Silicon dioxide SiO2 or silica



Silica is one of the most abundant substances, it is used widely in microelectronics and optics. The building blocks of silica crystals are SiO4 tetrahedra which combine in different polymorphic forms (the phase diagram is from

α-quartz 3* 154 P3221 enantiomorphic with P3121
β-quartz 3 180 P6222 C8, enantiomorphic with P6422
α-crystobalite 4 92 P41212 enantiomorphic with P43212
β-crystobalite 2 227 Fd-3m C9 (Si with O in bonds), see also cubic (8*, P213) and tetragonal (2, I-42d) distortions
α-tridymite 4 20 C2221 see also monoclinic (24, Cc) distortion
β-tridymite 3 194 P63/mmc C10
coesite 8 15 C2/c
stishovite 2 136 P42/mnm C4
α-moganite 6 15 C2/c
β-moganite 6 72 Ibam

* Number of units per primitive cell

Note that α and β forms are respectively low and high temperature forms. Other crystal structures include keatite (12, P41212), high pressure crystobalite (8, P21/c), next to stishovite high pressure polymorph (4, Pa-3). Other forms include lechatelierite (amorphous fused silica), vitreous silica (amorphous silica in glass phase), silica zeolites (example), silica aerogels, and other forms.

Band structure

Optical bandgap (fundamental absorption edge): 9.0 eV (α-quartz)

Band structure of α-quartz [Gritsenko95,DiPomponio95]:

9 eVdelocalized Si s,d and O s electrons
0 ÷ 9 eVbandgap
-4 ÷ 0 eVO 2p2 nonbonding electrons, perpendicular to O–Si–O plane
-10 ÷ -4 eVSi 3sp3 and O 2p2 bonding electrons, extend to 0 eV
-20 eVO 2s2 core electrons

Defects [Roma04]

Oxygen deficient centers (ODC):

neutral oxygen vacancy (NOV) or ODC(I)≡Si–Si≡2.68 Å vs. 2.35 Å in bulk Si, [Mukhopadhyay05b]
paramagnetic E' center≡Si·
E'γ, positively charged puckered (back projected) oxygen vacancy≡Si–Si≡ + h → ≡Si++ ·Si≡[Buscarino06,Mukhopadhyay05a]
E'δ (??), dimer configuration E' center≡Si–Si≡ + h → ≡Si·Si≡[Mukhopadhyay05a]
twofold-coordinated Si or ODC(II) (hole center)=Si:

Oxygen related centers:

nonbridging oxygen (NBO) or nonbridging oxygen hole center (NBOHC)≡Si–O·[Suzuki03]
peroxy radical (POR)≡Si–O–O·
peroxy linkage (POL)≡Si–O–O–Si≡

Excitons are self-trapped (STE) in the following configuration: has its electronic component in (≡Si·) and hole component in (≡Si–O·) [Ismail-Beigi05].

Optical absorption

Absorption bands inside bandgap (decomposed):

5-6  ≡Si·[Mukhopadhyay05a]
7.6  NOV[Mukhopadhyay05b,Agnello03,Cannas04]


Emission bands (decomposed):

 1.90.1-0.28-15 µsNBOHC[Cannas06,Bakos04,Glinka02]
γ2.7-2.80.75-10 µsNOV(Tσ*→σ)[Glinka02,Meinardi98]
β3.1-3.20.4 STE[Cannas02,Meinardi98]
α4.2-4.40.54-10 nsSTE,ODC(II)double structure, [Cannas02,Meinardi98,Agnello03]

Surface's NBOHC has 1.8 eV and decay times up to 1 ms. Lifetimes are typically grow up with disorder [Glinka02].