Transcript CHM 434F/1206F SOLID STATE MATERIALS CHEMISTRY

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
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
• 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
MAKING NANOCRYSTALLINE SILICON
LUMINESCENT: CAPPING
*(SiH)
CB
CB
VB
VB
capping Si cluster dangling
bond with H, F, O forms
bonding-antibonding orbitals, moves killer trap
states out of the gap
facilitates radiative
relaxation
(SiH)
Sin
HxSin
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
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-H:Si or nc-H:Si
• SiH4 + PH3 (RF)  n-Si
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
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 Si
Poly-domain texture
Useful for solar cell large
area devices
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
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 quantization 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, tailorable composition materials, do it
yourself quantum mechanics materials for the semiconductor
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-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 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
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
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
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
MOCVD surface chemistry of precursors,
nucleation and growth of product film
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
Schematic of cold wall MOCVD system
Single crystal substrate on inductively heated or resistively
heated susceptor
MOCVD deposited film
H2/AsH3/PH3
Water cooling
H2/InMe3/GaMe3
Thermocouple
H2/PEt3
Waste gases
H2/n-dope H2S/p-dope ZnMe2
REQUIREMENTS OF MOCVD PRECURSORS
• 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 decomposition
• HOMO : HETERO rates ~ 1 : 1000
• Heterogeneous reaction on substrate
• Greater than on other hot surfaces in reactor
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
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 thicknesses
• 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
• 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
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
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
BAND GAP
ENGINEERING
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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
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
potential can be used to enhance electron mobility in 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
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.
Cathode material to be
sputtered single or multi-target
+
Power supply
Inert gas inlet
Ar
M
M M
Substrate (anode)
Vacuum
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
PHYSICAL METHODS FOR MAKING THIN FILMS
• THERMAL VACUUM EVAPORATION
• High vac bell jar > 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
CONTROL OVER THIN FILM GROWTH AT THE
ATOMIC SCALE: MOLECULAR BEAM EPITAXY
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
MOLECULAR BEAM EPITAXY - MBE
• Million dollar thin film machine, ideal for preparing
high quality artificial semiconductor quantum
superlattices, ferroelectrics, superconductors
• Ultrahigh vacuum system >10-12 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
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