Research

Large scale fabrication and characterizations of 2D layered metal dichalcogenides towards device applications

We have been working on the project on the growth and characterization of transition metal dichalcogenides and oxides. We have successfully fabricated 2D single crystalline like Niobium disulfide (NbS₂), Sn disulfide (SnS₂) and Molybdenum disulfide (MoS₂) using Physical vapor growth using magnetron sputtering system and high vacuum chamber and also CVD method to synthesize in a programmable quartz tube furnace by co-evaporating the precursors to form the ultrathin transition metal disulfide films. For structure and texture analysis we used grazing incidence X-ray di.raction (GIXRD) and Reflectron High Energy Electron Diffraction (RHEED) pole figure analysis. For other characterizations, X-ray photoelectron spectroscopy (XPS), Raman scattering, atomic force microscopy (AFM), and ultraviolet-visible (UV-VIS) spectroscopy were used. The layered metal dichalcogenides .lms with intrinsic optical band gaps in the range of 1 to 3 eV have the potential applications in optoelectronics and photovoltaics. SnS_x is one of these promising materials that possess the advantages of non-toxic to the environment and earth-abundant. The SnS .lm has an optical band gap of 1.0 to 1.5 eV with a high absorption coefficient and can be used as an absorber in solar cells. The SnS₂ .lm has an optical band gap from 2.0 to 2.5 eV and has been demonstrated to have a fast photocurrent response time in a few micro sec. Recently we also have synthesized layered single phase SnS and SnS₂ on low-cost amorphous quartz and glass substrates. We can tune the phase and bandgap of SnSx films by controlling the thermodynamic parameters. The photo-response and other electronic characterizations were studied. We are also making the heterostructures of layered materials and in the process of a device application. Apart from the Transition metal disulfide layered materials, I have also worked in synthesizing the wide band gap oxide semiconductor like Nb₂O₅ and MoO₃.

Hetero-epitaxial growth of two-dimensional Transition metal Oxide layer

Two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (TMDs), have attracted extensive attention due to their unique physical properties. Nevertheless, 2D transition metal oxides (TMOs) have been less explored compared to other 2D materials due to lack of production method for layered oxides. However, it is expected that the TMOs have remarkable properties that are beneficial for electronic applications. Generally, they have wider band gap energies and low carrier concentrations in their stoichiometric states. In addition, large dielectric constant (κ) of TMOs can reduce the Coulomb scattering e.ect and their electronic properties can be manipulated by accommodation of ionic dopants. Among them, molybdenum trioxide (MoO₃) is one of the promising 2D oxides, which is a chemically stable n-type semiconductor with a high dielectric constant. We have demonstrated that layered α-MoO₃ films can be synthesized on other 2D layered substrates by van der Waals epitaxial growth, leading to good interfacial texture and fewer defects. We fabricated highly oriented α-MoO₃ layers (ranging from bilayers to few layers) on hexagonal Boron Nitride (hBN) and Graphene substrates using a cost-effective modified hot-plate method. The formed layers have a good texture having nearly single crystalline large grains with both out-of-plane and in-plane symmetry. Our optical and structural analyses reveal that the grown MoO₃ layers have phase. The stoichiometric composition of α-MoO₃ layers was investigated to relate the various compositional changes to the electrical and optical properties. The I-V characteristics for the MoO₃-hBN and MoO₃-Graphene heterostructures measured by conductive atomic force microscopy (C-AFM) showed a great potential of α-MoO₃ layers for electronic applications, which indicates that α-MoO₃ layers can be used as tunneling barriers, dielectrics, and channel materials for high power electronics. Our work shows a new way to fabricate 2D oxide layers, which can be categorized into new 2D family. Our finding may pave a way toward 2D-oxide-based electronics.

Metal enhanced Ge_1-xSn_x direct band gap alloy film growth on glass substrates using biaxial CaF₂ buffer layer

The Ge_1-xSn_x alloy film is an attractive candidate for silicon-based optoelectronic devices with active Ge layers. Ge has an indirect bandgap of 0.67 eV at room temperature and Sn as a metal has a bandgap of 0 eV. Ge_1-xSn_x alloys may bridge the need for small bandgap materials. Previous experimental work has shown that the optical energy gap of Ge can be tuned from 0.679 eV to 0.473 eV by varying the Sn concentration x ranging from 0.02 to 0.14 in Ge_1-xSn_x alloy films. In addition, the observed indirect to direct bandgap transition occurs for x ranging from 0.10 to 0.13. Besides the potential application in optoelectronics, Ge is a favored candidate for channel material used in the complementary metal oxide semiconductor. This is because of Ge's high electron mobility plus that the strain state of Ge_1-xSn_x can be adjusted by using a virtual substrate such as a Ge buffer layer on Si single crystal substrate. These potential applications have attracted intense interest in the synthesis of epitaxial Ge_1-xSn_x films on single crystal substrates. Researchers have grown Ge_1-xSn_x films on single crystal substrates of Si(100) or Ge(100) using molecular beam epitaxy, magnetron sputtering, or chemical vapor deposition. Various properties of these Ge_1-xSn_x films including their optical properties, optoelectronic properties, composition, and strain have been studied.

In this project we have reported a new way to grow biaxial Ge_1-xSn_x(111) alloy .film with a few percent Sn on a glass substrate instead of using a single crystal substrate at low temperatures. Ge_1-xSn_x alloyed films were grown on glass substrates by sequential physical vapor deposition of a biaxial CaF₂ buffer layer and a Sn heteroepitaxial layer at room temperature, followed by a Ge layer grown at low temperatures (200 - 350 .^oC). The predeposited Sn on the CaF₂ layer enhances the Ge diffusion and crystallization. The Sn substitutes into the Ge lattice to form a biaxial Ge_1-xSn_x alloyed film. The epitaxy relationships were obtained from x-ray pole figures from the samples with Ge_1-xSn_{x <-1 01 > ||} CaF₂ <-1 01 > and Ge_1-xSn_{x <-1 1 0> ||} CaF₂ <-1 1 0>. The crystallization and biaxial texture formation start at about 200 .C with the best biaxial Ge_1-xSn_x film grown at about 300 .C. The microstructure, texture and Sn concentration of the Ge_1-xSn_x films were characterized by X-ray diffraction, X-ray pole figure analysis, and transmission electron microscopy. The spatial chemical composition of Sn in Ge_1-xSn_x was measured by energy dispersive X-ray spectroscopy and was found nearly uniform throughout the thickness of the alloyed film. Raman spectra show shifts of Ge-Ge, Ge-Sn, and Sn-Sn vibration modes due to the percentage change of substitutional Sn in Ge as a function of growth temperature. This growth method is an alternative cost effective way to grow biaxial semiconductor films on amorphous substrates.

Figure 3: Schematic of the Oblique-angle PVD and fabrication of Ge-Sn alloy film.

Growth of Si-Ge Nanostructures on High Index Silicon surfaces using Molecular Beam Epitaxy and their Characterizations

This work was mainly focused on the molecular beam epitaxy (MBE) growth of self-assembled aligned Si1-xGex nanostructures on High index Silicon surfaces like Si(5 5 12), Si(5 5 7) and Si(5 5 3).The anisotropic surface reconstruction inherent to high-index silicon surfaces makes them potentially significant substrates for electronic device fabrication. These surfaces, consisting of periodic steps and terraces, have attracted renewed attention as templates for the controlled growth of aligned one-dimensional (1D) nanostructures. The implementation of Ge nanostructures into Si-based devices is of great potential for future high-speed devices, due to advantages like enhanced carrier mobilities and smaller band gap. Due to the indirect band gap, Si devices are not well suited for optoelectronic applications. New devices employing Ge/Si epitaxial layers are expected to overcome this restriction.

Si_1-xGe_x alloys show smaller fundamental band gaps compared to Si, because of larger lattice constant and altered lattice constituents and, due to the acquired tetragonal symmetry in pseudomorphic layers. Strained Si-Ge technology has been recognized as a promising solution for high-performance devices, because of its cost-effectiveness and high carrier mobility. In my thesis work, we showed that self-assembled growth at optimum thickness of the over layer leads to interesting shape transformations, namely, from nanoparticle to trapezoidal structures at higher thickness values. Thin films of Ge of varying thicknesses were grown under ultra-high vacuum conditions on a Si(5 5 12), Si(5 5 7) and Si(5 5 3) surfaces at a substrate temperature of 600 ^oC. The substrate heating was achieved by passing direct current (DC) through the samples. Well aligned trapezoid structures are found along step edge direction <1 -1 0>. A comparative study on the shape evolution of MBE grown Si_1-xGe_x islands on ultraclean reconstructed high index Si(5 5 12), Si(5 5 7) and Si(5 5 3) surfaces has been carried out experimentally and explained using a phenomenological kinetic Monte Carlo (kMC) simulation. We observe universality in the growth dynamics in terms of aspect ratio and size exponent, for all three high index surfaces, irrespective of the actual dimensions of Ge-Si structures. Scanning transmission electron microscopy (STEM) measurements suggested the mixing of Ge and Si. This has been confirmed with a quantitative estimation of the intermixing using high-resolution X-ray diffraction (HRXRD) and Rutherford backscattering spectrometry (RBS)

Figure 4: (a) Schematic of a vicinal surface and (b) crystallographic view of high index planes

• Heterostructure of 2D Layered Materials:

Based on my past research work during Ph.D. and postdoc, I had ample opportunities in handling various growth techniques including Molecular beam epitaxy (MBE) under ultra-high vacuum, oblique angle Physical vapor deposition (PVD), Chemical vapor deposition (CVD) and magnetron sputtering deposition. I have successfully fabricated and characterized layered materials NbS₂, MoS₂, SnS₂, etc during my postdoc period at RPI, USA and Yonsei University, South Korea. My past experiences will help me in contributing towards the future research projects on 2Dlayered materials. My main goal will be to grow large area 2D TMDC materials and make their heterostructures for device applications. My plan also includes the texture analysis in the layered structures for a .ne tuning in the crystal orientation. As I am trained in the characterization techniques like Scanning electron microscopy (SEM), Atomic Force Microscopy (AFM), X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and Raman scattering, I can analyze the structure, texture, morphology and compositions of the grown layered materials. I will also be able to work on the I-V and photo response measurements in this kind of materials. I aim to fabricate a contamination free hetero stacking of 2D TMDC and study the interface which may allow us to discover new physical phenomena. I will try to control the defects in grain boundary which is an important factor for the carrier mobility. The controlled fabrication methods and systematic transfer of the layered materials will lead us to design flexible and transparent electronic devices.

• New 2D Thermoelectric Materials for future energy:

The search for cleaner, more sustainable energy sources is an ever-growing global concern because of escalating energy costs and global warming associated with fossil fuel sources. With about two-thirds of all used energy being lost as waste heat, there is a compelling need for high-performance thermo-electric materials that can directly and reversibly convert heat to electrical energy. Among the viable technologies for this purpose, thermoelectric (TE) energy converters are of increasing interest because these solid-state devices can transform heat given o. from sources such as power plants, factories, motor vehicles, computers or even human bodies into electric power using the Seebeck e.ffect. The many advantages of this energy-conversion phenomenon include solid-state operation, the absence of toxic residuals, vast scalability, maintenance-free and a long life span of reliable operation. Conversely, solid-state thermoelectric devices can also change electrical energy into thermal energy for cooling or heating using the Peltier e.ffect. My future research includes the synthesis of new 2D metal chalcogenide and oxide materials for a reliable high e.eciency thermoelectric device. I have started some preliminary studies on 2D transition metal oxides and their heterostructures with other 2D semiconductors.