indexing the diffraction rings (see Table S1 for details), the crystal structure of the as-deposited cobalt carbide was determined to be the hexagonal Co3C crystal structure (PDF#43-1144). We also examined the annealed film by TEM. As shown in Figure 7(c), the annealed film was also polycrystalline and the crystallite size was considerably greater than that of the as-deposited film. Figure 7(d) shows the corresponding electron diffraction pattern, and the diffraction rings could be assigned to the hcp-Co structure (PDF#05-0727) (see Table S1 for details), which agreed well with the XRD results.

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FIGURE 7. (a,c) Representative TEM images and (b,d) electron diffraction patterns for (a,b) an as-deposited (80 C) cobalt carbide film (~26 nm) and (c,d) the film after 1 h annealing in N2 at 400 C.

FIGURE 8. Resistivity of the ALD cobalt carbide films as the functions of (a) the film thickness and (b) the reciprocal of the film thickness. The films were deposited at 80 C. We also measured the electrical resistivity of the ALD cobalt carbide films, as the resistivity is an important factor for applications in electronics and electrocatalysis. Figure 8(a) plots the resistivity of the as-deposited films (80 C) with respect to the film thickness. The lowest obtained resistivity was found to be 0.78 m cm for 40 nm thick films, and the resistivity increased as the films became thinner, because of the increased probability for the electrons to scatter with surfaces, interfaces, and grain boundaries. Based on a scattering-induced model,44, 52, 53 the resistivity  of the cobalt carbide may be expressed as  16


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= 0(1 + t0/t), where t is the film thickness, 0 is the bulk resistivity, and t0 is the characteristic electron scattering length to take into account the scattering effects from surfaces, interfaces, and grain boundaries. Accordingly, we plotted the resistivity  versus 1/t and conducted the linear fitting as shown in Figure 8(b). The fitted t0 and ρ0 were 17.9 ± 0.5 nm and 0.54 ± 0.01 mΩ cm, respectively, and the latter shows the conductive attribute of the ALD cobalt carbide.

FIGURE 9. (a) Room-temperature magnetic hysteresis loop and (b) the corresponding ZFC-FC curves for a ~80 nm ALD cobalt carbide film deposited at 80 C. The magnetic properties of the ALD cobalt carbide films were also characterized, using a SQUID magnetometer. Figure 9(a) shows the room-temperature magnetic hysteresis loop for a ~80 nm ALD cobalt carbide film deposited at 80 C. The as-deposited cobalt carbide film behaved as a ferromagnet at 300 K with saturation magnetization of 75 emu/g, remanent 17



magnetization of 48 emu/g, and small coercivity of 18 Oe, which suggests that the ALD cobalt carbide film could be used for magnetic applications such as data storage.54 Notably, the as-deposited film was nanocrystalline, and the crystallite size was much smaller than the typical size for ferromagnetic nanoparticles to be supermagnetic at room temperature.55 Therefore, we also carried out zero-field cooled (ZFC) and field cooled (FC) measurements (200 Oe) to study the magnetic blocking temperature of the cobalt carbide nanocrystallites in the film. As shown in Figure 9(b), the ZFC curve shows a maximum at ~80 K, which corresponds to an average blocking temperature of the nanocrystallites, and the two curves join at a much higher temperature of ~255 K, suggesting strong magnetic couplings between the cobalt carbide nanocrystallites.

FIGURE 10. SEM image showing a cross-section of an ALD cobalt carbide film deposited (at 80 C) inside a high-aspect-ratio (20:1) trench structure. The conformality of the ALD cobalt carbide films was evaluated by depositing the films into a deep narrow trench structure with a high aspect ratio of 20:1 (~14 μm in depth and 700 nm in width). Owing to the non-ideal Bosch fabrication process, the trench wall had some 18


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wavy structures. On this trench structure, we conducted the plasma-assisted ALD of cobalt carbide at 80 C using the aforementioned deposition recipe. Figure 10 shows the cross-sectional SEM image of the trench after the film deposition. The trench structure was conformally covered by a 60 nm thick cobalt carbide film throughout the entire trench. The uniform film coverage suggests that this plasma-assisted ALD process can be applied for the conformal film coatings of cobalt carbide on complex or porous 3D structures in general.

FIGURE 11. (a) Acquired QCM data for the plasma-assisted ALD of cobalt carbide at 80 C. (b,c) Enlarged views of the two boxed regions in panel a for (b) a normal deposition cycle and (c) a dummy cycle. (d,e) Baseline-subtracted QCM curves. Panels d and e display the baseline-subtracted curves shown in panels a and b, respectively. (f) Plot of m1, m2/m1, and m3/m1 with respect to the cycle number. The definitions of m1, m2, and m3 are illustrated in panel e. In this last, we employed the in-situ QCM technique to monitor the film growth behavior during ALD. The QCM technique is based on measuring the oscillation frequency of the 19



quartz crystal, and it is highly sensitive to the mass change of the film on it. However, the oscillation frequency is also influenced by temperature, and therefore any temperature fluctuation of the crystal, for instance due to the plasma heating effect, could cause artifacts in the measured curves.56 In addition, the charged plasma could also interfere with the oscillation circuits,57 and therefore special cares should be taken for the QCM measurements and data interpretation when plasma is used. Herein, we adopted a procedure previously used for this situation,43 by first running 100 dummy ALD cycles (without dosing Co(amd)2) prior to the normal deposition cycles. The dummy cycles allowed for the acquired QCM data to be stabilized without temperature drifting, and the dummy data also provided the baseline to be subtracted to account for the intermittent plasma effect. Figure 11(a) shows a set of the obtained QCM data for an ALD recipe consisting of sequentially 10 s Co(amd)2 pulse, 20 s purge, 10 s H2 plasma pulse, and 10 s purge. Figure 11(b,c) shows an enlarged view of the curves for a typical deposition cycle and a dummy cycle. Obviously, the plasma itself could cause apparent rise and fall in the QCM curve. To remove this plasma effect, the dummy cycle curve was used as the baseline to subtract from the original QCM data.43 Figure 11(d,e) shows the baseline-subtracted QCM curves, which allowed us to study the surface chemistry of the ALD process. As shown in Figure 11(e), during the first 10 s when Co(amd)2 was supplied, the QCM mass gain continuously increased to m3, implying the chemical reaction and possibly physisorption of Co(amd)2 on the surface. The mass gain then gradually decreased to m2 in the following 20 s purge, which possibly corresponded to the desorption of the weakly physisorbed Co(amd)2. After the plasma was switched on, the mass gain further decreased and reached m1 at the end of the cycle. The numbers of m1, m2, and m3 20


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were extracted for each cycle, and Figure 11(f) plots the obtained m1 as well as the ratios of m3/m1and m2/m1. Except for the first cycle (not shown in the plot), which was largely influenced by the initial substrate surface, all these numbers were fairly stable over the following ALD cycles, and the averaged values were m1 = 1.61±0.08 Hz, m2/m1 = 1.58±0.09, and m3/m1 = 5.42±0.25. Based on the previous ex-situ characterizations, the mass gain for a full ALD cycle (i.e., m1) should correspond to the growth of Co3C0.69 (or CoC0.23) in nominal stoichiometry. Assuming that the evolution of Co(amd)2 to afford CoC0.23 followed the two steps of

Co(amd)2 (g)  Co(CmNnHl) (s) + volatile species

(1)

Co(CmNnHl) (s) + H* (plasma)  CoC0.23 (s) + volatile species

(2)

in the Co(amd)2 and H2 plasma half-cycles, respectively, the effective molecular weight of the supposed surface intermediate Co(CmNnHl) could be calculated from m2/m1 to be 97.5±5.6, and this number corresponded to a fairly small molecular weight (38.6±5.6) for the supposed surface ligand CmNnHl. Indeed, CmNnHl was merely a nominal formula and could also be a statistically averaged formula, but its small molecular weight suggests that the structure of the amidinate ligand in Co(amd)2 largely fell apart upon its reaction with the surface. Previously, Ma et al. investigated the ALD surface chemistry of a similar metal amidinate compound of copper(I)-N,N’-di-sec-butylacetamidinate, and they showed that the copper amidinate compound could decompose on Ni or Cu metal at a fairly low temperature to afford chemisorbed

butene

from

the

terminal

sec-butyl

moieties

of

the

N,N’-di-sec-butylacetamidinate ligand via β-hydride elimination.58, 59 Given a similar ligand 21



structure in Co(amd)2, it was also possible for the N,N-diisopropylacetamidinate ligand of Co(amd)2 to undergo the β-hydride elimination to afford chemisorbed propene on the surface. Interestingly, the molecular weight of propene (i.e., 42) coincided with the measured value of CmNnHl within experimental error. In addition, the value of (m3 – m2)/m1 might reflect the

layers of the physisorbed Co(amd)2 molecules on each deposited Co atom, and based on our data, only 0.7 of such layer was expected, suggesting that no severe precursor condensation occurred during the deposition.

CONCLUSIONS

We reported a new plasma-assisted ALD process for cobalt carbide by using bis(N,N-diisopropylacetamidinato)cobalt(II) and H2 plasma. The process showed a good self-limiting ALD film growth behavior for a fairly wide temperature range of 70 − 160 C, and the growth rate was 0.066 nm/cycle for the deposition within the temperature range. The deposited cobalt carbide films were generally smooth and pure, and the film composition was found to be approximately Co3C0.7 for all the deposition temperatures from 80 − 200 C. Prolonged H2 plasma pulse length could reduce the carbon content in the films. Accordingly to high-resolution XPS, all the carbon in the as-deposited films formed cobalt carbide, and no carbon-carbon bonds were detected. Consistent results were also obtained from Raman spectroscopy, where no D and G signals of nanocrystalline graphite or amorphous carbon were observed for the as-deposited films. The films were nanopolycrystalline as deposited, and the crystal structure was determined by TEM electron diffraction to be the hexagonal Co3C structure. The films could decompose into hcp-Co metal and amorphous carbon upon 22


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400 C annealing in N2. The electrical resistivity of the as-deposited films was characterized for various thicknesses, and in particular the resistivity was 0.78 m cm for 40 nm films. The magnetic properties were also characterized, and the room-temperature magnetic hysteresis loop showed that the saturation magnetization, remanent magnetization, and coercivity were 75 emu/g, 48 emu/g, and 18 Oe, respectively. We further demonstrated that, using this plasma-assisted ALD process, highly conformal cobalt carbide films could be deposited into 20:1 high-aspect-ratio trenches, which indicates that this process is of broad applicability for conformal film coatings on complex or porous 3D structures in general. In the last, we studied the ALD growth chemistry by using the in-situ QCM technique, and the QCM results suggest that the structure of the amidinate ligand in the Co(amd)2 precursor largely fell apart upon its reaction with the surface during the ALD.

SUPPORTING INFORMATION

Analysis of TEM electron diffraction pattern and additional XPS plots.

ACKNOWLEDGMENTS This work is financially supported by NSFC (11775028, 51672011, and 11505013), Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No. 2015A030306036), Beijing Municipal Natural Science Foundation (4162024), BIGC Project (Ea201801 and 12000400001), and Shenzhen Science and Technology Innovation Committee (Grant No. JCYJ20170303140959031).

REFERENCES (1) Hwu, H. H.; Chen, J. G., Surface Chemistry of Transition Metal Carbides. Chem. Rev. 23



2005, 105, 185-212. (2) Gogotsi, Y., Transition metal carbides go 2D. Nat. Mater. 2015, 14, 1079. (3) Anasori, B.; Lukatskaya, M. R.; Gogotsi, Y., 2D metal carbides and nitrides (MXenes) for energy storage. Nat. Rev. Mater. 2017, 2, 16098. (4) Wan, W.; Tackett, B. M.; Chen, J. G., Reactions of water and C1 molecules on carbide and metal-modified carbide surfaces. Chem. Soc. Rev. 2017, 46, 1807-1823. (5) Xu, B.; Zhu, M.; Zhang, W.; Zhen, X.; Pei, Z.; Xue, Q.; Zhi, C.; Shi, P., Ultrathin MXene-Micropattern-Based Field-Effect Transistor for Probing Neural Activity. Adv. Mater. 2016, 28, 3333-3339. (6) Yu, J.; Yang, C.; Li, J.; Ding, Y.; Zhang, L.; Yousaf, M. Z.; Lin, J.; Pang, R.; Wei, L.; Xu, L.; Sheng, F.; Li, C.; Li, G.; Zhao, L.; Hou, Y., Multifunctional Fe5C2 Nanoparticles: A Targeted Theranostic Platform for Magnetic Resonance Imaging and Photoacoustic Tomography-Guided Photothermal Therapy. Adv. Mater. 2014, 26, 4114-4120. (7) Rodriguez-Gomez, A.; Holgado, J. P.; Caballero, A., Cobalt Carbide Identified as Catalytic Site for the Dehydrogenation of Ethanol to Acetaldehyde. ACS Catal. 2017, 7, 5243-5247. (8) Zhong, L. S.; Yu, F.; An, Y. L.; Zhao, Y. H.; Sun, Y. H.; Li, Z. J.; Lin, T. J.; Lin, Y. J.; Qi, X. Z.; Dai, Y. Y.; Gu, L.; Hu, J. S.; Jin, S. F.; Shen, Q.; Wang, H., Cobalt carbide nanoprisms for direct production of lower olefins from syngas. Nature 2016, 538, 84-87. (9) Rhee, H. S.; Ahn, B. T.; Sohn, D. K., Growth behavior and thermal stability of epitaxial CoSi2 layer from cobalt–carbon films on (100) Si substrate. J. Appl. Phys. 1999, 86, 3452-3459. (10) Carroll, K. J.; Huba, Z. J.; Spurgeon, S. R.; Qian, M.; Khanna, S. N.; Hudgins, D. M.; Taheri, M. L.; Carpenter, E. E., Magnetic properties of Co2C and Co3C nanoparticles and their assemblies. Appl. Phys. Lett. 2012, 101, 012409. (11) Zamanpour, M.; Bennett, S.; Taheri, P.; Chen, Y.; Harris, V. G., Magnetic properties and scale-up of nanostructured cobalt carbide permanent magnetic powders. J. Appl. Phys. 2014, 115, 17A747. (12) Li, S.; Yang, C.; Yin, Z.; Yang, H.; Chen, Y.; Lin, L.; Li, M.; Li, W.; Hu, G.; Ma, D., Wet-chemistry synthesis of cobalt carbide nanoparticles as highly active and stable electrocatalyst for hydrogen evolution reaction. Nano Res. 2017, 10, 1322-1328. (13) García-Torres, J.; Gómez, E.; Vallés, E., Modulation of magnetic and structural properties of cobalt thin films by means of electrodeposition. J. Appl. Electrochem. 2009, 39, 233-240. (14) Tajima, S.; Hirano, S.-I., Synthesis and properties of cobalt carbide film by radio-frequency magnetron sputtering. J. Mater. Sci. Lett. 1992, 11, 22-25. (15) Konno, T. J.; Shoji, K.; Sumiyama, K.; Suzuki, K., Structure and magnetic properties of co-sputtered Co–C thin films. J. Magn. Magn. Mater.. 1999, 195, 9-18. (16) Premkumar, P. A.; Turchanin, A.; Bahlawane, N., Effect of Solvent on the Growth of Co and Co2C Using Pulsed-Spray Evaporation Chemical Vapor Deposition. Chem. Mater. 2007, 19, 6206-6211. (17) Meng, X.; Wang, X.; Geng, D.; Ozgit-Akgun, C.; Schneider, N.; Elam, J. W., Atomic layer deposition for nanomaterial synthesis and functionalization in energy technology. Mater. Horiz. 2017, 4, 133-154. 24


Page 24 of 28

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


(18) Marichy, C.; Bechelany, M.; Pinna, N., Atomic Layer Deposition of Nanostructured Materials for Energy and Environmental Applications. Adv. Mater. 2012, 24, 1017-1032. (19) Palmstrom, A. F.; Santra, P. K.; Bent, S. F., Atomic layer deposition in nanostructured photovoltaics: tuning optical, electronic and surface properties. Nanoscale 2015, 7, 12266-12283. (20) George, S. M., Atomic layer deposition: An overview. Chem. Rev. 2010, 110, 111-131. (21) Gao, Y.; Shao, Y.; Yan, L.; Li, H.; Su, Y.; Meng, H.; Wang, X., Efficient Charge Injection in Organic Field-Effect Transistors Enabled by Low-Temperature Atomic Layer Deposition of Ultrathin VOx Interlayer. Adv. Funct. Mater. 2016, 26, 4456-4463. (22) Lu, J.; Fu, B.; Kung, M. C.; Xiao, G.; Elam, J. W.; Kung, H. H.; Stair, P. C., Coking- and Sintering-Resistant Palladium Catalysts Achieved Through Atomic Layer Deposition. Science 2012, 335, 1205-1208. (23) Shao, Y.; Guo, Z.; Li, H.; Su, Y.; Wang, X., Atomic Layer Deposition of Iron Sulfide and Its Application as a Catalyst in the Hydrogenation of Azobenzenes. Angew. Chem. Int. Ed. 2017, 56, 3226-3231. (24) Xiong, W.; Guo, Z.; Li, H.; Zhao, R.; Wang, X., Rational Bottom-Up Engineering of Electrocatalysts by Atomic Layer Deposition: A Case Study of FexCo1–xSy-Based Catalysts for Electrochemical Hydrogen Evolution. ACS Energy Lett. 2017, 2, 2778-2785. (25) Savin, H.; Repo, P.; von Gastrow, G.; Ortega, P.; Calle, E.; Garín, M.; Alcubilla, R., Black silicon solar cells with interdigitated back-contacts achieve 22.1% efficiency. Nat. Nanotechnol. 2015, 10, 624. (26) Guo, Z.; Wang, X., Atomic Layer Deposition of the Metal Pyrites FeS2, CoS2, and NiS2. Angew. Chem. Int. Ed. 2018, 57, 5898-5902. (27) Ahmed, B.; Xia, C.; Alshareef, H. N., Electrode surface engineering by atomic layer deposition: A promising pathway toward better energy storage. Nano Today 2016, 11, 250-271. (28) Li, H.; Guo, Z.; Wang, X., Atomic-layer-deposited ultrathin Co9S8 on carbon nanotubes: an efficient bifunctional electrocatalyst for oxygen evolution/reduction reactions and rechargeable Zn–air batteries. J. Mater. Chem. A 2017, 5, 21353-21361. (29) Miikkulainen, V.; Leskelä, M.; Ritala, M.; Puurunen, R. L., Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends. J. Appl. Phys. 2013, 113, 021301. (30) Sarr, M.; Bahlawane, N.; Arl, D.; Dossot, M.; McRae, E.; Lenoble, D., Tailoring the Properties of Atomic Layer Deposited Nickel and Nickel Carbide Thin Films via Chain-Length Control of the Alcohol Reducing Agents. J. Phys. Chem. C 2014, 118, 23385-23392. (31) Guo, Q.; Guo, Z.; Shi, J.; Xiong, W.; Zhang, H.; Chen, Q.; Liu, Z.; Wang, X., Atomic Layer Deposition of Nickel Carbide from a Nickel Amidinate Precursor and Hydrogen Plasma. ACS Appl. Mater. Interfaces 2018, 10, 8384-8390. (32) Xiong, W.; Guo, Q.; Guo, Z.; Li, H.; Zhao, R.; Chen, Q.; Liu, Z.; Wang, X., Atomic layer deposition of nickel carbide for supercapacitors and electrocatalytic hydrogen evolution. J. Mater. Chem. A 2018, 6, 4297-4304. (33) Sarr, M.; Bahlawane, N.; Arl, D.; Dossot, M.; McRae, E.; Lenoble, D., Atomic layer deposition of cobalt carbide films and their magnetic properties using propanol as a reducing 25



agent. Appl. Surf. Sci. 2016, 379, 523-529. (34) Moon, J.; Ahn, H. J.; Seo, Y.; Lee, T. I.; Kim, C.; Rho, I. C.; Kim, C. H.; Hwang, W. S.; Cho, B. J., The Work Function Behavior of Aluminum-Doped Titanium Carbide Grown by Atomic Layer Deposition. IEEE Trans. Electron Dev. 2016, 63, 1423-1427. (35) Ahn, H. J.; Moon, J.; Koh, S.; Seo, Y.; Kim, C.; Rho, I. C.; Kim, C. H.; Hwang, W. S.; Cho, B. J., Very Low-Work-Function ALD-Erbium Carbide (ErC2) Metal Electrode on High-K Dielectrics. IEEE Trans. Electron Dev. 2016, 63, 2858-2863. (36) Kim, D.-H.; Kim, Y. J.; Song, Y. S.; Lee, B.-T.; Kim, J. H.; Suh, S.; Gordon, R., Characteristics of Tungsten Carbide Films Prepared by Plasma-Assisted ALD Using Bis(tert-butylimido)bis(dimethylamido)tungsten. J. Electrochem. Soc. 2003, 150, C740-C744. (37) Kim, J. B.; Kim, S.-H.; Han, W. S.; Lee, D.-J., Atomic layer deposited nanocrystalline tungsten carbides thin films as a metal gate and diffusion barrier for Cu metallization. J.Vac. Sci. Technol. A 2016, 34, 041504. (38) Bertuch, A.; Keller, B. D.; Ferralis, N.; Grossman, J. C.; Sundaram, G., Plasma enhanced atomic layer deposition of molybdenum carbide and nitride with bis(tert-butylimido)bis(dimethylamido) molybdenum. J. Vac. Sci. Technol. A 2017, 35, 01B141. (39) Lee, H.-B.-R.; Kim, H., High-Quality Cobalt Thin Films by Plasma-Enhanced Atomic Layer Deposition. Electrochem. Solid-State Lett. 2006, 9, G323-G325. (40) Jae-Min, K.; Han-Bo-Ram, L.; Clement, L.; Christian, D.; Julien, G.; Hyungjun, K., Plasma-Enhanced Atomic Layer Deposition of Cobalt Using Cyclopentadienyl Isopropyl Acetamidinato-Cobalt as a Precursor. Jpn. J. Appl.Phys. 2010, 49, 05FA10. (41) Park, J.-H.; Moon, D.-Y.; Han, D.-S.; Kang, Y.-J.; Shin, S.-R.; Jeon, H.-T.; Park, J.-W., Plasma-enhanced atomic layer deposition (PEALD) of cobalt thin films for copper direct electroplating. Surf. Coat. Technol. 2014, 259, Part A, 98-101. (42) Lim, B. S.; Rahtu, A.; Gordon, R. G., Atomic layer deposition of transition metals. Nat Mater 2003, 2, 749-754. (43) Guo, Z.; Li, H.; Chen, Q.; Sang, L.; Yang, L.; Liu, Z.; Wang, X., Low-Temperature Atomic Layer Deposition of High Purity, Smooth, Low Resistivity Copper Films by Using Amidinate Precursor and Hydrogen Plasma. Chem. Mater. 2015, 27, 5988-5996. (44) Li, H.; Gao, Y.; Shao, Y.; Su, Y.; Wang, X., Vapor-Phase Atomic Layer Deposition of Co9S8 and Its Application for Supercapacitors. Nano Lett. 2015, 15, 6689-6695. (45) Wang, H.; Wong, S. P.; Cheung, W. Y.; Ke, N.; Wen, G. H.; Zhang, X. X.; Kwok, R. W. M., Magnetic properties and structure evolution of amorphous Co–C nanocomposite films prepared by pulsed filtered vacuum arc deposition. J. Appl. Phys. 2000, 88, 4919-4921. (46) Biesinger, M. C.; Payne, B. P.; Grosvenor, A. P.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C., Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni. Appl. Surf. Sci. 2011, 257, 2717-2730. (47) Grosvenor, A. P.; Wik, S. D.; Cavell, R. G.; Mar, A., Examination of the Bonding in Binary Transition-Metal Monophosphides MP (M = Cr, Mn, Fe, Co) by X-Ray Photoelectron Spectroscopy. Inorg. Chem. 2005, 44, 8988-8998. (48) Petitto, S. C.; Marsh, E. M.; Carson, G. A.; Langell, M. A., Cobalt oxide surface chemistry: The interaction of CoO(100), Co3O4(110) and Co3O4(111) with oxygen and water. J. Mol. Catal. A: Chem. 2008, 281, 49-58. 26


Page 26 of 28

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


(49) Xiong, J.; Ding, Y.; Wang, T.; Yan, L.; Chen, W.; Zhu, H.; Lu, Y., The formation of Co2C species in activated carbon supported cobalt-based catalysts and its impact on Fischer–Tropsch reaction. Catal. Lett. 2005, 102, 265-269. (50) Wang, H.; Wong, S. P.; Cheung, W. Y.; Ke, N.; Lau, W. F.; Chiah, M. F.; Zhang, X. X., Structural and magnetic properties of Co65C35 nanocomposite films prepared by pulsed filtered vacuum arc deposition. Mater. Sci. Eng., C 2001, 16, 147-151. (51) Bayer, B. C.; Bosworth, D. A.; Michaelis, F. B.; Blume, R.; Habler, G.; Abart, R.; Weatherup, R. S.; Kidambi, P. R.; Baumberg, J. J.; Knop-Gericke, A.; Schloegl, R.; Baehtz, C.; Barber, Z. H.; Meyer, J. C.; Hofmann, S., In Situ Observations of Phase Transitions in Metastable Nickel (Carbide)/Carbon Nanocomposites. J. Phys. Chem. C 2016, 120, 22571-22584. (52) Rossnagel, S. M.; Kuan, T. S., Alteration of Cu conductivity in the size effect regime. J. Vac. Sci. Technol., B 2004, 22, 240-247. (53) Wang, X. W.; Gordon, R. G., Smooth, Low-Resistance, Pinhole-Free, Conformal Ruthenium Films by Pulsed Chemical Vapor Deposition. ECS J. Solid State Sci. Technol. 2013, 2, N41-N44. (54) El-Gendy, A. A.; Qian, M.; Huba, Z. J.; Khanna, S. N.; Carpenter, E. E., Enhanced magnetic anisotropy in cobalt-carbide nanoparticles. Appl. Phys. Lett. 2014, 104. (55) Zhang, H.; Liang, C.; Liu, J.; Tian, Z.; Shao, G., The formation of onion-like carbon-encapsulated cobalt carbide core/shell nanoparticles by the laser ablation of metallic cobalt in acetone. Carbon 2013, 55, 108-115. (56) Rocklein, M. N.; George, S. M., Temperature-Induced Apparent Mass Changes Observed during Quartz Crystal Microbalance Measurements of Atomic Layer Deposition. Anal. Chem. 2003, 75, 4975-4982. (57) Heil, S. B. S.; Hemmen, J. L. v.; Sanden, M. C. M. v. d.; Kessels, W. M. M., Reaction mechanisms during plasma-assisted atomic layer deposition of metal oxides: A case study for Al2O3. J. Appl. Phys. 2008, 103, 103302. (58) Ma, Q.; Guo, H.; Gordon, R. G.; Zaera, F., Surface Chemistry of Copper(I) Acetamidinates in Connection with Atomic Layer Deposition (ALD) Processes. Chem. Mater. 2011, 23, 3325-3334. (59) Ma, Q.; Zaera, F.; Gordon, R. G., Thermal chemistry of copper(I)-N,N'-di-sec-butylacetamidinate on Cu(110) single-crystal surfaces. J. Vac. Sci. Technol. A 2012, 30, 01A114.

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Atomic Layer Deposition of Cobalt Carbide Thin Films from Cobalt

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