Presented by K. Kamalakkannan

OutlineOutline Introduction

History

Application

Structure and Polytype

Doping

Sample notation and Cutting

Conclusion

All the electronic equipments needs Semiconducting materials. At an early stage Si & Ge was dominate. Later GaAs used.

Looking for alternate materials for High temp, High power & High frequency applications is the current field of research interest. So that we need to develop wide Bg materials.

For this case, SiC materials are very useful, because of its exceptional properties, like Wide band gap, High saturation electron drift velocity, etc.,

The History of SiCSiC was first observed in 1824 by the Swedish scientist Jons Jacob

Berzelius, when attempt to synthesis of diamond.

Naturally occurring SiC has been discovered by Henri Moission in 1905 in a meteorite- called Moissanite.

First man made SiC- starts in 1891, by Edward G. Acheson, when he was running an experiment on the synthesis of diamonds.

Heating a mixture of clay (Aluminium silicate) & Coke (Carbon) in an iorn bowl- Acheson got blue crystal named Carborundum.

Continue…Lely in 1955 introduced a crystal growth technique to produce high quality bulk SiC. But the problem of producing large area defect free single crystal is difficult- So, device fabrication is also difficult.

Modified Lely method was developed by Tairov & Tsvetkov using seeded SiC.

Recent and most common method is Vapor- phase epitaxy (VPE)- used for epitaxial growth of SiC- in a CVD reactor.

Why we use SiC?…

Silicon carbide is a wide band gap IV- IV semiconducting material and it is an attractive material among researchers for more than a century due to its hardness next to diamond.

SiC is widely used because of it’s,1. Extreme hardness (mohs hardness is around 9.2)2. Light wear resistance3. High thermal conductivity (3.4 W/cm k)4. Wide energy band gap (3.2 ev)5. High electric field breakdown strength (2.22 X106 V/cm)6. High saturated electron drift velocity (2.2 X107 cm/s)

Wide band gap (3.2 ev)- Enables SiC to operate at extremely high temp, high voltage.

Cont.,High breakdown electric field- Enables the fabrication of high voltage and power devices such as power transistors & diodes.

High thermal conductivity- SiC can operate at extremely high power levels.

High saturated electron drift velocity- Can operate at high frequency ( RF and Microwave).

Cont.,At an early stage, the more attention is given to the mechanical property of SiC – Used as an abrasive- such as sand paper & cutting tools .

It is a very hard material because of Its shorter bond length 1.89 Ao

between Si and C results in high bond strength and hardness.

Wide band gap materials are superior to Si due to their physical, chemical properties. For this it is more difficult to excite electrons from VB to CB.

This causes reduction in leakage of currents & an increase in thermal stability in a device- high temp applications.

Efficiency of ZnO : Rajkumar et al (2011) - 0.142 % Kanmani et al (2012) - 0.163 % Prabhu (2014) - 0.366 %

Efficiency of ZnSe QDs : G.Sfyri et al (2014) - 0.59 % (Using FTO/TiO2/CdS/ZnSe/P3HT) Bang et al (2009) - 5.6 X 10-4%

So, here an attempt will be made to synthesize ZnSe thin film on glass substrate and studied for solar efficiency where other interesting properties will be analyzed simultaneously.

Number of thin film deposition methods are available - Among them, CBD provides a more promising option - large scale controllability, and low cost method[2]

Properties of some wide bandgap semiconductors

Micro electronic applicationSiC is the only wide Bg material having SiO2 as native oxide- SiO2 as a

dielectric is needed for gate material in MOSFET. This can formed by simple wet dry oxidation of SiC.

Inertness & high temp. stability of SiC is used in furnaces and combustion engines.

SiC is also used as a sensor to detect nitric oxide, hydrocarbons and oxides.

SiC is used in high frequency operations- microwave applications. It is also used as a substrate for material synthesis.

Crystal Structure and

poly typeSiC – a binary compound, having equal Si & C, where Si-C bonds are nearly co-valent with ionic contribution of 12% (Si is +ve, C is –ve charged).

Smallest building element of SiC is a tetrahedron of a Si(C) atom surrounded by four C(Si) atom in sp3 bonds.

Distance between Si-Si or C-C atom is ~3.08 Ao , Si-C atom is ~1.89 Ao . So the distance between Si-Si and Si-C ratio is 3:1.

Basic elements ofSiC crystals: Tetrahedronscontaining (a) one C and four Si(b) one Si and four C atoms.

Cont.,Most important property of SiC is polytypism- a 1D type polymorphism. SiC structurally differ by the stacking sequence of a close packed Si-C bi-planes along the c-axis.

More than 200 different polytypes of SiC are known to exist. Among them- Cubic, Hexagonal, Rhombohedra polytypes of SiC are common.

Only one cubic(C) polytype referred as- β- SiC is the available poly type, all other α- SiC (either a hexagonal (H) or rhombohedral(R)) symmetry.

The Si- atom lying exactly above the C- atom along the stacking axis (C-axis). Each type characterized by the stacking sequence of layers A, B and C.

Cont.,Stacking the layers in ABC order will produce Zinc blend (Cubic) structure and the staking sequence of AB is wurtzite structure (Hexagonal).

The crystalline structure of SiC: (a) the three possible hexagonal positions (A, B, and C) for Si and C atoms

Cont.,Commonly used nomenclature for describing the SiC polytypes is Ramsdell notation, describing each polytype with a number followed by a letter.

ex: 2H- SiC = Wurzite crystal structure. 3C- SiC = Zinc blend structure. 15R- SiC = Rhombohedral.

The number denotes the counts of the double layer in one period, the letter stands for symmetry.

Most common available forms in SiC are: 3C- SiC, 2H- SiC, 4H- SiC, 6H- SiC and 15R- SiC.

Cont.,

3C SiC poly type 6H- SiC poly type

Cont.,

3C- SiC 2H- SiC

4H- SiC 6H- SiC

Cont.,The stacking sequence of the common form of SiC are:

ex: 2H- SiC = AB. 3C- SiC = ABC. 4H- SiC = ABCB 6H- SiC = ABCACB 15R- SiC = ABCACBCABACABCB.

The path of covalent bonds between Si & C atoms on the projected atomic positions onto a (11-20) plane is traced for determination of polytype.

The turning points of Zig-Zag patterns in a Si-C pair is said to be hexagonal(h), between the turning points is called cubic (k).

Cont.,

The stacking sequences of four common SiC polytypes. The different cubicand hexagonal lattice sites are marked.

Cont.,It is also possible to derive the ‘Hexagonality, of a Si-C polytype, which is the percentage of the hexagonal sites out of whole crystal..

Notations and hexagonality of SiC polytypes with the corresponding numbers ofinequivalent lattice sites.

As the hexagonality increases from 0 to 100% the indirect Bg of SiC varies from 2.3 (3C) to 3.33 (2H) eV.

Doping of SiCSiC is a semi-insulator, which can made n &/or p type by doping with impurities. n-SiC can be achieved by Nitrogen or Phosphorus doping and p-SiC is achieved by doping with Aluminum, Boron, or gallium.

Metal like conductivity has been achieved by heavy doping with Al & Boron.

Superconductivity is also achieved in 3C-SiC:Al, 3C-SiC:B & 6H-SiC:B at 1.5K. .

Doping of SiC Comparison of SiC polytypes (Electrical parameters)

4H- SiC 6H- SiC 3C- SiCBand gap

(eV)3.2 3.0 2.3

Intrinsic carrier concentration (cm-3 )

10-7 10-5 10

Electron mobility at ND = 1016

cm2/V s

II- C, aixs= 800 II C- axis: 60 750

Hole mobility at ND = 1016 cm2/V s

115 90 40

Donor (Nitrogen) dopant

45 85 45

Sample notation- SiC

Carbon face up orientation

The primary flat is the {1010} plane with the flat face parallel to the <1120> direction.

Primary and secondary flat locations, and marking orientation, silicon face up for carbon face polished wafers

Silicon face up

Cuttinng of SiC

ConclusionSiC – IV-IV semiconducting material and widely used among researchers for its excellent properties of wide range application

Wide band gap, High saturation electron drift velocity, High electrical field breakdown strength, High maximum current density, Extreme hardness and High thermal conductivity are the interesting factors of SiC- Used in High power devices.

SiC- more than 200 known crystal structure- polymorphism. Common forms are 3C-SiC, 2H-SiC, 4H-SiC, 6H-SiC & 15R-SiC.

Electrical parameters shows 4H-SiC is better for device application.

SiC- hard material, Cutting of the sample is done with diamond tipped cutter.

Thank you