Here, we report on the precise analysis of the atomic structure of inversion domain boundaries (IDBs) in GaN by scanning transmission electron microscopy.  IDBs are a common defect in GaN that traps carriers and leads to a slightly modified luminescence wavelength [1,2].

Our analysis of IDBs in MOCVD grown nanowires confirms recent coherent Bragg imaging (CBI) results [3] stating that the atomic structure of this IDB is different or slightly different from the one determined in 1996 by first-principle calculations (IDB*) [4].  CBI experiments measured a 8 pm shift of the c-planes of the two domains [3], whereas  first-principle calculations predicted no shift. A previous study by STEM [5] found a shift of “ca. 0.6 Å” (60pm), corresponding roughly to the switch of the Ga and N positions without any additional shift. Here in addition to a shift along  c, we show that the interface configuration corresponds qualitatively to the IDB* model (cf. Fig. 1) and that there is a 10 pm dilatation perpendicular to the interface (shown in Fig. 3) in agreement with this model, while CBI did not find a dilatation.

To facilitate the measurement of atom positions across the IDB with picometre-precision, we use HAADF-STEM to avoid coherent effects leading to artefacts. Scanning and drift artefacts are being suppressed by acquiring series of rapid STEM images and aligning them using the newly developed Zorro code. This algorithm is based on calculating estimated drift positions by correlating every frame to multiple frames and minimizing the error of the overdetermined system to obtain a best estimate for the frame positions relative to each other. The sub-pixel aligned frames are then averaged and the peak positions are determined via TeMA (template-matching algorithm).

Our quantitative analysis of experimental and simulated STEM images shows that when atomic columns are very close to each other the measured distance can be slightly different from the real value.  For instance, when the distance between atomic columns becomes smaller than 0.1 nm, the difference between the measured and real values can account for several picometres. This effect can be well observed when an IDB kinks perpendicular to the observation direction leading to closely projected columns in the overlap region of the two domains as seen in Fig. 2. Electron scattering simulations show that the apparently wider distance between atoms is a channeling effect.

These results have provided elements to revisit previous theoretical models of IDBs in GaN.

[1] T. Auzelle et al., Appl. Phys. Lett. 107, 051904 (2015).

[2] R. Kirste et al., J. Appl. Phys., 110, 093503(2011).

[3] S. Labat et al., ACS Nano 9, 9210 (2015).

[4] J. E. Northrup et al., Phys. Rev. Lett. 77, 103 (1996).

[5] F. Liu et al., Adv. Mater. 20, 2162 (2008).

Figures:

Fig. 1: High-resolution ABF-STEM image showing an inversion domain boundary in GaN along the [11-20] zone axis. Atomic columns are black. The structure is qualitatively in agreement with the IDB* model [1] which is depicted as an overlay. Ga and N atoms are respectively blue and yellow.

Fig. 2: (a) HAADF image of an IDB in GaN kinking perpendicular to the [11-20] observation direction. (b) Line profile across the dumbbells with a Gaussian fit of the two atom columns in blue and red. The black vertical lines depict the distance in c-direction between the columns of the two pure domains (72pm).

Fig. 3: (a) HAADF image of an IDB in GaN in [11-20] zone axis. Shifts of the column positions relative to the reference (marked R) are depicted in (b) and (c) for the c-direction and perpendicular, respectively. The shift in (b) of 72pm corresponds to a switch of the Ga and N planes (c/8 = 65pm) plus 7pm. According to the IDB* model [1] there should be no additional shift (dashed red line). In (c) a dilatation of around 10pm can be seen, which is in accordance with the IDB* model (dashed red line) but has not been found by CBI [3]. The fact that the shift at the interface is not atomically sharp is owed to a kink in projection, which can be seen at the bottom of (a).

« Back to The 16th European Microscopy Congress 2016

EMC Abstracts - https://emc-proceedings.com/abstract/picometre-precision-atomic-structure-of-inversion-domain-boundaries-in-gan/