Rachel S. Goldman

Maria Goeppert Mayer Collegiate Professor


2094 H.H. Dow
T: (734) 647-6821




Research Facilities


Synthesis of Low-Dimensional Semiconductor Structures: Semiconductor Quantum Dots

Collaborators: J. Kieffer (MSE), K. Pipe (ME); D. G. Cahill, UIUC
Sponsor: National Science Foundation DMR-0210714 and CCF-0508225; Army Research Office DAAD 19-01-1-0462
Recently, strain-induced self-assembled quantum dots (QDs) have enabled the development of high-performance light-emitters and detectors. Further advances in optoelectronics and quantum computing will require a narrowing of the density of states and achievement of periodic charge distributions, both of which necessitate the fabrication of high density, nearly monodispersed, highly ordered QD arrays. Various efforts have been made to achieve laterally ordered InAs/GaAs QDs. However, the mechanisms of lateral ordering of QDs are the subject of continued debate. For example, the formation of QD arrays is generally driven by elastic relaxation of strain; yet, their perfection and stability are often determined by additional processes, such as diffusion and segregation, occurring during growth and/or annealing. In the InAs/GaAs QD system, we have investigated the mechanisms of lateral ordering of stacks of QDs, and propose a new model that relies upon a combination of island nucleation plus subsequent bulk diffusion (ref). An additional remaining question concerns the effects of surface patterning on lateral QD positioning. Therefore, we are presently investigating the roles of modified surfaces in the patterning of QD superlattices. For example, we recently showed that the controlled formation of "mounds" on a GaAs surface may be used to preferentially align QDs along the mound length. This anisotropic QD alignment is explained by a patterning mechanism based upon strain-enhanced In segregation. We are also in the process of exploring the effects of artificial topographical patterns on the growth of QDs, including focused-ion-beam (FIB) patterning, laser-texturing, and patterning using twist-bonded and nanotemplated substrates. In these studies, QDs preferentially nucleate within or near GaAs, resulting in the largest sizes and highest densities of QDs within the dimples, presumably due to an anisotropic surface energy. Our future plans include incorporating QD arrays into a variety of novel devices, including bipolar thermoelectric devices and intermediate-band solar cells.
Highlights (Click an image for more information)
  • Buffer Layer Patterning of QD Superlattices

    We have investigated the patterning effects of GaAs buffers during the growth of InAs/GaAs quantum dot (QD) superlattices (SLs). The figures show the morphologies of buffer layers grown and annealed at different conditions, shown in (a), (b) and (c), and 10-layer QD SLs grown on correspondent buffers, as shown in (d), (e), and (f). High temperature grown buffers consist of relatively flat surfaces, as shown in (a), while low temperature grown buffers contain “mound-like” features elongated along the [1-10] direction, as shown in (b) and (c). Isotropic distributions of QDs are observed for QD growth on flat buffers, as shown in (d). Interestingly, QD alignment along the [1-10] direction is observed for QD SL growth on buffers containing mounds. This anisotropic QD alignment is enhanced as the number of QD SLs increases and as the density mounds on buffer increases. The mechanism proposed for this lateral ordering of QDs is based upon buffer layer patterning leading to an undulated In-enriched GaAs spacer layer following the initial layer of QDs.