Solid Phase Epitaxy of Single Phase Two-Dimensional Layered InSe Grown by MBE

Abstract

Single-phase two-dimensional (2D) indium monoselenide (γ-InSe) film is successfully grown via solid phase epitaxy in the molecular beam epitaxy (MBE) system. Having high electron mobility and high photoresponsivity, ultrathin 2D γ-InSe semiconductors are attractive for future field-effect transistor and optoelectronic devices. However, growing single-phase γ-InSe film is a challenge due to the polymorphic nature of indium selenide (γ-InSe, α-In₂Se₃, β-In₂Se₃, γ-In₂Se₃, etc.). In this work, the 2D α-In₂Se₃ film was first grown on a sapphire substrate by MBE. Then, the high In/Se ratio sources were deposited on the α-In₂Se₃ surface, and an γ-InSe crystal emerged via solid-phase epitaxy. After 50 min of deposition, the initially 2D α-In₂Se₃ phase was also transformed into a 2D γ-InSe crystal. The phase transition from 2D α-In₂Se₃ to γ-InSe was confirmed by Raman, XRD, and TEM analysis. The structural ordering of 2D γ-InSe film was characterized by synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS).

1. Introduction

Recently, two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted strong interest from the scientific community because of their unique electrical and optical properties, making them promising candidates for future electronic and optoelectronic applications [1,2]. In addition to 2D TMDs, 2D III-VI metal chalcogenides such as In₂Se₃ and InSe were also shown potential applications in electronic devices [3,4,5] and optoelectronic devices [6,7,8]. Particularly, InSe with a six monolayer device exhibiting ultra-high mobility of 1000 cm²V⁻¹s⁻¹ at room temperature reported by Denis A. Bandurin et al. [9] shows a promising candidate to surpass silicon technology in the future. Although the exfoliation methods have been widely used to investigate 2D III-VI semiconductors’ exceptional electrical properties, synthesizing large-area high-quality layered materials still requires bottom-up strategies, which are more suitable to be realized for industrial applications. To date, epitaxial growth by chemical vapor deposition (CVD), pulsed laser deposition (PLD), or molecular beam epitaxy (MBE) has been used for the synthesis of 2D III-VI metal chalcogenides [10,11,12,13,14,15]. However, the synthesis of single-phase indium selenide is still a challenge by chemical vapor deposition (CVD). Because indium selenide has multiple structures (γ-InSe, α-In₂Se₃, β-In₂Se₃, γ-In₂Se₃, etc.) that co-exist at room temperature [16,17,18]. To be implemented for industrial applications, the growth of indium selenide film with a single-phase conducted by precise control of In and Se sources is required. Among the techniques used to fabricate 2D semiconductors, MBE is widely considered one of the most competitive, precisely controlling the ultra-low growth rate and making the grown film clean for device fabrication [19,20,21].

In this study, the MBE system successfully demonstrated the epitaxial 2D γ-InSe thin film via solid-phase epitaxy. The Raman, X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HR-TEM) image show that the 2D α-In₂Se₃ fully converted to 2D γ-InSe. We believe that the 2D γ-InSe growth mechanism is solid-phase epitaxy due to similar in-plane lattice and space groups between 2D α-In₂Se₃ and 2D γ-InSe.

2. Materials and Methods

Indium selenide films were deposited on c-plane sapphire substrates using a homemade MBE system in an ultra-high vacuum chamber with a base pressure of 2 × 10⁻⁸ Torr. In and Se sources were evaporated using Knudsen cells (K-cells), and the growth processes were monitored using in-situ RHEED(R-DEC Co., Ltd., Ibaraki, Japan). Before the deposition, the sapphire substrates were degreased in acetone by ultrasonic cleaning for 2 min, and then a mixture of phosphoric acid and sulfuric acid of 1:3 for 15 min. Finally, the substrates were immersed in deionized water for 2 min. After the chemical cleaning process, the substrates were introduced to the growth chamber, and the sapphire substrate was thermal cleaned at 650 °C for 30 min. To identify the growth conditions for α-In₂Se₃ growth, the K-cells temperature of In (T_In) was varied at 720 °C, 730 °C, 740 °C, and 750 °C, and Se (T_Se) was fixed at 198 °C, denoted In/Se ratio 0.05, 0.06, 0.07, and 0.11, respectively. Detailed calculation of In/Se ratio has been shown in Appendix A. The growth time of all samples was 2 h and the growth process schematic was shown in Figure 1a. The optimal α-In₂Se₃ film was achieved with In (T_In) at 740 °C and substrate temperature (T_sub) of 560 °C, resulting in growth rates of 0.58 nm/min. After having single-phase α-In₂Se₃ film, the In and Se sources were deposited on the α-In₂Se₃ surface by closing the Se shutter as a high In/Se ratio at T_In of 740 °C, T_Se of 198 °C, T_sub of 560 °C The crystallographic properties of the α-In₂Se₃ and γ-InSe were examined using XRD (D8, Bruker corp. Billerica, MA, USA) system and synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS) at Taiwan Photon Source (TPS) TPS25A (National Synchrotron Radiation Research Center, Hsinchu, Taiwan). The GIWAXS data were collected with an area detector. High-resolution transmission electron microscopy (HR-TEM) (ARM200F, JEOL Ltd., Tokyo, Japan) was used to probe the α-In₂Se₃ and γ-InSe microstructure and obtain atomic-scale images. The Raman spectra were obtained using a 532 nm wavelength of solid-state laser (70 mW) as the excitation source and a triple grating spectrometer (iHR-550, HORIBA, Ltd., Kyoto, Japan) system as a signal detector.

3. Results and Discussion

Figure 1b presents the in-situ RHEED patterns of In₂Se₃ with T_In varied from 720 °C to 750 °C, and T_Se fixed at 198 °C. As seen in the RHEED pattern, the stripe patterns were obtained along with [10$\overline{1}$0] and [11$\overline{2}$0] azimuth, indicating that In₂Se₃ was in layer-by-layer growth mode. When the T_In was lower than 740 °C, both the ring and streaky lines were exhibited in the RHEED pattern, indicating the layered and polycrystalline structures of In₂Se₃ were formed. Raman spectroscopy and XRD were used to identify the various indium selenide structures [22,23,24,25,26,27]. Figure 1c shows the Raman spectra of In₂Se₃ thin film on the sapphire substrate grown by T_In varied from 720 °C to 750 °C. The vibration modes of In₂Se₃ at T_In at 740 °C exhibit three peak features at 112, 176, and 207 cm⁻¹, which were characteristic of α-In₂Se₃ [11,12,22,23,24,25]. When the T_In was up to 750 °C, the vibrational mode appeared with two additional peaks at 146 cm⁻¹ and 244 cm⁻¹, which were characteristic of γ-In₂Se₃ [11,12,22,23]. Figure 1d shows the XRD θ-2θ scans of In₂Se₃ grown by T_In varied from 720 °C to 750 °C. The (002) and (004) α-In₂Se₃ diffraction peaks are at 9.43° and 18.89°, and the peak of (006) γ-In₂Se₃ is at 27.03°. As T_In at 740 °C, the optimal α-In₂Se₃ films were achieved because of the relatively high diffraction intensity and narrower full width at half maximum (FWHM)of 0.16° of the α-In₂Se₃ peaks. The crystal grain size is estimated as 52.6 nm from the Scherrer equation. However, the T_In at 750 °C leads to the co-existence of both α-In₂Se₃ and γ-In₂Se₃ structures, which confirmed our Raman analysis that the film grown by T_In set at 750 °C had both α-In₂Se₃ and γ-In₂Se₃ vibration modes.

To gain more insight into the α-In₂Se₃ crystal configurations and microstructure, one may examine the streaky line spacing in the RHEED pattern. Figure 2a,b shows the RHEED pattern and intensity profile of optimal α-In₂Se₃ films, and $\stackrel{\rightharpoonup }{a}$ and $\stackrel{\rightharpoonup }{m}$ are (10$\overline{1}$0) and (11$\overline{2}$0). The calculated $\stackrel{\rightharpoonup }{a}/\stackrel{\rightharpoonup }{m}$ is close to $\sqrt{3}$, where the $\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{a}$ spacing is 678 pixels and $\stackrel{\rightharpoonup }{m }$−$\stackrel{\rightharpoonup }{ m}$ spacing is 1200 pixels, revealing that the α-In₂Se₃ crystalline layer is six-fold symmetry on the sapphire substrate. Figure 2c,d shows the SEM image and cross-sectional HR-TEM image of the α-In₂Se₃ microstructure. The shape of the α-In₂Se₃ grains exhibited a hexagonal pattern, and the microstructure of α-In₂Se₃ exhibited a layered structure on the sapphire, where the α-In₂Se₃ monolayer thickness is 1.2 nm. The growth mechanism of the α-In₂Se₃ on the sapphire substrate may be similar to quasi-van der Waals (vdW) epitaxy [28,29,30].

From the above observations, we found that the single-phase α-In₂Se₃ could rely on simply controlling In/Se ratio, and we noted that the in-plane lattice of γ-InSe (4.01 Å) is close to that of α-In₂Se₃ (4.03 Å) and has the same space groups. Therefore, we proposed the solid-phase epitaxy method to grow single-phase γ-InSe films, which uses a single crystalline α-In₂Se₃ film as a template, followed by depositing In and Se sources for γ-InSe crystal growth on the α-In₂Se₃ and then heating the film to crystallize it. Figure 3a shows the process flow of InSe growth via solid-phase epitaxy. The optimal α-In₂Se₃ films were achieved with T_In set at 740 °C and a growth temperature of 560 °C. Then, the T_In and T_Se were kept, and the K-cell of the Se shutter was closed to become a high In/Se ratio with deposition times of 10 min to 50 min. Figure 3b presents the Raman spectra of InSe growth with high In/Se ratios for various deposition times. The vibration modes of γ-InSe appeared on the α-In₂Se₃ surface after the high In/Se ratio was deposited for 10 min, but when the deposition time reached 50 min, only the γ-InSe vibration signal remained. Figure 3c shows the XRD θ-2θ scans of InSe growth with high In/Se ratios for 10 min to 50 min deposition times. The (002) and (004) γ-InSe diffraction peaks emerged while the deposition time exceeded 10 min, and the peak intensity of γ-InSe gradually increased with the deposition time. In contrast, the peak intensity of α-In₂Se₃ gradually decreased with the deposition time. When the deposition time reached 50 min, the sample had only γ-InSe peaks. The FWHM of (004) γ-InSe peaks is around 0.36°, and the grain size is estimated as 23.5 nm. The Raman and XRD results show that solid-phase epitaxy on the α-In₂Se₃ surface effectively grows single-phase γ-InSe. Interestingly, α-In₂Se₃ also phase transitions to γ-InSe, probably because of the high diffusivity of Indium. Figure 3d presents the RHEED pattern of γ-InSe with high In/Se ratios for 50 min deposition time. The inset of Figure 3d shows the intensity profile of γ-InSe. In addition to showing the stripe pattern of γ-InSe in the RHEED pattern, the $\stackrel{\rightharpoonup }{a}$−$\stackrel{\rightharpoonup }{a}$ and $\stackrel{\rightharpoonup }{m}$−$\stackrel{\rightharpoonup }{m}$ spacing of γ-InSe are 672 pixels and 1150 pixels, which is similar to that of α-In₂Se₃ in Figure 2b. This confirmed the similar lattice spacing of γ-InSe and α-In₂Se₃. Figure 4a shows the cross-sectional HR-TEM image of γ-InSe with high In/Se ratios for 50 min deposition time. The microstructure of γ-InSe exhibited a layered structure and well-defined crystal lattice, in which the monolayer thickness of 0.8 nm is only two-thirds of α-In₂Se₃. Even the α-In₂Se₃, initially on the sapphire substrate, was transformed into the γ-InSe phase. To verify these local microstructure observations, we utilized GIWAXS measurement to investigate the structure of low-dimension materials and provide information on the structural morphology of an entire sample [31,32]. Figure 4b presents the 2D reciprocal space map of the γ-InSe films via the solid-phase epitaxy. The lateral face of (10$\overline{1}$0) γ-InSe is at q_r = 1.88 Å⁻¹. Since the γ-InSe is layered in nature and the crystal dimension is confined along the out-of-plane direction, the γ-InSe shows a vertical stripe pattern along the crystal truncation rod (CTR) in reciprocal space. Figure 5 illustrates the growth of γ-InSe via solid-phase epitaxy as a model. The single-phase α-In₂Se₃ film grown via MBE system at 560 °C, followed by the high In/Se ratio deposition on the α-In₂Se₃ film, the films underwent a thermodynamic transformation into γ-InSe crystal growth on the α-In₂Se₃ surface. The formation energy for InSe and In₂Se₃ are −0.527 and −0.322 eV/atom [33,34]. The practical indium selenide growth requires kinetic considerations such as growth temperature and concentration of sources [22]. During the γ-InSe crystal growth as a function of deposition times, the transformation mechanism involved the α-In₂Se₃ to the γ-InSe transition, and the whole structure finally became a single-phase γ-InSe structure.

4. Conclusions

This work demonstrated the single-phase γ-InSe epitaxy via solid-phase epitaxy in an MBE system. The α-In₂Se₃ was first grown on a sapphire substrate by MBE as a template, and then the γ-InSe was formed by depositing In and Se at a high In/Se ratio on the α-In₂Se₃ films. The growth mechanism of γ-InSe was facilitated by a close in-plane lattice match between the α-In₂Se₃ and γ-InSe. Raman and XRD measurement revealed that γ-InSe emerged on the α-In₂Se₃ structure, and the α-In₂Se₃ phase transformed into γ-InSe during the high In/Se ratio deposition. Furthermore, HR-TEM and GIWAXS measurements showed that the whole structure finally becomes the single-phase 2D γ-InSe structure. The solid-phase epitaxy method opens the door to the development of 2D materials and the possibility of making more 2D materials available for electronic device applications.