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Properties of amorphous silicon thin films synthesized by reactive particle beam assisted chemical vapor deposition
Thin Solid Films 518 (2010) 7372–7376
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Properties of amorphous silicon thin films synthesized by reactive particle beam assisted chemical vapor deposition Sun Gyu Choi a, Seok-Joo Wang a, Hyeong-Ho Park a, Jin-Nyoung Jang b, MunPyo Hong b, Kwang-Ho Kwon c, Hyung-Ho Park a,⁎ a b c
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea Department of Display and Semiconductor Physics, Korea University, Chungnam 339-700, Republic of Korea Department of Control and Instrumentation Engineering, Korea University, Chungnam 339-700, Republic of Korea
a r t i c l e
i n f o
Available online 9 May 2010 Keywords: Amorphous silicon Reactive particle beam ICP CVD Reflector Bias voltage
a b s t r a c t Amorphous silicon thin films were formed by chemical vapor deposition of reactive particle beam assisted inductively coupled plasma type with various reflector bias voltages. During the deposition, the substrate was heated at 150 °C. The effects of reflector bias voltage on the physical and chemical properties of the films were systematically studied. X-ray diffraction and Raman spectroscopy results showed that the deposited films were amorphous and the films under higher reflector voltage had higher internal energy to be easily crystallized. The chemical state of amorphous silicon films was revealed as metallic bonding of Si atoms by using X-ray photoelectron spectroscopy. An increase in reflector voltage induced an increase of surface morphology of films and optical bandgap and a decrease of photoconductivity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Amorphous silicon thin film transistors (TFTs) are widely used as backplane devices in displays and image sensors. The hydrogenated amorphous silicon (a-Si:H) deposited on glass substrate generally by plasma enhanced chemical vapor deposition (PECVD) with processing temperatures over 300 °C [1]. The deposition technology by reactive particle beam (RPB) assisted inductively coupled plasma (ICP) type chemical vapor deposition (CVD) system which can control the energy of being deposited particles in range of 1–100 eV. [2,3] The RPB generation system consists of internal antenna for an plasma generation and a silicon reflector. When the working gases are inserted into the plasma which was generated by internal antenna, they became ionized. The ions in the plasma sheath between plasma and reflector are accelerated to metal reflector. The accelerated ions run into the reflector and they are reflected at the surface of reflector. After reflection, these ions are neutralized mainly through the Auger neutralization [4]. The neutralized particle energy and neutralization and recoil efficiency are varied as a function of the reflector materials, surface roughness and impinging angle, etc [5]. Finally, the ionized particles in the plasma and neutralized particles which are neutralized by Auger reflection consist together reactive particle beam source. Then this
⁎ Corresponding author. E-mail address: [email protected] (H.-H. Park). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.006
energy-controlled reactive particle beam source is deposited on the substrate and forms thin films through ICP type CVD process. The conventional CVD processes supply the required energy for reaction by substrate heating. This kind of process may cause substrate deformation from high substrate temperature and surface damage with charged particles. But the RPB with proper kinetic energy can supply sufficient heating energy underling thin film for reaction without additional substrate heating and reduce charged particle damage at the surface of films. In this research, to understand the effects of reflector bias voltage change of RPB assisted CVD system, the structural, chemical, optical and electrical properties of amorphous silicon thin films were investigated by varying the bias voltage of reflector from 0 to −120 V. 2. Experimental procedures The amorphous silicon films were deposited by reactive particle beam assisted inductively coupled plasma type CVD system. The process chamber was evacuated to a base pressure of 2 × 10− 6 Torr by a turbo-molecular pump. The working gases were 5 sccm of silane (SiH4), 45 sccm of helium (He), 10 sccm of hydrogen (H2) and 5 sccm of argon (Ar). The working pressure is 2.7 mTorr during deposition. The RF bias for ICP antenna was 600 W and Si3N4 antenna shield tube was used. 0, −40, −80 − 120 V biases were loaded for reactive particle beam generation to n-type silicon reflector. During the deposition of thin films, the substrate temperature was maintained at 150 °C. Thicknesses of deposited Si thin films were measured by an alpha step surface profiler (Tencor P-2) and scanning electron
S.G. Choi et al. / Thin Solid Films 518 (2010) 7372–7376
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conductor analyzer. Surface morphology was studied by means of the atomic-force microscopy (AFM) using a NanoScope IIIa Dimension 3100 with tapping mode. Optical band gap was measured by UV visible spectrometer (V-570, Jasco). The amorphous silicon films were deposited on two different types of substrates. (100) oriented single crystal silicon substrate for X-ray Photoelectron Spectroscopy (XPS) and corning 1737 glass substrate for X-ray diffraction (XRD), Raman spectroscopy, AFM, electrical conductivity and optical transmittance analysis. 3. Results and discussion
Fig. 1. XRD patterns of amorphous silicon films deposited on glass substrates as a function of reflector bias voltages.
microscopy (SEM) using 15 kV of operating voltage. Film thickness was 350 nm and deposition rate was 0.97 Å/s. The crystal structure of the films was analyzed by thin film X-ray diffraction (TFXRD) with Cu Kα radiation using 40 kV and 40 mA and incident angle of θ was 0.5°. The chemical states of films were analyzed by X-ray photoelectron spectroscopy (XPS) with Al Kα X-ray source (1486.6 eV). Raman measurement was conducted by a UV-Micro Raman spectrometer (Renishaw) using 632.8 nm line of a He–Ne laser for excitation. Electrical conductivity of Si films was measured by HP 4156C semi-
The crystal structure of the RPB assisted amorphous silicon films was examined by thin film X-ray diffraction. Fig. 1 shows the XRD patterns of the deposited amorphous silicon films with reflector bias of 0 to −120 V. The large hump at around 23° was resulted from the amorphous nature of the samples [6]. The absence of normal diffractions of crystalline silicon such as 28.4°/(111), 47.3°/(220) and 56.1°/(311), revealed fully amorphous nature of the deposited silicon [7]. A negatively increase in reflector bias voltage means that reactive particles could have the higher energy. However, with negative increase of bias voltage from 0 to −120 V, the XRD patterns do not change and this means that all films deposited on glass substrate with various reflector bias voltages were amorphous. In case of reactive particle beam assisted ICP type CVD system, the accelerating energy is determined as the sum of the plasma potential and reflector biased voltage. For instance, when the bias voltage is −30 V with a plasma potential of 10 eV, the impinging ions can be
Fig. 2. Raman spectra of in-situ laser annealed amorphous silicon films deposited at different reflector bias voltages of 0 V, − 40 V, − 80 V and − 120 V.
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accelerated up to 40 eV and then the energy of reflected particles decreases at about 20 eV [8]. So the reactive particle beam energies are decided about a half of the impinging ion energies [9]. To further analysis of crystalline state of RPB assisted CVD deposited silicon films and examination of internal energy state of the films with various bias voltages, the Raman spectroscopy measurement proceeded. Fig. 2 shows the Raman spectra of the silicon films deposited at different reflector bias voltage ranging from 0 to −120 V. 14 mW of He–Ne laser power was used to check the phase state (crystalline or amorphous) of RPB assisted CVD deposited silicon films under various bias voltages and the laser power was increased up to 20 mW to supply laser energy to the amorphous silicon film to crystallize through laser induced phase transformation [10]. With an increase of laser power up to 20 mW, the amorphous phase changed to crystalline phase except amorphous silicon films deposited without reflector bias voltage. The broad peak centered around 470 cm− 1 is attributed to the transverse optical (TO) mode of Si–Si vibrations in the amorphous phase. The −40 V of reflector bias voltage applied silicon film was crystallized at 20 mW of laser power. The sharp peak centered around 500 cm− 1 corresponds to an intermediate phase between amorphous and crystalline phases. This peak is attributed to a defective part of nano crystals of diameter below 10 nm, a silicon wurtzite phase resulted from twins, or bond dilatation at grain boundaries [11–13]. In Fig. 2, the silicon film deposited under −80 V shows peak shift from 470 to 480 cm− 1 at 16 mW and 18 min, respectively. It could be considered that amorphous films did not crystallize with 16 and 18 mW of laser powers, but crystallization was in progress. However in case of − 120 V of bias voltage, the amorphous silicon film started to be crystallized with 16 mW of laser power. From the above results, it can be said that the crystalline nature of RPB assisted amorphous silicon films could be changed with reflector bias voltage. The films deposited under high reflector bias voltage (the energy of deposited particle becomes high) has high internal energy to be crystallized easier than low reflector bias voltage. The chemical state of amorphous silicon film was resolved by XPS analysis to confirm the effect of reflector bias voltages. Fig. 3 shows the X-ray photoelectron spectra of Si 2p, N 1s and O 1s of amorphous silicon films which were deposited under different reflector bias voltages. All film's surface were etched by Ar+ sputtering for surface cleaning before analysis, because generally amorphous silicon has native silicon oxide layer with carbon contamination at the surface. Fig. 3 shows Si 2p XPS core level spectra of RPB assisted amorphous silicon films. Normally the binding energy of Si 2p would be shifted to 101.6 or 103.6 eV when Si–N bond in Si3N4 or Si–O bond in SiO2 were present in the amorphous silicon films [14,15]. But the strong peak at 99 eV originated from metallic silicon was mainly found in the amorphous silicon films. N 1s core level spectra showed 397 eV of binding energy and it represented that N was mostly bound to Si because N 1s binding energy in Si3N4 is 397.5 eV [16]. This N–Si bonding contribution was appeared as an asymmetric peak broadening of Si 2p at the high binding energy side. However in the O 1s spectra, the main binding energy was found to be 531.3 eV. This binding energy is different from the O 1s binding energy in SiO2 (532.9 eV) but corresponded to the binding energy of oxygen which forms hydroxide or is physically absorbed on silicon surface [17]. These nitrogen and oxygen impurities were come from Si3N4 antenna tube during the deposition. Therefore from these above observations, it could be said that the chemical bonding state was not changed by the variation of reflector bias voltage, but the internal energy state of the film was changed. The influence of reflector bias voltage on the surface morphology of film was studied by using AFM. The surface morphology was measuring by tapping mode. In case of tapping mode, AFM tip is vertically oscillated near its resonance frequency and intermittently touches sample surface. Phase imaging goes beyond simple topographical mapping to reflect variations in the composition, friction, adhesion, and other properties [18]. Fig. 4(a) and (b) shows the AFM height
images of the amorphous silicon films which were deposited without reflector bias voltage and with −120 V, respectively. The root mean square (RMS) roughness values of amorphous films were 0.119 nm and 0.171 nm, respectively. This difference in RMS roughness came from the energy difference by reflector bias voltage. Fig. 4(c) and (d) is the phase image of Fig. 4(a) and (b), respectively. Fig. 4(d) shows more phases on the surface than (c) and it means that high energetic particles react and generate various phases like amorphous silicon clusters at the surface by regulating energy of particles [19,20]. The optical bandgap of silicon films has been deduced from the optical transmission and the reflectance spectra. Optical bandgap, E0 is commonly estimated by an extrapolation of linear plot of (αE)1/2 versus E − E0, where E is photon energy, to the α = 0 value. Fig. 5 shows optical band gap of amorphous silicon films deposited with
Fig. 3. X-ray photoelectron spectra of Si 2p, N 1s and O 1s of amorphous silicon films deposited at different reflector bias voltages.
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Fig. 4. AFM height images and phase images of amorphous silicon film (a, c) without reflector voltage and (b, d) with − 120 V, respectively.
Fig. 5. Optical bandgap of amorphous silicon films deposited as a function of reflector bias voltages.
Fig. 6. Photoconductivity of amorphous silicon films deposited as a function of reflector bias voltages.
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various reflector bias voltages. The optical bandgap increased with increasing reflector bias voltage because a change in the optical bandgap can be associated with the small cluster of silicon particles [21]. The increase in reflector voltage is seemed to enhance the formation of small a-Si clusters by creating more nucleation sites. Generally optical bandgap of a-Si is increased when the size of cluster is decreased [22,23]. From the above results of AFM, it could be confirmed that the amorphous silicon films deposited under high bias voltage showed many clusters observed through the phase image analysis. Fig. 6 shows the photoconductivity of amorphous silicon films with reflector bias voltage change. The conductivity of amorphous silicon films without reflector bias voltage shows 4.17 × 10− 8 (S/cm), but conductivity values with increasing reflector voltage from −40 to −120 V were decreased from 1.37 to 1.12 × 10− 8 (S/cm), respectively. The small size amorphous silicon clusters increased with increasing bias voltage would interrupt the conduction of electrons. From the above results, the deposition technology by RPB assisted ICP type CVD system was found to be effective for the deposition of silicon film with controlled internal energy by varying the reflector bias voltage. This internal energy reduced the activation energy of Si-nucleation however resulted in an increase in the optical bandgap and a decrease in the photo conductivity of amorphous film. 4. Conclusions We formed silicon films at 150 °C of substrate temperature by an RPB assisted CVD technique using various reflector voltages. All films showed amorphous state and it was confirmed by XRD observation. But the films deposited with high reflector bias voltage were revealed to be crystallized easier than the film deposited with low voltage due to higher accumulated internal energy during the process. The chemical bonding state of amorphous silicon films was not changed according to different reflector bias voltages but the surface roughness was increased as increasing reflector voltage. Although an increase in the optical bandgap and a decrease in the photoconductivity of amorphous film were observed, it was confirmed that RPB assisted de-
position could reduce the activation energy of Si-nucleation by controlling reflector bias voltage. Acknowledgment This work was supported by the IT R&D program of MKE/KEIT. [KI002182, TFT backplane technology for next generation display]. References [1] C.-S. Yang, L.L. Smith, C.B. Arthur, G.N. Persons, J. Vac. Sci. Technol. B 18 (2000) 683. [2] T. Lee, N.-K. Min, H.W. Lee, J.-N. Jang, D.H. Lee, M.P. Hong, K.-H. Kwon, Thin Solid Films 517 (2009) 3999. [3] Y.J. Lee, J.H. Kim, J.-N. Jang, I.H. Yang, S.N. Kwon, M.P. Hong, D.C. Kim, K.S. Oh, S.J. Yoo, B.J. Lee, W.-G. Jang, Thin Solid Films 517 (2009) 4019. [4] M.A. Cazalilla, N. Lorente, R.D. Muino, J.-P. Gauyacq, D. Teillet-Billy, P.M. Echenique, Phy. Rev. B 58 (1998) 13991. [5] B.A. Helmer, D.B. Graves, J. Vac. Sci. Technol. A 16 (1998) 3502. [6] H. Gamo, S. Kanemaru, J. Itoh, Jpn. J. Appl. Phys. 35 (1996) 6620. [7] Q. Cheng, S. Xu, K. Ostrikov, Nanotechnology 20 (2009) 215606. [8] S.J. Yoo, D.C. Kim, M. Joung, J.S. Kim, B.J. Lee, K.S. Oh, K.U. Kim, Y.H. Kim, Y.W. Kim, S.W. Choi, H.J. Son, Y.C. Park, J.-N. Jang, M.P. Hong, Rev. Sci. Instrum. 79 (2008) 02C301. [9] J.W. Cuthbertson, W.D. Langer, R.W. Motley, J. Nucl. Mater. 196/198 (1992) 113. [10] G. Viera, S. Huet, L. Boufendi, J. Appl. Phys. 90 (2001) 4175. [11] X.L. Wu, G.G. Siu, S. Tong, X.N. Liu, F. Yan, S.S. Jiang, X.K. Zhang, D. Feng, Appl. Phys. Lett. 69 (1996) 523. [12] Q. Cheng, S. Xu, J. Long, K.K. Ostrikov, Appl. Phys. Lett. 90 (2007) 173112. [13] Q. Cheng, S. Xu, S. Huang, K.K. Ostrikov, Cryst. Growth Des. 9 (2009) 2863. [14] B.S. Sahu, A. Kapoor, P. Srivastava, O.P. Agnihotri, S. MShivaprasad, Semicond. Sci. Technol. 18 (2003) 670. [15] S.P. Singh, P. Srivastava, S. Ghosh, S.A. Khan, G.V. Prakash, J. Phys.: Condens. Matter 21 (2009) 095010. [16] T. Hashizume, K. Ikeya, M. Mutoh, H. Hasegawa, Appl. Sur. Sci. (1998) 599. [17] S. Ohno, J.T. Yates, J. Vac. Sci. Technol. A 23 (2005) 475. [18] P. Jiang, S.-S. Xie, S.-J. Pang, H.-J. Gao, Appl. Sur. Sci. 191 (2002) 240. [19] D. Song, E.-C. Cho, G. Conibeer, Y.-H. Cho, Y. Huang, S. Huang, C. Flynn, M.A. Green, J. Vac. Sci. Technol. B 25 (2007) 1327. [20] W. Wu, D.H. Chen, W.Y. Cheung, J.B. Xu, S.P. Wong, R.W.M. Kwok, I.H. Wilson, Appl. Phys. A: Mater. Sci. Process. 66 (1998) S539. [21] C. Delerue, G. Allan, M. Lannoo, J. Lumines. 80 (1999) 65. [22] S. Furukawa, T. Miyasato, Phys. Rev. B 38 (1988) 5726. [23] N.-M. Park, T.-S. Kim, S.-J. Park, Appl. Phys. Lett. 78 (2001) 2575.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Properties of amorphous silicon thin films synthesized by reactive particle beam assisted chemical vapor deposition Sun Gyu Choi a, Seok-Joo Wang a, Hyeong-Ho Park a, Jin-Nyoung Jang b, MunPyo Hong b, Kwang-Ho Kwon c, Hyung-Ho Park a,⁎ a b c
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea Department of Display and Semiconductor Physics, Korea University, Chungnam 339-700, Republic of Korea Department of Control and Instrumentation Engineering, Korea University, Chungnam 339-700, Republic of Korea
a r t i c l e
i n f o
Available online 9 May 2010 Keywords: Amorphous silicon Reactive particle beam ICP CVD Reflector Bias voltage
a b s t r a c t Amorphous silicon thin films were formed by chemical vapor deposition of reactive particle beam assisted inductively coupled plasma type with various reflector bias voltages. During the deposition, the substrate was heated at 150 °C. The effects of reflector bias voltage on the physical and chemical properties of the films were systematically studied. X-ray diffraction and Raman spectroscopy results showed that the deposited films were amorphous and the films under higher reflector voltage had higher internal energy to be easily crystallized. The chemical state of amorphous silicon films was revealed as metallic bonding of Si atoms by using X-ray photoelectron spectroscopy. An increase in reflector voltage induced an increase of surface morphology of films and optical bandgap and a decrease of photoconductivity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Amorphous silicon thin film transistors (TFTs) are widely used as backplane devices in displays and image sensors. The hydrogenated amorphous silicon (a-Si:H) deposited on glass substrate generally by plasma enhanced chemical vapor deposition (PECVD) with processing temperatures over 300 °C [1]. The deposition technology by reactive particle beam (RPB) assisted inductively coupled plasma (ICP) type chemical vapor deposition (CVD) system which can control the energy of being deposited particles in range of 1–100 eV. [2,3] The RPB generation system consists of internal antenna for an plasma generation and a silicon reflector. When the working gases are inserted into the plasma which was generated by internal antenna, they became ionized. The ions in the plasma sheath between plasma and reflector are accelerated to metal reflector. The accelerated ions run into the reflector and they are reflected at the surface of reflector. After reflection, these ions are neutralized mainly through the Auger neutralization [4]. The neutralized particle energy and neutralization and recoil efficiency are varied as a function of the reflector materials, surface roughness and impinging angle, etc [5]. Finally, the ionized particles in the plasma and neutralized particles which are neutralized by Auger reflection consist together reactive particle beam source. Then this
⁎ Corresponding author. E-mail address: [email protected] (H.-H. Park). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.006
energy-controlled reactive particle beam source is deposited on the substrate and forms thin films through ICP type CVD process. The conventional CVD processes supply the required energy for reaction by substrate heating. This kind of process may cause substrate deformation from high substrate temperature and surface damage with charged particles. But the RPB with proper kinetic energy can supply sufficient heating energy underling thin film for reaction without additional substrate heating and reduce charged particle damage at the surface of films. In this research, to understand the effects of reflector bias voltage change of RPB assisted CVD system, the structural, chemical, optical and electrical properties of amorphous silicon thin films were investigated by varying the bias voltage of reflector from 0 to −120 V. 2. Experimental procedures The amorphous silicon films were deposited by reactive particle beam assisted inductively coupled plasma type CVD system. The process chamber was evacuated to a base pressure of 2 × 10− 6 Torr by a turbo-molecular pump. The working gases were 5 sccm of silane (SiH4), 45 sccm of helium (He), 10 sccm of hydrogen (H2) and 5 sccm of argon (Ar). The working pressure is 2.7 mTorr during deposition. The RF bias for ICP antenna was 600 W and Si3N4 antenna shield tube was used. 0, −40, −80 − 120 V biases were loaded for reactive particle beam generation to n-type silicon reflector. During the deposition of thin films, the substrate temperature was maintained at 150 °C. Thicknesses of deposited Si thin films were measured by an alpha step surface profiler (Tencor P-2) and scanning electron
S.G. Choi et al. / Thin Solid Films 518 (2010) 7372–7376
7373
conductor analyzer. Surface morphology was studied by means of the atomic-force microscopy (AFM) using a NanoScope IIIa Dimension 3100 with tapping mode. Optical band gap was measured by UV visible spectrometer (V-570, Jasco). The amorphous silicon films were deposited on two different types of substrates. (100) oriented single crystal silicon substrate for X-ray Photoelectron Spectroscopy (XPS) and corning 1737 glass substrate for X-ray diffraction (XRD), Raman spectroscopy, AFM, electrical conductivity and optical transmittance analysis. 3. Results and discussion
Fig. 1. XRD patterns of amorphous silicon films deposited on glass substrates as a function of reflector bias voltages.
microscopy (SEM) using 15 kV of operating voltage. Film thickness was 350 nm and deposition rate was 0.97 Å/s. The crystal structure of the films was analyzed by thin film X-ray diffraction (TFXRD) with Cu Kα radiation using 40 kV and 40 mA and incident angle of θ was 0.5°. The chemical states of films were analyzed by X-ray photoelectron spectroscopy (XPS) with Al Kα X-ray source (1486.6 eV). Raman measurement was conducted by a UV-Micro Raman spectrometer (Renishaw) using 632.8 nm line of a He–Ne laser for excitation. Electrical conductivity of Si films was measured by HP 4156C semi-
The crystal structure of the RPB assisted amorphous silicon films was examined by thin film X-ray diffraction. Fig. 1 shows the XRD patterns of the deposited amorphous silicon films with reflector bias of 0 to −120 V. The large hump at around 23° was resulted from the amorphous nature of the samples [6]. The absence of normal diffractions of crystalline silicon such as 28.4°/(111), 47.3°/(220) and 56.1°/(311), revealed fully amorphous nature of the deposited silicon [7]. A negatively increase in reflector bias voltage means that reactive particles could have the higher energy. However, with negative increase of bias voltage from 0 to −120 V, the XRD patterns do not change and this means that all films deposited on glass substrate with various reflector bias voltages were amorphous. In case of reactive particle beam assisted ICP type CVD system, the accelerating energy is determined as the sum of the plasma potential and reflector biased voltage. For instance, when the bias voltage is −30 V with a plasma potential of 10 eV, the impinging ions can be
Fig. 2. Raman spectra of in-situ laser annealed amorphous silicon films deposited at different reflector bias voltages of 0 V, − 40 V, − 80 V and − 120 V.
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accelerated up to 40 eV and then the energy of reflected particles decreases at about 20 eV [8]. So the reactive particle beam energies are decided about a half of the impinging ion energies [9]. To further analysis of crystalline state of RPB assisted CVD deposited silicon films and examination of internal energy state of the films with various bias voltages, the Raman spectroscopy measurement proceeded. Fig. 2 shows the Raman spectra of the silicon films deposited at different reflector bias voltage ranging from 0 to −120 V. 14 mW of He–Ne laser power was used to check the phase state (crystalline or amorphous) of RPB assisted CVD deposited silicon films under various bias voltages and the laser power was increased up to 20 mW to supply laser energy to the amorphous silicon film to crystallize through laser induced phase transformation [10]. With an increase of laser power up to 20 mW, the amorphous phase changed to crystalline phase except amorphous silicon films deposited without reflector bias voltage. The broad peak centered around 470 cm− 1 is attributed to the transverse optical (TO) mode of Si–Si vibrations in the amorphous phase. The −40 V of reflector bias voltage applied silicon film was crystallized at 20 mW of laser power. The sharp peak centered around 500 cm− 1 corresponds to an intermediate phase between amorphous and crystalline phases. This peak is attributed to a defective part of nano crystals of diameter below 10 nm, a silicon wurtzite phase resulted from twins, or bond dilatation at grain boundaries [11–13]. In Fig. 2, the silicon film deposited under −80 V shows peak shift from 470 to 480 cm− 1 at 16 mW and 18 min, respectively. It could be considered that amorphous films did not crystallize with 16 and 18 mW of laser powers, but crystallization was in progress. However in case of − 120 V of bias voltage, the amorphous silicon film started to be crystallized with 16 mW of laser power. From the above results, it can be said that the crystalline nature of RPB assisted amorphous silicon films could be changed with reflector bias voltage. The films deposited under high reflector bias voltage (the energy of deposited particle becomes high) has high internal energy to be crystallized easier than low reflector bias voltage. The chemical state of amorphous silicon film was resolved by XPS analysis to confirm the effect of reflector bias voltages. Fig. 3 shows the X-ray photoelectron spectra of Si 2p, N 1s and O 1s of amorphous silicon films which were deposited under different reflector bias voltages. All film's surface were etched by Ar+ sputtering for surface cleaning before analysis, because generally amorphous silicon has native silicon oxide layer with carbon contamination at the surface. Fig. 3 shows Si 2p XPS core level spectra of RPB assisted amorphous silicon films. Normally the binding energy of Si 2p would be shifted to 101.6 or 103.6 eV when Si–N bond in Si3N4 or Si–O bond in SiO2 were present in the amorphous silicon films [14,15]. But the strong peak at 99 eV originated from metallic silicon was mainly found in the amorphous silicon films. N 1s core level spectra showed 397 eV of binding energy and it represented that N was mostly bound to Si because N 1s binding energy in Si3N4 is 397.5 eV [16]. This N–Si bonding contribution was appeared as an asymmetric peak broadening of Si 2p at the high binding energy side. However in the O 1s spectra, the main binding energy was found to be 531.3 eV. This binding energy is different from the O 1s binding energy in SiO2 (532.9 eV) but corresponded to the binding energy of oxygen which forms hydroxide or is physically absorbed on silicon surface [17]. These nitrogen and oxygen impurities were come from Si3N4 antenna tube during the deposition. Therefore from these above observations, it could be said that the chemical bonding state was not changed by the variation of reflector bias voltage, but the internal energy state of the film was changed. The influence of reflector bias voltage on the surface morphology of film was studied by using AFM. The surface morphology was measuring by tapping mode. In case of tapping mode, AFM tip is vertically oscillated near its resonance frequency and intermittently touches sample surface. Phase imaging goes beyond simple topographical mapping to reflect variations in the composition, friction, adhesion, and other properties [18]. Fig. 4(a) and (b) shows the AFM height
images of the amorphous silicon films which were deposited without reflector bias voltage and with −120 V, respectively. The root mean square (RMS) roughness values of amorphous films were 0.119 nm and 0.171 nm, respectively. This difference in RMS roughness came from the energy difference by reflector bias voltage. Fig. 4(c) and (d) is the phase image of Fig. 4(a) and (b), respectively. Fig. 4(d) shows more phases on the surface than (c) and it means that high energetic particles react and generate various phases like amorphous silicon clusters at the surface by regulating energy of particles [19,20]. The optical bandgap of silicon films has been deduced from the optical transmission and the reflectance spectra. Optical bandgap, E0 is commonly estimated by an extrapolation of linear plot of (αE)1/2 versus E − E0, where E is photon energy, to the α = 0 value. Fig. 5 shows optical band gap of amorphous silicon films deposited with
Fig. 3. X-ray photoelectron spectra of Si 2p, N 1s and O 1s of amorphous silicon films deposited at different reflector bias voltages.
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Fig. 4. AFM height images and phase images of amorphous silicon film (a, c) without reflector voltage and (b, d) with − 120 V, respectively.
Fig. 5. Optical bandgap of amorphous silicon films deposited as a function of reflector bias voltages.
Fig. 6. Photoconductivity of amorphous silicon films deposited as a function of reflector bias voltages.
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various reflector bias voltages. The optical bandgap increased with increasing reflector bias voltage because a change in the optical bandgap can be associated with the small cluster of silicon particles [21]. The increase in reflector voltage is seemed to enhance the formation of small a-Si clusters by creating more nucleation sites. Generally optical bandgap of a-Si is increased when the size of cluster is decreased [22,23]. From the above results of AFM, it could be confirmed that the amorphous silicon films deposited under high bias voltage showed many clusters observed through the phase image analysis. Fig. 6 shows the photoconductivity of amorphous silicon films with reflector bias voltage change. The conductivity of amorphous silicon films without reflector bias voltage shows 4.17 × 10− 8 (S/cm), but conductivity values with increasing reflector voltage from −40 to −120 V were decreased from 1.37 to 1.12 × 10− 8 (S/cm), respectively. The small size amorphous silicon clusters increased with increasing bias voltage would interrupt the conduction of electrons. From the above results, the deposition technology by RPB assisted ICP type CVD system was found to be effective for the deposition of silicon film with controlled internal energy by varying the reflector bias voltage. This internal energy reduced the activation energy of Si-nucleation however resulted in an increase in the optical bandgap and a decrease in the photo conductivity of amorphous film. 4. Conclusions We formed silicon films at 150 °C of substrate temperature by an RPB assisted CVD technique using various reflector voltages. All films showed amorphous state and it was confirmed by XRD observation. But the films deposited with high reflector bias voltage were revealed to be crystallized easier than the film deposited with low voltage due to higher accumulated internal energy during the process. The chemical bonding state of amorphous silicon films was not changed according to different reflector bias voltages but the surface roughness was increased as increasing reflector voltage. Although an increase in the optical bandgap and a decrease in the photoconductivity of amorphous film were observed, it was confirmed that RPB assisted de-
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