Research


Materials


II-IV Nitrides

The Group II-IV nitrides have emerged as promising materials for optoelectronic applications in combination with the Group III nitrides. Group II-IV nitrides can be derived from Grop III nitrides by substituting pairs of group III (Al, Ga or In) atoms for a single group II (Be, Mg, Ca or Zn) atom or single group IV (C, Si, Ge or Sn) atom, which maintains the local valence requirements. Some transition metals in the +2 or +4 oxidation state can also be used, e.g. Mn or Cr.

The similar metal-nitrogen bond lengths in II-IV nitrides to III-nitrides means that it should be possible to fabricate epitaxial heterostructures using both materials, e.g. [001]-orientated MgSiN2 on [0001]-orientated GaN.

Read more: "Band gap and electronic structure of MgSiN2" Applied Physics Letters 105 (11) 112108 (2014).

Graph of band gap against lattice parameter for common III-nitrides and MgSiN2

Sc-based Nitrides

Many traditional III-nitride-based devices have limited efficiences due to a variety of factors including the quantum-confined Stark effect, which produces a spatial separation of electrons and holes due to a mismatch in internal polarisation of different materials at interfaces. Many techniques have been developed to try to reduce this effect, but ultimately there is a limit to what can be achieved using existing materials.

Sc-based nitride alloys (ScGaN and ScAlN) provide an attractive alternative by offering significantly higher exciton binding energies and piezoelectric coefficients than traditional III-nitrides while still having a tunable, direct band gaps between those of GaN and AlN.

Read more: "ScGaN and ScAlN: emerging nitride materials" Journal of Materials Chemistry A 2 6042 (2014).

The phase transition between rock-salt and hexagonal structures for Sc-based nitrides.

Defects in III-Nitrides

Dislocations are a one-dimensional line defect that typically exist in very high densities in nitrides-based devices, far higher than in other semiconductor devices. These are associated with a wide range of undesirable properties such as reduced efficiences and lifetimes. While dislocations have been extensively studied, they are still not well understood especially in regards to how they interact with other species – such as native point defects, impurities, dopant atoms and alloying additions.

We perform a wide range of combined theoretical and experimental studies to better understand the dynamics of how dislocations behave during growth, how concentrations of different species change close to dislocations, and the impact this ultimately has on device performance. Through better understanding of dislocations in the nitrides, we might be able to take advantage of them to be able to manipulate composition profiles on small length scales and to help design new devices.

Read more: "Segregation of In to Dislocations in InGaN" Nano Letters 15 (2) 923-930 (2015) and "Mg doping affects dislocation core structures in GaN" Physical Review Letters 111 (2) 923-930 (2015).

Segregation of In to an edge dislocation in an InGaN alloy.

UV Emitters

Ultraviolet light-emitting diodes have a wide range of potential applications including water treatment, lithography, UV curing and "solar-blind" communication. Current UV-LEDs based on AlGaN typically have limited efficiencies, and so the search for alternative wide band gap materials is very important. Part of our group's research is searching for new candidate UV emitters.

Using UV LEDs for water treatment is also a specific focus for our group, with research being performed on the optical wavelength of light for water steralisation and on the optimum geometries for water treatment units.

Read more: "Optical fluence modelling for ultraviolet light emitting diode-based water treatment systems" Water Research 66 338 (2014).

Array of UV LEDs used in one of our water-treatment experiments.

Techniques


Crystal Growth

We use the EPSRC electron beam physical vapour deposition (EB-PVD) facility which spans two separate ultra high vacuum systems: one dedicated to growing thin film nitrides, the other to oxides and oxynitrides.

The nitrides system boasts three EB-PVD sources (up to 10 kW capability), in-situ Reflection High Energy Electron Diffraction (50 keV source) with a detector for Total Reflection Angle X-ray Spectroscopy (RHEED-TRAXS), and a nitrogen RF atom source. The system also possesses an integrated Raman spectrometer which can perform measurements in or out of vacuum with the option of 532 nm or 785 nm lasers.

The oxides system comprises two EB-PVD sources and two RF atom sources: one for oxygen, the other for nitrogen. Common to both systems are a rotating substrate table with a heater capable of reaching 1000°C, and a tilt controller on each EB-PVD source. We also have a two colour pyrometer which can be utilised in situ on either system to accurately determine surface temperature, reflectance and emissivity during the growth process.

Our nitrides deposition system.
Our oxides deposition system.

Characterisation

At Imperial, we have a wide range of characterisation techniques available to our group, including:

  • High-resolution, aberration-corrected transmission electron microscopy (TEM/STEM)
  • Surface analysis including Time-of-Flight Secondary Ion Mass Spectroscopy (SIMS) and Low-Energy Ion Scattering (LEIS)
  • Atomic force microscopy (AFM)
  • High-resolution X-ray diffraction (XRD)

We also make use of synchrotron facilities at the Diamond Light Source, and also participate in many collaborations.

Imperial's FEI Titan 80-300 Transmission Electron Microscope

Theory

We perform a wide range of electronic structure calculations using the CASTEP and VASP density functional theory codes, both here at Imperial's HPC facility and on the UK's National Supercomputing Service, ARCHER.

We also perform a range of classical dynamics and Monte Carlo simulations over a range of length scales, as well as dislocation dynamics calculations using our custom code PANIC.

ARCHER - the UK National Supercomputing Service

Department of Materials
Imperial College London
Royal School of Mines
Exhibition Road
London, SW7 2AZ - UK