Paper EM-ThA2
InAs Lateral Quantum Dot Molecules with Controllable Configurations

Thursday, November 12, 2009, 2:20 pm, Room B1

A quantum dot molecule (QDM) is formed when two or more quantum dots are close enough so that the electronic properties of each dot are affected by the presence of the other. Well controlled, vertically-stacked QDMs are now routinely grown for optical investigations.[1] [#_ftn1] For device applications, laterally coupled dots offer compatibility with existing gate technologies and advantages in scalability. However, growth of complex laterally coupled QDMs is more challenging than the vertically stacked dots.

One method for influencing the lateral position of self-assembled QDs is to introduce features on the substrate which will act as preferred nucleation sites for dot growth.[2] [#_ftn2] One promising technique is to use gallium droplet epitaxy to form a template for further dot growth. In droplet epitaxy, gallium is deposited without arsenic overpressure, forming metallic islands without a wetting layer. When these droplets are exposed to arsenic and annealed they crystallize into homoepitaxial mounds. When InAs is grown on these mounds, the islands appear to grow only on the sloping edges of these mounds. Previous structures grown with this technique have demonstrated flexibility in QDM configuration and excellent uniformity.[3] [#_ftn3]

In this presentation, we will describe new growth techniques that can be used to control the configurations of lateral InAs QDMs. By fixing the direction of the incident indium flux, the indium beam is shadowed by the GaAs mounds, so that InAs dots will only form on the sides of the mound which face the indium source. This allows new configurations of QDMs to be grown that cannot be formed using a rotating substrate. In addition, by capping first layer QDMs with GaAs or AlGaAs and growing additional strain-coupled dots, we demonstrate flexible and uniform three dimensional QDM configurations.

With atomic force microscopy it is not possible to determine the structure of the InAs dots once they are buried under a GaAs capping layer. Cross sectional scanning tunneling microscopy (XSTM) is an ideal technique to examine the final structure with atomic resolution. For bi-molecules, the interdot separation is 8 nm and the center to center distance is 30 nm, which makes them excellent candidates for investigations of electron tunneling using photoluminescence spectroscopy.