Atomic layer deposited beryllium oxide as a gate dielectric or interfacial Layer for Si and III-V MOS devices
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The continuous improvement in the semiconductor industry has been successfully achieved by the reducing dimensions of CMOS (complementary metal oxide semiconductor) technology. For the last four decades, the scaling down of physical thickness of SiO₂ gate dielectrics has improved the speed of output drive current by shrinking of transistor area in front-end-process of integrated circuits. A higher number of transistors on chip resulting in faster speed and lower cost can be allowable by the scaling down and these fruitful achievements have been mainly made by the thinning thickness of one key component - Gate Dielectric - at Si based MOSFET (metal-oxide-semiconductor field effect transistor) devices. So far, SiO2 (silicon dioxide) gate dielectric having the excellent material and electrical properties such as good interface (i.e., Dit ~ 2x10¹⁰ eV⁻¹cm⁻²), low gate leakage current, higher dielectric breakdown immunity (≥10MV/cm) and excellent thermal stability at typical Si processing temperature has been popularly used as the leading gate oxide material. The next generation Si based MOSFETs will require more aggressive gate oxide scaling to meet the required specifications. Since high-k dielectrics provide the same capacitance with a thicker film, the leakage current reduction, therefore, less the standby power consumption is one of the huge advantages. Also, it is easier to fabricate during the process because the control of film thickness is still not in the critical range compared to the same leakage current characteristic of SiO₂ film. HfO₂ based gate dielectric is considered as the most promising candidate among materials being studied since it shows good characteristics with conventional Si technology and good device performance has been reported. However, it has still many problems like insufficient thermals stability on silicon such as low crystallization temperature, low k interfacial regrowth, charge trapping and so on. The integration of hafnium based high-k dielectric into CMOS technology is also limited by major issues such as degraded channel mobility and charge trapping. One approach to overcome these obstacles is using alternative substrate materials such as SiGe, GaAs, InGaAs, and InP to improve channel mobility. High electron mobility in the III-V materials has attracted significant attention for a possible application as a channel material in metal/oxide/semiconductor (MOS) transistors. One of the main challenges is that III-V MOSFETs generally lack thermodynamically stable insulators of high electrical quality, which would passivate the interface states at the dielectric/substrate interface and unpin the Fermi level. To address this issue, various dielectric, such as Si/SiO₂, Ge, SiGe, SiN and Al₂O₃, were considered as an interface passivation layer (IPL). Atomic Layer Deposited (ALD) Al₂O₃ has demonstrated superior IPL characteristics compared to the other candidates due to its high dielectric constant and interface quality. However, defect density in Al₂O₃ is still too high even as several cleaning methods such as NH₄OH, (NH₄)₂S and F treatment have been developed, which limits the performance of III-V MOSFETs. In the first part of this study, theoretical approaches to understand the motivation and requirements as an high-k gate dielectric or interfacial layer, and properties of ALD beryllium oxide (BeO) for Si and III-V MOS devices have been investigated. The second part of this study focuses on the precursor synthesis and fundamental material characterization of ALD BeO thin film using physical, optical and electrical analysis. Film properties such as self-cleaning reaction and oxygen diffusion barrier will be presented. At the third part, depletion mode transistor and self-aligned MOSFETs using ALD BeO on Si and InP high mobility substrates have been investigated. And as for the final part of this study, the density functional theory of Be(CH₃)₂ precursor, electromagnetics, and thermodynamics were investigated to understand the reaction mechanism and self-cleaning reaction, and to evaluate the gate dielectrics such as Al₂O₃, BeO, SiO₂, and HfO₂.