Our Research | johnsongroup

Our Research

Research within the Johnson group is broad and looks at precursors for materials with a wide range of applications. We also collaborate within a range of people from academic and industry, details of our collaborators are shown below

Precursor Development for the ALD Growth of Metal Oxide Thin Films

Atomic layer deposition (ALD) is a film growth technique that is expected to be broadly implemented by the semiconductor industry to meet future microelectronics manufacturing demands. What makes ALD unique is the surface—limited film growth mechanism which provides the atomic level control over the thin film growth. Unfortunately, for ALD to work we require volatile precursor molecules with excellent thermal stability and high reactivity toward a second reagent such as water or ammonia. Few existing chemical vapour deposition (CVD) precursors meet the requirements for use in ALD, so there is an urgent need for new ALD precursors with optimised properties.

Our focus is on the development of NEW ALD precursors that contain labile ligands, with the right balance of reactivity and stability. We have discovered a range of oxygen and nitrogen base donor ligands that provide volatile, thermally stable, and reactive precursors for a range of metals, beyond the usual Ti, Al and Zn. We are exploring the ALD growth of thin films from these precursors. Our laboratory is equipped with a Beneq TF200 ALD reactor, and we also collaborate with a number of groups across the UK and Europe in ALD film growth and characterisation.

Thin Film Precursor Development for “Difficult” Elements

For thin film growth by ALD and CVD it's a general requirement that the metal-containing precursor is sufficiently volatile such that an acceptable film growth rate can be achieved. For "big" elements such as strontium, barium, and the lanthanides, the inherent high atomic weight of these metals reduces the volatility of any compound, relative to compounds containing lighter metals. As a result, precursors containing strontium, barium, and the lanthanides often need to be heated to high temperatures to achieve reasonable vapour transport. Excessive heating can lead to precursor decomposition, which makes ALD and CVD thin film growth processes incredibly difficult. To address this problem, we are exploring the use of AACVD to circumnavigate the use of high temperatures and to avoid the need for incredibly complicated precursor ligands.

Deposition of Metastable Materials Using ALD

Controlling the oxidation state of the metal you are depositing has a huge effect on the properties of the final product (conductivity, carrier concentration and optical properties, etc). We have recently discovered that the ALD process, employing new Sn(II) precursors we have deployed in our lab, and water, affords crystalline thin film of SnO between 130 and 190 °C. This deposition process is remarkable because it affords the sparsely documented SnO, and also because it demonstrates that controlling the oxidation state of the precursor can be used to control the oxidation state in the thin film material.

Such powerful control over thin film properties is sparsely documented in ALD and CVD film growth. We are exploring the ALD growth of many metastable oxides, and chalcogenide phases using low- and mid-valent precursors. At the heart of this work is the development and synthesis and of new low- and mid-valent complexes that targeted precursor properties. Target materials include metastable main group oxide and sulfide phases as well as the low valent metals of group 5—6.

Carborane Chemistry

The dicarbollide ligand [C2B9H11] has been widely exploited in the chemistry of middle and late transition metals in low oxidation states. The nido—carborane dianion, [C2B9H11] , and its derivatives are ligands which are able to stabilise high oxidation state early transition metal and lanthanide organometallic complexes, in addition to main group elements. These complexes provide a contrast in electronic properties to the analagous cyclopentadienyl complexes; indeed there is a strong relationship between the dicarbollide anion, cyclopentadienide and also with metal imido chemistry. Specific areas of current interest include (but are not limited to) the chemistry of main group metal and early transition metal dicarbollide complexes and exploring their novel chemistries.

Thin Films with Novel Mechanical Properties

Automotive lubricants contain a range of chemicals that are designed to decompose and form inorganic films that are either low friction or prevent mechanical wear. The current generation of chemicals contain significant amounts of elements that poison catalytic converters (such as Zn, S and P), therefore limiting their use. Our research aims to design a range of film-forming precursors without these elements, we then test the mechanical properties in collaboration with the University of Leeds.

Collaborators

We collaborate with a range of different people, both within academia and the University of Bath, as well as with some industrial partners (see below). If you're interested in collaborating with us please get in touch!

University of Bath

Dr Petra Cameron, Department of Chemistry
Prof Michael Hill, Department of Chemistry
Dr Matthew Jones, Department of Chemistry
Prof Paul Raithby, Department of Chemistry
Prof Michael Whittlesey, Department of Chemistry
Dr Salvador Eslava, Department of Chemical Engineering
Dr Duncan Allsopp, Department of Electronic & Electrical Engineering
Dr Daniel Wolverson, Department of Physics

Academia

Dr Ibrahim Ahmet, Helmholtz—Zentrum Berlin
Dr Mark Fox, University of Durham
Prof Anne Neville, University of Leeds
Prof Philip Power, Univeristy of California, Davis
Dr Yann Sarazin, University of Rennes 1

Industry

Infineum UK Ltd
Pilkingtons NSG Ltd
Pragmatic Printing