Transcript Document

PROPERTIES OF NONTHERMAL CAPACITIVELY
COUPLED PLASMAS GENERATED IN NARROW
QUARTZ TUBES FOR SYNTHESIS OF SILICON
NANOPARTICLES*
Sang-Heon Songa), Romain Le Picardb), Steven L. Girshickb),
Uwe R. Kortshagenb), and Mark J. Kushnera)
a)University
of Michigan, Ann Arbor, MI 48109, USA
[email protected], [email protected]
b)University
of Minnesota, Minneapolis, MN 55455, USA
[email protected], [email protected], [email protected]
40th IEEE International Conference on Plasma Science (ICOPS)
San Francisco, USA, June16-21, 2013
*
Work supported by National Science Foundation and DOE Plasma Science Center.
AGENDA
 Plasma nanoparticle synthesis
 Description of the model
 Typical Ar/SiH4 plasma properties
 Nanoparticle density
 Power
 Pressure
 Flow rate
 SiH4 fraction
 Concluding remarks
ICOPS_2013
University of Michigan
Institute for Plasma Science & Engr.
NANOCRYSTALS (QUANTUM DOT)
 Size-dependent photoluminescence from Si nanocrystals
 Si nanocrystals fluoresce
with properties akin to
direct band-gap
semiconductors. The
emission wavelength is a
function of the size of the
nanocrystal.
 Applications
 Photovoltaic device
 Light emitting device
 Quantum computing
 Biological imaging
Ref: I. L. Medintz et al., Nature Material 4, 435 (2005).
ICOPS_2013
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PLASMA-SYNTHESIZED
SILICON NANOCRYSTALS
 Gas-phase plasma processes for Si nanocrystal production are environmentally
friendly without producing liquid effluents.
 The silicon nanoparticles (SiNP) are formed
by clustering of the dissociation products of
SiH4 passing through the plasma zone.
 Exothermic reactions of H-atoms on the
surface of nanoparticles likely produce
temperatures sufficient to anneal
amorphous particles to crystals.
 The quality of silicon nanocrystal (SiNC) can
be controlled by injecting additional gases
downstream of the primary plasma.
Ref: R. J. Anthony et al., Adv. Funct. Mater. 21,
4042 (2011).
ICOPS_2013
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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
 Fluid Kinetics Module:
 Heavy particles – Continuity, momentum, and
energy equations
 Electron – Continuity and energy equations
 Poisson’s equation
 Electron Monte-Carlo Module (eMCS):
 Secondary electron emission
 HPEM is parallelized using OpenMP
 Parallel successive over relaxation (SOR) utilized
red-black scheme for electron energy, gas
temperature, and Poisson’s equations.
 eMCS optimized for parallel execution
ICOPS_2013
University of Michigan
Institute for Plasma Science & Engr.
GLOBAL CHEMISTRY MODEL (GLOCHE)
Time Dependent
Kinetics
Gas Phase Reaction
Mechanism
 Plug flow reactor model
 Reaction mechanism is compatible with HPEM.
 Time dependent gas phase reaction kinetics are calculated using
predictor-corrector scheme (Adams-Bashforth-Moulton method).
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REACTOR GEOMETRY: CCP TUBE
 2D, cylindrically symmetric
 Tube radius = 0.3 cm
 Electrode separation = 2.2 cm
 Operating conditions
 Ar/SiH4 = 95/5 (range 99/1 – 90/10)
 Pressure = 2 Torr (range 0.5 – 4 Torr)
 Flow rate = 50 sccm (range 10 – 100 sccm)
 Frequency = 25 MHz
 Power = 1 W (range 1 – 10 W)
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NUCLEATION REACTIONS BY NEUTRALS
 84 species are included in the mechanism
 Medium sized silicon hydride = 63 species
 Reaction hierarchy up to Si10H20. Higher silanes are “particles”
 Nucleation reactions with neutrals
 28 reactions: Silyl formation by H abstraction.
 SinH2m + H → SinH2m-1 + H2
k =2.44×10–16T1.9exp(–2190/T) cm3/s
 106 reactions for making higher silanes
 SinH2m-1 + SijH2k-1 → Sin+jH2m+2k-2
k = 3.32 × 10−9 cm3/s
 321 reactions for making particle (n + j ≥ 11)
 SinH2m-1 + SijH2k-1 → particle
Ref: U. V. Bhandarkar et al., J. Phys. D: Appl.
Phys. 33, 2731 (2000)
ICOPS_2013
k = 2.66 × 10-11Tg0.5 cm3/s
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 [e]
 Te
 Tgas
 SiH4
PLASMA DENSITY and
TEMPERATURES
 Highest quality nano-crystals
are produced with only a few W
of power deposition.
 Moderate gas heating to 364 K
with 90% depletion of SiH4
indicates electron impact
dominates dissociation.
 Gas heating is dominantly by
Franck-Condon processes.
 Electron density (4 x 1010 cm-3)
is moderated by high rates of
diffusion loss but low rates of
attachment.
 1 W, 2 Torr, Ar/SiH4=95/5, 50 sccm
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MAX
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 SiH3
 SiH3–
 H
 SiH2
 SiNP
SILICON
HYDRIDES
 Exothermic recombination
of H atoms on nanocrystals is believed to be
important in annealing.
 Negative ions are confined
at the peak of the time
average plasma potential at
the center of the tube.
 Silicon nanoparticles
(SiNP) grow by successive
radical addition, and so
accumulate downstream.
 1 W, 2 Torr, Ar/SiH4=95/5, 50
sccm
MIN
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MAX
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DENSITIES vs POWER
 In spite of low rates of
attachment,
confinement of
negative ions produces
largely electronegative
plasmas.
 Depletion of SiH4 and
consumption of
radicals to form
nanoparticles limits
increase of SiHx with
power.
 HPEM, 2 Torr, Ar/SiH4=95/5, 50 sccm
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NANO PARTICLE vs POWER
 More silyl radicals are
produced by hydrogen
abstraction reaction
due to increased
density of hydrogen
radicals at higher
power.
 As a result, silyl
species are more likely
to find higher silyl
partners to form
nanoparticles and
saturated silanes.
 GLOCHE, 2 Torr, Ar/SiH4=95/5, 50 sccm
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DENSITIES vs PRESSURE
 Electron density
decreases with
increasing pressure
due more efficient
power deposition.
 Due to longer
residence time at
higher pressure there
is more accumulation
of dissociation
products.
 HPEM, 1 W, Ar/SiH4=95/5, 50 sccm
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University of Michigan
Institute for Plasma Science & Engr.
NANO PARTICLE vs PRESSURE
 Due to increased
residence time at
higher pressure silyl
density increases but
saturates by forming
nanoparticles.
 Since nanoparticle
particle formation is
irreversible at low
temperature, the
density of particles
increases in this
pressure range,
provided sufficient
silyl radicals.
 GLOCHE, 4 W, Ar/SiH4=95/5, 50 sccm
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DENSITIES vs FLOW RATE
 SiH4 dissociation
fraction decreases
with increasing flow
rate at constant
power.
 Electron density
decreases due to
larger average density
of SiH4.
 H, SiH3, and SiH3–
increase but saturate
due to the shorter
residence time at
higher flow rate.
 HPEM, 1 W, 2 Torr, Ar/SiH4=95/5
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NANO PARTICLE vs FLOW RATE
 Due to smaller
electron density and
shorter residence
time at higher flow
rate, the production
of silyl radicals
capable of forming
nanoparticles is
limited.
 GLOCHE, 4 W, 2 Torr, Ar/SiH4=95/5
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DENSITIES vs SiH4 FRACTION
 Plasma density
decreases with SiH4
fraction due to
electronegativity,
while SiH3 and SiH3–
increase due to larger
average density of
SiH4.
 H increases but
quickly saturates due
to the smaller
electron density at
higher fraction of
SiH4.
 HPEM, 1 W, 2 Torr, 50 sccm
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NANO PARTICLE vs SIH4 FRACTION
 The nanoparticle
density increases by
increasing SiH4
fraction due to the
increasing density of
silyl species provided
by electron impact
and H abstraction.
 Due to the smaller
electron density at
higher SiH4 fraction
the nanoparticle
density decreases
with SiH4 fraction.
 GLOCHE, 4 W, 2 Torr, 50 sccm
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CONCLUDING REMARKS
 As power increases, the electron density increases and
nanoparticle density increases due to more silyl species produced
by H radicals.
 As pressure increases, the electron density decreases but the
nanoparticle density increases due to the increased concentration
and residence time of H, SiH3, and SiH3–
 As flow rate increases, the electron density decreases and the
nanoparticle density decreases due to the reduced residence time.
 As SiH4 fraction increases, the electron density decreases but the
nanoparticle density is maximized at optimum fraction of SiH4 due
to trade off between electron and silyl production from SiH4.
ICOPS_2013
University of Michigan
Institute for Plasma Science & Engr.