Electron backscatter diffraction and electron channeling contrast imaging of tilt and dislocations in nitride thin films

In this paper we describe the use of electron backscatter diffraction (EBSD) mapping and electron channeling contrast imaging—in the scanning electron microscope—to study tilt, atomic steps and dislocations in epitaxial $\mathrm{GaN}$ thin films. We show results from a series of $\mathrm{GaN}$ thin films of increasing thickness and from a just coalesced epitaxial laterally overgrown $\mathrm{GaN}$ thin film. From our results we deduce that EBSD may be used to measure orientation changes of the order of 0.02°, in $\mathrm{GaN}$ thin films. As EBSD has a spatial resolution of $\approx 20\mathrm{nm}$, this means we have a powerful technique with which to quantitatively map surface tilt. We also demonstrate that electron channeling contrast images may be used to image tilt, atomic steps, and threading dislocations in $\mathrm{GaN}$ thin films.

Figure 1

(a) HKL Technology EBSD and forescatter detector. (b) Schematic diagram illustrating formation of EBSD patterns. (c) Example EBSD pattern from wurtzite $\mathrm{GaN}$. (d) Schematic diagram illustrating formation of electron channeling contrast images. (e) Example electron channeling contrast image from wurtzite $\mathrm{GaN}$ thin film. (The “spots” correspond to threading dislocations.)

Figure 2

(a) EBSD tilt map from a $70\mathrm{nm}\mathrm{GaN}$ thin film. (b) EBSD tilt map from a $520\mathrm{nm}\mathrm{GaN}$ thin film. (c) EBSD tilt map from a $1600\mathrm{nm}\mathrm{GaN}$ thin film. The grey scale represents tilt away from the ideal $⟨0001⟩$ orientation (in degrees). (i) shows the EBSD tilt maps overlying the EBSD pattern quality. As the EBSD pattern quality is dependent on sample topography, it provides an estimate of the sample topography. (ii) shows the EBSD tilt maps with the EBSD pattern quality information removed.

Figure 3

(a) Relative tilt distribution for $\mathrm{GaN}$ thin films of increasing thickness. (b) Same as (a) but with expanded $y$ axis. (c) Maximum tilt from the $⟨0001⟩$ orientation as a function of film thickness.

Figure 4

Electron channeling contrast image of $320\mathrm{nm}\mathrm{GaN}$ thin film.

Figure 5

(a) EBSD relative tilt map from a $1600\mathrm{nm}\mathrm{GaN}$ thin film. (b) Electron channeling contrast image of a $1600\mathrm{nm}\mathrm{GaN}$ thin film.

Figure 6

(a) Electron channeling contrast image from $1600\mathrm{nm}\mathrm{GaN}$ thin film. (b) Electron channeling contrast image from same region as (a) but with backscattered electron detector placed closer to the sample. (c) Schematic diagram illustrating that as the backscattered electron detector is moved closer to the sample: (i) it will intersect a wider angular range of backscattered electrons. (ii) it will intersect lower angle backscattered electrons. (d) Intensity profiles across dislocations situated on either side of an atomic step.

Figure 7

(a) Electron channeling contrast image from $1600\mathrm{nm}\mathrm{GaN}$ thin film with a sample tilt of 69.7°. (b) Electron channeling contrast image from $1600\mathrm{nm}\mathrm{GaN}$ thin film with a sample tilt of 71.0°. Regions $A$ and $B$ are outlined for comparison.

Figure 8

Dislocation density versus 1/film thickness for $\mathrm{GaN}$ thin films.

Figure 9

Schematic diagram of cross-section of ELOG thin film. The dimensions of the ELOG thin film under study are shown.

Figure 10

(a) Electron channeling contrast image from ELOG thin film. (b) Secondary electron image from same region of ELOG film as (a). The inset in (a) is an intensity profile across the electron channeling contrast image.

Figure 11

Variation in tilt across the ELOG thin film.

Figure 12

High magnification electron channeling contrast image from ELOG thin film (acquired at $15\mathrm{kV}$).