Embedded dielectric microstructures in molecular beam epitaxy : high-quality planar coalescence toward enhanced optoelectronic materials

Seamless integration of embedded dielectric microstructures in III-V crystal growth is a continued area of research due to its numerous high-impact applications. Historically, investigations into embedded dielectric microstructures within existing crystal growth techniques were focused on blocking dislocations at the III-V/dielectric interface in the production of low defect relaxed high mismatched heteroepitaxy. However, recent efforts have broadened the use of embedded dielectric microstructures for enhancement of optoelectronic device functionality and development of monolithic growth schemes toward integrated photonic circuits. The central challenge of embedding dielectric microstructures in III-V materials is achieving single-crystal high-quality planar coalescence within existing conventional III-V crystal growth techniques without defect. While prevalent in the field of III-V crystal growth, solid-source Molecular Beam Epitaxy (MBE) has a well-known "coalescence problem," historically lacking approaches that achieve planar coalescence over dielectric microstructures. Limited coalescence is in large part due to low diffusion of III-adatoms on dielectric surfaces, typically below 300nm, readily forming polycrystalline deposition on dielectric surfaces exceeding this diffusion length. Several solid-source MBE highly-selective growth and lateral epitaxial overgrowth (LEO) growth approaches have been reported; however, none demonstrating complete planar coalescence over dielectric microstructures. In this dissertation, to overcome the "coalescence problem," we demonstrate for the first time a general methodology for an all-MBE growth of high-quality planar coalescence over a variety of embedded dielectric microstructures. Underpinning the approach, we developed a two-stage all-MBE growth approach for GaAs and InAs on (001) substrates, producing highly selective LEO and planarization, returning the growth front to the (001) surface. Characterization of the growth approach demonstrates for the first time an all-MBE approach to planar coalescence. In application of the two-stage all-MBE growth approach towards photonics, we demonstrate enhancement of quantum emitters using buried silica gratings arrays and develop several methodologies for embedded high-contrast photonic materials through self-formed air voids and molded air channel processes. Lastly, in application to high-quality relaxed high mismatch heteroepitaxy, we demonstrate for the first time an all-MBE approach to III-V metamorphic heteroepitaxy, demonstrating threading dislocation reduction in InAs/GaAs metamorphics with high fill factor embedded silica gratings. Thus, from the material presented here, we provide several significant advances to the long-standing challenge of marrying high-quality semiconductor crystal growth with dielectric microstructures, unlocking several high-impact applications, including high-quality material pathways for enhanced quantum emitters and embedded metasurfaces as well as an all-MBE approach toward heterogeneous III-V integration on silicon.Electrical and Computer Engineerin

Highly tensile-strained Ge/InAlAs nanocomposites

Self-assembled nanocomposites have been extensively investigated due to the novel properties that can emerge when multiple material phases are combined. Growth of epitaxial nanocomposites using lattice-mismatched constituents also enables strain-engineering, which can be used to further enhance material properties. Here, we report self-assembled growth of highly tensile-strained Ge/In[subscript 0.52]Al[subscript 0.48]As (InAlAs) nanocomposites by using spontaneous phase separation. Transmission electron microscopy shows a high density of single-crystalline germanium nanostructures coherently embedded in InAlAs without extended defects, and Raman spectroscopy reveals a 3.8% biaxial tensile strain in the germanium nanostructures. We also show that the strain in the germanium nanostructures can be tuned to 5.3% by altering the lattice constant of the matrix material, illustrating the versatility of epitaxial nanocomposites for strain engineering. Photoluminescence and electroluminescence results are then discussed to illustrate the potential for realizing devices based on this nanocomposite material.National Science Foundation (U.S.) (DMR 1506371