SiCl3CCl3 as a novel precursor for chemical vapor deposition of amorphous carbon films

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S

iCl CCl as a novel precursor for chemical vapor deposition

₃

of amorphous carbon films

a a a ,

_*

b ,

_*

Yu-Hsu Chang , Lung-Shen Wang , Hsin-Tien Chiu

, Chi-Young Lee

a

Department of Applied Chemistry, National Chiao Tung University, Hsinchu, 30050, Taiwan, ROC b

Materials Science Center, National Tsing Hua University, Hsinchu, 30043, Taiwan, ROC Received 2 November 2002; received in revised form 20 January 2003; accepted 20 January 2003

Abstract

Amorphous carbon films, characterized by XRD, AFM, SEM and Raman, were deposited from SiCl CCl on quartz3 3

substrates at 773–1273 K by low pressure chemical vapor deposition using a hot-wall reactor. XPS studies showed that the films grown at 773 K contained 90% C and 10% Cl, while the films grown at 1273 K contained 100% C. SiCl , CCl and4 4

Cl C₂ =CCl₂ were detected by on-line FT-IR studies. The extrusion of dichlorocarbene, :CCl , from SiCl CCl₂ ₃ ₃ should provide the source of carbon in the reaction. On Si substrates, an etching process at the film-substrate interface assisted the lift-off of the films from the substrates. The C films curled and formed rolls.

Keywords: A. Carbon films; B. Chemical vapor deposition

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. Introduction solid carbon and gaseous CCl₄ is calculated to be an

exothermic reaction. The estimated Hreactionis 2139 kcal / The field of disordered carbon materials is important and mol [19,21]. Since the divalent species is highly reactive, covers a wide range, including carbon fiber [1], glassy we predict that carbon materials can be produced from carbon [2], microcrystalline carbon [3], amorphous carbon SiCl CCl . In this report, we demonstrate the deposition of₃ ₃ [4], and hydrogenated amorphous carbon [5]. Recently, amorphous carbon thin films using this strategy.

there has been considerable interest in applying carbon thin films to photovoltaic cells [6,7], cold cathode devices and

flat panel displays [8–10]. Chemical vapor deposition 2 . Experimental (CVD) methods [11–15], including hot-filament CVD,

plasma-enhanced CVD and laser-assisted CVD, have been SiCl CCl (Aldrich, 97%) was used as the precursor to₃ ₃ employed to deposit carbon thin films using various deposit thin films. The deposition experiments were carried 21 hydrocarbons as the precursors. out using a hot-wall reactor with a base pressure of 10

It has been shown that thin films and wires of Group IV, Pa. SiCl CCl3 3 was evaporated at 273 K without carrier such as Si and Ge, can be deposited using divalent gas. The typical deposition pressure was at |10 Pa.

Silica-2

precursors SiF [16], SiO [17] and GeI [18], respectively.2 2 glass and n-Si(100) substrates, approximately 10 mm , We would like to demonstrate that the strategy can be were used. The deposition temperatures were between 773 extended to deposit carbon thin films. According to and 1273 K.

literature reports [19,20], extrusion of :CCl , dichlorocar-₂ X-ray diffraction (XRD) studies were carried out using a bene, from SiCl CCl can be performed by pyrolyzing the3 3 diffractometer with CuKa radiation. Images of the films silane in vacuum. Disproportionation of gaseous :CCl into₂ were taken using a scanning electron microscope (SEM) equipped with an energy-dispersive spectra (EDS) attach-ment and an atomic force microscope (AFM). The growth *Corresponding authors. Tel.: 3-513-1524; fax:

1886-rates were calculated from the SEM cross-section views. 3-572-3764.

E-mail address: [email protected](H.T. Chiu). X-ray photoelectron spectra (XPS) were measured using a

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amorphous. The surface morphology of the films was characterized by SEM and AFM. Fig. 1a and b are the SEM micrographs of the films deposited on quartz at substrate temperatures of 773 and 1173 K, respectively. The deposition rate, 0.35–1.16 nm / min, was estimated from the cross-sectional SEM images. For a deposition temperature of 773 K, the film did not adhere well to the substrates and formed a rough and fluffy surface. When deposition occurred above 973 K, smooth films with good adhesion to the substrates were prepared. A selected AFM image of the film grown at 1173 K is shown in Fig. 2. From the images, the R_rms(root mean square roughness) of the surface was estimated to be 3–21 nm. The film deposited at 773 K showed an R_rms value of 21 nm since the film curved and lifted from the substrate.

The surface composition of the films was characterized by XPS (Fig. 3). The surface contains more than 90% C (Fig. 3a). The film deposited at 1073 K contains more than 97% C on the surface. For the films grown at temperature above 1073 K, the Cl concentration is near the detection limit of XPS. The Cl concentration increased with decreas-ing the temperature of deposition. For the deposition

temperature of 773 K, the Cl concentration on the surface Fig. 1. SEM images of film prepared at (a) 773 K and (b) 1173 K. is 10%. As shown in Fig. 3b, the high-resolution signals of

C 1s electron are observed at 284.5 eV, which is close to high-resolution spectra (Fig. 3c) are observed at 200.2 and the value of a graphite-like environment [22]. The bulge at 201.8 eV, respectively. These values correspond to the 286 eV can be assigned to a C–Cl bonding environment on C–Cl bonds linkage, as reported in the literature [22]. The 1 the surface. The Cl 2p_{3 / 2} and Cl 2p_{5 / 2} peaks of the oxygen concentration is low on the surface. After Ar

Table 1

Summary of the deposition conditions and characteristics of the deposits on silica glass

Deposition Deposition Growth Roughness Composition Resistivity

temperature time rate Rrms by XPS (mV cm)

1 (K) (h) (nm / min) (nm) (w / o Ar etching) 6 773 3 0.35 21 C 90% Cl 10% 1.25?10 973 3 0.29 3 C 92% Cl 2% O 6% 4920 1073 3 0.64 10 C 97% Cl,1% O 2% 3800 1173 3 0.69 6 C 99% Cl,1% 2470 1273 2 1.16 7 C 100% 1970

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Fig. 2. AFM image of an amorphous carbon film deposited on quartz at 1173 K, Rrms56 nm.

sputtering for 30 s, the O concentration decreased below the XPS detection limit. This suggests that the oxygen atoms were adsorbed on the surface as the films were exposed to air, which is a classical contamination.

Raman scattering spectra, shown in Fig. 4, were col-lected for the films deposited at 773, 1073 and 1273 K on quartz substrates. They were dominated by peaks at 1600

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and 1327 cm which are assigned to the G and D bands of graphitic carbon materials [23]. For each sample, both peaks are broad and the D peak is stronger than the G peak. This observation is consistent with the phenomenon observed for amorphous carbon films [23–25]. For the films deposited at 1273 K, as shown in Fig. 4c, the signals are sharper than for lower deposition temperature. This suggests that the film structure is more graphite-like at higher deposition temperatures.

A four-point probe was employed to measure the resistivity of the films prepared at various temperatures (Fig. 5). For the film deposited at 773 K, the resistivity is

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high, 1.25?10 mV cm. This probably reflects the high Cl concentration in the film. As the temperature of deposition increases, the resistivity decreases significantly. At 1273 K, the resistivity is 1970 mV cm, close to the value of a-C

Fig. 3. XPS spectra of films grown on quartz at 773 and 1073 K: film prepared by CVD (1000 mV cm) [7]. Compared to the

(a) survey, (b) high-resolution C 1s electrons and (c) high-res-reported data of other carbon materials, the value of 1970

olution Cl 2p3 / 2 and Cl 2p5 / 2 electrons. mV cm is higher than graphite (40 mV cm) [26] and

nanocrystalline diamond film (200 mV cm) [8], but much

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lower than evaporated a-C (|10 mV cm) or a-C:H (|10 expansion property, probably originated from the

differ-mV cm) films [27]. ence in the elemental composition within the film, might

We also studied the deposition process from SiCl CCl₃ ₃ cause the film to curl into a roll-like structure. EDS on Si substrates. The SEM image of the film that was analysis indicated that the composition was 76% C, 20% deposited at 1073 K for 25 min is shown in Fig. 6. It does Si and 4% O.

not adhere to the substrate. It curled into a roll of C film. In order to understand the reaction pathway of the We speculate that the Cl atoms from the precursor etched precursor SiCl CCl , an on-line FT-IR experiment was₃ ₃ the thin film–substrate interface and lifted the film from performed to identify the volatile products generated at the substrate. The uneven distribution of the thermal different temperatures. The spectra are shown in Fig. 7.

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(a) 773, (b) 1073 and (c) 1273 K.

Fig. 7. On-line FT-IR spectra of the vapor phase products obtained at (a) 300 K, (b) 473 K, (c) 573 K, (d) 773 K, (e) 973 K and (f) 1173 K.

The absorptions of the precursor, at 512, 610, 631and 754 Fig. 5. Resistivity of the films prepared at 773–1273 K. ₂₁

cm [28], are shown in Fig. 7a. The precursor starts to decompose above 473 K (Fig. 7b). Above 573 K, as shown in Fig. 7c–f, the signals of the precursor become negligible

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while the signals of SiCl4 (620 cm ) [29], CCl4 (794

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cm ) [30] and Cl C2 =CCl (916, 803 and 779 cm2 ) [30] dominate the spectra. As the deposition temperature in-creases, the signal of CCl decreases while the signals of4

Cl C₂ =CCl₂ increase. The volatile byproducts were also collected in a U-trap at liquid nitrogen temperature and studied by GC–MS. The generation of SiCl , CCl₄ ₄ and Cl C2 =CCl were confirmed. This agrees with the literature2

reports [19,20,31,32]. In Fig. 8, a reaction pathway is proposed to summarize and rationalize the experimental observation. Based on the formation of SiCl₄ and Cl C2 =CCl , it is proposed that a carbene, :CCl , was2 2

extruded from the precursor above 473 K. The divalent species may undergo a disproportionation reaction to deposit carbon films and to release CCl . Between 773 and₄ 1073 K, the stripping of Cl atoms from the solid appeared Fig. 6. SEM image of a carbon roll deposited on n-Si(100)

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. Conclusion

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A

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