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Investigations of thermal annealing role on the optical properties of Zn-In-Se thin films
Accepted Manuscript Title: Investigations of Thermal Annealing Role on the Optical Properties of Zn-In-Se Thin Films Authors: H.H. Gull ¨ u, ¨ E. Cos¸kun, M. Parlak PII: DOI: Reference:
S0030-4026(17)30775-1 http://dx.doi.org/doi:10.1016/j.ijleo.2017.06.106 IJLEO 59369
To appear in: Received date: Revised date: Accepted date:
23-2-2017 10-6-2017 25-6-2017
Please cite this article as: H.H.Gull ¨ u, ¨ E.Cos¸kun, M.Parlak, Investigations of Thermal Annealing Role on the Optical Properties of Zn-In-Se Thin Films, Optik - International Journal for Light and Electron Opticshttp://dx.doi.org/10.1016/j.ijleo.2017.06.106 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigations of Thermal Annealing Role on the Optical Properties of Zn-In-Se Thin Films H.H. Güllü1,4,*, E. Coşkun2,3,4, M. Parlak2,4 1 Central Laboratory, Middle East Technical University (METU), 06800 Ankara, Turkey Department of Physics, Middle East Technical University (METU), 06800 Ankara, Turkey 3 Department of Physics, Çanakkale Onsekiz Mart University, 17100 Çanakkale, Turkey 4 Center for Solar Energy Research and Applications (GÜNAM), METU, Ankara 06800, Turkey 2
*Corresponding author: Tel: +90 312 2106440; Fax: +90 312 2106425 Email: [email protected]
Abstract
Zn-In-Se (ZIS) thin films were prepared by sequential evaporation of its elemental sources on the glass substrates. The effect of thermal annealing under nitrogen environment on the optical properties of the films was discussed. In addition to the comparative study of three different annealing temperatures, the results were analyzed by relating with their structural and compositional characteristics. The optical analyses were based on the observed interference fringes on their transmission spectra of the films. The refractive indices were calculated by means of envelope method (EM) and the continuity of the refractive indices was evaluated by three-term Cauchy fitting process. From the results of the refractive index calculations, the real and imaginary part of the dielectric constant were determined. The optical absorption coefficients of the films were found in the range of 103-104 cm-1 over the visible and near-infrared region and by using these values, the extinction coefficients were calculated. Moreover, the band gap values were calculated from the corresponding Tauc plots, and the refractive index dispersion over the measured wavelength range was investigated with single-oscillator model (SOM) and the related parameters were obtained.
1. Introduction
In thin film materials technology, most of the recorded photovoltaic structures are fabricated with a thin CdS buffer layer [1-5]. Apart from the success of the hetero-structures including a CdS buffer layer, there are environmental safety concern on usage of Cd [1, 5]. In addition, alternative materials to this layer can provide a large band gap which allows transmission of higher energy photons than the band gap of CdS. Among these materials, ZnSe has been an attractive buffer layer with band gap larger than CdS and the lattice characteristics is also in a good match with the popular chalcopyrite and kesterite thin film absorbers [6, 7]. This binary structure belongs to II-VI family and crystallizes in the cubic zinc blende structure. Although ZnSe is a very promising compound for the thin film applications of solar cells [8-11], there are some difficulties in the control of atomic ratios of the constituent elements and as a result of the nature of the defects in the structure, and common high resistivity problem of the ZnSe films obstruct their photovoltaic applications. Therefore, alloying polycrystalline wide band gap thin film semiconductors with elements in group III is a familiar application to optimize the resistivity of these materials [12, 13]. In view of the fact that interests on ZnSe, Zn-In-Se (ZIS) has a high band gap value with low resistivity values to meet the expectations as a promising buffer layer. This polycrystalline structure is an n-type ternary chalcopyrite semiconductor belongs to the group of II-III-VI compounds with the interest of ZnSe structure [14]. In addition, it is frequently named as a ternary defect chalcopyrite semiconductor [15-17]. The defective characteristic in this compound arises from the percentage vacancies of Zn elements in the structure [4] and it is the main factor on the physical characteristics of this film structure. In the research on alternative buffers for thin film photovoltaics, it can be used as a Cd-free buffer due to their potential applications in various optical and electronic devices and as buffer material for thin film heterojunction solar cells [1, 2, 3, 15].
In the reported works on ZIS thin films, the results of structural, material characterizations and device applications were presented depending on the film thickness, substrate temperature, annealing processes, and also sample temperature. In these works, their crystal and film characteristics, and also applications in solar cells and optoelectronic devices were presented [1630]. The material and device characteristics of this type of compounds were studied as a heterojunction partner for p-type absorber layers. Generally, the ZIS ingot was prepared to analyze
the properties of the ZIS in crystal form [17] and it was also used to deposit ZIS thin films [1621]. In addition, vapor-phase chemical transport was used to grow ZIS crystal [21, 22]; and the coevaporation [24], evaporation of precursors and selenization [25], RF sputtering [26], spray deposition [26, 27] and electrodeposition [29] techniques were used to prepare thin film samples. Among these deposition processes, in order to reach a polycrystalline nature with the preferred orientation direction, heat treatment is required either during the deposition with applying substrate heating or post-thermal heating process. According to the reported results, the properties of thin films were determined under the effect of deposition technique and also conditions. Depending on the presence of high density of states near the band edges, there was a contradiction on the determination of the band gap nature (direct or indirect) [16, 20, 21]. Therefore, there is a difference in literature about the type of electronic transition and corresponding value of optical energy gap. In the arguments on optical characterization, although their transmittance behaviors were modelled by dispersion analysis to obtain optical parameters [21], investigation of the existence of band splitting and also evaluation of the tail states are required to complete understanding of the optical characteristics of these compounds.
In order to analyze the physical properties of ZIS thin films structure deposited by physical vapor evaporation (PVD) technique, the studies were initially focused on their crystalline structure, composition and surface characteristics in the previous work [31]. In our previous works, the establishment of the parametrized process to fabricate ZIS thin films and their structural analysis under the effect of three different annealing temperatures were presented by the authors. As mentioned in this paper, they were found to have Zn rich behavior in as-grown form and after annealing processes. However, Se re-evaporation on the film surface were observed after every heating treatment. Moreover, on the as-grown ZIS sample, there were some Zn agglomerations observed on the surface and they were found in irregularly distributed on the surface. With increasing the annealing temperature, their size distribution was decreased due to either possible diffusion of these atoms from the surface to the bulk or segregation of the other atoms on the surface of the films. All of the samples were found to be in polycrystalline behavior with (112) main crystalline orientation direction with and without annealing process. The crystallinity of the samples was also analyzed by Raman measurements. Although there was an improvement in the crystal structure of the sample by applying annealing processes, ZnSe secondary phase formation
was detected in the (101) orientation direction in the sample annealed at 500 °C. As a remarkable result of XPS analysis, the photoelectron peak of In core level was decomposed to the peaks corresponding to the In bonding with all constituent elements. However, after annealing process at 400 °C, the existence of the In-Se and In-Zn was disappeared.
In this work, the optical properties of ZIS samples were defined as the interaction between electromagnetic radiation and these semiconducting materials, including transmission, refraction and absorption analyses. The characterization and optimization of the optical properties of the films are important part for device applications, such as solar cell fabrication. Under the aim of their device applications, to progress in the characterization of the ZIS films, the studies were directed to analyze the distinct transitions in the high absorption region and investigate the splitting energies of crystal-field and spin–orbit splitting under the evaluation of Urbach energy calculation and width of the tail states. Application of Cauchy model and dispersion relation lead to get the detailed information about their optical properties, such as electronic band structure and optical constants.
2. Experimental Details
The polycrystalline ZIS thin films were deposited by thermal evaporation technique from the pure evaporation sources, In2Se3, Zn and Se, by using Vaksis Midas-PVD system. Chemically and ultrasonically cleaned conventional soda lime glass substrates were used as substrates for the film deposition. During the deposition process, the substrate temperature was kept at about 200°C and the system base pressure under 10-6 Torr. The ZIS thin films were evaporated layer by layer with the deposition rates about 0.3 to 1.0 Å/sec, which were measured by Inficon XTM/2 deposition monitor connected to the quartz thickness crystal inside the vacuum chamber. Following the deposition of the samples, the post-annealing treatments were applied to the films under nitrogen atmosphere in the temperature range of 300 - 500°C by 100°C step for 30 minutes to see the effect of annealing on the optical properties of thin films. This furnace heating process was found to improve the crystallinity in the film structure, whereas the binary ZnSe phase was observed in the XRD spectrum with annealing at 500°C [31]. Therefore, considering the structural modifications, the optical properties of the thin films subjected to an annealing process were discussed.
For the optical analysis of ZIS thin film samples, transmission measurements were carried out at room temperature by using Perkin-Elmer LAMBDA 950 UV/Vis/NIR spectrophotometer in the 300-2000 nm wavelength region.
3. Results and Discussion
In order to analyze the optical characteristics of the samples, transmission measurements were carried out at room temperature. Since the interaction of electromagnetic radiation with the material can be described by optical parameters, as, absorption coefficient, refractive index, band gaps, the wavelength dependence of transmission spectrum was investigated to determine the optical constants of the samples. Fig.1 shows the spectral behavior of the transmittance values of the ZIS films with respect to the annealing process. As seen from Fig.1, there are interference fringes in the transmittance spectrum of all films with a sharp fall at the band edge in the region 600–2000 nm; whereas the interference effects disappear below this region. A similar interference behavior was reported previously [20-22] with different deposition techniques and structure formations. Initially, these uniform sequential maxima and minima of the fringes at the transparent wavelength region indicate optical flatness and thickness uniformity of the deposited films even if the annealing processes were applied [21, 22, 32]. Uniformity was also checked together with the homogeneity by SEM and AFM measurements [31]. The optical transmission values of the films were between 80 and 90% in the transparent region. Although both as-grown film and the film annealed at 300°C showed similar transmittance behavior, with the further annealing processes, this optical characteristic was changed. There was a wavelength shift in the transmission spectrum of all samples and a remarkable change in the transmission of the 400 and 500 °C annealed films. As the surface of the samples become more reflective due to the morphological changes depending on the compositional changes, the decrease in the transmittance was the expected optical behavior with these annealing steps [32, 33]. On the other hand, due to the change in the thickness of the films, this effect can slightly be observed on the optical measurements. As expected, the structural and surface modifications on the films with annealing could trigger this result in transmittance values [31].
As observed in Fig.1, there are interference patterns in the transmission spectra of the films, therefore the envelope model (EM) can be used to determine their refractive indices [34, 35]. With taking care of all possibilities on the reflection and transmission events in the boundaries due to ̃ (λ), can be written as [36, 37], the local fields, the complex refractive index, N ̃ (λ) = n(λ) − iκ(λ) N
(1)
̃ (𝜆), respectively. In this expression, where 𝑛(𝜆) and κ(𝜆) are the real and imaginary part of 𝑁 ̃(𝜆) and its components 𝑛 and κ are these parameters are real and positive numbers; and also, 𝑁 wavelength dependent [38]. In this analysis, the imaginary part of the complex refractive index, κ, called as extinction coefficient or attenuation index, denotes absorption of optical energy by the semiconductor material [39] and 𝑛 is the ordinary refractive index. The complex refractive index for each sample was analyzed in the wavelength region 800-2000 nm in the weak and medium absorption regions [28]. This model deals with the interference effects therefore the strong absorption region, where the interference fringes are not observed, is out of interest for this analysis. In order to carry out this approach, it is important to work on at least two interference fringes in their weak absorption and transparent spectral region by constructing envelope curves of transmission spectra [34, 35]. By applying EM, the ordinary refractive index (n) can be determined as the following expression [35]: 𝑛 = [N + (𝑁 2 + 𝑛𝑠2 𝑛02 )1/2 ]1/2
(2)
where
𝑁=
𝑛02 +𝑛𝑠2 2
+ 𝑛𝑠 𝑛0
𝑇𝑀 −𝑇𝑚 𝑇𝑀 𝑇𝑚
(3)
From the Eq.2 and 3, 𝑛 can be calculated with the maxima 𝑇𝑀 and minima 𝑇𝑚 of the envelopes in the transmission spectra; the refractive index of the glass substrate, 𝑛𝑠 ; and the refractive index of the surrounding environment, 𝑛0 at the same wavelength value. In this case, 𝑛0 is the refractive index of the air and approximately equals to 1, and for used soda-lime glass substrate 𝑛𝑠 is about 1.53 at λ~547 nm [40]. The values of 𝑛𝑠 can be related to its transmission spectrum [41] given as;
1
1
𝑠
𝑠
𝑛𝑠 = 𝑇 + (𝑇 2 − 1)
(4)
where 𝑇𝑠 is the maximum transmittance of the substrate in the measured spectra. The refractive indices found by EM were shown in Fig.2. As a result of EM, 𝑛 values were between 1.8 and 2.6 depending on the annealing temperature and wavelength region. The calculated n values were found in decreasing behavior with increasing wavelength similar to the results in the literature works and the same behavior was also observed when comparing the wavelength dependent values with respect to the increase in annealing temperature [20, 21]. The annealing process can trigger re-evaporation and/or segregation of the elements, inactive in the formation of the compound structure, from the surface of the films. With increasing annealing temperature, especially Se atoms can re-evaporate from the surface and material loss could cause a decrease in thickness of the films. Therefore, the decrease in the density of the films may be attributed to this type of change in n values. To get a wavelength dependent continuity relation, these values can also be evaluated by the Cauchy dispersion relation [42];
𝑛(𝜆) = A +
𝐵
+
𝜆2
𝐶 𝜆4
+⋯
(5)
The expression given in Eq.5 can be used when the substances are transparent in visible region and Cauchy parameters, A and B, are all coefficients in fitting process depend on the optical characteristics of the film as coefficient of refraction and coefficient of dispersion, respectively. In our case, three-term Cauchy fitting was sufficient to provide a reasonable fit (Fig.2). The calculated values of the Cauchy parameters were given in Table 1 for all samples. From the refractive indices (𝑛1 and 𝑛2 ) of two adjacent maxima or minima (𝜆1 and 𝜆2 ) in the measured spectra, the thickness approximation of the samples can be done with the following relation [34, 43]; 𝑑 = 2(𝜆
𝜆1 𝜆2
1 𝑛2 −𝜆2 𝑛1 )
(6)
Although this equation gives approximations, without very sensitive and accurate results [18], this can be used to check the correctness of the applied EM [32, 43]. By using Eq.6, the mean values of 𝑑 were calculated as 584, 577, 550 and 520 nm, for as-grown and annealed films, respectively. The calculated and measured thicknesses (given in ref. 11) of the films were found to be in a good agreement with each other in the corresponding about 10 nm measurement error interval. Since the thickness values were in the estimating error limits with the profilometer, the obtained 𝑛 values by applying EM, can be accepted with small deviations. The other optical constant, extinction coefficient, κ(𝜆) can be calculated from the absorption coefficient α(𝜆) as [37]; α(𝜆) =
4𝜋 𝜆
κ(𝜆)
(7)
where α(𝜆) is related with the transmission values by means of the relation [45]; 1
𝐼
α(λ) = 𝑑 ln ( 𝐼0 )
(8)
In this calculation, 𝐼0 is the incident light perpendicular to the surface of the film, 𝐼 is the intensity of transmitted light. In fact, the ratio 𝐼 ⁄𝐼0 is the normalized transmittance value of the sample. The obtained κ(𝜆) values were shown in Fig.3, and they were found in decreasing behavior with increasing wavelength. Although there was a sharp decrease in κ(𝜆) values depending on wavelength and also an observable difference occurred with applying high annealing temperatures (400 and 500°C), there was no remarkable change in κ(𝜆) values for as-grown and annealed at 300°C.
The complex refractive index of the material is related to complex dielectric function as given below [37]; ̃ (λ))2 𝜀(𝜆) = (N
(9)
where 𝜀(𝜆) is evaluated in terms of its real and imaginary parts as; 𝜀(𝜆) = 𝜀𝑟𝑒 (𝜆) + 𝑖𝜀𝑖𝑚 (𝜆)
(10)
Therefore, knowing n(𝜆) and κ(𝜆) values, these components of 𝜀(𝜆) can be obtained from the following relations [36, 37]; 𝜀𝑟𝑒 (𝜆) = n2 (𝜆) − κ2 (𝜆)
(11)
𝜀𝑖𝑚 (𝜆) = 2n(𝜆)κ(𝜆)
(12)
and
The variations of these optical constants 𝜀𝑟𝑒 (𝜆) and 𝜀𝑖𝑚 (𝜆) in the wavelength interval of 800-2000 nm, can be evaluated in similar behavior with n(𝜆) and κ(𝜆). In this analysis, the calculated κ(𝜆) values should be very small as compared with n(𝜆) values. Fig.4 illustrates the calculated 𝜀𝑟𝑒 (𝜆) values in terms of change in wavelength.
To get a further knowledge about the ordinary refractive index, its dispersion in the semiconducting material can be calculated by a semi-empirical single-effective-oscillator model (SOM) [45, 46]. This model gives reasonable results for photon energies below the interband absorption edge. It relates the refractive index of the sample with the interband optical transitions and electronic band structure as [36]; 𝐸𝑝 𝐸
𝑛(𝐸)2 − 1 = 𝐸2 −𝐸𝑑2 𝑝
𝑑
(13)
where 𝐸𝑑 gives a measure of the strength of the interband optical transitions, 𝐸𝑝 , called as oscillatory energy, is related the energy gap value, and 𝐸 denotes the photon energy dependence in the form of ℎ𝜈. By using the linear part of (𝑛2 − 1)−1 versus (ℎ𝜈)2 plot (Fig.5) at low energy, 𝐸𝑝 and 𝐸𝑑 values were calculated listed in Table 2. According to the analysis of 𝐸𝑑 values, the strongest optical transition probability is dominant for the as-grown samples and this probability is in decreasing behavior with increasing annealing temperature.
Static and high frequency response of the samples can also be estimated from this dispersion relationship [37]. By extrapolation of the linear part of the plots given in Fig.5, the static refractive index n(0) and from the corresponding intercept values, the static dielectric constant 𝜀(0) can be calculated. Moreover, these values can be found by 𝑛2 (0) = 1 + 𝐸𝑑 ⁄𝐸𝑝 and 𝜀(0) = 𝑛2 (0) [39]. The obtained SOM relations in Fig.5 gives a linear behavior between 1.4 and 2.1 eV, these
parameters were calculated from the fitting process in this region. Apart from these low-frequency parameters, the high frequency dielectric constant 𝜀∞ can be obtained from the 𝜀𝑟𝑒 (𝜆) values of the films [37]. This relation can be analyzed from the Spitzer–Fan model [48, 49] as; 𝑒2𝑁
𝜀𝑟𝑒 (𝜆) = 𝜀∞ − (𝜋𝑐 2 𝑚∗) 𝜆2
(14)
where 𝑁 is the number of unit cells per unit volume, 𝑚∗ is the effective mass of the material, 𝑒 is the electronic charge and 𝑐 is the speed of light. The real part of the dielectric function asymptotically approaches to 𝜀(0) below the Reststrahlen region, and also it is connected to 𝜀∞ in the reststrahlen-near-IR range in the optical spectra [37]. These constants are also related to each other with long-wavelength lattice-optical modes by Lyddane-Sachs-Teller relation [49]. This relation gives the information about the amount of the polar characteristics in the chemical bonds of these structures [37]. The results for the deposited ZIS films obtained from the detailed SOM analysis are given in Table 2. From SOM analysis 𝜀∞ was found as 6.69 for as-grown films which is very close to the value reported by Zeyada et al. [20]. Since 𝜀(0)~𝜀∞ , as reported in ZnSe structures [51], the hetero-polar character is dominant with respect to homo-polar character in these samples. Moreover, the variation of the obtained optical constants with annealing temperature is in a good agreement with the work reported by El-Nahass et al. [21].
In the visible and near-infrared region, the absorption coefficient of the as-grown and annealed samples was found between 103 and 104 cm-1 (Fig.6), and there is no significant difference in these values in terms of thermal annealing. The transparent region in the wavelength range 400–700 nm can be used to evaluate the type of optical transition and the corresponding value of energy gap. Basically two types of optical transition, direct and indirect, can occur at the fundamental edge of crystalline semiconductors. The absorption coefficient α(λ) at the optical absorption edge varies with the photon energy (ℎ𝜈) according to the expression [39];
(𝛼ℎ𝜈) = 𝐴(ℎ𝜈 − 𝐸𝑔 )
𝑚
(15)
where 𝐴 is an energy-independent constant, but it depends on the transition probability, and 𝐸𝑔 is the energy gap value. In this relation, the power m is related to the band gap characteristics of the
structure; it can be 𝑚 = 1/2 or 𝑚 = 2 for direct and indirect transition in the band gap, respectively. Then, the plot of the optical absorption coefficient 𝛼 versus photon energy (ℎ𝜈) showed principle regions as reported in the work of Wood and Tauc [50]. In the spectral region where absorption is strongly effective, the absorption coefficient can be analyzed related to the band gap energy. Therefore, the band-gap values of the samples were calculated from transmission data using absorption coefficient and the band gap relation given in Eq.15 above. The data obtained from the absorption analysis of the films showed the existence of allowed direct transitions while in this structure, two absorption mechanisms, direct and indirect transitions are possibly expected for this material [16, 19, 21, 27]. Based on the band gap analysis, the corresponding figures for direct band gap transition, (αhν)2 versus hν curves, were shown in Fig. 6 (a-d). This type of semiconductors is the structural analogs of II-VI compounds, and therefore direct band gap can be expected in the visible region of the spectrum [51-53]. In addition, the case of Zn-rich behavior may be accepted as the reason of this type of transition in the deposited film structure [53]. In general, there are contradictions on the nature of the optical absorption in this material. The difference between the reported works was evaluated due to the difficulty in determination of band gap by optical spectroscopy [53], deposition techniques and variations in the elemental composition [27]. Since there were three linear contributions in the direct band gap transition curves, the effect of spin-orbit and crystal-field were taken into consideration in the analysis of ZIS structures [34, 43]. However, there is no other work considering the splitting on valence band. The calculated optical band gap values were assigned as the band splitting correspondence of the energy levels. The crystal-field splitting (CF) and the spin-orbit splitting values were determined by the following quasicubic relation [54]. 1
1
8
1/2
𝐸1,2 = − 2 (∆𝐶𝐹 + ∆𝑆𝑂 ) ± 2 ((∆𝐶𝐹 + ∆𝑆𝑂 )2 − 3 (∆𝐶𝐹 ∆𝑆𝑂 ))
(16)
where 𝐸1,2 energy differences between the direct transitions obtained from band gap analysis. The results obtained from this absorption process observed in the optical spectra of these films at room temperature were listed in Table 3. The band gap values revealed that the energy gap values for all transition levels decreases under the effect of increasing annealing temperature. It can the result of the annealing effect on the increase of the crystallinity and decrease in the absorption edge [ 21].
The optical absorption coefficient just below the absorption edge was found to vary exponentially with photon energy and this relation is referred as Urbach’s tail [56]. In this low energy region, the exponential of the absorption coefficient can be written as [37]: ℎ𝜈−𝐸𝑔
𝛼 = 𝛼0 𝑒𝑥𝑝 (
𝐸𝑈
)
(17)
where 𝐸𝑈 is the energy correspondence of Urbach’s tail, 𝐸𝑔 is an energy close to the band gap energy and 𝛼0 is a characteristic parameter for this relation. Urbach energy 𝐸𝑈 is an indication of the structural perfection of the materials and it appears because disordered and amorphous materials produce localized states extended in the bandgap [57]. By using Eq.17, these energy values were calculated from ln(𝛼) versus ℎ𝜈 plots given in Fig.8. Applying linear fitting analysis, the obtained values were listed in Table 3. 𝐸𝑈 values were found in decreasing behavior with increasing annealing temperature that may be related to the improvement in the structure [31].
4. Conclusion
In this work, the optical properties of the ZIS samples prepared by thermal evaporation of stacked elemental layers and heated under nitrogen atmosphere with different annealing temperatures were investigated. In the optical measurements, the interference fringes were observed in the transmission spectra of the all ZIS films. Therefore, by applying possible EM fitting process, these results were analyzed in order to determine the optical parameters of the samples. These results were also detailed by using the Cauchy dispersion relation. Optical constants were investigated with the differences occurred in composition, crystal structure and surface characteristics of the films under the effect of annealing temperatures. In addition, the optical absorption coefficients of the films were determined from the transmittance values and were found about 104 cm-1 in the visible and near-infrared region. From the Tauc plots, ZIS samples were found to have three distinct direct optical transitions depending on the crystal-field and spin-orbit splitting in the valence band of the structure. The fundamental band gap values were calculated in between 2.52 and 2.26 eV with increase in the annealing temperature. The optical absorption coefficient just below the absorption edge was identified by the energy width of the band tail states related with
the structural characteristics. It was found in consistent result with the improvement in the structure that there is a decreasing behavior with increasing annealing temperature.
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Figure Captions
Figure 1 The transmittance spectra of the ZIS samples Figure 2 The refractive indices of the ZIS samples calculated by EM method (dot) and by three term Cauchy relation (line) Figure 3 Calculated extinction coefficients for the ZIS samples Figure 4 Real part of the dielectric constants for the ZIS samples Figure 5 Plots of (𝑛2 − 1)−1 vs (ℎ𝜈)2 for ZIS samples Figure 6 The variation of (𝛼ℎ𝜈)2 with the photon energy for (a) as-grown, (b) 300 °C -annealed, (c) 400 °C-annealed and (d) 500°C-annealed ZIS thin films Figure 7 Plots of ln(α) versus hν for ZIS samples
Figure1
Figure2
Figure3
Figure4
Figure5
Figure6
Figure7
List of Tables
Table 1 Three term Cauchy parameters of the ZIS samples Table 2 SOM parameters of the ZIS samples Table 3 Band gap and Urbach tail values of the ZIS samples (in terms of eV)
Table 1: Three term Cauchy parameters of the ZIS samples
Sample
A
B (nm2)
C (nm4)
As-grown
2.43
9.96x104
1.57x1010
300 °C Annealing
2.12
2.28x105
5.46x106
400 °C Annealing
1.96
4.66x105
-7.88x1010
500 °C Annealing
1.63
6.64x105
-1.45x106
Table 2: SOM parameters of the ZIS samples
Sample
𝐸𝑝 (eV)
𝐸𝑑 (eV)
𝑛(0)
𝜀(0)
𝜀(∞)
As-grown
3.796
18.546
2.514
6.319
6.701
300 °C Annealing
2.649
9.361
2.293
5.256
5.332
400 °C Annealing
2.140
6.581
2.285
5.219
5.138
500 °C Annealing
1.796
3.622
2.084
4.345
4.292
Table 3: Band gap and Urbach tail values of the ZIS samples
Sample
𝐸𝑑,1
𝐸𝑑,2
𝐸𝑑,3
𝐸𝑖𝑛𝑑
𝐸𝑈
As-grown
2.00
2.29
2.52
1.62
0.261
300 °C Annealing
1.98
2.26
2.46
1.49
0.172
400 °C Annealing
1.95
2.14
2.45
1.42
0.110
500 °C Annealing
1.85
2.09
2.26
1.40
0.109
S0030-4026(17)30775-1 http://dx.doi.org/doi:10.1016/j.ijleo.2017.06.106 IJLEO 59369
To appear in: Received date: Revised date: Accepted date:
23-2-2017 10-6-2017 25-6-2017
Please cite this article as: H.H.Gull ¨ u, ¨ E.Cos¸kun, M.Parlak, Investigations of Thermal Annealing Role on the Optical Properties of Zn-In-Se Thin Films, Optik - International Journal for Light and Electron Opticshttp://dx.doi.org/10.1016/j.ijleo.2017.06.106 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigations of Thermal Annealing Role on the Optical Properties of Zn-In-Se Thin Films H.H. Güllü1,4,*, E. Coşkun2,3,4, M. Parlak2,4 1 Central Laboratory, Middle East Technical University (METU), 06800 Ankara, Turkey Department of Physics, Middle East Technical University (METU), 06800 Ankara, Turkey 3 Department of Physics, Çanakkale Onsekiz Mart University, 17100 Çanakkale, Turkey 4 Center for Solar Energy Research and Applications (GÜNAM), METU, Ankara 06800, Turkey 2
*Corresponding author: Tel: +90 312 2106440; Fax: +90 312 2106425 Email: [email protected]
Abstract
Zn-In-Se (ZIS) thin films were prepared by sequential evaporation of its elemental sources on the glass substrates. The effect of thermal annealing under nitrogen environment on the optical properties of the films was discussed. In addition to the comparative study of three different annealing temperatures, the results were analyzed by relating with their structural and compositional characteristics. The optical analyses were based on the observed interference fringes on their transmission spectra of the films. The refractive indices were calculated by means of envelope method (EM) and the continuity of the refractive indices was evaluated by three-term Cauchy fitting process. From the results of the refractive index calculations, the real and imaginary part of the dielectric constant were determined. The optical absorption coefficients of the films were found in the range of 103-104 cm-1 over the visible and near-infrared region and by using these values, the extinction coefficients were calculated. Moreover, the band gap values were calculated from the corresponding Tauc plots, and the refractive index dispersion over the measured wavelength range was investigated with single-oscillator model (SOM) and the related parameters were obtained.
1. Introduction
In thin film materials technology, most of the recorded photovoltaic structures are fabricated with a thin CdS buffer layer [1-5]. Apart from the success of the hetero-structures including a CdS buffer layer, there are environmental safety concern on usage of Cd [1, 5]. In addition, alternative materials to this layer can provide a large band gap which allows transmission of higher energy photons than the band gap of CdS. Among these materials, ZnSe has been an attractive buffer layer with band gap larger than CdS and the lattice characteristics is also in a good match with the popular chalcopyrite and kesterite thin film absorbers [6, 7]. This binary structure belongs to II-VI family and crystallizes in the cubic zinc blende structure. Although ZnSe is a very promising compound for the thin film applications of solar cells [8-11], there are some difficulties in the control of atomic ratios of the constituent elements and as a result of the nature of the defects in the structure, and common high resistivity problem of the ZnSe films obstruct their photovoltaic applications. Therefore, alloying polycrystalline wide band gap thin film semiconductors with elements in group III is a familiar application to optimize the resistivity of these materials [12, 13]. In view of the fact that interests on ZnSe, Zn-In-Se (ZIS) has a high band gap value with low resistivity values to meet the expectations as a promising buffer layer. This polycrystalline structure is an n-type ternary chalcopyrite semiconductor belongs to the group of II-III-VI compounds with the interest of ZnSe structure [14]. In addition, it is frequently named as a ternary defect chalcopyrite semiconductor [15-17]. The defective characteristic in this compound arises from the percentage vacancies of Zn elements in the structure [4] and it is the main factor on the physical characteristics of this film structure. In the research on alternative buffers for thin film photovoltaics, it can be used as a Cd-free buffer due to their potential applications in various optical and electronic devices and as buffer material for thin film heterojunction solar cells [1, 2, 3, 15].
In the reported works on ZIS thin films, the results of structural, material characterizations and device applications were presented depending on the film thickness, substrate temperature, annealing processes, and also sample temperature. In these works, their crystal and film characteristics, and also applications in solar cells and optoelectronic devices were presented [1630]. The material and device characteristics of this type of compounds were studied as a heterojunction partner for p-type absorber layers. Generally, the ZIS ingot was prepared to analyze
the properties of the ZIS in crystal form [17] and it was also used to deposit ZIS thin films [1621]. In addition, vapor-phase chemical transport was used to grow ZIS crystal [21, 22]; and the coevaporation [24], evaporation of precursors and selenization [25], RF sputtering [26], spray deposition [26, 27] and electrodeposition [29] techniques were used to prepare thin film samples. Among these deposition processes, in order to reach a polycrystalline nature with the preferred orientation direction, heat treatment is required either during the deposition with applying substrate heating or post-thermal heating process. According to the reported results, the properties of thin films were determined under the effect of deposition technique and also conditions. Depending on the presence of high density of states near the band edges, there was a contradiction on the determination of the band gap nature (direct or indirect) [16, 20, 21]. Therefore, there is a difference in literature about the type of electronic transition and corresponding value of optical energy gap. In the arguments on optical characterization, although their transmittance behaviors were modelled by dispersion analysis to obtain optical parameters [21], investigation of the existence of band splitting and also evaluation of the tail states are required to complete understanding of the optical characteristics of these compounds.
In order to analyze the physical properties of ZIS thin films structure deposited by physical vapor evaporation (PVD) technique, the studies were initially focused on their crystalline structure, composition and surface characteristics in the previous work [31]. In our previous works, the establishment of the parametrized process to fabricate ZIS thin films and their structural analysis under the effect of three different annealing temperatures were presented by the authors. As mentioned in this paper, they were found to have Zn rich behavior in as-grown form and after annealing processes. However, Se re-evaporation on the film surface were observed after every heating treatment. Moreover, on the as-grown ZIS sample, there were some Zn agglomerations observed on the surface and they were found in irregularly distributed on the surface. With increasing the annealing temperature, their size distribution was decreased due to either possible diffusion of these atoms from the surface to the bulk or segregation of the other atoms on the surface of the films. All of the samples were found to be in polycrystalline behavior with (112) main crystalline orientation direction with and without annealing process. The crystallinity of the samples was also analyzed by Raman measurements. Although there was an improvement in the crystal structure of the sample by applying annealing processes, ZnSe secondary phase formation
was detected in the (101) orientation direction in the sample annealed at 500 °C. As a remarkable result of XPS analysis, the photoelectron peak of In core level was decomposed to the peaks corresponding to the In bonding with all constituent elements. However, after annealing process at 400 °C, the existence of the In-Se and In-Zn was disappeared.
In this work, the optical properties of ZIS samples were defined as the interaction between electromagnetic radiation and these semiconducting materials, including transmission, refraction and absorption analyses. The characterization and optimization of the optical properties of the films are important part for device applications, such as solar cell fabrication. Under the aim of their device applications, to progress in the characterization of the ZIS films, the studies were directed to analyze the distinct transitions in the high absorption region and investigate the splitting energies of crystal-field and spin–orbit splitting under the evaluation of Urbach energy calculation and width of the tail states. Application of Cauchy model and dispersion relation lead to get the detailed information about their optical properties, such as electronic band structure and optical constants.
2. Experimental Details
The polycrystalline ZIS thin films were deposited by thermal evaporation technique from the pure evaporation sources, In2Se3, Zn and Se, by using Vaksis Midas-PVD system. Chemically and ultrasonically cleaned conventional soda lime glass substrates were used as substrates for the film deposition. During the deposition process, the substrate temperature was kept at about 200°C and the system base pressure under 10-6 Torr. The ZIS thin films were evaporated layer by layer with the deposition rates about 0.3 to 1.0 Å/sec, which were measured by Inficon XTM/2 deposition monitor connected to the quartz thickness crystal inside the vacuum chamber. Following the deposition of the samples, the post-annealing treatments were applied to the films under nitrogen atmosphere in the temperature range of 300 - 500°C by 100°C step for 30 minutes to see the effect of annealing on the optical properties of thin films. This furnace heating process was found to improve the crystallinity in the film structure, whereas the binary ZnSe phase was observed in the XRD spectrum with annealing at 500°C [31]. Therefore, considering the structural modifications, the optical properties of the thin films subjected to an annealing process were discussed.
For the optical analysis of ZIS thin film samples, transmission measurements were carried out at room temperature by using Perkin-Elmer LAMBDA 950 UV/Vis/NIR spectrophotometer in the 300-2000 nm wavelength region.
3. Results and Discussion
In order to analyze the optical characteristics of the samples, transmission measurements were carried out at room temperature. Since the interaction of electromagnetic radiation with the material can be described by optical parameters, as, absorption coefficient, refractive index, band gaps, the wavelength dependence of transmission spectrum was investigated to determine the optical constants of the samples. Fig.1 shows the spectral behavior of the transmittance values of the ZIS films with respect to the annealing process. As seen from Fig.1, there are interference fringes in the transmittance spectrum of all films with a sharp fall at the band edge in the region 600–2000 nm; whereas the interference effects disappear below this region. A similar interference behavior was reported previously [20-22] with different deposition techniques and structure formations. Initially, these uniform sequential maxima and minima of the fringes at the transparent wavelength region indicate optical flatness and thickness uniformity of the deposited films even if the annealing processes were applied [21, 22, 32]. Uniformity was also checked together with the homogeneity by SEM and AFM measurements [31]. The optical transmission values of the films were between 80 and 90% in the transparent region. Although both as-grown film and the film annealed at 300°C showed similar transmittance behavior, with the further annealing processes, this optical characteristic was changed. There was a wavelength shift in the transmission spectrum of all samples and a remarkable change in the transmission of the 400 and 500 °C annealed films. As the surface of the samples become more reflective due to the morphological changes depending on the compositional changes, the decrease in the transmittance was the expected optical behavior with these annealing steps [32, 33]. On the other hand, due to the change in the thickness of the films, this effect can slightly be observed on the optical measurements. As expected, the structural and surface modifications on the films with annealing could trigger this result in transmittance values [31].
As observed in Fig.1, there are interference patterns in the transmission spectra of the films, therefore the envelope model (EM) can be used to determine their refractive indices [34, 35]. With taking care of all possibilities on the reflection and transmission events in the boundaries due to ̃ (λ), can be written as [36, 37], the local fields, the complex refractive index, N ̃ (λ) = n(λ) − iκ(λ) N
(1)
̃ (𝜆), respectively. In this expression, where 𝑛(𝜆) and κ(𝜆) are the real and imaginary part of 𝑁 ̃(𝜆) and its components 𝑛 and κ are these parameters are real and positive numbers; and also, 𝑁 wavelength dependent [38]. In this analysis, the imaginary part of the complex refractive index, κ, called as extinction coefficient or attenuation index, denotes absorption of optical energy by the semiconductor material [39] and 𝑛 is the ordinary refractive index. The complex refractive index for each sample was analyzed in the wavelength region 800-2000 nm in the weak and medium absorption regions [28]. This model deals with the interference effects therefore the strong absorption region, where the interference fringes are not observed, is out of interest for this analysis. In order to carry out this approach, it is important to work on at least two interference fringes in their weak absorption and transparent spectral region by constructing envelope curves of transmission spectra [34, 35]. By applying EM, the ordinary refractive index (n) can be determined as the following expression [35]: 𝑛 = [N + (𝑁 2 + 𝑛𝑠2 𝑛02 )1/2 ]1/2
(2)
where
𝑁=
𝑛02 +𝑛𝑠2 2
+ 𝑛𝑠 𝑛0
𝑇𝑀 −𝑇𝑚 𝑇𝑀 𝑇𝑚
(3)
From the Eq.2 and 3, 𝑛 can be calculated with the maxima 𝑇𝑀 and minima 𝑇𝑚 of the envelopes in the transmission spectra; the refractive index of the glass substrate, 𝑛𝑠 ; and the refractive index of the surrounding environment, 𝑛0 at the same wavelength value. In this case, 𝑛0 is the refractive index of the air and approximately equals to 1, and for used soda-lime glass substrate 𝑛𝑠 is about 1.53 at λ~547 nm [40]. The values of 𝑛𝑠 can be related to its transmission spectrum [41] given as;
1
1
𝑠
𝑠
𝑛𝑠 = 𝑇 + (𝑇 2 − 1)
(4)
where 𝑇𝑠 is the maximum transmittance of the substrate in the measured spectra. The refractive indices found by EM were shown in Fig.2. As a result of EM, 𝑛 values were between 1.8 and 2.6 depending on the annealing temperature and wavelength region. The calculated n values were found in decreasing behavior with increasing wavelength similar to the results in the literature works and the same behavior was also observed when comparing the wavelength dependent values with respect to the increase in annealing temperature [20, 21]. The annealing process can trigger re-evaporation and/or segregation of the elements, inactive in the formation of the compound structure, from the surface of the films. With increasing annealing temperature, especially Se atoms can re-evaporate from the surface and material loss could cause a decrease in thickness of the films. Therefore, the decrease in the density of the films may be attributed to this type of change in n values. To get a wavelength dependent continuity relation, these values can also be evaluated by the Cauchy dispersion relation [42];
𝑛(𝜆) = A +
𝐵
+
𝜆2
𝐶 𝜆4
+⋯
(5)
The expression given in Eq.5 can be used when the substances are transparent in visible region and Cauchy parameters, A and B, are all coefficients in fitting process depend on the optical characteristics of the film as coefficient of refraction and coefficient of dispersion, respectively. In our case, three-term Cauchy fitting was sufficient to provide a reasonable fit (Fig.2). The calculated values of the Cauchy parameters were given in Table 1 for all samples. From the refractive indices (𝑛1 and 𝑛2 ) of two adjacent maxima or minima (𝜆1 and 𝜆2 ) in the measured spectra, the thickness approximation of the samples can be done with the following relation [34, 43]; 𝑑 = 2(𝜆
𝜆1 𝜆2
1 𝑛2 −𝜆2 𝑛1 )
(6)
Although this equation gives approximations, without very sensitive and accurate results [18], this can be used to check the correctness of the applied EM [32, 43]. By using Eq.6, the mean values of 𝑑 were calculated as 584, 577, 550 and 520 nm, for as-grown and annealed films, respectively. The calculated and measured thicknesses (given in ref. 11) of the films were found to be in a good agreement with each other in the corresponding about 10 nm measurement error interval. Since the thickness values were in the estimating error limits with the profilometer, the obtained 𝑛 values by applying EM, can be accepted with small deviations. The other optical constant, extinction coefficient, κ(𝜆) can be calculated from the absorption coefficient α(𝜆) as [37]; α(𝜆) =
4𝜋 𝜆
κ(𝜆)
(7)
where α(𝜆) is related with the transmission values by means of the relation [45]; 1
𝐼
α(λ) = 𝑑 ln ( 𝐼0 )
(8)
In this calculation, 𝐼0 is the incident light perpendicular to the surface of the film, 𝐼 is the intensity of transmitted light. In fact, the ratio 𝐼 ⁄𝐼0 is the normalized transmittance value of the sample. The obtained κ(𝜆) values were shown in Fig.3, and they were found in decreasing behavior with increasing wavelength. Although there was a sharp decrease in κ(𝜆) values depending on wavelength and also an observable difference occurred with applying high annealing temperatures (400 and 500°C), there was no remarkable change in κ(𝜆) values for as-grown and annealed at 300°C.
The complex refractive index of the material is related to complex dielectric function as given below [37]; ̃ (λ))2 𝜀(𝜆) = (N
(9)
where 𝜀(𝜆) is evaluated in terms of its real and imaginary parts as; 𝜀(𝜆) = 𝜀𝑟𝑒 (𝜆) + 𝑖𝜀𝑖𝑚 (𝜆)
(10)
Therefore, knowing n(𝜆) and κ(𝜆) values, these components of 𝜀(𝜆) can be obtained from the following relations [36, 37]; 𝜀𝑟𝑒 (𝜆) = n2 (𝜆) − κ2 (𝜆)
(11)
𝜀𝑖𝑚 (𝜆) = 2n(𝜆)κ(𝜆)
(12)
and
The variations of these optical constants 𝜀𝑟𝑒 (𝜆) and 𝜀𝑖𝑚 (𝜆) in the wavelength interval of 800-2000 nm, can be evaluated in similar behavior with n(𝜆) and κ(𝜆). In this analysis, the calculated κ(𝜆) values should be very small as compared with n(𝜆) values. Fig.4 illustrates the calculated 𝜀𝑟𝑒 (𝜆) values in terms of change in wavelength.
To get a further knowledge about the ordinary refractive index, its dispersion in the semiconducting material can be calculated by a semi-empirical single-effective-oscillator model (SOM) [45, 46]. This model gives reasonable results for photon energies below the interband absorption edge. It relates the refractive index of the sample with the interband optical transitions and electronic band structure as [36]; 𝐸𝑝 𝐸
𝑛(𝐸)2 − 1 = 𝐸2 −𝐸𝑑2 𝑝
𝑑
(13)
where 𝐸𝑑 gives a measure of the strength of the interband optical transitions, 𝐸𝑝 , called as oscillatory energy, is related the energy gap value, and 𝐸 denotes the photon energy dependence in the form of ℎ𝜈. By using the linear part of (𝑛2 − 1)−1 versus (ℎ𝜈)2 plot (Fig.5) at low energy, 𝐸𝑝 and 𝐸𝑑 values were calculated listed in Table 2. According to the analysis of 𝐸𝑑 values, the strongest optical transition probability is dominant for the as-grown samples and this probability is in decreasing behavior with increasing annealing temperature.
Static and high frequency response of the samples can also be estimated from this dispersion relationship [37]. By extrapolation of the linear part of the plots given in Fig.5, the static refractive index n(0) and from the corresponding intercept values, the static dielectric constant 𝜀(0) can be calculated. Moreover, these values can be found by 𝑛2 (0) = 1 + 𝐸𝑑 ⁄𝐸𝑝 and 𝜀(0) = 𝑛2 (0) [39]. The obtained SOM relations in Fig.5 gives a linear behavior between 1.4 and 2.1 eV, these
parameters were calculated from the fitting process in this region. Apart from these low-frequency parameters, the high frequency dielectric constant 𝜀∞ can be obtained from the 𝜀𝑟𝑒 (𝜆) values of the films [37]. This relation can be analyzed from the Spitzer–Fan model [48, 49] as; 𝑒2𝑁
𝜀𝑟𝑒 (𝜆) = 𝜀∞ − (𝜋𝑐 2 𝑚∗) 𝜆2
(14)
where 𝑁 is the number of unit cells per unit volume, 𝑚∗ is the effective mass of the material, 𝑒 is the electronic charge and 𝑐 is the speed of light. The real part of the dielectric function asymptotically approaches to 𝜀(0) below the Reststrahlen region, and also it is connected to 𝜀∞ in the reststrahlen-near-IR range in the optical spectra [37]. These constants are also related to each other with long-wavelength lattice-optical modes by Lyddane-Sachs-Teller relation [49]. This relation gives the information about the amount of the polar characteristics in the chemical bonds of these structures [37]. The results for the deposited ZIS films obtained from the detailed SOM analysis are given in Table 2. From SOM analysis 𝜀∞ was found as 6.69 for as-grown films which is very close to the value reported by Zeyada et al. [20]. Since 𝜀(0)~𝜀∞ , as reported in ZnSe structures [51], the hetero-polar character is dominant with respect to homo-polar character in these samples. Moreover, the variation of the obtained optical constants with annealing temperature is in a good agreement with the work reported by El-Nahass et al. [21].
In the visible and near-infrared region, the absorption coefficient of the as-grown and annealed samples was found between 103 and 104 cm-1 (Fig.6), and there is no significant difference in these values in terms of thermal annealing. The transparent region in the wavelength range 400–700 nm can be used to evaluate the type of optical transition and the corresponding value of energy gap. Basically two types of optical transition, direct and indirect, can occur at the fundamental edge of crystalline semiconductors. The absorption coefficient α(λ) at the optical absorption edge varies with the photon energy (ℎ𝜈) according to the expression [39];
(𝛼ℎ𝜈) = 𝐴(ℎ𝜈 − 𝐸𝑔 )
𝑚
(15)
where 𝐴 is an energy-independent constant, but it depends on the transition probability, and 𝐸𝑔 is the energy gap value. In this relation, the power m is related to the band gap characteristics of the
structure; it can be 𝑚 = 1/2 or 𝑚 = 2 for direct and indirect transition in the band gap, respectively. Then, the plot of the optical absorption coefficient 𝛼 versus photon energy (ℎ𝜈) showed principle regions as reported in the work of Wood and Tauc [50]. In the spectral region where absorption is strongly effective, the absorption coefficient can be analyzed related to the band gap energy. Therefore, the band-gap values of the samples were calculated from transmission data using absorption coefficient and the band gap relation given in Eq.15 above. The data obtained from the absorption analysis of the films showed the existence of allowed direct transitions while in this structure, two absorption mechanisms, direct and indirect transitions are possibly expected for this material [16, 19, 21, 27]. Based on the band gap analysis, the corresponding figures for direct band gap transition, (αhν)2 versus hν curves, were shown in Fig. 6 (a-d). This type of semiconductors is the structural analogs of II-VI compounds, and therefore direct band gap can be expected in the visible region of the spectrum [51-53]. In addition, the case of Zn-rich behavior may be accepted as the reason of this type of transition in the deposited film structure [53]. In general, there are contradictions on the nature of the optical absorption in this material. The difference between the reported works was evaluated due to the difficulty in determination of band gap by optical spectroscopy [53], deposition techniques and variations in the elemental composition [27]. Since there were three linear contributions in the direct band gap transition curves, the effect of spin-orbit and crystal-field were taken into consideration in the analysis of ZIS structures [34, 43]. However, there is no other work considering the splitting on valence band. The calculated optical band gap values were assigned as the band splitting correspondence of the energy levels. The crystal-field splitting (CF) and the spin-orbit splitting values were determined by the following quasicubic relation [54]. 1
1
8
1/2
𝐸1,2 = − 2 (∆𝐶𝐹 + ∆𝑆𝑂 ) ± 2 ((∆𝐶𝐹 + ∆𝑆𝑂 )2 − 3 (∆𝐶𝐹 ∆𝑆𝑂 ))
(16)
where 𝐸1,2 energy differences between the direct transitions obtained from band gap analysis. The results obtained from this absorption process observed in the optical spectra of these films at room temperature were listed in Table 3. The band gap values revealed that the energy gap values for all transition levels decreases under the effect of increasing annealing temperature. It can the result of the annealing effect on the increase of the crystallinity and decrease in the absorption edge [ 21].
The optical absorption coefficient just below the absorption edge was found to vary exponentially with photon energy and this relation is referred as Urbach’s tail [56]. In this low energy region, the exponential of the absorption coefficient can be written as [37]: ℎ𝜈−𝐸𝑔
𝛼 = 𝛼0 𝑒𝑥𝑝 (
𝐸𝑈
)
(17)
where 𝐸𝑈 is the energy correspondence of Urbach’s tail, 𝐸𝑔 is an energy close to the band gap energy and 𝛼0 is a characteristic parameter for this relation. Urbach energy 𝐸𝑈 is an indication of the structural perfection of the materials and it appears because disordered and amorphous materials produce localized states extended in the bandgap [57]. By using Eq.17, these energy values were calculated from ln(𝛼) versus ℎ𝜈 plots given in Fig.8. Applying linear fitting analysis, the obtained values were listed in Table 3. 𝐸𝑈 values were found in decreasing behavior with increasing annealing temperature that may be related to the improvement in the structure [31].
4. Conclusion
In this work, the optical properties of the ZIS samples prepared by thermal evaporation of stacked elemental layers and heated under nitrogen atmosphere with different annealing temperatures were investigated. In the optical measurements, the interference fringes were observed in the transmission spectra of the all ZIS films. Therefore, by applying possible EM fitting process, these results were analyzed in order to determine the optical parameters of the samples. These results were also detailed by using the Cauchy dispersion relation. Optical constants were investigated with the differences occurred in composition, crystal structure and surface characteristics of the films under the effect of annealing temperatures. In addition, the optical absorption coefficients of the films were determined from the transmittance values and were found about 104 cm-1 in the visible and near-infrared region. From the Tauc plots, ZIS samples were found to have three distinct direct optical transitions depending on the crystal-field and spin-orbit splitting in the valence band of the structure. The fundamental band gap values were calculated in between 2.52 and 2.26 eV with increase in the annealing temperature. The optical absorption coefficient just below the absorption edge was identified by the energy width of the band tail states related with
the structural characteristics. It was found in consistent result with the improvement in the structure that there is a decreasing behavior with increasing annealing temperature.
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Figure Captions
Figure 1 The transmittance spectra of the ZIS samples Figure 2 The refractive indices of the ZIS samples calculated by EM method (dot) and by three term Cauchy relation (line) Figure 3 Calculated extinction coefficients for the ZIS samples Figure 4 Real part of the dielectric constants for the ZIS samples Figure 5 Plots of (𝑛2 − 1)−1 vs (ℎ𝜈)2 for ZIS samples Figure 6 The variation of (𝛼ℎ𝜈)2 with the photon energy for (a) as-grown, (b) 300 °C -annealed, (c) 400 °C-annealed and (d) 500°C-annealed ZIS thin films Figure 7 Plots of ln(α) versus hν for ZIS samples
Figure1
Figure2
Figure3
Figure4
Figure5
Figure6
Figure7
List of Tables
Table 1 Three term Cauchy parameters of the ZIS samples Table 2 SOM parameters of the ZIS samples Table 3 Band gap and Urbach tail values of the ZIS samples (in terms of eV)
Table 1: Three term Cauchy parameters of the ZIS samples
Sample
A
B (nm2)
C (nm4)
As-grown
2.43
9.96x104
1.57x1010
300 °C Annealing
2.12
2.28x105
5.46x106
400 °C Annealing
1.96
4.66x105
-7.88x1010
500 °C Annealing
1.63
6.64x105
-1.45x106
Table 2: SOM parameters of the ZIS samples
Sample
𝐸𝑝 (eV)
𝐸𝑑 (eV)
𝑛(0)
𝜀(0)
𝜀(∞)
As-grown
3.796
18.546
2.514
6.319
6.701
300 °C Annealing
2.649
9.361
2.293
5.256
5.332
400 °C Annealing
2.140
6.581
2.285
5.219
5.138
500 °C Annealing
1.796
3.622
2.084
4.345
4.292
Table 3: Band gap and Urbach tail values of the ZIS samples
Sample
𝐸𝑑,1
𝐸𝑑,2
𝐸𝑑,3
𝐸𝑖𝑛𝑑
𝐸𝑈
As-grown
2.00
2.29
2.52
1.62
0.261
300 °C Annealing
1.98
2.26
2.46
1.49
0.172
400 °C Annealing
1.95
2.14
2.45
1.42
0.110
500 °C Annealing
1.85
2.09
2.26
1.40
0.109