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EXPLORING QUANTUM DOTS
By
Amit Kumar Sharma
Sai Krishna
Priyanka Gupta
What are QDs
Quantum dots are nano sized semiconductor embedded in another semiconductor.
They are capable of trapping discreet number of electron in a point structure.
They are approx. of the size of 30nm. They have energy band gaps smaller than that of
surrounding materials, hence they trap charge carriers.
The number can be changed by changing the surrounding electric field.
Types of Quantum Dots
Quantum dots are basically of two types:-
a) Gated quantum dots.
b) Self growing quantum dots.
Gated Quantum Dots
These are quantum dots that are at present fabricated using lithography followed by etching.
Success of this method is limited because of technological requirement of producing ultra small structure that are defect free.
Self growing Quantum Dots
In this type a layer of semi conductor over another thin layer of semiconductor material is spread.
Due to difference in lattice, stress is produced which lead to formation of QDs called ‘self growing quantum dots’.
They eliminate the limitation of ‘Gated -QDs’
How
Quantum Dots
Work
Fabrication of Quantum Dots
Gated QDs Fabrication
Silicon nano structures are used . Using of Silicon on insulator (SOI) bring the
size down to 5nm. Using high resolution low energy electron
beam Lithography we prepare Silicon dot structure.
Simultaneously the in plane side gate is formed in thin SOI film.
Gated QDs Fabrication
The etched Silicon is passivated at low temperature.
Contact holes opened in source and drain region by etching.
In this way gated quantum dots are fabricated
Self grown QDs fabrication
Several layers of silicon germanium are arranged in regular arrays.
Due to lattice mismatch between different layers regularly spaced or sized clusters are formed.
They form Si-Ge quantum dots pyramid shaped clusters, which are 100nm wide and 3-10 nanometer tall.
Once the process is initiated they develop self growing tendency hence the name.
Applications
Quantum dots find a number of application in various fields, the most important of
them is in field of telecommunication and computing.
Applications
Applications
A number of QDs can be used to form a cell which is the basic unit.
Each cell will have 4 QDs . In each cell there will be two electrons which occupy the dots in such a way that they are in
lowest energy states. These two states can be used as two states
of digital signal (0 and 1).
Quantum Cells - The Basic Unit
Applications
Quantum Cells - The Basic Unit
The two states can be used as two states of digital signal (0 and 1).
Quantum Cells - The Basic Unit
Applications
The Quantum-cells can be used to
transmit signals just as in a wire.
Applications
Quantum Wires
Applications
Direction of signal propogation
Applications
Direction of signal propogation
Applications
Direction of signal propogation
Applications
Direction of signal propogation
Applications
Direction of signal propogation
Applications
Direction of signal propogation
Applications
Direction of signal propogation
1
Quantum Wires
Advantages: Practically no heat dissipation. Signal remains unaffected by surrounding
EM fields. Hence no signal attenuation takes place
No insulation required. Massive reduction in size.
Applications
Implementing Logic Gates
Logic gates can be implemented using QDs. QD logic gates -three input gates. Majority gates
Applications
THE AND GATE
THE OR GATE
Advantages
Having the three basic gates realized, any logic circuit can be implemented.
The size of logic circuits, thereby, is drastically reduced.
Due to lesser heat dissipation the longevity of the circuits increases.
Applications
Implementing Logic Gates
Single Electron Tunneling (SET) Devices
The best example being electron tunneling transistors.
Reduces the size of transistor to ~20nm.
Applications
Advantages This reduction in size leads to-
1. decrease in switching delays
2. increase in calculation speed
3. drastic decrease in size of transistor based devices such as microprocessors, microcontrollers, ICs, etc
Applications
Single Electron Tunneling (SET) Devices
Conclusion
Thus we conclude that with the advent of Quantum Dots we are about to witness an era where the handhelds will possibly replace the present PCs, IC’s ‘ll shrink to micro dimensions, yielding processing speed comparable or even greater than today’s most advanced processors. Let us all hope for the best.
