Preparation, Chemical Composition, and Optical Properties of (β–Ga2O3 Composite Thin Films)/(GaSxSe1−x Lamellar Solid Solutions) Nanostructures

GaSxSe1−x solid solutions are layered semiconductors with a band gap between 2.0 and 2.6 eV. Their single crystals are formed by planar packings of S/Se-Ga-Ga-S/Se type, with weak polarization bonds between them, which allows obtaining, by splitting, plan-parallel lamellae with atomically smooth surfaces. By heat treatment in a normal or water vapor-enriched atmosphere, their plates are covered with a layer consisting of β–Ga2O3 nanowires/nanoribbons. In this work, the elemental and chemical composition, surface morphology, as well as optical, photoluminescent, and photoelectric properties of β–Ga2O3 layer formed on GaSxSe1−x (0 ≤ x ≤ 1) solid solutions (as substrate) are studied. The correlation is made between the composition (x) of the primary material, technological preparation conditions of the oxide-semiconducting layer, and the optical, photoelectric, and photoluminescent properties of β–Ga2O3 (nanosized layers)/GaSxSe1−x structures. From the analysis of the fundamental absorption edge, photoluminescence, and photoconductivity, the character of the optical transitions and the optical band gap in the range of 4.5–4.8 eV were determined, as well as the mechanisms behind blue-green photoluminescence and photoconductivity in the fundamental absorption band region. The photoluminescence bands in the blue-green region are characteristic of β–Ga2O3 nanowires/nanolamellae structures. The photoconductivity of β–Ga2O3 structures on GaSxSe1−x solid solution substrate is determined by their strong fundamental absorption. As synthesized structures hold promise for potential applications in UV receivers, UV-C sources, gas sensors, as well as photocatalytic decomposition of water and organic pollutants.


Introduction
Gallium oxide (Ga 2 O 3 ) is an ultra-wide band gap emerging semiconductor material, showing a well-marked polymorphism [1,2]. Currently, there are six confirmed Ga 2 O 3 polymorphs with different crystal structures and crystallization temperatures: α-Ga 2 O 3 with rhomboidal lattice, β-Ga 2 O 3 -monoclinic, γ-Ga 2 O 3 -cubic defective spinel-type structure, δ-Ga 2 O 3 -cubic, ε-Ga 2 O 3 -orthorhombic, and k-Ga 2 O 3 polytype, also with that ε and β phases predominate in 0 ≤ x ≤ 0.01 and 0.5 ≤ x ≤ 1 composition, respectively, while the γ phase is characteristic for single crystals with the composition 0.05 ≤ x ≤ 0. 40. In this work, the chemical and elemental composition, surface morphology, light absorption in the region of the fundamental absorption edge, photoluminescence (PL), and photoconductivity of the layer formed by the heat treatment of single crystalline GaS x Se 1−x solid solutions in a water vapor-rich atmosphere (AVH 2 O) at a temperature of 900 • C are studied.

Materials and Methods
The samples under investigation are micro-and nanocomposite structures based on β-Ga 2 O 3 on single crystal GaS x Se 1−x solid solutions (0 ≤ x ≤ 1) as substrates. Single crystals with x = 0, 0.17, 0.50, 0.80, 0.95, and 1 compositions were prepared by a modified Bridgmann-Stockbarger technique. The synthesis of compounds from chemical elements Ga (5N), Se (5N), and spectrally pure S, taken in stoichiometric amounts, and the preparation of single crystals were performed in a three-zone (zone I, zone II, and zone III) furnace. Initially, the furnace axis is tilted at~30 degrees towards the horizontal, and the foil with Ga, S, and Se is placed in sectors I and II. The temperature in sector II was increased slowly at a rate of 100 • C/h, up to 1100 • C, while in sector I, it is maintained at the melting point of Se (S). As the amount of condensate at the outer, cold end of the ampoule decreases, the temperature in sector I increases at a rate of~50 • C/h up to~1200 • C. At this temperature, for 20 g of substance in the ampoule, the synthesis took about 6 h. Throughout the synthesis, the ampoule was subjected to mechanical vibrations with a frequency of~50 Hz.
Thereafter, the melting furnace was moved vertically, its sector III set to~700 • C, and the melt ampoule proceeded through the temperature gradient between sectors II and III, with a speed of~1 mm/h. After the melt solidified, the temperature in the furnace was decreased at a rate of~100 • C/h down to ambient temperature.
From as-synthesized bulk single crystal ingots, through mechanical splitting perpendicularly to the C 6 axis, plane-parallel plates with thicknesses between 20 and 500 µm have been obtained. They were heat treated in an AVH 2 O, using a tube furnace with a MoSi 2 heating element at temperatures from 850 to 920 • C for 1-6 h.
The structure, composition, and surface morphology of the samples under study were investigated by XRD, Energy Dispersive X-ray Spectroscopy (EDXS), and Raman spectroscopy, as well as Scanning Electron Microscopy (SEM).
The XRD patterns (CuKα radiation, λ = 1.5406 Å) of the samples were recorded in the 2θ angular range of 20-90 • with an angular resolution of 0.02 • , using a PANalytical Empyrean diffractometer, in Bragg-Brentano (θ-2θ) geometry. Raman spectroscopy measurements were performed at room temperature in the backscatter configuration (180 deg), with a WITec alpha300 R spectrometer using an Ar + ion laser (wavelength λ = 480 nm and power of~2 mW) as excitation light source. The spectral frequency resolution of measurements was~2 cm −1 .
The morphology of the samples surface was examined by SEM-EDX, using a Zeiss Ultra Plus electron microscope (7 kV, 10 µA) equipped with an EDX analysis system [a Si/Li detector (Noran, Vantage System)]. Diffuse reflectance spectra of the surface of micro-and nanostructured β-Ga 2 O 3 layer on single crystal substrate (GaS, GaSe, and GaS x Se 1−x solid solutions) have been recorded with a Specord M-40 spectrophotometer with a spectral energy resolution of 0.5 meV, equipped with accessories for diffuse reflectance measurements at an angle of 90 • to the incident beam.
Photoluminescence spectra were recorded with a specialized photometric setup which included a high-power MDR-2 monochromator with diffraction gratings (600 and 1200 mm −1 ), equipped with a photomultiplier with multi-alkali[(Na 2 K)Sb + Cs] photocathode with a quartz window. A 200 W argon vapor lamp (UV-C light source) and a DRS-500 filtered mercury lamp (wavelength λ = 546 nm) were used as excitation sources of photoluminescence and photoconductivity of the samples under study. The photocurrent in the circuit was recorded with a V7-30 microvoltmeter-electrometer.

Structural Studies
The crystal structure and phase purity of GaS and GaSe single crystals and related solid solutions were investigated using X-ray diffractograms, recorded in the 2θ angular range between 20 and 90 • . Figure 1a shows the XRD diagram of GaS, in which the intense lines positioned at 2θ angles of 22 filtered mercury lamp (wavelength λ = 546 nm) were used as excitation sources of photoluminescence and photoconductivity of the samples under study. The photocurrent in the circuit was recorded with a V7-30 microvoltmeter-electrometer.

Structural Studies
The crystal structure and phase purity of GaS and GaSe single crystals and related solid solutions were investigated using X-ray diffractograms, recorded in the 2θ angular range between 20 and 90°. Figure 1a shows the XRD diagram of GaS, in which the intense lines positioned at 2θ angles of 22.985°, 34.781°, 46.840°, and 73.928° are emphasized; they correspond to (004), (006), (008), and (0012) crystal planes, respectively, parallel to the C6 axis.  According to (ICDD-JCPDS) PDF 840499 card, these XRD peaks can be identified as diffraction lines of β-GaS polytype with lattice constants a = 3.592 Å and c = 15.465 Å. In the XRD diagram of GaSe single crystals obtained by the Bridgman technique, in the 2θ range from 20 to 90 • , four peaks are clearly emphasized [ Figure 1e]. The highest intensity lines can be ascribed to (002), (004), (008), and (0012) crystal planes parallel to the C 6 axis. According to PDF card no. 00-037-0931, these peaks correspond to the hexagonal ε-GaSe phase with crystal lattice constants a = 3.749 Å and c = 15.907 Å.
The ionic radius of selenium (Se 6+ ) is 1.45 times greater than that of sulfur (S 6+ ). Consequently, selenium substitution by sulfur results in crystal lattice deformation of solid solutions. This process obviously leads to the change of the arrangement of S/Se-Ga-Ga-S/Se elementary packings with respect to each other in the unit cell, therefore also to the polytypism of respective crystal structures. As demonstrated by XRD measurements in [32], the constant C increases monotonically with the composition (x) of a single crystalline solid solution in the ranges of 0.0 ≤ x ≤ 0.3 and 0.4 ≤ x ≤ 1. Figure 1b shows the XRD pattern of GaS x Se 1−x solid solution crystals with the composition x = 0.80. According to Serizawa and coworkers [33], single crystals with this composition correspond to the γ-GaSe polytype. The diffraction lines centered at 2θ angles equal to 22 are in good correlation with the characteristic reflections of γ-GaSe single crystals (PDF card no. 811974). The small increase in the values of 2θ angles corresponds to the diffraction on atomic planes of the solid solution for the compositions x = 0.80 and x = 0 (pure GaSe). It is determined by the contraction of the crystal lattice caused by the replacement of the Se ions with S ions (the ionic radius of S 6+ is~1.45 times smaller than that of Se 6+ ). Further contraction of the crystal lattice of GaS x Se 1−x compound occurs together with increasing x (Figure 1d,e).
In Figure 1e (inset), the variation of 2θ diffraction angle on the (0012) atomic planes is schematically presented, from which the increase of 2θ values together with the composition (x) of the GaS x Se 1−x solid solution can be clearly ascertained.
The XRD patterns, in the 2θ range of 20-90 • , of the material obtained by the heat treatment (at 900 • C, for 6 h) of GaSe and GaS single crystals in AVH 2 O are shown in Figure 2a-e, respectively.
By comparing these two figures, one can clearly ascertain the good correlation between XRD peaks which, according to the PDF card 741-776, can be identified as diffraction lines corresponding to β-Ga 2 O 3 single crystals with the cell parameters: a = 12.23 Å, b = 3.04 Å, c = 5.80 Å, and β = 103.7 • .
Using Raman spectroscopy analysis, in [27], it was demonstrated that by oxidizing the GaSe single crystal, a composite of Ga 2 O 3 crystallites and amorphous Se is obtained. At the oxidation temperature of 800-950 • C, the β-Ga 2 O 3 layer on the surface of the GaS/GaSe single crystals sublimes, forming β-Ga 2 O 3 vapors on the sample surface. In [30], it is shown that β-Ga 2 O 3 nanoformations (nanowires, nanoribbons, nanotowers, etc.) are formed by vapor condensation on the surface of the sample.
The contour of X-ray diffraction lines from nanowires/nanoribbons broadens together with decreasing nanometer crystallite sizes. The average sizes of the crystallites can be approximated using the Debye-Sherrer formula [33]: where k is the Sherrer constant, equal to 0.94, λ is X-ray wavelength (1.54060 Å), θ hkl denotes the Bragg diffraction angle for the family of lattice planes with Miller indices (hkl), and β represents the angular full width at half maximum (FWHM) intensity of the (hkl) peak. The average sizes of the β-Ga 2 O 3 crystallites formed on the surface of GaSe and GaS plates have been estimated using Equation (1), considering the diffraction lines at 2θ = 57.56 and 57.85 • and were found to be equal to 19 and 20 nm, respectively. .087 Å; c = 17,020 Å; and γ = 120 • (PDF card no. 01-071-0505). When the heat treatment temperature is increased by 50 • , the surface of GaS and GaSe lamellae and GaS x Se 1−x solid solutions (0 ≤ x ≤ 1) is covered with a nanostructured layer of gallium oxide. atomic planes of the solid solution for the compositions x = 0.80 and x = 0 (pure GaSe). It is determined by the contraction of the crystal lattice caused by the replacement of the Se ions with S ions (the ionic radius of S 6+ is ~1.45 times smaller than that of Se 6+ ). Further contraction of the crystal lattice of GaSxSe1−x compound occurs together with increasing x (Figure 1d,e).
In Figure 1e (inset), the variation of 2θ diffraction angle on the (0012) atomic planes is schematically presented, from which the increase of 2θ values together with the composition (x) of the GaSxSe1−x solid solution can be clearly ascertained.
The XRD patterns, in the 2θ range of 20-90°, of the material obtained by the heat treatment (at 900 °C, for 6 h) of GaSe and GaS single crystals in AVH2O are shown in Figure 2a-e, respectively.  By comparing these two figures, one can clearly ascertain the good correlation between XRD peaks which, according to the PDF card 741-776, can be identified as diffraction lines corresponding to β-Ga2O3 single crystals with the cell parameters: a = 12.23 Å, b = 3.04 Å, c = 5.80 Å, and β = 103.7°.
Using Raman spectroscopy analysis, in [27], it was demonstrated that by oxidizing the GaSe single crystal, a composite of Ga2O3 crystallites and amorphous Se is obtained. At the oxidation temperature of 800-950 °C, the β-Ga2O3 layer on the surface of the GaS/GaSe single crystals sublimes, forming β-Ga2O3 vapors on the sample surface. In [30],  of the primary material, intense peaks with maxima positioned at 2θ angles of 15.60, 18.90, 30.00, 31.70, 35.00, 54.40, 59.80, 60.80, and 64.60° are shown, which according to PDF card no. 431012, correspond to β-Ga2O3 polytype (monoclinic lattice) with parameters a = 12.23 Å, b = 3.04 Å, c = 5.80 Å, and β = 103.7°. At the same time, this diagram shows three diffraction peaks with maxima located at (2θ angles of) 28.00, 38.30, and 62.64°, which according to PDF card no. 76-0975, can be ascribed to Ga2Se3-monoclinic lattice with parameters a = 6.660, b = 11.650, c = 6.649 Å, and β = 108.84°. The average crystallite sizes of Ga2Se3 and Ga2O3 were determined using Formula (1) from the broadening of diffraction peaks located at 28.33 and 54.40°, respectively, and were found to be equal to 10 nm and 17 nm, respectively.
The average crystallite sizes of Ga 2 Se 3 and Ga 2 O 3 were determined using Formula (1) from the broadening of diffraction peaks located at 28.33 and 54.40 • , respectively, and were found to be equal to 10 nm and 17 nm, respectively.
The XRD patterns of GaS x Se 1−x single crystals (solid solutions with the compositions x = 0.5 and 0.8), primary and after heat treatment at 900 • C for 6 h, are presented in Figure 1c,b and Figure 2c,d, respectively. The peaks of X-ray diffractograms in Figure 2c

Morphological and Compositional Analysis
The SEM images of the surface of the β-Ga 2 O 3 layer, obtained by a heat treatment in a normal atmosphere at 850/900 • C for 6 h, of single crystalline GaS, GaSe, and GaS x Se 1−x lamellae with the compositions x = 0.05, 0.17, 0.50, and 0.85, are shown in Figure 3b-g.
As can be seen in Figure 3(b1), by a heat treatment at 750 • C in AVH 2 O, a smooth layer of white material is formed on the (0001) surface of GaS lamella.
In GaS and GaSe single crystals, the valence bonds at the (0001) surface of elementary S/Se-Ga-Ga-S/Se stratified packings are practically closed. Therefore, the surface defects of elementary III-VI packings can constitute, most likely, nucleation germs of the β-Ga 2 O 3 layer. The defect density on the (0001) surface of GaSe plates is of the order of 10 10 -10 12 cm −2 [26]. On the other hand, the valence bonds on the edge of GaS/GaSe plates are open, which facilitates the formation of the β-Ga 2 O 3 layer on the edge. Under normal atmospheric conditions, the oxidation process of GaSe plates is long-lasting [26], and at temperatures over 750 • C, edge oxidation [(1000) surface] is especially favored.
The formation mechanisms of both β-Ga 2 O 3 and related nanoformations through the heat treatment of gallium chalcogenides in an oxygen-enriched atmosphere have been studied in Refs. [28,29,[34][35][36][37]. Formation reactions of gallium oxide (β-Ga 2 O 3 ) by hightemperature heat treatment of GaSe lamellae in an Ar/O 2 atmosphere can proceed with the formation of Ga 2 Se 3 compound and/or through direct transformation, as follows [35,37]: GaSe Analogously, β-Ga 2 O 3 compound is formed by the heat treatment of single crystalline GaS lamellae [29] in AVH 2 O. In addition, the process of obtaining β-Ga 2 O 3 oxide by photo-oxidation of GaSe single crystals is influenced by the amount of water vapor in the atmosphere [37,38]. In mentioned works, the photo-oxidation reaction takes place in two stages through the reactions: Figure 3c shows a typical SEM image of the nanoformations formed on the (0001) ε-GaSe surface of the plates subjected to heat treatment at 900 • C for 6 h in AVH 2 O. The nanoformations display chaotically oriented needles with micrometric-scale lengths of (1-5) µm.
As can be seen from the SEM image (Figure 3g), the β-Ga 2 O 3 layer obtained by a 6 h heat treatment of GaS x Se 1−x (x = 0.17) solid solution in AVH 2 O at 900 • C is formed by nanowires with lengths of 5-7 µm. In addition to β-Ga 2 O 3 , the composition of this material is likely to include an O 2 surplus of~6% (Table 1). By heat treatment (at 900 • C, for 6 h in AVH 2 O) of GaS 0.5 Se 0.5 and GaS 0.8 Se 0.2 solid solutions (Figure 3e,f), a homogeneous assembly of β-Ga 2 O 3 nanowires is obtained. As can be seen from this image, the surface of the primary material plate undergoes major deformations and is covered with nanowire assemblies, the length of which increases together with the duration of the heat treatment. These materials are probably composed of β-Ga 2 O 3 nanowires with a surplus of~1.2% and 0.5% atomic oxygen, respectively. Therefore, it can be observed that with the increase in the sulfur concentration x of the primary solid solution, the excess oxygen concentration in the layer of β-Ga 2 O 3 nanoformations increases.
The elemental composition of the layer penetrated by the 20 keV electron beam was determined from the EDXS spectra. The thickness of the β-Ga 2 O 3 layer (r) penetrated by the electron beam can be approximated using the Kanaya-Okayama empirical formula [39]: where A is the mass number, Z denotes the atomic number, ρ is the density (0.89 g/cm 3 ), and E 0 -the electron energy, in keV; for E 0 > 5 keV, the power coefficient is γ = 1.67. For The EDXS spectrum of β-Ga 2 O 3 layer on the GaS substrate is shown in Figure 4, from which it can be seen that the penetrated layer contains 59.33% oxygen, 40.43% gallium, and 0.23% sulfur. Consequently, the O/Ga atomic ratio in this layer is 3/2, which corresponds to the stoichiometric compound Ga 2 O 3 .   Table 1. As can be seen from this table, with a 30 min heat treatment of GaS plates at 900 °C, a deficit of oxygen can be found in the as-formed β-Ga2O3 layer, while in the β-Ga2O3 layer formed on the solid solutions with x = 0.80 and x = 0.50, a surplus of oxygen is attested. The highest oxygen deficit of 11% is found in the β-Ga2O3 layer formed on the surface of γ-GaS0.17Se0.83 polytype.
Raman spectra in the wavenumber range of 60-800 cm −1 of the material obtained by the heat treatment of GaS single crystals in AVH2O at temperatures of 750 and 900 °C are shown in Figure 5a   As the heat treatment temperature in AVH 2 O increases from 750 to 900 • C, a homogeneous layer of nanoribbons and nanowires is formed on the surface of the GaS lamellae (Figure 3(b1)). A field of β-Ga 2 O 3 nanowires with lengths of tens of micrometers is formed on the surface of GaS plate subjected to the heat treatment at 900 • C for 30 min (Figure 3(b2)). The elemental composition of the nanowires/nanolamellae structures was determined from the EDXS spectra (Figure 4a,b). If we accept that following the thermal treatment of GaS plates in AVH 2 O at temperatures of 850 and 900 • C, the compound β-Ga 2 O 3 is formed, then a surplus of gallium should result in the material obtained at 900 • C, which is emphasized in Table 1. This surplus of gallium, equal to~2%, in the β-Ga 2 O 3 layer formed on the surface of the GaS plate at a temperature of 850 • C, is increasing to~10% in the nanowire layer formed at 900 • C and may be caused by different formation times of metallic Ga and β-Ga 2 O 3 .
Concentrations (in at. %) of oxygen (C O ) and gallium (C Ga ) in the β-Ga 2 O 3 layers obtained by the heat treatment (0.5 and 6 h at 900 • C in AVH 2 O) of single crystal plates of GaS x Se 1−x solid solutions with the compositions x = 1, x = 0.80, x = 0.50, x = 0.17, and x = 0.05 are presented in Table 1. As can be seen from this table, with a 30 min heat treatment of GaS plates at 900 • C, a deficit of oxygen can be found in the as-formed β-Ga 2 O 3 layer, while in the β-Ga 2 O 3 layer formed on the solid solutions with x = 0.80 and x = 0.50, a surplus of oxygen is attested. The highest oxygen deficit of 11% is found in the β-Ga 2 O 3 layer formed on the surface of γ-GaS 0 . 17 Se 0 . 83 polytype.
Raman spectra in the wavenumber range of 60-800 cm −1 of the material obtained by the heat treatment of GaS single crystals in AVH 2 O at temperatures of 750 and 900 • C are shown in Figure 5a Table 2.
In this table, the symmetry of the vibration modes according to Ref. [5] was also included. One can conclude that by the heat treatment of GaS x Se 1−x (0 ≤ x ≤ 1) solid solution single crystals in AVH 2 O at temperatures of 850 and 900 • C with a duration from 30 min to 6 h, layers composed of β-Ga 2 O 3 nanowires and nanoribbons are formed on the lamellae surface.   AVH2O of single crystal plates of GaSxSe1−x solid solutions with x = 0.00, 0.17, 0.50, 0.80, and 1, are included in Table 2.

Optical and Photoluminescence Properties
The absorption threshold of micro-and nanostructured β-Ga 2 O 3 layers, obtained by high-temperature heat treatment in AVH 2 O of single crystalline GaS x Se 1−x solid solution lamellae, was studied from diffuse reflectance (R d ) measurements as a function of wavelength using the Kubelka-Munk function [45]: where R d = (I/I 0 ) dif (with I and I 0 denoting the intensities of diffuse reflected radiation by sample surface and BaSO 4 powder standard), α is the diffusion coefficient (usually expressed in cm −1 ), and S (in cm −1 ) represents the diffusion coefficient-a wavelength-independent parameter for particles with average sizes greater than the incident wavelengths. As clearly revealed by the SEM images of β-Ga 2 O 3 layers, their surface consists of micrometer-sized grains covered with nanoformations. It is considered that inhomogeneities with micrometric sizes behave as diffuse scattering centers of incident light, so one can admit that F(R d )~α.
In the case of parabolic energy bands, the band gap (E g ) and the absorption coefficient (α) of the semiconductor material are related by Equation (9) [46]: where A is a characteristic parameter for respective transitions, direct (n = 1/2) or indirect (n = 2), which is independent of photon energy, hν. Taking into account Equations (8) and (9), the relation between the function F(R d ) and the direct band gap energy E gd can be expressed by: with B denoting a new constant, also independent of photon energy. Figure 6 shows

wavelengths.
As clearly revealed by the SEM images of β-Ga2O3 layers, their surface consists of micrometer-sized grains covered with nanoformations. It is considered that inhomogeneities with micrometric sizes behave as diffuse scattering centers of incident light, so one can admit that F(Rd) ~ α.
In the case of parabolic energy bands, the band gap (Eg) and the absorption coefficient (α) of the semiconductor material are related by Equation (9) [46]: where A is a characteristic parameter for respective transitions, direct (n = 1/2) or indirect (n = 2), which is independent of photon energy, hν. Taking into account Equations (8) and (9), the relation between the function F(Rd) and the direct band gap energy Egd can be expressed by: with B denoting a new constant, also independent of photon energy. Figure 6 shows  The band gap of nanostructured β-Ga 2 O 3 layers obtained by calcination at temperatures of 870-900 • C in an AVH 2 O, determined from the spectral reflectance analysis, varies from 4.50 eV to 4.70 eV [47,48]. As demonstrated in Refs. [49,50], the forbidden bandwidth, E g , is influenced by the growth technology of β-Ga 2 O 3 . The band gap of β-Ga 2 O 3 layers obtained by Ar/O 2 flow transport of Ga vapors is 4.83 eV [51]. The optical band gap of β-Ga 2 O 3 layers prepared by the PLD technique on a heated support varies from 5.09 eV to 5.11 eV [52]. In the case of β-Ga 2 O 3 single crystals, E g is equal to 4.68 eV and is weakly influenced by the crystallographic direction of the UV radiation flux [53].
In Figure 7, the PL spectra of β-Ga 2 O 3 films prepared by heat treatment (at 900 • C for 6 h) in AVH 2 O of single crystalline GaS (1) and GaSe (6), as well as of GaS x Se 1−x (x = 0.17, 0.50, 0.80, and 0.95) solid solution lamellae at the temperature 80 K. Photoluminescence was excited using a 405 nm wavelength radiation.
Gallium sulfide (GaS) is known as an indirect semiconductor with a band gap of 2.57 eV. Also, the direct band gap in GaSe is 1.90 eV. In addition, the E g value in GaS x Se 1−x (0 ≤ x ≤ 1) solid solutions lies in the range between 1.90 and 2.57 eV [54]. The PL spectrum of β-GaS crystals at 80 K is composed of two bands, one in the green spectral region and the other in the yellow region. optical band gap of β-Ga2O3 layers prepared by the PLD technique on a heated support varies from 5.09 eV to 5.11 eV [52]. In the case of β-Ga2O3 single crystals, Eg is equal to 4.68 eV and is weakly influenced by the crystallographic direction of the UV radiation flux [53].
In Figure 7, the PL spectra of β-Ga2O3 films prepared by heat treatment (at 900 °C for 6 h) in AVH2O of single crystalline GaS (1) and GaSe (6), as well as of GaSxSe1−x (x = 0.17, 0.50, 0.80, and 0.95) solid solution lamellae at the temperature 80 K. Photoluminescence was excited using a 405 nm wavelength radiation. Gallium sulfide (GaS) is known as an indirect semiconductor with a band gap of 2.57 eV. Also, the direct band gap in GaSe is 1.90 eV. In addition, the Eg value in GaSxSe1−x (0 ≤ x ≤ 1) solid solutions lies in the range between 1.90 and 2.57 eV [54]. The PL spectrum of β-GaS crystals at 80 K is composed of two bands, one in the green spectral region and the other in the yellow region.
The PL bands with weakly asymmetric contour, located near the fundamental absorption edge of GaS and GaSe single crystals, peaked at 481 nm (2.578 eV) and 592 nm (2.094 eV) and are well known in the literature. In Refs. [55,56], they are interpreted as radiative emission of indirect excitons located in GaS and of direct excitons located in GaSe crystals (Figure 7, curves 1 and 6), respectively. The energy position of the particularity by 620 nm (2.00 eV) (curve 6) coincides with the energy of indirect excitons in GaSe. The band with a maximum at 542 nm predominates in the PL spectrum of undoped GaS crystals and is of an impurity nature. The PL spectra of the crystals with the compositions x = 0.95, 080, and 0.17 consist of broad, asymmetric bands with maxima positioned at 526 nm (2.36 eV), 572 nm (2.17 eV), and 614 nm (2.02 eV), respectively. These bands, together with the band with a maximum at 562 nm from the PL spectrum of the GaSxSe1−x crystals with the composition x = 0.80, are located in the optical transparency region of respective crystals and are probably of an impurity nature. In Ref. [57], the absorption and PL spectra at room temperature of 2D GaSxSe1−x solid solutions are studied, from which a good correlation was attested between the PL peak positions and the indirect band gap determined from the diffuse reflectance spectra. The PL bands with weakly asymmetric contour, located near the fundamental absorption edge of GaS and GaSe single crystals, peaked at 481 nm (2.578 eV) and 592 nm (2.094 eV) and are well known in the literature. In Refs. [55,56], they are interpreted as radiative emission of indirect excitons located in GaS and of direct excitons located in GaSe crystals (Figure 7, curves 1 and 6), respectively. The energy position of the particularity by 620 nm (2.00 eV) (curve 6) coincides with the energy of indirect excitons in GaSe. The band with a maximum at 542 nm predominates in the PL spectrum of undoped GaS crystals and is of an impurity nature. The PL spectra of the crystals with the compositions x = 0.95, 080, and 0.17 consist of broad, asymmetric bands with maxima positioned at 526 nm (2.36 eV), 572 nm (2.17 eV), and 614 nm (2.02 eV), respectively. These bands, together with the band with a maximum at 562 nm from the PL spectrum of the GaS x Se 1−x crystals with the composition x = 0.80, are located in the optical transparency region of respective crystals and are probably of an impurity nature. In Ref. [57], the absorption and PL spectra at room temperature of 2D GaS x Se 1−x solid solutions are studied, from which a good correlation was attested between the PL peak positions and the indirect band gap determined from the diffuse reflectance spectra. Figure 8 shows the PL spectra at 80 K of the β-Ga 2 O 3 layers formed by a 6 h heat treatment at 900 • C in AVH 2 O of single crystalline GaS (1) and GaSe (6)   The PL spectra of the β-Ga2O3 layer on GaSxSe1−x solid solution substrate was excited with the radiation of an emission band with a maximum at 254 nm and a half-width of 15 nm selected from the spectrum of a ~400 W Ar-arc lamp using a SiO2 prism monochromator that was intensity-modulated at 95 Hz. The PL spectra of β-Ga2O3 layers on GaS and GaSe plates, as well as on GaSxSe1−x (0 < x < 1) solid solution crystals, contain The PL spectra of the β-Ga 2 O 3 layer on GaS x Se 1−x solid solution substrate was excited with the radiation of an emission band with a maximum at 254 nm and a halfwidth of 15 nm selected from the spectrum of a~400 W Ar-arc lamp using a SiO 2 prism monochromator that was intensity-modulated at 95 Hz. The PL spectra of β-Ga 2 O 3 layers on GaS and GaSe plates, as well as on GaS x Se 1−x (0 < x < 1) solid solution crystals, contain two bands: a narrow one with the maximum at the UV-violet frontier and another one with a broad contour, displaying peak intensity in the green-red region. The UV-violet PL band is present in both β-Ga 2 O 3 thin films and nanostructured layers [54,58,59]. A PL band with a maximum of 390 nm was also observed in the cathodoluminescence spectrum of an assembly of nanowires obtained by the CVD technique [60]. In Ref. [59], it was observed that the structure of the PL spectrum of the nanostructured β-Ga 2 O 3 depends on the excitation wavelength. If for an excitation wavelength of 254 nm, the PL spectrum of the assembly of β-Ga 2 O 3 nanoribbons is composed of two poorly outlined bands with maxima positioned at 375 nm and 414 nm, then upon a 325 nm excitation, in the emission spectrum, two intense bands with maxima at 455 and 528 nm are emphasized.
In the PL spectrum (at the temperature 80 K) of the nanostructured β-Ga 2 O 3 layer formed on the surface of GaS lamella (Figure 8, curve 5), two bands with maxima positioned at 395 nm (3.14 eV) and 549 nm (2.28 eV) are clearly visible. The PL band peaked at the wavelength of 395 nm is well studied in [60][61][62]. In mentioned papers, it is assumed that this band is formed as a result of electron-hole recombination in the pair vacancy (V O -V Ga ) defect. In Refs. [63,64], it is admitted that oxygen defects (V O ) form donor levels in the energy range of 0.60-2.16 eV, while gallium vacancies (V Ga ) form two acceptor levels, located at~0.19 eV and 0.62 eV with respect to the valence band top.
The forbidden band gap of the nanostructured β-Ga 2 O 3 layer on the GaS substrate is equal to 4.82 eV ( Figure 6, curve 1). The energy diagram (deep levels) of β-Ga 2 O 3 crystals was established from Deep-Level Transient Spectroscopy (DLTS) and Deep Level Optical Spectroscopy (DLOS) analyzed in [63,64]. According to them, the emission band with a maximum at 395 nm can be interpreted as donor-acceptor (DA) radiative emission in the vacancy pair (V O , V Ga ) by recombination of electrons from the donor level with energy of 0.80 eV (relative to the conduction band minimum), with the holes on the acceptor level with energy E c = 3.94 eV (with respect to the valence band maximum). Additionally, the PL band with a maximum of 549 nm can be interpreted as the recombination of electrons from the deep level with an energy of 1.66 eV and holes from the acceptor level located at 0.88 eV above the valence band top. The PL spectrum of the β-Ga 2 O 3 layer on the GaSe substrate is composed of two bands with maxima located at 390 nm (3.18 eV) and 460 nm (2.70 eV) wavelengths. The violet emission band in the β-Ga 2 O 3 layers on GaS and GaSe substrates probably has the same nature and is determined by the presence of oxygen and gallium vacancies. An emission band with a maximum of 460 nm was observed in the PL spectrum of the nanostructured β-Ga 2 O 3 layer (nanoribbons and nanowires) obtained by a heat treatment at 950 • C of gallium in water vapor. This band is interpreted in [65] as electron-hole recombination in the donor-acceptor pair. As seen in Figure 8, curve 2, the band at 460 nm predominates and can be interpreted, based on the energy level diagram of β-Ga 2 O 3 compound [63,64], as the radiative transition of electrons from the conduction band on the energy level located at 2.71 eV below the conduction band minimum. We note that the PL spectrum of the β-Ga 2 O 3 powders studied in the Refs. [66,67] consists of a broad band with a poorly outlined maximum at 476 nm (2.60 eV). The structure of the PL spectrum of nanostructured β-Ga 2 O 3 depends on several (technological) factors (nature of the material, synthesis method, and temperature, etc.). As demonstrated in [68], the presence of a broad and poorly emphasized emission maximum at wavelengths of 510-530 nm is characteristic of the luminescence of β-Ga 2 O 3 nanoparticles.
In the PL spectra, at 80 K, of the β-Ga 2 O 3 layer formed on the substrate of GaS x Se 1−x (x = 0.17) solid solutions, the violet band with peak intensity in the wavelength range of 380-410 nm, characteristic for the PL spectrum of the nanostructured β-Ga 2 O 3 , and the green band with maximum at 540-560 nm are well marked. At the same time, in the PL spectrum of the layer formed by the heat treatment in AVH 2 O at 900 • C of GaS 0 . 17 Se 0 . 83 crystals with a high selenium concentration, an orange band is clearly emphasized. At high temperatures, the probability of S evaporation from S and Se solutions is substantially increased. In [69], it was demonstrated that Se can substitute oxygen atoms in β-Ga 2 O 3 with the formation of diluted Ga 2 (O 1−x Se x ) 3 solutions with x ≤ 0.16, whose indirect band gap is greatly influenced by Se concentration. Thus, one can admit that the structure of both violet and green-orange PL bands in the gallium oxide layer formed by high-temperature heat treatment in air at single crystal lamellae of GaS x Se 1−x solid solutions (0.05 ≤ x ≤ 0.95) is determined by the presence of selenium impurities in β-Ga 2 O 3 .
Single crystals of GaS, GaSe, and their solid solutions are formed by elementary S/Se-Ga-Ga-S/Se stratified packings with interlayer spacings of~0.3 nm [70], through which oxygen atoms may easily diffuse. Thus, at high temperatures, the formation process of β-Ga 2 O 3 takes place in the entire volume of the plate. By 6 h of heat treatment (in AVH 2 O, at 900 • C) of the 35 µm thick GaS plate, a homogeneous layer of nanostructured β-Ga 2 O 3 was obtained. This sample served as the photogenerating element of electron-hole pairs in a UV-C photodetector.
The Au and Au/Ti thin films are used as electrode material in β-Ga 2 O 3 -based photodetectors [71][72][73][74]. In Ref. [75], the current-voltage characteristics are analyzed in photodetectors based on thin films of β-Ga 2 O 3 with Au and Al electrodes, from which the linear dependence in the range of 0 ± 10 V under dark and illumination conditions is clearly seen for the photodetector with Al electrodes. The linearity of the current-voltage characteristic in photodetectors with Au electrodes after illumination is observed at voltages greater than ±2.14 V. The photoresistor studied in this work was obtained by vacuum deposition (10 −6 Torr) of two Al ribbons at a distance of~2 mm from each other. The structure thus obtained was subjected to a heat treatment at~400 • C for 2 h in vacuum. Figure 9 shows the spectral distribution of the photocurrent upon illumination with radiation provided by a filtered Xe lamp. As can be seen from the last figure, in the spectral range between 280 and 245 nm, the photocurrent through the photoresistor increases by more than two orders of magnitude, from ~10 −11 to ~10 −9 A. The maximum photocurrent is obtained for λ = 250 nm. The photoresponse of the detector at 250 nm wavelength is 0.9 nA. Figure 9 (Inset) shows the current-voltage dependencies upon excitation with a 254 nm wavelength radiation, the power density of which was 1.8, 3.0, and 4.1 µW/cm 2 . In the voltage range from 5 to 40 V, the photocurrent was found to increase linearly with the applied voltage.

Conclusions
The lamellar semiconductors in the series of GaSxSe1−x solid solutions exhibit a bandgap in the range of 1.98-2.58 eV. By using a heat treatment of respective single crystals in AVH2O at temperatures between 750 and 900 °C, homogeneous layers consisting of β-Ga2O3 nanowires/nanosheets with micrometric lengths are obtained. Depending on the heat treatment temperature, β-Ga2O3 layers with monoclinic crystal As can be seen from the last figure, in the spectral range between 280 and 245 nm, the photocurrent through the photoresistor increases by more than two orders of magnitude, from~10 −11 to~10 −9 A. The maximum photocurrent is obtained for λ = 250 nm. The photoresponse of the detector at 250 nm wavelength is 0.9 nA. Figure 9 (Inset) shows the current-voltage dependencies upon excitation with a 254 nm wavelength radiation, the power density of which was 1.8, 3.0, and 4.1 µW/cm 2 . In the voltage range from 5 to 40 V, the photocurrent was found to increase linearly with the applied voltage.

Conclusions
The lamellar semiconductors in the series of GaS x Se 1−x solid solutions exhibit a bandgap in the range of 1.98-2.58 eV. By using a heat treatment of respective single crystals in AVH 2 O at temperatures between 750 and 900 • C, homogeneous layers consisting of β-Ga 2 O 3 nanowires/nanosheets with micrometric lengths are obtained. Depending on the heat treatment temperature, β-Ga 2 O 3 layers with monoclinic crystal lattice are obtained, but also nanocomposite layers formed by β-Ga 2 O 3 and Ga 2 Se 3 /Ga 2 S 3 crystallites with submicrometric dimensions. The direct band gap of the β-Ga 2 O 3 layer depends on the composition (x) of GaS x Se 1−x solid solution and was found in the range of 4.52-4.82 eV. The β-Ga 2 O 3 and β-Ga 2 O 3 /Ga 2 Se 3 /Ga 2 S 3 nanocomposite layers are photoluminescent materials in the UV-blue region. Resistive detectors based on nanostructured β-Ga 2 O 3 layers show high sensitivity in the UV-C region (230-270) nm. The photocurrent obtained by irradiating the surface with a 254 nm beam is higher than the dark current by more than two orders of magnitude.