- Email: [email protected]
Controlling the geometrical orientation of hot-wire chemical vapor process grown silicon nanowires
Accepted Manuscript Controlling the geometrical orientation of hot-wire chemical vapor process grown silicon nanowires
Ankur Soam, Nagsen Meshram, Nitin Arya, Alka Kumbhar, Rajiv Dusane PII: DOI: Reference:
S0040-6090(16)30821-5 doi: 10.1016/j.tsf.2016.12.008 TSF 35665
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
1 October 2016 30 November 2016 6 December 2016
Please cite this article as: Ankur Soam, Nagsen Meshram, Nitin Arya, Alka Kumbhar, Rajiv Dusane , Controlling the geometrical orientation of hot-wire chemical vapor process grown silicon nanowires. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tsf(2016), doi: 10.1016/ j.tsf.2016.12.008
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 Controlling the Geometrical Orientation of Hot-wire Chemical Vapor Process Grown Silicon Nanowires Ankur Soam1,2, Nagsen Meshram1, Nitin Arya1, Alka Kumbhar1 and Rajiv Dusane1* 1
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Semiconductor Thin Films and Plasma Processing Laboratory, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai-400076, India 2 Present address: Nanomaterials and Device Fabrication Laboratory, Center for Nanoscience and Nanotechnology, Siksha O Anusandhan University, Bhubaneswar-751030, Odisha.
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Email: [email protected]
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Abstract In this work, the effect of chamber pressure on the morphology of hot wire chemical vapor processed silicon nanowires (SiNWs) using Sn as catalyst has been studied. It is observed that
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their geometrical orientation can be controlled as per requirement by adjusting the growth pressure. SiNWs synthesized at low pressure of 0.67 Pa grow preferentially perpendicular to the
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substrate. If the pressure is increased to 1.3 Pa, SiNWs become tilted to the substrate and have bending type structure with random distribution. Further increase in the chamber pressure to 4 Pa
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very few wires are seento grow and at 5.3 Pa no-growth of SiNWs is observed. Transmission electron microscopy study shows that the straight SiNWs have crystalline structure whereas the
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bent ones show polycrystalline structure.
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Keywords: silicon nanowires, Sn catalyst, VLS mechanism, hot-wire CVP, geometrical orientation.
ACCEPTED MANUSCRIPT Introduction Fabrication of SiNWs with well-controlled dimensions has been the focus of intensive research for their usage in nano-electronic and nano-photonic systems, including metal-oxidesemiconductor field-effect transistor, energy storage, biochemical sensors and solar cells [1-4]. In recent studies, it has been observed that SiNWs based solar cells exhibited a great improvement in the performance against the planar solar cell because SiNWs increase the light
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absorption by trapping the incoming light over the solar cell devices [5]. Although, the nanowire
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(NW) diameter and length have been controlled by altering the process parameters, not much
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effort has been given to control their alignment over a substrate [6-9]. This is a very important aspect in the development of SiNWs based devices such as photovoltaic cells in which light
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absorption is affected by the alignment of SiNWs. The main focus of this work is to get well aligned SiNWs on the substrate. Effect of pressure on the growth rate of SiNWs has been shown
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previously, however the effect of chamber pressure on the geometrical orientation of SiNWs grown by Hot-wire chemical vapor process (HWCVP) has not been reported before. We report
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here that the geometrical orientation of SiNWs has a strong correlation with the chamber pressure and SiNWs with the desired orientation can be fabricated either vertical or slanted by
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adjusting the chamber pressure which could be useful for their application in various devices.
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There are two approaches being observed for the SiNWs fabrication i.e. top-down and bottom up. In the top-down approach the bulk silicon wafer etched by the etchant solution results in long
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vertical SiNWs [10]. This process is limited for the Si substrate and it is not possible to transfer the NWs on a flexible substrate. Moreover, low throughput and high cost of the substrate are the
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other aspects of this approach [5]. However, the bottom-up approach allowing NWs fabrication on flexible as well as on a variety of substrates is very effective. In bottom-up process, the catalyst plays an important role through which the NW growth occurs at elevated temperatures. SiNWs growth is possible at low temperatures i.e. at 200-400oC with plasma enhanced chemical vapor deposition or HWCVP [11, 12] which is an added advantage of synthesizing SiNWs on low-cost polymer substrates for future flexible devices. The growth temperature of the SiNWs is decided by the eutectic temperature of the catalyst and silicon. Gold catalyst is widely used to synthesize the SiNWs with eutectic temperature of 363 ºC [1, 9, 13]. Gold creates deep level electrical defects in silicon degrading the electronic properties of the SiNWs [1]. In this context,
ACCEPTED MANUSCRIPT Sn has been considered to be an alternative to Au having electronic compatibility with Si and also assists in bringing down the growth temperature of SiNWs up to 240 oC [11].
Experimental Details SiNWs were synthesized on stainless steel substrate (SS 304) at temperatures of 400 oC. A film of Sn, thickness 20-30 nm was annealed for 10 min in presence of atomic hydrogen to form the
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catalyst, also called nano-template for the growth of SiNWs[14]. Atomic hydrogen was
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generated by flowing hydrogen on the heated filament in HWCVP chamber. The flow of
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hydrogen was maintained at 50 sccm and the hydrogen pressure inside the chamber was adjusted to 6.7 Pa. During the nano-template formation the filament was kept at constant temperature of
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1700oC. The nano-template of Sn was then exposed to Si species by introducing silane (SiH4) over the Tantalum filament heated at 1700oC. The chamber pressure during the growth of SiNWs
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was varied from 0.67 to 5.3 Pa. The desired value of chamber pressure in the reactor was maintained by adjusting the throttle valve between the chamber and the pumping stage. The flow
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of SiH4 was controlled to 5 sccn or 10 sccm by mass flow controller (MKS). The SiNWs morphology was observed by Field emission gun scanning electron microscopy (FEG-SEM,
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Model Jeol, JSM-7600F, operated voltage 0.1 to 30 kV as well as Dual beam FIB, from Carl Zeiss). High resolution transmission electron microscopy (HR-TEM, Model Jeol, JEM 2100F,
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Results and discussion
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operating voltage 200 kV) was also performed to evaluate their structure.
Fig. 1 shows the typical FEG-SEM images of the as-grown SiNWs on the SS substrate. SiNWs
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were synthesized at different chamber pressures of 0.67 Pa, 1.3 Pa, 2.7 Pa, 4 Pa and 5.3 Pa keeping other growth parameters constant. SiNWs prepared at 0.67-1.3 Pa chamber pressure seem to be vertically aligned over the substrate. As the chamber pressure was increased to 2.7 Pa, SiNWs are observed to be oriented randomly and bent. It is observed that the growth rate of the SiNWs continuously reduces when the chamber pressure is increased further to the extent that at 5.3 Pa no SiNWs are observed. The NWs synthesized at 2.7 Pa chamber pressure are longer than the others. There could be an error of about 5-10 % in the measurement of the length of SiNWs due to the slanting of the wires. Also the SiNWs have conical shapes because of the radial deposition of amorphous Si film on the NWs circumferential surface.
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At low chamber pressure growth rate is limited by the lower Si flux generation. The larger supply of Si by increasing the chamber pressure (due to the increased residence time of the SiH4 in the chamber) results in enhancement of the growth rate [15] as seen in Fig. 1. It is observed that the SiNWs grown at 2.7 Pa have almost two times larger growth rate than the wires grown at 0.67 Pa. It has been reported that the growth on smaller size catalyst needs high chamber
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pressure since the degree of supersaturation decreases as the size of catalyst is decreased [16].
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The increase in the chamber pressure results in increase in the supersaturation of the droplet [1,
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17], which in turn enables the growth even through smaller size of the particles. Thus the density of SiNWs is also increased with the chamber pressure. As the working pressure is increased to 4
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Pa, very few SiNWs were found on the substrate and no growth is seen at 5.3 Pa. Recently, J. Tang et al. [18] have studied the effect of the growth time on the SiNWs growth characteristics.
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They have observed that not only the NWs length changes with the growth time, but the NWs become conical from cylindrical then bent-conical and finally bent inverted conical [18]. In our
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study it is observed that the NWs growth pattern and morphology depend significantly on the chamber pressure and vertical NWs can be grown at low chamber pressure. The vertical growth
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of SiNWs also avoids the formation of inverted conical shaped NWs.
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In a previous report of SiNWs grown by thermal CVD, the decomposition of SiH4 at the catalyst surface has been considered to be rate limiting step. K. K. Lew and J. M. Redwing [19] have
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observed a linear increase in the growth rate with increasing SiH4 partial pressure from ~17 to ~86 Pa. On that basis, they concluded that the Si crystallization is not the rate-limiting step and
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the catalytic decomposition of SiH4 at the catalyst surface is the only rate determining step. In our case the same trend of increasing growth rate is observed upto 2.7 Pa chamber pressure (Fig. 1) while at chamber pressure larger than 2.7 Pa the growth rate decreases drastically indicating that the aforementioned assumption is not valid. In thermal CVD the decomposition and the growth occur on the substrate which is heated to 600oC whereas in the case of HWCVP the dissociation of SiH4 gas occurs at a heated filament and the Si atoms move towards the substrate where growth takes place of SiNWs takes place on the Sn nano-particles. Therefore, in HWCVP growth should be limited additionally by the dissociation rate of SiH4 molecules over the heated filament rather than the catalytic dissociation at the Sn nano-particle surface.
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At a high pressure of 5.3 Pa, the Si starts accumulating on the catalyst particle surface which means that the flux reaching the catalyst is too large to be crystallized entirely. It means that at higher pressure the growth rate limiting step is the crystallization or precipitation rather than the Si flux. For a stable growth the adsorbing amount of Si at the catalyst surface must be equal or lesser than the precipitation rate. At 5.3 Pa, the Si flux reaching the catalyst surface is much
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larger than the precipitation which causes an amorphous silicon film deposition rather than NWs
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growth.
It can be seen that the NWs grown at 4 Pa have length of less than 1 µm and these are covered
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with a thick amorphous layer of Si:H. Sn nano-particles have a wide size distribution and the growth depends on catalyst size. This template might be having some particles with bigger size
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through which the growth is still possible at that high chamber pressure, as a result very few NWs are observed at 4 Pa pressure. Increase in chamber pressure further (to 5.3 Pa) causes a
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large amount of Si impingement on the Sn catalyst and even bigger Sn particles are now unable to start the growth of SiNW. So, at higher chamber pressure where a large amount of Si is
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generated from the hot filament and gets adsorbedon the metal catalyst, the growth rate is limited
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by the crystallization or the precipitation rate.
The most important effect of the chamber pressure which is seen here is on the geometrical
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orientation of the SiNWs. At high pressure they are not perpendicular to the substrate but kinking is seen in many wires. The kinking mechanism in SiNWs has been explained in the literature
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[20-23]. The change in process conditions such as temperature and pressure were shown to produce kinking in SiNWs [24]. The kinking in NWs synthesized by thermal CVD via Au catalyst was shown to be caused by the sudden change in the growth pressure [23].In our case we have maintained these parameters constant for the growth and the SiNWs exhibit bending type structure rather than kinking. It means the above statement may not be valid and an explanation is needed for the observed bending of the SiNWs grown by HWCVP.
In SiNWs grown by vapor liquid solid mechanism, the presence of kinks could be due to instability at the liquid/solid interface [25]. There should be a proper balance between capillary
ACCEPTED MANUSCRIPT forces acting on this line for a stable growth. The growth rate is decided by the difference in chemical potential of Si in droplet and in the growing solid phase of Si. Increase in the chamber pressure results in production of a large amount of Si which in turn increases the supersaturation of Si in the Sn-Si droplet. In this situation, the growth rate is accelerated which can generate a lot of defects in the SiNWs [26]. These defects form all over the growth resulting in a bent structure of SiNWs. Buildup of strain during the growth could also be one of the possible reasons for the
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NW bending. On the other hand, low pressure in HWCVP produces lesser amount of Si flux
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which decreases the growth rate of SiNWs enabling a defect free growth of vertical straight
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SiNWs.
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As seen in HR-TEM image Fig. 2, SiNWs grown at high pressure consists of multiple small grains. Defects occurring during growth might inhibit the single crystal growth and as a
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consequence formation of polycrystalline structure takes place. In this case, selected area electron diffraction (SAED) pattern taken on a bent NW consists of spots distributed in circular
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rings (Fig. 3) which manifests a polycrystalline structure at high pressure. Fig. 4 (a) shows the TEM image of a SiNW grown at low pressure which is straight. Fig. 5 is HR-TEM image taken
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at the end on a straight SiNW which exhibits single crystalline structure of the SiNW. SAED pattern was also taken on the SiNW showing the periodic arrangement of the spots (Fig. 4 (b))
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which confirms the single crystal structure of the SiNW. We can also see some rings in the
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SAED pattern which may be from the amorphous shell present on the crystalline core. It is now clear that employing intermediate chamber pressure can lead to dense and long SiNWs.
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However, they are slanted and polycrystalline in structure. The growth of SiNWs is proportional to the amount of Si flux reaching to the catalyst. Therefore, in next experiment, we have increased the flow of SiH4 from 5 sccm to 10 sccm, and the chamber pressure was maintained at 0.4 Pa which is the minimum chamber pressure which is achievable with 10 sccm flow of SiH4 and the full speed of the turbo pump. In this experiment, the growth of SiNWs was carried out for 30 min. The substrate temperature was also increased to 450 oC in order to maintain the stable growth. Fig. 6 (a) shows the SEM image of the resulting SiNWs. SiNWs prepared under such conditions are found to be vertically aligned perpendicular to the substrate. The crosssectional SEM image is also depicted in Fig. 6 (b). Moreover, SiNWs are dense with length in
ACCEPTED MANUSCRIPT the range of 6-7 µm. For these SiNWs growth rate was determined to be ~220 nm/min. Previously, vertical SiNWs have been grown by using a nano-porous template which confines the growth of SiNWs along the pores [27]. After the growth this template is removed in a solution. However, this approach is not easy and requires multiple steps and it is a time consuming process whereas in our case vertical SiNWs can be grown without any additional
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complex step.
Conclusions
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We studied the effect of chamber pressure on the growth characteristics and morphology of SiNWs synthesized by HWCVP using Sn nanotemplate as catalyst. A proper control of SiNWs
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orientation is possible by tuning the growth pressure. Our results indicate that the change in growth rate may be responsible for the change in orientation of SiNWs. This proposed
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mechanism remains under active debate and needs further understanding of the various aspects observed. At high pressure, tilting and bending patterns of SiNWs are observed which could be
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due to the formation of defects in SiNWs. The defects are mostly generated by the high growth rate which is also responsible for the polycrystalline structure of SiNWs synthesized with such
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high chamber pressure. Whereas growth at low pressure produces single crystalline SiNWs which are straight perpendicular to the substrate and free from defects. It is also noted that the
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growth rate is not limited by a single step; it is the interplay between Si generation rate over the
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hot filament, incorporation rate and the crystallization rate.
Acknowledgment
Crompton Greaves, Mumbai is gratefully acknowledged to providethe financial support to AS. The authors also thank SAIF, IIT Bombay for providing TEM and SEM facilities. FIST facility (Dual beam FIB, Carl Zeiss Microscope) in ME & MS was also used for this work.
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Figure 1 (a)-(e) SEM micrographs of SiNWs grown for 10 min at chamber pressure of varying from 0.67 Pa to 5.3 Pa and substrate temperature of 400 °C, and (f) associated SiNWs growth
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Figure 2 HR-TEM image of a slanted SiNW grown at 2.7 Pa chamber pressure. Figure 3 (a) TEM image of a bent SiNWs and (b) its SAED pattern.
Figure 5 HR-TEM image of a straight SiNW
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Figure 4 (a) TEM image of a straight SiNW and (b) its SAED patterned.
Figure 6 (a) SEM image of the SiNWs synthesized by using 10 sccm of SiH4 with 0.4 Pa chamber
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ACCEPTED MANUSCRIPT Highlights
SiNWs were synthesized by HWCVP using Sn film as a catalyst layer
Control of the geometrical orientation of HWCVP grown SiNWs
SiNWs grown at high chamber pressure exhibited polycrystalline structure
SiNWs grown with low chamber pressure are long and have single crystalline structure
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Ankur Soam, Nagsen Meshram, Nitin Arya, Alka Kumbhar, Rajiv Dusane PII: DOI: Reference:
S0040-6090(16)30821-5 doi: 10.1016/j.tsf.2016.12.008 TSF 35665
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
1 October 2016 30 November 2016 6 December 2016
Please cite this article as: Ankur Soam, Nagsen Meshram, Nitin Arya, Alka Kumbhar, Rajiv Dusane , Controlling the geometrical orientation of hot-wire chemical vapor process grown silicon nanowires. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tsf(2016), doi: 10.1016/ j.tsf.2016.12.008
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 Controlling the Geometrical Orientation of Hot-wire Chemical Vapor Process Grown Silicon Nanowires Ankur Soam1,2, Nagsen Meshram1, Nitin Arya1, Alka Kumbhar1 and Rajiv Dusane1* 1
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Semiconductor Thin Films and Plasma Processing Laboratory, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai-400076, India 2 Present address: Nanomaterials and Device Fabrication Laboratory, Center for Nanoscience and Nanotechnology, Siksha O Anusandhan University, Bhubaneswar-751030, Odisha.
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Email: [email protected]
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Abstract In this work, the effect of chamber pressure on the morphology of hot wire chemical vapor processed silicon nanowires (SiNWs) using Sn as catalyst has been studied. It is observed that
AN
their geometrical orientation can be controlled as per requirement by adjusting the growth pressure. SiNWs synthesized at low pressure of 0.67 Pa grow preferentially perpendicular to the
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substrate. If the pressure is increased to 1.3 Pa, SiNWs become tilted to the substrate and have bending type structure with random distribution. Further increase in the chamber pressure to 4 Pa
ED
very few wires are seento grow and at 5.3 Pa no-growth of SiNWs is observed. Transmission electron microscopy study shows that the straight SiNWs have crystalline structure whereas the
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bent ones show polycrystalline structure.
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Keywords: silicon nanowires, Sn catalyst, VLS mechanism, hot-wire CVP, geometrical orientation.
ACCEPTED MANUSCRIPT Introduction Fabrication of SiNWs with well-controlled dimensions has been the focus of intensive research for their usage in nano-electronic and nano-photonic systems, including metal-oxidesemiconductor field-effect transistor, energy storage, biochemical sensors and solar cells [1-4]. In recent studies, it has been observed that SiNWs based solar cells exhibited a great improvement in the performance against the planar solar cell because SiNWs increase the light
T
absorption by trapping the incoming light over the solar cell devices [5]. Although, the nanowire
IP
(NW) diameter and length have been controlled by altering the process parameters, not much
CR
effort has been given to control their alignment over a substrate [6-9]. This is a very important aspect in the development of SiNWs based devices such as photovoltaic cells in which light
US
absorption is affected by the alignment of SiNWs. The main focus of this work is to get well aligned SiNWs on the substrate. Effect of pressure on the growth rate of SiNWs has been shown
AN
previously, however the effect of chamber pressure on the geometrical orientation of SiNWs grown by Hot-wire chemical vapor process (HWCVP) has not been reported before. We report
M
here that the geometrical orientation of SiNWs has a strong correlation with the chamber pressure and SiNWs with the desired orientation can be fabricated either vertical or slanted by
ED
adjusting the chamber pressure which could be useful for their application in various devices.
PT
There are two approaches being observed for the SiNWs fabrication i.e. top-down and bottom up. In the top-down approach the bulk silicon wafer etched by the etchant solution results in long
CE
vertical SiNWs [10]. This process is limited for the Si substrate and it is not possible to transfer the NWs on a flexible substrate. Moreover, low throughput and high cost of the substrate are the
AC
other aspects of this approach [5]. However, the bottom-up approach allowing NWs fabrication on flexible as well as on a variety of substrates is very effective. In bottom-up process, the catalyst plays an important role through which the NW growth occurs at elevated temperatures. SiNWs growth is possible at low temperatures i.e. at 200-400oC with plasma enhanced chemical vapor deposition or HWCVP [11, 12] which is an added advantage of synthesizing SiNWs on low-cost polymer substrates for future flexible devices. The growth temperature of the SiNWs is decided by the eutectic temperature of the catalyst and silicon. Gold catalyst is widely used to synthesize the SiNWs with eutectic temperature of 363 ºC [1, 9, 13]. Gold creates deep level electrical defects in silicon degrading the electronic properties of the SiNWs [1]. In this context,
ACCEPTED MANUSCRIPT Sn has been considered to be an alternative to Au having electronic compatibility with Si and also assists in bringing down the growth temperature of SiNWs up to 240 oC [11].
Experimental Details SiNWs were synthesized on stainless steel substrate (SS 304) at temperatures of 400 oC. A film of Sn, thickness 20-30 nm was annealed for 10 min in presence of atomic hydrogen to form the
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catalyst, also called nano-template for the growth of SiNWs[14]. Atomic hydrogen was
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generated by flowing hydrogen on the heated filament in HWCVP chamber. The flow of
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hydrogen was maintained at 50 sccm and the hydrogen pressure inside the chamber was adjusted to 6.7 Pa. During the nano-template formation the filament was kept at constant temperature of
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1700oC. The nano-template of Sn was then exposed to Si species by introducing silane (SiH4) over the Tantalum filament heated at 1700oC. The chamber pressure during the growth of SiNWs
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was varied from 0.67 to 5.3 Pa. The desired value of chamber pressure in the reactor was maintained by adjusting the throttle valve between the chamber and the pumping stage. The flow
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of SiH4 was controlled to 5 sccn or 10 sccm by mass flow controller (MKS). The SiNWs morphology was observed by Field emission gun scanning electron microscopy (FEG-SEM,
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Model Jeol, JSM-7600F, operated voltage 0.1 to 30 kV as well as Dual beam FIB, from Carl Zeiss). High resolution transmission electron microscopy (HR-TEM, Model Jeol, JEM 2100F,
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Results and discussion
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operating voltage 200 kV) was also performed to evaluate their structure.
Fig. 1 shows the typical FEG-SEM images of the as-grown SiNWs on the SS substrate. SiNWs
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were synthesized at different chamber pressures of 0.67 Pa, 1.3 Pa, 2.7 Pa, 4 Pa and 5.3 Pa keeping other growth parameters constant. SiNWs prepared at 0.67-1.3 Pa chamber pressure seem to be vertically aligned over the substrate. As the chamber pressure was increased to 2.7 Pa, SiNWs are observed to be oriented randomly and bent. It is observed that the growth rate of the SiNWs continuously reduces when the chamber pressure is increased further to the extent that at 5.3 Pa no SiNWs are observed. The NWs synthesized at 2.7 Pa chamber pressure are longer than the others. There could be an error of about 5-10 % in the measurement of the length of SiNWs due to the slanting of the wires. Also the SiNWs have conical shapes because of the radial deposition of amorphous Si film on the NWs circumferential surface.
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At low chamber pressure growth rate is limited by the lower Si flux generation. The larger supply of Si by increasing the chamber pressure (due to the increased residence time of the SiH4 in the chamber) results in enhancement of the growth rate [15] as seen in Fig. 1. It is observed that the SiNWs grown at 2.7 Pa have almost two times larger growth rate than the wires grown at 0.67 Pa. It has been reported that the growth on smaller size catalyst needs high chamber
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pressure since the degree of supersaturation decreases as the size of catalyst is decreased [16].
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The increase in the chamber pressure results in increase in the supersaturation of the droplet [1,
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17], which in turn enables the growth even through smaller size of the particles. Thus the density of SiNWs is also increased with the chamber pressure. As the working pressure is increased to 4
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Pa, very few SiNWs were found on the substrate and no growth is seen at 5.3 Pa. Recently, J. Tang et al. [18] have studied the effect of the growth time on the SiNWs growth characteristics.
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They have observed that not only the NWs length changes with the growth time, but the NWs become conical from cylindrical then bent-conical and finally bent inverted conical [18]. In our
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study it is observed that the NWs growth pattern and morphology depend significantly on the chamber pressure and vertical NWs can be grown at low chamber pressure. The vertical growth
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of SiNWs also avoids the formation of inverted conical shaped NWs.
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In a previous report of SiNWs grown by thermal CVD, the decomposition of SiH4 at the catalyst surface has been considered to be rate limiting step. K. K. Lew and J. M. Redwing [19] have
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observed a linear increase in the growth rate with increasing SiH4 partial pressure from ~17 to ~86 Pa. On that basis, they concluded that the Si crystallization is not the rate-limiting step and
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the catalytic decomposition of SiH4 at the catalyst surface is the only rate determining step. In our case the same trend of increasing growth rate is observed upto 2.7 Pa chamber pressure (Fig. 1) while at chamber pressure larger than 2.7 Pa the growth rate decreases drastically indicating that the aforementioned assumption is not valid. In thermal CVD the decomposition and the growth occur on the substrate which is heated to 600oC whereas in the case of HWCVP the dissociation of SiH4 gas occurs at a heated filament and the Si atoms move towards the substrate where growth takes place of SiNWs takes place on the Sn nano-particles. Therefore, in HWCVP growth should be limited additionally by the dissociation rate of SiH4 molecules over the heated filament rather than the catalytic dissociation at the Sn nano-particle surface.
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At a high pressure of 5.3 Pa, the Si starts accumulating on the catalyst particle surface which means that the flux reaching the catalyst is too large to be crystallized entirely. It means that at higher pressure the growth rate limiting step is the crystallization or precipitation rather than the Si flux. For a stable growth the adsorbing amount of Si at the catalyst surface must be equal or lesser than the precipitation rate. At 5.3 Pa, the Si flux reaching the catalyst surface is much
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larger than the precipitation which causes an amorphous silicon film deposition rather than NWs
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growth.
It can be seen that the NWs grown at 4 Pa have length of less than 1 µm and these are covered
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with a thick amorphous layer of Si:H. Sn nano-particles have a wide size distribution and the growth depends on catalyst size. This template might be having some particles with bigger size
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through which the growth is still possible at that high chamber pressure, as a result very few NWs are observed at 4 Pa pressure. Increase in chamber pressure further (to 5.3 Pa) causes a
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large amount of Si impingement on the Sn catalyst and even bigger Sn particles are now unable to start the growth of SiNW. So, at higher chamber pressure where a large amount of Si is
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generated from the hot filament and gets adsorbedon the metal catalyst, the growth rate is limited
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by the crystallization or the precipitation rate.
The most important effect of the chamber pressure which is seen here is on the geometrical
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orientation of the SiNWs. At high pressure they are not perpendicular to the substrate but kinking is seen in many wires. The kinking mechanism in SiNWs has been explained in the literature
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[20-23]. The change in process conditions such as temperature and pressure were shown to produce kinking in SiNWs [24]. The kinking in NWs synthesized by thermal CVD via Au catalyst was shown to be caused by the sudden change in the growth pressure [23].In our case we have maintained these parameters constant for the growth and the SiNWs exhibit bending type structure rather than kinking. It means the above statement may not be valid and an explanation is needed for the observed bending of the SiNWs grown by HWCVP.
In SiNWs grown by vapor liquid solid mechanism, the presence of kinks could be due to instability at the liquid/solid interface [25]. There should be a proper balance between capillary
ACCEPTED MANUSCRIPT forces acting on this line for a stable growth. The growth rate is decided by the difference in chemical potential of Si in droplet and in the growing solid phase of Si. Increase in the chamber pressure results in production of a large amount of Si which in turn increases the supersaturation of Si in the Sn-Si droplet. In this situation, the growth rate is accelerated which can generate a lot of defects in the SiNWs [26]. These defects form all over the growth resulting in a bent structure of SiNWs. Buildup of strain during the growth could also be one of the possible reasons for the
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NW bending. On the other hand, low pressure in HWCVP produces lesser amount of Si flux
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which decreases the growth rate of SiNWs enabling a defect free growth of vertical straight
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SiNWs.
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As seen in HR-TEM image Fig. 2, SiNWs grown at high pressure consists of multiple small grains. Defects occurring during growth might inhibit the single crystal growth and as a
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consequence formation of polycrystalline structure takes place. In this case, selected area electron diffraction (SAED) pattern taken on a bent NW consists of spots distributed in circular
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rings (Fig. 3) which manifests a polycrystalline structure at high pressure. Fig. 4 (a) shows the TEM image of a SiNW grown at low pressure which is straight. Fig. 5 is HR-TEM image taken
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at the end on a straight SiNW which exhibits single crystalline structure of the SiNW. SAED pattern was also taken on the SiNW showing the periodic arrangement of the spots (Fig. 4 (b))
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which confirms the single crystal structure of the SiNW. We can also see some rings in the
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SAED pattern which may be from the amorphous shell present on the crystalline core. It is now clear that employing intermediate chamber pressure can lead to dense and long SiNWs.
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However, they are slanted and polycrystalline in structure. The growth of SiNWs is proportional to the amount of Si flux reaching to the catalyst. Therefore, in next experiment, we have increased the flow of SiH4 from 5 sccm to 10 sccm, and the chamber pressure was maintained at 0.4 Pa which is the minimum chamber pressure which is achievable with 10 sccm flow of SiH4 and the full speed of the turbo pump. In this experiment, the growth of SiNWs was carried out for 30 min. The substrate temperature was also increased to 450 oC in order to maintain the stable growth. Fig. 6 (a) shows the SEM image of the resulting SiNWs. SiNWs prepared under such conditions are found to be vertically aligned perpendicular to the substrate. The crosssectional SEM image is also depicted in Fig. 6 (b). Moreover, SiNWs are dense with length in
ACCEPTED MANUSCRIPT the range of 6-7 µm. For these SiNWs growth rate was determined to be ~220 nm/min. Previously, vertical SiNWs have been grown by using a nano-porous template which confines the growth of SiNWs along the pores [27]. After the growth this template is removed in a solution. However, this approach is not easy and requires multiple steps and it is a time consuming process whereas in our case vertical SiNWs can be grown without any additional
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Conclusions
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We studied the effect of chamber pressure on the growth characteristics and morphology of SiNWs synthesized by HWCVP using Sn nanotemplate as catalyst. A proper control of SiNWs
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orientation is possible by tuning the growth pressure. Our results indicate that the change in growth rate may be responsible for the change in orientation of SiNWs. This proposed
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mechanism remains under active debate and needs further understanding of the various aspects observed. At high pressure, tilting and bending patterns of SiNWs are observed which could be
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due to the formation of defects in SiNWs. The defects are mostly generated by the high growth rate which is also responsible for the polycrystalline structure of SiNWs synthesized with such
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high chamber pressure. Whereas growth at low pressure produces single crystalline SiNWs which are straight perpendicular to the substrate and free from defects. It is also noted that the
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growth rate is not limited by a single step; it is the interplay between Si generation rate over the
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hot filament, incorporation rate and the crystallization rate.
Acknowledgment
Crompton Greaves, Mumbai is gratefully acknowledged to providethe financial support to AS. The authors also thank SAIF, IIT Bombay for providing TEM and SEM facilities. FIST facility (Dual beam FIB, Carl Zeiss Microscope) in ME & MS was also used for this work.
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Figure 1 (a)-(e) SEM micrographs of SiNWs grown for 10 min at chamber pressure of varying from 0.67 Pa to 5.3 Pa and substrate temperature of 400 °C, and (f) associated SiNWs growth
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Figure 2 HR-TEM image of a slanted SiNW grown at 2.7 Pa chamber pressure. Figure 3 (a) TEM image of a bent SiNWs and (b) its SAED pattern.
Figure 5 HR-TEM image of a straight SiNW
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Figure 4 (a) TEM image of a straight SiNW and (b) its SAED patterned.
Figure 6 (a) SEM image of the SiNWs synthesized by using 10 sccm of SiH4 with 0.4 Pa chamber
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SiNWs were synthesized by HWCVP using Sn film as a catalyst layer
Control of the geometrical orientation of HWCVP grown SiNWs
SiNWs grown at high chamber pressure exhibited polycrystalline structure
SiNWs grown with low chamber pressure are long and have single crystalline structure
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