Session Q20: Frontiers in Growth for Applications II

Controllable fabrication of wafer-scale MoS2 thin films and the flexible integrated circuit

As a new two-dimensional (2D) semiconductor material, monolayer molybdenum disulfide (MoS2) has wide application potential in advanced electronics technologies beyond silicon due to its atomic thickness and no dangling bond surface. In addition to the monolayer, multilayers MoS2 have narrower band gaps but improved carrier mobilities and current capacities. To realize the application of MoS2 in integrated circuits, the most important thing is the low power consumption and high performance of the device, which depends heavily on the quality of the material and the processing technology of the device. Controllable preparation of wafer-scale high-quality MoS2 films is the material basis for device applications, and higher requirements are also put forward for device manufacturing technology of ultra-thin two-dimensional materials. So far, high-quality monolayer MoS2 wafers have been available and various demonstrations from individual transistors to integrated circuits have also been shown. However, achieving high-quality multi-layer MoS2 wafers remains a challenge. Firstly, based on the self-developed oxygen-assisted chemical vapor deposition method, we have grown high-quality, uniform 4-inch wafer-scale single-layer and multi-layer MoS2 films via the layer-by-layer epitaxy process on sapphire substrates. The epitaxy leads to well-defined stacking orders between adjacent epitaxial layers and offers a delicate control of layer numbers up to six. Further, we developed a metal-buried gate combined with ultra-thin gate dielectric layer deposition process. By preferential processing of the gate electrode and deposited ultra-thin high-κ dielectric film on the gate electrode, we successfully reduce the high dielectric constant HfO2 gate dielectric layer thickness to 5 nm and the corresponding equivalent oxide thickness (EOT) to 1 nm. The FET devices on the prepared hard substrate have low power consumption, high current density and ultra-low leakage current with negligible hysteresis. At the same time, by optimizing the metal deposition process, the contact between the metal electrode and MoS2 can be made without any damage avoiding Fermi level pinning, and the contact resistance can be reduced to Rc< 600Ω ·μm. Then this process was applied to the manufacture of flexible devices. Due to the improvement of gate voltage regulation efficiency, the full-function large-scale flexible integrated circuit of MoS2 integrated on the flexible substrate can be operated at a voltage lower than 1V. Our research work provides a technical reserve for the development of 2D semiconductor-based integrated circuits in flexible portable, wearable, and implantable electronics.   

Growth of wurtzite AlScN thin films on commercial Silicon and SOI (111) substrates using PAMBE

AlScN is a rapidly developing ferroelectric nitride material for applications spanning wide-bandwidth filters, high-electron-mobility transistors, MEMS, among other things. However, a large lattice mismatch between AlScN and Silicon presents challenges for epitaxial integration of AlScN films with Silicon substrates.  We report the effects of temperature, Scandium composition and III-V ratio on the epitaxial growth of high quality AlScN monocrystalline thin films. By using a thin AlN seed layer, we are able to obtain 100-200nm AlScN films with rocking curve FWHM’s as low as 1800 arcsec, with low surface RMS roughness.   

Machine Learning-Assisted Analysis of Graphene Configurational Tuning and Atomic Patterning

The remarkable electronic, topological, and structural properties of graphene can be tuned by modifying configurations such as stacking, twist angle, defects, dopants, and strain. Using atomistic modeling and scanning transmission electron microscopy, we have studied the top-down atomic patterning of different dopants in graphene. Our results indicate that vacancy motion in bilayer graphene is constrained, which is supported by both experimental data and first-principles/molecular dynamics studies. Consequently, bilayer graphene emerges as an optimal template for property manipulation via doping. Understanding the relationship between different tuning parameters and desired properties is critical for practical applications of graphene. We used machine learning to predict these properties using high-throughput training datasets. Our study enhances our understanding of graphene’s subtle responses to different modifications.   

Synthesis of BaTiO3 Thin Films for Electro-Optic Applications

BaTiO3 thin films present exciting opportunities for use in photonics. In addition to their high electro-optic coefficient, BaTiO3 has a  high refractive index (~ 2.4) and wide bandgap (> 3 eV). We can integrate these films onto silicon based substrates to make integrated optical devices.  Using reactive-oxide molecular beam epitaxy (MBE), we synthesize high quality thin films of BaTiO3 on a number of oxide substrates. We explore the role of thin film deposition conditions and strain on the electro-optic properties. We also pursue the transfer of BaTiO3 to silicon substrates. We propose that this could be a potentially straightforward, scalable, and efficient method for the nanofabrication of BaTiO3 thin films for integration into optical devices.   

Solid Phase Epitaxy of SrRuO3 encapsulated by SrTiO3 membranes

Solid phase epitaxy (SPE) has been extensively studied for various thin film materials. This process involves annealing a pre-deposited amorphous layer on an epitaxial substrate, resulting in the formation of a single-crystalline thin film. Such films have found widespread use in industrial applications including metals and semiconductor thin films. In the case of complex oxides, achieving precise stoichiometric control during the growth of SPE-produced oxide thin films can be particularly challenging, especially when volatile phases are present at elevated temperatures.

In this study, we present our recent efforts to investigate the impact of encapsulation during the SPE process, using SrRuO₃ (SRO) as a model system. Amorphous SRO layers were deposited onto SrTiO₃ (STO) substrates at room temperature. Next, single-crystalline STO membranes were transferred onto the films, serving as encapsulation layers to prevent the evaporation of volatile species during the SPE process. Both unencapsulated and encapsulated SRO layers were successfully crystallized via SPE. However, we observed that the unencapsulated films experienced a substantial loss in Ru concentration (exceeding 20%) in comparison to the encapsulated counterpart. The study demonstrates that the encapsulation provided by oxide membranes effectively minimized stoichiometric loss during the traditional SPE procedure and enhanced surface quality, suggesting a promising avenue for the synthesis of functional oxide thin films.   

Evaluating Interfacial Free Energy Calculation Methods Using Classical and Ab Initio Molecular Dynamics

Interfacial free energies are directly linked to the growth speed and shape of crystalline materials. Several methods exist to estimate the interfacial free energy. In our work we focus on comparing interfacial free energies found through the cleaving method, test area method, and the use of capillary-wave theory. These theories are applied to data generated through classical and ab initio molecular dynamics simulations of NaCl slabs composed of the (001), (011) and (111) faces exposed to a layer of liquid water using the CP2K software suite. Finite size effects were explored by modeling each slab with surface areas of approximately 1 nm² and 4 nm².   

Gamma Radiation Induced Structural Changes in Two Dimensional MXenes

High electronic conductivity, structural diversity, and hydrophilicity of two dimensional MXenes opened broad prospects for their applications in variety of industrial and technological areas including energy storage, optoelectronics, spintronics, catalysis, and sensing. In this work, we studied the effect of gamma radiation on surface characteristics of mild etched Ti₃C₂T_X MXene using Raman spectroscopy. Ti₃C₂T_X MXene was synthesized by adding Ti₃AlC₂ powders into the LiF/HCl solution and was etched for 7 days at 70 °C. There are several spectral features apparent in the Raman Spectra.  The peak ~200  cm⁻¹  peak is related to A1g(Ti, O, C)  band whereas peak ~ 720 corresponds to  cm⁻¹   A1g(C).  These two Raman bands are highly diminished with the 1MGy dose of gamma radiation. The 150 cm⁻¹ was enhanced in gamma irradiated sample indicating that gamma radiation activates the oxidation of surface titanium atoms.  In addition, the different intensity of D (1350 cm⁻¹) and G ( 1570  cm⁻¹ ) bands between pristine and gamma irradiated MXenes indicates that the extent of amorphous carbon can also be tuned by gamma irradiation.   

Simulating Vapor Deposition of Nb3Sn on SRF Cavities: Insights from Genetic Algorithms Surface Structure Search

Nb₃Sn-coated superconducting radiofrequency (SRF) particle accelerator cavities offer a promising route for developing next-generation accelerator cavities due to their high critical fields and ability to operate at relatively high temperatures. However, its widespread adoption is constrained by challenges in effectively coating Nb cavities with Nb₃Sn. The surface properties of Nb₃Sn, especially its outermost layers, play an important role in determining the superconducting behavior of the coated cavity. To gain insights into the surface structure of Nb₃Sn, we employ genetic algorithms coupled with density functional theory (DFT) calculations to simulate the vapor deposition methods typically used for Nb₃Sn coating. Using the generated data, we construct a surface phase diagram for Nb₃Sn(100), delineating stable surface configurations as a function of temperature and Sn partial vapor pressure. Interestingly, the optimal conditions for Nb₃Sn growth identified through our surface phase diagram align well with experimentally used parameters, underscoring the efficacy of our approach. Additionally, our findings indicate that Sn anti-site defects that are particularly detrimental to superconductivity are stable predominantly near the surface.   

High-performance solid-state electrochromic device for emissivity control

This research presents a solid-state electrochromic device with exceptional emissivity control. The device's architecture includes thin films deposited on a glass substrate through magnetron sputtering: Au, NiO, Ta₂O₅, WO₃, and ITO.  The NiO layer stores ions, while Ta₂O₅ serves as an ion-conducting and electron barrier layer, enabling ion intercalation within WO₃ upon voltage application. This process induces significant changes in optical properties. The device shifts from a high reflecting state (95% reflectance) to a low reflecting state (1% reflectance) under positive bias, relative to the Au electrode, at 660 nm. Importantly, a negative bias restores the original high reflectance. The device's standout features include a rapid reflectance modulation with high contrast and an average switching time of approximately 5 seconds. This switching time is a significant improvement compared to similar devices in previous studies. Furthermore, optimization of NiO and ITO layers during fabrication yielded resistivity in the range of a few milli-Ohms per centimeter (mΩ·cm). This optimization is the key to its exceptional performance. This study highlights the immense potential of the device in applications such as energy-efficient optical systems, thermal regulation, and emissivity-driven radiative cooling.   

Structure Refinement of Al-Si Interface at the Atomic Level for Identification of Two-level Systems in Superconducting Qubits

Superconducting qubits are promising platforms for quantum computing. However, they are susceptible to decoherence caused by two-level system (TLS) defects. The interface between the superconducting metal and the Si substrate is one of the possible origin sites for TLS defects. Possible candidate sites include interface steps, metal film grain boundaries, mixing across the interface, or strain fields associated with any of these microstructures. We have investigated Al-Si interfaces in Josephson junctions using aberration-corrected atomic-resolution scanning transmission electron microscopy (STEM) imaging for precise structural characterization. The STEM data were used as targets for the refinement of structural models using FANTASTX (Fully Automated Nanoscale To Atomistic Structure from Theory and eXperiments) software which also minimizes the formation energy calculated using density functional theory. Candidate TLS structures derived from FANTASTX models will be discussed.

This work was supported by DOE DE-SC0020313.   

Emergent Physics at the Metal / Layered Semiconductor Interface

In most cases, metals grown upon non-reactive surfaces will tend to ball up into nanoclusters to minimize surface free energy. However, for noble metals grown on layered dichalcogenide semiconductors, film gorwth follows a very different set of rules Despite weak bnding and a lattice mismatch exceeding 8%, noble metals form well-deinfeed and quantized nanostructures. The sizes of these feaatures are not only discrete, but correlated with the electronic structure of the given metal. The sturctural sizes, which correspons to integer multiples of a Fermi wavelength, induce energy gaps and reduce the overall electronic energy in these systems. Usually, such energy savings are trivial compared to other rowth parameters like strain, interfacial bonding, and surface free energy. In order to achieve such electornic growth modes, the normal energetics fo film growth must be minimal in these noble metal / layered semiconductor systems. We find that bonding is almost equally viable at multiple surface sites, leading to a drastic reduction of strain which enables quantum effects to actually control feature sizes. Despite the overall weak bonding, we find that there is significnat hybridization with the surface molecular layer, which leads to semi-metallic behavior in a sinlge layer of the substrate. While weak, this hybridization leads to novel interface propoerties, particularly for ferromagnetic films. These discoveries can lead to the development of better electrical contact formation with layered materials and also induce novel 2D states at the interface.     

Surface Structure of α-Quartz Prepared by Heating and Chemical Etching

Crystalline silica (SiO₂) surfaces are used as growth substrates for carbon nanotubes and nanowires, in sensing applications, and as insulators in nanoelectronics applications. Despite its importance, the atomic surface structure of quartz still remains poorly understood, especially on the experimental side. Surface structures depend heavily on the preparation approach. Here we present a survey of common preparation approaches for α-quartz(0001) and characterize the resulting surface structures. We prepared mirror polished α-quartz (0001) by high-temperature annealing, cleaning with common solvents, and chemical etching with NaOH and KOH. Using atomic force microscopy, we find that etching leads to surface roughening while heating at high temperatures produces smooth, flat terraces on the surface. We observed a large-scale surface termination with a reconstruction periodicity of 5 nm from high temperature annealing in air, consistent with previous literature [1,2]. The study provides a pathway towards tailored surface structures for specific applications.

[1] Bart, F. & Gautier, M. Surf. Sci. 311, L671–L676 (1994).

[2] Eder, S. D. et al. Sci. Rep. 5, 14545 (2015)   

Surfactant-Mediated Sliding of Aqueous Droplets on Smooth Solid Surfaces

Slippery surfaces on which liquid droplets display high mobility have attracted great interest due to their wide range of applications. Immense research efforts have been devoted to the development of novel slippery surfaces with tailored functions. Prior studies have focused mainly on super-repellent surfaces and lubricant-infused surfaces, which possess superior slipperiness. However, these surfaces typically require creation of micro/nano surface textures and surface chemistry modification through complex processes. Unlike prior work, this study introduces a novel strategy to induce the sliding of aqueous droplets on smooth solid surfaces. We show that droplets containing ionic surfactants display high mobility on the solid surfaces without morphology and chemistry modification. The sliding behavior of the droplet is regulated by the surfactant concentration. Furthermore, we demonstrate the controlled manipulation of the surfactant-laden droplets.   

Film Dynamics Over a Topographical Surface Using Lattice Boltzmann Method

In this work, the dynamics of a spreading liquid film on a planar and topographical substrate are numerically modeled using the phase-field lattice Boltzmann Method (PFLBM). A two-phase interface is inherently mesoscopic in nature, making the PFLBM a suitable technique for modeling. Interfacial patterns generated using PFLBM perfectly match the experimental and analytical results obtained within the lubrication assumption. PFLBM simulations uncovered that steady-state solutions are not possible for large topographies and the fluid-fluid interface results in a series of droplets, leaving the topographical feature in the downstream direction. This unsteady pattern is found to be highly dependent on the advancing contact angle. A decrease in viscosity ratio (bottom to top fluid) increases the height of the capillary ridge formed, making the film more prone to instability. We also explore the effect of multiple obstacles on the capillary ridges formed by each and obtain the condition for independent ridges. Finally, a detailed analysis will be presented for the effect of aspect ratio (film thickness away from contact point versus capillary length) on planar surfaces with contact-line spreading. Our study unveils that at a critical value of the aspect ratio, the maximum value of dimensionless capillary ridge height reaches unity, and this critical value is found to be independent of the inclination angle. On further increasing the value of this parameter, a nose-like structure appears near the contact point, which is strongly dependent on contact angle values.