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Atomistic aspect of silicon surfaceprocesses studied by in-situ electron
microscopy and atomic force microscopy
Institute of Semiconductor Physics, Novosibirsk,
RUSSIA
Autumn School ««Electron microscopy for tomorrow industrial materials», Berlin 2005
Laboratory of Electron Microscopy and Submicron Structures
Alexander Latyshev
•• Diagnostics Diagnostics of Lowof Low--Dimension Dimension System with Atomic ResolutionSystem with Atomic Resolution(HREM, SEM, AFM, STM, unique (HREM, SEM, AFM, STM, unique UHVUHV--REM, exREM, ex-- and in situ and in situ characterizations) characterizations)
•• Methods of Atomic Methods of Atomic ManipulationsManipulations ((atomic processes at atomic processes at surface, interface and bulk, MBE, selfsurface, interface and bulk, MBE, self--organization et alorganization et al..))
OUTOUT--LINELINE
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HRTEMHRTEM SEMSEM
STMSTMAFMAFM
Imaging techniquesImaging techniques
Si(111)Si(111)
((7x7) 7x7) reconstructionreconstruction,,
Omicron,Omicron,((S.TeysS.Teys))
Au particles Au particles on graphite,on graphite,
LEOLEO--1430,1430,((T.GavrilovaT.Gavrilova))
Si(111),Si(111),
2D2D--islandsislands
NTNT--MDT,MDT,((E.RodyakinaE.Rodyakina))
Silicon,Silicon,JEMJEM--4000EX,4000EX,
((A.GutakovskiiA.Gutakovskii))
High resolution electron microscopy image
of cross-section of SiO2 - Si(111) interface. Monatomic steps, 0.31nm in height, are shown.
TEM Studies of Surface structure
Transmission electron microscopy
image of a carbon replicadecorated by gold particles from
Si(111) surface. Surface morphology including monatomic steps and step pinning centers can be defined from analyses of gold island distribution.
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Scattering process of electron beam at resonance conditions
incident beam
The incidence angle is usually a few degrees in order to restrict the penetration depth into the bulk of crystal.
TETEММ
screenscreen
REMREM samplesample
screen
objectiveobjective
apertureaperture
UHV Reflection Electron Microscopy
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Monatomic steps on Si(111)
0,4 0,4 µµmm
1010
T=T=1010000000CC
Typical REM-image and schematicalrepresentation of atomically clean silicon (111) surface with system of monatomic steps, 0.31nm in height.
9500C
1280128000CC1 1 µµmm
200C
Foreshortening effect x4
5
22 3311
T=1190T=119000CC
Step Pinning, Step Motion and 2D Nucleation
Step Pinning CentersStep Pinning Centers
Step MotionStep Motion
1 1 µµmm
w
wd
wd
w
2D Nucleation2D Nucleation(pits)(pits)
Study of surface dynamics in continuum models allows to calculate the step free energy cost and the
effective interaction between neighboring steps, step-nuclear, nuclear-nuclear and step-pinning center.
MONATOMIC STEP MOTION
Velocity of straight stepsVelocity of straight steps: VVstepstep((∆µ∆µ)=)=ΘΘ[exp([exp(∆µ∆µ//kTkT) ) --1]1]wherewhere ΘΘ =2=2νλνλss exp(exp(--W/kT)tanh(x/2W/kT)tanh(x/2λλss)) is kinetic coefficient,is kinetic coefficient,
λλss is the adatom diffusion length,is the adatom diffusion length,WW is the sublimation energy,is the sublimation energy,νν is the frequency of atom vibration,is the frequency of atom vibration,x is the is the interstepinterstep distance,distance,∆µ∆µ is a shift of the chemical potential and is a shift of the chemical potential and k is is BoltzmannBoltzmann’’ss constant.constant.
Velocity of a curved step (Velocity of a curved step (at a spatially nonat a spatially non--uniform chemical uniform chemical potentialpotential):):
VVcurvcurv((∆µ∆µ,K,U,K,U)=)=ΘΘ[exp([exp(∆µ∆µ//kTkT) ) -- exp(exp(ΩΩU/kTU/kT + + ΩβΩβK/K/kTkT)])]wherewhere UU is the density of the surface strain energy, is the density of the surface strain energy, ΩΩ is atomic area,is atomic area,
ββ is the step stiffness and K is the step stiffness and K is the step curvature.is the step curvature.
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Determination of basic parameters of atomic processes on silicon surface
Sublimation energy, Sublimation energy, W=4,2W=4,2±±0,20,2эВэВDiffusion coefficient, DDiffusion coefficient, Dss(T)(T)Adatom density, nAdatom density, nss(T)(T)Diffusion length, Diffusion length, λλss(T)(T)Migration energy, EMigration energy, Ess=1,2=1,2±±0,20,2эВэВShcwoebel’sShcwoebel’s barrier,barrier, ∆∆EEss=0,1=0,1÷÷0,20,2эВэВStep edge stiffness,Step edge stiffness, ββ≈≈1010--1212 J/cmJ/cm
1200120000CC 1170117000CC
1130113000CC1090109000CC
Step
rate
, nm
/c
Terrace width, µm
10000/T
Step
rate
, A/c
W=4,2±0,2eV
Adaptation of BCF theoryAdaptation of BCF theory..
Optimization treatments of MBE Optimization treatments of MBE
Computer simulationsComputer simulations..
Atomic Force Microscopy SOLVER P-47H (NT-MDT))
• For stability reasons, stiff cantilevers were preferable minimizing the effects of tip-substrate forces.
• To reduce noise contribution of external electromagnetic fields, the AFM apparatus have been positioned inside of a metal box having a good electric connection to ground.
• Additional rubber bearers for this box have been installed reducing mechanical vibration noise.
• The both temperature and humidity of atmosphere inside of the box were controlled during AFM scanning.
The both contact and semi-contact (frequency-
modulation) modes were performed.
scan 47µm x 47µm
Scan 47µm x 47µm
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SEMSEM
SEMSEM
10 nm
HREMHREM
0,31 nm0,31 nm
NoncontactNoncontact Silicon CantileverSilicon Cantilever
NSG20NSG20
Stepped Silicon SurfaceStepped Silicon Surface
terrace width = 0,314nm × arctg(α)
typical AFM-image
αα
α, degree
terrace, nm
5 12 1 18
0,5 36 0,1 180
0,01 1799
Laboratory of Electron Microscopy and Submicron StructuresLaboratory of Electron Microscopy and Submicron Structures
Si(111)Si(111)
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GaAs(001)GaAs(001)
MacrostepsMacrosteps on on Si(111)Si(111)
Analysis of step distribution on Si(111)
Typical ACM images and schematical representation of the surface with steps in height of one interplanes distances. To model the step motion in the frames of a linear kinetics approximation, one can take into account the changes in free energy.
7х7мкм7х7мкм22
Si(111)Si(111)
44хх44мкммкм22 55хх55мкммкм22
ступени эшелоны ступениµµmm0.50.5
Kinetic instability ofKinetic instability of Si(111)Si(111)
830 1050 1250 1350
bunchingbunching
bunchingbunchingbunchingbunching
stepssteps
stepssteps
stepssteps
regular
regular regular
The surface morphology depends on the direction of direct electric current which used for sample heating.
Temperature ( 0C)
T=1280T=128000CC
Real timeReal time
0,5 0,5 µµmm
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1,8 nm1,8 nm
height spectra
Temporal Dependence of Step Bunching
1
10
100
0,1 1 10 100
t I m e (m I n)
n um
b er
of
s t e
ps
The solid line represents the best fitted power law function with an exponent
equalled to 0.45±0.5.
JJstepstep--downdown
JJstepstep--upup
JJstepstep--down down ≠≠ JJstepstep--upup
bunc
hbu
nch
antianti--bunch
bunch
1 1 µµmm
LL
20 nm < L < 20 µm
dd
bunch
antianti--bunchbunchOne-dimensional
nanostructure fabrication through self-ordering of monatomic steps to anti-
bunches
aa
bb
cc
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Regular steps Step bunches Anti-bunches
Dislocation at the surface
Si(111)
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Large Terrace on the Silicon Surface
•• Surface : 5842.1 nm x 5895.9 nmSurface : 5842.1 nm x 5895.9 nm•• Peak to peak, Peak to peak, RRmaxmax = 5.952 = 5.952 AAoo
•• Mean, Mean, RRmeanmean = 5.581 = 5.581 AAoo
•• Roughness, RRoughness, Raa = 0.587 = 0.587 AAoo
•• RootRoot--MeanMean--Sq, Sq, RRqq = 0.723 = 0.723 AAoo
•• SkewnessSkewness, , RRsksk = = --0.0790.079•• Kurtosis , Kurtosis , RRkuku = 2.738= 2.738
Laboratory of Electron Microscopy and Submicron StructuresLaboratory of Electron Microscopy and Submicron Structures
Phys. LowPhys. Low--Dim.Structure 5 (2002) 231.Dim.Structure 5 (2002) 231.
Typical AFM image of Si(111) surface with
extremely wide terraces.
Huge step-free area after quenching and annealing
at 5600C
11
22
12 x 12 µm2
2211
• two-dimensional islands
• two types of islands:positive and negative
• number of positive islands is larger than negative ones
0,3 nm
0,3 nm0,3 nm
• positive islands have circular shape
• negative ones have triangular
• negative islands reveal their equilibrium shape
[110]
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enlargingenlarginginitialinitial nucleationnucleation
Analysis of structural-morphological peculiarities of low-dimensional system
9900х00х9900нм00нм2244хх44мкммкм22 44хх44мкммкм22
АFM-images of initial stages of epitaxial growth germanium islands on silicon substrate. Size dependence and islands distribution were studied in details. Typical TEM-image of germanium island on the silicon (111) surface is presented also.
TEMTEM
Statistical analysis of islands distribution on vicinal Si(111)
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0 1 2 3 4
step-step spacing, µm
isla
nd s
ize,
µm
The The characteristicalcharacteristicallength of changing the length of changing the mechanisms of mechanisms of epitaxial growth from epitaxial growth from stepstep--flow to twoflow to two--dimensional dimensional nucleation.nucleation.
0,001
0,01
0,1
0,8 0,9 1 1,1 1,2
1000 / T (1/K)
criti
cal l
engt
h
EEss=1,25=1,25±±0,15eV0,15eV
d >γλs / kBT
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Chemical reaction betweenChemical reaction between SiSi and Oand O22
Thermal etching by means of oxidation reactions at the surface for nanosructure formation method
Step motionisland
dкр=2λvd<dкрd>dкр
λv
Step flowStep flow 2D nucleation of pits2D nucleation of pits
Si + O(ads) Si + O(ads) →→ SiO(gasSiO(gas) ) ---- etchingetching
(1x1)(1x1)
(7x7)(7x7)
ААFMFM--imageimage (8х8(8х8mmmm22) ) of negative islands (pits) of negative islands (pits) on the silicon (111) surface, which shape is on the silicon (111) surface, which shape is depended on the surface reconstructiondepended on the surface reconstruction..
Initial stages of silicon oxidation as method of formation of dielectric wires
degreasing intensity of RHEED patterndegreasing intensity of RHEED patternpinning step motionpinning step motion
««erosionerosion» » of step edge of step edge
OutOut--phase boundariesphase boundarieserosion oferosion of step edgestep edge““noise” on terracesnoise” on terraces
O2(gas) → O(ads) + O(ads) Si + O(ads) → SiO(gas) -- травлениеSi + 2O(ads) → SiO2 -- окисление
AFMAFM--imagesimages (4х4(4х4µµmm22) ) of Si (111) of Si (111) surface at initial stages of surface at initial stages of oxidationoxidation..
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phase contrastphase contrasttopographytopography enlarged fragmentenlarged fragment
3D3D--image of TFT treatmentimage of TFT treatment
StepStep--flow growthflow growth
at initial stagesat initial stages
of of GeGe depositiondeposition
Step wandering and local step curvature
0
5
10
15
20
25
0 0,2 0,4step-step spacing
RM
S de
viat
ion
Statistic analysis ofStatistic analysis of amplitude and shorthort--&&--long wavelong wavelength fluctuations allows to evaluate the stepfluctuations allows to evaluate the step--step interaction and step interaction and step edge stiffness ( <X step edge stiffness ( <X 22> = kTL/12> = kTL/12ββ).).
Laboratory of Electron Microscopy and Submicron Structures
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Stress induced by screw dislocation at the surface
Laboratory of Electron Microscopy and Submicron Structures
The size of twoThe size of two--dimensional germanium islands is smaller around outdimensional germanium islands is smaller around out--point of dislocation emerging at the surface probably due to locpoint of dislocation emerging at the surface probably due to local al deformation having asymmetric shape at the dislocation coredeformation having asymmetric shape at the dislocation core..
topographictopographic phase contrastphase contrast
0
100
200
300
400
500
0 1 2 3 4 5 6 7 8 9top terrace (µм)
denu
ded
zone
(nм
)
Denuded zones and sizeDenuded zones and size--dimension of islands allows dimension of islands allows to deduce the distribution of to deduce the distribution of
germanium adatoms near germanium adatoms near steps and along terraces. steps and along terraces.
Dependence of denuded zones on terrace widthDependence of denuded zones on terrace width
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Shape and size of germanium twoShape and size of germanium two--dimensional islandsdimensional islands
Laboratory of Electron Microscopy and Submicron Structures
The islands reveal their equilibrium shape at high temperature.The islands reveal their equilibrium shape at high temperature.
Equilibrium for small islands can be achieved easily. Equilibrium for small islands can be achieved easily.
T = 490T = 49000CC T = 670T = 67000CC T = 790T = 79000CC
Multilayer growth
•Barrier-limited growth of multilayer islands.
•The driving force is the local gradient of chemical potential.
•Ehrlich-Schwoebel barrier for incorporation.
•Interlayer transport mechanism.
•Out-phase boundaries and denuded zones
Laboratory of Electron Microscopy and Submicron Structures
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Laboratory of Electron Microscopy and Submicron Structures
Germanium on Silicon
• denuded zones near steps
• triangular islands on terrace
• the same orientation of triangular islands
• equilibrium shape
• step edge barrier-limited growth
STM image of a Si(001) surface. STM image of a Si(001) surface. Scale is ~110 nm square. Scale is ~110 nm square.
Published by Prof. Max Published by Prof. Max LagallyLagally..
Structure of silicon (001) surfaceStructure of silicon (001) surface
•• atomic steps (a/2 in height) ~ 0,28 nm atomic steps (a/2 in height) ~ 0,28 nm
•• atomic steps (a/4 in height) ~ 0,14 nmatomic steps (a/4 in height) ~ 0,14 nm
where a is a lattice parameter.where a is a lattice parameter.
Typical AFM-images of silicon (001) surface: topography and phase contrast.
Si(001)Si(001)
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Stages of step bunching on Si(001)
t = 0 ct = 0 c t = 8 ct = 8 c t = 34 ct = 34 c t = 120 ct = 120 c
T = 860T = 86000CC
Step bunching on the silicon (001) surface at step-down direction of direct current using for sample heating.
Monatomic steps on Si(001) and step anisotropy Monatomic steps on Si(001) and step anisotropy induced by dimer row of reconstruction induced by dimer row of reconstruction
Laboratory of Electron Microscopy and Submicron Structures
19
Step bunching on silicon (001) surface
Fragments of surface morphology of silicon (001) Fragments of surface morphology of silicon (001) surface during sample heating by direct electric surface during sample heating by direct electric current flowing through the sample.current flowing through the sample.
Schematic presentation of influence of heating Schematic presentation of influence of heating current on the step pairing during thermal current on the step pairing during thermal annealing.annealing.
The average terrace width in the bunch depends The average terrace width in the bunch depends on the number steps: on the number steps: (W(W11 + W+ W22) > W) > W3 3 andand (W(W11 + W+ W22) = (W) = (W44 + W+ W55) )
W1
W5
W2
W3
W4
AFM “movie” for stepAFM “movie” for step--step interaction step interaction
1,6x1,61,6x1,6µµmm22
2,5x2,52,5x2,5µµmm22 2,5x2,52,5x2,5µµmm22
3,5x3,53,5x3,5µµmm22
3,5x3,53,5x3,5µµmm22
•• step pairing instability step pairing instability
•• nanonano--bridges between bridges between step pairsstep pairs
•• size modulation along size modulation along bridgebridge
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Attractive interaction between Attractive interaction between monolayer pit and atomic steps monolayer pit and atomic steps
on Si(001)on Si(001)
f
ff
f
Stress anisotropyStress anisotropy
The opposite sign of steps – attractive force.
The same sign of steps – repulsion force.
•• Basic studies of semiconductor surfaces Basic studies of semiconductor surfaces and interfaces by means of diagnostics and interfaces by means of diagnostics with with nanoscalenanoscale and atomic resolution and atomic resolution (HREM, SEM, AFM, STM, unique UHV(HREM, SEM, AFM, STM, unique UHV--REM, REM, exex-- and in situ characterizationsand in situ characterizations..
• Development of diagnostics and methods of atomic manipulations (atomic processes at surface, interface and bulk, self-organization, ordering et al.).
Conclusion :
