Semiconductor Quantum Dots Self-Assembled into 2D and 3D Ordered Lattices
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obtained. They concluded that “the control of the Fermi level should be important in future applications of colloidal semiconductor nanocrystals and the electron transfer method may be the viable approach in the nanometer length scale with strong confinement.” ROBERTSON ANSAH BILL
Semiconductor Quantum Dots Self-Assembled into 2D and 3D Ordered Lattices Researchers in the Materials Department at the University of California— Santa Barbara have developed a technique for growing semiconductor self-assembled quantum dots (QDs) into ordered lattices. Typically, in self-assembled QD systems, the QDs are isolated or are arranged as ensembles that are randomly distributed within a structure. It is thought, however, that self-assembled QDs, when arranged in an ordered array or lattice, will exhibit attributes related to the electronic or photonic quantum dot coupling within the array. While a number of techniques for producing spontaneous long-range order in QDs have been employed, none have resulted in systems that demonstrate coupling effects between individual dots. In the January 1 issue of Applied Physics Letters, P.M. Petroff and co-workers describe a method for self-assembling QDs (InAs surrounded by GaAs) into two- and three-dimensional periodic lattices using a coherently strained layer of In0.2Ga0.8As deposited by molecular beam epitaxy (MBE) over a semiconductor substrate. Quantum dots are formed by the epitaxial deposition of coherently strained islands. When the area over which the QDs are deposited is large and uniform, the nucleation process is random. To reduce this randomness, the researchers nucleated the QDs on a limited surface area, or, specifically, a mesa top with nanometer dimensions. A square lattice of mesas was patterned on a {100} MBEdeposited GaAs film using optical holography. The surface mesas had a square base with ~170 nm sides and ~25 nm height, and the two-dimensional (2D) square mesa lattice had a periodicity of ~250 nm along the unit cell primitive vectors. The oxide layer on the GaAs was thermally desorbed and a 60-nm thick GaAs layer was deposited to remove the surface damage induced by the oxide. To obtain a periodic lattice of nucleation sites, the research team used a periodic strain pattern induced by a coherently strained subsurface stressor layer of In0.2Ga0.8As re-grown on the GaAs patterned surface. InAs was then deposited and islands formed on the mesa tops. MRS BULLETIN/JANUARY 2001
Finally, a 10-nm thick capping layer of GaAs was deposited, transforming the islands into quantum dots. The researchers’ atomic force microscope (AFM) images illustrate three different island lattices formed on top of the mesas, each with a unit cell of a different in-plane orientation. Data show that over 90% of the InAs QDs are on top of the mesas and that the lattice periodicity of the QDs matches that of the mesa array. They also demonstrate that, by adjusting the mesa lattice, the lattice period and unit cell structure can be tuned. The number of QDs within the lattice can be adjusted by v
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