MBE - Studenti di Fisica

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
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