Color Centers in Silicon Carbide

1. Color Centers in Silicon Carbide Dan Maser

2. Background • Also known as “carborundum” • Occurs naturally (although very rare) as moissanite • While rare naturally on Earth, common form of stardust • First synthetically created in 1893 • Initially used as an abrasive moissanite

3. Role in Electronics • Used as a detector in early radios • 1907 - First LED • Semiconductor • Can be doped n-type with N, P • p-type with Al, B, Ga, Be • B substitutes C, while Al substitutes Si (produce different type semiconductors) SiCmonocrystal

4. Structure and Properties • 250 crystalline forms! • Three most common polytypes: (α) 6H-SiC (hexagonal), (β) 3C-SiC (lone cubic), and 4H-SiC (hexagonal) • α-SiC is the most common • Pure SiC is colorless – black color comes from Fe impurities, rainbow shine is from a passivation layer of SiO2

5. Structure and Properties (α) 6H-SiC (β) 3C-SiC 4H-SiC

6. Electrical and Optical Characterization of SiC G. Pensl and W. J. Choyke

7. Luminescence measurements of 6H-SiC • Comparison of n-type CVD film grown on the C-face of 6H-SiC to p-type CVD film grown on Si-face of 6H-SiC • Both samples show ZPL’s P0, R0, S0 characteristic of N-doped 6H-SiC • Come from recombination radiation of exciton (hole-electron pair) in a four-particle neutral donor complex at three inequivalent donor sites

8. Luminescence measurements of 6H-SiC • In C-face film, prominent features below ZPL are phonon replicas of P center • Indicates strong N-doping • In Si-face spectrum, three features denoted 4A, I, A0 • 4A – acceptor four-particle neutral complex • A0 associated with Ti in 6H-SiC

9. Comparison of photoluminescence spectra (410-434 nm)

10. Luminescence measurements of 6H-SiC (higher wavelength) • In C-face, lines due to two phonon replicas of the P line • Combinations of the P line phonon replicas and a center of the zone TO phonon are seen • In Si-face, ZPL of Ti center, A0, B0, C0, and phonon replicas are marked

11. Comparison of photoluminescence spectra (430 nm to 480 nm)

12. Comparative optical investigations of sintered and monocrystallineblack and green silicon carbide (SiC) • H. Werheit and K.A. Schwetz

13. Sintered SiC • Solid-state sintering: making a substance from powder by heating it to just below its melting point • Liquid-phase sintering: uses an additive that will melt before matrix phase • Capillary action pulls liquid into pores, grains rearrange into a better packing arrangement • Atoms preferentially go into solution and precipitate in areas of lower chemical potential – called “contact flattening”

14. Sintered SiC • S-SiC (solid-state sintered) and LPS-SiC(liquid-phase sintered) ordinarily only seen in black only • New LPS process can create green LPS-SiC • Uses α-SiC powder • S-SiC: doped with 0.2% B • LPS-SiC: doped with 1.2% Al, 0.3% N, 0.1% O • Mixed SiC(Al,N,O) crystal shell, pure SiC core • Green is obtained from removal of free carbon

15. Green SiC

16. Black SiC

18. References • G. Pensl and W.J. Choyke, “Optical and Electrical Characteristics of SiC”. Physica B, 185, 264-283 (1993). • H. Werheit and K. A. Schwetz, “Comparative optical investigations of sintered and monocrystalline black and green silicon carbide (SiC)”. Journal of Solid State Chemistry, 177, 580-585 (2004).