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Beta-phase gallium oxide (β-Ga2O3), with its ultrawide band gap energy (~4.8 eV), high predicted breakdown field strength (6-8 MV/cm), controllable n-type doping and availability of large area, melt-grown, differently oriented native substrates, has spurred substantial interest for future applications in power electronics and ultraviolet optoelectronics. The ability to support bandgap engineering by alloying with Al2O3 also extends β-(AlxGa1-x)2O3 based electronic and optoelectronic applications into new regime with even higher critical field strength that is currently unachievable from SiC-, GaN- or AlxGa1-xN- (for a large range of alloy compositions) based devices. However, the integration of β-(AlxGa1-x)2O3 alloys into prospective applications will largely depend on the epitaxial growth of high quality materials with high Al composition. This is considerably important as higher Al composition in β-(AlxGa1-x)2O3/Ga2O3 heterojunctions can gain advantages of its large conduction band offsets in order to simultaneously achieve maximized mobility and high carrier density in lateral devices through modulation doping. However, due to the relative immaturity of β-(AlxGa1-x)2O3 alloy system, knowledge of the synthesis and fundamental material properties such as the solubility limits, band gaps, band offsets as well as the structural defects and their influence on electrical characteristics is still very limited. Hence, this research aims to pursue a comprehensive investigation of synthesis of β-(AlxGa1-x)2O3 thin films via metal organic chemical vapor deposition (MOCVD) growth methods, building from the growth on mostly investigated (010) β-Ga2O3 substrate to other orientations such as (100), (001) and (-201), as well as exploring other polymorphs, such as alpha (α) and kappa (κ) phases of Ga2O3 and (AlxGa1-x)2O3 to provide a pathway for bandgap engineering of Ga2O3 using Al for high performance device applications. Using a wide range of material characterization techniques, experiments have been undertaken to investigate the physical structure, electronic and optical properties of Ga2O3 and (AlxGa1-x)2O3 films, with the correlation to theoretical studies. Al compositions in (AlxGa1-x)2O3 films are varied for the whole range in order to understand the phase segregation at higher Al compositions as predicted by equilibrium phase diagram as well as theoretical studies. The Si doping in β-(AlxGa1-x)2O3 thin films is investigated as a function of Al compositions, with a goal to identify the major challenges associated with MOCVD epitaxy β-(AlxGa1-x)2O3 films, such as the formation of cracks, increase of the concentrations of planer defects, carbon and hydrogen densities, which are predicted to act as compensating acceptors in β-(AlxGa1-x)2O3. In addition, the band discontinuities at β-(AlxGa1-x)2O3/Ga2O3 heterojunctions with various Al compositions are also determined along different orientations to understand the influence of substrate orientations on the band offsets. The growth of in-situ MOCVD Al2O3 dielectric on differently oriented β-(AlxGa1-x)2O3 films have been investigated to gain insights into how varying orientations and Al compositions influence the interfacial quality and the band offsets at Al2O3 dielectric/β-(AlxGa1-x)2O3 heterojunctions. Apart from β-(AlxGa1-x)2O3 epitaxy, the crystalline structure and quality of MOCVD grown single phase α-(AlxGa1-x)2O3 and κ-Ga2O3 films are also investigated for potential applications in high-power and high-frequency electronics. Such findings will greatly assist in designing and fabricating future high power heterostructure based electronic and optoelectronic devices.