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Vacuum 129 (2016) 23e30

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Structural evolution and optical properties of hydrogenatedgermanium carbonitride films

Chaoquan Hu a, b, *, Yuan Tian a, Jianbo Wang c, Sam Zhang d, Diyi Cheng a, You Chen a,Kan Zhang a, b, Weitao Zheng a, e, **

a School of Materials Science and Engineering and Key Laboratory of Mobile Materials, MOE, Jilin University, Changchun 130012, Chinab State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, Chinac School of Science, Changchun University of Science and Technology, Changchun 130022, Chinad School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singaporee State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130025, China

a r t i c l e i n f o

Article history:Received 29 January 2016Received in revised form5 April 2016Accepted 6 April 2016Available online 8 April 2016

Keywords:Germanium carbonitride filmsCompositionStructureOptical properties

* Corresponding author. School of Materials ScienLaboratory of Mobile Materials, MOE, Jilin University,** Corresponding author. School of Materials ScienLaboratory of Mobile Materials, MOE, Jilin University,

E-mail addresses: cqhu@jlu.edu.cn (C. Hu), wtzhen

http://dx.doi.org/10.1016/j.vacuum.2016.04.0070042-207X/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

Although the control of bond structure and optical properties in hydrogenated amorphous germaniumcarbonitride films (a-GeC1�xNx:H) is important for technological applications, the composition depen-dence of chemical bonds, especially hydrogen-containing bonds, is not yet well explored. The evolutionin refractive index (n) and Urbach tail width (E0) remains unclear. Here, we show that nitrogen content(CN) exerts a significant effect on bonding structure and optical properties of a-GeC1�xNx:H films. As CNincreases, the fraction of NeH increases, whereas that of CeH and GeeH bonds reduces, and GeeN bondsform at expense of GeeC bonds. The replacement of carbon by nitrogen induces a substantial decrease inn from 3.0 to 2.3 because of decrease in electronic polarizability. With increasing CN, a significant increasein E0 from 198.8 to 327.9 meV takes place. This behavior arises from decrease in dielectric coefficient (ε),rather than the change in the degree of disorder previously believed. The change in E0 is proportional tothe variation in 1/ε2, which agrees well with hydrogen-like atom model. This study discovers that a-GeC1�xNx:H films have the apparent tunability of n and E0 over a wide range, which is useful in con-trolling the optical transmission and absorption characteristics of these films.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, researches on ternary semiconductor filmsincluding germanium carbonitride (GeC1�xNx) and silicon car-bonitride (SiC1�xNx) attracted much attention [1e4] because oftheir wide optical gap [5], strong oxidation [6] and corrosionresistance [7,8], thermal stability (up to 1350 �C) [9,10], highhardness and low friction [11]. These properties make thempromising candidates in applications of antireflective and protec-tive coatings on infrared windows as well as using as antireflectivecoatings on solar cell panels [12e14]. In these applications, thestructural and optical properties are of the utmost importance, as

ce and Engineering and KeyChangchun 130012, China.ce and Engineering and KeyChangchun 130012, China.g@jlu.edu.cn (W. Zheng).

these properties of the coatings determine the ultimate perfor-mance of the devices. Therefore, it is crucial to understand how tocontrol them.

Studies have revealed that in GeC1�xNx and SiC1�xNx films[15,16], there are GeeC (SieC), GeeN (SieN), CeC and CeN bonds.Carbon and nitrogen content dramatically affect the bondingstructure and optical properties of the films. For instance, thenumber of GeeN (SieN) bonds increases, while that of GeeC (SieC)bonds decreases as nitrogen content increases in GeC1�xNx andSiC1�xNx films [17,18]. The SieC bonds gradually form at theexpense of SieN and CeC bonds with increasing of carbon, and thefraction of CeN first increases and then decreases in SiC1�xNx films[19]. Furthermore, researches have shown that GeC1�xNx andSiC1�xNx films are transparent in the region of visible to infraredwavelengths. By controlling the carbon and nitrogen content, therefractive index of SiC1�xNx films can be modulated from 1.7 to 2.2,and the optical gap from 3.5 to 5.0 eV. These tailored opticalproperties are favored in applications in solar cell panels and lowwavelength emitting diodes.

C. Hu et al. / Vacuum 129 (2016) 23e3024

Although studies on the bonding structures and optical prop-erties of GeC1�xNx and SiC1�xNx films have been reported, thefollowing three questions remain: (i) Hydrogen always appears inthe films because of the introduction of hydrogen-containing gasessuch as methane and acetylene during film deposition. However,how hydrogen atoms are bonded to other atoms in GeC1�xNx andSiC1�xNx films remains unanswered. (ii) Although the refractiveindex of SiC1�xNx films with different carbon and nitrogen contenthas been reported, the research on the refractive index of GeC1�xNx

films has not been attempted yet. How does the refractive index ofGeC1�xNx films evolvewith composition? (iii) The Urbach tail width(E0) is an important parameter in determining the optical trans-mission and optical absorption of amorphous semiconductor films.Although E0 of unitary (a-Si, a-Ge) and binary (a-SiC, a-SiN, a-GeC)semiconductor films have been reported [20], the studies on E0 ofthe ternary films are still lacking. How is the E0 affected when thefilm becomes ternary?

With the above three questions in minds, we sputteredGeC1�xNx:H films with different nitrogen content (CN) by control-ling the nitrogen flow rate, and carried out studies of the followingthree aspects: (i) identifying the evolution of structure and chem-ical bonds, especially hydrogen-containing bonds; (ii) elucidatingthe variation in the refractive indexwith CN; (iii) clarifying themaincontributing factor to E0 in the ternary GeC1�xNx films.

2. Experimental details

The germanium carbon films were prepared using radio-frequency (RF) magnetron sputtering single crystal Ge (111)target (100 mm in diameter and 3 mm in thickness) in the mixeddischarge gases of Ar (99.99%) and CH4 (99.99%), wherein the flowrate of CH4 and Ar were kept at 7.2 and 57.6 sccm at a ratio of 1/8.The incorporation of nitrogen into the germanium carbon films wasrealized through addition of nitrogen gas in the sputtering atmo-sphere, where the flow rate of N2 increased from 0 to 24 sccm, withgradual decreasing flow of CH4 and Ar while maintaining the ratioof 1/8. During the deposition process, the deposition time, substratetemperature, RF power and the total chamber pressurewere kept at20 min, 200 �C, 150 W and 0.9 Pa, respectively. No bias voltage wasapplied. The films were deposited on single-crystal Si (001) waferand optical glass at the same time in the same chamber. The dis-tance between the target and substrate holder was fixed at 80 mm.The chamber was evacuated by a turbomolecular pump to6 � 10�4 Pa prior to deposition. Before introduction into the vac-uum chamber, the Si wafer and glass substrates were cleaned ul-trasonically in acetone and petroleum ether, consecutively. Theflow rates of Ar, CH4 and N2 were accurately controlled by D08-1 A/ZM mass flow controllers. The film thickness was 993, 996, 1003,802, 824, 938 and 772 nm, determined using a Dektak3 surfaceprofile measuring system, for the GeC1�xNx:H films with the ni-trogen content CN ¼ 0, 1.7, 4.5, 8.4, 9.7, 11.1 and 17.6 atomic %.

The films on Si (001) wafer were examined using Atomic ForceMicroscopy (AFM, Dimension Icon, Bruker, Germany), ScanningElectron Microscopy (SEM, JSM-6700F, JEOL Inc., USA), FourierTransform Infra-red spectroscopy (FTIR, Spectrum One B type,Perkin Elmer, USA), X-ray Photoelectron Spectroscopy (XPS, ESCA-LAB 250, Thermo Electron, USA) for surface morphology, chemicalbonding and composition measurements. Those on glass weretested for Ultraviolet-visible-near infrared transmittance (UV-VIS-NIR, Lambda 900, Perkin Elmer, USA) (to avoid absorption renderedby silicon substrate). AFM images were obtained using RTESP probewith a resonance frequency of 300 kHz. The scanning range was2 � 2mm. Each measurement was conducted for three times and anaverage of roughness was calculated with error less than 0.2 nm.The SEM observations were obtained at an acceleration voltage of

8 kV and working distance of 9.1 mm. The IR transmittance wasmeasured with an FTIR spectrometer at a resolution of 4 cm�1. Theabsorption spectra were calculated from transmission spectra byapplying Lambert-Beer's law [21]. XPS measurements were carriedout using a monochromatized Al Ka (1486.6 eV) X-ray source (VGESCA LAB MKII). During XPS experiments, the energy of the Arþ

beam was set at 3 keV, and an electron flood gun was used forcharge compensation. All spectra were collected using the C1sspectra of contaminated carbon with a binding energy of 284.6eV.We used Origin 8.5 software for peak fitting, where the backgroundtype was User Defined mode. The position of Gaussian peaks wasconstrained during the whole fitting process. Arþ cleaning proce-dure was applied to all samples for 180 s prior to XPS quantitativeanalysis. The content of nitrogen (CN), carbon (CC) and germanium(CGe) were obtained from the integrated area of the correspondingnarrow-scanning peaks of N1s, C1s and Ge3d, which are calculatedfrom the following equations:

CN ¼IN1sSN

IGe3dSGe

þ IN1sSN

þ IC1sSC

;CC ¼IC1sSC

IGe3dSGe

þ IN1sSN

þ IC1sSC

;CGe ¼IGe3dSGe

IGe3dSGe

þ IN1sSN

þ IC1sSC

(1)

Where, IN1s, IC1s and IGe3d represent the integrated area of N1s,C1s and Ge3d peaks, respectively. SN, SC and SGe denote sensitivityfactors of nitrogen, carbon and germanium, which are 0.42, 0.25and 0.38 [22], respectively. Since H atoms are bonded with Ge, Cand N atoms, the total concentration of bonded H per unit volume(CH) in the film should be the sum (CGe�H þ CC�H þ CNeH) of theconcentration of H bonded to Ge, C and N per unit volume. Todescribe the variation of bonded H quantitatively, we applied therelation CH ¼ AIa, where Ia is the integrated intensity of the ab-sorption band and the constants A are 1 � 1020 cm�2 [23] for theGeeH bands, 1 � 1021 cm�2 [24] for CeH bands, and4.7 � 1020 cm�2 [25] for NeH bonds. Wide-angle X-ray diffractionmeasurements (XRD, D8tools, Bruker, Germany) were carried out ata scan speed of 0.2�/s. The transmittance in the range of300e2500 nm was obtained with a UV-VIS-NIR spectrometer. Ac-cording to the envelope method [26], the refractive index of weakabsorption band (1300e1700 nm) was calculated by the followingequations:

n ¼�N þ

�N2 � S

�1=2�1=2(2)

N ¼ 2sTM � TmTMTm

þ s2 þ 12

(3)

Where n is the refractive index of films, s is the refractive indexof substrate, TM and Tm represent the upper and lower trans-mittance, respectively. Dielectric constant ε was calculated byε¼ n2. Urbach tail width was extrapolated from the inverse slope oflna~hn line in the region of exponential absorption, where a and hnbeing the absorption coefficient and incident photon energy,respectively.

3. Results and discussion

3.1. Composition and structure

Fig. 1a plots a typical full-scanning XPS spectrum of GeC1�xNx:Hfilms, where the appearances of Ge3d, Ge3p, Ge3s, Ge Auger, C1sand N1s peaks suggest germanium, carbon and nitrogen exist in thefilm. Fig. 1b plots the content of nitrogen, carbon and germanium(CN, CC and CGe) in the films as a function of nitrogen flow rate (RN2).

Fig. 1. (a) The typical full-scanning XPS spectrum of the GeC1�xNx:H film (CN ¼ 8.4 atomic %), and (b) CN, CC and CGe in GeC1�xNx:H films as a function of RN2.

C. Hu et al. / Vacuum 129 (2016) 23e30 25

For RN2 ¼ 0 sccm, the CC and CGe in the film are 21.0 atomic % and79.0 atomic %, respectively. As RN2 increases from 0 to 24 sccm, theCN and CC in the film experience a linear increase and decrease,respectively, while the CGe remains almost unchanged. The increasein CN and decrease in CC are due to an enhancement of the nitrogengas supply and the reduction of the methane gas supply. CGe re-mains unchanged, which is attributed to the constant sputteringyield of germanium target under the invariant sputtering pressureand RF power. These XPS results suggest that carbon atoms arereplaced by nitrogen atoms, when nitrogen is introduced into thefilms.

Fig. 2a plots a typical XRD pattern of the films. The absence of

Fig. 2. The typical (a) XRD pattern and (b) SEM image of the GeC1�xNx:H film (CN ¼ 8.4 atomof GeC1�xNx:H samples with (d) CN ¼ 0 atomic %, (e) 8.4 atomic % and (f) 17.6 atomic %.

sharp peaks except that from the single-crystal silicon substrateand appearance of a broad peak centered at 2q ¼ 32� indicate thatthe GeC1�xNx:H films are amorphous. Fig. 2b shows a typical SEMimage, showing relatively smooth and featureless film surface.Fig. 2def shows typical 3D AFM images, in which flat and smoothsurface with island-like features are observed. Fig. 2c plots the rootmean square (RMS) roughness of GeC1�xNx:H films with differentCN. As nitrogen is introduced, the RMS drops sharply from 3.32 nmto 0.78 nm, indicating that the incorporation of nitrogen smoothsthe film surface. The XRD and AFM analysis are in good agreement,confirming all the films are amorphous and smooth. Amorphousand Smooth characteristic can reduce the optical loss from light

ic %); (c) RMS roughness of GeC1�xNx:H films with different CN; The typical AFM images

C. Hu et al. / Vacuum 129 (2016) 23e3026

scattering at the surface of the films, hence is favored in investi-gation of optical properties such as transmission and absorption.

3.2. Chemical bonds

Fig. 3a and b plot the FTIR absorption spectra in the low-wavenumber region of 400e1000 cm�1. For nitrogen-free GeC:Hfilms, there are only two absorption peaks at 610 and 770 cm�1

(Fig. 3b), which are assigned to GeeC [27] and Ge-CH [14] bonds.However, when nitrogen is incorporated, GeeC and Ge-CH peaksget weaker, and new GeeN peak at ~730 cm�1 [22] emerges andbecomes strong (Fig. 3a). When CN reaches 8.4 atomic %, Ge-CHpeak disappears. It can also be seen that as CN increases from 0 to17.6 atomic %, the dominant absorption peak shifts from ~610 cm�1,related to GeeC bonds, to ~730 cm�1, corresponding to GeeNbonds. The variation in intensity of these absorption peaks and theshifting of the dominant peak confirm that GeeN bonds are grad-ually formed at the expense of GeeC bonds as nitrogen isincorporated.

Fig. 3c displays FTIR absorption spectra of the films in the high-wavenumber region of 1000e4000 cm�1. Without nitrogen, thereexists mainly CeC stretching mode at 1480 cm�1 [28], C-Hn sym-metric/asymmetric stretching mode at 2800 cm�1 and Ge-Hnbending mode at 1930 cm�1 [29]. With nitrogen, however, N-Hnbending mode [29] appears and becomes stronger and stronger

Fig. 3. FTIR absorption spectra of GeC1�xNx:H films in (a) & (b) low-wavenumber regionconcentration of bonded H as a function of CN.

with increasing N, and at the same time, the CeN stretching modealso appears at 1480 cm�1. As CN reaches 8.4 atomic %, C-Hn sym-metric/asymmetric stretching mode and Ge-Hn bending modealmost totally disappeared, while CeN stretching mode at1130 cm�1 and C^N stretching mode at 2075 cm�1 becamenoticeable [29]. These variations of the vibration modes furtherconfirm that nitrogen atoms have replaced carbon atoms to formNeGe and NeH bonds. Fig. 3d plots the concentration of bonded Has a function of CN. The concentration of bonded H decreases withincreasing N, as expected, owing to increase in partial pressure ofnitrogen gas and reduction in partial pressure of hydrogen-containing methane gas.

To better understand the chemical bonding states in the films,detailed XPS core level spectra of Ge3d, C1s and N1s are plotted inFig. 4aec, respectively. The features of Ge3d, C1s and N1s curvesgradually change as CN increases, implying the chemical environ-ment surrounding the Ge, C and N atoms is influenced by nitrogenincorporation. Fig. 4def display the spectra obtained by peak-fitting, utilized to analyze the content of chemical bonds. InFig. 4d, without N, a relatively symmetrical peak attributed to GeeCbond locates at 30.7 eV [22]. As N is incorporated, however, peakbroadening is observed at the higher binding energy region, due toformation of GeeN bond at 31.8 eV [22]. Fig. 4g displays theincreasing ratio of integrated intensities between GeeN and GeeC(IGe�N/IGe�C) as a function of CN, suggesting that the GeeN bonds

of 400e1000 cm�1, and (c) high-wavenumber region of 1000e4000 cm�1; (d) The

Fig. 4. The XPS (a) Ge 3d, (b) C 1s and (c) N 1s core level spectra for GeC1�xNx:H films with different CN, the components of (d) Ge 3d, (e) C 1s and (f) N 1s peaks obtained by peak-fitting, and (g) & (h) & (i) the ratio between integrated intensities of these components.

C. Hu et al. / Vacuum 129 (2016) 23e30 27

form at the expense of the GeeC bonds, consistent with the FTIRresults discussed afore. Fig. 4b, e and h show the variation ofchemical bonds related to carbon. In Fig. 4e, without N, peaksattributed to GeeC bond locating at 284.2 eV [30] and CeC/CeHbond locating at 285.0 eV [18] are observed. With N, however, theCeN peak, at 286.2 eV [18], appears, rises and then drops. Fig. 4hplots the ratio of CeC and/or CeH to the sum of GeeC and CeNbonds with varying CN. The CeH bonds keep decreasing with N,agreeing well with the FTIR analysis. In the XPS N1s core energyspectra (Fig. 4c and f), the GeeN peak (398.1 eV) and the CeN peak(399.5 eV) appear upon incorporation of N. Fig. 4i displays the ratioof the GeeN to the CeN bonds as a function of CN, illustrating theGeeN bonds increasing steadily with nitrogen.

In summary, the results of XPS and FTIR are consistent and all ingood agreement to confirm that the majority of the bonds innitrogen-free GeC:H films are GeeC, CeH and GeeH. When N

atoms are incorporated into the GeC:H network, nitrogen atomsbond with Ge, C and H atoms. With increasing N, GeeN and NeHbonds increase while GeeC, CeH and GeeH bonds decrease.

3.3. Optical properties

Fig. 5a displays the transmission spectra of the films withdifferent CN, demonstrating that all the films possess superior op-tical transmittance at infrared wavelengths. The optical trans-mittance obtained is as high as 91%, close to the transmittance ofglass substrate, suggesting that these films are infrared-transparent. However, optical transmittance of these films de-clines abruptly at visible wavelengths, ascribed to the absorption bythe interband transition. With incorporation of nitrogen, the cutoffof the transmission spectra shifts towards shorter wavelengths,meanwhile the streaks in the transmission spectra become more

Fig. 5. (a) Transmission spectra of GeC1�xNx:H films with different CN, and (b) a typical envelope obtained by the transmission spectrum (CN ¼ 8.4 atomic %); The refractive index (c)at different wavelengths and (d) at a fixed wavelength of l ¼ 1450 nm for GeC1�xNx:H films as a function of CN.

C. Hu et al. / Vacuum 129 (2016) 23e3028

dispersed, indicating that the transmission spectra are dependentof CN. In Fig. 5b, TM and Tm represent the upper and lower trans-mittance, respectively. According to the envelope method [26], wecalculate the refractive index (n) of weak absorption band(1300e1700 nm), and the results are plotted in Fig. 5c. As CN in-creases from 0 to 17.6 at atomic %, n drops from 3.0 to 2.3. Fig. 5ddisplays n at the wavelength of 1450 nm, indicating an almostlinear reduction of the refractive index with increasing CN.

According to Lorentz-Lorenz equation [31]:

n2 � 1n2 þ 2

¼�rA0

M

�4pa3

(4)

where r is the mass density, a is the molecular electronic polariz-ability, Ao is the Avogadro's number, andM is the molecular weight,n depends strongly on a. Studies have shown that the a of N atom(aN ¼ 1.10 � 10�24 cm3 [32]) is less than that of carbon atom(aC ¼ 2.21 � 10�24 cm3 [33]). Hence, the decrease of nwe observed(Fig. 5d) may be attributed to the decrease of average polarizabilityof the films, as carbon is replaced by nitrogen (Fig. 1b).

Fig. 6a plots Urbach tail width (E0) of the films, showing that E0significantly increases from 198.8 to 327.9MeV as CN increases from0 to 17.6 atomic %. As earlier researches on unitary and binary films

[34,35] have found, the degree of disorder is one of the mostimportant factors in determining E0 of these films. Thus, weinvestigate the compositional disorder and structural disorder ofthe samples, respectively. Considering compositional disorder,when N is incorporated into the films, the original binary GeC:Hnetwork first transforms into the ternary GeC1�xNx:H networkupon addition of N atoms. Then as nitrogen becomes sufficientlyhigh, ternary GeC1�xNx:H network ultimately transforms into thebinary GeN:H network. From this perspective, with incorporatingN, compositional disorder of the films first increases and then de-creases. As for the degree of structural disorder, we characterize thefilms via FTIR spectra, which is an effective method to evaluatestructural disorder [36]. Fig. 6b displays FWHM (Full Width of HalfMaximum) of dominant absorption peak at ~700 cm�1, whereinFWHM first increases as CN increases from 0 to 4.5 atomic % fol-lowed by decrease as CN continues to increase from 4.5 to 17.6atomic %, echoing the increase and then decrease of the structuraldisorder. However, at the same time, E0 keeps increasing, indicatingthat the degree of disorder does not seem to be a critical factor toaffect E0.

Other factors may play an important role in determining E0.Earlier studies revealed that the dielectric coefficient (ε) is a sig-nificant factor in determining E0 [36,37]. According to hydrogen-

Fig. 6. (a) E0, (b) FWHM of FTIR absorption peak at ~700 cm�1 and (c) the reciprocal of square of dielectric coefficient (1/ε2) for GeC1�xNx:H films with different CN, and (d) the lineardependence between E0 and 1/ε2.

C. Hu et al. / Vacuum 129 (2016) 23e30 29

like atommodel, E0 is proportional to the reciprocal of square of thedielectric coefficient (1/ε2), which was proved in amorphous binarysemiconductor alloys such as a-GeC, a-SiC and a-SiN [36]. Wepropose that the relationship between E0 and 1/ε2 may exist internary semiconductor alloy films. To confirm this, we obtained thevalue of 1/ε2 in Fig. 6c, which illustrates that 1/ε2 increases with CN.Fig. 6d displays a good linear relationship between E0 and 1/ε2,suggesting that E0 in GeC1�xNx:H films indeed depends strongly onε, in good agreement with the prediction from the hydrogen-likeatom model. Thus, the increase of E0 is attributed to the decreaseof ε. In conclusion, amorphous GeC1�xNx:H films possess a highdegree of structural disorder because of the lack of long-range or-der, which means the change in CN only has a limited effect on thedegree of structural disorder. In contrast, ε is sensitive to the vari-ation in CN and significantly affects E0 of the GeC1�xNx:H films. Thisfinding is of significant importance in modulating E0 of amorphousternary semiconductor alloy films.

4. Conclusions

(1) In amorphous hydrogenated germanium carbonitride(GeC1�xNx:H) films, there are GeeN, GeeC, CeN andhydrogen-containing bonds (GeeH, CeH and NeH). As ni-trogen content increases, carbon and hydrogen content

decrease, which promotes the formation of NeH and GeeNbonds at the expense of GeeH, CeH and GeeC bonds.

(2) The refractive index of the films decreases from 3.0 to 2.3with increasing nitrogen content as a result of decrease inelectronic polarizability. The apparent tunability of therefractive index over a wide range is useful for the potentialapplications of GeC1�xNx:H films as multilayer antireflectionand protection coating of IR windows [23], as well as themultilayer optoelectronic system [38]. Thus, it merits furtherinvestigations.

(3) The significant increase of Urbach tail width (E0) with ni-trogen results from decrease of the dielectric coefficient (ε),rather than the increase of the degree of disorder that pre-viously proved to be an important factor contributing to E0.The change in E0 is proportional to the variation in 1/ε2,agreeing well with hydrogen-like atom model. This discov-ery is important in modulating the optical absorption prop-erties of amorphous ternary semiconductor alloys.

Acknowledgments

The authors gratefully acknowledge the financial support fromNational Natural Science Foundation of China (Grant Nos.51572104, 51102110 and 51372095), National Major Project forResearch on Scientific Instruments of China (2012YQ240264),

C. Hu et al. / Vacuum 129 (2016) 23e3030

Technology Development Project (2015220101000836), Programfor studying abroad of China Scholarship Council. Also, the authorsare grateful for the support from the Academic Research Fund ofSingapore (Tier 1, RG187/14).

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