The origin of 2.78 eV emission and yellow coloration in bulk AlN substrates

Abstract

The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light with wavelengths shorter than that of yellow. This yellow impurity limits UV transparency and hence restricts the applications of AlN substrates for deep UV optoelectronic devices. Here, the optical properties of AlN epilayers, polycrystalline AlN, and bulk AlN single crystals have been investigated using photoluminescence ͑PL͒ spectroscopy to address the origin of this yellow appearance. An emission band with a linewidth of ϳ0.3 eV ͑at 10 K͒ was observed at ϳ2.78 eV. We propose that the origin of the yellow color in bulk AlN is due to a band-to-impurity absorption involving the excitation of electrons from the valence band to the doubly negative charged state, ͑V Al 2− ͒, of isolated aluminum vacancies, ͑V Al ͒ 3−/2− described by V Al 2− +h =V Al 3− +h + . In such a context, the reverse process is responsible for the 2.78 eV PL emission.

Status of Growth of Group III-Nitride Heterostructures for Deep Ultraviolet Light-Emitting Diodes

“…The underlying mechanisms for the formation of these bands are still under discussion. [35,39,[44][45][46][47][48][49][50] Specifically, at least three absorption bands at 2.8 eV, 3.3-4.3 eV and 4.7 eV were found and discussed for PVT-grown AlN. The origin of the 2.8 eV band is still an issue of intense debate.…”

“…Some researcher ascribed it to transition between the valence band and Al vacancy (VAl) level. [50] However, others attributed to transitions between oxygen on N sites (ON) and VAl (ON-VAl). [44] The broad 3.3-4.3 eV band is presumably associated with VAl and/or oxygen related complexes, and reduction of oxygen contaminations in the source material was found to lead to a significant decrease of UV absorption in both colorless and yellowish areas of bulk AlN.…”

Status of Growth of Group III-Nitride Heterostructures for Deep Ultraviolet Light-Emitting Diodes

“…The origin of the 2.8 eV band is still an issue of intense debate. Some researcher ascribed it to transition between the valence band and Al vacancy (V Al ) level [53]. However, others attributed it to transitions between oxygen on N sites (O N ) and V Al (O N -V Al ) [47].…”

“…These absorption bands are attributed to residual impurities such as silicon, oxygen (mainly from the source materials), and carbon (mainly from the reactor parts such as TaC crucible) and intrinsic point defects, and Al vacancies. The underlying mechanisms for the formation of these bands are still under discussion [38,42,[47][48][49][50][51][52][53].…”

Development of 2" AlN Substrates Using SiC Seeds

“…Al , the isolated aluminum vacancies (V Al ) 3−/2− (Hung et al, 2004;Sedhain et al, 2009) or the Al vacancy-impurity complexes (Lu et al, 2008). …”