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Abstract
The main application area for GaSe crystals is nonlinear optics, but there are problems with machining and antireflection coatings due to the layered structure of GaSe. Large GaSe crystals with dominant ε-modification were grown. Using various harmonics of a femtosecond laser, antireflection microstructures (ARM) were fabricated by laser ablation on the plate surface and modes providing increased transmission up to 90% in the mid-IR were selected. Using SEM and optical microscopy, as well as EDX and optical spectroscopy, defects on the GaSe surface and their effect on the spectral region and the degree of transmission increase were studied.
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1. Introduction
AIIIBVI type semiconducting compounds have become very attractive due to their structural, optical and electrical properties. One of the members of this family is gallium selenide GaSe. This crystal was characterized in detail both experimentally and theoretically during last three decades. GaSe demonstrates great possibilities in the areas of optoelectronic and semiconducting devices [1,2,3], as well as of laser [4,5] and nonlinear optical technology for mid-IR [6,7] and terahertz [8] applications.
The layer semiconductor compound GaSe contains four monoatomic sheets in the sequence Se-Ga-Ga-Se. The intralayer chemical bondings of the atoms are predominantly covalent, while the interlayer forces are relatively weak: these layers are held together via van der Waals forces. In the literature, four principal forms of GaSe structure are described: these are β, ε, γ, and δ modifications [9,10]. The centrosymmetric β-GaSe consists of two layers per unit cell and has the space group D46h; noncentrosymmetric ε-modification is the main component obtained from the melt: it consists of two layers and crystallizes with the space group D13h. The γ -type with space group C53v contains one layer and often exists as stacking fault in the melt grown ε -type crystals. The δ–type GaSe contains four layers per unit cell, with space group C46v. The ε and β phases are 2H hexagonal polytypes, while the modification γ has 3R trigonal structure.
In this paper, we focus on the applications of GaSe in nonlinear optics. GaSe crystals of ε-modification demonstrate an optimal combination of parameters for efficient conversion: a wide range of transparency (0.62–20 µm) and phase synchronism of the conversion, high damage threshold (0.03 GW/cm2 for 10.6 µm 125 ns pulses), and high nonlinear conversion coefficient (54 pm/V for 10.6 µm) [11]. However, because of high refractive index (average n ∼2.63 in the 2–16 µm range) the reflection losses are significant (∼35%) for GaSe. For quantum electronics, bulk GaSe ε-modification crystals of high optical quality are required: with a minimum content of defects, both extended and point; with high transparency in the transmission range. Proven methods of applying antireflective coatings are also needed. To characterize the crystals grown by the Bridgman method, we used optical spectroscopy methods (absorption, luminescence and Raman spectroscopies), as well as optical and electron scanning (SEM) microscopy.
The main problem of GaSe is the layered nature and strong cleavage, i.e. the tendency to chip along the plane perpendicular to (001), as well as weak (Van der Waals) bonds and, as a consequence, the plasticity of the material with Mohs hardness ≈ 0 [7]. Thus, optical elements can be manufactured only by chipping crystals perpendicular to the optical axis (001) and it is impossible to manufacture polished plates of the desired geometry in accordance with the available phase synchronisms. The conventional method to increase the surface transmittance is the use of single-layer or multi-layer antireflection coatings (ARC). However, ARCs cannot be reliably applied to GaSe surfaces because of the adhesion problems. Owing to the highly uneven surface, applied coatings tend to delaminate and fail to provide antireflection properties. An alternative and comparatively novel approach is to fabricate antireflection microstructures (ARMs) on the crystal surface [12]. According to the effective medium theory [13], for wavelengths λ larger than λ=n×p (where n is the refractive index and p is the period of the microstructure), ARM acts as a layer with a refractive index gradient, which leads to reduced reflection [14].
Using optical spectroscopy, as well as XRD and SEM, the grown GaSe crystals were evaluated. Antireflection microstructures were fabricated on GaSe surface via different laser ablation modes, and their influence on GaSe optical properties was studied.
2. Experimental
2.1 Crystal growth
The GaSe single crystals have been grown by the Bridgman technique. The charge for growing GaSe single crystals was obtained by pyrosynthesis from elementary, high purity gallium (6N) and selenium (4N). The components of the compound taken in stoichiometric ratio were placed in a silica ampoule, pumped out up to 10−2 mm Hg and hermetically sealed. The ampoule prepared in this way was placed in a two-zone horizontal furnace for synthesis. The temperature was 1100°C and 500°C for the “hot” and “cold” zones, respectively. The synthesis time varied from 1 to 6 hours, depending on the size of the sample. At the end of the synthesis, the furnace was cooled in the power-off mode. Silica ampoules for growing crystals were carbon coated from the inside to prevent the crystals from adhering to the walls of the ampoule and their destruction during the post-growth temperature drop. As the crystal grows, the ampoule moves from the “hot” zone to the “cold” one at a rate of 10 mm/day. The temperature gradient in the crystallization zone ranged from 2 to 4°C/cm. A boule of GaSe is shown in Fig. 1(a). The size of the boule is about 70 mm long and its diameter is about 25 mm in the wide part.
Fig. 1. (a) A GaSe boule and (b) the plate 1 mm thick. (c,d) Fragments of the image of GaSe plates with ARM structures obtained by using a HUNI HTC-383 CCD camera sensitive to 1.2 µm in the near IR. The area with ARM looks like a dark square with dimensions of about 2×2 mm2. Features in the morphology or shape of the plates are visible: These are the ragged layers formed when chipping (c), and the gradual deviation of the shape from the plane (d).
2.2 X-ray diffraction (XRD)
X-ray phase analysis of the GaSe sample was carried out according to the procedure described in [15]. The essence of the technique consists in obtaining and summing several debaegrams with different orientations of the sample relative to the primary beam. A particle with dimensions less than 0.1 mm was split off from the initial single crystal with dimensions of ∼10х10х1 mm3. The experiment was carried out on a Bruker D8 Venture diffractometer (Incoatec IμS 3.0 microfocus tube, CuKa radiation, three-circle goniometer, PHOTON III CPAD detector, resolution 768×1024, pixel size 135×135 µm2) at T = 298 K. The introduction of amendments to the external standard (Si - SRM-640) and the transition to the standard form was carried out according to the Dioptas program [16]. Diffractograms from the face of GaSe single crystal in the Bragg-Brentano scheme were obtained using a Bruker D8 Advance diffractometer, CuKa radiation and linear position-sensitive detector LYNXEYE XE-T.
2.3 ARM performing
The typical size of GaSe plates for applying ARM structures was 10×10 mm2 with a thickness of about 1 mm. Local micro-cavities as elements of the ARM structure on the surface of the GaSe plate were made using laser ablation. This technique is based on local material removal due to the high energy density of a femtosecond pulse. The energy from the pulse is transferred to the super-heated electron plasma induced in the GaSe volume and then to the atomic lattice of the material. This process takes place over an extremely short period of time (on the sub-picosecond scale) [17]. Laser beam intensity exceeds the ablation threshold of the material and local material removal takes place. In this work, two different installations were used for the manufacture of ARM structures, localized 1. at Bauman Moscow State Technical University (BMSTU) in Moscow and 2. at the Institute of Automation and Electrometry SB RAS (IA&E SB RAS) in Novosibirsk. In both cases, a Pharos Yb:KGW femtosecond pulse laser from Light Conversion, Lithuania was used as a radiation source for ARM fabrication. The parameters of the modes used are summarized in Table 1. This laser system allows generating radiation in the base harmonic (1026 nm) and the second harmonic (513 nm), at a repetition frequency from 1 to 200 kHz and pulse duration from 200 to 1000 fs. A 100× objective lens (Mitutoyo Corporation, Japan) and a 100× Mitutoyo Plan Apo NIR HR objective with NA = 0.7 were used. Laser beam was focused slightly underneath the sample surface in order to increase the cavity depth in ARM. The focal spot size at different wavelengths was about 0.9 µm. In BMSTU only single pulse was used to form a single microcavity and the repetition rate was 200 kHz. In Novosibirsk the pulse picker of laser was controlled to generate a pulse train of several (up to four) consequential pulses was used for producing a single cavity. This required a reduction in the repetition rate, which was only 10 kHz. Mode parameters are given in Table 1. The movement velocity of stages was adjusted to produce ARMs with a period of 3.4 µm both in BMSTU and IA&E. The areas of ARM structures were up to 2×2 mm2 and 0.5×0.5 mm2, respectively.
Table 1. Mode parameters when applying ARM to chipped GaSe plates
2.4 Optical spectroscopy
Transmission spectra were recorded on a UV-2501PC Shimadzu spectrometer in the ranges from visible to near IR and on an Infralum FT-801 Fourier transform spectrometer in the mid-IR. In the case of ARM structures, transmission spectra in the mid-IR were measured using a Bruker Vertex 70 FTIR spectrometer combined with a microscope Hyperion 2000, with beam diameter of about 100 µm and spectral resolution of 4 cm-1. Raman and photoluminescence (PL) spectra at 325 nm and 532 nm excitations were recorded using a LabRAM HR800 confocal microRaman spectrometer combined with a LINKAM THMS 300 heating/cooling stage and a CT94 temperature controller.
2.5 Optical and SEM microscopy
Images of grown GaSe boules and plates made from them, including those with ARM structures, were obtained using MBS-10 and Olympus BX53M optical microscopes. The sample surface morphology was examined with a Pioneer Ultra High Resolution Scanning electron microscope manufactured by Raith. Surface chemical composition was determined with energy-dispersive X-ray spectroscopy (EDX spectra) using a SEM manufactured by HITACHI (SU 8220).
3. Results
3.1 GaSe crystals
The size of the as-grown GaSe boule is about 70 mm long and the diameter is about 25 mm in its wide part. The X-ray diffractogram obtained for GaSe (see Fig. S1a) corresponds to the diffractogram presented in the PDF card No. 01-080-093. All reflexes are indicated within the spatial group P-6m2 (No. 187), i.e. ε-modification of GaSe. This conclusion is also confirmed by Raman and luminescent spectroscopy. Figure S1b shows a diffractogram obtained from the face of a single crystal in the Bragg-Brentano scheme. The insert shows the reflection profile (0010): the width of the reflexes is about 0.04°. All crystals investigated were chipped along the cleavage to obtain platelets of about 1 cm2 area and of thickness of about 1 mm. A typical GaSe boule and a (001) plate knocked out of it are shown in Fig. 1. When recording transmission spectra, the beam is directed along [001]: this is an ordinary beam with E┴[001].
The main defects on the surface of GaSe plates are steps, sometimes of complex shape, formed during chipping, and representing ragged GaSe layers. They are clearly visible in optical microscopy (Fig. 1(c),(d)) and SEM (Fig. 2(a),(b)). The presence of ragged layers on the surface of the GaSe plate is very important in the manufacture of ARM: when passing through such a step, the focus of the laser beam and the geometry of the microcavity produced by laser ablation change abruptly. As a result, the antireflection effectiveness is also changing. This is clearly seen by the heterogeneity of the color within the ARM in Fig.1c. In contrast to the rather abrupt changes caused by ragged layers, in some cases there is a fairly smooth change in color within the ARM (Fig. 1(d)). It can be assumed that in this case there is a smooth deviation of the plate shape from the plane due to the plasticity of the GaSe crystal. In addition, teardrop-shaped formations with a size of 180-300 nm are also observed in the SEM (Fig. 2(a),(b)). These formations can be confined to the boundaries of ragged layers (Fig.2a) and it can be assumed that it is the rows of such inclusions that are the cause of the layers breakage. Using the Hitachi SU 8020 SEM, the composition of GaSe was determined in the main volume and in inclusions observed in SEM experiments. These results are presented in Supplement 1 both in the form of EDX-spectra (Fig. S2) and in tabular form (Table S2). Four elements are fixed in these spectra: the main ones are gallium and selenium, as well as additional ones - carbon and oxygen. It can be seen that the maximum value of the Ga/Se ratio occurs for the original GaSe plates, in the points far from the inclusions: it is 1.08. In the inclusions the Ga/Se ratio is about 0.06. This means that the inclusions observed by SEM in GaSe consist almost entirely of Se.
Fig. 2. (a,b) SEM images show defects on the surface of the freshly split GaSe plate. Main defects on the GaSe surface are boundaries of the layers (a) and the formations of a teardrop shape (a,b). (c,d) SEM image of the ARM structures performed using 513 nm (c) and 1026 nm (d) laser pulses, respectively.
3.2 Antireflection microstructures on the GaSe surface
The morphology of the GaSe surface in the ARM produced using laser radiation of 513 and 1026 nm is shown in Fig. 2(c) and 2(d), respectively. It can be seen that the cavities turn out to be deeper in the case of short-wave radiation 513 nm, although at 1026 nm the pulse energy at is higher compared to the 2nd harmonic and 4 pulses were used to obtain single micro-cavity. This is due to the band-to-band electronic transitions for 513 nm light, whereas the radiation at the fundamental frequency of 1026 nm is in the region of the GaSe transparency. When using radiation of 513 nm, the cavities turn out to be deeper, there is practically no undisturbed space between them (Fig.2c), unlike radiation of 1026 nm (Fig. 2). Figures S3a and S3b show images obtained using an optical Olympus microscope for ARM structures performed by 513 nm and 1026 nm pulses, respectively.
The composition was determined for GaSe plates with ARMs both directly in the microcavity formed during laser ablation and at points between adjacent cavities. Obtained results are given in Tables S2c and S2d, respectively. The Ga/Se ratio in the cavity is (0.827), while in the points around the cavity this ratio is (0.94). Thus, after laser ablation, an increased selenium content in GaSe is recorded everywhere! The carbon contribution in Table S2 is associated with a carbon film applied to the inner surface of quartz ampoules used in the synthesis and growth experiments.
3.3 Absorption spectra
Figure 3 shows the transmission spectra of a 1 mm thick GaSe plate chipped from the as-grown boule (curve 1) and that of the same plate with ARM (2). GaSe is transparent in the range from 0.62 to 20 µm. For comparison, in the same spectral range transmission spectra for ordinary (To) and extraordinary (Te) rays in GaSe are constructed using dispersion curves from [6], under the assumption of multiple reflections and zero absorption. In the range of 2-18 µm, the maximum permissible transmission increases from 63.8 to 65.8% with increasing wavelength for an ordinary beam and from 69.6 to 71.5 for an extraordinary one.
Fig. 3. Transmission spectra of a 1 mm thick GaSe plate after chipping (1) and in the ARM zone (2). T =300 K. Curve 2a shows the spectrum (2), with magnification ×20. For comparison, curves To and Te show the maximum transmission level in GaSe calculated under the assumption of multiple reflections and zero absorption.
The fact that the actual transmission of GaSe crystal is higher than expected for an ordinary beam can be explained by the difference in refractive indices in the volume that was taken during calculations and on the GaSe surface. The application of the ARM structure leads to transparency increase in the region of 5-17 µm. On the other hand, the transmission is reduced at lower wavelengths. In the visible region of the spectrum, the crystal becomes almost opaque in the ARM zone. That is why the c area turns out to be dark against the background of a lighter area without ARM in Fig. 1(c),(d).
In the transmission spectra of the mid-IR for GaSe, some features are observed (Fig. 4). According to the characteristic vibrations, they can be attributed to the manifestation of certain structural fragments and they are indicated in Fig. 4. By analogy with chalcogenide glasses [18], some of the features are associated with their native defects: so, the absorption bands in the range of 10.5 -14 µm refers to Se-Se vibrations (in the case of an excess of selenium). In some GaSe crystals the weak absorption bands of an impurity nature are observed. These are bands in the range of 2.5-2.7 µm and a band of about 6.0 µm that are associated with OH groups and H2O, a band of 4.3 µm with CO2, while the band of 8.0 µm is interpreted as a manifestation of Ga-O vibrations [17]. It is assumed that impurity atoms or groups are trapped in the space between layers weakly bound due to the Van der Waals interaction. A more intense absorption band near 16 µm-1 is associated with the second harmonic from oscillations near 307 cm-1 of the GaSe matrix, which is also manifested in Raman spectra (A21G mode).
Fig. 4. Detail of the GaSe transmission spectrum in the mid- IR for a plate 5 mm thick. The identification of the main absorption bands is given following to [15].
The analysis of the shape of the edge of fundamental absorption allows us to obtain information about the type of electronic band-to-band transitions. Following the Tauc approach [19], the dependence (α×hν)n = f(hν) was constructed and analyzed for GaSe. We found that straightening is achieved quite well both at n = 2 (the case of direct electronic band-to-band transitions) and at n = 0.5 (indirect transitions). In this case, at 80 K the approximation of the rectilinear section to the abscissa axis gives the values of the band gap width Eg= 2.11 eV for the case of direct transitions and Eg= 2.087 eV for indirect ones. At room temperature the corresponding Eg values are 1.981 eV and 1.962 eV. This is a fairly rare case: usually a certain variant of these two is preferred. Theoretical consideration from the first principles for the energy spectrum for GaSe [20,21] showed that with indirect transitions, the minimum of the conduction band is located at the point M of the Brillouin zone, about 25 meV below the direct minimum at point G. Thus, GaSe should be attributed to indirect semiconductors. Similar conclusions were made, for example, in [22–24].
Figure 5 shows the Tauc plot for the case of indirect transitions, and the temperature dependence for the band gap width Eg is shown in the insert. It can be seen that this dependence can be described by the semi-empirical Varshni equation [25]:
Fig. 5. Tauc plot for GaSe plate 150 μm thick at different temperatures in the 80-300 K range, for the case of indirect band-to-band electronic transitions. The insert shows the temperature dependence for the band gap width in GaSe and the results of approximation in the framework of semi-empirical Varshni equation.
The constant α1 is related to the electron/exciton-phonon interaction and β1 is associated with the Debye temperature of material, which is 251 K for GaSe [25]. Figure 6(a) shows the absorption spectrum for a GaSe plate with a thickness of 40 µm, at 80 and 300 K. The main feature of this spectrum is a narrow line of 592.4 nm (2.092 eV) and 620.3 nm (1.998 eV) at 80 and 300 K, respectively. Similar narrow lines are observed in the photoluminescence spectra at 532 nm excitation and they were previously attributed to direct free excitons (DFE) in GaSe [26].
Fig. 6. (a) Absorption spectra for GaSe at 80 K(1) and 300 K(2). (b) PL spectra at 532 nm excitation, at 80 K (3) and 300 K (4). (c) PL spectrum for GaSe in the ARM structure at 80 K (5) and the same spectrum with 10 times multiplication (5a).
3.4 Photoluminescence (PL)
The PL in GaSe has been studied in sufficient detail. It is known that undoped GaSe crystals are characterized by intense photoluminescence in the 580-630 nm region [27–32]. The authors distinguished a narrow line of 592.4 nm (2.093 eV) caused by direct free excitons, as well as lines 596.8, 609.3 and 619.4 nm, attributed to direct bound (DBE), indirect free (IFE) and indirect bound excitons (IBE), respectively [29,30]. In addition to these rather narrow lines, with a width of about 10 MeV for DFE, 19 MeV (DBE) and 25 MeV (IFE, IBE), broader features were also observed at larger wavelengths in especially activated GaSe crystals. These are bands of 700 and 900 nm in Mn-doped GaSe crystals [31] and bands of 635, 708 and 765 nm in Cd-doped ones [32]. In most of the obtained GaSe crystals, we observed a single 592.4 nm line due to DFE (Fig. 6(b)), and this indicates the high quality of GaSe crystals, the absence of intrinsic and impurity defects on which excitons could be localized. In GaSe with ARM structures similar to those shown in Fig. 2 c,d, the intensity of the DFE line decreases and additional lines 608.5 (IFE), 640.9, 646.6, 652.8 673.3 and 704.3 nm (with photon energies at 2.037, 1.934, 1.917, 1.989, 1.898, 1.841 and 1.760 eV, respectively) appear supposedly as a result of the appearance of additional structural defects (Fig. 6(c)). Temperature dependences of PL parameters are represented in Supplement S5.
3.5 Raman spectroscopy
In Fig. 7, curves 1 and 2 show Raman spectra for a freshly chipped GaSe plate, recorded at 325 and 532 nm excitations, respectively. At 532 nm excitation (spectrum 2), as well as at 1.06 µm excitation, three relatively narrow lines dominate the spectrum. The ε-GaSe has two out-of-plane phonon modes, A11G at ∼133 cm-1 and A21G at ∼307 cm-1, and one in-plane E22G mode at ∼212 cm-1, in line with previous reports [33,34]. Thus, the noncentrosymmetric polytype ε-GaSe, which has a very high χ coefficient, is predominant in grown GaSe. At short-wave excitation (325 nm, curve 1), the spectrum is dominated by a broad band of 251.8 cm-1, against which rather weak features characteristic of spectrum 2 are manifested. The entire spectrum is shifted to the high-energy side, up to 650 cm-1 and a band of about 500 cm-1 is well expressed. It is assumed that spectrum (2) corresponds to GaSe in the crystal volume, whereas spectrum (1) refers to a thin near-surface layer. The broadband nature of spectrum 1 may indicate the material amorphization. In particular, a broad band of 251 cm-1 agrees well with the signal for amorphous Se [24]. Se droplets on the surface of the chipped GaSe were also observed in SEM (Fig. 2 a,b). The shift of the broadband Raman spectrum to high energies with the manifestation of a band of about 500 cm-1 may be due to the presence of amorphous gallium oxide Ga2O3 on the surface [35].
Fig. 7. Raman spectra for GaSe at excitation of 325 nm (1) and 532 nm (2-4) for a perfect freshly chipped GaSe (1) crystal, as well as for GaSe with ARM (3, 4). Spectrum 3 was measured at some point between the ARMs, spectrum 4 was recorded directly in the ARM zone. T = 300 K. The spectra are shifted vertically for convenience. The intensity in the spectrum (2) is attenuated by a factor of 100.
Spectra 3 and 4 show Raman scattering in GaSe with ARM. Spectrum 4 is obtained directly in ARM, whereas spectrum 3 corresponds to some point between two neighboring ARMs. It can be seen that in the area exposed to the laser beam, the intensity of three GaSe lines weakens and a broadband background appears in the range of 50-350 cm-1. One can see at-least two additional broad bands with maxima of about 150 and 250 cm-1. A similar broadband background, although weaker, is observed in spectrum 3. As for the additional bands 150 and 250 cm-1, they can be associated with Ga2Se3 and amorphous selenium α-Se [33]. As shown by experiments using Raman, Auger and photoelectron spectroscopy, the oxidation of GaSe occurs when stored in the atmosphere. This process is especially fast, almost instantaneous at a temperature of more than 450°C. It is obvious that much higher temperatures are realized at laser ablation. The authors of [33] suggest that two main reactions are realized in GaSe, namely: (1) GaSe + ¼O2 → 1/3Ga2Se3+1/6Ga2O3 and (2) GaSe + ¾ O2 → ½ Ga2O3 + 3Se [36,37]. Further oxidation takes place via: Ga2Se3+3/2O2 → Ga2O3+3Se. During these reactions, selenium atoms are replaced by oxygen resulting in the formation of amorphous selenium in conjunction with the oxide. Thus, after laser evaporation in the process of creating an ARM structure, part of the material is deposited again both in the ARM region and in some neighborhood around the ARM zone as combination of Ga2Se3, α-Se and Ga2O3.
3.6 Changes in GaSe transmission after producing ARM
The parameters at which the antireflection structures were fabricated on laser set-ups at BMSTU (Moscow) and IA&E SB RAS (Novosibirsk) are given in Table 1. On the GaSe plate in BMSTU, 4 ARMs with a size of about 2×2 mm2 were produced using radiation of 513 nm. All these structures in Fig. 8 are made in approximately the same mode. The choice of the radiation wavelength was determined primarily by the requirement to ensure good absorption of the incident laser beam and effective ablation (Table 1). The surface of the GaSe plate in the ARM region is shown in Fig. 2(c). As can be seen in Fig. 3 (curve 2), the ARM producing significantly changes the transmission spectrum of GaSe. If as-grown GaSe demonstrates good transmittance at a level of 65% in the 0.64-8 µm range, then after producing ARM, strong transmittance reducing takes place in the visible and near IR ranges. The transmittance is about 5% at the edge of the fundamental absorption. As wavelength grows transmittance gradually increases to the maximum characteristic of GaSe without ARM at about 5 µm and then turns out to be even higher. Due to the strong losses in the visible part of the spectrum, the regions with ARM turn out to be dark and the intensity of this coloration correlates with the degree of increased transmission relative to the GaSe region without ARM. Figure 9 shows the transmission spectra measured at the darkest points in Fig. 8, where transmission in the mid-IR should be maximum. The analysis of the color distribution within different ARMs shows a good correlation with the existing extended defects. Maximum transmission in the mid-IR was registered in the visually darkest areas in Fig. 11, namely, in zone A in ARM1 and zones C in ARM3, whereas in ARM2 and for the most part ARM4, the effect is distributed fairly uniformly. At the same time, transmission increase in ARM2 is significantly stronger compared to ARM4.
Fig. 8. The image for the GaSe with ARM structures performed in BMSTU (Moscow). This image is obtained in transmitted white light. The light squares with letters show the characteristic points at which the transmission spectra were measured in a spot of 100 x100 µm2.
Fig. 9. Transmission spectra for three points with maximum transparency in the mid-IR after ARM performing on a GaSe plate 1 mm thick (see Fig. 8). Curves 2-4 correspond to points ARM1A, ARM2A and ARM3C. Curve 1 shows transmission spectrum for GaSe outside the ARM.
The transmission spectra for three points with the maximum effect are shown in Fig. 9, while the transmission values at 8 µm are given in Table 2. It can be seen that when performing ARM on one of the opposite planes of GaSe, it is possible to increase the transmission by about 10%. Another important parameter is the crosspoint between the spectrum of GaSe outside the ARM and the spectra recorded in the ARM region. The position of this point determines the shortwave edge of the spectrum range where transmission is increased after ARM producing. As can be seen from Table 2 and Fig. 9, that position of this point varies in the 3.9-4.7 µm region, but maximum transmission values are observed at somewhat larger wavelengths, approximately around 5 µm. For GaSe applications in nonlinear optics, it is advisable to expand the range with increased transmission in the short-wave direction, so that the pumping wavelength in optical parametric oscillators also falls here. According to the effective medium theory [13], the boundary wavelength is determined by the formula λ=n×p and, thus, a further reduction of the structure period in ARM is required. For this experiment this period is about 3.4 µm.
Table 2. Transmission parameters in GaSe with and without ARM for sample from BMSTU
In IA&E SB RAS (Novosibirsk) ARM structures are created using the radiation of a similar femtosecond laser, but with the 1026 nm wavelength. This wavelength falls into the GaSe transparency region and, as a result, this radiation is absorbed much weaker compared to the radiation of the second harmonic (513 nm). Figure 10 shows a plate of GaSe c 11 ARMs obtained using pulses with different energies (E = 56, 67, 84 and 112 nJ). In this case, one to four pulses were used to produce a single microcavity. In these experiments, ARM was performed only on one side of the GaSe plate.
Fig. 10. The image obtained with side lighting for a GaSe plate 1 mm thick, with 11 ARMs, each about 0.5×0.5 mm2, performed in IA&E SB RAS, Novosibirsk. Here the energies in one pulse are indicated on the top, on the left there are the numbers of pulses used to produce a single microcavity. The ARM period is 3.4 µm.
Figure 11 shows the spectra measured for points within the ARMs obtained under different conditions. It can be seen that, depending on the energy in the pulse and the number of pulses, the transmission spectra vary both in transmission level and shape. With an increase in the values of these parameters, the maximum transmission increases, which at a wavelength of 10 µm at maximum parameter values (N = 4, E = 112 nJ) reaches 77% compared to the initial value of about 65.7%. On the other hand, the range with increased transmission gradually shifts to the long-wavelength side and, as a result, the same displacement is experienced by the crosspoint. At low energies, the crosspoint is in the short-wave region of about 3.5 µm and with increasing both parameters this point shifts to 6.6 µm. These correlations are presented graphically in Fig. 12 (a, b) and tabular, in Tables 3, 4.
Fig. 11. Transmission spectra for GaSe plate (black curve) and for GaSe with ARM performed at laser ablation using 1026 nm pulses of different energy: 56 nJ (a), 67 nJ (b), 84 nJ (c) and 112 nJ (d) energy. The numbers near the curves show the number of pulses used to produce a single cavity on one of the surfaces.
Fig. 12. GaSe transmission (a) and crosspoint position (b) versus pulse energy and number when performing ARM with 1026 nm pulses.
Table 3. Transmission (%) at 10 μm in the ARM versus energy E and number N of laser pulses
Table 4. Position of the crosspoint (μm) versus energy E and number N of laser pulses
Further, ARMs about 500×500 µm2 in size, with the same step of 3.4 µm were applied to both opposite sides of the GaSe plate, at a pulse energy of 84 nJ and using 2 pulses to create one microcavity. Figure 13 compares the transmission spectra recorded for GaSe outside the ARM (spectrum 1) with spectra for GaSe with ARM on one side of the plate (2) and with the two similar ARMs on both opposite faces. It can be seen that when applying ARM on both sides, the GaSe effect of transmission increase doubles and is about 8%. At the same time, the position of the crosspoint point of the spectra remains unchanged, about 6 µm. Thus, for practical application, it is advisable to apply ARM simultaneously on both opposite sides of the crystal.
Fig. 13. Transmission spectra for GaSe plate (1) and for GaSe with ARM performed on one (2) and on two opposite faces (3). Wavelength 1026 nm, pulse energy 84 nJ, number of pulses 2.
4. Discussion
It is known that the degree of increase in transmission, the spectral range where transmission increases when ARM is performed, as well as the features of the transmission spectrum after ARM fabricating depend on many parameters such as the distribution and shape of the microcavities, their depth and filling factor [38]. In the case of single-pulse mode, wavelength 513 nm and high energy in the pulse cavities are almost adjacent to each other and the filling factor is 0.76 [39]. According to SEM data, cavities with a diameter of about 2.6 µm and a depth of about 2.5 µm are obtained and the aspect ratio is about 0.96 [39]. A distinctive feature of such ARMS is the position of the crosspoint at relatively short wavelengths (about 3.5 µm) and a fairly flat top of the spectrum in the range of 5-15 µm in Fig. 9. When using 1026 nm radiation, relatively narrow (with a diameter of 1.7 µm) and deeper (judging by SEM) at a period of about 3.4 µm are obtained. It turns out a greater increase in transparency (about 12%), but the range of increased transmission is narrower: the crosspoint is near 6 µm. It is assumed that fabricating ARM to both opposite surfaces of the GaSe plate will increase transmission by up to 90%. It should be noted that the use of 513 nm radiation can significantly reduce the time for manufacturing an ARM structure due to the use of a single-pulse mode. In the most successful experiments, when fabricating such an ARM, it has already been possible to increase the transmission on one surface to 92% [39]. Already now, GaSe plates with ARM should provide reasonably good antireflection properties at 10.6 µm as well as for the second harmonic at 5.3 µm. In the case of using GaSe in the optical parametric oscillator it is necessary to move to lower crosspoint wavelengths reducing the period in ARM array. Currently, research in the direction of optimizing ARM parameters continues. Another important parameter of such designs is the threshold of optical damage to GaSe crystals with ARM, and so far this question remains open.
5. Conclusion
Large GaSe crystals with a diameter of up to 25 mm and a length of 70 mm were grown. The only method of mechanical processing of GaSe at present is chipping along (001). SEM and EDX spectroscopy revealed characteristic defects: ragged (001) planes and drops of amorphous selenium. The main impurities are carbon and oxygen. Optical absorption spectroscopy revealed vibrations associated with OH, CO2, Ga-O, Se-Se bonds. XRD analysis shows ε-modification of GaSe.
Tauc analysis showed that GaSe is an indirect semiconductor. At 80 K, the band gap is 2.087 eV, whereas the Eg for a direct transition is 2.11 eV. The temperature dependence for Eg is described by the semi-empirical Varshni equation with parameters E0 = 2.097 eV, α1 = 2×10−3 and β1 = 1000 K.
The photoluminescence spectra at 80 K are dominated by a narrow line of 592.4 nm (2.093 eV) due to direct free excitons: this indicates the high quality of crystals and the dominant modification of the ε-GaSe. The PL quenching by an order of magnitude in the range from 80 to 300 K is described by Mott's law.
Fabrication of antireflection microstructure (ARM) on the GaSe surface by femtosecond 1026 nm and 513 nm laser ablation allows one to increase the transmission by about 12% when applying ARM on one face, and double this value for ARM on both (opposite) faces. The advantage of 513 nm laser radiation is a smaller boundary wavelength (about 3.8 µm) compared to 6 microns at 1026 nm.
After the ARM producing, additional lines associated with structural defects appear in the PL spectra whereas Raman spectra record an amorphous film with components Ga2Se3, Ga2O3 and Se on the GaSe surface. The process of GaSe oxidation is significantly accelerated at elevated temperatures (T > 450°C) under laser ablation conditions.
Funding
Russian Science Foundation (20-72-10027); Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences (FUUF-2019-0008); Russian Federation Budget (IA&E SB RAS project №121030500067-5).
Acknowledgments
A.A. Bushunov, A.A. Teslenko, M.K. Tarabrin, A.P. Yelisseyev, L.I. Isaenko and A.A. Goloshumova acknowledge the Russian Science Foundation according to the research project No. 20-72-10027 for the support of crystal growth, ARM performing, optical studies.
S.I. Lobanov acknowledges the state assignment of IGM SB RAS (FUUF-2019-0008) for the support of pyrosynthesis.
A.V. Dostovalov and S.A. Babin acknowledge Russian Federation Budget (IA&E SB RAS project №121030500067-5) for ARM performing in IA&E SB RAS.
Part of experiments was carried out using the equipment of the collective use centers ``NANOSTRUKTURY'‘ IFP SBRAN and “VTAN” in the ATRC department of NSU.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
Supplemental document
See Supplement 1 for supporting content.
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