Hexagonal boron nitride based hybrid nanomaterials for flexible electronic and optoelectronic devices

Abstract

Hexagonal boron nitride (h-BN) is an attractive insulating two-dimensional (2D) nanomaterial with excellent physical properties such as high thermal conductivity, high mechanical strength, and great thermal/chemical stability. Recently, 2D nanomaterial-based hybrid material system are receiving a lot of interests for fabricating next-generation electronic and optoelectronic devices with additional functionalities such as flexibility or transferability. However, the lack of insulating properties in the 2D nanomaterial have raised multiple issues, requiring sophisticated device structures. Consequently, h-BN can be an ideal platform for the novel hybrid material system, when integrated with conventional semiconductor nano/microstructures. Nevertheless, the h-BN based hybrid nanomaterial system has not yet been investigated in detail. In this dissertation, systematic studies on the growth, characterization and device applications of hybrid nanomaterials based on h-BN are discussed. <br/> Large-scale, epitaxial h-BN few-layer films was synthesized on Ni(111) single crystal substrates using atmospheric pressure chemical vapor deposition (APCVD) with ammonia-borane single precursor. The grown films were transferred to arbitrary substrates via an electrochemical delamination technique, and the remaining Ni(111) substrates were repeatedly re-used. Various physical characterizations confirmed that the grown films exhibited typical characteristics of hexagonal boron nitride layers over the entire area. Furthermore, the heteroepitaxial relationship between h-BN and Ni(111), as well as the overall crystallinity of the film have been thoroughly investigated using synchrotron radiation x-ray diffraction (SR-XRD) analysis and transmission electron microscopy (TEM) based techniques.<br/> Next, the mechanism and the microstructural properties of heteroepitaxial growth on h-BN layers have been thoroughly studied. First, the heteroepitaxy of zinc oxide (ZnO) nanostructures on h-BN was investigated. The van der Waals (vdW) surface feature of the h-BN, due to free of dangling bonds, typically results in low density random nucleation–growth in the epitaxy. The difficulty in control of nucleation sites was resolved by artificially formed atomic ledges prepared on h-BN substrates, which promoted preferential vdW nucleation–growth of ZnO specifically along the designed ledges. Electron microscopy revealed crystallographically domain-aligned incommensurate vdW heteroepitaxial relationships, even though the ZnO/h-BN is highly lattice mismatched. The first-principles theoretical calculations exhibited the weakly bound, noncovalent binding feature of ZnO/h-BN heterostructure. Furthermore, shape- and morphology-controlled epitaxy of ZnO nanostructures on h-BN was demonstrated. The study was then extended to the ZnO nanostructures on large-scale epitaxial h-BN layers. Large-scale ZnO nanostructure grown on the epitaxial h-BN exhibited similar properties to the ZnO on mechanically exfoliated h-BN, an ordered orientation over long-range.<br/> In addition to the nanostructures, structural properties of gallium nitride (GaN) thin-films grown on h-BN was investigated as well. The heteroepitaxial relationship between GaN and h-BN lead to the growth of single crystalline GaN thin-films over the entire area. Especially, the defect structure was analyzed by two-beam dark field (DF) imaging. Screw-type dislocations was dominantly observed, different to the GaN thin-films on the conventional substrates. The density of the threading dislocations was found to be comparable to those of other GaN thin-films grown on 2D nanomaterials or Si(111) substrates.<br/> The functional device application of the h-BN was demonstrated by fabrication of the flexible ultra violet (UV) sensor. The photocurrent of the ZnO nanostructures on h-BN was first studied on the mechanically exfoliated h-BN. The flexible UV sensor was then fabricated using the large-scale ZnO/h-BN heterostructures patterned as a microdisk array, followed by the mechanical transfer onto flexible substrates thanks to the weak interlayer bonding of h-BN. The fabricated flexible UV sensors exhibited excellent performance such as a low dark current, a high on-off ratio and short response/recovery times, even under highly bent conditions.<br/> Finally, bottom-up integration of a 2D based hybrid semiconductor nanostructure for flexible electronics was investigated. Here ZnO nanotubes on graphene film was used to fabricate the vertical field-effect transistors (VFET). It should be noted that the study can be readily integrated with the h-BN layers. Due to the high quality of the single crystal ZnO nanotubes and the unique one-dimensional (1D) device structure, the fabricated VFET exhibited excellent electrical characteristics. For example, it had a small subthreshold swing of 110 mV/dec, a high Imax/Imin ratio of 106 and a transconductance of 170 nS/m. The electrical characteristics of the nanotube VFETs were validated using three-dimensional transport simulations. Furthermore, the nanotube VFETs fabricated on graphene films could be easily transferred onto flexible plastic substrates. The resulting components were reliable, exhibited high performance, and did not degrade significantly during testing. <br/>