Hole Trapping at Acceptor Impurities and Alloying Elements in AlN

Abstract

Hole localization is a root cause of difficulties in p-type doping of aluminum nitride. Herein, the stability of localized holes in AlN and their susceptibility to self-trapping within bulk material or in the presence of isovalent elements and acceptor impurities are calculated. It is found that self-trapped holes are metastable in bulk AlN, and also exhibit a very low barrier to detrapping. However, holes become trapped in the presence of acceptor impurities as well as isovalent elements such as B, In, and Sc, and may contribute to the broad luminescence observed in AlN alloys. It is also found that hole trapping contributes to the large ionization energies for acceptors in AlN. Most acceptors can also trap a second hole, becoming positively charged and further exacerbating difficulties in doping aluminum nitride p type. The stability of trapped holes is found to scale with atomic size mismatch to Al.

A p-type dopable ultrawide-bandgap oxide

“…The small size of B impurity allows it to displace into a planar configuration, bonding to three oxygen atoms, allowing a hole to localize on a fourth oxygen neighbor representing a B• • • O broken bond. Similar behavior has been observed for B impurities in AlN [33]. The ionization energies of Al Sn (1.06 eV), Ga Sn (0.91 eV), and In Sn (0.87 eV) in SnO 2 are smaller, and are consistent with prior calculations [15,16].…”

Strain and crystal field splitting inversion in III-Nitrides

“…Finally, we have shown, from a careful analysis of our DFT results, that internal strains effects together with spin orbit effects play a significant role in the energetic order of the top valence states. As an example, concerning to difficulties to obtain p-type AlN related to the GaN and InN, is directly related to the relative position of Γ ⊥ 7 state (CH) compared with both the Γ 9 (HH) and Γ 7 (LH) ones at the top of valence bands.In this case, the CH state behaves as the hole trap mentioned in previous theoretical works 13,15 . We hope that our results give guidance for future experiments on this subject.…”

“…It is interesting to remark, when we consider the results of the presence of the acceptors in III-Nitrides 13,15 , that the CH state is the aforementioned hole trap and, its relative position to the HH and LH ones explain why is difficult to have p-type AlN related to the GaN and InN as well.…”

“…In these new complex nanostructures, there is an enhancement in hole concentration of over five orders of magnitude at reduced temperatures, as compared with to that of doped bulk samples [9][10][11][12] . However, as pointed out by Lyons and Van de Walle 13 , these new techniques could work for acceptors in GaN and InN, but not for AlN, in which hole localization presents a main problem, since the self-trapped hole is a locally stable configuration in this material.…”

“…Within this method, transition charge levels for deep impurities were obtained and interpreted with success. E. g., due to the limitation of the computational efforts caused by their long range impurity interaction, only recently quantitative predictions of shallow impurities features within this model were published 13,15 .…”