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Optical properties of (Ga2Se3)0.75 – (Ga2S3)0.25 single crystals by spectroscopic ellipsometry
Physica B: Condensed Matter 560 (2019) 6–10
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Optical properties of (Ga2Se3)0.75 – (Ga2S3)0.25 single crystals by spectroscopic ellipsometry
T
M. Isika,∗, N.M. Gasanlyb,c, L. Gasanovad a
Department of Electrical and Electronics Engineering, Atilim University, 06836 Ankara, Turkey Department of Physics, Middle East Technical University, 06800 Ankara, Turkey c Virtual International Scientific Research Centre, Baku State University, 1148 Baku, Azerbaijan d Department of Physics, Baku State University, 1148 Baku, Azerbaijan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ellipsometry Ga2Se3 Ga2S3 Optical properties
Structural and optical properties of 75 mol % Ga2Se3 – 25 mol % Ga2S3 system of single crystals were investigated by experimental techniques of x-ray diffraction (XRD), energy dispersive spectroscopy, Raman spectroscopy and ellipsometry. XRD pattern indicated that the studied compound has crystalline nature with cubic structure. Vibrational modes in the crystal were revealed using Raman spectroscopy experiments in the 90450 cm−1 frequency range and nine modes were observed in the spectrum. Ellipsometry measurements were utilized in the 1.2–6.2 eV range to get spectral dependencies of optical constants; complex dielectric function, refractive index and extinction coefficient. Under the light of fundamental expressions and models, refractive index and extinction coefficient spectra were analyzed to get various optical parameters of the single crystal.
1. Introduction Ga2Se3 and Ga2S3 are two attractive materials of III2–VI3 (III = Ga, In, Tl and VI = S, Se, Te) type semiconducting compounds which get an important position especially in photovoltaic devices [1–3]. These gallium chalcogenides have been successfully used for chemical and electrical passivation of the surface of III–V type semiconducting and IR/visible optoelectronic devices [4]. The studies on these materials indicated that Ga2Se3 can be used in nuclear particle detection applications [5] whereas Ga2S3 having attractive optoelectronic characteristics has been used in various devices [6–11]. The structural and optical characteristics of Ga2Se3 and Ga2S3 have been investigated for since years. Ga2Se3 has cubic structure with lattice constant of a = 0.5444 nm [12]. The optical characterization methods showed that Ga2Se3 have indirect and direct gap energies of 2.06 and 2.65 eV, respectively [13]. Ga2S3 has three main polytype phases; monoclinic α-Ga2S3 with lattice parameters of a = 1.1094 nm, b = 0.9578 nm, c = 0.6395 nm and γ = 141°, hexagonal β-Ga2S3 with wurtzite type of lattice constants a = 0.36785 nm, c = 0.60166 nm and cubic γ-Ga2S3 with lattice parameter of a = 0.517 nm [14]. These three various phases present different band gap energies as 3.44 eV (α-type), 2.48 eV (β-type) and 2.96 eV (γ-type) [14–16]. Spectroscopic ellipsometry experiments were carried out on Ga2S3 compound by our research group [17]. The spectra of complex dielectric function and
∗
refractive index of this binary crystal were reported in the range of 1.2–6.2 eV. Interband transition energies of Ga2S3 crystals were found as 3.20, 3.75, 4.20 and 4.65 eV. Analyses performed on absorption coefficient resulted in gap energy of 2.48 eV for the compound. The intermediate series of (Ga2Se3)x - (Ga2S3)1-x semiconductor alloys are formed from Ga2Se3 and Ga2S3 binary materials. There are a few studies on the characterization of these mixed compounds. The optical properties of thin film structures of alloys for concentration of x = 0.5 were investigated by transmission measurements and indirect band gap energy of the crystals was found to be 2.33 eV [18]. Spectral dependency of refractive index and analyses results of this dependency in the below band gap energy region was also given in Ref. [18]. The vibrational spectra of (Ga2Se3)x - (Ga2S3)1-x solid solutions were reported by Musaeva et al. for compositions of 0 ≤ x ≤ 1 [19]. Thermally evaporated (Ga2Se3)0.5 - (Ga2S3)0.5 thin films were studied to get information about their ac conductivity and dielectric properties [20]. The measurements were performed in the temperature range of 306–403 K and in the frequency range of 102–105 Hz. The photovoltaic characteristics of n(Ga2S3-Ga2Se3)/p-Si heterojunction were studied by Abd El-Rahman [21]. The present paper aims to expand research studies on characterization of (Ga2Se3)x - (Ga2S3)1-x crystals by investigating structural and optical properties of compound for composition of x = 0.75. The optical methods of spectroscopic ellipsometry and Raman measurements
Corresponding author. E-mail address: [email protected] (M. Isik).
https://doi.org/10.1016/j.physb.2019.02.023 Received 19 November 2018; Received in revised form 12 February 2019; Accepted 13 February 2019 Available online 16 February 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 560 (2019) 6–10
M. Isik, et al.
were carried out while structural characterization was accomplished by x-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) experiments. 2. Experimental details (Ga2Se3)0.75 - (Ga2S3)0.25 semiconducting crystals were grown by Bridgman method in evacuated (10−5 Torr) silica tubes. The ampoule was moved in a vertical furnace through a thermal gradient of 30 °C/cm between temperatures 1000 and 650 °C at a rate of 0.5 mm/h. The synthesized single crystals were structurally characterized by XRD and EDS techniques. XRD experiment was carried out on powder forms of crystals using Rigaku miniflex diffractometer with CuKα radiation (λ = 0.154049 nm). The diffractometer operated by a scanning speed of 0.02°/s. The diffraction angel (2θ) range for measurements was chosen as 20–80°. EDS experiments were accomplished using scanning electron microscope (Nova NanoSEM 430) to get information about the chemical composition of the studied compound. Raman scattering spectrum was obtained by 532 nm line of YAG:Nd3+ laser. The spectrum was recorded in the frequency range of 90–450 cm−1 with Horiba Yvon RMS-550 Raman spectrometer with resolution of 1 cm−1. The spectroscopic ellipsometry measurements on (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals were performed in the 1.2–6.2 eV spectral range by SOPRA GES-5E rotating polarizer ellipsometer. The incident angle (φ) of the light was applied as 70°. The thickness of the crystals used for ellipsometry measurements was measured as ∼2.6 mm by standard micrometer.
Fig. 2. Energy dispersive spectrum of (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals.
of GaSe and GaS compounds. The lattice parameter of crystal structure of (Ga2Se3)0.75 - (Ga2S3)0.25 were obtained by powder diffraction indexing program of DICVOL04. The analyses presented the lattice parameter of the crystal as a = 0.5350 nm which is slightly smaller than that of Ga2Se3. This smaller value may be related to reduction of Ga2Se3 lattice due to substitution of larger selenium elements (radius 0.117 nm) with sulfur elements (radius 0.104 nm). The atomic compositions of the elements constituting the (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals were identified by EDS analyses. Fig. 2 presents the EDS spectrum in which peaks are related to characteristic x-rays emitted from the constituent elements. EDS spectrum exhibits four peaks which are associated with elements given on the peak according to characteristic emission energies of these elements. The atomic compositions of the elements (Ga: S: Se) were determined as (42.2: 13.5: 44.3) which is well correlated with (8: 3: 9) ratio presented in 75 mol % Ga2Se3 – 25 mol % Ga2S3. Optical properties of the crystals were studied by Raman spectroscopy and ellipsometry measurements. Fig. 3 presents the unpolarized Raman spectrum of single crystals recorded in the frequency range of 90–450 cm−1. The spectrum exhibits nine peaks at 122, 146, 172, 239, 256, 294, 346, 375 and 402 cm−1 as indicated by arrows on the figure. Most of the revealed frequencies may be associated with internal and external vibrations of tetrahedral GaS(Se)4 groups [24]. At this point it would be worthwhile to discuss observed frequencies under the light of
3. Results and discussion Structural characterization of (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals were investigated using XRD and EDS measurements. Fig. 1 indicates XRD diffraction patterns of the studied crystal and for the Ga2Se3 compound. The presence of sharp intensive peaks signs for good crystallinity of the samples. As seen from the figure, XRD patterns are similar to each other from the point of view of peak positions and relative intensities. The crystalline structure of the Ga2Se3 is given in JCPDS card as cubic with lattice parameter of a = 0.5429 nm. Although XRD patterns of (Ga2Se3)0.75 - (Ga2S3)0.25 and Ga2Se3 are detected around the same angles, it was observed that peak positions of studied sample are slightly larger. This shift in the diffraction angle was previously observed for many materials [22,23]. In Ref. [22], shift of the peaks to higher angles is associated with increase of sulfur composition in the GaSe1-xSx mixed crystals which are formed from the combination
Fig. 1. XRD pattern of (Ga2Se3)0.75 - (Ga2S3)0.25 and Ga2Se3 single crystals.
Fig. 3. Raman spectrum of (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals. 7
Physica B: Condensed Matter 560 (2019) 6–10
M. Isik, et al.
Fig. 4. Spectra of amplitude ratio (ψ) and phase difference (Δ).
results of previously reported papers on similar compound structures. The dominant peaks at 172 and 294 cm−1 in the spectra of (Ga2Se3)0.75 - (Ga2S3)0.25 crystals were assigned according to Julien et al. [25] to F2 and A1 modes of a Ga(Se,S)4 molecular unit, respectively. A detailed study on the Raman spectroscopy of sulfur and sulfur-selenium mixture was reported by A.T. Ward [26]. In the Raman spectrum of crystalline sulfur, two peaks which are closer to our revealed values were observed at 147 and 252 cm−1 at 25 °C. Author assigned these peaks to E2 symmetry (147 cm−1) and E3 (252 cm−1) species. When the Raman spectrum of S-Se mixture (S0.67Se0.33) was recorded, three peaks which are in good agreement with some of our frequencies were observed at 122, 344 and 380 cm−1. Taking into consideration reported these values, peaks at 122, 346 and 375 cm−1 are thought due to S-Se vibrations. Spectroscopic ellipsometry experiments were conducted on the samples in the 1.2–6.2 eV region. Fig. 4 shows the ellipsometric data of ψ ─ E and Δ ─ E plots. ψ and Δ symbolize the amplitude ratio of and phase difference between parallel and perpendicular parts of the reflected light from the irradiated crystal by a polarized light. The complex dielectric function of the sample was obtained using ellipsometric data by the help of air/sample optical model which is expressed as [27]
Fig. 5. Spectra of real (ɛ1) and imaginary (ɛ2) components of complex dielectric function of (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals.
The refractive index (n) and extinction coefficient (k) spectra were plotted as shown in Fig. 6 using ɛ1 and ɛ2 spectra and expressions [33]
2
1 − ρ⎞ ⎡ ⎤ tan2 (ϕ) ⎥, ε = ε1 + iε2 = sin2 (ϕ) ⎢1 + ⎜⎛ ⎟ 1 + ρ ⎝ ⎠ ⎣ ⎦
1/2
k = [ (−ε1 + (ε12 + ε22)1/2)/2] (1)
1/2
n = [(ε1 + (ε12 + ε22)1/2)/2]
where ρ = tan (ψ) eiΔ is the complex reflectance ratio of the polarized light and φ is the angle of incidence. Fig. 5 shows ɛ1 and ɛ2 spectra derived from Eq. (1). The peak energy value of 4.1 eV recorded in ɛ2spectrum is related to the energy of interband transition in which an electron occupying in a state in the valence band is excited to an unoccupied state in the conduction band [27,28]. Moreover, there exists a change of curvature in the ɛ2-spectrum around 2.15 eV. In literature there are some reported ellipsometry papers in which a sharp or slight change in the ɛ2-spectrum was observed [29,30]. Taking into consideration these previously reported studies, this energy value may be related to gap energy (Eg) of the compound. Moreover, the imaginary part theoretically should be zero in the below band gap energy region. However, our ɛ2-spectrum does not conform to this condition. At the present time the reason of this below-band gap behavior for the studied crystal is not clear. Previously, similar absorption tails below the fundamental gap edge were also reported in the papers on optical characterization of CuIn5Se8, CuGa5Se8 [31] and CuAlxIn1-xSe2 [32] materials investigated by ellipsometry method. Authors attributed this nonzero behavior of the imaginary component to the intrinsic contributions (alloy disorder) and deviation from stoichiometry. Taking into consideration these papers and authors thought, one of the possible reasons for (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals may be the deviation from stoichiometry during the crystal growth process.
,
.
(2) (3)
Extinction coefficient and absorption coefficient (α) are related to each other by the well-known fundamental equation of α = 4πk/λ. Band gap energy of the compound can be determined using the relation [33]
(αhν ) = A (hν − Eg ) p,
(4)
where constant A depends on transition probability and index p takes values of either 2 for indirect transition or 1/2 for direct transition. The plot of (αhν)1/2 vs. hv given in Fig. 7 presented the suitable behavior according to Eq. (4). The direct gap energy was determined from interception of fitted straight line on energy axis as 2.15 eV. Eq. (4) can also be re-written mathematically [34]
p d ln(αhv ) = . d (hv ) hv − Eg
(5)
According to this equation d(ln(αhv))/d(hv) vs. (hv) plot presents a peak having its maximum intensity at gap energy. Inset of Fig. 7 shows the corresponding plot exhibiting a peak around 2.17 eV. This value is in good agreement with 2.15 eV obtained according to Eq. (4). At this point it would be beneficial to compare this revealed energy with those of Ga2Se3 and Ga2S3 crystals. Band gap energies of Ga2Se3 and Ga2S3 compounds were reported as 2.06 eV [13] and 2.48 eV [17], 8
Physica B: Condensed Matter 560 (2019) 6–10
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Fig. 6. Spectra of refractive index and extinction coefficient of (Ga2Se3)0.75 (Ga2S3)0.25 single crystals. Fig. 7. The dependency of (αhν)1/2 on photon energy. The solid line is fitted linear line according to Eq. (5). Inset shows the photon energy dependency of d (ln(αhv))/d(hv).
respectively. The revealed gap energy of 2.15 eV for (Ga2Se3)0.75 (Ga2S3)0.25 single crystals lies between these energy values. If a linear variation of gap energy with composition was considered for (Ga2Se3)x (Ga2S3)1-x materials, ∼2.17 eV value corresponds to gap energy of compound with x = 0.75 composition. This energy is very close to revealed energy value for studied crystal. The refractive index of (Ga2Se3)0.75 - (Ga2S3)0.25 semiconducting crystals gets value of 2.52 around the band gap energy (∼2.15 eV). The refractive index and gap energy values are related to each other by various expressions [35]. One of these relations is given by Moss as
n4Eg = 95 eV .
(6)
Refractive index value was calculated as 2.58 for gap energy of 2.15 eV using Eq. (6). The experimentally determined value of 2.52 is in good agreement with calculated value. Single effective oscillator model states the energy dependency of refractive index in the below band gap energy region by the expression [36]
n2 (hν ) = 1 +
Eso Ed Eso2 − (hν )2
(7)
where Eso and Ed abbreviate for single oscillator energy and dispersion energy, respectively. According to this expression, (n2−1)−1 vs. (hv)2 plot can be utilized to get Eso described as a measure of the intensity of interband optical transition and Ed defined as average band gap energy. Eso and Ed energies were found as 7.53 and 36.1 using the slope and intercept of fitted line (see Fig. 8). The zero-frequency (v = 0) refractive index (n0) and dielectric constant (ε0) were determined using formula of n0 = (1 + Ed/Eso)−1 and ε0 = n02 as 2.41 and 5.79, respectively. Spitzer-Fan model expresses the real part of the dielectric function as [37]
e2 N ⎞ λ2 ε1 = n2 − k 2 = ε∞ − ⎡ 2 ⎤ ⎛ ⎢ ⎥ πc m ∗⎠ ⎝ ⎣ ⎦
Fig. 8. Graph of (n2-1)−1 vs. (hv)2. Inset: Plot of n2 – k2 versus λ2. Solid lines in each plot indicates the linear fits.
Here, ε∞ is the high-frequency dielectric constant, N is carrier concentration and m* is effective mass. According to Eq. (8), the linear dependency of ε1 - λ2 can be used to determine ε∞ and N/m* which were found as 7.26 and 3.78 × 1051 kg−1 cm−3, respectively (see inset of Fig. 8).
(8) 9
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4. Conclusion
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The structural and optical properties of 75 mol % Ga2Se3 – 25 mol %Ga2S3 single crystals were investigated in the present paper. XRD analyses indicated that studied semiconducting sing crystals have cubic crystalline structure with lattice parameters of a = 0.5350 nm. EDS analyses resulted in atomic composition of (Ga: S: Se) ≡ (42.2: 13.5: 44.3) which is well correlated with 75 mol % Ga2Se3 – 25 mol %Ga2S3 composition. Raman spectrum exhibited nine peaks at 122, 146, 172, 239, 256, 194, 346, 375 and 402 cm−1. Optical parameters of the crystals were obtained from the analyses of ellipsometric data. The spectral dependencies of refractive index, extinction coefficient and components of complex dielectric function were plotted in the 1.2–6.2 eV region. Indirect band gap energy of the crystals was found as 2.15 eV from the photon energy dependency of absorption coefficient. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.physb.2019.02.023. References [1] B. Asenjo, C. Guillen, A.M. Chaparro, E. Saucedo, V. Bermudez, D. Lincot, J. Herrero, M.T. Gutierrez, Properties of In2S3 thin films deposited onto ITO/glass substrates by chemical bath deposition, J. Phys. Chem. Solid. 71 (2010) 1629–1633. [2] N. Balakrishnan, E.D. Steer, E.F. Smith, Z.R. Kudrynskyi, Z.D. Kovalyuk, L. Eaves, A. Patane, P.H. Beton, Epitaxial growth of γ-InSe and α, β and γ-In2Se3 on ɛ-GaSe, 2D Mater. 5 (2018) 035026. [3] M.J. Zhang, X.M. Jiang, L.J. Zhou, G.C. Guo, Two phases of Ga2S3: promising infrared second-order nonlinear optical materials with very high laser induced damage thresholds, J. Mater. Chem. C 1 (2013) 4754–4760. [4] B.I. Sysoev, N.N. Bezryadin, G.I. Kotov, B.L. Agapov, V.D. Strygin, Passivation of the GaAs (100) surface by III2-VI3 (110) gallium chalcogenides, Semiconductors 29 (1995) 12–16. [5] Y.G. Gurevich, V.M. Koshkin, I.N. Volovichev, The heterocontact of two intrinsic semiconductors and radiation stable electronics, Solid State Electron. 38 (1995) 235–242. [6] C.S. Yoon, F.D. Medina, L. Martinez, T.Y. Park, M.S. Jin, W.T. Kim, Blue photoluminescence of α-Ga2S3, α-Ga2S3:Fe2+ single crystals, Appl. Phys. Lett. 83 (2003) 1947–1949. [7] S.E. Al Garni, A.F. Qasrawi, Design and characterization of the Ge/Ga2S3 heterojunction, J. Electron. Mater. 46 (2017) 4848–4856. [8] M.Z. Kovalyuk, V.I. Vitkovskaya, M.V. Tovarnitskii, p-GaSe–n-Ga2S3 heterojunctions, Tech. Phys. Lett. 23 (1997) 385-385. [9] Z. Huang, J.G. Huang, K.A. Kokh, V.A. Svetlichnyi, A.V. Shabalina, Y.M. Andreev, G.V. Lanskii, Ga2S3: optical properties and perspectives for THz applications, 2015 40th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2015, pp. 1–2. [10] M. Zervos, A. Othonos, V. Gianneta, A. Travlos, A.G. Nassiopoulou, Sn doped βGa2O3 and β-Ga2S3 nanowires with red emission for solar energy spectral shifting, J. Appl. Phys. 118 (2015) 194302. [11] C.H. Ho, M.H. Lin, Y.P. Wang, Y.S. Huang, Synthesis of In2S3 and Ga2S3 crystals for oxygen sensing and UV photodetection, Sensor Actuat. A-Phys. 245 (2016) 119–126.
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Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Optical properties of (Ga2Se3)0.75 – (Ga2S3)0.25 single crystals by spectroscopic ellipsometry
T
M. Isika,∗, N.M. Gasanlyb,c, L. Gasanovad a
Department of Electrical and Electronics Engineering, Atilim University, 06836 Ankara, Turkey Department of Physics, Middle East Technical University, 06800 Ankara, Turkey c Virtual International Scientific Research Centre, Baku State University, 1148 Baku, Azerbaijan d Department of Physics, Baku State University, 1148 Baku, Azerbaijan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ellipsometry Ga2Se3 Ga2S3 Optical properties
Structural and optical properties of 75 mol % Ga2Se3 – 25 mol % Ga2S3 system of single crystals were investigated by experimental techniques of x-ray diffraction (XRD), energy dispersive spectroscopy, Raman spectroscopy and ellipsometry. XRD pattern indicated that the studied compound has crystalline nature with cubic structure. Vibrational modes in the crystal were revealed using Raman spectroscopy experiments in the 90450 cm−1 frequency range and nine modes were observed in the spectrum. Ellipsometry measurements were utilized in the 1.2–6.2 eV range to get spectral dependencies of optical constants; complex dielectric function, refractive index and extinction coefficient. Under the light of fundamental expressions and models, refractive index and extinction coefficient spectra were analyzed to get various optical parameters of the single crystal.
1. Introduction Ga2Se3 and Ga2S3 are two attractive materials of III2–VI3 (III = Ga, In, Tl and VI = S, Se, Te) type semiconducting compounds which get an important position especially in photovoltaic devices [1–3]. These gallium chalcogenides have been successfully used for chemical and electrical passivation of the surface of III–V type semiconducting and IR/visible optoelectronic devices [4]. The studies on these materials indicated that Ga2Se3 can be used in nuclear particle detection applications [5] whereas Ga2S3 having attractive optoelectronic characteristics has been used in various devices [6–11]. The structural and optical characteristics of Ga2Se3 and Ga2S3 have been investigated for since years. Ga2Se3 has cubic structure with lattice constant of a = 0.5444 nm [12]. The optical characterization methods showed that Ga2Se3 have indirect and direct gap energies of 2.06 and 2.65 eV, respectively [13]. Ga2S3 has three main polytype phases; monoclinic α-Ga2S3 with lattice parameters of a = 1.1094 nm, b = 0.9578 nm, c = 0.6395 nm and γ = 141°, hexagonal β-Ga2S3 with wurtzite type of lattice constants a = 0.36785 nm, c = 0.60166 nm and cubic γ-Ga2S3 with lattice parameter of a = 0.517 nm [14]. These three various phases present different band gap energies as 3.44 eV (α-type), 2.48 eV (β-type) and 2.96 eV (γ-type) [14–16]. Spectroscopic ellipsometry experiments were carried out on Ga2S3 compound by our research group [17]. The spectra of complex dielectric function and
∗
refractive index of this binary crystal were reported in the range of 1.2–6.2 eV. Interband transition energies of Ga2S3 crystals were found as 3.20, 3.75, 4.20 and 4.65 eV. Analyses performed on absorption coefficient resulted in gap energy of 2.48 eV for the compound. The intermediate series of (Ga2Se3)x - (Ga2S3)1-x semiconductor alloys are formed from Ga2Se3 and Ga2S3 binary materials. There are a few studies on the characterization of these mixed compounds. The optical properties of thin film structures of alloys for concentration of x = 0.5 were investigated by transmission measurements and indirect band gap energy of the crystals was found to be 2.33 eV [18]. Spectral dependency of refractive index and analyses results of this dependency in the below band gap energy region was also given in Ref. [18]. The vibrational spectra of (Ga2Se3)x - (Ga2S3)1-x solid solutions were reported by Musaeva et al. for compositions of 0 ≤ x ≤ 1 [19]. Thermally evaporated (Ga2Se3)0.5 - (Ga2S3)0.5 thin films were studied to get information about their ac conductivity and dielectric properties [20]. The measurements were performed in the temperature range of 306–403 K and in the frequency range of 102–105 Hz. The photovoltaic characteristics of n(Ga2S3-Ga2Se3)/p-Si heterojunction were studied by Abd El-Rahman [21]. The present paper aims to expand research studies on characterization of (Ga2Se3)x - (Ga2S3)1-x crystals by investigating structural and optical properties of compound for composition of x = 0.75. The optical methods of spectroscopic ellipsometry and Raman measurements
Corresponding author. E-mail address: [email protected] (M. Isik).
https://doi.org/10.1016/j.physb.2019.02.023 Received 19 November 2018; Received in revised form 12 February 2019; Accepted 13 February 2019 Available online 16 February 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
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were carried out while structural characterization was accomplished by x-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) experiments. 2. Experimental details (Ga2Se3)0.75 - (Ga2S3)0.25 semiconducting crystals were grown by Bridgman method in evacuated (10−5 Torr) silica tubes. The ampoule was moved in a vertical furnace through a thermal gradient of 30 °C/cm between temperatures 1000 and 650 °C at a rate of 0.5 mm/h. The synthesized single crystals were structurally characterized by XRD and EDS techniques. XRD experiment was carried out on powder forms of crystals using Rigaku miniflex diffractometer with CuKα radiation (λ = 0.154049 nm). The diffractometer operated by a scanning speed of 0.02°/s. The diffraction angel (2θ) range for measurements was chosen as 20–80°. EDS experiments were accomplished using scanning electron microscope (Nova NanoSEM 430) to get information about the chemical composition of the studied compound. Raman scattering spectrum was obtained by 532 nm line of YAG:Nd3+ laser. The spectrum was recorded in the frequency range of 90–450 cm−1 with Horiba Yvon RMS-550 Raman spectrometer with resolution of 1 cm−1. The spectroscopic ellipsometry measurements on (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals were performed in the 1.2–6.2 eV spectral range by SOPRA GES-5E rotating polarizer ellipsometer. The incident angle (φ) of the light was applied as 70°. The thickness of the crystals used for ellipsometry measurements was measured as ∼2.6 mm by standard micrometer.
Fig. 2. Energy dispersive spectrum of (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals.
of GaSe and GaS compounds. The lattice parameter of crystal structure of (Ga2Se3)0.75 - (Ga2S3)0.25 were obtained by powder diffraction indexing program of DICVOL04. The analyses presented the lattice parameter of the crystal as a = 0.5350 nm which is slightly smaller than that of Ga2Se3. This smaller value may be related to reduction of Ga2Se3 lattice due to substitution of larger selenium elements (radius 0.117 nm) with sulfur elements (radius 0.104 nm). The atomic compositions of the elements constituting the (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals were identified by EDS analyses. Fig. 2 presents the EDS spectrum in which peaks are related to characteristic x-rays emitted from the constituent elements. EDS spectrum exhibits four peaks which are associated with elements given on the peak according to characteristic emission energies of these elements. The atomic compositions of the elements (Ga: S: Se) were determined as (42.2: 13.5: 44.3) which is well correlated with (8: 3: 9) ratio presented in 75 mol % Ga2Se3 – 25 mol % Ga2S3. Optical properties of the crystals were studied by Raman spectroscopy and ellipsometry measurements. Fig. 3 presents the unpolarized Raman spectrum of single crystals recorded in the frequency range of 90–450 cm−1. The spectrum exhibits nine peaks at 122, 146, 172, 239, 256, 294, 346, 375 and 402 cm−1 as indicated by arrows on the figure. Most of the revealed frequencies may be associated with internal and external vibrations of tetrahedral GaS(Se)4 groups [24]. At this point it would be worthwhile to discuss observed frequencies under the light of
3. Results and discussion Structural characterization of (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals were investigated using XRD and EDS measurements. Fig. 1 indicates XRD diffraction patterns of the studied crystal and for the Ga2Se3 compound. The presence of sharp intensive peaks signs for good crystallinity of the samples. As seen from the figure, XRD patterns are similar to each other from the point of view of peak positions and relative intensities. The crystalline structure of the Ga2Se3 is given in JCPDS card as cubic with lattice parameter of a = 0.5429 nm. Although XRD patterns of (Ga2Se3)0.75 - (Ga2S3)0.25 and Ga2Se3 are detected around the same angles, it was observed that peak positions of studied sample are slightly larger. This shift in the diffraction angle was previously observed for many materials [22,23]. In Ref. [22], shift of the peaks to higher angles is associated with increase of sulfur composition in the GaSe1-xSx mixed crystals which are formed from the combination
Fig. 1. XRD pattern of (Ga2Se3)0.75 - (Ga2S3)0.25 and Ga2Se3 single crystals.
Fig. 3. Raman spectrum of (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals. 7
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Fig. 4. Spectra of amplitude ratio (ψ) and phase difference (Δ).
results of previously reported papers on similar compound structures. The dominant peaks at 172 and 294 cm−1 in the spectra of (Ga2Se3)0.75 - (Ga2S3)0.25 crystals were assigned according to Julien et al. [25] to F2 and A1 modes of a Ga(Se,S)4 molecular unit, respectively. A detailed study on the Raman spectroscopy of sulfur and sulfur-selenium mixture was reported by A.T. Ward [26]. In the Raman spectrum of crystalline sulfur, two peaks which are closer to our revealed values were observed at 147 and 252 cm−1 at 25 °C. Author assigned these peaks to E2 symmetry (147 cm−1) and E3 (252 cm−1) species. When the Raman spectrum of S-Se mixture (S0.67Se0.33) was recorded, three peaks which are in good agreement with some of our frequencies were observed at 122, 344 and 380 cm−1. Taking into consideration reported these values, peaks at 122, 346 and 375 cm−1 are thought due to S-Se vibrations. Spectroscopic ellipsometry experiments were conducted on the samples in the 1.2–6.2 eV region. Fig. 4 shows the ellipsometric data of ψ ─ E and Δ ─ E plots. ψ and Δ symbolize the amplitude ratio of and phase difference between parallel and perpendicular parts of the reflected light from the irradiated crystal by a polarized light. The complex dielectric function of the sample was obtained using ellipsometric data by the help of air/sample optical model which is expressed as [27]
Fig. 5. Spectra of real (ɛ1) and imaginary (ɛ2) components of complex dielectric function of (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals.
The refractive index (n) and extinction coefficient (k) spectra were plotted as shown in Fig. 6 using ɛ1 and ɛ2 spectra and expressions [33]
2
1 − ρ⎞ ⎡ ⎤ tan2 (ϕ) ⎥, ε = ε1 + iε2 = sin2 (ϕ) ⎢1 + ⎜⎛ ⎟ 1 + ρ ⎝ ⎠ ⎣ ⎦
1/2
k = [ (−ε1 + (ε12 + ε22)1/2)/2] (1)
1/2
n = [(ε1 + (ε12 + ε22)1/2)/2]
where ρ = tan (ψ) eiΔ is the complex reflectance ratio of the polarized light and φ is the angle of incidence. Fig. 5 shows ɛ1 and ɛ2 spectra derived from Eq. (1). The peak energy value of 4.1 eV recorded in ɛ2spectrum is related to the energy of interband transition in which an electron occupying in a state in the valence band is excited to an unoccupied state in the conduction band [27,28]. Moreover, there exists a change of curvature in the ɛ2-spectrum around 2.15 eV. In literature there are some reported ellipsometry papers in which a sharp or slight change in the ɛ2-spectrum was observed [29,30]. Taking into consideration these previously reported studies, this energy value may be related to gap energy (Eg) of the compound. Moreover, the imaginary part theoretically should be zero in the below band gap energy region. However, our ɛ2-spectrum does not conform to this condition. At the present time the reason of this below-band gap behavior for the studied crystal is not clear. Previously, similar absorption tails below the fundamental gap edge were also reported in the papers on optical characterization of CuIn5Se8, CuGa5Se8 [31] and CuAlxIn1-xSe2 [32] materials investigated by ellipsometry method. Authors attributed this nonzero behavior of the imaginary component to the intrinsic contributions (alloy disorder) and deviation from stoichiometry. Taking into consideration these papers and authors thought, one of the possible reasons for (Ga2Se3)0.75 - (Ga2S3)0.25 single crystals may be the deviation from stoichiometry during the crystal growth process.
,
.
(2) (3)
Extinction coefficient and absorption coefficient (α) are related to each other by the well-known fundamental equation of α = 4πk/λ. Band gap energy of the compound can be determined using the relation [33]
(αhν ) = A (hν − Eg ) p,
(4)
where constant A depends on transition probability and index p takes values of either 2 for indirect transition or 1/2 for direct transition. The plot of (αhν)1/2 vs. hv given in Fig. 7 presented the suitable behavior according to Eq. (4). The direct gap energy was determined from interception of fitted straight line on energy axis as 2.15 eV. Eq. (4) can also be re-written mathematically [34]
p d ln(αhv ) = . d (hv ) hv − Eg
(5)
According to this equation d(ln(αhv))/d(hv) vs. (hv) plot presents a peak having its maximum intensity at gap energy. Inset of Fig. 7 shows the corresponding plot exhibiting a peak around 2.17 eV. This value is in good agreement with 2.15 eV obtained according to Eq. (4). At this point it would be beneficial to compare this revealed energy with those of Ga2Se3 and Ga2S3 crystals. Band gap energies of Ga2Se3 and Ga2S3 compounds were reported as 2.06 eV [13] and 2.48 eV [17], 8
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Fig. 6. Spectra of refractive index and extinction coefficient of (Ga2Se3)0.75 (Ga2S3)0.25 single crystals. Fig. 7. The dependency of (αhν)1/2 on photon energy. The solid line is fitted linear line according to Eq. (5). Inset shows the photon energy dependency of d (ln(αhv))/d(hv).
respectively. The revealed gap energy of 2.15 eV for (Ga2Se3)0.75 (Ga2S3)0.25 single crystals lies between these energy values. If a linear variation of gap energy with composition was considered for (Ga2Se3)x (Ga2S3)1-x materials, ∼2.17 eV value corresponds to gap energy of compound with x = 0.75 composition. This energy is very close to revealed energy value for studied crystal. The refractive index of (Ga2Se3)0.75 - (Ga2S3)0.25 semiconducting crystals gets value of 2.52 around the band gap energy (∼2.15 eV). The refractive index and gap energy values are related to each other by various expressions [35]. One of these relations is given by Moss as
n4Eg = 95 eV .
(6)
Refractive index value was calculated as 2.58 for gap energy of 2.15 eV using Eq. (6). The experimentally determined value of 2.52 is in good agreement with calculated value. Single effective oscillator model states the energy dependency of refractive index in the below band gap energy region by the expression [36]
n2 (hν ) = 1 +
Eso Ed Eso2 − (hν )2
(7)
where Eso and Ed abbreviate for single oscillator energy and dispersion energy, respectively. According to this expression, (n2−1)−1 vs. (hv)2 plot can be utilized to get Eso described as a measure of the intensity of interband optical transition and Ed defined as average band gap energy. Eso and Ed energies were found as 7.53 and 36.1 using the slope and intercept of fitted line (see Fig. 8). The zero-frequency (v = 0) refractive index (n0) and dielectric constant (ε0) were determined using formula of n0 = (1 + Ed/Eso)−1 and ε0 = n02 as 2.41 and 5.79, respectively. Spitzer-Fan model expresses the real part of the dielectric function as [37]
e2 N ⎞ λ2 ε1 = n2 − k 2 = ε∞ − ⎡ 2 ⎤ ⎛ ⎢ ⎥ πc m ∗⎠ ⎝ ⎣ ⎦
Fig. 8. Graph of (n2-1)−1 vs. (hv)2. Inset: Plot of n2 – k2 versus λ2. Solid lines in each plot indicates the linear fits.
Here, ε∞ is the high-frequency dielectric constant, N is carrier concentration and m* is effective mass. According to Eq. (8), the linear dependency of ε1 - λ2 can be used to determine ε∞ and N/m* which were found as 7.26 and 3.78 × 1051 kg−1 cm−3, respectively (see inset of Fig. 8).
(8) 9
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4. Conclusion
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The structural and optical properties of 75 mol % Ga2Se3 – 25 mol %Ga2S3 single crystals were investigated in the present paper. XRD analyses indicated that studied semiconducting sing crystals have cubic crystalline structure with lattice parameters of a = 0.5350 nm. EDS analyses resulted in atomic composition of (Ga: S: Se) ≡ (42.2: 13.5: 44.3) which is well correlated with 75 mol % Ga2Se3 – 25 mol %Ga2S3 composition. Raman spectrum exhibited nine peaks at 122, 146, 172, 239, 256, 194, 346, 375 and 402 cm−1. Optical parameters of the crystals were obtained from the analyses of ellipsometric data. The spectral dependencies of refractive index, extinction coefficient and components of complex dielectric function were plotted in the 1.2–6.2 eV region. Indirect band gap energy of the crystals was found as 2.15 eV from the photon energy dependency of absorption coefficient. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.physb.2019.02.023. References [1] B. Asenjo, C. Guillen, A.M. Chaparro, E. Saucedo, V. Bermudez, D. Lincot, J. Herrero, M.T. Gutierrez, Properties of In2S3 thin films deposited onto ITO/glass substrates by chemical bath deposition, J. Phys. Chem. Solid. 71 (2010) 1629–1633. [2] N. Balakrishnan, E.D. Steer, E.F. Smith, Z.R. Kudrynskyi, Z.D. Kovalyuk, L. Eaves, A. Patane, P.H. Beton, Epitaxial growth of γ-InSe and α, β and γ-In2Se3 on ɛ-GaSe, 2D Mater. 5 (2018) 035026. [3] M.J. Zhang, X.M. Jiang, L.J. Zhou, G.C. Guo, Two phases of Ga2S3: promising infrared second-order nonlinear optical materials with very high laser induced damage thresholds, J. Mater. Chem. C 1 (2013) 4754–4760. [4] B.I. Sysoev, N.N. Bezryadin, G.I. Kotov, B.L. Agapov, V.D. Strygin, Passivation of the GaAs (100) surface by III2-VI3 (110) gallium chalcogenides, Semiconductors 29 (1995) 12–16. [5] Y.G. Gurevich, V.M. Koshkin, I.N. Volovichev, The heterocontact of two intrinsic semiconductors and radiation stable electronics, Solid State Electron. 38 (1995) 235–242. [6] C.S. Yoon, F.D. Medina, L. Martinez, T.Y. Park, M.S. Jin, W.T. Kim, Blue photoluminescence of α-Ga2S3, α-Ga2S3:Fe2+ single crystals, Appl. Phys. Lett. 83 (2003) 1947–1949. [7] S.E. Al Garni, A.F. Qasrawi, Design and characterization of the Ge/Ga2S3 heterojunction, J. Electron. Mater. 46 (2017) 4848–4856. [8] M.Z. Kovalyuk, V.I. Vitkovskaya, M.V. Tovarnitskii, p-GaSe–n-Ga2S3 heterojunctions, Tech. Phys. Lett. 23 (1997) 385-385. [9] Z. Huang, J.G. Huang, K.A. Kokh, V.A. Svetlichnyi, A.V. Shabalina, Y.M. Andreev, G.V. Lanskii, Ga2S3: optical properties and perspectives for THz applications, 2015 40th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2015, pp. 1–2. [10] M. Zervos, A. Othonos, V. Gianneta, A. Travlos, A.G. Nassiopoulou, Sn doped βGa2O3 and β-Ga2S3 nanowires with red emission for solar energy spectral shifting, J. Appl. Phys. 118 (2015) 194302. [11] C.H. Ho, M.H. Lin, Y.P. Wang, Y.S. Huang, Synthesis of In2S3 and Ga2S3 crystals for oxygen sensing and UV photodetection, Sensor Actuat. A-Phys. 245 (2016) 119–126.
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