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 ID: 153392
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Slide1
Crystals and Crystal GrowingSlide2
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?Slide3
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: LiNbO
3
, BaTiO
3
, BaNaNiO
3
Piezoelectric microphone sources: quartz
4. Radiation detectors: HgI
2
,
NaI:Tl
,
CsI:Tl
,
LiI:Eu
, Si,
Ge
, III-V, II-VI,
PbSSlide4
5. Micromechanical devices: Si Utah Neural Array (SEM image)
6. Research: everything. Why?
7. Artificial gems: sapphire, ruby, TiO
2
, ZrO
2
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.Slide5Slide6
Superconducting Ceramic Single Crystals
Aps.orgSlide7
Bulk Crystal Growth TechniquesSlide8
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)
CuInSe
2
, MCT, CdTe, ZnSe, GaSe Oxides-insulators: sapphire
Reasonable
K
eff
Seeded: predetermined orientation
Crucible can be a problem
Czochralski
("
Cz
")
Windows
: sapphire
Scintillators
: BGO, CdWO
4
NLO materials: LiNbO
3
, CsLiB
6
O
10
Alkali
scintillators-CsI:Tl
Halides
: windows-wide transmitting filters
Always the technique of choice
Dopants
can be volatile
ContaminationSlide9
Technique
Examples
Advantages
Disadvantages
Vapor
Physical
vapor transport
(evaporation
& condensation)
HgI
2
,
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 controlChemical vapor deposition(open flow)Refractories: SiC, PBNSemiconductor epitaxy!Chemical vapor transport(closed system)TiO2, EuS, "halogen lamps" SnO2, In2S3Very slow; batch processSolution/FluxADP, KDP, RefractoriesHydrothermal quartzDiamond: 1450 C, 742 kpsi, Ni fluxProteins, Minerals, Mo2CHigh Tc superconductors; BiSrCaCuOMorton’s tablesaltLarge sizes possiblePotentially low cost, large scaleReduced temperatureless container contaminationCan be inexpensiveVery slowTemperature control very importantKeff often very smalldoping difficultSlide10
Digression on Segregation and Purification
Electronic materials are only interesting when doped
Carrier type: “n”
Dopant
: “P”
“Res”:
“1-20 ohms”Slide11
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
μ
n
= 1700 cm
2
/V-sec
n
P
= 3.7x1014/cm3nSi = 2.33 gm/cm3) x(6.02x1023 atoms/mole) ÷(28.068 gm/mole) = 4.997x1022 atoms per molenP / nSi = 7x10-9 = 7 ppb!Background impurity level must be small on this scale!Slide12
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!Slide13
Origin of Segregation: Binary Phase Diagram
W. G.
Pfann
, Zone MeltingSlide14
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 MeltingSlide15
Impurity
Distribution after Normal Freezing
W. G.
Pfann
, Zone MeltingSlide16
Concept of Zone Refining
Molten zone of length l is passed through ingot of length L
Also the process used to make “float zone silicon”
W. G.
Pfann
, Zone MeltingSlide17
Impurity Distribution after Single Pass of Zone
(Less efficient than
normal freezing)
W. G.
Pfann
, Zone MeltingSlide18
Impurity Distribution from Multi-pass Zone Refining
n.b
.: k = 0.9524, l/L = 0.01
W. G.
Pfann
, Zone MeltingSlide19
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 purificationSlide20
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 HSiCl
3
→ 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.Slide21
Czochralski
Growth
Synthesis may or may not be part of growth
GaAs
may
be pre-synthesized or a
pre- measured
quantity
As may be bubbled through Ga metalLi H synthesized from Li and H2 (or D2) Typical sizes: Si - 12" φ, 200 kg charge; GaAs - 4" φWe have grown from a 2 g melt of isotopically pure K13C15N Typical growth rates: cm/hrwww.people.seas.harvard.eduSlide22
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