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SILICON NANOWIRE GROWTH BY LPCVD USING MULTIPLE
GOLD CATALYST DEPOSITION METHODS AND AN
APPLICATION AS HYDROGEN SENSOR
Siva Penmetsa, Alison Viegas, Savitha P
Centre for Nanoscience and Engineering, Indian Institute of Science Bangalore 560012
Email: [email protected]
Abstract: Silicon nanowires (SiNWs) were grown by VLS method in LPCVD, using Au deposited by sputtering and electron beam evaporation as catalyst. Gold deposition was also carried out by a new solution-based method using AuCl3 in water.
Thickness of the Au catalyst layer was seen to effect the nanowire growth, with thick films giving short wires with higherdiameters. Au catalyst layer with higher thicknesses failed to produce nanowires due to improper separation of Au/Si eutecticdroplets on annealing as observed by scanning electron microscopy. Thicker Au depositions obtained using concentrated
solutions of AuCl3 were also unable to effectively catalyse the nanowire growth. Longer nanowires with smaller diameters
were produced with thin Au films and dilute AuCl3 solutions. A palladium coated silicon nanowire was further used tofabricate a hydrogen sensor .
Keywords: Nanowires, VLS method, LPCVD, Gas Sensor
1. INTRODUCTION
Silicon nanowires (SiNWs) are gaining great interest due totheir electrical and optical properties. SiNWs basednanodevices like field effect transistors have potentialapplications in electronics and biosciences. They are alsovery attractive for bio and gas sensors, solar cells, lithium battery etc [1]. Vapor-Liquid-Solid (VLS) mechanism has
been extensively investigated for the growth of SiNWs. InVLS method, metal particles like Au act as catalyst for thegrowth and Si based gas sources like SiH4 or SiCl4 are usedas precursors at temperatures typically around 500°C to
1000°C. The Au forms a eutectic with Si and forms moltenAu/Si alloy. Si from gas source diffuses into liquid alloy andsuper saturates at the liquid/solid interface forming SiNWs
[2]. Deposition of the Au catalyst could be carried out byseveral methods including sputtering [3], evaporation [4] orsolution based methods using Au nanocrystals [5].
SiNWs have high sensitivity due to large surface to volumeratio and so are very attractive for use as gas sensors [1]. H 2
is highly flammable and becomes explosive when theconcentration is more than 4% in air, which makes detectionof H2 a critical safety factor in applications like hydrogenfuelled cars. Palladium (Pd) can absorb H2 up to 900 times its
own volume at room temperature and atmospheric pressure[6] which makes it a good material for H2 sensing. SiNWsfunctionalized with Pd have been used as H2 sensors in the
recent past [7].
In the present work, a new method of Au catalyst deposition,with an aqueous gold chloride (AuCl3) solution, for growingSiNWs by VLS technique in LPCVD, is demonstrated andcompared with traditional methods like sputtering and
evaporation. Thickness of the Au films deposited is
correlated to the Au droplet size and the final diameter of theSiNW formed. Further, a device is fabricated and used for H2
sensing using single SiNW coated with Palladium.
2.
EXPERIMENTAL
SiNWs were grown on p-type Si (100), Si (111) and SiO2 substrates using Au as catalyst layer. Commercially available
AuCl3 (5% in water) solution was used for Au deposition.Also, thin Au films were deposited by sputtering or e-beamevaporation on samples of sizes 1cm x 1cm. A KI:I2 gold
etchant solution was used to etch gold to reduce the thicknessin the range of 10-30nm. These samples were annealed informing gas (FG: N2 and H2 in the ratio of 9:1) at 450 °C for
60 minutes. The samples were placed in LPCVD to growSiNWs at a temperature of 600°C and a pressure of 400millitorr for 60 or 120 minutes. The samples werecharacterized using Scanning Electron Microscopy (SEM).
Fabrication and measurements of the SiNW hydrogen sensoris explained in section 3.6.
3.
RESULTS AND DISCUSSION
3.1 SiNWs from AuCl3 solution
AuCl3 (5% gold chloride in water) solution was used for
depositing Au catalyst by wet chemical process. Threeconcentrations – a 100% original solution, a 10% and 5%
solutions prepared by further dilution of the original solutionwith water were taken. Samples of size 1 cm x 1cm were placed on hot plate maintained at 250°C and the solutions
dropped on the samples such that maximum surface of thesamples were covered. The samples were heated until thewater gets evaporated leaving Au particles behind.
All samples were used to grow SiNWs in the LPCVDfurnace. It was seen that the 100% and 10% diluted solutiondoes not support considerable growth of SiNWs as the Au particles are densely packed. The Si sample treated with 5%
diluted solution produced SiNWs at places where Au isdispersed loosely. Gold particles of diameter in the range of30-130 nm produced SiNWs of length around 3000nm after
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60 minutes of growth in LPCVD. The diameter of these wireswere in the range of 70-230nm (Fig. 1).
(a) (b)
Figure 1. SEM top view images of samplestreated with 5% AuCl3 solution on a) Si
and b) SiO2
(a)
(b)
Figure 2. SEM cross-sectional images after
LPCVD growth on a) Si and b) SiO2
Same process was also carried out with a 5% solution on athermally grown 100nm SiO2 on Si sample (Fig.2). As seenfrom Fig. 2(b), NW growth on SiO2 was sparse compared to
that of pure Si substrate.
3.2
SiNWs using sputtered Au catalyst layer
Au films of thickness 35 nm were sputtered on Si (100)samples. Gold etchant was used to thin down these films to
thickness of 7, 17 and 22nm. Au/Si eutectic droplets wereformed by FG annealing, with the droplets growing todiameters of 17-200 nm depending on the initial thickness of
the film. These samples when used for SiNW growth gave NWs of diameter in the range of 70-390 nm.
(a)
(b)
Figure 3. SEM images (top view) of Au
catalyst layer after FG anneal at 450°Ca) Au of thickness 7nm b) Au of thickness22nm
As seen from Table 1 and Fig.3, the diameter of Au dropletsformed increases with higher thicknesses of Au catalyst layer.
Also, inter droplet distance was seen to decrease drasticallywith the increasing thickness of the Au layer.
The diameter and the variation in diameter of the SiNWs alsoincreased with increasing thickness of the Au layer (Table 1).This could be due to the higher variation in the Au dropletsformed after FG annealing, for thicker Au catalyst layers.
Table 1. Characteristics of SiNWs obtained using Audeposited by sputtering
Substrate
Au
filmthickness
(nm)
Au-Si
dropletdiameter
(nm)
SiNWdiameter
(nm)
Average
length(nm) for60 min
Si(100) 7 17 - 160 36 - 175 3000
17 30 - 230 40 -290 2900
22 25 - 200 35 - 210 2300
Substrates with thicker Au layer (Fig 4.b) produced shorter NWs, whereas less slanted and thinner NWs were obtainedwith a lower Au thickness (Fig.4.a)
(a)
(b)
Figure 4. SEM cross-sectional images of
SiNWs produced with a) Au of thickness
7nm b) Au of thickness 22 nm
3.3
SiNWs from E-Beam evaporated Au
Electron beam evaporation was used to deposit about 40nm
of Au on Si (100), Si (111) and SiO2 (100nm) on Sisubstrates. Ellipsometry measurements confirmed thethicknesses of 44nm on Si (100), 39nm on Si (111) and 33nm
on SiO2 sample.
(a)
(b)
(c) (d)
Figure 5. SEM (top view) images of e-Beam evaporated Au films after FGannealing: a) Au, 44nm on Si(100) b) Au,8.2 on Si(100) c) Au, 21.2 on Si(111)
d) Au, 19 nm on SiO2
All the samples were etched for 2 sec and 4 sec using Au
etchant to reduce the thickness of the deposited film. The Authin films thus obtained, had thicknesses as follows: for Si(100), after the 2 sec etch process, thickness was 21.8nm and
after 4 sec etch, thickness was 8.2nm; for Si (111), after the 2
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sec etch, thickness was 26.8nm and the 4 sec etch gave a filmthickness of 21.2nm. In the case of SiO2, the 2 sec etch
reduced the Au thickness to 31nm, and the 4 sec etch gave19nm thickness. All the samples along with ‘as deposited’samples were cut around 1cm x 1cm size and FG annealed.
Fig 5 shows the SEM images of the Au films on Si samplesafter the FG annealing step. The ‘as deposited’ samples with
Au thicknesses of 44nm on Si (100), 39nm on Si (111) and31nm on SiO2 surface did not produce Au/Si eutecticdroplets, maybe due to the higher thickness of the Au film
leading to improper separation of droplets. Hence, thesesamples were not further processed for SiNWs in LPCVD. Si(100), Si (111) and SiO2 samples after Au etching for 2 secand 4 sec were used to grow SiNWs for 120 min (Fig 6).
(a)
(b)
(c) (d)
Figure 6. SEM cross section images ofSiNWs grown after FG anneal of a) Au 8.2nm on Si(100) b) Au 21.8 on Si(100) c) Au
21.2 on Si(111) d) Au 19nm on SiO2
As observed with sputtered samples, it can be seen thatthicker Au catalyst layer gave NWs of higher diameters.Compared to Si (100) substrates, NWs grown on Si (111)
were less slanted and densely packed. In contrast to the behavior with AuCl3 catalyst layer, evaporated Au layer onSiO2 substrate gave dense NWs with higher diameter. Acomparison of Au/Si droplet diameters and NW
characteristics obtained with evaporated Au layer is given inTable 2.
Table 2 Characteristics of SiNW's obtained using Au
deposited by evaporation
Substrate
Aufilm
thickness(nm)
Au/Sidroplet
diameter(nm)
SiNW
diameter(nm)
Average
length (nm)120 min
Si(100) 21.8 30 - 205 40 - 140 6200
8.2 10 - 100 55 - 190 6500
Si (111) 26.8 40 - 270 70 - 290 6700
21.2 110- 260 60 - 390 6600
SiO2 31 200- 590 60 - 350 6000
19 60 - 410 80 - 450 5500
3.4 E-beam patterned Au catalyst layer
Samples with square patterns of dimensions 250 nm and 500nm, spaced at 750nm and 1um respectively, were fabricatedusing e-beam lithography. Au was deposited on the samples
using e-beam evaporation for a thickness of 40 nm, and lift-off process was carried out. SEM images after lift-off andafter LPCVD process for SiNWs growth for duration of 120
minutes is shown in Fig 7. It can be seen that 250 nm size patterns did not produce SiNWs whereas 500 nm sized ones produced SiNWs of diameter around 200nm. The lack of
SiNWs formation in the 250 nm sized patterns could be dueto their shrinkage while forming Au/Si eutectic liquid dropletduring annealing which reduces the effective surface area andrenders it insufficient as a catalyst for SiNW growth.
(a) (b)
(c) (d)
Figure 7. SEM images (top view) of e-Beam patterned a) Au patterns of 250nm b)
Au, 250nm after LPCVD processing c) Au patterns of 500nm d) Au, 500nm afterLPCVD processing
3.5
Effect of Au catalyst layer on SiNW growth
SiNWs grown in LPCVD using Au layer deposited byvarious means were compared. Irrespective of the method ofAu deposition, higher thickness of Au film resulted in the
formation of wider Au droplets after annealing. This, in turn,led to SiNWs with shorter length and higher diameter, maybedue to the higher deposition of Si on Au droplets leading to a
collapse of the growing NW. Au deposited using electron beam evaporation gave longer, more vertical nanowirescompared to sputtering method. E-beam patterned wafers
gave nanowires when the size of the Au patterns was 500 nm.
AuCl3 solution was also used to produce catalyst layer for
SiNW growth by LPCVD. For these samples, SiNW growthwas not observed at higher concentrations, maybe due tocoagulation of the Au particles deposited. SiNWs grown on
SiO2 were few, maybe due to the difficulty in the formationof Au/Si eutectic.
3.6 SiNW hydrogen sensor: Fabrication flow and
measurements
Fig. 8 depicts the steps involved in the fabrication of thesensor. After growing the SiNWs in LPCVD using Silane,Palladium (Pd) of thickness 20 nm was deposited on SiNWs
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by sputtering. The presence of Pd on SiNW was confirmed by EDS as seen in Fig 9. Platinum (Pt) pads (100 x 100 um)
were made on SiO2 grown on Si wafer by photolithographyand lift-off. The Pd deposited SiNWs were transferred ontosamples with Pt contact pads using solution based scratch,dip and disperse method. The SiNWs that were close to the Pt
pads were connected using FIB (Fig 10.a). For electricalmeasurements, the Pt pads were wire bonded on to a PCB forrobustness (Fig 10.b). Characterization of the device wascarried out using 1% Hydrogen.
Figure 8. Animation depicting the
fabrication flow 1. Clean the Si wafer. 2.Deposit thin Au film on Si. 3. FG anneal toform Au droplets. 4. SiNWs growth inLPCVD by VLS method. 5. Palladium
deposition using sputtering. 6. Pd deposited
SiNWs transferred to Pt pads on SiO2grown Si sample. 7. Pd deposited SiNWsconnected to Pt pads with FIB 8. Pt padswired bonded onto PCB
(a)
(b)
Figure 9. EDS measurement taken from FE
SEM depicting of Pd on SiNW
The resistance of the Pd coated SiNW was 24 kilo-ohm, asmeasured using IV characterization by probing on the Pt pads. The device was wire bonded onto PCB and was placed
in a closed chamber with an inlet and outlet, at roomtemperature. 1% H2 gas and 99.9997% pure N2 gas, 1000sccm each, were passed alternately through the inlet.
A constant voltage of 0.1 V was applied to the device withKeithley source measure unit (SMU). The change in current
was acquired using a Matlab program. The change in
resistance graph is plotted as given in Fig 11(b). The performance can be further improved by process
optimization.
(a)
(b)
Figure 10. (a) Electroscopic image of Pd
coated SiNW connection with Pt made byusing FIB and, (b) Optical image of wire bonded device onto PCB
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0.0
10.0µ
20.0µ
30.0µ
40.0µ
50.0µ
C u r r e n t ( A )
Voltage (V) 0 30 60 90
21.8k
22.0k
22.2k
22.4k
H2
H2
N2
N2
R e s i s t a n c e ( O h m )
Time (minute)
Voltage: 0.1 V
N2
Figure 11. (a) IV graph of Pd coated singleSiNW and (b) Resistance variation withtime by alternate flow of N2 and H2
4.
CONCLUSIONS
SiNWs were grown in LPCVD using Au seeds deposited by
various methods including sputtering, evaporation andelectroless deposition using AuCl3 solution. Thicker Audepositions produced wider Au/Si droplets leading to theformation of shorter NWs. Patterned seed layer deposition
using electron beam lithography had NW growth for 500nm
Au square patterns. In comparison, thicker Au depositions byevaporation and AuCl3 solution did not produce NWs due to
insufficient separation of the droplets/particles. The LPCVDgrown nanowires had diameters ranging from 17nm to590nm and length from 2 um to 8 um. A device was also
fabricated for H2 sensing using single SiNW.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the funding supportfrom Ministry of Communication and InformationTechnology under a grant for the Centre of Excellence in Nanoelectronics, Phase II. They also thank Prof. Navakanta
Bhat and Dr. Palash Kumar Basu for helpful discussions andDr.M.M Nayak and Manjunath M.S. for helping with thewire bonding of the sensor. They acknowledge Micro and Nano Characterization Facility (MNCF) and National NanoFabrication Center (NNFC) at the Centre for Nanoscienceand Engineering, Indian Institute of Science for the usage of
the facility.
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