Silicio Per massa il Silicio è circa il 26% della crosta terrestre, (principalmente nella forma di silice o quarzo cristallino (SiO2) ) , ed è il secondo elemento per abbondanza, dopo l’ossigeno. Molto raro è il cristallo di silicio. Purificazione Sabbia (SiO2) e carbone (C) in una fornace SiO2+2C→Si+2CO Metallurgic Grade Silicon (MGS) 98% Il Silicio è poverizzato e fatto Costo regire circa con HCl (gas) per fare trichlorosilane SiHCl3 3$/Kg un liquido ad alta pressione di vapore (bolle a 32°C ). Il trucco è che molte impurezze reagiscono con Cl e formano vari cloruri, ciascuno con diverso punto di ebollizione. Si+(Al,C) +3HCl(gas)→SiHCl3+H2+(Al,C,cloruri) Mediante distillazione frazionata si ottiene SiHCl3 di alta purezza 10-9. SiHCl3+H2→2Si+3HCl Electronic Grade Silicon (EGS) 10-9 Growth Techniques • Czochralski Method (LEC) (Bulk Crystals) • Chemical Vapor Deposition (CVD) (Thin films; epitaxial film growth) – Metal-Organic Chemical Vapor Deposition (MOCVD) • Molecular Beam Epitaxy (MBE) (Thin films) • Liquid Phase Epitaxy (LPE) (Thin films) Czochralski Method Bridgeman Method a temperature gradient along the crucible growth speed ~ 2 - 3 mm/minute O, C are contaminants! Czochralski growth (1916) 32 inch , 80 cm Ora si cresce in modo assai più raffinato… Heterointerfaces AlAs AlAs Thin Film Growth (General) • High Quality Film (1µm or less thickness) deposited on high quality substrate. • To minimize strain, need crystal structure of film & substrate to be ~ same (at least very similar) • Epitaxy: “in an ordered way” Homoepitaxy: same structure as substrate Heteroepitaxy: different structure than substrate Epitaxial growth: crescita ordinata Chemical Vapor Deposition (CVD) • Example reaction: SiH4 (heat) (Silane gas) Si + (On substrate) 2 H2 (gas) • Reaction occurs in a sealed container (reactor) • NOTE!! Silane gas is highly toxic & highly explosive!! • NOTE!! Hydrogen gas is highly explosive!!!! Metal-Organic Chemical Vapor Deposition (MOCVD) • Example reaction: Ga(CH3)3 + (Metal-organic gas) 3CH4 (Methane gas) AsH3 (Arsene gas) + GaAs (on substrate) • Reaction occurs in a sealed container (reactor) • NOTE!! Arsene gas is highly toxic and highly flamable!! Methane gas is highly explosive! MOCVD Dopants are introduced in precisely controlled amounts! Molecular Beam Epitaxy (MBE) • Thin film growth under ultra high vacuum. • Reactants introduced by molecular beams. • Create beams by heating source of material in an effusion (or Knudsen) cell. • Several sources, several beams of different materials aimed at substrate Can deposit 1 atomic layer or less! • A very precisely defined mixture of atoms to give EXACTLY the desired material com MBE RHEED: Used with MOCVD & MBE electron beam probe to monitor surface film quality One period of oscillation growth of one atomic layer of GaAs (or whatever material) MOCVD vs. MBE MBE • Mainly useful for research lab experiments. Not efficient for mass production! • High quality • Low growth rate MOCVD • Useful for lab experiments and for mass production! • Good-high quality • High growth rate Liquid Phase Epitaxy (LPE) (GaAs and other III-V materials) • Group III metal utilized as solvent for As • Solvent cooled in contact with (GaAs) substrate. Becomes saturated with As. Nucleation of GaAs on substrate. • Slider, containing different solutes, can grow precise compositions of material LPE Heterointerfaces AlAs AlAs Leghe ternarie AlGaN, GaAsN Controllo dell’energia del gap proibito: dispositivi selettivi sull’energia dei fotoni ( rivelatori o emettitori luce in regioni spettrali definite ) Controllo omogeneità lega______controllo omogeneità proprietà Struttura a bande delle leghe GaAs AlAs Struttura a bande delle leghe P=Energy gap, lattice constant, etc. Legge di Vegard P(AxB1-xC)=xP(AC)+(1-x)P(BC) Struttura a bande delle leghe QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.