【Member Papers】The defect-related anisotropic strain induced saw-like cracking in the (001) β-Ga₂O₃ homoepitaxial film

      A research paper titled “The defect-related anisotropic strain induced saw-like cracking in the (001) β-Ga₂O₃ homoepitaxial film” has recently been published in Applied Surface Science by the group of Professor Hongwei Liang and Associate Professor Hezhi Zhang at the School of Integrated Circuits, Dalian University of Technology, in collaboration with the State Key Laboratory of Crystal Materials, Shandong University. This work reveals the lattice distortion characteristics beneath streaky groove defects in (001) β-Ga₂O₃ homoepitaxial films, and explains how defect-induced anisotropic strain causes saw-like cracking in vertical SBD devices.

Background 

      β-Ga₂O₃, as an ultra-wide bandgap (UWBG) semiconductor material, exhibits a bandgap of 4.8–4.9 eV, a theoretical breakdown electric field as high as 8 MV/cm, and a Baliga figure of merit (BFOM) of 3444, demonstrating tremendous potential in power electronic devices, deep-ultraviolet photodetectors, and extreme-environment applications. However, during the epitaxial growth of β-Ga₂O₃, defects that extend from the substrate into the epitaxial layer and those generated during the epitaxial process not only act as non-radiative recombination centers, reducing carrier lifetime, but also deteriorate the surface quality of the epitaxial layer, leading to a reduced breakdown voltage and compromised long-term operational reliability of power devices. Therefore, a deep understanding of the intrinsic relationships among stress accumulation, lattice distortion, defect generation, and surface morphology evolution in β-Ga₂O₃ epitaxial layers is critically important for optimizing epitaxial processes and improving device performance.

Abstract

1.Markedly reduced carrier lifetime in the streaky groove region

      The homoepitaxial film was grown on a Sn-doped β-Ga₂O₃ (001) substrate by the oxide vapor phase epitaxy (OVPE) method. Scanning electron microscopy (SEM) characterization revealed streaky groove defects elongated along the [010] direction on the (001) epitaxial layer surface. The photoluminescence (PL) spectra demonstrate that the deep-level emission peak positions in the streaky groove region are essentially identical to those in the nearby region (350, 385 and 434 nm), whereas time-resolved photoluminescence (TRPL) measurements reveal that the carrier lifetimes in the groove region at all three wavelengths of 350, 385 and 434 nm are markedly shorter than those in the nearby region. This indicates that a larger number of carriers participate in the defect-related recombination in the groove region, giving rise to a faster quenching rate, which is strongly associated with self-trapped exciton emission.

2.Presence of (-401) rotational domains and pronounced anisotropic strain beneath the streaky groove

      high-resolution transmission electron microscopy (HRTEM) and (SAED) analyses reveal that (-401) rotational domains are ubiquitous in both the streaky groove region and its nearby region, accompanied by stacking faults and tilted dislocation lines. The β angle of the unit cell in the region beneath the streaky groove increases to approximately 105.18°, and the (400) and (002) d-spacings along the a- and c- axes are both appreciably elongated , indicative of spontaneous compressive stress along the b-axis. In the nearby region, although (-401) rotational domains are likewise present, the (400) d-spacing remains close to the bulk value and the β angle decreases by approximately 0.75°, reflecting a considerably milder degree of lattice distortion. The specific parameters are summarized in Table I.

3.AFM verification that rotational domains give rise to an increased surface angle

      AFM characterization demonstrates that the angle between the (-401) plane and the (001) substrate surface in the nearby region is approximately 3-4°, in agreement with the theoretically calculated value, whereas this angle broadens to 6-8° in the streaky groove region. This result further corroborates that the defect-induced anisotropic strain accumulation in the groove region gives rise to more severe lattice distortion.

Highlights

      Defect induced (-401) rotational domains beneath the streaky groove region of β-Ga₂O₃ (001) epitaxial film.

      The rotational domains related lattice distortion produces anisotropic strain, which is stretching of a- and c- axes and a notable broaden β angle.

      Such anisotropic strain is equal to compressive stress happened along the b-axis.

      Saw-like cracking along the (100) and (001) planes for β-Ga₂O₃ (001) vertical SBD due to (-401) rotational domains with spontaneous anisotropic strain.

Conclusion

      The present study proposes a complete physical model: the emergence of (-401) rotational domains in the streaky groove region manifests strong anisotropic strain accumulation, expressed as elongation of the a- and c- axes together with an increase in the β angle, which is equivalent to compressive stress along the b-axis. Once the accumulated stress exceeds the critical threshold, cleavage preferentially occurs along the (100) and (001) planes of the rotational domain system, ultimately giving rise to saw-like cracks. This mechanism well accounts for the recently reported phenomenon of saw-like cracking upon breakdown in β-Ga₂O₃ (001) vertical SBDs, and is consistent with the nanoindentation experimental results, thereby providing an important theoretical basis for optimizing epitaxial processes and enhancing device reliability.