Marie Curie Actions
Project funded by the EC within the 6th Framework Programme
 
 
 
 
 
 
 
 
 
 
Mansic for all
 
Silicon carbide
Silicon carbide (SiC), also known as carborundum or moissanite (name given from the french researcher Dr. Henri MOISSAN who first identified it in 1905), is a solid compound made of 50% carbon and 50% silicon. Naturally occurring SiC is found only in minute quantities in exceptional geological places like diamond containing chimneys, kimberlitic volcanic openings and even in certain types of meteorite. Virtually all of the silicon carbide sold in the world is synthetic.
This material is at the same time a ceramic and a semiconductor with outstanding properties. It is very hard (almost as hard as diamond), inert chemically, resistant to high temperature (>1000°C), oxidation and harsh environment. It has a high thermal conductivity so that it can evacuate heat like metals. It has been used as abrasive for more than a century in industry (main product of the Carborundum company) but its applications are widening nowadays to aero-space (high temperature engines, brake disks), composites, particulate filters and heating elements.
SiC crystals are also recently used in jewellery under the name of moissanite because of its optical properties very close to those of diamond. But the main application for SiC crystals is in electronic. Indeed, the electronic devices made from SiC should work at higher temperature, higher power, higher frequency and in harsher environment than the actual ones made from other semiconducting materials (silicon, germanium, gallium arsenide...). But, in order to reach the intrinsic electronic properties of SiC, it is imperative to grow high quality crystals of this material.
Jewel stone made from a Moissanite crystal
Dr. Henri MOISSAN
(Sept. 28, 1852
to Feb. 20, 1907)
Cover of the Carborundum Company's 1894 prospectus for its new diamond-like product
SiC crystal growth

A crystal is a solid material composed of a regular and periodic stacking of atoms. Almost every solid material (metals, alloys, ceramics, salts, plastics...) can produce crystals if the conditions for their atomic organisation are reached. In most cases, some energy (heat or pressure) has to be given to the atoms to help their regular positioning in a three-dimensional network. The crystals found in the nature are usually formed spontaneously but this growth procedure creates a lot of defects (imperfect stacking, inclusions of other materials...). Since the electronic industry needs high quality crystals in order to enhance the performances of the fabricated devices, the best spontaneously grown crystal are generally used as seeds for further growth and/or improvement of the quality. Therefore, a replicating growth technique called epitaxy (from the Greek root "epi = above" and "taxis = in ordered manner") is applied using some reactants and a seed.
Basic unit cell of diamond crystal
Basic unit cell of silicon carbide crystal

 
Cross sectional view of atomic stacking at the interface between crystalline silicon and silicon carbide epitaxial film
Basic scheme of the epitaxial growth from a vapor phase

It is important to note that the stacking periodicity (the 3D atomic network) of the atoms within the crystals often depends on the conditions of crystallization. The most famous example is given by the carbon atoms which can produce graphite (coal) under moderate conditions of temperature and pressure while it can also lead to diamond if the temperature and the pressure are extremely high. This ability to crystallize under different forms is called polymorphism.

SiC presents a special case of polymorphism, called polytypism, where the change of the stacking sequences of the atoms only occurs in one specific direction of the 3D network. Due to this specific property which allows almost infinite variation of the network, tens of SiC polytypes have been discovered.
However, only three of them are commonly produced because more stable. They are called 6H-SiC, 4H-SiC and 3C-SiC (the letters H and C refer to the crystal symmetry, H for hexagonal and C for cubic). Most of the physical properties of these polytypes are identical, except the electronic ones. For the electronic devices, each polytype has its specific advantages. For example, 4H is better suited for high power (ex. high voltage electricity distribution) and high temperature (ex. car or plane engines) while 3C should be better for high frequency applications (ex. radar).
Atomic stacking difference between the main polytype of SiC

Due to the high thermal stability of SiC, one has to give a high amount of energy to the material in order to allow its crystal growth. As a consequence, the technique commonly used for such growth requires temperatures above 2200°C. This need of high temperatures makes the growth process very difficult to control. Furthermore, these conditions only allow the crystallization of the hexagonal polytypes (6H and 4H), the 3C one being stable at lower temperatures (below 2000°C) where the energy given to the atoms is not enough for good crystal growth.
That is why up to now there are no commercially available crystals of 3C-SiC with sufficient quality for electronic applications, while high quality 4H and 6H crystals can be purchased from different companies.
 
Cubic SiC crystal growth
Two crucial points have to be solved in order to be able to elaborate 3C-SiC crystals usable for electronic applications:
1) to find an adequate seed for 3C epitaxy (since there is no available 3C crystal of good quality)
2) to develop new growth techniques which could promote good 3C crystalline quality (low defects density) even at low temperatures (for 3C stabilization).
Concerning the seed, crystalline silicon has been extensively studied for SiC epitaxy but the resulting 3C material is always of poor quality. In specific conditions, 6H and 4H polytypes can also be used as seeds for 3C epitaxy but the layers usually contain a high density of defects.
Recently, new growth techniques which target the 3C polytype are being developed in Europe. They have demonstrated very promising and complementary results since one allows the deposition of good 3C layers (thickness of few µm, 10-6 m) on 6H (or 4H) substrates while the other allow growing much thicker 3C material (several hundreds of µm to few mm thick) while keeping the quality of the underneath 3C seed.
There is no real competitor on the international level using similar techniques which means that Europe is in advance on this specific subject and has a major role to play.
Highly defected 3C-SiC epitaxial layer grown on 6H-SiC seed
High quality 3C-SiC epitaxial layer grown on 6H-SiC seed
MANSiC objectives

The present MANSiC consortium gathers the teams developing such new growth techniques and other European laboratories with an internationally recognised expertise in the field of SiC for electronic applications. This joint effort would surely allow developing an alternative and European commercial source of 3C-SiC crystals with better crystalline quality than the actual commercial product. Such improved material would be first characterized and tested (from surface polishing to device fabrication) with the aim of fabricating innovating and/or improved electronic devices.

Such a complex and interrelated research project is an excellent base to assemble a training project for young researchers in the field of solid state physics and materials science. PhD students and young researchers will be trained in various fields: from new growth techniques, to semiconductor material characterization and fabrication of electronic devices. Workshops and training schools will be organized to provide the young researchers with the essential scientific knowledge background and to give them the opportunity to present their work and results to the scientific community.
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