Influence of high-temperature processing on the surface properties of bulk AlN substrates

• The origin of deep-level luminescence in RT-PL spectra of HVPE-AlN was studied.
• Deep-level luminescence was disappeared after surface polishing of AlN.
• AlN surfaces grown by HVPE at 1450 °C and cooled with NH3 are contaminated by Si.
• Surface contamination by Si generates Al vacancies (VAl) near the AlN surface.
• Recombination of the shallow donor (Si) and VAl resulted in an emission at 3.3 eV.

Deep-level luminescence at 3.3 eV related to the presence of Al vacancies (VAl) was observed in room temperature photoluminescence (RT-PL) spectra of homoepitaxial AlN layers grown at 1450 °C by hydride vapor-phase epitaxy (HVPE) and cooled to RT in a mixture of H2 and N2 with added NH3. However, this luminescence disappeared after removing the near surface layer of AlN by polishing. In addition, the deep-level luminescence was not observed when the post-growth cooling of AlN was conducted without NH3. Secondary ion mass spectrometry (SIMS) studies revealed that although the point defect density of the interior of the AlN layers remained low, the near surface layer cooled in the presence of NH3 was contaminated by Si impurities due to both suppression of the surface decomposition by the added NH3 and volatilization of Si by decomposition of the quartz reactor walls at high temperatures. The deep-level luminescence reappeared after the polished AlN wafers were heated in presence of NH3 at temperatures above 1400 °C. The surface contamination by Si is thought to generate VAl near the surface by lowering their formation energy due to the Fermi level effect, resulting in deep-level luminescence at 3.3 eV caused by the shallow donor (Si) to VAl transition.