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Curing Quantum Dots using Inductively Coupled Argon Plasma

T. Mei, D. Nie, H.S. Djie and B.S. Ooi
Nanyang Technological University, SG

quantum dot, argon plasma, photoluminescence

Great advancement achieved in quantum dot (QD) growth using Molecular Beam Epitaxy or Metal-Organic Vapor Phase Epitaxy (MOVPE) has made QD-based devices demonstrate superior performances.1 However, QD structures are highly-strained material systems grown under low temperature such that large number of defects inherently present in the materials. These grown-in defects may behave as non-radiative recombination centers reducing quantum efficiency and lead to poor thermal stability2 as well such that bandgap shift is enhanced in annealing process.
In our previous study of argon inductively coupled plasma (ICP) process for quantum well (QW) intermixing enhancement, we have observed significant photoluminescence (PL) enhancement in InGaAsP/InP QW samples.3 It is further and more clearly evidenced in an experiment using an AlGaAs/GaAs 5-QW structure,4 where the PL peak of a defect-rich QW was recovered in contrast to the PL peaks of the rest QWs.
In this work, we demonstrate the reduction of low temperature grown-in defects using inductively coupled argon plasma process in samples of an In0.5Ga0.5As/GaAs quantum dot (QD) structure. After 3 min plasma exposure, the PL peak intensity of QDs is increased to 2.7 times of the original (see Fig. 1). The subsequent rapid thermal annealing (RTA) at 750 ºC for 60 s increases the PL peak intensity up to 5.5 times, in contrast to less than 20% PL enhancement observed in the annealed-only sample (see Fig. 2). This shows that the ICP treatment cures the QD samples by removing defects more efficiently than a conventional RTA process. Furthermore, the PL peak wavelength blueshift is suppressed by 26 nm in the ICP treated sample in comparison to that in the annealed-only sample. The suppression in bandgap blueshift increases with the plasma exposure duration and RTA time. This phenomenon denotes unambiguously that the thermal stability of the In0.5Ga0.5As/GaAs QD structure is improved by the plasma treatment. An excitation-dependent PL experiment was also done in the as-grown and plasma-treated samples as shown in Fig. 3. The results shows the state-filling effect becomes more appreciable in the latter sample (Fig. 3(b)), which is plasma treated, showing less amount of defects for non-radiative recombination.
This work suggests a novel approach to QD material quality improvement by grown-in defect reduction which implicates device performance improvement. It is more efficient compared to the conventional RTA process for material quality recovery. Since the plasma treatment is implemented at near room temperature, this method should be further explored for recovering other types of QD materials which are not tolerable to high temperature process.
1. D. L. Huffaker, G. Park, Z. Zou, O. B. Shchekin, and D. G. Deppe, Appl. Phys. Lett. 73, 2564 (1998).
2. A. Babinski, J. Jasinski, R. Bozek, A. Szepielow, and J. M. Baranowski, Appl. Phys. Lett. 79, 2576 (2001).
3. H. S. Djie, T. Mei, and J. Arokiaraj, Appl. Phys. Lett. 83, 60 (2003).
4. H.S. Djie, T. Mei, J. Arokiaraj, Semicond. Sci. & Tech., 20, 244 (2005).

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