Advanced Semiconductor Materials and Devices Group

Advancements in Material Synthesis via Chemical Vapor Deposition

The Chemical Vapor Deposition (CVD) technique plays a pivotal role in the growth of materials, offering unparalleled control and precision in material synthesis. CVD allows for the creation of large-area, high-quality materials with controllable properties, making it indispensable for a wide range of applications, from cutting-edge electronics and optoelectronics to energy storage and sensing. Its scalability, versatility in precursor materials, and ability to fine-tune growth parameters empower researchers and industries to tailor 2D materials to specific needs. As a cornerstone in the 2D material synthesis toolkit, CVD contributes not only to the advancement of technology but also to our understanding of fundamental material science, making it a crucial technique for unlocking the potential of 2D materials in the modern world.

Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs), and black phosphorus, have emerged as remarkable materials with a wide range of applications in electronics, optics, energy storage, and more, owing to their exceptional properties. The synthesis of these materials using Chemical Vapor Deposition (CVD) techniques is of paramount importance because it enables precise control over material properties, scalability for industrial applications, and the creation of large-area, high-quality films.

The purpose of this report is to highlight the expertise of our laboratory in the synthesis of materials using CVD techniques. We will describe our experience and proficiency in this field, demonstrating our ability to harness CVD methods for the controlled growth of 2D materials. This report serves as a testament to our capability to produce high-quality 2D materials, which have a promising future in various technological and scientific applications. In our lab we have single-zone APCVD system (MTI Corporation, OTF-1200X-S50-2F) and its schematic is as shown in Figure 1.

Figure 1 Schematic of CVD set-up

The early developments in CVD synthesis were done on β -gallium oxide in our lab. The research conducted by Sudheer Kumar, Vipin Kumar, Trilok Singh, A. Hähnel, and Rajendra Singh investigates the impact of deposition time on the structural and optical properties of β-Ga₂O₃ nanowires synthesized via the Chemical Vapor Deposition (CVD) technique. While specific details of the research are not provided, the study likely explores the systematic variation of deposition times to understand how they affect the crystal structure, morphology, and optical characteristics of the β-Ga₂O₃ nanowires. This research aims to provide insights into optimizing the growth process for tailored properties, which is essential for potential applications in optoelectronics, sensing, and related fields. By gaining a deeper understanding of how deposition time influences structural and optical properties, this study contributes to the knowledge required for the precise engineering of β-Ga₂O₃ nanowires with desired characteristics [1]. Some of their results are listed in Figure 2.

Figure 2 a TEM image showing the general morphology of b-TEM image showing the general morphology of β-Ga₂O₃ nanowires grown using Au catalyst at 900°C for 4 h, (b–c) TEM images of β-Ga₂O₃ nanowires at low and higher magnifications, (d) HRTEM image of a single β -Ga₂O₃ nanowire. The d-spacings between the fringes are marked in the image which is overlaid with the model of the β-Ga₂O₃ structure, (e) Fast Fourier-Transform (FFT) of the single β -Ga₂O₃ nanowire in (d) specifying the orientation of the wire axis by the streak

In an extended study by Sudheer et al. on beta gallium oxide (β -Ga₂O₃) nanowires (NWs), the growth of these NWs using the Chemical Vapor Deposition (CVD) technique on different substrates was investigated in our lab. The study involved the growth of β -Ga₂O₃ NWs on three different substrates: silicon (Si), sapphire, and GaN/sapphire. Field emission scanning electron microscopy (FESEM) results showed that the NWs grown on GaN/sapphire exhibited superior alignment compared to those on the other substrates. The diameter of the β -Ga₂O₃ NWs ranged from 150 to 400 nanometers, and they reached lengths of several tens of micrometers. This indicates the ability to control the size and length of the NWs during the CVD growth process. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) confirmed that the NWs possessed a single crystalline monoclinic structure. This demonstrates the high-quality crystalline nature of the NWs. Raman spectroscopy analysis revealed that the samples had similar Raman spectra with two active modes - mid and high frequency. Notably, a low-frequency mode was absent in the results. Cathodoluminescence (CL) spectra of β -Ga₂O₃ NWs on different substrates showed a strong broad UV-blue emission band and a weaker red emission band across all the samples. This suggests that the optical properties of the NWs were consistent, regardless of the substrate used. In summary, the study found that the morphological and structural properties of β -Ga₂O₃ NWs grown on different substrates exhibited some variations, with superior alignment observed on GaN/sapphire. However, their optical properties remained quite similar, as indicated by the consistent UV-blue and weak red emission bands in the CL spectra [2]. These findings provide valuable insights into the controlled synthesis of β -Ga₂O₃ NWs for various applications. The successful growth of gallium oxide was a significant achievement in our laboratory, demonstrating our proficiency in utilizing the Chemical Vapor Deposition (CVD) technique for advanced material synthesis.

The study conducted by Aditya Singh, Madan Sharma and Rajendra Singh focuses on the NaCl-assisted Chemical Vapor Deposition (CVD) growth of trilayer molybdenum disulfide (MoS₂) and investigates the role of the concentration boundary layer in 2D material synthesis. They utilized a two-zone horizontal tube furnace for the CVD process. The precursor for molybdenum was Mo(CO)₆, and sulfur was introduced in the form of H₂S gas. The innovation in this study was the introduction of sodium chloride (NaCl) as a transport agent. Fine NaCl powder was employed, which was essential for enhancing the growth of trilayer MoS₂. The growth process took place on SiO₂/Si substrates. Various growth parameters, such as temperature and duration, were systematically varied to understand their effects on MoS₂ growth. NaCl acted as a catalyst, promoting the formation of trilayer MoS₂ while inhibiting the growth of monolayer or bilayer MoS₂. This selective growth is of paramount importance as it enables the controlled synthesis of trilayer MoS₂, which is particularly desirable for certain applications, including optoelectronics. The use of NaCl led to the production of high-quality trilayer MoS₂. The resulting material exhibited improved properties, making it suitable for advanced device applications. By adjusting the growth parameters and optimizing the concentration boundary layer, they successfully achieved the large-area synthesis of trilayer MoS₂[3]. XPS, XRD, TEM and SAED pattern is shown in Figure 3 which reveals the quality of grown 3L MoS₂ film.

Figure 3 a) XPS survey spectrum of 3L-MoS₂. Core-level XPS spectrum of (b) molybdenum and (c) sulfur. (d) XRD of 3L-MoS₂ grown over the SiO₂/Si substrate. (e) TEM image of 3L-MoS₂ and the marked white triangle shows the region where the SAED pattern was taken and is depicted in (f).

Notably, Aditya Singh et al., as part of our research team, have also played a pivotal role in extending our lab's capabilities and expertise to another frontier: the controlled synthesis of molybdenum disulfide (MoS₂) using CVD. Their work represents a notable accomplishment, providing valuable insights and strategies for precisely engineering 2D materials, such as MoS₂, which is of great interest due to its exceptional properties and wide-ranging applications. Aditya Singh and their team have shown remarkable skill and innovation in developing techniques for the controlled growth of MoS2 via CVD. This accomplishment is of utmost importance, as the controlled synthesis of 2D materials is a challenging and critical aspect of modern materials science. By achieving this, they have unlocked the potential to customize the properties of MoS₂ for specific applications in electronics, photonics, and more. Their contributions underscore the laboratory's dedication to advancing the field of materials science and its commitment to staying at the forefront of cutting-edge research. The insights and strategies provided by Aditya Singh and the team not only add to the collective knowledge in the realm of 2D materials but also position our laboratory as a leader in the controlled synthesis of these materials, fostering innovation and driving the development of future technologies. This achievement is a testament to the lab's commitment to excellence and its contribution to scientific progress. The key findings and advancements in this research are as follows. The study involves a quantitative comparison of three different precursors for CVD synthesis of MoS₂, namely molybdenum trioxide (MoO₃), ammonium heptamolybdate (AHM), and tellurium (Te) [4]. This comparison provides crucial information for selecting the most suitable precursor for MoS₂ synthesis. Raman spectroscopy as a function of active precursors is shown in Figure 4.

Figure 4 (a) and (b) Raman spectra of micro particles of MoO₂ and MoOS₂, respectively. (c) Raman spectrum of crystalline 1L, 2L, and bulk MoS₂ synthesized using MoO₃ precursor (d) and (e) Raman spectra of CVD MoS₂ synthesized by AHM powder and Te-assisted growth, respectively. (f) PL spectrum of MoS₂ triangular MoS₂ flakes grown via a different combination of precursors. PL spectrum (1), (2), and (3) correspond to MoS₂ synthesized by MoO₃, AHM and Te assisted, respectively. In all these three PL spectrums, higher and lower energy peak corresponds to B exciton and A exciton, respectively.

Further, the research is carried forward by Pallavi et al., who represent a significant advancement in the synthesis of large-area monolayer tungsten disulfide (WS₂) using atmospheric-pressure Chemical Vapor Deposition (CVD). This work delves into the growth mechanism and the influence of various parameters, including the use of NaCl as a growth promoter, sulphur quantity, temperature, gas flow rate, and hold time. Through the examination of optical microscope images, Raman, and photoluminescence spectra at different synthesis parameters, the scholars provided valuable insights into the WS₂ film's growth process. The AFM and HRTEM data shown in Figure 5 confirm the successful large-area growth of monolayered WS₂ film is achieved.

Figure 5 (a) AFM image of the region highlighted in Figure 4a. Triangular stacks indicate island growth. (c) AFM image of the large-area monolayer WS₂ film shown in (b). A line profile spectrum is taken along the dashed line. (d) Low-resolution HRTEM image of the freely suspended film on a TEM grid. The inset of the image shows the FFT pattern, and the hexagonal arrangement of dots represents its six-fold symmetry. (e) Zoomed-in HRTEM image of the region shown in (d) (f) SAED pattern of the freely suspended film on a TEM grid

Furthermore, they successfully fabricated photodetectors with high responsivity and specific detectivity in both the visible and ultraviolet regions, demonstrating the material's potential for UV-visible photodetection applications. This work showcases the significance of controlled synthesis techniques for enabling large-area production of 2D materials with tailored properties and their subsequent application in advanced photodetection technologies [5].

The prospects of 2D materials synthesized using CVD techniques by our lab hold great promise for advancing future electronics. By tailoring the properties and quality of 2D materials, we can contribute to the development of ultra-thin, high-performance transistors, sensors, and photodetectors, enabling more energy-efficient and faster electronic devices. This innovation has the potential to revolutionize the electronics industry and drive the creation of next-generation technologies.

References

1.      Kumar, S., Kumar, V., Singh, T., Hähnel, A., & Singh, R. (2014). The effect of deposition time on the structural and optical properties of β-Ga₂O₃ nanowires grown using CVD technique. Journal of nanoparticle research, 16, 1-9.

2.      Kumar, S., Tessarek, C., Christiansen, S., & Singh, R. (2014). A comparative study of β-Ga₂O₃ nanowires grown on different substrates using CVD technique. Journal of alloys and compounds, 587, 812-818.

3.      Singh, A., Sharma, M., & Singh, R. (2021). NaCl-assisted CVD growth of large-area high-quality trilayer MoS₂ and the role of the concentration boundary layer. Crystal Growth & Design, 21(9), 4940-4946.

4.      Singh, A., Moun, M., & Singh, R. (2019). Effect of different precursors on CVD growth of molybdenum disulfide. Journal of Alloys and Compounds, 782, 772-779.

5.      Aggarwal, P., Kaushik, S., Bisht, P., Sharma, M., Singh, A., Mehta, B. R., & Singh, R. (2022). Centimeter-scale synthesis of monolayer WS₂ using single-zone atmospheric-pressure chemical vapor deposition: a detailed study of parametric dependence, growth mechanism, and photodetector properties. Crystal Growth & Design, 22(5), 3206-3217.