Structural and electrical properties of flash memory cells with HfO2 tunnel oxide and with/without nanocrystals


Thesis Type: Postgraduate

Institution Of The Thesis: Middle East Technical University, Faculty of Arts and Sciences, Department of Physics, Turkey

Approval Date: 2009

Thesis Language: English

Student: Döndü Şahin

Supervisor: RAŞİT TURAN

Abstract:

In this study, flash memory cells with high-k dielectric HfO2 as tunnel oxide and group IV (Si, Ge) nanocrystals were fabricated and tested. The device structure was grown by magnetron sputtering deposition method and analyzed by various diagnostic techniques such as X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy. The use of HfO2 tunnel oxide dielectric with high permittivity constant was one of the main purposes of this study. The ultimate aim was to investigate the use of Si and Ge nanocrystals together with HfO2 tunnel oxide in the memory elements. Interface structure of the fabricated devices was studied by XPS spectroscopy. A depth profile analysis was performed with XPS. Nanocrystal formations were verified using Raman spectroscopy technique. The final part of the study includes electrical characterization of memory devices fabricated using Si and Ge floating gate. C-V (Capacitance Voltage) and G-V (Conductance-Voltage) measurements and charge storage behaviour based on C-V measurements were performed. For comparison, structure of Si and Ge layers either in thin film or in the nanocrystal form were studied. A comparison of the C-V characteristics of these two structures revealed that the memory device with thin films do not confine charge carriers under the gate electrode as should be expected for a continuous film. On the other hand, the device with nanocrystals exhibited better memory behavior as a result of better confinement in the isolated nanocrystals. Trace amount of oxygen was found to be enough to oxidize Ge nanocrystals as confirmed by the Raman measurements. The charge storage capability is weakened in these samples as a result of Ge oxidation. In general, this work has demonstrated that high-k dielectric HfO2 and group IV nanocrystals can be used in the new generation MOS based memory elements. The operation of the memory elements are highly dependent on the material and device structures, which are determined by the process conditions.