Epitaxy and epitaxial methods

What do we want to do?

What do we want to do?

Review: A.D. Yoffe, Advances in Physics 51, 799 [2002]

bulk 3D

thin films, layer structures,

quantum wells 2D

linear chain structures,

quantum wires 1D

colloids, quantum dots

0D

Dimensionality

Review: A.D. Yoffe, Advances in Physics 51, 799 [2002]

3D

2D

1D

0D

Dimensionality

Epitaxy

nucleation

surface processes during growth

What can we grow by MBE? What can be grown by epitaxy?

Tunneling magnetoresistance

With metals

GaN-based multiple quantum well laser diode

With semiconductors

Epitaxy:

growth of solid film on crystalline substrate

arrangement of growing atoms mimics substrate structure

Homoepitaxy:

growing layer and substrate same material

Heteroepitaxy:

growing layer differs from substrate in:

- lattice constant

- and/or crystal structure

Vocabulary

P. Jungwirth, Nature 474, 168 [2011] W. Deracke et al. , Nature Materials 2, 253 [2003]

water SiC(100)

Dangling bonds

Roughness

Atomically rough/smooth surface

Lennard-Jones potential

stress-strain

stress = force / unit area

strain = amount of

deformation an object

experiences compared to

its original size and shape

stress-strain

Lattice mismatch/misfit and related stress/strain

strain in layer

due to lattice mismatch

𝜀 =𝑎𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 − 𝑎𝑙𝑎𝑦𝑒𝑟

𝑎𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒

Poisson‘s effect

material compressed in one direction,

usually tends to expand in the other

two directions perpendicular to the

direction of compression.

Poisson's ratio

= -dεtrans/dεaxial

measure of the Poisson effect

= measure of the

ratio of the fraction (or percent) of

expansion

divided by the fraction (or percent) of

compression,

for small values of these changes.

Auguste Bravais

[1845]

William Hallowes Miller

Crystallography [1839]

U F = U - TS Internal energy Helmholtz free energy U = energy needed to create a system F = energy needed to create a system - the energy you can get from the environment

H = U + PV G = U + PV -TS

Enthalpy Gibbs free energy H = energy needed to create a system G = total energy needed to create a + the work needed to make room for it system and make room for it – the energy you can get from the environment

thermodynamic potentials

- TS

+ P

V

wetting layer

equilibrium shapes

NaCl at 710 C

Au at 1000 C

NaCl at 620 C

equilibrium shape [facetting]

near[est] neighbors

diamond structure

K Jakobi, Prog. Surf. Sci. 71, 185 [2003]

equilibrium shape vs. substrate orientation

InAs on GaAs

equilibrium shape vs. substrate orientation - InAs on GaAs (-1-13)B

equilibrium shape vs. substrate orientation - InAs on GaAs (113)A

Molecular Beam Epitaxy [MBE] and the case study of SiGe/Si

Molecular Beam Epitaxy [MBE]:

▪ growth technique

▪ for fabrication of epitaxiyal layers

▪ with monolayer [ML] control

MBE principle:

▪ atoms [or clusters of atoms]

▪ produced by heating up a solid source

▪ which migrate in vacuum environment

▪ and impinge on a hot substrate

▪ where they can diffuse and eventually incorporate into the growing film

MBE

▪ vacuum deposition technique

▪ carried out in Ultra High Vacuum [UHV]:

p < 10-7 Pa

growth conditions far from thermodynamic equilibrium

▪ governed by kinetics of surface processes:

interaction impinging atoms/uppermost substrate layer

▪ precise control of beam fluxes and growth conditions

MBE MBE

MBE process MBE process

Interaction potential Interaction potential

University of Cambridge

Semiconductor Physics Group

Epitaxy and on-line monitoring

University of Cambridge

Semiconductor Physics Group

Epitaxy and on-line monitoring

Molecular Beam Epitaxy [MBE] and the case study of SiGe/Si

When special requirements are needed:

▪ interfaces abruptness and control

▪ exact doping profiles

thanks to:

▪ lower growth temperature

▪ lower growth rate

control on vacuum and sources ensures:

▪ higher purity [vs. non-UHV techniques)

▪ use of electron diffraction methods for growth control

Why to [ar not to] employ MBE Why MBE

Mass production:

MBE lower yield compared to e.g.

metalorganic Vapor Phase Epitaxy [MOVPE] beacuse of:

▪ lower growth rates

▪ lower wafer capability:

GaAs MBEmax : 4 x 6“

GaAs MOVPEmax : 5 x 10“

[up to 96 x 2“ for Si]

Why to [ar not to] employ MBE Why not MBE

1960‘s: MBE evolves from

▪ „three temperature method“

▪ surface kinetic studies of interaction Ga-As2 with GaAs

1970‘s:

▪ study of MBE surface chemical processes

▪ thermal accomodation coefficients, surface lifetimes,

desorption energies, reaction order

▪ in-situ electron diffraction techniques [RHEED]

1980‘s:

▪ introduction of gas sources

History History

1980‘s:

▪ RHEED oscillations

▪ pulsed mode

▪ multichamber systems

1990‘s-2000‘s:

▪ low dimensional heterostructures:

▪ multiple quantum wells

▪ quantum dots

▪ laser structures

History History

W.P.MacCray, Nature Nanotech. 2, 259 [2007]

W.P.MacCray, Nature Nanotech. 2, 259 [2007]

W.P.MacCray, Nature Nanotech. 2, 259 [2007]

W. Wiegmann

A. Gossard H. Störmer

R. Dyngle

Also MOVPE deserves a place in the history books

Physics Nobel 2014

Also MOVPE deserves a place in the history books

Physics Nobel 2014

Also MOVPE deserves a place in the history books

Physics Nobel 2014

K. von Klitzing et al., Phys. Rev. Lett. 45, 494 [1980] – Nobel 1985 [quantum Hall effect]

A new era

Tsui, Strömer, and Gossard – Nobel 1998 [fractional quantum Hall effect]

K. von Klitzing et al., Phys. Rev. Lett. 45, 494 [1980] – Nobel 1985 [quantum Hall effect]

A new era

The growth chamber Growth chamber

Vacuum system:

▪ stainless-steel growth chamber

▪ UHV-connected to preparation chamber

[for substrate outgassing]

▪ load-lock module for transfer to air

All components of growth chamber must stand bake-out temperatures up to 200 °C

▪ necessary after each opening to minimize

outgassing from walls

MBE system MBE system

Pumping system:

▪ to reduce residual impurities to a minimum

▪ elements:

ion pumps, Ti-sublimation pumps, etc...

Characteristic feature of MBE:

beam nature of mass flow towards substrate

To preserve beam

mean free path > [distance

(source orifice) – substrate]

MBE system MBE system

Liquid N2 cryopanels:

▪ surround both chamber walls and source flange

▪ MBE cold wall technique

▪ cryopanels:

prevent re-evaporation from parts other than sources

provide thermal isulation among different sources

provide additiona pumping of residual gas

MBE system MBE system

Effusion cells Effusion cells

Effusion cells [1]:

(1) Crucible pyrolitic boron nitride, stable up to 1300 °C

(2) Ta filament for heating

(3) heat shielding Ta foils

(4) Thermocouple to measure material temperature

MBE system Effusion cells

Effusion cells [2]:

idealized Knudsen cells

▪ isothermal enclosure with small orifice

▪ evaporating surface large compared to orifice

▪ inner equilibrium pressure peq

▪ orifice < 1/10 mean free path at equilibrium

pressure

Total effusion rate:

in UHV:

MBE system Effusion cells

Effusion cells [3]:

Knudsen-type cells

small orifice

to ensure thermodynamic equilibrium melt/vapour in the cell

Langmuir-type cells [normally used]

larger orifice

wanted flux onto substrate reached with

lower cell temperature

▪ lower power consumption

▪ reduced thermal generation of impurities

MBE system Effusion cells

Inside the growth chamber Inside the growth chamber

First zone [Knudsen-like or Langmuir-like effusion cells]:

▪ molecular beams generated

▪ choice of source, source temperature (flux) and substrate temp.

epitaxial layers of desired chemical composition

▪ uniform beam fluxes

uniform thickness and composition [sample rotation improves homogeneity]

MBE system: zones Zones of the process

Second zone [mixing zone]:

▪ molecular beams intersect each other

▪ mean free path of the species long enough to prevent

ineraction between different beams

Third zone [substrate surface]:

▪ site of the epitaxial growth

▪ interaction impinging species – substate

▪ adsorbtion, migration, incorporation, thermal desorption,...

MBE system: zones Zones of the process

Surface processes Processes during [MB]epitaxy

layer-by-layer

Frank-van der Merwe

layer + islands

Stransky-Krastanov

islands

Volmer-Weber

Growth modes Growth modes

▪ Epitaxy from sequentially controlled surface conditions

1 atomic layer (ML) deposited per each reaction sequence

▪ digital rate control of growing surface

▪ attractive for fabrication of complex layered structures

▪ based on saturating surface reactions substrate/species

▪ each sequence: full ML or partial ML

Atomic layer epitaxy [ALE] Atomic layer epitaxy - ALE

e.g. CdTe epilayers on CdTe(111) polar surface

Atomic layer epitaxy [ALE] Atomic layer epitaxy - ALE

ALE: surface processes

Atomic layer epitaxy [ALE] Atomic layer epitaxy - ALE

Deposition timing scheme for Cd1-x(Zn,Mn) x Te ALE

Atomic layer epitaxy [ALE] Atomic layer epitaxy - ALE

diffusion implantation epitaxy

▪ simple process

▪ everything

diffuses

▪ high

concentrations

▪ lattice damage

▪ intact lattice

▪ low

concentrations

Doping doping

What can we grow by MBE? What can be grown by MBE?

What can we grow by MBE? What can be grown by MBE?

What can we grow by MBE? What can be grown by MBE?

What can we grow by MBE? What can be grown by MBE?

M. Kalahdouz et al., Appl. Phys. Lett. 96, 213516 [2010]

QD in the structure

improved performance of IR thermal detectors

And combined structures And combined structures

Z.M.Wang et al., Appl.Phys.Lett. 84, 1931 [2004]

InGaAs quantum dots on GaAs

G.Springholz et al., Science 282, 734 [1998]

Self assembling and self ordering in PbSe

[vertically aligned]

quantum dots

ZnTe and ZnMnTe nanostructures on CdTe

A.Bonanni et al., Appl.Phys.Lett. 74, 3732 [1999]

L.Wang et al., New J.Phys. 10, 045010 [2008]

Combination of MBE and in situ etching

Coupled semiconductor quantum dots Coupled semiconductor quantum dots