Physica B 407 (2012) 2340–2343

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Physica B

0921-45

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Structure and optical properties of Ge–Ga–S films depositedby thermal evaporation

Jing Fu n, Xiang Shen, Guoxiang Wang, Qiuhua Nie n, Fen Chen, Jun Li, Wei Zhang, Shixun Dai, Tiefeng Xu

Faculty of Information Science and Engineering, Ningbo University, Ningbo 315211, China

a r t i c l e i n f o

Article history:

Received 31 December 2011

Received in revised form

9 March 2012

Accepted 11 March 2012Available online 17 March 2012

Keywords:

Thin films

Thermal evaporation technique

Optical properties

Local structure

26/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.physb.2012.03.031

esponding authors. Tel.: þ86 574 8760 0947

ail address: fujingsuny@163.com (J. Fu).

a b s t r a c t

Amorphous Ge–Ga–S thin films have been successfully deposited onto glass slides at room temperature

by the thermal evaporation technique. The structural units of the films were studied using Raman

spectroscopy. In addition to the basic structural units of GeS4 tetrahedra, there are some S–S and Ge–Ge

homopolar bonds which exist in the films. The increase in Ge atoms leads to the replacement of S–S

bonds by Ge–Ge bonds, and the isolated Ge(Ga)S4 tetrahedra units transform into corner-sharing or

edge-sharing Ge(Ga)S4 tetrahedra units in the films. The refractive index and optical band gap were

derived from transmission spectra of films. The values of optical band gap decrease while the refractive

indices increase with increasing Ge content. Composition dependence of optical band gap and refractive

index has also been interpreted in terms of the variation in the structure of films based on Raman

spectra.

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1. Introduction

Chalcogenide glasses (ChGs) have attracted much attention fortheir excellent infrared transparency, high linear and non-linearrefractive index, photosensitivity which enables direct opticalpatterning of planar waveguides, and the properties that arecontinuously tunable through compositional tailoring [1–4]. Theproperties found in chalcogenide glasses make them very inter-esting for technological applications, such as optoelectronics andchemical sensors [5,6]. In the recent years, chalcogenide filmshave been used as an alternative platform for the development ofultra-fast all-optical signal processing photonic chips, whichmonolithically integrate several functions and are capable ofhandling high data rate signal [7,8]. Several of these functionshave been demonstrated, such as wavelength conversion, demul-tiplexing and regeneration [9–12]. The optical band gap andrefractive index are the most significant parameters for amor-phous semiconducting thin films in optical devices. It is knownthat modifications of chemical bonds result in changes in thephysical properties, such as the optical band gap and index ofrefraction. Therefore, the studies of physical properties and localstructure of chalcogenide thin films are necessary for the fabrica-tion of ultra-fast all-optical signal processing devices.

Recently, the Ge–Ga–S ternary system has been proposed as akind of As-free material for the active optical planar waveguide

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for their relatively low phonon energy, high rare-earth solubility,and good thermal and chemical durability [13,14]. Additionally, ithas wide transparency in the visible, permitting a greater range ofexcitation wavelengths. This paper is concerned with the opticalproperties and local structure of Ge–Ga–S films prepared bythermal evaporation in vacuum. The index of refraction and opticalband gap were derived from the optical transmission spectra usingSwanepoel’s method and Tauc extrapolation, respectively. Thevariation of these properties is discussed on the basis of changesin local structure of investigated samples, which were character-ized by Raman spectra. Furthermore, the structural changes inGe–Ga–S films have been interpreted in terms of the chemical-bondapproach.

2. Experimental

The Ge–Ga–S films were thermally evaporated onto chemicallycleaned microscope glass slides at room temperature frombulk glasses with a stoichiometry of GexGa10S90�x (x¼20, 25 and30 at%), using a conventional coating unit (JGP-450). The bulkglasses were prepared from their constitutional elements by theusual melt-quenching technique. The pressure inside the chamberwas about 6.4�10�4 Pa. The substrates were fixed onto a rotatableholder (�30 rpm) to obtain homogeneous films at a distance of25 cm above the evaporator boat. The deposition rate as well as thefilm thickness was controlled using a film thickness monitor duringthe deposition process. The deposition rate was at about 8 A/s, andthe thickness of the films was �1 mm.

J. Fu et al. / Physica B 407 (2012) 2340–2343 2341

The chemical compositions of the investigated films have beenmeasured using an Energy Dispersive Spectrometer (EDS); theresults indicate that the following chemical compositions wereobtained: Ge23.03Ga5.77S71.20, Ge28.74Ga5.25S66.01 and Ge34.60 Ga5.16

S60.24, and the uncertainty of EDS measurement is within 71.50%.It shows that the compositional deviation from correspondingbulk glass is about 5%, which could be attributed to the equili-brium vapor of Ga being much lower than that of Ge and S. Theamorphous nature of as-deposited films was confirmed by theX-ray diffraction method with a diffractometer (BRUKER D2GMBH) using CuKa radiation (l¼0.154 nm, 36 kV, 20 mA).Raman spectra of the films were recorded at room temperatureusing a Renishaw InVia Raman spectroscope equipped with an Arion laser with a wavelength of 488 nm. The power of theexcitation beam used for the measurement is about 5 mW, inorder to avoid affecting the surface of the films. The resolution ofthe Raman spectra is 1 cm�1. The optical transmission spectrawere obtained using a dual beam UV–vis–NIR PerkinElmerLambda 950 spectrophotometer from 300 to 2500 nm.

3. Results and discussion

Fig. 1 shows the Raman spectra of Ge–Ga–S films. Ramanspectra of the films present a broad band in the range 300–450 cm�1 with a shoulder at a higher frequency composedof three bands at about 370, 400 and 420 cm�1. The main bandat about 340 cm-1 is ascribed to the symmetric stretching mode ofisolated Ge(Ga)S4 units, indicating that the basic structural unitsof the films are isolated Ge(Ga)S4 structural units. The shouldersnear 370 and 400 cm�1 have been attributed to the T2 modeof edge-sharing and corner-sharing GeS4, while the band at420 cm�1 is related to the anti-symmetric stretching mode oftwo tetrahedra connected through one bridging sulfur atom, asin S3Ge–S–GeS3 [15]. Two bands of low amplitude in the100–160 cm�1 and 450–500 cm�1 ranges can also be observed.The origin of the band in the low frequency 100–160 cm�1 rangeis not totally clear. In accordance with Ivanova et al. [16], thisband may be associated with the asymmetrical bending vibrationof GeS4 units. The band in the range of 450–500 cm�1 is formedby superimposed bands at 475 and 490 cm�1, corresponding tothe vibration of S–S bonds in S8 rings and Sn chains, respectively[15,17]. In addition, the Raman bands between 175 and 260 cm�1

are associated with the presence of Ge–Ge bonds in structural

200 300 400 500 600

Ge23.03Ga5.77S71.20

Ge28.74Ga5.25S66.01

Ge34.60Ga5.16S60.24

Nor

mal

ized

Inte

nsity

(a.u

.)

Raman shift (cm-1)

Fig. 1. Raman spectra of Ge–Ga–S films.

entities containing fewer than four S atoms around Ge ofGexS(3�x)Ge–GeS(3�x)Gex type [18,19]. As the content of Geincreases, the main band at about 340 cm�1 becomes broader,meaning a conversion of isolated Ge(Ga)S4 tetrahedra units tocorner-sharing or edge-sharing Ge(Ga)S4 tetrahedra units inthe films network. The center of the band in the range of450–500 cm�1 shifts to a slightly higher frequency for films withcomposition Ge23.03Ga5.77S71.20 and Ge28.74Ga5.25S66.01, whichmay confirm that the S–S bonds in S-rich films are more likelyin the form of Sn chains. Additionally, the intensity of this banddecreases progressively with increasing Ge content and eventotally disappears in Ge34.60Ga5.16S60.24 films, which is related tothe vanishing of S–S bonds. On the other hand, the bands inthe range of 175–275 cm�1 arising from Ge–Ge bonds appear inGe34.60Ga5.16S60.24 films. It confirms the presence of Ge–Ge bondsin S-deficient films. It must be noted that the amplitudes of thebands in 175–275 cm�1 and 300–450 cm�1 are comparable inGe34.60Ga5.16S60.24 films, suggesting that the network appears tobe composed of GexS(3�x)Ge–GeS(3�x)Gex ethane-like units andGeS4 tetrahedra units, which form the majority of the network.

Abe et al. [13] had given structural interpretations of theGe–Ga–S glass, and reported that the glasses with o15 at% Gaexhibited a similar variation of structure with S content such asthe formations of Ge–Ge bonds and S–S bonds in the Ge–S system.Similar structural changes can be observed in the Raman spectraof Ge–Ga–S films. Moreover, the structural evolution can beinterpreted in terms of the chemical-bond approach proposedby Bicermo and Ovshinsky [20], who assume that heteropolarbonds have preeminence over homopolar bonds and bonds areformed in the sequences of decreasing bond energy until all theavailable valences of the atoms are saturated. According to them,the Ge and Ga atoms more favorably combine with S atomsforming Ge(Ga)S4 tetrahedra units in Ge–Ga–S films and theremaining sulfur atoms can form Sn chains or S8 rings, inagreement with the Raman bands in the range of 300–500 cm�1

as shown in Fig. 1. Increasing Ge content at the expense of Sresults in a sulfur deficient network, promoting the substitutionof isolated Ge(Ga)S4 units with corner- or edge-shared tetrahedraunits and the formation of weaker Ge–Ge bonds compared toS–S bonds. This behavior is mainly due to the formation ofcorner- or edge-shared tetrahedra units and ethane-like unitswhich provides the extra sulfur needed for the formation oftetrahedra units.

300 600 900 1200 1500 1800 2100 24000

20

40

60

80

100

Ge23.03Ga5.77S71.20

Ge28.74Ga5.25S66.01

Ge34.60Ga5.16S60.24

Tran

smis

sion

(%)

wavelength (nm)

Fig. 2. Optical transmission spectra of the Ge–Ga–S films.

800 1000 1200 1400 1600

2.1

2.2

2.3

2.4

2.5

2.6

Ge23.03

Ga5.77

S71.20

Ge28.74

Ga5.25

S66.01

Ge34.60

Ga5.16

S60.24

Ref

ract

ive

inde

x

wavelength (nm)

Fig. 4. Refractive indices of Ge–Ga–S films.

J. Fu et al. / Physica B 407 (2012) 2340–23432342

Fig. 2 shows the optical transmission spectra of the Ge–Ga–Sfilms. According to Kosa et al. [21], the absence of shrinkage ofinterference fringes in the transmission spectra confirms thehomogeneity of the studied films. It can be observed that theabsorption edges of the films shift to a higher wavelength withincreasing Ge content. Following the fundamental Kramers–Kronig relation [22], the red-shift in the spectrum must necessa-rily give an increased refractive index value (justified later).

It is known that the optical absorption edge of amorphoussemiconductors generally contains three distinct regions. In thestrong absorption region (aZ104 cm�1), the optical band gap ofthe films has been determined from absorption coefficient data asa function of photon energy, according to the generally accepted‘‘non-direct transition’’ model for amorphous semiconductors,proposed by Tauc [23]:

a¼ Bðhn�EgÞ2=hn ð1Þ

where Eg is the optical band gap and B is the band tailingparameter. Linear (ahn)1/2 vs hn plots in Fig. 3 indicate that the‘‘non-direct transition’’ model should be responsible for theoptical absorption in Ge–Ga–S films. The values of optical bandgap Eg are obtained by extrapolating (ahn)1/2 to zero. The values ofoptical energy gap and Tauc slope B1/2 are listed in Table 1.

According to Table 1, the Tauc slope B1/2 increases while theoptical band gap decreases with increasing Ge content. Theenhancement of B1/2 indicates a higher topological disorder, whichis responsible for the observed red-shift of the optical band gap inthe as-deposited films. On the other hand, the variation in opticalband gap could also be understood on the basis of the change ofthe average bond energy. It is known that the decrease in opticalenergy gap is related to the decrease in average bond energy. Thebond energy of Ge–Ge bonds (260 kJ/mol) is much lower than thatof Ge–S bonds (557 kJ/mol) and S–S (425.3 kJ/mol) bonds [24].Hence, the formation of Ge–Ge bonds reduces the average energy

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.60

100

200

300

400

500

600 Ge23.03

Ga5.77

S71.20

Ge28.74

Ga5.25

S66.01

Ge34.60

Ga5.16

S60.24

h(c

m-1

/2eV

1/2 )

h (eV)

Fig. 3. Determination of the optical band gaps in terms of Tauc’s equation as linear

extrapolation of the strong absorption data of Ge–Ga–S films.

Table 1Values of the optical band gap (Eg) and Tauc slope (B1/2) for Ge–Ga–S films.

Film Eg (eV) B1/2 (cm�1/2 eV�1/2)

Ge23.03Ga5.77S71.20 2.48 341.34

Ge28.74Ga5.25S66.01 2.40 358.43

Ge34.60Ga5.16S60.24 1.95 534.82

of the system, thus leading to a decrease in the optical energy gapwith increasing Ge content.

Fig. 4 shows the spectral distribution of refractive index of theGe–Ga–S films. The values of refractive index are derived fromoptical transmission spectra using Swanepoel’s method, which isbased on the envelopes of interference maxima and minima [25].It is easily seen that the refractive index increases with increasingGe content, corresponding to the red-shift in the absorption edgesin shown Fig. 2.

Following Lucovsky [26], the refractive index as a function ofdensity, coordination number and average bond susceptibility whas the form

e¼ n2 ¼ 1þ4pN0NTs/wSrA, /wS¼X

oiwi ð2Þ

where e and n are the dielectric constant and refractive index,respectively, N0 is Avogadro’s number, NT is the total number ofbonds per atom (NT¼(CN)/2), o is the fraction of bonds formingthe network, w is the bond susceptibility and rA is the atomicdensity (the volume density r divided by an average atomicmass). It can be concluded that the refractive index has a positivecorrelation with the density of materials and bond susceptibility.Considering the heavier atomic weight of Ge atom compared toS atom, the introduction of Ge should enhance the density offilms. Furthermore, the enhancement of refractive index can bealso explained by average bond susceptibility. The S–S bonds(6.55�10�24 cm3) can be replaced by higher susceptibility bondsof Ge–Ge (1.35�10�23 cm3) when the content of Ge increases[26,27]. Consequently, the average bond susceptibility of Ge–Ga–Sfilms can also be increased, resulting in an increase in refrac-tive index.

4. Conclusions

Amorphous Ge–Ga–S films have been successfully deposited atroom temperature by the thermal evaporation technique. Based oninterpretation of the Raman spectra, it can be found that in additionto the basic structure of Ge(Ga)S4 tetrahedra units, there are Sn chainsor S8 rings in Ge23.03Ga5.77S71.20 and Ge28.74Ga5.25S66.01 films butS3Ge–GeS3 ethane-like units in S-deficiency Ge34.60Ga5.16S60.24 films.Increasing Ge content at the expense of S results in a sulfur deficientnetwork, promoting a conversion of isolated GeS4 to corner- or edge-sharing GeS4 tetrahedra units and GexS(3�x)Ge–GeS(3�x)Gex units,with a corresponding reduction in S–S bonds. The optical band gap

J. Fu et al. / Physica B 407 (2012) 2340–2343 2343

and refractive index are derived from transmission spectra of films,and the results show that the optical band gap decreases whilerefractive index of the films increases with increasing Ge content. Thepresence of Ge–Ge bonds, which have lower bond energy and higherbond susceptibility than those of S–S bonds, is responsible for thedecrease in optical band gap and enhancement of refractive index.

Acknowledgments

This work was financially supported by the Natural ScienceFoundation of China (Grant nos. 61008041, 61107047 and60978058), the Natural Science Foundation of Zhejiang Province,China (Grant no. Y1090996), the Natural Science Foundation ofNingbo City, China (Grant nos. 2011A610092 and 2011A610189),the Ningbo optoelectronic materials and devices creative team(Grant no. 2009B21007), and the Open Research Fund of State KeyLaboratory of Transient Optics and Photonics, Chinese Academy ofSciences (Grant no. SKLST201010), and sponsored by K.C. WongMagna Fund in Ningbo University.

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