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Issue #3/2024
M. A. Kudryashov, L. A. Mochalov, Yu. P. Kudryashova, E. A. Slapovskaya, M. A. Vshivtsev, R. N. Kriukov
The Effect of Substrate Temperature on the Optical Properties of GaSe Thin Films Obtained by PECVD
The Effect of Substrate Temperature on the Optical Properties of GaSe Thin Films Obtained by PECVD
DOI: 10.22184/1993-7296.FRos.2024.18.3.246.254
The Effect of Substrate Temperature on the Optical Properties of GaSe Thin Films Obtained by PECVD
M. A. Kudryashov1,2, L. A. Mochalov1,2, Yu. P. Kudryashova1,2, E. A. Slapovskaya2, M. A. Vshivtsev1,2, R. N. Kriukov2
Nizhny Novgorod State Technical University n. a. Alekseev, Nizhny Novgorod, Russia
Lobachevsky University, Nizhny Novgorod, Russia
GaSe thin films were first obtained by plasma-enhanced chemical vapor deposition (PECVD), where high-purity elemental gallium and selenium were used as starting materials. The interaction between the elements was initiated by HF discharge (40.68 MHz) at a reduced pressure of 0.1 Torr. The composition, surface morphology, structural and optical properties of gallium selenide films were investigated as a function of substrate temperature. The deposited polycrystalline GaSe films were obtained on sapphire substrate at temperatures of 250 °C and 350 °C, and amorphous GaSe films were obtained at 150 °C.
Keywords: thin films, gallium selenide, PECVD
Article received: 28.03.2014
Article accepted: 15.04.2024
INTRODUCTION
Two-dimensional (2D) materials are attractive for both basic physical research and high-tech applications in electronics, optoelectronics, photonics, and flexible devices [1–5]. Gallium selenide (GaSe), a typical representative of group III metal chalcogenides, is currently of great interest among 2D materials because of its thickness-dependent optoelectronic properties. GaSe has a layered hexagonal structure consisting of two Ga and two Se sublayers in a Se-Ga-Ga-Se sequence, where the Se-Ga and Ga-Ga bonds in the layers are covalent and the Se-Se bond between neighbouring four atomic layers is due to van der Waals forces. Depending on the packing sequence, there are several polytypes of gallium selenide, which results in the formation of ε-, β-, δ-, γ-, and γ′-phases of the material [6].
Bulk GaSe is a semiconductor with a direct bandgap of about 2 eV, which increases with the transition to multiple monolayers [7]. Studies have shown that it is attractive for applications in photodetectors [8] as well as devices that emit visible light [9]. GaSe exhibits large optical nonlinearity and broad wavelength transparency, which makes it very promising in nonlinear optics [10]. It has also been used as a radiation detector operating at room temperature [11]. GaSe has been reported to be applicable as a passivation layer on Si and GaAs [12, 13].
Numerous studies have been conducted on the preparation of GaSe thin films by methods such as mechanical exfoliation from bulk material [7], liquid-phase exfoliation [14], chemical vapour deposition [15], electrochemical deposition [16], epitaxial growth [17], pulsed laser deposition [18], thermal evaporation [19] and magnetron sputtering [20]. However, no reports have been found on plasma-enhanced chemical vapour deposition (PECVD) of gallium monoselenide films.
Since deposition conditions significantly affect the properties of the resulting films, the aim of this work was to obtain GaSe thin films by PECVD at different substrate temperatures, where high-purity elemental gallium and selenium were used as percursors. It should be noted that PECVD from elemental substances is a relatively new method [21–26]. In this case, the growth rate can be varied by adjusting the precursor temperature, carrier gas flow rate and plasma discharge power, which is useful for both thin film studies and thick layer growth. Thus, the PECVD method offers the potential to grow high quality films at substrate temperatures that are relatively lower than in other methods.
EXPERIMENTAL
The schematic of the experimental setup is shown in Fig. 1. This setup was described earlier in [27–29]. High-purity (6N) elemental gallium and selenium were loaded into special containers made of high-purity quartz equipped with external resistive heating elements and thermocouples for temperature control. The gallium source is positioned in close proximity to the plasma zone. Chalcogen vapours are transported through heated (300 °C) quartz lines to the reaction zone by a stream of high-purity argon (99.9999 vol. %), which is also used as a plasma-forming gas. The temperature of the gallium source was 850 °C, the temperature of the selenium source was 175 °C.
The plasma discharge power in the experiments was 70 W. Sapphire (0001) plates measuring 10 × 10 mm were used as substrates. The temperature of the substrates varied in the range between 150 and 350 °C. The overall pressure in the system was kept constant at 0.1 Torr during the experiments. The average growth rate calculated from the measured film thicknesses (50 nm) using a Taylor Hobson microinterferometer was about 100 nm/hour. Deposition was performed during 30 min.
Microphotographs of the samples were obtained by scanning electron microscopy (SEM) on an AURIGA CrossBeam microscope (Carl Zeiss Group) with a resolution of 0.8 nm at optimum working distance (operating voltage 15 kV) with an electron probe diameter of about 2 nm and a probe current below 0.3 nA. The macrocomposition of the obtained films was studied using an X–MaxN 20 energy dispersive X-ray spectroscopy detector (Oxford Instruments) attached to a JSM IT‑300LV scanning electron microscope (JEOL). Transmission spectra were recorded on a UV‑1800 dual beam spectrophotometer (Shimadzu, Japan) in the wavelength range of 190–1 100 nm with a step of 1 nm.
RESULTS AND DISCUSSION
The composition of the obtained gallium selenide films depended on the substrate temperature (see Table). At the minimum temperature, an excess of gallium was observed, which was apparently related to the incomplete reaction of the interacting precursors. Increasing the temperature to 250 °C improved the degree of conversion, which favoured the formation of the gallium monoselenide film. The highest selenium content was observed at the highest substrate temperature.
Fig. 2 shows the effect of substrate temperature on the structure of the obtained gallium selenide films. At the minimum substrate temperature (150 °C), only an X-ray amorphous film could be obtained. Increasing the substrate temperature up to 250 °C led to the formation of polycrystalline δ-phase GaSe [96-210-6699 COD] with the main reflex (008). The half-width of the reflex (008) decreased with further increase in substrate temperature up to 350 °C, indicating an increase in crystallite size. The transition from amorphous to crystalline film with increasing substrate temperature was also observed in [20].
The lattice parameters of the hexagonal unit cell were determined as a = 3.77 Å and c = 32.12 Å. A comparison of these values with the crystallographic data of 96-210-6699 COD (a = 3.76 Å and c = 31.99 Å) shows an increase in the parameters. Thus the films experience a slight mechanical stretching in the longitudinal and transverse direction. The lattice constants of the sapphire substrate are a = 4.76 Å and c = 12.99 Å. A lattice mismatch of 21% can be expected to contribute to the degree of transverse strain in the layers.
The substrate temperature also affects the surface morphology of the obtained gallium selenide films (Fig. 3). The minimum temperature (150 °C) results in a rather developed surface with different fragments merging into each other, thus testifying to the amorphous nature of the film. Raising the temperature of the sapphire substrate to 250 °C significantly changes the surface morphology. In this case, a homogeneous film with a smooth surface with ~20 nm grains is formed. Subsequent temperature increase to 350 °C causes the crystalline grains to grow to ~60 nm.
The atomic force microscopy images of the film surface confirm the changes in morphology depending on the substrate temperature (Fig. 4). At low temperature (150 °C), the highest surface roughness of 22.5 nm is observed. It then decreases to 2.0 nm at 250 °C and then increases again to 7.24 nm at the highest substrate temperature due to the growth of grains.
Fig. 5 shows the transmission spectra of GaSe films plotted versus the substrate temperature, which clearly has an effect on the transparency edge of the samples. The width of the band gap was found from the relationship αhν = A(hν – Eg)n, where n = 1 / 2 in the case of the amorphous film obtained at the substrate temperature of 150 °C (Fig. 6a), and n = 2 in the case of crystalline gallium selenide films (Fig. 6b).
For the amorphous GaSe film, the band gap was found to be approximately 1.95 eV, which is in good agreement with literature data [30]. For crystalline gallium selenide films deposited at 250 and 350 °C, Eg was found to be 2.10 and 2.34 eV, respectively. Note that the optical band gap width of 2.34 eV is the maximum value experimentally reported in the literature for GaSe films. At the same time, this value agrees with theoretical calculations [31]. As it was already noted above, the deposition method and conditions significantly affect the properties of gallium selenide thin films, including their optical properties. For example, for GaSe films obtained by mechanical cleavage (exfoliation) from bulk gallium selenide onto a polyethylene terephthalate substrate, the direct and indirect band gap energies were 2.2 and 1.92 eV [32], respectively. Amorphous films deposited by magnetron sputtering on fused quartz [20] and by thermal evaporation on glass [19] have Eg = 1.8 eV. Furthermore, two indirect and one direct band gap optical transitions with energies of 1.1, 1.44, and 1.92 eV, respectively, were observed for GaSe films thermally deposited on glass [33]. It should be added that due to quantum confinement effects, the band gap of gallium selenide increases with decreasing material thickness [9,31]. Thus, our synthesis method makes it possible to obtain GaSe films with a band gap width close to the theoretical limit, when dimensional effects are still absent.
CONCLUSIONS
Thin GaSe films on sapphire were obtained from high-purity elements under conditions of low-temperature non-equilibrium plasma HF discharge. The effect of substrate temperature on the composition and properties of GaSe films was investigated. Gradual increase in temperature leads to a decrease in gallium content from 53.3±1 to 49.7±1 at. % due to the enhanced degree of conversion of the interacting precursors. Along with this, the structure of gallium selenide films changes from X-ray amorphous to polycrystalline. However, the surface morphology of the films behaves non-monotonically. At the lowest substrate temperature (150 °C), the films have a rather developed surface, where merged fragments of different sizes and shapes can be observed. At higher substrate temperatures, a fairly homogeneous film is formed, and the temperature increase from 250 to 350 °C contributes to an increase in grain size and surface roughness. All films are quite transparent (60–80%) in the range of 500–1 100 nm, and their band gap width increases from 1.95 to 2.34 eV.
FUNDING
The research was supported by the Russian Science Foundation, grant No. 22-19-20081, https://rscf.ru/en/project/22-19-20081/.
CONTRIBUTION OF THE AUTHORS
M. A. Kudryashov: conducting an experiment, processing the results, discussions
L. A. Mochalov: idea, suggestions and comments, discussions
Y. P. Kudryashova: conducting an experiment
E. A. Slapovskaya: experiment design
M. A. Vshivtsev: processing the results
R. N. Kriukov: conducting an experiment, processing the results
CONFLICT OF INTEREST
We inform you that we have no known conflicts of interest related to this publication. We confirm that the manuscript has been read and approved by all the named authors and that there are no other persons who meet the criteria of authorship, but are not listed. We also confirm that the order of the authors listed in the manuscript has been approved by all of us.
M. A. Kudryashov1,2, L. A. Mochalov1,2, Yu. P. Kudryashova1,2, E. A. Slapovskaya2, M. A. Vshivtsev1,2, R. N. Kriukov2
Nizhny Novgorod State Technical University n. a. Alekseev, Nizhny Novgorod, Russia
Lobachevsky University, Nizhny Novgorod, Russia
GaSe thin films were first obtained by plasma-enhanced chemical vapor deposition (PECVD), where high-purity elemental gallium and selenium were used as starting materials. The interaction between the elements was initiated by HF discharge (40.68 MHz) at a reduced pressure of 0.1 Torr. The composition, surface morphology, structural and optical properties of gallium selenide films were investigated as a function of substrate temperature. The deposited polycrystalline GaSe films were obtained on sapphire substrate at temperatures of 250 °C and 350 °C, and amorphous GaSe films were obtained at 150 °C.
Keywords: thin films, gallium selenide, PECVD
Article received: 28.03.2014
Article accepted: 15.04.2024
INTRODUCTION
Two-dimensional (2D) materials are attractive for both basic physical research and high-tech applications in electronics, optoelectronics, photonics, and flexible devices [1–5]. Gallium selenide (GaSe), a typical representative of group III metal chalcogenides, is currently of great interest among 2D materials because of its thickness-dependent optoelectronic properties. GaSe has a layered hexagonal structure consisting of two Ga and two Se sublayers in a Se-Ga-Ga-Se sequence, where the Se-Ga and Ga-Ga bonds in the layers are covalent and the Se-Se bond between neighbouring four atomic layers is due to van der Waals forces. Depending on the packing sequence, there are several polytypes of gallium selenide, which results in the formation of ε-, β-, δ-, γ-, and γ′-phases of the material [6].
Bulk GaSe is a semiconductor with a direct bandgap of about 2 eV, which increases with the transition to multiple monolayers [7]. Studies have shown that it is attractive for applications in photodetectors [8] as well as devices that emit visible light [9]. GaSe exhibits large optical nonlinearity and broad wavelength transparency, which makes it very promising in nonlinear optics [10]. It has also been used as a radiation detector operating at room temperature [11]. GaSe has been reported to be applicable as a passivation layer on Si and GaAs [12, 13].
Numerous studies have been conducted on the preparation of GaSe thin films by methods such as mechanical exfoliation from bulk material [7], liquid-phase exfoliation [14], chemical vapour deposition [15], electrochemical deposition [16], epitaxial growth [17], pulsed laser deposition [18], thermal evaporation [19] and magnetron sputtering [20]. However, no reports have been found on plasma-enhanced chemical vapour deposition (PECVD) of gallium monoselenide films.
Since deposition conditions significantly affect the properties of the resulting films, the aim of this work was to obtain GaSe thin films by PECVD at different substrate temperatures, where high-purity elemental gallium and selenium were used as percursors. It should be noted that PECVD from elemental substances is a relatively new method [21–26]. In this case, the growth rate can be varied by adjusting the precursor temperature, carrier gas flow rate and plasma discharge power, which is useful for both thin film studies and thick layer growth. Thus, the PECVD method offers the potential to grow high quality films at substrate temperatures that are relatively lower than in other methods.
EXPERIMENTAL
The schematic of the experimental setup is shown in Fig. 1. This setup was described earlier in [27–29]. High-purity (6N) elemental gallium and selenium were loaded into special containers made of high-purity quartz equipped with external resistive heating elements and thermocouples for temperature control. The gallium source is positioned in close proximity to the plasma zone. Chalcogen vapours are transported through heated (300 °C) quartz lines to the reaction zone by a stream of high-purity argon (99.9999 vol. %), which is also used as a plasma-forming gas. The temperature of the gallium source was 850 °C, the temperature of the selenium source was 175 °C.
The plasma discharge power in the experiments was 70 W. Sapphire (0001) plates measuring 10 × 10 mm were used as substrates. The temperature of the substrates varied in the range between 150 and 350 °C. The overall pressure in the system was kept constant at 0.1 Torr during the experiments. The average growth rate calculated from the measured film thicknesses (50 nm) using a Taylor Hobson microinterferometer was about 100 nm/hour. Deposition was performed during 30 min.
Microphotographs of the samples were obtained by scanning electron microscopy (SEM) on an AURIGA CrossBeam microscope (Carl Zeiss Group) with a resolution of 0.8 nm at optimum working distance (operating voltage 15 kV) with an electron probe diameter of about 2 nm and a probe current below 0.3 nA. The macrocomposition of the obtained films was studied using an X–MaxN 20 energy dispersive X-ray spectroscopy detector (Oxford Instruments) attached to a JSM IT‑300LV scanning electron microscope (JEOL). Transmission spectra were recorded on a UV‑1800 dual beam spectrophotometer (Shimadzu, Japan) in the wavelength range of 190–1 100 nm with a step of 1 nm.
RESULTS AND DISCUSSION
The composition of the obtained gallium selenide films depended on the substrate temperature (see Table). At the minimum temperature, an excess of gallium was observed, which was apparently related to the incomplete reaction of the interacting precursors. Increasing the temperature to 250 °C improved the degree of conversion, which favoured the formation of the gallium monoselenide film. The highest selenium content was observed at the highest substrate temperature.
Fig. 2 shows the effect of substrate temperature on the structure of the obtained gallium selenide films. At the minimum substrate temperature (150 °C), only an X-ray amorphous film could be obtained. Increasing the substrate temperature up to 250 °C led to the formation of polycrystalline δ-phase GaSe [96-210-6699 COD] with the main reflex (008). The half-width of the reflex (008) decreased with further increase in substrate temperature up to 350 °C, indicating an increase in crystallite size. The transition from amorphous to crystalline film with increasing substrate temperature was also observed in [20].
The lattice parameters of the hexagonal unit cell were determined as a = 3.77 Å and c = 32.12 Å. A comparison of these values with the crystallographic data of 96-210-6699 COD (a = 3.76 Å and c = 31.99 Å) shows an increase in the parameters. Thus the films experience a slight mechanical stretching in the longitudinal and transverse direction. The lattice constants of the sapphire substrate are a = 4.76 Å and c = 12.99 Å. A lattice mismatch of 21% can be expected to contribute to the degree of transverse strain in the layers.
The substrate temperature also affects the surface morphology of the obtained gallium selenide films (Fig. 3). The minimum temperature (150 °C) results in a rather developed surface with different fragments merging into each other, thus testifying to the amorphous nature of the film. Raising the temperature of the sapphire substrate to 250 °C significantly changes the surface morphology. In this case, a homogeneous film with a smooth surface with ~20 nm grains is formed. Subsequent temperature increase to 350 °C causes the crystalline grains to grow to ~60 nm.
The atomic force microscopy images of the film surface confirm the changes in morphology depending on the substrate temperature (Fig. 4). At low temperature (150 °C), the highest surface roughness of 22.5 nm is observed. It then decreases to 2.0 nm at 250 °C and then increases again to 7.24 nm at the highest substrate temperature due to the growth of grains.
Fig. 5 shows the transmission spectra of GaSe films plotted versus the substrate temperature, which clearly has an effect on the transparency edge of the samples. The width of the band gap was found from the relationship αhν = A(hν – Eg)n, where n = 1 / 2 in the case of the amorphous film obtained at the substrate temperature of 150 °C (Fig. 6a), and n = 2 in the case of crystalline gallium selenide films (Fig. 6b).
For the amorphous GaSe film, the band gap was found to be approximately 1.95 eV, which is in good agreement with literature data [30]. For crystalline gallium selenide films deposited at 250 and 350 °C, Eg was found to be 2.10 and 2.34 eV, respectively. Note that the optical band gap width of 2.34 eV is the maximum value experimentally reported in the literature for GaSe films. At the same time, this value agrees with theoretical calculations [31]. As it was already noted above, the deposition method and conditions significantly affect the properties of gallium selenide thin films, including their optical properties. For example, for GaSe films obtained by mechanical cleavage (exfoliation) from bulk gallium selenide onto a polyethylene terephthalate substrate, the direct and indirect band gap energies were 2.2 and 1.92 eV [32], respectively. Amorphous films deposited by magnetron sputtering on fused quartz [20] and by thermal evaporation on glass [19] have Eg = 1.8 eV. Furthermore, two indirect and one direct band gap optical transitions with energies of 1.1, 1.44, and 1.92 eV, respectively, were observed for GaSe films thermally deposited on glass [33]. It should be added that due to quantum confinement effects, the band gap of gallium selenide increases with decreasing material thickness [9,31]. Thus, our synthesis method makes it possible to obtain GaSe films with a band gap width close to the theoretical limit, when dimensional effects are still absent.
CONCLUSIONS
Thin GaSe films on sapphire were obtained from high-purity elements under conditions of low-temperature non-equilibrium plasma HF discharge. The effect of substrate temperature on the composition and properties of GaSe films was investigated. Gradual increase in temperature leads to a decrease in gallium content from 53.3±1 to 49.7±1 at. % due to the enhanced degree of conversion of the interacting precursors. Along with this, the structure of gallium selenide films changes from X-ray amorphous to polycrystalline. However, the surface morphology of the films behaves non-monotonically. At the lowest substrate temperature (150 °C), the films have a rather developed surface, where merged fragments of different sizes and shapes can be observed. At higher substrate temperatures, a fairly homogeneous film is formed, and the temperature increase from 250 to 350 °C contributes to an increase in grain size and surface roughness. All films are quite transparent (60–80%) in the range of 500–1 100 nm, and their band gap width increases from 1.95 to 2.34 eV.
FUNDING
The research was supported by the Russian Science Foundation, grant No. 22-19-20081, https://rscf.ru/en/project/22-19-20081/.
CONTRIBUTION OF THE AUTHORS
M. A. Kudryashov: conducting an experiment, processing the results, discussions
L. A. Mochalov: idea, suggestions and comments, discussions
Y. P. Kudryashova: conducting an experiment
E. A. Slapovskaya: experiment design
M. A. Vshivtsev: processing the results
R. N. Kriukov: conducting an experiment, processing the results
CONFLICT OF INTEREST
We inform you that we have no known conflicts of interest related to this publication. We confirm that the manuscript has been read and approved by all the named authors and that there are no other persons who meet the criteria of authorship, but are not listed. We also confirm that the order of the authors listed in the manuscript has been approved by all of us.
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