Sythesis of III - V

8/12/2019 Sythesis of III - V

1/46


2/46

Growth methods

Crystal Techniques:1. CZOCHRALSKI

2. BRIDGMAN & STOCKBARGER

3. ZONE MELTING

4. VERNEUIL

5. PVD

6. CVD


3/46

Crystal Growth

How do single crystals differ from polycrystalline samples?

Single crystal specimens maintain translational symmetry overmacroscopic distances (crystal dimensions are typically 0.1mm 10 cm).

Why would one go to the effort of growing a single crystal?

-Structure determination and intrinsic property measurementsare preferably, sometimes exclusively, carried out on single

crystals.

-For certain applications, most notably those which rely onoptical and/or electronic properties (laser crystals,semiconductors, etc.), single crystals are necessary.


4/46

GROWTH OF SINGLE CRYSTALSMICRONS TO METERS

Vapor, liquid, solid phase crystallization techniques

Single crystals - meaningful materials property measurements

Single crystals allow measurement of anisotropic phenomena in

crystals with symmetry lower than cubic (isotropic)

Single crystals important for fabrication of devices, like silicon

chips, yttrium aluminum garnet solid state lasers, beta-beryllium

borate for doubling and tripling the frequency of CW or pulsedlaser light, lithium niobate optoelectronic switch for guiding

light in miniature optical circuits, quartz crystal oscillators for

ultra-sensitive nanogram mass monitors


5/46

LET'S GROW CRYSTALS

Key point to remember when learning how to be a crystalgrower (incidentally, an exceptionally rare profession andextraordinarily well paid)

Many different techniques exist, hence one must thinkvery carefully as to which method is the most appropriatefor the material under consideration

Think also about size of crystal desired, stability in air,morphology or crystal habit required, orientation, doping,defects, impurities

So let's proceed to look at some case histories.


6/46

Bridgman

growth

A method used to grow single-crystal semiconductors typically III-V, e.g.

GaAs; uses multi-zone furnace in which Ga and As are contained in the

ampule and in contact with GaAs seed; the GaAs melt is passed from

higher to lower temperature zone; conceptually similar to float-zone

(FZ) crystal growth method.

Czochralski

crystal

growth, CZ

process of obtaining single-crystal solids; the most common method for

obtaining large diameter semiconductor wafers (e.g. 300 mm Si wafers);

single crystal material is pulled out of the melt in which single-crystal

seed is immersed and then slowly withdrawn; desired conductivity type

and doping level is accomplished by adding dopants to the melt.

float-zone

crystal

growth, FZ

method used to form single crystal semiconductor substrates;

alternative to CZ crystal growth process; polycrystalline material

(typically in the form of a circular rod) is converted into single-crystal by

zone heating (zone melting) starting at the plane where single crystal

seed is contacting polycrystalline material; used to make Si wafers;results in very high purity (i.e. very high resistivity) single crystal Si; does

not allow as large Si wafers as CZ does (200 mm and 300 mm); radial

distribution of dopant in FZ wafer not as uniform as in CZ wafer; wafers

used in high-end Si microelectronics are almost uniquely CZ grown.


7/46


8/46

Pulling direction of

seed on rod

Heater

CZOCHRALSKI

Crucible

Inert atmosphereunder

pressure prevents

material loss and

unwanted reactions

Layer of molten oxide

like B2O3prevents

preferential

volatilization of onecomponent - precise

stoichiometry control

Melt just above mp

High viscosity low

vapor pressure

Growing crystal

Crystal seed

Counterclockwise

rotation of melt and

crystal being pulledfrom melt, helps

maintain uniform T,

composition and

homogeneity of crystal

growth

1. CZOCHRALSKI


9/46

CZOCHRALSKI METHOD

Interesting crystal pulling technique (but can you pronounce

and spell the name!)

Single crystal growth from the melt precursor(s)

Crystal seed of material to be grown placed in contact with

surface of melt

Temperature of melt held just above melting point, highest

viscosity, lowest vapor pressure favors crystal growth

Seed gradually pulled out of the melt (not with your hands of

course, special crystal pulling equipment is used)


10/46


11/46


12/46

CZOCHRALSKI METHOD

Seed gradually pulled out of the melt (not with your hands

of course, special crystal pulling equipment is used)

Melt solidifies on surface of seed

Melt and seed usually rotated counterclockwise with

respect to each other to maintain constant temperature

and to facilitate uniformity of the melt during crystal

growth, produces higher quality crystals, less defects

Inert atmosphere, often under pressure around growing

crystal and melt to prevent any materials loss and

undesirable reactions like oxidation, nitridation etc


13/46

GROWING BIMETALLIC SINGLE CRYSTALSLIKE GaAs

REQUIRES A MODIFICATION OF THE CZOCHRALSKI METHOD

Layer of molten inert oxide like B2O3spread on top of the moltenfeed materialto prevent preferential volatilization of the morevolatile component of the bimetal melt

Critical for maintaining precise stoichiometry, e.g., Ga1+xAs andGaAs1+xwhen made rich in Ga and As, become p- and n-doped!!!

The Czochralski crystal pulling technique is invaluable for growingmany large single crystals as a rod, to be cut into wafers and

polished for various applications like silicon, germanium, lithiumniobate

Utility of some single crystals made by Czochralski listed below


14/46

EXAMPLES OF CZOCHRALSKI GROWN SCs

SOLIDIFICATION OF STOICHIOMETRIC MELT

LiNbO3- NLO material - Perovskite - temperature dependent tetragonal-cubic-ferroelectric-paraelectric phase transition at Curie T electricalcontrol of refractive indexuse electrooptical switch

SrTiO3- Perovskite substrate

used for epitaxial growth of high Tc defectPerovskite - YBa2Cu3O7superconducting films - SQUIDS

GaAlInP- quaternary alloy semiconductor - near IR diode lasers

GaAswafersred laser diodes- photonic crystal devices

NdxY3-xAl5O12neodynium YAG - NIR solid state lasers- 1.06 microns

Si - microelectronic chips, Ge - semiconductor higher electron mobilityfaster electronics than Si


15/46

2. BRIDGMAN AND STOCKBARGER METHODS

Controlled Crystallization of a Stoichiometric Melt

STOCKBARGER fixed temperature

gradient - moving crystal

BRIDGEMAN changing temperature

gradient - static crystal

T

TDistance

Distance

Crystallization of melt on seed as

crucible gradually displaced throughtemperature gradient from hotter to

cooler end

melt crystal

Furnace gradually cooled and

crystallization begins on seed at

cooler end of crucible

Tm

TmT1T2

T3

Temperature gradient


16/46

BRIDGMAN AND

STOCKBARGER METHOD


17/46

Bridgman method - Bridgman furnace - silicon crystal growth also used for GaAs

For the production of multicrystaline solar silicon the Bridgman method melts poly silicon

in a high pressure furnace. A crucible containing the silicon mold is moved form hot to

cold in order to enable crystal growth.

The American physicist Percy Williams Bridgmanhas a great share in developing

todays high pressure furnaces and contributed to crystallography, where he devised a

method of growing single crystals. In 1946 he received the noble pricefor the invention of

an apparatus to produce extremely high pressures, and for his discoveries within the field

of high pressure physics.

The Bridgman furnace works with three temperature zones. The upper zone with

temperatures above the melting point of silicon. The lower zone with a temperature below

melting point and an adiabatic zone as a baffel between the two.

The ampoule containg silicon is raised into the upper zone until only the lower portion of

the single crystal seed remain unmelted in the lower zone. After the temperature stabilizes,the ampoule is lowered slowly into the lower zone to initiate crystal growth from the seed.

Due to a directed and controlled cooling process of the cast, zones of aligned crystal lattices

are created.

Merely 60% of the polycrystal silicon can be processed to wafers for photovoltaics. The rest

gets lost in the sawing and cutting process.
http://nobelprize.org/nobel_prizes/physics/laureates/1946/bridgman-bio.htmlhttp://en.wikipedia.org/wiki/List_of_Nobel_laureates_in_Physicshttp://en.wikipedia.org/wiki/High_pressure_physicshttp://en.wikipedia.org/wiki/High_pressure_physicshttp://en.wikipedia.org/wiki/List_of_Nobel_laureates_in_Physicshttp://nobelprize.org/nobel_prizes/physics/laureates/1946/bridgman-bio.html


18/46

BRIDGMAN AND STOCKBARGER METHODS

Stockbarger method is based on a crystal growing from the melt,

involves the relative displacement of melt and a temperature

gradient furnace, fixed gradient and a moving melt/crystal

Bridgman method is again based on crystal growth from a melt,but now a temperature gradient furnace is gradually loweredand

crystallization begins at the cooler end, fixed crystal and changing

temperature gradient

Both methods are founded on the controlled solidification of a

stoichiometric meltof the material to be crystallized in a

temperature gradient


19/46

BRIDGMAN AND STOCKBARGER METHODS

Stockbarger and Bridgman methods both involve controlledsolidification of a stoichiometric meltof the material to becrystallized in a temperature gradient

Enables orientedsolidification

Melt passes through a temperature gradient

Crystallization occurs at the cooler end

Both methods benefit from seed crystals, predeterminedorientationand controlled atmospheres


20/46

T

Distance

Crystal or powder

Localized melt region - impurities

concentrated in meltenergetic

benefit

Crystal growing from seed

Temperature profile furnce

Pulling direction

Tm

3. ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF

SOLIDS


21/46

ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF

SOLIDS

Method related to the Stockbarger technique - thermalprofile furnace employed- material contained in a boat

Only a small region of the charge is melted at any one time- initially part of the melt is in contact with the seed

Boat containing sample pulled at a controlled velocitythrough the thermal profile furnace

Zone of material melted, hence the name of the method -oriented solidification of crystal occurs on the seed -simultaneously more of the charge melts


22/46

ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF

SOLIDS

Partitioning of impurities occurs between melt and crystal

Basis of the zone refining methods for purifying solids

Impurities concentrate in melt more than the solid phase

where structure-energy constraints of crystal sites more

severe than melt-impurities swept out of crystal by moving

the liquid zone

Used forpurifying materials like W, Si, Ge, Au, Pt to ppb

level of impurities, often required for device applications


23/46

Note: VERNEUIL method for information only


24/46

O2+ powdered precursor(s)

O2+ H2

Fusion flame

Liquid drops of molten precursor(s)

Growing crystal

Support for growing crystal

4. VERNEUIL FUSION FLAME METHOD


25/46


26/46

VERNEUIL FUSION FLAME METHOD

1904 first recorded use of the method, useful for growingcrystals of extremely high melting and refractory metaloxides, examples include:

Ruby red from Cr3+/Al2O

3powder, sapphire blue from

Cr26+/Al2O3powder, luminescent host CaO powder

Starting material fine powder form, passed throughO2/H2flame or plasma torch

Melting of the powder occurs in the flame, moltenmicrodroplets fall onto the surface of a seed or growingcrystal, leads to controlled crystal growth


27/46

RUBY - CRYSTAL PRESSURE SENSOR?

[Cr(3+)] d3determines Oh monatomic Cr(3+) or diatomic Oh(Cr(3+)-O-Cr(3+))sites in Al2O3corundum lattice

t2g to eg d-d electronic transition red shifts withconcentrationincrease in d orbital DOS and narrowing ofCF splitting - red to blue color of ruby and sapphire

t26 to eg d-d transitions sensitive to Cr-O distanceincreasein pressure decreases these distances and increases CFsplitting causing blue shifts proportional to pressure

hence senses pressure - useful for in situ highpressure diamond cell materials synthesis,spectroscopic and diffraction studies


28/46

Electronically isolated Oh

Cr(3+) d3 CrO6

Electronically coupled adjacent

Oh Cr(3+) d3 O5CrOCrO5

BASICS: RUBY REDTO SAPPHIRE BLUE

ELECTRONICALLY ISOLATED TO COUPLED Cr(3+) Oh CRYSTAL SITES IN CORUNDUM

LATTICE

Cr(3+) LOWER SYMMETRY HIGHER DOS

eg

t2g


29/46

CRYSTAL GROWING METHODS

CZOCHRALSKI, BRIDGMAN, STOCKBARGER, ZONE MELTING, VERNEUIL

All methods have the advantage of rapid growth rates of large crystalsrequired for many advanced device applications

BUTthe CRYSTAL QUALITYobtained by all of these techniques must bechecked for inhomogeneities in surface and bulk composition andstructure, gradients, domains, impurities, point-line-planar defects,twins, grain boundaries

THINKhow you might go about checking this if you were confrontedwith a 12"x12"x12" crystal - useful methods for small crystals include:confocal optical microscope, polarization optical microscopebirefringence, Raman microscope, spatially resolved OM, XRD, TEM, ED,EDX, AFMwhat does one use for large ones?


30/46


31/46

Chemical vapor deposition (CVD):

reaction mechanisms Mass transport of the reactant

in the bulk Gas-phase reactions

(homogeneous)

Mass transport to the surface

Adsorption on the surface

Surface reactions(heterogeneous)

Surface migration

Incorporation of film

constituents, island formation

Desorption of by-products

Mass transport of by-products

in bulk

CVD: Diffusive-convective transport of

depositing species to a substrate with

many intermolecular collisions-driven by a

concentration gradient

SiH4SiH4

Si


32/46

Chemical Vapor Deposition (CVD) : Overview

CVD (thermal)

APCVD (atmospheric)

LPCVD (


33/46

PLASMA ASSISTED CVD

Plasma-enhanced chemical vapor deposition (PECVD)is a process used to deposit thin films from a gas state(vapor) to a solid state on a substrate.

Chemical reactions are involved in the process, whichoccur after creation of a plasma of the reacting gases.

The plasma is generally created by RF (AC) frequency or

DC discharge between two electrodes, the spacebetween which is filled with the reacting gases.


34/46

PLASMA ASSISTED CVD


35/46

Chemical Vapor Transport

-A polycrystalline sample, A, and a transporting species, B, are

sealed together inside a tube.

-Upon heating the transporting species reacts with the sample toproduce a gaseous species AB.

-When AB reaches the other end, which is held at a different

temperature, it decomposes and redeposits A.

If formation of AB is endothermic crystals are grown in the cold

end of the tube.

A (powder) + B (g)AB (g) (hot end)

AB (g)A (crystal) + B (g) (cold end)

If formation of AB is exothermic, crystals are grown in the hot end

of the tube.

A (powder) + B (g)AB (g) (cold end)

AB (g)A (crystal) + B (g) (hot end)


36/46

Typical transporting agents include:

I2, Br2, Cl2, HCl, NH4Cl, H2, H2O, AlCl3, CO

Temperature gradient is typically created andcontrolled using a two-zone furnace.

Tubes are usually SiO2, unless reactive, in which case

metal tubes (Pt, Au, Nb, Ta, W) are used.

Examples :

Growth of Fe3O4 crystals

Fe3O4 (s) + 8HCl (g)FeCl2(g) + FeCl3 (g) + 4H2O (g)(Endothermic)

Growth of ZrNCl crystals


37/46

Growth of ZrNCl crystals

ZrNCl (s) + 3HCl (g)ZrCl4 (g) + NH3 (g)(Exothermic)

Growth of Ca2SnO4crystalsSnO2(s) + COSnO (g) + CO2(g)

SnO (g) + CO2(g) + 2CaO (s)Ca2SnO4(s) + CO (g)

Chemical Vapor Transport is a good method ofgrowing high quality crystals from powders. However,growth rates are usually quite slow (mg/hr) whichmakes this approach more attractive for research

than for industrial applications.


38/46


39/46

PHYSICAL VAPOUR DEPOSITION

Physical vapor deposition (PVD) is a variety of vacuum

deposition and is a general term used to describe any of avariety of methods to deposit thin films by thecondensation of a vaporized form of the material ontovarious surfaces (e.g., onto semiconductor wafers).

The coating method involves purely physical processes suchas high temperature vacuum evaporation or plasma sputterbombardment rather than involving a chemical reaction atthe surface to be coated as in chemical vapor deposition.

The term physical vapor deposition appears originally in the1966 book Vapor Depositionby CF Powell, JH Oxley and JMBlocher Jr, but Michael Faraday was using PVD to depositcoatings as far back as 1838.



40/46


Variants of PVD include, in order of increasing novelty:

Cathodic Arc Deposition: In which a high power arc discharged at the targetmaterial blasts away some into highly ionized vapor.

Electron beam physical vapor deposition: In which the material to bedeposited is heated to a high vapor pressure by electron bombardment in"high" vacuum.

Evaporative deposition: In which the material to be deposited is heated to ahigh vapor pressure by electrically resistive heating in "low" vacuum.

Pulsed laser deposition: In which a high power laser ablates material from thetarget into a vapor.

Sputter deposition: In which a glow plasma discharge (usually localized aroundthe "target" by a magnet) bombards the material sputtering some away as avapor.

PVD


41/46

PVD PVD is used in the manufacture of items including semiconductor devices,

aluminized PET film for balloons and snack bags, and coated cutting tools for

metalworking. Besides PVD tools for fabrication special smaller tools mainly for

scientific purposes have been developed. They mainly serve the purpose ofextreme thin films like atomic layers and are used mostly for small substrates. A

good example are mini e-beam evaporators which can deposit monolayers of

virtually all materials with melting points up to 3500C.

Some of the techniques used to measure the physical properties of PVD coatings

are:

Calotester: coating thickness test

Nanoindentation: hardness test for thin-film coatings

Pin on disc tester: wear and friction coefficient test

Scratch tester: coating adhesion test


42/46

Cathodic Arc Evaporation

The cathodic arc evaporator ispreferably used for vaporization ofthin layers because of its simpleconstruction and operation.

In this case the vapor arises in the rootof an arc which is burning (afterignition with a special ignition facility)within the vapor of the layer material.


43/46

Cathodic Vacuum Arc Evaporation

Substrate

Anode

Cathode (Target)

Cooling water

S i


44/46

Sputtering

Sputtering is a process whereby atoms areejected from a solid target material due to

bombardment of the target by energetic

particles. It is commonly used for thin-filmdeposition, etching and analytical

techniques.


45/46

Sputtering Schematic Diagram


46/46

http://www2.fkf.mpg.de/crystal/

Documents

Sythesis of III - V