(Invited) Growth and Characterization of α-, β-, and ε-Ga2O3 Epitaxial Layers on Sapphire
Authors: Yao Yao , Lyle Luke A. M. , Rokholt Johanne A. , Okur Serdal , Tompa Gary S. , Salagaj Tom , Sbrockey Nick , Davis Robert F. , Porter Lisa M. 
Published: February 2020

Increasing global demand for energy makes urgent the need for highly efficient electronics for energy conversion and transport. Power electronics are required for electrical switching within the electrical grid and for green modes of transportation such as the in-switched-mode power supplies now used for hybrid electric vehicles. Silicon devices have been traditionally used for power electronics. However, wide bandgap semiconductors are much more efficient and thus more useful for future energy applications, because they can withstand higher electric fields with less material and reduced energy loss. As an example, Toyota recently began trials of a new hybrid system using power electronics based on SiC and claims that power electronic devices based on SiC could increase fuel efficiency of hybrid vehicles by 10%. Almost all high-power electronic devices are fabricated in very thick films grown on low defect density semi-insulating substrates of the same composition and crystal structure. At present, 4H-SiC is the material of choice for both substrates and films for devices operating at and above 1200 volts. The upper limits of operation for power devices fabricated in GaN-based films are markedly lower. And the substrates of both materials are highly textured polycrystalline materials and still very expensive. A very promising alternative to SiC and GaN is gallium oxide, Ga2O3, which has an even larger bandgap than the former two materials. The availability of this material presents new possibilities for disruptive devices and technologies that could translate to even greater energy efficiencies at lower cost than predicted for SiC and GaN.
Ga2O3 is known to exist in five polymorphs, i.e., α-, β-, γ-, δ-, and ε-phases. The monoclinic β-phase is the most thermodynamically stable phase. Single crystal boules of this phase can be grown using inexpensive melt-growth methods. Polished 2-in diameter wafers diced from these boules and oriented homoepitaxial films have recently become commercially available. However, there is increasing interest in the other phases, particularly the metastable corundum-structured α- and hexagonal-structured ε-Ga2O3 phases. Both of these phases have been observed to grow epitaxially on oriented substrates. We have successfully grown epitaxial films of α-, β- and ε-phases on c-plane sapphire using different precursors flow rates and growth conditions. The α- and ε-phases have generally been reported in the literature to form at lower growth temperatures than the β-phase However, we observed a change in phase formation at the same growth temperature by changing our growth technique and Ga precursor from metalorganic chemical vapor deposition (MOCVD) and trimethlygallium to halide vapor phase epitaxy (HVPE) and gallium chloride. The HVPE method allowed a significantly higher growth rate than MOCVD and is, therefore, advantageous for growth of the very thick films needed in high-power devices. The α- and ε-phases are of particular interest because of their higher symmetry and simpler epitaxial relations to c-plane sapphire. Moreover, given their similar structures to other wide bandgap materials such as ZnO and AlN, it should be possible to produce functional heterostructures or tunable bandgaps through alloying. Frequently, a thin interfacial layer of the α-phase is observed to grow on the substrate, before it undergoes a phase transition to the ε-phase. By varying the total gas flow rate as well as the Ga:O flow ratio, we observed different relative thicknesses of the two competing phases. Data from x-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy will be presented to illustrate the different epitaxial films. The results of secondary ion mass spectroscopy of the various phases will also be presented, as these data suggest that compositional differences may exist among the phases. Additionally, α- and ε-Ga2O3 are metastable phases; however, we have observed these phases to be stable up to at least 750oC. Only after prolonged annealing at T> 800oC did they begin to undergo a transformation to the β-phase. These reasonably high working temperatures justify further investigation of α- and ε-Ga2O3-based devices. The authors wish to acknowledge the Office of Naval Research under contract no. N00014-16-P2059.