材料表征论文-GrowthofSiCNanorodsonSiSubstrate

Growth of SiC Nanorods on Si Substrate

Abstract

Silicon carbide (SiC) is a ™-™ compound semiconductor material

with a wide band gap. Semiconductor electronic devices and circuits

made from SiC are presently being developed for high-temperature,

high-power, and high-radiation conditions in which conventional

semiconductors cannot adequately perform. One-dimensional SiC, such

as nanowires and nanorods, is of great interests for many applications due

to their excellent properties, such as high mechanical strength, high

thermal stability, high thermal conductivity. Especially SiC nanorods are

widely considered as reinforcement materials for ceramic composites

providing very high strength and toughness due to their very high elastic

modulus and strength over their bulk-counterpart. In this study, the SiC

nanorods were fabricated by vapor-liquid-solid (VLS) mechanism on Si

substrate. The SiC nanaorods were characterized by scanning electron

microscopy (SEM), X-ray diffraction (XRD)

and energy diffraction

spectrometer (EDS).The factors which influenced the formation of SiC

nanorods were studied.

Keywords: SiC nanorods, VLS mechanism, CVD

Introduction

Silicon carbide (SiC) is a wide band gap semiconductor with many

super properties, such as high breakdown field，high thermal conductivity,

high saturation drift velocity, low relative dielectric constant and

excellent resistance to oxidation and corrosion

[1-2]

. These outstanding

properties make SiC a very attractive semiconductor material. For

example, SiC is commercially applied for optoelectronic devices

[3]

, such

as photodiodes and light-emitting diodes which emit throughout the

visible spectrum into the ultraviolet. The applications of SiC also cover

the area of high-temperature sensors, high-power devices, and microwave

devices (both avalanche diodes and field effect transistors).

In the meantime, since carbon nanotubes emerged into the scientific

world in 1991 and their exceptional excellent properties were introduced,

one-dimensional nanomaterials such as SiC, GaN, have attracted much

interest from researchers because the extreme geometry of the

nanomaterials is of importance to investigate the physical and chemical

properties of the materials such as their quantum size effect. These

nanosized materials are important for ceramic nanocomposite materials

[4,

5]

. They are also claimed to be promising raw materials for engineering

ceramic devices offering superplasticity and high strength at high

temperatures. Furthermore nanoscale filters or support for a catalytic

surface might be interesting application of SiC nanopowders.

2

A lot of methods have been developed to synthesize SiC nanorods

[6]

.

SiC nanorods can be fabricated without the metallic catalysts. For

instance, Zhou

[7]

fabricated SiC nanowires by the hot filament chemical

vapor deposition (CVD) method. B.-C. Kang synthesized SiC nanorods

by CVD method. Li

[8]

synthesized SiC nanowires by using a SiC rod as

the anode to arc-discharge. And Hyung Suk Ahn

[9]

synthesized SiC

nanorods by using LPCVD. SiC nanorods can be also fabricated with the

metallic catalysts. For example, B.-C. Kang fabricated SiC nanorods by

using nickel as a catalyst. And Zhang

[10]

et al. synthesized SiC nanorods

using Fe powders as the catalyst. Among these methods, carbothermal

reduction of silica-containing materials and the CVD method are the most

commonly employed.

In carbonthermal reduction process, three mechanisms are involved

to form SiC nanorods. They are called vapor-solid (VS) mechanism,

two-stage growth (TS) mechanism and vapor-liquid-solid (VLS)

mechanism.

[11]

By the VS mechanism, the nanorods are grown by direct

accommodation of silicon and carbon atoms to the growth plane from the

silicon- and carbon-carrying vapors. The nanorods are formed in the raw

materials containing metal impurities such as rice-hulls by the TS

mechanism. The impurities form discrete liquid droplets on the growth

plane. The droplets are quickly covered with vapor species because of

their high accommodation coefficient and act as nucleation sites for the

3

nanorods growth. It results in axial growth of nanaorods (first stage), and,

then in lateral thickening (second stage)

[11, 12]

. The essential features of

VLS mechanism can be expressed as the growth of nanorods via the

assistance of liquid solution containing the desired ingredient of the

nanorods to be grown. The processes are complex and the fundamental

issues remain to be ascertained. The growth of nanorods involves the

dissolution of solute at the vapor/liquid interface and its subsequent

precipitate at the liquid/solid interface during the VLS growth process. In

this paper, nickel was used as a metallic catalyst to deposit SiC nanorods

on Si substrate via the VLS mechanism.

Experiment

SiC nanorods were fabricated in metalorganic chemical vapor

deposition (MOCVD) system. The water-cooled reactor, as shown in

Fig. 1 schematic configuration of MOCVD reactor

4

Fig.1, was a horizontal quartz tube. First, nickel thin film with thickness

of 400∗500 nm, which acted as a catalyst in growing SiC nanorods, was

deposited on Si substrate by DC sputtering. The Si substrates covered

with nickel thin flim were set on a SiC-coated graphite susceptor, which

was heated by ratio frequency (RF) induction. According to Ni-Si and

Ni-C phasediagram

[13-15]

, the growth temperature was selected between

1250ε and 1380ε . Silane (SiH

4

) and acetylene (C

2

H

2

) were used as

source gas. Hydrogen (H

2

) gas purified by a Pd purifier was used as the

carrier gas. The flow rate of H

2

was fixed to be 500 sccm (standard cubic

centimeter per minute). And the growth pressure of SiC nanorods was

fixed to be 60 Torr.

Two processes were carried out to synthesize SiC nanorods. One

was called two-step process, in which only C

2

H

2

was first introduced into

the reactor to fabricate carbon nanotubes on the Si substrates covered

nickel thin film at 1150ε for several minutes. Then the growth

temperature increased to 1150ε ∗1350ε , and C

2

H

2

and SiH

4

were reacted

as the source gas to synthesize SiC nanorods. Another process was called

one-step process, in which C

2

H

2

and SiH

4

as the source gases were

introduced into the reactor at the same time.

The crystal structure of SiC nanorods was characterized by X-ray

diffraction (XRD). The morphology of SiC nanorods was characterized

by scanning electron microscopy (SEM). Energy dispersive spectrometer

5

(EDS) was carried out to identify their chemical composition.

Results and discussion

nanorods synthesized by two-step process

By the two-step process, carbon nanotubes were first synthesized on

the Si substrate in the carbonized process. The morphology and the

composition characterized by SEM and EDS, were shown in Fig.2 and

Fig.3 respectly. The Ag peak appeared in the EDS image was introduced

in the experiment during the SEM and EDS analysis. According to the

figure, high density of carbon nanotubes was grown on the Si substrate.

Fig. 2 SEM images of carbon naotubes grown on a Si substrate

Fig. 3 EDS spectrum of carbon nanotubes

grown on a Si substrate

6

The XRD spectrum of SiC nanorods synthesized by two-step process was

shown in Fig.4. In the XRD patterns, characteristic peaks from (111),

(200) and (220) plane of ß-SiC appeared at 35.68°, 47.68° and 60.16°,

respectively. Peaks from other polytypes of SiC were not observed, so the

SiC nanorods were zinc-blende structure. The morphology of SiC

nanorods was depicted in Fig.5. High density of nanorods was randomly

grown on the substrate. The diameters of SiC nanorods were almost the

same.

2. SiC nanorods synthesized by one-step process

The characters of the SiC nanorods fabricated by one-step process

were characterized by XRD, SEM and EDS. The results are shown in

Fig.6∗8 and table 1. In general, SiC nanorods were synthesized by

one-step process.

Fig. 4 XRD patterns of SiC nanorods

fabricated by two-step method

Fig. 5 SEM image of SiC nanorods

synthesized by two-step method

7

Although all the samples were growth by VLS mechanism, it was

clearly that the diameter of the SiC nanorods fabricated by one-step

process was much larger than that prepared by two-step process. The

reasons should be the confinement effect of carbon nanotubes in two-step

process. The size of carbon nanotubs limited the lateral growth of SiC

nanorods and led to the diameter of SiC nanorods almost the same as that

of carbon nanotubes. For the one-step process, however the main factors

which determined the diameter of SiC nanorods should be the volume of

liquid droplet and wetting behavior

[16-17]

, so the diameter of SiC nanorods,

Fig. 6 XRD pattern of SiC nanorods

fabricatedby one-step process

Fig. 7 SEM image of SiC nanorods

fabricated by one-step process

Table 1: the content of SiC nanorods

fabricated by one-step process

ElementWeight% Atomic%

C K 55.16 69.63

O K 18.36 17.39

Si K 22.34 12.06

Ni K 2.89 0.75

Ag L 1.25 0.18

Totals 100.00

Fig.8 EDS spectrum of SiC

nanorods by one-step process

8

was much larger.

The atomic content of carbon was higher than that of silicon for the

SiC nanorods made by one-step process as shown in table 1. It should be

originate from the result of C

2

H

2

activity. Because of its high activity,

C

2

H

2

should be decomposed very quickly at 1250ε . A lot of carbon

atoms deposited on Si substrate. However, not enough silicon atoms

reacted with them. So the redundant carbon atoms formed amorphous

carbon on the substrate.

Summary

SiC nanorods were successfully synthesized via VLS mechanism by

two-step process and one-step process. The structure, morphology and

composition were characterized by XRD, SEM and EDS. Factors which

affected the diameter of SiC nanorods were discussed.

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