Effect of substrate temperature on gold-catalyzed silicon nanostructures growth by hot-wire chemical vapor deposition (HWCVD)

Applied Surface Science 257 (2011) 3320–3324

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Effect of substrate temperature on gold-catalyzed silicon nanostructures growth by hot-wire chemical vapor deposition (HWCVD) Su Kong Chong a,∗ , Boon Tong Goh a , Zarina Aspanut a , Muhamad Rasat Muhamad a , Chang Fu Dee b , Saadah Abdul Rahman a a b

Low Dimensional Materials Research Centre, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

a r t i c l e

i n f o

Article history: Received 8 July 2010 Received in revised form 11 October 2010 Accepted 2 November 2010 Available online 10 November 2010 Keywords: Silicon nanostructures HWCVD Crystallinity HRTEM Microstructure parameter

a b s t r a c t The effect of substrate temperature on the structural property of the silicon nanostructures deposited on gold-coated crystal silicon substrate by hot-wire chemical vapor deposition (HWCVD) was studied. The uniformity and size of the as-grown silicon nanostructures is highly influenced by the substrate temperature. XRD, Raman and HRTEM measurements show the silicon nanostructures consist of small crystallites embedded within amorphous matrix. The crystallite size of the as-grown silicon nanostructures decreases with increases in substrate temperature. FTIR shows that these silicon nanostructures are highly disordered for sample prepared at substrate temperature above 250 ◦ C. The correlation of crystallinity and structure disorder of the silicon nanostructures growth at different substrate temperature was discussed. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Fabrication and characterization of silicon nanostructures which includes nano-wires, whiskers, pillars, tubes, cones and particles have been widely studied due to their unique physical properties and applications in future development in nanoelectronic (FET, diode, sensor) [1], nanophotonic [2,3] and optoelectronic devices [4]. Plenty of investigations to optimize the growth conditions of silicon nanostructures were done using variety fabrication techniques including magnetron sputtering [5], chemical vapor deposition [6,7], thermal evaporation [8], laser ablation [9,10], and molecular-beam epitaxy [11]. Compare to these deposition techniques, hot-wire chemical vapor deposition (HWCVD) which also called catalytic or initiated CVD is known as a new technique for silicon nanostructures deposition. High uniformity of silicon nanostructures films are believed to be deposited using HWCVD due to its advantages of high decomposition rate on precursor and free of ion bombardment [12]. In HWCVD, silane (SiH4 ) gas as a source for silicon nanostructures deposition usually can be decomposed into Si-Hx free radical at filament temperature between 1750 and 1950 ◦ C [13]. The chemical decompositions and reactions of SiH4 gas using HWCVD had been studied systematically [14,15].

∗ Corresponding author. Tel.: +60 3 7967 4147; fax: +60 3 7967 4146. E-mail address: [email protected] (S.K. Chong). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.11.012

In bottom up approach, metal catalysts play a main role in growth of silicon nanostructures. Metal such as Au, Fe, Cu, Co, Al, In, and Ga are used to form silicon-metal alloy, which has lower melting temperature than silicon (Tm of Si ∼1100 ◦ C). Among of these, Au becomes a favorable catalyst since it form Si–Au alloy (18.6%) with melting point of about 363 ◦ C and prevent from oxidation compare to Al and Fe. Besides, Au also acts as a promising candidate in metal induced crystallization (MIC) process which had promoting crystallization of a-Si [16,17], a-Ge [18] and even protein [19]. For common MIC process, a thin layer of metal is deposited directly on a-Si film and then annealed to obtain poly-Si structure [20]. During annealing, the interface diffusion of metal/Si layers induced the formation of crystalline Si structure at temperature below the intrinsic crystallization temperature of a-Si, which is 600 ◦ C [21]. Different types of metal can induce different degree of crystallization depends on annealing temperature that form a eutectic with Si [18]. Among of these, Au and Al are common used in MIC as they resist to silicide formation. From literature [16], Au is found to induce higher Si crystalline fraction compared to Al. However, the reported Au induced crystallization temperature of 500 ◦ C is still quite high for large area device manufacturing. In this work, we propose the Au induced growth and crystallization of silicon nanostructures at temperature lower than 400 ◦ C. The silicon nanostructures were grown on Au coated Si (1 1 1) substrates by decomposition of pure SiH4 using HWCVD at substrate temperatures, Ts of 200–400 ◦ C in an increment of 50 ◦ C. The effect of Ts on morphological, structural property and chem-

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Fig. 1. Side view of the schematic diagram of our home-built HWCVD reaction chamber.

ical bonding of the silicon nanostructures have been investigated using field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), micro-Raman scattering spectroscopy, high resolution transmission electron microscope (HRTEM), and Fourier transform Infrared (FTIR) spectroscopy. 2. Experimental Silicon nanostructures samples were deposited on p-type Si (1 1 1) substrates using a home-built HWCVD system as shown in Fig. 1. The cleaning process of substrates followed the RCA-I and II cleaning methods [22]. Prior to the deposition of the samples, a thin layer of Au about 7 nm was coated on the surface of substrates by using conventional DC sputter coater. Spiral tungsten with length and diameter of 30 mm and 3 mm, respectively, was used as a hotfilament for dissociation of the SiH4 gas. The distance between the filament and the substrate holder was fixed at about 4 cm. This distance is enough to provide mean free path for Si-Hx free radicals [23] and also to prevent the thermal effect on the substrates [24]. Five sets of samples were prepared at different Ts varied from 200 to 400 ◦ C with an interval of 50 ◦ C. The base pressure of the chamber was about 2 × 10−5 mbar. The SiH4 gas flow-rate and deposition pressure were maintained at 5 sccm and 0.18 mbar, respectively. The filament temperature, Tfil was measured by pyrometer (Reytek, Raynger 3i) and its temperature is controlled at about 1900 ◦ C. The deposition time were fixed at 15 min for all the samples. The surface morphological and the distribution of the nanostructures of samples were observed using FEI Quanta 200 FESEM. The XRD measurement was carried out using SIEMENS D5000 ˚ The specX-ray diffractometer (Cu K␣ X-ray radiation  = 1.5418 A). tra were taken at a grazing angle of 5◦ . The Raman spectra of the samples were recorded using a Horiba Jobin Yvon 800 UV micro-Raman spectrometer with a CCD detector. The excitation wavelength was selected at 514.5 nm by using Ar+ laser with the power of 20 mW. The laser was focus onto a spot of 1 ␮m in diameter to collect the backscattered light from the samples. HRTEM and selected area electron diffraction (SAED) images of the nanos-

Fig. 2. (a) FESEM images of the samples deposited at different Ts of 300 ◦ C, with inserting of the cross-sectional view image, and (b) the distribution of diameter size in percentage for silicon nanostructures deposited at different Ts .

tructure were obtained from a JEOL-JEM-2010F HRTEM operating at 200 kV. The Si–H bonding configurations, hydrogen content, CH , and microstructure parameter, R, of the samples were investigated by Perkin-Elmer System 2000 FTIR spectra in the range of 400–4000 cm−1 . 3. Results and discussion The surface and cross-sectional images of the sample deposited at Ts of 300 ◦ C obtained by FESEM are shown in Fig. 2(a). The sample reveals columnar like nanostructures which are evenly distributed on the surface. The percentage of diameter distribution of nanostructures estimated from the FESEM images in an area of 2.5 ␮m × 2.0 ␮m with Ts is illustrated in Fig. 2(b). Generally, the diameters of nanostructures of all samples are varied within 50–200 nm. However, the level of uniformity of these nanostructures is much depending on the Ts . The nanostructures prepared at Ts of 200 and 250 ◦ C exhibit diameter of equal or less than 100 nm. The percentage of nanostructures with diameter equal or less than 100 nm decrease to about 75% at Ts of 300 ◦ C and slightly increase to about 86% and 92% at Ts of 350 and 400 ◦ C, respectively. Wide range of diameter of nanostructures can be observed for samples prepared at Ts of 300 ◦ C and above. The irregular distribution of nanostructures grown at high temperatures is due to the surface heating effect which attributed to the removing of hydrogen from Si:H networks and re-clustering of the nanostructures.

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Fig. 3. Spectra of the background (bgd) and samples deposited by HWCVD at different Ts .

Fig. 3 shows the XRD spectra of the samples deposited at different Ts . The spectrum for the c-Si substrate as a background is also inserted in the figure as a reference. Appearance of sharp diffraction peaks at 2 = 28.4◦ , 47.3◦ , 56.4◦ , 69.6◦ and 76.8◦ which correspond to the crystalline silicon orientation planes of (1 1 1), (2 2 0), (3 1 1), (4 0 0) and (3 3 1), respectively, indicating that the films are highly crystalline in structure. Furthermore, appearance of the diffraction broadening at 38.2◦ corresponds to Au orientation plane of (1 1 1) confirms the presence of Au nanocrystallites in the films. The crystallite size of Au (DAu ) estimated from Scherrer’s equation [25] is in the range of 6–14 nm as depicted in Fig. 3. Surprisingly, the largest value of DAu (∼14 nm) is observed for sample prepared at the Ts of 250 ◦ C, which actually induce growth of the most uniform nanostructures with smallest diameter compared to other samples. Moreover, the existing of Au nanocrystallites had successfully induced the crystallization of the films by using pure SiH4 gas, which usually form amorphous nature of silicon films as reported by Jadkar et al. [26] using similar technique. In this case, the Au nanocrystallites are suspected to diffuse into silicon layer and induce different degree of silicon crystallization depending on Ts . The crystallite size of silicon estimated from Scherrer’s equation as a function of Ts is shown in Fig. 4. The crystalline silicon planes of (1 1 1) and (2 2 0) were used to calculate the crystallite size of samples, namely D1 1 1 and D2 2 0 , respectively, due to the high intensity of diffraction. The crystalline silicon plane of (3 1 1)

Fig. 5. Raman spectra of the samples deposited by HWCVD at different Ts and (inset is the deconvoluted Gaussian peaks of sample prepared at 400 ◦ C).

was not employed in calculation due to the influence of substrate effect as observed from the Fig. 3. Generally, the D1 1 1 and D2 2 0 varied from 25.2 to 20.0 nm and 12.2 to 10.0 nm, respectively. In the figure, both crystallites, D1 1 1 and D2 2 0 , decrease with increase in Ts up to 300 ◦ C. D1 1 1 increases slightly and D2 2 0 increases gradually with further increase in Ts to 400 ◦ C. At Ts of 200–300 ◦ C, decrease in crystallite size can be a reason of desorption of atoms due to the higher absorption energy than the surface energy [17]. At Ts above 300 ◦ C, higher surface heating effect results the increase in crystallite size which is obviously happened to the crystalline plane at (2 2 0). Fig. 5 shows the Raman spectra of samples deposited at different Ts . Appearance of sharp peaks at around 510 cm−1 corresponded to transverse optical (TO)-phonon modes for all the samples with shoulder indicates the films consist of nanocrystallites of silicon embedded within the amorphous matrix. The TO-phonon Raman peak can be deconvoluted into three Gaussian peaks corresponding to amorphous, grain boundary and crystalline components at 480 cm−1 , 500 cm−1 and 520 cm−1 , respectively. In these films, the downshift of peaks from 520 cm−1 can be due to the presence of the nanocrystallites. These nanocrystallites are usually surrounding by grains boundaries. The inserted figure shows the deconvoluted Gaussian peaks of the Raman spectrum of sample prepared at Ts of 400 ◦ C. According to the principle proposed in [27], the size of nanocrystallites can be estimated by the following equation:

 DR = 2

B , ω

where B is 2.24 cm−1 nm2 for silicon and ω is the shifting of crystalline peak from single crystal silicon peak located at 520 cm−1 . The crystalline volume fraction, XC of the samples can be calculated from the deconvolution data using following equation [28]:

XC =

I520 + I500 , ˇI480 + I520 + I500

where I520 , I500 and I480 are integrated intensities of the Raman peaks corresponding to crystalline, grain boundaries and amorphous components, respectively. The factor ˇ is the ratio of the cross-section of the amorphous phase to the crystalline phase, and is defined as: Fig. 4. Variation of crystallite sizes (D1 1 1 and D2 2 0 ) with Ts for the samples deposited by HWCVD.

ˇ = 0.1 + exp

 −d  250

,

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Fig. 6. Variation of DR and XC determined from the Raman spectra with Ts for the samples deposited by HWCVD.

where d is the grain size in nm. In the case of the ␮c-Si and nc-Si, with their small crystallites, ˇ is ∼ =1. The variation of DR and XC as a function of Ts for the samples is shown in Fig. 6. The DR decreases from 7 to 3.8 nm with increase in Ts from 200 to 300 ◦ C and slightly increases to 4.4 nm with further increase in Ts to 400 ◦ C. The variation of DR is agreed with the variation of crystallite sizes estimated from XRD spectra. Also, a well matched trend for D1 1 1 and DR indicates a preferential orientation of (1 1 1) plane for all the samples which are parallel to the Au induced crystallization study reported by Pereira et al. [29]. Both XRD and Raman results indicate the existing of nanocrystallites which surrounded by grains boundaries within the nanostructures. These nanocrystallites were clustering together forming different diameter of nanocolumnar structures as observed in Fig. 2. Generally, the XC decrease gradually from ∼72% to 58% with increase in Ts from 200 to 400 ◦ C. Slightly increase in XC for sample prepared at Ts of 350 ◦ C might due to the more compact structure compared to Ts of 300 ◦ C. Highest value of XC at Ts of 200 ◦ C is expected because of the high density of nanostructures with the largest crystallite size distributed on the surface of sample. At higher Ts , the crystallite size slowly increases but the amount of nanostructures decreases which contribute to the reduction of XC . HRTEM measurement was carried out on the as-grown silicon nanostructures to further confirm the crystalline and amorphous structures. Fig. 7(a) shows the TEM image of the silicon nanostructure prepared at Ts of 300 ◦ C. Large amount of nanoparticles with darker color which reveal highly crystalline structure compared to

Fig. 7. (a) TEM image, (b) high resolution TEM image, and (c) SAED pattern of the silicon nanostructure prepared at Ts of 300 ◦ C.

Fig. 8. FTIR spectra of (a) Si–H and Si–O bonds vibration modes in wavenumber of 500–1350 cm−1 and (b) Si–H stretching mode of the samples deposited at different Ts .

the brighter side of amorphous structure are distributed throughout the nanostructures. It clearly shows that the nanostructures actually consist of high density of Si nanocrystallites embedded in amorphous matrix and again agreed with the results predicted by XRD and Raman spectra. HRTEM image depicted in Fig. 7(b) shows the Si nanocrystallites with lattice fringes of 0.31 and 0.20 nm consistent with the {1 1 1} and {2 2 0} orientation planes. The crystallite size of the Si nanocrystallites estimated from HRTEM images is about 20.0 ± 0.7 nm and 8.3 ± 2.6 nm for {1 1 1} and {2 2 0} orientations, respectively. The result shows a well match with D1 1 1 (20.0 ± 2.2 nm) and D2 2 0 (9.9 ± 0.8 nm) calculated from XRD spectra. Furthermore, the crystalline to amorphous ratio estimated from 100 nm × 100 nm of TEM image is about 52.8 ± 4.3% which is slightly less than the XC (60.5 ± 1.2%) calculated from Raman peak. This is because the HRTEM image provides a two-dimensional view on the structural and crystal lattice of the nanostructures; but, the crystallites embedded in a few hundred nanometers beneath the nanostructures can be detected from Raman scattering. Diffraction rings with weak spots of crystal Si correspond to {1 1 1}, {2 2 0} and {3 1 1} planes are observed from SAED pattern as shown in Fig. 7(c). The brighter spotty background revealed in crystal Si {1 1 1} diffraction ring confirms the preferential growth of {1 1 1} plane. The Si–H and Si–O related vibration modes of FTIR spectra of the samples prepared at different Ts are illustrated in Fig. 8(a) and (b). The CH was determined from the absorption band of Si–H wagging mode at 630 cm−1 [30] and microstructure parameter, R is calculated through the ratio of the integrated intensity of the absorption

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nanostructures prepared at Ts of 200 ◦ C indicates that Au can induce crystallization of a-Si at low temperature. For nanostructures deposited at Ts of 250 ◦ C and above, the structure is highly disorder in amorphous silicon structure. Acknowledgements This work was supported by the Ministry of Higher Education under Fundamental Research Grant Scheme (FRGS) of FP008/2008C, University Malaya Research Grant (UMRG) of RG061/09AFR and University of Malaya Postgraduate Research Fund (PPP) of PS310/2009B. References

Fig. 9. Variation of CH and R calculated with different Ts for samples deposited by HWCVD.

peak at 2090 cm−1 to the sum of the integrated intensity of the absorption peaks at 2000 and 2090 cm−1 [31]. The variation of CH and R with Ts is shown in Fig. 9. The CH decreases gradually from about 3.5% to 1.6% with increase in Ts from 200 to 400 ◦ C. The reduction of CH with temperature is due to evolution of hydrogen from the nanostructures which occurred at higher temperatures. Moreover, it is important to note that the CH is quite low which is below 4% for all samples as observed by other researchers [26,32]. Low CH exhibited by HWCVD deposited Si films might eliminates the de-hydrogenation step before the crystallization. The R increases significantly from 0.74 to 0.93 with increase in Ts from 200 to 250 ◦ C and slightly increases to 0.95 with further increase in Ts to 400 ◦ C. High value of R indicates the nanostructures are highly disorder due to high density of SiH2 bond in the samples [33]. Presence of Au as a catalyst induced the formation of nanostructures, however, increases in Ts enhanced the size of nanostructures and these nanostructures are mainly disorder in structure with larger R. From literature [33], high concentration of SiH2 in nc-Si:H film can be correlated to the crystallinity where the SiH2 is formed at the grains boundaries. However, the XC decreases gradually but the R remained at high values with increase in Ts indicate that the disorder is formed by SiH2 within amorphous Si structure. 4. Conclusions Effect of Ts on the structural property of Au-induced silicon nanostructures growth by HWCVD was discussed. High yield and uniform morphology of columnar-like nanostructures with diameter equal or less than 100 nm have been produced at Ts below 300 ◦ C. The nanostructures consist of high density of nanocrystallites embedded in amorphous matrix with preferential orientation of (1 1 1) crystal silicon plane. Highest XC of 72% exhibited by

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