Highly efficient and stable photoluminescence from silicon nanowires coated with sic

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

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22 December 2000

Chemical Physics Letters 332 (2000) 215-218

CHEMICAL PHYSICS LETTERS

www.elsevier.nl/locate/cplett

Highly efficient and stable photoluminescence from silicon

nanowires coated with SiC

X.T Zhou, R.Q Zhang, H.Y Peng, N.G Shang, N Wang, I Bello, C.S Lee,

S.T Lee *

Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films (COSDAF),

City University of Hong Kong, Hong Kong, People’s Republic of China

Received 23 June 2000

Abstract

A reaction of silicon nanowires (SiINW) with methane and hydrogen has been performed to produce a thin coating layer of cubic silicon carbide (B-SiC) using an ion beam deposition technique High resolution transmission electron microscopy (HRTEM) showed that silicon oxide shells originally cladding the as-grown SiNW were removed and replaced by a thin layer of nano-sized crystals of B-SiC This has led to stable photoluminescence (PL) observed from the SIC-coated SINW with high efficiency almost tripled as compared with that before SiC coating © 2000 Elsevier Science B.V All rights reserved

In the past decade, intensive studies on porous

silicon related nano-technology have been stimu-

lated by its potential applications in silicon-based

optoelectronic devices [1-12] One-dimensional

silicon nanowire in the same family 1s considered

more promising for its inherent quantum confine-

ment effect in the other two dimensions [13]

Large-scale synthesis, which is always an essential

requirement for wide applications, of silicon

nanowire (SINW) has recently been achieved rou-

tinely [14,15] However, problems of degradation

[2,16,17] and low photoluminescence (PL) effi-

ciency [18] from the silicon nano-structures remain

unsolved The PL degradation, usually accompa-

nied with a blue-shift of the PL peak position, 1s

generally believed to originate from the chemical

“ Corresponding author Fax: +852-2784-4696

E-mail address: apannale@cityu.edu.hk (S.T Lee)

instability of the surfaces [2,16,17] Upon exposure

to the atmosphere, the thickness of the surface silicon oxide layer increases, and leads to increased absorption and reflection of the incident light This eventually leads to degradation of the PL intensity [16,17,19] At the same time, the oxidation process reduces the size of the core Si nano-crystallites and thus results in the blue shift [16,17] Before large- scale application 1s possible, this instability prob- lem has to be solved One possible solution is to passivate the silicon surface Among the existing passivation methods, a nitridation is generally believed to be an advantage [20] However, there 1s

a technological complication involving the re- moval of the surface oxide layer 1n order to achieve

a satisfactory nitriding effect In addition, a dia- mond-like carbon film has been used to coat po- rous silicon [21] Nevertheless, the problem of stability has not yet been satisfactorily solved and further effort is still needed to make the promising 0009-2614/00/$ - see front matter © 2000 Elsevier Science B.V All rights reserved

PII: S0009-2614(00)01145-3

216 X.T Zhou et al | Chemical Physics Letters 332 (2000) 215-218

silicon nano-structures applicable for the widely

desired nano-technology

Silicon carbon (SiC) is a semiconductor with

various merits including a wide energy band gap, a

high breakdown electric field, and a high satura-

tion electron drift velocity Furthermore, it is du-

rable at high temperature and possesses high

thermal conductivity and chemical stability In

addition, it has good mechanical properties, such

as high tensile strength and a high Young’s mod-

ulus which facilitate its application as a protective

coating material Such a material should be ideal

for the integration with silicon in optoelectronic

devices In this Letter, we show that SiNW can be

coated with B-SiC so as to integrate the various

advantages of the SiC with the important nano-

scaled materials in order to advance its applica-

tions in photophysics and device physics

SINW were synthesized by thermal evapora-

tion from a mixture of silicon and silicon dioxide

powder [14,15] The SiNWs deposited on a silicon

water have an average diameter of about 20 nm,

as shown in Fig | High resolution transmission

electron microscopy (HRTEM) shows that the

SINW consists of a core of crystalline silicon

clothed by a shell of silicon oxide with a thickness

up to half of the wire radius (Fig 1b) A micro-

Raman spectrometer (Ranishaw 2000, wavelength

of laser: 514 nm) was used to characterize the PL

properties of the sample Very weak PL intensity

was obtained from the as-grown SiINW sample

(Fig 2a) The PL peak centers at around 630 nm

(2.0 eV) No PL peak relating to the presence of

the silicon oxide was found The low PL intensity

is likely due to the presence of the silicon oxide

Shell, which covers the SiNWs The silicon oxide

can absorb and reflect both the excitation inci-

dent light as well as the emitted PL light Al-

though some kinds of silicon oxide can also give

visible PL, it is difficult to obtain a suitable

composition of silicon oxide required for efficient

luminescence Therefore, it is desirable to miuni-

mize or eliminate the silicon oxide in order to

obtain silicon nano-structures with efficient PL

To address such a need, the following experiment

has been done

The as-grown SiNW sample was transferred to

an ion beam deposition chamber with a base

a SẺ

1

>

.“.< V1) Để

"A! +

SORES

Fig 1 Typical TEM 1mages of SINW: (a) morphology at low magnification; (b) morphology at higher magnification Inset shows the corresponding electron diffraction pattern

1000

800 -

600 -

200 - 0 + | _

Wavelength (nm)

Fig 2 PL from: (a) as-grown; (b) 1on beam treated silicon nanowire sample

vacuum of 2 x 10~’ Torr A broad-beam Kaufman

ion source fed with a mixture of methane (>99.9%), hydrogen (>99.999%) and argon (>99.995%) was used to deposit thin films onto the

X.T Zhou et al | Chemical Physics Letters 332 (2000) 215-218 217

SiNW sample The three gases were mixed in a

ratio of CH4:H»:Ar= 1:50:173 The total flow rate

was 2 sccm and the deposition pressure was kept at

about 5x 10-* Torr Ions from the Kaufman

source were accelerated to bombard the SiINW

with an accelerating voltage of 200 V The ion dose

as measured by a Faraday cup placed next to the

SiNW sample was 3 x 10!° cm~* The substrate

temperature measured by an infrared pyrometer

was 700°C

The TEM image and selected area electron

diffraction (SAED) pattern as shown in Fig 3a

indicate that a cubic silicon carbide layer has been

formed just outside the silicon nanowire The

HRTEM images (Fig 3b,c) show that a few B-SiC

nanoparticles contact the core of the SiNW di-

rectly for some nanowires (Fig 3b), and that a

very thick outer layer composed of B-SiC nano-

particles exists for the other nanowires (Fig 3c)

There is no clear evidence of the presence of the

silicon oxide layer in the ion bombarded SiNW

after the ion beam deposition (Figs 2b and 3c) It

would be of great importance that the previously

observed silicon oxide layer covering the silicon

nanowire has been removed after the coating of

the silicon carbide

It is of particular interest that the undesirable

oxide layer was replaced by a thin layer of stable,

wide band gap semiconductor This new shell

would allow the high transmission of both the ex-

citation light and the PL emission with much less

loss It is believed that during the deposition pro-

cess, hydrocarbon and hydrogen ions react with

silicon oxide to form carbon oxide and silicon car-

bide The carbon oxide gas was pumped out while

silicon carbide remained on the silicon nanowire

Fig 2b shows the PL spectrum of the SiC-

coated SiNW sample It can be seen that the PL

intensity has been increased by about three times

The broadening of the PL peak towards higher

energy is probably due to the reduction of the di-

ameters of the SINW upon ion bombardment The

PL peak is in a similar region to that of the porous

silicon, showing no apparent change difference

when the dimension of the silicon nano-structure is

increased from zero to one

It is well known that SiC is relatively inert to air

and thus can prevent the silicon nanowire core

SiC(200) Peale 111)

a

i

SiC(311)

Fig 3 Typical TEM images of SiINW after ion beam treat-

ment: (a) morphology at lower magnification; (b) morphology

at higher magnification

from further oxidation Therefore, the coated SiNW are likely to be very stable We have mea- sured PL of the coated SiNW after prolonged

exposure to a laser and exposure to the atmo- sphere (75 days) No obvious PL degradation was

218 X.T Zhou et al | Chemical Physics Letters 332 (2000) 215-218

observed The present results indicate that the

ion beam SiC coating process may find practical

usage in fabricating silicon nano-devices or photo-

devices

Although there have been reports on the PL

from SiC nanostructures [22,23], the nano-scale

SiC layer on the silicon nanowire shows no no-

ticeable PL The absence of PL relating to SiC

nano-structures may be due to insufficient excita-

tion in our PL measurement

In conclusion, silicon carbide coating of SiNW

can be achieved by ion beam deposition The

coated SiNW show enhanced performance in

terms of both the intensity and stability of PL

Acknowledgements

Financial support from the Research Grant

Council of Hong Kong under Grant No 9040365

is gratefully acknowledged

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