Effect of film thickness on physical properties of RF ...

Surface & Coatings Technology 276 (2015) 587–594

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Effect of film thickness on physical properties of RF sputtered In2S3 layers Yaxin Ji a, Yufeng Ou a, Zhou Yu a,⁎, Yong Yan a,b, Dan Wang a, Chuanpeng Yan a, Lian Liu a, Yong Zhang a, Yong Zhao a,c a Superconductivity and New energy R&D Center (SNERDC), Key Laboratory of Advanced Technology of Materials, Ministry of Education of China, Mail Stop165#, Southwest Jiaotong University, Chengdu 610031, China b School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610031, China c School of Materials Science and Engineering, University of New South Wales, Sydney 2052 NSW, Australia

a r t i c l e

i n f o

Article history: Received 1 September 2014 Revised 5 May 2015 Accepted in revised form 7 June 2015 Available online 14 June 2015 Keywords: In2S3 layer Sputtering Optical constants Photoconductivity

a b s t r a c t Indium sulfide (In2S3) layers were deposited by RF magnetron sputtering process with different thickness. The effect of thickness on structural, morphological, optical and photo-conductive properties of the In2S3 layers has been studied. In2S3 layers exhibit near stoichiometric chemical composition and single tetragonal phase with (103) preferred orientation. Variation of surface morphology, Raman bands and optical transmittance of the films have been observed with the increase of film thickness. Energy band gap of the layers determined from transmittance spectra were 2.44–2.66 eV (direct) and 1.82–2.06 eV (indirect), respectively. Based on the measured optical constants (n and k), the Wemple–DiDomenico model was performed to determine values of single oscillator energy (Eo), dispersion energy (Ed), optical band gap (Eg) and high frequency dielectric constant (ε∞). The determined β parameter indicates In2S3 as an ionic compound. The ε∞ values agree well with that of calculated according to the Spitzer–Fan model. Photoconductivity properties of the films were analyzed relating to the rate competition among photo-generation, recombination and trapping of carriers. © 2015 Elsevier B.V. All rights reserved.

1. Introduction In recent years, much interest has been paid on indium sulfide (In2S3) due to its potential use in photovoltaic and optoelectronic devices. In2S3 is a possible replacement of toxic CdS buffer layer in Cu(In,Ga)Se2 based thin film solar cells. Using In2S3 as buffer layer, Cu(In,Ga)Se2 solar cells reached conversion efficiencies in the range of 15–16% [1–3]. Generally, In2S3 exhibits three different crystalline phases of α, β and γ depending on the growth temperature and pressure. Among them, the β-In2S3 phase was found to be the stable crystalline phase at room temperature till to 420°C. It is also the most frequently obtained structure in thin film form. β-In2S3 crystallizes in a defect spinel structure with a high degree of vacancies, ordering at tetrahedral cation sites [4]. Thus tetragonal β-In2S3 could be written as In6(In2□)S12, where □ indicates vacancies and the parentheses describe tetrahedral site. Here all octahedral sites and 8 of 12 tetrahedral sites are occupied by indium atom and the other 4 tetrahedral sites are orderly empty. A small fraction of indium atoms may easily leave their ordered positions and occupy ordered vacancies, resulting in a number of quasi-interstitial cations and an equal number of cation vacancies. Thus, even in a stoichiometric crystal of β-In2S3, a considerable number of disorders are always present [5]. ⁎ Corresponding author. E-mail address: [email protected] (Z. Yu).

http://dx.doi.org/10.1016/j.surfcoat.2015.06.011 0257-8972/© 2015 Elsevier B.V. All rights reserved.

Up to now, In2S3 films have been prepared through a number of wet chemical and vacuum techniques. Spray pyrolysis [6], chemical bath deposition (CBD) [7], ion layer gas reaction (ILGAR) [1], and successive ionic layer adsorption and reaction (SILAR) [8] are some of the prominent chemical techniques. The simplicity of the approach and the possibility of large area deposition are the two main merits of chemical techniques. However, the fabricated In2S3 films often exhibit significant composition deficiency of indium and sulfur, contain fairly large concentration of oxygen and possess poor crystalline quality. These films also tend to be non-uniform and have a rough surface. Physical methods are, generally, techniques for depositing uniform and good quality films. The carried out vacuum methods contain atomic layer deposition (ALD) [2], evaporation [9,10], modulated flux deposition [11], and metal-organic chemical vapor deposition (MOCVD) [12]. There are also several instances wherein In2S3 films prepared by vacuum approaches also possess, albeit to a lesser extent, nonstoichiometric composition, oxygen impurity and secondary phases. Generally, physical properties of the fabricated In2S3 films strongly depend on the deposition methods and conditions [13] as well as the film thickness [14]. Structure and surface morphology of the films change with the increase of film thickness, which will in turn affect the optical transmittance [15] and electronic properties [16]. Furthermore, accurate knowledge of the optical constants and absorption coefficients of In2S3 films is of great importance for the design and analysis of In2S3–CuInGaSe2 solar cell devices.

Y. Ji et al. / Surface & Coatings Technology 276 (2015) 587–594

2. Experimental details

60

1.50

58 1.45

56

S

54

1.40

52

S/In

50

1.35

48 1.30

46

In

44

1.25

42 40

Indium sulfide (In2S3) thin films were deposited on bare soda lime glass (SLG) substrates by one step radio frequency (RF) magnetron sputtering process. Glass substrates were ultrasonically cleaned with the sequence of acetone, alcohol and de-ionized water. A 5 cm diameter stoichiometric In2S3 ceramic target was used as sputtering source. Prior to deposition, the sputtering chamber was evacuated to a background pressure of 1 × 10−4 Pa. High purity argon was then introduced to the chamber and pressure was maintained at 0.5 Pa during the sputtering process. RF sputtering power was set at 100 W and the target to substrate distance was 5 cm. Substrates were heated to a temperature of 385 °C and kept constant throughout the film deposition process. For different deposition cycles, sputtering time was set at 5, 10, 20 and 40 min, respectively. The corresponding film thickness was 172, 380, 634 and 1425 nm established by spectroscopic ellipsometry (SE) method. Crystalline structure of the films was carried out by X-ray diffraction (XRD, PANalyticalX'Pert PRO XRD system, Cu Kα irradiation, λ = 1.5405 Å). Surface morphology and chemical composition of the films were analyzed by scanning electron microscope (FESEM JOEL 7001, Japan) and dispersive X-ray spectroscopy (EDS, Inca spectrometer). Optical transmission spectra were measured by a spectrophotometer (Perkin Elmer LAMBDA 900) in the wavelength range of 300–1100 nm with 1 nm step. Raman scattering was performed on a HORIBA LabRAM HR Raman spectrometer using 633 nm laser as the light source. Optical constants of refractive index (n) and extinction coefficient (k) were measured using ellipsometer spectrophotometer (SENTECH SENresearch) in the wavelength range of 300–2400 nm. The photoconductivity was determined under ambient environment at room temperature. Two parallel strip gold electrodes (DC sputtering, with a distance of 1 mm and length of 10 mm) was used to measure the electrical properties at bias voltage of 2 V. A 300 W (white light) Xenon lamp with light intensity adjusted to 1000 mW/cm2 was used as illumination source. The light was switched on and off every 10 s and a Keithley 2400 SMU was used for measuring the photoconductivity response with recording of data every 0.25 s. 3. Results and discussion 3.1. Composition and phase analysis of the In2S3 films Chemical composition of the RF sputtered In2S3 films was evaluated by EDS analysis. Fig. 1 shows the content of indium and sulfur in the films as well as the variation of S/In ratio as a function of film thickness. The sputtered In2S3 layers exhibit a slight change of chemical composition with the increase of film thickness. Indium and sulfur content in the films vary only 42–44 at.% and 56–58 at.%, respectively. Consequently, S/In ratio changes from 1.32 to 1.37 and reaches its maximum value of 1.37 at the thickness of 380 nm. In our sputtering process, composition of the In2S3 target is maintained in the deposited films, which is not consistent with the reported result of stoichiometric deviation in close space evaporated In2S3 films with different film thickness [10]. The formation of In2S3 films can be explained in the following way. Firstly, the In2S3 target

S/In ratio

There were few reports of fabricating In2S3 thin films through radio frequency (RF) magnetron sputtering process; therefore, the dependence of structure and physical properties on film thickness of the RF sputtered films is still unclear . In this work, we studied the effect of film thickness on structural, optical and photoconductive properties of the RF sputtered In2S3 films. In sputtering technique, as the In2S3 ceramic target is bombarded by high energy Ar+ ions, the sputtered molecules bear higher kinetic energy so that the ad-atoms have higher mobility when compared to the evaporation process. This results in an enhanced surface diffusion ability of the ad-atoms that can migrate for a longer distance and react much easier with other atoms, leading to produce films with unique physical properties.

Composition (at.%)

588

1.20

172

380

634

1425

Thickness (nm) Fig. 1. The content of indium and sulfur and variation of S/In ratio in the sputtered films as a function of film thickness.

is bombarded by high energy Ar plasma and In2S3 dissociates according to the reaction [17]: In2 S3 ðsÞ→In2 SðgÞ þ S2 ðgÞ:

ð1Þ

Secondly, the formed In2S and S2 molecules condense on the substrate and then quickly react with each other to form InxSy compound. In the condensation step, considering the partial pressure of In2S (g) is about 103 times higher than that of S2 (g), the formation of nearly stoichiometric In2S3 film suggests that the sticking coefficients of In2S and S2 molecules on the substrate are nearly identical. In the reaction step, a certain bonding between indium and sulfur occur soon after the atoms arrive on the substrate. It can be deduced from the fact of chemical composition stability feature with the increase of film thickness. The vapor pressure of sulfur is very high; it is about 1 Pa at 523 K. Therefore, in the event of sulfur existing in elemental form in the initially formed In2S3 films, loss of sulfur is expected to take place during the subsequent high temperature deposition process (658 K). The quick reaction of In2S and S2 atoms to form InxSy compounds in sputtering process might attribute to high energy of the sputtered atoms (several eV as compared to evaporated atoms of 0.1–0.3 eV), which provide the atoms with high mobility and strong reaction ability. Fig. 2 shows the X-ray diffraction (XRD) patterns of the RF sputtered In2S3 films with different thickness. All sputtered films exhibit polycrystalline feature. Diffraction peaks of the films can be assigned to tetragonal β-In2S3 phase (JCPDS 25–390, a = 7.619 Å, c = 32.329 Å, α = β = γ = 90°). From Fig. 2, the thinnest film (172 nm) shows several broad diffraction peaks with low peak intensity, indicating poor crystalline quality of this film. The diffraction peaks are corresponding to (h 03 h) reflection of β-In2S3 phase, indicating preferred orientation along (103) plane of this film. For thicker films, diffraction peaks show stronger intensity and sharper feature, suggesting crystallinity improvement of the films. XRD peaks of these layers can be indexed to (103), (109), (206), (0012), (1015) and (4012) reflections of β-In2S3 phase. These layers present similar preferred orientation along (103) plane. Table 1 shows full width half maximum (FWHM) values and calculated grain size of the films. Grain size was calculated through the Debye–Scherer formula [18]. The grain size calculated based on (103) and (206) planes shows similar tendency that the thicker the In2S3 film is, the larger the In2S3 grains are. The calculated grain size based on the strongest peak increases from 17.8 nm to 38.5 nm as film thickness elevates from 172 nm to 634 nm, and then decreases to 30.2 nm in the thickest film.

7k

(4012)

(1015) (316)

(116)

Intensity (a.u.)

8k

(2212)

9k

β− In2S3 (0012)

10k

(109)

(103)

11k

(206)


1425 nm

6k 5k

634 nm

4k 3k

(309)

(206)

1k

(4012)

380 nm

(103)

2k

172 nm

0 10

15

20

25

30

35

40

45

50

55

60

65

70

o

2θ ( ) Fig. 2. X-ray diffraction (XRD) patterns of the sputtered In2S3 films with different film thickness.

589

atoms from the formed film surface, making the film surface smooth [20]. The formed smooth In2S3 layer enhances surface diffusion ability of the adsorbed atoms and then improves crystallization of the small grains [10], therefore, increasing deposition time to 40 min leads to larger grains in the upper layer of the film. Fig. 5 shows Raman vibration modes of the sputtered films. The thinnest In2S3 film (172 nm) exhibits an extremely broad Raman band centered around 280 cm− 1; all the phonon bands of In2S3 are very weak, even the strongest one centered at about 250 cm−1. By increasing film thickness, intensity of Raman phonon modes elevate; three A1g model located at 250, 307 and 364 cm−1 can be observed while the Eg mode at 266 cm−1 appears as shoulder [21,22]. The Raman peaks located at 250 and 307 cm−1 are attributed to the vibrations of InS6 octahedral and InS4 tetrahedral [23]. The Raman band centered at 170 cm−1 might associate with the red shift of the F2g mode located at 180 cm−1. It is well known that Raman vibrations are affected by local atomic arrangement, including stress, defects and structural disorder [24]. As film thickness elevates from 380 nm to 1425 nm, the Raman peak position and the band width remain nearly invariant, indicating unconspicuous change of the local chemical environment of the sputtered films as thickness increased.

3.2. Microstructure and Raman analysis of the In2S3 films 3.3. Optical properties and single oscillator model analysis of the In2S3 films Fig. 3 shows FESEM images of the sputtered In2S3 layers with different thickness. All the In2S3 layers present uniform and dense surface morphology without voids or cracks. For the thinnest film (Fig. 3a), island like grains with the size of several tens of nm distribute on the film surface. Increasing film thickness to 380 nm (Fig. 3b) leads to grain growth into granular structure with size range from 150 to 200 nm. Further increase of film thickness to 634 nm improves film surface morphology. Fig. 3c exhibits satisfactory film compactness as well as smooth surface. Nevertheless, the thickest film (1425 nm) shows coarse grains with size of approximately 500–700 nm. Cross section FESEM measurement was carried out to observe microstructure of the deposited films. As shown in Fig. 4, thickness of the films deposited at 10 min, 20 min and 40 min are 320 ± 80 nm, 590 ± 40 nm, 1300 ± 120 nm, respectively. Thickness values measured by FESEM are consistent with the spectroscopic ellipsometry (SE) results. In contrast to polyhedral and cornered grains shown in the 10 min deposited film, compact and ambiguously shaped grains were observed in the 20 min films. It is worthy to see in Fig. 4c that the film deposited at 40 min shows a double layered feature. The upper part of the film consists of compact grown grains with clear boundaries and the bottom part is comprised of packed fine particles. Nevertheless, border of the double layer is not very obvious. Combined with the above results, growth process of the In2S3 layer can be analyzed as follows: at the initial stage of deposition, nucleation centers of In2S3 generate dispersedly on the substrate, absorbing nearby In2S and S2 ad-atoms to form small crystallites. The strong interaction between substrate and atoms restricts mobility of ad-atoms and therefore the growth of small crystallites, resulting film with island like particles after 5 min of deposition [19]. Increasing film thickness benefits the three dimensional growth of these small particles, thus larger cornered grains present in 10 min deposited film. Further increasing film thickness results in the connectivity between grains, combining with the occurrence of re-evaporation of energetically unstable S2

Fig. 6 shows the transmittance (T) spectrum of the In2S3 films as a function of thickness. The transmittance value at 800 nm decreases from 85% to 60% as film thickness increases from 172 to 1425 nm. This is because of the enhanced surface scattering, surface roughness and thickness induced absorption [10]. Transmittance properties are considered highly sensitive to both the distribution of grains and their height variation on the layer surface [25]. Interference patterns can be observed in the figure as the film thickness is larger than 380 nm, indicating good thickness uniformity of these films [26]. Besides, the absorption edge of the sputtered films shifts towards longer wavelength (red-shift) with increase in In2S3 film thickness, indicating band gap narrowing. The absorption coefficient α, which is a function of photon energy hυ, was evaluated by using the relation: α ¼ ln ð1=T Þ=d

ð2Þ

where d is the thickness of the film, and T is the percentage transmittance. Near the fundamental absorption edge, the variation of α with hυ is determined by Tauc relation: αhυ ¼ B hυ−Eg

m

ð3Þ

where B is a parameter that depends on the transition probability, m = 1/2 and 2 denote the allowed direct and indirect transition nature of the film. Since there are reports about both direct and indirect allowed band gaps of In2S3 [27,28], typical (αhν)2 vs. hν and (αhν)1/2 vs. hν plots for the 683 nm film were shown in Fig. 7. Band gap can be calculated through extrapolating the straight line portion of the curves to the energy axis. The calculated indirect and direct band gaps of the films are shown in Table 1. Both type of values increase and then decrease with the increase of thickness. Direct and indirect

Table 1 The FWHM values, calculated grain size along (103) and (206) planes, indirect and direct band gaps of the sputtered films with different thickness, and band gaps calculated through transmittance spectra. Film thickness

FWHM of (103)

Grain size of (103) (nm)

FWHM of (206)

Grain size of (206) (nm)

Band gap (eV)-indirect

Band gap (eV)-direct

172 nm 380 nm 634 nm 1425 nm

0.704 0.258 0.208 0.342

11.4 31.0 38.5 23.4

0.461 0.246 0.236 0.272

17.8 33.3 34.7 30.2

2.02 2.06 1.88 1.82

2.62 2.66 2.54 2.44

590


Fig. 3. FESEM images of the In2S3layers fabricated with different film thickness of (a) 172 nm, (b) 380 nm, (c) 634 nm, and (d) 1425 nm.

band gaps lie in 2.44–2.66 eV and 1.82–2.06 eV, respectively. The reason for the band gap variation may attribute to the disorders resulted from the movement of tetrahedrally coordinated indium

atoms in the deposition process, as a large number of vacant tetrahedral sites presented in the structure [27]. Density of disorders is determined by the degree of composition deviation from stoichiometry.

Fig. 4. Cross section FESEM images of the In2S3 films deposited at (a) 10 min, (b) 20 min and (c) 40 min; thickness of the films measured by SE corresponds to 380 nm, 634 nm and 1425 nm.


9k

250 cm-1

1.6

307 cm-1

1.8

591

2.0

2.2

2.4

2.6

2.8

3.0

3.2 5

600

4k 3k

400 3 300

2

5k

4

1/2

6k

Intensity(a.u.)

500

1425 nm 634 nm 380 nm 172 nm

364 cm-1

2 200

2k

1

100

1k 0 100 150 200 250 300 350 400 450 500 550 600

(αhυ) * 1010(eV/cm)

268 cm-1

1 /2

171 cm-1

(αhυ) (eV/cm)

7k

2

8k

0

0 1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

h υ (eV)

Raman Shift (cm-1)

The formed disorders lead to new energy states in the forbidden gap [27], resulting in band gap narrowing. Therefore, the 380 nm film presents the largest band gap for its nearest stoichiometric composition. The corresponding fewest vacant tetrahedral sites restrict the generation of disorder. For other In2S3 films with composition deviation from stoichiometry, high degree of disorder exists in the films and thus narrows the band gaps. Many researchers obtain optical constants of thin films via calculating refractive index (n) and extinction coefficient (k) through the Swanepoel method by using transmission spectra in the interference zone [10,29,30]. However, this procedure suffers from the seriestermination error as well as the difficulty of making corrections for the presence of micro-rough surface layers or micro-structural defects, similar with the analysis of reflectance spectroscopy through Kramers– Kronig relation to determine dielectric function of material. On the other hand, spectroscopic ellipsometry (SE) measurements yield much more accurate values of the dielectric function and optical constants for the measurements are free of these sources of error [31]. SE uses polarized light to characterize ellipsometric parameters of Ψ and Δ, RP (parallel) and RS (perpendicular to the plane of incidence), through [32]: tan ϕ ¼ RP =RS ¼ Ψ expðiΔÞ

Fig. 7. Calculation of the direct and indirect band gaps of the 634 nm In2S3 films using (αhν)2 vs. hν and (αhν)1/2 vs. hν plots.

semiconductors, the Tauc–Lorentz (TL) model is used to fit the experimental SE data. Fig. 8a and b shows the changes of refractive index (n) and extinction coefficient (k) of the In2S3 films as a function of film thickness. From Fig. 8a, n value increases firstly, then decreases, and finally remains almost constant with the increase of measured wavelength. The initial rapid increase of n indicates a rapid change in the absorption

a

2.9

172nm 380nm 634nm 1425nm

2.8

2.7

n

Fig. 5. Raman scattering spectra of the In2S3 layers sputtered at different film thickness.

2.6

2.5

2.4

ð4Þ 2.3 300

where ϕ is the Brewster angle, Ψ is magnitude and takes value of 0–90° and Δ is the phase difference. For the calculation of optical parameters of

90

70

172 nm 380 nm 634 nm 1425 nm

1200

1500

1800

2100

2400

1.2

172nm 380nm 634nm 1425nm

1.0 0.8

60

k

Transmittance(T%)

80

900

Wavelength(nm)

b

100

600

50 40

0.6 0.4

30

0.2

20 10 0 300

0.0 300 400

500

600

700

800

900

1000

400

500

600

700

800

Wavelength(nm)

Wavelength/nm Fig. 6. Transmittance (T) spectra of the In2S3 layers with different film thickness.

Fig. 8. (a) Refractive index (n) and (b) extinction coefficient (k) of the In2S3 films as a function of film thickness measured by spectroscopic ellipsometry.

592


energy of the material [33]. At longer wavelength, the step of n value decrease indicates normal dispersion of the In2S3 films. The n value at 1500 nm changes from 2.35 to 2.38. The small variation of n value might relate to the difference of film composition [34]. In addition, with the increase of measurement wavelength (λ), k initially decreases sharply and then reduces slowly until it reaches zero at the wavelength around 650 nm (Fig. 8b). Zero value of k indicates transport feature of the In2S3 films which can be clearly observed in the transmittance spectra. Otherwise, the k value decreases slightly with the increase of film thickness. The single-oscillator model proposed by Wemple and DiDomenico is widely used to formalize the dispersion function in the visible region of the film [35]. The approach combines the higher-order contribution with the first-resonant oscillator and retains terms to order (hυ)2 and then yields the single-oscillator approximation: n2 ðhυÞ−1 ¼

E d E0 E20 −ðhυÞ

ð5Þ

2

where Eo is the single oscillator energy which simulates all the electronic excitations involved and Ed is the dispersion energy of the oscillator strength. The oscillator energy, Eo is an ‘average’ energy gap and can be related to the optical band gap Eg in close approximation by Eo ≈ 2 Eg. By plotting (n2 − 1)− 1 versus (hυ)2 and fitting straight lines as shown in Fig. 9, Eo and Ed can be determined from the intercept which equal to Eo/Ed, while the slope of this line is (EoEd)− 1. Furthermore, extrapolating the linear parts towards long wavelengths, the points of interception with the ordinate at (hν)2 = 0 yield the value of n2∞ = ε∞. Results of Eo, Ed, and high frequency dielectric constant ε∞ are shown in Table 2. Eg values of the In2S3 films are approximately 2.6 eV which agrees with the above calculated direct band gap values. The calculated dispersion energy (Ed) values of our sputtered In2S3 films are in the range of 21–26 eV. These Ed results coincide with thermal evaporated In2S3 films ranging from 18 to 29 eV through analyzing transmittance and reflectance spectra of the films [36]. According to the prospect of Wemple and DiDomenico, Eo and Ed have a significant association with the crystalline structure and ionicity of materials. Ed was found to be closely related to the chemical bonding. The observed simple dependence on coordination number and chemical valency suggests further that nearest-neighbor atomic-like quantities strongly influence the optical properties of materials [35,37]. The following empirical relation for Ed is suitable in crystals containing a single anion species [35,38]: Ed ¼ β N c Z a N e

Table 2 The calculated optical parameters of the sputtered In2S3 layers with different thickness through the Wemple–DiDomenico model, parameters including the single oscillator energy (Eo), dispersion energy (Ed), optical band gap (Eg), high frequency dielectric constant (ε∞), carrier concentration to the effective mass ratio (N/m*), band gap parameter (Ea) and plasma frequency (ħωp). Film thickness

Eo (eV)

Ed (eV)

Eg (eV)

ε∞ Fig. 9

ε∞ Fig. 10

β

N/m* 1048/cm3 g

Ea (eV)

ħωp (eV)

172 nm 380 nm 634 nm 1425 nm

4.82 5.67 5.32 5.63

21.70 25.58 24.40 24.98

2.41 2.84 2.66 2.82

5.50 5.51 5.58 5.44

5.66 5.63 5.71 5.56

0.23 0.27 0.25 0.26

2.00 1.52 1.49 1.59

1.21 1.42 1.33 1.41

5.11 6.03 5.70 5.94

where the parameter β is a constant and takes value of 0.26 ± 0.04 eV for ionic compounds and 0.37 ± 0.05 eV for covalent compounds. Nc, Za, and Ne are the coordination number of the cation nearest to the anion, the formal chemical valency of the anion and the efficient number of valence electrons per anion (usually Ne = 8). For In2S3 films, Nc, Za and Ne takes the value of 6, 2 and 8. The calculated β values shown in Table 2 are approximately 0.26 eV in terms of the fitted Ed. Clearly, In2S3 film appears to be an ionic compound. For further analysis of the optical data, the contribution from the free carrier electric susceptibility χe to the real dielectric constant is discussed according to the Spitzer–Fan model by [39]: 2 e N n2 ¼ ε∞ − λ2 ¼ ε∞ þ 4πχe λ2 πε0 c 2 m

ð7Þ

where ε∞ is the high-frequency dielectric constant without any contribution from free carrier, εo is the permittivity of free space, N/m* is the carrier concentration to the effective mass ratio, e is the electronic charge, and c is the velocity of light. Plotting n2 versus λ2 (Fig. 10) can estimate the values of ε∞ and N/m* by extrapolating the linear part to intercept the n2 axis and the slope, respectively. The calculated parameters ε∞ and N/m* are shown in Table 2. Comparing the ε∞ values achieved from the Wemple–DiDomenico model (Fig. 9) and the Spitzer–Fan model (Fig. 10), their results show satisfactory agreement. The petty difference of ε∞ values is attributed to the contribution from the free electrons. The free carrier susceptibility increases with the wavelength and becomes sufficiently large to reduce dielectric constant in the near-infrared region [40].

ð6Þ 6.50

0.230

172nm 380nm 634nm 1425nm

0.225

172nm 380nm 634nm 1425nm

6.25

0.220

n2

0.215

2

(n -1)

-1

6.00

5.75 0.210

5.50

0.205

0.200

5.25 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

hυ2 (eV )2 Fig. 9. (n2 − 1)−1 versus (hυ)2 plots of In2S3 layers deposited at different thickness.

0

1

2

3

4

5

6

λ2 (μm)2 Fig. 10. Variation of n2 according to λ2 of the In2S3 layers with different thickness.


b -1

d

0.2

-1

1

-1

-1

"380nm"

0.4

.cm .s

0

"172nm"

-1

.cm .s

2

dσp/dt

-1

3

593

dσp/dt

0.0

-1

-0.2 8.4

a

4.0

c

3.5

.cm p

-1

3.0

p

-1

.cm

-1

-1

8.2

8.0

2.5

172nm

2.0 0

10

20

30

40

50

380nm

7.8

60

70

80

0

10

20

Time(s)

30

40

50

60

70

80

Time(s)

Fig. 11. Photoconductivity (σp) response and dσp/dt of the sputtered In2S3 films as a function of time with film thickness ranging from 172 nm to 1425 nm.

The band gap parameter Ea and plasma frequency ħωp are calculated in the range 1.21–1.43 eV and 5.11–6.03 eV, respectively. And the values are shown in Table 2. These indicated that the single oscillator model was appropriate for the optical properties of In2S3 films.

domination of photo-generation for the relatively slow rates of recombination and capture at that moment [43]. Subsequently, the immediately sharp fall of dσp/dt is caused by significant increase of recombination rate which is proportional to the density of photo-generated carriers. The succeeding slower decrease of dσp/dt is due to carrier capture by trapping states which has a much longer time constant resulting in a slower change of carrier density [44]. After turning off the light, the σp decayed slowly for the same reason as discussed in the slow growth of σp. The slow decay of σp results in the σp not decreasing to its dark value at the end of the first decay, and thus σp reaches higher values at the end of the subsequent second illumination period. The increase of σp is clearly shown in the 380 nm and 634 nm samples for high density of trapping states in the two samples.

3.4. Photoconductivity of the In2S3 films

4. Conclusions

The photoconductivity (σp) response of the sputtered In2S3 films with different thickness is illustrated in Fig. 11a, c, e and g. The dark conductivity values (σd) determined just before the onset of the illumination are 1.8, 7.75, 3.9 and 6.7 ∗ 10−4 Ω−1 cm−1 for the 172, 380, 634 and 1425 nm samples, respectively. The σd value of the thickest In2S3 film is 1000 times smaller than that of other In2S3 samples for its low carrier density. This might attribute to less density of thermal activated energy states close to conduction band caused by grain boundaries as a result of larger grain size shown in the upper part of the thickest film [27]. Under light illumination, the σp values of all these four In2S3 samples show a quick rise at the early stage of illumination followed by sublinear increasing behavior, which indicates competition between the rates of photo-generation, recombination and capture by trapping states [42]. Under illumination, σp of 380 and 634 nm films do not show significant rise, only 10% larger than its dark conductivity, indicating numerous disorders presented in the film acting as recombination centers and trapping states. No indication of σp saturation was observed in all the sputtered In2S3 films after 10 s of illumination. To analyze the above competition process, dσp/dt of the photoconductivity curves are shown in Fig. 11b, d, f, g, respectively. At the early stage of illumination, dσp/dt of the In2S3 samples increases rapidly and quickly reaches a maximum value. This phenomenon indicates the

In2S3 layers with different thickness were deposited by RF magnetron sputtering process. The effect of thickness on compositional, structural, optical and photoconductive properties of the films has been studied. The fabricated films exhibit a slightly sulfur deficient composition and a tetragonal phase with preferred orientation along the (103) plane. Change of surface morphology, Raman vibration bands and optical transmittance are observed with the increase of film thickness. Both direct and indirect band gaps were calculated through transmittance spectra. Optical constants were measured by spectroscopic ellipsometry and single oscillator Wemple–DiDomenico model was used to determine single oscillator energy (Eo), dispersion energy (Ed), optical band gap (Eg) and high frequency dielectric constant (ε∞). An empirical relationship with the fitted dispersion energy indicates In2S3 film belonging to ionic class. The ε∞ values show satisfactory agreement with that calculated through the Spitzer–Fan model. Photoconductivity properties of the films were related to rate competition among photo-generation, recombination and trapping of carriers.

In addition, the band gap parameter Ea introduced by Hopfield [41] provides the connection between the single oscillator parameters (Eo, Ed) and the Phillips static–dielectric constant parameters (Eg, ħωp), i.e. Eo Ea ¼ E2g

ð8Þ

2 Ea Ed ¼ ħωp :

ð9Þ

Acknowledgment The authors are grateful for the financial support of the National Natural Science Foundation of China (Nos. 51271155, 51377138, 61404109); the Foundation of National Magnetic Confinement Fusion

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