Blue Luminescent Properties of Silicon Nanowires Grown by a Solid-Liquid-Solid Method

Blue Luminescent Properties of Silicon Nanowires Grown by a Solid-Liquid-Solid

Method*

PENG Ying-Cai(彭英才)1,2**,FAN Zhi-Dong(范志东)1,BAI Zhen-Hua(白振华)1,ZHAO Xin-Wei(赵新为)3,

LOU Jian-Zhong(娄建忠)1,CHENG Xu(程旭)1

1College of Electronic and Informational Engineering,Hebei University,Baoding071002 2Key Laboratory of Semiconductor Material Science,Institute of Semiconductors,Chinese Academy of Sciences,

Beijing100083

3Department of Physics,Tokyo University of Science,Tokyo162-8601,Japan

(Received26January2010)

Silicon nanowires(SiNWs)were grown directly from n-(111)single-crystal silicon(c-Si)substrate based on a solid-liquid-solid mechanism,and Au film was used as a metallic catalyst.The room temperature photoluminescence properties of SiNWs were observed by an Xe lamp with an exciting wavelength of350nm.The results show that the SiNWs exhibit a strongly blue luminescent band in the wavelength range400–480nm at an emission peak position of420nm.The luminescent mechanism of SiNWs indicates that the blue luminescence is attributed to the oxygen-related defects,which are in SiO x amorphous oxide shells around the crystalline core of SiNWs.

PACS:73.21.Hb,78.55.Ap DOI:10.1088/0256-307X/27/5/057305

Quasi-one dimensional nanostructures-SiNWs have been the subject of great interest because of their unique physical properties and potential ap-plications in optoelectronic devices,which are?eld emission devices,single electron memory,e?cient lasers and solar cells.[1?3]To date,many methods for fabricating SiNWs have been reported,such as ther-mal chemical vapor deposition(CVD),laser ablated deposition(LAD),physical evaporation and electron beam evaporation(EBE).[4?6]At the same time,in-?uences of experimental conditions on their structural charateristics have also been studied.More recently, our group have grown the SiNWs with diameters of 30–60nm by solid-liquid-solid(SLS)method.[7] During the past decade,photoluminescence(PL) properties of SiNWs have been investigated exten-sively.In the early works,Feng et al.[8]reported the synthesis and PL properties of SiNWs,and found the blue emission of420nm,green emission of450nm and red emission of786nm,respectively.Holmes et al.[9] studied the PL properties of SiNWs,and indicated that the blue luminescence of3.75eV is attributed to the oxygen-related defects.The red luminescence at SiNWs has con?rmed that the light-emission band in the range1.70–1.73eV is dominated by oxygen-related defects near SiO x/Si interface,while light-emission band in the range1.8–2.10eV is due to nonbridging oxygen hole center(NBOHC)in the SiO x layer and on surface of SiNWs.[10]Qi et al.[11]grew SiNWs with a diameter of～20nm by laser ablated deposition, and showed that the blue emission of peak position at 455nm and green emission of peak position at525nm are independent of quantum con?nement.The silver-capped SiNWs were fabricated via electroless metal deposition on a silicon wafer,and a strong ultraviolet-emitting property with an emission peak at330nm was observed.[12]In this Letter,room-temperature PL properties of SiNWs are observed by an Xe lamp with an emission wavelength at350nm,and the origin of blue luminescence is also discussed.

The n-type(1.0–1.5?·cm)single silicon substrates with(111)orientation were cleaned in a solution of 2%HF for5min.The cleaned samples were dried and transferred into a vacuum coater,where Au cata-lyst?lms of5–20nm were thermally evaporated.The samples were then placed in a quartz tube for an-nealing.Puri?ed N2was?owed into the quartz tube throughout the annealing process.Typical synthetic conditions for SiNWs were N2?ow rate of1.5L/min, annealing temperature of1100°C and growth time of 60min.The morphology and microstructure of the as-grown SiNWs were observed by an S-4800scanning electron microscope(SEM).The chemical composi-tions presented in the SiNW S were analyzed using the energy-dispersive x-ray spectroscopy(EDS).The room temperature PL properties of SiNWs were ob-served by the Xe lamp with an exciting wavelength of 350nm and an exciting power of450W.

Figure1shows the EDS spectra of SiNWs fabri-cated by the SLS method.It is found that Si peak is originated from the body of the wires,rather than from the silicon substrate.The O peak came from the silicon oxide layer.Although N2was?owed through-out the experiment,small amounts of oxygen were

*Supported by the Natural Science Foundation of Hebei Province(E2008000626)and the Key Laboratory of Semiconductor Material Science,Institute of Semiconductors,Chinese Academy of Science(KLSM05-03).

**Email:ycpeng2002@https://www.360docs.net/doc/d515367643.html,

c○2010Chinese Physical Society and IOP Publishing Ltd

believed still presented in the chamber and caused the formation of silicon oxide.Growth of a native oxide layer on the SiNW surface after the experiment also contributed to this signal.Figure 2shows the SEM image of typical SiNWs grown at annealing temper-ature of 1100°C N 2?ow rate of 1.5L/min,Au ?lm thickness of 15nm and growth time of 60min.

0.00.8 1.6 2.4 3.2 4.0 4.8

5.6

I n t e n s i t y (a r b .u n i t s )

Energy (eV)Fig.1.EDS spectra of the SiNWs.

N 2

2 m m

Fig.2.SEM image of the SiNWs.

Figure 3shows the PL spectrum from the as-grown SiNWs,which exhibits a strong blue luminescent band around 400–480nm (2.6–3.1eV),and its emission peak is located at 420nm (2.95eV).In addition,the SiNWs also have a weak red luminessence around 600–650nm (1.91–2.10eV),and its emission peak is located at 625nm (2.0eV).We believe that the strongly blue luminescent band is attributed to oxygen-related de-fects,which are oxygen vacancies located at SiO x amorphous oxide shells,because of small amounts of oxygen contained in our SiNWs samples.It is well known that the SiNWs are two-dimensional quantum con?ned systems similar to silicon nanometer crys-tallites (nc-Si)or silicon quantum dots (Si-QD).The structural characteristics of these silicon-based nanos-tructures can directly in?uence their PL properties.In general,the SiNWs grown by SLS methods are of a core-shell structure,which are a silicon crystalline core,an amorpous SiO x shell layer,and an interfa-

cial region between the silicon core and the SiO x shell layer.Oxygen vacancies in SiO x amorphous oxide shells act as light-emission centers,leading to the blue luminescence of SiNWs.[8,13]However,the weak red luminescent band results from the Si =O double band states or NBOHC located at interfacial regions.[14]It is interesting to note that after the SiNWs were set in air for one month,initial strong and wide blue lumi-nescent band was separated into two narrow lumines-cent peaks of 420nm and 450nm.We believe that the former is due to oxygen vacancy defects in the SiO x amorphous,while the latter comes from the hydrox-ides existing at the surface of SiNWs,for example,H-O or Si-O-H bonds,in which bonds were formed from water molecules presented in air.On the other hand,the initial weak red luminescent band naturally disappeared,as shown in insert Fig.3.[15]

380480580680

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

400500600700

1216

As -growed

SiNW

P L i n t e n s i t y (a r b .u n i t s )

Wavelength (nm)

450nm

420

P L i n t e n s i t y (a r b .u n i t s )

Wavelength (nm)

Fig.3.PL properties of the as-grown SiNWs.

380480580680

P L i n t e n s i t y (a r b .u n i t s )

Wavelength (nm)

Fig.4.Dependence of PL properties on oxidation treat-ment.

The in?uence of oxidation treatment on the PL properties has been studied.The as-grown SiNWs were oxidized in dry oxygen at 1000°C.Figures 4(a)and 4(b)show the PL spectra of SiNWs after ther-mal oxidation at 1000°C for 2min and 4min,respec-

tively.It is found that with increasing thermal oxida-tion times,the PL intensity is signi?cantly decreased.This is due to the fact that oxidation treatment can decrease the numbers or concentration of oxygen va-cancies in the SiO x phase.

380480580680

0.0

0.5

1.0

1.5

2.0

2.5

3.03.5

(b) B -doped SiNW(30min)

(a) As -growed SiNW

(c)

(b)

(a)

P L i n t e n s i t y (a r b .u n i t s )

Wavelength (nm)

Fig.5.Dependence of PL properties on B-doping.

The B-doped SiNWs were obtained by annealing the SiNWs in N 2atomsphere at 1000°C and used B 2O 3pellets (20mm in diameter)as a solid stated doping source.During the annealing process,B atoms are considered to be incorporated into SiNWs.Figure 5shows the e?ect of B-doping on the PL properties of SiNWs.We demonstrated that PL intensity decreases rapidly with the increasing B-doping times,this is due to a nonradiative Auger recombination process result-ing in the decrease of the PL intensity.If B atoms were doped into the SiNWs,Auger process becomes possible,the recombination energy of an electron-hole pair can be transferred to a hole in a neutral acceptor by exciting the hole deep into the valence band.[16]

In conclusion,we have successfully grown the SiNWs on crystal silicon substrates using Au ?lms as a metallic catalyst.The room temperature PL properties of SiNWs have been studied.The results show that the SiNWs exhibit a strong blue lumines-

cent band at 400–480nm with an emission peak at 420nm.The origin of the photoluminescence is due to the oxygen-related defects,which are oxygen vacan-cies in the amorphous SiO x .No luminescence based on quantum con?ned modes is found in our samples.We also examine the in?uence of oxidation treatment and B-doping on the PL properties.The results in-dicate that both oxidation treatment and B-doping would decrease the PL intensity of SiNWs.In order to deeply understand the origin of photoluminescence,it is necessary to study the correlation between lumines-cence and structural characteristics in future works.

References

[1]Bjork M T,Knoch J,Schmid H,Riel H and Riess 2008

Appl.Phys.Lett.92193504

[2]Fang H,Li X D,Song S,Xu X and Zhu J 2009Nanotech-nology 19255703

[3]Peng Z L,Liang S and Deng L G 2009Chin.Phys.Lett .26

127301

[4]Lew K K and Redwing J M 2003J.Crystal Growth 25414[5]Sivakov V,Heroth F and Falk F 2007J.Crystal Growth

300288[6]Fukuda N,Oshima T and Tsurui T 2005

Sci.Technol.Adv.Mater.6628

[7]Peng Y C,Fan Z D,Bai Z H and Ma L 2010Acta Phys.Sin.

591169(in Chinese)

[8]Feng S Q,Yu D P,Zhang H Z,Bai Z G and Ding Y 2000

J.Crystal Growth 209513

[9]Holmes J D,Johnston K P and Doty R C 2000Science 287

1471

[10]Torchynska T V,Rodriguez M M,Espinoza F G B,

Khomenkova L Y,Korsunska N E and Scherbina L V 2001Phys.Rev .B 65115313

[11]Qi J F,White J M and Belcher A M 2003Chem.Phys.Lett.

372763

[12]Qiu T,Wu X L,Yang X,Huang G S and Zhang Z Y 2004

Appl.Phys.Lett.843867

[13]Liao L S,Bao X M,Zheng X Q,Li N S and Ming N B 1996

Appl.Phys.Lett.68850

[14]Wolkin M V,Jorne J,Fouchez P M,Allan G and Delerue

C 1999Phys.Rev.Lett.82197

[15]Prokes S M 1996Appl.Phys.Lett.623244

[16]Mimura A,Fujii M,Hayashi S and Yamamoto K 1999Solid

State Commun.109561