9th October 2020

Materials Approaches

The approach at Tampere uses seeks to pattern metal films in an environment containing super-critical CO2 (>100 Bar) in order to achieve controllable localised oxidation of the metal in the regions patented by the laser interference. This is an entirely new approach and many parameters/variables have need to be explored, including the laser pulse length, number of pulses, laser wavelength, CO2 pressure etc. the laser interference in this case is formed using a diffractive optical element which produces a square array. The angle of incidence is around 6 degrees.

Initial studies have focused on titanium (left) and zinc right) surfaces using four beam interference at 1064nm and shows a good patterning capability

The MBE approach at Sheffield seeks to use in-situ laser interference to direct the growth of semiconductor structures, including quantum dots and nanowires. It uses a free space 4 beam interference set up connected to an MBE reactor. The four beams come together on the heated substrate at 58 degrees to produce a square pattern. We visualise the UV laser spots using a InGaN wafer, which helps us with the alignment.

One of the most significant findings we have observed is the ability to induce both holes and islands sometimes even on the same GaAs wafer with the same laser exposure, but at different regions of the wafer. The effect, we believe, is as a consequence of different laser fluence, with islands generated by diffusion towards the cool regions of the pattern (interference minima) and the holes at interference maxima due to diffusion from region of high local heat

When we attempt to grow InAs quantum dots on this patterned surface we are able to nucleate these at the edges of the nanoislands. We have proposed a mechanism to explain this as due to migration of indium under the influence of the thermal pattern such that the indium moves towards the island where it reaches a step edge which is a preferential site for nucleation

We have subsequently been able to demonstrate excellent control over site occupancy and to extend this work to include patterned droplet nucleation on Silicon substrates

Work at CIET has focussed on the development of a new technique we call Laser Enhanced Aerosol Assisted Chemical Vapor Deposition (LEAACVD). This uses an interference laser pattern to enhance CVD growth.  Since this approach is very new, it has required much experimentation with the laser conditions, but is now showing good results.

The approach uses a small reaction vessel to which laser light is applied to the substrate (typically a metal film or silicon substrate) via a large glass window. The approach uses a DOE to achieve the interference pattern. Th reactant gas is passed through an atomiser and fed into the reaction vessel where its reaction rate at the substrate is controlled by the laser pattern induced temperature gradient.

The figure shows the reaction vessel after growth and the results of ZnO growth (note, this is laser assisted, but without the interference.