Basic epitaxial crystal growth methods and it's detailed analysis including the major defects in epitaxial layer formation.

1. EPITAXIAL CRYSTAL GROWTH:
METHODS AND ANALYSIS
KANNAN RAJEEV
20304009
PONDICHERRY UNIVERSITY

2. EPITAXY
• Epitaxy is a type of crystal growth or material deposition process in which new crystalline
layers are formed with one or more well-defined orientations with respect to the crystalline
substrate. The deposited crystalline film is called an epitaxial film or epitaxial layer.
• Epitaxial growth is broadly defined as the condensation of gas precursors to form a film on a
substrate. Liquid precursors are also used, although the vapor phase from molecular beams is
more in use.
• Epitaxy is used in silicon-based manufacturing processes for bipolar junction transistors (BJTs)
and modern complementary metal–oxide–semiconductors (CMOS), but it is particularly
important for compound semiconductors such as gallium arsenide.
• For most thin film applications – hard or soft coatings, or optical coatings – it is of little
importance but it is critical in semiconductor thin film technology

3. There are various types of epitaxy
• Homoepitaxy – The film and the substrate are the same material. This is often used to grow films that
are purer than the substrate, and which can be doped independently of it.
- Si on Si
• Heteroepitaxy – This is performed with different materials and often used to grow films of materials for
which crystals can’t otherwise be obtained e.g. silicon on sapphire, or graphene on hexagonal boron
nitride. This method allows for optoelectronic structures and bandgap engineered devices.
- GaAs on Si
- Si on Sapphire(Al2O3)

4. MOTIVATION
• Epitaxial growth is useful for applications that place stringent demands on a deposited layer:
- High purity
- Low defect density
- Abrupt interfaces
- Controlled doping profiles
- High repeatability
- Safe, efficient operation
• It can create clean, fresh surface for device fabrication.
PURPOSE
• Barrier layer for bipolar transistor
- Reduce collector resistance while keeping high breakdown voltage
- Only available with epitaxy layer
• Improve device performance for CMOS and DRAM because much lower oxygen, carbon concentration than
the wafer crystal

5. EPITAXY APPLICATION
Bipolar Transistor CMOS

6. EPITAXIAL GROWTH
• 1. Liquid Phase Epitaxy
• 2. Vapour Phase Epitaxy
• 3. Molecular Beam Epitaxy

7. LIQUID PHASE EPITAXY
• Reactants are dissolved in a molten solvent at high temperature.
• Substrate dipped into solution while the temperature is held constant.
• Example:- SiGe on Si
• Bismuth used as solvent.
• Temperature held at 800°C.
• High quality layer.
• Fast, inexpensive.
• Not ideal for large area layers or abrupt interfaces.
• Thermodynamic driving force relatively very low.

8. Factors influencing quality and properties of epilayer :-
• Initial equilibrium temperature and cooling rate of melt.
• Volume ratio of the melt and contact area between melt and the substrate.
• Nature of solvent and solute.
• Substrate surface condition.
ADVANTAGES
• High growth rates.
• Ability to produce very flat surfaces and excellent structural perfection.
• Wide selection of dopants.
• Low capital equipment and operating costs.
DISADVANTAGES
• Difficulties in producing very thin epitaxial layers with planar morphology.
• Not all compositions of ternary and quaternary alloys can be deposited.

9. VAPOUR PHASE EPITAXY
• Specific form of chemical vapour deposition (CVD).
• Reactants introduced as gases.
• Material to be deposited bound to ligands.
• Ligands dissociate, allowing desired chemistry to reach surface.
• Some desorption, but most adsorbed atoms find proper
crystallographic position.
• Example: Deposition of silicon
- SiCl4 (g) + 2H2 <--> Si (s) + 4 HCL (g)
- SiCl4 introduced with hydrogen
- Forms silicon and HCl gas
- SiH4 breaks via thermal decomposition
- Reversible and possible to do negative (etching)

10. TYPES OF REACTORS
Precursors for VPE
• Must be sufficiently volatile to allow acceptable growth rates.
• Heating to desired T must result in pyrolysis.
• Less hazardous chemicals preferable
• VPE techniques distinguished by precursors used.
Horizontal Reactor Vertical Reactor Barrel Reactor
Flow of Gas

11. ADVANTAGES
• Ability to grow very good quality layers.
• High growth rate.
• Principle is relatively simple.
• Allows great flexibility (chance in doping level or type of doping)
• Can handle several large wafers.
• It can be carried out in atmospheric pressure.
• Substrate penetration are not as stringent as in MBE.
DISADVANTAGES
• High purity of substrate is very important.
• Once the layer has been made and the process is complete, there may be a requirement of needing to
remove excess precursors from the final product.

12. MOLECULAR BEAM EPITAXY
• Molecular Beam Epitaxy is a method for thin film deposition of single crystals.
• MBE is widely used in the manufacture of semiconductors.
• It is considered as one of the fundamental tools for the development of
nanotechnologies.
• Takes place in a high or ultra-high vaccum.
• The UHV environment allows for the use of diagnostic techniques during the
growth as the RHEED and the integration of the growth system with UHV
surface characterization techniques.
• Growth rate is as small as 0.01 um/min upto 0.3 um/min.
• Development of structures where the electrons can be confined in space,
giving quantum wells or quantum dots.

13. • Beams created by evaporating solid source in UHV.
• Evaporated beam of particle travel through very high vaccum and then condense to shape the layer.
• Doping is possible by adding impurity to source gas by (eg: Arsine and Phosphors)
• No boundary layer or stagnant layer problem.
• MBE is very costly and sophisticated equipment.
• Thickness of each layer can be controlled to that of a single atom.
APPLICATIONS
• Engineered wafers
- clean, flat layer on top of less ideal Si substrate
- Higher purity layer on lower quality substrate (SiC)
• CMOS structures
- Layers of different doping.
- p- layer on top of p+ substrate to avoid latch-up.
• Bipolar Transistor
- Needed to produce buried layer
• III-V Devices
- interface quality key
- Heterojunction Bipolar Transistor
- LED
- Laser

14. PROCESS
• Very high vaccum condition to minimize the contamination problem.
• Evaporate the particular species.
• By using an electron gun, an electron beam is focused on the silicon
sourse and silicon is evaporated.
• Two pumps : Rotory pump and Diffusion pump.
• Crucible in that silicon is placed.
• Next to this, the electron gun is kept.
• E-beam is focused on to the silicon source aand from there silicon is
getting evaporated in this conical shape.
• In order to introduce dopants, one uses an effusion cell through
which a jet of evaporated dopants come out.
• Shutters to place substrate.

15. ADVANTAGES
• Clean surfaces
• Independent vaporization of each material.
• Multiple sources are used to grow alloy films and heterostructures.
• Deposition is controlled at sub monolayer level.
• Extremely flexible technique since growth parameters are varied independently.
DISADVANTAGES
• This process is very expensive as compared to CVD process.
• For overall perfect and pure film, it is necessary to maintain a very low pressure of the order of 10-10 Tor, which
is slightly difficult.
• The growth rate in MBE process is 0.01 – 0.3 gm/min which is very small compared to the growth rate of
1 µm/min in CVD process.

16. EPITAXIAL CRYSTAL GROWTH MODES
• a) Layer by layer – Frank-van der Merve growth mode
• b) Layer by layer – continuously
• c) Layer and island on the wetting layer – Stransky-Krastanow mode
• d) Island on the substrate – Volmer-Weber mode.
• e) Columnar growth type mode.

17. AUTO DOPING
• Process of manipulating the doping concentration.
Gas Phase Auto doping
• Dopents which are in higher concentration in the surrounding of the substrate will penetrate inside the substrate
and changing the doping concentration.
Solid state Diffusion
• Different concentration at the junction between n type and p type substrate creates diffusion of atoms inside the
substrate.
Lateral Auto doping
• It happens in the sidewise direction, n+ atoms going into p substrate rather than in upper direction.
• Very hazardous and this should be controlled.
Pattern shift Distortion

18. SINGLE WAFER REACTOR
• Sealed chamber, hydrogen ambient.
• Capable for multiple chambers on a
mainframe.
• Large wafer size (to 300 mm).
• Better uniformity control.
• Hydrogen purge, clean, temperature ramp up.
• Epitaxial layer grows.
• Hydrogen purge, heating power off.
• Wafer unloading, reloading.
• In-situ HCl clean.
Why Hydrogen Purge?
• Most systems use nitrogen as purge gas.
• Nitrogen is a very stable adundant.
• SiN in wafer surface affects epi deposition.
• H2 is used for epitaxy chamber purge.
• Clean wafer surface by hydrides formation.

19. DEFECTS IN EPITAXIAL LAYER
• Impurity particle
- Particles which come during the process of epitaxial layer formation.
• Dislocation
- It is present till the bottom of the substrate, in future it will create dislocation of Ics.
• Stacking fault
- The impurity particle present in the top layer of the substrate leads to nucleation and
thus creating empty portions.

20. REFERENCES
• S. M Sze, “VLSI Technology”, McGraw Hill Publication, 2nd Edition 2017.
• S.K. Gandhi, “VLSI Fabrication Principles”, Willy-India Pvt. Ltd , 2008.
• Semiconductor optical communication devices : Lecture-Epitaxial Growth, Prof. Utpal Das.
• Y. Cho, "Growth of III–V semiconductors by molecular beam epitaxy and their properties," Thin Solid
Films, vol. 100.
• Molecular Beam Epitaxy (MBE) Lorenzo Morresi, University of Camerino-School of Science and
Technology-Physics Division, Via Madonna delle Carceri, Italy.