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.
Highlights
Self-assembled GaN quantum dots (QD) stacked in superlattices (SL) with AlN spacer layers are known to be important nanostructures for optoelectronic applications in the UV/visible and infrared spectral regions [1,2,3]
This behaviour is driven by the combined effects of the quantum confinement (QC) of the carriers and the quantum confined Stark effect (QCSE), which is influenced by the strain state of the stacked heterostructures [4,8,9]
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]
Summary
Self-assembled GaN quantum dots (QD) stacked in superlattices (SL) with AlN spacer layers are known to be important nanostructures for optoelectronic applications in the UV/visible and infrared spectral regions [1,2,3]. It is well established that the GaN QD excitonic recombination can occur at photon energies above and below the GaN bulk bandgap [1,2,3,4,5,6,7,8] This behaviour is driven by the combined effects of the quantum confinement (QC) of the carriers and the quantum confined Stark effect (QCSE), which is influenced by the strain state of the stacked heterostructures [4,8,9]. The #987 and #989 GaN QD/AlN SL were implanted with high (1 × 1014-15 ions cm-2) and low (1 × 1013 ions cm-2) fluences of Europium ions; the SL structures were further submitted to post-implantation thermal annealing in order to achieve Eu3+ optical activation [17,18]. X-ray reflection (XRR) was performed on a high-resolution system using a Göbel mirror to focus the beam and CuKa1,2 radiation
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