Ultra-wide bandgap semiconductors such as diamond, AlN and Ga2O3 are emerging as strong contenders to beat the limits of current power electronics, based on SiC and GaN. Ga2O3 is particularly interesting, with its 4.9 eV bandgap, high (> 8 MV/cm) breakdown electric field and large Baliga’s figure of merit. The growth of Ga2O3 using atomic layer deposition (ALD) and molecular beam epitaxy (MBE) remain largely unexplored, and present interesting potential. On the one hand, ALD, as a self-limiting growth method, enables sub-nm thickness control and excellent conformality, on both flat and high-aspect-ratio structures, making it particularly suitable for ultra-thin layers and interface engineering, while using a low thermal budget. On the other hand, MBE provides highly controlled growth under ultra-high vacuum conditions, allowing for abrupt interfaces, precise control of composition and doping, and detailed in-situ monitoring of growth processes, making it particularly well suited for the growth of heterostructures. In this context, the Ga2O3/AlN heterojunction was identified as a particular target of these studies. This thesis is a contribution the development of Ga2O3-based devices for power electronics, through the exploration of the deposition of Ga2O3 thin films by ALD and MBE. The fabrication of efficient devices requires high-quality thin films in order to ensure carrier mobility. With this purpose, we developed processes for both ALD and MBE in order to optimise the deposition of Ga2O3 thin films. The research carried out in this work was strongly focused on the optimisation of the growth conditions, in order to improve the films’ crystalline quality.

            First, an ALD process using TEGa and O3 is designed, yielding a growth rate of 0.42 ± 0.02 Å/cycle, with an ALD window found between 250 and 400 °C. The growth of high-crystalline quality, epitaxial (001) κ-Ga2O3 thin films on c-plane sapphire is then achieved for ultra-thin films (< 20 nm), and the epitaxial relationships between film and substrate are established as [001] κ-Ga2O3 || [0001] α-Al2O3, [010] κ-Ga2O3 || [10-10] α-Al2O3 and [100] κ-Ga2O3 || [11-20] α-Al2O3. The growth mechanisms enabling the formation of the metastable κ-Ga2O3 phase are discussed. The effects of an increasing thickness on the crystalline structure of the films are then investigated by increasing the number of ALD cycles. Finally, the Ga2O3 ALD layers are annealed and undergo phase transition to (-201) β-Ga2O3.

            In the scope of assessing the feasibility of an AlN/Ga2O3 heterojunction, we studied the MBE growth of AlN on Ga2O3. The nucleation of Al-polar AlN is optimised under N-rich conditions, which result in high-quality layers with a sharp monoclinic-to-wurtzite transition. The epitaxial relationships are identified as [0001] AlN || ⟂ (-201) β-Ga2O3 and [2-1-10] AlN || [020] β-Ga2O3, with an interface consisting of two monolayers of AlGaN. The challenging MBE growth of Ga2O3 is then investigated on AlN. The growth of single-phase (-201) β-Ga2O3 is achieved for temperatures above 630 °C, and optimal structural properties are obtained for relatively low gallium fluxes. The epitaxial relationship identified as [020] β-Ga2O3 || <11-20> AlN, with three rotational domains. As in the case of AlN on β-Ga2O3, the interface is crystallographically sharp, but it contains two AlGaN monolayers.

These results demonstrate the potential of ALD and MBE as growth techniques for future ultra-wide-bandgap power electronics, and represent an important step towards the integration of Ga₂O₃ into advanced heterostructures.

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