Upplägg och planering för NanoIntro’15; Lars Samuelson

Upplägg och planering för NanoIntro’15; Lars Samuelson ([email protected]):

Måndag 31/8: Presentationer av deltagarna8-10 Sal F Generellt om kursen/utbildningen. Exempel på “nanofenomen” runt oss

Torsdag 3/9: Viktiga grunder: energistruktur, atomer-molekyler-kristaller10-12 Sal F Metaller-halvledare-isolatorer. Bandgap hos halvledare (& isolatorer)

Måndag 7/9: Nanofysik: kvantfysik & unika fenomen på nanoskalan8-10 Sal F Partikel-våg dualitet, “konstgjorda atomer”, tunnlingsfenomen

Torsdag 10/9: Materialvetenskap/teknik – syntes på nanoskalan, funktionella material 10-12 Sal F Epitaxi, nanomaterial, sveptunnel- och atomkraftmikroskop mm mm

Måndag 28/9: Nanoelektronik och -optik, Nano-energi 8-10 Sal F Transistorer, lysdioder, solceller mm

Onsdag 2/10: Övning & Frågestund10-12 Sal H421

Micro/Nanoelectronics och technology for the fabrication of integrated circuits and advanced heterostructure devices

EpitaxyEpitaxy allows layer-by-layer deposition of mono-crystalline materials. Basis for fabrication of low-dimensional structures: quantum wells (QWs), quantum wires (QWRs)& quantum dots (QDs).Extremely good (ML) control of thicknesses

Types of epitaxy:•liquid phase epitaxy (LPE)•vapor phase epitaxy (VPE, MOVPE)•molecular beam epitaxy (MBE, also CBE)

The MOVPE, MBE and CBE methods are used widely for nanostructure/low-dimensional structure growth.

Example of MOVPE process (T≈600 °C):

TMGa + AsH3 ----> GaAs +by-productsTMIn + PH3 ----> InP +by-products

MBE: frequently used for III-V materials1- RHEED screen2-effusion oven shutters3-effusion cells for several elements4-cryo shrouds5-RHEED electron gun6-main shutter7-substrate

0

0,5

1

1,5

2

2,5

3

3,5

4

4 4,5 5 5,5 6 6,5 7Lattice parameter (Å)

GaN

SiCBP

Si

GaPAlP

GaAs

Ge

ZnSe

AlAsCdS

InP

PbS

CdSe

InAs

GaSb

ZnTe

PbSe

AlSb

SnTe

HgTeInSb

CdTe

400

500

600700

1000

2000

10000∞

IR

UV

© 1999

InN

AlN (6.2 eV)

Band gaps of different semiconductors

IVIII V

För lysdioder (LEDs) i UV - blått - grönt dominerar idag AlGaInN

B-A-B

A-B-A

A-B-A-B-A

Growingcrystalsurface

First layerof AlGaAsgrown on a substrateof GaAs

Desorption ofexcess molecules

Supplied sourceatoms/molecules

So - how do you do epitaxial growth and how do you form heterostructures?


Thin layerof GaAs










Thin layerof GaAs


Thin QWof GaAs




Top layerof AlGaAs



Thin QWof GaAs




Top layerof AlGaAs


Thin QWof GaAs


Top layerof AlGaAs


Thin QW

of GaAsFirst layer

of AlGaAs

grown on a

substrate

of GaAs

Top layer

of AlGaAs


Quantizedenergy levels

Growth direction

Ener

gy

Conduction band(for electrons)

Valence band(for holes)



Growth direction

Ener

gy



Heterostructures to confine carriers in quantum wells, wires and dots




Growth direction

Ener

gy

Heterostructures to confine carriers in quantum wells, wires and dots




Growth direction

Ener

gy

Growth direction

Ener

gyConduction band(for electrons)


IN CLASSICAL PHYSICS:The electron approaches whatacts like a barrier which stops(blocks) the transport

Heterostructures to create tunnel barriers

Growth direction

Ener

gy

IN QUANTUM PHYSICS:If the barrier is ”thin” the wave-function can appear on the other side: the electron passes throughby a quantum mechanical process, called TUNNELING



Heterostructures to create tunnel barriers

Kvantprick

~ 0,3 eV

Väteatom

13,6 eV elektronenstillåtnaenerginivåer ljus

ljus

protonproton

Jämförelse mellan

en ”riktig” atom och en ”artificiell” atom

What’s a Quantum Dot Like?

10nm

10nm

[110]

[110]

AFM

Contains ~10000 atoms

InP dots grown on GaInP/GaAs

K. Georgsson et al., Appl. Phys. Lett. 67, 2981 (1995).

Aerosol particles of III-V semiconductors

Formation ofultrafine group-III aerosol particles

Size selection Adding group-V precursor

Formation of III-V semiconductor

nanocrystals

K. Deppert et al., J. Aerosol Sci. 29 (1998) 737Deppert and Samuelson, Appl. Phys. Lett. 68 (1996) 1409

10 nm

InP

10 nm

GaAs

Semiconductor nanoparticles

K. Deppert et al., J. Aerosol Sci. 29 (1998) 737

A top-down approach to making one-dimensional quantumdevices. like resonant tunneling via quantum dots. Method pioneered by Randall and Reed at Texas instruments in the late 1980s. However, rather unsatisfactory device properties due to fabrication induced damage and poor lateral control.

TOP-DOWN fabrication of 1D devices

Alternative No. 2: BOTTOM-UP fabricationPlant a seed and control bottom-up growthof a perfectly functioning Bonsai tree.

Alternative No. 1: TOP-DOWN fabricationStart with a block of wood and carve a small wooden mini-tree with trunk and branches.

Comparison between top-down & bottom-up fabrication of complex structures

A forest of nanotrees with multiply seeded trunks, branches and leaves, with the entire tree being single-crys-talline and monolithic.

Each level of branches is seeded by Au aerosol nanoparticles, allowingcontrol of: – diameter – length – compositionincluding formation ofheterostructures insidebranches or at branch-leaf interfaces.

Kimberly Dick et al.

Top view

Side view

Au aerosol particlesdeposited on <111>B- oriented nanowires(low density)

Kimberly Dick et al.

AEROTAXY growth w/o substrate Would it be possible (a thought exp!)to initiate NW growth directly froma catalytic gold-particle, which wesomehow hold in a nano-tweezer?

If so, new possibilities could emerge for production of semiconductors w/o the need for expensive substrates, for areas like solar cells, LEDs, Batteries etc..

New initiative for substrate-free NW-growth

AEROTAXY: a revolutionary new way to grow NWs

TraditionalNW growth

Generally accepted notion:NWs grow guided by the substrate on top of which the NW nucleates.

The crystalline structure & orientation then governs the structure & orientation of the resulting NWs!

(111)

50nm Au seedsTg = 525°C

- The growth rate is extremely high, >1µm/s, which is up to 1000 times faster than for normal epitaxial growth!

From HRTEM+FFT we can say:- The NWs are perfect ZB, and virtually defect free- Growth direction is <111>B.

New initiative for substrate-free NW-growth

AEROTAXY: a revolutionary new way to grow NWs

Heurlin et al., “Continuous gas-phase synthesis of nanowires with tunable properties”, NATURE 492, 90, 6th Dec. 2012

Aerotaxy - Present status

1"µm" 0.5"µm"

In our presently operated Gen 3.0 we produce perfectly straight and untapered GaAs nanowires of length 2-4µm

Lund Nano Lab

Second floor

• Cleanroom class ISO 7 “class 10,000”• Semiconductor growth• Connected with Berzelius Laboratory

First floor

• Cleanroom class ISO 5 “class 100”• Semiconductor processing• 3 individual anti-vibration platforms

Lithography: from Greek “writing on stone”

Lithography: pattern transfer into recording media (resist) and its subsequent

transfer to a desired device structure (metallization, etching, ion implantation).

Different types:

1. Optical lithography: contact, proximity and projection printing

2. X-ray lithography

3. Electron beam lithography

4. Ion beam lithography

5. Imprint lithography

Lithographic techniques

EBL exposure strategy

Dedicated EBL system or modified SEM

Exposure: sequential writing, pixel-by-

pixel (exceptions: shaped beam, cell

projection lithography, SCALPEL

systems)

What is needed:

1. Source of e-beam

2. Pattern generator

3. Alignment system

Pattern transfer after lithography: lift-

e-

(a) (b) (c)

ResistSubstrate

Exposure Developed

backward leaning profile in resist (key requirement!)

evaporation of metal dissolution of resist (acetone)

metalmetal etching

contactsion impl.etc

Nanoimprint lithography (NIL)

New lithographic technique, S. Chou et al (1995)

NIL processing:

1. Deposition of a polymer onto substrate.

2. Physical contact between stamp and substrate. Application of pressure (50-80 bar) and heating above Tg of the polymer.

3. Cooling and release of stamp from the substrate.

4. Oxygen plasma ashing to remove resist residues on substrate.

CONFERENCES

+ EXPO

Site control & morphology of NW growth induced by Au patterns• Many applications require a high degree of control over site and morphology…• Site control - how ideal can we make it?• We can determine the growth site by controlling the site of the seed particle.

Metallization, 1-50 nm Au

Transferred to growth system, Au particles alloy with the substrate

Lift-off defines gold nanoparticle seeds

NW growth begins when precursors are introduced

Bare waferEBL opens up apertures in the resist

For details of EBL- and NIL-defined nanowire arrays, see for instance:T. Mårtensson et al., “Fabrication of individually seeded nanowire arrays by VLS growth”, Nanotechnology 14, 1255 (2003) T. Mårtensson et al., “Nanowire arrays defined by nanoimprint lithography”, Nano Letters 4, 699 (2004)

InP NW array grownby Thomas Mårtenssonusing MOVPE

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Upplägg och planering för NanoIntro’15; Lars Samuelson