Impact of Thickness on the Optical Properties of Selenium Thin Films*

Impacto del espesor sobre las propiedades ópticas de las películas finas de selenio

Ingeniería y Universidad, vol. 25, 2021

Pontificia Universidad Javeriana

Shiva Udachan ^a shivaudachan8@gmail.com

Rani Channamma University, India

Narasimha Ayachit

Rani Channamma University, India

Lingappa Udachan

Channabasaveshwar Arts, Science and Commerce College, India

Raghunanda Halembre

Government First Grade College, India

Esta obra está bajo una Licencia Creative Commons Atribución 4.0 Internacional.

Received: 05 May 2019

Accepted: 06 June 2020

Published: 17 June 2021

DOI: https://doi.org/10.11144/Javeriana.iued25.itop

Abstract: Objectives: The primary objective of this investigation is to make a systematic study on the impact of thickness on optical properties, such as energy gap, absorption coefficient, optical density etc., for selenium thin films. Understanding of the band gap energy and its influence on film thickness is of utmost importance in acquiring the intended electrical characterization of semiconducting films. Materials and methods: Ultra-purity selenium (99.99 %) was deposited on glass substrates. During deposition, the glass substrate with its holder were rotated with constant speed to have a smooth coating. Results and discussions: The XRD findings indicate that selenium is amorphous in nature. The optical band gap energy is found to be decreasing form (2.3 to 2eV) with the rise of film thickness in interval (200 to 1000 nm). The band gap energy obeys inverse square law with respect to thickness. Conclusion: We have properly grown thin films of Se below the De Broglie wavelength limit by thermal evaporation in vacuum. The optical density varies directly with film thickness. The absorption coefficients were in the interval (0.5 to 4) × 107m-1. The AFM results confirmed that the Se nano-size increases with the increase in thickness. Both the grain boundaries and sub-grain regions are clearly visible in the SEM micrographs

Keywords:Absorption coefficient, band gap, optical properties of Se thin films.

Resumen: Objetivos: El objetivo principal de esta investigación es el de realizar un estudio sistemático sobre el impacto que tiene el espesor sobre las propiedades ópticas tales como la banda prohibida, coeficiente de absorción, densidad óptica, entre otras, para las películas finas de selenio. La comprensión de la banda prohibida y su influencia en el espesor de la película es de suma importancia para la adquisición de la caracterización eléctrica deseada de las películas semiconductoras. Materiales y métodos: Se depositó selenio de ultra pureza (99,99%) sobre sustratos de vidrio. Durante la deposición, se hizo girar el sustrato de vidrio en su respectivo soporte a velocidad constante para obtener un recubrimiento uniforme. Resultados y discusión: Los hallazgos de XRD indican que el selenio es de naturaleza amorfa. Se encuentra que la banda prohibida de la banda óptica toma forma descendente (2,3 a 2eV) con el aumento del espesor de la película en el intervalo 200 a 1000 nm. La banda prohibida de la banda obedece la ley del cuadrado inverso respecto al espesor. Conclusión: Hemos generado correctamente películas finas de Se por debajo del límite de longitud de onda de De Broglie mediante evaporación térmica en vacío. La densidad óptica varía directamente con el espesor de la película. Los coeficientes de absorción se ubicaron en el intervalo (0,5 a 4) × 107 m-1. Los resultados de AFM confirmaron que el tamaño nanométrico de Se aumenta con el incremento del espesor. Tanto los límites del grano como las regiones subgranulares son claramente visibles en las micrografías SEM.

Palabras clave: coeficiente de absorción, banda prohibida, propiedades ópticas de películas finas de Se.

Introduction

Thin-film technology is used in many applications such as microelectronics, optics, magnetic-, hard-, and corrosion-resistant coatings, and micro-mechanics [1]. Advances of thin film growth technology over the past two decades has led to oxide heterostructures synthesis by a variety of techniques, including sputter techniques and molecular beam epitaxial for use in electro-optical devices, space science, solar cell energy utilization etc., [1, 2, 3, 4]. Engineers made a promising material stable enough for use in solar cells: simply changing to a perovskite surface removes barriers to its functionality [5], [6].

Optical measurements on thin films provide a good technique for assessing the properties of semiconductors. Particularly, the measurement of absorption coefficient for various energies gives a lot of information about the band gaps of the material. Knowledge of the band gap and its dependence on the film thickness is extremely important in achieving the required electrical properties of a semiconductor for specific applications. In particular, the measurement of absorption coefficients for various incident energies of photons on Se films throws light on the band gap of materials. Knowledge of optical constants of a material such as optical band gap, refractive index and extinction coefficient is quite essential to examine material’s potential opto-electronic applications [5]. Understanding and knowledge of these band gaps is of primary importance for awareness of the electrical resistivity and carrier mobility of semiconductors [6]. In addition, it is widely used in the production of photocells, exposure meters for photographic use, and in reprography [5] for reproducing and copying documents.

Electrical and optical properties are very much influenced by various deposition parameters such as film thickness [7], rate of evaporation [8], substrate temperature [9], crystalline structure [10], base pressure [11], substrate material [12], and the ambient gas [13] present in the vacuum chamber.

Apart from this, selenium is used as a material in electro-photography [14], x-ray photoconductors [15], high-definition digital video cameras [16], pigments [17], semiconductors [18], filament detector arrays [19], medical imaging sensors [20], to improve the thermal stability of photosensors [21], optical applications in the IR region [22], photonic devices [23], and superconductors [24].

In view of the above, interesting properties and little work have been reported on the optical properties of thin selenium films [25], [26] and these background facts prompted us to undertake this study. Hence, we have chosen Se as it is, a nonmetal, in order to study its optical properties in thin film form.

Theoretical Section

Analysis of optical absorption spectra is one of the most productive tools for understanding and developing the band structure and energy gap. The optical absorption coefficient (α) is related to the transmission (T) of a specimen [27] is given by equation (1).

(1)

Where,

t = thickness of the films.

Since Se, a non-metal, is a direct band gap material and for direct allowed transitions, the absorption coefficient (α) is related to band gap energy by the relation [28] and is shown in equation (2).

(2)

Where,

hν is the energy of the incident photon,

Eg is the optical band gap energy of the material,

and B is a constant, which has different values for different transitions.

The energy gap is evaluated for different thicknesses (t) and the band gap variation with the film thickness follows the relation [29] is mentioned in the equation (3).

(3)

Where,

m* is the effective mass of the charge carriers,

(t) is the thickness,

(ћ) is the Planck’s constant,

and ΔE is the kinetic energy contribution due to motion normal to the film plane.

In the present investigation, Se is a semiconducting material and it becomes necessary to estimate the De Broglie wavelength in order to select the thickness range of the film, where the investigation is to be carried out. The De Broglie wavelength (λ') can be estimated using equation (4).

(4)

Where,

h = Planck’s constant,

c = Speed of light,

Ef = Fermi energy = 1.0643 eV = half of the average band gap energy.

Experimental Section

Selenium of 99.99 % purity (obtained from Leico Industries, New York, USA) was thermally evaporated by resistive heating at room temperature (22 °C) under a pressure of (2 x 10^-6) Torr using Hind High Vacuum coating unit model 12A4D. The rate of deposition was around (1-2) nms^-1.

The distance between the substrate and the evaporation source was approximately 20 cm. Before deposition, the substrates were treated with various cleaning techniques including ultrasonically agitation method. Lastly, the substrate was cleaned through the ionic bombardment method in the vacuum chamber before deposition. During deposition, the substrate holder was rotated with a constant speed as to have a smooth surface of the sample as required in optical studies.

The film thickness and deposition rate were controlled by means of an built-in quartz crystal digital thickness monitor (Model DTM-101), which could resolve thickness up to 0.1 % in the vacuum coating unit system. The transmittance spectra (T %) were taken at room temperature by using a spectrophotometer in the wavelength (λ) spectral range of 200 to 800 nm.

Nowadays, there are several thin film coating methods available [29, 30, 31, 32]. Among them, thermal evaporation in vacuum happens to be the best method because it gives films with no impurities as required in many scientific investigations. Glass slides were used as substrates in the present investigation because of their di-electric nature.

Results and Discussions Section

Many experiments have been conducted to study the optical properties of Se thin films. Figure 1 shows a typical XRD pattern for one of the deposited Se thin film of thickness 210 nm. The intensity data were collected over a range of 2θ is 15° to 85°. The pattern shows that the film was amorphous in structure with peaks that started to grow and appear, which exhibit a prominent reflection angle 2θ = 23.5° and 2θ = 29.5°.

There were similar results obtained in other studies [33], [34] which are in a good agreement with the results obtained by Roy et al. for selenium films [15].

The x-ray diffraction studies of selenium film grown at room temperature reveal that they are fine grained and polycrystalline. X-ray diffraction data of selenium films well agrees with that of trigonal selenium peaks [JCPDS: 0362] [15], [17]. The XRD result showed that the selenium film is amorphous in nature due to this arrangement of atoms in thin films [35].

Figure 1.
Plot of Intensity (cps) versus 2θ, The XRD pattern of Se thin films of thickness 210 nm

Source: Own elaboration

Figure 2.
Plot of Transmittance (T %) versus wave-length (λ) of incident photon at different Se films thicknesses (t)*

* Series 1, t1 = 200 nm; Series 2, t2 = 350 nm; Series 3, t3 = 500 nm; Series 4, t4 = 600 nm; Series 5, t5 = 700 nm; Series 6, t6 = 850; and Series 7, t7 = 1000 nm (table A1)

Source: Own elaboration

It is observed that transmittance increases with a decrease in film thickness. According to figure 2, the optical transmission spectrum [T %] of Se thin films, the spectrum shows the interference pattern. As it can be seen from this figure, the results obtained for Se films are characterized with transmission greater than 55 % for wavelength values greater than 500 nm. Similar results were obtained by Harpreet Singh [23] for Se films, Jassim, Zumaila, and Waly [27] for CdS thin films and Anandh et al. [35] for CdSe films. It is observed that all the films are highly transparent, and the maximum transmittance is about 75 %.

Figure 3 shows the plot of optical absorption coefficient (α) against photon energy (hν) at different films thicknesses (t). Series 1, t₁ = 200 nm; Series 2, t₂ = 350 nm; Series 3, t₃ = 500 nm; Series 4, t₄ = 600 nm; Series 5, t₅ = 700 nm; Series 6, t₆ = 850; and Series7, t₇ = 1000 nm.

Figure 3.
Plot of Absorption co-efficient (α) versus photon energy, hν in eV at different Se films thicknesses (t)*

* See table A2.

Source: Own elaboration

Since absorption is a function of incident photon energy [26], based on figure 3, it appears that initially the absorption coefficient decreases from (1.85 to 0.45 x 10^-7m^-1) as the photon energy increases from (0.5 to 2.25 eV). Moreover, the absorption coefficient is high at the low region of incident photon energy while it becomes (0.45 x 10^-7m^-1) at high photon energy of (2.25 eV). Further, the absorption increases from [(0.75 to 3.3) x 10^-7m^-1] with the increase in the photon energy from (2.25 to 5 eV). Similar behavior between absorptions and incident photon energy has been reported for selenium thin films by Mutar and Hemed [34]. In the case of tin oxide thin films, a similar trend in the curve has been noticed [36].

Figure 4 shows the typical plot of (αhν)² against (hν) for Se thin films. The optical band gap is calculated from the intercept (hν = 0) of the straight line. For allowed direct transitions, by extrapolating the straight-line portion of the plots towards low energy, the optical band gap energy can be obtained as shown in figure 4.

Figure 4.
Plot of (αhν)2 versus photon energy, hν in eV at different Se films thicknesses (t)

* Series 1, t₁ = 200 nm; Series 2, t₂ = 350 nm; Series 3, t₃ = 500 nm; Series 4, t₄ = 600 nm; Series 5, t₅ = 700 nm; Series 6, t₆ = 850; and Series 7, t₇ = 1000 nm (table A3).

Source: Own elaboration

The extrapolation of plots of (αhν)² versus (hν) onto energy axis will give the energy band gap. The energy band gap of films varied in the range (2.0 to 2.3) eV [37]. The value of the optical band gap energy is found to be decreasing from (2.3 to 2.0 eV) [35] as the film thickness increases from (200 to 1000 nm), which shows its capabilities to be used as an absorber layer in photovoltaic applications and wide band gap materials (Cdo), suitable for solar cells [38].

In general, density of localized state in the film increases with the thickness, resulting in the decrease of band gap. Such a variation in energy band gap with the increase in thickness in ZnO [39], Sn Se [40], Bi₂ Te _3-x Se_x [41], Sb₂ S₃ [42], ZnS PbS Zn_1-xPb_xS, Zn_x Pb_1-x S, Pb Zn_x S_1-x [43], and LiNbO₃ [44] thin films has been reported. Moreover, the band gap energy for Se estimated by [25], [33] is consistent with the value obtained in this investigation.

From the results obtained in our experiment, the band gap mainly depends on the thickness of the film and the shift in the bandgap energy is attributed to the nanocrystalline quantum size effect [44].

The values of the optical band gap energy (Eg) investigated for Se are given in table 1.

Table 1.
Optical band gap and thickness of Se films

Source: Own elaboration

The plot of (Eg) versus (1/ t²) for Se films is given in figure 5, which is in good agreement with the above relation.

Semi-conductors and semi-metal films are expected to exhibit the quantum size effect [45] when the thickness of the films is found to be comparable with or less than the mean free path or less than the effective De Broglie wavelength of the carriers. Under such circumstances, the quantization of the transverse component of the quasi-momentum occurs because of finite size and thickness of the films. This allows the transverse components of the electron-states to occupy quasi-discrete energy values. Because of this quantization effect, an additional gap in the energy will be generated between the top of the valence band and the bottom of conduction band, by an amount ∆E, which is given by equation (3). Hence, the increase in the band gap energy in the present reference may be due to an additional contribution of energy to the energy band because of quantization effect.

Figure 5.
Plot of band gap energy (Eg) versus (1/ t2) for Se thin films of various thicknesses (t)*

* See table A4.

Source: Own elaboration

From the slope of the curve of figure 5, we get the value for effective mass m* and found it to be (0.3375 x 10^-4) times m_o, where m_o is the rest mass of carriers (electrons), i.e., 9.1 x 10^-31Kg, which is comparable to that obtained for In_0.1Bi₁ Te₃ thin films [46].

The optical absorption coefficient (α) has high values for higher thickness films compared to that of lower thickness films. On the other hand, the optical density (OD) or the absorption is proportional to the thickness of the films [47] and is given by (α x t). The variation of OD with the incident photon energy, hν in eV is depicted in figure 6.

Figure 6.
Plot of optical density (OD) x 10-14 versus the photon energy of Se at various films thicknesses (t)

* Series 1, t₁ = 200 nm; Series 2, t₂ = 350 nm; Series 3, t₃ = 500 nm; Series 4, t₄ = 600 nm; Series 5, t₅ = 700 nm; Series 6, t₆ = 850; and Series7, t₇ = 1000 nm (table A5)

Source: Own elaboration

Optical Intensity (OD) measures the amount of attenuation, or intensity lost, when light passes through an optical component. It also tracks attenuation based on the scattering of light, whereas the absorbance considers only the absorbance of light within the optical component. We obtained a minimum OD at the photon energy of 2.25 eV for all selenium films, as it mainly depends on the incident photon energy. Initially as the incident photon energy increases from (1.5-2.25) eV, the OD decreases for all the thicknesses. Afterwards, the OD increases as the film thickness increases and with that, an increase in the photon energy from (2.5-5) eV. The maximum value of OD achieved in the present investigation is around 8.5 x 10 ^-14 as seen in figure 6. Our results are comparable with those reported for non-crystalline Cd_50.50-XSe. thin films [47].

It is a customary to estimate electron conduction mean free path or De Broglie wavelength (λ'), while making a study on electrical/optical properties with metals or non-metallic films, respectively.

The De Broglie wavelength of electrons or holes, estimated by Fermi-energy taken to be half of the average band gap energy (1.0643 eV), turns out to be about 1167 nm. Hence, we need to select the thickness range around 1167 nm or less than that. Thus, the quantum size effect is expected to be shown by the films in the thickness range of 200-1000 nm [29], [41], [44], [45]. Such an increase in the band gap which is inversely proportional to the square of the film thickness, is attributed to the quantum effect.

In this study, a decrease in band gap energy with increase in film thickness is noticed which has been depicted in the figure 7. A similar trend has also been observed in CdS thin films [9].

Figure 7.
Plot of energy gap (Eg) versus thickness (t) for Se thin films of various thicknesses

* Series 1, t₁ = 200 nm; Series 2, t₂ = 350 nm; Series 3, t₃ = 500 nm; Series 4, t₄ = 600 nm; Series 5, t₅ = 700 nm; Series 6, t₆ = 850; and Series7, t₇ = 1000 nm (table A6)

Source: Own elaboration

The band gap in the case of CdS thin films decreases with film thickness. In this study, the optical band gap decreases with the increase in the thickness of the film.

This variation is due to quantum size effect occurring in semi-conducting thin films. This is verified by figure 7, in which optical band gap shows as a straight line up to the 850 nm thickness, which is comparable with mean free path and effective De Broglie wavelength of the carrier and then shows the constant value of band gap. Furthermore, the optical band gap decreases with thickness, which is explained in terms of quantum size effect [46].

The morphology of Se films of thicknesses 157 and 210 nm were observed by SEM and the images are presented in figure 8 (a-b). The SEM micrographs obtained on the Se thin films are shown for the same magnification, 10,000 for different thicknesses. The marker represents 10 µm in all the 2-micrographs. Similar types of grains were observed in the case of tin films as seen through TEM [48].

The scanning electron micrographs are helpful in elucidating the incipient growth of Se film on glass substrate. Figure 8 (a) shows SEM images of grains and sub-grains oriented in random directions with non-uniform and dense microstructures. The average grain size was around 1430 nm for Se of a 157 nm thickness, which is depicted in figure 8 (a). Similar structures have been reported by Gode et al. [49] for amorphous Zinc Sulphide thin films. For tin films of thickness 30 nm by using TEM, figure 8 (b) shows grains were small with non-uniform shape and well-defined grain boundaries exist. The largest grain has a size of 4.4 µm.

(a) SEM images of Se thin films of magnification 10,000 for thicknesses 157 nm, (b) SEM images of Se thin films of magnification 10,000 for thicknesses 210 nm

Figure 8.
(a) SEM images of Se thin films of magnification 10,000 for thicknesses 157 nm, (b) SEM images of Se thin films of magnification 10,000 for thicknesses 210 nm

Source: Own elaboration

Atomic force microscopy (AFM) is a technique for analyzing the surface morphology and texture of a rigid material all the way down to the level of the atom. AFM utilizes a mechanical cantilever probe to magnify surface features up to 100,000,000 times and produces 3-D images of the surface of the sample.

If the film is continuous, homogeneous, and has no discontinuities, the AFM image is a reflection of that area. AFM measurements were performed to obtain the surface roughness of the Se films. Figure 9 (a-b) shows the AFM images with a scanning area of 5 × 5 µm² of the films. It is clearly observed that the numerous protruded terraces and holes are randomly distributed on the surface of the films and these surfaces are not homogeneous and uniform. The roughness was estimated by root mean square (RMS value). The root mean square (RMS) roughness was about 182, -115 nm and 64, -3.2 nm, respectively, for Se films of thicknesses 157 and 210 nm. It is noticed that the roughness of films decreases as the thickness increases as evidenced by the AFM images. Similar images were obtained for CdS and Ag films [50], [51]. It is known that the increase in the surface roughness may cause deterioration of the electrical and optical properties [52], [53] for ZnO and Tungsten doped ZnO thin films.

Figure 9.
(a) AFM images of Se thin films for thickness 157 nm, (b) AFM images of Se thin films for thickness 210 nm

Source: Own elaboration

Electron Dispersive Spectroscopy (EDS) analysis yields information on the quantitative analysis of the films, which was carried out using Scanning Electron Microscopy (SEM) measurements in order to study the stoichiometry of the films. Figure 10 shows the EDS pattern for Se film of thickness 210 nm with relative analysis and it confirms that the material used in the present investigation is Se. H Metin [54] has obtained a similar graph for the identification of CdS films and Senthil Kumaran et al. obtained an EDS pattern which exactly coincides with our results [55] for pure selenium films as indicated by the prominent peak in our spectra.

Figure 10.
EDS spectra of Se films of thickness 210 nm

Source: Own elaboration

Conclusion

The following conclusions may be drawn from the results obtained.

We have successfully grown the thin films of Se on glass substrates held at room temperature below the De Broglie wave-length limit by thermal evaporation in vacuum. The XRD analysis reveals that structure of Se is amorphous. The direct band gap energy calculated from absorption data are in the range (2.0-2.3) eV, which is in good agreement with the expected value. The results were found to be in consistent with other studies that used different procedures. The direct band gap decreases from 2.3 eV to 2.0 eV as the thickness increases. The optical band gap energy of thin Se films has been found to obey the inverse square law with respect to thickness. The optical density studied is found to be a function of film thickness. The effective electron mass estimated from this investigation (0.3375 x 10^-4) times m. is comparable to that obtained by other authors. The absorption coefficients were in the range (0.5-4.0) × 10.m^-1. The AFM results confirmed that the Se nanosize increases with the increase in the film thickness. Both the grain boundaries and sub-grain regions are clearly visible in the SEM micrographs. We observe that the grain and sub-grain microstructure is a result of the bombardment of growth surface by energetic ions during deposition. The EDS spectra confirms the material used in the present investigation is pure selenium.

Acknowledgments

The authors Prof. Dr. L. A. Udachan and Prof. Dr. N. H. Ayachit acknowledge the Vision Group on Science & Technology, Government of Karnataka, India, for providing the necessary research facility for carrying out the research work and the instruments procured under grant sanctioned, bearing GRD No. 185 & GRD No. 539, respectively.

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Appendices

Table A1.
Plot of transmittance (T %) versus wave-length (λ) of incident photon at different Se films thicknesses (t)

Source: Own elaboration

Table A2.
Plot of absorption co-efficient (α) versus photon energy, hν in eV at different Se films thicknesses (t)

Source: Own elaboration

Table A3.
Plot of (αhν)2 versus photon energy, hν in eV at different Se films thicknesses (t)

Source: Own elaboration

Table A4.
Plot of band gap energy Eg in eV versus (1\t2) for Se thin films of various thicknesses (t)
$Plot of band gap energy Eg in eV versus (1\t2) for Se thin films of various thicknesses (t)$

Source: Own elaboration

Table A5.
Plot of optical density [(OD) (α x t) x 10-14] versus the photon energy at Se various films thicknesses (t)

Source: Own elaboration

Table A6.
Plot of Energy gap (Eg) versus thickness (t) for Se thin films of various thicknesses

Source: Own elaboration

Notes

* Research article

Author notes

^a Corresponding author. E-mail: shivaudachan8@gmail.com

Additional information

How to cite this article: S. Udachan, N. Ayachit, L. Udachan, and R. Halembre, “Impact of thickness on the optical properties of selenium thin films,” Ing. Univ., vol. 25, 2021. https://doi.org/10.11144/Javeriana.iued25.itop