Table of Contents

Quantum Dot Molecules M. H. Na,(a,b) H. Luo,(a,b) and Y. Liang Supported by EMSL Operations. (a) EMSL User. (b) Physics Department, State University of New York–Buffalo. A 3-dimensional semiconductor structure on the scale of a few to a few tens of nanometers can store electrons that exhibit properties of particles in zero dimension. Such structures are referred to as nanostructures or quantum dots (QDs), which are the ultimate quantum structure of semiconductors—the key element to further miniaturization and quantum computing. While QDs can be fabricated with state-of-the-art electron lithography techniques, they also can be obtained by a self-assembly growth process, which has much higher yield and lower cost. Self-assembled quantum dots can be synthesized and formed randomly in single dot form and exhibit similarities to atoms, e.g., discrete states, shell structures, and effects related to the Hund’s rule. The next step is to produce quantum dot arrays or combinations in order to further tailor the electronic states for practical applications. Two types of molecules with different levels of complexity have been synthesized and formed using CdTe. The results presented here demonstrate the possibility of forming QD molecules through self-assembly, which enhances the ability to manipulate electronic states. The first type of molecule (Figure 3.7) is the analogue of the simplest hydrogen molecule and has double dot formation. This molecule was form by growing CdTe on vicinal ZnSe surfaces. Figure 3.7. An AFM image showing pairs of CdTe QDs grown on a ZnSe substrate. The second type of molecule (Figure 3.8) resembles a "benzene" structure, with CdTe quantum dots self-organized in a ring formation. This molecule forms when the structure is subjected to post-growth annealing at 400°C. Figure 3.8. An AFM image reveals a ring consisting of eight individual QDs. CdTe quantum dot rings shown here (Figure 3.9a and b) are the first compound semiconductor structures that have self-passivated surfaces with a Te-rich cap layer. Chemical mapping of the rings show the increased Te in the top layer, and its resistance to C and O, two of the most common contaminants.   Figure 3.9. Scanning Auger microscopy images. (a) Te distributions of a quantum dot ring. (b) C distributions of a quantum dot ring. William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: June 12, 2001 Security & Privacy PNNL-13147