HfO2 as gate dielectric on Si and Ge substrate

Hafnium oxide HfO_2 has been considered as an alternative to silicon dioxide SiO_2 in future nano-scale complementary metal-oxide-semiconductor (CMOS) devices since it provides the required capacitance at the reduced device size because of its high dielectric constant. HfO_2 films are currently deposited by various techniques. Many of them require high temperature annealing that can impact device performance and reliability. In this research, electrical characteristics of capacitors with HfO_2 as gate dielectric deposited by standard thermal evaporation and e-beam evaporation on Si and Ge substrates were investigated. The dielectric constant of HfO_2 deposited by thermal evaporation on Si is in the range of 18-25. Al/HfO_2/Si MOS capacitors annealed at 450 C show low hysteresis, leakage current density and bulk oxide charges. Interface state density and low temperature charge trapping behavior of these structures were also investigated. Degradation in surface carrier mobility has been reported in Si field-effect-transistors with HfO_2 as gate dielectric. To explore the possibility of alleviating this problem we have used germanium (Ge) substrate as this semiconductor has higher carrier mobility than Si. Devices fabricated by depositing HfO_2 directly on Ge by standard thermal evaporation were found to be too leaky and show significant hysteresis and large shift in flatband voltage. This deterioration in electrical performance is mainly due to the formation of unstable interfacial layer of GeO_2 during the HfO_2 deposition. To minimize this effect, Ge surface was treated with the beam of atomic nitrogen prior to the dielectric deposition. The effect of surface nitridation, on interface as well as on bulk oxide, trap energy levels were investigated using low temperature C-V measurements. They revealed additional defect levels in the nitrided devices indicating diffusion of nitrogen from interface into the bulk oxide. Impact of surface nitridation on the reliability of Ge/HfO_2/Al MOS capacitors has been investigated by application of constant voltage stress at different voltage levels for various time periods. It was observed that deeper trap levels in nitrided devices, found from low frequency and low temperature measurements, trap the charge carrier immediately after stress but with time these carriers detrap and create more traps inside the bulk oxide resulting in further devices deterioration. It is inferred that though nitrogen is effective in reducing interfacial layer growth it incorporates more defects at interface as well as in bulk oxide. Therefore, it is important to look into alternative methods of surface passivation to limit the growth of GeO_2 at the interface.

Name thesis:degree_name

Doctor of Philosophy in Computer Engineering - (Ph.D.)

Discipline thesis:degree_discipline

Electrical and Computer Engineering

Year

2006

Author dc:creator

• Garg, Reenu

Contributors dc:contributor

• Durgamadhab Misra
• Leonid Tsybeskov
• Marek Sosnowski

• Hafnium oxide
• Germanium
• Constant voltage stress
• Traps behavior
• Low temperature characterization
• Interfacial layer
• Computer Engineering

Repository record dc:identifier

https://digitalcommons.njit.edu/dissertations/768

OAI identifier oai:identifier

oai:digitalcommons.njit.edu:dissertations-1823