properties of nonthermal capacitively coupled plasmas generated in narrow quartz tubes for synthesis...

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, [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

University of MichiganInstitute for Plasma Science & Engr.

ICOPS_2013

NANOCRYSTALS (QUANTUM DOT)


Ref: I. L. Medintz et al., Nature Material 4, 435 (2005).

· 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

ICOPS_2013

PLASMA-SYNTHESIZED SILICON NANOCRYSTALS

· Gas-phase plasma processes for Si nano-crystal 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

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

University of MichiganInstitute for Plasma Science & Engr.ICOPS_2013


GLOBAL CHEMISTRY MODEL (GLOCHE)

· 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).

Gas Phase Reaction Mechanism

Time Dependent Kinetics

· 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)


REACTOR GEOMETRY: CCP TUBE

NUCLEATION REACTIONS BY NEUTRALS

Ref: U. V. Bhandarkar et al., J. Phys. D: Appl. Phys. 33, 2731 (2000)

· 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 k = 2.66 × 10-11Tg0.5 cm3/s



· [e] · Te · TgasPLASMA 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

MIN MAXICOPS_2013

· SiH4

SILICON HYDRIDES


· Exothermic recombination of H atoms on nano-crystals 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

· SiH3 · SiH3– · H · SiNP

ICOPS_2013MIN MAX

· SiH2

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

NANO PARTICLE vs POWER


· GLOCHE, 2 Torr, Ar/SiH4=95/5, 50 sccm

· 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.

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.

ICOPS_2013

· HPEM, 1 W, Ar/SiH4=95/5, 50 sccm

NANO PARTICLE vs PRESSURE


· GLOCHE, 4 W, Ar/SiH4=95/5, 50 sccm

· 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.

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.

ICOPS_2013

· HPEM, 1 W, 2 Torr, Ar/SiH4=95/5

NANO PARTICLE vs FLOW RATE


· GLOCHE, 4 W, 2 Torr, Ar/SiH4=95/5

· Due to smaller electron density and shorter residence time at higher flow rate, the production of silyl radicals capable of forming nanoparticles is limited.

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.

ICOPS_2013

· HPEM, 1 W, 2 Torr, 50 sccm

NANO PARTICLE vs SIH4 FRACTION


· GLOCHE, 4 W, 2 Torr, 50 sccm

· 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.

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.


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