Semiconductor surfaces_

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					Semiconductor surfaces:
Group IV (Si, Ge) : diamond structure II-V compounds (GaAs, InP ..): Zincblende (ZnS) structure II-VI compound( ZnTe, CdSe..): wurzite structure Review article: C.B. Duke, Chem. Rev. 96, 1237 (1996) G.P Srivatava, Rep. Prog. Phys. 60, 561 (1997) J.A. Kuby, J.J. Boland, Surface Science Report 26, 61 (1996) Why do semiconductor surfaces reconstruct ? Principles 1. Surface energy may be lowered by atomic relaxation leading to the semiconducting nature of the system 2. Surface reconstruction may lower the energy by forming new bonds leading to a semiconductiong nature. 3. Surface relaxation or reconstruction obeys the electron counting rule: empty cation and filled anion for compound semiconductors . chemical bonding charge neutrality growth kinetics Fermi surface instability strain

Group IV surfaces
Daimond structure

Si (100) and Ge(100) surfaces Reconstructions: 2x1, 2x2, c(4x2), c(4x4)

Dangling bonds

* 

* 

*
D B dow n D B up



Energy Gain due to Asymmetric Dimer Formation
P.Kruger and J. Pollmann, Phys.Rev. Lett. 74,1155 (1995)

  
Symmetric dimer Dimer Easy.(eV): C ….. Si 0.14



Easy.
Asymmetric dimer Ge 0.30

Thermally induced dimer flipping on Si(100), but suppressed on Ge(100) - symmetric dimer: band overlap, metallic at room temperature

- asymmetric dimer: open band gap, semiconducting
Si(100)-2x1 Ge(100)-2x1

Comparision of C, Si and Ge Surface Dimers
  Ge

Surface dimer

c

c

Si

Si
Ge

HOMO diagram
- BE(kcal/mol)  H2X=XH2 Dimers C Si Ge 18 22 25 10-28 5-10 5

H3X-XH3 1.53 2.33 2.40

Bond length (A) H2X=XH2 Dimers 1.34 2.15 2.30 ~1.4 2.2 2.41

Tilted angle Dimers 0 4.6 16.4

Tilted surface dimers have weak - bonding and Zwitterion di-radical characters
C. Mui et al, JACS 124, 4027(2002), J . Phys. Chem. A. 104, 2457,(2000)

Si(100)-2x1

STM image of Si(100)

500x500 nm2 Ge (100)-2x1 reconstruction

20x20 nm2

- two domain structure - anisotropic stress along dimer: tensile across dimer: compressive - Chadi notation SA step: parallel to the dimer rows SB step: perpendicular the dimer rows DA,DB: doubel height steps

How to get a single domain 2x1 surface ? - 4o vicinal Si(100) surface - extended high temperature annealing (900~1000oC) - externally applied stress.

SI(111) surfaces: the cleavage plane of Si crystal reconstructions: 1x1, 2x1,2x2, 3x3, 5x5, 7x7,9x9, c(4x2)
cleaved 350C 400C

Si

2x1

5x5

7x7

Si(111)-2x1 Hanman (1961): bucking model Pandey (1981):  bonded chain model Haneman (1988): three bond scission model

- energy gap due the tilted -chainstructure: semiconducting

Si(111)-7x7 - 1983 G. Binning , H. Rohrer, the first STM image Phy. Rev. Lett. 50, 120 (1983) - 1985 Takayanagi: DAS (dimer adatom stacking fault) model J. Vac. Sci. Tech, A3, 1502 (1985), Surf. Sci. 164, 367 (1985) - Hallmark of successful piece of science. - Dangling bond removal via adatom decoration vs the energy cost in stacking fault and corner holes - 7x7 is the consequence of the ease of forming stacking fault (0.02eV per 1x1 unit mesh) and corner hole formation - not all dangling bonds are removed: remaining electrons from nonbonding 2D surface state bonds : metallic surface - 49 (7x7) dangling bonds reduced to 19 dangling bonds per 7x7 unit cell - surface free energy: 7x7 is lower by 0.403 eV per 1x1 cell than the ideal Si(111)-1x1 surface. Ge(111) surface cleaved Ge(111): 2x1 annealing at 600K: c(2x8) Ge(111)-c(2x8) surface - 4 adatoms saturated all the substrate dannging bond except that of remaining rest atoms: semiconducting nature - 16 dangling bonds - Why do Ge(111) and Si(111) show different reconstructions ? Strain related: the lattice constant of Ge is 4% larger than Si (Si: 5.43A, Ge: 5.66A, C: 3.57A) - the energy cost for making corner hole and stacking faults is relatively higher than Si

STM image of Si(111)-7x7

- 12 adatoms in T4 sites in a 2x2-like arrangement - 6 rest atoms, 3-fold coordinated, between the adatoms in the 2nd layer - 9 dimers along the boundary of the faulted half of the surface unit cell (in the 3rd layer) - 1 corner hole (no atoms in the top three layer) - Total 102 Si atoms in the top three layers of the 7x7 unit cell 12 adatoms+42 atoms in the rest atom layers+48 atoms in the layers containing the stacking fault - Among 102 atoms, only 19 atoms(12 adatoms, 6 rest atoms, 1 corner

hole) possess dangling bonds

Ge(111)-c(2x8)

STM image of Ge(111)c(2x8) surface

III-V compound semiconductor GaAs, InP, InSb : Zincblende (ZnS) structure

GaAs(110), InP(110), InSb(110) surfaces - cleavage plane - unrelaxed surface zigzag chain As-Ga-As… metallic - relaxed surface As moves outward and Ga inward semiconducting: electrons move from Ga (3/4e)(empty) to As(5/4e) (filled)

VSEPR model predicts GaH3 is planar and :AsH3 pyramidal

GaAs(111) surface polar surfce Ga rich: (111A) or (111): exothermic 2x2 As rich: (111B) or (-1-1-1): endothermic 2x2,√3x√3, 3x3, √19x√19 GaAs(111)-(2x2) reconstruction - p(2x2) Ga vacancy structure is faovored - # of electrons =(3e/4 for each Ga bond)x3 +(5e/4 for each As bond)x3 = 6 enough to fill the anion dangling bonds and form semiconducting surface - rehybridization induced lowering energy stabilize the surfacce 3Ga-As bond energy: 3x1.7 eV = 5.1 eV filled As derived states 3xEg(band gap) = 4.5 eV

GaAs(-1-1-1)-2x2 - As trimer model - the surface contain completely occupied anion dangling bonds only

- Annealing above 530C, 2x2 is converted into √19x√19

GaAs(100) surface - a vaiety of structures as function of processing conditions and surface stoichiometry - the ratio[As]/[Ga] decreases: c(4x4),c(2x8), 1x6, 4x6, c(8x2) As rich Ga rich - subject of controversy

Adsorbate induced reconstruction
References V.G. Lifshits, A.A. Saranin, and A.V. Zotov, Surface Phases on Silicon, (Wiley,1994) G.P Srivatava, Rep. Prog. Phys. 60, 561 (1997) Adsorbates on diamond structure semiconductor Adsorbates on (100) surfaces Group I on Si(100), Ge(100) H on Si(100): 2x1, 3x1, 1x1 H on Ge(100): 2x1, 1x1 Alakali on Si(100): 2x1

H yd ro g en A d so rp ti n o n S i o (100)

2x1 Monohydride

3x1 Mono + Dihydride H2

1x1 Dihydride

300 k

400 k

Transition Mechanism Transition Mechanism

Group IIIA (Ga, In) on Si(100): 3x2, 2x2, 2x1 - parallel addimer model

Group IV (Ge ) on Si(100): 1x2 - asymmetric dimer, symmetric dimer Group V (As,Sb) on Si(100): 2x1 - Si dimers are broken apart

Group VI (S, Se) on Si(100): 1x1 Group VII (F, Cl, Br): 2x1 - covalent bonding - tilted dimer to the symmetric configuaration Adsorbates on (111) surfaces Group I on Si(111) and Ge(111) H on Si(111), Ge(111) - cleaved 2x1 →1x1 - 7x7 →1x1 Alkali metal on SI(111) - √3x√3-R30o , 3x1 Ag, Au on Si(111) - √3x√3-R30o, 5x2 Group III (Al, Ga, In, B) on Si(111) - √3x√3-R30o - two adatom geometry 3-fold symmetry hollow (H3), 4-fold atop sites(T4)

- B induced √3x√3-R30o is substitutional nature

Group IV (Ge, Pb)on Si(111)

- √3x√3-R30o - T4 is more stable than the three fold H3 geometry Group V (As,Sb, Bi) on Si(111) - As/Si(111)-1x1, As/Ge(111)-1x1 - Sb/SI(111)-√3x√3-R30o , Sb/Ge(111)-1x1 - Bi/Si(111)-1x1, Bi/Ge(111)-1x1 Group VII (F,Cl, Br)on Si(111) - 1x1 - one fold coordinated covalent site geometry J.J. Boland et al, Phys. Rev. Lett. 78, 98 (1997)