Photoluminescence characteristics from amorphous SiC thin films with various structures deposited at low temperature

Solid State Communications 133 (2005) 565–568 www.elsevier.com/locate/ssc

Photoluminescence characteristics from amorphous SiC thin films with various structures deposited at low temperature Jun Xua,b,*, Ling Yanga, Yunjun Ruia, Jiaxin Meia, Xin Zhanga, Wei Lia, Zhongyuan Maa, Ling Xua, Xinfan Huanga, Kunji Chena a

National laboratory of Solid State Microstructures, Department of Physics and Jiangsu Provincial Key Laboratory of Photonic and Electronic Materials Science and Technology, Nanjing University, Hankou Road 22, Nanjing 210093, China b National Laboratory of Infrared Physics, Shanghai Institute for Technical Physics, CAS, Shanghai, China Received 9 November 2004; received in revised form 19 December 2004; accepted 22 December 2004 by H. Takayama Available online 11 January 2005

Abstract Hydrogenated amorphous SiC thin films deposited at low substrate temperature (100 8C) show the different bonding configurations and microstructures which depend on the carbon concentrations in the films controlled by the gas ratio R of methane to silane during the deposition. Photoluminescence characteristics are investigated for these samples with different structures. A strong luminescence in red light region can be observed for samples deposited with low gas ratio R which is significantly reduced its intensity with increasing the carbon concentrations in the films. On the other hand, the luminescence bands located at blue-green light region are detected under UV light excitation for samples deposited with high gas ratio R, which can be associated with the existence of amorphous SiC clusters in the films. q 2004 Elsevier Ltd. All rights reserved. PACS: 78.55.Qr; 81.05.Gc; 81.15.Gh Keywords: A. Silicon carbide; C. Structures; E. Luminescence

Hydrogenated amorphous and nanocrystalline silicon carbide (a-SiC:H and nc-SiC:H) thin films are attracted much attention in the recent years because of their potential applications in many kinds of optoelectronic devices, such as solar cells, image sensors and photodiodes [1–5]. By varying the carbon concentrations in a-SiC or nc-SiC films, the optical band gap can be continuously tuned in a wide range which makes it useful for device design and performance [6]. However, the chemical bonding configurations and film microstructures of SiC materials are very

* Corresponding author. Address: Department of Physics, Nanjing University, Nanjing 210093, China. Tel.: C86 2583594836; fax: C86 2583595535. E-mail address: [email protected] (J. Xu). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.12.036

complicated due to the incorporation of carbon and hydrogen since carbon atom can be in a form of sp2 or sp3 configuration when it is bonded to Si and H. It is well accepted that the film structures and in turn the physical properties of SiC materials are strongly influenced by the preparation techniques and conditions [6–9]. Therefore, it is interesting and important to elucidate the relationship between the optical properties and the SiC film structures from the viewpoint of actual applications. In our previous work [10], the influence of growth conditions on the structural and optical properties of aSiC:H thin films prepared by using radio frequency (r.f.) plasma enhanced chemical vapor deposition (PECVD) technique were investigated. During the deposition, the substrate temperature was kept at 320 8C. It has been shown that by choosing high r.f. power, more carbon atoms can be

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bonded to Si and H atoms to form the Si–CH2, Si–CH3 entities. The optical band gaps of these films were increased with increasing the carbon concentrations in the films. On the other hand, by choosing low r.f. power, the inhomogeneous structures were formed which contain Si-rich clusters and graphite-like carbon clusters. The Si–C bonds acted as the bridge bonds to link the inhomogeneous phases. The optical band gap was decreased with increasing the carbon concentration due to the formation of more graphitelike clusters [10]. Since atomic hydrogen plays an important role in controlling the film network and in turn the physics properties of amorphous semiconductors [11,12], it is interesting to investigate the film microstructures and bonding configurations of a-SiC:H deposited at low substrate temperature which will results in the incorporation of more hydrogen into the materials. Therefore, in the present work, a low deposition temperature at 100 8C is used to deposit a-SiC:H films under low r.f. power density. It is found that the film structures are changed with changing the carbon concentrations in the films by controlling the gas ratio during the deposition process. In contrast with the previous results, the present films contain much more hydrogen and the optical band gap is increased with increasing the carbon concentrations in the films. It is very interesting to see the change of the luminescence characteristics from the SiC films with various carbon concentrations which reflect the change of the film microstructures. A series of a-SiC:H films are prepared by using conventional PECVD system by using Methane (CH4) and Silane (SiH4) as reaction gas sources. The gas ratio R, which is defined as [CH4]/[SiH4], is changed from 2 to 15 to prepare a-SiC:H films with different carbon concentrations. During the deposition process, the substrate temperature and r.f. power density is kept at 100 8C and 65 mW/cm2, respectively. Crystalline Si (c-Si) wafers and fused quartz plates are used as substrates for different measurements. The film structures and chemical bonding configurations are investigated by using Fourier-transform Infrared (FT-IR) spectroscopy and Raman scattering technique. The optical properties and films thickness are determined from the spectra measured by UV–VIS-NIR spectrophotometers. The optical band gap, deduced from Tauc plot, is increased from 2.05 to 2.65 eV with increasing the gas ratio R as shown in Fig. 1. The film thickness is estimated about 400–600 nm. Photoluminescence (PL) measurements are carried out at room temperature for samples deposited on c-Si wafers under the excitation both of ArC laser (488 nm) and Xe lamp. Fig. 2 gives the FT-IR spectra for samples deposited with gas ratio RZ2 and 15. The stretching mode of Si–C (780 cmK1), the bending mode of Si–CH2 (1000 cmK1) and wagging mode of Si–CH3 (1250 cmK1) can be clearly observed in both spectra. For sample deposited with RZ2, the Si–H wagging mode (640 cmK1) and stretching mode (2100 cmK1) can be found but these bonding signals become weaker for sample deposited with RZ15 [6], especially the vibration band of Si–H wagging mode. The

Fig. 1. Optical band gap of a-SiC:H films deduced from Tauc plot as a function of gas ratio RZ[CH4]/[SiH4].

vibration bands around 3000 cmK1 can be ascribed to C–Hn (nZ2, 3) stretching mode [13]. It was reported in our previous work that, the C–Hn bands are lack in FT-IR spectra when the samples deposited under low r.f. power density (65 mW/cm2) at substrate temperature of 320 8C [10]. The present FT-IR results indicate that the more hydrogen atoms are incorporated into the SiC film network to form Si–H and C–H bonds due to the low substrate temperature (100 8C), which makes the film structures quite different from that deposited at high substrate temperature. It is also demonstrated that the Si-rich phases which is surrounded with CHn bonds, dominate the film structures for

Fig. 2. FT-IR spectra of prepared a-SiC:H films deposited with gas ratio RZ2 and RZ15.

J. Xu et al. / Solid State Communications 133 (2005) 565–568

sample deposited with RZ2, however, the Si–C bonds are obviously enhanced with increasing the carbon concentrations in the films to form Si–CHn structures. The film structures are further investigated by using Raman scattering technique which is consistent with the FT-IR results. As shown in Fig. 3(a) and (b), a strong Si–Si TO mode (480 cmK1) dominates the Raman spectrum for sample deposited with RZ2 and no C–H trace can be identified. For sample deposited with RZ15, it is found that the signals related to SiC vibration band around 500 cmK1 can be observed. The similar spectrum was reported in 1000 8C annealed a-SiC:H films with high carbon concentration [14]. The weak bands at 610 and 790 cmK1 can be attributed to the SiC bonding configurations with 4H–SiC structures [15]. It is worth noting that the low temperature process in the present study makes all the films to be completely amorphous phase. However, the appearance of 4H–SiC Raman bands indicates the existence of amorphous SiC clusters with 4H–SiC short range structures. The lack of C–C bands at 1300–1600 cmK1 suggests that all of the carbon atoms are bonded to Si and H

Fig. 3. Raman spectra of prepared a-SiC:H films deposited with gas ratio RZ2 and RZ15. (a) 300–1200 cmK1, (b) 2800–3100 cmK1.

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instead of forming the graphite-like structures which is considered as one of the main factors to induce the deterioration of the optoelectronic properties of SiC films [6, 16]. Consequently, a strong double C–H bands at 2900 and 2960 cmK1 can be observed in Raman spectrum for sample deposited with RZ15 as given in Fig. 3(b). The carbon content in a-SiC:H films can be roughly estimated from the chemical shift of Si–C bands in FT-IR spectra according to the empirical equation proposed by Lucovsky [11]. The carbon concentration of sample deposited with RZ2 is about 30% and that of sample deposited with RZ15 is about 42%. It is shown that the radicals in the plasma play important roles in the formation of SiC films during the deposition. When low gas ratio R is used, SiH radicals dominant the growth process so that the Si atoms are aggregated to form Si-rich clusters and CHn is mainly existed in the interfacial regions between the Si clusters. When high gas ratio R is used, CHn (nZ2, 3) radicals are increased. Therefore, more H radicals are bonded to C due to the higher electronegativity of carbon, meanwhile, Si–C bonds are enhanced due to the increment of carbon content. Therefore, the structures can be described as the amorphous SiC clusters embedded in CHn matrix. Generally, a strong room temperature PL can be detected from a-SiC:H films. Fig. 4 is the PL spectra measured from a-SiC:H films deposited with various gas ratio R. ArC laser with wavelength of 488 nm is used as an excitation source. It is shown that the PL intensity is obviously enhanced in aSiC:H films deposited with low gas ratio R. The PL band is very broad with full width at half maximum (FWHM) of

Fig. 4. Room temperature Photoluminescence spectra for a-SiC:H films deposited with different gas ratio R. The excitation light is ArC laser with wavelength of 488 nm.

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150 nm. It seems that the PL spectrum contains multi-bands. The similar PL results have been reported by other groups [9,14]. The origin of the PL was attributed to the defect states related recombination process. For samples deposited with high gas ratio R, the PL intensity is significantly reduced and the PL peak energy is shifted to short wavelength due to the increases of the optical band gap. It is interesting to find that the PL band located at bluegreen light region can be detected by using UV light excitation (350 nm from Xe lamp) from the samples deposited with high gas ratio (RZ10 and 15) while it can not be found from the samples deposited with low gas ratio (RZ2 and 5). As shown in Fig. 5, the PL peak is centered at 525 and 445 nm with FWHM about 70 and 55 nm, respectively. The blue-green light emission has also been reported in nanocrystalline-SiC and spark-processed SiC films but scarcely reported in amorphous SiC films deposited by using methane as carbon source [3,17]. Xu et al. attributed this emission to the radiative recombination from some direct transitions such as self-trapped excitons in the surface of nanoclusters [17]. In our case, the existence of amorphous SiC clusters as mentioned before is believed to be responsible to the blue-green light emission. Since this emission band can not be detected by using 488 nm (2.54 eV) light excitation, it indicates the size of the clusters is quite small. The carriers excited by 350 nm (3.54 eV) in these amorphous SiC clusters are recombined radiatively through luminescence centers which may associated with surface states on SiC clusters as reported in the visible light emission from a-Si clusters.

Fig. 5. Photoluminescence spectrum for sample deposited with gas ratio RZ15 measured at room temperature. Xe Lamp with wavelength of 350 nm is used as an excitation light source.

In conclusion, a-SiC:H films are deposited at low substrate temperature with various gas ratio of RZ [CH4]/[SiH4]. It is found that the chemical bonding configurations and film microstructures are quite different from that deposited at high substrate temperature which can be attributed to the more hydrogen incorporation into the film network. The change of the film structures is investigated as a function of gas ratio R. Moreover, photoluminescence characteristics are studied for the samples with different carbon concentrations. The green light emission was found under UV light excitation besides the conventional red light emission band, which can be attributed to the existence of the amorphous SiC nanoclusters in films with high carbon concentrations. Acknowledgements This work is supported by NSF of China under grant No. 10374049, 90301009, 60425414 and 50472066). The authors also acknowledge the partly financial support from state key program for basic research of china (Grant no. 2001CB610503).

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