Details of the development of MBE-grown MgS thin films, including the growth optimization, crystal-structure and chemical characterization and the studies on the scientific issues involved in these research works were addressed in this thesis. The fabrication and characterization of a novel MgS-based UV-detector system, which is suitable for flame sensing applications, was also described in this thesis.

High quality MBE-grown single-crystalline MgS thin films on GaAs substrates of two different orientations were successfully prepared. The respective optimized conditions of growth were identified. The prepared thin films were characterized by both in-situ and ex-situ techniques of RHEED, AES, EBSD and HRXRD to confirm their chemical compositions and crystal structures. It was found tha...[ Read more ]

Details of the development of MBE-grown MgS thin films, including the growth optimization, crystal-structure and chemical characterization and the studies on the scientific issues involved in these research works were addressed in this thesis. The fabrication and characterization of a novel MgS-based UV-detector system, which is suitable for flame sensing applications, was also described in this thesis.

High quality MBE-grown single-crystalline MgS thin films on GaAs substrates of two different orientations were successfully prepared. The respective optimized conditions of growth were identified. The prepared thin films were characterized by both in-situ and ex-situ techniques of RHEED, AES, EBSD and HRXRD to confirm their chemical compositions and crystal structures. It was found that the rocksalt (RS) phase is favored for MgS grown on zinc-blende (ZB) GaAs(100) substrates. This can be attributed to the lacking of required interface species for forming ZB MgS as MgAs has not been shown to exist and GaS was only reported to exist in hexagonal phase. MgS, being ionic in nature, therefore favors its most thermally stable phase of RS structure instead of the lattice-matched ZB phase when it was grown on a ZB GaAs(100) surface. This is similar to ionic thin films of RS NaCl, RbF and MgO forming on diamond and ZB substrates. Deviation of the above expectation was also addressed for wurtzite (WZ) MgS resulted in its growth on GaAs(111)B substrates. This is due to the rather small (~0.8%) lattice mismatch, existence of hexagonal GaS and Mg₂As₃ interface species and the non-thermal equilibrium growth condition in MBE. These underlying mechanisms addressed provide insight in the preparation of non-thermally-stable phases of thin-film material when a non-thermal-equilibrium growth technique is used.

Photoresponse curves of RS MgS-based and WZ MgS-based UV detectors display 4 to 5 orders of visible rejection with cutoff wavelengths at 235nm and 245nm respectively. These values correspond to photon energies of 5.28eV and 5.06eV. The estimated bandgap of 5.06eV for WZ MgS is also consistent with the results obtained from the reflectance measurement conducted, in which a band-edge absorption dip at 5.10eV was found. These experimental values of estimated bandgaps for both RS MgS and WZ MgS are found to be much larger than the theoretical values previously predicted.

Packaging of MgS-based UV detectors in both transistor-outline can and surface-mount-device chip packages was demonstrated. Possibilities for the integration of a metal-dielectric-metal (MDM) filter into a fire-sensing unit based on a MgS-based UV detector were also examined through the fabrication and analysis of MDM filters.