PORE FORMING PROCESS IN ANODICALLY OXIDIZED SILICON WAFERS

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PORE FORMING PROCESS IN ANODICALLY OXIDIZED
SILICON WAFERS

• Basics of electrochemical cell - p-Si wafer
  anode in contact with aqueous HF electrolyte
• 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

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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 quantum-spatial
  confinement, siloxenes, and SiOH
• Luminsecent 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
• Efficient e-h charge-injection required for
  practical LED

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MAKING NANOCRYSTALLINE SILICON LUMINESCENT
CAPPING
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THERMAL OXIDATION FORMATION OF METAL OXIDE AND
NITRIDE THIN FILMS

• Anodic layers, metal exposed to a glow discharge
• Ti O2 ? TiO2 thickness 3-4 nm
• Similar method applicable to other metals, Al, V,
  W, Zr
• Not restricted to oxides, nitrides too,
  exceptionally hard, high temperature protective
  coating
• Ti NH3 ? TiN
• Al NH3 ? AlN

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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
• Et4Si (thermal, air) ? SiO2
• SiCl4 or SiH4 (thermal T, H2) ? a-HSi or nc-HSi
  
• SiH4 PH3 (RF) ? n-Si

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CHEMICAL VAPOUR DEPOSITION

• Si2H6 B2H6 (RF) ? p-Si
• SiH3SiH2SiH2PH2 (RF)? n-Si
• Me3Ga (laser photolysis, heating) ? Ga
• Me3Ga AsH3 H2 ? GaAs CH4
• Si (laser evaporation, supersonic jet) Sin (size
  selected cluster deposition) ? Si

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Amorphous hydrogenated silicon a-HSi, 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
Si Poly-domain texture Useful for solar cell
large area devices
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METAL ORGANIC CHEMICAL VAPOR DEPOSITION, MOCVD

• Invented by Mansevit in 1968
• Recognized high volatility of metal organic
  compounds as sources for semiconductor thin film
  preparations
• Enabling chemistry for electronic and optical
  quantum devices
• Quantum wells and superlattices
• Occurs for 5-500 angstrom layers
• Known as artificial superlattices

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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 n2p2h2/2mL2
• Tunable thickness, tailorable composition
  materials, do it yourself quantum mechanics
  materials for the semiconductor industry

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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
• Known as artificial superlattices, designer
  periodicity of layers, quantum confined lattices,
  thin layers, epitaxially grown
• Example GaxAl1-xAsGaAsGaxAl1-xAs

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MOCVD

• Example GaxAl1-xAsGaAsGaxAl1-xAs
• n- and p-doping achievable by having excess As or
  Ga respectively in a GaAs layer
• Composition and carrier concentration controls
  refractive index and electrical conductivity,
  thus TIR achieved in a semiconducting
  superlattice
• Enables quantum and photon confinement for
  electronic and optoelectronic and optical
  devices, multiple quantum well laser, quantum
  cascade laser, distributed feedback laser,
  resonant tunneling transistor, high mobility
  ballistic transistor, laser diode

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BAND GAP ENGINEERING OF SEMICONDUCTORS

• The MOCVD, LPE, CVD, CVT, MBE are 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
• The key thing is to achieve epitaxial lattice
  matching of the film with the underlying
  substrate
• This avoids things like lattice strain at the
  interface, elastic deformation, misfit
  dislocations, defects
• All of these problems serve to increase carrier
  scattering and quenching of e-h recombination
  luminescence (killer traps), thereby reducing the
  efficacy of the material for advanced device
  applications

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MOCVD PRECURSORS, SINGLE SOURCE MATERIALS

• Me3Ga, Me3Al, Et3In
• NH3, PH3, AsH3
• H2S, H2Se
• Me2Te, Me2Hg, Me2Zn, Me4Pb, Et2Cd
• Example for IR detectors
• Me2Cd Me2Hg Me2Te (H2, 500oC) ? CdxHg1-xTe
• All pretty toxic materials

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MOCVD PRECURSORS, SINGLE SOURCE MATERIALS

• Specially designed MOCVD reactors, hot and cold
  wall designs, controlled flow of precursors using
  mass flow meters directing them to heated
  substrate single crystal, induction heater,
  silicon carbide coated graphite susceptor for
  mounting substrate
• Chemistry of this type creates a problem for
  semiconductor manufacturers in terms of safe
  disposal of toxic waste
• Most reactions occur in range 400-1300oC,
  complications of diffusion at interfaces,
  disruption of atomically flat epitaxial
  surfaces/interfaces occurs during deposition
• Photolytic processes (photoepitaxy) help to bring
  the deposition temperatures to more reasonable
  temperatures

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REQUIREMENTS OF MOCVD PRECURSORS

• RT stable
• No polymerization, decomposition
• Easy handling
• Simple storage
• Not too reactive
• Vaporization without decomposition

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REQUIREMENTS OF MOCVD PRECURSORS

• Vaporization without decomposition
• Modest lt 100oC temperatures
• Low rate of homogeneous pyrolysis, gas phase, wrt
  heterogeneous decomposition
• HOMO HETERO rates 1 1000
• Heterogeneous reaction on substrate
• Greater than on other hot surfaces in reactor

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REQUIREMENTS OF MOCVD PRECURSORS

• Not on supports, vessel
• Ready chemisorption of precursor on substrate
• Detailed surface and gas phase studies of
  structure of adsorbed species, reactive
  intermediates, kineticss, vital for quantifying
  film nucleation and growth processes
• Electronic and optical films synthesized in this
  way
• Semiconductors, metals, silicides, nitrides,
  oxides, mixed oxides (e.g., high Tc
  superconductors)

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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, gt 100 cm2
• Precise reproducibility

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CRITICAL PARAMETERS IN MATERIALS PREPARATION FOR
SYNTHESIS OF THIN FILMS

• Growth rate, thickness control
• 2-2000 nm layer thicknesses
• Precise control of film thickness
• Accurate control of deposition, film growth rate

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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

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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
• Challenge for chemistry, purifying and analyzing
  at the ppb level
• Demands exceptionally clean growth system
  otherwise defeats the object of controlled doping
  of films for device applications

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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 thicknesses required with atomic level
  precision
• All of the above has been more-or-less perfected
  in the electronics and optics industries

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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

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BAND GAP ENGINEERING

• Designer semiconductors
• Zinc blende lattice
• Lattice constant
• Composition
• Doping
• Thickness
• Multilayers
• Epitaxial lattice matching
• Control of band gap and refractive index
• Operating wavelengths for optical
  telecommunication systems labeled in purple

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6InP/3GaAs/6InP EPITAXIALLY MATCHED SUPERLATTICE
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TAILORED BAND GAPS - DESIGNER MOCVD GRADED
COMPOSITION POTENTIAL WELLS
AlxGa1-xAs graded composition-gap superlattice
CB AlAs wide gap
e
CB GaAs narrow gap
Tunable h?
VB GaAs narrow gap
h
VB AlAs wide gap
Designer quantum well architecture - band gap
engineering - graded potential can be used to
enhance electron mobility in HEMTs or build a
quantum cascade laser
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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
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QUANTUM CASCADE LASER A NICE EXAMPLE OF BAND GAP
ENGINEERING BY MOCVD The white bands in the TEM
are QWs made of GaInAs, which are sandwiched
between barrier layers of AlInAs ranging in
thickness from atomic to 12 atomic layers . All
the wells are part of a quantum cascade laser.
When a voltage is applied to the device,
electrons move down the potential barrier from
narrow to wide band QWs and emit a photon between
the two thickest QWs. Then the electron moves on
to the next stage to the right where the process
repeats.
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PHYSICAL METHODS FOR MAKING THIN FILMS

• CATHODE SPUTTERING
• Bell jar equipment, 10-1 to 10-2 torr of Ar, Kr,
  Xe
• Glow discharge created, positively charged rare
  gas ions, accelerated in a high voltage to
  cathode target, high energy ions collide with
  cathode
• Sputter material from cathode, deposits on
  substrate opposite cathode to form thin film
• Multi-target sputtering also possible, creates
  composite or multi-layer films

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PHYSICAL METHODS FOR MAKING THIN FILMS

• THERMAL VACUUM EVAPORATION
• High vac bell jar gt 10-6 torr, heating e-beam,
  resistive, laser
• Gaseous material deposits on substrate, film
  nucleates and grows
• Containers must be chemically inert, W, Ta, Nb,
  Pt, BN, Al2O3, ZrO2, Graphite
• Substrates include insulators, metals, glass,
  alkali halides, silicon, sources include metals,
  alloys, semiconductors, insulators, inorganic
  salts

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CONTROL OVER THIN FILM GROWTH AT THE ATOMIC
SCALE MOLECULAR BEAM EPITAXY
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MOLECULAR BEAM EPITAXY
Structure of thin film
Vapor phase species control
Ar ion gun for cleaning substrate surface or
depth profiling
Surface analysis
Elemental sources in shuttered Knudsen cells
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MOLECULAR BEAM EPITAXY - MBE

• Million dollar thin film machine, ideal for
  preparing high quality artificial semiconductor
  quantum superlattices, ferroelectrics,
  superconductors
• Ultrahigh vacuum system gt10-12 torr, what's in
  the chamber?
• Elemental or compound sources in shutter
  controlled Knudsen effusion cells, Ar ion gun
  for cleaning substrate, surface or depth
  profiling sample using Auger analyzer, high
  energy electron diffraction for surface structure
  analysis, mass spectrometer for control and
  detection of vapor species, e-gun for heating the
  substrate

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PHOTOEPITAXYMaking atomically perfect thin
films under milder and more controlled
conditionsEt2Te Hg H2 (h?, 200oC) ? HgTe
2C2H6