Aluminum Content Tunability of Structural and Optical Properties in Wide-Gap Semiconducting (AlxGa1-x)2O3
Krueger, Benjamin William
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(AlxGa1-x)2O3 alloys have attracted renewed interest due to their potential for use in deep-ultraviolet optical devices and high-powered transistors, but realization of these technologies depends on the ability to tune the band gap and crystal phase of alloy films for specific applications. In this work, fundamental relationships between structural and optical properties of (AlxGa1-x)2O3 were explored in a series of experiments on single-phase sintered powders produced by solution combustion synthesis, and continuous-composition-spread (CCS) thin films deposited at room temperature on silicon and sapphire by pulsed-laser deposition and post-annealed to successively higher temperatures in air, to see how crystallinity develops with alloy composition and measure the corresponding change in band gap. As the annealing temperature is raised (to a maximum of 1000°C), amorphous (AlxGa1-x)2O3 films transition first to the metastable gamma phase, and then to the beta phase, for alloy compositions in the range 0.1 < x < 1 with the specific (am.) -> gamma and gamma -> beta transition temperatures increasing steadily with Al content. A different phase was identified in the Ga-rich region 0 < x < 0.1 after annealing to 400°C-800°C, tentatively assigned to a kappa/gamma mix; at higher temperatures these films also relax to the beta phase. A maximum solubility limit of x = 0.8 was found for Al in the monoclinic beta-(AlxGa1-x)2O3 structure after sintering at 1600°C for 12 hours, followed by a transition to corundum alpha-Al2O3 above x > 0.8. Within each phase the band gap increases steadily with Al concentration, with greater bowing in the amorphous films than crystalline films, and linear variation with no bowing in 1600°C sintered powders. No discontinuity in the band gap was observed through the beta -> gamma transition, but a 0.1-0.2 eV jump to lower band gap occurs when films become amorphous. (AlxGa1-x)2O3 films were also grown on silicon and sapphire at elevated temperature, with and without a Ga2O3 buffer layer, to determine limits on epitaxy with these two substrates and to see if films grown at high temperature exhibit similar properties as post-annealed films grown at room temperature. Epitaxial (AlxGa1-x)2O3 CCS films on sapphire are initially [-201]-oriented on the Ga-rich side, with an in-plane epitaxial relationship Ga2O3  || sapphire [-210] and six-fold rotational symmetry. Films with thickness greater than 100 nm start to lose their orientation by twinning on the (101) plane, parallel to one of the faces of the distorted oxygen fcc sublattice, with the twinned regions growing faster than [-201]-oriented regions as the thickness increases further. As the Al concentration is increased, films on sapphire immediately begin to lose crystallinity, due to a relatively low growth temperature of 500°C-600°C, but a Ga2O3 buffer layer extends the solubility limit to x = 0.15. (AlxGa1-x)2O3 films on silicon are initially unoriented on the Ga-rich side due to the oxidization of the Si surface (SiOx), but develop a spontaneous / orientation as the Al content is raised above x > 0.1, which grows stronger with film thickness. Above x = 0.35 a transition to the gamma phase occurs, which is extended to x = 0.4 by the presence of a Ga2O3 buffer layer. The higher solubility of Al in CCS films in SiOx than sapphire is attributed to a higher substrate temperature of 800°C during growth, and is consistent with the solubility limit of post-annealed room-temperature-deposited films at the same temperature.
- Physics