Silicio Per massa il Silicio circa il 26

Silicio Per massa il Silicio è circa il 26% della crosta terrestre, (principalmente nella forma di silice o quarzo cristallino (Si. O 2) ) , ed è il secondo elemento per abbondanza, dopo l’ossigeno. Molto raro è il cristallo di silicio.

Purificazione Sabbia (Si. O 2) e carbone (C) in una fornace Si. O 2+2 C→Si+2 CO Metallurgic Grade Silicon (MGS) 98% Il Silicio è poverizzato e fatto Costo regire circa con HCl (gas) per fare trichlorosilane Si. HCl 3 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) +3 HCl(gas)→Si. HCl 3+H 2+(Al, C, cloruri) Mediante distillazione frazionata si ottiene Si. HCl 3 di alta purezza 10 -9. Si. HCl 3+H 2→ 2 Si+3 HCl 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 Al. As

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: Si. H 4 (heat) (Silane gas) Si + (On substrate) 2 H 2 (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(CH 3)3 + (Metal-organic gas) 3 CH 4 (Methane gas) + As. H 3 (Arsene gas) Ga. As (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 Ga. As (or whatever material)

MOCVD vs. 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) (Ga. As and other III-V materials) • Group III metal utilized as solvent for As • Solvent cooled in contact with (Ga. As) substrate. Becomes saturated with As. Nucleation of Ga. As on substrate. • Slider, containing different solutes, can grow precise compositions of material

LPE

Heterointerfaces Al. As

Leghe ternarie Al. Ga. N, Ga. As. N 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 Ga. As Al. As

Struttura a bande delle leghe Legge di Vegard P=Energy gap, lattice constant, etc. P(Ax. B 1 -x. C)=x. P(AC)+(1 -x)P(BC)

Struttura a bande delle leghe