Interplay between phonon confinement effect and anharmonicity in silicon nanowires

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

Physica E 38 (2007) 109–111

Interplay between phonon confinement effect and anharmonicity in

silicon nanowires M.J Konstantinovic´a,b,

a SCK CEN, Studiecentrum voor Kernenergie/Centre d’Etude de l’Energie Nucle´aire, Boeretang 200, B-2400 Mol, Belgium

b Institute of Physics, P.O Box 68, 11080 Belgrade, Serbia

Available online 16 December 2006

Abstract

Getting light out of silicon is a difficult task since the bulk silicon has an indirect energy electronic band gap structure It is expected that this problem can be circumvented by silicon nanostructuring, since the quantum confinement effect may cause the increase of the silicon band gap and shift the photoluminescence into the visible energy range The increase in resulting structural disorder also causes the phonon confinement effect, which can be analyzed with a Raman spectroscopy The large phonon softening and broadening, observed in silicon nanowires, are compared with calculated spectra obtained by taking into account the anharmonicity, which is incorporated through the three and four phonon decay processes into Raman scattering cross-section This analysis clearly shows that the strong shift and broadening of the Raman peak are dominated by the anharmonic effects originating from the laser heating, while confinement plays a secondary role

r2007 Elsevier B.V All rights reserved

PACS: 78.30.Am; 78.20.e; 78.66.Db

Keywords: Nanowires; Silicon; Raman

1 Introduction

For the past 10 years, researchers have tried to coax light

out of silicon, with varying degrees of success The main

problem is that the indirect energy band gap electronic

structure of bulk silicon makes it not suitable for

optoelectronic applications It is expected that this problem

can be circumvented by silicon nanostructuring, since the

quantum confinement effect may cause the increase in the

silicon band gap and shift the photoluminescence into the

visible energy range The expectation that reducing

dimensions of silicon structures would turn this material

from indirect into direct band gap system triggered a lot of

research in the field of optoelectronics However, despite a

large amount of research, the exact origin of the increased

luminescence and a strong Raman phonon softening,

reported in previous works on Si clusters [1–8], are not

fully understood Recently, it was shown [12] that anharmonicity, due to the local heating effect, represents the main source of phonon softening and broadening, while the phonon confinement plays a secondary role Here, I extend this investigation to the silicon nanowires, reanalyze the local heating effect that is always present in these kind of experiments, and compare the results with those in silicon nanoclusters

2 Experiment The sample used in this investigation is made of an array

of silicon nanowires (nanopillars, nanorods) obtained by electrochemical etching process[9].Fig 1shows a scanning electron micrograph of a typical part of the sample Nanowires are vertically aligned with a typical length of about 10 mm and a diameter of about 50–500 nm Some nanorods are found to be detached from the non-reacted part of the silicon crystal, lying in the horizontal position

on the top of the sample Micro-Raman spectra were taken

in ambient conditions with excitation from the 514.5 nm

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doi: 10.1016/j.physe.2006.12.011

Institute of Physics, P.O Box 68, 11080 Belgrade, Serbia.

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E-mail address: konst@phy.bg.ac.yu

line of an Ar laser, using powers at the sample surface that

varied from 10 to 500 mW The Raman spectra were

measured in the backscattering configuration and analyzed

using a DILOR triple spectrometer with

liquid-nitrogen-cooled charge-coupled-device detector

3 Results and discussion

Fig 2 shows typical Raman spectra taken from silicon

nanowires A dramatic change in the spectra is observed

for the moderate increase in the laser power There is a

strong red shift of the first-order phonon mode at

520 cm1, which is accompanied by a substantial

broad-ening, as evident from a series of Stokes and anti-Stokes

Raman spectra taken with laser powers ranging from 10 to

500 mW The softening and broadening are large, up to 30

and 20 cm1, respectively It can be also seen inFigs 2 and

3 that the intensity ratio between the Stokes and

anti-Stokes part of the spectrum decreases as the laser power

increases This implies that a dramatic change in the local

temperature of the nanowires takes place during the

measurements, as expected in micro-Raman experiments

where the laser light is focused on the micrometer-size area

Typically, the silicon bulk samples do not exhibit any shifts

and broadening of the first-order phonon mode at

520 cm1 for the laser powers in the range used in

experiment Moreover, the Raman spectra show the

existence of the symmetric phonon line shapes, regardless

of the frequency shifts and the broadening This

observa-tion is in clear contradicobserva-tion with a strong asymmetric line

shape expected in the case of quantum phonon

confine-ment[10]

A comparison between calculated and measured Raman

spectra is shown inFig 3 The calculated curve is obtained

by taking the Lorentz line shape that includes the

anharmonic effects via three and four phonon decay

processes[11,12]:

oðk; T Þ ¼ oðkÞ þ DðTÞ,

DðTÞ ¼ A 1 þ 2

e_o=2kB T1

e_o=3kB T1þ

3

ðe_o=3kB T1Þ2

,

oðkÞ ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1:7 þ cosðpk=2ÞÞ105

q

, where o(k) is the silicon phonon dispersion at 300 K The phonon line width is given by

e_o=2kB T1

e_o=3kB T1þ

3

ðe_o=3kB T1Þ2

, where A, B, C and D are anharmonic constants The temperature difference between spectra 1 and 2 presented

Fig 1 Scanning electron microscopy picture of the Si-nanowires.

-540 -520 -500 -480 -460 460 480 500 520 540

Raman shift (cm)-1

Fig 2 The Stokes and anti-Stokes Raman spectra of silicon nanowires measured with different laser line power densities.

anti-Stokes

Fig 3 The Stokes and anti-Stokes Raman spectra of silicon nanowires measured at two different laser powers The full lines are calculated spectra.

in Fig 3 is estimated from the Stokes and anti-Stokes

intensity ratio to be around 600 K

The agreement between calculation and experimental

data is very good, showing that the shift and broadening

arise mainly due to local laser heating effect The expected

peak asymmetry, due to phonon confinement effect, is not

observed

Similar results are obtained in the case of silicon

nanoclusters [12] The small peak asymmetry observed in

the case of silicon nanoclusters at the low-frequency side of

the peak, is in the spectra ofFig 3 represented by a

low-intensity hump The fact that this feature is suppressed in

nanowires in comparison to nanoclusters suggests that it

might originate from the Raman scattering of amorphous

silicon This can be understood as being the consequence of

the difference between the preparation techniques The

silicon nanoclusters were produced by the laser

vaporiza-tion technique, which resulted in the formavaporiza-tion of

nanoclusters on the top of the amorphous film On the

other hand, the nanowires are produced by starting from

the silicon crystalline material (eching of the crystalline

bulk sample) so the amorphous signal is expected to be

much smaller

The Raman spectra of silicon nanowires point to the

main problem related to the optical characterization of

nanostructures: the heat dissipation during the experiment

It is, however, expected that the heat dissipation depends

on the actual size of the wire Moreover, the size

irregularity of the wire sample might enhance the

contribution of the anharmonic decay as well In this type

of experiments, one usually measures the averaged signal from various nano-sized structures, consistent with certain temperature distribution for different wires Because of that, the question of individual wire contribution cannot be addressed since the laser spot size is much larger than the size of a single wire

4 Conclusion This work shows that strong anharmonic effects exist in the silicon sample consisting of an array of nanowires It is found that the shift and broadening of the first-order Raman peak are dominated by the local heating effect, while the confinement plays a secondary role

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