SiNW Hydrogen Sensor

7/23/2019 SiNW Hydrogen Sensor

http://slidepdf.com/reader/full/sinw-hydrogen-sensor 1/5

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



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



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



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.

REFERENCES

[1]

Yuting Wan, Jian Sha, Bo Chen, Yanjun Fang, ZongliWang, and Yewu Wang, “Nanodevices Based on Silicon

Nanowires”, Recent Patents on Nanotechnology, 3,1-9(2009).



[2]

J. Westwater, D. P. Gosain, S. Tomiya, S. Usui and H.Ruda, “Growth of silicon nanowires via gold/silane

vapor–liquid–solid reaction”, Journal of VaccumScience and Technology B 15, 554 (1997).

[3]

Kolb, F. M. , H. Hofmeister , R. Scholz , M. Zacharias ,

U. Gösele , D. D. Ma , and S. T. Lee, “Analysis ofsilicon nanowires grown by combining SiO evaporationwith the VLS mechanism”, Journal of theElectrochemical Society 151 (7) , G472-G475 (2004).

[4]

Koto, Makoto; Shimizu, Tomohiro; Shingubara, Shoso,

"Improved control of silicon nanowire growth by thevapor-liquid-solid method using a diffusion barrier layer between catalyst and substrate", Journal of Crystal

Growth, Volume 369, 1-7 (2013).

[5]

P R Bandaru and P Pichanusakorn, "An outline of thesynthesis and properties of silicon nanowires",

Semiconductor Science and Technology, 25, 024003(2010).

[6] S. H. Lim, B. Radha, J. Y. Chan, M. S. M. Saifullah, G.U. Kulkarni and G. W. Ho, Flexible “Palladium-BasedH2 Sensor with Fast Response and Low Leakage

Detection by Nanoimprint Lithography”, ACS AppliedMaterials Interfaces, 5, 7274 – 7281 (2013).

[7] Z. H. Chen, J. S. Jie, L. B. Luo, H. Wang, C. S. Lee andS. T. Lee, “Applications of silicon nanowiresfunctionalized with palladium nanoparticles in hydrogen

sensors”, Nanotechnology, 18, 345502 (2007).

SiNW Hydrogen Sensor

Documents

Transcript of SiNW Hydrogen Sensor