1. PROPERTIES OF NONTHERMAL CAPACITIVELY COUPLED PLASMAS GENERATED IN NARROW QUARTZ TUBES FOR SYNTHESIS OF SILICON NANOPARTICLES* Sang-Heon Song a), Romain Le Picard b), Steven L. Girshick b), Uwe R. Kortshagen b), and Mark J. Kushner a) a) University of Michigan, Ann Arbor, MI 48109, USA ssongs@umich.edu, mjkush@umich.edu b) University of Minnesota, Minneapolis, MN 55455, USA rlepicar@umn.edu, slg@umn.edu, kortshagen@umn.edu 40 th IEEE International Conference on Plasma Science (ICOPS) San Francisco, USA, June16-21, 2013 * Work supported by National Science Foundation and DOE Plasma Science Center.

2. AGENDA  Plasma nanoparticle synthesis  Description of the model  Typical Ar/SiH 4 plasma properties  Nanoparticle density  Power  Pressure  Flow rate  SiH 4 fraction  Concluding remarks University of Michigan Institute for Plasma Science & Engr. ICOPS_2013

3. NANOCRYSTALS (QUANTUM DOT) University of Michigan Institute for Plasma Science & Engr. 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

4. 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 SiH 4 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. University of Michigan Institute for Plasma Science & Engr. Ref: R. J. Anthony et al., Adv. Funct. Mater. 21, 4042 (2011). ICOPS_2013

5. 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 Michigan Institute for Plasma Science & Engr. ICOPS_2013

6. University of Michigan Institute 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

7.  2D, cylindrically symmetric  Tube radius = 0.3 cm  Electrode separation = 2.2 cm  Operating conditions  Ar/SiH 4 = 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) University of Michigan Institute for Plasma Science & Engr. ICOPS_2013 REACTOR GEOMETRY: CCP TUBE

8. 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 Si 10 H 20. Higher silanes are “particles”  Nucleation reactions with neutrals  28 reactions: Silyl formation by H abstraction.  Si n H 2m + H → Si n H 2m-1 + H 2 k =2.44×10 –16 T 1.9 exp(–2190/T) cm 3 /s  106 reactions for making higher silanes  Si n H 2m-1 + Si j H 2k-1 → Si n+j H 2m+2k-2 k = 3.32 × 10 −9 cm 3 /s  321 reactions for making particle (n + j ≥ 11)  Si n H 2m-1 + Si j H 2k-1 → particlek = 2.66 × 10 -11 T g 0.5 cm 3 /s University of Michigan Institute for Plasma Science & Engr. ICOPS_2013

9. University of Michigan Institute for Plasma Science & Engr.  [e] TeTe  T gas 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 SiH 4 indicates electron impact dominates dissociation.  Gas heating is dominantly by Franck-Condon processes.  Electron density (4 x 10 10 cm -3 ) is moderated by high rates of diffusion loss but low rates of attachment.  1 W, 2 Torr, Ar/SiH 4 =95/5, 50 sccm MIN MAX ICOPS_2013  SiH 4

10. SILICON HYDRIDES University of Michigan Institute for Plasma Science & Engr.  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/SiH 4 =95/5, 50 sccm  SiH 3  SiH 3 – HH  SiNP ICOPS_2013 MIN MAX  SiH 2

11. DENSITIES vs POWER  In spite of low rates of attachment, confinement of negative ions produces largely electronegative plasmas.  Depletion of SiH 4 and consumption of radicals to form nanoparticles limits increase of SiH x with power. University of Michigan Institute for Plasma Science & Engr. ICOPS_2013  HPEM, 2 Torr, Ar/SiH 4 =95/5, 50 sccm

12. NANO PARTICLE vs POWER University of Michigan Institute for Plasma Science & Engr. ICOPS_2013  GLOCHE, 2 Torr, Ar/SiH 4 =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.

13. DENSITIES vs PRESSURE University of Michigan Institute for Plasma Science & Engr.  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/SiH 4 =95/5, 50 sccm

14. NANO PARTICLE vs PRESSURE University of Michigan Institute for Plasma Science & Engr. ICOPS_2013  GLOCHE, 4 W, Ar/SiH 4 =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.

15. DENSITIES vs FLOW RATE University of Michigan Institute for Plasma Science & Engr.  SiH 4 dissociation fraction decreases with increasing flow rate at constant power.  Electron density decreases due to larger average density of SiH 4.  H, SiH 3, and SiH 3 – increase but saturate due to the shorter residence time at higher flow rate. ICOPS_2013  HPEM, 1 W, 2 Torr, Ar/SiH 4 =95/5

16. NANO PARTICLE vs FLOW RATE University of Michigan Institute for Plasma Science & Engr. ICOPS_2013  GLOCHE, 4 W, 2 Torr, Ar/SiH 4 =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.

17. DENSITIES vs SiH 4 FRACTION University of Michigan Institute for Plasma Science & Engr.  Plasma density decreases with SiH 4 fraction due to electronegativity, while SiH 3 and SiH 3 – increase due to larger average density of SiH 4.  H increases but quickly saturates due to the smaller electron density at higher fraction of SiH 4. ICOPS_2013  HPEM, 1 W, 2 Torr, 50 sccm

18. NANO PARTICLE vs SIH 4 FRACTION University of Michigan Institute for Plasma Science & Engr. ICOPS_2013  GLOCHE, 4 W, 2 Torr, 50 sccm  The nanoparticle density increases by increasing SiH 4 fraction due to the increasing density of silyl species provided by electron impact and H abstraction.  Due to the smaller electron density at higher SiH 4 fraction the nanoparticle density decreases with SiH 4 fraction.

19. 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, SiH 3, and SiH 3 –  As flow rate increases, the electron density decreases and the nanoparticle density decreases due to the reduced residence time.  As SiH 4 fraction increases, the electron density decreases but the nanoparticle density is maximized at optimum fraction of SiH 4 due to trade off between electron and silyl production from SiH 4. University of Michigan Institute for Plasma Science & Engr. ICOPS_2013