陳帥,謝燈,丘志仁,Tin Chin-Che,王洪朝,梅霆,萬玲玉,馮哲川

(1.物理科學(xué)與工程技術(shù)學(xué)院,廣西相對論天體物理重點實驗室,光電子材料與探測技術(shù)實驗室,廣西大學(xué),南寧　530004;2.華南師范大學(xué)光電子材料與技術(shù)研究所,廣州　510631;3.中山大學(xué)光電材料與技術(shù)國家重點實驗室,物理科學(xué)與工程技術(shù)學(xué)院,廣州　510275;.Department of Mechanical Engineering,University of Malaya,50603 Kuala Lumpur,Malaysia;5.Department of Physics,Auburn University,Auburn,Alabama 36849,U.S.A.)

基于CVD方法生長在硅基底上立方碳化硅的拉曼散射研究

陳帥1,謝燈2,丘志仁3*,Tin Chin-Che4,5,王洪朝2,梅霆2,萬玲玉1,馮哲川1

(1.物理科學(xué)與工程技術(shù)學(xué)院,廣西相對論天體物理重點實驗室,光電子材料與探測技術(shù)實驗室,廣西大學(xué),南寧530004;2.華南師范大學(xué)光電子材料與技術(shù)研究所,廣州510631;3.中山大學(xué)光電材料與技術(shù)國家重點實驗室,物理科學(xué)與工程技術(shù)學(xué)院,廣州510275;.Department of Mechanical Engineering,University of Malaya,50603 Kuala Lumpur,Malaysia;5.Department of Physics,Auburn University,Auburn,Alabama 36849,U.S.A.)

摘要:立方碳化硅(3C-SiC)薄膜通過化學(xué)氣相沉積(CVD)制備在Si(100)襯底上。本論文主要通過橢偏光譜儀(SE)和拉曼散射儀對3C-SiC薄膜的微觀結(jié)構(gòu)和光學(xué)性能進行進一步的研究。根據(jù)SE的分析獲得3C-SiC薄膜厚度；根據(jù)拉曼散射的分析:可從TO模式和LO模式的線形形狀的擬合得到樣品的相關(guān)長度和載流子濃度。結(jié)果表明:該碳化硅(3C-SiC)薄膜質(zhì)量隨膜厚度增加而得到提高,同時分析了外延層厚度對薄膜特性的影響。

關(guān)鍵詞:3C碳化硅；光譜橢偏儀；拉曼散射；厚度

1Introduction

With the rapid development of the electronics industry and technology,Si as the representative of semiconductor materials in the first generation and GaAs,GaP,InP as the representative of compound semiconductor material in the second generation,have been unable to meet the more demanding needs of the military and the high power segments of our industries.Therefore,silicon carbide(SiC)as the representative of compound semiconductor material in the third generation is developing rapidly.Silicon carbide(SiC)is a wide(2.3～3.3 eV)semiconductor material.At the temperature of 930℃,it still maintains the low intrinsic carrier concentration,high break down electric field(4×10 V/cm),high saturated electron drift velocity(2×10 cm/s),high thermal conductivity(4.9 W/(cm·k))and other features.These features of the SiC are better than those of Si[1].So silicon carbide(SiC)material is an ideal material to replace Si for high temperature,high frequency,and high power applications[2].In this paper,3C-SiC films on Si(100)substrate were prepared by chemical vapor deposition(CVD).We used the method of Raman Scattering to analyze four samples in this paper to study the LO-phonon-plasmon coupled mode in 3C-SiC and to determine the free-carrier concentration and the damping constants in 3C-SiC.Various techniques are discussed in great detail,such as spectroscopic ellipsometry(SE)and Raman scattering which were used to characterize these films.

2Experimental details

The 3C-SiC films considered in this study were grown on Si(100)substrates at Auburn University under normal atmospheric pressure environment by using the CVD system which employs a vertical reactor configuration with a rotating susceptor[3-4].In order to perform the spectroscopic ellipsometery measurements on four of our samples,we have used a variable angle phase-modulated SE system [Sanco SC6200] to assess two parametersΨandΔas a function of the wavelengthλ.The experimental observations were carried out between the spectral range from 250 to 1700 nm(0.73 to 5 eV)in 1 nm steps,at three angles of incident(60°,65°,70°).The confocal Raman microprobe spectroscope instrument made by Renishaw Company in United Kingdom was employed to characterize the 3C-SiC epilayers samples.The micro-Raman measurements were done using the 325 nm line of an Ar-ion laser at room temperature.

3Result and discussion

3.1Spectroscopic ellipsometry and film thickness

In order to accurately determine the thickness of 3C-SiC films,spectroscopic ellipsometry was measured from 250 to 1700 nm(0.73 to 5 eV)at three angles of incident(60°,65°,70°).Here,we utilized the Complete EASE software to fit our SE data.The fitting model used in the analysis can be expressed as:Si(100)substrate // 3C-SiC layer // surface roughness.The complex dielectric function e of 3C-SiC versus photon energy (E) is modeled using Tauc-Lorentz multiple oscillator mode[5]:

εT-L(E)=εn1+iεn2

(1)

Where

(2)

Ampn,Brn,EonandEgnare fit parameters corresponding to amplitude,broadening,center energy and bandgap energy of oscillatorn,respectively.The real part of the dielectric functionnis obtained by exploiting the Kramers-Kronig integrations i.e.

(3)

In Eq.(3)the termPstands for the Cauchy principal part of the integral.Two oscillators were sufficient to obtain a satisfactory fit over the entire spectral range.The best fit results of our simulations for sample 3C713 displayed in Fig.1 compare favorably well with the SE data.The similar results were also derived from other 3C-SiC films(sample 3C121,3C714,3C713,3C414B)with different thicknesses.The thicknesses obtained for 3C-SiC films were given in Table 1.From Table 1,it is obvious that the thicknesses of 3C-SiC films from SE.

Fig.1　SE experimental data, best fit curves, model and result of three 3C-SiC/Si samples

Sample3C1213C7143C7133C414BSE/μm2.1664.9425.6216.120

Fig.2Raman spectra from the three structures of SiC thin film materials

3.2Raman Scattering

Fig.2 shows the Raman spectra from the three structures of SiC thin film materials.The sharp Raman peak at 520 cm-1corresponds to the optical phonon mode of the Si substrate.The Raman peaks appear at near 794 cm-1and 970 cm-1are ascribed to TO and LO phonons of 3C-SiC,respectively[6]; For 4H-SiC the TA,E2(TO),E1(TO)and A1(LO)bands are observed at 200 cm-1,778 cm-1,800 cm-1and 968 cm-1,respectively[7]; For 6H-SiC the TA,TO6/6,TO2/6和A1(LO)bands are observed at 146 cm-1,770 cm-1、792 cm-1and 972 cm-1,respectively[8].The 3C-SiC has no the TA mode[9].Therefore,we can judge the structure of the SiC material by the characteristic peaks of Raman spectra.

Fig.3 shows the room temperature(RT)first-order Raman spectra from four 3C-SiC samples,excited by the 325 nm line from an Ar+laser.The principal peaks are labeled as the active phonon modes.The sharp Raman peak at 520 cm-1corresponds to the optical phonon mode of the Si substrate.The Raman peaks appear at near 794 cm-1and 970 cm-1are ascribed to transverse optical(TO)and longitudinal optical(LO)phonons of 3C-SiC,respectively.Raman lines from Si substrate and 3C-SiC longitudinal optical(LO)and transverse optical(TO)phonon modes are observed.Their relative intensities are varied with the variation of the growth time,i.e.the film thickness,d.The intensities of TO mode and LO mode increase with respect to that of Si mode with increasing film thickness,except sample 3C414B.It may have a higher doping concentration in the sample 3C414B.

Fig.3Raman spectra of four 3C-SiC/Si(100)samples,under the 325 nm excitation

3.3The Raman TO mode and theoretical fit

Raman spectroscopy has merits of non-destructiveness,fast analysis and requiring no special preparation for samples.The Raman parameters such as intensity,width,and peak frequency provide plenty of information on thecrystal quality.In terms of SiC,due to the strong covalence of the bonding,its Raman efficiency is high and the Raman signals can be easily detected.The analysis of the line shape of major phonon modes from Raman spectral measurements can obtain the useful information regarding the material properties,which has been performed using a spatial correlation model.This model was proposed to describe the crystalline quality by introducing a parameter called the correlation length,which is defined as the average size of the region of material homogeneity.According to this spatial correlation model,the first-order Raman spectrumI(ω) can be expressed as[10-11]

(4)

whereqis expressed in units of 2π/a,ais the lattice constant,Lis the correlation length in units ofa,and Γ0is the natural or intrinsic line width.The dispersion relationω(q) for optical phonons can be represented by two analytical forms:

ω2(q)=A+{A2-B[1-cos(πq)]}1/2

(5)

or

ω(q)=A-Bq2

(6)

with A and B as adjustable parameters,and in the paper we use the (6) formula to calculate.Consequently,the calculated Raman spectrum,including the line width Γ(FWHM),can be obtained and fitted to the experimental data.

Fig.4　The Raman TO mode and theoretical fit from 3C-SiC samples

Tab.2Fitted parameters on TO modes for samples 3C121,3C714,3C713 and 3C414B

Sample121714713414BA/cm-1795.5795.5795.3797.0B/cm-1120120120120L/?1316.316.412Γ0/cm-17.94.84.55.0Thickness/μm2.1664.9425.6216.120

Fig.4 shows line shape fit for the TO mode from 3C-SiC samples.Fitted parameters from 3C-SiC samples are listed in Table 2.As can be seen,the correlation length L increases and the line width Γ(FWHM)decreases as film thickness increases,indicating an improvement of the film quality with increasing film thickness.

3.4Raman line shape analysis for the coupling of the LO-phonon

Raman spectral line shape analysis can also lead to an optical determination of an electrical property of SiC,free carrier concentration,i.e.,the doping level.When doped with impurities in SiC,the lattice vibration will be affected by free carriers through their interactions,leading to coupling of the LO-phonon and plasma(LOPC).According to theory of scattering cross section,Raman scattering intensity of LOPC mode can be expressed as[12-13]

(7)

Where

ω2η(ω2+γ2)}

(8)

(9)

andωLis the longitudinal optical mode frequency;ωTis transverse optical mode frequency;ηis phonon damping constant;γis plasma damping constant;n1andn2are refractive indices at incident frequency and scattering frequencyw2respectively;Cis Faust-Henry coefficient,here the value is about 0.35;αis polarizability;Eis macroscopic electric field;nwis Bose-Einstein factor.Dielectric function is described as

(10)

(11)

wherewpis plasma frequency;nis free carrier concentration;m*is effective mass;eis unit charge;ε∞is high frequency dielectric constant,here the value is about 6.38.

Tab.3Fitted parameters on LO-phonon for samples 3C121,3C714,3C713 and 3C414B

Sample121714713414Bωp/cm-1207187187380γ/cm-15.05.05.0100.0η/cm-17.06.26.234.0n/×1017cm-218.33917.95317.95374.132

Fig.5 presents the theoretical fits on LOPC phonon modes of 3C-SiC samples.Fitted parameters from 3C-SiC samples are listed in Table 3.The obtained values of plasma frequencywp,and carrier concentrationn,for four experimental samples are given in the Table 3.It is clear that sample 3C414B has a higher doping concentration than other samples.This causes that the LO mode peak of sample 3C414B is different with other samples.

4Conclusion

In summary,we have grown a series of 3C-SiC films on Si(100)substrate with different growth time by chemical vapor deposition(CVD).The microstructures and optical properties of the films were investigated by SE and Raman scattering as a function of film thickness.From the analysis of SE we can obtain thickness of 3C-SiC films.Raman scattering measurements showed all the characteristic 3C-SiC phonon modes which identify the SiC epilayers as 3C poly-type.By analysis of the line shape of TO mode and LO mode,we obtained the correlation length and carrier concentration,respectively.The correlation length result shows that the film quality improves with increasing film thickness.Raman data reveal that the crystalline quality is expected to improve with film thickness.

Fig.5　Raman line shape analysis for the coupling of the LO-phononand free carrier

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Raman Scattering Studies on CVD Grown Cubic SiC Thin Films on Si

CHEN Shuai1,XIE Deng2,QIU Zhi-ren3*,TIN Chin-che4,5,WANG Hong-chao2,MEI Ting2,WAN Ling-yu1,FENG Zhe-chuan1

(1.CollegeofPhysicsScience&Technology,GuangxiKeyLaboratoryfortheRelativisticAstrophysics,LaboratoryofOptoelectronicMaterials&DetectionTechnology,GuangxiUniversity,Nanning530004,China;2.InstituteofOptoelectronicMaterialandTechnology,SouthChinaNormalUniversity,Guangzhou510631,China;3.StateKeyLaboratoryofOptoelectronicMaterialsandTechnologiesandSchoolofPhysicsandEngineering,SunYat-SenUniversity,Guangzhou510275,China;4.DepartmentofMechanicalEngineering,UniversityofMalaya,50603KualaLumpur,Malaysia;5.DepartmentofPhysics,AuburnUniversity,Auburn,Alabama36849,U.S.A.)

Abstract:Cubic(3C)-SiC films were grown on Si(100)substrate by chemical vapor deposition(CVD).The microstructures and optical properties of 3C-SiC films have been studied by spectroscopic ellipsometry(SE)and Raman scattering.Raman scattering and SE were used to characterize 3C-SiC materials.From the analysis of SE we can obtain thickness of 3C-SiC films.Also from the analysis of Raman spectra,we can see the line shape of TO mode and LO mode from the fitting result,and the correlation length and carrier concentration were obtained.The result of correlation length reveals that the crystalline quality is expected to improve with film thickness increasing and Raman scattering spectra also shows the effects of the epilayer thicknesses.

Key words:3C-SiC；spectroscopic ellipsometry；Raman scattering；thickness

收稿日期:2015-06-10; 修改稿日期:2015-09-12

基金項目:中山大學(xué)光電材料與技術(shù)國家重點實驗室開放課題和國家自然科學(xué)基金(11474365，61377055，61176085);廣西相對論天體物理重點實驗室廣西自然科學(xué)基金創(chuàng)新團隊項目(2013GXNSFFA019001)和國家自然科學(xué)基金“基于鐵電體微疇結(jié)構(gòu)的平板集成高速電光器件研究”(AE0520088)

作者簡介:陳帥(1989-),女,碩士,從事光電材料工作。E-mail:chenshuai7377@163.com 通訊作者:丘志仁(1963-),男,副教授,主要從事超快激光脈沖產(chǎn)生技術(shù)和時間、空間及光譜分辨技術(shù)與應(yīng)用,半導(dǎo)體材料光物理及器件研究等；E-mail:stsqzr@mail.sysu.edu.cn

文章編號:1004-5929(2016)02-0125-06

中圖分類號:O433.3,O484.5

文獻標(biāo)志碼:A

doi:10.13883/j.issn1004-5929.201602006