T'Vijaykumar

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Application to key materials
- T.Vijaykumar
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Outline

• Introduction
• Technical data
• Effusion cells
• Growth mechanism
• RHEED
• Application
• Quantum dots
• HEMT
• High Tc Superconductors
• GMR
• Summary

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Invented in late 1960s at Bell Laboratories by
J. R. Arthur and A. Y. Cho.
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Diagram of a typical MBE system growth chamber
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Molecular beam epitaxy (MBE) is performed with
different types of semiconducting materials
like i) Group IV elemental semiconductors
like Si, Ge, and C ii) III-V-semiconductors
arsenides (GaAs, AlAs, InAs), antimonides
like GaSb and phosphides like InP iii) II-VI-
semiconductors ZnSe, CdS, and HgTe
Electrons move through GaAs five times faster
than through silicon.
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• Features of MBE
• very low deposition rates typically 1um/hr or
  1A?/sec
• typically in ultra-high vacuum
• Uses high purity elemental charge materials.
• very well controlled growth
• films with good crystalline structure
• often use multiple sources to grow alloy films
• deposition rate is so low that substrate
  temperature does not need to be as high .

Epitaxy Growth of film with a crystallographic
relationship with the substrate Types
Homoepitaxy Heteroepitaxy.
For good epitaxy deposition rate -
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Types of MBE
The Gas-Source MBE (GS-MBE) III-V
semiconductors, group-V materials are
hydrides such as arsine (AsH3) or phosphine (PH3)
Metalorganic MBE (MO-MBE) group-III
materials are metalorganic compounds.
e.g., TEGa and TMIn
Solid-Source MBE (SS-MBE) group-III
and -V molecular beams.
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Effusion cells
                                      Cylindrica
l crucible offers good charge material capacity,
but uniformity decreases as charge material is
depleted. It offers excellent long-term flux
stability, but permits large shutter flux
transients.
                                      Conical
crucible offers reduced charge material capacity,
excellent uniformity, and poor long-term flux
stability, and permits large shutter flux
transients.
                                      SUMO
crucible offers excellent charge material
capacity, excellent uniformity, excellent
long-term flux stability, and minimal
shutter-related flux transients.
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- made of Ta, Mo, and pyrolytic boron nitride
(PBN) - do not decompose or outgas impurities
even when heated to 1400ºC.
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Valved cracker Effusion cell
- combines the evaporation and cracking of
elements like P,S, As, Se, Te etc.
Technical Data
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MBE growth mechanism.

• Atoms arriving at the substrate surface may
  undergo
• absorption to the surface,
• surface migration,
• incorporation into the crystal lattice,
• thermal desorption.
• depends strongly on the temperature of the
  substrate..

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Growth modes
At very high temperature of substrate, there are
many different possible surface diffusion
mechanisms
Ehrlich-Schwoebel barrier
epitaxial growth is ensured by-

• very low rates of impinging atoms,
• migration on the surface and
• subsequent surface reactions

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Depending on the migration rates, different
growth modes can result

• high migration rate (IBAD is followed)
  Frank vander merwe growth mode.
• high rate of incoming atoms and high
  Ehrlich-Schwoebel barrier, island
• growth will occur.
  Volmer-Weber or Stranski-Krastanov growth modes.
• - Stranski-Krastanov growth is possible in a
  heteroepitaxial system.

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• Reflection high-energy electron diffraction
  (RHEED).
• sensitive to surface changes, either due to
  structural changes or due to adsorption.
• to calibrate growth rates,
• observe removal of oxides from the surface,
• calibrate the substrate temperature,
• monitor the arrangement of the surface atoms,
• determine the proper arsenic overpressure,
• give feedback on surface morphology,
• provide information about growth kinetics
• A high energy beam (3-100 keV) is directed at
  the sample surface at a grazing
• angle.
• - distance between the streaks - surface lattice
  unit cell size.
• - atomically flat surface - sharp RHEED patterns.
• - rougher surface - diffused RHEED pattern.

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RHEED is sensitive for surface structures and
reconstructions.
a) GaAs(100) - 1x1
b) GaAs(100) - 2x1
electron beam is incident in the (110) with 8.6
keV
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Interpretation of some RHEED patterns
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Different stages of layer-by-layer growth by
nucleation of 2D islands and the corresponding
intensity of the diffracted RHEED beam.
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(a) Transmission through high and wide crystal
(b) Transmission through high and narrow
crystal (c) Transmission through short and wide
crystal (d) Diffraction from nearly flat
asperities.
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RHEED intensity oscillations 

• - direct measure of growth rates in MBE.
• - oscillation frequency corresponds to the
  monolayer growth rate.
• incident angle dependence of the oscillations
  suggest that interference between
• electrons scattering from the underlying layer
  and the partially grown layer contribute
• to these oscillations.
• magnitude of the RHEED oscillations damps
  because as the growth progresses,
• islands nucleate before the previous layer is
  finished.

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• Beam Equivalent Pressure  - To measure the
  growth rate.
• Proportional to the flux at the sample surface
  and hence the growth rate.
• - The biggest difficulty is, the gauge gets
  coated.

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Epitaxial Growth of AlxGa1-xAs  Substrate
temperatures - 580ºC-650ºC. Requires an As
overpressure to prevent the surface from becoming
Ga rich. GaAs, there is a large window for
which there is both unity sticking and sufficient
mobility. Ternary or quaternary compounds - the
window becomes smaller - differences in
the relative bond strengths of the different
group III adatoms. RHEED can be used to
determine the minimum amount of As required to
maintain the proper stoichiometry by measuring
the incorporation ratio. Values of the
incorporation ratio for GaAs is 1.3 to 1.8.
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(100) is the predominant substrate orientation
for MBE growth of compound semiconductors.
Growth of InGaAlAs on InP   Incorporating In
into AlxGa1-xAs will decrease the
bandgap. In.52Al.48As - 1.49 eV ? 0.74 eV -
In.53Ga.47As - small enough bandgap to
detect light at the important wavelengths of 1.3
mm and 1.55 mm.
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Quantum dots structures based on highly lattice
mismatched materials. mean free path and the de
Broglie wavelength of free carriers exceed the
critical sizes of structures. carriers experience
a three-dimensional quantum confinement.
InAs grows layer-by-layer till critical coverage
- wetting layer (WL). After ?c1.6 ML (w0.5
nm), Stranski-Krastanow 3D growth
occurs. relaxation of the elastic energy which
builds up as the thickness of mismatched
epilayers increases.
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AFM image of InAs/GaAs Quantumdots.
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Mismatched heteroepitaxial systems for quantum
dots III-V compounds arsenides
(InGaAs/AlGaAs ,InAs/InGaAs, InAlGaAs /AlGaAs )
phosphides (InAs/InP, InP/InGaP ),
antimonides and nitrides (GaN/AlN), IV-IV
compounds Ge/Si and SiGe/Si II-VI
compounds CdSe/ZnSe and
Mixed-group compounds InAs/Si.
Bennett, Magno, and Shanabrook Appl. Phys. Lett.
68 (4), 22 (1996)
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E Eg Ee Eh E - emission energy Eg -
quantumdot bandgap energy Ee - electron
confinement energy Eh - hole confinement energy
Nt Exciton binding energy is
neglected.
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Tuning QD emission
C. K. Chia et al., J. Crystal. Growth, 288, 57-60
(2006)
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High Tc Superconductivity
increases in the Tc in thin films of copper oxide
superconductors hydrostatic
pressures compressive epitaxial
strain. Under compressive epitaxial strain a much
larger increase in Tc is possible. Requires the
choice of a suitable system and substrate
combination

• The two essential structural features in copper
  oxide superconductors
• CuO2 planes
• charge reservoirs (CR)

• The possible control knobs are
• the distance between the magnetic (black arrows)
  Cu atoms dab,
• the corrugation of the CuO2 planes alpha
• the thickness of CR dCR
• the interlayer distance between two consecutive
  CuO2 planes dIL
• the distance between the charge reservoir and the
  CuO2 plane dCT.

the lattice deformations associated with the
strain fundamentally modify the energy scales,
leading to the formation and condensation of the
superconducting pairs.
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"214" film on a SrTiO3 substrate, the Cu
and O atoms of the CuO2 planes are
expanded. "214" thin film on a SrLaAlO4 substrate
in-plane compression and an
out-of-plane expansion.
- redox reaction at interface is a serious
problem fabricating tunnel junctions using
high-Tc Cuprates,
J. P. Locquet et al., Nature 394, 453 (1998)
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MgB2 a novel material for high Tc superconductor.
The graphite-like array of boron (shown in black)

• has great promise for superconducting electronics
  
• operated at T 20 K.

Josephson tunnel junctions (MgB2/AlOx/MgB2)
fabricated on sapphire -C substrates below 300
C.
K. Ueda, S. Saito, K. Semba, T. Makimoto and M.
Naito, APL 86 , 172502 (2005)
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Gallium Nitride (GaN) HEMT
Dr. Takashi Mimura - inventor of HEMT ( - FET
with a junction between two materials with
different band gaps as the channel instead of an
n-doped region.
Features of GaN HEMT high frequency power
transistors for RF transmission applications
covering the 1-50 GHz band. inherently higher
transconductance, good thermal management and
higher cutoff frequencies. prime candidate for
high power microwave applications.
M. Aslf Khan, J. N. Kuznia, et al, Appl. Phys.
Lett., 65(9)1121
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extremely low noise device in terrestrial and
space telecommunications systems, radio
telescopes in the area of astronomy, DBS
receivers and a car navigation receivers.
In 1985, HEMT was announced as a unique microwave
semiconductor device with the lowest noise
characteristics in the world.
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- can receive TV programs from satellites
36,000Km above the ground. - The antenna is less
than 30 centimeters in diameter,
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GMR heterostructures
- prepared in 1986, using MBE
- magnetoresistance is much larger than that of
the intrinsic magnetoresistance of the Fe layers
themselves
Oscillatory coupling - a very general property of
almost all transition-metal magnetic multilayered
systems the nonferromagnetic layer comprises
one of the 3d, 4d, or 5d transition metals or one
of the noble metals
The oscillation period was found to be just a few
atomic layers, typically about 10 Å, but varying
up to 20 Å.
- ferromagnetic cobalt layers separated by thin
copper layers, was found to exhibit very large
GMR effects even at room temperature
GMR sandwiches can achieve sensitivities to such
fields of perhaps as much as five hundred times
greater than conventional materials.
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For copper thickness (typically 1.9 nm) -
anti-parallel magnetic coupling between
successive cobalt layers in zero applied field.
the magnitude of the interlayer exchange coupling
decreases much more rapidly with increasing Cu
thickness.
"spin valve" effect
GMR read heads allows the reading of extremely
small magnetic bits at an areal density of 2.69
gigabits per square inch.
One minute please..
IBM J . RES. DEVELOP. VOL 42 NO. 1 JANUARY 1998
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Summary

• MBE is a versatile technique for device
  fabrication.
• A good lab for studying Molecular dynamics and
  surface mechanism.
• The primary application for MBE - grown layers
  is the fabrication of
• electronic devices .

www.research.ibm.com/research/demos/gmr/1.swf www.
research.ibm.com/research/demos/gmr/2.swf
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Appropriate other meanings of MBE

• Mind Boggling Experiment
• Mostly Broken Equipment
• Mega-Buck Evaporator
• Medieval Brain Extractor
• Money Buys Everything
• Management Bullshits Everyone

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