sic epitaxy

INDO GERMAN WINTER ACADEMY 2005

EPITAXIAL GROWTH OF SILICON CARBIDE

THIN FILMS

G. SREECHAKRA

Indian Institute of Technology, Kharagpur

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SYNOPSIS

• Introduction to Epitaxy– What?... Why?... Types

• Homoepitaxy– Vapour Phase Epitaxy

• Hot-Wall Concept – You’ll see…• Where does CVD take place?• How exactly does Epitaxial Growth take place?• Polytype control in Epitaxy• Unintentional Doping

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SYNOPSIS Continued…

• Intentional Doping– C:Si ratio

• Dealing with the defects - I• Heteroepitaxy• Dealing with the defects – II• Selective Epitaxial Growth• Summary

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EPITAXY

• The stage is set – courtesy Tairov and Tsvetkov

• Problems with Bulk GrowthThe unique SiC properties, superior in comparison to standard semiconductors, can be utilized only when the material is of high quality.

• Epitaxy – here we come…• Greek: epi = upon AND taxis = ordered• Why epitaxy?• Homoepitaxy and Heteroepitaxy

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Types of Epitaxy

• Sublimation epitaxy – Vodakov et al.• Vapour Phase Epitaxy

– Growth more controllable• Liquid Phase Epitaxy• Molecular Beam Epitaxy

– Very thin epitaxial layers – nm/h growth rate!– Growth temperature quite low– Extremely precise control

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VAPOUR PHASE EPITAXY

• Feed precursor gases : SiH4, C3H8

• Diluted in a carrier gas : H2

• Into a reaction chamber – different types• Growing on a heated seed crystal

CHEMICAL VAPOUR DEPOSITION PRINCIPLE

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C.V.D.

• Why CVD?• Redesign of III-V semiconductors’ reactor• Variety of SiC-CVD processes – Your call!• Typical SiC-CVD

– Deposition temperature: 1500 - 1650°C– Pressure: 1 to 960 mbar– Temperature vs growth rate– Atmospheric or low pressure– Substrate rotation

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The Hot-Wall Concept

• Cold-wall reactor – graphite block, RF heated from below

• Hot-wall reactor – graphite chamber• Horizontal hot-wall reactor first introduced

in 1993.• Hot hot hot – crack crack crack!• The low thermal gradient takes care of it

all… of what?• Thermal insulation

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Hot Wall Reactors

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Schematic diagram of an atmospheric-pressure CVD growth

system

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Reactor Design Aspects• Demands – layer uniformity, process control• Boundary layer problem – higher gas-flow rate• Buoyancy-caused convection problems:

– Reduction of layer uniformity– Memory effects

• Remedies:– Low-reactor pressure– High carrier-gas flow-velocity– Low reactor-cell height

• Choice of material:– Growth temperature: 1450 – 1650°C– Radiation – effective shielding– Thermal insulation

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Schematic

Of Some

Horizontal

Reactions

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Horizontal Reactors

• Hot-wall reactor – rotating substrate holder• Bigger reactor – gas-insert / graphite liner• Drawback

– limited possibility of multiple substrate growth– Rotational symmetry needed

• Solutions– Planetary multi-wafer reactor (Aixtron, Epigress) –

can handle seven 50-mm diameter substrates– Vertical reactor (Emcore) – high gas-flow velocity,

high susceptor rotation-speed

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Schematics of the Multi-Batch Reactors

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Epitaxial Growth Process

• Growth temperature – 1450 - 1650°C• Carrier gas – Hydrogen• H2 also acting as an etchant… so?• Flow rates between 3 and 80 slm• Growth rate vs silane flow• Is reactor pressure a growth parameter?• Effect of growth parameters on C:Si ratio• Supersaturation

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Typical Growth Sequence

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You better be off the axis!

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Step-controlled Epitaxy

• Terraces and steps on SiC {0001} surfaceAccording to a classical growth theory, adsorbed species migrate on the surface and are incorporated into the crystal at steps and kinks where the surface potential is low. [ B.R.Pamplin, Crystal Growth, Pergamon press, Oxford, 1975 ]

• Doesn’t nucleation occur on terraces?• On-axis {0001} faces – Twinning 3C-SiC!!• Off-axis – the steps shall guide thee!• Off-orientation towards <11.0> direction

preferred.

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But Sir, I was not doping it!

• Unintentional doping• Site-competition epitaxy• C:Si ratio – the Ruler of Dopingham!• Nitrogen is there in Si epitaxy too, isn’t it?• What about C-faces?• Decreasing system pressure stops N

incorporation too!

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In Situ Intentional Doping

• (CH3)3Al, B2H6, N2, PH3 – Precursors• Why Aluminium? Abrupt doping profiles!• Phosphine or Nitrogen? You decide!• The house of a dopant atom – Si or C.• Si: 1.17, C: 0.77, P: 1.10, N: 0.74, Al:

1.26, B: 0.82 Å (non-polar covalent radii)• Equivalent size of B-H compound: 1.10 Å

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Impurity-Incorporation Mechanism C:Si ratio

• C:Si ratio measurements• High C:Si ratio – N dopant hampered• Low C:Si ratio – N dopant enhanced• Passivation of B acceptor – Anneal it!• 1 < C/Si ratio < 2 is Si-rich condition• 3 < C/Si ratio < 6 is C-rich condition

Through systematic growth experiments under various C/Si ratio and TMA flow rate conditions, heavily doped p+ layers (p > 1019 cm-3) can be obtained on Si-faces only.

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Look at the C-face!

Unlike on the Si-face, dopant incorporation on C-faces is independent of the C/Si ratio and almost constant. Why?

How do adsorbed dopants reach the steps?

C-rich surface not so rich after all…Si-rich surface not justifying its name either…

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To err is crystal!

• High supersaturation• Low temperatures• Unstable step flow• Propagation of defects from substrate• Substrate surface preparation• CVD process

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Crystalline Defects

• Classification – 0-, 1-, 2- and 3-D defects• Intrinsic Point Defects• Extrinsic Point Defects• Not always ‘defects’, you know!• Line Defects – further classified• Edge and Screw Dislocations• Area defects• Volume defects

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An edge dislocation; note the insertion of atoms in the upper part of the lattice

A screw dislocation; note the screw-like 'slip' of atoms in the upper part of the lattice

Photo of a Stacking Fault; Image Source: http://lmass.uah.edu- J. A. Gavira-Gallardo, J. D. Ng and M.A. George

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Basal Plane Terrace

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A 20 μm thick 4H-SiC epilayer with various defects: (a) micropipe; (b) triangle defect; (c) growth

pits; (d) carrot like defect (Nomarski photograph)

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Micropipes

• What is a micropipe?Recently it has been shown that the deposition of epitaxial layers on the seeds can lead to a reduction in the micropipe density.

• Ignition of microplasma• Micropipe conversion vs C:Si ratio• Substrates with epitaxially closed micropipes could be

used as seeds for bulk sublimation growth in order to decrease the micropipe density!

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Surface images of an epitaxial layer in Nomarski contrast (a, c) and micropipes in the (000-1) 4H-SiC

substrate at the same positions in transmission light (b, d) (C/Si = 0.9 (a, b); C/Si = 1.5 (c, d)).

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Step bunching

Poly-type/Face

Step height >= 70% of bilayers

Step height ~ 20% of bilayers

6H/Si 3 -

6H/C 1 3

4H/Si 4 2

4H/C 1 2

Step height on off-cut substrates, in number of Si-C bilayers at highest and next highest probability

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HETEROEPITAXY

• Growth of 3C SiC on Si substrates• Conventional reactor

– Carbonization– Growth

• Horizontal hot-wall low-pressure furnace– Efficiency measure

• Buffer-layer optimizing parameters– Void formation – a problem

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The alternative heteroepitaxy chamber

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Illustration of crystal defects in 3C SiC: (right) twin boundaries; (below) anti-phase boundaries

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An upcoming alternative…

• Selective Epitaxial Growth (SEG) – why do we need it?

• SEG and ELO go hand-in-hand!• Attractive features:

– May reduce need for SiC etching– May reduce film tensile stress due to defects– ELO makes film forget the substrate!– Patterned SiC– Way to prevent leakage current

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Selective Epitaxy

• Key factors– Growth temperature – 1100 to 1400 °C– Choice of mask material

• Limiting factors– Oxide stability– Deposition time

• The biggest challenge in SEG of SiC is based on the two conflicting requirements: high growth temperatures and low growth temperatures!

• Masks

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Schematic diagram of SEG followed by ELO process; (a-c) different stages of the

overgrowth

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Conventional SEG followed by ELOG

• First reported by Ohshita et al.• SEG of 3C-SiC/Si (100) using HCDS,

propane, H2 and HCl gas by CVD at 1350°C – by Jacob et al.

• Lower growth temperature – use of HMDS

• Pyramidal Growth – Okui et al.

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High magnification TEM image of SiC in the window region showing the bending of the

defects.

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Schematic of the procedure for pyramidal growth in mode A:

(a)CVD of thin SiC layer at windows,

(b)Wet etching of SiO2 mask,

(c) Dry etching of Si, and

(d)Regrowth of SiC.

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Schematic of the procedure for pyramidal growth in another mode B:

(a)CVD of thin SiC layer at windows after etching,

(b)Wet etching of SiO2mask, and

(c) Regrowth of 3C-SiC.

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Outlook

• Lower off-axis angle substrates– development of a 3 inch epitaxial growth process at Infineon

Technologies AG for 4 off-cut substrates– Deleting the past glory?

• CVD made more effective and moving ahead… 50 μm crossed!

• CVD – useful for investigation of defects in SiC and improving bulk crystal substrates by micropipe healing

• Hot-wall CVD – fundamental now and great potential• SEG technique is a promising approach for

heteroepitaxial growth of 3C-SiC films, which are useful for devices and MEMS applications.

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Summary

• Epitaxy – its origin, types and ways• Homoepitaxy – Chemical Vapour

Deposition• The concept of hot-wall – reactors• Site-competition and Step-controlled

epitaxy• Playing with defects• Heteroepitaxy – Selective Epitaxial

Growth

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REFERENCES• Akira Itoh and Hiroyuki Matsunami ‘Single Crystal

Growth of SiC and Electronic Devices’ [Critical Reviews in Solid State and Materials Sciences]

• G. Wagner, D. Schulz, D. Siche ‘Vapour phase growth of epitaxial silicon carbide layers’ [Progress in Crystal Growth and Characterization of Materials 47 (2003) pages139-165]

• Aparna Gupta, Chacko Jacob ‘Selective epitaxy and lateral overgrowth of 3C-SiC on Si: A review’ [Progress in Crystal Growth and Characterization of Materials 51 (2005) pages 43-69]

• Book: Process Technology for Silicon Carbide Devices– edited by Carl-Mikael Zetterling

• Book: VLSI Technology – S. M. Sze