Effects of sputtering pressure and annealing temperature on the characteristics of indium selenide thin films

Indium selenide is a significant two-dimensional lamellar semiconductor with excellent physical properties whose enormous potential utilization in optoelectronic devices has been practically hindered by the lack of suitable thin film deposition techniques. Herein, γ-In2Se3 thin films were successfully fabricated from an InSe-target via magnetron sputtering combined with subsequent annealing process. The effects of sputtering pressure and annealing temperature on the characteristics of as-deposited thin films were investigated. The x-ray diffraction (XRD) patterns reveal that the pristine thin films are amorphous in nature, whereas transform into polycrystalline and are identified as γ-In2Se3 phase after annealing treatment. The growth mechanism of as-deposited layers combines a two-dimensional lateral growth and a three-dimensional island growth. The scanning electron microscopy (SEM) and atomic force microscopy (AFM) images indicate that all the samples show uniform and compact structures with no evident holes and crevices. The UV–vis-NIR spectrophotometer was employed to measure the optical transmittance and band gap of the synthesized thin films. The results show an obvious decrease in the band gap from 2.56 eV to 1.88 eV with annealing temperature increased from 400 °C to 600 °C, respectively. In addition, the difficult reasons for preparing monophase InSe thin films by magnetron sputtering method were discussed. These intriguing findings in this study may shed light on the growth of indium selenide thin films with well-crystallized and high quality.


Introduction
Indium selenide has attracted great attention as a typical two-dimensional (2D) layered semiconductor material belonging to a complex family of compounds with distinct stoichiometries, wherein comprises InSe, In 2 Se 3 , In 4 Se 3 , In 6 Se 7 , etc. [1].Among these indium selenide compounds, In 2 Se 3 has been extensively utilized in photovoltaic solar cells [2], high-performance photodetectors [3], ionic batteries [4], water splitting applications [5], and phase-change memory [6], owing to its tunable bandgap, high carrier mobility as well as good ambient stability.In 2 Se 3 as a lamellar structure crystal has two different bonding ways, in-plane atomic covalent bonding and interlayer weak van der Waals interaction, and may consist of at least five distinguishable phases, including α [7], β [8], γ [9], δ, and κ [10], depending upon the alignment of atomic configurations.Nevertheless, it should be noted that the practical application for optoelectronic devices based on In 2 Se 3 material has been limited, mostly due to the lack of cost-effective and large-area thin film synthesis techniques.Many efforts and attempts have been made to achieve the growth of In 2 Se 3 thin films, such as thermal evaporation [11], chemical bath deposition (CBD) [12], electrodeposition [13], metal-organic chemical vapor deposition (MOCVD) [14], solidstate reaction (SSR) [15], molecular beam epitaxy (MBE) [16] and pulsed laser deposition (PLD) [17], etc.However, all these methods mentioned above have inevitable demerits in terms of the preparation of indium selenide thin films, for example, poor substrate adhesion, difficulties in overcoming the coexistence of different phases, or being too expensive to mass manufacture.In contrast, magnetron sputtering as a widely used thin film deposition method possesses two apparent advantages, namely, it allows low-temperature deposition and high deposition rate.Additionally, it enables precise control in chemical composition and dopant species.Based on these merits, the magnetron sputtering technique is regarded as a potential method to obtain In 2 Se 3 thin films.
The sputtering parameters and annealing conditions have an important impact on the structure and physical properties of In 2 Se 3 thin films prepared by sputtering technology.For instance, Li et al [18] have successfully synthesized the monophase γ-In 2 Se 3 thin films with a strong c-axis preferential orientation by adjusting the RF power and substrate temperature during film deposition.Recently, they demonstrated a substrate-directed method with an aim to achieve the phase control of In 2 Se 3 thin films [19].Yan et al [20] mainly studied the influence of growth pressure on the characteristics of In 2 Se 3 thin films and concluded that the γ-In 2 Se 3 could be observed only if the working pressure was less than 4.0 Pa.An attempt has been made by Ashish Waghmare et al [16] to understand the underlying correlations between deposition time and the characteristics of In 2 Se 3 thin films.The effect of annealing temperature on structural and optoelectronic properties of γ-In 2 Se 3 thin films was investigated by Yogesh Hase et al [21] and proved that γ-In 2 Se 3 could be a promising material for photodetection applications.
Currently, some work on the growth and performance studies of In 2 Se 3 thin films deposited from an In 2 Se 3 -target has been reported, while the utilization of InSe-target in terms of preparing indium selenide thin film is rarely investigated.In addition, there is a lack of adequate understanding with regard to the challenges in the growth of monophase InSe thin films with well-crystallized through magnetron sputtering method.
With this motivation, an InSe-compound target was employed in this study to grow indium selenide thin films by adjusting the sputtering pressure and annealing temperature.At the same time, based on our experimental results and previous literature reports, the reasons for the difficulties in obtaining monophase InSe thin films via magnetron sputtering have been discussed from three aspects.The relevant findings in this research not only present a novel thought concerning the fabrication of In 2 Se 3 thin films, but also provide insight into the difficulty reasons for fabricating InSe thin films.

Experimental
2.1.Thin film preparation Indium selenide thin films were deposited onto soda lime glass (SLG) substrates with an area of 1 cm 2 by radiofrequency magnetron sputtering.The sputtering target used in the follow-up experiments is a 2-inch diameter target with the In: Se element ratio close to 1:1, exhibiting a hexagonal crystal structure corresponding to the ε-InSe phase, as identified by XRD analysis (see figure S1 in the supplementary material).The pre-treatment of the substrates was carried out with acetone, isopropanol, ethanol and de-ionized water successively in an ultrasonic instrument for 10 min, then dried in nitrogen gas and introduced into the sputtering chamber.A fixed distance of target-to-substrate was kept at 10 cm for all the thin film deposition.SLG substrates were horizontally put on a plane steel susceptor and rotated throughout the sputtering process to obtain uniform thin film.Furthermore, the sputtering chamber was baked for 1 h before each film deposition and evacuated to a high-vacuum pressure of 2.0 × 10 −4 Pa to remove the possible formation of metal oxides.With the aim to explore the effect of sputtering pressure, an automated butterfly valve was employed to adjust it to vary from 1 Pa to 3 Pa; the sputtered films were subject to post-annealing treatment at a temperature of 400 °C for 1 h in argon atmosphere.Some of the samples were kept in their as-grown form for comparison.Then as-deposited indium selenide thin films were annealed at different temperatures with the working pressure maintaining at 1.5 Pa, in order to investigate the role of annealing temperatures on the growth of thin films.The details of the sputtering parameters used to grow indium selenide thin films are listed in table 1.

Thin film characterization
The crystal structure of as-deposited indium selenide thin films was characterized by an x-ray diffractometer (D/max 2550 V) with Cu-Ka (λ = 1.54056Å) radiation.Surface morphology was observed by field emission scanning electron microscopy (FESEM, SU8220, Japan) with an operating voltage of 10 kV.The film thickness was measured by taking cross-section figures.The studies of three-dimensional (3D) topography and growth mechanism of thin films were carried out by virtue of atomic force microscopy (AFM, NTEGRA).UV-vis-NIR (Lambda 950) spectrophotometer was employed to obtain optical transmission and absorption spectra.

Effect of growth pressure
Growth pressure, as one of the most basic sputtering parameters, plays an important role in the nucleation and growth of the indium selenide thin films, which in turn affects its quality and performance.In the initial study, indium selenide thin films were grown at different sputtering pressures, while keeping other growth parameters constant.The crystal structure of thin films, as well as phase and preferential orientation, were analyzed by XRD diffractometer.From the results, it can be seen that all the samples without annealing treatment are amorphous regardless of the sputtering pressure [see figure S2 in the supplementary material].In addition, for the samples annealed at 400 °C for 1 h, it can be obviously observed from figure 1 that the indium selenide thin films grown at relatively low sputtering pressure show good crystallinity, whereas those thin films prepared at high growth pressure exhibit amorphous characteristics.
As can be observed that all the diffraction peaks for the indium selenide thin films grown at 1 Pa are accountable to hexagonal γ-In 2 Se 3 as verified by comparison with the standard data card (PDF# 71-0520).Specifically, the prominent crystalline peaks are observed at 2θ = 25.1°and27.6°, corresponding to (110) and (006) crystal planes, respectively.As the sputtering pressure is increased to 1.5 Pa, there exhibits an obvious increase in the intensity for most diffraction peaks except for the (006) plane and the preferred growth direction has changed with well-oriented along (110) plane.Additionally, a further increase in the sputtering pressure leads to a significant decrease in the crystalline quality.No obvious intense peaks appear in the thin films grown at 2 Pa and 3 Pa.Generally speaking, overhigh growth pressure will shorten the mean free path of the sputtered species and increase its collision probability with other particles, thus reducing the number and energy of the reactant atoms arrived at the substrate; as a result, the reduction in diffusion ability and mobility of the adatoms makes it unable to relax and nucleate at the certain position of thermodynamic equilibrium, thereby resulting in a decrease in the crystallinity.
SEM was employed to investigate the surface morphology of the indium selenide thin films deposited at different pressures, as shown in figure 2. At a sputtering pressure of 1 Pa, the as-deposited thin film is covered with small closely packed grains.For the sample grown at 1.5 Pa, those smaller grains begin to coalesce and aggregate with each other to form irregularly shaped cluster particles, resulting in an increase in the average roughness of the film surface.As growth pressure rises to 2 Pa, it can be seen from its corresponding SEM image that the degree of agglomeration between granules as well as the surface roughness of the film further increases.
As can be seen from the sample grown at 3 Pa, there exists a tendency to form a continuous thin film by means of secondary nucleation and growth of subsequent sputtered particles.Figure 3 presents a relationship between the 3D topography of the indium selenide thin films and growth pressure with the help of atomic force microscopy.As can be seen, all the deposition thin films are composed of tightly packed island-liked granules without obvious holes and crevices.When the thin films were grown at a sputtering pressure of 1 Pa, small island-shaped granules with comparable size randomly spread over the thin film surface.For the sample grown at 1.5 Pa, it can be observed from its 3D AFM topography that the aggregation and coalescence of these small island grains take place, forming a certain amount of large island  granules.As the growth pressure was increased to 2 Pa, more relatively large island-like granules appeared on the surface.A further rise in the sputtering pressure to 3 Pa results in an increase in the average roughness of the thin film surface and a decrease in the density.Film growth can be simply divided into four stages, including the absorption of sputtered particles, nucleation, the appearance of maze network structure and a continuous film formed, as demonstrated in figure 4.
For the indium selenide thin films deposited by magnetron sputtering, its growth mechanism combines a 2D lateral growth and a 3D island growth, as both revealed by the SEM and AFM characterizations.Overall, the growth mode of the films mainly depends on the wettability between the adatoms and the substrate.When the film reaches a critical layer thickness, the adopted process parameters start to play a dominant role.

Effect of annealing temperatures (Ta)
In addition to growth pressure, the structural and optical properties of the indium selenide thin films are critically dependent on annealing temperatures.Given this, a further investigation was conducted by changing the annealing temperatures from 400 °C to 600 °C, at the optimized growth pressure of 1.5 Pa.The crystal structure and crystalline quality of the indium selenide thin films annealed at different temperatures were determined by XRD spectra, as shown in figure 5.
It can be seen from the XRD patterns that the as-grown indium selenide thin films are amorphous in nature, whereas transform into polycrystalline after the post-annealing process.Specifically, the identification of the diffraction peaks conforms the existence of one major phase in the annealed thin films, which is identified as hexagonal γ-In 2 Se 3 (PDF# 71-0250).No detectable impurity phases are observed in the samples annealed at 400 °C and 500 °C, implying that the films grown under such process conditions are of high purity.As the annealing temperature is elevated to 600 °C, the crystalline quality gets deteriorated with a dramatic decrease in the intensity of the major diffraction peaks of γ-In 2 Se 3 , meanwhile, two low-intensity peaks located at 2θ = 21.4°and 35.4°likely attributed to the SiO 2 phase appear.This result may be associated with the softening of the substrate and the decomposition of as-deposited In 2 Se 3 occurred at overhigh annealing temperature.
The crystallize size (D) of γ-In 2 Se 3 thin films has been calculated from the full-width at half-maximum (FWHM) of the (110) plane located at around 25°via Debye-Scherrer's equation: where l is the wavelength of the x-ray source, b is the FWHM of diffraction peak measured in radians, and q represents the Bragg's angle.The average crystallize size values for the annealed thin films are listed in table 2.
Figure 6 shows the energy-dispersive x-ray spectroscopy (EDS) spectra of the pristine thin film and the sample annealed at 400 °C.It can be seen that the element composition of the sputtered thin films solely contains In and Se regardless of whether annealing treatment is performed, indicating the high purity of the resulting films.In addition, the atomic % ratio of Se: In is approximately 3:2, as revealed by EDS data analysis, confirming that the indium selenide thin film deposited at such process conditions is stoichiometric.
The surface morphology presented in figure 7 shows a strong dependence on the annealing temperatures.The as-grown indium selenide thin film [figure 7(a)] is covered with tiny and uniform grains with well-defined boundaries, showing a flat and compact surface structure.However, these small grains tend to congregate and combine when the samples are annealed at 400 °C [figure 7(b)], resulting in an obvious increase in the surface roughness.As an increase in the annealing temperature to 500 °C [figure 7(c)], distinct lamellar horizontal grains show up with some randomly distributed black spots observed simultaneously.The black spots may be    , some large grain clusters with sizes ranging from 100 nm to 300 nm appear on the thin film surface.Furthermore, it can be found from the EDS analysis (see figure S4 in the supplementary material) that the concentration of indium in these isolated clusters is higher than that of the rest of the film.The micro-droplets of metal indium formed and condensed on the film surface may be responsible for this result.In fact, this phenomenon is not uncommon in group III element thin film materials, especially those that have undergone high-temperature annealing [22].
It is well known that the thickness of thin films has a strong influence on their fundamental properties.In this study, the thickness of as-grown and annealed indium selenide thin films was determined by taking their corresponding cross-sectional SEM images, as presented in figure 8 and listed in table 2. For all samples, its thickness was measured to be around 360 nm (denoted by the yellow line), implying that the measured thickness is almost insensitive to the annealing temperature.
An UV-vis-NIR spectrophotometer was employed to evaluate the influence of annealing temperature on the optical properties of the indium selenide thin films.The transmittance spectra for all the samples were measured in the wavelength range 300-1000 nm at ambient temperature, as displayed in figure 9. Except for the  sample annealed at 600 °C, the other samples exhibit considerable transmittance, suggesting that peeling off In 2 Se 3 thin film from substrates and the decomposition of as-deposited layers induced by annealing at high temperature will do harm to its optical properties.
Indium selenide, as a direct band gap semiconductor material, its energy band gap can be deduced according to the following expression: where a is the absorption coefficient, d is the thickness of thin films, T % is the transmittance; A is a constant relying on transition probability, hv is the photon energy, E g is the band gap, n is the index and often referred to as the transition coefficient (1/2 for the direct allowed transition).In order to calculate the band gap, the curves related to the absorption coefficient were replotted in the form of hv 2 a ( ) versus hv, then extrapolated the linear portion yielded by these typical plots on the hv axis at hv 2 a ( ) = 0.As can be seen from the plots shown in figure 10, the band gap varies in the range from 2.56 eV to 1.88 eV with the increases in annealing temperature.In general, when the sample is annealed within an appropriate temperature range, the density of structural defects inside the film as well as the localized states introduced in the forbidden band will decrease, thereby causing an increase in the optical band gap of the film [23].However, contradictory results may be observed in samples annealed at high temperatures, which may be explained by the decomposition of the film.As for the reduction in band gap with increasing the annealing temperature, it might be induced by the deterioration of the crystalline quality, which can be revealed by the variation of the intensity of the (110) diffraction peak or its FWHM value.The calculated band gaps for the sputtered In 2 Se 3 thin films are in good agreement with the reported value [20], as tabulated in table 2. At the same time, a similar change in E g with annealing temperature has been observed in the previous literature [24].

Difficulties and challenges
The reasons for the difficulty in the preparation of monophase InSe thin films from an InSe-target through the magnetron sputtering method were briefly discussed.It can be speculated that three factors are responsible for this dilemma.(i) The target composition may deviate from the initial stoichiometric ratio after a period of sputtering, likely due to the precipitation of low melting point indium from the interior of the target (see figure S5 in the supplementary material).(ii) The higher thermal stability of In 2 Se 3 also presents an obstacle to achieving the fabrication of monophase InSe films [1].(iii) The elemental composition of the resulting indium selenide thin films deviated from the InSe-target used, which may be the consequence of the preferential sputtering.According to the sputtering theory proposed by Sigmund [25], the sputtering yield of different types of target atoms can be calculated using the following equation: where Y is the sputtering yield, a is a dimensionless factor that is positively correlated with the atomic mass of target atoms [26], S E n ( ) is the nuclear stopping power per atom and U 0 is the surface binding energy per atom (the heat of sublimation per atom is often equated to U 0 ).Some physical properties related to indium and selenium are summarized in table 3.
Taking the effect of surface binding energy per atom and its atomic mass on sputtering yield into consideration, it can be concluded that Se possesses a much higher sputtering yield compared to In.According to the theoretical calculation results, the as-deposited indium selenide thin film will be Se-rich and In-poor even sputtered from the 1:1 InSe target, which is consistent with the EDS characterization data.Based on the experimental results and previous literature [27], it seems that the priority formation of the In 2 Se 3 phase takes place when the as-deposited film is In-deficient.Consequently, we developed a co-deposition scheme using both an InSe and In target to ensure that the indium is stable and sufficient during the growth of indium selenide thin films.The current research progress has confirmed that single-phase InSe thin films can be prepared by this codeposition scheme via magnetron sputtering.

Conclusion
In summary, polycrystalline γ-In 2 Se 3 thin films were successfully grown from an InSe-target via magnetron sputtering deposition technique followed by post-annealing treatment.Relevant correlations between process parameters and the characteristics of indium selenide thin films have been established.In particular, a relatively low growth pressure and an appropriate annealing temperature may contribute to the well-crystallized γ-In 2 Se 3 thin films, while high growth pressure and overhigh annealing temperature will lead to a dramatic decrease in the crystallinity and optical transmittance.It was evident that the growth mechanism of the indium selenide thin films deposited combines a 2D lateral growth and a 3D island growth.The reactant particles on the thin film surface tend to aggregate and combine with increasing the growth pressure and annealing temperature.A red shift in optical absorption edge can be observed and the band gap decreases from 2.56 eV to 1.88 eV when the annealing temperature is increased from 400 °C to 600 °C, respectively.In addition, this research suggests that it is still challenging to obtain InSe thin films via magnetron sputtering, which may be attributed to the preferential sputtering of target atoms, and the higher thermal stability of In 2 Se 3 also presents a huge barrier to achieving the fabrication of InSe films.

Figure 4 .
Figure 4. Schematic diagram of indium selenide thin films deposited by magnetron sputtering.

Figure 5 .
Figure 5. XRD spectra of as-grown and annealed indium selenide thin films deposited by magnetron sputtering.

Figure 9 .
Figure 9. Optical transmittance for samples annealed at different temperatures from 400 °C to 600 °C.

Figure 10 .
Figure 10.Optical band gap for In 2 Se 3 thin films deposited at different annealing temperatures.

Table 1 .
Process parameters employed for the deposition of indium selenide thin films.
Figure 1.XRD patterns of indium selenide layers deposited at different sputtering pressures.

Table 2 .
The calculated parameters for γ-In 2 Se 3 films at various annealing temperatures.

Table 3 .
Some physical parameters of indium and selenium.