Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa, Israel
The aggressive downscaling of nano-electronic devices demands the incorporation of new high k materials into this industry. One family of materials suggested is rare-earth oxides (REOs), since these materials present high dielectric constants and good chemical and thermal stability. The objective of this research was to study the electrical properties of REOs and associate them with their structure and the underlying substrates and interfaces. REOs can be deposited both epitaxially and as poly crystalline films on a variety of substrates. Epitaxial films are particularly of interest because of the stability of the interface with the substrate. Understanding how their electrical behavior changes is a crucial step in the integration of such oxides in nano-electronic devices that are fabricated on various substrates. The first step was to understand how these films are affected by strain, induced by the underlying substrate when deposited epitaxially. In order to study that, epitaxial nanometers thick (GdxNd1-x)2O3 films were deposited by MBE on Si (111) substrates. It was found that the dielectric constant of such layers was enhanced when strain was introduced, i.e. in the case of dissimilar film and substrate lattice parameters. In order to understand this phenomenon of the effect of different substrate lattice parameters on the film properties, Gd2O3 films were deposited on both Si and Ge (111) substrates. The dielectric constant enhancement, in the case of growth on Si, arises from the formation of a highly distorted phase that has a dielectric constant similar to the monoclinic phase of Gd2O3 and not the commonly observed cubic phase, obtained also on Ge. In addition, it was found that carbon incorporation into Gd2O3 films when grown on Si could create a significant shift in the flat band voltage of MOS capacitors based on this combination. This can be used to tune the effective work function of without the addition of steps in the process of these films, giving them viability for both p- and n-type MOS transistors. We also studied the properties of REOs on SrTiO3; in this case, the ternary oxide of LaLuO3 was used. The conductive 2D electron gas (2DEG) that was expected to form between the two oxide crystals was replaced by a nonconductive interface. It was found that the perovskite-perovskite interface includes zero dimensional positions that show a metallic-like behavior. The discontinuous nature of the metallic-like behavior was established as the reason for the interface not being conductive, and not the lack of carriers at the interface. Finally, LaLuO3 and GdScO3 were deposited on GaN and AlGaN in order to study the behavior of REOs on nitrides commonly used in the high power electronics industry. Unlike other systems, the REOs-nitrides systems presented a staggered, asymmetric band alignment. The current-voltage characteristics of a MOS structure based on this combination proved that the staggered band structure resulted in high electrical currents on one bias polarity, while efficiently preventing it at the opposite polarity. This gives the REO-nitride system the ability to perform as a diode for high power electronics applications.
PhD advisor: Prof. Moshe Eizenberg