Growth and investigation of different gallium oxide polymorphs

In spite of its proven applications in the fields of deep UV detection and power electronics, Ga2O3 is still a relatively new material and many of its crystalline and semiconducting properties are still unexplored, while the growth and deposition techniques require further development and optimisation. The research activity carried out in the frame of this thesis ranges from technological aspects of ?- and ?-phase thin film epitaxy, to their comprehensive characterisation and first proven applications. Further, a big effort was made in order to highlight the anisotropic properties of the ? bulk material. A chemical vapour deposition process was developed for reproducible growth of ?-Ga2O3 thin films on c-sapphire at low temperature. The deposited layers display good crystalline and morphological quality, with intense and sharp X-ray diffraction (XRD) peaks and low surface roughness. The growth mechanism and the relationship between deposition parameters and final film quality are discussed. Crystal structure of ?-Ga2O3 was accurately investigated by transmission electron microscopy and XRD; the determined crystallographic structure at the nanoscale reconciles the contradictory results previously reported in literature, i.e. the orthorhombic (?) and hexagonal (?) symmetry. Depending on the resolution of the employed technique and on the lateral dimension of the columnar twin-domains, one gets the impression of dealing with an hexagonal (macroscopic view) or an orthorhombic (atomic-scale view) phase. An important consequence of the real structure of the material is its ferroelectricity, which was verified experimentally. Thermal stability studies of the deposited ?-Ga2O3 layers indicate that this phase is stable up to at least 650 °C. The films exhibited spontaneously high resistivity and a pronounced photoconductivity, which allowed fabricating a solar-blind UV-detector with properties comparable to those of ?-based photodetectors. Some modifications to the initial reactor setup are shown, which allow to reduce heat dissipation and to increase the homogeneity of the deposited layers. Preliminary results obtained by operating in atomic layer deposition mode are also discussed, and confirm the potential of this technique to provide crystalline growth at lower temperature, as well as to increase thickness uniformity. As for ?-Ga2O3 films, it is shown how a variant of the pulsed laser deposition technique, the so-called †œoff-axis†� configuration, can be employed to limit film damage during the growth process, thus increasing the final surface quality and allowing for successful fabrication of field-effect transistors based on very thin ?-Ga2O3 layers. Another consistent part of the thesis was devoted to the investigation of some anisotropic properties of bulk ?-Ga2O3. Raman normal modes with Ag and Bg symmetry were selectively observed by varying the experimental geometry. From the angular behaviour of Raman intensity, a, b and d Raman tensor elements were determined for each Ag mode, exploring different possible models for data fitting. Optical absorption of ?-Ga2O3 was seen to be anisotropic as a function of incident light polarisation, thus confirming previous reports and further extending the measurements to unexplored orientations. Theoretical analysis supports these findings and provides a physical interpretation of absorption anisotropy, in terms of a suppression of the transition matrix elements of the three top valence bands when the polarisation is parallel to the b-axis. The lattice parameters of ?-Ga2O3 were determined by powder diffraction in the temperature range 300†"700 K. The estimated expansion coefficients are in good agreement with previously published data below room temperature. This result is of great technological interest for heteroepitaxy, as it allows to evaluate the thermal mismatch between ?-Ga2O3 and any other material used as substrate (and vice versa).