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

Fabrication methods

(a) 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.

Typical QD structures

Non-isotropic etching

Damage on sidesdue to RIE

Limited size

Limited size

Integration problem

Best

Other techniques

1.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 required2. Cleaved edge overgrowth quantum dots form at the junction of three orthogonal quantum wells a. complicated process b. difficult to control size and shape

Quantum dot

Barrier

Barrier

AlGaAs

AlGaAsGaAs001

Quantum wires

Other techniques

1.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 required2. Cleaved edge overgrowth quantum dots form at the junction of three orthogonal quantum wells a. complicated process b. difficult to control size and shape

Quantum dot

Barrier

Barrier

AlGaAs

AlGaAsGaAs001

Quantum wires

Growth mode for QD2 + 12 <? 1 Surface + Strain energy

Stranski-Krastanow growth mode

What happen when together?

Shape evolution

Recent Achievements

Ordering of QD (recently achieved)

InAs QD

APL, 78, 105 (2001)

PbSe QD

Science, 282, 734 (1998)

Our experimental Results

0 0.50 1.00 1.50m

0.0 nm

25.0 nm

50.0 nm

m0 0.50 1.00 1.50

0.0 nm

17.5 nm

35.0 nm

Co magnetic Nanoparticles prepared by PVD

Without Electron Charging With Electron Charging

Size: 10~100nm Size: 10~20nm

Ge/Si(001)• Self-assembly• by MBE or CVD

20nm

Ge quantum dots on Si(001) substrate

PyramidDome

Dome Superdome

Stability of Ge quantum dot against water vapor

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

Nanocluster characterization with TEM

Pyramid Dome

Strain

Composition

Shape

Size

Application in quantum devices

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

Inverter Majority Gate

Parallel

Opposite

4 dot cellt :energy barrier a :spacing

The extra two electrons will move around until the lowest energy configuration depending on the Schrödinger equation

By University of Notre Dame

Field-effect Spin Resonance Transistor

Prof. Kang L. Wang, Electrical Engineering Department, UCLA

by NC State

Silicon quantum dot quantum computation

Single electron is trapped at each quantum dot at low temperatureZeeman spin states of these electrons constitute the qubitsExchange coupling between electron spins

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