Temperature dependent Raman and photoresponse studies of Bi₂Te₃ thin films annealed at different temperatures for improved optoelectronic performance

Abstract

Nanocrystalline Bi₂Te₃ thin films of 800 nm thickness synthesized via a thermal evaporation method were annealed at various temperatures, which induced notable changes in both structural and optical characteristics. Structural analysis confirmed the presence of the rhombohedral Bi₂Te₃ phase in the material. The crystallinity got enhanced at increased annealing temperature. Temperature-dependent Raman studies revealed a red shift in the vibrational modes. Photoluminescence data revealed a blue shift of the emission edge, primarily attributed to the annealing-induced alteration in crystallinity, resulting in greater electron–hole emission efficiency. Morphological analysis provides insights into the slight agglomeration of the films at higher annealing temperatures. The uniform distribution of constituent elements in the films was confirmed through EDX and surface mapping. The optical study indicated a reduction in transmittance values, with increase in the absorption coefficient. The red shift of the absorption edge implied a decrease in the direct optical bandgap value from 0.528 to 0.506 eV. The estimated static refractive index and linear dielectric constant exhibited an increasing trend at higher annealing temperatures. The enhanced non-linear refractive index ranging from 3.62 × 10⁻⁹ to 3.96 × 10⁻⁹ esu resulted in the increase in non-linear susceptibility with annealing. Surface wettability studies demonstrated high hydrophilicity at increased annealing temperature. Photo-response studies indicated an increment in the photo-current for higher temperature annealed films. The observed optimized properties of the films are suitable for various optoelectronic applications, such as absorber layers for solar cells, infrared related devices, and other photo-response applications.

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1. Introduction

Recently, semiconducting thin films based on chalcogenides have emerged as prominent candidates for a diverse array of applications in optoelectronics, electronics, and energy-related fields.^1,2 Their extensive range of properties have led to their utilization in microelectronics and various coating materials. Among materials based on group V and VI elements, including bismuth selenide (Bi₂Se₃), bismuth telluride (Bi₂Te₃), and bismuth sulphide (Bi₂S₃), Bi₂Te₃ stands out as an established material, particularly renowned for its thermoelectric properties at room temperature.^3–5 Bi₂Te₃, due to its unique material characteristics, has been pivotal in the development of thermoelectric generators and coolers. Its thin films possess a layered crystalline structure, comprised of quintuple layers of Bi₂Te₃ units. This unique structure plays a crucial role in their thermoelectric behaviour, significantly impacting their charge transport properties and ultimately contributing to their excellent thermoelectric performance.⁶ Apart from their thermoelectric prowess, Bi₂Te₃ thin films exhibit intriguing optical properties driven by their distinctive electronic band structure and quantum confinement effects. Bi₂Te₃ is classified as a topological insulator, due to its unique electronic band structure. The bandgap of Bi₂Te₃ thin films falls within the mid-infrared range.^7,8 Bi₂Te₃ thin films display strong absorption within their optical bandgap, a property advantageous for infrared detectors. Their efficient absorption of specific wavelengths of light makes them suitable for applications in thermal radiation sensing and other optical detection technologies.⁹ Furthermore, the ability to modulate their optical properties upon application of external stimuli, like temperature and electric field, enhances their utility in photonic devices. Due to their mid-infrared absorption capabilities, Bi₂Te₃ thin films serve as effective materials for infrared photodetectors. They can efficiently convert incident infrared radiation into electrical signals, offering potential applications in thermal imaging, remote sensing, and other fields.¹⁰

Various external energy treatments have produced notable modifications in the thin film, for different application purposes. Photoinduced conversion, thermal annealing, ion irradiation, etc. are considered as different energy treatments for many prepared thin films.^11–13 Among these, thermal annealing is a widely employed post-deposition process that induces structural, chemical, and physical changes in thin films.¹⁴ It is essential for tailoring the properties and enhancing the performance of thin film materials in various applications. Thermal annealing has the potential for the recrystallization and refinement of crystal structure within thin films. It can help eliminate defects, grain boundaries, and dislocations, resulting in larger and more ordered crystal grains.¹⁵ This structural enhancement can significantly improve the electrical, optical, and mechanical properties of films. Annealing can promote surface diffusion and smoothen the roughness of thin films, making them more suitable for applications that require precise surface topography, such as integrated circuits and optical coatings.^16,17

Several pioneering studies have investigated the transformative effects of annealing on Bi₂Te₃ thin films, revealing enhancements in their structural, optical, and electrical characteristics. These studies have utilized various deposition methods and annealing techniques to tailor the properties of Bi₂Te₃ thin films, paving the way for diverse applications. Zheng et al.¹⁸ explored the influence of rapid heat treatment on Bi₂Te₃ films, prepared on polymer substrates using single-source evaporation. The process led to improved crystallinity and controlled grain size, resulting in the dominance of the pure Bi₂Te₃ phase. This enhancement in crystal quality was reflected in an elevated power factor and a notable increase in the ZT value, highlighting the significance of rapid annealing in optimizing thermoelectric performance. Norimasa et al.¹⁹ focused on radio-frequency magnetron sputtering based Bi₂Te₃ films prepared on polyimide substrates. Post-annealing enhanced the crystallite size and crystal orientation, positively influencing electrical conductivity. However, it also caused substrate stretching, mitigating strain accumulation in the films and promoting crystal growth. This study sheds light on the complex interplay between annealing, substrate, and film properties. Guo et al.²⁰ introduced a scalable method for obtaining high-quality Bi₂Te₃ films using magnetron sputtering. The enhanced annealing conditions facilitated the formation of large size grains with a smooth surface and highly oriented Bi₂Te₃ films. This study emphasized the temperature-dependent nature of film growth and its implications for film quality. He et al.²¹ investigated the in situ annealing of n-type Bi₂Te₃ films during magnetic sputtering. The study revealed that higher sputtering power led to an increased grain size but reduced electrical conductivity, along with a decrease in carrier concentration. This work demonstrated the intricate relationship between annealing, sputtering conditions and film properties. Naumochkin et al.²² reported the influence of annealing on sputtered Bi₂Te₃ thin films, highlighting its role in improving crystallinity and reducing defect states. This work underscored the significance of annealing in enhancing film quality and performance. Fan et al. adopted a rapid laser annealing method to produce Bi₂Te₃ films with superior structural, chemical, and thermoelectric properties compared to those achieved using conventional annealing techniques.²³ This innovative approach showcased the potential of advanced annealing methods in tailoring materials' properties. Eguchi et al.²⁴ synthesized Bi₂Te₃ thin films through the electrodeposition method, observing a crystalline phase transition from Bi₂Te₃ to Bi₄Te₅ during annealing. This transition led to an increase in thermoelectric performance, highlighting the role of annealing in modifying the material's composition and properties. Electron-beam synthesis of Te-rich Bi₂Te₃ thin films at room temperature, followed by annealing at various temperatures, resulted in n-type semiconductor behavior and improved electrical properties.²⁵ Adam et al.²⁶ have reported the optical behaviour of Bi₂Te₃ films prepared through various techniques such as vacuum thermal evaporation and pulsed laser deposition. The direct optical bandgaps of 0.91 and 0.95 eV were found for films prepared through vacuum and pulsed laser deposition methods, respectively. These two methods exhibit quite different results from each other in terms of optical, structural, and other properties. Jian et al.²⁷ observed modification in the wettability and nanomechanical behaviour of Bi₂Te₃ films after annealing. Bi₂Te₃ films deposited by a pulsed laser method over the SiO₂/Si substrate showed increased crystallinity, but decreased hardness. Similarly, the contact angle decreased with increasing annealing temperature, which increased the surface energy of the material. The increment in the hydrophilicity occurs due to the increased surface roughness of the material upon annealing. Bi₂Te₃ materials have evolved to become versatile materials with profound implications for modern optoelectronics, electronics, and energy-related applications. Their unique properties, both in the thermoelectric and optical domains, continue to fuel research and innovation across a range of technological fields. Owing to the importance of Bi₂Te₃ films in different application sectors based on their unique properties, our aim is to optimize their optoelectronic properties upon thermal annealing and investigate their surface wettability and photo-response behaviour under different annealing conditions.

In the current study, the profound effect of annealing process on thermally evaporated Bi₂Te₃ thin films was studied thoroughly. By subjecting the as-prepared Bi₂Te₃ films to annealing at various temperatures, specifically at 473 K (200 °C) and 523 K (250 °C), various alterations in their optical, microstructural, and morphological properties are investigated. The as-grown and annealed films were studied for their structural change by temperature dependent Raman and X-ray diffraction (XRD) analyses. Photoluminescence data were recorded for investigating the shift of emission edge upon annealing.

The contact angle measurement reveals the surface wettability nature, while the photocurrent–voltage response was recorded using a Keithley 2450 meter. The change in optical properties was determined from the UV-visible data, while the elemental composition was determined from the energy dispersive X-ray study (EDX). Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) were used to observe the surface structural change due to annealing.

2. Synthesis and characterization procedures

2.1. Thin film preparation

Bi₂Te₃ films were fabricated using the thermal evaporation process with an HHV Smart Coat: 3.0 coating unit. High-purity Bi₂Te₃ powder (99.999%, Sigma Aldrich) was employed for the synthesis. Prior to film preparation, the glass substrate was cleaned properly with dilute HNO₃, aqueous NaOH, and distilled water. The drying of the substrates was done at 373 K (100 °C) inside a hot air oven. Using molybdenum boats, the requisite Bi₂Te₃ powder samples were loaded into the deposition chamber. The material was deposited to a thickness of approximately 800 nm on the glass substrates. The room temperature conditions and the pressure of 5 × 10⁻⁵ torr within the deposition chamber were maintained. To ensure even and uniform deposition, the substrate holder was in continuous motion at a slow speed. The attached quartz crystal thickness controller monitored the film thickness. Thermal annealing was done for the prepared films at 473 K and 523 K for 2 h in a vacuum oven. Scheme 1 presents the schematic representation of the deposition and thermal annealing of the Bi₂Te₃ thin film.

Scheme 1 Schematic representation of deposition and annealing processes of Bi₂Te₃ thin films.

2.2. Characterisation

XRD data were recorded for the as-grown and annealed Bi₂Te₃ films using a Bruker D8 Advance machine equipped with a Cu K_α X-ray source (wavelength = 1.54 Å). The operating voltage of 30 kV and current of 40 mA were the operating conditions for XRD. The scan was taken over angles ranging from 20° to 60° at a glancing angle of 1° to obtain the structural information. In addition to XRD, temperature dependent Raman and photoluminescence (PL) measurements have also been done using a Renishaw, RE 04 spectrometer equipped with a 532 nm excitation green laser. Optical measurements were performed using a JASCO-660 UV-vis-NIR spectrophotometer over the wavelength range of 1600–2500 nm with a spectral resolution of 1 nm. The morphology and composition of Bi₂Te₃ films were determined by using a JEOL, JSM-7601FPLUS FESEM with an attached EDX setup. The FESEM instrument was operated at voltages between 20 and 200 kV during characterization. High-resolution TEM (JEM-2100-HRTEM) was used to estimate the inter-planar spacing and determine the phase composition. The line spacing was accurately calculated from selected area electron diffraction (SAED) and HRTEM images. TEM characterisation was done on the 523 K annealed thin films. The surface wettability test was performed using a DME-211 Plus contact angle measurement unit.

2.3. Photo-response measurement

To obtain the photo-current data, both the as-grown and annealed films were deposited on substrates of 5 × 5 mm² area. To establish conductive contact, a small quantity of Ag paste was applied to the two parallel sides of the films. These samples were then affixed to a sample holder at room temperature. For the photo-response measurement, the materials were illuminated by the 532 nm green laser. Scheme 2 presents the photocurrent measurement set up for the current thin films. The photo-response study was carried out by varying the laser power over the material. Current measurements were performed using a Keithley 2450 meter across a specific voltage varying from −10 V to 10 V. The step size was 0.03 V.

Scheme 2 Schematic representation of the photo-current measurement set up.

Current–voltage measurements were performed under two distinct conditions: light conditions and dark conditions. Under light conditions, measurements were performed by illuminating the sample with laser powers of 20 mW, 40 mW and 60 mW, respectively. Under dark conditions, measurements were performed with the sample placed under complete darkness. This dual approach allowed for a comprehensive assessment of the sample's response to varying voltage levels in both illuminated and non-illuminated states. On the other hand, the variation of photo-response of the material with time was also determined through the current versus time plot at a particular bias voltage under different laser power conditions.

3. Results and discussion

3.1. XRD analysis

Fig. 1(a) illustrates the XRD pattern of as-grown and annealed Bi₂Te₃ films showing the formation of rhombohedral phase. The XRD pattern showed the rhombohedral phase (ICDD code: 00-015-0863) of Bi₂Te₃ films. For the as-grown Bi₂Te₃ film, the diffraction peaks found around 23.64°, 27.62°, 37.79°, 40.20°, 49.82° and 56.98° correspond to the (hkl) values of (1 0 1), (0 1 5), (1 0 10), (0 1 11), (2 0 5) and (0 2 10), respectively. Meanwhile, the 473 K annealed thin film exhibits a similar pattern to that of the as-prepared thin film, but one more peak at 62.91°, which corresponds to (hkl) (0 2 13). However, the 523 K annealed Bi₂Te₃ thin film shows greater crystallinity as compared to 473 K annealed and as-grown films. Several other diffraction peaks appeared for the higher temperature annealed films. Peaks around 17.48°, 32.97°, 41.04°, 44.33°, 53.75°, 61.99°, 65.92°, 69.72° and 72.47° correspond to the (hkl) values of (0 0 6), (0 1 8), (1 1 0), (0 0 15), (1 0 16), (1 1 15), (0 2 16) and (2 1 10). The crystallite peak shifted towards a higher diffraction angle upon annealing from the as-grown state. Fig. 1(b) shows the peak shift of (0 1 5) towards a higher theta value. Various structural parameters were also calculated from the crystallite peaks of the as-grown and annealed materials. The average crystallite size, number of crystallites per unit area, dislocation density, and strains in the lattice were determined from the XRD data using the Scherrer equation, full width at half maximum (FWHM) (β) and diffraction angle (θ). The crystallite size (D) refers to the dimensions of the individual crystalline domains in a material. In other words, it is the small, well-ordered regions within a larger material that exhibit crystalline structure. The size of crystallites can have significant effects on the material's properties, such as mechanical strength, electrical conductivity, and optical characteristics. The crystallite size can also be calculated using the following relation:²⁸Here, FWHM β is in radians, wavelength of Cu K_α-line (λ = 1.54 Å), Bragg's angle is θ. The increase in the annealing temperature leads to the increment in the crystallinity of the prepared thin films. Lattice strain, also known as crystal lattice strain, is a deformation or distortion in the regular geometric arrangement (lattice) of atoms within a crystalline material. In a perfect crystal lattice, atoms are arranged in a specific and repeating pattern. When a crystal lattice is subjected to external forces, temperature changes, or other factors, it can undergo strain. This can be estimated using the following relation:²⁹Dislocation density is a measure of the concentration of crystallographic defects known as dislocations within a crystal lattice. Dislocations are linear defects or disruptions in the regular arrangement of atoms in a crystalline material. They can occur due to various factors such as plastic deformation, thermal stresses, or the presence of foreign particles. Dislocation density can also be defined as follows:³⁰The number of crystallites is contingent upon structural factors such as crystallite dimensions and the extent of clustering or agglomeration. Specifically, the number of crystallites is highly influenced by the size of the crystallites within the material. Hence, the number of crystallites per unit volume (N_C) can be estimated by using the ‘D’ value,³⁰

Fig. 1 (a) Illustration of XRD patterns, (b) peak shift of (015) and (c) increment in the crystallite sizes of as-grown and annealed Bi₂Te₃ thin films.

Table 1 shows the values of crystallite size as 14.74, 14.97 and 35.51 nm for as-grown and 473 K and 523 K annealed films, respectively. From the estimated values of crystallite size, it is clearly seen that the D value increases monotonically after the annealing process as shown in Fig. 1(c). Lattice strain and dislocation density values reduced with increment in annealing temperature. The reduction in the dislocation density of the material is from 3.12 × 10⁻⁴ to 0.23 × 10⁻⁴ nm⁻². The improvement in crystallinity resulting from the annealing process contributes to a decrement in disordered states and, consequently, lowers dislocation density.³¹ Likewise, the value of N_c demonstrates a decreasing trend during the annealing process. The overall change in structural parameters within the crystal can be attributed to crystallite size refinement and the elimination of dislocations within the films. These changes indicate an enhancement in the overall crystalline quality of the films.^32,33

Table 1 Evaluated structural parameters of Bi₂Te₃ films

Films	D (nm)	(ε)	Δ (nm^−2) (×10^−4)	N c
Asp	14.74	0.0074	3.12	0.249
473 K	14.97	0.0079	2.97	0.238
523 K	35.51	0.0030	0.23	0.178

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3.2. Raman analysis

Raman spectroscopy is one of the important characterisation methods, which provides detailed information regarding the microstructural modifications of the material. In the current study, Raman peaks are found at 108 cm⁻¹, 116 cm⁻¹ and 132 cm⁻¹, as presented in Fig. S1 (ESI†). For the as-grown thin film, the Raman spectrum shows only one peak, i.e., at 118 cm⁻¹. However, upon annealing, the Raman spectrum becomes more prominent. For the 473 K annealed film, it exhibits one prominent peak around ∼116 cm⁻¹, which obviously corresponds to the Bi₂Te₃ vibrational modes.³⁴ Furthermore, for the 523 K annealed one, it shows three peaks: a less intense peak at 108 cm⁻¹ and 118 cm⁻¹ and a prominent peak at 132 cm⁻¹. As the Bi₂Te₃ thin film is rhombohedral in nature with the R3̄m space group, it consists of five atoms in its primitive cells. This results in 15 lattice vibrational modes that include 12 optical and 3 acoustic modes at the Brillouin zone.³⁴ The peak around 108 cm⁻¹ corresponds to the vibrational frequencies of the smaller crystallites present in the Bi₂Te₃ thin films.³⁵ The peaks at 118 and 132 cm⁻¹ correspond to the A_1u and A²_1g modes of Bi₂Te₃ thin films.³⁶ The peak intensity of the A_1g mode is more than those of the other two peaks, indicating symmetric out of plane stretching of Bi–Te atoms.³⁷

In the current report, we have performed the temperature dependent Raman study of the 523 K annealed Bi₂Te₃ thin film. Fig. 2(a) shows all the prominent peaks observed for the material. During Raman characterization, the previously annealed film was subjected to annealing over a temperature range varying from 298 K to 523 K. The Raman spectra were recorded at these temperatures for the higher temperature annealed film. With the enhancement of the temperature, the smaller crystallites disappeared, leading to decreased intensity of the peak. Under annealing conditions, the Raman peak shifts towards a lower wavenumber region (red shift). Fig. 2(b) presents the Raman shift of three different modes at different temperatures. It clearly indicates the change in the bond strength of the material. This is due to the thermal expansion occurring in the thin films, which affects the interatomic distances and vibrational frequencies of the lattice modes. Furthermore, the increment in the temperature during Raman measurement leads to a significant change in the phase, mostly creating a new phase due to this structural deformation.³⁸

Fig. 2 (a) Temperature dependent Raman spectra of Bi₂Te₃ thin films and (b) plot between Raman shift and temperature.

Fig. 3(a)–(c) present the change in FWHM of the corresponding 108, 118 and 132 cm⁻¹ peaks with the temperature, respectively. The increment in the FWHM value with the temperature indicated the peak broadening of the different modes. As the temperature increases, the material tends to expand, leading to a change in the lattice parameters.³⁹ Also, the elevated temperature enhances phonon–phonon interactions, resulting in more complex and broadened spectral features. These interactions also involve harmonic and anharmonic processes.³⁸

Fig. 3 Plot of shift in FWHM vs. temperature for (a) 108 cm⁻¹, (b) 118 cm⁻¹ and (c) 132 cm⁻¹ peaks observed for Bi₂Te₃ thin films.

3.3. FESEM and EDX analyses

FESEM pictures of the material illustrates the morphology, surface texture and other alterations in the material due to various energy treatments. Fig. 4(a)–(c) show the FESEM pictures of as-grown and annealed Bi₂Te₃ films. Fig. 4(a) shows the FESEM image of the as-deposited sample. It shows the uniformity in the structure of the material. However, after the annealing process, there is a slight change in the material's surface. Due to the heating process some of the materials form aggregates and this tends to increase the particle size. Again, the increase in the surface area actually enhanced the available area of the material for use as the absorber layer. Meanwhile, the enhanced particle size also results in the reduction of optical bandgap, thus leading to better absorbance. However, the thermal annealing process also influences the surface roughness and porosity of the films. The EDX spectra present the elemental composition of the sample, as shown in Fig. S2 (ESI†). The data were collected from different portions of the films, which show the presence of Bi and Te elements. The peak found around 2.43 keV corresponds to Bi–M_α. The energy spectra showed peaks around 0.65 keV and 3.75 keV, indicating the presence of Te–L_α. For experimental purposes, carbon tape is used for the conduction process, and hence the peak around 0.27 keV indicates the presence of C–L_α. Fig. S3 (ESI†) illustrates the elemental mapping of the 523 K annealed film. It also confirms the distribution of all the constituent elements in the sample. Fig. S3(a)–(c) (ESI†) show the elemental mapping of Bi and Te in the studied films.

Fig. 4 FESEM pictures of (a) as-grown and (b) 473 K and (c) 523 K annealed Bi₂Te₃ films.

3.4. TEM analysis

TEM is considered as one of the powerful tools to analyse the exact crystal morphology as well as structural parameters. Fig. 5(a) and (b) illustrate the TEM morphology of the 523 K annealed film. Fig. 5(c) shows the particle size distribution of the material, which is ∼36.33 nm. This is also very close to the crystallite size calculated from the XRD pattern. Fig. 5(d) and (e) present the HRTEM images of the material. Fig. 5(d) shows the presence of the Bi₂Te₃ (015) plane with the d spacing of 3.29 Å in the sample, whereas Fig. 5(e) confirms the presence of the (101) plane with d = 3.63 Å. Fig. 5(f) shows the SAED pattern, confirming the presence of the rhombohedral Bi₂Te₃ (015) plane in the material, which is in good agreement with the XRD study.

Fig. 5 (a and b) TEM pictures, (c) particle size distribution plot, (d and e) HRTEM images and (f) SAED pattern of the 523 K annealed Bi₂Te₃ film.

3.5. Optical study

3.5.1. PL analysis. The PL measurements of the as-grown and annealed Bi₂Te₃ films were performed at 550–850 nm with laser excitation at 532 nm. PL involves the recombination of the electron–hole pairs and leads to the emission of photons. Fig. 6(a) shows the PL spectra of both the films. There is a shift in the PL peak position towards a lower wavelength with annealing, as clearly seen in Fig. 6(a). The peaks found around 712, 704 and 694 nm correspond to as-grown and 473 K and 523 K annealed films, respectively. This blueshift observed in this case may be because of increase in crystallinity and reduction in the defect states. The reduction in the defect-related energy level can shift the PL peaks towards higher energies. Similarly, annealing improves the carrier mobility, which changes the dynamics of electron–hole recombination contributing to a blue shift in the PL spectra.^40,41 For various practical application purposes, it is necessary to understand the chromaticity of the material. Fig. 6(b) presents the 1931 Commission Internationale de l’Éclairge (CIE) diagram of the spectra shown in Fig. 6(a). The coordinates of (0.628, 0.370), (0.619, 0.379) and (0.614, 0.385) are observed for as-grown and 473 K and 523 K annealed films, respectively, from the CIE diagram.⁴²

Fig. 6 (a) PL spectra and (b) CIE chromaticity coordinates with the CCT scale of as-grown and 473 K and 523 K annealed Bi₂Te₃ films.

In the current case, the emission occurred in greater energy ranges than optical bandgaps. This behaviour occurs mostly due to the following reasons, such as excitonic effects, defects and impurities and other Auger recombinations. Such emission in our case may be due to the smaller crystallites present in the material.^40,42

3.5.2. Transmittance (T), absorption coefficient (α) and optical bandgap (E_g). Optical properties are some of the important parameters determining the applicability of the films. Fig. 7(a) shows the change in the transmittance of the as-grown and annealed Bi₂Te₃ films. There is reduction in transmittance upon thermal annealing over a higher wavelength regime. For this particular case, the transmittance is observed in the infra-red regime, and hence the annealed films can be considered to have possible applications in infra-red related devices. As the annealing temperature increases, the crystallinity shows a pronounced increase, which leads to increased light scattering and a reduced transmittance value.

Fig. 7 (a) Change in transmittance, (b) change in ‘α’ and (c) (αhν)²vs. (hν) plot for bandgap calculation of as-grown and annealed Bi₂Te₃ films.

The absorption coefficient (α) of a thin film quantifies its capability to absorb light at a particular wavelength, playing a pivotal role in comprehending the material's optical characteristics. Fig. 7(b) illustrates the absorption coefficient vs. wavelength plot. The ‘α’, which is estimated from the transmittance and thickness of the film, is given by:⁴³

Here, the film thickness is denoted as ‘d’, which is ∼800 nm. The change in ‘α’ with ‘λ’ is shown in Fig. 7(b). The α values are of 10⁴–10⁵ cm⁻¹ order over the 1600–1800 nm IR range. As the annealing temperature increases, there is an observable shift in the absorption edge towards longer wavelengths. This red shift is primarily attributed to increment in the material's absorption properties. Furthermore, the improvement in crystallinity also contributes to alterations in the optical behaviour.⁴⁴

Chalcogenide materials exhibit distinct absorption characteristics, with absorption edges categorized into three main regions: low absorption region (α < 10² cm⁻¹); medium absorption (α ∼ 10²–10⁴ cm⁻¹); and high absorption regimes (α > 10⁴ cm⁻¹). The α ≥ 10⁴ cm⁻¹ regime is known as the high absorbance region, where conduction and valence band transitions occur. It is also known as the Tauc region, which generally helps to estimate the bandgap energy, E_g, given by:⁴³

αhν = B(hν − E_g)^m (3)

Here the symbols ν and h have their usual meaning of frequency and Planck's constant. The exponent ‘m’ determines the nature of transition occurring in the sample and it is called the transition factor. It also signifies the electron-density profile over valence and conduction bands. It has different values like 1/2 and 3/2 for direct allowed and direct forbidden transitions. The m = 2 and 3 refer to the indirect allowed and forbidden transition cases. The so-called Tauc parameter is denoted as B whose numerical value represents the degree of disorder in the bandgap regime. The studied material showed high crystalline nature confirmed by the XRD pattern, and so here m = 1/2 has been considered for the bandgap estimation. The change in (αhν)²vs. hν is shown in Fig. 7(c), where the linear fitting of the data points allowed the estimation of the optical bandgap. The slope determines the Tauc parameter ‘B’ and the intercept on the X-axis measures the ‘E_g’ value. The estimated E_g and B² values for both the films are presented in Table 2. The direct bandgap of the material exhibits a decreasing trend, measuring 0.528 eV for the as-prepared sample, 0.510 eV for the 473 K annealed thin film, and 0.506 eV for the 523 K annealed film. The Tauc parameter values are 3.23 × 10¹⁰ cm⁻² eV², 3.19 × 10¹⁰ cm⁻² eV² and 3.10 × 10¹⁰ cm⁻² eV² for as-prepared and 473 K and 523 K annealed thin films, respectively. The Mott and Davis “density of state model” states that the annealing process enhances the crystallization of the material.⁴⁵ This results in an increment in the number of dangling bonds around the surface of the crystallites, leading to a higher concentration of localized states. The widening of these localized states contributes to a reduction in the bandgap. The alterations in the bandgap are notably impacted by surface morphology, specifically influenced by varying annealing temperatures.⁴⁶

Table 2 Estimated optical parameters of as-grown and annealed Bi₂Te₃ films

Optical parameters	As-prepared film	473 K annealed film	523 K annealed film
Optical bandgap (Eg), eV	0.528	0.510	0.506
B ^2 × 10^10 cm^−2 eV^2	3.23	3.19	3.10
E u (eV)	0.394	0.440	0.456
Steepness parameter (σ) × 10^−3	65.22	58.40	56.35
(Ee–p) × 10^−3	10.22	11.41	11.83
NEAR	0.978	0.980	0.982
Static refractive index (n0)	4.05	4.09	4.10
High frequency dielectric constant ε∞	16.46	16.78	16.86
n 2 × 10^−9 esu	3.62	3.90	3.96
χ ^(1) in esu	1.23	1.25	1.26
χ ^(3) × 10^−10 esu	3.90	4.24	4.32
Optical electronegativity (ηopt)	1.583	1.580	1.579

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In examining the low absorption regime of the material, the Urbach rule is predominantly employed to investigate its nature. This is because, in this region, the absorption coefficient follows an exponential change with photon energy (hν). The Urbach energies (E_u) of the as-grown and annealed thin films are obtained using the relation:⁴⁷

where α₀ is a constant factor. The E_u value provides valuable insights into the level of disorder within the material. It serves as an indicator of band tail width associated with localized states within the gap region. Eqn (4) shows that the E_u can be computed by taking the inverse of the slope. The transition between one band's extended states and the other bands' exponential tails primarily occurs in this lower absorption region. Table 2 tabulates the expected values of E_u. The E_u value shows an increasing trend from 0.394 to 0.456 eV. The electron–phonon interaction (E_e–p) and steepness of absorption edge (σ) are two essential features that only provide information about the structural behavior of the material at the absorption edge. The two parameters are determined as follows:⁴⁸

σ = k_BT/E_u and E_e–p = 2/3σ (5)

where k_B is the Boltzmann constant and T is the absolute temperature. The σ values of as-grown and annealed films are 65.22 × 10⁻³, 58.40 × 10⁻³ and 56.35 × 10⁻³, respectively. The calculated values are presented in Table 2. By increase annealing temperature, σ values reduce with increased crystallinity of films. The estimated values of E_e–p are 10.22 × 10⁻³, 11.41 × 10⁻³ and 11.83 × 10⁻³ for as-grown and 473 K and 523 K annealed films, respectively (Table 2). This also explains why the ionicity and anion valence of thin film samples changed.

Urbach tails are caused by defects in the chalcogenide material and are close to the band-edge. The near edge absorptivity ratio (NEAR) in the Tauc region determines the Urbach tail's magnitude. The NEAR is defined as:⁴⁹

The NEAR values vary from 0.978 to 0.982 (Table 2), which indicates the change in the defect states in the film structure.

3.5.3. Skin depth (δ), extinction coefficient (k) and optical density (OD). The skin depth (δ) is the path distance at which the incident radiation intensity is reduced to the 1/e value during its movement inside the sample. It is calculated as 1/α, i.e. the reciprocal of α.⁵⁰ The change in ‘δ’ as a function of energy is presented in Fig. 8(a), which shows the reduction of δ with hν. Different multimode optical fiber applications are made possible for such decreased δ values at a specific wavelength. The extinction coefficient (k) characterizes the energy loss experienced as electromagnetic waves propagate within a material, also recognized as the loss factor. The value of ‘k’ is determined according to ref. 51, . The relationship between ‘k’ and hν reveals an escalating trend in the k value, which is shown in Fig. 8(b). The change in ‘k’ also tends to affect the nonlinearity of the material. Similarly, the material's optical density (OD) is related to the absorption ability of the material during its exposure to electromagnetic radiation. It is estimated as the product of absorption coefficient and film thickness,⁵²i.e., OD = α × t. Fig. 8(c) presents the OD variation with λ, which changes like α. Following annealing, the OD values exhibit an increased trend, reaching saturation in the higher wavelength region.

Fig. 8 Variation of (a) skin depth, (b) extinction coefficient and (c) optical density for as-grown and annealed Bi₂Te₃ films.

3.5.4. Linear static refractive index (n₀) and high-frequency dielectric constant (ε_∞). The refractive index (n) at hν → 0, named the static refractive index (n_o), can be estimated for all synthesized samples from the Dimitrov and Sakka empirical formula,⁵³The estimated values of n₀ for all samples by considering bandgap values are given in Table 2. The values of n₀ are 4.05, 4.09 and 4.10 for as-grown and 473 K and 523 K annealed films, respectively. According to Moss's rule, the refractive index is inversely proportional to the optical bandgap value, i.e., E_gn⁴ ∼ constant.⁵⁴ The high-frequency dielectric constant, ε_∞ = n_o², shows the values of 16.46, 16.78 and 16.86 for as-grown and 473 K and 523 K annealed films, respectively, which are tabulated in Table 2.

The electronegativity (η_opt) of the as-grown and annealed bilayer films has been estimated as,⁵⁵ with ‘C’ as a constant (C = 25.54). It defines the attraction ability of the positive radicals to the electrons to form ionic bonds. The electronegativity values are 1.583, 1.580 and 1.579 for as-grown and 473 K and 523 K annealed films, respectively (Table 2).

3.5.5. Refractive index determined from different models. The suitability and applicability of any material solely depend on its two fundamental parameters: bandgap and refractive index (n). The transparency of the sample is tuned by its refractive index, whereas the bandgap measures the threshold of electromagnetic absorption for any semiconductor material. There exists a relationship between these two parameters as proposed by various theoretical models. The inverse relationship between ‘n’ and ‘E_g’ makes the material more suitable for various electronic applications. Numerous ways were used to establish a comprehensive correlation between ‘n’ and ‘E_g’. Based on the energy level concept of semiconductors, Moss⁵⁴ put forward the following relation: E_gn⁴ = k, which can also be written as: , with the ‘k’ value being 95 eV. The refractive loss is estimated using this relation in order to improve the solar cells' conversion parameter.⁵⁵ The modified relation was proposed by Ravindra,⁵⁶ according to which the difference between the average and optical energy gaps is constant. It can be expressed as: n_R = 4.084 − [0.62 × E_g]. Moss confirmed the relationship and stated that it holds valid for bandgaps smaller than 4 eV. This will display erroneous values for both extremely low and high bandgap values. However, for low energy gap materials, Herve–Vandamme⁵⁷ put forward the model based on oscillator theory. The relation is expressed as: , with A being the ionization energy of hydrogen = 13.6 eV and B = 3.74 eV. The rewritten form of the equation is: . Another relationship between n and E_g was also suggested by Tripathy.⁵⁸ It is based on the exponential decay relation between ‘n’ and ‘E_g’ as: n_T = 1.73 × [1 + 1.9017 × e^{−0.539×E_g}].

The graphical relationship of refractive index calculated using the theoretical model for the as-grown and annealed films is shown in Fig. S4(a) (ESI†). The refractive index ranges from 2.06 to 2.80.

3.5.6. Nonlinear optical parameters.
3.5.6.1. First-order nonlinear susceptibility (χ⁽¹⁾), third-order nonlinear susceptibility (χ⁽³⁾) and non-linear refractive index (n₂). Chalcogen-based films are particularly promising for a range of nonlinear optical applications due to their distinctive characteristics, including higher nonlinearity and a significant interaction domain. Unlike linear behaviour, the polarization of these films responds to an applied electric field in a nonlinear manner, further enhancing their suitability for diverse applications in nonlinear optics. The exposure of the sample to intense electromagnetic radiation/light results in the nonlinear phenomena in the sample. The linear polarization is lost in the case of high intensity radiation interaction of the sample and in this case, polarization varies with multi-power of electric field. In a linear system, the response is directly proportional to the applied stimulus (e.g., an electric field), following a linear relationship, P = ε₀χE. However, in the case of chalcogenide materials, the relationship becomes nonlinear under certain conditions. This modifies the induced polarization (P) vector by series expansion in terms of applied electric field (E) and susceptibility (χ) as follows:⁵⁹

P = ε₀[χ⁽¹⁾·E + χ⁽²⁾·E² + χ⁽³⁾·E³ + …] (8)

with linear polarization being ‘χ⁽¹⁾’ and the remaining part contributing to the nonlinear response. However, for centrosymmetric media, the 2nd order nonlinear susceptibility becomes zero due to the absence of inversion symmetry. So, the major contribution towards nonlinear phenomena is governed by the 3rd order nonlinear susceptibility (χ⁽³⁾) in many semiconducting materials. The values of both χ⁽¹⁾ and (χ⁽³⁾) are calculated as:^60,61with ‘A’ being a constant factor of ∼1.7 × 10⁻¹⁰ and not the static refractive index. The χ⁽¹⁾ value varies from 1.23 to 1.26 esu and χ⁽³⁾ varies from 3.90 × 10⁻¹⁰ esu to 4.32 × 10⁻¹⁰ esu, for as-grown and annealed thin films (Table 2). Such tuning of χ⁽³⁾ values is useful for fiber fabrication and optical communication purposes. The change in χ⁽³⁾ with bandgap under annealing conditions is shown in Fig. S4(b) (ESI†).

The quantity Δn (change in the refractive index) originates from the nonlinear response as given by Δn = n₂|E|², where ‘n₂’ is the nonlinear refractive index and ‘E’ is the applied electric field. The n₂ values of the as-grown and annealed films were determined using Ticha–Tichy and Miller's formula as:⁶¹ and are given in Table 2. The values are 3.62 × 10⁻¹⁰ esu, 3.90 × 10⁻¹⁰ esu and 3.96 × 10⁻¹⁰ esu for as-grown and 473 K and 523 K annealed films, respectively. The increased n₂ value upon annealing is suitable for nonlinear device fabrication. The existence of significant nonlinearities presents an opportunity to develop compact and energy-efficient devices for telecommunications. The feasibility of creating small, low-power telecommunications devices is enhanced, as larger nonlinearity requires less energy and results in shorter interaction lengths.^62,63

3.6. Surface wettability study

Adhesion related to wetting is a crucial element and a widely employed technique for assessing the surface characteristics of materials, which is often accomplished through contact angle measurement. The assessment of wetness of the surfaces involves studying the solid and liquid interaction, and this process is termed contact angle measurement. The microscopic parameter defining a material's wettability is known as the contact angle (θ). For high wettability, the contact angle (θ) is small, indicating the hydrophilic nature of the material Conversely, the contact angle (θ) is high for low wettability, indicating the hydrophobic nature of the material's surface. This measurement provides valuable insights into the affinity of a material for liquids, facilitating a nuanced understanding of its surface properties.⁶⁴

Fig. 9(a)–(c) present the contact angle measurement of as-grown and annealed Bi₂Te₃ samples. The as-prepared sample shows a contact angle of around 72°, which indicates its hydrophilic nature. 473 K and 523 K annealed films show 48° and 40° contact angles, respectively. There is gradual enhancement in the hydrophilic nature of the films with annealing temperature. The surface free energies (γ) of these as-grown and annealed films were calculated using the relation:⁶⁵ with γ_w = 71.99 mN m⁻¹ (surface tension of water). The calculated values of γ_se are 0.58, 6.47 and 5.99 for as-grown and 473 K and 523 K annealed films. The thermal induced hydrophilicity of the material mostly increased due to the surface structure, change in the surface roughness and sometimes due to the oxygen vacancy sites. In the current case, the surface structure and surface roughness seemed to remain the same for annealed films, which shows the healing of atomic impairment. Fig. 9(d) shows a comparative plot between the contact angle and surface energy of the as-grown and annealed thin films.⁶⁶

Fig. 9 Contact angle measurement of (a) as-prepared and (b) 473 Kand (c) 523 K annealed films and (d) comparative plot between contact angle and surface energy of the as-grown and annealed Bi₂Te₃ samples.

3.7. Photo-response study

Group V–VI Bi₂Te₃ thin films are mostly considered as semiconducting materials.⁶⁷ The performance of the Bi₂Te₃ thin films as photodetectors is greatly affected by the photo-response properties of the materials. Firstly, the current versus voltage measurement (I–V characteristics) was performed for both as-prepared and annealed films. Fig. 10(a)–(c) illustrate the I–V plot of as-prepared and 473 K and 523 K annealed Bi₂Te₃ thin films under dark and illumination conditions, respectively. For illumination, the prepared material was illuminated under 20, 40 and 60 mW powers of 532 nm green laser. With the increment in the laser power of the material, there is a gradual increment in the current value for all the materials. With the enhancement of the annealing temperature, the annealed Bi₂Te₃ thin films exhibit good photo-conductivity compared to the as-prepared one. The resistances of the as-prepared and annealed Bi₂Te₃ samples are shown in Fig. S5 (ESI†). The figure clearly shows the reducing trend of the resistance value, which also indicates the increment in the conductivity of the material. The I–V measurement of the material clearly shows a linear plot, with the current in the mA range.

Fig. 10 I–V characteristics of as-prepared and 473 K and 523 K annealed Bi₂Te₃ thin films under OFF (dark) and ON illumination conditions.

For further investigation of the photo-response properties of the material, it is illuminated under the green laser. With the variation in the laser power from 20 to 40 to 60 mW, the active region of the sample gets illuminated. Fig. 11(a)–(c) present the clear photo-switching behaviour of the as-prepared and annealed thin films upon switching ON/OFF the laser illumination. The measurement is taken at a 60 second time interval. The current of the material gradually increased with the enhancement of the laser power. The photo-response mechanisms of both as-prepared and annealed Bi₂Te₃ thin films follow quite similar procedures to those of bismuthine and tellurene.⁶⁸ This response occurred due to (i) formation of an electron–hole pair upon photoexcitation and (ii) photoinduced charge transportation. Here, the photocurrent exhibits a positive dependence on the incident power, which is attributed to the enhancement of the number of photogenerated carriers.^69,70 Several parameters, including responsivity (R), detectivity (D) and external quantum efficiency (EQE), can be used to verify the device performances.

Fig. 11 Time dependent photocurrent under different illumination conditions of (a) as-prepared and (b) 473 K and (c) 523 K annealed Bi₂Te₃ thin films.

Responsivity is defined as the photocurrent generated by the excitation of unit incident power. It can be estimated using the relation:

where I_p is the photocurrent, P is the power density and A is the active area. Detectivity refers to the ability of the material to detect the weak optical signal. It can be evaluated according to the equation:where I_d is the dark current. For the as-prepared Bi₂Te₃ thin film sample, the R and D values are 40 A W⁻¹ and 8 × 10⁹ Jones under 60 mW power of 532 nm laser irradiation. Similarly, for 473 K and 523 K annealed films, the R values are 80 A W⁻¹ and 100 A W⁻¹, respectively, under 60 mW laser illumination. On the other hand, the D values are 9.12 × 10¹⁰ Jones and 9.88 × 10¹⁰ Jones, respectively. In comparative Table S1 (ESI†), all the related figure of merits of Bi₂Te₃ based photodetectors are given. Other important factors for investigating the photodetector are response time (t_res) and recovery time (t_rec). Here, the response time (t_res) and recovery time (t_rec) were ascribed to the time interval for the rise and decay from 10% to 90% and from 90% to 10% of the peak value of the current, respectively. Fig. 12 illustrates the response and recovery time plots of as-prepared and other annealed Bi₂Te₃ films. From these plots, it can be concluded that for the as-prepared one, t_res and t_rec are 20 s and 21 s, respectively. Similarly, from Fig. 12(b), the t_res and t_rec are 23 s and 20 s for the 473 K annealed one. Lastly, Fig. 12(c) presents the lowest t_res and t_rec of 16 s and 18 s for the 523 K annealed one, respectively.

Fig. 12 The t_res and t_rec of (a) as-prepared, (b) 473 K and (c) 523 K annealed Bi₂Te₃ thin films at 60 mW laser power.

4. Conclusion

In the current report, the annealing induced modifications in the as-grown and annealed Bi₂Te₃ films were studied thoroughly by different characterization methods. The XRD study confirmed the presence of rhombohedral phase and the increment in the crystallinity due to thermal annealing. The temperature dependent Raman study showed a red shift in the vibrational modes of the material. This clearly shows the enhanced thermal conductivity of the material, due to the increment in the phonon–phonon interactions. The FESEM images show increment in the surface porosity and agglomeration of the material. The TEM study confirmed the different crystal planes and the average particle size of ∼36 nm for the 523 K annealed sample. The PL spectra confirm a blue shift in the emission peak, and the chromaticity diagram also confirms the shift in the colour coordinates. The optical study showed the reduction in the transmittance value and good optical properties in the infra-red region with the increase in annealing temperature. The absorption edge shows a red shift leading to decrease in the optical bandgap from 0.528 to 0.506 eV. The non-linear susceptibility and refractive index were enhanced by annealing treatment, as required for nonlinear optical devices. The surface wettability of the thin film shows higher hydrophilicity with the increase in the annealing temperature. The photo-response study of the material shows increased photo-current compared to that observed under dark conditions. With the illumination of green laser, the photo-response study revealed the possibility of using the material as a photodetector device. All these key results made the material suitable for infra-red related devices and non-linear optoelectronic applications.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts of interest for this manuscript.

Acknowledgements

The author Dr R. Naik thanks the Central Instrumentation Facility of ICT-IOC for different characterizations.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ma01044a

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