sublimation growth of wide band-gap materials (sic …bickermann/public/isscg-2012-slides.pdf ·...

Leibniz Institute for Crystal Growth (IKZ) Berlin

Sublimation Growth of Wide Band-Gap Materials (SiC and AlN)

Matthias BickermannLeibniz Institute for Crystal Growth (IKZ), Berlin, GERMANY

Lecture for the International Summer School in Crystal Growth (ISSCG) 2012in Brasov, Romania


Outline

Definitions: – Growth from Vapour– Sublimation

Materials: – Silicon Carbide (SiC)– Aluminium Nitride (AlN)

Technique: – Equipment– Materials– Processes– Seeding

Results: – Crystal Habit– Impurities– Structural Defects

Status and outlook

Sublimation Growth of Wide Band-Gap

Materials (SiC and AlN)Matthias Bickermann

Source: www.hexatechinc.com


Growth from vapour

Species are transported as vapour to the nucleation area.(The source consists of gases, liquids and/or solid material.)

PhysicalVapour

ChemicalVapour

Sublimationand/or

Evaporation

– Sputtering– Pulsed laser

deposition (PLD)– and others

Reversible reactionwith transport agent(iodine, chlorine, …)

Transport speciesdecompose at theinterface into growth species and waste


What is Sublimation

Sublimation is the process of transformation directly from the solid phase to the gaseous phase. Sublimation is an endothermic phase transition that occurs at temperatures and partial pressures below a substance's triple point in its phase diagram.

Source: Wikipedia (english)


How Does Sublimation Work

Answer: Cold air at T < 0°C is quite dry.Its relative humidity (water partial pressure) is below the triple point of water/ice.Therefore, on cold sunny days with somewind, wet clothes may actually dry faster ifyou hang them outside than in your house.

Question1:Do clothes dry outside even in winter frost?

A case study in three questions.



Question 2:What happens when you put this in your fridge?

Answer: The lid cooles faster than thepot as it has less thermal mass.This establishes a temperature gradientbetween pot and lid.As a result, water evaporates from thepot to the lid and condenses there.



Vapor pressures of water and ice: B.J. Mason, The Physics of Clouds(Clarendon Press, 1971) [from http://www.its.caltech.edu/~atomic/snowcrystals/ice/ice.htm]

The vapour pressure p(T) over a liquid or solid increases roughly exponentially with temperature T.

Question 3: If we apply a constant temperature gradient to a container and place the same solid material at the hot end and the cold end, what will happen?

ΔTT2

(colder)T1(hotter)

Answer: As T1 > T2, also p(T1) > p(T2) ,inducing a net species (mass) transportfrom the hotter to the colder end.


Sublimation Growth Temperature

Material transport depends on– thermal gradient between source and seed ΔT = (T1 – T2)– process temperature (T1)– total pressure (inert gas)– growth cell design (flow pattern, advection?)– growth rate (Stefan flow)Surface diffusion depends on– interface temperature (T2)

Growth Temperature should be chosen to balance the rate of material transporttowards the interface and the rate of species diffusion on the growing interface.

The preferred growth temperature in sublimation growth is at the discretion of the grower.In high-temperature growth, reasonablegrowth windows are mostly limited byexternal constraints.


Introducing AlN and SiC

The substrate itself posesses no functionality for the device!But semiconductor devices need the substrate for:– mechanical basis– heat dissipation– structural basis for epitaxy

We'll examine sublimation growth on two crystals used as semiconductor substrates.

Silicon Carbide

SiCfor High-Power and

High-FrequencyElectronics

GTO

ThyristorGTO

IGBT

IGBT

MOSFET

MOSFET

HBT

GaAsSiGeMESFET

GaN SiCMESFET

10 Hz 100 1 kHz 10 100 1MHz 10 100 1GHz 10 100

Si

SiC

Frequency

1 GW

100

10

1 MW

100

10

1 kW

100

Pow

erSource: Infineon AG


Introducing AlN and SiC

The substrate itself posesses no functionality for the device!But semiconductor devices need the substrate for:– mechanical basis– heat dissipation– structural basis for epitaxy

We'll examine sublimation growth on two crystals used as semiconductor substrates.

Water disinfection Optical data storage Dermatology

Aluminium Nitride

AlNfor Deep-UV

OptoelectronicsSource: M. Kneissl, PARC


Sublimation Growth of AlN

Aluminum nitride (AlN) decomposes prior to

melting at ~2500°C and 1 bar

⇒ Melt growth not possible!

Al NAlN

660

25202550

T [°C]

Nitrogen content [at %]

AlN + Ns 2g

Al + AlNs s

Al + AlNliq s

Al + Ng 2g

Al + AlNg s

AlN-N2 system

B. M. Epelbaum, M. Bickermann, et al., J. Crystal Growth 305 (2007) 317Phase diagram: L. Siang-Chung, Mater. Sci. Lett. 16 (1997) 759

AlN ↔ Al (g) + ½ N2(g)

Main constraints:– High partial pressures

(as high T2 is required for N2 dis-sociation and surface diffusion)

– Congruent dissociative sublimation(the source is completely sublimed)

– Excess of nitrogen in the gas phase(N2 as additional inert gas,Al/N ratio depends on T1 and ptotal)

– Materials compatibility critical(which crucible/insulation material to use to withstand Al vapour?)


AlN Sublimation products at Different Temperatures

Source: B.M. Epelbaum Optimum Temperature


AlN Supersaturation under N2 Excess

Source: B.M. Epelbaum, M. Bickermann et al., Mater. Sci. Forum 457-460 (2004) 1537


AlN Supersaturation under N2 Excess

Source: B.M. Epelbaum

Thermal gradients ofΔT = 50 K lead to a supersaturation of 50% almost independent of total pressure (N2 excess)

Low ΔT is required.


Sublimation Growth of SiC

Silicon Carbide (SiC) decomposes prior to

melting at ~2800°C and 1 bar

⇒ Melt growth not possible!

SiC ↔ Si(g) + Si2C(g) +SiC2(g) + C(s)

S.K. Lilov, Mater. Sci. Eng. B 21 (1993) 65Phase Diagram: Tairov 1988, http://www.ioffe.rssi.ru/SVA/NSM/Semicond/SiC

Main constraints:– Low partial pressures

(as surface diffusion is sufficient)– Incongruent dissociative sublimation

(solid carbon stays behind)– Excess of silicon in the gas phase

(Ar or He as inert gases,Si/C ratio depends on T1 only)

– Materials compatibility excellent(as different carbon materials can beused as heater, crucible, insulation)


Sublimation Growth Set-Up: Equipment

Crucible

Heater

Sourcematerial

Nucleation(Seed) Area

Growthroom

ΔTT2 (colder)

T1 (hotter)

Equipment High-Temp. Sublimation Growth

Facility dimensions 1–2 m² platform, 2 m height

Crucible dimensions ∅ 80–200 mm, height 100–250 mm (for 2" to 4" boules)

Heating system Resistive (30 V, 500A) or induction (10-20 kHz)Power 10-20 kW depending on insulation

Additional gases N2 (AlN), He or Ar (SiC), dopants

Temp. measurement Pyrometers (through holes drilled into insulation)


Sublimation Growth Set-Up: Equipment

Pyrometer

High vacuum

Gas inlet

Growth control

Growth chamber

Induction heaterwith moveable coil

Resistive heater

Research furnace at


Sublimation Growth Set-Up: Materials



Crucible

Heater

Sourcematerial

Nucleation(Seed) Area

Growthroom

ΔTT2 (colder)

T1 (hotter)

Materials AlN SiC

Heater Tungsten or graphite Graphite

Crucible Tungsten or TaC Graphite

Seed AlN or SiC SiC

Source Polycrystalline AlN Polycrystalline SiC

Thermal insulation Tungsten or graphite Graphite

25 mm

Dense sinteredTaC crucible



AlN materials compatibility:Most refractories, carbides and nitrides react with Al(g)Graphite crucible leads to heavycarbon contaminationW (tungsten) passivates againstAl(g) for p(N2) > 200 mbar– less contamination of growing

crystal– not compatible with graphite,

use W heat shields as insulationTaC (ceramic) crucible tends to crack, but is chemically stable– can be used with graphite

susceptor and insulation– can even be used together with

tungsten (e.g. as inner crucible)


Sublimation Growth Set-Up: Process

Sublimation growth stages1.Pumping2.Vacuum (and pre-bake)3.Heating4.Growth start bypressure reduction5.Growth6.Growth end bypressure increase7.Cool down

The whole process takesabout 96 hours (including72 hours of growth atrates of 100–200 µm/h).

Stage No.

growth time

15


Sublimation Growth Set-Up: Process

High-Temperature Heat TransportHeat transport by radiation dominatesat T > 2000°C.Thermal field and ΔT depend decisively on growth cell geometry:– crucible geometry and wall thickness– pyrometer holes– tailored voids for temp. balancing– insulation/near growth cell geometryLatent heat, Stefan flow, and diffusiondo not contribute significantly.Insulation degradation is an issue.

Due to fixed set-up geometry, thermal field changes as the crystal evolves.This limits the acievable crystal height.


Seeding Techniques

ΔT =20–30 K

ΔT < 10 K ΔT =20–30 K

no seed with seed

Deposition of polycrystalline materialon the crucible lidEasy growth rate testingNo single crystal

Spontaneous nucleationand free-standing growthof several single crystalsSingle crystals of perfectstructural qualitySmall & undefined size

Deposition on the seedSingle crystal growth at defined orientationSingle crystal at seed size(enlargement possible)Defects inherited from seedand seeding process


Growth Results (No Seed)

SiC

AlN


Growth Results (Spontaneous Nucleation)

AlNSiC


20.0 20.2 20.4 20.6 20.8 21.0 21.2 21.4

FWHM = 15 arcsec

I [a.

u.]

ω [deg]

(0002) DCRC

Current status @ IKZDislocation density ~ 100 cm²Homogeneous optical propertiesSubstrates up to 10 x 10 mm² in size

Growth Results (Spontaneous Nucleation)


Growth Results (With Seed)


T = 2080°CR ~ 50…200µm/h(depends on orientation)

T = 2150°CR ~ 30…250µm/h(depends on orientation)

T = 2200°CR ~ 250µm/h

Crystal Habit Changes With Growth Temperature

Crystal habit changes with growthtemperature (at constant supersaturation)in AlN growth

polycrystalline AlN deposit

freeestanding AlN crystals

TaC crucible

AlN source powder

grid


Crystal Habit Changes With Impurity Content

AlN grown in TaC cruciblesingle (0001) facet on top,full prismatic (1010) facets

AlN grown in W crucible6-fold pyramid with small (0001) facet

© CrystAl-N

[C] = 1 x 1019 cm–3

[O] = 2 x 1019 cm–3

[C] = 2 x 1017 cm–3

[O] = 5 x 1018 cm–3


Impurity Incorporation Depends on Growth Facet

SiCAlN

M. Bickermann et al., Physica Status Solidi C 3 (2006) 1902


Impurities Govern Deep-UV Transparency

M. Bickermann et al., Physica Status Solidi C 3 (2006) 1902


Structural Defects in High-Temp. Sublimation Growth

6H4H

15R

Source: H. P. Strunk

seed

crystal

D. Hofmann, M. Bickermann et al., Mater. Sci. Eng. B 61-62 (1999) 48

SiC– Micropipes

– Macropipes

– Dislocations

– Hollow defects

– Mosaicity

– Polytypes



Problem: Improper seed fixation leads to extended defects

Further reading: O. Filip, M. Bickermann et al., J. Crystal Growth 318 (2011) 427



AlN– Low-angle grain boundaries

– Mosaicity

– Dislocations

– Hollow defects

Low-Angle grain boundaries (lines)and dislocation piercing points (dots)

Reciprocal space mapshows tilted domains(up to 0.3° tilt), butalmost no strain.

(102) plane

M. Bickermann et al., Phys. Status Solidi C 5 (2008) 1502 and C 8 (2011) 2235

L. Kirste, Fraunhofer IAF


Current Development

Source: www.hexatechinc.com Source: www.cree.com

SiC growth is much more mature∅ 100 mm wafers are widely employed,∅ 150 mm is available commercially.Technology development moved to industry.

AlN growth is catching upTechnology is driven by spin-off companieswith support from research institutes.10 x 10 mm² wafers are available,∅ 50 mm is expected commercially for 2014.


Conclusions & Outlook

High-temperature sublimation growth is very lively dueto successful growth of SiC and AlN bulk single crystals.Main issues of growth technology are solved,even though the growth window is small andtransient effects (insulation degradation) are hard to control.

Structural defects and impurity levels have been brought downby careful seed selection and growth parameter adjustment.

There are challenges remaining (see below);also, size and cost (low growth rate) remain critical issues.

ChallengeCrystal enlargement

control of faceting

ChallengeHomogeneous properties

avoid zone structure

ChallengeKeep structural quality

control of nucleation


Credits

Contributors:

IKZ Berlin – AlN group- Dr. Jürgen Wollweber- Carsten Hartmann- Dr. Andrea Dittmar- Dr. Christo Guguschev- Frank Langhans- Sandro Kollowa

Contributors:

Univ. ErlangenCrystAl-N GmbH- Prof. Dr. A. Winnacker- Dr. Boris Epelbaum- Dr. Octavian Filip- Dr. Paul Heimann

sublimation growth of wide band-gap materials (sic …bickermann/public/isscg-2012-slides.pdf ·...

Documents