Progress of Semiconductor Quantum Dots Chuan-Pu Liu ( 劉全璞 ) Department of Materials Science and

1. Progress of Semiconductor Quantum Dots Chuan-Pu Liu (劉全璞) Department of Materials Science and Engineering, National Cheng-Kung University Taiwan

2. Outline • Introduction • Fabrication methods • Recent achievements • Our achievements • Application in quantum devices

5. Fabrication methods

6. 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

7. 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

8. 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

9. Growth mode for QD g2 + g12 <? g1 Surface + Strain energy

10. Stranski-Krastanow growth mode together? What happen when Shape evolution

11. Recent Achievements

12. Ordering of QD (recently achieved) PbSe QD InAs QD APL, 78, 105 (2001) Science, 282, 734 (1998)

14. Our experimental Results

15. 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

16. Ge quantum dots on Si(001) substrate Pyramid • Ge/Si(001) • Self-assembly • by MBE or CVD Dome Dome Superdome 20nm

17. Stability of Ge quantum dot against water vapor

18. 40nm 40nm • Si/Ge(111) • Self-assembly • by MBE or CVD • InAs/GaAs(001) • Self-assembly • by MOCVD

19. Nanocluster fabrication by UHV-Sputtering • Ge / Si (001) • By UHV–Sputtering • Size shrinkage • 4 quantum dot a cell

20. Dome Pyramid Nanocluster characterization with TEM Shape Strain Composition Size

21. Application in quantum devices

22. 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

23. 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

24. Field-effect Spin Resonance Transistor Prof. Kang L. Wang, Electrical Engineering Department, UCLA

25. 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

26. 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