An investigation of the oxides and silicates of Hf and Zr

Computers are vital in every aspect of our lives. One of the main planks of the current technology, the complementary metal-oxide-silicon circuit, is reaching the end of their lifespan. In order to increase the speed at which a circuit works, the circuits have been aggressively scaled. As the circuit shrinks, all the components within that circuit also shrink. This scaling has lead to a problem with one particular component. This component is the gate oxide layer. The purpose of this layer is to insulate the gate from the channel within the circuit. Currently this layer is composed of SiO2, which has traditionally worked well. As the layer gets thinner, the SiO2 can no longer effectively insulate the gate from the channel. This leads to leakage, which compromises the effectiveness of the circuit.

For some time now, the search has been on for a replacement for SiO2 in the gate oxide layer. The requirements for this layer are very stringent and an ideal candidate has not been found. Among the most promising of materials are those based on the oxides and silicates of hafnium and zirconium. One of the major obstacles in successfully developing a replacement material is a lack of understanding of the candidate materials behaviour when they are present in very small layers rather than in the bulk.

In order to understand the behaviour of these materials better, the properties of the oxides of hafnium and zirconium have been investigated through experimental and theoretical work. The experimental methods are primarily performed using an electron microscope and include high magnification imaging and electron energy-loss spectroscopy (EELS). The theoretical methods are calculations of the density-of-states (DOS) and x-ray absorption near-edge structure (XANES) of the compounds. These calculations are carried out using the simulation package FEFF8.

Chapter 1 is an introduction. It contains an overview of the current technology and its problems.

Chapter 2 contains information on the structure and previous research of the materials studied in this thesis.

Chapter 3 details the instrumentation and techniques used in this thesis. It includes descriptions of the Tecnai F20 electron microscope. In addition, the basic theory behind EELS is included.

Chapter 4 contains information regarding scripts written to process data in Gatan’s Digital Micrograph.

Chapter 5 is a theoretical investigation into the edge shapes of the oxides and silicates of hafnium and zirconium. It investigates the three most common phasese of the oxides and the only phase of the silicates. How the environment of the central atom affects the shape of the edge is investigated.

Chapter 6 looks at, by X-ray diffraction and electron microscopical means, the development of a sol-gel “amorphous” HfSiO4 into its crystalline form

Chapter 7 examines the possibility that monoclinic hafnia has anisotropic properties which affect the shape of the EELS edge. This is undertaken using EELS and Kikuchi patterns.

Chapter 8 draws conclusions and comparisons with literature.