Transcript Bulk Crystal Growth

Crystals and Crystal Growing
Why Single Crystals
• What is a single crystal?
• Single crystals cost a lot of money.
• When and why is the cost justified?
– Current semiconductor devices on an IC have
characteristic dimensions of ¼ micron.
– What happens if grain size is on the scale of microns?
– What makes optical materials look translucent?
– What happens when a “weapons grade laser beam”
hits an inhomogeneity in an optical component?
Applications of Single Crystals
For what applications are single crystals necessary?
1. Semiconductor optoelectronics (substrate materials)
Transistors, diodes, integrated circuits: Si, Ge, GaAs, InP
LEDs and lasers: GaAs, GaInAs, GaInP, GaAsP, GaP:N, ruby
Solar cells: Si, GaAs, GaInP/GaAs tandems
Microwave sources: GaAs
2. Non-glass optics (see previous lecture for transmission ranges): alkali
halides, alkaline earth halides, thallium halides, Ge, sapphire
3. Electromechanical transducers
Ultrasonic generators, sonar: ADP, KDP
Strain gauges: Si
Optical modulators: LiNbO3, BaTiO3, BaNaNiO3
Piezoelectric microphone sources: quartz
4. Radiation detectors: HgI2, NaI:Tl, CsI:Tl, LiI:Eu, Si, Ge, III-V, II-VI, PbS
5. Micromechanical devices: Si Utah Neural Array (SEM image)
6. Research: everything. Why?
7. Artificial gems: sapphire, ruby, TiO2, ZrO2
Why are they necessary for those applications? (Numbers correspond)
1. Electrical homogeneity on the length scale of the device; minimum
carrier scattering
2. Optical homogeneity on the length scale of the light being transmitted;
minimum light scattering
3. Mechanical strength and homogeneity; availability of processing
technology: nickel-based super alloy turbine blades
4. Purity; well-defined material
In all cases: optical, electronic or mechanical properties superior to nonsingle crystal competition.
Superconducting Ceramic Single Crystals
Aps.org
Bulk Crystal Growth Techniques
Technique
Examples
Advantages
Disadvantages
Melt:
”Directional
Solidification”
Elemental & Compound
semiconductors: Si, Ge,
GaAs, InP
FAST
Large sizes possible
Often energy intensive
some materials decompose
before melting
Bridgman
(horizontal/vertical)
CuInSe2, MCT, CdTe, ZnSe,
GaSe Oxides-insulators:
sapphire
Reasonable Keff
Seeded: predetermined
orientation
Crucible can be a problem
Czochralski ("Cz")
Windows: sapphire
Scintillators: BGO, CdWO4
NLO materials: LiNbO3,
CsLiB6O10
Alkali scintillators-CsI:Tl
Halides: windows-wide
transmitting filters
Always the technique of choice
Dopants can be volatile
Contamination
Technique
Vapor
Examples
Physical vapor
transport
(evaporation &
condensation)
HgI2, CdS, ZnS, NH4X
Hg2Cl2, CdS
Molecular Organics!
Chemical vapor
deposition
(open flow)
Refractories: SiC, PBN
Semiconductor epitaxy!
Chemical vapor
transport
(closed system)
TiO2, EuS, "halogen lamps"
SnO2, In2S3
Advantages
Disadvantages
Can be used with materials that
decompose or have excessive
vapor pressure at melting point or
with destructive phase transitions
or extremely high melting points
or which react with containers.
Materials must have
reasonable vapor pressure at
temperature where surface
kinetics is adequate
Typically slow
Difficult to control
Very slow; batch process
Solution/Flux
ADP, KDP, Refractories
Hydrothermal quartz
Diamond: 1450 C, 742 kpsi,
Ni flux
Proteins, Minerals, Mo2C
High Tc superconductors;
BiSrCaCuO
Morton’s tablesalt
Large sizes possible
Potentially low cost, large scale
Reduced temperature
less container contamination
Can be inexpensive
Very slow
Temperature control very
important
Keff often very small
doping difficult
Digression on Segregation and Purification
• Electronic materials are only interesting when
doped
• Carrier type: “n”
• Dopant: “P”
• “Res”:
“1-20 ohms”
Typical Numbers
• On previous label, ρ = 1-20 Ohm (presumably 120 Ω-cm)
• As you know: σ = 1/ρ = ne μ
• For silicon at 10 Ω-cm with μn = 1700 cm2/V-sec
• nP = 3.7x1014/cm3
• nSi = 2.33 gm/cm3) x(6.02x1023 atoms/mole)
÷(28.068 gm/mole) = 4.997x1022 atoms per mole
• nP / nSi = 7x10-9 = 7 ppb!
• Background impurity level must be small on this
scale!
Segregation
• Coefficient can be greater or less than unity
• Nutrient volume is finite
– Causes major problems with dopant uniformity
– Can be resolved by adding dopant to melts during
growth
• Only works for K>1!
Origin of Segregation: Binary Phase Diagram
W. G. Pfann, Zone Melting
Using Segregation for Purification:
“Normal Freezing”
n.b.: exactly the same process is used
to grow large single crystals “from the melt”!
W. G. Pfann, Zone Melting
Impurity
Distribution after
Normal Freezing
W. G. Pfann, Zone Melting
Concept of Zone Refining
W. G. Pfann, Zone Melting
Molten zone of length l is passed through ingot of
length L
Also the process used to make “float zone silicon”
Impurity
Distribution
after Single
Pass of
Zone
(Less efficient than
normal freezing)
W. G. Pfann, Zone Melting
Impurity
Distribution
from Multi-pass
Zone Refining
n.b.: k = 0.9524,
l/L = 0.01
W. G. Pfann, Zone Melting
Take Away Lessons
• Segregation of impurities/dopants is a fact
that you must deal with as an aspect of
materials preparation
• Segregation can be used as part of an elegant
purification process
• Zone refining can be very effective for
materials purification
Current Purification of Silicon
(Wikipedia)
• Siemens process: high-purity silicon rods are
exposed to trichlorosilane at 1150 °C. The
trichlorosilane gas decomposes and deposits
additional silicon onto the rods, enlarging
them:
• 2 HSiCl3 → Si + 2 HCl + SiCl4
• Silicon produced from this and similar
processes is called polycrystalline silicon.
Polycrystalline silicon typically has impurity
levels of less than 10−9.
Czochralski
Growth
Synthesis may or may not be part of
growth
GaAs
may be pre-synthesized or a premeasured quantity
As may be bubbled through Ga
metal
Li H
synthesized from Li and H2 (or D2)
Typical sizes: Si - 12" φ, 200 kg
charge; GaAs - 4" φ
We have grown from a 2 g melt of
isotopically pure K13C15N
Typical growth rates: cm/hr
www.people.seas.harvard.edu
Vertical Bridgman Technique
Melting point isotherm is directionally
translated through an ingot from a spatially
confined region.
Typically unseeded  no seed necessary
Can be seeded: quality as high as
Czochralski
High yield: all starting material is recovered
as single crystal
Diameters to 22 inches; 40 cm2 square KDP
Used extensively for alkali halide
scintillators, transducers and windows