Chromatic aberration in homogeneous diameter expansion growth of AlN crystals by the PVT method

Abstract Homogeneous diameter expansion growth is the preferred method for preparing large-sized AlN single crystals using the PVT method. However, chromatic aberration between the expansion and seed regions, as well as localized chromatic aberrations within the expansion region, are commonly observed, but not deeply investigated yet. This paper investigates the homoepitaxial lateral growth of AlN crystals using Al-polar face seeds and analyzes the crystal structure, composition, and optical properties of both the expansion and seed regions. XRD and Raman spectroscopy indicate no significant differences in crystallization quality between the expansion and seed regions. XPS and photoluminescence characterizations reveal that the light yellow color of the seed region is mainly due to $$V_{Al}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mi>Al</mml:mi> </mml:mrow> </mml:msub> </mml:math> defects. In contrast, the deeper yellow color in the expansion region is caused by $$V_{Al}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mi>Al</mml:mi> </mml:mrow> </mml:msub> </mml:math> – $$O_N$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>O</mml:mi> <mml:mi>N</mml:mi> </mml:msub> </mml:math> complex defects. Additionally, etching results exhibit a polarity reversal from the Al-polar to N-polar growth face in localized chromatic aberration region. The bonding angle of approximately $${90}^\circ$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mn>90</mml:mn> </mml:mrow> <mml:mo>∘</mml:mo> </mml:msup> </mml:math> at the $$B_{1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>B</mml:mi> <mml:mn>1</mml:mn> </mml:msub> </mml:math> bond (formation by the half-filled orbitals of the Al and N atoms) site, which occurs during the bonding of Al and N atoms, results in the polarity reversal during m-plane lateral growth. This work provides implications for the preparation of large-sized AlN single crystals.