2, Imperial College London, London, , United Kingdom
In recent years, much effort has been made to find alternative materials and geometries to achieve ultra-small stable electronic devices. Among all, high electron mobility transistors (HEMTs) are very promising in terms of both high carrier mobility and high breakdown voltage. However, the commonly used AlGaN/GaN interface still shows limitations, such as high defect concentrations and gate leakage, that need to be overcome. For this reason, the exploration of novel ternary and quaternary nitrides, as well as the engineering of their lattice parameters, band gaps and their relationship, have been encouraged.
Transition metal nitrides (TMN) have recently received increasing attention due to their unique electronic and structural properties . The optoelectronic properties of ScxGa1-xN are very interesting for HEMTs: ScN and GaN are stable in two different structures, therefore, as x increases from 0 (GaN) to 0.5 (Sc0.5Ga0.5N) the wurtzite structure distorts, the c/a ratio decreases, eventually producing a structural phase transition to a non-polar 5-fold coordinated hexagonal crystal structure. In the composition range around this phase transition, ScxGa1-xN shows unique properties, such as huge piezoelectric constant, high electron mobility, and good lattice match with GaN.
Another interesting TMN material for electronic application is FeGaN . If GaN is doped with small concentration of iron, the thin film shows semi-insulating properties with high quality structure and high resistivity. When FeGaN is employed in HEMTs, its semi-insulating behaviour will better isolate the device by lowering the leakage from both the gate and the substrate interfaces.
In addition, the quaternary material FeScGaN is expected to show intermediate properties between the two ternary TMN materials. Thus, if it is employed in ScGaN/GaN HEMTs, the gate leakage will decrease without introducing lattice-mismatch defects at the FeScGaN/ScGaN interface.
To successfully employ these novel materials in HEMT devices, it is then necessary to achieve deep knowledge of both their electronic and structural properties. For this reason, ScGaN, FeGaN and FeScGaN thin films are grown on sapphire using Electron Beam Epitaxy technique and then fully characterized. HR-TEM, STEM, XRD are used to explore the change in the microstructure of all the thin films for different TM content. SIMS is employed to calculate both the TM element content and the level of impurities in each film. Finally, the electron mobility, band gap and Raman shift are reported to investigate the electronic and optical properties of these materials and their dependence on the TM concentration.
These TMNs show high-quality crystal structures and larger band gap for higher TM content; from these preliminary results ScGaN, FeGaN and FeScGaN seem to be promising materials for enhanced electronic devices.
 M.A. Moram S. Zhang, J Mater Chem A, 2014,2, 6042-6050
 A. Bonanni, Semicond Sci Technol, 2007, 22, R41-R56