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The effect of dopants on the microstructure of polycrystalline silicon thin film grown by MILC method
ARTICLE IN PRESS
Journal of Crystal Growth 290 (2006) 379–383 www.elsevier.com/locate/jcrysgro
The effect of dopants on the microstructure of polycrystalline silicon thin film grown by MILC method Ji-Su Ahn, Yeo-Geon Yoon, Seung-Ki Joo School of Materials Science and Engineering, Seoul National University, San 56-1, Shinrim-Dong, Kwanak-gu, Seoul 151-742, Republic of Korea Received 13 December 2003; received in revised form 13 November 2005; accepted 27 January 2006 Communicated by M. Schieber
Abstract The effect of dopants on the crystal growth and the microstructure of poly-crystalline silicon (poly-Si) thin film grown by metal induced lateral crystallization (MILC) method was intensively investigated. PH3 and B2H6 were used as source gases in ion mass doping (IMD) process to make n-type and p-type semiconductor respectively. It was revealed that the microstructure of MILC region varies significantly as the doping type of the samples varied from intrinsic to n-type and p-type, which was investigated by field emission (FE)SEM. The microstructure of MILC region of the intrinsic was bi-directional needle network structure whose crystal structure has a (1 1 0) preferred orientation. For p-type doped sample, the microstructure of MILC region was revealed to become unidirectional parallel growth structure more and more as MILC growth proceed, which was led by unidirectional division of needlelike grain at the front of MILC region. And for n-type doped sample, the microstructure was random-directional needlelike growth structure. These phenomena can be explained by an original model of Ni ion and Ni vacancy hopping in the NiSi2 phase and its interface at the front of MILC region. r 2006 Elsevier B.V. All rights reserved. Keywords: A1. Dopant effect; A1. FE-SEM; A3. MILC; A3. Unidirectional parallel growth; B1. a-Si; B1. Poly-Si
1. Introduction Low temperature (below 500 1C) crystallization process of amorphous silicon (a-Si) thin-film is essential for flat panel display devices such as high-resolution thin film transistor (TFT)—liquid crystal display (LCD) and organic electro-luminescence display (OLED) which use poly-Si TFTs. Excimer laser annealing (ELA) methods and metalinduced lateral crystallization (MILC) process have been developed in order to obtain high performance poly-Si TFTs at low temperatures [1,2]. MILC process is relatively simple and needs no additional high cost equipments such as laser equipments. With the structure modifications such as Ni-offset [3] or long-Ni-offset structure [4], high performance poly-Si TFTs with low leakage current have been successfully fabricated. There were also scientific studies of crystallization of a-Si in order to find out the mechanism of MILC. The studies on the crystallization Corresponding author. Tel.: +82 2 880 7442; fax: +82 2 887 8791.
E-mail address: [email protected] (S.-K. Joo). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.01.060
mechanism [5,6] and the dopant effects on MILC [4,7] were reported. However, the systematic studies on the changes of microstructures with the type of dopant are not made and the crystallization mechanism is not yet clear. In this study, we investigate the microstructures of poly-Si which was fabricated by MILC method with optical microscopy and field emission (FE)-SEM, varying the type and concentration of dopant, and we propose a crystallization mechanism which can describe the aspect of MILC rate and microstructural change. 2. Experimental procedures A 300-nm-thick SiO2 thin film for a buffer layer was deposited on the glass (Corning 1737) by plasma-enhanced chemical vapor deposition (PECVD). Then, a 50-nm-thick a-Si thin film was deposited by low-pressure chemical vapor deposition (LPCVD). Disilane (Si2H6) was used as Si source gas. The deposition pressure and temperature were 200 mTorr and 450 1C, respectively. After a-Si deposition, the samples were doped with B2H6 (p-type) or PH3 (n-type)
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by ion mass doping (IMD). After then, a 5-nm-thick Ni thin film was deposited selectively on a-Si film by sputtering and lithography process. The samples were then annealed at 550 1C for 5 h in vacuum electric furnace for crystallization. The region where the nickel thin film was selectively deposited was crystallized by metal induced crystallization (MIC) and the other region where the nickel thin film was not deposited was laterally crystallized by MILC from several to several tens of micron-meter. The microstructures of MILC region were analyzed by optical microscopy and FE-SEM. Before the FE-SEM observation, the MILC samples were etched appropriately by Secco etchant. 3. Results and discussion 3.1. Dopant effects on MILC growth rate The MILC length of the B2H6 and PH3 doped sample after 5 h heat treatment is plotted on Fig. 1 as a function of doping time. For the n-type sample doped with PH3, MILC growth rate was drastically decreased as the doping time increased, while for the p-type sample doped with B2H6 MILC rate was found to increase rapidly and then to be saturated soon to 5075 mm as the doping time increased. Fig. 2 shows the optical microscopic images of MILC region of the intrinsic, B2H6 (for 5 min) and PH3 (for 4 min) doped sample. As shown in Figs. 2(a) and (b), B2H6 doped sample has the wavy interface between MILC and a-Si region while intrinsic sample has flat interface between them. And for PH3 doped sample, the interface between MILC and a-Si area is very rugged with many needlelike Si crystallites protruding.
Fig. 2. Optical-micrographs showing the MIC and MILC area of Si films crystallized by MILC (a) intrinsic (b) B2H6 (5 min) and (c) PH3 (4 min) doped samples after 550 1C 5 h annealing.
3.2. Dopant effects on the microstructure of the MILC region
Fig. 1. The MILC length as a function of IMD process time (min) with B2H6 and PH3 after 550 1C 5 h annealing.
After the intrinsic, B2H6 (for 5 min) and PH3 (for 4 min) doped sample was Secco-etched until the amorphous Si and c-Si grain boundary were etched appropriately, the morphology and the microstructure of MILC region of each sample were analyzed by FE-SEM. As shown in Fig. 3(a), that the microstructure of MILC region of intrinsic sample has a network structure of branched crystallites whose primary and secondary arms’
ARTICLE IN PRESS J.-S. Ahn et al. / Journal of Crystal Growth 290 (2006) 379–383
directions have the regular angles of 70 or 1101 (we call this structure as bi-directional needle network structure), which shows us that the MILC region has (1 1 0) preferredorientation [1]. And the thickness of the needle-like crystallites is about 60 nm. This result is in perfect accordance with the previous report [1] which had carried out TEM analyses of that structure.
381
For B2H6 doped sample, the microstructure of MILC region was revealed that unidirectional-parallel growth structure where branching secondary arm is highly suppressed as showed in Fig. 3(b). This result is also in agreement with the previous report [4] with TEM analyses. But the thickness of each crystallites is about 40–50 nm which is thinner than that of intrinsic samples and the result somewhat differs from the previous report [4]. For PH3 doped sample, the microstructure of MILC area was randomly needle-like crystallites grown without preferred direction (see Fig. 3(c)). The thickness of the crystallites was about 100 nm, which is much thicker than that of intrinsic samples. From a closer look on the FE-SEM micrograph of the sample doped with B2H6, it was revealed that the microstructure of MILC region become unidirectional parallel growth structure more and more and bi-directional needle network structure disappear gradually as the MILC growth proceeded (Fig. 4(a)). This is attributed to the fact that unidirectional division of MILC needlelike grain at the front of MILC region is dominant and two-directional division is relatively restrained in the case of B2H6 doped sample. The characteristics of the samples with doping type are seen in Table 1. In the case of intrinsic Si, though MILC poly-Si is composed of narrow needles in the image, the interface between poly-Si and a-Si is observed to be flat by optical microscope, because lengths and directions of needles are uniform and widths of needles are narrower than that of needles in n-type doped Si. In the case of ptype Si, its front is wavy though the interface is observed to be flat interface with unidirectional parallel growth of polySi needles. This difference is attributed to the fact that bidirectional growth regions exist between the unidirectional growth regions. It was also observed that the widths of poly-Si needles in n-type Si is wider and that in p-type Si narrower than that in intrinsic Si. It is therefore inversely proportional to the MILC growth rate according to the doping type of Si. It can be explained that the slower crystallite growth enhances solid phase epitaxy (SPE) growth so that polySi needles are relatively thick. Moreover, since most of grains are unidirectional in ptype Si and that poly-Si needles can be formed with broader width in n-type Si than in intrinsic Si as listed in Table 1, novel process can be developed for the fabrication of improved MILC poly-Si TFTs [4]. 3.3. MILC growth mechanism: Ni ion and Ni vacancy (NINV) hopping mechanism
Fig. 3. FE-SEM images showing the microstructure of Si films crystallized by MILC (a) intrinsic (b) B2H6 and (c) PH3 doped samples after annealing.
The phenomena of the dopant effects on MILC growth can be explained by an original model which introduces Ni ion and Ni vacancy hopping mechanism. NiSi2 precipitates which exist at the front of MILC region migrate through the a-Si thin film leaving a trail of epitaxial c-Si [1,3]. In that case, Ni atom in NiSi2 precipitate is thought to repeat
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the process of bond-breaking and recombination in the Si matrix to migrate to MILC growth direction. It can be shown schematically in Fig. 5. MILC growth process can be divided into a three-step process. As shown in Fig. 5, the first step is the coherent adsorption step of Si atom in the NiSi2/a-Si interface with atomic bond breaking and hopping, the second step is the Ni vacancy migration by hopping in the NiSi2 phase which result in net migration of Ni atoms toward MILC growth direction and the third is the atomic rearrangement step of Si atoms which were left by Ni atoms so that NiSi2 phase can proceed toward MILC growth direction leaving a trail of epitaxial c-Si. NiSi2 phase can form coherent interface with c-Si and form Schottky contact with c-Si [8]. Also since Ni atom in NiSi2 phase can be negatively charged because of negative Mulliken charge [9] and act as acceptor in c-Si matrix [10]. Therefore acceptor B and donor P in the depletion region at the NiSi2/c-Si interface made by Schottky contact can induce repulsive or attractive force [11] on the charged Ni+ vacancy or Ni ion (see Fig. 6). Especially, these forces are thought to mainly act on the Ni ion which can easily be left behind at the c-Si/NiSi2 interface. Because of such repulsive or attractive force the velocity of the c-Si/NiSi2 interface can be speeded up or down so that MILC growth rate is accelerated by P-type doping, and is decelerated by N-type doping.
Fig. 4. FE-SEM images showing (a) the microstructure of Si films crystallized by early state of MILC of B2H6 doped sample and (b) first and second unidirectional division. Fig. 5. Schematic diagram which shows a three-step MILC mechanism with Ni atom and Ni vacancy hopping model.
Table 1 Characteristic structures and microstructures of MILC growth region Dopant
Interface shape observed by optical microscope
Microscopic interface shape
Dominant microstructure
Thickness of needle-like crystallite (nm)
Intrinsic B2H6
Flat Wavy
Rugged Flat and cascade
60–70 40–50
PH3
Rugged
Very rugged
Bi-directional needle network Unidirectional parallel growth structure Needle-like growth structure without preferred direction
90–100
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possibility to develop a process for the fabrication of improved MILC poly-Si TFTs using the microstructural change of poly-Si by dopant effect.
Acknowledgments The authors wish to thank PT-PLUS Inc. for their support and incorporation.
Fig. 6. Schematic diagram showing Ni ion and Ni+ vacancy hopping mechanism which explains the enhancement and retardation of MILC rate in the case of p-type and n-type doping, respectively.
4. Conclusion The effect of dopants on the crystal growth and the microstructure of poly-crystalline silicon thin film grown by MILC method was investigated. For the n-type sample doped with PH3, MILC growth rate was found to be decreased as the doping time increased, while for the p-type sample doped with B2H6 MILC rate was found to increase rapidly and then to be saturated soon as the doping time increased. The microstructure of MILC region of the intrinsic was bi-directional needle network structure while for P-type doped sample, the microstructure was revealed to become unidirectional-parallel-growth structure as MILC growth proceed, which was led by unidirectional division of MILC needlelike grain. And for n-type doped sample, the microstructure was a needlelike growth structure without preferred direction. These phenomena can be explained by an original model of Ni ion and Ni+ vacancy hopping in the NiSi2 phase and its interface at the front of MILC region. Moreover, the results point to the
References [1] S.W. Lee, S.K. Joo, Low temperature poly-Si thin-film transistor fabrication by metal-induced lateral crystallization, IEEE Electron Device Lett. 17 (4) (1996) 160. [2] M. Miyasaka, J. Stoemenos, Excimer laser annealing of amorphous and solid-phase-crystallized silicon films, J. Appl. Phys. 86 (10) (1999) 5556. [3] T.H. Ihn, T.K. Kim, B.I. Lee, S.K. Joo, A study on the leakage current of poly-Si TFTs fabricated by metal induced lateral crystallization, Microelecron. Reliab. 39 (1999) 53. [4] G.-B. Kim, Y.-G. Yoon, M.-S. Kim, H. Jung, S.-W. Lee, S.-K. Joo, The electrical characteristics of MILC poly-Si TFTs with long Nioffset structure, IEEE Trans. Electron Device 50 (12) (2003) 2344. [5] C. Hayzelden, J.L. Batstone, Silicide formation and silicide-mediated crystallization of nickel-implanted amorphous silicon thin films, J. Appl. Phys. 73 (12) (1993) 8279. [6] A.R. Jopshi, T. Krishnamohan, K. Saraswat, A model for crystal growth during metal induced lateral crystallization of amorphous silicon, J. Appl. Phys. 93 (1) (2003) 175. [7] T. Ma, M. Wong, Dopant and thickness dependence of metalinduced lateral crystallization of amorphous silicon films, J. Appl. Phys. 91 (3) (1993) 1236. [8] D. Cherns, G.R. Anstis, J.L. Hutchison, Atomic structure of the NiSi2/(1 1 1)Si interface, Philos. Mag. A 46 (5) (1982) 849. [9] J. Jang, J.Y. Oh, S.K. Kim, Y.J. Choi, S.Y. Yoon, C.O. Kim, Electric-field-enhanced crystallization of amorphous silicon, Nature 395 (1998) 481. [10] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [11] C. Licoppe, Y.I. Nissim, Impurity-induced enhancement of the growth rate of amorphized silicon during solid-phase epitaxy: a free carrier effect, J. Appl. Phys. 59 (2) (1986) 432.
Journal of Crystal Growth 290 (2006) 379–383 www.elsevier.com/locate/jcrysgro
The effect of dopants on the microstructure of polycrystalline silicon thin film grown by MILC method Ji-Su Ahn, Yeo-Geon Yoon, Seung-Ki Joo School of Materials Science and Engineering, Seoul National University, San 56-1, Shinrim-Dong, Kwanak-gu, Seoul 151-742, Republic of Korea Received 13 December 2003; received in revised form 13 November 2005; accepted 27 January 2006 Communicated by M. Schieber
Abstract The effect of dopants on the crystal growth and the microstructure of poly-crystalline silicon (poly-Si) thin film grown by metal induced lateral crystallization (MILC) method was intensively investigated. PH3 and B2H6 were used as source gases in ion mass doping (IMD) process to make n-type and p-type semiconductor respectively. It was revealed that the microstructure of MILC region varies significantly as the doping type of the samples varied from intrinsic to n-type and p-type, which was investigated by field emission (FE)SEM. The microstructure of MILC region of the intrinsic was bi-directional needle network structure whose crystal structure has a (1 1 0) preferred orientation. For p-type doped sample, the microstructure of MILC region was revealed to become unidirectional parallel growth structure more and more as MILC growth proceed, which was led by unidirectional division of needlelike grain at the front of MILC region. And for n-type doped sample, the microstructure was random-directional needlelike growth structure. These phenomena can be explained by an original model of Ni ion and Ni vacancy hopping in the NiSi2 phase and its interface at the front of MILC region. r 2006 Elsevier B.V. All rights reserved. Keywords: A1. Dopant effect; A1. FE-SEM; A3. MILC; A3. Unidirectional parallel growth; B1. a-Si; B1. Poly-Si
1. Introduction Low temperature (below 500 1C) crystallization process of amorphous silicon (a-Si) thin-film is essential for flat panel display devices such as high-resolution thin film transistor (TFT)—liquid crystal display (LCD) and organic electro-luminescence display (OLED) which use poly-Si TFTs. Excimer laser annealing (ELA) methods and metalinduced lateral crystallization (MILC) process have been developed in order to obtain high performance poly-Si TFTs at low temperatures [1,2]. MILC process is relatively simple and needs no additional high cost equipments such as laser equipments. With the structure modifications such as Ni-offset [3] or long-Ni-offset structure [4], high performance poly-Si TFTs with low leakage current have been successfully fabricated. There were also scientific studies of crystallization of a-Si in order to find out the mechanism of MILC. The studies on the crystallization Corresponding author. Tel.: +82 2 880 7442; fax: +82 2 887 8791.
E-mail address: [email protected] (S.-K. Joo). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.01.060
mechanism [5,6] and the dopant effects on MILC [4,7] were reported. However, the systematic studies on the changes of microstructures with the type of dopant are not made and the crystallization mechanism is not yet clear. In this study, we investigate the microstructures of poly-Si which was fabricated by MILC method with optical microscopy and field emission (FE)-SEM, varying the type and concentration of dopant, and we propose a crystallization mechanism which can describe the aspect of MILC rate and microstructural change. 2. Experimental procedures A 300-nm-thick SiO2 thin film for a buffer layer was deposited on the glass (Corning 1737) by plasma-enhanced chemical vapor deposition (PECVD). Then, a 50-nm-thick a-Si thin film was deposited by low-pressure chemical vapor deposition (LPCVD). Disilane (Si2H6) was used as Si source gas. The deposition pressure and temperature were 200 mTorr and 450 1C, respectively. After a-Si deposition, the samples were doped with B2H6 (p-type) or PH3 (n-type)
ARTICLE IN PRESS 380
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by ion mass doping (IMD). After then, a 5-nm-thick Ni thin film was deposited selectively on a-Si film by sputtering and lithography process. The samples were then annealed at 550 1C for 5 h in vacuum electric furnace for crystallization. The region where the nickel thin film was selectively deposited was crystallized by metal induced crystallization (MIC) and the other region where the nickel thin film was not deposited was laterally crystallized by MILC from several to several tens of micron-meter. The microstructures of MILC region were analyzed by optical microscopy and FE-SEM. Before the FE-SEM observation, the MILC samples were etched appropriately by Secco etchant. 3. Results and discussion 3.1. Dopant effects on MILC growth rate The MILC length of the B2H6 and PH3 doped sample after 5 h heat treatment is plotted on Fig. 1 as a function of doping time. For the n-type sample doped with PH3, MILC growth rate was drastically decreased as the doping time increased, while for the p-type sample doped with B2H6 MILC rate was found to increase rapidly and then to be saturated soon to 5075 mm as the doping time increased. Fig. 2 shows the optical microscopic images of MILC region of the intrinsic, B2H6 (for 5 min) and PH3 (for 4 min) doped sample. As shown in Figs. 2(a) and (b), B2H6 doped sample has the wavy interface between MILC and a-Si region while intrinsic sample has flat interface between them. And for PH3 doped sample, the interface between MILC and a-Si area is very rugged with many needlelike Si crystallites protruding.
Fig. 2. Optical-micrographs showing the MIC and MILC area of Si films crystallized by MILC (a) intrinsic (b) B2H6 (5 min) and (c) PH3 (4 min) doped samples after 550 1C 5 h annealing.
3.2. Dopant effects on the microstructure of the MILC region
Fig. 1. The MILC length as a function of IMD process time (min) with B2H6 and PH3 after 550 1C 5 h annealing.
After the intrinsic, B2H6 (for 5 min) and PH3 (for 4 min) doped sample was Secco-etched until the amorphous Si and c-Si grain boundary were etched appropriately, the morphology and the microstructure of MILC region of each sample were analyzed by FE-SEM. As shown in Fig. 3(a), that the microstructure of MILC region of intrinsic sample has a network structure of branched crystallites whose primary and secondary arms’
ARTICLE IN PRESS J.-S. Ahn et al. / Journal of Crystal Growth 290 (2006) 379–383
directions have the regular angles of 70 or 1101 (we call this structure as bi-directional needle network structure), which shows us that the MILC region has (1 1 0) preferredorientation [1]. And the thickness of the needle-like crystallites is about 60 nm. This result is in perfect accordance with the previous report [1] which had carried out TEM analyses of that structure.
381
For B2H6 doped sample, the microstructure of MILC region was revealed that unidirectional-parallel growth structure where branching secondary arm is highly suppressed as showed in Fig. 3(b). This result is also in agreement with the previous report [4] with TEM analyses. But the thickness of each crystallites is about 40–50 nm which is thinner than that of intrinsic samples and the result somewhat differs from the previous report [4]. For PH3 doped sample, the microstructure of MILC area was randomly needle-like crystallites grown without preferred direction (see Fig. 3(c)). The thickness of the crystallites was about 100 nm, which is much thicker than that of intrinsic samples. From a closer look on the FE-SEM micrograph of the sample doped with B2H6, it was revealed that the microstructure of MILC region become unidirectional parallel growth structure more and more and bi-directional needle network structure disappear gradually as the MILC growth proceeded (Fig. 4(a)). This is attributed to the fact that unidirectional division of MILC needlelike grain at the front of MILC region is dominant and two-directional division is relatively restrained in the case of B2H6 doped sample. The characteristics of the samples with doping type are seen in Table 1. In the case of intrinsic Si, though MILC poly-Si is composed of narrow needles in the image, the interface between poly-Si and a-Si is observed to be flat by optical microscope, because lengths and directions of needles are uniform and widths of needles are narrower than that of needles in n-type doped Si. In the case of ptype Si, its front is wavy though the interface is observed to be flat interface with unidirectional parallel growth of polySi needles. This difference is attributed to the fact that bidirectional growth regions exist between the unidirectional growth regions. It was also observed that the widths of poly-Si needles in n-type Si is wider and that in p-type Si narrower than that in intrinsic Si. It is therefore inversely proportional to the MILC growth rate according to the doping type of Si. It can be explained that the slower crystallite growth enhances solid phase epitaxy (SPE) growth so that polySi needles are relatively thick. Moreover, since most of grains are unidirectional in ptype Si and that poly-Si needles can be formed with broader width in n-type Si than in intrinsic Si as listed in Table 1, novel process can be developed for the fabrication of improved MILC poly-Si TFTs [4]. 3.3. MILC growth mechanism: Ni ion and Ni vacancy (NINV) hopping mechanism
Fig. 3. FE-SEM images showing the microstructure of Si films crystallized by MILC (a) intrinsic (b) B2H6 and (c) PH3 doped samples after annealing.
The phenomena of the dopant effects on MILC growth can be explained by an original model which introduces Ni ion and Ni vacancy hopping mechanism. NiSi2 precipitates which exist at the front of MILC region migrate through the a-Si thin film leaving a trail of epitaxial c-Si [1,3]. In that case, Ni atom in NiSi2 precipitate is thought to repeat
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the process of bond-breaking and recombination in the Si matrix to migrate to MILC growth direction. It can be shown schematically in Fig. 5. MILC growth process can be divided into a three-step process. As shown in Fig. 5, the first step is the coherent adsorption step of Si atom in the NiSi2/a-Si interface with atomic bond breaking and hopping, the second step is the Ni vacancy migration by hopping in the NiSi2 phase which result in net migration of Ni atoms toward MILC growth direction and the third is the atomic rearrangement step of Si atoms which were left by Ni atoms so that NiSi2 phase can proceed toward MILC growth direction leaving a trail of epitaxial c-Si. NiSi2 phase can form coherent interface with c-Si and form Schottky contact with c-Si [8]. Also since Ni atom in NiSi2 phase can be negatively charged because of negative Mulliken charge [9] and act as acceptor in c-Si matrix [10]. Therefore acceptor B and donor P in the depletion region at the NiSi2/c-Si interface made by Schottky contact can induce repulsive or attractive force [11] on the charged Ni+ vacancy or Ni ion (see Fig. 6). Especially, these forces are thought to mainly act on the Ni ion which can easily be left behind at the c-Si/NiSi2 interface. Because of such repulsive or attractive force the velocity of the c-Si/NiSi2 interface can be speeded up or down so that MILC growth rate is accelerated by P-type doping, and is decelerated by N-type doping.
Fig. 4. FE-SEM images showing (a) the microstructure of Si films crystallized by early state of MILC of B2H6 doped sample and (b) first and second unidirectional division. Fig. 5. Schematic diagram which shows a three-step MILC mechanism with Ni atom and Ni vacancy hopping model.
Table 1 Characteristic structures and microstructures of MILC growth region Dopant
Interface shape observed by optical microscope
Microscopic interface shape
Dominant microstructure
Thickness of needle-like crystallite (nm)
Intrinsic B2H6
Flat Wavy
Rugged Flat and cascade
60–70 40–50
PH3
Rugged
Very rugged
Bi-directional needle network Unidirectional parallel growth structure Needle-like growth structure without preferred direction
90–100
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possibility to develop a process for the fabrication of improved MILC poly-Si TFTs using the microstructural change of poly-Si by dopant effect.
Acknowledgments The authors wish to thank PT-PLUS Inc. for their support and incorporation.
Fig. 6. Schematic diagram showing Ni ion and Ni+ vacancy hopping mechanism which explains the enhancement and retardation of MILC rate in the case of p-type and n-type doping, respectively.
4. Conclusion The effect of dopants on the crystal growth and the microstructure of poly-crystalline silicon thin film grown by MILC method was investigated. For the n-type sample doped with PH3, MILC growth rate was found to be decreased as the doping time increased, while for the p-type sample doped with B2H6 MILC rate was found to increase rapidly and then to be saturated soon as the doping time increased. The microstructure of MILC region of the intrinsic was bi-directional needle network structure while for P-type doped sample, the microstructure was revealed to become unidirectional-parallel-growth structure as MILC growth proceed, which was led by unidirectional division of MILC needlelike grain. And for n-type doped sample, the microstructure was a needlelike growth structure without preferred direction. These phenomena can be explained by an original model of Ni ion and Ni+ vacancy hopping in the NiSi2 phase and its interface at the front of MILC region. Moreover, the results point to the
References [1] S.W. Lee, S.K. Joo, Low temperature poly-Si thin-film transistor fabrication by metal-induced lateral crystallization, IEEE Electron Device Lett. 17 (4) (1996) 160. [2] M. Miyasaka, J. Stoemenos, Excimer laser annealing of amorphous and solid-phase-crystallized silicon films, J. Appl. Phys. 86 (10) (1999) 5556. [3] T.H. Ihn, T.K. Kim, B.I. Lee, S.K. Joo, A study on the leakage current of poly-Si TFTs fabricated by metal induced lateral crystallization, Microelecron. Reliab. 39 (1999) 53. [4] G.-B. Kim, Y.-G. Yoon, M.-S. Kim, H. Jung, S.-W. Lee, S.-K. Joo, The electrical characteristics of MILC poly-Si TFTs with long Nioffset structure, IEEE Trans. Electron Device 50 (12) (2003) 2344. [5] C. Hayzelden, J.L. Batstone, Silicide formation and silicide-mediated crystallization of nickel-implanted amorphous silicon thin films, J. Appl. Phys. 73 (12) (1993) 8279. [6] A.R. Jopshi, T. Krishnamohan, K. Saraswat, A model for crystal growth during metal induced lateral crystallization of amorphous silicon, J. Appl. Phys. 93 (1) (2003) 175. [7] T. Ma, M. Wong, Dopant and thickness dependence of metalinduced lateral crystallization of amorphous silicon films, J. Appl. Phys. 91 (3) (1993) 1236. [8] D. Cherns, G.R. Anstis, J.L. Hutchison, Atomic structure of the NiSi2/(1 1 1)Si interface, Philos. Mag. A 46 (5) (1982) 849. [9] J. Jang, J.Y. Oh, S.K. Kim, Y.J. Choi, S.Y. Yoon, C.O. Kim, Electric-field-enhanced crystallization of amorphous silicon, Nature 395 (1998) 481. [10] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [11] C. Licoppe, Y.I. Nissim, Impurity-induced enhancement of the growth rate of amorphized silicon during solid-phase epitaxy: a free carrier effect, J. Appl. Phys. 59 (2) (1986) 432.