Quantum dot structures
Researchers: Abuduwayiti Aierken
When a narrow-bandgap material is completely surrounded by a larger bandgap material, the electrons and holes are confined into discrete quantum states and their movement is restricted in three dimensions. This kind of structure is known as quantum dot (QD) structure. Due to these discrete energy states, QDs can be considered as artificial atoms of a kind. The electronic properties of QDs, e.g., the high density of states, make them promising for use in many device applications, such as the QD laser.
The metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) techniques are used to fabricate QD structures. The common way to fabricate QDs is to bury self-assembled islands in a matrix of another semiconductor having a larger band gap. An alternative way to create QDs is to use the self assembled islands as stressors. When self assembled islands were grown on the top of a near surface quantum well (NSQW) structure, the tensile strain underneath the islands is utilized to locally reduce the band gap of the NSQW. The resulting lateral confinement potential is nearly parabolic for both electron and holes, and the vertical confinement is achieved by the high quality interfaces of the QW. The versatility of this approach is that the emission wavelength of the SIQD can be tuned by adjusting the QW composition and island size. This structure is called strain-induced QD (SIQD). Fig.1 shows the schematic diagram of quantum confinement and photoluminescence (PL) spectrum of a SIQD.
Fig.1. (a) Schematic diagram of quantum confinement and (b) photoluminescence (PL) spectrum of a SIQD.
While fabricating buried QDs, self-assembled islands are capped by fully overgrowing them. On the other hand, partial capping can be used to transform the morphology of the islands into volcano-like quantum rings (QRs). These nanostructures present an interesting topic in nanotechnology due to the potential offered by the unique topology. Moreover, investigation of the island-to-ring transformation helps to further refine the growth of self-assembled islands especially when fabricating buried QDs.
Figs. 2.-5. show some experimental results obtained in the group.
Fig. 2. PL spectrum of SIQD sample with In0.1Ga0.9As QW and InP stressors as the excitation intensity increased.
H. Lipsanen, et al., Solid-State Electronics 40, 601 (1996)
Fig. 3. Photoluminescence spectra showing the wavelength tunability of InGaAs(P)/InP SIQDs.
J. Riikonen, et al., JJAP 44 L976 (2005)
Fig. 4. Atomic force micrographs (0.5x0.5 μm2) of (a) as-grown InAs island, and islands annealed in TBP at (b) 520, (c) 540, and (d) 555 oC.
J. Sormunen, et al., JJAP 44 L1323 (2005)
Fig. 5. 3D AFM images (1x1 μm2) of QRs transformed by partially capped InAs islands fabricated at different temperature (a) 500, (b) 530, (c) 550, and (d) 570 oC. The thickness of the capping layer is 2 nm, annealing time with TBAs is 60 s.
A.Aierken, et al., Nanotechnology 19, 245304 (2008)
PhD thesis on QD structures
M. Sopanen, 1997, Helsinki University of Technology, “Self-organized growth and optical spectroscopy of semiconductor nanostructures”
J. Sormunen, 2006, Helsinki University of Technology, “Growth and modification of planar and self-assembled semiconductor nanostructures” http://lib.tkk.fi/Diss/2006/isbn9512280388/
J. Riikonen, 2006, Helsinki University of Technology, “Self-assembled nanorings and stressor quantum dots” http://lib.tkk.fi/Diss/2006/isbn9512281821/
H. Koskenvaara, 2008, Helsinki University of Technology, “Photoluminescence spectroscopy and carrier dynamics modeling of quantum dot structure” http://lib.tkk.fi/Diss/2008/isbn9789512294145/