Chemical Vapor Deposition of Hexagonal Boron Nitride and Its Use in Electronic Devices

Download or read book Chemical Vapor Deposition of Hexagonal Boron Nitride and Its Use in Electronic Devices written by Fei Hui and published by . This book was released on 2018 with total page 175 pages. Available in PDF, EPUB and Kindle. Book excerpt: Dielectrics are insulating materials used in many different electronic devices (e.g. capacitors, transistors, barristors), and play an important role in all of them. In fact, the dielectric is probably the most critical element in most devices, as it is exposed to electrical fields that can degrade its performance. In this PhD thesis I have investigated the use of monolayer and multilayer hexagonal boron nitride (h-BN) as dielectric for electronic devices, as it is a 2D material with a band gap of ̃5.9 eV. My work has mainly focused on the synthesis of the h-BN using chemical vapor deposition, the study of its intrinsic morphological and electrical properties at the nanoscale, and its performance as dielectric in different electronic devices, such as capacitors and memristors. We observe that monolayer and multilayer h-BN can be growth by CVD on Pt, Cu and Fe substrates. The main parameters affecting the growth of the h-BN are: i) a proper temperature determines the decomposition of the precursor. Lower temperatures will produce remaining particles and more defects in BN layer. ii) The flow rate of precursor/H2 influences the density of seeds. Excessive precursor will give rise to the formation of h-BN multilayer islands. iii) High vacuum and low pressure help to remove impurities in the tube furnace (e.g. oxygen, carbon), and therefore it produces better quality h-BN, i.e. uniform thickness with less defects. h-BN sheets grown on polycrystalline Pt substrates show different thicknesses depending on the crystallographic orientation at the surface of each Pt grain. This produces an undesired fluctuation on the leakage current from one Pt grain to another. However, the leakage current across the h-BN on the same Pt grain is very uniform, much more than that observed across amorphous HfO2 and TiO2 thin films. This phenomenon doesn't take place when growing the h-BN on Cu or Fe substrates. For example, the leakage current across h-BN grown on Cu substrates display small current variability among different Cu grains. The dielectric breakdown behavior in multilayer h-BN shows surface extrusion, similar to what happens in SiO2, HfO2 and Al2O3. However, monolayer h-BN keeps unaltered its structure even for harder breakdown events. The reason may be the extremely high thermal conductivity of monolayer h-BN. Multilayer h-BN shows random telegraph noise signals when applying constant voltage stresses, both at the device level and at the nanoscale. This strongly indicates the trapping and de-trapping of charges during the stress. This observation has been confirmed by the detection of charges at the dielectric breakdown location. The breakdown spot shows a singular ring-like structure that contains fixed negative charges, mobile negative charges, and positive fixed charges. The synthesis of h-BN on polycrystalline Fe substrates required longer cooling down times than when using Pt and Cu substrates. The reason is that the growth of h-BN on Fe substrates mainly takes place by surface precipitation mechanism, while on Pt and Cu substrates the mechanism is by surface-mediated reaction. Memristors with Ag/h-BN/Fe structure show both threshold resistive switching when the set is induced by applying positive voltage to the Ag electrode, and bipolar resistive switching when the set/reset processes are induced by applying negative/positive voltage to the Ag electrode. The reason should be that in threshold mode the filament is formed by Ag+ ions that penetrate in the h-BN stack, while in bipolar mode Fe+ ions penetrate in the h-BN stack. Ag+ ions show higher diffusivity than Fe+ ions and produce volatile switching.