Researchers devise new approach to anchor metals to metal oxides

A new method of depositing metal atoms on very thin oxide layers may help next-generation computers boot up instantly, making entire memories immediately available for use.

A newly patented way to deposit metal atoms on very thin oxide layers may help next-generation computers boot up instantly, making entire memories immediately available for use.

The technique also may help fabricate less expensive catalysts for chemical reactions and lead to better nanotechnology devices and ceramic or metal seals.

The method is said to anchor ultrathin metallic layers to metal oxides by using a chemical reaction discovered at the Pacific Northwest National Laboratory and understood and generalised by theoretical scientists at Sandia National Laboratory in Albuquerque.

The inexpensive trick bypasses the hurdle created when metal atoms cluster together into three-dimensional islands when deposited on oxide surfaces. These ultrasmall islands of metal produce discontinuous, non-crystalline metal films. The new smooth interfaces achieve crystallinity by only a few atomic layers and should also produce greater durability in electronic devices.

The findings may have the most immediate bearing on magnetic tunnel junctions, slated for use in magnetoresistive random access memory, or MRAM. MRAM will allow computers to store information in a non-volatile fashion. As a result, MRAM will allow computers to boot up instantly once turned on, rather than slowly retrieving information during the boot-up stage.

In a magnetic tunnel junction, an ultrathin layer of insulator, typically aluminium oxide with a thickness of less than one nanometer, is sandwiched between thin layers of magnetic metal, such as cobalt or nickel-iron.

Current flows through the device and the magnetic orientation of the two metal layers can be switched, resulting in different values of the tunnelling current, thereby creating an environment in which ‘bits’ of computer memory can be stored.

Yet difficulties in growing an atomically flat, ultrathin film of metal on top of any insulator material have been well documented for years. In order to achieve ferromagnetism, thick layers of the top metal must be made. The new discovery should allow for much thinner layers of metal and lower currents needed to switch the direction of the magnetic field.

The new method uses a chemical reaction to embed metal atoms at scattered points within the top layer of the oxide, amounting to about one anchor for every ten oxygen atoms in the top layer. These anchoring atoms are then able to bind other metallic atoms just above the oxide surface.

‘Many advanced technologies rely on strong interfaces between metals and oxides,’ said Scott Chambers, PNNL chief scientist. ‘These findings are very exciting because they may provide the molecular insight industry needs to create better materials for microelectronics and sensors.’

Chambers worked in partnership with PNNL’s Tim Droubay, who helped with the experiments, and Dwight Jennison, a solid-state theorist from the US Department of Energy’s Sandia National Laboratories.

‘The process Scott tested concerns growing cobalt on aluminium oxide. Cobalt’s interaction with the oxide is so weak that it would normally ball up when deposited,’ said Jennison. ‘However, if the surface of the oxide is first completely hydroxylated, i.e. is terminated by a layer of hydrogen and oxygen atoms bound together, cobalt atoms which hit two hydroxyl groups at once can react to release a hydrogen gas molecule. These cobalt atoms then become oxidised themselves and end up in the top layer of the oxide, surrounded by negative ions to which they bind strongly. These are the anchors.’

‘For industry, a solution may be as simple as exposing the thin aluminium oxide films to a low pressure of water vapour before adding a final cobalt layer,’ said Chambers. The entire process may be done at room temperature, while it is often important to avoid high temperatures in manufacturing.

Jennison, who first found which chemical reactions would be energetically favourable, collaborated at Sandia with Thomas Mattsson, who has long experience in first principle based diffusion and reaction studies and in computing critical reaction barriers. Their theoretical first principles calculations predicted some and validated other experimental results.

Notably, the calculations provided insight into what reaction is taking place, where it occurs, the energy barrier for it to happen, and the time it needs to be completed versus the time it takes arriving cobalt atoms to lose energy while in contact with the surface. In particular, the latter is important because if the reaction were slow, the rapidly diffusing cobalt atoms could find a growing island first.

However, because hydrogen molecules are being made, the reaction can be very fast, of the order of tenths of a picosecond and well before the arriving cobalt atoms can assume the temperature of the substrate. ‘Otherwise the experimental result would be impossible to explain,’ said Jennison. ‘However, here we have a wonderful joining of theory and experiment.’

While the experiment was conducted using cobalt, Jennison’s calculations predict the method also would be effective for iron and nickel, two other metals under consideration for MRAM, as well as metals such as copper, ruthenium, and rhodium, the latter two having catalysis applications.