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 CrystalsFor 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 non-
single 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
FASTLarge sizes possible
Often energy intensivesome 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: sapphireScintillators: BGO, CdWO4
NLO materials: LiNbO3, CsLiB6O10
Alkali scintillators-CsI:TlHalides: windows-wide transmitting filters
Always the technique of choice Dopants can be volatile Contamination
Technique Examples Advantages DisadvantagesVapor
Physical vapor transport(evaporation & condensation)
HgI2, CdS, ZnS, NH4X Hg2Cl2, CdSMolecular Organics!
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 slowDifficult to control
Chemical vapor deposition(open flow)
Refractories: SiC, PBNSemiconductor epitaxy!
Chemical vapor transport(closed system)
TiO2, EuS, "halogen lamps" SnO2, In2S3
Very slow; batch process
Solution/Flux
ADP, KDP, RefractoriesHydrothermal quartzDiamond: 1450 C, 742 kpsi, Ni fluxProteins, Minerals, Mo2CHigh Tc superconductors; BiSrCaCuOMorton’s tablesalt
Large sizes possiblePotentially low cost, large scaleReduced temperatureless container contaminationCan be inexpensive
Very slowTemperature control very important
Keff often very smalldoping 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 1-20
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
Molten zone of length l is passed through ingot of length LAlso the process used to make “float zone silicon”
W. G. Pfann, Zone Melting
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 GrowthSynthesis may or may not be part of
growth
GaAs
may be pre-
synthesized or a pre-
measured 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 TechniqueMelting 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