Bulk Crystal Growth

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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: 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, PbS

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 nonsingle crystal competition.

Superconducting Ceramic Single Crystals

Aps.org

Bulk Crystal Growth Techniques

Technique

Melt:

”Directional

Solidification”

Examples

Elemental & Compound semiconductors: Si, Ge,

GaAs, InP

Advantages

FAST

Large sizes possible

Disadvantages

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

Contamination

Vapor

Technique Examples Advantages Disadvantages

Physical vapor transport

(evaporation & condensation)

HgI

Hg

2

2

, CdS, ZnS, NH

Cl

2

, CdS

4

X

Molecular 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 slow

Difficult to control

Chemical vapor deposition

(open flow)

Chemical vapor transport

(closed system)

Solution/Flux

Refractories: SiC, PBN

Semiconductor epitaxy!

TiO

2

, EuS, "halogen lamps"

SnO

2

, In

2

S

3

Very slow; batch process

ADP, KDP, Refractories

Hydrothermal quartz

Diamond: 1450 C, 742 kpsi,

Ni flux

Proteins, Minerals, Mo

2

C

High T c 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

K eff 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 1-

20 Ω-cm)

• As you know: σ = 1/ρ = ne μ

• For silicon at 10 Ω-cm with μ

= 3.7x10

14 /cm 3 n

= 1700 cm 2 /V-sec

• n n

P

Si

= 2.33 gm/cm 3 ) x(6.02x10

23 atoms/mole)

÷(28.068 gm/mole) = 4.997x10

22 atoms per mole

• n

P

/ n

Si

= 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 HSiCl

3

→ Si + 2 HCl + SiCl

4

• Silicon produced from this and similar processes is called polycrystalline silicon .

Polycrystalline silicon typically has impurity levels of less than 10 −9 .

Czochralski

Growth

www.people.seas.harvard.edu

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 H

2

(or D

2

)

Typical sizes: Si 12" φ, 200 kg charge; GaAs 4" φ

We have grown from a 2 g melt of isotopically pure K 13 C 15 N

Typical growth rates: cm/hr

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 cm 2 square KDP

Used extensively for alkali halide scintillators, transducers and windows

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