Multiple optical centers in Eu-implanted AlN nanowires for solid-state lighting applications

Abstract

A detailed spectroscopic analysis of Eu3+ implanted and annealed AlN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy is presented by using micro-Raman, temperature-dependent steady-state photoluminescence, and time-resolved photoluminescence. Two different annealing temperatures (1000 °C and 1200 °C) were used. Such annealing conditions achieved a recovery of the original AlN crystalline structure as confirmed by Raman analysis. For both samples, the red Eu3+ intra-4f 6 luminescence was demonstrated, where the 5D0 → 7F2 transition at 624 nm is the most intense. Two well-resolved Eu optically active centers were observed in the present AlN NWs and designated as Eu1 and Eu2, due to their similar spectral shape when compared to those observed in GaN layers [Bodiou et al., Opt. Mater. 28, 780 (2006); Roqan et al., Phys. Rev. B 81, 085209 (2010)]. Their behavior was found to depend on the annealing temperature. Photoluminescence studies reveal that at 14 K, Eu2 is dominant for the lower annealing temperature, while Eu1 is dominant for the highest annealing temperature. Moreover, at room temperature, Eu1 center was found to be the dominant for both samples. Indeed, the luminescence intensity of the 5D0 → 7F2 transition exhibits a lower thermal quenching for the samples annealed at the highest temperature (∼80% for the sample annealed at 1200 °C and ∼50% for the sample annealed at 1000 °C) boosting their potential use as efficient red emitters.

Eu3+ optical activation engineering in Al Ga1-N nanowires for red solid-state nano-emitters

“…Thus, by annealing AlxGa1-xN (x > 0) NWs at 1200 ℃, it is not expected to strongly damage the GaN NW template, as it is protected by the AlxGa1-xN top-section [57]. Similar annealing conditions were reported to successfully achieve the recovery of the lattice damage introduced by the implantation and the optical activation of RE 3+ ions in III-N layers [66,67] and nanostructures [26][27][28].…”

“…In fact, ion implantation might induce a high concentration of cation and nitrogen vacancies (VIII and VN), leading to the incorporation of Eu 3+ ions in cation sites and to the formation of complexes with the generated point defects, which can provide energetic levels within the host's bandgap able to assist Eu 3+ excitation. Recently, two dominant optically active Eu 3+ centers (Eu1 and Eu2) were identified by photoluminescence in Eu 3+ -implanted AlN NWs, with similar spectral shape to the corresponding Eu1 and Eu2 centers in GaN [26].…”

“…To the best of our knowledge, studies on ternary AlxGa1-xN:Eu 3+ over the entire range of AlN content were only performed in layers and revealed a reduced thermal quenching with enhanced Eu 3+ luminescence efficiency for high AlN content hosts [18,51,52]. Besides, Eu 3+ -implantation in III-N NWs can play a key role for improving Eu 3+ luminescence efficiency [26][27][28]: on the one hand, NWs are expected to be less affected by residual implantation-induced damage compared to 2D structures [25]; and on the other hand, Eu 3+ -implantation occurs not only on the upper surface (c-plane) but also on side facets. Although this increases the complexity of implantation damage build-up since crystallographic surface planes can lead to distinct damage in GaN during implantation [53,54], it provides an additional parameter to control the distribution of Eu 3+ and defects within the NW via the implantation geometry [25].…”

“…The incorporation of the rare-earth (RE) trivalent europium ions (Eu 3+ ) into III-N layers [16][17][18][19][20][21][22][23] and nanostructures [24][25][26][27][28][29][30][31][32] was proved as an excellent strategy to obtain red emission characterized by the sharp and stable Eu 3+ intra-4f 6 transitions. This approach was successfully implemented for in situ Eu 3+ -doped GaN-based emitting devices [31,[33][34][35][36][37][38], with Eu 3+ -related luminescence external quantum efficiency (EQE) values reaching ~29% at room temperature (RT) and ~48% at 77 K [39].…”

“…As an alternative to in situ RE incorporation, ex-situ ion implantation constitutes a suitable method to obtain a controlled doping profile with concentration even beyond solubility limits [18,25,26,32,[42][43][44][45][46][47]. This technique requires post-implantation thermal annealing to recover the implantation-induced damage and to optically activate the RE ions.…”