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Effect of Eu-implantation and annealing on the GaN quantum dots excitonic recombination.

Peres M, Magalhães S, Fellmann V, Daudin B, Neves AJ, Alves E, Lorenz K, Monteiro T - Nanoscale Res Lett (2011)

Bottom Line: In Eu-implanted SL structures, the GaN QD recombination was found to be dependent on the implantation fluence.Despite the fact that different deexcitation processes occur in undoped and Eu-implanted SL structures, the excitation population mechanisms were seen to be sample-independent.Two main absorption bands with maxima at approx. 4.1 and 4.7 to 4.9 eV are responsible for the population of the optically active centres in the SL samples.

View Article: PubMed Central - HTML - PubMed

Affiliation: Departamento de Física e I3N, Universidade de Aveiro, Campus de Santiago, Aveiro, 3810-193, Portugal. tita@ua.pt.

ABSTRACT
Undoped self-assembled GaN quantum dots (QD) stacked in superlattices (SL) with AlN spacer layers were submitted to thermal annealing treatments. Changes in the balance between the quantum confinement, strain state of the stacked heterostructures and quantum confined Stark effect lead to the observation of GaN QD excitonic recombination above and below the bulk GaN bandgap. In Eu-implanted SL structures, the GaN QD recombination was found to be dependent on the implantation fluence. For samples implanted with high fluence, a broad emission band at 2.7 eV was tentatively assigned to the emission of large blurred GaN QD present in the damage region of the implanted SL. This emission band is absent in the SL structures implanted with lower fluence and hence lower defect level. In both cases, high energy emission bands at approx. 3.9 eV suggest the presence of smaller dots for which the photoluminescence intensity was seen to be constant with increasing temperatures. Despite the fact that different deexcitation processes occur in undoped and Eu-implanted SL structures, the excitation population mechanisms were seen to be sample-independent. Two main absorption bands with maxima at approx. 4.1 and 4.7 to 4.9 eV are responsible for the population of the optically active centres in the SL samples.

No MeSH data available.


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Normalised 14 K PL and PLE spectra for the as-grown and Eu-implanted GaN QD/AlN SL structures. Black full lines: PL spectra obtained with excitation with photons of 3.81 eV energy (He-Cd laser line); lines and closed symbols: PL spectra obtained with excitation of 4.7 eV. Dashed lines: PLE spectra monitored at the maxima of the low energy broad emission band. (i) stands for sample 987 which was implanted with 1 × 1015 atoms.cm-2 and annealed at 1000°C (a), 1100°C (b) and 1200°C (c). (ii) stands for sample 989 which was implanted with 1 × 1013 atoms.cm-2 (a) and 1 × 1014 atoms.cm-2 (b) and annealed at 1000°C.
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Figure 2: Normalised 14 K PL and PLE spectra for the as-grown and Eu-implanted GaN QD/AlN SL structures. Black full lines: PL spectra obtained with excitation with photons of 3.81 eV energy (He-Cd laser line); lines and closed symbols: PL spectra obtained with excitation of 4.7 eV. Dashed lines: PLE spectra monitored at the maxima of the low energy broad emission band. (i) stands for sample 987 which was implanted with 1 × 1015 atoms.cm-2 and annealed at 1000°C (a), 1100°C (b) and 1200°C (c). (ii) stands for sample 989 which was implanted with 1 × 1013 atoms.cm-2 (a) and 1 × 1014 atoms.cm-2 (b) and annealed at 1000°C.

Mentions: Two other sets of 20 periods GaN QD/AlN SL (#987 and #989) with larger quantum dots (average QD heights of 3.7 and 4.2 nm according to [9]) were implanted with different fluences of Eu3+ ions. The SL structures were further submitted to thermal annealing treatments between 1000 and 1200°C in order to achieve Europium optical activation. The Eu3+ emission from GaN QD as well as from AlN layers were identified previously [17,18]. The structural analysis by X-ray diffraction (XRD) of the implanted and annealed SL structures showed that high implantation fluences (1014 and 1015 ions cm-2) lead to higher lattice damage causing an expansion of the SL structure in the [0001] direction, while lower fluence does not change the XRD characteristics of the sample [17]. For these samples, and besides the Eu3+ luminescence, additional broad emission bands can be identified on the high energy side, as shown in Figure 2i, ii. Independently of the annealing temperature, a very broad emission band peaked at approx. 2.7 eV could be observed under excitation with photons of 3.81 eV energy for samples implanted with high fluence. The similarity of the spectral shape and peak position of the broad band with the emission detected under the same excitation conditions in the as-implanted sample indicates that it arises from large, 'blurred' GaN QD present in the damaged region of the implanted SL. The PLE spectra monitored at 2.7 eV is similar to the one shown in Figure 1a for the non-implanted SL samples suggesting that the optically active defects in the implanted SL are excited via the same paths. This is also confirmed with wavelength-dependent PL studies as seen in Figure 2. Exciting the samples in the wetting layer and/or oxygen-related AlN defect absorption bands (approx. 4.7 eV) makes the 2.7-eV PL always observable. Besides the 2.7-eV emission band, the GaN QD/AlN SL structures implanted with high fluence show an additional emission band peaked at 3.9 eV under 4.7 eV excitation. The observation of two GaN QD emission bands suggests the presence of a bimodal size distribution in the studied SL. Bimodality of GaN QD population in similar SL structures was previously reported by Adelmann et al. [16] and they found that such distribution occur at high GaN coverage and/or substrate temperature, which is the case of the analysed SL samples.


Effect of Eu-implantation and annealing on the GaN quantum dots excitonic recombination.

Peres M, Magalhães S, Fellmann V, Daudin B, Neves AJ, Alves E, Lorenz K, Monteiro T - Nanoscale Res Lett (2011)

Normalised 14 K PL and PLE spectra for the as-grown and Eu-implanted GaN QD/AlN SL structures. Black full lines: PL spectra obtained with excitation with photons of 3.81 eV energy (He-Cd laser line); lines and closed symbols: PL spectra obtained with excitation of 4.7 eV. Dashed lines: PLE spectra monitored at the maxima of the low energy broad emission band. (i) stands for sample 987 which was implanted with 1 × 1015 atoms.cm-2 and annealed at 1000°C (a), 1100°C (b) and 1200°C (c). (ii) stands for sample 989 which was implanted with 1 × 1013 atoms.cm-2 (a) and 1 × 1014 atoms.cm-2 (b) and annealed at 1000°C.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211471&req=5

Figure 2: Normalised 14 K PL and PLE spectra for the as-grown and Eu-implanted GaN QD/AlN SL structures. Black full lines: PL spectra obtained with excitation with photons of 3.81 eV energy (He-Cd laser line); lines and closed symbols: PL spectra obtained with excitation of 4.7 eV. Dashed lines: PLE spectra monitored at the maxima of the low energy broad emission band. (i) stands for sample 987 which was implanted with 1 × 1015 atoms.cm-2 and annealed at 1000°C (a), 1100°C (b) and 1200°C (c). (ii) stands for sample 989 which was implanted with 1 × 1013 atoms.cm-2 (a) and 1 × 1014 atoms.cm-2 (b) and annealed at 1000°C.
Mentions: Two other sets of 20 periods GaN QD/AlN SL (#987 and #989) with larger quantum dots (average QD heights of 3.7 and 4.2 nm according to [9]) were implanted with different fluences of Eu3+ ions. The SL structures were further submitted to thermal annealing treatments between 1000 and 1200°C in order to achieve Europium optical activation. The Eu3+ emission from GaN QD as well as from AlN layers were identified previously [17,18]. The structural analysis by X-ray diffraction (XRD) of the implanted and annealed SL structures showed that high implantation fluences (1014 and 1015 ions cm-2) lead to higher lattice damage causing an expansion of the SL structure in the [0001] direction, while lower fluence does not change the XRD characteristics of the sample [17]. For these samples, and besides the Eu3+ luminescence, additional broad emission bands can be identified on the high energy side, as shown in Figure 2i, ii. Independently of the annealing temperature, a very broad emission band peaked at approx. 2.7 eV could be observed under excitation with photons of 3.81 eV energy for samples implanted with high fluence. The similarity of the spectral shape and peak position of the broad band with the emission detected under the same excitation conditions in the as-implanted sample indicates that it arises from large, 'blurred' GaN QD present in the damaged region of the implanted SL. The PLE spectra monitored at 2.7 eV is similar to the one shown in Figure 1a for the non-implanted SL samples suggesting that the optically active defects in the implanted SL are excited via the same paths. This is also confirmed with wavelength-dependent PL studies as seen in Figure 2. Exciting the samples in the wetting layer and/or oxygen-related AlN defect absorption bands (approx. 4.7 eV) makes the 2.7-eV PL always observable. Besides the 2.7-eV emission band, the GaN QD/AlN SL structures implanted with high fluence show an additional emission band peaked at 3.9 eV under 4.7 eV excitation. The observation of two GaN QD emission bands suggests the presence of a bimodal size distribution in the studied SL. Bimodality of GaN QD population in similar SL structures was previously reported by Adelmann et al. [16] and they found that such distribution occur at high GaN coverage and/or substrate temperature, which is the case of the analysed SL samples.

Bottom Line: In Eu-implanted SL structures, the GaN QD recombination was found to be dependent on the implantation fluence.Despite the fact that different deexcitation processes occur in undoped and Eu-implanted SL structures, the excitation population mechanisms were seen to be sample-independent.Two main absorption bands with maxima at approx. 4.1 and 4.7 to 4.9 eV are responsible for the population of the optically active centres in the SL samples.

View Article: PubMed Central - HTML - PubMed

Affiliation: Departamento de Física e I3N, Universidade de Aveiro, Campus de Santiago, Aveiro, 3810-193, Portugal. tita@ua.pt.

ABSTRACT
Undoped self-assembled GaN quantum dots (QD) stacked in superlattices (SL) with AlN spacer layers were submitted to thermal annealing treatments. Changes in the balance between the quantum confinement, strain state of the stacked heterostructures and quantum confined Stark effect lead to the observation of GaN QD excitonic recombination above and below the bulk GaN bandgap. In Eu-implanted SL structures, the GaN QD recombination was found to be dependent on the implantation fluence. For samples implanted with high fluence, a broad emission band at 2.7 eV was tentatively assigned to the emission of large blurred GaN QD present in the damage region of the implanted SL. This emission band is absent in the SL structures implanted with lower fluence and hence lower defect level. In both cases, high energy emission bands at approx. 3.9 eV suggest the presence of smaller dots for which the photoluminescence intensity was seen to be constant with increasing temperatures. Despite the fact that different deexcitation processes occur in undoped and Eu-implanted SL structures, the excitation population mechanisms were seen to be sample-independent. Two main absorption bands with maxima at approx. 4.1 and 4.7 to 4.9 eV are responsible for the population of the optically active centres in the SL samples.

No MeSH data available.


Related in: MedlinePlus