Part 1' Thin Films and Layered Structure

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Part 1. Thin Films and Layered Structure
? Thin films and layered structure are widely
used in the present technologies IC,
optoelectronics, recording media, etc.
Examples Silicide layer formed by various
methods (SSR), Light emitting diode, laser
(heteroepitaxial structure), ITO (Indium
Tin Oxide) as transparent conducting layer,
CoPtCr layer as HD magnetic layer ? Films and
layers could be prepared using different
methods. The most important one is vapor phase
deposition (including physical and chemical
vapors) gt Atoms in vapor phase, surface,
atoms on surface
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? Growth and Deposition For purity reason
films are typically deposited in vacuum gt
impinging flux of atoms from the background gas
pressure
P pressure (N/m2) V molar volume, the volume
of one mole of gas R gas constant 8.31
J/K-mole NA Avogadros number 6.02?1023
molecules/mole kB the Boltzmanns constant
1.38?10-23 J/K T absolute temperature (K)
Unit of pressure 1 mm Hg 1 Torr 133.3 N/m2
133.3 Pa
For P 1 atm at RT the molar volume of an ideal
gas is
J/k-mole ? k
N/m2
(J N.m)
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gt Gas density is
molecules/cm3
For P 1 atm gt n 2.45 ? 1019 molecules/cm3
P 1 Torr gt n 3.22 ? 1016
molecules/cm3 P 10-7 Torr gt n 3.22 ?
109 molecules/cm3 P 10-10 Torr gt n
3.22 ? 106 molecules/cm3
? The flux of atoms impinging on a surface is
given by the product of particle
concentration and velocity.
/cm3 (concentration) ? cm/s (velocity)
/cm2-s of molecules impinging on a unit
surface per unit time
For ideal gas, the mean kinetic energy of a
molecule
Ek mean kinetic energy Va velocity of a
molecule m mass of the molecule
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M molar weight of the molecule
Take N2 for example, M 28 g/mole
J (work F?x ma?x) N.m kg m/s2?m kg
m2s-2 gt (J/g)1/2 (1000 m2s-2)1/2 31.623 m/s
sound velocity of a gas
For a time interval of t, number of molecules
with velocity Va that could hit the surface of
area A can be estimated as the following
Va
If a molecule is at a distant larger than Va?t
away from the surface (say l1 gt Va?t), the
molecule wont Reach the surface at a time
interval of t. Total number of molecules that
could reach the surface at a time interval of t
is roughly n?Va?t?A.
Va?t
l1
A
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The impinging rate of molecules per unit area per
unit time (rc) is
Using this relation, one could estimate the time
for a monolayer of atoms to be collected on a
substrate surface when we expose it to an gas at
certain pressure. (Assuming the sticking
coefficient of the gas to be 1)
Take Va for 500 m/s (50000 cm/s) At 1 atm, n
2.45 ? 1019 molecules/cm3, rc 1.225 ?
1024 molecules/cm2-s gt to complete one monolayer
( 1015 atoms/cm2) takes 1015/(1.225?1024)
10-9 sec! At 10-10 Torr, n 3.22 ? 106
molecules/cm3, rc 1.61 ? 1011 molecules/cm2-s
gt to complete one monolayer takes 1015/(1.61?1011
) 6.2?103 sec! 17 hr.
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For epitaxial growth rate of one monolayer/sec
and with purity of 99.9, the impinging rate of
residual gas should be less than 10-3 times the
deposition rate, i.e. 1012 molecules/cm2-s, gt
the corresponding vacuum is 10-9 Torr!
? In the above estimation, the velocity
distribution of the gas molecules and the
concentration gradient of the gas are not
considered. A more complete derivation shows
that the flux is given by
rc is over-estimated by a factor of 4 to 5 times
larger than the actual case!
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? Surfaces The surface of a semiconductor is
the typical starting places for growth. There
are microscopic (atomic displacements) and
macroscopic structure (steps, ledges, etc.)
on a surface. ? Atomically Surface
could reconstruct to minimize the surface
energy. For example, a ball and stick model of Si
(100) surface
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From http//www.chem.qmw.ac.uk/surfaces/scc/scat1
_6a.htm or http//www.fhi-berlin.mpg.de/th/person
al/hermann/pictures.html
Si (100) surface
reconstructed
unreconstructed
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? Macroscopically Surface can contain
terraces, steps, and defects (vacancy).
adatom
kink
cluster
step
?
L0
h
vacancy
terrace
Average step spacing (L0) is related to
miscut of the crystal (miscut angle ?). ?
tan(h/L0) h/L0
For ? 0.1o, monolayer step of Si 100 surface h
a/4 0.543 nm/4 gt L0 77.8 nm For miscut of
Si 100 surface by 0.5o, gt L0 77.8/5 nm 15.6
nm
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? Steps is far from perfect! Steps and
terraces form because low index planes,
such as 100 plane, are more stable than
higher index planes.
Two types of steps (a) Straight low energy steps
SA (b) Rough high energy step SB
Si (100) surface
SB
SA
From Prof. Max Lagally, MSE, Wisconsin
Madison http//mrgcvd.engr.wisc.edu/lagallygroup/

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? Surface layer of compound semiconductor
must be further characterized by their
atomic composition. For example, the (100)
and (111) surface of GaAs contain either
all Ga atoms (A face Ga terminated
surface) or all As atoms (B face As terminated
surface).
? Atoms on a surface An atom on a solid
surface sees an array of binding sites or
potential wells formed by substrate atoms.
Adatoms can hop from one well to another
(surface diffusion) and can escape from the
well to vacuum (desorption). ? Representation
of the kinetics of the surface diffusion
and desorption
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? Desorption is defined as
Probability of successful attempt
Number of attempts
vs vibration frequency of surface atoms ( 10-13
sec-1) ?Gdes change in free energy associated
with desorbing one atom
gtthe residence time ?s of an atom on the
surface is
During the residence time ?s, an atom can
move from one site to a neighboring site
(diffusion) with a frequency
?GmS energy required to move to a
neighboring site.
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The surface diffusivity is defined as
? the jump distance between two neighboring
sites
In typical growth condition, desorption is
less likely than surface diffusion due to
larger activation energy. For example For
the (111) surface of a FCC noble metal we
typically have ?Gdes and ?GmS to be about 1 eV
and 0.5 eV, respectively.
Using the concepts gt describe the behavior
of an atom on a stepped surface with mean
step spacing of L0.
For the atoms to reach the step before
desorption gt
Assume stable site at step or kinks
cm2/s ? s1/2 cm
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The diffusion time for an atom to reach the
step is
Define the sticking coefficient to be the
ratio of
For Sc ? 1, an atom will diffuse to the step and
become bound on the surface. For Sc lt 1,
desorption can occur.
Using the criteria and using ?2 10-15
cm2, L0 10-5 cm, ?Gdes 1 eV , and ?Gms
0.5 eV gt at T 300 K ? Sc 2.47 ?
103 at T 600 K ? Sc 1.57 ? 10-1
? Actual case is much more complex than the
simplest estimation! For example,
interaction between atoms to form clusters
(islands). Other factors including surface
energy and strain energy, etc.