Infrared spectra of 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

Infrared spectra of silicon nanowires Junjie Niu a,⁎ , Deren Yang b

, Jian Shab,c, Jian Nong Wang a, Ming Lib a

School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, People's Republic of China

b

State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People's Republic of China

c

Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China

Received 8 November 2005; accepted 7 June 2006 Available online 28 June 2006

Abstract

Infrared (IR) spectra of the silicon nanowires (SiNWs) with oxide layer are analyzed by introducing the disorder-induced mechanical coupling between the optically active oxygen asymmetric stretch (AS) and inactive oxygen asymmetric stretch (I-AS) modes in terms of the transverse-optic (TO) and longitudinal-optic (LO) vibrational modes The shapes of the IR spectra are similar to that of the reported SiO2, indicating that the SiNWs possess an oxide layer outside The TO frequencies of coupled AS and I-AS are experimentally observed as peak at approximately 1085 cm− 1and its shoulder of 1200 cm− 1, respectively The other TO absorption peaks of∼468 cm− 1,∼480 cm− 1, and∼808 cm− 1are also observed Furthermore, the

intensity of the AS-mode TO band centered at∼1085 cm− 1decreases while those of silicon lattice absorption peaks are enhanced with the crystalline

quality increased

© 2006 Elsevier B.V All rights reserved

Keywords: Infrared spectra; Nanowires; Silicon

1 Introduction

Silicon nanowires (SiNWs), as an excellent candidate material

for smart nano-devices, have raised extensive interests[1–5] In

general, the oxide layer as the sheath of SiNWs is generated

during fabrication[6] Because the oxide layer would undertake

electronic interaction in future nano-devices especially in MOS,

understanding of the structural and compositional characteristics

of oxide sheath on SiNWs becomes more significant In fact, IR

absorption spectra in amorphous SiO2(a-SiO2) or in silicon wafer

with oxide layer have been detailedly studied for many years[7–

11] Three obvious IR absorption peaks of 460 cm− 1, 810 cm− 1,

and 1070 cm− 1in a-SiO2have been confirmed The 1070 cm− 1

peak was much stronger when the disorder degree of a-SiO2

was improved[10] In the analysis of the a-SiO2on silicon wafer,

Kirk detailedly explained the IR spectra by the disorder-induced

mode[8]

For SiNWs, IR absorption of SiO2on SiNWs has been

report-ed[12,13] Hu et al showed an enhanced absorption of SiO2/Si

nanowires around 1130 cm− 1and 1160 cm− 1(LO mode)

com-pared with that of SiO2nano-particles [12] Sun et al studied the stabilities and reactivity of hydrogen-terminated surfaces of SiNWs by FTIR spectroscopy[13] In this letter, we discussed the TO frequencies of∼468 cm− 1,∼480 cm− 1,∼808 cm− 1,

∼1085 cm− 1, and∼1200 cm− 1in IR absorption spectra of SiNWs with oxide layer using the disorder-induced mode indicated by Kirk[8] The IR data demonstrated that SiNWs were covered with

an oxide layer and undertook a similar phenomenon with SiO2 Moreover, the main AS TO band of∼1085 cm− 1was weakened while those of silicon lattice were strengthened with an increase in crystalline quality

2 Experimental procedure SiNWs were prepared by methods of chemical vapor depo-sition (CVD) and evaporation methods as reported previously

[3,14] Sample-I was synthesized by deposition of silane in a CVD system at∼620 °C[3] Sample-II was grown on an Al2O3

substrate at the position of lower temperature zone by the evaporation of SiO particles (purity: 99.99%)[14] Sample-III was grown on a p-type (111) silicon wafer with a resistivity of

∼0.001 Ω cm at the position of higher temperature in the same system of Sample-II The diameter of those SiNWs varied from

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E-mail address: jjniu@sjtu.edu.cn (J Niu).

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doi: 10.1016/j.matlet.2006.06.017

∼30 nm to ∼100 nm and the length was about several

micro-meters The thickness of the oxide layer on a single crystal SiNW

core was about several nanometers The original oxide layer

thickness decreased from sample-I to sample-III, which have

been confirmed by the previous work[3] The morphology and

structure of SiNWs were investigated by scanning electron

microscopy (SEM), transmission electron microscopy (TEM),

selected area electric diffraction (SAED), and X-ray diffraction

(XRD), respectively The IR absorption measurements were

conducted in a Bruker IFS 66v/S Fourier transform infrared

spectrometer (FTIR) with an absorption mode SiNWs on Al2O3

templates were mashed and mixed with high-purity KBr to make

a flat pellet for measurements The data were processed in a

vacuum environment of 4 mBar The spectral resolution was

∼1 cm− 1and the signal-to-noise ratio was 10,000:1 pp in 5 s in

range of 400 cm− 1–4000 cm− 1 The spectra with subtracted

background were used for analysis

3 Results and discussion

Fig 1shows a typical SEM image of the as-grown SiNWs on an

Al2O3template The as-synthesized SiNWs with a uniform diameter

and high purity are observed In addition, SAED and XRD data

indicated that the crystal nature is improved from I to

sample-III with increased temperature [3,14] These data are not shown

because the improvement of crystal quality with increasing temperature

is easily understood[15] The TO bands of sample-I and sample-II are presented inFig 2 The IR spectrum of the float-zone (FZ) silicon with free oxygen in the bottom part of Fig 2 is used to be a standard reference of silicon lattice absorption The peak of∼1085 cm− 1with

high intensity is observed in both samples This peak is characterized as Si–O AS mode in which the adjacent O atoms execute the AS motion

in phase with each other From the data inTable 1, it is clearly seen that the integrated area and peak width decreased from I to

sample-II This can be explained from the analysis of oxide layer on SiNWs grown under different conditions The crystalline quality of sample-II

is better than that of sample-I because of the improvement of tempera-ture Thus, the enhanced crystallinity induced the reducing of amor-phous oxide layer Furthermore, the diameter of sample-II (∼30 nm) is smaller than that of sample-I (∼60 nm) Therefore, the oxide layer area

in sample-II is less than that in sample-I The less oxide layer induces that active asymmetric stretching oxygen decreases and then contrib-utes to the reducing of∼1085 cm− 1 The improvement of crystalline

quality can also be indicated from the change of silicon lattice absorp-tion peaks Compared with the IR spectra of sample-I, the absorpabsorp-tion bands of silicon lattice in sample-I, such as 611 cm− 1, 738 cm− 1, and

891 cm− 1, are obviously improved This demonstrates that the crystal nature of sample-II is better than that of sample-I On the shoulder of

∼1085 cm− 1of sample-I and sample-II, there is a weak peak centered

at∼1200 cm− 1 This vibrational behavior of the TO band is regarded

as the contribution of I-AS mode in which adjacent O atoms execute the AS motion 180° out of phase with each other[16,17] Although the peaks of 1107 and 1224 cm− 1 related to interstitial and precipitate oxygen similar to the bulk might have contribution on peaks of around

∼1085 cm− 1and∼1200 cm− 1, they are blanketed and the value is so

small that could be ignored here Furthermore, the enhancement of

∼1130 cm− 1is not found in our experiments as the previous report

[12] It is well-known that LO modes cannot be observed in normal-incidence infrared absorption spectra because the infrared wave cannot interact directly with longitudinal phonons except for the

oblique-Fig 1 SEM image of SiNWs on an Al 2 O 3 template.

Fig 2 IR absorption spectra of sample-I and sample-II Sample-I was fabricated

in a CVD process at 620 °C, while sample-II was fabricated in a thermal

evaporation process at 1100 °C.

Table 1 Integrated area and peak width of 1085 cm− 1

Fig 3 TO bands of IR absorption spectra for the SiNWs growth on silicon wafer

at 1100 °C The bottom is TO bands of the SiNWs with silicon lattice peaks subtracted.

incidence absorption spectra by using p-polarized light, according to the

Berreman effect[18] Therefore, in our measurements, there are no LO

peaks to appear, such as 1160 cm− 1, 1256 cm− 1, etc It is interesting to

note that our TO AS and TO I-AS absorption peaks of∼1085 cm− 1and

1200 cm− 1are the same as the results of Kirk[8] That means that the

behavior of oxide layer on SiNWs is similar to that of oxide layer on

silicon wafer Therefore, it demonstrates that the as-received SiNWs

contain a quantity of oxide layer on the surface Additionally, the peak

of∼1632 cm− 1inFig 2might be related to Al

2O3template (Al–O stretch) which is used as a substrate or interference of KBr prepared

during the measurements One thing that must be mentioned is the

reproducibility and comparison of FTIR spectra used in experiments A

number of samples with different ratios of SiNWs and KBr are

pro-cessed and display a similar repeated result For comparison, the purity

of SiNWs is very high and both samples contained almost the same

backgrounds including template, KBr, and other impurities Therefore,

the FTIR spectra could be used as a relative comparison to study the

structure of oxide layer on SiNWs

The other TO bands of∼480 cm− 1and∼808 cm− 1are also found in

sample-I and sample-II (Fig 2) Compared with sample-I, the intensity of

peaks of∼480 cm− 1and∼808 cm− 1in sample-II is decreased which is

similar to the∼1085 cm− 1 The decrease is equally due to the reducing of

oxide layer thickness The low-frequency TO band of∼480 cm− 1can be

described as a vibrational mode of rocking of O atom about an axis

through the two Si atoms (R) While the middle-frequency TO band of

∼808 cm− 1can be characterized by symmetrical stretching of the O atom

along a line bisecting the axis formed by the two Si atoms in silicon oxide

(SS) As for the high-frequency TO band of∼1085 cm− 1, the O atom

moves back and forth along a line parallel to the axis through the two Si

atoms Although the LO bands have not been observed, it is still

considered that the TO–LO frequency pairs coupled with AS and I-AS

mode will take a compositive effect on IR absorption bands of around

∼480 cm− 1, ∼808 cm− 1, and 1085 cm− 1, respectively The

corre-sponding vibrational modes of the main TO absorption peaks in SiNWs

are marked in theFigs 2 and 3, which are similar to those in silica[8]

Fig 3shows the IR absorption spectra of SiNWs on a silicon wafer

fabricated by evaporation of SiO at 1100 °C Similar to the above

results, the AS TO band of∼1085 cm− 1is the strongest related to Si–

O, and the integrated area and peak width decreased sharply according

to those of sample-I and sample-II which can be seen in Table 1

Differently, the low-frequency TO band shifts down to the position of

∼468 cm− 1, which is the same as the previous result[12] This should

be caused by the different rocking distance of O atom about an axis

through the two Si atoms induced by different growth processes

4 Conclusion

To summarize, the IR absorption spectra of SiNWs with

oxide layer are analyzed and indicate that the as-received

SiNWs contain a quantity of oxide layer The three major TO bands of∼480 cm− 1,∼808 cm− 1, and ∼1085 cm− 1 are ob-served and analyzed by introducing disorder-induced mechanical coupling between the optically active oxygen asymmetric stretch and inactive oxygen asymmetric stretch modes in terms of the transverse-optic and longitudinal-optic vibrational modes Acknowledgements

This work was supported by the National Natural Science Foundation of China, key project of the Education Ministry of China, and Youth Teacher Fund of Shanghai Jiaotong University (A2306B) We would like to thank Instrumental Analysis Center

of Shanghai Jiaotong University, for their great help in the measurements

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