SYNTHESIS OF THIN FILMS
FILMS - FORM?
• Supported - substrate type - effect of interface – single
crystal (oriented) or glassy
• Free standing - synthetic strategy - air/liquid or
liquid/liquid interface or substrate lift-off
• Epitaxial - lattice matching and tolerance with
underlying substrate
• Superlattice - artificial multilayer – periodic or aperiodic
• Patterned - chemical or physical lithography – bottomup synthesis or top-down physical methods
FILMS - WHEN IS A FILM THICK OR THIN?
• Monolayer - atomic, molecular thickness
• Multilayer - compositional superlattice - scale - periodicity
• Bulk properties - scale - thickness greater than l(e,h)
• Quantum size effect - 2D spatial confinement – quantum
confined along z, free electron behavior along x,y – called
quantum wells – enable range of quantum devices
THIN FILMS VITAL IN MODERN TECHNOLOGY
• Protective coatings
• Optical coatings - antireflection coatings
• Electrochromic windows – photochromic spectacles
• Dielectric film – low k circuit packaging – high k
transistor gate insulation
• Optical filters
• Microelectronic devices and circuits
• Optoelectronic devices and circuits
• Photonic devices and circuits
THIN FILMS VITAL IN MODERN TECHNOLOGY
• Electrode surfaces – solar cells, fuel cells, lithum
solid state batteries
• Xerography, photography
• Electrophoretic, electrochromic,
electrodewettability displays
• Catalytic and photocatalytic surfaces
• Nanoporous membranes – gas separation
• Information storage - magnetic, magnetoresistant, magneto-optical, optical, flash
FILM PROPERTIES - ELECTRICAL, OPTICAL,
MAGNETIC, MECHANICAL, ADSORPTION,
PERMEABILTY, CHEMICAL
• Thickness and Surface : Volume ratio
• Surface vs bulk structure - surface reconstruction,
dangling bonds – unsatisfied valencies, surface
roughness
• Hydrophobicity - hydrophilicy - wettability
• Composition – surface vs bulk
• Texture - single crystal, microcrystalline, orientation,
glassy
• Form - supported or unsupported (free-standing) nature of substrate - patterned or un-patterned
METHODS OF SYNTHESIZING THIN FILMS
• ELECTROCHEMICAL, PHYSICAL, CHEMICAL
• Cathodic or anodic
• Electroless deposition
• Laser ablation
• Cathode sputtering, vacuum evaporation, e-gun
• Thermal oxidation, nitridation, sulfidation
METHODS OF SYNTHESIZING THIN FILMS
• ELECTROCHEMICAL, PHYSICAL, CHEMICAL
• Liquid/melt phase epitaxy
• Self-assembly - surface molecule anchoring,
organization, close-packing, monolayers or multilayers
• Discharge (plasma) techniques - RF, microwave
• Chemical vapor deposition CVD, metal organic chemical
vapor deposition MOCVD
• Molecular beam epitaxy, supersonic cluster beams,
aerosol deposition
ANODIC OXIDATIVE DEPOSITION OF FILMS
• Deposition of metal oxide films, such as
alumina, titania by oxidation of metal
electrode in aqueous salts or acids
• Deposition of conducting polymer films by
oxidative polymerization of monomer, such
as thiophene, pyrrole, aniline, acetylene
ANODIC OXIDATION OF Al IN OXALIC OR
PHOSPHORIC ACID TO FORM ALUMINUM OXIDE
• Pt|H3PO4, H2O|Al
ECCell
• Al  Al3+ + 3e-
Anode
• PO43- +2e-  PO33- + O2-
Cathode
• 2Al3+ + 3O2-  g-Al2O3 (annealing)  a-Al2O3
• Voltage control of oxide thickness
• Al3+/O2- diffuse through growing layer of Al2O3
ANODIC OXIDATION OF PATTERNED Al DISC TO
MAKE PERIODIC NANOPOROUS Al2O3 MEMBRANE
SiC patterned master
harder than Al to make
nanoimprint replica
How to remove residual Al
and Al2O3 barrier layer???
2Al + 3PO43-  Al2O3 + 3PO332Al + 3C2O42-  Al2O3 + 6CO + 3O2-
ANODIC OXIDATION OF PATTERNED Al DISC TO
MAKE PERIODIC NANOPOROUS Al2O3 MEMBRANE
Aqueous HgCl2 dissolves Al to give Hg and Al(H2O)63+
and H3PO4 dissolves Al2O3 barrier layer to give
Al(H2O)63+ - yields open channel membrane
ANODIC OXIDATION OF LITHOGRAPHIC
PATTERNED Al TO PERIODIC NANOPOROUS Al2O3
Not bad for chemistry!!!
Hexagonal close packed nanochannel membrane
ANODIC OXIDATION OF LITHOGRAPHIC
PATTERNED Al TO PERIODIC NANOPOROUS Al2O3
40V
Voltage control of channel diameter
50-500 nm accessible
60V
80V
PROPOSED MECHANISM OF ALUMINA PORE FORMATION
IN ANODICALLY OXIDIZED ALUMINUM
SELF ORGANIZED
SELF LIMITING
GROWTH OF PORES
Electric and
strain fields guide
and organize hcp
channel growth
Templated synthesis of
metal barcoded nanorods
Collection of multi-metal nanorods imaged in an optical
microscope by the different reflectivity’s of different metal
segments, Science 2001, 294, 137
•
•
Optical (A)
and field
emission
scanning
electron
microscopy
FE-SEM (B)
images of an
Au-Ag multistripe
nanorods
550-nm Au
stripes and Ag
stripes of 240,
170, 110, and
60 nm -top to
bottom
240 nm
550 nm
170 nm
110 nm
60 nm
Orthogonal assembly on nanorods. Butyl
isonitrile is bound non-selectively to Pt
and Au surfaces. Aminoethanethiol
displaces isonitriles selectively on gold but
not on platinum. Rhodamine isocyanate is
reacted with terminal amino groups to
fluorescently label gold segments.
-NH-CS-NHthiourea linkage of
rhodamine fluorescent
dye to Au segment
DNA sandwich hybridization
assay on metal barcode nanorods
- Science 2001, 294, 137
SYNTHESIS OF CHEMICALLY POWERED
NANOROD MOTORS
?
Ozin et al Chem Comm, AdvMater 2006, Mallouk et al JACS 2005
ANODIC OXIDATION OF Si TO FORM POROUS Si:
THROWING SOME LIGHT ON SILICON
• Typical electrochemical cell to
prepare PS by anodic
oxidation of heavily doped p+type Si
• PS comprised of
interconnected nc-Si with
H/O/F surface passivation
• nc-Si right size for QSEs and
red light emission observed
during anodic oxidation –
electroluminescence
ELECTRONIC BAND STRUCTURE OF
DIAMOND SILICON LATTICE
•
•
•
•
•
•
band structure of Si computed using density
functional theory with local density and
pseudo-potential approximation
diamond lattice, sp3 bonded Si sites
VB maximum at k = 0, the G point in the
Brillouin zone, CB minimum at distinct k
value
indirect band gap character, very weakly
emissive behavior
absorption-emission phonon assisted
photon-electron-phonon three particle
collision very low probability, thus band gap
emission efficiency low, 10-5%
SEMICONDUCTOR BAND STRUCTURE:
CHALLENGE, EVOKING LIGHT EMISSION FROM Si
• Effective Mass Approximation Rexciton ~ 0.529e/mo where e =
dielectric constant, reduced mass of exciton mo = memh/(me + mh)
• Note exciton size within the bulk material defines the size regime
below which significant QSEs on band structure are expected to
occur, clearly < 5 nm to make Si work
REGULAR OR RANDOM NANNSCALE CHANNELS
IN ANODICALLY OXIDIZED SILICON WAFERS
• Anodized forms of p+type Si wafer
• Showing formation of
random (left) and
regular (right) patterns
of pores
• Lithographic pretexturing directs
periodic pore formation
PORE FORMING PROCESS IN ANODICALLY
OXIDIZED SILICON WAFERS
•
Basics of electrochemical cell - p+-Si
wafer anode in contact with aqueous
HF electrolyte – simplified
electrochemistry:
•
•
•
Si  Si(4+) + 4e
Si(4+) + 6F(-)  [SiF6]24H(+) + 4e  2H2
•
Mechanism of natural self-limiting
process for regular pore formation
based on wider band gap of PS
compared to bulk Si and respective
redox potentials for anodic oxidation
KEY ISSUES: ORIGIN OF PHOTO- AND
ELECTROLUMINESCENE OF POROUS SILICON
• Origin of luminescence key point- as bulk Si is
indirect band gap semiconductor with very
weak light emission
• Models for light emission include quantumspatial confinement, siloxenes, and SiOH
• Luminescent nc-Si structure requires SiO, SiH
surface bonds - caps dangling bonds -removes
killer traps in band gap
• Size dependence of k, m selection rules, scaling
laws determine light emission properties
• Mechanical, photochemical, chemical stability
are key factors for devices – safety too - care
with humidity control and toxic silane evolved
Si[H]surface + H2O  SiH4
• Efficient e-h charge-injection required for
practical LED
MAKING NANOCRYSTALLINE SILICON
LUMINESCENT: CAPPING
*(SiH)
CB
CB
VB
VB
capping Si cluster dangling
bond with H, F, O forms
bonding-antibonding SiH
-orbitals, moves killer
trap states out of the gap
facilitates radiative over
non-radiative relaxation
(SiH)
Sin
HxSin
LIGHT WORK BY THE SILICON SAMURAI
WHERE IT ALL BEGAN AND WHERE IT IS ALL GOING???
FROM CANHAM’S 1990 DISCOVERY OF PL AND EL ANODICALLY
OXIDIZED p+-DOPED Si WAFERS, TO NEW LIGHT EMITTING SILICON
NANOSTRUCTURES, AND DREAM OF SILICON OPTOELECTRONICS AND
PHOTONIC COMPUTING – ACTUALLY BIOSENSORS EVENTUALLY
CHEMICAL VAPOUR DEPOSITION
• Pyrolysis, photolysis, chemical reaction, discharges RF, microwave facilitated deposition processes
• Epitaxial films, correct matching to substrate lattice
• CH4 + H2 (RadioF, MicroW)  C, diamond films
(perfect non-stick frying pan – inert, hard, transparent,
non-stick, high thermal conductivity)
• Et4Si (thermal, air)  SiO2
• SiCl4 or SiH4 (thermal T, H2)  a-H:Si or nc-H:Si
• SiH4 + PH3 (RF)  n-Si (ppm P)
CHEMICAL VAPOUR DEPOSITION
• Si2H6 + B2H6 (RF)  p-Si (ppm B)
• Single source precursor SiH3SiH2SiH2PH2 (RF) n-Si
• Me3Ga (laser photolysis, heating)  Ga
• Me3Ga + AsH3 + H2 (T,P)  GaAs + CH4
• Si (laser evaporation, molecular beam, high to low P
supersonic jet, ionization) Sin+ (size selected MS - cluster
deposition)  Si
H H
H
H H H
H H
HH
H
H
H
H
H
H
H
H
H H H H H
H H H
H
H
H
H
H
H
H
H H
Amorphous hydrogenated
silicon a-H:Si, easy to form thin
film by CVD
Hydrogen capping of dangling
surface sp3 bonds
Reduces surface electron killer
traps
Enhances electrical conductivity
compared to a-Si but less than
bulk c-Si
Poly-domain texture
Useful for pn and pin junction
large area solar cell devices
REMOVING DANGLING BONDS BY Si-H CAPPING
*(SiH)
CB
CB
VB
VB
capping Si cluster dangling
bond with H, F, O forms
bonding-antibonding SiH orbitals, moves killer trap
states out of the gap facilitates
charge transport and
radiative relaxation
(SiH)
Sin
HxSin
METAL ORGANIC CHEMICAL VAPOR DEPOSITION
MOCVD
• Invented by Mansevit in 1968
• Recognized high volatility and chemical reactivity of
metal organic compounds as sources for semiconductor
thin film preparations
• Enabling chemistry: electronic, optical quantum devices
• Quantum wells and superlattices
• Occurs for 5-500 Angstrom layers
• Known as artificial superlattices
Schematic energy band diagram of a quantum well structure showing
confined electron and hole states produced by large Eg GaAlAs layers
sandwiching small Eg GaAs depicting quantum size effects and some
possible optical transitions
CB edge
GaAlAs
CB/VB
edges GaAs
L
En = n2p2h2/2m*L2
VB edge
GaAlAs
METAL ORGANIC CHEMICAL VAPOR DEPOSITION,
MOCVD
• Quantum confined electrons and holes when thickness of quantum
well L is comparable to the wavelength of an electron or hole at the
Fermi level of the material, band diagram shows confined particle
states and quantization effects for electrical and optical properties
• Discrete electronic energy states rather than continuous bands, given
by solution to the simple particle in a box equation, assuming infinite
barriers for the wells, m* is the effective mass of electrons and holes
• En = n2p2h2/2m*L2
• Tunable thickness, tailored composition materials, do it yourself
quantum mechanics materials for the semiconductor industry
METAL ORGANIC CHEMICAL VAPOR
DEPOSITION, MOCVD
• Quantum well structure synthesized by depositing a
controlled thickness superlattice of a narrow band
gap GaAs layer sandwiched by two wide band gap
GaxAl1-xAs layers using MOCVD
• Ga(Al)Me3 + AsH3 (H2, T)  Ga(Al)As + CH4
• Artificial superlattices, designer periodicity of
layers, quantum confined lattices, thin layers,
epitaxially grown, x determines electronic band gap
• Example: GaxAl1-xAs|GaAs|GaxAl1-xAs
MOCVD
• Example: GaxAl1-xAs|GaAs|GaxAl1-xAs
• n- and p-doping achievable by having excess As or Ga
respectively in a GaAs layer
• Composition and carrier concentration controls refractive
index (low RI cladding, TIR optical confinement) and
electrical conductivity (p-n and p-n-p junction devices), in a
semiconducting superlattice
• Enables electron (quantum) and photon (RI) confinement
for electronic, optoelectronic and optical devices
• Multiple quantum well laser, quantum cascade laser,
distributed feedback laser, resonant tunneling transistor, high
electron mobility ballistic transistor (HEMT), laser diode
Resonant tunneling transistor
BAND GAP ENGINEERING OF SEMICONDUCTORS
• MOCVD, LPE, CVD, CVT, MBE all
deposition techniques that provide angstrom
precise control of film thickness
• Together with composition control one has a
beautiful synthetic method for fine tuning
the electronic band gap and hence most of
the important properties of a semiconductor
quantized film
BAND GAP ENGINEERING OF SEMICONDUCTORS
• Key is to achieve epitaxial lattice matching of
film with underlying substrate
• Avoids interfacial lattice strain, elastic
deformation, misfit dislocations, defects - all
of these problems serve to increase carrier
scattering, decrease charge-transport,
increased quenching of e-h recombination
luminescence (killer traps), thereby reducing
the efficacy of the material for advanced
device applications
MOCVD SINGLE SOURCE PRECURSORS
• Me3Ga, Me3Al, Et3In (synthesis GaCl3 + MeLi/R2Mg/RMgI)
• NH3, PH3, AsH3 (synthesis Mg3As2/HCl)
• H2S, H2Se
• Me2Te, Me2Hg, Me2Zn, Me4Pb, Et2Cd
• E.g. synthesize an IR detector based on p-n photodiode
• Me2Cd + Me2Hg + Me2Te (H2, 500oC)  HgxCd1-xTe
• p-HgxCd1-xTe/n-HgxCd1-xTe
•
•
p- and n-doping requires precise control of Hg/Cd and Te stoichiometry
x determines the electronic bandgap – tuned to IR wavelength range for detector
• Toxic materials – safe handling and disposal of toxic waste!!!
Schematic of cold wall MOCVD system
Single crystal substrate on inductively heated or resistively
heated susceptor – mass flow control of precursors
MOCVD deposited film
H2/AsH3/PH3
Water cooling
H2/InMe3/GaMe3
Thermocouple
H2/PEt3
Waste gases
H2/n-dope H2S/p-dope ZnMe2
MOCVD surface chemistry of precursors,
nucleation and growth of product film on substrate
CH4
Me
Me
Me
Ga
Me
Me
Me
Me
Ga
Me
H
Me
Al
H
H H
As
As Al As Al As Al As Al As Al As Al As Al As Al As Al As
Precursor adsorption on single crystal oriented substrate - lattice
matching epitaxy criteria - surface physisorption - chemisorption surface diffusion - dissociative chemisorption - reaction - desorption
Different models for film nucleation and growth - depends whether
surface diffusion involved - fixed vs mobile crystal nuclei
MOCVD SINGLE SOURCE PRECURSORS
• Specially designed MOCVD reactors, hot and cold wall
designs, controlled flow of precursors using digital mass
flow meters directing precursors to heated single crystal
substrate, induction or resistive heater, silicon carbide
coated graphite susceptor for mounting substrate
• This chemistry creates problems for semiconductor
manufacturers wrt safe handling and disposal of toxic waste
• Most reactions occur in range 400-1300oC, complications of
diffusion at interfaces, disruption of atomically flat epitaxial
surfaces/interfaces may occur during deposition
• Photolytic processes (photoepitaxy) help
to bring the deposition temperatures to
more reasonable temperatures
PHOTOEPITAXY
Making atomically perfect thin films under milder and more controlled conditions
Et2Te + Hg + H2 (h, 200oC)  HgTe + 2C2H6
Bottom graphite,
middle substrate, top
HgTe film
H2 gas window
Hg pool
H2/Et2Te
Exhaust gases
UV illumination
PHOTOEPITAXY
Making atomically perfect thin films under milder and more controlled conditions
• Mullin and Tunnicliffe 1984
• Et2Te + Hg (pool) + H2 (h, 200oC)  HgTe + 2C2H6
• Et2Te/Me2Cd + Hg (pool) + H2 (h, 200oC)  HgxCd1-xTe +
2C2H6
• MOCVD preparation requires 500oC using Me2Te +
Me2Hg/Me2Cd
• Advantages of photo-epitaxy
• Lower temperature operation, multi-layer formation, less
damage of layers - ternaries HgxCd1-xTe, n- and p-doping, Te
and Hg/Cd rich, p-n diodes, IR photodetectors, multi-layers,
quantum size effect devices HgxCd1-xTe|HgTe|HgxCd1-xTe
PHOTOEPITAXY
Making atomically perfect thin films under
milder and more controlled conditions
• Lower interlayer diffusion, easy to fabricate multilayers
• Abrupt boundaries, less defects, strain and
irregularities at interfaces
• Note that H2 gas window in apparatus prevents
deposition of HgTe on observation port
• In this way CdTe can be deposited onto GaAs at 200250oC even with a 14% lattice mismatch
• Key consideration - GaAs is susceptible to damage
under MOCVD conditions 650-750oC
REQUIREMENTS OF SUCCESSFUL MOCVD PRECURSOR
• RT stable
• No polymerization, decomposition
• Easy handling
• Simple storage
• Not too reactive
• Vaporization without decomposition
REQUIREMENTS OF MOCVD PRECURSORS
• Vaporization without decomposition
• Modest < 100oC temperatures
• Low rate of homogeneous pyrolysis, gas phase, wrt
heterogeneous, surface, decomposition
• HOMO : HETERO rates ~ 1 : 1000
• Heterogeneous reaction preferred on substrate
• Greater than on other hot surfaces in reactor
REQUIREMENTS OF MOCVD PRECURSORS
• Not on supports or reaction chamber/vessel
• Ready chemisorption of precursor on substrate
• Detailed surface and gas phase studies of structure of
adsorbed species, reactive intermediates, kinetics, vital
for quantifying film nucleation and growth processes
• Electrical, magnetic, optical films made in this way
• Semiconductors, metals, silicides, nitrides, oxides,
mixed oxides (e.g., high Tc superconductors), sulfides,
selenides
CRITICAL PARAMETERS IN MATERIALS
PREPARATION FOR SYNTHESIS OF THIN FILMS
• Composition control - precise command over
stoichiometry and adventitious carbonaceous deposits
• Variety of materials to be deposited
• Good film uniformity
• Large areas to be covered, > 100 cm2
• Precise reproducibility
CRITICAL PARAMETERS IN MATERIALS
PREPARATION FOR SYNTHESIS OF THIN FILMS
• Growth rate, thickness control
• 2-2000 nm layer thickness
• Precise control of film thickness
• Accurate control of deposition, film growth rate
CRITICAL PARAMETERS IN MATERIALS
PREPARATION FOR SYNTHESIS OF THIN FILMS
• Crystal quality, epitaxy
• High degree of film perfection
• Defects degrade device performance
• Reduces useable wafer yields
CRITICAL PARAMETERS IN MATERIALS
PREPARATION FOR SYNTHESIS OF THIN FILMS
• Purity of precursors
• Usually less than 10-9 impurity levels
• Stringent demands on starting material purity
• Chemistry challenge, purifying, analyzing
precursors at ppb level
• Demands exceptionally clean growth system
otherwise defeats the object of controlled
doping of films for device applications
CRITICAL PARAMETERS IN MATERIALS
PREPARATION FOR SYNTHESIS OF THIN FILMS
• Interface widths
• Abrupt changes of composition, dopant concentration
required, vital for quantum confined structures
• 30-40 sequential layers often needed
• Alternating composition and graded composition films
• 0.5-50 nm thickness required with atomic level precision
• All of the above has been more-or-less perfected in the
electronics and optics industries – amazing achievement!!!
III-V BAND GAP
ENGINEERING
1500nm
•
•
•
•
•
•
•
•
•
•
•
•
Designer semiconductors
Single crystal substrate
Single crystal layers
Zinc blende lattice
Lattice constant
Composition
Doping
Thickness
Multilayers
Epitaxial lattice matching
Control of Eg band gap
and RI refractive index
Operating wavelengths for
optical telecommunication
systems labeled in purple
TECHNIQUES USED TO GROW SEMICONDUCTOR
FILMS AND MULTILAYERED FILMS
• MOCVD
• Liquid phase epitaxy
• Chemical vapor transport
• Molecular beam epitaxy
• Laser ablation
• Used for band gap engineering of semiconductor
materials that function at 1.5 microns in near IR integrating with glass fiber optics and waveguides
6InP/3GaAs/6InP EPITAXIALLY
MATCHED SUPERLATTICE
TAILORED BAND GAPS - DESIGNER MOCVD
GRADED COMPOSITION POTENTIAL WELLS
AlxGa1-xAs graded composition-gap superlattice
e
Tunable h
h
CB AlAs wide gap
CB GaAs narrow gap
VB GaAs narrow gap
VB AlAs wide gap
Designer quantum well architecture - band gap engineering - graded
composition enables gradient potential – speeds mobility of electrons
injected into channel - used to enhance performance in high electron
mobility transistors HEMTs or build a quantum cascade laser
Federico Capasso co-inventor of the quantum cascade laser imagined
small things when he used size and dimensionality of materials to
tailor their properties for electronic and optical devices
QUANTUM CASCADE LASER - A
NICE EXAMPLE OF BAND GAP
ENGINEERING BY MOCVD
h
h
h
White bands in the TEM are QWs made of
narrow band gap GaInAs, which are
sandwiched between barrier layers of wide
band gap AlInAs ranging in thickness
from atomic to 12 atomic layers
All wells are part of a QCL
Voltage applied to device, electrons
move down potential barrier from wide
to narrow Eg QWs (tunnel from QW to
QW) and emit a photon between two
thickest QWs.
Electrons move on (tunnel) to the next
stage to the right where the process
repeats – hence cascade laser.