PROBING THE GAS PHASE CHEMISTRY INVOLVED IN DIAMOND CHEMICAL VAPOUR DEPOSITION (CVD)

1. PROBING THE GAS PHASE CHEMISTRY INVOLVED IN DIAMOND CHEMICAL VAPOUR DEPOSITION (CVD) Mike Ashfold School of Chemistry University of Bristol Bristol BS8 1TS http://www.chm.bris.ac.uk/pt/laser/

2. Chemical vapour deposition of diamond films • Activation of gas mixture by • Hot filament (Tfil~2450 K) • Microwave plasma • DC arc jet • Polycrystalline films grow on Si, Mo, • W, … substrates; Tsub > 700°C. • Growth of single crystal diamond by CVD demonstrated(Isberg et al, • Science, 297, 1670 (2002)) • Properties: • High thermal conductivity • Optical transparency (UV  mid IR) • Chemically inert • Electrical insulator – can be doped

3. SEM images of polycrystalline CVD diamond

4. Film characterisation by Raman spectroscopy 1332 cm-1 Intensity / arb. units 600 800 1000 1200 1400 1600 1800 Raman Shift / cm-1

5. Recent studies by the Bristol Diamond Group • Hydrocarbon / H2 mixtures in a hot filament (HF) reactor • Molecular beam mass spectrometry • Laser probing H atoms, CH3 radicals andC2H2 molecules • Modelling CH4 / H2 and C2H2 / H2 gas mixtures • (Ashfold et al., Phys. Chem. Chem. Phys. (2001), 3, 3471) • Probing and modelling CH4/NH3/H2 gas mixtures • (Smith et al., J. Appl. Phys. (2002), 92, 672) • CO2 / CH4 mixtures in a microwave (MW) reactor • Molecular beam mass spectrometry • Modelling H/C/O gas phase chemistry • Effect of S additions (as H2S or CS2) • (Petherbridge et al., J. Appl. Phys. (2001), 89, 1484, 5219) • CH4 / H2 / Ar mixtures in a DC arc jet reactor • Cavity ring down measurements of C2H2 and of C2 and CH radicals • Modelling plasma activated CH4 / H2 gas mixtures • (Wills et al., J. Appl. Phys. (2002), 92, 4213 • Rennick et al., Chem. Phys. Lett. (2004), 383, 518 • Rennick et al., Diam. Rel. Mater. (2004), 14, 561)

6. Diamond film growth in a hot filament reactor 0.5-1% CH4 in H2 Total flow rate = 100 sccm Pressure = 20 Torr Film growth rate ~ 1m/hr

7. In situ molecular beam mass spectrometry, I 1%CH4 / H2 Gas mixture sampled through a skimmer by a differentially pumped quadrupole mass spectrometer, equipped with variable energy electron ioniser. X[CH4] ~ X[C2H2]

8. Predictions from equilibrium thermodynamics [CH4] ~ [C2H2] at Tgas ~ 1500 K Gas temperature / K

9. In situ molecular beam mass spectrometry, II 0.5%C2H2/H2 Equilibrium thermodynamics predicts no C2H2 CH4 conversion! X[CH4] ~ X[C2H2]

10. MBMS probing HF-CVD reactors: Conclusions • Near HF (i.e. where Tgas > 1400 K) the hydrocarbon is present as • a CH4C2H2 mixture, irrespective of original source gas used. • High [hydrocarbon]  increased growth rates, but poorer • diamond ‘quality’ (as assessed by morphology, Raman etc.) • High [H]  reduced growth rates, but improved diamond ‘quality’. • Large T gradients near HF  preferential diffusion of heavier • species (i.e. hydrocarbons) from hotter regions - Soret effect. • CH3is a key diamond growth species in weakly activated • hydrocarbon/H2 gas mixtures. • Narrow range of Tfil for optimal diamond growth: • - Low Tfil: insufficient H2  H dissociation, • - High Tfil: CH4C2H2 equilibrium shifts far to right, and • [CH3] in the growth region falls.

11. ‘Ideal’ requirements of a diagnostic • Species selectivity • High sensitivity • Spatial (and temporal) resolution • Minimal intrusion • Ease of implementation • Readily interpretable results • laser spectroscopy Resonance enhanced multiphoton ionisation (REMPI)for H atoms and CH3 radicals in HF-CVD reactor – spatially resolved (relative) number densities. Cavity ring down spectroscopy (CRDS) for CH(X),C2(a) and C2(X) radicals, and C2H2 molecules, in DC-arc jet reactor – spatially resolved (absolute) column densities

12. Hydrogen atoms, I • H atoms are crucial in most diamond CVD environments. They: • Initiate gas phase chemistry ( reactive C containing species) • Terminate the growing diamond surface, preventing reconstruction • Abstract surface H to create vacant sites for C-radical attachment • Etch non-diamond deposits such as graphite. ionisation continuum • Probe by 2+1 REMPI in HF activated • hydrocarbon / H2 gas mixtures • Move filament and substrate relative to • laser beam focus to obtain spatially resolved number density distributions • Collect H+ ions with a biased Pt probe n = 3 n = 2 n = 1

13. Hydrogen atoms, II • Measure H+ ion yield as a function • of laser excitation wavenumber. • Laser bandwidth makes negligible • contribution to measured 2s 1s • lineshape. • Area of lineshape  local number • density of H atoms, NH. • Local gas temperature obtained • from FWHM of Doppler broadened • (Gaussian) lineshape.

14. Hydrogen atoms, III Tgasvs d, fixed Tfil. Tgas (from FWHM values) declines monotonically with distance, d, from the HF. NH vs d, fixed Tfil. NH falls with increasing d, butmuch more slowly than would be predicted by assuming that the H2 2H equilibrium was determined by the local Tgas.

15. Hydrogen atoms, IV NH increases rapidly with Tfil NH independent of p(H2)– zero order kinetics Measured NH dependences are consistent with H atom formation by dissociative chemisorption of H2 on HF surface, and subsequent diffusion throughout reactor. Gas phase H atom recombination is very inefficient when H2 is the main ‘third body’.

16. Methyl radicals, I • CH3 radicals can be detected by 2+1 REMPI via the v = 0 level of the 3pz,2A2” Rydberg state. • The two photon transition from the ground state is dominated by an intense Q branch. • To convert measured REMPI intensities into CH3 radical number densities we must correct for the Tgas dependence of: • (i) the vibrational partition function • (ii) the rotational band contour. Simulation Experimental 59600 60000 60400 Two-photon wavenumber / cm-1

17. Methyl radicals, II • Corrected  Raw Data 1% CH4 / H2 0.5% C2H2 / H2 [CH3] vs d from HF activated CH4/H2 and C2H2/H2 mixtures

18. Methyl radicals, III • [CH3] vs %C and vs Tfil for 1% CH4/H2 ( ) and 0.5% C2H2/H2(o), probed at d = 4 mm, plotted to match at 0.5% C and at Tfil = 2575 K. • All measured [CH3] dependences are explicable in terms of gas phase chemistry.

19. Gas phase reaction mechanism - qualitative CH4/H2 chemistry: Radical formation: H + CH4  CH3 + H2 (‘H-shifting’ reactions)H + CH3  CH2+ H2, etc. C1  C2 conversion: CH3 + CH3 + M C2H6 + M C2H6 + H  C2H5 + H2 …… C2H2 + H2 C2H2/H2 chemistry: Analogous H + C2H2  C2H+ H2 radical initiation step is endothermic. Earlier models often invoked a role for heterogeneous chemistry (on surface of HF, or on the growing diamond film), but spatially resolved [CH3] profiles led us to suggest that gas phase addition processes like C2H2 + H + M  C2H3 + M may occur in cooler regions of the reactor.

20. Gas phase reaction mechanism - quantitative • 3-D modelling of the Bristol HF-CVD reactor. • (Mankelevich and Suetin, Moscow State University) • Model consists of 3 blocks that describe: • activation (gas heating, H2 dissociation on filament) • gas phase processes (heat and mass transfer, reaction kinetics) • gas-surface processes at the substrate • Gas phase reaction kinetics and thermochemistry from GRIMECH 3.0 detailed reaction mechanism for C/H/(O/N) mixtures. • Conservation equations for mass, momentum, energy and number • densities are integrated numerically until steady-state is achieved. • Model outputs include spatial distributions of gas temperature, • flow field and species number densities.

21. Summary of elementary reaction rates Reaction Rate / cm-3 s-1 Reaction 730 K 1200 K 1750 K 2000 K H + C2H2 + M  C2H3 + M 1.85E +16 4.00E +15 4.07E +14 1.64E +14C2H3 + M  H + C2H2 + M 3.72E +12 1.23E +15 1.93E +16 3.23E +16H + C2H3  H2 + C2H2 1.05E +16 3.34E +15 5.24E +15 6.07E +15H2 + C2H2  H + C2H3 7.15E +1 2.54E +9 8.58E +12 6.97E +13Total: C2H2  C2H38.00E +15 5.73E +14 2.41E +16 3.82E +16 Total: C2H3 C2H47.87E +15 1.10E +14 2.42E +16 4.25E +16 Total: C2H4 C2H54.97E +15 3.45E +14 1.47E +15 1.53E +15 Total: C2H6/C2H5 CH34.97E +15 4.51E +15 3.75E +15 5.00E +15 Total: C2H6 C2H5 4.23E +14 4.45E +15 4.44E +15 3.48E +15 Entries in red confirm the suggestion that, at low Tgas, there is net C2  C1conversion via C2H2  C2H3  C2H4  C2H5  CH3.

22. CH4 C2H2 interconversion high Tgas; net CH4C2H2 lower Tgas; net C2H2CH4

23. Experiment and theory compared expt. model • 1% CH4 / H2 • 0.5% C2H2 / H2 Tfil = 2475 K

24. Conclusions from studies of HF-activated CH4/H2 and C2H2/H2 gas mixtures • With CH4/H2 input mixtures, gas phase H abstraction reactions initiate the overall CH4 C2H2 conversion in regions of high Tgas near the HF. • Diffusion rates are much faster than gas replacement rates in these reactors (typical gas flow rate ~100 sccm), so much of the C2H2 formed by CH4 C2H2 conversion near the HF will diffuse to cooler regions. (confirmed by cw CRDS monitoring of C2H2 rotational temperature and the time constants for its build up and decay – • Wills et al., Diamond Rel. Mater. (2003), 12, 1346) • H additionreactions in cooler regions of the reactor drive the reverse C2H2 CH4 conversion, offering a means of regenerating methane and eventual CH4 C2H2 equilibration. • Purely gas phase processes can account for the observed C2 C1 interconversion in CH4/H2 and C2H2/H2 input gas mixtures.

25. Recent gas phase diagnostics studies in Bristol • Hydrocarbon / H2 mixtures in a hot filament (HF) reactor • Molecular beam mass spectrometry of stable species • REMPI laser probing H atoms and CH3 radicals • Modelling CH4 / H2 and C2H2 / H2 gas mixtures • (Ashfold et al., Phys. Chem. Chem. Phys. (2001), 3, 3471) • Probing and modelling CH4/NH3/H2 gas mixtures • (Smith et al., J. Appl. Phys. (2002), 92, 672) • CO2 / CH4 mixtures in a microwave (MW) reactor • Molecular beam mass spectrometry • Modelling H/C/O gas phase chemistry • CH4 / H2 / Ar mixtures in a DC arc jet reactor • Cavity ring down measurements of C2H2 and of C2 and CH radicals • Modelling plasma activated CH4 / H2 gas mixtures • (Wills et al., J. Appl. Phys. (2002), 92, 4213 • Rennick et al., Chem. Phys. Lett. (2004), 383, 518 • Rennick et al., Diam. Rel. Mater. (2004), 14, 561)

26. Diamond film growth in a DC arc jet 10 kW DC arc jet 1%CH4 in Ar/H2 at 50 Torr Growth rates ~100 m hr-1 Aggressive activation: much higher gas temperatures and flow rates than in HF or MW reactors. How to probe gas phase chemistry and composition? Optical emission spectroscopy (OES) and cavity ring down spectroscopy (CRDS).

27. DC arc jet in operation diamond film growingon Mo substrate plasma jet CH4 injection ring

28. Diamond films grown with DC arc jet SEM images of polycrystalline diamond films grown in the DC arc jet

29. Optical emission from the arc jet plume What are primary growth species in this highly activated environment? C atoms? C2 radicals? Latter show strongly in optical emission. Spatially resolved C2(d-a) emission C2 Swan system(d3g  a3u)

30. Proposed mechanism for diamond growth by C2 C2 addition to H-terminated and to bare diamond (110) surfaces has been calculated to be barrierless and exothermic. (D.A. Horner et al. Chem. Phys. Lett. 233 (1995) 243)

31. In situ diagnosis of the arc jet plume, I • Optical emission spectroscopy (OES) • Only fluorescent species can be observed. • Provides information about the (minor) electronically excited components in plume – how to relate to ground state concentrations, properties, etc? • Spatially resolved measurements difficult. • Resonance enhanced multiphoton ionisation (REMPI) • Used successfully to probe ground state H atoms and CH3 radicals in HF reactor, but ion probe will not survive harsh plasma environment and background ion/electron signal would be a problem. • Laser induced fluorescence (LIF) • Species of interest must have fluorescent excited state. • Need to quantify excited state quenching characteristics in order to relate measured LIF signal intensities to ground state populations of interest. • Detector likely to be overwhelmed by intense spontaneous emission from plume.

32. In situ diagnosis of the arc jet plume, II Absorption spectroscopy • Beer-Lambert behaviour • I = I0 exp{-s [X] L} • Advantages: • Straightforward • General • Quantitative • Disadvantages: • Insensitive • Non-selective Fractional absorption per pass I = (I0 – I)/I0  10-4

33. In situ diagnosis of the arc jet plume, III Intra-cavity absorption spectroscopy • Build cavity around sample • Multipass a light pulse • Detect rate of loss of light • Cavity ring-down spectroscopy I(t) = I0 exp{-k0t -ct} ;  =  [X] ; I min ~ 10-8 Change in ring-down rate as a function of excitation wavelength gives the absorption spectrum

34. Cavity Ring Down Spectroscopy in the DC arc jet • Variables include: • CH4 flow rate • power into plasma • distance from substrate

35. C2(a) radical detection Portion of C2 d3Pga3Pu(0,0) band integrated absorption coefficient of measured line • A00 = Einstein A coefficient for vibronic transition of interest. • p = fraction of total oscillator strength within probed rovibrational transition • (T dependent). • C2(a, v = 0) column density. • C2 (a) number density IF we know Tgas (and thus qvib) and the absorbing column length, L (from OES).  [C2(a3Pu)] ~ 1.1  1013cm-3 for 3.3%CH4/H2 gas mixture, 6 kW input power, assuming Tgas = 3300 K and L = 1 cm.(

36. C2(a) radical detection – gas temperature determination Boltzmann plots of C2(a) rotational state population distribution measured in the plume (2 < z < 25 mm) give Trot = 3300  200 K. ‘Doppler’ linewidth analyses give similar Tgas for z > 5mm, but overestimate Tgas close to the substrate – a consequence of plume flaring in the boundary layer. probe

37. C2(X) radical detection Portion of C2 (D1uX1g) spectrum recorded in free plume at  ~ 235 nm Tvib = 3000  500 K [C2(X1g )] = (3.00.9)1012 cm-3 again assuming L = 1 cm. = 0.270.08 c.f. 0.23 if the a and X states of C2 were in thermal equilibrium at 3300 K – implies intersystem crossing is faster than reaction (with e.g. H2) under operational conditions.

38. CH(X) radical detection Portion of CH A2X2 (0,0) band ~ 427 nm [CH(X)] = (7.01.3)  1012 cm-3 in the free plume under normal operating conditions. (again assuming L = 1 cm). Non-zero absorbance between peaks probably attributable to C3 radicals.

39. C2(a) and CH(X) radical column densities as fn(z) C2(a) CH(X) 3% CH4/H2 ,6 kW input power, range of probe transitions

40. C2(a) and CH(X) radical column densities as fn[CH4] C2(a) CH(X) x sccm CH4 / 1.8 slm H2 / 12.2 slm Ar Arc jet power 6 kW, range of probe transitions

41. cw CRDS probing of C2H2 in the DC arc jet reactor ECDL: Littman configuration extended cavity diode laser AOM: acousto-optic modulator

42. cw CRDS probing of C2H2 in the DC arc jet reactor R(22) line of 1 + 3 combination band of C2H2 •  = 0.022  0.003 cm-1 • (650  90 MHz). • pressure broadening: ~200 MHz at 50 Torr • laser bandwidth: ~4 MHz • Tgas = 550  150 K C2H2 present along whole viewing column? [C2H2] = 1.2  0.2 1014 cm-3 for 0.83% CH4/H2 feed (i.e. only 25% of our ‘standard’ CH4 flow rate) and assuming L = 100 cm

43. CRDS in DC arc jet: summary of experimental findings • Probing in the free plume region of the • arc jet, with a CH4 flow of 60 sccm: • [C2(a)] ~ 1.1 x 1013 cm-3, • [C2(X)] ~ 3 x 1012 cm-3, • [CH(X)] ~ 7 x 1012 cm-3 (all assuming L = 1 cm ) • Tgas = 3300  200 K • [C2H2] ~ 1.2 x 1014 cm-3 (using a reduced • (15 sccm) CH4 flow, assuming L =100 cm) • Tgas ~ 550 K • There is a boundary regionclose to the substrate, where C2 and CH column • densities increase – due to plume flaring and the longer L? • Increased linewidths at small z mainly due to plume flaring. Internal • quantum state population distributions of radical species suggest Tgas • relatively insensitive to z. • [C2H2], and Tgas value (average over all L?) • is insensitive to z in range 2 – 25 mm.

44. Modelling of the DC arc jet plume (Mankelevich) • 2-D (r,z) model, comprising of three blocks, describing: • (i) activation of the reactive mixture (i.e. gas heating, ionisation, H2 dissociation in arc jet and intermediate chamber, H atom loss and H2 production on nozzle exit walls), • (ii) gas-phase processes (heat and mass transfer, chemical kinetics), • (iii) gas-surface processes at the substrate. • Thermochemical data and the reduced chemical reaction mechanism builds on Yu.A. Mankelevich et al., Diam. Rel. Mater. (1996), 5, 888. • Chemical kinetics scheme involves 23 species (H, H2, Ar, C, CH, 3CH2, 1CH2, CH3, CH4, C2(X), C2(a), C2Hx (x = 1-6), C3Hx (x = 0-2), C4Hx (x = 0-2)) and 76 reversible reactions. • Set of conservation equations for mass, momentum, energy and species concentrations, with appropriate initial and boundary conditions, thermal and caloric equations of state, are integrated numerically in cylindrical (r,z) coordinate space until attaining steady state conditions. • Model output includes spatial distributions of Tgas, the flow field, and the various species number densities.

45. Modelling of the DC arc jet plume: Tgas • Gas temperature distribution, Tgas H2/Ar plasma enters here methane injection ring substrate

46. Modelling of the DC arc jet plume: H • Tgas • H: H2 >90% dissociated; high • [H] at substrate.

47. Modelling of the DC arc jet plume: CH4 • Tgas • H: H2 >90% dissociated; high • [H] at substrate. • CH4 injected through ring

48. Modelling of the DC arc jet plume: C2H2 • Tgas • H: H2 >90% dissociated; high • [H] at substrate. • CH4 injected through ring • rapidly converted to C2H2

49. Modelling of the DC arc jet plume: C4H2 • Tgas • H: H2 >90% dissociated; high • [H] at substrate. • CH4 injected through ring • rapidly converted to C2H2 • and to larger CxHy compounds • (e.g. C4H2)

50. Modelling of the DC arc jet plume: C3 • Tgas • H: H2 >90% dissociated; high • [H] at substrate. • CH4 injected through ring • rapidly converted to C2H2 • and to larger CxHy compounds • (e.g. C4H2) and C3 radicals