CoFe2O4/buffer layer ultrathin heterostructures on Si(001)

Abstract

Epitaxial films of ferromagnetic CoFe 2 O 4 (CFO) were grown by pulsed laser deposition on Si(001) buffered with ultrathin yttria-stabilized zirconia (YSZ) layers in a single process. Reflection high-energy electron diffraction was used to monitor in real time the crystallization of YSZ, allowing the fabrication of epitaxial YSZ buffers with thickness of about 2 nm. CFO films, with thicknesses in the 2-50 nm range were subsequently deposited. The magnetization of the CFO films is close to the bulk value. The ultrathin CFO/YSZ heterostructures have very flat morphology (0.1 nm roughness) and thin interfacial SiO x layer (about 2 nm thick) making them suitable for integration in tunnel (e.g., spin injection) devices. V C 2011 American Institute of Physics.[doi:10.1063/1.3651386] Device downsizing has been the causing factor of the continuous progress in microelectronics. But scaling is approaching a limit, and further improvements will finally require the integration of new materials.1 Oxides with remarkable properties are good candidates. An example is the replacement of SiO x by high-k oxides such as HfO 2 in MOSFETs. Other complex oxides are also of high interest. However, in most of the targeted applications, they have to be crystalline and oriented (i.e., epitaxial) on silicon platforms, which is a challenging task regarding the dissimilarities between oxides and silicon, both structurally and chemically. Ferroelectric (FE) oxides are now starting to be used for FE random access memories as potential alternatives to flash memories.2 In contrast to FE oxides and despite the expected bright future of spintronics in semiconductor technology, [3][4][5] the use of ferromagnetic (FM) oxides in microelectronic devices appears to be distant. This is probably due to the elusive integration of epitaxial FM oxides with silicon. Therefore, epitaxial growth on silicon wafers with controlled characteristics and properties comparable to those of films on oxide single-crystalline substrate is crucial in the development of new functional oxide-based heterostructures. A recent example is the successful preparation of the twodimensional electron gas at interfaces between LaAlO 3 and SrTiO 3 (STO) on Si(001).6 In this case, the STO film is a part of the functional heterostructure but can also act as a buffer layer. The use of a buffer layer is a common requisite for the integration of most of the complex oxides. STO can be grown by molecular beam epitaxy on Si(001) using wellestablished methods.7-10 However, to integrate complex oxides with large lattice mismatch and/or chemical interaction with STO, different buffers have to be considered. In that case, yttria-stabilized zirconia (YSZ) is an alternative.11-15 YSZ has been grown epitaxially on Si(001), with crystallization after reduction of the native oxide. 16,17 However, it had been found that in ultrathin YSZ films islands are formed in the early growth stages, and although coalescence and flattening were observed for thicker YSZ films, 16 the roughness at the initial stage ...

Epitaxial integration of CoFe2O4 thin films on Si (001) surfaces using TiN buffer layers

“…However Mg inter-diffuses into the CoFe 2 O 4 thin films at the high temperatures required for good quality growth [24,25]. Yttria-stabilized zirconia (YSZ) template layers or double CeO 2 /YSZ buffer layers also provide a good platform to integrate CoFe 2 O 4 with silicon (001) [26,27]. For the integration on Si (111) wafers, buffer layers of Sc 2 O 3 have also explored [28].…”

Atomically Flat Ultrathin Cobalt Ferrite Islands

“…Strategies for fabricating high quality CFO nanostructures while preserving the ability to tune its physical properties are thus highly appealing even though extremely demanding. Substantial progress has been made in the development of high‐quality CFO thin films through a variety of methods, recently also at low growth temperatures . However, the issue of APBs remains.…”

Integration of functional complex oxide nanomaterials on silicon

“…Following the same philosophy, Yttria-stabilized zirconia (YSZ) grown by PLD has also been used as an effective buffer layer to develop the technology of oxide films on Si(001) [27]. This resulted in the integration of functional ferromagnetic spinel oxides [28] or ferroelectric perovskite oxides [29].…”