|Ph.D Student||Lipp Eran|
|Subject||Investigation of the Properties of Metal/High-K (Gd2O3)|
Interfaces for Advanced MOS Devices
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Moshe Eizenberg|
|Full Thesis text|
Future downscaling of metal-oxide-semiconductor (MOS) devices relies on the successful introduction of high-k dielectrics and metal electrodes, which will replace the traditional poly-Si/SiO2 gate-stack. Gd2O3 is one of the only materials that have been reported to suit the industry’s requirements for 2016. In addition, Gd2O3 has a low lattice mismatch to Si, which enables the growth of a single-crystalline oxide layer. Additional oxide structures can be obtained by changing the substrate orientation or the oxide growth temperature.
The aim of this research was to understand the effect of oxide structure on the electrical properties and on the thermal stability of metal/Gd2O3/Si stacks. This goal was achieved by depositing three different structures of Gd2O3 on Si by molecular-beam epitaxy (MBE). Amorphous oxide layers were deposited at 90°C, while layers deposited at 600°C were crystalline. The structure of crystalline Gd2O3 layers was observed to depend on the Si orientation. Single-crystalline layers were obtained on Si(111), whereas on Si(100), domain-structured layers were obtained. In the case of domain-structured Gd2O3, a silicate-like interfacial layer was observed at the oxide/Si contact.
The electrical properties of Gd2O3 were observed to be almost independent of the oxide structure. The oxide k-value was similar for all structures, however, the k-value of domain-structured Gd2O3 (17.7±0.8) was somewhat higher than those of single-crystalline (16.3±0.7) and amorphous (16.9±0.8) layers. The electron effective mass in crystalline Gd2O3 was found to be (0.1±0.02)·me, while for amorphous layers, a value of (0.5±0.1)·me was obtained. The conduction mechanism through crystalline Gd2O3 was found to be contact-limited in most of the measured conditions, with a barrier height of (0.6±0.1)eV at the Pt/Gd2O3 interface. This value is explained by the existence of a defect-related energy band in the oxide.
The extent of Fermi-level pinning at metal/Gd2O3 interfaces was found to be significantly affected by the oxide structure. At metal/single-crystalline Gd2O3 interfaces, Fermi-level pinning was negligible, while at metal/amorphous Gd2O3 interfaces, a dominant pinning effect could be observed. These results are explained in terms of the metal-induced gap states model.
The thermal stability of Pt/crystalline Gd2O3/Si stacks was found to be almost independent of the oxide structure, however the effect of treatment ambient on the thermal stability was quite pronounced. Thermal instability resulted from Gd out-diffusion, which was observed after forming-gas or vacuum annealing at 550°C for 30min. The ambient effect is explained by the concentration of O in the Pt layer during the heat treatment.