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Ion-Beam Modification of Metastable Gallium Oxide Polymorphs
TL;DR: In this article, the structural changes under the action of Al+ ion irradiation have been investigated for a polymorphic gallium oxide layers grown by hydride vapor phase epitaxy on c-plane sapphire.
Abstract: Gallium oxide with a corundum structure ({\alpha}-Ga2O3) has recently attracted great attention in view of electronic and photonic applications due to its unique properties including a wide band gap exceeding that of the most stable beta phase (\b{eta}-Ga2O3). However, the lower thermal stability of the {\alpha}-phase at ambient conditions in comparison with the \b{eta}-phase requires careful investigation of its resistance to other external influences such as ion irradiation, ion doping, etc. In this work, the structural changes under the action of Al+ ion irradiation have been investigated for a polymorphic gallium oxide layers grown by hydride vapor phase epitaxy on c-plane sapphire and consisting predominantly of {\alpha}-phase with inclusions of {\alpha}(\k{appa})-phase. It is established by the X-ray diffraction technique that inclusions of {\alpha}(\k{appa})-phase in the irradiated layer undergo the expansion along the normal to the substrate surface, while there is no a noticeable deformation for the {\alpha}-phase. This speaks in favor of the different radiation tolerance of various Ga2O3 polymorphs, especially the higher radiation tolerance of the {\alpha}-phase. This fact should be taken into account when utilizing ion implantation to modify gallium oxide properties in terms of development of efficient doping strategies.
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TL;DR: In this paper, high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga2O3 epilayers grown at low temperature (650 °C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale.
Abstract: A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga2O3 epilayers grown at low temperature (650 °C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called e phase; i.e. they exhibited hexagonal P63mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga2O3 layers deposited at low temperature at the nanoscale: TEM investigation demonstrates that the Ga atoms and vacancies are not randomly distributed, but actually possess ordering, with (110)-twinned domains of 5–10 nm size. Each domain has orthorhombic structure with Pna21 space group symmetry, referred to as κ-Ga2O3. Further XRD analysis carried out on thicker samples (9–10 μm) confirmed this finding and provided refined structural parameters. The six (110)-type twinned ordered domains together – if the domain size falls below the actual resolution of the probing techniques – can be misinterpreted as the disordered structure with its P63mc space group symmetry usually referred to as e-Ga2O3 in the current literature. The crystal structure of these Ga2O3 layers consists of an ABAC oxygen close-packed stacking, where Ga atoms occupy octahedral and tetrahedral sites in between, forming two types of polyhedral layers parallel to (001). The edge-sharing octahedra and the corner-sharing tetrahedra form zig-zag ribbons along the [100] direction. Anti-phase boundaries are common inside the domains. The polar character of the structure is confirmed, in agreement with the characteristics of the Pna21 space group and previous observations.
193 citations
TL;DR: The crystal structure and ferroelectric properties of ε-Ga2O3 deposited by low-temperature MOCVD on (0001)-sapphire were investigated by single-crystal X-ray diffraction and the dynamic hysteresis measurement technique.
Abstract: The crystal structure and ferroelectric properties of e-Ga2O3 deposited by low-temperature MOCVD on (0001)-sapphire were investigated by single-crystal X-ray diffraction and the dynamic hysteresis measurement technique. A thorough investigation of this relatively unknown polymorph of Ga2O3 showed that it is composed of layers of both octahedrally and tetrahedrally coordinated Ga3+ sites, which appear to be occupied with a 66% probability. The refinement of the crystal structure in the noncentrosymmetric space group P63mc pointed out the presence of uncompensated electrical dipoles suggesting ferroelectric properties, which were finally demonstrated by independent measurements of the ferroelectric hysteresis. A clear epitaxial relation is observed with respect to the c-oriented sapphire substrate, with the Ga2O3 [10–10] direction being parallel to the Al2O3 direction [11–20], yielding a lattice mismatch of about 4.1%.
167 citations
TL;DR: In this article, the state-of-the-art technologies of β-Ga2O3 and α-Ga 2O3 for future power device applications are compared in the context of comparing material properties, bulk crystal growth, epitaxial growth, device fabrication, and resulting device performance.
Abstract: Ga2O3 is an ultrawide bandgap semiconductor with a bandgap energy of 4.5–5.3 eV (depending on its crystal structure), which is much greater than those of conventional wide bandgap semiconductors such as SiC and GaN (3.3 eV and 3.4 eV, respectively). Therefore, Ga2O3 is promising for future power device applications, and further high-performance is expected compared to those of SiC or GaN power devices, which are currently in the development stage for commercial use. Ga2O3 crystallizes into various structures. Among them, promising results have already been reported for the most stable β-Ga2O3, and for α-Ga2O3, which has the largest bandgap energy of 5.3 eV. In this article, we overview state-of-the-art technologies of β-Ga2O3 and α-Ga2O3 for future power device applications. We will give a perspective on the advantages and disadvantages of these two phases in the context of comparing the two most promising polymorphs, concerning material properties, bulk crystal growth, epitaxial growth, device fabrication, and resulting device performance.
139 citations
TL;DR: In this paper, a 3-fold increase in the growth rate of pure β-Ga2O3 was achieved by tuning the flow rate of HCl along with other precursors in an MOCVD reactor.
Abstract: Precise control of the heteroepitaxy on a low-cost foreign substrate is often the key to drive the success of fabricating semiconductor devices in scale when a large low-cost native substrate is not available. Here, we successfully synthesized three different phases of Ga2O3 (α, β, and e) films on c-plane sapphire by only tuning the flow rate of HCl along with other precursors in an MOCVD reactor. A 3-fold increase in the growth rate of pure β-Ga2O3 was achieved by introducing only 5 sccm of HCl flow. With continuously increased HCl flow, a mixture of β- and e-Ga2O3 was observed, until the Ga2O3 film transformed completely to a pure e-Ga2O3 with a smooth surface and the highest growth rate (∼1 μm/h) at a flow rate of 30 sccm. At 60 sccm, we found that the film tended to have a mixture of α- and e-Ga2O3 with a dominant α-Ga2O3, while the growth rate dropped significantly (∼0.4 μm/h). The film became rough as a result of the mixture phases since the growth rate of e-Ga2O3 is much higher than that of α-Ga2O3...
134 citations