Abstract
The present work encloses structural and optical characterization of copper (I) selenide (Cu2Se) thin films. The films having thickness 85 nm have been deposited using thermal evaporation technique in initial step of work. The structural and morphological studies of deposited thin films are then done by X-ray diffraction (XRD), scanning electron microscope (SEM), and surface profilometer measurements. Later on, ultraviolet-visible-near-infrared (UV-VIS-NIR) spectrophotometer and Raman spectroscopic measurements are performed for optical characterization of films. The structure and morphology measurements reveal that deposited material of films is crystalline. The optical band gap estimated from the optical transmission spectra of the film has been found 1.90 eV. The mean values of refractive index, extinction coefficient, real and imaginary dielectric constant are received 3.035, 0.594, 9.623, and 3.598, respectively. The obtained results are compared and analyzed for justification and application of Cu2Se thin films.
1 Introduction
The copper chalcogenide thin films have generated a wide interest among the researchers due to their various applications in optoelectronic devices such as solar cells, photo detectors, photo conductors, photo thermal conversion, electro conductive electrodes, sensor, laser diodes, thin film transistor etc. [1], [2], [3], [4]. It is a semiconducting material, which has electrical and optical properties suitable for optoelectronic application [5]. The copper selenide compositions exist in different structural forms such as stoichiometric (CuSe, CuSe2, Cu2Se, and Cu3Se) and non-stoichiometric compositions (Cu2-xSe) [6], [7].
The structural and optical properties of the Cu2Se thin films have been investigated in detail by different researchers. The current and voltage equation for Cu2Se nanowires are reported to be non-linear and rectifying in nature while conduction in it is followed by different mechanisms in different voltage regions [7]. Mane and co-workers studied the p-type copper (I) selenide (Cu2Se) thin films and found its potential application in p-n hetro-junction solar cells [8]. The synthesis Cu2Se nanoparticles through electrochemical procedure and its optical/electrical characterization are reported by Rong et al. [9]. It is described in literature that Cu2Se composite having P-Cu2Se heterojunction comprises gas-sensing properties towards acetone gas [10]. Yue et al. synthesized Cu2Se electrode on copper grid substrate and his study indicates that Cu2Se is suitable cathode material for sodium ion batteries [11].
The semiconducting material TiO2 of energy band gap ∼3.4 eV and refractive index ∼2.5 is used in removal of organic impurity under ultra-violet photo catalysis by oxidation [12], [13]. When the ultraviolet (UV)-light incidents on TiO2, electron–hole pairs are generated, that oxidizes organic pollutants in CO2 and H2O. The material TiO2 anatase having high refractive index (>3.0) has been reported to be suitable for visible light photo catalysts as due to its low band gap (<2.0 eV) [14]. The surface area enhances and band gap reduces at nanoscale. Due to increase in surface area, more pollutants come across the surface of material and hence, the oxidation rate enhances. The reduction in band gap makes the material to be active in visible region. The nano material of Cu2Se having particle sizes (75 nm) are reported to have 1.94 eV band gap [9]. Therefore, nano material of Cu2Se and its thin film can be used as visible light photo catalysts with improved efficiency for removal of organic pollutants of water.
Even though, seldom works have been done on structural, electrical and optical properties of Cu2Se thin films but the studies on wavelength dependent variation of refractive index, extinction coefficient and dielectric constant of nano Cu2Se thin films are not much reported in literature. The present work is, therefore, concentrated on synthesis and characterization of Cu2Se nano thin films. In initial step, thin films of Cu2Se having thickness 85 nm are prepared using thermal evoporation technique. The deposited films are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and surface profilometer measurements for its structural and morphological properties. Later on, UV-visible-near-infrared (VIS-NIR) and Raman spectroscopic measurements are performed for the optical characterization of the films. The obtained results of deposited films are discussed and analyzed to explore its inherent structural, optical and dielectric properties.
2 Synthesis and measurements
For the preparation of films, Cu2Se powder of 99.9% purity was purchased from Sigma-Aldrich. After cleaning glass substrate with acetone and isopropyl alcohol, the deposition of Cu2Se on glass substrate was carried out by thermal evaporation technique using vacuum coating unit model 12A4D-T “HIND HIVAC” under vacuum 5 × 10−5 mbar. In this process, roughing and backing vacuum were maintained at 6 × 10−2 mbar while primary and secondary current were settled at 6.0–6.5 A and 110–120 A, respectively. Furthermore, the deposition on substrate was taken only for 30 s to form the films. After the formation of films, surface profilometer (Veecco dektak 150) apparatus was used to measure the thickness of thin films. Later on, XRD measurements of deposited film was carried out with X’ Pert Powder PANalytical using Cu-Kα radiation (λ = 1.5405Å). The diffracted intensity was measured for angular variation of 10°–80°. The SEM (Carl Zeiss EVO 40 Cambridge UK) was used to record the surface morphology of films. The UV-VIS-NIR spectrometer lambda 750 was used to measure the transmission of thin film in the spectral range 300–1100 nm. A Raman spectrum was monitored by using Renishaw in Via Raman microscope in the range of 100–1500 cm−1.
3 Results and discussion
3.1 Structural and morphological study
3.1.1 Thickness and XRD measurements
The measurements made on deposited thin films with surface profilometer reveal that the Cu2Se films (having deposition time 30 s) have thickness 85 nm. This confirms that the thickness of prepared films is of the order of nanometer. The XRD is an important investigative technique for phase identification and structure analysis of grown film. The XRD pattern of Cu2Se thin film is shown in Figure 1. A comparison with JCPDS file no. 65-2982 confirms that the obtained diffraction peaks of the film at 2θ = 27.11°, 44.72°, 52.92°, 64.90°, and 71.52° can be assigned as (111), (220), (311), (400), and (331), respectively. It also confirms that the phase of the film is cubic with lattice parameters a = 5.7600 Å, and space group Fmˉ3m {225}. The crystallite size (D) of grown film is determined using Debye-Scherrer’s formula from the full width at half maximum (β) of peaks expressed in radians [15].
where, constant K is equal to 0.94 and λ is the X-ray wavelength while θ is diffraction angle. The calculation with Equation (1) provides that the crystallite size of the film which is found equal to 41 nm. From Figure 1, it also confirms that Cu2Se thin film is polycrystalline in nature. Sudha et al. have reported 84 nm crystallite size of Cu2Se thin film having thickness 134 nm [1]. The synthesis and non-destructive characterization of zinc selenide thin films crystallite size increases with increasing thickness/deposition time [16]. Yet, the crystallite size of film is dependent of various parameter during deposition of films such as temperature of substrate deposited on glass, deposition current, deposition time, concentration of structural defect, increasing thickness of the film etc. However, the integrated study of present and Sudha et al. works reveals that crystallite size of Cu2Se thin films increases with increasing thickness.
X-ray diffraction (XRD) patterns of the Cu2Se thin film.
3.1.2 SEM and EDS measurements
The SEM has been used to explore the surface morphology of the deposited nano thin film. Figure 2 represents the SEM micrograph of the film under study which predicts that deposited particles on glass substrate are randomly distributed over the surface with an average size ∼70 nm having approximately spherical shape. The deposited film surfaces are also confirmed by SEM micrograph. Xue et al. have also found the spherical shape of particles in Cu2Se thin films [17]. Figure 3 shows the energy- dispersive spectroscopy (EDS) spectrum of the prepared thin film sample by the emergence of their respective peaks which confirms the presence of Cu and Se elements in the thin film. The spectrum also shows that the prepared thin film does not contain any additional impurity elements.
Scanning electron microscope (SEM) micrograph of the deposited Cu2Se thin film.
The EDS spectrum of the deposited Cu2Se thin film.
3.2 Optical characterization
3.2.1 Optical transmission measurements
The optical transmittance spectra of the Cu2Se thin film is recorded with UV-VIS-NIR spectrophotometer in transmittance mode for the range of 300–1100 nm as shown in Figure 4. The intensity of transmitted light for mid-VIS to IR region is received to vary 20–60% (Figure 4). The optical transmittance is found to increase with wavelength. The optical transmission of the light in the IR region is obtained as 40–60%. The Fresnel’s theory of electromagnetic wave deduced that the refractive index of concern medium enhances with decay in amplitude ratio of transmitted and incident electromagnetic waves or transmittance of the concern medium [18]. Thus, the gradual increase of transmittance from visible to infra-red regions for Cu2Se film reveals the decay in refractive index of film.
Optical transmission spectra of the Cu2Se thin film.
We have also calculated the optical band gap for the deposited thin film using Wood and Tauc formula [19]:
where, hυ, Eg and B are energy of radiation, optical band gap and tailoring band constant, respectively. The constant m has value 1/2 for the direct band gap of allowed transition. The obtained curves between (αhυ)2∼ hυ for the deposited thin film is shown in Figure 5. The estimated value of optical band gap with optical transmission spectra (Figure 5) for the present deposited Cu2Se film (85 nm thickness) is found 1.90 eV. This value of band gap is quite close to the earlier reported values for the Cu2Se films [8], [9]. Sudha et al. have reported 1.75 eV band gap for Cu2Se thin film having thickness 134 nm [1]. The band gap of thin films have been reported to decrease with increase in film thickness/crystallite size [16]. The present finding along with Sudha et al. work supports the same characteristics of band gap for Cu2Se thin film. Since, the wavelength corresponding to 1.9 eV is 653.3 nm which belongs to visible region therefore; the present thin film can be used in production of laser light. The reported values of energy band gap for CdSe, CuSe, ZnSe, ZnO, and ZnS, nano thin films are 1.7 eV, 2.03 eV, 2.6 eV, 3.3 eV, and 3.5 eV, respectively [6], [16], [20], [21], [22]. Hence, the present nano thin film has low band gap in comparison to other oxide/sulphide semiconduting nano thin films while posses approximatetly same band gap as for CdSe and CuSe.
Plot (αhν)2 vs. hν for Cu2Se thin film.
3.2.2 Refractive index, extinction coefficient and dielectric constant calculations
The refractive index is an important property of the thin film that signifies the amount of light transmitting through it. During the transmission of electromagnetic wave through thin film having negligible absorbance, the light propagates through several layers (1st, 2nd, 3rd to tth layer) of a transparent medium. The transmittance light of tth layer can be written as [23].
For normal incidence, the reflectance ‘R’ is function of refractive index ‘n’ and extinction coefficient ‘k’ [23].
On solving Equation (4) using componendo and dividendo method, the following expression of refractive index n is obtained.
The extinction coefficient (k) measures the absorption of light by the medium. Numerically, it is equal to αλ/4π[24]. Here, α is absorption coefficient and can be obtained with the help of transmittance as α = d−1lnT−1. Here, d is thickness of the film. The quantities k and n for the deposited Cu2Se film are estimated using the measured transmittance under wavelength variation. The estimated extinction coefficient and refractive index with wavelength variation are shown in Figure 6. The mean value of extinction coefficient and refractive index of present Cu2Se thin film are found 0.594 and 3.035 for wavelength range 300–100 nm, respectively. The refractive index of ZnO, ZnS, CuSe, ZnSe, and CdSe nano thin films have been reported to be in between 2.0 and 2.8 in visible region [25], [26], [27], [28], [29]. Thus, the present nano thin film encompasses large refractive index in comparison to other semiconducting sulphide/selenide/oxide nano thin films. The polynomial curve fit analysis of n and k indicates that these physical quantities are eighth order polynomial function of wavelength
Refractive index and extinction coefficient of the Cu2Se thin film.
Polynomial fit of refractive index and extinction coefficient of the Cu2Se thin film.
| Fit equation → | n = B0 + Β1λ1 + B2λ2 + B3λ3 + B4λ4 + B5λ5 + B6λ6 + B7λ7 + B8λ8 | k = B0 + Β1λ1 + B2λ2 + B3λ3 + B4λ4 + B5λ5 + B6λ6 + B7λ7 + B8λ8 | ||
|---|---|---|---|---|
| Constants ↓ | Value | Error | Value | Error |
| B0 | 6.259 × 10+2 | 0.212 × 10+2 | 5.179 × 10+1 | 0.178 × 10+1 |
| B1 | −7.439 × 10+0 | 0.271 × 10+0 | −6.220 × 10−1 | 0.229 × 10−1 |
| B2 | 3.842 × 10−2 | 0.149 × 10−2 | 3.240 × 10−3 | 0.125 × 10−3 |
| B3 | −1.116 × 10−4 | 0.046 × 10−4 | −9.427 × 10−6 | 0.383 × 10−6 |
| B4 | 1.990 × 10−7 | 0.085 × 10−7 | 1.683 × 10−8 | 0.071 × 10−8 |
| B5 | −2.233 × 10−10 | 0.100 × 10−10 | −1.888 × 10−11 | 0.084 × 10−11 |
| B6 | 1.539 × 10−13 | 0.072 × 10−13 | 1.300 × 10−14 | 0.060 × 10−14 |
| B7 | −5.968 × 10−17 | 0.290 × 10−17 | −5.029 × 10−18 | 0.244 × 10−18 |
| B8 | 9.964 × 10−21 | 0.504 × 10−21 | 8.374 × 10−22 | 0.425 × 10−22 |
The dielectric constant is a well-known fundamental intrinsic property of material. Since, the wave propagation vector is a complex number (n + ik) for anisotropic medium or thin film. Therefore, the dielectric constant of the medium/thin film reduces to (n + ik)2 with a real component ε1=n2−k2 and imaginary component ε2 = 2nk. Both of these quantities inflict the property of slowing down the speed of light in thin film with respect to free space. The dielectric constant ε1 and ε2 for the deposited Cu2Se thin film are estimated using the measured quantities n and k under wavelength variation. The estimated real dielectric constant ε1 and imaginary dielectric constant ε2 with wavelength variation are shown in Figure 7.
Real and imaginary dielectric constants of the Cu2Se thin film.
Both the component of dielectric constants has found to decay with wavelength of incident light. Besides this, the real part of ε has also found to be larger than its imaginary part. The similar characteristic of dielectric constant for thin film has been reported in literature [30]. The decay in dielectric constant/refractive index reveals the enhancements of phase velocity/transmittance with respect to increase in wavelength of incident light for the present thin film. The mean value of ε1 and ε2 of present Cu2Se thin film are found 9.623 and 3.598 for wavelength range 300–1100 nm, respectively. The polynomial curve fit analysis of ε1 and ε2 indicates that these physical quantities are eighth order polynomial function of wavelength
Polynomial fit of real and imaginary dielectric constants of the Cu2Se thin film.
| Fit equation → | ε1 = B0 + Β1λ1 + B2λ2 + B3λ3 + B4λ4 + B5λ5 + B6λ6 + B7λ7 + B8λ8 | ε2 = B0 + Β1λ1 + B2λ2 + B3λ3 + B4λ4 + B5λ5 + B6λ6 + B7λ7 + B8λ8 | ||
|---|---|---|---|---|
| Constants ↓ | Value | Error | Value | Error |
| B0 | 7.164 × 10+3 | 0.250 × 10+3 | 1.325 × 10+3 | 0.046 × 10+3 |
| B1 | −8.559 × 10+1 | 0.320 × 10+1 | −1.593 × 10+1 | 0.060 × 10+1 |
| B2 | 4.419 × 10−1 | 0.175 × 10−1 | 8.270 × 10−2 | 0.329 × 10−2 |
| B3 | −1.280 × 10−3 | 0.053 × 10−3 | −2.408 × 10−4 | 0.100 × 10−4 |
| B4 | 2.287 × 10−6 | 0.100 × 10−6 | 4.304 × 10−7 | 0.188 × 10−7 |
| B5 | 2.565 × 10−9 | 0.118 × 10−9 | −4.835 × 10−10 | 0.221 × 10−10 |
| B6 | 1.768 × 10−12 | 0.085 × 10−12 | 3.336 × 10−13 | 0.159 × 10−13 |
| B7 | −6.856 × 10−16 | 0.343 × 10−16 | −1.293 × 10−16 | 0.064 × 10−16 |
| B8 | 1.144 × 10−19 | 0.059 × 10−19 | 2.161 × 10−20 | 0.111 × 10−20 |
3.2.3 Raman measurements
Figure 8, shows Raman spectra of the Cu2Se thin film. The sample gives various bands centered at 259, 928 and 1115 cm−1. In these spectra, the band at 259 cm−1 is more intense compared to the other bands, which corresponds to Cu-Se vibration. The two other modes of longitudinal optical phonon frequency have been observed at 928 and 1115 cm−1 with relatively weak intensity in the Raman spectra. The other reported works related to this study of Cu2Se thin films having different thickness have also found main peak in Raman spectra either at 259 cm−1 or 260 cm−1 or 262 cm−1 [31], [32], [33]. Since, the main peak in Raman spectra of Cu2Se thin film are found approximately same wavenumber therefore, Cu-Se bond length and force constant for Cu2Se will be independent of film thickness.
Raman spectra of the Cu2Se thin film.
4 Conclusions
The Cu2Se thin films were deposited on a glass substrate using thermal evaporation techniques. The XRD pattern of the film verifies the nano-crystalline nature having cubic structure, while SEM micrographs of film confirm the formation of spherical particles in its surface. The EDS measurement shows the presence of Cu and Se elements in the thin film. The optical transmittance is found to increase with wavelength. The estimated value of optical band gap with optical transmission spectra for the present deposited Cu2Se film (85 nm thickness) is found 1.90 eV. Since, the wavelength corresponding to 1.9 eV is 653.3 nm which belongs to visible region therefore the present thin film can be used in production of laser light. The mean value of refractive index, extinction coefficient, real and imaginary dielectric constant of present Cu2Se thin film are found 3.035, 0.594, 9.623, and 3.598, respectively for wavelength range 300–1100 nm. The band at 259 cm−1 in Raman spectra is more intense compared to the other bands, which corresponds to Cu-Se vibration. The present study opens a new base for further study and application of Cu2Se thin films.
Acknowledgements
The authors wish to acknowledge Prof. Vinay Gupta, Department of Physics, University of Delhi, New Delhi, India for providing films sample deposition and UV-VIS-NIR spectroscopy facility. The authors are grateful to Prof. K. Srinivas, Department of Physics, University of Delhi, New Delhi, India for providing Raman spectroscopy measurement facility. The authors express high gratitude to Dr Satish Chandra, Department of Physics, P.P.N. (P.G.) College, Kanpur, for his support in language improvement of the manuscript.
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: None declared.
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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