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Focused Electron Beam Deposition of Nanowires from Cobalt Tricarbonyl Nitrosyl (Co(CO)3NO) Precursor Gian Carlo Gazzadi,*,† Hans Mulders,‡ Piet Trompenaars,‡ Alberto Ghirri,† Marco Affronte,†,§ Vincenzo Grillo,† and Stefano Frabboni†,§ †
S3 Center, Nanoscience Institute CNR, Via Campi 213/a, 41125 Modena, Italy FEI Electron Optics, Achtseweg Noord 5, 5600 KA Eindhoven, The Netherlands § Dipartimento di Fisica, Universita di Modena e Reggio Emilia, Via Campi 213/a, 41125 Modena, Italy ‡
ABSTRACT: Nanowires deposited by focused electron beaminduced deposition (FEBID) of cobalt tricarbonyl nitrosyl (Co(CO)3NO) precursor have been thoroughly characterized from an electrical, magnetic, and structural point of view. Deposit composition and deposition yield have been studied as a function of beam energy and current. Atomic concentrations are weakly dependent on beam parameters and have average values of 49 at % for Co, 27 at % for O, 14 at % for N, and 10 at % for C. Deposition yield decreases as the beam energy increases, and strong enhancement (67) is observed for deposition at 130 C substrate temperature. FEBID nanowires are highly resistive (F = 6.3 mΩ cm at RT), and resistivity increases as T decreases with a power-law behavior typical of metalinsulator (MI) nanogranular systems. Nonmagnetic behavior is revealed by magnetoresistance (MR) measurements. After 400 C vacuum annealing, conductivity of the nanowire is greatly improved (F = 62 μΩ cm at RT), and a metallic-like resistivity is fully recovered. MR angular plots display a (cos θ)2 dependence typical of anisotropic MR (AMR) in ferromagnets, with AMR values of 0.85%. Co concentration in the deposit is not significantly increased though. TEM structural analysis reveals that before annealing the deposit has a CoO fcc structural phase with nanograins size around 1 nm. After annealing, a new Co hcp phase shows up beside CoO fcc, and coarsening (10 15 nm) and interconnection of the Co nanograins are observed, providing the conditions for ferromagnetism and metallic electrical transport.
1. INTRODUCTION Device nanoprototyping, nanostructures fabrication, and in situ experiments on nanosized samples (e.g., nanoparticles and nanotubes) are typical cases in nanoscience research where an additive nanolithography is required that can be applied directly on the sample in a single-step, maskless approach. Focused electron beam- and focused ion beam-induced depositions (FEBID and FIBID, respectively)1,2 meet this requirement and are being increasingly employed thanks to the easy availability on common lab equipment like dual beam systems, focused ion beamscanning electron microscope (FIB-SEM), which also offer the advantage of high resolution navigation on the sample. Deposition on the nanoscale is obtained through the dissociation of precursor gas molecules adsorbed on the substrate under irradiation of the focused primary beam. (See Figure 1a.) Ideally, for a metallorganic precursor, the metallic part of the molecule is deposited, and the volatile organics are pumped out. To extend the applicability and improve the performance of these techniques, the characterization of new gas precursors is one key point. Among the variety of deposited materials, those having magnetic properties are of great interest from both an applicative and fundamental viewpoint. Cobalt FEBID from cobalt carbonyl precursor (Co2(CO)8), in particular, has been employed to fabricate magnetic force microscopy (MFM) tips,3 crossbar Hall r 2011 American Chemical Society
nanosensors,4 and, recently, the study of magnetization dynamics in high-purity Co ferromagnetic nanowires fabricated by FEBID has been reported.5 A major drawback of the cobalt carbonyl molecule is the poor stability under vacuum storage due to the low decomposition temperature (6070 C). This causes spontaneous dissociation and polymerization through release of 4(CO) and formation of Co4(CO)12, building up a high pressure in the gas container with severe bursts at the opening. Polymerization residues can also damage the high voltage parts of the equipment. To explore alternative candidates for Co deposition, we have recently investigated the FEBID and FIBID of cobalt tricarbonyl nitrosyl (Co(CO)3NO), a precursor with higher thermal stability (decomposition temperature 130140 C) compared with cobalt carbonyl.6 A preliminary electrical and magnetic characterization7 revealed that the as-deposited FEBID nanowires were highly resistive and had an increasing trend of resistivity at low T, whereas the FIBID ones were more conductive and metallic. Both nanowires showed nonmagnetic response under MR measurements. After performing 400 C vacuum annealing, dramatic changes were observed, especially in the FEBID nanowires, becoming much more Received: July 11, 2011 Revised: August 30, 2011 Published: September 01, 2011 19606
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Table 1. EDX Atomic Concentrations for the As-Deposited FEBID Material electron energy
beam current
Co
O
N
C
(keV)
(nA)
(at %)
(at %)
(at %)
(at %)
5 5
0.2 0.9
49 49
23 28
15 13.5
13 9.5
Figure 1. (a) Sketch of the FEBID process: electrons decompose the precursor molecules; the metallic part is deposited and the volatile parts are pumped out. (b) SEM micrograph of a typical FEBID deposit used for the EDX analysis.
5
4.1
49
28
14
9
15
0.3
51
24
14
11
15
1.3
51
26
14
9
15
4.3
48
30
14
8
20
3.5
48
30
13
9
conductive and metallic. A transition to ferromagnetic behavior was observed for both nanowires. In the present Article, we extend the characterization of the precursor focusing onto the FEBID process because it showed the most pronounced changes after annealing. First, we investigate deposit composition and deposition rates as a function of beam parameters; then, the different electrical transport regimes in the as-deposited and annealed nanowires are determined by quantitative fitting of the resistivity data. Finally, a transmission electron microscopy (TEM) structural analysis of the FEBID deposit is performed before and after thermal annealing, correlating the deposit nanostructure with the changes observed in the electrical and magnetic properties.
2. EXPERIMENTAL SECTION Co depositions were carried out at RT in a FEI Nova 600i dual beam system, combining a Schottky field-emission SEM and a Ga-ion FIB. Substrates were heated to 130 C to desorb air exposure contaminants and allowed to cool to RT before starting deposition runs. The liquid Co(CO)3NO precursor was contained in a gas injector operated at RT and equipped with a 10 μm limiting-aperture to obtain a gas pressure in the chamber of 3.2 106 mbar. To study the deposit composition and deposition yield, we performed FEBID depositions on a Si substrate over a 0.5 0.5 μm2 area, for 450 s with 10 μs of dwell time, varying the electron beam parameters (energy and current). The typical deposit obtained, a square trapezoid 0.6 μm high, is shown in Figure 1b. Elemental composition was checked in situ by energy dispersive X-ray analysis (EDX) with 3 keV primary beam energy. For the electrical and magnetic characterization, Co nanowires with dimensions of 0.55 0.25 12 μm3 (width thickness length) were deposited across four gold electrodes patterned on SiO2/Si substrates. Electron beam parameters were 5 keV energy, 4.1 nA current, and 10 μs of dwell time. Electrical and magnetic characterizations were performed through four-probe resistivity measurements in a Quantum Design PPMS-7T cryomagnetic system, at temperatures down to 2 K and magnetic field intensity (B) up to 7 T. Thermal annealing of the nanowires was performed in a vacuum furnace (base pressure 2.0 105 mbar) at T = 400 C for 30 min. TEM analysis was performed with a JEOL 2200 FX microscope at 200 keV, preparing the samples by FIB lift-out method with a FEI Strata 235 M dual beam. 3. RESULTS 3.1. Deposit Composition and Deposition Yield. To study the deposit composition and deposition yield of the FEBID process,
Figure 2. (a) Typical EDX spectrum from the FEBID deposit. (b) EDX atomic concentrations versus electron beam power for the as-deposited FEBID material.
we performed depositions varying the electron energy (520 keV) and beam current (0.24.3 nA). In Table 1, the atomic concentrations measured by EDX are listed and, in Figure 2, a typical EDX spectrum is shown (Figure 2a) along with the concentration data plotted as a function of the electron beam power (Figure 2b). It is evident that there is a very weak dependence of atomic concentrations on energy and current in the explored range. Co concentration, around 50 at %, slowly decreases as the beam power increases, and O concentration, between 25 and 30 at %, slightly increases with beam power. N and C values are almost constant around 15 and 10 at %, respectively. Average measured values are: 49 at % for Co, 27 at % for O, 14 at % for N, and 10 at % for C. In Figure 3a, the FEBID yield as a function of electron energy is shown, the deposited volume being measured from the SEM micrographs. The deposition yield starts at 3 104 μm3/nC at 5 keV and linearly decreases to 1 104 μm3/nC at 15 keV, where it becomes stable for higher energies. In Figure 3b, the effect of substrate temperature on the deposition yield is shown. The two pillars were grown with the same beam parameters (5 keV electron energy, 4.1 nA 19607
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The Journal of Physical Chemistry C beam current) but different substrate temperatures: 130 C for the tall pillar and RT for the short pillar. The high-temperature (HT) pillar (with menhir-like shape) is 14.5 μm tall and has an average diameter of 1.75 μm, whereas the RT pillar is 1.45 μm tall and has a square base of 0.6 μm. This corresponds to a 67 enhancement of the HT deposition yield (189 104 μm3/nC) with respect to RT (2.8 104 μm3/nC). A different surface topography is also evident: the walls of the HT pillar are much rougher, suggesting the presence of crystallites typical of autocatalytic thermal growth. The leaning growth of both structures is related to sample drift during deposition, possibly due to the insulating
Figure 3. (a) FEBID yield as a function of electron energy for RT deposition. (b) Comparison between two pillars deposited at RT and at 130 C substrate temperature by FEBID. The two structures are displayed on the same scale.
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character of the deposit, as explained later. EDX analysis on the HT pillar returned similar concentrations to those measured at RT: 51 at % for Co, 25 at % for O, 18 at % for N, and 6 at % for C. 3.2. Electrical and Magnetic Properties. In Figure 4, the results of electrical and magnetic characterizations on the asdeposited and annealed FEBID nanowires are presented. 3.2.1. As-Deposited Nanowires. In Figure 4a, the SEM micrograph of the FEBID nanowire, deposited across four gold electrodes for the resistivity measurement, is shown. In Figure 4b, the resistivity (F) data measured as a function of temperature are shown. The curve starts from ∼6 mΩ cm at 300 K and then increases as the temperature is lowered, finishing at a value of 14 mΩ cm at 2 K. To determine the electrical transport type, we analyzed the logarithmic derivative of resistivity (w = d(ln F)/d(ln T)), shown in the inset of Figure 4b, and found that it is negative and fairly constant around an average value of 0.16 over the whole T range. A constant w trend is expected from nanogranular metallic systems close to the metalinsulator (MI) transition but not yet described by a variable range hopping (VRH) model (that would give a linearly decreasing w with T).8 The temperature dependence of resistivity, in this case, follows a power law behavior: F(T) = CTw. The best fit curve, shown in Figure 4c, was obtained for C = 15.79 ( 0.05 mΩ cm and w = 0.160 ( 0.001. Magnetic characterization was performed through MR measurements,9 a suitable method to study the magnetic properties of nanowires.10 Resistance is measured as a function of the external magnetic field intensity and orientation, and MR(%) = 100*(RB R0)/R0 data (where RB and R0 are the resistance measured in field B and in zero field, respectively) display characteristic trends, especially in the case of ferromagnetic materials. In Figure 4d, the MR data measured as a function of the angle between the normal axis and an external field B = 5 T (see drawing) are shown for the
Figure 4. (a) SEM micrograph of a FEBID nanowire deposited across four gold electrodes. (b) Resistivity versus temperature data and fitting curve of the as-deposited nanowire. In the inset the logarithmic derivative of resistivity is shown. (c) Resistivity versus temperature data and fitting curve of the 400 C annealed nanowire. (For definitions of the fitting functions, see the text.) (d) Magnetoresistance versus magnetic field angle for as-deposited and 400 C annealed nanowire. 19608
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Figure 5. (a) TEM micrograph and (b) SAED pattern of the as-deposited FEBID material. Arrows in panel a indicate the Co-containing dark nanograins. In panel b, the CoO fcc diffraction pattern is shown in the inset for comparison with data. (c) HRTEM micrograph and (d) SAED pattern of the FEBID nanowire after 400 C vacuum annealing. Diffraction spots marked as 1, 2, and 3 in panel d correspond to {1011}, {1120}, and {1013} reflections of the Co hcp structure.
as-deposited nanowire: they basically show no angular dependence, indicating a nonmagnetic behavior. 3.2.2. 400 C Annealed Nanowires. To improve the electrical and magnetic properties of the nanowire, thermal annealing was performed at 400 C for 30 min in a vacuum furnace. This procedure is aimed to remove the volatile impurities (C, N, and O) through outgassing and also to improve the structural quality of the deposit by increasing the size and crystallinity of the metal nanograins.11 In Figure 4c, the F versus T curve of the annealed nanowire is shown. Two radical changes are evident from the asdeposited data: F now decreases linearly as T is decreased, recovering a metallic behavior, and the value at 300 K has dropped by two orders of magnitude, to 62 μΩ cm. Data are well-fitted by the BlochGruneisen (BG) formula, describing the classical temperature dependence of resistivity in metals12 5 Z TD =T T x5 dx FðTÞ ¼ F0 þ α x TD ðe 1Þð1 ex Þ 0 where F0 is the zero-temperature resistivity, α is a constant, and TD is the Debye temperature. Best-fit values of 37.34 ( 0.01 μΩ cm and 288 ( 1 K are obtained for F0 and TD, respectively.
In Figure 4d, the MR angular plot for the annealed nanowire is shown. It is completely different from the as-deposited case: MR is oscillating between MR|| (maximum) and MR^ (minimum) saturation values with a period of 180 following a cos2(θ) behavior. This angular dependence is the one expected from the anisotropic MR (AMR) effect in ferromagnets.9,10 The measured AMR value (MR|| MR^) is 0.85%. A further evidence of the ferromagnetic transition that occurred after annealing was provided by MFM analysis of the FEBID wire, showing that at RT a magnetization vector was aligned along the easy axis of the wire.7 EDX analysis after annealing returned the following atomic concentrations (compared with those before annealing in parentheses): 46% (42%) for Co, 30% (28%) for O, 12% (15%) for N, and 12% (15%) for C. The values before annealing are different from those measured in situ probably because of air exposure of the samples. A modest increase in Co concentration is observed; therefore, it can be concluded that vacuum annealing does not produce a significant purification of the deposit. 3.3. TEM Structural Analysis. To investigate the impact of annealing on the nanostructure of the deposit, we performed high-resolution TEM (HRTEM) analysis before and after the thermal procedure. In Figure 5a,b, the cross-sectional TEM 19609
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Figure 6. High-resolution TEM micrograph of the 400 C annealed FEBID nanowire showing two neighboring CoO fcc and Co hcp nanograins.
micrograph and the corresponding selected area electron diffraction (SAED) pattern of the as-deposited material are shown, respectively. TEM image in Figure 5a was taken under brightfield condition so that the heavier elements (stronger scatterers) appear darker. The typical nanogranular structure of metallorganic deposits can be noticed, with dark metal nanocrystals (arrowed) embedded in a matrix of lighter elements. The size of dark nanograins is very small (∼1 nm) and homogeneously distributed in the analyzed region. More insight into the material structure comes from the SAED pattern in Figure 5b), showing a sequence of diffused rings that match quite well the structure of fcc CoO. In particular, the first three rings belonging to (111), (200), and (220) atomic planes can be recognized. In Figure 5c,d, the HRTEM micrograph and the corresponding diffraction pattern of the 400 C annealed deposit are shown, respectively. The bright-field HRTEM image shows a polycrystalline structure, with nanocrystallites 10 to 15 nm large, displaying interference fringes of variable spacing and orientation. The enlarged grains are closely packed, and many are touching. The SAED pattern now consists of a superposition of sharper fcc CoO diffraction rings with new spots closely matching the Co hcp pattern. In particular, the spots marked as 1, 2, and 3 belong to Co hcp {1011}, {1120}, and {1013} planes, respectively. In Figure 6, a close-up HRTEM image allows us to resolve two neighboring CoO and Co nanograins: on the left, the linear interference fringes belonging to CoO fcc {111} planes (0.241 nm atomic spacing) and on the right, the hexagonal pattern of Co hcp {1010} planes (0.216 nm atomic spacing).
4. DISCUSSION The average atomic composition of the FEBID deposits is dominated by Co (50 at %) and O (2530 at %) content. The Co amount is aligned with the best results obtained using cobalt carbonyl precursors13 with similar beam parameters, although almost pure Co deposits were recently obtained by FernandezPacheco et al.14 The nondependence of atomic concentrations on beam current and power is different from what is observed
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with cobalt carbonyl13 and other metallorganic precursors,15 where an increase in these parameters usually improves the precursor molecule dissociation and hence the metal content. This is due to thermally assisted decomposition induced by beam heating, and the higher decomposition temperature of our precursor with respect to Co carbonyl is one reason to explain our results. It could be interesting to explore the FEBID methodology recently reported by Bernau et al.16 that obtains metal-rich deposits in a nonthermal regime by properly choosing the dwell and refresh time parameters. The decreasing FEBID yield with increasing electron energy has the expected trend, and it is related to the larger secondary electron yield at lower primary energy. The huge effect of substrate temperature on the deposition yield (67) is in agreement with a recent study6 and can be explained by an autocatalytic growth process, where precursor decomposition is activated by temperature, either through substrate heating, as in this case, or through beam-induced heating of the growing structure. The combination of EDX and TEM results on the as-deposited nanowire, showing a CoO fcc phase in the SAED patterns, suggests that FEBID deposit consists of cobalt monoxide mixed with a metallic Co component (the amount in excess of the CoO concentration balance). The presence of Co nanograins embedded in an insulating matrix (CoO) is also suggested by the resistivity data that are well-described by an MI transition behavior. The presence of insulating material in the deposit is confirmed by charging effects like the lateral drift of deposited structures, evident in the sloped growth of pillars in Figure 3b, and the glowing contrast observed during slow-scan SEM imaging on the same pillars. This overall picture is in remarkable agreement with results of Ivanova et al.17 on chemical vapor deposition (CVD) of cobalttricarbonylnitrosyl. For low deposition temperatures (1 mOhm cm, of the same order of magnitude of our value of 6 mOhm cm. The formation of CoO at these temperatures was explained by a thermodynamically favorable path for oxygen to recombine with cobalt and form the oxide. C, N, and O contaminations were found to persist up to 210, 250, and 350 C deposition temperatures, respectively, and the metallic Co hcp phase appeared at 270 C. On the contrary, the lower decomposition temperature of Co carbonyl results in pure Co hcp phase already at 200 C CVD,18 confirming the higher dissociation energies for the cobalttricarbonyl nitrosyl molecule. The nonmagnetic behavior of the as-deposited nanowire can be ascribed essentially to the very small size (∼1 nm) of Co nanograins, as revealed by TEM analysis. In fact, the threshold size above which Co nanoparticles can sustain a stable magnetization (superparamagnetic limit) is between 7 and 8 nm,19 and the much smaller size observed here, combined with a large intergrain separation, prevents any ordered magnetic state to build up. Concerning the CoO component, the bulk material is antiferromagnetic, but this property is generally weakened in nanoparticles as the size decreases, leading to paramagnetic or even ferromagnetic behavior.20 Anyway, the size of CoO nanograins is similar to Co nanograins; therefore, for the same reason, they cannot contribute a defined magnetic state. Thermal annealing at 400 C in vacuum does not substantially purify the Co deposit because only a modest increment of Co 19610
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The Journal of Physical Chemistry C concentration, ∼10%, is measured. To improve the metal concentration, H-containing gaseous atmosphere, typically employed to reduce CoO,21 should be used instead of vacuum. On the contrary, the annealing radically changes the electrical and magnetic properties as a consequence of nanostructural transformations. As revealed by TEM analysis, there is a coarsening and interconnection of the Co nanograins that increases their size above the superparamagnetic limit and creates a continuous path to the electron transport. The good fitting of resistivity data with the BG function stands as a confirmation of this picture. The best-fit TD value (288 K) is lower than the pure metal value (TD(Co) = 385 K)22 by the same amount observed in annealed Pt-FIBID23 wires and can be interpreted as a consequence of the structural disorder in the system. The RT resistivity (62 μΩcm) and AMR value (0.85%) measured on the annealed nanowire are comparable to those obtained for the pure-Co FEBID nanowires from cobalt carbonyl,14 indicating a similar electrical and magnetic quality. Moreover, the AMR value is also comparable to the one reported for electron beam lithography (EBL)-fabricated Co nanowires.10
5. CONCLUSIONS Electrical, magnetic, and structural characterization of nanowires deposited by FEBID of cobalt tricarbonyl nitrosyl precursor (Co(CO)3NO) has been performed. Deposit composition and deposition yield have been studied as a function of beam energy and current. Co content in the deposit is ∼50 at %, independent of the beam parameters employed. Deposition yield has the expected decreasing trend with beam energy, and an autocatalytic/ thermal enhancement of 67 is observed when deposition is performed at 130 C substrate temperature. From an electrical and magnetic point of view, as-deposited nanowires are highly resistive, nonmetallic at low T, and nonmagnetic. After 400 C vacuum annealing, resistivity is reduced by two orders of magnitude, metallic behavior is recovered, and evidence of AMR, typical of ferromagnets, is clearly observed. Co concentration in the deposit is not significantly increased though. TEM analysis on the nanowires reveals that before annealing the deposit consists of a CoO fcc phase with nanograins size around 1 nm. After annealing, a polycrystalline structure of Co hcp and CoO fcc nanograins, with size increased to 1015 nm, is observed. Coarsening and interconnection of the Co nanograins provide the condition for ferromagnetic behavior and metallic electrical transport.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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