2018 15th International Conference on Electrical Engineering, Computing Science and Automatic Control (CCE) Mexico City, Mexico September 5-7, 2018
Luminescent Silicon Oxycarbide Thin Films via Hot-wire CVD using Tetraethyl Orthosilicate: Role of the Chamber Pressure and Post-deposition Annealing J. R. Ramos-Serrano
Y. Matsumoto
C. Morales
Programa de Nanociencias y Nanotecnolog´ıa CINVESTAV-IPN M´exico City, M´exico Email:
[email protected]
Departamento de ingenier´ıa el´ectrica CINVESTA-IPN M´exico City, M´exico Email:
[email protected]
Centro de investigaci´on en dispositivos semiconductores BUAP Puebla, M´exico Email:
[email protected]
Abstract—We report the obtention of luminescent silicon oxycarbide thin films deposited by Hot-wire chemical vapor deposition technique using tetraethyl orthosilicate as precursor. Additionally, we study the effect of the chamber pressure and the post-deposition thermal annealing in oxygen and hydrogen environments on the properties of the films. All the as-deposited samples showed an intense and wide emission band centered in the blue region. The samples deposited at 0.3 and 0.5 Torr presented a high surface uniformity. The X-ray diffraction measurements did not show the presence of nanocrystalline phase, so, we attributed the emission bands to defects in the silicon oxycarbide matrix, mainly related to oxygen deficiency centers, and to hydrogen and carbon-related defects. After the thermal annealing in oxygen and hydrogen, the samples showed a significative reduction in the emission intensity because of radiative defects passivation. Keywords—HWCVD, TEOS, visible PL, pressure, annealing.
I. I NTRODUCTION Currently, the investigation on the use of silicon-based materials for optoelectronic devices has aroused interest due to the enormous advantages that might represent, as the direct integration of optoelectronic devices with the actual technologies at low costs. However, the development of siliconbased optoelectronic devices represents a problem because of the bulk silicon has a very low radiation-emission efficiency due to its indirect band-gap transition. From the discovery of visible luminescence in porous silicon [1], many reports on luminescent silicon-based materials, mainly silicon oxide, and silicon nitride have been described [2], [3]. Nowadays, the silicon oxycarbide (SiOC) is focus of numerous studies due to its properties, as diffusion barrier, low-k dielectric and more recently as silicon-based luminescent thin film, where has been possible to obtain a strong white luminescence [4], [5]. The SiOC has been obtained by different methods [6]. For the deposition of SiOC commonly a mixture of silane and methane is used. As an alternative for the SiOC thin films deposition the use of organic-based precursors as monometyl-silane and tetraethyl orthosilicate (TEOS) have been studied [7], [8].
c 978-1-5386-7033-0/18/$31.00 2018 IEEE
A previous report showed the obtention of intense white luminescence in SiOC films deposited by Hot-wire chemical vapor deposition (HW-CVD) using TEOS as precursor without the need of post-deposition thermal annealing [9]. In this work, we report the obtention of luminescent SiOC thin films deposited by HW-CVD technique at low substrate temperature using TEOS as precursor. The obtained films did not require a post-deposition thermal annealing to induce the luminescence. Additionally, we study the effect of the chamber pressure and the post-deposition thermal annealing in oxygen and hydrogen environments on the properties of the films. II. E XPERIMENT A. Sample preparation SiOC thin films were deposited by the HW-CVD technique using TEOS (Sigma-Aldrich, reagent grade, 98%) as precursor and argon as carrier gas. A tungsten wire of 0.75 mm diameter at 1800 o C with 5 cm as wire-substrate distance was used as catalyzer. The films were deposited on n-type mirror-polished crystalline silicon with orientation (100) and on Corning glass, both heated at 200 o C. The argon flow was kept constant at 30 standard cubic centimeters per minute (sccm) meanwhile the chamber pressure was varied at 0.1, 0.3, and 0.5 Torr. Furthermore, thermal annealing were carried out on samples deposited at 0.3 Torr at different environments for 30 minutes. Thermal annealing in 30% oxygen diluted in nitrogen at atmospheric pressure at 600 and 750 o C were done. Besides, other sample was annealing in hydrogen plasma at 300 o C. The hydrogen plasma was formed in the HW-CVD system with a 20 sccm hydrogen flow through a tungsten wire at 2000 o C and a chamber pressure of 0.1 Torr. B. Characterization The refractive index and film thickness were obtained in a Gaertner ellipsometer with a variable angle and a 632.8 nm wavelength. X-ray diffraction (XRD) measurements were carried out in a PANalytical XPERT-PRO diffractometer with
suggest that the concentration of these in the films are very low. 0.5 Torr
Intensity (a.u.)
0.3 Torr
monochromatic Cu-Kα radiation in order to determinate the presence of nanocrystalline material in the films. To identify the different types of bonds in the films Fourier Transform Infrared (FTIR) spectra were obtained in a Thermo-Nicoletnexus-470. Photoluminescence (PL) spectra were obtained in a Nanolog from Horiba Jobin Yvon using an excitation wavelength of 350 nm at room temperature. III. R ESULTS AND D ISCUSSION The samples deposited at different chamber pressures show difference on the surface appearance, where the sample deposited at 0.1 Torr displays the formation of concentric rings, while, the other samples display an uniform deposition as the figure 1 shows. Nevertheless, the sample deposited at 0.5 Torr had a very low growth rate and it shows the formation of silicates as fine powders on to the chamber and the substrate. The formation of these powders may be related with an increment of the reactions in homogeneous phase due to the chamber pressure, because of this, the precursors reacted in vapor phase before reaching the substrate precipitating as solids. Table 1 shows the thickness and the refractive index obtained by ellipsometry. For the sample deposited at 0.1 Torr the measurement was carried out on the inner point. The thickness has an inverse relation with the chamber pressure being this at 0.1 Torr close to 8 times thicker than sample deposited at 0.5 Torr. On the other hand, the refractive index remains practically at the same value at different chamber pressures. Figure 2 shows the XRD patterns of the samples deposited at different chamber pressures on glass. All samples present a broad peak related with amorphous material, and it was not possible to observe any diffraction peak corresponding to crystalline material. Although the XRD measurements are not enough to discard the formation of nanocrystals, the results TABLE I R EFRACTIVE INDEX AND THICKNESS BY ELLIPSOMETRY OF
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2 theta (degrees)
Fig. 2. XRD patterns of the as-deposited samples at different chamber pressures.
A variation of several orders of magnitude in the emission intensities was observed, where the as-deposited samples display intensities of 107 ,106 , 104 counts per second (CPS) for 0.1, 0.3 and 0.5 Torr, respectively . This variation may be due to the differences in the samples thicknesses. Figure 3 shows the PL spectra of the samples deposited at different chamber pressures. The PL spectra are normalized for a better comparation. All the samples show a wide band in the visible region, centered in the blue-green region between 380 and 420 nm. The presence of nanocrystals in a dielectric matrix is reported as the base of the quantum confinement (QC) effects in silicon-based materials [10]. So, because the nanocrystalline phases could not be observed by XRD measurements, the QC effects might not play a main role in the film emission. Normally, this visible emission-band is commonly related with a different kind of defects related with the SiOC matrix, as neutral oxygen vacancy (NOV), self-trapped exciton (STE), and E’ centers [11], [12]. Furthermore, this band can be attributed to the formation of carbon nanoclusters C=C which
Normalized Intensity (a.u.)
Fig. 1. Morphological features of the samples deposited at a) 0.1, b) 0.3, and c) 0.5 Torr.
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AS - DEPOSITED SAMPLES OBTAINED AT DIFFERENT CHAMBER PRESSURE . 400
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Refractive index 1.49 1.46 1.44
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Fig. 3. Normalized PL spectra of the as-deposited samples at different chamber pressures.
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Fig. 4. FTIR spectra of the as-deposited samples at different chamber pressures.
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could provide luminescence centers in the visible spectrum. Also, the blue emission could be due to transitions in the socalled C-related NOV (O3 ≡ C − Si ≡ O3 ) [13]. On the other hand, the sample deposited at 0.1 Torr shows an additional emission-band in the green-orange region centered at 500 nm. This band may be related to defects too, mainly with nonbridging oxygen hole centers (NBOHC) and hydrogen-related defects [14], [15]. Figure 4 shows the FTIR spectra of the films. All the samples show absorption bands at 440, 800 and 1064 cm−1 , these bands correspond to the rocking, bending and stretching modes for the Si–O–Si bonds, respectively. The stretching mode showed a shift from the stoichiometry value for silicon dioxide (1080 cm−1 ). This shift indicates an oxygen deficiency in the films. This is according with the PL measurements, where the emission bands were related to a different kind of defects in the SiOC matrix. The absorption band observed at 800 cm−1 may corresponds to Si–C bonds too [16]. In addition to these absorption bands, it is also observed other bands related to Si–Hn , Si–OH and Si–CHn bonds at 670, 920, 1250 cm−1 , respectively [17], [18]. Also, a shoulder at 1100 cm−1 related with Si–O–C bond can be observed [19]. The spectrum of the sample deposited at 0.1 Torr shows a higher intensity in the bands related with Si–H, Si–OH, and Si–O–C bonds. This bands could be related with the intense emission band in the green-orange region [14], [20]. The samples deposited at 0.1 and 0.3 Torr showed a good emission intensity. However, at 0.1 Torr the film shows a poor
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Fig. 5. PL spectra of the samples annealed in oxygen and hydrogen environments.
quality on the surface uniformity. For this reason, the samples for thermal annealing were deposited at 0.3 Torr. Figure 5 shows the PL spectra of the samples annealed in oxygen and hydrogen environments. The sample treated at 600 o C in oxygen shows a slightly increasing in the emission interval and a reduction in the intensity. By contrast, the samples treated at 750 o C showed an important reduction in the emission intensity of about two orders of magnitude. At 750 o C the oxygen can be incorporated in to the films, due to this, much of the radiative defects related with an oxygen deficiency were eliminated. On the other hand, in the sample treated in hydrogen plasma, the luminescence was almost completely extinguished. Given that the annealing temperature was low, the reduction of the PL intensity could not be due to a collapse of the defects in the film. This phenomenon may be related to a hydrogen incorporation in the films as defect passivator [21]. Figure 6 shows the FTIR spectra of the samples with thermal annealing in oxygen and hydrogen. In the samples treated in oxygen we are observed a reduction in the Si–H and Si–CHn bondings. Additionally, at 750 o C the shoulder related with Si–O–C bond is more defined. The stretching band shows a shift to 1080 cm−1 , this value corresponds to the stoichiometric silicon oxide. This indicates a phase separation between the different phases in the films and the oxygen was incorporated in the vacancy centers [22]. Th sample treated in hydrogen plasma shows an increment in the 940 cm−1 wavenumber, which is related to Si–H and Si–OH bonds. This can be correlated with the PL results where it was proposed that the hydrogen incorporation acts as passivator for many radiative defects [23].
Si-O
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Fig. 6. FTIR spectra of the samples annealed in oxygen and hydrogen environments.
IV. C ONCLUSIONS SiOC thin films were obtained by HW-CVD using TEOS and was correlated its photoluminescence spectra as a function of the deposition chamber pressure. The sample deposited at 0.3 Torr presented a better surface quality, and all of the samples showed an intense and wide emission band in the visible region. No crystalline phases were detected by XRD measurements, so that, we have attributed the emission bands to the defects with H- and C-related bonds and an oxygen deficiency centers as showed in the FTIR spectra. On the other hand, the thermal annealing induces a significant reduction in the emission intensity, probably because of the radiative defects passivation. ACKNOWLEDGMENT We appreciate the assistant help of Miguel A. Luna in the sample preparations. We also want to thank to PhD. ´ Alejandro Avila, PhD. Gabriel Romero and M.Sc. Adolfo Tavira from of Sees-Cinvestav for FTIR, ellipsometry and XRD characterizations and PhD. Antonio Mendez-Blas of IFUAP-BUAP for PL measurements. Also, the authors thank to PhD. Jos´e Sa´ul Arias for his contribution in the development of this work. R EFERENCES [1] L. T. Canham, “Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers,” Applied Physics Letters, vol. 57, no. 10, pp. 1046–1048, sep 1990.
[2] S. A. Caba˜nas-Tay, L. Palacios-Huerta, J. A. Luna-L´opez, M. AcevesMijares, S. Alc´antara-Iniesta, S. A. P´erez-Garc´ıa, and A. MoralesS´anchez, “Analysis of the luminescent centers in silicon rich silicon nitride light-emitting capacitors,” Semiconductor Science and Technology, vol. 30, no. 6, pp. 1–10, 2015. [3] A. Dutt, S. Godavarthi, Y. Matsumoto, G. Santana-Rodriguez, A. Avila, V. Sanchez, and G. Raina, “HW-CVD Deposited Nanocrystalline Silicon Thin Films at Low Substrate Temperature with White-Blue Luminescence,” Current Nanoscience, vol. 11, no. 5, pp. 621–626, jun 2015. [4] A. Majumdar, G. Das, N. Patel, P. Mishra, D. Ghose, and R. Hippler, “Microstructural and chemical evolution of CH3 incorporated (low-k) SiCO (H) films prepared by dielectric barrier discharge plasma,” Journal of The Electrochemical Society, vol. 155, no. 1, pp. D22—-D26, 2008. [5] S. Gallis, V. Nikas, H. Suhag, M. Huang, and A. E. Kaloyeros, “White light emission from amorphous silicon oxycarbide (a-SiCx xOy ) thin films: Role of composition and postdeposition annealing,” Applied Physics Letters, vol. 97, no. 8, pp. 1–4, 2010. [6] K. Lu, “Porous and high surface area silicon oxycarbide-based materials - A review,” Materials Science and Engineering R: Reports, vol. 97, pp. 23–49, 2015. [7] H. Nakayama and T. Hata, “Low-temperature growth of Si-based organicinorganic hybrid materials, SiOC and SiNC, by organic Cat-CVD,” Thin Solid Films, vol. 501, no. 1-2, pp. 190–194, apr 2006. [8] M. Nakaya, S. Yasuhara, T. Maeda, and A. Hotta, “Impact of hot wire and material gas species on the Cat-CVD coating of gas barrier SiOC thin films onto PET bottles,” Surface and Coatings Technology, vol. 344, pp. 21–29, jun 2018. [9] A. Dutt, Y. Matsumoto, S. Godavarthi, G. Santana-Rodr´ıguez, J. Santoyo-Salazar, and A. Escobosa, “White bright luminescence at room temperature from TEOS-based thin films via catalytic chemical vapor deposition,” Materials Letters, vol. 131, pp. 295–297, 2014. [10] L. Vaccaro, L. Spallino, A. F. Zatsepin, E. A. Buntov, A. V. Ershov, D. A. Grachev, and M. Cannas, “Photoluminescence of Si nanocrystals embedded in SiO2 : Excitation/emission mapping,” Physica Status Solidi (B) Basic Research, vol. 252, no. 3, pp. 600–606, 2015. [11] D. L. Griscom, “Self-trapped holes in amorphous silicon dioxide,” Physical Review B, vol. 40, no. 6, pp. 4224–4227, aug 1989. [12] L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” Journal of NON-crystalline Solids, vol. 239, no. 1-3, pp. 16–48, 1998. [13] V. Nikas, S. Gallis, M. Huang, A. E. Kaloyeros, A. P. Nguyen, A. Stesmans, and V. V. Afanas’Ev, “The origin of white luminescence from silicon oxycarbide thin films,” Applied Physics Letters, vol. 104, no. 6, pp. 1–5, 2014. [14] Y. D. Glinka, S. H. Lin, L. P. Hwang, and Y. T. Chen, “Photoluminescence from mesoporous silica: Similarity of properties to porous silicon,” Applied Physics Letters, vol. 77, no. 24, pp. 3968–3970, dec 2000. [15] R. Salh, “Defect Related Luminescence in Silicon Dioxide Network: A Review,” in Crystalline Silicon - Properties and Uses. InTech, jul 2011. [16] J.-M. Costantini, S. Miro, and O. Pluchery, “FTIR study of silicon carbide amorphization by heavy ion irradiations,” Journal of Physics D: Applied Physics, vol. 50, no. 9, p. 95301, 2017. [17] U. Kroll, J. Meier, A. Shah, S. Mikhailov, and J. Weber, “Hydrogen in amorphous and microcrystalline silicon films prepared by hydrogen dilution,” Journal of Applied Physics, vol. 80, no. 9, pp. 4971–4975, 1996. [18] W. S. Lau, Infrared characterization for microelectronics. World scientific, 1999. [19] Q. Liu, W. Shi, F. Babonneau, and L. V. Interrante, “Synthesis of Polycarbosilane/Siloxane Hybrid Polymers and Their Pyrolytic Conversion to Silicon Oxycarbide Ceramics,” Chemistry of Materials, vol. 9, no. 11, pp. 2434–2441, 1997. [20] E. J. Friebele, D. Griscom, and M. Marrone, “The optical absorption and luminescence bands near 2 ev in irradiated and drawn synthetic silica,” Journal of Non-Crystalline Solids, vol. 71, no. 1-3, pp. 133–144, 1985. [21] S. Basu, Crystalline Silicon Properties and Uses, 2011. [22] A. Morales-S´anchez, M. Aceves-Mijares, A. A. Gonz´alez-Fern´andez, K. Monfil-Leyva, J. Juvert, and C. Dom´ınguez-Horna, “Blue and red electroluminescence of silicon-rich oxide light emitting capacitors,” Proceedings of SPIE, vol. 7719, p. 77190N, 2010. [23] E. Cartier, J. H. Stathis, and D. A. Buchanan, “Passivation and depassivation of silicon dangling bonds at the Si/SiO2 interface by atomic hydrogen,” Applied Physics Letters, vol. 63, no. 11, pp. 1510–1512, sep 1993.
