007) 2949–2953www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Fabrication of nanocrystalline silicon carbide thin film by helicon waveplasma enhanced chemical vapour deposition

Wei Yu, Wanbing Lu ⁎, Yanbin Yang, Chunsheng Wang, Li Zhang, Guangsheng Fu

College of Physical Science and Technology, Hebei University, Baoding 071002, PR China

Received 15 November 2005; received in revised form 21 July 2006; accepted 30 August 2006Available online 12 October 2006

Abstract

Nanocrystalline cubic silicon carbide thin films have been fabricated by helicon wave plasma enhanced chemical vapour deposition on Sisubstrates using the mixture of SiH4, CH4, and H2 at a low substrate temperature of 300 °C. The infrared absorption spectroscopy analyses andmicrostructural characteristics of the samples deposited at various magnetic fields indicate that the high plasma intensity in helicon wave mode is akey factor to the success of growing nanocrystalline silicon carbide thin films at a relative low substrate temperature. Transmission electronmicroscopy measurements reveal that the films consist of silicon carbide nanoparticles with an average grain size of several nanometers, and thelight emission measurements show a strong blue photoluminescence at room temperature, which is considered to be caused by the quantumconfine effect of small size silicon carbide nanoparticles.© 2006 Elsevier B.V. All rights reserved.

Keywords: Silicon carbide; Nanostructures; Plasma processing and deposition

1. Introduction

Since the discovery of visible photoluminescence (PL) fromporous Si at room temperature [1], many studies have focusedon the fabrication of Si nanostructures with blue-emittingproperties. Compared to silicon, the high band gap energy ofSiC makes nanocrystalline SiC a better candidate for blue lightemission. Using a similar technique already applied forfabrication of porous Si, porous SiC has been prepared andexhibits improved PL than that of starting bulk material [2].However, the fabrication of porous SiC is not compatible withtraditional Si planar techniques. So, recently more attention hasbeen focused on the fabrication of nanocrystalline SiC, whichcan possibly emit efficiently short wavelength light because ofthe combination of wide band gap and quantum confinementeffect. Many techniques, such as sputtering, pulsed laserdeposition, ion implantation and plasma enhanced chemicalvapor deposition have been used for fabricating the nanocrystal-line SiC [3–7].

⁎ Corresponding author.E-mail address: luwb_202@163.com (W. Lu).

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.08.038

Compared to conventional plasma sources, helicon waveplasma (HWP) source has many advantages, such as the higherplasma density, lower ion energy and lower working pressure,which can lead to a greater efficiency in gas dissociation such asthe breaking of C–H and Si–H bonds in the feedstocks [8].Furthermore, the HWP can be excited apart from the processingchamber, which provides the possibility of controlling sepa-rately the reactant activation and the surface reaction process. Inthis paper, we report the synthesis of nanocrystalline 3C siliconcarbide films by using helicon wave plasma enhanced chemicalvapor deposition (HW-PECVD) in H2, SiH4 and CH4 gasmixtures at a low temperature of 300 °C. The structural andoptical emission properties of the obtained films have beeninvestigated by Fourier transform infrared absorption (FTIR),atomic force microscopy (AFM), transmission electron micros-copy (TEM), and PL spectroscopy.

2. Experimental details

The hydrogenated SiC thin films were deposited by an HW-PECVD system which has been detailed elsewhere [9,10]. Itconsists of a stainless steel cylinder chamber as the film

Fig. 1. (a) FTIR absorption spectra and (b) deconvolution of the SiC band intoLorentzian and Gaussian lines for the films deposited at magnetic fields of 0 T,1.0×10−2 T, 1.5×10−2 T, and 2.0×10−2 T.

2950 W. Yu et al. / Thin Solid Films 515 (2007) 2949–2953

deposition chamber and an upper quartz tube as the plasmageneration source. The base pressure was less than 3×10−4 Pa.The HWP was excited by a radio frequency (30 MHz) wavefield in a quartz tube with a 7 cm diameter Nagoya III typeexternal helicon antenna. Hydrogen was used as an excitationgas and is introduced at the top of the plasma source, while theSiH4 and CH4 were introduced into the deposition chamber viaa dispersal ring with a diameter of 12.5 cm.

All the films were deposited on (100)-oriented single crystalsilicon substrates. The substrates were first cleaned with acetoneand methanol, then cleaned successively with NH4OH:H2O2:H2O (1:2:5), de-ionized H2O, HF: H2O (1:10) and de-ionizedH2O, and finally were immediately placed on the sample holder.The flow rates of H2 were kept at 60 sccm. Considering thehigher stability of C–H bond than that of Si–H, the flow rates ofCH4 and SiH4 were set at 1.2 and 0.6 sccm, respectively, atwhich a near-stoichiometric SiC thin film can be fabricated [11].The working pressure, substrate temperature, and RF powerwere kept at about 0.7 Pa, 300 °C, and 700 W, respectively. Themagnetic field was varied from 0 to 2.0×10−2 T. The depositionduration was 2 h for all samples.

Infrared absorptions were measured with a Perkin–Elmer2000 Fourier transform infrared spectrophotometer (FTIR)within the range 400–4000 cm−1. The film thickness wasdetermined from the UV–VIS transmittance spectra using theSwanepoel treatment [12]. Atomic force microscopy observa-tions of the samples were performed with a Dimension 3000Instrument (Nanoscope IIIa, Digital Instruments, Santa Barbara,CA) in the tapping mode. The TEM specimens were preparedfrom nanocystalline SiC film, which were scraped from thefilm, triturated into powder, and then sonically dispersed inalcohol. A carbon TEM microgrid was immersed into thealcohol to collect the fragment of samples. The HRTEM andelectron diffraction pattern (EDP) images were obtained using aJEOL Electron microscopy 2010 instrument and operating at200 kV. Photoluminescence (PL) measurements were carriedout at room temperature using a He–Cd laser at an emissionwavelength of 325 nm.

3. Results and discussion

3.1. FTIR

Fig. 1(a) shows the infrared absorption spectra of the SiCfilms deposited by HW-PECVD at different magnetic fields.The absorption bands detected at around 790, 2100, and2900 cm−1 are due to the stretching modes of Si–C, Si–H, andC–H bonds, respectively [13,14]. In addition, a shoulder ataround 1000 cm−1 is observed, which could be attributed to therocking and wagging mode of Si–CHn bonds [15]. As can beseen, the Si–C stretching absorption bands of the filmsdeposited with static magnetic field become narrower com-pared to the one of the film deposited without applying staticmagnetic field. The shape evolution of Si–C bands is a possiblesignature of the occurrence of the SiC phase transition fromamorphous to crystalline state. In general, the Lorentzian peakreflects a narrow bond length and low bond angle distortion

corresponding to a crystalline state, while the Gaussian onerepresents a broad distribution of bound lengths and bond anglecharacterizing the amorphous state [15]. In order to verify this,the deconvolution of this band in the range of 500–1200 cm−1

is performed, by considering the complete band to be thecontribution of C–Hn rocking/wagging modes located at about1000 cm−1, Si=H2 bending mode of about 900 cm−1 and theSi–Hn wagging or rocking mode of about 640 cm−1 except forthe Gaussian and the Lorentzian peaks for Si–C stretchingmode at around 800 cm−1. The deconvolutions for all the curvesin Fig. 1(b) reveal that the main difference of Si–C absorptionbetween the films deposited with and without magnetic fields isthat the former can only be fitted by a Gaussian peak, while forthe later both Gaussian and Lorentzian peaks are needed. This

Fig. 2. The deposition rate as a function of the magnetic field.

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result shows that crystalline SiC phase is formed when magneticfields is applied. In addition, a first increase and then decrease ofthe fraction of Lorentzian component is obtained from the curvedeconvolution.

Above results can be explained by the high plasma density ofthe HWP. When no magnetic field is applied, the plasma canonly work at a capacitive plasma mode [16], at which both theplasma density and the dissociation efficiency of the reactantsare low. Only amorphous SiC can be grown since the substrate

Fig. 3. AFM images (2 μm×2 μm) of the nanocrystalline SiC films deposited at dif

temperature is maintained at a low value in the experiment [11].After the static magnetic fields are applied, the resonancecoupling of helicon wave and the antenna can rapidly transferwave energy to the electron due to the Landau dampingmechanism [17], which will lead to a transition of the plasmafrom capacitive to helicon wave mode. When the plasma worksin the helicon wave mode, the plasma density is several ordersof magnitude larger than the density in the capacitive mode. Theresult has been widely reported in the research of helicondischarge process [16] and has been also confirmed by ourpreviously probe measurement. Subsequently, the density ofactive silicon and carbon precursors and hydrogen atoms isincreased, which will improve the probability of crystalnucleation and the growth rate of the crystalline SiC.

3.2. The deposition rate

Fig. 2 shows the deposition rate of SiC thin films as afunction of the magnetic field. With an increase in magneticfield from 0 to 1.0×10−2 T, the deposition rate shows a sharpincrease beyond which there is a slightly decrease. The variationof the deposition rate is believed to be a result of the competitionbetween the processes of active radical deposition and hydrogenion etching. When the plasma works in the helicon wave mode,the function of the plasma density and outer magnetic field canbe determined by dispersion of the helicon wave. From the

ferent magnetic field: (a) 0 T; (b) 1.0×10−2 T; (c) 1.5×10−2 T; (d) 2.0×10−2 T.

Fig. 4. HRTEM micrograph of the film deposited at 2.0×10−2 T magnetic field.The inset shows corresponding EDP.

Fig. 5. PL of the film deposited at 2.0×10−2 T magnetic field, which excited atroom temperature using a He–Cd laser at an emission wavelength of 325 nm.

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dispersion of the helicon wave, we know that the electronnumber density is proportional to the magnetic field when theRF power field is constant [18]. The increase of magnetic fieldwill result in the concentration increment of active radicals inthe plasma due to the dissociation by electron impact.Furthermore, according to the global model for high densitylow pressure discharges [19], the electron temperature isdetermined by balancing the production rate of electron-ionpairs in the bulk of the plasma to the loss rate of particles to thewalls. Applying a confining magnetic field will result in thedecrease of ion loss and consequently a higher plasma density,as also confirmed by the experimental results [20]. Thesubsequent increased density of active silicon and carboncontaining radicals can account for the initial increase of filmdeposition rate. When higher magnetic field is applied, theavailable reactive species contributing to the film growth couldbe limited by the low flow rates of SiH4 and CH4 despite thehigher ionization rate of the plasma. On the other hand, due tothe high H2 dilution, there is a strong dissociation of H2 thatincreased with the magnetic field. The large amount ofhydrogen atoms could etch off weaker sp2 bonded carbonincorporated in the films, leading to a decrease in the depositionrate.

3.3. AFM

Fig. 3 shows the AFM images of the samples deposited atdifferent magnetic fields. It can be seen that the surface ofsample deposited at zero magnetic field is very smooth, which istypical amorphous characteristic. The surfaces of all thesamples deposited with magnetic fields consist of many smallcubic particles, which are a sign of cubic symmetry of thecrystallites. The average grain size of these cubic particlesshows a decrease trend with increasing the magnetic field. Thestatistic results show that the average grain size is around 120and 70 nm corresponding to the samples deposited at1.0×10−2 and 1.5×10−2 T magnetic fields, respectively. Forthe sample deposited at 2.0×10−2 T magnetic field, the grainsbecome even smaller as can be seen in the figure. But thestatistic average grain size of the sample is not obtained for the

resolution limit of AFM figure. The results are in agreementwith those obtained from FTIR and the deposition rate of thinfilms.

Since, as mentioned by above, the increase of magnetic fieldresults not only in a higher efficiency of SiH4 and CH4

molecules dissociation but also in an increased dissociation ofH2 molecules. The increasing concentration of hydrogen atomswill promote the probability of the nuclei generation and thecrystallite formation [21]. The decrease of the SiC crystal grainsizes in the films, as shown in Fig. 3, should be the combiningeffect between the increased nucleation density and thehydrogen etch effect. After magnetic field increased to2.0×10−2 T, the SiH4 and CH4 reactants have been depleted.The continuous increased hydrogen etching effect make the SiCcrystal grain sizes reduce further. Probably, since there is highstrain energy and a large ratio of volume to surface for smallgrains, which would need more hydrogen atoms to terminate thedangling bongs, the deconvoluting of Si–C bonds for filmsgrown at this magnetic field shows a low crystalline fraction. Infact, a visible increase of the Si=H2 and C–Hn absorptionswhich relate hydrogen content at grain boundary region isdisplayed in FTIR curve of this samples. From the aboveresults, we can see that the high plasma density of helicon waveplasma source and the high hydrogen dilution are both keyfactors for depositing nanocrystalline SiC films at a lowtemperature.

3.4. TEM

In order to obtain direct information about the microstructureof the samples, Fig. 4 shows a TEM image of the sampledeposited at 2.0×10−2 T magnetic field. As can be seen, theformation of nanosized crystallite is clearly evidenced by theobservation of the reflecting plane of well-oriented SiC grains.The size of nanocrystallites is ranging within severalnanometers. The interface regions between the grains have anamorphous structure, as can be seen from the irregularlyarranged atom images. The inset of Fig. 4 shows the electrondiffraction pattern (EDP) of the sample. The EDP reveals the

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presence of diffraction rings related to SiC crystalline phase.The first spread diffraction spot of the EDP indicates that thereare amorphous components in the film. The EDP fine rings,originating from the {111} and {220} SiC lattice planes, arecomposed of many diffraction spots, which reflect that there aredifferent orientation for the crystalline grains in the films. TEMobservations support that the microstructure of the filmdeposited by HW-PECVD is crystalline SiC grains embeddedin the amorphous counterpart, which is consistent with theanalysis of the FTIR results presented above. Both the TEM andthe corresponding EDP results confirm that the film consists ofpredominantly 3C–SiC polytypes.

3.5. PL

PL characteristics of the deposited films are performed atroom temperature using a He–Cd laser at an emissionwavelength of 325 nm. The results show that the samplesdeposited at the magnetic fields of 0, 1.0×10−2, 1.5×10−2 Texhibit no noticeable visible PL at room temperature, while thesample deposited at 2.0×10−2 T exhibit strong blue lightemission at room temperature, as can be seen in Fig. 5. The PLspectrum shows a broad peak with a maximum at 430 nm(approximately 2.9 eV) far above the band-gap energy of bulk3C–SiC 2.2 eV, implying that no evidence exists to ascribe thisPL band to a donor–acceptor transition. Comparing themicrostructure of the sample deposited at 2.0×10−2 T to thatof the samples deposited at the other magnetic field conditions,it can be noted that the average size of the cubic SiCnanocrystals embedded in the amorphous SiC matrix is smallerthan those in other samples. Furthermore, the size of mostnanocrystals in the sample deposited at 2.0×10−2 T is smallerthan double Bohr radius of 3C–SiC (approximately 5.4 nm),which will make their band-gap energy larger than 3C–SiC bulkmaterial [22]. Most importantly, visible light emission can nowbe observed at room temperature. In addition, it is veryanalogous on the peak central position and the full width at halfmaximum to the luminescence of the nanocrystalline 3C–SiCfilms prepared by electrochemical etching of polycrystallineSiC [23], which is explicitly ascribed to quantum confinementeffect. Therefore, it is reasonable to think that the roomtemperature PL observed from the sample is ascribed to thequantum confinement effect of SiC nanocrystallites. Theevolution of the PL at low temperature and different excitedwave length are underway to further clarify the detailed light-emitting mechanism.

4. Conclusion

In this work, HW-PECVD was successful used to grownanocrystalline 3C–SiC thin films on single crystalline Si

substrate at a low temperature of 300 °C with SiH4, CH4, and H2

as reactant. The detailed microstructure analyses indicate thatthe high intensity of helicon wave plasma is a key condition fordepositing nanocrystalline silicon carbide thin films at a relativelow temperature. By adjusting the magnetic field, one cancontrol the size of SiC nanocrystal and can obtain thenanocrystalline 3C–SiC thin films with an average grain sizeof several nanometers at 2.0×10−2 T magnetic field. The PL ofthese films presents a strong blue light emission at the roomtemperature when being excited by the He–Cd laser with325 nm wavelength.

Acknowledgement

This work was supported by Natural Foundation of Hebeiprovince, PR China (Grant No.503129).

References

[1] A.G. Cullis, L.T. Canham, Nature 353 (1991) 335.[2] R.L. Deleon, L. Sun, E. Rexer, J. Charlesbois, J.F. Garvey, E.W. Forsythe,

G.S. Tompa, Mater. Res. Soc. Symp. Proc. 441 (1997) 573.[3] C.M. Chen, X.Q. Liu, Z.F. Li, G.Q. Yu, D.Z. Zhu, J. Hu, M.Q. Li, W. Lu,

J. Vac. Sci. Technol., A, Vac. Surf. Films 18 (2000) 2591.[4] R. Reitano, G. Foti, C.F. Pirri, F. Giorgis, P. Mandracci, Mater. Sci. Eng.,

C, Biomim. Mater., Sens. Syst. 15 (2001) 299.[5] S.J. Xu, M.B. Yu, S.F. Yoon, C.M. Che, Appl. Phys. Lett. 76 (2000) 2550.[6] Y. Kamlag, A. Goossens, I. Colbeck, J. Schoonman, Appl. Surf. Sci. 184

(2001) 118.[7] T. Rajagopalan, X. Wang, B. Lahlouh, C. Ramkumar, Partha Dutta, S.

Gangopadhyay, J. Appl. Phys. 94 (2003) 5252.[8] T.Z. Fang, Chin. Phys. Lett. 18 (2001) 1098.[9] W. Yu, B.Z. Wang, Y.B. Yang, W.B. Lu, G.S. Fu, Acta Phys. Sin. 54 (2005)

2394.[10] W. Yu, B.Z. Wang, W.B. Lu, Y.B. Yang, L. Han, G.S. Fu, Chin. Phys. Lett.

21 (2004) 1320.[11] W. Yu, W.B. Lu, L. Han, G.S. Fu, J. Phys. D: Appl. Phys. 37 (2004) 3304.[12] R. Swanepoel, J. Phys. E 16 (1983) 1214.[13] W.G. Spitzer, D. Kleinman, D. Walsh, Phys. Rev. 113 (1959) 127.[14] A. Jean, M. Chaker, Y. Diawara, P.K. Leung, E. Gat, P.P. Mercier, H. Pépin,

S. Gujrathi, G.G. Ross, J.C. Kieffer, J. Appl. Phys. 72 (1992) 3110.[15] S. Kerdiles, A. Berthelot, F. Gourbilleau, R. Rizk, Appl. Phys. Lett. 76

(2000) 2373.[16] C.M. Franck, O. Grulke, T. Klinger, Phys. Plasmas 10 (2003) 323.[17] J.W. Davis, A.A. Haasz, P.C. Stangeby, J. Nucl. Mater. 145 (1987) 417.[18] S.H. Kim, I.H. Kim, K.S. Kim, J. Vac. Sci. Technol., A, Vac. Surf. Films 15

(1997) 307.[19] R.W. Boswell, Phys. Lett., A 33 (1970) 457.[20] A.D. Cheetham, J.P. Rayner, J. Vac. Sci. Technol., A, Vac. Surf. Films 16

(1998) 2777.[21] J. Sobol-Antosiak, W.S. Ptak, Mater. Lett. 56 (2002) 842.[22] X.L. Wu, J.Y. Fan, T. Qiu, X. Yang, G.G. Siu, P.K. Chu, Phys. Rev. Lett. 94

(2005) 026102.[23] J.Y. Fan, X.L. Wu, F. Kong, T. Qiu, G.S. Huang, Appl. Phys. Lett. 86

(2005) 171903.