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K.N.T University of technology Electronic Department

Crystal Growth

Instructor: Prof. F. Hossein-Babaei

Presented by:M. H. Jalalpour

P. Talebnia

Fall 2014

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Outline

• History• Examples• Crystal Growth Theories• Crystal Growth Classes

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ZnSe

img.directindustry.comQuartz

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Example single crystals

Semiconductors:• Electronic grade Ge and Si, doped and undoped. • II-VI semiconductors (CdTe, CdS, CdSe, ZnS, ZnSe, ZnTe, etc.) • III-V semiconductors (GaAs, GaP, GaSb, InAs, InP, InSb, etc.) • IV-VI semiconductors (PbS, PbSe, PbTe, SnTe, etc.)

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•Optical crystals: AgBr, AgCl, BaF2, CaF2, CdTe, CsI, Ge, KBr, KCl, KRS-5, LiF, MgF2, NaCl, sapphire, ruby, Si, ZnSe.

•Art crystals and jewelry : Amethyst

Amethyst crystal

A selection of both rough and cut Kashmir sapphires.

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Crystal growth classes

Growth from solid

Growth from vapor

Growth from solution

Growth from melt

Bridgman method

Czochralski method

Floating zone

Verneuil method

Solution

Vapor

Al from solid

Si ingot from melt

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Crystal growth classes

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History

• Industrial crystal production started with Verneuil 1902 who with the flame-fusion growth process named after him, for the first time achieved growth of single ruby and sapphire crystals with melting points above 2000oC.

• Czochralski process is named after Polish scientist Jan Czochralski, who invented the method in 1916.

• Bridgman, in 1940’s, used temperature gradients to grow single crystals by directional solidification.

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Growth from melt

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Growth from melt

Bridgman method

Czochralski method

Floating Zone method

Verneuil method

Heating methods

RF heating

Hydrogen torch

Resistance heating

Optical heating

Chamber pressure

High pressure

Atmospheric pressure

Vacuum

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The method involves heating polycrystalline material in a container above its melting point and slowly cooling it from one end where a seed crystal is located. Single crystal material is progressively formed along the length of the container. The process can be carried out in a horizontal or vertical geometry.

Bridgman Method

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Vertical Bridgman Method

Temperature ProfileVertical Bridgman Tube Furnace

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Vertical Bridgman Method

Obtaining cadmium zinc telluride (CZT) crystals Obtaining zinc selenide (ZnSe) crystals

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Horizontal Bridgman-Stockbarger Method

Schematics of the furnace and crucible used for GaAs growth.

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Tem

pera

ture

ºC

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The growth from a single crystal seed

• With the necking technique shown, the grown crystal is in contact with a single grain of the polycrystalline material used as the seed.

• The growth will follow the structure of the grain that is in contact with.

• The selected crystal orientation is almost random.

Mel

tPo

lycr

ysta

lSi

ngle

cry

stal

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Horizontal Bridgman

http://www.krist.uni-freiburg.de/Forschung/Einrichtungen/einrichtungen_en.php

An example of industrial horizontal Bridgman system

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Photograph of a Cd(1-x)MnxTe single crystal ingot 30 mm in diameter and 120 mm in length

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Czochralski method

• By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single-crystal, cylindrical ingot from the melt.

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https://www.google.com/#q=Czochralski%2Bimage

• A highly developed method of crystal growth, widely used in semiconductor industry.

• High-purity, semiconductor-grade silicon (only with a few ppm of impurities) is melted down in a crucible, which is usually made of SiO2-lined graphite. A seed crystal, mounted on a rod, is dipped into the molten silicon. The seed crystal is pulled upwards and rotated at the same time.

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Czochralski method

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In order to prevent the oxidation of the melt and the crystal, the whole process must be performed in oxygen- and moisture-free atmosphere.

While the largest silicon ingots produced today are 400 mm in diameter and 1 to 2 meters in length, 200 mm and 300 mm diameter crystals are standard in industrial processes.

The pulling rate (usually a few mm/min) and the temperature profile determine the crystal diameter.

Czochralski Method

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Czochralski (CZ) crystal growth

1. Polysilicon charge in silica crucible placed in a graphite holder.

3. Shoulder growth, after neck is complete.

5. Body growth.

2. Start of necking. Seed is dipped to > 1415 °C melt.

4. Start of body.

6. Conical tail growth after completion of body.

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Crystal growth

• The raw material contains only < 1 ppb impurities. Pulled crystals contain oxygen≈ 1018 cm-3, and C ≈ 1016 cm-3, plus any added dopants.

Seed

Single Crystal Silicon

Quartz Crucible

Water Cooled Chamber

Heat Shield

Carbon Heater

Graphite Crucible

Crucible Support

Spill Tray

Electrode

• Essentially all Si wafers used for ICs today come from Czochralski grown crystals.

• Polysilicon material is melted, held at close to 1417 ˚C, and a single crystal seed is used to start the growth.

• Pull rate, melt temperature and rotation rate are all important control parameters.

→ Introduces O ≈1017-1018cm-3

Ar +H2 atmosphere

→ C ≈ 1015-1016 cm-3

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Crucible dissolving =>

Introduce noble gas plus hydrogen Seed dislocation

Seed/melt contact shock

Solution : Necking

Temperature and density gradients => convection

Solution : Seed rotation and magnetic Field

Dopant segregation coefficient

Solution : Growth Rate Control

Generating gas

Using graphite crucible

Generating gas; introducing impurity

=> in homogeneity

=> Crystal dislocation

Crystal growth some problems and solutions

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Czochralski method

An example industrial Czochralski growth system.

http://www.latticematerials.com/products/custom/seeds/FHB

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Czochralski method

Output of a Czochrolski system; single crystal Si ingot. http://cnfolio.com/ELMnotes15FHB

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Floating zone method

The basic idea in float zone (FZ) crystal growth is to move a liquid zone through the material. If properly seeded, a single crystal may result in.

The melt never comes into contact with anything but the inert atmosphere of the furnace.

The maximum diameter of the FZ-grown crystals is about 20 mm.

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http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_6/advanced/t6_1_3.html

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Floating zone method

RF heating Optical heating

Advantages:• No crucible; no impurity is introduced; can produce high resistivity Si;

http://www.lanl.gov/mst/crystal/floatzone.html

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Optical floating zone method

Optical heating of the zone. Photograph of an optical FZ growth system.www.mrsec.umd.edupeople.seas.harvard.eduFHB

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Lab floating zone puller with resistance heater(left part = transformer / right part = control system)

Floating zone method

http://www.surfacenet.de/html/floating_zone_pullers.htmlFHB

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Verneuil Method

A sketch of an early furnace used by Verneuil to synthesise rubies using the Verneuil process

http://en.wikipedia.org/wiki/Verneuil_process

• Verneuil process, also called flame fusion, was the first commercially successful method of manufacturing synthetic gemstones developed in 1902 by the French chemist Auguste Verneuil.

• For higher crystal quality, the produced ingots are annealed for hours at elevated temperatures close to the melting point.

Auguste Verneui

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Verneuil Method

O2 + Al2O3 + Cr2O3 inlet

O2 + H2 mix and ignite, T > 2000K

Molten drops fall onto “pedestal”

Xtal forms and growsExample of Al2O3 xtal (right end)

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http://www.allaboutgemstones.com/gemstones_ruby_enhancements.html

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Verneuil Method

• It is primarily used to produce ruby and sapphire varieties of corundum, as well as the diamond simulants rutile and strontium titanate. The method is still the least expensive way to make sapphire and ruby adequate for many applications.

• In principle, the process involves melting powdered substance using an oxyhydrogen flame. The melt droplets crystallize on a single crystal seed which grows to produce the crystal ingot. The process is considered to be the founding step of the modern industrial crystal growth technology. The technique remains in wide use to this day.

• A crucial factor in growth of good quality artificial gemstones is using highly pure starting material, with at least 99.9995% purity. The presence of sodium impurities is especially undesirable, as it makes the crystal opaque.

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Verneuil Method The starting material (alumina) is finely powdered and placed in a container above the Verneuil furnace.

Oxygen is supplied into the furnace, which carries the powder particles down a narrow tube.

Combustion occurs at the point where the narrow tube opens into a larger one. The flame, at least 2000 °C hot at its core, melts the powder into small droplets, which fall onto an earthen support rod placed underneath.

The seed crystal eventually forms. As more droplets fall onto the seed tip, a single crystal, called a boule, starts to form.

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Ruby production line according to the Verneuil technique in the Chemiekombinat Bitterfeld (around 1970)

Verneuil Method

Industrial Verneuil MethodFHB

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Kyropoulos method

Kyropoulos method (1926) is somewhat similar to Czochralski technique.It also starts from the contact of a seed crystal mounted in the holder with a molten alumina.However the crystallization front in a crystal pulled by Czochralski method is a meniscus located in the column connecting the surface of the molten alumina with the growing crystal.In Kyropoulos technique the growing crystal is surrounded by the melt. The crystal growth occurs within the crucible with the molten alumina.The crystal grows until its surface reaches the crucible walls. Then the crystal is pulled out (lifted) and the growing cycle repeats. There is also a version of the process with continuous pulling the crystal from the crucible.The diameter of the crystal grown by Kyropoulos method is limited by the diameter of the crucible.

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The main advantage of Kyropoulos method is the crystallization at low temperature gradients – below 10ºC/cm , which results in low thermal stresses in the crystal.

High optical quality large crystals (boules) with diameter exceeding 350 mm may be produced by Kyropoulos method.

Kyropoulos method is widely used for growing sapphire crystals of a very high optical quality. Kyropulos sapphire is suitable for manufacturing ingots and substrates for LED and RF applications.

Kyropoulos method

http://rmdinc.com/2012/07/page/6/

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•The gas coats the crystal, inhibiting growth. The ordered crystal phase has grown around a gas bubble impinging on its surface. •Slow

Examples: •Snowflakes

Crystal Growth from Gas Phase

http://www.scientificamerican.com

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Crystal Growth from Solution

1. Dissolving the material in the solvent→ saturation2. Cooling saturated solution slowly → Crystal growth

Methods:• Growth from Aqueous Solution: Solvent is water. Examples NaCl, Rock candy• Growth from Non-aqueous solution: Solvent is not water, e.g. alcohol• Flux Growth: Solvent is solid. First, solvent is melted. Then, material is dissolved.

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Advantages:-Grow congruently and incongruently melting materials- Need relatively simple equipment- Has short growth-time scales-Need small amounts of materials

Disadvantages:- Results not too large a crystal (mm to cm)

Crystal Growth from Solution

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1. Lecture notes on “semiconductor device fabrication technology” by Prof. F. Hossein-Babaei, 2013

2.Microchip Fabrication, peter van zant3.crystal Growth Technology ,Hans.J.Scheel4. http://www.memsnet.org/mems/processes/deposition.html5. Single crystal growth employing Czochralski method, adam pikul6. http://en.wikipedia.org/wiki/Verneuil_process7. www.mrsec.umd.edu8. http://www.cradley-crystals.com/CCinit.php?id=technologyam_49. www.substech.com10. http://en.wikipedia.org/wiki/Verneuil_process11. Flux Method for Preparing Crystals: Athena S. Sefat12. http://commons.wikimedia.org/wiki/File:SnowflakesWilsonBentley.jpg 13. http://www.scientificamerican.com 14. http://rmdinc.com/2012/07/page/6/ 15.http://www.allaboutgemstones.com/gemstones_ruby_enhancements.html

Sources

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Thank you for your attention

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Crystal Growth Theories

Surface Energy Theory

Surface Nucleation

Theory

Diffusion Theory

J. Willard Gibbs

Pierre Curie

Arthur Amos Noyes Max Volmer Ivan StranskiFHB

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Crystal growth

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