Oxide dielectric breakdown is an old problem that has been studied over decades. It causes power dissipations and irreversible damage to the electronic devices. The aggressive downscaling of the device size exponentially increases the leakage current density, which also raises the risk of dielectric breakdown. It has been proposed that point defects, current leakages, impurity diffusions, etc. all contribute to the change of oxide chemical composition and ultimately lead to the dielectric breakdown. However, the conclusive cause and a clear understanding of the entire process of dielectric breakdown are still under debate. In this research, the electronic structure at metal/oxide interfaces is studied using first-principle calculations within the framework of Density Functional Theory (DFT) to investigate any possible key signature that would trigger the dielectric breakdown.
A classical band alignment method, the Van de Walle method, is applied to the case study of the Al/crystal-SiO2 (Al/c-SiO2) interface. Point defects, such as oxygen vacancy (VO) and hydrogen impurity (IH), are introduced into the Al/c-SiO2 interface to study the effects on band offset and electronic structure caused by point defects at metal/oxide interfaces. It is shown that the bonding chemistry at metal/oxide interfaces, which is mainly ionic bond, polarizes the interface. It results in many interface effects such as the interface dipole, built-in voltage, band bending, etc. Charge density analysis also indicates that the interface can localize charge due to such ionic bonding. It is also found that VO at the interface traps metal electrons which closes the open -sp3 orbital. The analysis on local potential shows that the metal potential penetrates through a few layers of oxide starting from the interface, which metalizes the interfacial region and induces unoccupied states in the oxide band gap. In addition, it is shown that higher oxygen content at metal/oxide interfaces minimizes such metal potential invasion. In addition, an oxygen vacancy is created at multiple sites through the Al/c-SiO2 and Al/a-SiO2 interface systems, separately. The oxygen local pressure is also calculated before its removal using Quantum Stress Density theory. Correlations among electronic structure, stress density, and vacancy formation energy are found, which provide informative insights into the defect generation controlling and dielectric breakdown analysis.
A new band alignment approach based on the projection of plane-waves (PWs) into the space-dependent atomic orbital (LCAO) basis is presented and tested against classical band offset methods -- the Van de Walle method. It is found that the new band alignment approach can provide a quantitative and reliable band alignment and can be applied to the heterojunctions consisting of amorphous materials. The new band alignment approach reveals the real-space dependency of the electronic structure at interfaces. In addition, it includes all interface effects, such as the interface dipole, built-in voltage, virtual oxide thinning, and band deformation, which cannot be derived using classical band offset methods. This new band alignment approach is applied to the case study of both the Al/amorphous-SiO2 (Al/a-SiO2) interface and the Al/c-SiO2. We have found that at extremely low dimensions, the reduction of the insulator character due to the virtual oxide thinning is a pure quantum effect. I highlight that the quantum tunneling current leakage is more critical than the decrease of the potential barrier height on the failure of the devices. / PHD / Metal/oxide interfaces have many applications in electronic devices such as Field Effect Transistors (FETs), resistive/dynamic Random-Access Memory (RAM) devices, Tunnel Junctions (TJs), Metal Oxide Semiconductor (MOS) devices, or Back-End-of-Line (BEOL) on integrate-circuits. The downscaling of devices dimension is still following the Moore’s Law. However, it brings several reliability challenges, such as the electric current leakage that is significant for ultrathin oxide films (< 5 nm). At low dimensionality, the stress induced leakage currents (SILC) caused by quantum effects exponentially increases. These electric conductions harm devices and constantly degrade insulating materials, until the degradation reaches a critical level called dielectric breakdown that ultimately leads to the electronic failure of the materials. The insulating/conducting transition is a complex and irreversible very well-known process. Experimentally, the observation of sudden electric current increase is a typical sign of the breakdown. Many experimental works in past decades suggest that point defects are very important to the initiation of dielectric breakdown, however they cannot be the only cause. Many other factors such as the electric voltage, material imperfection, mechanical stress, humidity, and temperature are also critical to the final breakdown. Therefore, a comprehensive and theoretical study is necessary to better understand the mechanisms behind the dielectric breakdown. It benefits the semiconductor industry for inventing new materials and exploring advanced techniques to prevent the occurrence of dielectric breakdown.
In this dissertation, a set of theoretical case studies using the aluminum (Al) and silica (SiO₂) to explore correlations among different electronic, thermodynamic, and mechanical properties have been performed. This study reveals that all these material properties are intrinsically correlated and allow a clear understanding of the dielectric breakdown.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/95967 |
| Date | 19 June 2018 |
| Creators | Huang, Jianqiu |
| Contributors | Mechanical Engineering, Hin, Celine, von Spakovsky, Michael R., Heremans, Jean J., Asryan, Levon V., Priya, Shashank |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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