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Silicon Carbide Epitaxy (credit to Sreechakra, IIT Kgp)
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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