Catalyst-free synthesis of silicon nanowires by oxidationand reduction process

Sanjay K. Behura • Qiaoqin Yang • Akira Hirose •

Omkar Jani • Indrajit Mukhopadhyay

Received: 18 January 2013 / Accepted: 24 May 2013 / Published online: 11 February 2014

� Springer Science+Business Media New York 2014

Abstract A new process has been developed to grow

silicon (Si) nanowires (NWs), and their growth mecha-

nisms were explored and discussed. In this process, SiNWs

were synthesized by simply oxidizing and then reducing Si

wafers in a high temperature furnace. The process involves

H2, in an inert atmosphere, reacts with thermally grown

SiO2 on Si at 1100 �C enhancing the growth of SiNWs

directly on Si wafers. High-resolution transmission elec-

tron microscopy studies show that the NWs consists of a

crystalline core of *25 nm in diameter and an amorphous

oxide shell of *2 nm in thickness, which was also sup-

ported by selected area electron diffraction patterns. The

NWs synthesized exhibit a high aspect ratio of *167 and

room temperature phonon confinement effect. This simple

and economical process to synthesize crystalline SiNWs

opens up a new way for large scale applications.

Introduction

Silicon nanowires (SiNWs), a quasi-one-dimensional

structure, have attracted much research interest in the past

decade due to their exceptional physical properties and the

central role of silicon in integrated circuit technology and

electronic industry. They have wide potential applications,

including high performance field effect transistors [1],

nanoelectromechanical systems [2], high performance

lithium battery anodes [3], and photovoltaic devices [4–6].

Significant progress has been made for catalyst assisted

growth of SiNWs and their applications [7–10], but only a

few papers have been reported on the catalyst-free growth

of SiNWs [11–13]. So far, many methods have been

developed to grow SiNWs, but a universal growth tech-

nique fulfilling all the requirements for potential applica-

tions has not yet emerged. Chemical vapor deposition

(CVD) [14, 15], super-critical fluid and solution based

techniques [16], molecular beam epitaxy (MBE) [17], laser

ablation [18], and silicon monoxide (SiO) evaporation

techniques [13, 19] have been widely used to produce

substrate-bound arrays of SiNWs.

CVD allows controlled and selective growth of NWs

through prepatterning the metal catalysts on the substrate.

However, it requires costly metals such as gold, indium, or

platinum as catalysts. Supercritical-fluid based method

produces thin quality nanowires (NWs) with high yield, but

it is difficult to achieve controlled and epitaxial growth.

MBE can produce single crystalline NWs on predefined

positions on the substrate, but its low growth rate of a few

nanometers per minute results in NWs of limited aspect

ratios. Laser ablation technique is very simple, but difficult

to achieve epitaxial growth. SiO evaporation technique is

simple too, but it is difficult to control the diameters and

lengths of grown NWs. Moreover, SiO powder is harmful

S. K. Behura � Q. Yang (&)

Department of Mechanical Engineering, University of

Saskatchewan, Saskatoon, SK S7N 5A9, Canada

e-mail: qiaoqin.yang@usask.ca

S. K. Behura � O. Jani

Solar Energy Research Wing, Gujarat Energy Research and

Management Institute-Research, Innovation and Incubation

Centre, Gandhinagar 382 007, Gujarat, India

A. Hirose

Plasma Physics Laboratory, Department of Physics and

Engineering Physics, University of Saskatchewan, Saskatoon,

SK S7N 5E2, Canada

I. Mukhopadhyay

School of Solar Energy, Pandit Deendayal Petroleum University,

Gandhinagar 382 007, Gujarat, India

123

J Mater Sci (2014) 49:3592–3597

DOI 10.1007/s10853-013-7476-5

to health. Nevertheless, the limitations associated with

those growth methods have stimulated increasing interest

in developing catalyst-free synthesis of SiNWs without

using any metal catalyst and SiO powders. In this paper, we

report on a simple, single step and cost-effective technique

to synthesize SiNWs for large scale applications. Using this

technique, SiNWs can be synthesized on Si substrate

without any additional nano-sized metal catalyst seed layer

by simply oxidizing and reducing Si wafers through oxide-

assisted growth. Comparing with the previously reported

catalyst assisted methods, this process presents many

advantages, including (i) elimination of metal catalyst

contamination; (ii) avoiding the use of toxic precursor

gases such as SiH4 or SiCl4; and (iii) no need to transfer the

NWs for device manufacturing because NWs are directly

grown on Si wafers.

Experimental

The synthesis of SiNWs were conducted in a horizontal

quartz tube mounted in a high temperature furnace (Barn-

stead Thermolyne, USA). The quartz tube was connected

to a mechanical pump with mass flow controllers. The pre-

treated p-type silicon (1 0 0) wafers with a thickness of

500 lm were placed into the tube and thermally oxidized at

800 �C for 1 h under air. After the oxidization, the tube

was cooled to a temperature close to 100 �C and subse-

quently pumped down to 40 mTorr, then the furnace was

heated to 1100 �C under hydrogen flowing (200 sccm).

When the temperature reached 1100 �C, a mixture of

hydrogen (100 sccm) and argon (300 sccm) was introduced

into the chamber, maintaining a pressure of 60 mTorr.

After one hour, the power was turned off and the furnace

was cooled down in hydrogen and argon atmosphere. The

detailed thermal processing condition is given in Fig. 1.

The surface morphology of the as-grown samples was

studied using a field emission scanning electron micros-

copy (FESEM, HITACHI SU6600). High-resolution

transmission electron microscopy (HR-TEM) analyses

were performed using a FEI, TITAN 80-300 microscope

operated at 80 kV. The compositional analysis was per-

formed using an energy dispersive X-ray spectroscopy

(EDS, EDAX Instruments). Phase identification was car-

ried out by X-ray diffractometer (XRD, Philips PW1450/

70). Raman spectroscopy (Renishaw 2000) was used to

characterize the structural details. All the measurements

were carried out using a laser excitation wavelength of

514 nm (Ar? ion laser with maximum power of 3.4 mW

for 509 objective). For HR-TEM and selected area elec-

tron diffraction (SAED) studies, the as-grown SiNWs were

scratched off from the substrate and the obtained powder

samples were then dispersed in ethanol. A drop of this

suspension was poured onto the copper grids and dried for

HR-TEM and SAED analysis, whereas the as-grown SiN-

Ws on Si wafer was used for FESEM, EDS and Raman

measurements.

Results and discussion

As the average length and average diameter of SiNWs are

known to govern its optical and electrical properties, the

synthesized samples were subjected to detailed character-

ization by FESEM. The FESEM images of the as-grown

SiNWs in both low and high magnifications are shown in

Fig. 2. Detailed analysis shows that the NWs have an

average length of 5 ± 0.1 lm and an average diameter of

30 ± 5 nm, thus the average aspect ratio of the NWs is

167. Figure 2b shows a single NW of uniform diameter

throughout. The residual oxide flakes are also clearly seen

on the substrate (Fig. 2a). It should be noted that those

oxide flakes should be removed by selective chemical

etching before they can be used. The purifying of the NWs

for practical applications is ongoing in our group and will

be reported.

In present experiments for the synthesis of SiNWs, there

was no metal catalyst, suggesting that we found a new

mechanism governing the growth at 1100 �C, very differ-

ent from the other techniques. This new growth mechanism

does not require a metal catalyst, as indicated by the

absence of any metal catalyst seed layer. There are two

possibilities for the growth of SiNWs: growth directly from

elemental silicon or from a silicon compound through

catalyst particles. It is clear that for the latter case, a

chemical reaction has to take place. In present experiments,

Fig. 1 Thermal processing conditions for SiNWs synthesis using a

high temperature vacuum furnace

J Mater Sci (2014) 49:3592–3597 3593

123

the source for the growth of SiNWs is SiO2, which formed

from the oxidation of Si surface. As there are two typical

mechanisms governing SiNWs growth: (i) vapor–liquid–

solid [20] (VLS) and (ii) oxide-assisted growth [21] (OAG)

mechanism, VLS mechanism is not applicable here,

because this mechanism uses a liquid metal catalyst which

draws components out of a vapor phase and deposits them

in a solid phase. The OAG mechanism is originally pro-

posed for growth via laser ablation, where it was observed

that, nanowires could be catalyzed by SiO2 [22, 23]. In the

present SiNW synthesis experiments, there was no metal

catalyst, suggesting that OAG might be the growth mech-

anism. Herein, we propose the SiNW growth mechanism as

schematically shown in Fig. 3. The oxygen atoms in the

tetrahedral structure of SiO2 reacts with H2 forming water

vapors, and giving opportunity for Si–Si nucleation and

formation of NWs. The excess oxygen atoms in the SiO2

might be repelled by the silicon atoms to the edge and

forming a chemically inert SiO2 sheath, which restricts the

further increment in diameter of NWs and inducing the

one-dimensional growth of crystalline NWs. So the edge

side of the nucleus grows slower than the tip of the nucleus.

When high temperature is kept, the nucleus undergoes

recrystallization with phase separation into Si and Si oxide.

Crystallized Si stays inside and amorphous SiO2 diffuses

toward the nanowire surface. Subsequently, as the growing

edge of the recrystallizing region approaches the slower

growth edge, an amorphous Si oxide sheath is created. As

the surface of the SiO2 sheath is smooth and at a temper-

ature higher than the substrate surface, which is responsible

for NWs development, growth completes on the SiO2

sheath in the lateral direction. On the other hand, once the

crystalline core forms, it expands continuously in the axial

direction due to the rapid formation of nanoclusters. If the

reaction stops, a SiO2 layer forms at the wire edge. From

thermodynamic viewpoint, recrystallization with phase

separation is the effect of free energy minimization of the

system, which results in crystalline core and amorphous

shell. The amorphous shell helps to prevent mechanical or

radiation damage and thus suppresses chemical reactivity,

which could lead to the oxidation and contamination of

silicon nanowire. Our experiments have demonstrated that

the thermal oxidization of host Si substrate is essential for

the growth of SiNWs. Without oxidation, no nanowires

were formed on the Si wafers. Thus, the chemical reaction

for the NW nucleation is given below.

SiO2 þ 2H2 ! Siþ 2H2O ð1ÞFigure 4a illustrates a typical energy dispersive X-ray

spectrum (EDS) of the nanowires. The results show that the

NWs consist of silicon and oxygen with an atomic ratio of

Si/O = 90.95:9.05. The inset shows the spot area on a

single NW, where EDS was performed. The EDS elemental

mapping of silicon and oxygen of a selected NW is shown

in Fig. 4b. It is clearly observed (Fig. 4b) that the NW

consists of silicon in the confined regime and oxygen in the

surrounding area.

The structural and crystallographic characterizations of

the collected nanocluster powders, which are gray in color,

flexible, and sticky, were performed using HR-TEM and

SAED techniques. The sample for HR-TEM study was

prepared following the method given in the experimental

section. The typical diameters of NWs were observed to be

30 ± 5 nm, which is shown in Fig. 5a and b. It can be seen

Fig. 2 FESEM micrograph images of SiNWs grown on Si substrates a at low magnification and b at high magnification

Fig. 3 A schematic representation of growth mechanism for SiNW

on Si wafer

3594 J Mater Sci (2014) 49:3592–3597

123

that the SiNWs have a crystalline core of diameter 25 nm

and an amorphous SiO2 shell of 2 nm in thickness around

the wire. Figure 5a represents the low-magnification HR-

TEM image of the core–shell SiNW. The lattice of the

SiNW and the thickness of the crystalline Si core and

amorphous SiO2 shell around the NW has been clearly

shown in Fig. 5b [24, 25]. The corresponding SAED pat-

tern (inset of Fig. 5b) with concentric ring consists of

distinct bright spots confirming the crystalline structure of

silicon core. Beside the concentric ring and spot pattern, we

do notice the presence of weak scattered spots, which are

diffuse in nature. These diffuse scattered spots may be

attributed to the presence of amorphous SiO2 in the form of

shell. Based on the above results, we propose that the NWs

were grown by an oxide-assisted cluster-solid mechanism,

which can be explained by Fig. 3. Deposition of the na-

noclusters starts when nuclei form as a metastable

amorphous matrix at the substrate temperature around

1000 �C. The sticking coefficient of vapor phase na-

noclusters to the amorphous growth surface is as high as

unity, which can thus lead to a very high growth rate

from vapor phase. Both the stickiness of the nanoclusters

and the temperature dependence of deposition may be

important properties of nanoclusters for the formation of

nanowires.

Figure 6 shows the XRD pattern of the SiNWs on p-Si

(1 0 0) wafer. Due to crystalline structure, SiNW shows

distinct peaks corresponding to Si (1 1 1) and Si (3 1 1)

planes.

The room temperature electrical conductivity of the

SiNWs is 100 X-1 cm-1, as measured using a four-probe,

method, compared with electrical conductivity of

10 X-1 cm-1 for silicon wafer. The significantly higher

electrical conductivity of the NWs is probably due to the

Fig. 4 a EDS of NWs and the inset image shows the spot area on the wire and b elemental mapping of silicon and oxygen in the selected NW

Fig. 5 HR-TEM image of SiNW with Si as core and amorphous SiO2 as shell a at low magnification and b at high magnification with the

corresponding SAED pattern of the NWs in the inset

J Mater Sci (2014) 49:3592–3597 3595

123

size effect. In the SiNWs, electricity can be also conducted

through their large area of surfaces.

Raman spectroscopy is an important tool for the non-

destructive characterization of the SiNWs. The Raman

spectra of the nanowires measured at room temperature

using different laser powers are shown in Fig. 7. The strong

and narrow-band centered at around 520 cm-1 corresponds

to the first-order transverse optical phonon mode (TO) of

Si, further confirming the NWs are silicon. The high

intensity and symmetric nature of the peak indicate the

formation of highly crystalline NWs with uniform diameter

[26, 27]. Due to the Heisenberg uncertainty principle, the

fundamental q = 0 Raman selection rule is relaxed for a

finite-size domain, allowing the participation of phonons

away from the Brillouin-zone center. The phonon uncer-

tainty goes roughly as Dq = 1/d, where d is the diameter of

the NWs. This gives a downshift and asymmetric broad-

ening of the Si Raman peak at 520 cm-1. As shown in

Fig. 7, the NW peak at 520 cm-1 upon the irradiance of

laser having power of 0.17 mW moves to 521 cm-1 when

the laser power decreases to 0.0017 mW, is in good

agreement with the work done by Piscanec et al. [27]. The

NW peaks are also broadened comparing to the bulk Si-

peak at 520 cm-1 taken at 0.17 mW. The downshift in

frequency and broadening of peaks at high laser power may

be attributable to the phonon confinement effects in the

NWs [28, 29], further confirming the formation of SiNWs.

Summary and conclusions

A new method was used to synthesize SiNWs. HR-TEM

and SAED analyses clearly show the formation of single

crystalline SiNWs with core–shell morphology and Raman

spectroscopy analysis confirms the quantum confinement

effect. All the results show that SiNWs of high aspect ratio

and uniform size can be synthesized on Si wafers by a

simple oxidation and reduction process. This technique is

simple, cost-effective, easy to be scaled up, and has the

advantage of elimination of the use of metal catalysts, toxic

gases, and costly equipment, thus holding great potential

for practical applications.

Acknowledgements This work was supported by Foreign Affairs

and International Trade Canada (DFAIT) under Commonwealth

Graduate Student Exchange Program Scholarship (2011–2012). Q.Y.

and A.H. acknowledge the support from NSERC and Canada

Research Chair Program. S.K.B. and Q.Y. acknowledge the technical

assistance from Dave McColl, Plasma Physics Laboratory, Rob

Peace, Department of Mechanical Engineering and Jason Maley,

SSSC, University of Saskatchewan, Canada. S.K.B. and O.J.

acknowledges Prof. T. Harinarayana, Director, GERMI Research,

Innovation and Incubation Centre, India. The EM research described

in this paper was performed at the Canadian Centre for Electron

Microscopy at McMaster University, which is supported by NSERC

and other government agencies.

References

1. Cui Y, Zhong Z, Wang D, Wang WU, Lieber CM (2003) Nano

Lett 3:149

2. He R, Yang P (2006) Nature Nanotechnol 1:42

3. Chan CK, Peng H, Liu G, Mcllwrath K, Zhang XF, Huggins RA,

Cui Y (2008) Nature Nanotechnol 3:31035

4. Granett E, Yang P (2010) Nano Lett 10:1082

5. Pignalosa P, Lee H, Qiao L, Tseng M, Yi AY (2011) AIP

Advances 1:032124

6. Kalita G, Adhikari S, Aryal HR, Afre R, Soga T, Sharon M,

Koichi W, Umeno M (2009) J Phys D 42:115104

7. Schmidt V, Wittemann JV, Senz S, Gosele U (2009) Adv Mater

21:2681

Fig. 6 XRD pattern of SiNWs grown on p-Si (1 0 0) wafer

Fig. 7 Raman spectra of SiNWs using different laser powers

3596 J Mater Sci (2014) 49:3592–3597

123

8. Wu Y, Cui Y, Huynh L, Barrelet CJ, Bell DC, Lieber CM (2004)

Nano Lett 4:433

9. Ferry DK (2008) Science 319:579

10. Colli A, Hofmann S, Fasoli A, Ferrari AC, Ducati C, Dunin-

Borokowski RE, Robertson J (2006) Appl Phys A 85:247

11. Kim BS, Koo TW, Lee JH, Kim DS, Jung YC, Hwang SW, Choi

BL, Lee EK, Kim JM, Whang D (2009) Nano Lett 9:864

12. Wang N, Tang YH, Zhang YF, Lee CS, Lee ST (1998) Phys Rev

B 58:R16024

13. Pan ZW, Dai ZR, Xu L, Lee ST, Wang ZL (2001) J Phys Chem B

105:2507

14. Garnett EC, Liang W, Yang P (2007) Adv Mater 19:2946

15. Schmidt V, Wittemann JV, Gosele U (2010) Chem Rev 110:361

16. Holmes JD, Johnston KP, Doty RC, Korgel BA (2000) Science

287:1471

17. Schubert L, Werner P, Zakharov ND, Gerth G, Kolb FM, Long L,

Gosel U, Tan TY (2004) Appl Phys Lett 84:4968

18. Morales AM, Lieber CM (1998) Science 279:208

19. Niu J, Sha J, Yang D (2004) Physica E 23:131

20. Yang HJ, Yuan FW, Tuan HY (2010) Chem Commun 46:6105

21. Zhang RQ, Lifshitz Y, Lee ST (2003) Adv Mater 15:635

22. Wang N, Tang YH, Zhang YF, Yu DP, Lee CS, Bello I, Lee ST

(1998) Chem Phys Lett 283:368

23. Wang N, Tang YH, Zhang YF, Lee CS, Bello I, Lee ST (1999)

Chem Phys Lett 299:237

24. Menga F, Li J, Hong Z, Zhia M, Sakla A, Xianga C, Wua N

(2013) Catal Today 199:48

25. Lu J, Zeng X, Liu H, Zhang W, Zhang Y (2012) J Phys Chem C

116:23013

26. Dhar S, Giri PK (2011) Int J Nanosci 10:13

27. Piscanec S, Ferrari AC, Cantoro M, Hofmann S, Zapien JA,

Lifshitz Y, Lee ST, Robertson J (2003) J Mater Sci Eng C 23:931

28. Compaan A, Lee MC, Trott G (1985) Phys Rev B 32:6731

29. Jellison GE, Modine FA (1983) Phys Rev B 27:7446

J Mater Sci (2014) 49:3592–3597 3597

123