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Color Centers in Silicon Carbide. Dan Maser. 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.
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Color Centers in Silicon Carbide Dan Maser
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
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
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
Structure and Properties (α) 6H-SiC (β) 3C-SiC 4H-SiC
Electrical and Optical Characterization of SiC G. Pensl and W. J. Choyke
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
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
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
Comparative optical investigations of sintered and monocrystallineblack and green silicon carbide (SiC) • H. Werheit and K.A. Schwetz
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”
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
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).