Hafnium oxide breakthrough paves way for next gen electronic devices

The material is a new form of hafnium oxide - an electrical insulator used in optical coatings, capacitors and transistors – and was developed by Dr Andrew Flewitt's research group in the Department of Engineering at the University of Cambridge. According to Dr Flewitt, the material provides 'exceptionally high' dielectric constant compared with existing forms of hafnium oxide. Traditionally, metal oxides are produced on substrates by sputtering, a process by which some of the atoms of an electrode are ejected as a result of bombardment by heavy positive ions. However, the sputtering process makes it difficult to precisely control the energetic of the deposition process and, as a result, the material properties – such as defect density. In a bid to enable greater control, the researchers used a novel deposition technology known as High Target Utilisation Sputtering (HiTUS) to promote plasma sputtering. One of the first materials the Cambridge team looked at using HiTUS was hafnium oxide - a key material in the electronics industry. A number of companies are currently using hafnium oxide to replace silicon dioxide in transistors, due to its high ratio of electric displacement in a medium to the intensity of the electric field producing it, known as a dielectric constant. The higher the dielectric constant of a material, the higher its capacitance - the ability to store an electric charge. Hafnium oxide forms in different crystalline and polycrystalline structures: monoclinic, cubic and orthorhombic. However, according to Dr Flewitt, an amorphous form is preferable to polycrystalline forms due to the absence of grain boundaries, the point at which two crystals in a polycrystalline material meet. Grain boundaries act as conduction paths through thin films of the material. They not only reduce the resistivity, but lead to a non-uniformity in conductivity over a large area, which itself leads to spatial non-uniformity in device performance. Until now, amorphous hafnium oxide has had a relatively low dielectric constant of around 20. The form of hafnium oxide developed by Dr Flewitt has a dielectric constant higher than 30. "Most people thought that all amorphous hafnium oxide had to exist in the monoclinic-like phase," said Dr Flewitt. "What we've shown is that it can exist and does exist in a cubic like phase. This is similar to amorphous carbon, where you can get diamond like properties out of amorphous carbon material." According to Dr Flewitt, amorphous dielectrics are more homogenous than other forms, allowing improved uniformity from one device to another, and the absence of grain boundaries results in higher effective resistivity, as well as less optical scatter. The material is produced using a room temperature, high deposition rate process. This makes it particularly suitable for plastic electronics and high volume semiconductor manufacturing, while the absence of grain boundaries makes it ideal for optical coatings and more efficient solar cells. Cambridge Enterprise, the University's commercialisation group, is currently seeking commercial partners for collaborative development and licensing of this material.