【Member Papers】GAREN SEMI's Breakthrough in Gallium Oxide Single Crystal Growth were Published in the Crystal Growth&Design

      Recently, Hangzhou GAREN SEMI and the research team of Zhejiang University have made key technological breakthroughs in the field of β-Gallium Oxide (β-Ga₂O₃) single crystal growth! Published in Volume 25 / Issue 3 of the internationally renowned journal Crystal Growth & Design entitled " Highly Coherent Grain Boundaries Induced by Local Pseudomirror Symmetry in β-Ga₂O₃ ”

      β-Ga₂O₃ is a kind of material which is easy to produce twins in the growth and epitaxy of single crystal, which seriously restricts the improvement of the yield of single crystal growth and epitaxy. Based on the Symmetry principle, this study proposed Pseudo Symmetry in β-Ga₂O₃ for the first time, successfully analyzed the physical origin of the (100) twin boundary and the "Coherent Asymmetric Grain Boundaries", and solved the "gene" that makes β-Ga₂O₃ easy to form twins. It is helpful to improve the yield of single crystal growth and epitaxy from the level of scientific principle and engineering design, and clear the key obstacles for the development of high-performance devices.

      With reference to the research results, the R&D team of GAREN carried out optimization and iteration of the β-Ga₂O₃ growth process based on the self-developed Gallium Oxide special equipment, which initially solved the problem of twins in the industry.

[Figure 1] Standard symmetry in a β-Ga₂O₃ cell

      However, in addition to the strict symmetries described above, β-Ga₂O₃ also has special "Pseudo Symmetry". As shown in Figure 2 (a), β-Ga₂O₃ has an atomic layer structure of "quasi-specular symmetry," meaning that the position between the mirror image of the layer's atoms and itself does not strictly coincide, but is separated by only a negligible distance. The "quasi specular symmetry" plane of this atomic layer is perpendicular to the (100) plane, marked as a black dashed line in Figure 2 (a), and the spacing of this " pseudo symmetry plane" is:

      This "pseudo symmetry" makes the growth and epitaxy of single crystal of β-Ga₂O₃ prone to (100) face twins. As shown in Figure 2 (b), after the formation of the twin structure, there is almost no atomic displacement and bond distortion at the twin interface, and the twin boundary energy is only 0.008J/m² -- almost no!

[Figure 2] (a) pseudo symmetric atomic layer in β-Ga₂O₃; (b) the (100) face twin of β-Ga₂O₃ built on the pseudo symmetry

      At this point, another coincidence appears: there is a special "lattice coincidence" in the single-cell structure of β-Ga₂O₃, as shown in Figure 3 (a) :

      This "lattice coincidence" is superimposed with "pseudo-symmetry" so that there are two special "twin cores" in β-Ga₂O₃, as shown in Figure 3 (b) and (c).

[Figure 3] (a) Special "lattice coincidences" in β-Ga₂O₃; (b) A-type twin core in β-Ga₂O₃; (c) B-type twin core in β-Ga₂O₃

      A three-dimensional extension of the " twinning core" in Figure 3 (b) and (c) constitutes the series of twin boundary structures in Figure 4. On a microcosmic level, they can be viewed as twin chains strung by twin cores; On a macroscopic level, these interface structures appear as twins composed of different crystal face.

      For example, the interface structure in Figure 4 (b) shows that the (002) and (20-2) faces of β-Ga₂O₃ produce twins on a macro level! Here, we name it "Coherent Asymmetric Grain Boundaries" (CAGBs).

[Figure 4] (a) - (f) The twin core is expanded into a three-coherent asymmetric twin; (g) coherent asymmetric twins are macroscopically presented as twin boundaries composed of two different crystal face; (f) The surface energy of the crystal face constituting a coherent asymmetric twin

      Coherent Asymmetric Grain Boundaries plays an important role in the growth and epitaxy of single crystal of β-Ga₂O₃. As shown in Figure 5, when growing 2-inch β-Ga₂O₃ single crystal by CZ method, we observed the structure of the symmetrical asymmetric twin, and determined that Figure 4 (b) was the most stable β-Ga₂O₃ coherent asymmetric twin structure through the HAADF-STEM and first-principles calculations in Figure 5 (e).

[Figure 5] Coherent asymmetric twins in 2-inch β-Ga₂O₃ single crystal grown by Cz method were used by (a) Polarizing strain gauge; (b) optical microscope; (c) TEM; (d) the structure of coherent asymmetric twins observed by STEM and (e) HAADF-STEM.

      We take the step flow epitaxy of β-Ga₂O₃ (100) surface substrate as an example to illustrate the important role of coherent asymmetric twins on β-Ga₂O₃ epitaxy. A large number of studies have shown that the β-Ga₂O₃ (100) surface can achieve step flow growth through chamfer to improve epitaxial quality. However, the principle that the chamfer direction plays a decisive role in epitaxial mass remains unresolved (APL Mater. 7 022515 (2019)). Here, we need to emphasize the critical role of Coherent Asymmetric Grain Boundaries.

      As shown in Figure 6 (a), the (100) substrate chamfer exposes the step face of (001) along [001] and the chamfer exposes the step face of (-201) along [00-1]. At the (001) step surface exposed along the [001] chamfer, a twin defect is formed due to the presence of coherent asymmetric twins. Its energy density is 3.11J/m², which is even lower than the 3.26J/m² energy density of the defect without twins - which means that during epitaxy, twins form spontaneously! However, on the (100) substrate chamfer along [00-1], the (-201) step surface inhibits the formation of associated twin defect. Therefore, in order to improve the quality of epitaxy, it is essential to choose the direction of β-Ga₂O₃ substrate reasonably.

[Figure 6] (a) β-Ga₂O₃ (100) substrate chamfer exposes (001) step face along [001] direction and chamfer exposes (-201) step face along [00-1] direction. (b) On the (001) step surface, the defect of the twin at the step caused by a coherent asymmetric twin are lower in energy than the step without defect.