Characterisation of ScN and ScGaN alloys grown by Molecular Beam Epitaxy

The wurtzite III-nitrides AlN, GaN and InN are currently widely used in optoelectronic applications such as light-emitting diodes and this success is owed in large part to the possibility of band gap engineering by alloying between the constituent nitrides. Unfortunately, the restricted material properties currently limit the performance of such devices, and efficiencies of emitters in the green and ultraviolet spectral regions remain low. These challenges have motivated the search for alloying additions offering greater degrees of freedom for tuning the material properties. ScN is of particular interest in this regard, and ScGaN alloys have been predicted to remain stable in the wurtzite structure with direct band gaps for Sc contents of up to 27%. Of further interest is the predicted deviation from wurtzite symmetry towards a non-polar structure at higher Sc contents, which could lead to additional device functionalities such as ferroelectric switching. However, experimental data regarding the properties of ScGaN alloys are very limited, and further investigations of the growth are required if these materials are to be integrated into the existing III-nitride technology. This thesis is an in-depth characterisation of ScN and dilute ScGaN films grown by molecular beam epitaxy (MBE), with special emphasis on the microstructure, surface topography and local bonding environment of Sc in these materials. The results are based on a broad range of characterisation techniques including transmission electron microscopy, scanning probe microscopy, high-resolution X-ray diffraction and X-ray absorption spectroscopy (XAS). The effects of varying growth conditions on the properties of ScN and ScGaN films are discussed, including observed growth rates and defect densities. The introduction of Sc into the GaN system was found to decrease growth rates due to the likely presence of a ‘floating layer’, as has also been observed for other transition metal doped nitrides. Microstructural investigations found that at low Sc contents the formation of I1-type basal-plane stacking faults is promoted, while at higher contents lamellar inclusions of the cubic phase of up to 4 stacking repeats were observed. The local flattening of the wurtzite unit cell around substitutional Sc atoms was directly observed by XAS measurements, confirming theoretical predictions and giving confidence that related predictions for higher Sc content films are also correct