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Title: Chemical vapor deposition of hexagonal boron nitride and its use in electronic devices
Author: Hui, Fei
Director/Tutor: Lanza, Mario
Keywords: Nanoelectrònica
Nitrur de bor
Boron nitride
Issue Date: 23-Jul-2018
Publisher: Universitat de Barcelona
Abstract: [eng] 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.
[spa] Los dieléctricos son materiales aislantes utilizados en muchos dispositivos electrónicos (por ejemplo condensadores, transistores, baristores), en los que juegan un papel muy importante. En realidad, el dieléctrico es probablemente la parte más crítica en la gran mayoría de dispositivos electrónicos, ya que casi siempre está expuesto a campos eléctricos que pueden degradar sus prestaciones. El dióxido de silicio (SiO2) ha sido el material aislante tradicionalmente utilizado en la industria; sin embargo la miniaturización de los dispositivos requirió una reducción del grosor de los dieléctricos SiO2, lo que provocó un incremento dramático de la corriente de fugas y el fallo del dispositivo entero. Actualmente los dispositivos electrónicos más avanzados utilizan materiales aislantes con una constante dieléctrica alta (por ejemplo HfO2, Al2O3 y TiO2), y así no es necesario reducir tanto su grosor, lo que mantiene una baja corriente de fugas. Sin embargo, estos materiales muestran muchos problemas intrínsecos, y también una mala interacción con materiales adyacentes. Por lo tanto, la carrera para encontrar un material dieléctrico ideal para dispositivos electrónicos sigue abierta. En este contexto, los materiales bidimensionales se han convertido en una seria opción, no sólo por sus excelentes propiedades, sino también gracias al desarrollo de nuevos métodos de síntesis escalables. En esta tesis doctoral he investigado el uso de nitruro de boro hexagonal (h-BN), monocapa y multicapa, como material dieléctrico en dispositivos electrónicos, ya su banda de energías prohibidas es de ~5.9 eV. Mi trabajo se ha focalizado en la síntesis de h-BN mediante el método chemical vapor deposition, el estudio de sus propiedades morfológicas y eléctricas a escala nanométrica, y sus prestaciones como dieléctrico en diferentes dispositivos (condensadores y memristores). Nuestros experimentos indican que h-BN es un material dieléctrico muy fiable, y que es apto para su uso en dispositivos. Sus prestaciones dependen de diferentes parámetros, como el sustrato en el que ha sido crecido, su grosor, y los materiales usados como electrodos adyacentes. Además, h-BN muestra propiedades adicionales nunca observadas en dieléctricos tradicionales, como modulación de la resistividad volátil, lo que podría extender su uso a nuevas aplicaciones.
Appears in Collections:Tesis Doctorals - Facultat - Física

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