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Self-aggregated Si quantum dots in amorphous Si-rich SiC
Journal of Non-Crystalline Solids 358 (2012) 2126–2129
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Self-aggregated Si quantum dots in amorphous Si-rich SiC Tzu-Chieh Lo, Ling-Hsuan Tsai, Chih-Hsien Cheng, Po-Sheng Wang, Yi-Hao Pai, Chih-I Wu, Gong-Ru Lin ⁎ Graduate Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, No. 1, Roosevelt Road Sec. 4, Taipei 10617, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 5 August 2011 Received in revised form 27 December 2011 Available online 2 February 2012 Keywords: Si-QDs; Solar cells; Self-assembly; Nanostructure
a b s t r a c t Amorphous silicon quantum dots (Si-QDs) self-aggregated in silicon-rich silicon carbide are synthesized by growing with plasma-enhanced chemical vapor deposition on (100)-oriented Si substrate. Under the environment of Argon (Ar)-diluted Silane (SiH4) and pure methane (CH4), the substrate temperature and RF power are set as 350 °C and 120 W, respectively, to provide the Si-rich SiC with changing fluence ratio (R = [CH4 ]/[SiH4] + [CH4]). By tuning the fluence ratio from 50% to 70%, the composition ratio x of Si-rich Si1 − xCx film is varied from 0.27 to 0.34 as characterized by X-ray photoelectron spectroscopy (XPS), which reveals the component of Si2p decreasing from 66.3 to 59.5%, and the component of C1s increasing from 23.9% to 31% to confirm the formation of Si-rich SiC matrix. Annealing of the SiC sample from 650 °C to 1050 °C at 200 °C increment for 30 min induces the very tiny shift on the wavenumber of the crystalline Si (c-Si) related peak due to the precipitation of Si-QDs within the SiC matrix, and the Raman scattering spectra indicate a broadened Raman peak ranging from 410 to 520 cm − 1 related to the amorphous Si accompanied with the significant enhancement for Si\C bond related peak at 980 cm − 1. From the high resolution transmission electron microscopy images, the critical temperature for Si-QD precipitation is found to be 850 °C. The self-assembly of the crystallized Si-QDs with the size of 3± 0.5 nm and the volume density of (3± 1) ×1018 (#/cm3) in Si-rich SiC film with R= 70% are observed after annealing at higher temperature. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Silicon carbide (SiC) is an attractive material for fabricating versatile optoelectronic devices owing to its relatively high electrical resistivity and low dielectric constant [1]. The superior optical characteristics of SiC such as high power handling efficiency and the high-absorption coefficient (α > 10 5 cm − 1) makes it applicable on light-emitting diodes (LEDs) or solar cells [2,3]. Recently, many researches emphasize on utilizing the non-stoichiometric nano-scale Si1 − xCx film as the host matrix for self-aggregating the Si quantum dots (Si-QDs) under specific recipe. The Si-rich Si1 − xCx with tunable wide bandgap was successfully synthesized with versatile methods such as Si and SiC co-sputtering [4], ultrahigh-temperature plasma enhanced chemical vapor deposition (PECVD) [5] with complex fluence mixture of H2 carried SiH4 and Si(CH3)3H or Si2(CH3)6H2. In this study, we propose the synthesis of non-stoichiometric Si1 − xCx by PECVD with the hydrogen-free mixture gas using Argon (Ar)-diluted Silane (SiH4) and pure Methane (CH4). The fluence ratio of R =[CH4]/[SiH4] + [CH4] is detuned to obtain the variable composition ratio of C/Si in Si-rich Si1 − xCx. By increasing the annealing temperature, the excessive Si atoms in non-stoichiometric Si-rich Si1 − xCx start to diffuse and to self-aggregate within the Si1 − xCx matrix.
⁎ Corresponding author. Tel.: + 886 2 33663700x235; fax: + 886 2 33669598. E-mail address: [email protected] (G.-R. Lin).
The experimental results on growing the non-stoichiometric Sirich Si1 − xCx films are briefly explained and the data of composition ratio of the Si-rich SiC films measured are provided by using the XPS analysis. A micro-Raman spectroscopic analysis is employed to elucidate the formation of Si\Si and Si\C bonds and the presence of amorphous Silicon (a-Si), Si-QD, and amorphous SiC (a-SiC) phases within the Si1 − xCx films annealed at different temperatures. The quantity of Si-QDs is determined by high-temperature transmission electron microscopy (HRTEM) and their size distribution is fitted by Gaussian function. 2. Experimental Under the PECVD process conditions with Ar-diluted SiH4 (8%) and pure CH4 gaseous mixtures at substrate temperature of 350 °C, the Si1 − xCx film was deposited on (100)-oriented Si substrate. For removing the native surface oxide, the Si wafers were additionally dipped in classical BOE solution for 5 min. The chamber pressure, RF frequency and RF power were controlled at 0.67 Torr, 13.56 MHz, and 120 W during deposition time for 30 min. The flowing rate of Argon (Ar)-diluted SiH4 (8%) is fixed at 75 sccm (with pure SiH4 of 6 sccm), and the flowing rate of pure CH4 varies from 6 to 14 sccm. The fluence ratio is defined as R = [CH4]/([CH4] + [SiH4]) × 100%. The C/Si composition ratio is detuned by changing the gaseous fluence ratio R from 50% to 70% at increment of 10%.
0022-3093/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.01.013
T.-C. Lo et al. / Journal of Non-Crystalline Solids 358 (2012) 2126–2129
The Si-QDs are self-assembled within the non-stoichiometric Sirich Si1 − xCx by annealing in the argon flowing quartz furnace for 30 min with the annealing temperature changing from 650 °C to 1050 °C at 200 °C increment. The high resolution transmission electron microscopy (HRTEM, JEOL JEM 3010) was employed to characterize the size and distribution of the Si-QDs buried in Si1 − xCx film. The carbon composition ratio and the chemical bonding configuration of Si and C atoms in the alloys were characterized by using X-ray photoelectron spectroscopy (XPS) with Mg Kα line (1253.6 eV) as the source of radiation and a hemispherical energy at working pressure below 5 × 10 − 10 Torr. The surface oxide with thickness of 15 nm was removed by argon sputtering before detection. The Raman spectra were measured by Nd:YAG laser pumping at 532 nm. The system is calibrated with a single crystalline Si wafer to show a significant anti-stoke Raman scattering peak at 520 cm − 1. 3. Results By changing the fluence ratio R from 50 to 70%, the nonstoichiometric Si1 − xCx film with the variable fraction index of x changing from 0.27 to 0.34 is investigated. When ignoring the small amount of the oxygen atoms around ~9% due to the invaded oxygen atoms in SiC film during the annealing process, the C/Si composition ratio of Si1 − xCx film could be detuned from 0.36 to o.52 in this study, and the error estimation of C/Si composition ratio for each sample with its area size around 1 cm 2 is statistically summarized from different measuring points. On the other hand, from the crosssection HRTEM image, the thickness of Si-rich Si1 − xCx film with annealing treatment after 1050 °C was determined as 256 nm by high-resolution transmission electron microscopy in Fig. 1. When increasing the fluence ratio R, the decomposed XPS signal of Si1 − xCx film with variable C/Si composition ratio is shown in Fig. 2. The XPS spectra of the as-grown Si1 − xCx with variable fluence ratio changing from R = 70% to 50% are represented in Fig. 2, which are composed by Si, C, and O related bonds with corresponding binding energies of 99, 283, and 531 eV, respectively [6]. By changing the fluence ratio from 50% to 70%, the XPS intensity ratio of Si2p component is decreased from 66.3 to 59.5%, and the XPS intensity ratio of C1s component is increased from 23.9% to 31%. This is attributed to more carbon atoms dissociated in the SiC matrix to form the Si-rich SiC film. Therefore, the XPS spectra show a strong evidence for the change of C/Si composition ratio in Si-rich SiC film synthesized by changing the fluence ratio R. 4. Discussion To observe the presence of the Si-QDs self-aggregated in amorphous Si-rich SiC after annealing at critical temperature, both the
Fig. 1. The C/Si composition ratio with the fluence ratio varying from R = 50 to 70%, and the inserted figure is the cross-section TEM image of Si-rich SiC film after annealing at 1050 °C.
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Fig. 2. The XPS spectra of SiC film grown with variable fluence ratio from R = 50% to R = 70%.
Raman spectroscopy and the HRTEM analysis are employed to check the Si-QD related Raman scattering peak and their size distribution, respectively. The Raman spectroscopic analysis clearly shows the evidence for the existence of a-Si and a-SiC in the Si-rich SiC matrix grown with different fluence ratios. The Raman scattering spectra for the Si-rich SiC films grown with their fluence ratios changing from 50% to 70% and with their annealing temperature increasing from 650 °C to 1050 °C at 200 °C increment are shown in Fig. 3. All samples are analyzed by using a homemade micro-Raman spectroscope, in which the pumping source is a frequency-doubled Nd:YAG laser at 532 nm (second-harmonic generated and spectrally sliced by an Thorlabs FL532-1 optical band-pass filter with 3-dB linewidth of 1 ± 0.2 nm). The Raman scattering signal is collected by the 10 × or 100× objective lens and coupled into the optical fiber in connection with the a monochromator (CVI, DK480). The scattered light signal is spectrally resolved and passed through a slit prior to the optoelectronic conversion by the photomultiplier tube PMT (Hamamatsu, R928). The resolution of Raman spectrum is enhanced by shrinking the slit width and by reducing the wavenumber increment of the monochromator. By decreasing the slit width and the wavenumber increment from 500 μm to 300 μm and from 4 cm − 1 to 0.7 cm − 1, respectively, the linewidth of the crystalline Si (c-Si) related Raman scattering signal at 520 cm − 1 significantly reduces from 50 cm − 1 to 7 cm − 1. A very tiny shift on wavenumber of the c-Si related peak due to the precipitation of Si-QDs within the SiC matrix could be discriminated by Raman spectroscopy. The a-Si related Raman signal with an extremely large linewidth is clearly observed at all samples, which significantly distinguishes from the reference substrate signal shown at the bottom of each figure in Fig. 3. Similar experimental observation on the a-Si formation associated with a broadening Raman peak at wavenumber smaller than that of c-Si has previously been reported [7]. In that work, the broadened Raman spectrum of the as-grown sample ranging from the 250–475 cm − 1 represents the existed transverse-optical mode of a-Si [7]. After obtaining the Si-QDs by increasing the annealing temperature, the Raman spectrum shows a narrowing peak at wavelength 520 cm − 1 indicating the transformation from a-Si to c-Si phase. As a result, the linewidth of a-Si related Raman scattering signals is ranged between 410 cm− 1 and 510 cm− 1 for as-grown Si-rich SiC samples. After annealing up to 1050 °C, a slightly narrowed Raman spectrum ranging between 430 cm − 1 and 510 cm− 1 and a blueshifted Raman peak at 519 cm− 1 are concurrently observed, which are contributed by the a-Si and the Si-QDs, respectively [8,9]. Therefore, the phase transition of Si from amorphous to crystalline in SiC matrix can be confirmed from the observation of the gradually enhanced Raman peak at 519–520 cm − 1. In contrast, the a-Si related signal intensity increases with the decreasing fluence ratio R, as the
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Fig. 3. The Raman spectra of SiC film grown with fluence ratios R = 50%, R = 60%, and R = 70% at various annealing temperatures.
reducing CH4 environment facilitates the synthesis of a-Si in deposited SiC film. Nevertheless, the precipitation of Si-QD is not always beneficial from Si-rich condition especially when the excessive Si atoms
are sufficiently dense to form a-Si grain instead of Si-QDs. According to previous works and our observations, the optimized R recipe for Si-QD self-aggregation is set between 60% and 80% for obtaining Si-
Fig. 4. The cross-sectional HRTEM images and SAD pattern of the sample at R = 70% with the annealing temperature of 650 (upper left), 850 (lower left) and 1050 °C (right).
T.-C. Lo et al. / Journal of Non-Crystalline Solids 358 (2012) 2126–2129
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of HRTEM image and the volume density (V) of 2 × 10 18 (#/cm 3) can be simply calculated by Dvolume = [Dsurface] 3/2. Alternatively, the Si-QD quantity (NSi-QDs) is divided by the HRTEM monitored sample volume (Dvolume = NSi-QD/V) to obtain the comparable magnitude of volume density. There is a little deviation between the aforementioned calculations due to the overlapping images of the embedded Si-QDs at same position but different depth. Therefore, the trustable volume density of Si-QDs in the Si-rich SiC film is (3 ± 1) × 10 18 (#/cm 3). 5. Conclusion
Fig. 5. The corresponding size distribution of Si-QDs embedded in the Si-rich SiC film.
rich SiC with the C/Si composition ratio around 0.5 ± 0.02. The deposited SiC film with lower or higher C/Si composition ratio could inevitably lead to the enhanced growth of a-Si and a-SiC matrices, such that the growth of Si-QDs could be suppressed even after hightemperature and long-term annealing. In the case of the SiC sample prepared at R = 70% and Ta = 1050 °C recipe, it is apparent that the c-Si related Raman peak caused by the precipitated Si-QDs reveals a larger intensity than those contributed by the a-Si and the SiC host matrices. In this sample, the SiC bond correlated Raman signal at 980 cm − 1 also strengthens by increasing the annealing temperature. This observation is mainly attributed to the significant rebinding effect between Si and C atoms, such that the SiC matrix is formed to induce the isolation of Si nano-grains and/or clusters for the formation of Si-QDs. However, the contrast between c-Si and a-Si related Raman peaks shows a shrinkage when decreasing the fluence ratio to R = 50%, indicating that the suppression on the self-aggregation effect of Si-QDs in SiC matrix with overly Si-rich condition. At last, the cross-sectional HRTEM is performed to detect the SiQDs embedded in the Si-rich Si1 − xCx matrix. After annealing from 650 °C to 1050 °C at 200 °C increment for 30 min, the crosssectional bright-field images for the Si-rich SiC with composition ratio of Si0.66C0.34 are shown in Fig. 4. From the HRTEM images, the SiC film still shows an amorphous structure at annealing temperature of 650 °C (upper left of Fig. 4). When increasing the annealing temperature up to 850 °C (see lower left of Fig. 4), the HRTEM image confirms that the Si-rich Si0.66C0.34 contains few crystallized Si-QDs, which represent the self-aggregation of Si atoms initiating at such high annealing temperature. By annealing up to 1050 °C (see right part of Fig. 4), the evidence on the existence of Si-QDs is significant [10]. To estimate the size distribution of Si-QDs in addition to Fig. 4, the size and quantity statistics for all HRTEM images are obtained from different positions of the Si-rich SiC film annealed at 1050 °C. By plotting the quantity of Si-QDs as a function of size, the size distribution of the Si-QDs fitted by a Gaussian function is illustrated as shown in Fig. 5. The average size and standard deviation of the SiQDs buried in Si-rich SiC film are 3 ± 0.5 nm [11]. On the other hand, the volume density can also be obtained by the HRTEM analysis. For performing HRTEM analysis, the SiC sample with the geometric size of height × length × width = 80 (nm) × 35 (nm) × 35 (nm) is sliced by the focus-ion-beam. Afterwards, there are two methods used to estimate the volume density of Si-QDs[12]. The surface density (S) of 1.6× 1012 (#/cm2) could be calculated by utilizing the number of Si-QDs which were detected in the limit area (35 nm × 35 nm)
In conclusion, the mechanism for growing the Si-QDs by lowtemperature PECVD is investigated. By changing the annealing temperature, the Si-QDs with different sizes are self-aggregated due to the diffusion of the excessive Si atoms in SiC matrix. Under the environment of Argon (Ar)-diluted SiH4 and pure CH4 with the fluence ratio of R = [CH4]/[CH4] + [SiH4] changing from 50% to 70% at increment of 10%, the fraction index x of non-stoichiometric Si1 − xCx changes from 0.27 to 0.34, and the corresponding C/Si composition ratio is easily detuned from 0.36 to 0.52 as confirmed by XPS analysis. To induce the self-aggregation of Si atoms for Si-QD precipitation, the furnace annealing at temperature of 650–1050 °C at 200 °C increment for 30 min in Ar flowing quartz furnace is performed. According to the results of the Raman scattering spectroscopy, the phase transition from a-Si to c-Si can be confirmed from the observation of the gradually enhanced Raman peak at 519–520 cm − 1. The deposited SiC film with lower and higher C/Si composition ratio could inevitably lead to the enhanced growth of a-Si and a-SiC matrices, such that the growth of Si-QDs will be suppressed even after high-temperature and long-term annealing. The HRTEM analysis reveals that the excessive Si atoms self-assemble into numerous Si-QDs when annealing the Si-rich SiC up to 850 °C. By increasing the annealing temperature to 1050 °C, the Si-QDs with average diameter 3 ± 0.5 nm are obtained and the volume density could be controlled at (3 ± 1) × 10 18 (#/cm 3). Acknowledgments The authors appreciate the financial support from the National Science Council of Republic of China and the Excellent Research Projects of National Taiwan University under grants NSC 1002623-E-002-002-ET, NSC 99-2622-E-002-024-CC2, and NSC 982221-E-002-023-MY3. References [1] J. Huran, L. Hrubcin, A. P. Kobzev and J. Liday, Elsevier. 47 (1996) 1223 [2] Y. Tawada, K. Tsuge, M. Kondo, H. Okamoto, J. Appl. Phys. 53 (1982) 5273–5281. [3] D. Kruangama, T. Toyamaa, Y. Hattori, M. Deguchia, H. Okamotoa, Y. Hamakawaa, J. Non-Crys. Solids 97–98 (2003) 293–296. [4] J. Bullot, M.P. Schmidt, Phys. Status Solidi (a) 143 (1987) 345–418. [5] S. Zhang, L. Raniero, E. Fortunato, L. Pereira, N. Martins, P. Canhola, I. Ferreira, N. Nedev, H. Aguas, R. Martins, J. Non-Crys. Solids 338–340 (2004) 530–533. [6] I. Kusunoki, Y. lgari, Appl. Surf. Sci. 59 (1992) 95–104. [7] P. Pellegrino, B. Garrido, C. Garcia, J. Arbiol, J.R. Morante, J. Appl. Phys. 97 (2005) 074312-074312. [8] A. Vivas Hernandez, T.V. Torchynska, Y. Matsumoto, S. Jimenez Sandoval, M. Dybiec, S. Ostapenko, L.V. Shcherbina, Microelectron. J. 36 (2005) 510–513. [9] P. Bibhu, S. Swain, Afr. J. Sci. 105 (2009) 77–80. [10] G.-R. Lin, C.J. Lin, C.K. Lin, Opt. Express 15 (2007) 2555–2563. [11] G.-R. Lin, C.J. Lin, K.C. Yu, J. Appl. Phys. 96 (2004) 3025–3027. [12] G.-R. Lin, C.J. Lin, H.C. Kuo, Appl. Phys. Lett. 91 (2007) 093122.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Self-aggregated Si quantum dots in amorphous Si-rich SiC Tzu-Chieh Lo, Ling-Hsuan Tsai, Chih-Hsien Cheng, Po-Sheng Wang, Yi-Hao Pai, Chih-I Wu, Gong-Ru Lin ⁎ Graduate Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, No. 1, Roosevelt Road Sec. 4, Taipei 10617, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 5 August 2011 Received in revised form 27 December 2011 Available online 2 February 2012 Keywords: Si-QDs; Solar cells; Self-assembly; Nanostructure
a b s t r a c t Amorphous silicon quantum dots (Si-QDs) self-aggregated in silicon-rich silicon carbide are synthesized by growing with plasma-enhanced chemical vapor deposition on (100)-oriented Si substrate. Under the environment of Argon (Ar)-diluted Silane (SiH4) and pure methane (CH4), the substrate temperature and RF power are set as 350 °C and 120 W, respectively, to provide the Si-rich SiC with changing fluence ratio (R = [CH4 ]/[SiH4] + [CH4]). By tuning the fluence ratio from 50% to 70%, the composition ratio x of Si-rich Si1 − xCx film is varied from 0.27 to 0.34 as characterized by X-ray photoelectron spectroscopy (XPS), which reveals the component of Si2p decreasing from 66.3 to 59.5%, and the component of C1s increasing from 23.9% to 31% to confirm the formation of Si-rich SiC matrix. Annealing of the SiC sample from 650 °C to 1050 °C at 200 °C increment for 30 min induces the very tiny shift on the wavenumber of the crystalline Si (c-Si) related peak due to the precipitation of Si-QDs within the SiC matrix, and the Raman scattering spectra indicate a broadened Raman peak ranging from 410 to 520 cm − 1 related to the amorphous Si accompanied with the significant enhancement for Si\C bond related peak at 980 cm − 1. From the high resolution transmission electron microscopy images, the critical temperature for Si-QD precipitation is found to be 850 °C. The self-assembly of the crystallized Si-QDs with the size of 3± 0.5 nm and the volume density of (3± 1) ×1018 (#/cm3) in Si-rich SiC film with R= 70% are observed after annealing at higher temperature. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Silicon carbide (SiC) is an attractive material for fabricating versatile optoelectronic devices owing to its relatively high electrical resistivity and low dielectric constant [1]. The superior optical characteristics of SiC such as high power handling efficiency and the high-absorption coefficient (α > 10 5 cm − 1) makes it applicable on light-emitting diodes (LEDs) or solar cells [2,3]. Recently, many researches emphasize on utilizing the non-stoichiometric nano-scale Si1 − xCx film as the host matrix for self-aggregating the Si quantum dots (Si-QDs) under specific recipe. The Si-rich Si1 − xCx with tunable wide bandgap was successfully synthesized with versatile methods such as Si and SiC co-sputtering [4], ultrahigh-temperature plasma enhanced chemical vapor deposition (PECVD) [5] with complex fluence mixture of H2 carried SiH4 and Si(CH3)3H or Si2(CH3)6H2. In this study, we propose the synthesis of non-stoichiometric Si1 − xCx by PECVD with the hydrogen-free mixture gas using Argon (Ar)-diluted Silane (SiH4) and pure Methane (CH4). The fluence ratio of R =[CH4]/[SiH4] + [CH4] is detuned to obtain the variable composition ratio of C/Si in Si-rich Si1 − xCx. By increasing the annealing temperature, the excessive Si atoms in non-stoichiometric Si-rich Si1 − xCx start to diffuse and to self-aggregate within the Si1 − xCx matrix.
⁎ Corresponding author. Tel.: + 886 2 33663700x235; fax: + 886 2 33669598. E-mail address: [email protected] (G.-R. Lin).
The experimental results on growing the non-stoichiometric Sirich Si1 − xCx films are briefly explained and the data of composition ratio of the Si-rich SiC films measured are provided by using the XPS analysis. A micro-Raman spectroscopic analysis is employed to elucidate the formation of Si\Si and Si\C bonds and the presence of amorphous Silicon (a-Si), Si-QD, and amorphous SiC (a-SiC) phases within the Si1 − xCx films annealed at different temperatures. The quantity of Si-QDs is determined by high-temperature transmission electron microscopy (HRTEM) and their size distribution is fitted by Gaussian function. 2. Experimental Under the PECVD process conditions with Ar-diluted SiH4 (8%) and pure CH4 gaseous mixtures at substrate temperature of 350 °C, the Si1 − xCx film was deposited on (100)-oriented Si substrate. For removing the native surface oxide, the Si wafers were additionally dipped in classical BOE solution for 5 min. The chamber pressure, RF frequency and RF power were controlled at 0.67 Torr, 13.56 MHz, and 120 W during deposition time for 30 min. The flowing rate of Argon (Ar)-diluted SiH4 (8%) is fixed at 75 sccm (with pure SiH4 of 6 sccm), and the flowing rate of pure CH4 varies from 6 to 14 sccm. The fluence ratio is defined as R = [CH4]/([CH4] + [SiH4]) × 100%. The C/Si composition ratio is detuned by changing the gaseous fluence ratio R from 50% to 70% at increment of 10%.
0022-3093/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.01.013
T.-C. Lo et al. / Journal of Non-Crystalline Solids 358 (2012) 2126–2129
The Si-QDs are self-assembled within the non-stoichiometric Sirich Si1 − xCx by annealing in the argon flowing quartz furnace for 30 min with the annealing temperature changing from 650 °C to 1050 °C at 200 °C increment. The high resolution transmission electron microscopy (HRTEM, JEOL JEM 3010) was employed to characterize the size and distribution of the Si-QDs buried in Si1 − xCx film. The carbon composition ratio and the chemical bonding configuration of Si and C atoms in the alloys were characterized by using X-ray photoelectron spectroscopy (XPS) with Mg Kα line (1253.6 eV) as the source of radiation and a hemispherical energy at working pressure below 5 × 10 − 10 Torr. The surface oxide with thickness of 15 nm was removed by argon sputtering before detection. The Raman spectra were measured by Nd:YAG laser pumping at 532 nm. The system is calibrated with a single crystalline Si wafer to show a significant anti-stoke Raman scattering peak at 520 cm − 1. 3. Results By changing the fluence ratio R from 50 to 70%, the nonstoichiometric Si1 − xCx film with the variable fraction index of x changing from 0.27 to 0.34 is investigated. When ignoring the small amount of the oxygen atoms around ~9% due to the invaded oxygen atoms in SiC film during the annealing process, the C/Si composition ratio of Si1 − xCx film could be detuned from 0.36 to o.52 in this study, and the error estimation of C/Si composition ratio for each sample with its area size around 1 cm 2 is statistically summarized from different measuring points. On the other hand, from the crosssection HRTEM image, the thickness of Si-rich Si1 − xCx film with annealing treatment after 1050 °C was determined as 256 nm by high-resolution transmission electron microscopy in Fig. 1. When increasing the fluence ratio R, the decomposed XPS signal of Si1 − xCx film with variable C/Si composition ratio is shown in Fig. 2. The XPS spectra of the as-grown Si1 − xCx with variable fluence ratio changing from R = 70% to 50% are represented in Fig. 2, which are composed by Si, C, and O related bonds with corresponding binding energies of 99, 283, and 531 eV, respectively [6]. By changing the fluence ratio from 50% to 70%, the XPS intensity ratio of Si2p component is decreased from 66.3 to 59.5%, and the XPS intensity ratio of C1s component is increased from 23.9% to 31%. This is attributed to more carbon atoms dissociated in the SiC matrix to form the Si-rich SiC film. Therefore, the XPS spectra show a strong evidence for the change of C/Si composition ratio in Si-rich SiC film synthesized by changing the fluence ratio R. 4. Discussion To observe the presence of the Si-QDs self-aggregated in amorphous Si-rich SiC after annealing at critical temperature, both the
Fig. 1. The C/Si composition ratio with the fluence ratio varying from R = 50 to 70%, and the inserted figure is the cross-section TEM image of Si-rich SiC film after annealing at 1050 °C.
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Fig. 2. The XPS spectra of SiC film grown with variable fluence ratio from R = 50% to R = 70%.
Raman spectroscopy and the HRTEM analysis are employed to check the Si-QD related Raman scattering peak and their size distribution, respectively. The Raman spectroscopic analysis clearly shows the evidence for the existence of a-Si and a-SiC in the Si-rich SiC matrix grown with different fluence ratios. The Raman scattering spectra for the Si-rich SiC films grown with their fluence ratios changing from 50% to 70% and with their annealing temperature increasing from 650 °C to 1050 °C at 200 °C increment are shown in Fig. 3. All samples are analyzed by using a homemade micro-Raman spectroscope, in which the pumping source is a frequency-doubled Nd:YAG laser at 532 nm (second-harmonic generated and spectrally sliced by an Thorlabs FL532-1 optical band-pass filter with 3-dB linewidth of 1 ± 0.2 nm). The Raman scattering signal is collected by the 10 × or 100× objective lens and coupled into the optical fiber in connection with the a monochromator (CVI, DK480). The scattered light signal is spectrally resolved and passed through a slit prior to the optoelectronic conversion by the photomultiplier tube PMT (Hamamatsu, R928). The resolution of Raman spectrum is enhanced by shrinking the slit width and by reducing the wavenumber increment of the monochromator. By decreasing the slit width and the wavenumber increment from 500 μm to 300 μm and from 4 cm − 1 to 0.7 cm − 1, respectively, the linewidth of the crystalline Si (c-Si) related Raman scattering signal at 520 cm − 1 significantly reduces from 50 cm − 1 to 7 cm − 1. A very tiny shift on wavenumber of the c-Si related peak due to the precipitation of Si-QDs within the SiC matrix could be discriminated by Raman spectroscopy. The a-Si related Raman signal with an extremely large linewidth is clearly observed at all samples, which significantly distinguishes from the reference substrate signal shown at the bottom of each figure in Fig. 3. Similar experimental observation on the a-Si formation associated with a broadening Raman peak at wavenumber smaller than that of c-Si has previously been reported [7]. In that work, the broadened Raman spectrum of the as-grown sample ranging from the 250–475 cm − 1 represents the existed transverse-optical mode of a-Si [7]. After obtaining the Si-QDs by increasing the annealing temperature, the Raman spectrum shows a narrowing peak at wavelength 520 cm − 1 indicating the transformation from a-Si to c-Si phase. As a result, the linewidth of a-Si related Raman scattering signals is ranged between 410 cm− 1 and 510 cm− 1 for as-grown Si-rich SiC samples. After annealing up to 1050 °C, a slightly narrowed Raman spectrum ranging between 430 cm − 1 and 510 cm− 1 and a blueshifted Raman peak at 519 cm− 1 are concurrently observed, which are contributed by the a-Si and the Si-QDs, respectively [8,9]. Therefore, the phase transition of Si from amorphous to crystalline in SiC matrix can be confirmed from the observation of the gradually enhanced Raman peak at 519–520 cm − 1. In contrast, the a-Si related signal intensity increases with the decreasing fluence ratio R, as the
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Fig. 3. The Raman spectra of SiC film grown with fluence ratios R = 50%, R = 60%, and R = 70% at various annealing temperatures.
reducing CH4 environment facilitates the synthesis of a-Si in deposited SiC film. Nevertheless, the precipitation of Si-QD is not always beneficial from Si-rich condition especially when the excessive Si atoms
are sufficiently dense to form a-Si grain instead of Si-QDs. According to previous works and our observations, the optimized R recipe for Si-QD self-aggregation is set between 60% and 80% for obtaining Si-
Fig. 4. The cross-sectional HRTEM images and SAD pattern of the sample at R = 70% with the annealing temperature of 650 (upper left), 850 (lower left) and 1050 °C (right).
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of HRTEM image and the volume density (V) of 2 × 10 18 (#/cm 3) can be simply calculated by Dvolume = [Dsurface] 3/2. Alternatively, the Si-QD quantity (NSi-QDs) is divided by the HRTEM monitored sample volume (Dvolume = NSi-QD/V) to obtain the comparable magnitude of volume density. There is a little deviation between the aforementioned calculations due to the overlapping images of the embedded Si-QDs at same position but different depth. Therefore, the trustable volume density of Si-QDs in the Si-rich SiC film is (3 ± 1) × 10 18 (#/cm 3). 5. Conclusion
Fig. 5. The corresponding size distribution of Si-QDs embedded in the Si-rich SiC film.
rich SiC with the C/Si composition ratio around 0.5 ± 0.02. The deposited SiC film with lower or higher C/Si composition ratio could inevitably lead to the enhanced growth of a-Si and a-SiC matrices, such that the growth of Si-QDs could be suppressed even after hightemperature and long-term annealing. In the case of the SiC sample prepared at R = 70% and Ta = 1050 °C recipe, it is apparent that the c-Si related Raman peak caused by the precipitated Si-QDs reveals a larger intensity than those contributed by the a-Si and the SiC host matrices. In this sample, the SiC bond correlated Raman signal at 980 cm − 1 also strengthens by increasing the annealing temperature. This observation is mainly attributed to the significant rebinding effect between Si and C atoms, such that the SiC matrix is formed to induce the isolation of Si nano-grains and/or clusters for the formation of Si-QDs. However, the contrast between c-Si and a-Si related Raman peaks shows a shrinkage when decreasing the fluence ratio to R = 50%, indicating that the suppression on the self-aggregation effect of Si-QDs in SiC matrix with overly Si-rich condition. At last, the cross-sectional HRTEM is performed to detect the SiQDs embedded in the Si-rich Si1 − xCx matrix. After annealing from 650 °C to 1050 °C at 200 °C increment for 30 min, the crosssectional bright-field images for the Si-rich SiC with composition ratio of Si0.66C0.34 are shown in Fig. 4. From the HRTEM images, the SiC film still shows an amorphous structure at annealing temperature of 650 °C (upper left of Fig. 4). When increasing the annealing temperature up to 850 °C (see lower left of Fig. 4), the HRTEM image confirms that the Si-rich Si0.66C0.34 contains few crystallized Si-QDs, which represent the self-aggregation of Si atoms initiating at such high annealing temperature. By annealing up to 1050 °C (see right part of Fig. 4), the evidence on the existence of Si-QDs is significant [10]. To estimate the size distribution of Si-QDs in addition to Fig. 4, the size and quantity statistics for all HRTEM images are obtained from different positions of the Si-rich SiC film annealed at 1050 °C. By plotting the quantity of Si-QDs as a function of size, the size distribution of the Si-QDs fitted by a Gaussian function is illustrated as shown in Fig. 5. The average size and standard deviation of the SiQDs buried in Si-rich SiC film are 3 ± 0.5 nm [11]. On the other hand, the volume density can also be obtained by the HRTEM analysis. For performing HRTEM analysis, the SiC sample with the geometric size of height × length × width = 80 (nm) × 35 (nm) × 35 (nm) is sliced by the focus-ion-beam. Afterwards, there are two methods used to estimate the volume density of Si-QDs[12]. The surface density (S) of 1.6× 1012 (#/cm2) could be calculated by utilizing the number of Si-QDs which were detected in the limit area (35 nm × 35 nm)
In conclusion, the mechanism for growing the Si-QDs by lowtemperature PECVD is investigated. By changing the annealing temperature, the Si-QDs with different sizes are self-aggregated due to the diffusion of the excessive Si atoms in SiC matrix. Under the environment of Argon (Ar)-diluted SiH4 and pure CH4 with the fluence ratio of R = [CH4]/[CH4] + [SiH4] changing from 50% to 70% at increment of 10%, the fraction index x of non-stoichiometric Si1 − xCx changes from 0.27 to 0.34, and the corresponding C/Si composition ratio is easily detuned from 0.36 to 0.52 as confirmed by XPS analysis. To induce the self-aggregation of Si atoms for Si-QD precipitation, the furnace annealing at temperature of 650–1050 °C at 200 °C increment for 30 min in Ar flowing quartz furnace is performed. According to the results of the Raman scattering spectroscopy, the phase transition from a-Si to c-Si can be confirmed from the observation of the gradually enhanced Raman peak at 519–520 cm − 1. The deposited SiC film with lower and higher C/Si composition ratio could inevitably lead to the enhanced growth of a-Si and a-SiC matrices, such that the growth of Si-QDs will be suppressed even after high-temperature and long-term annealing. The HRTEM analysis reveals that the excessive Si atoms self-assemble into numerous Si-QDs when annealing the Si-rich SiC up to 850 °C. By increasing the annealing temperature to 1050 °C, the Si-QDs with average diameter 3 ± 0.5 nm are obtained and the volume density could be controlled at (3 ± 1) × 10 18 (#/cm 3). Acknowledgments The authors appreciate the financial support from the National Science Council of Republic of China and the Excellent Research Projects of National Taiwan University under grants NSC 1002623-E-002-002-ET, NSC 99-2622-E-002-024-CC2, and NSC 982221-E-002-023-MY3. References [1] J. Huran, L. Hrubcin, A. P. Kobzev and J. Liday, Elsevier. 47 (1996) 1223 [2] Y. Tawada, K. Tsuge, M. Kondo, H. Okamoto, J. Appl. Phys. 53 (1982) 5273–5281. [3] D. Kruangama, T. Toyamaa, Y. Hattori, M. Deguchia, H. Okamotoa, Y. Hamakawaa, J. Non-Crys. Solids 97–98 (2003) 293–296. [4] J. Bullot, M.P. Schmidt, Phys. Status Solidi (a) 143 (1987) 345–418. [5] S. Zhang, L. Raniero, E. Fortunato, L. Pereira, N. Martins, P. Canhola, I. Ferreira, N. Nedev, H. Aguas, R. Martins, J. Non-Crys. Solids 338–340 (2004) 530–533. [6] I. Kusunoki, Y. lgari, Appl. Surf. Sci. 59 (1992) 95–104. [7] P. Pellegrino, B. Garrido, C. Garcia, J. Arbiol, J.R. Morante, J. Appl. Phys. 97 (2005) 074312-074312. [8] A. Vivas Hernandez, T.V. Torchynska, Y. Matsumoto, S. Jimenez Sandoval, M. Dybiec, S. Ostapenko, L.V. Shcherbina, Microelectron. J. 36 (2005) 510–513. [9] P. Bibhu, S. Swain, Afr. J. Sci. 105 (2009) 77–80. [10] G.-R. Lin, C.J. Lin, C.K. Lin, Opt. Express 15 (2007) 2555–2563. [11] G.-R. Lin, C.J. Lin, K.C. Yu, J. Appl. Phys. 96 (2004) 3025–3027. [12] G.-R. Lin, C.J. Lin, H.C. Kuo, Appl. Phys. Lett. 91 (2007) 093122.