Buffer Layer Patterning of QD Superlattices

Rachel S. Goldman

Professor

rsgold@umich.edu

2094 H.H. Dow Building

T: (734) 647-6821

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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.