We fabricated quantum rings (QR) by capping InAs/(InGaAs)/InP quantum dots (QD) grown by metal-organic chemical vapor deposition (MOCVD) with 5 to 10 GaAs mono-layers (ML). The InAs/InGaAs QDs before GaAs capping had a lens-shape geometry with a 57.0 nm base length along the [1-10] direction (42.0 nm along the [110] direction) and a 4.6 nm height. After the deposition of 5 to 10 GaAs MLs on the QD, AFM image indicates that each QD evolved to a QR, as reported in InAs/GaAs system by the deposition of thin GaAs layer [1]. However, our QR is split into two adjacent mounds with 2.4 nm height which are also elongated along the [1-10] direction [2]. A 54 meV blue shift is observed in the photoluminescence (PL) spectrum after deposition of 10 GaAs MLs, compared to the spectra of QDs without GaAs layer.
We investigated the strain distribution and the electron energy spectrum of the QR by using a valence force field (VFF) method within an eight-band k•p technique. We assumed a composition grading that follows the indium profile reported in InAs/GaAs QRs [3] exists in our InAs/(InGaAs)/InP QDs, The in-plane strain (Exx+Eyy) values range from -7.37 % at the bottom of the QR to -3.07 % at the top of each mound of the QR, while the vertical strain (Ezz) ranges from +4.51 % to +1.78 % on the same locations. As a result, the bi-axial strain at the top of each mound is 40 ~ 50 % smaller than in the center of the QR. In solving for the QR energy spectrum, we followed Burt's method [4] to eliminate the spurious states arising from the k•p Hamiltonian. Depending on the degree of composition grading, the PL spectrum shows a 30 meV to 60 meV blue shift.
[1] Axel Lorke, R. Johannes Luyken, Alexander O. Govorov, and Jörg P. Kotthaus, Phys. Rev. Lett. 84, 2223 (2000).
[2] Kwangmin Park, Pilkyung Moon, Eungjin Ahn, Sukwon Hong, Euijoon Yoon, Jeong Won Yoon, Hyeonsik Cheong, and Jean-Pierre Leburton, Appl. Phys. Lett. 86, 223110 (2005).
[3] P. Offermans, P. M. Koenraad, J. H. Wolter, D. Granados, J. M. Garcia, V. M. Fomin, V. N. Gladilin, and J. T. Devreese, Appl. Phys. Lett. 87, 131902 (2005).
[4] M. G. Burt, J. Phys.: Condens. Matter 4, 6651 (1992). |