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## Creator

[Qinqiang Zhang](https://orcid.org/0000-0001-7242-1718), [Ryo Matsumura](https://orcid.org/0000-0003-2303-4978), [Naoki Fukata](https://orcid.org/0000-0002-0986-8485)

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This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Applied Nano Materials, copyright © 2025 American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acsanm.5c01552.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Aluminum-Catalyzed Lateral Growth of Spherulite-like GeS Thin Films on Insulating Substrates Using Vapor Transport: Implications for Electro-optic Applications](https://mdr.nims.go.jp/datasets/2687de79-84a6-4b4a-babb-16e2adb7b59a)

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11 Aluminum-Catalyzed Lateral Growth of Spherulite-2 Like GeS Thin Films on Insulating Substrates using 3 Vapor Transport: Implications for Electro-Optic 4 Applications5 *Qinqiang Zhang†, Ryo Matsumura†, and *Naoki Fukata†‡6 † Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials 7 Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan8 ‡ Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-9 8573, Japan10 ABSTRACT11 Due to the presence of lone-pair electrons, it is challenging to attain lateral growth of thin 12 films of Group IV monochalcogenides. In this study, lateral growth of germanium monosulfide 13 (GeS) thin films with a minimum thickness of about 20 nm was attained on SiO2/Si and quartz 14 substrates at a growth temperature of 420 ˚C, employing a combined method that uses an 15 aluminum (Al) catalyst and a pre-deposited amorphous GeS layer. Grown GeS thin films using a 16 20 nm-thick Al show dendrite structures, likely attributable to defects introduced in the Al layer 17 when deposited at room temperature. Dendrite structures virtually eliminated by annealing 18 substrates at 120 ˚C during Al deposition or by using a 5 nm-thick Al layer deposited at room 19 temperature. Time-dependent changes in grown GeS thin films using a 5 nm-thick Al on SiO2/Si 20 and quartz substrates were shown to exhibit a lateral growth rate of 2 - 3 µm/sec. The Page 1 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960221 birefringent properties of grown GeS with a Maltese extinction cross pattern were confirmed 22 using cross-polarized optical microscopy, indicating the formation of a spherulite-like structure. 23 It appears that Mullins-Sekerka instability dominates the growth front at the periphery of circular 24 domains, causing a dendrite morphology. A transition region dominated by kinetic-limited 25 diffusion occurs, forming the spherulite-like structure of GeS. X-ray diffraction and atomic force 26 probe investigation reveal the grown spherulite-like GeS, on insulating substrates, to consist of a 27 layered structure with a flat surface. The single-crystalline area of GeS appears to be larger than 28 200 nm in size as evaluated by selected area electron diffraction via transmission electron 29 microscopy. This study reveals a potential method of achieving lateral growth of GeS on 30 insulating substrates. This method may facilitate the use of GeS as functional semiconductors for 31 developing next-generation field-effect transistors with potential for all-optical manipulation, 32 such as in-memory sensing and computing devices.3334 KEYWORDS Lateral growth, germanium monosulfide, Al-catalyzed growth, birefringence, 35 spherulite-like structure 3637 1. INTRODUCTION38 Recently-rediscovered Group IV monochalcogenides, such as germanium monosulfide 39 (GeS), are promising layered semiconductors [1–3] ; they exhibit unique properties that include 40 non-toxicity, earth-abundancy [4], a moderate bandgap [5], high Curie temperature [6], Eshelby 41 twist [7], in-plane ferroelectricity [8], photostrictive properties [9] and other useful properties [10–42 14]. Tin monosulfide (SnS), which has a structure that is analogous to that of GeS, has been 43 shown elsewhere to exhibit bulk photovoltaic properties [15], in-plane ferroelectric domain 44 transitions [16], high carrier mobility [17] and other characteristics of interest [18, 19]. Lateral 45 growth techniques for creating large-area thin films of Group IV monochalcogenides remain a 46 challenge, likely due to the presence of lone-pair electrons, resulting in a higher charge energy 47 between van der Waal layers than that of other transition metal chalcogenides [20]. In our 48 previous investigations, a pre-deposited amorphous technique for taking Mullins-Sekerka 49 instability into account has been studied to achieve lateral growth of GeS thin films [21–23]. Page 2 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960350 Centimeter-scale GeS continuous film has been achieved; however, the observed minimum 51 thickness of the grown GeS is still approximately 100 nm, and bulk-like GeS prisms are 52 generally observed in the center of grown circular GeS domains. Growing GeS thin films 53 laterally is still an ongoing project [24–26].54 An effective method of achieving a self-limited growth mechanism or of minimizing 55 thickness in the growth of other two-dimensional materials, such as graphene [27, 28] and 56 transition metal chalcogenides [29–31], is to employ a metal catalyst. The first attempt to apply an 57 appropriate metal catalyst to grow GeS took into account the vapor pressure and eutectic 58 temperature of aluminum (Al) and germanium [32, 33]. The eutectic temperature of both is about 59 400 ˚C, and it is close to the growth temperature in our previous investigation that employed the 60 pre-deposited amorphous GeS method [34]. Aluminum is a commonly-used metal in the 61 semiconductor industry. It is capable of being formed on the target substrate by thermal 62 deposition and atomic layer deposition [35, 36]. Relaxation methods (e.g., post-annealing) for 63 reducing defects in Al layer when deposited under vacuum conditions, such as atomic voids, 64 internal stresses and so forth, are also applied [37–39]. To attain GeS thin films on insulators, Al as 65 the metal catalyst can be used to provide the appropriate activation energy that will minimize the 66 formation of bulk-like GeS prims [29] and to obtain centimeter-scale crystalline GeS thin films. 67 We believe this study to be the first investigation to use Al as a catalyst for the lateral growth of 68 centimeter-scale continuous GeS thin films. In addition, a slight modification of the susceptor 69 with a tilt angle is also implemented, analogous to the other investigation [40], since the boundary 70 layer [41, 42] that results from fluid dynamics on the target substrates using vapor transport 71 method is considered as an important factor to minimize thickness variation in the pre-deposited 72 amorphous GeS layer and the grown GeS thin films [34, 41, 42].73 Based on the aforementioned information, an Al layer as the catalyst, designed to be 74 synergistic with the pre-deposited amorphous GeS layer method, was applied to grow GeS thin 75 films on insulating substrates (SiO2/Si and quartz). Two substrates, i.e., SiO2/Si measuring 13 76 mm ×  13 mm and quartz measuring 15 mm ×  15 mm were both placed in a region which had 77 a growth temperature of 420 ˚C. The birefringent properties of the grown GeS thin films were 78 observed on both the substrates using an optical microscope with crossed polarizers. The 79 spherulite-like structure of the GeS thin films was confirmed by observation of the Maltese Page 3 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960480 extinction cross pattern [43, 44]. Spherulite crystals are found in a large number of materials 81 documented elsewhere, including polymers [45], salts [46, 47], metals [48, 49], and several human 82 diseases such as Alzheimer’s disease and kidney stones [50, 51]. A growth mechanism for 83 spherulite-like GeS thin films is considered and discussed later in detail in comparison with other 84 investigations [52, 53]. The presence of the spherulite-like structure of GeS thin films is likely 85 attributable to ultrahigh supersaturation-induced condensation at the growth front, resulting in 86 the lateral growth of spherulite-like GeS thin films. The lateral growth rate for nonlinear 87 spherulitic growth obtained here is analogous to the supercooling-like method [54, 55] and is in the 88 order of several micrometers per second (2 - 3 µm/sec). In addition to the observed birefringent 89 properties, grown GeS thin films were investigated in this study by Raman spectroscopy, X-ray 90 diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray detection 91 (EDX), transmission electron microscopy (TEM), and atomic force probe. Despite techniques for 92 growing spherulites being more an art than a science [51], emerging cylindrical symmetric 93 rotating crystals [56], and the potential to grow single-crystalline functional materials on 94 insulating substrates, as previously investigated for GeO2 [57], suggest that grown spherulite-like 95 GeS thin films hold the potential to achieve single-crystalline semiconductors on insulating 96 substrates such as SiO2/Si and quartz, taking advantage of the substrate engineering synergy that 97 benefits from the kink-pair nucleation mechanism (i.e., the Peierls energy barrier) [58, 59] and 98 paving the way for the development of next-generation GeS field-effect transistors (FETs) with 99 the potential advantage of use in programmable FETs using the optic control method [60–62].100101 2. RESULTS AND DISCUSSION102 Al as the catalyst layer was deposited on SiO2/Si and quartz substrates using a thermal 103 evaporation apparatus in a range of thicknesses from 5 - 20 nm, deposited at room and elevated 104 temperatures, followed by the vapor transport process for growth of GeS. A horizontal quartz 105 tube with two independently-controlled heating regions was then utilized to proceed with the 106 pre-deposited amorphous GeS layer and the growth of crystalline GeS thin films at elevated 107 temperatures. GeS powder was placed in the upstream heating region of the tube, while 108 substrates (300 nm-SiO2/Si and quartz) were positioned in the downstream heating region. The Page 4 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859605109 experimental heating profile is analogous to that used in our previous investigations [34]; in 110 addition, a further optimized strategy for locating the susceptor, analogous to other 111 investigations, [40–42] and a fixed growth temperature of 420 ˚C were utilized in the current study. 112 (See details in the Materials and Experimental Methods.) This was simply to minimize, as much 113 as possible, the variation in thickness of the grown GeS thin films on SiO2/Si and quartz 114 substrates to a greater extent than in our previous studies, in which the thickness of the grown 115 crystalline GeS films varied from 1 µm to 100 nm on a single substrate [34]. 116 In this, the first attempt to apply Al as the catalyst layer for growing GeS thin films, a 20 117 nm-thick Al layer for 15-min growth of GeS thin films was employed, as shown in Figure 1. The 118 growth of bulk-like GeS prisms and clusters were significantly suppressed as a result of use of 119 the Al catalyst, compared to that without employing Al catalyst, as shown in Supporting Figure 120 S1a-S1c. However, large numbers of dendrite structures were observed in the grown GeS films, 121 as shown in Figure 1a. Two types of dendrite structures were identified here. Type 1 dendrite 122 structures, which are dark in color, occupy a smaller area, whereas Type 2 dendrite structure are 123 pale in color and occupy a larger area. Type 1 dendrite structures are present at the center of the 124 Type 2 dendrite structures. Corresponding EDX mappings of dendrite structures are shown in 125 Figure 1b-f. Lower signal intensities for Ge and S and higher intensities for Si and O are 126 observed in accordance with the smaller dark areas present at the center of Type 2 dendrite 127 structures, indicating that the Type 1 dendrite structure is attributable to the loss of Ge and S 128 (Figures 1b,1c,1e,1f). Analogously, dendrite structures with lower signal intensities for Al are 129 observed in accordance with the large pale areas of dendrite structures as shown in Figures 1a 130 and 1d. The Type 2 dendrite structure is therefore attributable to the loss of Al catalyst. Position-131 dependent measurements of Raman spectra are shown in Figure 1g from the center (#1 spot) to 132 the edge (#3 spot) of the dendrite structure. All three spots show typical Raman phonon modes 133 for GeS [63] as indicated by B3g at 211 cm–1, Ag at 239 cm–1 and Ag at 269 cm–1, respectively 134 representing the vibration mode of the zigzag edge direction, the layer breathing mode and the 135 vibration mode of the armchair edge direction. A stronger peak intensity for the Si substrate at a 136 Raman signal of 521 cm–1 is observed at the center of the dendrite structure, due to the loss of 137 grown GeS in Type 1 dendrite structures. The Raman signal of Si is lower at the #2 and #3 spots, 138 likely attributable to the effect of Al. The typical surface morphology of the dendrite structures 139 was evaluated by AFM as shown in Figure 1i. The dendrite structures (darker areas) are thinner, Page 5 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606140 as shown in the AFM mapping of Figure 1h. The corresponding line profile exhibits two steps in 141 its thickness, from Type 1 to Type 2 dendrite structures. The presence of a lower step (with a 142 Type 1 dendrite structure) and a higher step (with a Type 2 dendrite structure) is therefore 143 attributable to grown GeS and the loss of Al (approximately 20 nm thick), respectively, in the 144 light of the preceding investigations. Grown GeS thin films located at areas without dendrite 145 structures, i.e., at the #3 spot, show a flat surface on SiO2/Si substrates (Figures 1a and 1h). It 146 thus appears that the dendrite structures are attributable to defects such as internal stress and 147 atomic voids that were already present in the thicker thermally-deposited Al [39, 64]. These defects 148 are aggravated by the elevated temperature of the argon carrier gas containing GeS. 149 Identification of the starting point of dendrite structures will require a great deal of investigation, 150 since it is likely related to the elevated temperature, conditional pressure, speed of argon carrier 151 gas flux, concentration of GeS and other variables. Synergy with simulation methods in fluid 152 dynamics and phase-field models will therefore be examined in future investigations.153 The XRD results confirm the orthorhombic structure of GeS to be in accordance with the 154 standard GeS reference (PDF#00-009-0231) as shown in Figure 1i. In the 2𝜃 scan mode, 155 diffraction peaks at 16.9˚ and 34.2˚ are obtained, representing (002) and (004) respectively, 156 indicating that the crystal orientation is along the c-axis (001), and normal to the van der Waals 157 stacking layers. However, additional (102), (012), (110) and (103) peaks are obtained. These can 158 likely be ascribed to the loss of Al, resulting in the formation of other crystal orientations. 159 Representative photographs and optical microphotographs are shown in Figure 1j-l, which reveal 160 almost complete coverage of grown GeS thin film on a 13 mm × 13 mm SiO2/Si substrate for a 161 growth time of 15 min. The grown GeS film exhibits various interference colors on the Al 162 catalyst without dendrite structures, and Type 1 and Type 2 dendrite structures as evaluated by 163 cross-polarized optical microscopy. The birefringent properties of grown GeS thin films are 164 likely attenuated by thick Al as described in Figure 1j-l and Supporting Information Figure S1d-165 e, whereas they exhibit stronger interference colors on dendrite structures. Analogous 166 birefringent properties have been noted elsewhere in an investigation of SnS flakes on a mica 167 substrate [15]. Since this study is on the growth of GeS thin films, a thin film relaxation strategy 168 [37, 38], namely annealing, is implemented to limit the formation of dendrite structures.169Page 6 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859607170171 Figure 1. Evaluation of grown GeS thin films on SiO2/Si substrate with 20 nm-thick Al deposited at room 172 temperature. (a) Top view of SEM observation. (b-f) EDX mappings for Ge, S, Al, Si, and O, respectively. (g) 173 Raman spectra of measured #1, #2 and #3 spots, respectively indicated in (a). (h) Surface morphology and depth 174 profile of grown GeS thin films with 20 nm-thick Al. (i) XRD spectra of grown GeS thin films with the standard 175 GeS reference of PDF#00-009-0231. (j-l) Photographs and microphotographs for 15-min growth of GeS using an 176 optical microscope with and without a crossed polarizer.177178 If annealing is applied at 120 ˚C during 20 nm-thick Al deposition, the grown GeS thin 179 film exhibits no dendrite structures, as demonstrated in Supporting Figure S2a and S2b. 180 Annealing during Al deposition to reduce defects (internal stresses, atomic voids, etc.) as Page 7 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859608181 evaluated here appears to be an effective method for growing GeS thin films. However, 182 annealing during Al deposition under vacuum may result in hillock growth, agglomeration and 183 other surface structures [39] with increasing film thickness, as seen in the SEM images of the 184 thickness-dependent surface morphology of Al (Supporting Figures S2d-f). The thickness-185 dependent surface morphology of Al is analogous to the structure between Zone 1 and the 186 Transition Zone [39], at deposition temperatures below 0.3Tmax (Tmax is the melting point of the 187 deposited metal) as described by the extended structure zone model proposed by Thornton [65]. 188 To prevent the formation of dendrite and surface structures in Al, we conducted a method using a 189 5 nm-thick Al deposited at room temperature. The as-deposited 5 nm-thick Al shows very few 190 surface structure, as seen in the SEM image in Supporting Figure S2g. Time-dependent changes 191 in the GeS growth process (3 and 5 min) with deposition at room temperature of 5 nm-thick Al 192 on both SiO2/Si and quartz substrates are shown in Figures 2a-h. Figures 2a and 2c are 193 photographs of 3-min growth and Figures 2e and 2g show 5-min growth of GeS on SiO2/Si and 194 quartz substrates. Dendrite structure-free GeS thin films are obtained on SiO2/Si and quartz 195 substrates with a 5 nm-thick Al layer deposited at room temperature, analogous to that for a 20 196 nm-thick Al layer deposited and then annealed at 120 ˚C. Since no vapor-liquid-solid growth 197 modes are observed, in the same way as seen with gold metal-catalyzed growth of GeS 198 nanowires [7], the Al appears to stabilize the pre-deposited amorphous GeS layer and suppress 199 the re-sublimation rate of the pre-deposited amorphous GeS layer.200 Optical microscopy using a crossed polarizer is utilized to evaluate the birefringent 201 properties of GeS thin films grown on SiO2/Si and quartz substrates with growth times of 3 min 202 (Figures 2b,2d) and 5 min (Figures 2f,2h), respectively. The darker regions, the so-called 203 Maltese extinction cross pattern, are a common characteristic of radial anisotropic bodies 204 between a crossed polarizer. The Maltese cross is parallel to the polarizer, and the analyzer 205 orientations of the microscope are independent of the stage orientation. The Maltese cross results 206 from the cancellation of birefringence every 90˚ with a crossed polarizer; for this reason, 207 crystalline GeS along the vertical and horizontal directions is dark in the observed optical image. 208 This suggests that the grown GeS thin film possesses a spherulite-like structure, since spherulite 209 substances exhibit a Maltese cross pattern when observed using cross-polarized optical 210 microscopy. The observed interference colors of spherulite-like GeS are likely affected by the 211 thickness of the film and the underlying substrate, likely due to the birefringent properties of the Page 8 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859609212 grown GeS thin film on the SiO2/Si substrate. The thicker of the GeS film, the longer the 213 reflected wavelength of the color observed, that is, moving it toward the red end of the spectrum. 214 Conversely, the thinner the GeS film, the shorter the wavelength observed, moving it toward the 215 blue end of the spectrum. In addition, if the polarizer is slightly rotated, GeS thin films show 216 optically enhanced (blueish color) and weakened (reddish color) areas with different interference 217 colors, as demonstrated in Supporting Figures S2h-k. It appears likely that the colored area 218 shows the same structural orientation along the armchair or zigzag edge as sub-domains of 219 crystallized GeS, present in the circular domain of spherulite-like GeS thin films. The formation 220 of spherulite-like GeS thin films is likely attributable to a sharp local phase gradient in the 221 adatom region of the growth front among the crystallized GeS, GeS vapor source and re-222 sublimated GeS from the Al catalyst [34, 66]. The ultrahigh supersaturation-induced condensation 223 at the growth front likely results in the lateral growth of spherulite-like GeS at the interface of 224 the sharp local phase gradient [57]. The diameter of one domain for spherulite-like GeS thin films 225 with a 3-min growth time is about 450 - 500 µm, as shown in Figures 2b and 2d, whereas that for 226 a 5-min growth time is about 700 µm, as shown in Figures 2f and 2h. The growth rate which 227 achieves one circular domain with a diameter of hundreds of micrometers within 3 - 5 minutes, 228 i.e., in the order of several micrometers per second (i.e., 2 - 3 µm/sec observed in this study), for 229 the nonlinear spherulitic growth of GeS thin films, is analogous to the supercooling-like method.230 Raman spectra of the grown spherulite-like GeS thin film on SiO2/Si and quartz 231 substrates are shown in Figure 2i, measured in four positions from the emanating center to the 232 circular periphery and outside of the circular domain, as respectively denoted as the Centre, 233 Middle, Fringe, and Remote areas. The Raman spectra noted at the emanated center (Centre), 234 Middle and Fringe areas of the circular domain show three typical Raman phonon modes for 235 GeS [63] as indicated by B3g at 211 cm–1, Ag at 240 cm–1 and Ag at 270 cm–1, respectively 236 representing the vibration mode of the zigzag edge direction, the layer breathing mode and the 237 vibration mode of armchair edge direction. The Raman spectrum at 521 cm–1 corresponds to the 238 Si substrate. Raman spectra of grown GeS thin films on quartz substrates are also obtained, 239 whereas they generally show a downshift compared to that of on SiO2/Si substrates. Raman 240 spectra of grown GeS thin films on both SiO2/Si and quartz substrates are not detected in the 241 Remote area outside the circular domain. The lateral growth of GeS thin film is therefore 242 attained by using the Al catalyst and pre-deposited amorphous GeS layers. Since AFM Page 9 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596010243 measurements at the outside of the circular domains might affected by remaining pre-deposited 244 amorphous GeS and Al, we employed cross-sectional SEM observations to evaluate thickness. 245 These are described in the next paragraph. Therefore, only the surface roughness is shown in 246 Figure 2j-k.247 AFM measurements with a surface roughness Ra, of about 0.7 nm confirm the lateral 248 growth of spherulite-like GeS thin films exhibiting a flat surface on both SiO2/Si and quartz 249 substrates. The fractal dendrite at the circular periphery is observed for grown GeS thin films on 250 a quartz substrate as shown in the represented SEM image (Supporting Figure S2l). This likely 251 confirms the Mullins-Sekerka instability at the growth front of the circular periphery [21–23], 252 which is limited to only the normal growth mechanism for diffusion-limited growth; however, 253 spherulite formation at the inner body of the grown GeS appears to be controlled by kinetic 254 growth, since spherulites do not form under diffusion control. Diffusion-limited growth starts 255 from the growth front at the periphery of the spherulite-like GeS after nucleation, following 256 which there may be a transition region that facilitates a transformation from diffusion-limited 257 growth [67] to kinetic-limited growth [68] for the body of spherulite-like GeS thin films. XRD 258 spectra of the Al-catalyzed lateral growth of GeS thin films on both SiO2/Si and quartz substrates 259 are highlighted by green and yellow lines in Figure 2l. The XRD results confirm the 260 orthorhombic lattice structure of GeS, in accordance with the standard GeS reference of 261 PDF#00-009-0231. In the 2𝜃 scan mode, the diffraction peaks that occur only at about 16.9˚ and 262 34.2˚ for (002) and (004) respectively are obtained, indicating that the crystal orientation is along 263 the c-axis (001), and normal to the van der Waals stacking layers. This suggests that GeS thin 264 films grown on SiO2/Si and quartz substrates using 5 nm-thick Al and pre-deposited amorphous 265 GeS consist of layered structures.266Page 10 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596011267268 Figure 2. Evaluations of temporal change after 3 and 5 min for grown GeS thin films on SiO2/Si and quartz 269 substrates with 5 nm-thick Al deposited at room temperature. (a-h) Top view of optical microscope observations 270 with and without applied crossed polarizer. r and d respectively represent the radius and diameter of one circular 271 domain. (i) Raman spectra of measured spots at the Centre, Middle, Fringe and Remote spots on SiO2/Si and quartz 272 substrates as indicated in (a) and (c). (j-k) Surface morphology of grown GeS thin films with a 5 nm-thick Al on 273 SiO2/Si and quartz substrates, respectively. (l) XRD spectra of grown GeS thin films on insulating substrates with 274 the standard GeS reference of PDF#00-009-0231.275Page 11 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596012276 Since a 15-min growth time is sufficient to fully cover the SiO2/Si substrates with GeS, 277 cross-sectional SEM measurements with EDX mappings were carried out to evaluate the 278 thickness of the grown GeS thin films. The thickness of a grown GeS thin film is closely related 279 to that of the pre-deposited amorphous GeS layer. To obtain various thicknesses of GeS thin 280 films using a 5 nm-thick Al, a pressure control with identical flow of inlet Ar gas flux to achieve 281 various thickness of pre-deposited amorphous GeS layer was applied. The observed various 282 thicknesses of grown GeS thin films using a 5 nm-thick Al from areas highlighted by dashed 283 circles are about 60 nm, 35 nm and 20 nm as shown in Figures 3a-c, attained by using pressures 284 of 6500 Pa, 7000 Pa and 7500 Pa respectively during the pre-deposited amorphous GeS layer 285 process. Insets of Figure 3a-c show overall photographs of Al-catalyzed grown GeS thin films on 286 SiO2/Si substrates. Cross-sectional EDX mappings for the elements Ge, S, Si and O in the grown 287 approx. 60 nm-thick GeS thin film (as referenced in Figure 3a) are shown in Figure 3d-g. Ge and 288 S are clearly obtained on top of the SiO2/Si substrate. The two represented thicknesses measured 289 using the provided software equipped with an SEM apparatus for the grown GeS thin film of 290 about 35 nm and 20 nm are illustrated in Supporting Figure S3. The Al-catalyzed grown GeS 291 thin films are much thinner than in our previous results (i.e., the observed minimum thickness of 292 GeS is about 100 nm) [34], with a much lower variation in thickness of several tens of 293 nanometers (in comparison with several micrometers in our previous results), facilitated by use 294 of the optimized tilt susceptor method [40–42].295296Page 12 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596013297 Figure 3. Cross-sectional SEM images for the thickness evaluation of 15-min growth of GeS thin films on SiO2/Si 298 substrates with 5 nm-thick Al deposited at room temperature. (a-c) Cross-sectional SEM observations with achieved 299 thicknesses of 60 nm, 35 nm and 20 nm. Insets are photographs of the corresponding samples. (d-g) Cross-section 300 EDX mappings for Ge, S, Si and O.301302 Since electron backscatter diffraction (EBSD) has been investigated to identify the 303 cylindrically symmetric-rotating crystal InSiO [56] and the spherulite GeO2 [57], it is considered 304 to evaluate the crystallinity of grown spherulite-like GeS thin films and the Al deposited at 305 elevated temperature in this study as well. However, it is extremely difficult to obtain any usable 306 data using EBSD to identify the crystallinity or crystalline size of grown GeS thin films and the 307 Al deposited at elevated temperatures. Observations of EBSD mappings, inverse pole figure 308 (IPF) and IPF with 2 degrees intensity plot are shown in Supporting Figures S4a-h. 309 Crystallinities of 5-nm and 20 nm-thick Al deposited at room-temperature and 120 ˚C on SiO2/Si 310 substrates exhibit amorphous-like states, analogous to that on bare SiO2/Si substrates. Only the 311 Al deposited on single-crystal Si (001) shows a single-crystal feature. In addition, crystallinities 312 of about 60 nm-thick GeS thin films using 5-nm and 20 nm-thick Al on SiO2/Si substrates show 313 quite a number of areas in amorphous-like states, whereas the crystalline orientation of GeS 314 (001) are slightly observed at certain areas. Observed amorphous-like crystallinities of Al-315 catalyzed grown GeS thin films are somewhat inconsistent with aforementioned XRD and 316 Raman spectra, likely attributable to the amorphous GeSx shell developed using the vapor 317 transport method as investigated elsewhere [69]. It may simply be attributable to the uncertain 318 factor that sulfur-containing compounds are not readily identifiable by the EBSD apparatus, as 319 well. Thus, evaluation using transmission electron microscopy (TEM) is implemented for 320 identifying the crystallinity of Al-catalyzed grown GeS thin films instead of further discussions 321 regarding EBSD measurements of Al-catalyzed grown GeS thin films and deposited Al on 322 SiO2/Si substrates at elevated temperature in the current study.323 Evaluation of the crystalline size of Al-catalyzed grown spherulite-like GeS thin films 324 was investigated using TEM with selected-area electron diffraction (SAED) and EDX element 325 mappings as shown in Figures 4a-h. As-grown 60 nm-thick GeS thin films with 5 nm-thick Al on 326 300-nm SiO2/Si substrates were immersed in diluted hydrofluoric acid solutions to dissolve the Page 13 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596014327 SiO2 and metallic Al catalyst. The GeS thin films were then transferred to another beaker filled 328 with deionized (DI) water. The delamination of GeS thin films from the as-grown SiO2/Si 329 substrate was observed immediately on the substrate being immersed in DI water, since the 330 surface tension of the water is strong enough to separate the GeS from the substrate, which then 331 floats on the surface of the DI water. An amorphous carbon-enhanced copper microgrid is then 332 utilized to support the transferred GeS lamellae for TEM evaluations. High-resolution TEM 333 (HRTEM) imaging shows the lattice fringe of the Al-catalyzed grown GeS, clearly exhibiting an 334 orthorhombic structure [70] with zigzag- and armchair-edge directions as illustrated in Figure 4b. 335 The fast Fourier transform pattern of the observed area in Figure 4b shows a clear, individual, 336 and distinct diffraction pattern, confirming the crystallization of Al-catalyzed grown GeS in the 337 limited observation area, as shown in the inset (Figure 4c). To identify the size of single-338 crystalline GeS, SAED with predetermined observation areas for aperture diameters of 200 nm to 339 16 µm was utilized. Observations of the crystalline size of the grown GeS as a function of these 340 diameters are shown in Figure 3d-g. Typical GeS lamellae tend to adopt a clear single-crystalline 341 structure of 200 nm in size with an individual and distinct diffraction pattern. Abnormal 342 diffraction patterns, not associated with the orthorhombic atomic structure, are likely ascribable 343 to the markedly different orientation of the GeS stitched at grain boundaries, or corrugated and 344 overlapping structures resulting from the wet transfer process. On increasing the aperture to 800 345 nm in diameter, a slightly tiled and separated SAED pattern is observed, as shown in Figure 4e. 346 At a diameter of 4 µm, the SAED pattern exhibits a slightly separated SAED pattern with a 347 strong signal, although the amorphous diffraction ring is distinct (Figure 4f). The SAED pattern 348 at a diameter of 16 µm shows discernible lathy diffraction patterns within the amorphous 349 diffraction ring (Figure 4g). TEM, high-angle annular dark field scanning TEM (HADDF-350 STEM) and EDX element mappings are shown in Figure 4h. Aluminum shows a much lower 351 signal than that of Ge and S, indicating that the Al should be removed so as to not affect the 352 SAED patterns. Even though typical spherulite lamellae are radial with a small degree of 353 divergence and observations of the Maltese extinction cross pattern under cross-polarized optical 354 microscope are confirmed as mentioned in preceding sections [43–48, 50–57], it is not possible to 355 state with full confidence that Al-catalyzed grown spherulite-like GeS thin films possess in-plane 356 twisted structures as suggested solely by the observed lathy SAED patterns, without the 357 confirmation of EBSD observations as investigated elsewhere [7, 46, 47, 56, 57]. Although the Page 14 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596015358 rotating degree of diffraction patterns is relatively small and lathy, and the grown GeS thin films 359 are confirmed by cross-polarized optical microscopy to possess a spherulite-like structure 360 emanating from the circular domain center, it would be premature for us to conclude that the 361 observed area of GeS lamellae indicates a pure polycrystalline structure. Further 362 characterizations to identify the atomic lattice structures of grain boundaries and emanated 363 centers in spherulite-like GeS thin films utilizing TEM and focused ion beam techniques will be 364 needed in subsequent investigations. We therefore solely report a single-crystalline area of Al-365 catalyzed grown GeS thin film exceeding 200 nm in size. 366367368 Figure 4. TEM evaluations of the crystallinity of GeS thin films grown for 15 min on SiO2/Si substrates with a 5 369 nm-thick Al deposited at room temperature. (a) Top view of TEM observation supported by an amorphous carbon-370 enhanced copper microgrid after the wet transfer process. (b-c) High-resolution TEM image of a selected area, and 371 its corresponding fast-Fourier transform (FFT) pattern. (d-g) SAED patterns of the observed area in (a) with aperture 372 diameters of about 200 nm, 800 nm, 4 µm and 16 µm. (h) HADDF-STEM observations and corresponding EDX 373 mappings of elemental C, Ge, S and Al.374375 3. CONCLUSIONSPage 15 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596016376 In this study, germanium monosulfide (GeS) thin films with a minimum thickness of 377 about 20 nm were laterally grown on SiO2/Si (13 mm × 13 mm) and quartz (15 mm × 15 mm) 378 substrates at a growth temperature of 420 ˚C, employing an Al catalyst synergistic with a pre-379 deposited amorphous GeS layer. Grown GeS thin films using a 20 nm-thick Al show dendrite 380 structures. When annealing at 120 ˚C is applied during 20 nm-thick Al deposition, the grown 381 GeS thin film exhibits no dendrite structures. Annealing the substrate during the thermal 382 deposition of Al appears to be an effective method of reducing introduced defects caused by 383 internal stress, atomic voids and so forth when growing GeS thin films. However, annealing 384 during Al deposition under vacuum may result in hillock growth, agglomeration and other 385 surface structures that result in increased film thickness, as predicted by Thornton’s extended 386 structure zone model. To prevent the formation of Al dendrite and surface structures, a method 387 using a 5 nm-thick Al deposited at room temperature is implemented for the lateral growth of 388 GeS thin films. The lateral growth rate of Al-catalyzed grown spherulite-like GeS thin films was 389 observed to be 2 - 3 µm/sec. 390 The birefringent properties of Al-catalyzed grown GeS thin films with a Maltese 391 extinction cross pattern were confirmed by using a cross-polarized optical microscope, indicating 392 that the grown GeS has a spherulite-like structure. It is likely that the ultrahigh supersaturation-393 induced condensation due to Mullins-Sekerka instability takes place at the growth front at the 394 edge of the circular domain, resulting in lateral growth of spherulite-like GeS thin films and 395 showing a fractal or dendritic morphology. A transition region dominated by kinetic-limited 396 diffusion likely occurs that creates the spherulite-like inner body of the GeS thin films, since 397 spherulites do not form under diffusion control; and Mullins-Sekerka instability is limited to the 398 normal growth mechanism of diffusion-limited growth. Moreover, the nonlinear spherulitic 399 growth obtained in this study is analogous to the supercooling-like method. XRD spectra and 400 AFM measurements show that the method applied in this study can result in the layered structure 401 seen in spherulite-like GeS thin films with (002) and (004) crystal orientation and on flat 402 surfaces on SiO2/Si and quartz substrates. The observed TEM-SAED patterns suggest the 403 crystals in the GeS to be larger than 200 nm. To summarize, this study proposes a potential 404 method of growing lateral spherulite-like GeS thin films on insulating substrates (SiO2/Si and 405 quartz). It may also suggest that spherulite-like GeS holds the potential to achieve non-epitaxial 406 single-crystalline functional semiconductors on insulating substrates, as previously investigated Page 16 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596017407 for GeO2 [57], for the development of next-generation GeS FETs (e.g., in-memory sensing and 408 computing devices) with the potential advantage of creating programmable FETs via all-optical 409 manipulation.410411 4. EXPERIMENTAL METHODS412 GeS thin films are grown on insulating substrates such as 300 nm-SiO2/Si and quartz 413 using a horizontal quartz tube furnace with two independently-controlled heating regions. 414 Approximately 20 mg GeS powder (99.99%, Sigma-Aldrich) is placed in a quartz boat in the 415 upstream heating region. The SiO2/Si (13 mm × 13 mm) and quartz (15 mm × 15 mm) substrates 416 are cleaned by sonication in acetone for 15 min, then by repeated rinsing in isopropyl alcohol 417 and deionized water. The substrates are then placed in the quartz tube in the downstream heating 418 region with a tiled susceptor in a similar way to that used in our previous investigation [34]. The 419 tube is evacuated to 0.015 Pa, followed by the supply of high-purity Ar carrier gas to reach the 420 base pressure of 90 Pa by adjusting the needle valve in the gas feed-in line. Fluxed Ar gas for 421 cleaning the growth environment of the quartz tube is maintained for at least 10 min. The growth 422 temperature of the upstream heating region is increased to 440 ˚C, and that of the downstream 423 heating region is increased to 420 ˚C 20 min later [34]. The three sets of pressures for the pre-424 deposited amorphous GeS layer are 6500, 7000, and 7500 Pa to grow GeS thin films about 60, 425 35, and 20 nm thick, respectively. The growth pressure is then maintained at around 6500 Pa by 426 adjusting the vacuum valve in the exhaust line. The growth times are set to at 3, 5, and 15 min to 427 evaluate the lateral growth and fully cover the substrates with GeS thin films. After growth, the 428 quartz tube is cooled down rapidly to stop the growth process as soon as possible. Aluminum 429 cut-wires (99.999% purity) are evaporated from an alumina-coated tungsten wire crucible and 430 deposited on the SiO2/Si and quartz substrates using a thermal evaporation apparatus with a 431 heating stage. The deposition rate for various Al thicknesses is calibrated as 0.5 Å/sec using a 432 quartz crystal sensor. 5 nm-thick Al is deposited at room temperature (RT) and 20 nm-thick Al is 433 deposited at both RT and at the calibrated temperature of 120 ˚C.434 Scanning electron microscopy (SEM) using a Hitachi SU8230 mounted with an energy-435 dispersive X-ray spectroscopy (EDX) detector were employed to investigate the defect structures Page 17 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596018436 of grown GeS thin films. A Nikon Eclipse ME600 optical microscope with an angle-resolved 437 polarizer was utilized to investigate the birefringent properties of the Al-catalyzed grown GeS 438 thin films. The Raman microscopy observations were conducted at an excitation wavelength of 439 532 nm at a low laser power of about 20 µW to prevent any damage to the crystallized GeS. A 440 PANalytical X’Pert PRO MRD X-ray diffractometer (XRD) with a Cu K𝛼 source were utilized 441 to collect XRD data in the range of 2𝜃 from 10˚ to 40˚ in 0.01˚ steps. Transmission electron 442 microscopy (TEM) operated at 200 kV with an EDX detector and selected area electron 443 diffraction (SAED) in an FEI Talos F200X field emission microscope was utilized to investigate 444 the crystallinity of the grown GeS thin films. The surface conditions were inspected using 445 Hitachi atomic force probe equipment and an SII SPA 400. Electron backscatter diffraction 446 (EBSD) measurements were conducted using a JEOL JSM-7900F with an EBSD detector.447448 ACKNOWLEDGMENTS449 This work was supported by JSPS Kakenhi (Grants no. JP20K14796, JP23K13370 and 450 JP24KF0164). Part of this work is also supported by the Advanced Research Infrastructure for 451 Materials and Nanotechnology in Japan (ARIM), and MEXT Japan, proposal numbers 452 24NM5064 and 24UT1037. The author is grateful for helpful conversations with Prof. Dr. 453 Nagashio of the University of Tokyo, and Drs. Tsukagoshi and Da of the National Institute for 454 Materials Science, which brought birefringence and spherulite to the authors’ attention.455456 AUTHOR INFORMATION457 Corresponding Author458 FUKATA.Naoki@nims.go.jp459 ZHANG.Qinqiang@nims.go.jp460 Present Addresses461 † Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials 462 Science (NIMS), Tsukuba, Ibaraki 305-0044, JapanPage 18 of 33ACS Paragon Plus EnvironmentACS Applied Nano Materials12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596019463 ‡ Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-464 8573, Japan465 Author contributions466 The manuscript is written with contributions from all the authors. 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