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.
In this work, Eu 3+ -implanted and annealed AlxGa1-xN (0 ≤ x ≤ 1) nanowires (NWs) grown on GaN NW template on Si (111) substrates by plasma-assisted molecular beam epitaxy are studied by µ-Raman, cathodoluminescence (CL), nano-CL, and temperature-dependent steady-state photoluminescence. The preferential location of the Eu 3+ -implanted ions is found to be at the AlxGa1-xN top-section. The recovery of the as-grown crystalline properties is achieved after rapid thermal annealing (RTA). After RTA, the red emission of the Eu 3+ ions is attained for all the samples with below and above bandgap excitation. The 5 D0 → 7 F2 transition is the most intense one, experiencing a redshift with increasing AlN nominal content (x) from GaN to AlN NWs. Moreover, AlN nominal content and annealing temperature alters its spectral shape suggesting the presence of at least two distinct optically active Eu 3+ centers (Eu1 and Eu2). Thermal quenching of the Eu 3+ ion luminescence intensity, I, is found for all the samples from 14 K to 300 K, being the emission of Eu 3+ -implanted AlN NWs after RTA at 1200 ℃ the most stable (I300 K/I14 K ~80%). The GaN/AlN interface in this sample is also found to have a key role in the Eu 3+ optical activation.
Europium
(Eu)-implanted AlN nanowire (NW) p–n junctions,
subjected to rapid thermal annealing at 1000 °C, were investigated
in view of application as red light-emitting diodes (LEDs). In a first
step, the structural and optical properties of NWs implanted with
two different fluences (1 × 1014 cm–2 and 5 × 1014 cm–2) were studied.
The luminescence of the trivalent Eu ions (Eu3+) was achieved
for both samples using below and above AlN bandgap energy excitation.
The excitation below the AlN bandgap occurs through two broad bands,
A1 (peaked at ∼270 nm) and A2 (peaked at ∼367 nm), associated
with lattice defects. In addition to Eu3+ luminescence,
other radiative channels linked to deep-level defects were identified
in photoluminescence (PL). The cathodoluminescence (CL) relative intensity
ratio between intra-ionic and defect-related emissions increases compared
to that of PL. In both PL and CL, the Eu3+ luminescence
intensity increases about three times for the highest fluence, while
the contribution from radiative recombination at defects decreases.
This study also allowed to map an in-depth profile of the optically
active Eu3+, revealing that it extends deeper than the
range predicted by Monte Carlo simulations. Based on these findings,
a proof-of-concept red LED is shown using the NWs implanted with the
highest fluence. The devices exhibited the typical rectifying behavior
of a p–n junction and an electroluminescence signal dominated
by the 5D0 → 7F2 transition (∼624 nm) starting at a threshold voltage of 12
V. The demonstration of red LEDs based on Eu-implanted AlN NWs highlights
the potential of such an approach for developing multi-color nano-emitters.
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