# The In-Plane-Two-Folders Symmetric a-Plane AlN Epitaxy on r-Plane Sapphire Substrate

^{1}

^{2}

^{3}

^{4}

^{5}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Experiment

#### 2.1. Synthesis

#### 2.2. X-ray Diffraction (XRD) Characterization

_{α1}X-ray, with λ = 0.154056 nm.

#### 2.3. Atomic Force Microscopy (AFM) Characterization

#### 2.4. Raman Spectroscopy Characterization

## 3. Result and Discussion

^{−1}. For AlN film, three vibration peaks were seen: the A

_{1}(TO), E

_{2}

^{H}, and E

_{1}(TO) modes at around 644.2, 663.2, and 675.8 cm

^{−1}; the corresponding vibration mode schemes are shown in Figure 3b. The appearance of all Raman signals indicates the good crystallinity of the annealed samples.

_{1}(TO) mode did not change very much, despite it being under the annealing operation, including both the FWHM and Raman shift. In addition, it can be observed that the A

_{1}(TO) vibration presents a redshift feature upon increasing thickness. A similar characteristic was observed in a-AlN with buffers grown at different temperatures, which resulted from the introduced strain [30,31]. However, the HTA had an obvious contribution to the E

_{2}

^{H}peak, and it can be noted that both the FWHM and Raman wavenumber were greatly decreased by HTA. The FWHM reduction mainly resulted from the improvement of crystalline quality; however, the decreased Raman wavenumber was caused by the strain resetting after annealing. When compared with the strain-free E

_{2}

^{H}signal at 657 cm

^{−1}in bulk AlN, our as-grown a-AlNs present a blueshift, while the annealed samples exhibit a redshift. According to previous studies [32], the phonon frequency reduction and increase are caused by the lattice expansion and shrinkage, respectively. The Raman results are in agreement with the XRD 2Theta-Omega scans.

## 4. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**XRD rocking curves of (

**a**) (110) and (

**b**) (100) planes of as-grown and annealed 500 nm thick samples; (

**c**) thickness-dependent FWHMs of (110) and (100) plane rocking curves of annealed AlN samples.

**Figure 2.**XRD 2Theta-Omega scans of as-grown and annealed samples, when the scanning directions are along the AlN (

**a**) out-of-plane $\left[11\overline{2}0\right]$ and (

**b**) specific $\left[1\overline{1}00\right]$ directions; (

**c**) the 2Theta-Omega scans along the $\left[11\overline{2}0\right]$ direction of annealed AlN samples with different thicknesses.

**Figure 3.**(

**a**) The full Raman spectra of annealed samples with different thicknesses, and the phonon vibration signals of the sapphire substrate and AlN epilayer are labelled; (

**b**) the scheme of different phonon vibration modes in the AlN lattice.

**Figure 4.**The region of Raman spectra from 630 to 680 cm

^{−1}of (

**a**) as-grown and (

**b**) annealed samples; the FWHM (blue open labels) and Raman shift (red solid labels) of (

**c**) A

_{1}(TO) and (

**d**) E

_{2}

^{H}modes, as dependent on the thickness of the as-grown (diamonds) and annealed (squares) samples.

**Figure 5.**(

**a**) The XRD phi-dependent polar figures of r-sapphire substrate (006) and a-AlN (100) film, it is clearly observed that the in-plane components of the AlN and sapphire are both vertical. (

**b**) The lattice scheme of the a-AlN and r-sapphire from the out-of-plane direction (r-direction of sapphire and a-direction of AlN); (

**c**) three-dimensional crystalline scheme of the epitaxial relationship between the sapphire substrate and AlN epilayer.

**Figure 6.**(

**a**) Chi scans of the (100) plane of AlN samples with 500 and 1000 nm thickness after calibrating the $\left[11\overline{2}0\right]$ direction of the sapphire substrates; (

**b**) the corresponding scheme of lattice distortion describe in (

**a**).

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**MDPI and ACS Style**

Zhang, F.; Huang, L.; Zhang, J.; Liang, Z.; Zhang, C.; Liu, S.; Luo, W.; Kang, J.; Cao, J.; Li, T.;
et al. The In-Plane-Two-Folders Symmetric *a*-Plane AlN Epitaxy on *r*-Plane Sapphire Substrate. *Symmetry* **2022**, *14*, 573.
https://doi.org/10.3390/sym14030573

**AMA Style**

Zhang F, Huang L, Zhang J, Liang Z, Zhang C, Liu S, Luo W, Kang J, Cao J, Li T,
et al. The In-Plane-Two-Folders Symmetric *a*-Plane AlN Epitaxy on *r*-Plane Sapphire Substrate. *Symmetry*. 2022; 14(3):573.
https://doi.org/10.3390/sym14030573

**Chicago/Turabian Style**

Zhang, Fabi, Lijie Huang, Jin Zhang, Zhiwen Liang, Chenhui Zhang, Shangfeng Liu, Wei Luo, Junjie Kang, Jiakang Cao, Tai Li,
and et al. 2022. "The In-Plane-Two-Folders Symmetric *a*-Plane AlN Epitaxy on *r*-Plane Sapphire Substrate" *Symmetry* 14, no. 3: 573.
https://doi.org/10.3390/sym14030573