Download pdf - SiC Epitaxy

INDO GERMAN WINTER ACADEMY 2005

EPITAXIAL GROWTH OF SILICON CARBIDE

THIN FILMS

G. SREECHAKRA

Indian Institute of Technology, Kharagpur

3/5/2006 7:21 PM Epitaxial Growth of SiC thin films 2

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


SYNOPSIS Continued…

• Intentional Doping– C:Si ratio

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


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


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


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


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


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


Hot Wall Reactors


Schematic diagram of an atmospheric-pressure CVD growth

system


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


Schematic

Of Some

Horizontal

Reactions


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


Schematics of the Multi-Batch Reactors


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


Typical Growth Sequence



You better be off the axis!


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.



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!


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


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.




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…



To err is crystal!

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


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


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

http://lmass.uah.edu/


Basal Plane Terrace


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

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


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!


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)).


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


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


The alternative heteroepitaxy chamber


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


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


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


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

overgrowth


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.


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

defects.


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.


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.


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.


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


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