Latent Order in High-Angle Grain Boundary of GaN

Sangmoon Yoon,Young-Kyun Kwon,Miyoung Kim,Seoung-Hun Kang,Hyobin Yoo

doi:10.1038/s41598-018-22603-3

Abstract

We report the existence of latent order during core relaxation in the high-angle grain boundaries (GBs) of GaN films using atomic-resolution scanning transmission electron microscopy and ab initio density functional theory calculations. Core structures in the high-angle GBs are characterized by two pairs of Ga-N bonds located next to each other. The core type correlates strongly with the bond angle differences. We identify an order of core relaxation hidden in the high-angle GBs by further classifying the 5/7 atom cores into a stable 5/7 core (5/7(S)) and a metastable 5/7 core (5/7(M)). This core-type classification indicates that metastable cores can exist at real high-angle GBs under certain circumstances. Interestingly, 5/7(M) exhibits distinct defect states compared to 5/7(S), despite their similar atomic configurations. We investigate the reconstruction of defect states observed in 5/7(M) by analyzing the real-space wave functions. An inversion occurred between two localized states during the transition from 5/7(S) to 5/7(M). We suggest an inversion mechanism to explain the formation of new defect states in 5/7(M).

Highlights

GaN films have been widely used in commercial devices, such as blue light-emitting diodes (LEDs)[1,2] and high-electron-mobility transistors[3], due to their remarkable physical properties including a direct wide band gap, high saturation velocity, and high two-dimensional electron gas densities
Studies based on high-resolution transmission electron microscopy (HRTEM) analysis indicated that isolated edge dislocations (EDs) have two types of core structure: 5/7 and 8 atom cores[18,19,20,21], whereas EDs obtained using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) without post-processing[22,23] consist of only 8 atom cores
There is no direct evidence that the two defects have the identical core structures, because high-angle grain boundaries (GBs) have never been investigated based on HAADF STEM

Summary

Boundary of GaN

SangmoonYoon[1,2], HyobinYoo[1], Seoung-Hun Kang[2,3], Miyoung Kim1 &Young-Kyun Kwon 2,3. This suggests the presence of a robust order for core relaxation in the high-angle GBs of GaN. The degree of freedom at the high-angle GBs results in unique physical properties compared to those of the isolated EDs. We further analyzed the defect states of 5/7(S) and 5/7(M) and found that during the transition between two 5/7 atom cores, defect-state reconstruction was caused by localized state inversion rather than rehybridization of dangling bonds

Author Contributions

Findings

Additional Information