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Patterning and lithography of QCA-like structures, and FIB guiding of heteroepitaxial growth

 

 

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contact person: Surajit Atha (sa2e@virginia.edu)

INTRODUCTION:

This project is part of the NSF funded MRSEC (Materials Research Science and Engineering Center) established in September 2000 at the University of Virginia.

 Epitaxial 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)

 Ex-situ 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.

 Preliminary 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.

 In-situ 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).

 

Text Box: Fig 2: (a) TEM image showing FIB patterned Si surface. (b) TEM image (g, 3g weak beam) of sample after annealing and growth of GE islands at 650 ºC. (c) Bright field TEM image (enlarged) of area similar to that shown in (b). The 90 nm diameter irradiated areas are clearly visible from their speckled contrast, and within each area there is a Ge island of only 20 nm in diameter. (d) Weak beam image of an area near to one shown in (b), where analysis of contrast within island (arrow) confirms presence of dislocations. (e) Bright field image showing islands formed after patterning and annealing using a longer irradiation time (0.5)ms) and a higher beam current (20 pA). These islands have formed at the periphery of the irradiated area, which covers the right half of the image. Scale bar 100 nm for all images  

 

 

 

 

 

 

 

 Quantum 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.

 

 

 

 

Text Box: Fig3: AFM image showing the QF structure showing the 4 edges surrounding the central pit. These four edges can be used as the four dots necessary in a QCA cell.  

 

 

 

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 devices.

 

Text Box: Fig4: Concept of QCA cells, with Pt connections from the dots for electrical contacting.

 

 

 

 

 

                              

 

 

 

 

 

 

 

Text Box: Fig 5: Images showing the step by step contacting of the QF-QCA like structures for electronic testing. From top: FIB Pt contacting, Pt E-Beam lithography, and Gold optical lithography.  

 

 

 

 

 

 

 

 

 

For more details about this work, contact Surajit Atha at surajit@virginia.edu