Advanced Epitaxy on 2D Materials for Bottom-up Heterointegration with Low-defects and Membrane Production with High-throughput

Conventional epitaxy has significantly advanced the semiconductor industry, driving remarkable progress in various application fields such as electronics and optoelectronics. However, several limitations of epitaxial techniques have impeded the development of next-generation electronic and optoelectronic devices. These limitations encompass the absence of cost-effective methods for producing functional membranes with high throughput, the need to reduce the high costs of non-silicon semiconductor wafers, and the challenge of effectively integrating multiple functional semiconductor layers without detrimental effects from defects or interfacial states caused by lattice mismatch and disparate thermal expansion coefficients. In this thesis, novel epitaxy techniques and an in-depth investigation of their underlying principles are introduced to tackle these limitations inherent in conventional epitaxy techniques, thus paving the way for the production of high-quality epitaxial membranes as well as their heterogeneous integration in a cost-effective and high-throughput manner. Firstly, a unique mechanism of relaxing misfit strain in lattice-mismatched heteroepitaxial systems is observed through the implementation of remote heteroepitaxy, which involves the process of conducting heteroepitaxy on graphene-coated substrates. This approach facilitates spontaneous relaxation of misfit strain and reduction of misfit dislocations in epilayers due to the slippery graphene surface, while preserving the single-crystalline properties of the epilayers by the penetrated atomic potential from the substrate through the graphene layer. It provides a new pathway towards the heterogeneous integration of largely lattice-mismatched systems with minimized dislocation density, which could eventually broaden the material spectrum for advanced electronics and photonics. Subsequently, a high-throughput layer transfer technique based on remote epitaxy with directly grown two-dimensional (2D) materials on wafers as an interlayer is 3 presented. This approach enables a pristine amorphous 2D-on-wafer template for epitaxy, addressing issues of degraded or contaminated semiconductor wafer surfaces after standard 2D materials growth or transfer processes. Consequently, it enables a scheme to produce multiple freestanding membranes from a single wafer without sacrificial layer etching or wafer polishing. Moreover, atomic-precision exfoliation at the 2D interface allows wafer recycling for subsequent membrane production, with the potential for substantial cost reduction in manufacturing processes involving nonsilicon wafers. Additionally, we demonstrate remote epitaxy and nanopatterned epitaxy of InP, along with large-scale flexible membrane exfoliation and InP wafer recycling. By employing ultra-low temperature boron nitride growth, we successfully implement these advanced epitaxy and layer transfer techniques on InP substrate, despite its low dissociation temperature and weak ionicity. This approach paves the way for new opportunities in InP thin film-based optoelectronics and novel heterostructures at a significantly reduced cost. Lastly, we delve into intricacies of remote epitaxy by elucidating the respective roles and impacts of the substrate material, 2D layer, 2D-substrate interface, and epitaxial material for electrostatic coupling of these materials, which governs cohesive ordering and can lead to single-crystal epitaxy in the overlying film. By exploring various material systems and processing conditions, we demonstrate that the rules of remote epitaxy vary significantly depending on the ionicity of material systems as well as the 2D-substrate interface and the epitaxy environment. These studies lay the theoretical foundation for all of the novel epitaxy on 2D techniques investigated in this thesis.Ph.D

Graphene-assisted spontaneous relaxation and direct CVD growth of graphene on III-V substrate

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, May, 2020Cataloged from the official PDF of thesis.Includes bibliographical references (pages 35-37).Although conventional homoepitaxy forms high-quality epitaxial layers, the limited set of material systems for commercially available wafers restricts the range of materials that can be grown homoepitaxially. At the same time, conventional heteroepitaxy of lattice-mismatched systems produces dislocations above a critical strain energy to release the accumulated strain energy as the film thickness increases. The formation of dislocations, which severely degrade electronic/photonic device performances, is fundamentally unavoidable in highly lattice-mismatched epitaxy. This thesis reports a unique mechanism of relaxing misfit strain in heteroepitaxial films that can enable effective lattice engineering. We have observed that heteroepitaxy on graphene-coated substrates allows for spontaneous relaxation of misfit strain owing to the slippery graphene surface while achieving single-crystalline films by reading the atomic potential from the substrate. This spontaneous relaxation technique could transform the monolithic integration of largely lattice-mismatched systems by covering a wide range of the misfit spectrum to enhance and broaden the functionality of semiconductor devices for advanced electronics and photonics. We also noticed the defective areas caused by graphene transfer process, which were observed at the interface of graphene-coated substrates and epitaxial films, clearly degraded the quality of grown epilayers. In order to solve this problem, we developed a CVD growth process that directly coats III-V semiconductor substrates with graphene having full coverage and thickness that is close to monolayer. This novel graphene growth process can further improve the materials quality and performance of lattice-mismatched systems that utilize the spontaneous relaxation mechanism.by Kuangye Lu.S.M.S.M. Massachusetts Institute of Technology, Department of Mechanical Engineerin

Influences of ALD Al2O3 on the surface band-bending of c-plane, Ga-face GaN

The recently demonstrated approach of grafting n-type GaN with p-type Si or GaAs, by employing ultrathin Al2O3 at the interface, has shown the feasibility to overcome the poor p-type doping challenge of GaN. However, the surface band-bending of GaN that could be influenced by the Al2O3 has been unknown. In this work, the band-bending of c-plane, Ga-face GaN with ultrathin Al2O3 deposition at the surface of GaN was studied using X-ray photoelectron spectroscopy. The study shows that the Al2O3 can help suppress the upward band-bending of the c-plane, Ga-face GaN with a monotonic reduction trend from 0.48 eV down to 0.12 eV as the number of Al2O3 deposition cycles increases from 0 to 20. The study further shows that the band-bending can be mostly recovered after removing the Al2O3 layer, concurring that the introduction of ultrathin Al2O3 is the main reason for the surface band-bending modulation

Role of transferred graphene on atomic interaction of GaAs for remote epitaxy

Remote epitaxy is a recently discovered type of epitaxy, wherein single-crystalline thin films can be grown on graphene-coated substrates following the crystallinity of the substrate via remote interaction through graphene. Although remote epitaxy provides a pathway to form freestanding membranes by controlled exfoliation of grown film at the graphene interface, implementing remote epitaxy is not straightforward because atomically precise control of interface is required. Here, we unveil the role of the graphene-substrate interface on the remote epitaxy of GaAs by investigating the interface at the atomic scale. By comparing remote epitaxy on wet-transferred and dry-transferred graphene, we show that interfacial oxide layer formed at the graphene-substrate interface hinders remote interaction through graphene when wet-transferred graphene is employed, which is confirmed by an increase of interatomic distance through graphene and also by the formation of polycrystalline films on graphene. On the other hand, when dry-transferred graphene is employed, the interface is free of native oxide, and single-crystalline remote epitaxial films are formed on graphene, with the interatomic distance between the epilayer and the substrate matching with the theoretically predicted value. The first atomic layer of the grown film on graphene is vertically aligned with the top layer of the substrate with these atoms having different polarities, substantiating the remote interaction of adatoms with the substrate through graphene. These results directly show the impact of interface properties formed by different graphene transfer methods on remote epitaxy.</p&gt

Impact of 2D–3D Heterointerface on Remote Epitaxial Interaction through Graphene

Remote epitaxy has drawn attention as it offers epitaxy of functional materials that can be released from the substrates with atomic precision, thus enabling production and heterointegration of flexible, transferrable, and stackable freestanding single-crystalline membranes. In addition, the remote interaction of atoms and adatoms through two-dimensional (2D) materials in remote epitaxy allows investigation and utilization of electrical/chemical/physical coupling of bulk (3D) materials via 2D materials (3D-2D-3D coupling). Here, we unveil the respective roles and impacts of the substrate material, graphene, substrate-graphene interface, and epitaxial material for electrostatic coupling of these materials, which governs cohesive ordering and can lead to single-crystal epitaxy in the overlying film. We show that simply coating a graphene layer on wafers does not guarantee successful implementation of remote epitaxy, since atomically precise control of the graphene-coated interface is required, and provides key considerations for maximizing the remote electrostatic interaction between the substrate and adatoms. This was enabled by exploring various material systems and processing conditions, and we demonstrate that the rules of remote epitaxy vary significantly depending on the ionicity of material systems as well as the graphene-substrate interface and the epitaxy environment. The general rule of thumb discovered here enables expanding 3D material libraries that can be stacked in freestanding form