September 26, 2013 at 11:36 am

CMSS Colloquium: Tunable Control over Individual Dopants in Semiconductors, Sept. 26

The Condensed Matter & Surface Sciences Colloquium Series presents Dr. Jay Gupta, Assistant Professor of Physics at Ohio State University, on “Tunable control over individual dopants in semiconductors via STM positioning of charged defects” on Thursday, Sept. 26, at 4:10 p.m. in Walter Lecture Hall 245.

CMSS Colloquia“The scaling of electronic devices such as transistors to nanometer dimensions requires more precise control of individual dopants in semiconductor nanostructures, as statistical fluctuations can impact device performance and functionality,” Gupta says in his abstract. “Toward this end, the scanning tunneling microscope (STM) is emerging as a useful tool for its capabilities of atomic manipulation, imaging and tunneling spectroscopy. I will discuss our STM studies of acceptors (Mn, Co, Zn) within the surface layer of a p-doped GaAs crystal 1,2,3,4,5.

“In addition to the native Zn acceptors, we introduce surface-layer acceptors (Mn, Co) by first sublimating adatoms (Coad, Mnad) onto the GaAs (110) surface, prepared by cleavage in ultrahigh vacuum. A voltage pulse applied with the STM tip allows us to replace a Ga atom in the surface with the metal atom, thus forming a single acceptor (CoGa, MnGa) and a Ga adatom (Gaad). We find that the properties of acceptors can be tuned by control of the local electrostatic landscape.

“For example, the STM tip can be used to position As vacancies (VAs) and other adatoms (e.g. Mn, Ga), all of which are positively charged. Direct Coulomb repulsion causes a reduction in the hole-Mn binding energy as these species are moved nearby. Tunneling spectroscopy allows us to quantify this effect, through the shift of an in-gap acceptor resonance toward lower energy. In addition, we find that defect-induced band bending provides a new, indirect method for locating occupied acceptor states, (i) through the appearance of ionization rings in STM images and (ii) non-Coulombic behavior at short distances,” he says.

“We have extended this work to dimers of Mn acceptors, in the hopes of tuning the spin-spin interaction between them. Tunneling spectra of the dimers reveal six peaks, representing bonding and anti-bonding combinations of Mn acceptor states. These states systematically shift in energy as charged defects are brought nearby. Comparison of these experimental results with density functional theory calculations provides further insight into the electronic states and properties of the defects. These studies show that tunable control over single dopants in semiconductors is becoming a realistic route for next-generation classical- and quantum-based information technologies, while at the same time informing the design of conventional nanoscale devices.”

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