Abstract
Aluminum nitride (AlN) has attracted increasing interest as an optoelectronic material in the deep ultraviolet spectral range due to its wide bandgap of 6.0 eV (207 nm wavelength) at room temperature. Because AlN bulk single crystals are ideal device substrates for such applications, the crystal growth of bulky AlN has been extensively studied. Two growth methods seem especially promising: hydride vapor phase epitaxy (HVPE) and sublimation. However, the former requires hazardous gases such as hydrochloric acid and ammonia, while the latter needs extremely high growth temperatures around 2000 °C. Herein we propose a novel vapor-phase-epitaxy-based growth method for AlN that does not use toxic materials; the source precursors are elementary aluminum and nitrogen gas. To prepare our AlN, we constructed a new growth apparatus, which realizes growth of AlN single crystals at a rate of ~18 μm/h at 1550 °C using argon as the source transfer via the simple reaction Al + 1/2N2 → AlN. This growth rate is comparable to that by HVPE, and the growth temperature is much lower than that in sublimation. Thus, this study opens up a novel route to achieve environmentally friendly growth of AlN.
Highlights
Because the reaction of Al + 1/2N2 → aluminum nitride (AlN) is seemingly inactive, especially due to the inert nature of N2 gas, AlN crystal growth usually involves processes to activate Al and N
HVPE uses NH3 and Al-trichloride generated via a reaction between Al and HCl, while sublimation requires AlN powders and extremely high growth temperatures around 2000 °C. [This is true for other thin film growth techniques
Similar to thin film growth, homoepitaxial and heteroepitaxial nucleations are possible for bulk AlN single crystals
Summary
Because the reaction of Al + 1/2N2 → AlN is seemingly inactive, especially due to the inert nature of N2 gas, AlN crystal growth usually involves processes to activate Al and N. We employed heteroepitaxy on sapphire substrates as a nucleation method because this study demonstrates proof-of-concept AlN growth and heteroepitaxy is easier and more cost effective than homoepitaxy for this purpose.
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