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Effect of substrate temperature on HWCVD deposited a-SiC:H film
Materials Letters 61 (2007) 4731 – 4734 www.elsevier.com/locate/matlet
Effect of substrate temperature on HWCVD deposited a-SiC:H film Bibhu P. Swain ⁎, Rajiv O. Dusane Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, India Received 20 October 2005; accepted 4 March 2007 Available online 15 March 2007
Abstract Hydrogenated amorphous silicon carbon (a-SiC:H) films were deposited using pure SiH4 and C2H2 without hydrogen dilution by hot wire chemical vapor deposition (HWCVD) technique. The photoluminescence, optical, and structural properties of these films were systematically studied as a function of substrate temperature (Ts). a-SiC:H films deposited at lower substrate temperature (Ts) show degradation in their structural, optical and network properties. The hydrogen content (CH) in the films was found to be increased with decrease of Ts studied. Photoluminescence spectra shift to higher energy and less FWHM at high Ts. Raman spectroscopic analysis showed that structural disorder increases with decrease in the Ts. © 2007 Elsevier B.V. All rights reserved. Keywords: a-SiC:H; HWCVD; FTIR; Raman; Photoluminescence
1. Introduction Hydrogenated amorphous silicon carbon (a-SiC:H) alloy thin films are important for various applications like thin film solar cells, light emitting diodes (LEDs), radiation detectors, antifuse technology protective layers in xerography and creation of a biocompatible surface for biomedical application [1–5]. Over the last 15 years different processing techniques have been employed to synthesize a-SiC:H thin films to get the desired physical and electronic properties. Also use of various source gases such as CH4, C2H2, Si(CH3)4 etc [6–8] has been made for making these films. A recent addition to the list of techniques is the hot wire chemical vapor deposition (HWCVD). This technique shows a lot of potential to replace the RF glow discharge method for the processing of all the Si based amorphous alloy thin films such as a-Si:H, a-SiN:H, a-SiC:H and micro-crystalline silicon [8–12]. The advantage of employing HWCVD comes from two aspects i. Absence of the deleterious electrons, ions and surface charges and ii. High dissociation rate of source gases. ⁎ Corresponding author. National Research Laboratory for Charged Nanoparticle, School of Materials Science and Engineering. Seoul National University, Seoul, South Korea. Tel./fax:+82 2 880 9152. E-mail address: [email protected] (B.P. Swain). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.029
The former relates to the avoiding of powder formation at high pressure and filament temperature and the latter leads to higher deposition rate [13,14]. Several previous attempts have been taken for PECVD, ECRCVD and other CVD deposited a-C:H, a-SiN:H, a-SiC:H and a-Si:H films [15–18]. We have made various attempts to optimally synthesize a-SiC:H layers with the desired optoelectronic properties, and we have been able to synthesize a working LED based on HWCVD deposited a-SiC:H layers [10]. However the efficiencies of the devices are quite low. The primary concern in fabricating the high band gap a-SiC:H layers is the drastic increase in the defect density of the material with increasing carbon [8]. In this paper, an attempt has been made to investigate the influence of Ts on optical, structural and photoluminescence properties of a-SiC:H films deposited by HWCVD method. It has been observed that these properties are greatly affected by the Ts. 2. Experimental details The films were deposited simultaneously on Corning 7059 glass and c-Si b100N wafers in a HWCVD system, details of which have been described elsewhere [1,9,10]. It consists of a stainless chamber coupled with a turbo molecular pump, which
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yields a base pressure of b 10− 6 Torr. Pure silane (SiH4) and acetylene (C2H2), Matheson semiconductor grade, were used as the source gas. The process pressure was maintained constant by using a manual throttle valve. A tungsten wire of 0.5 mm diameter was used as the filament. The process parameters maintained during the deposition parameter of the films are given in Table 1. The transmittance and reflectance of a-SiC:H were measured by using a Jasco V 530 UV-Vis spectrometer. The optical band gap was deduced from Tauc's equations. The infrared (IR) absorption spectra of the alloy films deposited on c-Si were recorded on a Nicolet 5000 Fourier-transform infrared (FTIR) spectrometer in the range 400–4000 cm− 1. The hydrogen concentration (NH) was calculated from the relation N = A × Ia, where Ia is the integrated absorption coefficient and A is a constant. The values of A used here were 1.4 × 1020 cm− 2 for the determination of hydrogen bonded to Si and 1.0 × 1021 cm− 2 for that of the hydrogen bonded to carbon [2]. The film thickness was measured using a Dektak II surface profilometer from a step height in a masked area on the substrate. Raman spectra of the samples were recorded by a Labram-1 (DilorJobin Yvon-Spex make) spectrometer in a back scattering geometry with a spectral resolution of 2 cm− 1. The 488.0 nm (approx. 2.54 eV) line of an Ar laser is used for excitation and the Raman scattered light is analyzed using a charge coupled device (CCD) camera for multi-channel detection. Steady state PL measurements were performed at room temperature with a SPEX-Fluorolog system. Ultraviolet — UV light from a Xe arc lamp is focused onto the sample through a 0.22-m monochromator with a 1.25-mm slit giving an excitation bandwidth of 5 nm. The sample is placed at an angle of approximately 50°– 75° with respect to the incident excitation light. The photoluminescence results are collected by a 0.22-mm double monochromator with a 1.25-mm slit giving an emission bandwidth of 2.25 nm coupled with a photomultiplier tube. To reduce the scattering at higher energy 400 nm slit is used after Xe arc lamp. 3. Results and discussion Disorder of films is inversely proportional to deposition rate of the film. Dissociation rate, gas phase scattering and sticking coefficient of the film forming species on the substrate surface are important parameters, which decide the chemical composition, structure and other physical properties. At higher substrate temperature the sticking coefficient of radicals are less and vice versa. So an optimum substrate temperature needs to be identified to get device quality a-SiC:H films. In the present work Ts was varied from 150 to 350 °C.
Fig. 1. ETauc and Eo4 band gap of a-SiC:H film as a function of Ts and Tauc's plot inserted in it.
3.1. Variation of band gap Fig. 1 shows Eo4 and ETauc as a function of Ts and the Tauc plot is inserted inside it. The band gap increases from 2.43 eV at 150 °C to 2.55 eV at 350 °C. Deposition rate of a-SiC:H film decreases with increases in substrate temperature. Deposition rate decreases from 0.65 to 0.44 Å/s at 150 °C and 350 °C respectively. Fig. 2 shows the B parameter of a-SiC:H films as a function of substrate temperature (Ts). B parameter increases with increase of Ts from 925 eV cm− 1/2 to 1145 eV cm− 1/2 at 150 °C and 350 °C respectively. It is observed that the B parameter increases with increase of substrate temperature while (Eo4 − ETauc) it slightly decreases with increase of Ts. It indicates that a-SiC:H films deposited at lower substrate temperature are more disordered than films deposited at higher substrate temperature (Ts). This may lead to the following reasons: 1. Sticking coefficient & surface mobility of radicals are key factors which control the deposition kinetics. An increase of the B parameter clearly indicates that the structural order in the film drastically increases with Ts. 2. High substrate temperature may lead to desorption of hydrogen and with further rearrangement of atom yield a denser material with Ts. 3. Reduced sp2 carbon in the films at higher Ts could result in the observed increases in Eg.
Table 1 SiH4 flow rate C2H2 flow rate Process pressure Substrate temperature (Ts) Filament temperature (TF) Substrate to filament distance
2 sccm 3 sccm 100 mTorr 150–350 °C 1800 °C 6 cm
Fig. 2. B parameter and (Eo4-ETauc) of a-SiC:H films as a function of substrate temperature.
B.P. Swain, R.O. Dusane / Materials Letters 61 (2007) 4731–4734
Fig. 3. FTIR spectra of HWCVD a-SiC:H films deposited with increasing substrate temperature. C2H2 = 3 sccm and SiH4 = 2 sccm are kept constant.
3.2. Infrared spectroscopy Fig. 3 shows the FTIR spectra of a-SiC:H films deposited at different substrate temperatures (Ts). To reveal the hydrogen bonding configuration and total hydrogen content in the film, FTIR spectroscopy was used. The major absorption bands observed are in the regions: 2800– 3000, 2000–2100, 960–1100, and 540–800 cm− 1 [2]. The band corresponding to 2800–3000 cm− 1 is attributed to the stretching vibration of C–Hn groups in sp2 (2880 cm− 1) and sp3 (2960 cm− 1) configurations [2], while that at 2000–2100 cm− 1 is attributed to the stretching vibration of SiHn groups. The band at 960–1100 cm− 1 corresponds to C–H wagging/bending modes or Si–CH3 wagging, while the strong band at 780 cm− 1 corresponds to the Si–C stretching mode [2]. The band at 1250 to 1450 cm− 1 corresponds to C–(C–H2). Fig. 4 shows the variation in the Si–H, Si–C and C–H bond densities as a function of substrate temperature. Interestingly it is observed that C–H and Si–C bonds increase monotonically with Ts while Si–H bonding decreases monotonically with Ts. Hydrogen content decreases from 15 to 5 at.% at 150 and 350 °C respectively. 3.3. Raman spectroscopy
4733
Fig. 5. Raman spectra of a-SiC:H films as a function of substrate temperature.
Si–Si TO mode located near 480 cm− 1, TA mode located at 180 cm− 1, LA mode located at 280 and LO modes located at 380 cm− 1 respectively indicates that all films are amorphous [2]. At low Ts the Raman spectrum is quite featureless, though the second order TO peak is quite prominent. This indicates a significant ordering in atomic ring structure in the network. However at high Ts, the topological arrangement is better which is reflected in a discernable spectral feature. The second order TO peak of Si shows prominence as a function of substrate temperature. At low substrate temperature the second order TO is comparable in intensity to the first order TO peak, while intensity slowly decreases at higher substrate temperature. This suggests that the a-SiC:H films are quite transparent at higher substrate temperature. The variation of the network as a function of Ts is depicted in Fig. 6 where we plot the ratio ITA/ITO and the frequency ωTO as a function of Ts. We see that ITA/ITO decreases with an increase of Ts from 0.65 to 0.47 at 150 °C and 350 °C respectively. This indicates that films are more ordered at high Ts and IRO improves with an increase in substrate temperature [2]. The improvement observed in the structural network at high Ts is a generally observed phenomenon even in the case of films deposited by other techniques. The general explanation for such improvement is a result of increased surface mobility of the incoming
Fig. 5 shows Raman spectra of a-SiC:H films deposited at various Ts by HWCVD technique. The broad and smooth nature of Raman
Fig. 4. Bond density of a-SiC:H films as a function of substrate temperature (Ts).
Fig. 6. ITA/ITO and ωTO as the function of substrate temperature of HWCVD deposited a-SiC:H films.
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of ∼ 50 at.% were deposited at deposition rate of N 0.5 Å/s at low Ts. However, a-SiC:H films deposited at low Ts show degradation in their network, optical and structural properties. The FTIR spectroscopic studies showed that the hydrogen content in the films was found to be ∼ 7 at.% over the entire range of Ts studied. This indicates that the growth of a-SiC:H films in HWCVD is mainly from the atomic species (Si and C) evaporated from the hot filament and the hydrogen gets incorporated in the film via gas phase reactions and substrate– gas interactions. PL peak gets narrow and shifts to higher energy due to rearrangement of atomic bonding. References
Fig. 7. PL spectra of a-SiC:H films as a function of Ts.
radical and subsequent densification of the films. Incidentally the H content in the film also decreases with an increase in Ts and the bonding of H is prominently monohybrid. All these effects lead to an improvement in the optoelectronic properties of the films as we shall see in the photoluminescence in Fig. 7. 3.4. Photoluminescence
[1] [2] [3] [4] [5] [6] [7] [8] [9]
Fig. 7 shows the continuous photoluminescence (PL) spectra as a function of substrate temperature (Ts). The spectra are asymmetric towards the lower energy. PL spectra peak shifts to higher energy with increase of Ts. The interesting feature in this case is that the PL spectra are more symmetric as well as narrow for the film deposited at higher Ts. This can be understood on the basis of increased structural and compositional ordering in these films which has been observed by Raman spectra. Moreover, we have seen from Fig. 2 that the B parameter improves indicating a lesser band tail width for high Ts films. This leads to decreases in the (Eo4 − EPL) for these films. Thus, though a small shift in the peak position is observed with increasing Ts, what is of higher significance is the narrowing of the PL peak.
[10]
4. Conclusion
[18]
An attempt has been made to ascertain the role of Ts on optical, structural and optoelectronic properties of a-SiC:H films deposited from pure SiH4 and C2H2 using the HWCVD technique. The device quality a-SiC:H films with carbon content
[11] [12] [13] [14] [15] [16] [17]
B.P. Swain, Surf. Coat. Techno. 201 (2006) 1589. B.P. Swain, Surf. Coat. Techno. 201 (2006) 1132. B.P. Swain, R.O. Dusane, Microelec. Engg. 83 (2006) 55. B.P. Swain, T.K.G. Rao, R.O. Dusane, J. Non-cryst. Solids, 352 (2006) 1416. B.P. Swain, Appl. Surf. Sci. 253 (2006) 2310. A. Kumbhar, S.B. Patil, S. Kumar, R. Lal, R.O. Dusane, Thin Solid Films 395 (2001) 244. S.B. Patil, Alka A. Kumbhar, Shweta Saraswat, R.O. Dusane, Thin Solid Films 430 (2003) 257. P.P. Ray, P. Chaudhuri, P. Chatterjee, Thin Solid Films 403–404 (2002) 275. B.P. Swain, S.B. Patil, A. Kumbhar, R.O. Dusane, Thin Solid Films 430 (2003) 186. S.B. Patil, A.V. Vairagar, A.A. Kumbhar, L.K. Sahu, V.R. Rao, N. Venkatramani, R.O. Dusane, B. Schroeder, Thin Solid Films 430 (2003) 63. P.C. Waghmare, S.B. Patil, A.A. Kumbhar, R. Rao, R.O. Dusane, Thin Solid Films 430 (2003) 189. S.B. Patil, A.A. Kumbhar, S. Saraswat, R.O. Dusane, Thin Solid Films 430 (2003) 257. A.H. Mahan, J. Carapella, B.P. Nelson, R.S. Crandall, I. Balberg, J. Appl. Phys. 69 (1991) 6728. N. Honda, A. Masuda, H. Matsumura, J. Non-Cryst. Solids 266–269 (2000) 100. S.F. Yoon, H. Yang, Rusli, J. Ahn, Q. Zhang, T.L. Poo, Diamond Relat. Mater. 6 (1997) 1683. A. Von. Keudell, W. Moller, R. Hytry, Appl. Phys. Lett. 62 (1993) 937. G. Capote, R. Prioli, P.M. Jardim, A.R. Zanatta, L.G. Jacobsohn, F.L. Freire Jr., J. Non-Cryst. Solids 338–340 (2004) 503. S.R. Jadkar, Jaydeep V. Sali, S.T. Kshirsagar, M.G. Takwale, J. Non-Cryst. Solids 299–302 (2002) 168.
Effect of substrate temperature on HWCVD deposited a-SiC:H film Bibhu P. Swain ⁎, Rajiv O. Dusane Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, India Received 20 October 2005; accepted 4 March 2007 Available online 15 March 2007
Abstract Hydrogenated amorphous silicon carbon (a-SiC:H) films were deposited using pure SiH4 and C2H2 without hydrogen dilution by hot wire chemical vapor deposition (HWCVD) technique. The photoluminescence, optical, and structural properties of these films were systematically studied as a function of substrate temperature (Ts). a-SiC:H films deposited at lower substrate temperature (Ts) show degradation in their structural, optical and network properties. The hydrogen content (CH) in the films was found to be increased with decrease of Ts studied. Photoluminescence spectra shift to higher energy and less FWHM at high Ts. Raman spectroscopic analysis showed that structural disorder increases with decrease in the Ts. © 2007 Elsevier B.V. All rights reserved. Keywords: a-SiC:H; HWCVD; FTIR; Raman; Photoluminescence
1. Introduction Hydrogenated amorphous silicon carbon (a-SiC:H) alloy thin films are important for various applications like thin film solar cells, light emitting diodes (LEDs), radiation detectors, antifuse technology protective layers in xerography and creation of a biocompatible surface for biomedical application [1–5]. Over the last 15 years different processing techniques have been employed to synthesize a-SiC:H thin films to get the desired physical and electronic properties. Also use of various source gases such as CH4, C2H2, Si(CH3)4 etc [6–8] has been made for making these films. A recent addition to the list of techniques is the hot wire chemical vapor deposition (HWCVD). This technique shows a lot of potential to replace the RF glow discharge method for the processing of all the Si based amorphous alloy thin films such as a-Si:H, a-SiN:H, a-SiC:H and micro-crystalline silicon [8–12]. The advantage of employing HWCVD comes from two aspects i. Absence of the deleterious electrons, ions and surface charges and ii. High dissociation rate of source gases. ⁎ Corresponding author. National Research Laboratory for Charged Nanoparticle, School of Materials Science and Engineering. Seoul National University, Seoul, South Korea. Tel./fax:+82 2 880 9152. E-mail address: [email protected] (B.P. Swain). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.029
The former relates to the avoiding of powder formation at high pressure and filament temperature and the latter leads to higher deposition rate [13,14]. Several previous attempts have been taken for PECVD, ECRCVD and other CVD deposited a-C:H, a-SiN:H, a-SiC:H and a-Si:H films [15–18]. We have made various attempts to optimally synthesize a-SiC:H layers with the desired optoelectronic properties, and we have been able to synthesize a working LED based on HWCVD deposited a-SiC:H layers [10]. However the efficiencies of the devices are quite low. The primary concern in fabricating the high band gap a-SiC:H layers is the drastic increase in the defect density of the material with increasing carbon [8]. In this paper, an attempt has been made to investigate the influence of Ts on optical, structural and photoluminescence properties of a-SiC:H films deposited by HWCVD method. It has been observed that these properties are greatly affected by the Ts. 2. Experimental details The films were deposited simultaneously on Corning 7059 glass and c-Si b100N wafers in a HWCVD system, details of which have been described elsewhere [1,9,10]. It consists of a stainless chamber coupled with a turbo molecular pump, which
4732
B.P. Swain, R.O. Dusane / Materials Letters 61 (2007) 4731–4734
yields a base pressure of b 10− 6 Torr. Pure silane (SiH4) and acetylene (C2H2), Matheson semiconductor grade, were used as the source gas. The process pressure was maintained constant by using a manual throttle valve. A tungsten wire of 0.5 mm diameter was used as the filament. The process parameters maintained during the deposition parameter of the films are given in Table 1. The transmittance and reflectance of a-SiC:H were measured by using a Jasco V 530 UV-Vis spectrometer. The optical band gap was deduced from Tauc's equations. The infrared (IR) absorption spectra of the alloy films deposited on c-Si were recorded on a Nicolet 5000 Fourier-transform infrared (FTIR) spectrometer in the range 400–4000 cm− 1. The hydrogen concentration (NH) was calculated from the relation N = A × Ia, where Ia is the integrated absorption coefficient and A is a constant. The values of A used here were 1.4 × 1020 cm− 2 for the determination of hydrogen bonded to Si and 1.0 × 1021 cm− 2 for that of the hydrogen bonded to carbon [2]. The film thickness was measured using a Dektak II surface profilometer from a step height in a masked area on the substrate. Raman spectra of the samples were recorded by a Labram-1 (DilorJobin Yvon-Spex make) spectrometer in a back scattering geometry with a spectral resolution of 2 cm− 1. The 488.0 nm (approx. 2.54 eV) line of an Ar laser is used for excitation and the Raman scattered light is analyzed using a charge coupled device (CCD) camera for multi-channel detection. Steady state PL measurements were performed at room temperature with a SPEX-Fluorolog system. Ultraviolet — UV light from a Xe arc lamp is focused onto the sample through a 0.22-m monochromator with a 1.25-mm slit giving an excitation bandwidth of 5 nm. The sample is placed at an angle of approximately 50°– 75° with respect to the incident excitation light. The photoluminescence results are collected by a 0.22-mm double monochromator with a 1.25-mm slit giving an emission bandwidth of 2.25 nm coupled with a photomultiplier tube. To reduce the scattering at higher energy 400 nm slit is used after Xe arc lamp. 3. Results and discussion Disorder of films is inversely proportional to deposition rate of the film. Dissociation rate, gas phase scattering and sticking coefficient of the film forming species on the substrate surface are important parameters, which decide the chemical composition, structure and other physical properties. At higher substrate temperature the sticking coefficient of radicals are less and vice versa. So an optimum substrate temperature needs to be identified to get device quality a-SiC:H films. In the present work Ts was varied from 150 to 350 °C.
Fig. 1. ETauc and Eo4 band gap of a-SiC:H film as a function of Ts and Tauc's plot inserted in it.
3.1. Variation of band gap Fig. 1 shows Eo4 and ETauc as a function of Ts and the Tauc plot is inserted inside it. The band gap increases from 2.43 eV at 150 °C to 2.55 eV at 350 °C. Deposition rate of a-SiC:H film decreases with increases in substrate temperature. Deposition rate decreases from 0.65 to 0.44 Å/s at 150 °C and 350 °C respectively. Fig. 2 shows the B parameter of a-SiC:H films as a function of substrate temperature (Ts). B parameter increases with increase of Ts from 925 eV cm− 1/2 to 1145 eV cm− 1/2 at 150 °C and 350 °C respectively. It is observed that the B parameter increases with increase of substrate temperature while (Eo4 − ETauc) it slightly decreases with increase of Ts. It indicates that a-SiC:H films deposited at lower substrate temperature are more disordered than films deposited at higher substrate temperature (Ts). This may lead to the following reasons: 1. Sticking coefficient & surface mobility of radicals are key factors which control the deposition kinetics. An increase of the B parameter clearly indicates that the structural order in the film drastically increases with Ts. 2. High substrate temperature may lead to desorption of hydrogen and with further rearrangement of atom yield a denser material with Ts. 3. Reduced sp2 carbon in the films at higher Ts could result in the observed increases in Eg.
Table 1 SiH4 flow rate C2H2 flow rate Process pressure Substrate temperature (Ts) Filament temperature (TF) Substrate to filament distance
2 sccm 3 sccm 100 mTorr 150–350 °C 1800 °C 6 cm
Fig. 2. B parameter and (Eo4-ETauc) of a-SiC:H films as a function of substrate temperature.
B.P. Swain, R.O. Dusane / Materials Letters 61 (2007) 4731–4734
Fig. 3. FTIR spectra of HWCVD a-SiC:H films deposited with increasing substrate temperature. C2H2 = 3 sccm and SiH4 = 2 sccm are kept constant.
3.2. Infrared spectroscopy Fig. 3 shows the FTIR spectra of a-SiC:H films deposited at different substrate temperatures (Ts). To reveal the hydrogen bonding configuration and total hydrogen content in the film, FTIR spectroscopy was used. The major absorption bands observed are in the regions: 2800– 3000, 2000–2100, 960–1100, and 540–800 cm− 1 [2]. The band corresponding to 2800–3000 cm− 1 is attributed to the stretching vibration of C–Hn groups in sp2 (2880 cm− 1) and sp3 (2960 cm− 1) configurations [2], while that at 2000–2100 cm− 1 is attributed to the stretching vibration of SiHn groups. The band at 960–1100 cm− 1 corresponds to C–H wagging/bending modes or Si–CH3 wagging, while the strong band at 780 cm− 1 corresponds to the Si–C stretching mode [2]. The band at 1250 to 1450 cm− 1 corresponds to C–(C–H2). Fig. 4 shows the variation in the Si–H, Si–C and C–H bond densities as a function of substrate temperature. Interestingly it is observed that C–H and Si–C bonds increase monotonically with Ts while Si–H bonding decreases monotonically with Ts. Hydrogen content decreases from 15 to 5 at.% at 150 and 350 °C respectively. 3.3. Raman spectroscopy
4733
Fig. 5. Raman spectra of a-SiC:H films as a function of substrate temperature.
Si–Si TO mode located near 480 cm− 1, TA mode located at 180 cm− 1, LA mode located at 280 and LO modes located at 380 cm− 1 respectively indicates that all films are amorphous [2]. At low Ts the Raman spectrum is quite featureless, though the second order TO peak is quite prominent. This indicates a significant ordering in atomic ring structure in the network. However at high Ts, the topological arrangement is better which is reflected in a discernable spectral feature. The second order TO peak of Si shows prominence as a function of substrate temperature. At low substrate temperature the second order TO is comparable in intensity to the first order TO peak, while intensity slowly decreases at higher substrate temperature. This suggests that the a-SiC:H films are quite transparent at higher substrate temperature. The variation of the network as a function of Ts is depicted in Fig. 6 where we plot the ratio ITA/ITO and the frequency ωTO as a function of Ts. We see that ITA/ITO decreases with an increase of Ts from 0.65 to 0.47 at 150 °C and 350 °C respectively. This indicates that films are more ordered at high Ts and IRO improves with an increase in substrate temperature [2]. The improvement observed in the structural network at high Ts is a generally observed phenomenon even in the case of films deposited by other techniques. The general explanation for such improvement is a result of increased surface mobility of the incoming
Fig. 5 shows Raman spectra of a-SiC:H films deposited at various Ts by HWCVD technique. The broad and smooth nature of Raman
Fig. 4. Bond density of a-SiC:H films as a function of substrate temperature (Ts).
Fig. 6. ITA/ITO and ωTO as the function of substrate temperature of HWCVD deposited a-SiC:H films.
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of ∼ 50 at.% were deposited at deposition rate of N 0.5 Å/s at low Ts. However, a-SiC:H films deposited at low Ts show degradation in their network, optical and structural properties. The FTIR spectroscopic studies showed that the hydrogen content in the films was found to be ∼ 7 at.% over the entire range of Ts studied. This indicates that the growth of a-SiC:H films in HWCVD is mainly from the atomic species (Si and C) evaporated from the hot filament and the hydrogen gets incorporated in the film via gas phase reactions and substrate– gas interactions. PL peak gets narrow and shifts to higher energy due to rearrangement of atomic bonding. References
Fig. 7. PL spectra of a-SiC:H films as a function of Ts.
radical and subsequent densification of the films. Incidentally the H content in the film also decreases with an increase in Ts and the bonding of H is prominently monohybrid. All these effects lead to an improvement in the optoelectronic properties of the films as we shall see in the photoluminescence in Fig. 7. 3.4. Photoluminescence
[1] [2] [3] [4] [5] [6] [7] [8] [9]
Fig. 7 shows the continuous photoluminescence (PL) spectra as a function of substrate temperature (Ts). The spectra are asymmetric towards the lower energy. PL spectra peak shifts to higher energy with increase of Ts. The interesting feature in this case is that the PL spectra are more symmetric as well as narrow for the film deposited at higher Ts. This can be understood on the basis of increased structural and compositional ordering in these films which has been observed by Raman spectra. Moreover, we have seen from Fig. 2 that the B parameter improves indicating a lesser band tail width for high Ts films. This leads to decreases in the (Eo4 − EPL) for these films. Thus, though a small shift in the peak position is observed with increasing Ts, what is of higher significance is the narrowing of the PL peak.
[10]
4. Conclusion
[18]
An attempt has been made to ascertain the role of Ts on optical, structural and optoelectronic properties of a-SiC:H films deposited from pure SiH4 and C2H2 using the HWCVD technique. The device quality a-SiC:H films with carbon content
[11] [12] [13] [14] [15] [16] [17]
B.P. Swain, Surf. Coat. Techno. 201 (2006) 1589. B.P. Swain, Surf. Coat. Techno. 201 (2006) 1132. B.P. Swain, R.O. Dusane, Microelec. Engg. 83 (2006) 55. B.P. Swain, T.K.G. Rao, R.O. Dusane, J. Non-cryst. Solids, 352 (2006) 1416. B.P. Swain, Appl. Surf. Sci. 253 (2006) 2310. A. Kumbhar, S.B. Patil, S. Kumar, R. Lal, R.O. Dusane, Thin Solid Films 395 (2001) 244. S.B. Patil, Alka A. Kumbhar, Shweta Saraswat, R.O. Dusane, Thin Solid Films 430 (2003) 257. P.P. Ray, P. Chaudhuri, P. Chatterjee, Thin Solid Films 403–404 (2002) 275. B.P. Swain, S.B. Patil, A. Kumbhar, R.O. Dusane, Thin Solid Films 430 (2003) 186. S.B. Patil, A.V. Vairagar, A.A. Kumbhar, L.K. Sahu, V.R. Rao, N. Venkatramani, R.O. Dusane, B. Schroeder, Thin Solid Films 430 (2003) 63. P.C. Waghmare, S.B. Patil, A.A. Kumbhar, R. Rao, R.O. Dusane, Thin Solid Films 430 (2003) 189. S.B. Patil, A.A. Kumbhar, S. Saraswat, R.O. Dusane, Thin Solid Films 430 (2003) 257. A.H. Mahan, J. Carapella, B.P. Nelson, R.S. Crandall, I. Balberg, J. Appl. Phys. 69 (1991) 6728. N. Honda, A. Masuda, H. Matsumura, J. Non-Cryst. Solids 266–269 (2000) 100. S.F. Yoon, H. Yang, Rusli, J. Ahn, Q. Zhang, T.L. Poo, Diamond Relat. Mater. 6 (1997) 1683. A. Von. Keudell, W. Moller, R. Hytry, Appl. Phys. Lett. 62 (1993) 937. G. Capote, R. Prioli, P.M. Jardim, A.R. Zanatta, L.G. Jacobsohn, F.L. Freire Jr., J. Non-Cryst. Solids 338–340 (2004) 503. S.R. Jadkar, Jaydeep V. Sali, S.T. Kshirsagar, M.G. Takwale, J. Non-Cryst. Solids 299–302 (2002) 168.