Functionality of Oxides

A Thin Film Approach to Engineering Functionality into Oxides


As a class of materials, oxides are renowned for their diverse optical, magnetic and electronic properties. Fabricating these materials at reduced dimensions, e.g. as ultrathin films, and interpreting the resultant changes in physical properties is the focus of a vast and dynamic field of research. Schlom and co-workers review the substantial progress made in the fabrication and understanding of thin film oxides in Journal of the American Ceramic Society. Precision growth techniques such as molecular beam epitaxy and pulsed laser deposition allow crystalline oxide layers to be grown to a thickness of just a few ?, facilitating both commercial applications and extra degrees of freedom for explorative research. Research into thin film oxide heterostructures has also revealed remarkable behavior at material interfaces and itself offers enormous scope for research.

http://www.materialsviews.com/matview/display/en/403/TEXT

Schlom et al., J. Am. Ceram. Soc., 2008, 91, 2429

The broad spectrum of electronic and optical properties exhibited by oxides offers tremendous opportunities for microelectronic devices, especially when a combination of properties in a single device is desired. Here we describe the use of reactive molecular-beam epitaxy and pulsed-laser deposition to synthesize functional oxides, including ferroelectrics, ferromagnets, and materials that are both at the same time. Owing to the dependence of properties on direction, it is often optimal to grow functional oxides in particular directions to maximize their properties for a specific application. But these thin film techniques offer more than orientation control; customization of the film structure down to the atomic-layer level is possible. Numerous examples of the controlled epitaxial growth of oxides with perovskite and perovskite-related structures, including superlattices and metastable phases, are shown. In addition to integrating functional oxides with conventional semiconductors, standard semiconductor practices involving epitaxial strain, confined thickness, and modulation doping can also be applied to oxide thin films. Results of fundamental scientific importance as well as results revealing the tremendous potential of utilizing functional oxide thin films to create devices with enhanced performance are described.



Introducing crystalline rare-earth oxides into Si technologies

It becomes obvious that SiO2 will very soon reach its limit as gate dielectric for all kind of low power applications. Although higher power dissipation may be tolerable with some high-performance processors, it quickly leads to problems for mobile devices. Eventually, another strong limitation for thin oxides might come from their reduced lifetime. In addition, the increased operation temperature
considerably increases the gate leakage through thin oxides and reduces lifetime further. The oxide reliability thus remains one of the other major issues in CMOS scaling. It may be another driving force for a replacement of SiO2 with alternative high-K dielectrics.

It is very difficult to obtain in a production environment oxides thinner than 1 nm corresponding to the thickness of a native oxide. Note that such thin oxides are forecasted for the CMOS technology node within the next 10 years. Due to the large band gap of SiO2, ~9 eV, and the low density of traps and defects in the bulk of the material, the carrier current passing through the dielectric layer is normally very low. For ultrathin films this is no longer the case. When the physical thickness between the gate electrode and doped Si substrate becomes thinner than ∼2 nm, direct tunneling through the dielectric barrier dominates leakage current.

According to fundamental quantum mechanical laws, the tunneling current increases exponentially with decreasing oxide thickness. For silicon dioxide, at a gate bias of ∼1 V, the leakage current changes from ∼10–6 A/cm2 at ∼3 nm to ∼10 A/cm2 at ∼1.5 nm: seven orders of magnitude in current for a thickness change of only a factor of 2!

.........As an alternative to oxide/nitride systems, much work has been done on high-K metal oxides as a means to provide a substantially thicker (physical thickness) dielectric for reduced leakage and improved gate capacitance. Many
materials systems are currently under consideration as potential replacements for SiO2 as the gate dielectric material for future sub-0.1 μm CMOS technology. A systematic consideration of the required properties of gate dielectrics indicates that the key guidelines for selecting an alternative gate dielectric are (a) permittivity, band gap, and band alignment to silicon, (b) thermodynamic stability, (c) film morphology, (d) interface quality, (e) compatibility with the current or expected materials to be used in processing for CMOS devices, (f) process compatibility, and (g) reliability.

Many dielectrics appear favorable in some of these fields, but very few materials are promising with respect to all of these guidelines. The most promising of
these are the simple binary metal oxides. Unfortunately, a number of these materials are not thermally stable on silicon.

The upcoming generation of high-K dielectrics will be most likely formed by hafnium-based alloys. Interface engineering schemes have been developed to form oxynitrides and oxide/nitride reaction barriers between these high-K metal oxide materials and Si in an attempt to prevent or at least minimize reaction with the underlying Si. The passivating properties of such reaction barriers have been widely investigated. In most cases, this amounts to further scaling the approaches used to form oxynitrides. These barrier layers have been shown to reduce the extent of reaction between the high-K dielectric and Si, as well as to help maintain high channel carrier mobility........... Thus, any interfacial lower-permittivity layer should be minimized in future generations. However, it is essential to keep channel
carrier mobilities high. Without any amorphous interfacial layer spacing the defects and the soft phonons of the high-K oxide away from the channel electrons, there might be severe reduction in mobility and trapping.

.....Generally, there are two groups of possible candidates for epitaxial growth on Si, namely (a) perovskite-type structures and (b) binary metal oxides, in particular lanthanide oxides.

Introducing crystalline rare-earth oxides into Si technologies

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