Growth and Characterization of Ga2O3 (Ga1-xGdx)2O3 and (Ga1-xFex)2O3 Thin Films by Pulsed Laser Deposition
Mia, Md Dalim
<p>Thin film wide bandgap Ga2O3 based alloys are important for applications in high power electronic, deep UV photonic devices and nuclear detectors. Such alloys can be deposited epitaxially by pulsed laser deposition on c-plane sapphire substrates with varied compositions. This work enables the development of heterostructures with Ga<sub>2</sub>O<sub>3</sub> and bandgap tunable device structures with a variety of functionalities. Examples include the use of Gd in the alloy for higher bandgap material which can be used as a gate dielectric in MOSFET devices, and due to the high nuclear cross section of Gd, for nuclear radiation detectors. Alloys containing Fe can be used for the development of high bandgap magnetic devices and high resistive buffer layers for FET applications. Combinations of these thin film alloys open the possibility of heterostructure devices that includes HEMT and quantum well UV optoelectronics. Being motivated by these reasons, this thesis work focused on the growth optimization and characterization of (Ga<sub>1-x</sub>Gd<sub>x</sub>)<sub>2</sub>O<sub>3</sub> & (Ga<sub>1-x</sub>Fe<sub>x</sub>)<sub>2</sub>O<sub>3</sub> thin films using pulsed laser deposition.</p> <p>Epitaxial (Ga<sub>1-x</sub>Gd<sub>x</sub>)<sub>2</sub>O<sub>3</sub> & (Ga<sub>1-x</sub>Fe<sub>x</sub>)<sub>2</sub>O<sub>3</sub> thin films with varied x were successfully grown on c-plane sapphire substrates to tune the optical and structural properties using variations in the growth parameters. High growth temperatures favor the formation of the monoclinic (Ga<sub>1-x</sub>Gd<sub>x</sub>)<sub>2</sub>O<sub>3</sub> phase; the higher the Gd composition, the greater the growth temperature required for high quality crystalline thin films. UV-vis measurements demonstrate a slight red shift of the bandgap (4.99eV~4.82eV) in comparison with the pure β-Ga<sub>2</sub>O<sub>3</sub>.</p> <p>Successfully grown, both spinel and monoclinic (Ga<sub>1-x</sub>Fe<sub>x</sub>)<sub>2</sub>O<sub>3</sub> thin films exhibit room temperature ferromagnetism, which can be used for the fabrication of nonvolatile semiconductor memories, magneto-optic devices, and microelectronics. Incorporation of Fe into Ga<sub>2</sub>O<sub>3</sub> expands the lattice constant eventually transforming the crystal structure from the monoclinic to the spinel phase for x>0.1. Optical transmission measurements show that with increasing Fe content the absorption edge moves toward longer wavelengths with the introduction of an intermediate band. Similarly, high oxygen pressure introduces intermediate bands, while low oxygen pressure eliminates defects as well as induces a phase transformation from the monoclinic to the spinel phase. On the other hand, the thermally induced spinel structures transform to the monoclinic phase during a high temperature anneal in an oxygen environment. Chemical composition, surface states, optical properties, and crystal structure were systematically evaluated by several techniques. XRD and pole figure analysis were used to distinguish the crystal phase. X-ray photoelectron spectroscopy measurements suggest that the (Ga<sub>1-x</sub>Fe<sub>x</sub>)<sub>2</sub>O<sub>3</sub> alloy contains a mixture of Fe<sup>3+</sup> and Fe<sup>2+</sup> valence states in the films. However, higher oxygen pressure during growth and annealing favors formation of Fe<sup>3+</sup> over Fe<sup>2+</sup>.</p>
Wide bandgap, Alloys of gallium oxide, Gallium gadolinium oxide, Gallium iron oxide, Gamma phase, Monoclinic phase, Phase transformation, Ferromagnetism
Mia, M. D. (2020). <i>Growth and characterization of Ga2O3 (Ga1-xGdx)2O3 and (Ga1-xFex)2O3 thin films by pulsed laser deposition</i> (Unpublished dissertation). Texas State University, San Marcos, Texas.