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Materials for Energy efficient ICT

We work on designing and nanoengineering oxide thin film to create improved energy efficient ICT devices.

We work on designing and nanoengineering oxide thin film to create improved energy efficient ICT devices. Some of the work is basic science (i.e. understanding interface effects in electronic oxide films) while other parts are more applied.  Below we discuss recent example results on magnetoelectrics, magnetics, and oxides for CMOS flexlogic.

a) Nanocomposite magnetoelectrics

Electric field control of magnetism could provide for low power, ultra-high density non-volatile memory. However, despite intensive research efforts, no practical materials systems have emerged. Interface-coupled, composite systems containing ferroelectric and ferri-/ferromagnetic elements have been the most promising, but they have many problems, e.g. substrate clamping, large unwanted currents (leakage), and they cannot be miniaturized to give high density recording. Through careful materials selection, design, and nanoengineering, we have demonstrated a high-performance room temperature magnetoelectric system. A vertically aligned nanocomposite structure in which the strain coupling is independent of the substrate was used and a new, low leakage ferroelectric material was employed. A large converse magnetoelectric coefficient was achieved of >10-9 s m-1

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Figure 1. ME effect in the VAN film. (a) AFM image of the sample surface. (b) Schematic illustration of the strain mediated ME effect VAN films. (c) Magnetic hysteresis loops along the OOP direction and (d) magnetic hysteresis loops along the IP direction with and without applying in-situ voltages.

b) New Science of Oxide Interfaces

Unique opportunities exist to enhance and create emergent physical properties of complex oxide interfaces by controlling the interactions between adjacent layers. Examples include the formation of a 2DEG, novel ferroelectric effects, magnetoelectric coupling. Charge transfer, atomic transfer and orbital reconstruction effects can all play a role. The Driscoll group grows films and superlattice structures using advanced PLD, and analyses the chemical and electronic states across the interfaces using XPS and synchrotron techniques such a polarized neutron reflectivity, XAS, XMCD, etc.

Example work in this area is:

- Hidden interface effects in ferromagnetic La1-xAxMnO3 (A = Sr, Ca; x = 0.2, 0.3), e.g. LSMO, LCMO thin films on SrTiO3 (Fig. 2). Iinterface driven exchange coupling effects was observed. An interfacial antiferromagnetic (AFM) layer arising from non-uniformity of Mn3+/Mn3+ and Mn3+/Mn4+ in the film plane was determined, as well as a canted spin layer below the intrinsic ferromagnetic layer. The work was a collaboration between Los Alamos National Lab (with our close collaborator, Aping Chen: http://www.lanl.gov/expertise/profiles/view/aiping-chen

as well as with Argonne National lab, Oak Ridge National Lab, SUNY, Buffalo and ourselves.

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Figure 2. Spin configuration of nominally ferromagnetic manganite oxide heterostructures with an interfacial layer formed between a manganite layer and a SrTiO3 substrate. Reproduced with permission from A. Chen et al., 10.1002/adma.201700672.

c) Magnet Property Tuning Using a Nanocomposite Approach. 

Ferromagnetic insulating oxides (FMI) are of great research interest because of rare combination of ferromagnetism and insulating characteristics which are needed for oxide spintronics and multiferroics.  However, current materials don’t exhibit the required properties at room temperature. A further problem is that because of nanoscale interfacial effects, e.g. as shown in b) above, film properties are unintentionally different (and often in a detrimental way) to bulk properties.

A way is needed to induce the required properties in films and to do it in simple and reproducible way. We have shown that by using nanocomposite films, with the right materials choices, and with a clear understanding of the materials science, it is possible to radically change the magnetic properties for the better.  We highlight a couple of examples for Sm0.34Sr0.66MnO3 (SSMO) and for ZnFe2O4 (ZFO).

The first example is PLD-grown, self-assembled nanocomposite Sm0.34Sr0.66MnO3 (SSMO) + Sm2O3 films. Here, we have demonstrated that the AF properties of plain films can be switched to 140K ferromagneting insulating properties (see Fig. 3 below). The reason for the observed phenomenon is that nanocomposite films have different and unusual strain states.

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Figure 3. Nanocomposite films of Sm1-xSrxMnO3 (SSMO, x=0.63) + Sm2O3 from DOI: 10.1039/c6nr01037g

The second example is PLD-grown, self-assembled ZFO films confined in a mesoscopic STO matrix. Here, we demonstrated that magnetic properties can be switched from antiferromagnetic to ferrimagnetic. This is different to the antiferromagnetic behavior shown by bulk ZFO and plain ZFO films, including a much enhanced saturation magnetization and a Curie Temperature of 500K. The result is contrast to the LSMO work discussed in a), since instead of inducing an interfacial AF layer in a magnetic film, here we induce interfacial magnetism in a magnetic film (see Fig. 4 below).

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Figure 4. Epitaxial nanocomposite film of ZnFe2O4 (ZFO) in a SrTiO3 (STO) mesoscopic matrix, and corresponding magnetic properties compared to a plain film. Figures from Park C, Wu R, Lu P, Zhao H, Zhan B, Li W, Yun C, Wang H, J. L. MacManus-Driscoll JL*, Cho S*, Use of A Mesoscopic Host Matrix to Induce Ferrimagnetism in An Antiferromagnetic Spinel Oxide, Advanced Functional Materials, in press, 2018. More info. on Dr. Seungho Cho’s work can be found here: http://scho.unist.ac.kr

d) Atmospheric pressure spatial ALD (AP-SALD) for CMOS printed logic

High quality, low-temperature-grown thin film metal oxides are urgently needed for a wide range of emerging electronic applications relating to the Internet of Things. Low power consumption is key, particularly in devices for energy harvesting and for RFID. Reducing power consumption would greatly expand the technology’s market opportunities.  We are particularly interested in printed logic on flexible substrates. Owing to the low availability of simple p-type materials with sufficient carrier concentration and mobility, the focus to date has been on nMOS logic. CMOS is highly preferable, however, owing to the low power requirement. However, CMOS requires both n-type and p-type materials, but simple p-type materials with the required carrier concentrations are scare. Hence, we are developing p-type oxides grown at low temperature using AP-SALD.

 

A recent review we have written on the prospects and challenges of ALD for oxide electronics will be published in the “Roadmap of Oxide Technologies for Electronic Applications 2018” (MacManus-Driscoll JL and Napari M, Atomic Layer Deposition of Oxides:  Benefits, Challenges and Future Directions, Applied Surface Science, 2018). Some example information on our recent growth of p-type NiO films is shown in Fig. 5 below.

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Figure 6. AP-SALD system schematic with precursors shown for growth of p-type NiO films, AFM image of film grown on glass with RMS roughness of 0.67 nm, and Mott Shottky plot indicating hole carrier concentration of 1018cm-3, courtesy of Lana and Mari Napari (unpublished).

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