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Progress of Semiconductor Quantum Dots Chuan-Pu Liu ( 劉全璞 ) Department of Materials Science and Engineering, National Cheng-Kung University Taiwan. Outline Introduction Fabrication methods Recent achievements Our achievements Application in quantum devices.
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Progress of Semiconductor Quantum Dots Chuan-Pu Liu (劉全璞) Department of Materials Science and Engineering, National Cheng-Kung University Taiwan
Outline • Introduction • Fabrication methods • Recent achievements • Our achievements • Application in quantum devices
Typical QD structures • metal and metal oxide systems patterned by lithography. • (b) metallic dots out of chemical suspensions. • (c) lateral quantum dots through electrical gating of heterostructures. • (d) vertical quantum dots through wet etching of quantum well structures. • (e) pyramidal quantum dots through self-assembled growth. • (f) trench quantum wire. Damage on sides due to RIE Limited size Best Integration problem Non-isotropic etching Limited size
Barrier Quantum dot Barrier Other techniques • Patterned substrate: V-grooves or • inverted pyramids. But • a. growth is complex, such as corrugation of facet surfaces • tilting of facets, non-uniform growth rate • b. understanding of complex surface, interfacet kinetics and • energetics is required • 2. Cleaved edge overgrowth • quantum dots form at the junction • of three orthogonal quantum wells • a. complicated process • b. difficult to control size and shape 001 GaAs AlGaAs AlGaAs Quantum wires
Barrier Quantum dot Barrier Other techniques • Patterned substrate: V-grooves or • inverted pyramids. But • a. growth is complex, such as corrugation of facet surfaces • tilting of facets, non-uniform growth rate • b. understanding of complex surface, interfacet kinetics and • energetics is required • 2. Cleaved edge overgrowth • quantum dots form at the junction • of three orthogonal quantum wells • a. complicated process • b. difficult to control size and shape 001 GaAs AlGaAs AlGaAs Quantum wires
Growth mode for QD g2 + g12 <? g1 Surface + Strain energy
Stranski-Krastanow growth mode together? What happen when Shape evolution
Ordering of QD (recently achieved) PbSe QD InAs QD APL, 78, 105 (2001) Science, 282, 734 (1998)
35.0 nm 50.0 nm 17.5 nm 25.0 nm 0.0 nm 0.0 nm 0 0 0.50 0.50 1.00 1.00 1.50 1.50 m m Co magnetic Nanoparticles prepared by PVD With Electron Charging Without Electron Charging Size: 10~100nm Size: 10~20nm
Ge quantum dots on Si(001) substrate Pyramid • Ge/Si(001) • Self-assembly • by MBE or CVD Dome Dome Superdome 20nm
40nm 40nm • Si/Ge(111) • Self-assembly • by MBE or CVD • InAs/GaAs(001) • Self-assembly • by MOCVD
Nanocluster fabrication by UHV-Sputtering • Ge / Si (001) • By UHV–Sputtering • Size shrinkage • 4 quantum dot a cell
Dome Pyramid Nanocluster characterization with TEM Shape Strain Composition Size
Advantages of implementing quantum dot • for quantum computation • Compactness and Robustness • Large number of qubits • No statistical mixture of pure quantum states • like in NMR • compatible with current Si based technology
Wireless logic devices 4 dot cell t :energy barrier a :spacing Parallel The extra two electrons will move around until the lowest energy configuration depending on the Schrödinger equation Opposite Majority Gate Inverter By University of Notre Dame
Field-effect Spin Resonance Transistor Prof. Kang L. Wang, Electrical Engineering Department, UCLA
Silicon quantum dot quantum computation Single electron is trapped at each quantum dot at low temperature Zeeman spin states of these electrons constitute the qubits Exchange coupling between electron spins by NC State
III - V Pillar Quantum Computer • Asymmetric dots produce • a large dipole moment • Dephasing due to electron- • phonon scattering and • spontaneous emission is • strongly minimized. • Strong dipole-dipole • coupling and long • dephasing time by NC State