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Correlation study of photoluminescence properties and mechanism of hydrogenated amorphous silicon carbide films
Accepted Manuscript Correlation study of photoluminescence properties and mechanism of hydrogenated amorphous silicon carbide films Mingming Li, Lihua Jiang, Yihua Sun, Tao Wang, Yu Peng, Haiyan Tian, Ting Xiao, Peng Xiang, Xinyu Tan PII:
S0022-2313(18)31778-2
DOI:
https://doi.org/10.1016/j.jlumin.2019.04.020
Reference:
LUMIN 16415
To appear in:
Journal of Luminescence
Received Date: 1 October 2018 Revised Date:
17 February 2019
Accepted Date: 11 April 2019
Please cite this article as: M. Li, L. Jiang, Y. Sun, T. Wang, Y. Peng, H. Tian, T. Xiao, P. Xiang, X. Tan, Correlation study of photoluminescence properties and mechanism of hydrogenated amorphous silicon carbide films, Journal of Luminescence (2019), doi: https://doi.org/10.1016/j.jlumin.2019.04.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Correlation study of photoluminescence properties and mechanism of hydrogenated amorphous silicon carbide films
Mingming Lia,c, Lihua Jianga,b,c,**, Yihua Suna,c, Tao Wanga, Yu Penga, Haiyan Tiana,
a
College of Materials and Chemical Engineering, China Three Gorges University, 8
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Daxue Road, Yichang, Hubei 443002, China b
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Ting Xiaoa,b,c, Peng Xianga,c, Xinyu Tana,b,c,*
Hubei Provincial Collaborative Innovation Center for New Energy Microgrid,
c
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China Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China College of Materials and Chemical Engineering, Key Laboratory of Inorganic
Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang 443002, China
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** Corresponding author. E-mail addresses: [email protected] (Lihua Jiang).
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* Co-corresponding author. E-mail addresses: [email protected] (Xinyu Tan).
ACCEPTED MANUSCRIPT Abstract Hydrogenated amorphous silicon carbide (a-SiCx:H) films have been deposited by plasma enhanced chemical vapor deposition (PECVD) system with different radio-frequency (RF) powers. The microstructure and optical properties of the films deposited at different RF powers have been studied. The results show that the sp2/sp3
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ratio and the average size of sp2-C rich domains increase while the hydrogen content and the density of sp2-C rich domains in the film decreases with increasing RF power. In addition, the sp2 carbon domains are observed by HR-TEM, and a possible
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photoluminescence (PL) mechanism is also suggested: the strong PL emission originates from radiative recombination of excitons within sp2-C rich domains and C–Si/C–C random network. As RF power increases, the change of the PL properties
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can be attributed to the influence of RF power on the sp2-C rich domains and the C–Si/C–C random network. The corresponding color coordinates of PL spectra also show that the film has great application potential in white light emission. Keyworks: silicon carbide films; photoluminescence properties; sp2 carbon; PECVD;
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radio-frequency power
ACCEPTED MANUSCRIPT 1. Introduction Nowadays, silicon-based optoelectronic integrated technology is still the dominant for the development of optoelectronic integrated circuit. However, in components and devices for light emission applications, silicon is very poor light emission due to its
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indirect band gap nature. A variety of light emissions from silicon, such as porous silicon and nanocrystalline silicon have been studied and show a sufficiently strong luminescence, however they present several drawbacks as stability, the tuning of emission color and particularly in the use of the flexible devices [1-2]. In the past
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decade, the hydrogenated amorphous silicon carbide (a-SiCx:H) film has aroused great research interest to the researchers of optoelectronic material since it
shows
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excellent chemical stability, tunable emission color and more important is the fact that it can be used on flexible devices [3-4]. Thus, various physical or chemical deposition technologies have been employed to deposited a-SiCx:H films such as D.C./RF magnetron sputtering [5], hot wire chemical vapor deposition (HWCVD) [6], electron-cyclotron resonance plasma-enhanced chemical-vapor deposition (CVD) [7],
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laser ablation [8] and PECVD technologies [9-10]. Compared with other techniques, PECVD technology is one of the suitable techniques for the deposition of optical a-SiCx:H film because it allows the growth at low substrate temperature (T ≤ 100 ℃)
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with high deposition rate, and there is not necessity of thermal treatments to produce high levels of light emission [1].
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Researches show that the optical properties of a-SiCx:H film can be well tailored by microstructure of the film. As silicon content increases, the optical band gap of a-SiCx:H film gradually decreases and closes to the optical band gap of amorphous silicon, and in the carbon-rich a-SiCx:H film (x > 0.5), the structural model and the optical properties are similar to those of hydrogenated amorphous carbon film [11-15]. It is well known that sp2 carbon phase in a-SiCx:H film is related to the ion/particle energy in the reactor. In our previous study, a-SiCx:H films with carbon nanostructures have been deposited by PECVD technology, and carbon nanostructures of the films gradually grow up with increasing substrate temperature [16]. Moreover, the PL
ACCEPTED MANUSCRIPT properties are related to the sp2 carbon content, and the more sp2 carbon content leads to the narrower optical band gap, which can effectively regulate the light emission color of a-SiCx:H film [17-18]. So far, however, there is few study about the influence of RF power on microstructure and PL properties, and the PL mechanism of the
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a-SiCx:H film is still unclear.
Herein, in this work, the a-SiCx:H films have been deposited by PECVD technology at different RF powers, and the influence of RF power on microstructure and PL
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properties has also been studied. The results show that microstructure and optical properties can be efficiently tailored by RF power. The PL emission peak show a
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series of red-shifts with increasing RF power, which can be attributed to the influence of the RF power on sp2 carbon phase of the film. The PL emission peak intensity gradually decreases, which results from produced defects through dehydrogenation with increasing RF power. In addition, based on the relationship between structure and optical properties, a possible PL emission mechanism is suggested: the PL
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originates from radiative recombination of excitons within sp2-C rich domains and C–Si/C–C random network.
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2. Experimental section
2.1. Materials and methods
In this work, a-SiCx:H films are deposited by a double chamber three cathode physical
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and chemical vapor deposition system (JGP450+CVD) manufactured by Shenyang scientific instruments co., LTD., Chinese academy of sciences. In order to ensure the different measurement purposes, 4 inches 200 µm thick n-type crystalline silicon wafer (100), 2 mm thick crystalline KBr (100) and 1 mm thick microscope glass slide are selected as three different substrates, and the crystalline silicon substrate is polished by chemical mechanical polishing technology. Briefly, a plasma discharge is formed from a hot filament from the methane (CH4) (purity 99.995%) and a gaseous mixture of silane (SiH4) diluted to 10 % in hydrogen. The pure CH4 and SiH4 gaseous mixture flow rates are fixed at 14 and 7 stand-state cubic centimeter per minute
ACCEPTED MANUSCRIPT (sccm), respectively. The substrate temperature and chamber pressure are fixed at around 200 ℃ and 60 Pa, respectively. Only RF power varies from 100 to 180 W (the corresponding RF power density varies from 289 to 520 mW/cm2), and the step is 20 W. The deposition time is maintained at about 40 minutes in order to obtain the
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appropriate thickness.
2.2. Characterizations
To prevent pollution during the sample preservation and characterization, optical
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properties are straightway characterized after deposition. The transmission and absorption spectra of the as deposited a-SiCx:H film samples on microscope glass
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slide substrates are studied at normal incidence by an ultraviolet visible (UV-Vis) spectrometer (UV-2550v, Shimadzu Corporation, Japan) over the spectral range of 300-800 nm. The room temperature PL spectra of the samples deposited on silicon substrates are measured by a spectro-fluorimeter (Hitachi Model F-4500). The PL emission color of the sample deposited at different RF powers is predicted with the
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help of CIE color co-ordinates. The thicknesses of the as deposited films at different RF powers are measured through observing cross-sectional views of the films by scanning electron microscopy (SEM). The structure properties of the films are
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characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman scattering spectroscopy and high resolution scanning electron microscopy (HR-TEM). The details of the FTIR, XPS spectroscopy and
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HR-TEM have been shown in [19], and the Raman scattering spectra are observed by a dispersive Raman microscope (Senterra R200-L) with a 532 nm excitation and 10 s integration time.
3. Results and discussion 3.1. Film thickness As a basic characteristic, the thickness of the film has a great influence on the analysis of optical and structural properties. So, the thicknesses of the films deposited at different RF powers are firstly measured by the cross-sectional view of SEM, and the
ACCEPTED MANUSCRIPT clean and tidy cross-sections have been prepared in advance as shown in Fig. 1 (a-e). By measuring different positions of each sample, the change curve of film thickness with RF power is shown in Fig. 1 (f), the corresponding error bar of every sample has also been presented. By fixing other deposition parameters and only increasing RF
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power from 100 to 180 W, the thickness of the films increases from about 310 to 520 nm. Considering to the same deposition time, the increasing thickness with RF power can be explained by the fact that more CHn/SiHn radicals are generated at high RF power density [20-21]. In addition, the error bar increases gradually with the RF
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power, which indicates that the film is less uniform.
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3.2. FTIR
The characteristics of chemical bonds of the a-SiCx:H film on KBr substrate are studied by FTIR absorption spectra. The FTIR spectra after normalized by the film thickness are shown in Fig. 2 (a). There are three obvious absorption peaks at around 1254, 1650 and 2110 cm-1, and two absorption bands at 600-1200 and 2800-3000 cm-1.
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In generally, each FTIR absorption band consists of different vibration modes. Therefore, the absorption band at 600-1200 cm-1 is decomposed into three absorption peaks at around 800, 890 and 1030 cm-1 with the best Gaussian fit as shown in Fig. 2 (a). According to the references [1, 22-27], the vibration modes at around 800, 890,
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1030, 1254, 1650, 2110 cm-1 and 2780-3015 cm-1 can be attributed to Si–C stretching vibration, Si–O–Si bending vibration, Si-CH2 rocking vibration, Si–CH3 bending
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vibration, C=C/O–H vibration, CSi–Hn bending vibration and C–Hn stretching vibration, respectively. Each chemical bond content can be estimated by the integral intensity of each vibration mode. The trends of each relative chemical bond content of the films deposited at different RF powers are shown in Fig. 2 (b). The weak Si–O–Si vibration mode can be attributed to the surface oxidation and/or the using of hygroscopic KBr substrates [28-30]. It is obvious that with increasing RF power the Si–C bond content first decreases and then increases slightly when the Si–CH3, CSi–Hn and C–Hn bond contents decrease slightly with increasing RF power. In addition, the Si–CH2 bond content is oscillating with increasing RF power.
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3.3. XPS The studies of chemical composition and carbon hybridization structure have been implemented by XPS. The Si2p, C1s and O1s peaks can be observed at around 101.1,
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284.5 and 531.8 eV, respectively. The carbon, silicon and oxygen atom concentrations are calculated by taking into their sensitivity factors and the area below the measured curves [31], as shown in Table 1. The C1s peaks of the films deposited at different RF powers are decomposed into three mode peaks with the best Gaussian fit as shown in
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Fig. 3 (a-e). Refer to the literatures [31-33], the three mode peaks located at 283.5, 284.6 and 285.2 eV correspond to C–Si, sp2 C=C and sp3 C–C, respectively. The
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percent contents of these three bonds are calculated by the integral intensity of each mode peak and shown in Table 1. To study the change of microstructure with increasing RF power, the changes of Si, C–Si content and sp2/sp3 ratio are shown in Fig. 3 (e). It is found that the sp2/sp3 ratio gradually increases and the Si and C–Si
3.4. Raman
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contents seem to decrease with increasing RF power.
Based on the study [34], the PL properties of the a-SiCx:H film can be determined by the concentration and dimension of sp2-C rich domains because π and π* states of sp2
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carbon form band edges. In a-SiCx:H films, sp2-C rich domains can be characterized well by Raman spectra, and the Raman spectra of the films deposited on glass
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substrates have been implemented as shown in Fig. 4 (a). It is obvious a D peak at around 1356 cm-1 and a G peak at around 1580 cm-1 [35]. After deducting the baseline, the normalized Raman spectra are shown in the inset of Fig. 4 (a). The G peak signifies the presence of sp2 carbon in the material, and the D peak signifies various defects in the material like stacking faults, open edges etc. in the material [36-37]. Therefore, the intensity ratio of the D and G peaks (ID/IG) is strictly connected to the sp2/sp3 ratio and sp2-C rich domains [38]. As RF power increases, the ID/IG decreases gradually, which indicates that the sp2/sp3 ratio and the average size of sp2-C rich domains increase gradually, the density of sp2-C rich domains in the film decreases
ACCEPTED MANUSCRIPT with increasing RF power [18, 39]. The Raman results are in good agreement with the XPS. Moreover, in order to further study the sp2-C rich domains, the sample deposited at RF power = 100 W is selected to observe to typical plan-view HR-TEM image. The size of sp2-C rich domains is only about 15-20 nm, smaller than that reported in the
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literature [19].
The reason that the structural properties of the films change with increasing RF power can be attributed to the different decompositions of precursor gas or/and the film
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growth process. In methane plasma, the main ionization processes are: CH4+CH4+→CH5++CH3 and CH4+CH3+→C2H5++H2 [40]. When the silane gas is the
main
ionization
processes
will
include
simultaneously
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introduced
SiH4+CH4+→CH5++SiH3 and SiH4+CH3+→CH3-SiH2++H2. Because the C-H bond (the bond energy is about 414 kj/mol) is stronger than the Si-H bond, the silane molecule will be preferentially decomposed than methane molecules at same RF power. Thus, the enhancement of RF power may improve the decomposition rate of
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methane. During the decomposition of the precursor gas, the concentration of carbon free radicals (–CHn, –C2H5 and etc.) increases and the concentration of silicon free radicals (–SiHn, –SiH2CH3 and etc.) decreases with increasing RF power, which leads
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to the reduction of silicon content in the film. The reduction of silicon content may also be a reason for the reduction of C–Si bonds. In addition, the higher energy ion/free-radicals promotes the sputtering of hydrogen atoms from the films [40].
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According to reference [34], three basic chemical reactions are proposed: (i) some strained C–C bonds (sp3 carbon) broke up with consecutive reforming into C=C bonds (sp2 carbon); (ii) two methyl groups are dehydrogenated to form a C–C bonds (sp3 carbon); (iii) reaction of residual allyl groups containing C=C double bonds as: –C/SiHn–CH2–CH2–C/SiHn– → –C/SiHn–CH=CH–C/SiHn–+H2↑ (i) –C/SiHn–CH3+CH3–C/SiHn– → –C/SiHn–CH2–CH2–C/SiHn–+H2↑ (ii) –C/SiHn–CH=CH2+CH2=CH–C/SiHn– → –C/SiHn–CH=CH–CH=CH–C/SiHn–+H2↑ (iii)
(n=0,1,2) In these processes, as the RF power increases, the reforming of sp3 carbon to sp2
ACCEPTED MANUSCRIPT carbon promotes the graphitization of the film and the growing up of sp2 carbon domains. The sputtering of hydrogen atoms leads to diminution of C–Hn, CSi–Hn and Si–CH3 bond (as FTIR results). Oscillating Si–CH2 bond contents can be attributed to the combined effect of (i) and (ii). In addition, hydrogen atoms play a role of reducing
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the density of dangling bonds [40-41]. The sputtering of hydrogen atoms from the films must result in more C/Si dangling bonds, which provides more landing points for oxygen atoms and brings about a large influence on optical properties.
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3.5. Optical properties
To explore the effect of the microstructure on the optical properties, the raw
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transmission and absorption spectra of the a-SiCx:H films have been measured as shown in Fig. 5. As a reference, the transmission and absorption spectra of blank glass substrate are also given. From the transmission spectra, it is found that the transmittances of the samples have a sharp decrease in the range of 320-550 nm. In the absorption spectra of Fig. 5 (b), there is an obvious absorption in the range of
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320-550 nm, which corresponds to transmission spectra. As RF power increases the transmission and absorption spectra show a series of red shift, which can be attributed to the graphitization of the film. Refer to the research of V. Drínek et al. [34], the
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UV-vis absorption peak of ethylene is at 162 nm, and the substitution of a hydrogen atom by an alkyl group causes a red shift of the absorption peak. The absorption is associated with some imperfections and irregularities deviating from an ideal structure.
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However, our absorption peaks are in the range of 320-550 nm and show a series of red shift with increasing RF power, which also indicates that the high RF power promotes the graphitization of the film by implementing the (i) and (iii).
Moreover, it is believed that the strong PL emission is related to the microstructure of the a-SiCx:H films. In order to explore the origin of the PL emission, the normalized PL spectra excited by selected 330 nm light are shown in Fig. 6 (a). As the RF power increases from 100 to 180 W, the PL emission band intensity gradually decreases and the PL band position shows a series of red shifts. For the sample deposited at 100 W,
ACCEPTED MANUSCRIPT the PL excitation (PLE) spectrum recorded with the strongest luminescence (at 467 nm emission) shows two sharp peaks at around 265 and 330 nm as shown in Fig. 6 (b). The PL emissions excited by 265 nm and 330 nm also are the strongest PL as shown in Fig. 6 (b), and they have similar peak shapes and different emission intensities. The
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330-nm PLE peak corresponds to the absorption band of 320-500 nm. The 265-nm PLE peak should also have a corresponding absorption band that can hide in the strong background absorption of glass substrate. Refer to the studies [42-43], the two electron transitions of 330 nm (3.57 eV) and 265 nm (4.68 eV) shown in the PLE
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spectra can be attributed to electrons transition from the σ and π orbitals (highest occupied molecular orbital, HOMO) to lowest unoccupied molecular orbital (LUMO).
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The PLE spectrum clearly demonstrates that the room temperature PL from the a-SiCx:H films is directly correlated with the two electrons transitions at 330 nm and 265 nm. When the excitation wavelength is less than 265 nm, photon energy (< 4.68 eV) is not enough to urge the electrons transition from the σ orbitals to LUMO. So after excited by 330 nm, lots of excitons will be generated in the π orbital (HOMO)
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and π* orbital (LUMO) through electrons transitions.
The change of the PL emission band with RF power can be attributed to the difference
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of radiative recombination mechanism. In order to further explore the radiative recombination mechanism of the excitons, the normalized PL spectra are decomposed into three main peaks with the best Gaussian fit, as shown in Fig. 7 (a-e). The peak
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positions of mode 1, mode 2 and mode 3 are fixed at around 475, 570, 660 nm, respectively. The change of the intensity of each mode with increasing RF power is shown in Fig. 7 (f). It is found that the intensity of the mode 3 is almost constant with increasing RF power, and so the mode 3 can be attributed to the double frequency scattering peak. As the RF power increases from 100 W to 140 W, the intensity of the mode 1 decreases sharply while the intensity of the mode 2 first remains constant. As the RF power increases from 140 W to 180 W, the intensity of the mode 2 decreases sharply and the mode 1 disappear due to too weak. The mode 1 and the mode 2 are strictly dependent on RF power, which can be attributed to different radiative
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According to the Pauli’s exclusion principle, electrons with parallel spins are forbidden to sit at the same position. As sp2-C clusters in size, more molecular orbitals
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(MO) will be create, i.e., the MO’s become bands [45]. In the amorphous carbon films, the strong PL emission can be associated with the broad band tail states. Because the broad band tail states give rise to highly localized and polarized intrasp2-cluster PL emission [18]. In this work, the carbon hybridization structure and the density of sp2
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rich domains of the a-SiCx:H films deposited at different RF powers have been investigated by XPS and Raman spectra. The density of sp2-C rich domains decreases
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with decreasing RF power, which leads to a consequence of carrier localization within an decreasing number of sp2 rich domains. As RF power increases, the carrier localization within an decreasing number of sp2-C rich domains must contribute weak PL emission. So, the the strong PL emission of the mode 2 can be attributed to the
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radiative recombination of the excitons within sp2-C rich domains.
For the mode 1, the origin of PL emission is thought to be related to the C–Si/C–C random network. It is studied that the silicon atoms and network defects induce some
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localized gap states into the middle of the band gap [46]. On the one hand, the localized gap states into the middle of the band gap can cause strong PL emission through the radiative recombination in the band gap. On the other hand, the localized
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gap states induced by network defects will become electron or hole capture centers, which may cause non-radiative recombination of the electron-hole pairs [40, 47] In this investigation, lots of silicon atoms and network defects have been found in the films, which may cause the localized gap states. Moreover, with increasing RF power, higher ion impact energy promotes the sputtering of hydrogen atoms from the films, which leads to more C/Si dangling bonds. After excitation, as a defect within the C–Si/C–C random network, the unstable C/Si dangling bonds in the films will contribute non-radiative recombination. Generally, there is a competition between these two recombination processes, the enhanced non-radiative recombination must
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In addition, the corresponding color coordinates of PL excited by 330 nm have been given in CIE 1931 chromaticity diagram as shown in Fig. 8. It is seen that the
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characteristic colors of the PL emission spectra change from light blue to light yellow as RF power increases from 100 to 180 W. The PL emission of the film deposited at 120 W is positioned at x = 0.31 and y = 0.36 in the chromaticity diagram, which is very close to the values of “white” light (x = 0.325 and y = 0.335) [2]. Therefore,
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application potential in white light emission [48].
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through reasonable control of deposition parameters, the a-SiCx:H films have great
ACCEPTED MANUSCRIPT 4. Conclusions In conclusion, we have provided experimental evidence for the change of the structural and optical properties of a-SiCx:H films deposited at different RF powers. The effect of the RF power on structural and PL properties of the films has also been
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studied. The results confirm that the microstructure and optical properties are very sensitive for the RF power. As RF power increase, on the one hand, the silicon content and the sp2/sp3 ratio gradually increase, which results in a enriched sp2-C domains. On the other hand, with increasing RF power high ion impact energy promotes the
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sputtering of hydrogen atoms, which contributes lots of defects within the band gap. In the PL process, the electrons transition from the π orbital (HOMO) to the π* orbital
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(LUMO), after the thermalization processes, some electrons return the ground state through the radiative recombination of electron-hole pairs within the sp2-C rich domains and the C–Si/C–C random network. So, the change of the PL properties can be attributed to the influence of RF power on the sp2-C rich domains and the
Acknowledgement
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C–Si/C–C random network.
This work is supported by the National Natural Science Foundation of China (No.
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61604087 and U1765105), the State Scholarship fund of the China Scholarship Council (CSC201708420285) and the Research Fund for Excellent Dissertation of
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China Three Gorges University (2018SSPY058).
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ACCEPTED MANUSCRIPT [29] P.B. Lukins, D.R. McKenzie, 13C NMR and FTIR study of thermal annealing of amorphous hydrogenated carbon, Carbon 31 (1993) 569–575. [30] Ö.D. Coşkun, T. Zerrin, Optical, Structural and bonding properties of diamond-like amorphous carbon films deposited by DC magnetron sputtering, Diam.
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[39] M.A.A. Rahman, B.T. Goh, W.S. Chiu, C.Y. Haw, M.R. Mahmood, P.S. Khiew, S.A. Rahman, Aging- and thermal-annealing effects on the vibrational- and microstructural- properties of PECVD grown hydrogenated amorphous silicon carbon nitride thin films, Vib. Spectrosc. 94 (2018) 22-30. [40] E. Tomasella, C. Meunier, S. Mikhailov, a-C:H thin films deposited by
ACCEPTED MANUSCRIPT radio-frequency plasma: influence of gas composition on structure, optical properties and stress levels, Surf. Coat. Technol. 141 (2001) 286-296. [41] A. Morimoto, T. Miura, M. Kumeda, T. Shimizu, Defects in hydrogenated amorphous silicon‐carbon alloy films prepared by glow discharge decomposition
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Photoluminescence studies of hydrogenated amorphous carbon and its alloys, J. Appl.
[45] M. Li, L. Jiang, Y. Sun, T. Xiao, P. Xiang, X. Tan, Correlation study of microstructure and photoluminescence properties of a-C:H thin films deposited by plasma-enhanced chemical vapor deposition technique, Diam. Relat. Mater. 80 (2017)
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[46] Z. Li, J. Zhang, H. He, J. Bian, X. Zhang, G. Han, Blue-green luminescence and SERS study of carbon-rich hydrogenated amorphous silicon carbide films with
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multiphase structure, Phys. Stat. Sol. (a) 207 (2010) 2543-2548. [47] W. Siebert, R. Carius, W. Fuhs, K. Jah, Photoluminescence in a-Si1−xCx:H films, Phys. Stat. Sol. (b) 140 (1987) 311-321.
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[48] Z. Khatami, P.R.J. Wilson, J. Wojcik, P. Mascher, On the origin of white light emission from nanostructured silicon carbonitride thin films, J. Lumin. 196 (2018) 504-510.
ACCEPTED MANUSCRIPT Table 1 The percent contents of carbon, silicon and oxygen in the films and the percent contents of each mode peak obtained by decomposed C1s peak. C content
Si content
O content
C-Si
sp2 C=C
sp3 C-C
100W 120W 140W 160W 180W
63.14% 72.51% 72.92% 77.78% 73.59%
25.63% 16.17% 14.34% 11.58% 15.27%
11.23% 11.31% 12.74% 10.64% 11.14%
content 18.92% 16.62% 16.58% 7.51% 6.03%
content 47.19% 36.38% 50.14% 63.98% 69.71%
content 33.89% 47.00% 33.28% 28.51% 24.26%
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RF Power
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(c)
100nm
100nm
100nm
600
(e) 426nm
550 Thickness (nm)
(d)
336nm
315nm
313nm
512nm
100nm
450 400 350 300
(f)
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100nm
500
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(a)
250
100
120 140 160 RF Power (W)
180
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Fig. 1. SEM images of as deposited a-SiCx:H films with different RF powers; (a-e) Cross-sectional view and (f) the thicknesses of the films deposited at different RF
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powers. (The scale bars of (a-e) are 100 nm.)
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(800)
CSi-Hn(2110)
Intensity (a.u.)
Si-CH3 C=C/O-H (1254) (1650)
C-Hn (2780-3015)
(b)
3.0
180W
2.5
160W
2.0
140W
Si-C Si-O-Si Si-CH2 Si-CH3 C=C/O-H CSi-Hn C-Hn
1.5
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Si-O-Si Si-CH2 Si-C (890) (1030)
Intensity(a.u.)
(a)
1.0 120W 0.5 0.0
500
1000
1500
2000
2500
3000
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100W
100
120
140
160
180
RF power (W)
-1
Wavenumber (cm )
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Fig. 2. (a) FTIR spectra of the a-SiCx:H film deposited at different RF powers; (b) the content changes of each chemical bond with increasing RF power. (The colorful version of this figure can be viewed online.)
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290 280
282
280
282
284
286
288
286
288
Intensity (a.u.)
290
280
282
50
(e)
290 280
Binding Energy (eV)
282
284
286
288
Binding Energy (eV)
284
286
288
290
Binding Energy (eV)
C1s Peak sum Baseline sp2 C=C sp3 C-C C-Si
Intensity (a.u.)
Intensity (a.u.)
C1s Peak sum Baseline sp2 C=C sp3 C-C C-Si
(d)
284
Binding Energy (eV) 40
4
(f)
2
2
30 20
290
3
sp /sp ratio Si content
0
C-Si content
-2
10
0
100
120
140
-4 160
sp 2/sp 3 composition ratio
288
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286
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284
Binding Energy (eV)
Pencent content (%)
282
C1s Peak sum Baseline sp2 C=C sp3 C-C C-Si
(c)
C1s Peak sum Baseline sp2 C=C sp3 C-C C-Si
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Intensity (a.u.) 280
(b) Intensity (a.u.)
C1s Peak sum Baseline sp2 C=C sp3 C-C C-Si
(a)
180
RF Power (W)
Fig. 3. (a-e) C1s peak of the a-SiCx:H films deposited at different RF powers; (f) the percent contents of the Si and C-Si bonds as well as the sp2/sp3 ratio obtained by
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online.)
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decomposing XPS C1s peak. (The colorful version of this figure can be viewed
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G
100W 120W 140W 160W 180W
(b)
1000 1250 1500 1750 2000
HR-TEM: 100W
1.6 1.4 1.2
10nm 1.0 0.8 0.6
500
1000
1500
2000
2500
3000
-1
Wavenumber (cm )
100
120
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D
ID/IG
Intensity (a.u.)
(a)
140
160
180
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RF power (W)
Fig. 4. (a) Raman spectra of the a-SiCx:H films deposited at different RF powers and
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the inset of (a) shows the D and the G peaks after subtract baseline; (b) the change of ID/IG with increasing RF power and the inset of (b) shows the HR-TEM image of the
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film deposited at 100 W. (The colorful version of this figure can be viewed online.)
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(a)
60
glass substrate 100W 120W 140W 160W 180W
40
20
400
500
600
700
800
2.0
glass substrate 100W 120W 140W 160W 180W
1.5 1.0 0.5
900
0.0 300
400
500
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80
0 300
(b)
2.5
Absorbance (a.u.)
Transmittance (%)
100
600
700
800
900
Wavelength (nm)
Wavelength (nm)
Fig. 5. (a) Transmission spectra and (b) absorption spectra of the as deposited
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a-SiCx:H films at different RF powers. (The colorful version of this figure can be
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viewed online.)
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(b) Si substrate 100 W 120 W 140 W 160 W 180 W
500
600
700
800
800
Em Ex265 Em Ex330
330 nm
0 500
700
265 nm
500
400
600
260
280
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1000
wavelength (nm)
400
Intensity (Em 467)
Intensity (a.u.)
1500
300
Intensity (Ex265/330nm)
(a)
300
320
340
Excitation wavelength (nm)
Wavelength (nm)
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Fig. 6. (a) Room temperature PL spectra of the film deposited at different RF powers by 330 nm excitation; (b) blue curve: PL excitation spectrum at 467 nm emission; Red
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curve: PL emission spectra of the film deposited at 100 W excited by 265 and 330 nm
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excitation. (The colorful version of this figure can be viewed online.)
ACCEPTED MANUSCRIPT rate date mode 1 mode 2 mode 3
(b)
120 W
2.0x105
2.0x105
0.0 700
800
2.0x105
0.0 400
500
600
700
800
400
rate date mode 2 mode 3
(e)
1x105
rate date mode 2 mode 3
180 W
2x105
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Intensity (a.u.)
160 W
2x105
rate date mode 1 mode 2 mode 3
1x105
(f)
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600
140 W
4.0x105
Intensity (a.u.)
500
Wavelength (nm)
Intensity (a.u.)
(c)
0.0 400
(d)
rate date mode 1 mode 2 mode 3
4.0x105
Intensity (a.u.)
100 W
4.0x105
Intensity (a.u.)
Intensity (a.u.)
(a)
mode 1 mode 2 mode 3
400000 300000 200000
0
0 500
600
700
800
Wavelength (nm)
0
400
500
600
700
Wavelength (nm)
800
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400
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100000
100
120
140
160
180
RF power (W)
Fig. 7. (a-e) Multi-peak Gauss fit of the PL spectra using 330 nm excitation and (f) the intensity of each PL mode with increasing RF power. (The colorful version of this
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Fig. 8. The color coordinate of PL emission in CIE 1931 chromaticity diagram. (The
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S0022-2313(18)31778-2
DOI:
https://doi.org/10.1016/j.jlumin.2019.04.020
Reference:
LUMIN 16415
To appear in:
Journal of Luminescence
Received Date: 1 October 2018 Revised Date:
17 February 2019
Accepted Date: 11 April 2019
Please cite this article as: M. Li, L. Jiang, Y. Sun, T. Wang, Y. Peng, H. Tian, T. Xiao, P. Xiang, X. Tan, Correlation study of photoluminescence properties and mechanism of hydrogenated amorphous silicon carbide films, Journal of Luminescence (2019), doi: https://doi.org/10.1016/j.jlumin.2019.04.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Correlation study of photoluminescence properties and mechanism of hydrogenated amorphous silicon carbide films
Mingming Lia,c, Lihua Jianga,b,c,**, Yihua Suna,c, Tao Wanga, Yu Penga, Haiyan Tiana,
a
College of Materials and Chemical Engineering, China Three Gorges University, 8
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Daxue Road, Yichang, Hubei 443002, China b
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Ting Xiaoa,b,c, Peng Xianga,c, Xinyu Tana,b,c,*
Hubei Provincial Collaborative Innovation Center for New Energy Microgrid,
c
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China Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China College of Materials and Chemical Engineering, Key Laboratory of Inorganic
Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang 443002, China
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** Corresponding author. E-mail addresses: [email protected] (Lihua Jiang).
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* Co-corresponding author. E-mail addresses: [email protected] (Xinyu Tan).
ACCEPTED MANUSCRIPT Abstract Hydrogenated amorphous silicon carbide (a-SiCx:H) films have been deposited by plasma enhanced chemical vapor deposition (PECVD) system with different radio-frequency (RF) powers. The microstructure and optical properties of the films deposited at different RF powers have been studied. The results show that the sp2/sp3
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ratio and the average size of sp2-C rich domains increase while the hydrogen content and the density of sp2-C rich domains in the film decreases with increasing RF power. In addition, the sp2 carbon domains are observed by HR-TEM, and a possible
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photoluminescence (PL) mechanism is also suggested: the strong PL emission originates from radiative recombination of excitons within sp2-C rich domains and C–Si/C–C random network. As RF power increases, the change of the PL properties
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can be attributed to the influence of RF power on the sp2-C rich domains and the C–Si/C–C random network. The corresponding color coordinates of PL spectra also show that the film has great application potential in white light emission. Keyworks: silicon carbide films; photoluminescence properties; sp2 carbon; PECVD;
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radio-frequency power
ACCEPTED MANUSCRIPT 1. Introduction Nowadays, silicon-based optoelectronic integrated technology is still the dominant for the development of optoelectronic integrated circuit. However, in components and devices for light emission applications, silicon is very poor light emission due to its
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indirect band gap nature. A variety of light emissions from silicon, such as porous silicon and nanocrystalline silicon have been studied and show a sufficiently strong luminescence, however they present several drawbacks as stability, the tuning of emission color and particularly in the use of the flexible devices [1-2]. In the past
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decade, the hydrogenated amorphous silicon carbide (a-SiCx:H) film has aroused great research interest to the researchers of optoelectronic material since it
shows
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excellent chemical stability, tunable emission color and more important is the fact that it can be used on flexible devices [3-4]. Thus, various physical or chemical deposition technologies have been employed to deposited a-SiCx:H films such as D.C./RF magnetron sputtering [5], hot wire chemical vapor deposition (HWCVD) [6], electron-cyclotron resonance plasma-enhanced chemical-vapor deposition (CVD) [7],
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laser ablation [8] and PECVD technologies [9-10]. Compared with other techniques, PECVD technology is one of the suitable techniques for the deposition of optical a-SiCx:H film because it allows the growth at low substrate temperature (T ≤ 100 ℃)
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with high deposition rate, and there is not necessity of thermal treatments to produce high levels of light emission [1].
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Researches show that the optical properties of a-SiCx:H film can be well tailored by microstructure of the film. As silicon content increases, the optical band gap of a-SiCx:H film gradually decreases and closes to the optical band gap of amorphous silicon, and in the carbon-rich a-SiCx:H film (x > 0.5), the structural model and the optical properties are similar to those of hydrogenated amorphous carbon film [11-15]. It is well known that sp2 carbon phase in a-SiCx:H film is related to the ion/particle energy in the reactor. In our previous study, a-SiCx:H films with carbon nanostructures have been deposited by PECVD technology, and carbon nanostructures of the films gradually grow up with increasing substrate temperature [16]. Moreover, the PL
ACCEPTED MANUSCRIPT properties are related to the sp2 carbon content, and the more sp2 carbon content leads to the narrower optical band gap, which can effectively regulate the light emission color of a-SiCx:H film [17-18]. So far, however, there is few study about the influence of RF power on microstructure and PL properties, and the PL mechanism of the
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a-SiCx:H film is still unclear.
Herein, in this work, the a-SiCx:H films have been deposited by PECVD technology at different RF powers, and the influence of RF power on microstructure and PL
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properties has also been studied. The results show that microstructure and optical properties can be efficiently tailored by RF power. The PL emission peak show a
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series of red-shifts with increasing RF power, which can be attributed to the influence of the RF power on sp2 carbon phase of the film. The PL emission peak intensity gradually decreases, which results from produced defects through dehydrogenation with increasing RF power. In addition, based on the relationship between structure and optical properties, a possible PL emission mechanism is suggested: the PL
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originates from radiative recombination of excitons within sp2-C rich domains and C–Si/C–C random network.
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2. Experimental section
2.1. Materials and methods
In this work, a-SiCx:H films are deposited by a double chamber three cathode physical
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and chemical vapor deposition system (JGP450+CVD) manufactured by Shenyang scientific instruments co., LTD., Chinese academy of sciences. In order to ensure the different measurement purposes, 4 inches 200 µm thick n-type crystalline silicon wafer (100), 2 mm thick crystalline KBr (100) and 1 mm thick microscope glass slide are selected as three different substrates, and the crystalline silicon substrate is polished by chemical mechanical polishing technology. Briefly, a plasma discharge is formed from a hot filament from the methane (CH4) (purity 99.995%) and a gaseous mixture of silane (SiH4) diluted to 10 % in hydrogen. The pure CH4 and SiH4 gaseous mixture flow rates are fixed at 14 and 7 stand-state cubic centimeter per minute
ACCEPTED MANUSCRIPT (sccm), respectively. The substrate temperature and chamber pressure are fixed at around 200 ℃ and 60 Pa, respectively. Only RF power varies from 100 to 180 W (the corresponding RF power density varies from 289 to 520 mW/cm2), and the step is 20 W. The deposition time is maintained at about 40 minutes in order to obtain the
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appropriate thickness.
2.2. Characterizations
To prevent pollution during the sample preservation and characterization, optical
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properties are straightway characterized after deposition. The transmission and absorption spectra of the as deposited a-SiCx:H film samples on microscope glass
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slide substrates are studied at normal incidence by an ultraviolet visible (UV-Vis) spectrometer (UV-2550v, Shimadzu Corporation, Japan) over the spectral range of 300-800 nm. The room temperature PL spectra of the samples deposited on silicon substrates are measured by a spectro-fluorimeter (Hitachi Model F-4500). The PL emission color of the sample deposited at different RF powers is predicted with the
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help of CIE color co-ordinates. The thicknesses of the as deposited films at different RF powers are measured through observing cross-sectional views of the films by scanning electron microscopy (SEM). The structure properties of the films are
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characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman scattering spectroscopy and high resolution scanning electron microscopy (HR-TEM). The details of the FTIR, XPS spectroscopy and
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HR-TEM have been shown in [19], and the Raman scattering spectra are observed by a dispersive Raman microscope (Senterra R200-L) with a 532 nm excitation and 10 s integration time.
3. Results and discussion 3.1. Film thickness As a basic characteristic, the thickness of the film has a great influence on the analysis of optical and structural properties. So, the thicknesses of the films deposited at different RF powers are firstly measured by the cross-sectional view of SEM, and the
ACCEPTED MANUSCRIPT clean and tidy cross-sections have been prepared in advance as shown in Fig. 1 (a-e). By measuring different positions of each sample, the change curve of film thickness with RF power is shown in Fig. 1 (f), the corresponding error bar of every sample has also been presented. By fixing other deposition parameters and only increasing RF
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power from 100 to 180 W, the thickness of the films increases from about 310 to 520 nm. Considering to the same deposition time, the increasing thickness with RF power can be explained by the fact that more CHn/SiHn radicals are generated at high RF power density [20-21]. In addition, the error bar increases gradually with the RF
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power, which indicates that the film is less uniform.
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3.2. FTIR
The characteristics of chemical bonds of the a-SiCx:H film on KBr substrate are studied by FTIR absorption spectra. The FTIR spectra after normalized by the film thickness are shown in Fig. 2 (a). There are three obvious absorption peaks at around 1254, 1650 and 2110 cm-1, and two absorption bands at 600-1200 and 2800-3000 cm-1.
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In generally, each FTIR absorption band consists of different vibration modes. Therefore, the absorption band at 600-1200 cm-1 is decomposed into three absorption peaks at around 800, 890 and 1030 cm-1 with the best Gaussian fit as shown in Fig. 2 (a). According to the references [1, 22-27], the vibration modes at around 800, 890,
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1030, 1254, 1650, 2110 cm-1 and 2780-3015 cm-1 can be attributed to Si–C stretching vibration, Si–O–Si bending vibration, Si-CH2 rocking vibration, Si–CH3 bending
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vibration, C=C/O–H vibration, CSi–Hn bending vibration and C–Hn stretching vibration, respectively. Each chemical bond content can be estimated by the integral intensity of each vibration mode. The trends of each relative chemical bond content of the films deposited at different RF powers are shown in Fig. 2 (b). The weak Si–O–Si vibration mode can be attributed to the surface oxidation and/or the using of hygroscopic KBr substrates [28-30]. It is obvious that with increasing RF power the Si–C bond content first decreases and then increases slightly when the Si–CH3, CSi–Hn and C–Hn bond contents decrease slightly with increasing RF power. In addition, the Si–CH2 bond content is oscillating with increasing RF power.
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3.3. XPS The studies of chemical composition and carbon hybridization structure have been implemented by XPS. The Si2p, C1s and O1s peaks can be observed at around 101.1,
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284.5 and 531.8 eV, respectively. The carbon, silicon and oxygen atom concentrations are calculated by taking into their sensitivity factors and the area below the measured curves [31], as shown in Table 1. The C1s peaks of the films deposited at different RF powers are decomposed into three mode peaks with the best Gaussian fit as shown in
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Fig. 3 (a-e). Refer to the literatures [31-33], the three mode peaks located at 283.5, 284.6 and 285.2 eV correspond to C–Si, sp2 C=C and sp3 C–C, respectively. The
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percent contents of these three bonds are calculated by the integral intensity of each mode peak and shown in Table 1. To study the change of microstructure with increasing RF power, the changes of Si, C–Si content and sp2/sp3 ratio are shown in Fig. 3 (e). It is found that the sp2/sp3 ratio gradually increases and the Si and C–Si
3.4. Raman
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contents seem to decrease with increasing RF power.
Based on the study [34], the PL properties of the a-SiCx:H film can be determined by the concentration and dimension of sp2-C rich domains because π and π* states of sp2
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carbon form band edges. In a-SiCx:H films, sp2-C rich domains can be characterized well by Raman spectra, and the Raman spectra of the films deposited on glass
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substrates have been implemented as shown in Fig. 4 (a). It is obvious a D peak at around 1356 cm-1 and a G peak at around 1580 cm-1 [35]. After deducting the baseline, the normalized Raman spectra are shown in the inset of Fig. 4 (a). The G peak signifies the presence of sp2 carbon in the material, and the D peak signifies various defects in the material like stacking faults, open edges etc. in the material [36-37]. Therefore, the intensity ratio of the D and G peaks (ID/IG) is strictly connected to the sp2/sp3 ratio and sp2-C rich domains [38]. As RF power increases, the ID/IG decreases gradually, which indicates that the sp2/sp3 ratio and the average size of sp2-C rich domains increase gradually, the density of sp2-C rich domains in the film decreases
ACCEPTED MANUSCRIPT with increasing RF power [18, 39]. The Raman results are in good agreement with the XPS. Moreover, in order to further study the sp2-C rich domains, the sample deposited at RF power = 100 W is selected to observe to typical plan-view HR-TEM image. The size of sp2-C rich domains is only about 15-20 nm, smaller than that reported in the
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literature [19].
The reason that the structural properties of the films change with increasing RF power can be attributed to the different decompositions of precursor gas or/and the film
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growth process. In methane plasma, the main ionization processes are: CH4+CH4+→CH5++CH3 and CH4+CH3+→C2H5++H2 [40]. When the silane gas is the
main
ionization
processes
will
include
simultaneously
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introduced
SiH4+CH4+→CH5++SiH3 and SiH4+CH3+→CH3-SiH2++H2. Because the C-H bond (the bond energy is about 414 kj/mol) is stronger than the Si-H bond, the silane molecule will be preferentially decomposed than methane molecules at same RF power. Thus, the enhancement of RF power may improve the decomposition rate of
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methane. During the decomposition of the precursor gas, the concentration of carbon free radicals (–CHn, –C2H5 and etc.) increases and the concentration of silicon free radicals (–SiHn, –SiH2CH3 and etc.) decreases with increasing RF power, which leads
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to the reduction of silicon content in the film. The reduction of silicon content may also be a reason for the reduction of C–Si bonds. In addition, the higher energy ion/free-radicals promotes the sputtering of hydrogen atoms from the films [40].
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According to reference [34], three basic chemical reactions are proposed: (i) some strained C–C bonds (sp3 carbon) broke up with consecutive reforming into C=C bonds (sp2 carbon); (ii) two methyl groups are dehydrogenated to form a C–C bonds (sp3 carbon); (iii) reaction of residual allyl groups containing C=C double bonds as: –C/SiHn–CH2–CH2–C/SiHn– → –C/SiHn–CH=CH–C/SiHn–+H2↑ (i) –C/SiHn–CH3+CH3–C/SiHn– → –C/SiHn–CH2–CH2–C/SiHn–+H2↑ (ii) –C/SiHn–CH=CH2+CH2=CH–C/SiHn– → –C/SiHn–CH=CH–CH=CH–C/SiHn–+H2↑ (iii)
(n=0,1,2) In these processes, as the RF power increases, the reforming of sp3 carbon to sp2
ACCEPTED MANUSCRIPT carbon promotes the graphitization of the film and the growing up of sp2 carbon domains. The sputtering of hydrogen atoms leads to diminution of C–Hn, CSi–Hn and Si–CH3 bond (as FTIR results). Oscillating Si–CH2 bond contents can be attributed to the combined effect of (i) and (ii). In addition, hydrogen atoms play a role of reducing
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the density of dangling bonds [40-41]. The sputtering of hydrogen atoms from the films must result in more C/Si dangling bonds, which provides more landing points for oxygen atoms and brings about a large influence on optical properties.
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3.5. Optical properties
To explore the effect of the microstructure on the optical properties, the raw
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transmission and absorption spectra of the a-SiCx:H films have been measured as shown in Fig. 5. As a reference, the transmission and absorption spectra of blank glass substrate are also given. From the transmission spectra, it is found that the transmittances of the samples have a sharp decrease in the range of 320-550 nm. In the absorption spectra of Fig. 5 (b), there is an obvious absorption in the range of
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320-550 nm, which corresponds to transmission spectra. As RF power increases the transmission and absorption spectra show a series of red shift, which can be attributed to the graphitization of the film. Refer to the research of V. Drínek et al. [34], the
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UV-vis absorption peak of ethylene is at 162 nm, and the substitution of a hydrogen atom by an alkyl group causes a red shift of the absorption peak. The absorption is associated with some imperfections and irregularities deviating from an ideal structure.
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However, our absorption peaks are in the range of 320-550 nm and show a series of red shift with increasing RF power, which also indicates that the high RF power promotes the graphitization of the film by implementing the (i) and (iii).
Moreover, it is believed that the strong PL emission is related to the microstructure of the a-SiCx:H films. In order to explore the origin of the PL emission, the normalized PL spectra excited by selected 330 nm light are shown in Fig. 6 (a). As the RF power increases from 100 to 180 W, the PL emission band intensity gradually decreases and the PL band position shows a series of red shifts. For the sample deposited at 100 W,
ACCEPTED MANUSCRIPT the PL excitation (PLE) spectrum recorded with the strongest luminescence (at 467 nm emission) shows two sharp peaks at around 265 and 330 nm as shown in Fig. 6 (b). The PL emissions excited by 265 nm and 330 nm also are the strongest PL as shown in Fig. 6 (b), and they have similar peak shapes and different emission intensities. The
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330-nm PLE peak corresponds to the absorption band of 320-500 nm. The 265-nm PLE peak should also have a corresponding absorption band that can hide in the strong background absorption of glass substrate. Refer to the studies [42-43], the two electron transitions of 330 nm (3.57 eV) and 265 nm (4.68 eV) shown in the PLE
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spectra can be attributed to electrons transition from the σ and π orbitals (highest occupied molecular orbital, HOMO) to lowest unoccupied molecular orbital (LUMO).
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The PLE spectrum clearly demonstrates that the room temperature PL from the a-SiCx:H films is directly correlated with the two electrons transitions at 330 nm and 265 nm. When the excitation wavelength is less than 265 nm, photon energy (< 4.68 eV) is not enough to urge the electrons transition from the σ orbitals to LUMO. So after excited by 330 nm, lots of excitons will be generated in the π orbital (HOMO)
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and π* orbital (LUMO) through electrons transitions.
The change of the PL emission band with RF power can be attributed to the difference
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of radiative recombination mechanism. In order to further explore the radiative recombination mechanism of the excitons, the normalized PL spectra are decomposed into three main peaks with the best Gaussian fit, as shown in Fig. 7 (a-e). The peak
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positions of mode 1, mode 2 and mode 3 are fixed at around 475, 570, 660 nm, respectively. The change of the intensity of each mode with increasing RF power is shown in Fig. 7 (f). It is found that the intensity of the mode 3 is almost constant with increasing RF power, and so the mode 3 can be attributed to the double frequency scattering peak. As the RF power increases from 100 W to 140 W, the intensity of the mode 1 decreases sharply while the intensity of the mode 2 first remains constant. As the RF power increases from 140 W to 180 W, the intensity of the mode 2 decreases sharply and the mode 1 disappear due to too weak. The mode 1 and the mode 2 are strictly dependent on RF power, which can be attributed to different radiative
ACCEPTED MANUSCRIPT recombination after thermalization processes [44].
According to the Pauli’s exclusion principle, electrons with parallel spins are forbidden to sit at the same position. As sp2-C clusters in size, more molecular orbitals
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(MO) will be create, i.e., the MO’s become bands [45]. In the amorphous carbon films, the strong PL emission can be associated with the broad band tail states. Because the broad band tail states give rise to highly localized and polarized intrasp2-cluster PL emission [18]. In this work, the carbon hybridization structure and the density of sp2
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rich domains of the a-SiCx:H films deposited at different RF powers have been investigated by XPS and Raman spectra. The density of sp2-C rich domains decreases
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with decreasing RF power, which leads to a consequence of carrier localization within an decreasing number of sp2 rich domains. As RF power increases, the carrier localization within an decreasing number of sp2-C rich domains must contribute weak PL emission. So, the the strong PL emission of the mode 2 can be attributed to the
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radiative recombination of the excitons within sp2-C rich domains.
For the mode 1, the origin of PL emission is thought to be related to the C–Si/C–C random network. It is studied that the silicon atoms and network defects induce some
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localized gap states into the middle of the band gap [46]. On the one hand, the localized gap states into the middle of the band gap can cause strong PL emission through the radiative recombination in the band gap. On the other hand, the localized
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gap states induced by network defects will become electron or hole capture centers, which may cause non-radiative recombination of the electron-hole pairs [40, 47] In this investigation, lots of silicon atoms and network defects have been found in the films, which may cause the localized gap states. Moreover, with increasing RF power, higher ion impact energy promotes the sputtering of hydrogen atoms from the films, which leads to more C/Si dangling bonds. After excitation, as a defect within the C–Si/C–C random network, the unstable C/Si dangling bonds in the films will contribute non-radiative recombination. Generally, there is a competition between these two recombination processes, the enhanced non-radiative recombination must
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In addition, the corresponding color coordinates of PL excited by 330 nm have been given in CIE 1931 chromaticity diagram as shown in Fig. 8. It is seen that the
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characteristic colors of the PL emission spectra change from light blue to light yellow as RF power increases from 100 to 180 W. The PL emission of the film deposited at 120 W is positioned at x = 0.31 and y = 0.36 in the chromaticity diagram, which is very close to the values of “white” light (x = 0.325 and y = 0.335) [2]. Therefore,
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application potential in white light emission [48].
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through reasonable control of deposition parameters, the a-SiCx:H films have great
ACCEPTED MANUSCRIPT 4. Conclusions In conclusion, we have provided experimental evidence for the change of the structural and optical properties of a-SiCx:H films deposited at different RF powers. The effect of the RF power on structural and PL properties of the films has also been
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studied. The results confirm that the microstructure and optical properties are very sensitive for the RF power. As RF power increase, on the one hand, the silicon content and the sp2/sp3 ratio gradually increase, which results in a enriched sp2-C domains. On the other hand, with increasing RF power high ion impact energy promotes the
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sputtering of hydrogen atoms, which contributes lots of defects within the band gap. In the PL process, the electrons transition from the π orbital (HOMO) to the π* orbital
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(LUMO), after the thermalization processes, some electrons return the ground state through the radiative recombination of electron-hole pairs within the sp2-C rich domains and the C–Si/C–C random network. So, the change of the PL properties can be attributed to the influence of RF power on the sp2-C rich domains and the
Acknowledgement
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C–Si/C–C random network.
This work is supported by the National Natural Science Foundation of China (No.
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61604087 and U1765105), the State Scholarship fund of the China Scholarship Council (CSC201708420285) and the Research Fund for Excellent Dissertation of
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China Three Gorges University (2018SSPY058).
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ACCEPTED MANUSCRIPT Table 1 The percent contents of carbon, silicon and oxygen in the films and the percent contents of each mode peak obtained by decomposed C1s peak. C content
Si content
O content
C-Si
sp2 C=C
sp3 C-C
100W 120W 140W 160W 180W
63.14% 72.51% 72.92% 77.78% 73.59%
25.63% 16.17% 14.34% 11.58% 15.27%
11.23% 11.31% 12.74% 10.64% 11.14%
content 18.92% 16.62% 16.58% 7.51% 6.03%
content 47.19% 36.38% 50.14% 63.98% 69.71%
content 33.89% 47.00% 33.28% 28.51% 24.26%
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RF Power
ACCEPTED MANUSCRIPT (b)
(c)
100nm
100nm
100nm
600
(e) 426nm
550 Thickness (nm)
(d)
336nm
315nm
313nm
512nm
100nm
450 400 350 300
(f)
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100nm
500
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(a)
250
100
120 140 160 RF Power (W)
180
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Fig. 1. SEM images of as deposited a-SiCx:H films with different RF powers; (a-e) Cross-sectional view and (f) the thicknesses of the films deposited at different RF
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powers. (The scale bars of (a-e) are 100 nm.)
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(800)
CSi-Hn(2110)
Intensity (a.u.)
Si-CH3 C=C/O-H (1254) (1650)
C-Hn (2780-3015)
(b)
3.0
180W
2.5
160W
2.0
140W
Si-C Si-O-Si Si-CH2 Si-CH3 C=C/O-H CSi-Hn C-Hn
1.5
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Si-O-Si Si-CH2 Si-C (890) (1030)
Intensity(a.u.)
(a)
1.0 120W 0.5 0.0
500
1000
1500
2000
2500
3000
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100W
100
120
140
160
180
RF power (W)
-1
Wavenumber (cm )
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Fig. 2. (a) FTIR spectra of the a-SiCx:H film deposited at different RF powers; (b) the content changes of each chemical bond with increasing RF power. (The colorful version of this figure can be viewed online.)
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290 280
282
280
282
284
286
288
286
288
Intensity (a.u.)
290
280
282
50
(e)
290 280
Binding Energy (eV)
282
284
286
288
Binding Energy (eV)
284
286
288
290
Binding Energy (eV)
C1s Peak sum Baseline sp2 C=C sp3 C-C C-Si
Intensity (a.u.)
Intensity (a.u.)
C1s Peak sum Baseline sp2 C=C sp3 C-C C-Si
(d)
284
Binding Energy (eV) 40
4
(f)
2
2
30 20
290
3
sp /sp ratio Si content
0
C-Si content
-2
10
0
100
120
140
-4 160
sp 2/sp 3 composition ratio
288
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286
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284
Binding Energy (eV)
Pencent content (%)
282
C1s Peak sum Baseline sp2 C=C sp3 C-C C-Si
(c)
C1s Peak sum Baseline sp2 C=C sp3 C-C C-Si
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Intensity (a.u.) 280
(b) Intensity (a.u.)
C1s Peak sum Baseline sp2 C=C sp3 C-C C-Si
(a)
180
RF Power (W)
Fig. 3. (a-e) C1s peak of the a-SiCx:H films deposited at different RF powers; (f) the percent contents of the Si and C-Si bonds as well as the sp2/sp3 ratio obtained by
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online.)
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decomposing XPS C1s peak. (The colorful version of this figure can be viewed
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G
100W 120W 140W 160W 180W
(b)
1000 1250 1500 1750 2000
HR-TEM: 100W
1.6 1.4 1.2
10nm 1.0 0.8 0.6
500
1000
1500
2000
2500
3000
-1
Wavenumber (cm )
100
120
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D
ID/IG
Intensity (a.u.)
(a)
140
160
180
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RF power (W)
Fig. 4. (a) Raman spectra of the a-SiCx:H films deposited at different RF powers and
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the inset of (a) shows the D and the G peaks after subtract baseline; (b) the change of ID/IG with increasing RF power and the inset of (b) shows the HR-TEM image of the
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film deposited at 100 W. (The colorful version of this figure can be viewed online.)
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(a)
60
glass substrate 100W 120W 140W 160W 180W
40
20
400
500
600
700
800
2.0
glass substrate 100W 120W 140W 160W 180W
1.5 1.0 0.5
900
0.0 300
400
500
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80
0 300
(b)
2.5
Absorbance (a.u.)
Transmittance (%)
100
600
700
800
900
Wavelength (nm)
Wavelength (nm)
Fig. 5. (a) Transmission spectra and (b) absorption spectra of the as deposited
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a-SiCx:H films at different RF powers. (The colorful version of this figure can be
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viewed online.)
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(b) Si substrate 100 W 120 W 140 W 160 W 180 W
500
600
700
800
800
Em Ex265 Em Ex330
330 nm
0 500
700
265 nm
500
400
600
260
280
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1000
wavelength (nm)
400
Intensity (Em 467)
Intensity (a.u.)
1500
300
Intensity (Ex265/330nm)
(a)
300
320
340
Excitation wavelength (nm)
Wavelength (nm)
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Fig. 6. (a) Room temperature PL spectra of the film deposited at different RF powers by 330 nm excitation; (b) blue curve: PL excitation spectrum at 467 nm emission; Red
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curve: PL emission spectra of the film deposited at 100 W excited by 265 and 330 nm
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excitation. (The colorful version of this figure can be viewed online.)
ACCEPTED MANUSCRIPT rate date mode 1 mode 2 mode 3
(b)
120 W
2.0x105
2.0x105
0.0 700
800
2.0x105
0.0 400
500
600
700
800
400
rate date mode 2 mode 3
(e)
1x105
rate date mode 2 mode 3
180 W
2x105
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Intensity (a.u.)
160 W
2x105
rate date mode 1 mode 2 mode 3
1x105
(f)
RI PT
600
140 W
4.0x105
Intensity (a.u.)
500
Wavelength (nm)
Intensity (a.u.)
(c)
0.0 400
(d)
rate date mode 1 mode 2 mode 3
4.0x105
Intensity (a.u.)
100 W
4.0x105
Intensity (a.u.)
Intensity (a.u.)
(a)
mode 1 mode 2 mode 3
400000 300000 200000
0
0 500
600
700
800
Wavelength (nm)
0
400
500
600
700
Wavelength (nm)
800
M AN U
400
SC
100000
100
120
140
160
180
RF power (W)
Fig. 7. (a-e) Multi-peak Gauss fit of the PL spectra using 330 nm excitation and (f) the intensity of each PL mode with increasing RF power. (The colorful version of this
AC C
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figure can be viewed online.)
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Fig. 8. The color coordinate of PL emission in CIE 1931 chromaticity diagram. (The
AC C
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colorful version of this figure can be viewed online.)