Abstract text (incl. figure legends and references)

Introduction

As one of the most successful methods for synthesizing bulk AlN crystal, PVT can also be used to synthesize a high-quality micro- and nanoscale 1D single crystal with unintentionally doped impurity due to its special growth environment. CL spectroscopy is a spectroscopic technique based on electron-excited luminescence. It uses a highly focused electron beam to generate electron-hole pairs in the sample, which are through radiative or non-radiative processes for diffusion and recombination. Micro-area characterizations of micro-scale crystals by CL spectroscopy are an advantageous means to understand the correlation between structural and optical properties of the sample. The study of defect properties is also important for understanding the formation or elimination of various defect centers of bulk aluminum nitride materials.

Objective

To understand the influence of the growth habit on defect-related luminescence properties in m-oriented AlN

Materials and methods

The large-scale AlN microwires were synthesized in a homemade RF-heated furnace. A high-purity AlN source material is obtained by sublimation of commercially available AlN powder (TOKUYAMA). The growth was performed at a background nitrogen pressure of 100 mbar and nitrogen gas was used as protective gas at the flow rate of 100 sccm at high temperature. The growth temperature was kept at 2000 °C for about 0.5 h.

Results

Fig. 1a and b show the SEM morphology of the top surface of the single UW and the corresponding panchromatic CL spectra. The near band edge emission at 200 nm cannot be detected due to the limitation of the detector used at an effective wavelength range from 250 to 600 nm. It is found that there is strong CL emission in the region near both sidewalls of the UW, and this symmetrically distributed CL emission pattern extends into the semipolar plane. Interestingly, the strong CL emission makes the Si substrate outside the UW also show an elevated background emission. As shown in Fig. 1c, the CL emission of the UW is around the wavelengths of 320 nm (VL-1), 345 nm (VL-2), and 450 nm (BL). The typical SEM morphology and panchromatic CL spectra of the N-polar c-plane of UW are shown in Fig. 1d and e. The localized CL luminescence appears as a symmetrical "sandwich structure". Fig. 1f shows the point mode CL spectra for the positions marked in Fig. 1e.

Fig. 1. CL characterization of the AlN UW. SEM image of the top surface of the UW (a) and simultaneously recorded panchromatic CL spectra (b). SEM image of the bottom surface of the UW (d) and simultaneously recorded panchromatic CL spectra (e). CL spectra were obtained in point mode on different positions of the UW, as shown in (c) and (f).

Conclusion

In summary, the high-quality nonpolar AlN microwires with rectangular cross-sections were synthesized by the PVT. The difference in impurity incorporation efficiency on different facets is responsible for the peculiar CL patterns in the AlN UW. In addition, it is confirmed that the radiative recombination mechanism is determined by the O/C content ratio. It may contribute to a more thorough understanding of bulk AlN properties and growth.

Reference

[1] Chuang Wang, Xiaodong Gao, Jiafan Chen, Luhua Wang, Xionghui Zeng*, et al. Morphology evolution and cathodoluminescence properties of non-polar aluminum nitride microwires, Journal of alloys and compounds, 2023, 937, 168407