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Capacitively Coupled RF Plasmas for Silicon Nanocrystals Synthesis

This paper discusses the use of low temperature, low pressure plasmas for synthesizing silicon nanocrystals (SNCs). The size and crystallinity of SNCs are controlled by temperature and residence time in the plasma. Computer modeling of plasma production of SNCs is also discussed.

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Capacitively Coupled RF Plasmas for Silicon Nanocrystals Synthesis

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  1. CAPACITIVELY COUPLED RF PLASMAS FOR THE SYNTHESIS OF SILICON NANOCRYSTALS* R. Le Picard1, A.H. Markosyan2, D.H. Porter3, M.J. Kushner2, S.L. Girshick1 1Univ. of Minnesota, Dept. Mech. Engr., Minneapolis, MN USA 2Univ. of Michigan, Electrical & Comp. Engr., Ann Arbor, MI USA 3U. of Minnesota Supercomputing Inst., Minneapolis, MN USA * Work supported by DOE (DE-SC0001939), NSF (CHE-124752) and the Minnesota Supercomputing Institute.

  2. University of Michigan Institute for Plasma Science & Engr. NANODUSTY PLASMAS • Low temperature, low pressure plasmas are used to synthesize silicon nanocrystals (SNCs). • Systems are flowing plasmas in Ar/SiH4 mixtures. • Size and crystallinity of SNCs are controlled by temperature and residence time in plasma. • Silicon nanocrystals • Growth of SNCs involves aerosol kinetics bridging clustering of radicals to agglomeration. • Mechanisms of SNC growth are poorly understood. • In this paper, computer modelling of plasma production of SNCs will be discussed. • Model combines 2-plasma hydrodynamics model with aerosol sectional model. ISPC2015

  3. University of Michigan Institute for Plasma Science & Engr. APPLICATIONS OF PLASMA-SYNTHESIZED NCs Si NC PV cells ZnO films with high mobility Si NC LED, EQE>9% URK et al., Nano Letters 2009 URK et al., Nano Letters 2010, 2011 ET, ESA, URK et al., Nat. Comm. 2014 Metal-insulator transition inNC solids Nucleation of nanodiamond Si NC anodes for Li ion batteries Shklovskii, URK, et al., Nat. Mater. under review Sankaran et al., Nat. Comm. 2013 Mangolini et al., J. Power Sources 2015 ISPC2015

  4. University of Michigan Institute for Plasma Science & Engr. TYPICAL EXPERIMENTAL SNC SYNTHESIS • Model system is based on capacitively coupled RF plasma reactor developed by Mangolini et al.* • Quartz tube: 0.8 cm inner and 1.0 cm outer diameter. • Ring electrodes: 1 cm separation • Upper electrode: 25 MHz. • Lower electrode: Grounded • Power deposition in plasma: a few Watts. • Ar/Si = 99.75/0.25, 2 Torr, 10 - 100 sccm. *L. Mangolini et. al, Nano Lett. 5, 655-659 (2005) • Silicon nanocrystals ISPC2015

  5. University of Michigan Institute for Plasma Science & Engr. HPEM: HYBRID PLASMA EQUIPMENT MODEL *M.J. Kushner, J. Phys. D., 42 194013 (2009) ISPC2015

  6. University of Michigan Institute for Plasma Science & Engr. AEROSOLS INTEGRATED IN PLASMA MODEL • Integration of HPEM with aerosol module including: • Population balance equations for nanoparticle size and charge • Momentum equation • Ion and neutral drag forces • Nanoparticle charging • Growth mechanisms (nucleation, coagulation and surface growth) ISPC2015 Plasma Chemistry Aerosol

  7. University of Michigan Institute for Plasma Science & Engr. AEROSOL MODULE • An aerosol sectional model is used. • The continuity equation for nanoparticles (aerosol general dynamics equation): • where iis the section index and k is its charge. • 12 nucleation reactions are considered: • Si2H4- with SiH4, Si2H6, SiH2, Si2H4, • Si2H5- with SiH4, Si2H6, SiH2, Si2H4, SiH2, Si2H3, Si2H5 • Silicon hydride chemistry is taken from Bhandarkaret al.* *U.V. Bhandarkaret al., JPD, 33 2731 (2000) ISPC2015

  8. University of Michigan Institute for Plasma Science & Engr. AEROSOL MODULE • Silicon hydride chemistry is taken from Bhandarkaret al.* *U.V. Bhandarkaret al., JPD, 33 2731 (2000) ISPC2015

  9. Min Max University of Michigan Institute for Plasma Science & Engr. PLASMA PROPERTIES [e] 5x108 - 9x1011cm-3 Te 0.5 - 5.3 eV P-Pot -260 - 60 V • X axis is scaled by factor of 3. • [e] peaks at 9x1011 cm-3above the powered electrode where Te ≈ 4 eV. • Electrons dissociate SiH4 above the electrode as the gas flows downward from the inlet nozzle. Particles grow as they flow through the tube. ISPC2015

  10. Min Max University of Michigan Institute for Plasma Science & Engr. NUCLEATION REACTIONS [Si2H5-] 5x109 - 6x1011cm-3 [SiH3] 1x1011 - 2x1013cm-3 [H] 1x1011 - 7x1013cm-3 • Si2H5- is the main precursor responsible for nucleation. • SiH3 has highest impact on surface growth. • Reaction mechanism should be extended to higher Si5Hx. • Predicted hydrogen density is comparable to experimental observation (≈1013 cm-3) by N.J. Kramer et al.* • Hydrogen may be responsible for particle crystallization as shown in experiments* *N.J. Kramer et. al, J. Phys. D., 47 075202 (2014) ISPC2015

  11. Min Max University of Michigan Institute for Plasma Science & Engr. NANO-PARTICLE DENSITY Tot. Particles 1x1010 - 2x1012cm-3 Nucleation 5x1013 - 6x1015cm-3 Surf. Growth 1 - 40 • Total nanoparticle density is the highest where the tube expands. • SNCs nucleate and grow near the electrodes and flow downstream reaching a density of 2× 1012 cm-3. • SNCs nucleation begins few centimeters above the powered electrode where electron impact dissociation of the feedstock SiH4 becomes important. • Particle nucleation peaks between the two electrodes at ≈ 5×1015 cm-3 s-1. • In this simulation, particles grow by coagulation and surface growth. ISPC2015

  12. University of Michigan Institute for Plasma Science & Engr. NANOPARTICLE SIZE DISTRIBUTION Gas flow Powered Grounded • The particle size distribution is shown at the center-line along the tube • Nanoparticles nucleate as small clusters of size ≈ 0.5 nm in diameter. • Nanoparticles start growing at 5 cm from the inlet. • At the outlet, the larger nanoparticles are ≈ 1 nm in diameter. ISPC2015

  13. University of Michigan Institute for Plasma Science & Engr. NANOPARTICLE CHARGE DISTRIBUTION • The nanoparticle charge distribution for 1-nm diameter particle 4 cm above the outlet on the centerline. • Particles are mostly negative, though a considerable fraction are neutral. • We expect coagulation to make an important contribution to particle growth*. *L. Ravi et. al, Phys. Rev. E 79, 026408 (2009) ISPC2015

  14. University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS • Current model explains many experimental observations, such as nanoparticle deposition on the wall above the electrodes. • Electron density is high up to 1 cm below grounded electrode, as observed experimentally. Predicted H-atom density also corresponds to experimental observations*. • Preliminary results show surface growth rate peaks at 40 nm/s in the vicinity of the powered electrode. • Based on comparisons to experiment, we speculate that coagulation may be a major growth mechanism for these experimental conditions. • To relax assumptions on nucleation and surface growth rates, we will include a more detailed chemistry. *L. Mangolini et. al, Nano Lett. 5, 655-659 (2005) ISPC2015

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