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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


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