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