Metal Oxides for Thin Film Transistors
In recent years, metal oxides have emerged as a viable alternative to amorphous silicon in thin film transistors for display applications. This is due to their significantly higher carrier mobility and potentially improved stability. Furthermore, these materials are more suited to plastic substrates, and so open up a range of new plastic electronics aplications. Our work in this field, much of which has been performed through the Cambridge Integrated Knowledge Centre, has focussed on two general themes. The first is manufacturability. Magnetron sputtering, which is most commonly used to deposit these materials, suffers from problems in terms of reproducibility and scale-up. We have been investigating the use of other deposition technologies, including a High Target Utilisation (HiTUS) sputtering system, which are more suited to scale-up. Using the HiTUS system we have demonstrated a new phase of amorphous hafnium oxide - cubic-like amorphous hafnium oxide (ca-HfOx) - which has a fully amorphous structure and a dielectric constant of over 30, and is a signficiant improvement over the state-of-the-art. These materials have been incorporated into a variety of devices.
Current research is focused on understanding the instability mechanisms in metal oxide thin film transistors, developing more stable n-type materials and investigating p-type materials.
I am also investigating high speed testing regimes for large-area electronics as part of the EPSRC Centre for Innovating Manufactiuring in Large-Area Electronics.
Acoustic Wave Sensors
Our work on acoustic wave sensors is based on both surface acoustic wave (SAW) devices and film bulk acoustic resonator (FBAR) devices. In both cases, we most commonly use zinc oxide that is sputtered with a very low stress (less than 100 MPa) using our HiTUS sputtering system (see above). This has allowed us to produce FBAR devices with operating frequencies between 1 and 2 GHz and with a Q-factor of ~1500. Most notable, we have shown that carbon nanotubes can be used to form the top electrode of these devices. This allows a higher surface area and hence a signficnat increase in sensitivity. We have also developed a new 'dual-mode' device in which both mass and temperature changes can be simultaneously measured with just one device. This obviates the need for either a control device or a regulated temperature environment, making sensing at the point of use possible. Mass sensitivit of our devices is below 1 fg, which is around the mass of a single virus. These devices are being developed for biosensing and gas sensing applications.
Metal Oxides for Solar Cells
We have also started to use our metal oxide materials for solar cells. In particular, we can produce a high mobility cuprous oxide, which is p-type and has a Hall mobility of ~10 cm2 V-1 s-1. Cuprous oxide has a band gap ~2 eV, and so is ideal for the absorber layer in solar cells.
We also work on zinc oxide nanowires for tactile surfaces. This again uses the piezoelectric properties of zinc oxide to both detect deflection and produce deflection. We use thin films of nanowires to make touch-sensiotive surfaces for future generations of touch-sensitive displays and surfaces.