The effect of radio frequency power on the structural and optical properties of a-CH films prepared
ARTICLES
The effect of radio frequency power on the structural and optical
properties of a-C:H?lms prepared by PECVD
Yequan Xiao
College of Materials and Chemical Engineering,Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials(CTGU),China Three Gorges University,Yichang443002,China;and School of Materials
Science and Engineering,Key Laboratory of Advanced Materials(MOE),Tsinghua University,Beijing100084,China Xinyu Tan,a)Lihua Jiang,b)Ting Xiao,and Peng Xiang
College of Materials and Chemical Engineering,Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials(CTGU),China Three Gorges University,Yichang443002,China
Wensheng Yan
Institute of Microstructure Technology(IMT),Karlsruhe Institute of Technology,Karlsruhe76344,Germany
(Received16August2016;accepted20December2016)
Hydrogenated amorphous carbon(a-C:H)?lms with a designed buffer layer of amorphous
hydrogenated silicon carbide on the substrates were fabricated by plasma enhanced chemical
vapor deposition(PECVD).The effect of radio frequency(RF)power on the structural and
optical properties of a-C:H?lms was investigated.The ratios of sp3to sp2of carbon atoms
and hydrogen contents in the RF power range of75–175W are determined and a similar trend as a function of power.The increase of sp3to sp2ratio leads to the increase of transmittance
and optical gap of a-C:H?lms.a-C:H?lm under an RF power of175W possesses high
transmissive ability(.80%)in the visible wave length,even the highest transmittance value of about94.2%is achieved at the wave length550nm.These results show the optimal a-C:H?lms which are promising for the applications in the area of solar cells acting a window layer and
antire?ection layer.
I.INTRODUCTION
Hydrogenated amorphous carbon(a-C:H)?lms consist of carbon atoms of both graphitic type bonding(sp2)and diamond type tetrahedral bonding(sp3).Similar to their counterparts,like diamond,graphite,and hydrocarbon polymers,The hydrogenated amorphous carbon?lms demonstrate some extraordinary properties such as excellent mechanical hardness,heat conduction,high electrical resistance,high transmittance of infrared(IR) and ultraviolet(UV)light-cut?lter,and optical trans-parency in the infrared region.1–4These properties enable a-C:H?lms to be of wide applications in magnetic media, optical lens,biomedical and micro-/nano-electromechanical devices,and optical coating.5–9In addition,a-C:H?lms are promising as antire?ection coating,p-type window layer for solar cells,10,11and active layer for light emitting diodes(LED),12,13etc.
The hydrogenated amorphous carbon?lms can be prepared by various techniques,such as plasma enhanced chemical vapor deposition(PECVD),14pulsed laser deposition,15ion beam assisted deposition,16sputtering,17and?ltered cathodic vacuum arc.18However,PECVD is the most suitable and widely used technique,which enables the growth of a-C:H?lms on a variety of sub-strates,and in a wide temperature range,especially at room temperature.Moreover,PECVD also produces the uniform deposition of a-C:H?lms over large area.19To our knowledge,the effect of deposition parameter such as substrate temperature,gas?ow,deposition pressure on the structural and optical properties have been widely stud-ied.20,21However,the reports about the effect of radio frequency(RF)power on chemical bonding and optical properties are few.More importantly,the chemical bond-ing and optical properties are useful to the optimization of process parameters and application of a-C:H?lms in different areas.Therefore,the effect of RF power on the structural and optical properties of a-C:H?lms deposited by PECVD has been researched in this work.
In this work,the a-C:H?lms were deposited on three kinds of substrates for different measurements (for example,FTIR test on crystalline KBr and UV–Vis spectrometry test on glass).However,different types of substrate may provide different physical properties and structure of a-C:H?lms.22To exclude the in?uence of different substrates on the structural and optical properties of a-C:H?lms,a buffer layer of a-SiC:H is designed and adopted prior to the deposition of a-C:H ?lms,which can also improve the adherence of a-C:H
Contributing Editor:Mauricio Terrones Address all correspondence to these authors.
a)e-mail:husttanxin@https://www.360docs.net/doc/6e925539.html,
b)e-mail:jlihua107@https://www.360docs.net/doc/6e925539.html,
DOI:10.1557/jmr.2016.522
?lms on the substrates.23,24The fabrications are also conducted by PECVD.As a result,the effect of the RF power on the chemical bonding properties such as sp 3/sp 2,H concentration on the optical properties such as FTIR and transmittance is investigated.In addition,the excellent transmittance results indicate that optimal a-C:H ?lms is suitable for the applications,for example,acting window layer for solar cells.
II.EXPERIMENTS
Via RF-PECVD technique (frequency f 513.56MHz),bilayer a-SiC:H/a-C:H ?lms were deposited on single-crystal silicon (100),crystalline KBr and glass (microscope slides)substrate,respectively [Fig.1(a)].a-SiC:H ?lms were deposited on the three substrates using silane and methane under the conditions of sub-strate temperature of 250°C,(RF power of 150W)and the pressure of 60Pa.The deposition time is 30min.Then,a-C:H ?lms were subsequently deposited on a-SiC:H thin ?lms at different RF power from 75W to 175W the step of 25W.The deposition settings are given in Table I.All depositions were carried out under the optimal param-eters of 250°C and the chamber pressure of approximately 60Pa.Prior to depositions,the background pressure of the chamber is about 3.0?10à4Pa.
Before deposition,to obtain good adhesion of the ?lms,the silicon and glass substrates were cleaned by the following procedure:ultrasonic cleaning in acetone,rinsing with deionized water,ultrasonic cleaning in alcohol,and rinsing with deionized water.The silicon wafers were additionally etched by 25%HF to remove the oxide layer on the substrate surface.
The morphology and thickness of the ?lms were examined by scanning electron microscopy (SEM;JSM-7500F,JEOL Ltd.,Tokyo,Japan)and atomic force microscopy (AFM;INNOVA,Bruker Corporation,Billerica,Massachusetts).Raman spectrometry (HR800,Horiba Jobin Yvon Company,France,Paris)and
Fourier transform infrared spectroscopy (FTIR;NEXUS,ThermoNicolet Corporation,Madison,Wisconsin)were used to characterize the microstructure and bond con ?g-urations of the ?lms.The optical properties of the ?lms were investigated by UV –Vis spectrometry (UV-2550v,Shimadzu Corporation,Kyoto,Japan).
III.RESULTS AND DISCUSSION A.Microstructure analysis
The cross-section image of n-Si/a-SiC:H/a-C:H which a-C:H ?lms prepared at RF power of 75W is shown in Fig.1(b),where one can clearly see the presence of three well-de ?ned regions including n-Si,a-SiC:H,and a-C:H.The respective thicknesses of the a-SiC:H layer and a-C:H layers are approximately 320and 70nm.Figure 2shows the typical top view surface of SEM and typical AFM images of the ?lms prepared at RF power of 175W.The results of SEM show that the a-C:H ?lms have uniform distribution,considerable smooth,homo-geneous morphology,and carbon particle with nanometer in size.As is seen in Fig.2(b),the scale bar is as small as 100nm and the surface morphologies demonstrate that all fabricated a-C:H ?lms homogeneous and compact.There are no visible difference from SEM and AFM images for these a-C:H ?lms deposited at different powers from 75W to 175W.However,the RF power produces an obvious in ?uence on chemical bonding and optical properties as seen in the next
sections.
FIG.1.(a)Schematic view of a-C:H ?lm and a-SiC:H layer on substrate;(b)the cross-section view of n-Si/a-SiC:H/a-C:H.
TABLE I.The deposition parameter for a-C:H.
Parameter
Value
Substrate temperature (°C)250
Background pressure (Pa)Low 5.0?10à4Vessel pressure (Pa)
Approximately 60Radio-frequency power (W)75,100,125,150,175
Methane ?ow (sccm)15Deposition time (min)
45
B.Fourier transform infrared spectroscopy (FTIR)analysis
To study the bonding structure of the ?lms and increase the signal/noise ratio of FTIR measurements,the a-C:H ?lms on the KBr substrates were used to FTIR measurements.Because KBr is transparent from the near ultraviolet to long-wave infrared wavelengths and therefore it has no signi ?cant optical absorption in its high transmission region.The FTIR measurement was conducted in the wave-number interval from 400to 4000cm à1with a resolution of 2cm à1.Figure 3shows the typical FTIR spectra of the a-SiC:H ?lm deposited on KBr substrates as well as the a-C:H ?lms grown on the a-SiC:H/KBr substrates at different RF powers.From Fig.3,it is observed that the characteristic bands of the ?lms:a broad contribution of Si –C bonds is around 780cm à1,a band is around 998cm à1,which is attributed to Si –(CH 2)n –Si bonds,as well as the band is located around 1245cm à1referring to the different con ?gurations of C (sp 2,sp 3),the band corresponding to Si –H bonds is around 2100cm à1.In addition,one corresponding to the stretching vibrations of C –H bonds is in the 2800–3000cm à1area,one connecting with the skeleton vibrations of C –C bonds and with bending vibrations of C –H bonds is in the 1280–1680cm à1area.The measurement results indicate that a-SiC:H ?lm and a-C:H ?lms are successfully achieved,which are further supported by the relevant literature.1,25–28
Figure 4shows the FTIR absorption spectra of the fabricated ?lms.The skeleton vibrations of C –C bonds and bending vibrations of C –H bonds corresponding to absorption peaks in the wavenumbers 1280–1700cm à1region are shown in Fig.4(a).The four main peaks are located at 1350,1400,1450,and 1620cm à1,which correspond to sp 3CH 3,sp 3(CH 3)3,sp 3CH 2,and sp 2C 5C aromatic or ole ?nic,respectively.1,28,29Fig.4(b)exhibits the stretching vibrations of C –H bonds related to absorption peaks in the 2750–3012cm à1region.By the ?tting,it is found that the four Gaussian peaks are at 2855,2890,2920,and 2955cm à1,respectively.
The absorption peaks at 2855and 2890cm à1correspond to sp 3CH 2and sp 3CH 3symmetrical modes whereas the peaks observed at 2955cm à1ascribes to sp 3CH 3asymmetrical mode.In addition,the peak at 2920cm à1refers to sp 3CH or the sp 3CH 2asymmetric mode.1,28–30Figs.4(c)and 4(d)represent the deconvoluted FTIR absorbance spectra of a-SiC:H ?lm and a-SiC:H/a-C:H ?lms at RF powers 175W in the wave number range of 2750–3012cm à1.It shows that the a-SiC:H ?lm has less sp 3CH bonding content,which indicates that the carbon of a-SiC:H ?lm serve as end groups in the bond chain.From the areas of sp 2,sp 3C –C bonds,and C –H bonds peaks,we can estimate the binding ratio of sp 3to sp 2.28,29The total content of H bonded to C,N H ,was estimated from the integrated intensity of the stretching vibrations of C –H bonds,using the following equation:
N H cm à3àá?A Z
a x eT
d x ;
e1Twhere A is a normalization factor proportional to the inverse of the absorption strength and changes on the position of the absorption peak,and a (x )is
the
FIG.2.(a)The typical top view surface of SEM and (b)the typical AFM images of the ?lms prepared at RF power of 175
W.
FIG.3.FTIR transmission spectra of the a-SiC:H ?lm deposited on KBr substrates and a-C:H ?lms grown on the a-SiC:H/KBr substrates at various RF powers.
absorption coef ?cient at each frequency x .Here A 51.35?1021cm à3has been assumed to estimate N H for all the ?lms.30,31The estimated sp 3to sp 2binding ratio and H-content are plotted in Fig.5.It is seen that the sp 3to sp 2ratio value and H-content present an irregular change,which can be described by using the sublantation model.32,33
For deposition of a-C:H from CH 4plasma,the situa-tion is complex since many molecular (CH 4,C 2H 6,C 2H 4,and H 2),radicals and atom (CH 3,CH 2,CH,C 2H 5,and H),ions species (CH 41,CH 31,CH 51,and C 2H 51)interact with the growing ?lm.34,35Among these species,the CH 3radical has a large amount of number density and plays a major role as precursors in the carbon ?lm deposition.At low RF power (or low ion energies),the ?lm deposition process is attributed to the CH 3radicals adsorption physically on the growing ?lm surface (weakly bonded precursor state)and then transfer to the chemically bonded state by means of reactions with hydrocarbon ions.36So the H-content of the ?lms and sp 3to sp 2binding ratio increases at low RF power.With the RF powers increasing,the surface coverage decreases and the content of ions species increases.The ions can dehydrogenate and leave sp 3site.Along with the increase of RF power,H-content of the ?lms and sp 3to sp 2binding ratio begin to decrease.While higher RF power (higher ion energies)lead to the direction incorporation into mainstream,sp 2to sp 3conversion by compression at constant H content and hydrogenation gives sp 3sites.
As a result,the H-content and sp 3to sp 2binding ratio increases again.34,35
C.Raman spectrum analysis
Raman spectroscopy is one of the most popular nondestructive and principal characterization tools to characterize the structural change of carbon materials.For amorphous carbon ?lms,the Raman-spectrum anal-ysis includes deconvolution of the spectrum with two main Gaussian peaks:the G-peak places in the range 1555–1600cm à1,as it does arise from the C 5C sp 2stretch vibrations of aromatic rings and ole ?nic
or
FIG.4.FTIR absorption spectra of the ?lms in the region of (a)1280–1700cm à1,(b)2750–3012cm à1,(c)the deconvoluted FTIR absorption spectra of a-SiC:H ?lm deposited on the KBr substrates at region 2750–3012cm à1;(d)the deconvoluted FTIR absorption spectra of a-C:H ?lms grown on KBr/a-SiC:H complex substrates at r.f.-powers 175W in the wavenumbers 2750–3012cm à1
region.
FIG.5.The variation of H-content and sp 3/sp 2ratio with r.f.-power.
conjugated carbon chains;the D-peak around1350cmà1 is due to the breathing modes of sp2atoms in ring.37,38 The linewidths,positions of the G graphite peak and D disordered peak,and their intensity ratios can reliably predict hydrogenation,optical gap,and other mechan-ical properties.39,40I(D)and I(G)are used to describe the intensity of G graphite peak and D disordered peak.
Figure6shows the Raman spectra of a-SiC:H?lm and a-C:H?lms with different powers on the SiC:H and glass complex substrate.To extract the information from Raman spectra and determine the values of the I(D)/I(G) ratio,D,and G-peak positions,the spectra were decon-taminated with Gaussian function and highlighted with dashed line peaks.It should be mentioned that the rather high nonlinear background(or a wide spectral band) centered approximately near1450cmà1is displayed in all spectra simultaneously with G-and D-bands.It may arise from vibrations of C–H39,41or amorphous graph-ite,42the later line denotes hereafter as A-band.The size of the graphite cluster(La)depends on the intensity ratio of the D band to the G band[I(D)/I(G)]based on the following relationship1,43:
IeDT
IeGT
?c La2;e2Twhere c is equal to;0.0055(for La in Angstrom).
44,45 FIG.6.Raman spectra of a-SiC:H and a-SiC:H/a-C:H?lms deposited on glass with different powers of(a)a-SiC:H?lms,(b)a-SiC:H/a-C:H?lms 75W,(c)100W,(d)125W,(e)150W,and(f)175W.
Figure7(a)shows the I(D)/I(G)integral intensities ratio versus RF power.This ratio values demonstrate an increase with the power from75W to100W reaching the maximum value of1.61.But,the values decrease when the powers are increased from100W to175W. Using the expression(2),the crystallite size La changes from0.5nm to1.75nm as seen from the green line in Fig.7(a).Figure7(b)shows that the G-peak position moves from;1582cmà1to;1568cmà1by increasing the RF power till125W.By this enhancement,the half-width of G-peak increases with the increasing power. At the higher power,the G peak position moves back to;1573cmà1and the half-width values of G-peak reduce with increasing RF power.For the Raman spectra of carbon based materials,the G-peak position,move to higher wave number due to sp2content or cluster size increases.46The half-width of G-peak increases linearly with increasing order of carbon?lms.40,47In combination with FTIR results,it can be concluded that(i)at the initial stage(namely,the power is less than100W),the graphitic clusters size increases and the G-peak position moves to greater wave number whereas the sp2content decreases;(ii)in the middle range of100W–150W,the graphitic clusters size is decreased while the sp2content and the number of graphitic clusters are increased. (iii)from150W to175W,all of the graphitic clusters size,number of graphitic clusters,and the sp2content decreases.The G-peak position moves down.The trend is consistent with the literature.48–50The ratio of integral intensities I(A)/I(G)show a decrease?rst and then an increase with the increasing RF power,which indicates a structural transition from sp2-bonding amorphous graph-ite to crystalline graphite resulting in increased graphitic clusters size at lower power.In contrast,for higher RF power the graphitic clusters size decrease and the number of graphitic clusters increase result in A-peak intensity increase.
D.Optical properties analysis
Figure8(a)shows the transmittance of a-SiC:H?lm, and a-SiC:H/a-C:H?lms with a-C:H?lms prepared at RF power of100W and175W,respectively.The optical and structural properties of a-SiC:H?lm and a-C:H?lms prepared at various RF power are list in Table II.
In contrast to a-SiC:H?lm,the transmittance of a-SiC:H/a-C:H?lms are enhanced.Particularly,the transmittance of a-SiC:H/a-C:H?lms with a-C:H?lms prepared at RF powers175W,reaches94.2%at the absorption wave length550nm(as shown in Table II). It indicates that a-C:H?lms can be grown onto silicon carbide layer as an antire?ection coating.Figure
8(b) FIG.7.(a)The integral intensity ratio I(D)/I(G),I(A)/I(G),and calculated graphitic cluster size La versus r.f.-power;(b)positions and FWHM of G peak versus
r.f.-power.
FIG.8.(a)Transmittance spectra for a-SiC:H?lm and a-SiC:H/a-C:H?lms deposited on glass;(b)optical gap of a-C:H?lms prepared at different r.f.-powers.
shows the optical energy gap of a-C:H?lms deposited on a-SiC:H/glass prepared at different RF powers.The optical gap of a-C:H?lms are calculated by the well-known Tauc equation.1,51Table II lists the optical and structural properties of the fabricated a-SiC:H?lm and a-SiC:H/a-C:H?lms.It can be seen that the optical gaps of a-C:H?lms are in the range of2.89–3.71eV and increase with the increasing of RF power.Based on the discussion above,we can conclude that the transmittance of a-SiC:H/a-C:H?lms have the same variation trend as the increasing the sp3to sp2ratio and H-content. According to the cluster model proposed by Robertson and O’Reilly,52a-C:H contains both sp2and sp3sites, with sp2sites segregated into clusters embedded in a sp3-bonded matrix.The difference between the p–p*gap of the sp2sites and the r–r*gap of the sp3sites creates very strong static disorder?uctuations.The optical gap of a-C:H?lms depends primarily on the sp2fraction, followed by the number of6-fold rings in the graphitic clusters.53The in?uence of RF power on the optical gap of a-C:H here is due to the percentage of sp2coordinates, which changes along with the variations of the RF power. V.CONCLUSIONS
a-C:H?lms/a-SiC:H/substrates were fabricated to investigate the effect of the RF power on the structural and optical properties of a-C:H?lms.The ratios of sp3to sp2of carbon atoms and H contents are determined in the power range of75–175W.The ratios of sp3to sp2of carbon atoms and hydrogen contents in the a-C:H?lms changes as a function of RF powers.The increase of sp3 to sp2ratio leads to the increase of transmittance and optical gap of a-C:H?lms.At the optimal power of175W,the optical transmittance of a-C:H?lms demonstrate the best value with average value of90% in the wave length range of350–900nm,which is higher than the transmittances of a-C:H?lms depos-ited at other powers as well as SiC thin?lms.The highest transmittance value is up to about95%at the wave length550nm.These results show the optimal a-C:H?lms,which are promising for the applications in solar cells acting a window layer and antire?ection layer.ACKNOWLEDGMENTS
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