Growth and characterization of tungsten carbide nanowires by thermal annealing of sputter-deposited WCx films

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Growth and characterization of tungsten carbide nanowires by thermal annealing of

sputter-deposited WC x films

Shui-Jinn Wang, Chao-Hsuing Chen, Shu-Cheng Chang, Kai-Ming Uang, Chuan-Ping Juan, and Huang-Chung Cheng

Citation: Applied Physics Letters 85, 2358 (2004); doi: 10.1063/1.1791322 View online: http://dx.doi.org/10.1063/1.1791322

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/85/12?ver=pdfcov

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Growth and characterization of tungsten carbide nanowires by thermal

annealing of sputter-deposited WC

_x

films

Shui-Jinn Wang,a) Chao-Hsuing Chen, Shu-Cheng Chang, and Kai-Ming Uang

Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China

Chuan-Ping Juan and Huang-Chung Cheng

Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan, Republic of China

(Received 22 January 2004; accepted 21 July 2004)

In this letter, the growth of dense W₂C nanowires by a simple thermal annealing of sputter-deposited WC_xfilms in nitrogen ambient is reported. Straight nanowires with a density of 250– 260␮m−2_and

length/diameter in the range of 0.2– 0.3␮m / 13– 15 nm were obtained from the 700° C-annealed samples, which exhibit good electron field emission characteristics with a typical turn-on field of about 1.7 V /␮m. The self-catalytic growth of W2C nanowires is attributed to the formation of

American Institute of Physics. [DOI: 10.1063/1.1791322]

Electron field emission(FE) from durable emitter mate-rials has received much attention due to their potential appli-cations as cold cathodes in flat panel display and nanoelec-tronics. Though researchers are still actively looking for alternative materials, diamonds,1 diamond-like carbon,2,3 amorphous carbon,4,5carbon nanotubes(CNTs),6,7etc., have been widely studied as possible candidates for FE emitters.8 In general, a qualified material used as FE emitter should be made of highly conductive film with high mechanical strength, high emission site density, high thermal conductiv-ity, and low FE threshold fields.9,10

Refractory metal carbides (RMCs), such as TaC, TiC, WC, etc., have been applied as diffusion barrier layers in Al and Cu metallization due to their good electrical conductiv-ity, high chemical inertness, and low atomic diffusion at high temperatures.11–14It is expected that the growth of nanosized structures from RMC films, if possible, would be beneficial to the improvement of electron FE properties for actual de-vice performance. However, there have been very limited reports on the electron field emission of RMC films.8,15 In this letter, we report on the growth of tungsten carbide nanowires by a simple thermal annealing of sputter-deposited WCxfilms. The self-catalytic growth behavior and microstructures of the thermally annealed WC_x films were investigated. FE properties of the nanowires were examined. Through material and structural analysis, a possible self-catalytic growth mechanism is proposed.

In experiments, WCxfilms with thickness in the range of 15– 60 nm were sputter-deposited on n-type Si共100兲, 10

⍀ cm substrates. A dc magnetron sputtering system using a

water-cooled WC target(50:50 wt %) with 99.5% purity was used. The deposition was carried out under an argon flow rate of 24 sccm, a dc power of 200 W, and a pressure of 7.6 m Torr at room temperature. The typical deposition rate was measured to be around 0.36 Å / s. The as-deposited samples were subsequently subjected to thermal annealing in quartz tube furnace with a temperature ranging from 600 to 850° C in N2ambient for 30 min.

Figure 1 shows the typical scanning electron microscopy

(SEM) image for the 700°C-annealed sample and insets of

the figure for the 650 and 750° C-annealed samples. All samples are with 60 nm in thickness. Similar results were also found on samples with other thicknesses. It is seen that nanosized white protrusions appear on the surface of the 650° C-annealed samples, while samples annealed at lower temperatures 共艋600°C兲 show a smooth and clean surface. After thermal annealing at 700° C, the small extrusions have developed into dense and randomly oriented nanowires with a density of 250– 260␮m−2. After annealing at 750° C, how-ever, most of the nanowires have been transformed into dark-colored grains with a typical size of 6.8– 7.5⫻10−3 _␮_m2_.

Upon further increasing of the annealing temperatures to 800 or 850° C (not shown), the grain size was enlarged and a relatively smoother surface appeared.

Figure 2 presents the transmission electron microscopy

(TEM) image of the nanowires synthesized on the

700° C-annealed samples. Essentially, the nanowires are all with a straight body. The typical diameter and length of the nanowires are of 13– 15 nm and 0.2– 0.3␮m, respectively. Also shown in the inset to Fig. 2 is the selected area diffrac-tion(SAD) pattern for an individual nanowire. The inner and outer rings indicated by an arrow are with a d space of 2.36 and 1.35 Å, which corresponds to the phase of ␣

-a)_{Electronic mail: [email protected]}

FIG. 1. Top view SEM image of the 700° C-annealed WCxfilms. Insets are

the SEM images of samples annealed at 650 and 750° C. The scale bar is applied to all figures. Thermal annealing was performed in N2ambient for 30 min.

APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 12 20 SEPTEMBER 2004

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W2C共002兲 and ␣-W2C共103兲, respectively. Between the

above two rings, WO2共2¯20兲 with d space of about

1.72– 1.73 Å was observed. As compared to tungsten (W) nanowires synthesized from pure W films reported by Lee et

al.,15 the W2C nanowires have a much higher density than

that of W nanowires, in addition, the temperature required for the growth of W2C nanowires is around 700° C, which is

about 150° C lower than that used in their work.

The FE characteristics of the WCxnanowires were mea-sured in a parallel plate configuration with a gap spacing of 210␮m and a contact area of 1.5⫻10−2_cm2_{at a pressure of}

1⫻10−7_{Torr. Since the WC}

x films are highly conductive with a typical resistivity of 200 – 230␮⍀ cm as measured by four-point probe technique, essentially, the FE characteristics of the prepared samples are not sensitive to the film thick-ness in the range of 15– 60 nm. Figure 3 illustrates the typi-cal Fowler–Nordheim(FN) ln共J/E2_{兲 versus 共1/E兲 plots for}

the WC_x films annealed at 650 and 700° C. Samples an-nealed at 750 and 800° C show poor FE properties. The lin-ear FN characteristics indicated that electron emission of W2C nanowires should be dominated by the FN process.

According to the FN model16and assuming a work function of 4.5– 5.0 eV,15,17,18 the extracted enhancement factors ␤ for the 650 and 700° C-annealed samples are of 4267– 4998 and 5413– 6339, respectively. Though these values are com-parably higher than some reported values for CNTs,18,19to further improve FE properties, efforts including additional

treatment to achieve alignment and elongating the length of the nanowires for FE applications are still necessary. The turn-on field at an emission current density of 1␮A / cm2for the 650 and 700° C-annealed samples is about 2.1 and 1.7 V /␮m, respectively.

Figure 4 is a typical x-ray diffraction(XRD) pattern of the prepared samples, taken with a Rigaku D/MAX2500 dif-fractometer at a scan step of 0.01°, using Cu K␣(wavelength of 1.5418 Å) radiation. For the as-deposited sample, a single broad peak corresponding to WCxphase was observed. After thermal annealing at 600 and 650° C, the peak was seen to shift to a value of 38.03° corresponding to the ␣-W2C共002兲 phase. It indicates that a finite amount of

car-bon was decomposed and carcar-bon depletion together with the formation of␣-W2C structure occurred. The same

phenom-enon has been reported by Romanus et al.20With increasing the annealing temperature to 700° C, the original ␣-W₂C peak was observed to broaden and get an asymmetrical shape. This anomalous peak can be deconvoluted into six Gaussian peaks as shown in Fig. 4(b). Note that the main peak centered at ⬃38.03° corresponds to ␣-W2C共002兲,

while the other peaks correspond to WO2, W5Si3,␤-W, and

␣-W phases. The appearance of␤-W and ␣-W phases indi-cates that a further amount of carbon depletion occurred in the 700° C-annealed films. Further increasing the annealing temperature above 700° C, the main structure of the annealed films was seen to be dominated by␤-W and then by ␣-W phase. Comparing the XRD patterns for the 750, 800, and 850° C-annealed samples, the intensity of␣-W2C and ␤-W

peak is seen to decrease with increasing annealing tempera-ture, while the intensity of␣-W peak exhibits a reverse situ-ation. It is noted that samples annealed at 700° C and above show a significant portion of W5Si3 共321兲 phase. However,

FIG. 2. TEM images of W2C nanowires obtained from the 700° C-annealed samples. Inset is the SAD diffraction pattern for individual nanowires.

FIG. 3. The Fowler Nordheim(FN) plots for the 650 and 700°C-annealed films. The inset shows the typical I-V curve for the 700° C-annealed samples.

FIG. 4. (a) XRD patterns of the as-deposited and annealed WCxfilms.(b)

XRD spectra of the 700° C-annealed WCxfilm in the range of 33° – 45°.

This anomalous peak has been deconvoluted into six Gaussian peaks corre-sponding to W2C, WO2, W5Si3,␤-W, and␣-W phases.

Appl. Phys. Lett., Vol. 85, No. 12, 20 September 2004 Wanget al. 2359

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as revealed by AES depth profiles(not shown), no sign of Si appears on the surface and the W5Si3phase only accumulates

at the WC_x/ Si interface. The influence of W₅Si₃ phase on nanowire formation is limited.

As reflected from XRD analysis, thermal annealing has rendered the main structure of the WCx films undergoing a sequential change from WCx phase 共⬍600°C兲 to

␣-W2C 共⬃600–700°C兲, to ␤-W 共⬃700–800°C兲, and

finally, to ␣-W phase (⬃800 and 850°C). Judging from SEM images the onset of the appearance of nanostructures at 650° C, the growth of dense nanowires at 700° C, and the disappearance of nanowires at 750– 800° C, it is very inter-esting to find that this trend was parallel to the variation of the intensity of the␣-W₂C共002兲 peak as revealed by XRD analysis. Obviously, carbon plays an important role in the growth and collapse of nanowires on annealed WCx films. The self-catalytic growth of W2C nanowires should be

attrib-uted to the formation of␣-W2C structures caused by carbon

depletion in the WCxfilms during thermal annealing. In ad-dition, the collapse of W2C nanowires in samples annealed at

temperatures艌700°C is due to the nanowires experiencing a structure transformation from ␣-W2C共002兲 phase into

grains of␤-W phase.

In conclusion, self-catalytic synthesis of W2C nanowires

from a simple thermal annealing of sputter-deposited WC_x films in N2 ambient and their FE properties have been

re-ported. Dense W2C nanowires with a typical diameter and

length of 13– 15 nm and 0.2– 0.3␮m, respectively, have been obtained from the 700° C-annealed WCx films, which exhibit good FE properties with a turn-on field of 1.7 V /␮m. According to material analysis, the possible mechanism gov-erning the self-catalytic growth of W2C nanowires should be

attributed to the phase change from WCx to W2C during

thermal annealing. The low turn-on field paired with high packing density and simple growth process indicates that W2C nanowire could be a potential material for field

emitters.

This work was supported by the National Science Council(NSC) of Taiwan, Republic of China, under Contract No. NSC 92-2218-E-006-033. The authors are indebted to Dr. W. J. Chen, Dept. of E. E., National Huwei Institute of Technology, for the technical support in TEM measurements.

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