G.J. Salamo, J.B. Smathers, C.L. Workman, and H. Yang

Department of Physics, University of Arkansas, Fayetteville, AR

Our work has focused on the Stranski-Krastanov growth mode to form quantum dots and wires. For this growth mode, i.e., small interface energy but large lattice mismatch, initial growth is layer by layer. As a result, deposited materials first form a two-dimensional (2D) wetting layer on the substrate. As material is initially deposited it is energetically favorable for material to add to the step edges of the 2D islands as opposed to adding to the top to begin 3D-island formation. In this way, the 2D islands grow laterally in size. However, this is done at energy cost as the strain increases. Soon it becomes energetically unfavorable for new atoms to continue the lateral growth and they diffuse to the top of the 2D island initiating 3D growth. Layer-by-layer, the 3D island develops while atoms at the top are strain relieved compared to atoms at the bottom.

Once quantum dots or wires form on the surface we must be able to examine and observe them with high resolution if we are to probe and understand their growth. The two tools we have found essential for surface characterization are in-situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). For example, using RHEED data we can very quickly, and with 0.1-monolayer accuracy, identify the critical thickness for a surface transition from 2D islands to 3D islands. This is because 3D islands are distinguished by a 3D-diffraction pattern that is strikingly different than the typical 2D-diffraction pattern.

While RHEED data is important, most of our investigations center around our in-situ STM. A scanning tunneling microscope picture of quantum dots grown in our MBE facility is shown to the right. It beautifully reveals the structural details associated with the two island shapes seen when InAs 3D islands, or dots, form on a GaAs substrate. These pictures reinforce our expectation that research on defining an atom-by-atom scale offers tremendous payoff in the evolution of our understanding of materials and the revolutionary development of new devices.

At this workshop, we will discuss the growth and annealing studies of InAs 3D islands on GaAs (001) using a combined molecular beam epitaxy and scanning tunneling microscopy facility. The studies reveal how the island volume, shape, and intermixing with the substrate relate to ripening as the annealing time is increased. For example, STM studies indicate two distinct island shapes, one for small volumes (type-I) and one for large island volumes (type-II). In addition, we have also observed a substantial fraction of a hybrid shape for islands falling roughly in the middle of the volume distribution. With increasing ripening, the islands are observed to transition from a type-I shape to a type-II shape and then back to a type-I shape. Eventually, all islands are of the type-I shape.

If time permits we will also discuss similar observations with other material systems.