Innovation and Development of Study Field Nanomaterials at the Technical University … ·...

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Innovation and Development of Study Field Nanomaterials at the Technical University of Liberec

nano.tul.cz

http://nano.tul.cz/projekt-esf-na-podporu-oboru-nanomaterialy

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Studijní program: Nanotechnologie

Studijní obor: Nanomateriály (organizuje prof. J. Šedlbauer, FPP TU v Liberci)

Preparation of semiconductor

nanomaterials

2013/2014

(prof. E. Hulicius, FZÚ AV ČR, v.v.i.,)

3. Epitaxial techniques in general

(Not only semiconductor nanostructures preparation)

It is crystallic growth on (usually) monocrystallic wafer (substrate) . It

is possible to prepare high-quality hetero- a nano-structures of

different materials. Principles, modes and types of growth. Types of

epitaxy – Solid state and Liquid phase epitaxy, variants. Epitaxial

growth from material point of view. SPE, LPE.

This is the fundamental chapter, understanding of epitaxial growth

will be examined!

4. Molecular Beam Epitaxy (MBE)

Explanation of basic principles of technology. MBE technology

parameters, properties and reasons of its limits.

5. Metalorganic Vapour Phase Epitaxy (MOVPE)

Explanation of basic principles of technology. MBE technology

parameters, properties and reasons of its limits.

These two chapters are also fundamental. Description and scheme of

MBE or MOVPE will be in each set of questions.

Name epitaxy origins from Greek epi-taxis which means „arranged on" was introduced by L.

Royer at 1936.

It is monocrystalic growth on (usually) monocrystalic substrate (wafer). Growth is not

(usually) epitaxial when lattice constant difference is bigger than 15%.

Explanation of importance and principles and comparison with other

monocrystal preparation methods

Why so monstrous, expensive, danger and demanding technology equipments

Preparation and properties of the bulk crystals.

Epitaxial growth – advantages, new possibilities, limits. Homo- and hetero-

epitaxy.

Equation of minimum of energy: μ = ΔG = ΔH - TΔS

chemical potential – μ, free Gibbs energy - δG, μ = δG/δn/T,p, Enthalpy - ΔH and

Entropy – ΔS.

Principle of the epitaxial growth.

Atoms or molecules of the compound, which we would like to deposit on suitable

substrate, are transported to its surface, which have to be atomically clean – cleaned

from oxides and sorbants - and atomically smooth (only with atomic steps due to

disorientation of the monocrystalic substrate). On the surface the atoms will be

physisorbed, and after that chemisorbed to the crystal structure. By this way atomic

layers and all structure are grown.

Epitaxy technologies

Epitaxial growth of monocrystalic layers (on the bulk

monocrystal wafers = substrates) is realised at lower

temperature than growth of monocrystals from melted

material, which is substantial for:

Influence of the enthropy (native defects),

lower solutibility of the unintentional impurities into the

prepared layers.

Nevertheless this lower temperature (usually around

500°C) is high enough for creation of atomically clean

and flat surface and enable atoms jump over the energy

barriers for physisorption and chemisorption.


Epitaxial crystal growth modes a) Layer by layer – Frank-van der Merwe growth mode

b) Layer by layer - continuously

c) Layer and island on the wetting layer - Stransky-Krastanow mode

d) Island on the substrate – Volmer-Weber mode.

e) Columnar growth type mode

Types of epitaxial growths explanation of used abbreviations:

SPE (Solid Phase Epitaxy)

LPE (Liquid Phase Epitaxy)

LPEE (Liquid Phase Electroepitaxy)

VPE (Vapour (Vapor) Phase Epitaxy)

CVD (Chemical Vapour Deposition)

PVD (Physical Vapour Deposition)

Main types of VPE epitaxial growths

Molecular Beam Epitaxy - MBE (Molecular beam epitaxy)

SSMBE = Solid Source MBE,

CBE = Chemical Beam Epitaxy,

GSMBE = Gas Source MBE (Hydride Source MOMBE or Metal Organic

MBE),

UHV ALE = Ultra High Vacuum Atomic Layer Epitaxy

Vapour Phase Epitaxy from organometallic

MOVPE (MetalOrganic Vapour Phase Epitaxy)

MOCVD (MetalOrganic Chemical Vapour Deposition)

Photo-MOVPE (Nonthermal, light activated)

Plasma-MOVPE (Nonthermal, plasma activated)

Semiconductor (mono)crystals

and their structure – crystallographic, electron, optical

Crystallographic lattices

Band structure



Crystallographic lattices

Why crystals exist? Minimum of energye

Why crystalline in different structures? Conditions during creation

What lattice influences? Nearly all - electrical and optical

Elemental cells

Seven crystallic systems

Bravais lattices (14)

Miller indexes



Elemental cells

Composed Elemental cells (trick) symmetry, simple coordinate system.

The most squeeze arranged atoms.



Seven crystallin systems

Elements of symetry



Bravais lattices - 14 why just 14?

krystalová soustava minimální symetrie

triklinická (trojklonná) žádná

monoklinická (jednoklonná) jedna 2četná osa podél c

ortorombická

(rombická, kosočtverečná)

tři 2četné osy podél a, b , c

tetragonální (čtverečná) jedna 4četná osa podél c

kubická (izometrická) čtyři 3četné osy podél

tělesových úhlopříček

krychle

hexagonální (šesterečná) jedna 6četná osa podél c

trigonální

(romboedrická, klencová)

jedna 3četná osa

podél hexagon. Buňky

Bravais lattices

(1850)

For explanation you have to

use theory of groups



Miller indexes?

Describes specific planes of crystals



Band Structure

of electron (and hole) energy states.

BS is created by atoms, but mainly by lattice.

BS is possible to calculate (not easy, up-initio – only for limited

amount of atoms).

BS is possible to determine from optical and electrical measurements.

BS is fundamental for optical and electrical properties of monocrystls.

One approach how to describe creation of the band structure:

Band structure in the k-space

The first

approximation of

the Perturbation

theory , without

spin-orbital

interaction

The first

approximation of

the Perturbation

theory , with spin-

orbital interaction

The second

approximation of

the Perturbation

theory , with spin-

orbital interaction


and their structure – crystallic, electron, optical

Band structure of electron (and hole) energy states

is possible to presented at

„high school level“: on y-axe is energy and on x-axe space

It is sufficient for majority of descriptions – heterostructures as well as

p-n junction

Or at

“university level“: on y-axe is energy and on x-axe momentum (at k-

space)

Only by this way we can understand direct and indirect semiconductors,

effective mass of electrons and holes, Auger recombinations etc.



Space and time for discussion on band structure

((which you would know from previous lectures))



Defects of crystallic lattice and their influence on band structure

Grains, microcrystals, inclusions no in semiconductors!

Plane defects (grain boundaries, …) also not too much.

Line defects (dislocations,… ) mainly harmful

Point defects majority applications are

(intrinsic, impurities, dopants)based on them, but also not

desired nonradiative

recombination, electron diffraction, …

All can be harmful or useful

Name epitaxy origins from Greek epi-taxis which means „arranged on" was introduced by L.

Royer at 1936.

It is monocrystalic growth on (usually) monocrystalic substrate (wafer). Growth is not

(usually) epitaxial when lattice constant difference is bigger than 15%.

Explanation of importance and principles and comparison with other

monocrystal preparation methods

Why so monstrous, expensive, danger and demanding technology equipments

Preparation and properties of the bulk crystals.

Epitaxial growth – advantages, new possibilities, limits. Homo- and hetero-

epitaxy.

Equation of minimum of energy.

Principle of the epitaxial growth.

Atoms or molecules of the compound, which we would like to deposit on suitable

substrate, are transported to its surface, which have to be atomically clean – cleaned

from oxides and sorbants - and atomically smooth (only with atomic steps due to

disorientation of the monocrystalic substrate). On the surface the atoms will be

physisorbed, and after that chemisorbed to the crystal structure. By this way atomic

layers and all structure are grown.


Vysvětlení významu, principu a zasazení do souvislostí

Proč tak monstrózní, drahé, nebezpečné a náročné technologické aparatury?

Monokrystaly vznikají protože systém atomů má minimum energie. To ale

přesně platí jen při T = 0 K! Při nenulových teplotách má rovnovážný sytém

minimum energie s určitou koncentrací defektů (obvykle bodových –

vakance, intersticiály, výměnné defekty (antisite Ga↔As) a jejich

kombinace). Člen –TS v rovnici pro minimum energie (G=H-TS). Entropie S

= ln(n). Více viz

http://cs.wikipedia.org/wiki/Gibbsova_voln%C3%A1_energie .

Objemové monokrystaly rostou z taveniny, při teplotě tání, ta je mnohem

vyšší než teplota epitaxního růstu.

Epitaxní technologie - konkrétně

http://cs.wikipedia.org/wiki/Gibbsova_voln%C3%A1_energie

The old method with new applications.

Metastable amorphous phase of solid state material, which is in touch

with monocrystal, gradually assume crystalline form starting from the

junction, copy monocrystal lattice, but with much lower point defect

concentration.

Growth speed – usually nm per second – is controlled by activation

energy SPE Ea:

v = v0 exp (-Ea/kT) Applications:

Preparation of thick semiconductor epitaxial layers with high doping.

Low temperature epitaxy (Ga(Mn)As – spinotronics?).

Growth of the buffer layers for improving of properties of heterostructures – decreasing dislocation densities - Nitrides!

Silicide layers for electric contacts and Schottky barriers for Si devices.

Solid State Epitaxy - SPE

Steps of the Solid State Epitay:

The old method with new applications.

Metastable amorphous phase of solid state material, which is in touch with monocrystal, gradually

assume crystalline form starting from the junction, copy monocrystal lattice, but with much lower point

defect concentration.

Growth speed – usually nm per second – is controlled by activation energy SPE Ea:

v = v0 exp (-Ea/kT)

Applications:

Preparation of thick semiconductor epitaxial layers with high doping.

Low temperature epitaxy (Ga(Mn)As – spinotronics?).

Growth of the buffer layers for improving of properties of

heterostructures – decreasing dislocation densities - Nitrides!

Silicide layers for electric contacts and Schottky barriers for Si

devices.

Solid State Epitaxy - SPE

The most important epitaxial method during seventies and

eighties of the last century.

Still using in the industry (cheap LEDs and when tens of microns are

necessary).

Important for thermodynamical equilibrium grown structures.

Principle of the LPE:

Saturated solution of suitable materials (e.g. As in Ga) is cooling (or

the liquid part is evaporated – this is not realistic for Ga (it has low

vapour pressure)) and thus starts to be oversaturated, thus As is going

out from solution and created GaAs on the reasonable bulk or epitaxial

substrate.

Liquid Phase Epitaxy - LPE

Solutibility – simple in the binary system.

more complex dependencies in ternary,

quaternary, ... Systems, on the pressure, there are, dynamical

processes, ...)

Doping (including amphoteric)

Calculations of the growth speed, diffusion limits, …

Growth of ultrathin layers (under100 nm)

LPE

Thin layers under 100 nm are possible by LPE, but contact of substrate and

solution have to be shorter than ms. Problems with reproducibility

homogeneity etc.

Liquid Electroepitaxy

Modification of LPE, which is controlled by current, which flow

through boundary solution-substrate. Peltier effect and electro

migration are created.

It is used for several mm thick homogenous (better than 1%) ternary

layers. E.g.: InGaAs on InP or GaAs; AlGaSb on GaSb, …

LPE

Todayand at least next ten years it will be the most important semiconductor

preparation technology not only for industry but also for research.

In principle it is possible to describe it as physical (PVD - Physical Vapour

Deposition) and chemical (CVD - Chemical Vapour Deposition), according the

transport of material from the source to the substrate.

At first –PVD – it is evaporation of atoms or molecules (using heat, sputtering,

ablation, discharge, etc.) without their chemical changes.

At second – CVD – it is transport of volatile chemical compounds (precursors)

using some transport gases (H2, N2) to the heated substrate near of its surface their

are decomposed.

Epitaxial growth on atomically clean and flat surface of usually monocrystalical

substrate (wafer) is then similar. Also parameters of prepared layers are similar.

In both cases we need extreme „semiconductor“ cleanness – vacuum (10-10 torr) or

transport gas H2 či N2 (at the level of fractions of ppb).

Vapour phase Epitaxy - VPE

Main types of VPE growths

Molecular epitaxy - MBE (Molecular beam epitaxy)

SSMBE = SolidSource MBE,

CBE = ChemicalBeamEpitaxy,

GSMBE = GasSource MBE (HydrideSource MOMBE, MetalOrganic MBE),

UHV ALE = UltraHighVacuum AtomicLayerEpitaxy

Vapour epitaxy from organometalic compounds -


MOCVD (MetalOrganic Chemical Vapour Deposition)

Photo-MOVPE (Nonthermal, light activated)

Plasma-MOVPE (Nonthermal, plasma activated)

Ohřev substrátu (kvůli jeho dokonalému očištění a atomárnímu vyrovnání - viz

výše principy epitaxe) se, vzhledem k těmto extrémním požadavkům na čistotu,

provádí nepřímo – vysokofrekvenčním ohřevem, světlem (optickou výbojkou -

MOVPE), nebo nepřímým odporovým ohřevem (MBE).

VPE umožňuje i růst jednotlivých atomárních rovin ( Ultra High Vacuum

Atomic Layer Epitaxy).

PVD

Vypařování: Teoreticky je počet molekul dNe vypařujících se z plochy Ae za čas dt roven

dNe/Aedt = (peq - p) sqr(NA/2πMkBT) [m-2s-1]

kde M je molekulární hmotnost vypařované látky, peq je rovnovážný tlak, p je

hydrostatický tlak vypařované látky v plynném stavu, kB je Boltzmannova konstanta a

NA je Avogadrovo císlo.

Epitaxe z plynné fáze - VPE

Ve skutečnosti musíme zavést koeficient vypařování av (neboť část vypařených molekul

(1 - av) přispívá k jen tlaku, nikoliv toku molekul. Dostáváme tak obecnou rovnici pro

vypařování z volné plochy - Hertzovu-Knudsenovu

dNe/Aedt = av(peq - p) sqr(NA/2πMkBT) [m-2s-1]

Vypařujeme-li z Knudsenovy efusní cely, která má otvor podstatně menší než je

povrch vypařované látky a má nejméně desetkrát menší průměr než je volná dráha

vypařovaných molekul, dostáváme po úpravách pro celkovou efusní rychlost Γc rovnici

Γc = dNe/dt = 3,51 x 1022 pAe/sqr(MT) [molekul-1]

Pro vlastní PVD růstové procesy má adsorpčně-desorpční

kinetika na růstovém povrchu zásadní význam. Poměrně snadno

lze růst modelovat a počítat v případě (kvazi-) rovnovážného

stavu; horší je to v nerovnovážném stavu, nebo při přechodových

jevech.

Příklad PVD je Molekulární epitaxe - MBE

Můžeme ji dělit podle toho z čeho získáváme molekulární svazky:

Solid Source MBE,

Gas Source MBE (neboli Chemical Beam Epitaxy)

Hydride Source MBE, MetalOrganic MBE,

Další varianty MBE

Ultrahigh Vacuum Atomic Layer Epitaxy

Migrací urychlená MBE

UV zářením stimulovaná MBE

Plasmou aktivovaná MBE

Dotování MBE vrstev pomocí iontů

CVD

Chemický stav daného systému dobře popisuje chemický potenciál μ. Pro danou fázi

je definován jako vzrůst volné Gibbsovy energie δG když se přidá jeden mol látky

při konstantní teplotě a tlaku

μ = δG/δn/T,p

Vyjádříme-li molarní Gibbsovu energii ΔG pomocí enthalpie ΔH a entropie ΔS

μ = ΔG = ΔH - TΔS

což lze po dosazení používat k výpočtům.

Examples:

Halide epitaxy

Metals or elementary semiconductors - (WF6 → W + ..., SiCl4 → Si + ...)

Compound semiconductors - (GaCl + AsH3 → GaAs + ...)

Granits of rare earths - (YCl3 + FeCl2 + O2 → Y3Fe5O12 + ...)

Oxide epitaxy

Compound semiconductors - (GaO2 + PH4 → GaP + ...)

Hydride epitaxy

Elementary semiconductors, very important silicon - (SiH4 → Si + ...)

Izolating layers: oxides, nitrides - (SiH4 + H2O → SiO2 + ...;

SiH4 + NH3 → Si3N4 + ...)

Organometalic epitaxy

Compound semiconductors - (Ga(CH3)3 + AsH3 → GaAs + ...)

Metals - (Al(C4H9)3 → Al + ...)

High-temperature supraconductors YBaCuO

4. Molecular Beam Epitaxy (MBE)

Explanation of basic principles of technology. MBE

technology parameters, properties and reasons of its

limits.

This chapter is also fundamental. Description and

scheme of MBE or MOVPE will be in each set of

questions.

We can sort it according the sources o the molecular beams:

Solid Source MBE,

Gas Source MBE (or Chemical Beam Epitaxy)

Hydride Source MBE, MetalOrganic MBE,

Schema and photos of different equipments

produced by some producers:

The most important for nanotechnology are MBE and MOVPE

Molecular beam epitaxy - MBE

Zdroj: http://www.fzu.cz/oddeleni/povrchy/mbe/mbe.php

Molecular beam epitaxy

Principle of method:

Substrate(s) is heated in the high vacuum (10-(9-10) torr) up to so high

temperature when native oxides and surface impurities are desorbed

and surface of the substrate is atomically clean. Then preheated

Knudsen (effusion) cell will be open and atoms or molecules fly

several tens centimetres without collisions through growth chamber

on the substrate (they are also evaporated at the vicinity of

substrate). Atoms of future epitaxial layer sit on the surface

(physisorption) moving to the proper crystallographic sites where

they are bounded (chemisorption). By this way the epitaxil layer is

created.

Substrate is monocrystallic semiconductor wafer with diameter from

2 to 8 inches, which is 300 – 500 μm thick.

Questions?

Expected questions:

Why this substrate size? How input and také out substrate? What

is maximal size of substrate(s)?





several tens centimetres without collisions through growth chamber on

the substrate (they are also evaporated at the vicinity of substrate).

Atoms of future epitaxial layer sit on the surface (physisorption)

moving to the proper crystallographic sites where they are bounded

(chemisorption). By this way the epitaxil layer is created.

Substrate is monocrystallic semiconductor wafer with diameter from 2

to 8 inches, which is 300 – 500 μm thick.

Expected questions:



Why substrate rotates?












Expected questions:




Why so high vacuum is necessary?












Expected questions:

Why tihis substrate size? How input and také out substrate? What



Why so high vacuum is necesary?

How open and close effusion cells? Influence on vacuum?












Expected questions:

Why this substrate size? How input and take out substrate? What




How high substrate temperature is necessary?

How recharge, heat, open and close effusion cells? Influence on

vacuum?









(chemisorption). By this way the epitaxil layer is created

Expected questions:







vacuum?

Are there difference between atomic and molecular beam?






the substrate (they are also evaporated at the vicinity of substrate). …

…

Expected questions:







vacuum?


How is MBE influence by growth chamber size?







…

Expected questions:







vacuum?



What influenced growth speed?







…

Expected questions:







vacuum?




Why epitaxial layer is only on the substrate?

Expected questions:







vacuum?





How stop the growth?

Expected questions:







vacuum?






Financial tasks? Cost of one growth, structure, chip, equipment?

Expected questions:







vacuum?







Main troubles of growths?

Expected questions:







vacuum?


How is MBE influence by growing chamber size?





Main troubles of growths?

In-situ diagnostics?

MBE is vacuum (exactly “highvacuum“) method, so we can use majority of the

electron characterisation techniques.

Scheme of in-situ measuring technique RHEED,

picture on the screen is created by electron

reflected by the crystal surface. Dependence of

signal on the growth time is shown.

“Size“ of electrons (probability of their position in the space = de Broglieho

wavelength) is comparable with the crystal (= with size of atoms).

The most important for nanotechnology are MBE and MOVPE

Molecular beam epitaxy - MBE



5. Metalorganic Vapour Phase Epitaxy

(MOVPE)

Explanation of basic principles of

technology. MBE technology parameters,

properties and reasons of its limits.

This chapter is also fundamental. Description

and scheme of MBE or MOVPE will be in

each set of questions.

Today and at least next ten years it will be the most important

semiconductor preparation technology not only for industry but

also for research.

In principle it is possible to describe it as physical (PVD - Physical Vapour

Deposition) and chemical (CVD - Chemical Vapour Deposition), according the

transport of material from the source to the substrate.

At first –PVD – it is evaporation of atoms or molecules (using heat, sputtering,

ablation, discharge, etc.) without their chemical changes.

At second – CVD – it is transport of volatile chemical compounds (precursors)

using some transport gases (H2, N2) to the heated substrate near of its surface their

are decomposed.

Epitaxial growth on atomically clean and flat surface of usually monocrystalical

substrate (wafer) is then similar. Also parameters of prepared layers are similar.

In both cases we need extreme „semiconductor“ cleanness – vacuum (10-10 torr) or

transport gas H2 či N2 (at the level of fractions of ppb).

Vapour phase Epitaxy - VPE

This technology is not so controllable and „exact“ for

research as MBE, (which has better control of growth

driving, it is able to prepare sharper hetero-boundaries and

lower growth temperatures), but it is fundamental for

industry, mainly for optoelectronic devices. MOVPE is

cheaper, with higher productivity and it is more suitable for

nitrides.

The most important for nanotechnology are MBE and MOVPE:

Organometalic Vapour Phase Epitaxy


Organometalic Vapour Phase Epitaxy - MOVPE

The most important industrial and very importatnt research technology

Principle of method:

Substrate(s) is heated in the ultra clean gas (hydrogen, nitrogen) up to so high

temperature when native oxides and surface impurities are desorbed and surface of

the substrate is atomically clean. Then we will send to the quartz reactor suitable

precursors (organometals, hydrides) they will be thermally decomposed at the

vicinity of preheated substrate. Atoms of future epitaxial layer sit on the surface

(physisorption) moving to the proper crystallographic sites where they are bounded

(chemisorption). By this way the epitaxial layer is created.

Qestions??

Basic summary equation for GaAs growth from trimethylgallium (TMGa) and arsine

Ga(CH3)3 + AsH3 → GaAs + 3CH4

and similar for ternary semiconductor compounds

xGa(CH3)3 + (1-x)Al(CH3)3 + AsH3 → GaxAl(1-x)As + 3CH4

Examples of organometalic molecules:

TMGa + AsH3 GaAs + 3 CH4

TMGa AsH3 TBAs

CCl4

SiH4

DETe

Equiations for growth of GaAs are not so

simple:

2Ga(CH3)3 → 3CH3 + Ga(CH3)2 + Ga(CH3)

CH3 + AsH3 → AsH2 + CH4

Ga(CH3) + AsH2 → GaAs + CH4 + H

Detail description is more komplex (Stringfellow):

Organometalic Vapour Phase Epitaxy - MOVPE

Brief History:

Ruhrwein – US patent (1968)

Manasevit – first experiments (1968)

Hall, Stringfellow – importatnt developement of method

Dupois, Dapkus – clean organometals (1977/78)

The most important industrial semiconductor technology (1990

- …)

Photo of equipment

Schema

Examples of organometalic molecules

„Bubler“ – botle for organometals

Zdroj: http://www.fzu.cz/texty/brana/movpe/movpe.php

Description of growth parameters:

Description of MOVPE reactor:

MOVPE SiO2 reactor

with vf heating

Bubbler for

organometallic

precursors

Growth out of thermodynamical equilibrium: SPE and LPE no, MBE a MOVPE

yes.

It can be usefull for preparation of the strained layers.

Lattice not-matched strained layers – nanostructures OK, thicker layers relax –

dislocations are created no luminescence.

Strained layers (nanostructures) can have new desired properties

- change of material type (direct × indirect semiconductor),

- separation of light and heavy holes (fundamental increase of limit frequency),

- moving of levels in quantum wells (laser wavelength tuning).

Epitaxial transverse overgrowth (ELO – Epitaxial Lateral Overgrowth)

Very successful for nitride growth! (blue LEDs, lasers, ...)

Epitaxial specialities


Hard Heteroepitaxy

Epitaxial layer is rather different from substrate:

- lattice constant

- crystallographic structure

- chemical bonds

Examples:

CdTe on GaAs, Si on sapphire


Graphoepitaxy or Artificial Epitaxy

Growth of crystalic layers on amorphous surfaces (ceramics, glass, polycrystals,

oxides). Potentially interesting applications in microelectronics, micromechanices

optics a optoelectronics.

It is necessary to prepare surface.


Role of surfaces

The exact and complex understanding of epitaxial

processes need quantum mechanical approach.

It is very difficult.

For MOVPE is not able to use electron

diagnostic techniques (the growth is running close to

atmospheric preasure of hydrogen or nitrogen).

Photons are much bigger than crystal lattice constant, but

when the light is polarised and can interag with larger

part of surface which is changed during the growth, we

can use them.

RAS and its explanation:

In situ monitoring – RAS (Reflection Anisotropy Spectroscopy)

allows to monitor and control processes taking part during the

epitaxial growth, such as the formation of QDs during the InAs

deposition and during the waiting time or the 3D object dissolution

during the QD overgrowth.

It is therefore easier to optimise the amount of deposited InAs and

the waiting time or even other technological parameters (V/III ratio,

growth temperature, growth rate, SRL composition).

Using these data it is possible to control parameters of the prepared

structures through the technological parameters during the growth.

In situ growth monitoring and controling I

In situ monitoring – RAS (Reflection Anisotropy Spectroscopy) Linearly polarised light is shone on a sample under perpendicular

incidence. The elliptically polarised reflected light runs through a

photoelastic modulator and a second polarising prism. The modulated

intensity is analysed by a monochromator and a detector

In situ growth monitoring and controling II

Quantum dot growth

In situ growth monitoring and controling III

In situ monitoring – RAS (Reflection Anisotropy Spectroscopy)

Types of imagine .

In situ growth monitoring and controling I

Spectroscopic Colorplot Time resolved

Exact and full complex understanding of the

epitaxial growths needs quantum mechanical

approach.

This is rather difficult.

Thank you for your attention

Questions and answers Heterostructures: Semiconductor heterostructures in some of devices (you can choose one)?

New effects – tunnel diode, quantum cascade laser, Ohm normal (quantum Hall effect), ...

Fundamental improvement of parameters – LD (CW at room temperature), LED (high efficiency,

colours), planar waveguides, ...Localisation of electrons and holes and light, ...

Technology in general: Name three main reasons for using of epitaxial technologies!

It is possible to prepare material of better quality than from melted material – the lower temperature

the lower entropy and the lower solutibility of undesired impurities).

Higher reproducibility of the heterostructure preparation – more controlled structure= better

devices Possibility of nanostructure preparation – new effects.

Possibility of separation of photons and electrons in the device structure – LD, LED.

Possibility of separation of electrons and their donors – high mobility = HF devices.

In-situ Nanocharacterization a diagnostics: Describe difference between mezi RAS a RHEED!

RAS = Reflectance Anisotropy Spectroscopy, is optical non vacuum method, which is suitable

despite much bigger size of photons than lattice constant. It is working because of „full surface“

atom arrangement during different stages of layer or structure growth and polarized photons can

„see“ surface arrangement of atoms. RAS can monitor growth of individual monolayers via ML

oscillation. It can give information about QD and QW growth. But it has lower resolution than

vacuum RHEED.

RHEED = electron vacuum method of the surface study, it cam work only in the ultrahigh vacuum,

there are possibility of surface study by electron methods, possibility of monitoring of growth of

individual monolayers via ML oscillation. It is possible to prepare sharp and define heterojunctions

layer thickness is controlled with accuracy of fractions of ML, because the electron size (= de

Broglieho wavelength – space of probability of electron location) is comparable with crystal lattice

constant.

Stranského-Krastanowův typ růstu QD

Při heteroepitaxním růstu tenké vrstvy se na monokrystalický substrát (např. Si)

nanáší monokrystalická tenká vrstva jiného chemického složení (např. Ge). Je-li

vrstva dostatečně tenká, má tzv. pseudomorfní strukturu, tj. její mřížkový parametr

ve směru rovnoběžném s rozhraním je roven mřížkovému parametru substrátu.

Taková struktura je ovšem elasticky deformována, elastická energie deformace je

úměrná tloušťce vrstvy a čtverci mřížkového nepřizpůsobení.

kde jsou mřížkové parametry substrátu a nedeformovaného materiálu vrstvy.

Celková vnitřní energie vrstvy je součtem této elastické energie a povrchové

energie vrstvy, ta je ovšem konstantní během růstu, pokud je rostoucí povrch

vrstvy stále rovinný. S rostoucí tloušťkou vrstvy celková energie vrstvy roste a od

jisté tloušťky je energiově výhodnější zvlněný povrch vrstvy. Tímto zvlněním se

sice zvětší povrchová energie, poklesne ovšem energie elastická, protože

krystalová mřížka v okolí zvlněného povrchu může elasticky relaxovat.

Appendix 1

S

SL

a

aaf

Tento tzv. Stranski-Krastanowův (SK) růstový mód se tedy skládá ze dvou fází;

v první fázi roste pseudomorfní tenká vrstva s rovinným povrchem a v druhé fázi se

povrch vrstvy zvlní, krystalová mřížka elasticky relaxuje a amplituda zvlnění

postupně roste [6-8].

Numerická analýza procesu zvlnění ukázala, že existuje kritická vlnová délka

zvlnění [8]

která se ve spektru vlnových délek zvlnění vyskytuje zdaleka nejčastěji. V rovnici

(2) g označuje povrchové napětí, G a n jsou smykový modul a Poissonův poměr

materiálu vrstvy. SK růstový mód vytváří proto téměř periodickou soustavu

ostrůvků na povrchu rostoucí vrstvy.

Ve vztahu (2) jsme zanedbali závislost povrchového napětí g na

krystalografické orientaci povrchu. Ukazuje se však, že tato závislost je podstatná

pro porozumění výsledné morfologie povrchu vrstvy. Během depozice vrstvy

v druhé fázi SK růstového módu roste amplituda zvlnění a tedy i střední kvadratický

sklon drsného povrchu. Dosáhne-li lokální orientace zvlněného povrchu hodnoty

odpovídající minimu funkce g, vytvoří se krystalografická faseta.

Podle V. Holého, publ. v ČsČasFyz 2005

22crit)1(2

1

fG

g

n

n

Appendix 2

Augerova nezářivá rekombinace

Dvoustupňový proces Augerovy nezářivé

rekombinace. V této jednoduché pásové

struktuře musí být přechody šikmé (zachování

hybnosti k) (CHCC)

Energie z rekombinace je využita pro

přechod mezi těžkými a lehkými děrami

(CHHL)

Obvyklý případ pro AIIIBV polovodiče -

je excitována díra ze spinorbitálně

odštěpeného pásu (CHHS)

Rezonanční Augerův proces v křemíku

vyžaduje účast dvou elektronů (typ CHCC)

Appendix 3

End of the fifth part

Next:

6.1. In situ obecné

6.1.1. Měření vakua

6.1.2. Měření teploty

6.1.3. Hmotnostní spektroskopie

6.1.4. Absorpční spektroskopie

6.1.5. Ramanův rozptyl

6.1.6. Laserem indukovaná fluorescence

6.2. In situ povrchové analýzy

6.2.1. Difrakční techniky

6.2.2. Optické metody

6.2.3. Sondové rastrovací metody

6.3. Ex situ


6.3.2. Elektrické (transportní)

6.3.3. RTG difrakce

6.3.4. Mikroskopie

Charakterizace a diagnostika epitaxního růstu a nanostruktur

6.2. In situ povrchové analýzy

6.2.1. Difrakční techniky

6.2.1.1. LEED - Difrakce nízkoenergetických elektronů

6.2.1.2. RHEED - Difrakce vysokoenergetických elektronů odrazem

6.2.1.3. GIXS (Grazing Incidence X-ray scattering)


6.2.2.1. Reflektance

Polarizovaného světla

Anizotropická spektroskopie

Elipsometrie

Polarizovaná spektroskopie

Povrchová fotoabsorpce

Reflektometrie

6.2.2.2.Rozptyl

Laserového světla

Ramanův

6.2.3. Sondové rastrovací metody

6.2.3.1. AFM

6.2.3.2. STM


7.3. Ex situ


7.3.2. Elektrické (transportní)

7.3.3. RTG difrakce

7.3.4. Mikroskopie

7.3.4.1. Elektronové

SEM

TEM

7.3.4.2. Nanoskopické

HRTEM

X-STM

X-AFM


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