High‐κ Dielectric Nanofilms Fabricated from Titania Nanosheets

Abstract

In the semiconductor industry, increasing density requirements continue to motivate the production of microelectronic devices that function reproducibly at ever-smaller dimensions (a trend known as Moore’s law). The need for insulating films with nanometer-scale thickness that show good compatibility with semiconductor technology is a critical issue for future device development. As the film thickness scales down to 10 nm, conventional SiO2 gate dielectrics show a poorly insulating character with high leakage currents of 10– 10 A cm due to quantum tunneling, thereby causing large energy consumption and deteriorated device reliability. Continued device scaling will therefore require substitution of SiO2 with high-dielectric-constant (high-j) metal oxide gate dielectrics that afford high capacitance without reliance on film thickness, thus allowing for efficient charge injection and also reducing tunneling leakage currents. Here, we show that 2D titania (Ti0.87O2) nanosheets function as a high-j nanomaterial. We have successfully fabricated high-j nanofilms by forming multilayer assemblies of titania nanosheets via a layer-by-layer approach through solution-based processes at room temperature. The use of an atomically flat (001) SrRuO3 (SRO) epitaxial film as the bottom electrode enables the fabrication of atomically uniform, highly dense, and void-free nanofilms that retain the superior electrical properties of the titania nanosheets. These films exhibit a reduced leakage current density (10–10 A cm) with a high relative dielectric constant of ∼ 125, even for thicknesses as low as 10 nm. These results, as well as the facile molecular-scale assembly of the nanostructures, suggest that titania nanosheets are attractive candidates for use in nanocomponent insulating high-j dielectric films. In the area of high-j dielectrics, there is currently an intense research effort focused on the synthesis and device integration of various oxide materials. Current candidates are placed into two categories according to the j value: simple metal oxides with j= ∼ 10–20, such as Ta2O5, HfO2 and ZrO2, and materials with j> 100, such as rutile-type TiO2 [7] and (Ba,Sr)TiO3 (BST). [8] In addition to intrinsic properties such as a high dielectric constant and good insulating properties, high-j gate oxides have to fulfill a number of other requirements for device integration: i) they must be chemically stable in contact with the bottom electrodes (Si and/or metals), ii) for optimal performance, they should have an atomically defined interface with the bottom electrodes, without a low-j interfacial layer, and iii) they should have a simple metal oxide system and be accessible through a low temperature ( 600 °C), which often causes both a reduction in the polarizability due to non-stoichiometry, as well as degradation of the bottom electrode. Thus, the development of new high-j oxides fulfilling these requirements represents a critical challenge for materials science. In searching for alternative materials, we have focused on titania (Ti0.87O2) nanosheets, [4] a new class of nanomaterials that represent a modification of nanometer-sized titanium oxide prepared by delaminating layered titanates into their molecular single sheets (Fig. 1a). Compared with rutile and other high-j oxides, titania nanosheets have some unique features: i) they are completely 2D nanomaterials, with a thickness of 0.7 nm, and ii) due to their colloidal and negatively charged character, they can be self-assembled layer-by-layer via a solution-based process to construct ultrathin films. Therefore, the use of titania nanosheets as building blocks for high-j films could provide a potential route to the room-temperature manufacture of high-j devices, eliminating the degradation of the bottom electrode during the annealing process. Here, we report the performance of high-j nanofilms fabricated from titania nanosheets by a solution-based layer-by-layer approach. Multilayer films of large-sized titania nanosheets (with an average lateral size of 30 lm) have been fabricated by an electrostatic self-assembly technique, using poly(diallyldimethylammonium) (PDDA) as a counter polycation. As the bottom electrode, we use an atomically flat conducting SRO substrate, consisting of a 50 nm thick (001)-oriented epitaxial SRO layer on a (001) SrTiO3 single crystal. The as-deC O M M U N IC A IO N S