contact person: Surajit Atha (email@example.com)
This project is part of the NSF funded MRSEC (Materials
Research Science and Engineering Center) established in September 2000 at
the University of Virginia.
growth of Si-Ge over Si (001) has been studied over the decades. The lattice
mismatch between Si-Ge layer and Si (substrate) leads to a Stranski -Krastinov
growth mode. The strain build up due to this lattice mismatch is relieved
either by islanding or formation of dislocations, or both. When islanding
occurs, the final surface morphology consists of small dot like structures
(of the order of 10s to 100s of nanometers), which are often referred to as
quantum dots (due to possible quantum confinement of electrons/holes in
them). For using these q-dots for any technological application, it is
necessary to get a control over the alignment of the dots, which poses a
fundamental problem. Part of the aim of this project is to study the
possibility of guiding the formation of the dots using the Focused Ion Beam
(FIB). The project also derives its motivation from the promising concept of
Quantum Cellular Automata (QCA), which is described in brief below.
Four quantum dots placed at four corners of a square make up
the basic unit cell of QCA.
The unit cell has two extra electrons which are free to tunnel between the
four dots. Columbic repulsion would keep the electrons at the two opposite
corner dots, thus giving the two states of existence of the cell, “0” and
“1” as shown. The state of one cell directly affects the state of the cell
next to it. Cells can be arranged together to form logic gates to be used in
complex circuits. Thus information about the state can be transmitted at
extremely low currents/power dissipation. The QCA technology has these
advantages over the present CMOS technology: Low power dissipation, High
speed operation, Miniaturization.
For developing functional semiconductor QCA, organization of
the quantum dots with respect to position and size is of utmost importance.
To achieve this, here we are exploring two distinctive methods, namely FIB
pre-patterning of the surface (Ex-situ/in-situ patterning), and use of a
unique Quantum Fortress (QF) structure (see work of J. Gray)
FIB pre-patterning: The Si surface is patterned
using the FIB prior to growth of Ge/Si-Ge in the MBE chamber, the pattern
dimensions ranging from a few microns to a few nanometers. The substrate
then undergoes normal cleaning and growth in the MBE chamber. These patterns
affect the positioning of the Ge/Si-Ge quantum dots, by altering the
conditions (energy, chemical, etc.) at the surface and subsurface regions.
Fig 1: FIB patterned Si (001) surface showing 0.7 micron mesas (left) and
one single mesa (out of the same group) after epitaxial growth of 10 ML of
Ge at 650ºC (right), showing the preferential growth of q-dots on the mesa
region, and at the edges.
work suggests that the q-dots preferentially form at the edge of a patterned
mesa, which is expected from energy considerations. They are also more
likely to form on the untouched mesa surface than the FIB-milled hole.
Patterning (by Martin Kammler at IBM):
Working on a UHV–CVD-TEM chamber, Martin used in-situ patterning on Si
substrates, before annealing and growing on them. His results show that even
though no surface morphological effect of the pattern remains after the
annealing, the q-dots preferentially grow at the FIB touched sites. This
result may be attributed to the chemical effect of the Ga atoms implanted in
the subsurface region during the FIB patterning, and the effect is dependent
of the dose of Ga implant (see Fig. 2).
Fortress: Unique 4-walled fortress type structures have been observed.
while growing SiGe on Si under certain conditions; these are termed as
quantum fortress (QF). These structures are being investigated as a possible
structure for a unit QCA cell, for these naturally form a stable four dot
(ridge) structure. For more details on these see work by J.Gray.
Besides formation of QCA like structures, lithography work is
also being done towards contacting these nanometer scale cells to the “outer
world”. A combination of FIB deposition and Electron beam & optical
lithography is being used to contact these structures, and low temperature
measurements are being carried out for the electronic testing of these
structures, to test the feasibility of their being used as semiconductor QCA
For more details
about this work, contact Surajit Atha at