Silicio

1. 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.

2. Purificazione Sabbia (SiO2) e carbone (C) in una fornace SiO2+2C→Si+2CO Metallurgic Grade Silicon (MGS) 98% Il Silicio è poverizzato e fatto regire con HCl (gas) per fare trichlorosilane SiHCl3 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. Costo circa 3$/Kg 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

3. 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)

4. Czochralski Method Bridgeman Method a temperature gradient along the crucible  growth speed ~ 2 - 3 mm/minute O, C are contaminants!

5. Czochralski growth (1916)

6. 32 inch , 80 cm

7. AlAs AlAs Ora si cresce in modo assai più raffinato… Heterointerfaces

8. 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

11. Epitaxial growth: crescita ordinata

13. Chemical Vapor Deposition(CVD) • Example reaction: SiH4(heat)  Si + 2 H2 (Silane gas) (On substrate) (gas) • Reaction occurs in a sealed container (reactor) • NOTE!!Silane gas is highly toxic & highly explosive!! • NOTE!!Hydrogen gas is highly explosive!!!!

14. Metal-Organic Chemical Vapor Deposition(MOCVD) • Example reaction: Ga(CH3)3 + AsH3  (Metal-organic gas) (Arsene gas) 3CH4 + GaAs (Methane gas) (on substrate) • Reaction occurs in a sealed container (reactor) • NOTE!!Arsene gas is highly toxic and highly flamable!! Methane gas is highly explosive!

15. MOCVD Dopants are introduced in precisely controlled amounts!

16. 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

17. MBE

18. 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)

19. 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

20. 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

21. LPE

22. AlAs AlAs Heterointerfaces

23. 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

24. Struttura a bande delle leghe GaAs AlAs

25. Struttura a bande delle leghe P=Energy gap, lattice constant, etc. Legge di Vegard P(AxB1-xC)=xP(AC)+(1-x)P(BC)

26. Struttura a bande delle leghe