Theoretical Investigations of the Reactions of N- and O-Containing Species on a C(100):H 2 × 1 Reconstructed Diamond Surface.

Quantum mechanical and hybrid quantum mechanical/molecular mechanical cluster models were used to investigate possible reaction mechanisms whereby gas-phase NHx (x = 0-2), CNHx (x = 0, 1), and OH radicals can add to, and incorporate in, a C-C dimer bond on the C(100):H 2 × 1 diamond surface during chemical vapor deposition (CVD) from microwave-activated C/H containing gas mixtures containing trace amounts of added N or O. Three N incorporation routes are identified, initiated by N, NH, and CN(H) addition to a surface radical site, whereas only OH addition was considered as the precursor to O incorporation. Each is shown to proceed via a ring-opening/ring-closing reaction mechanism analogous to that identified previously for the case of CH3 addition (and CH2 incorporation) in diamond growth from a pure C/H plasma. On the basis of the relative abundances of N atoms and NH radicals close to the growing diamond surface, the former is identified as the more probable carrier of the N atoms appearing in CVD grown diamond, but fast H-shifting reactions postaddition encourage the view that NH is the more probable migrating and incorporating species. CN radical addition is deemed less probable but remains an intriguing prospect, since, if the ring-closed structure is reached, this mechanism has the effect of adding two heavy atoms, with the N atom sitting above the current growth layer and thus offering a potential nucleation site for next-layer growth.

Microwave Plasma-Activated Chemical Vapor Deposition of Nitrogen-Doped Diamond. II: CH4/N2/H2 Plasmas.

We report a combined experimental and modeling study of microwave-activated dilute CH4/N2/H2 plasmas, as used for chemical vapor deposition (CVD) of diamond, under very similar conditions to previous studies of CH4/H2, CH4/H2/Ar, and N2/H2 gas mixtures. Using cavity ring-down spectroscopy, absolute column densities of CH(X, v = 0), CN(X, v = 0), and NH(X, v = 0) radicals in the hot plasma have been determined as functions of height, z, source gas mixing ratio, total gas pressure, p, and input power, P. Optical emission spectroscopy has been used to investigate, with respect to the same variables, the relative number densities of electronically excited species, namely, H atoms, CH, C2, CN, and NH radicals and triplet N2 molecules. The measurements have been reproduced and rationalized from first-principles by 2-D (r, z) coupled kinetic and transport modeling, and comparison between experiment and simulation has afforded a detailed understanding of C/N/H plasma-chemical reactivity and variations with process conditions and with location within the reactor. The experimentally validated simulations have been extended to much lower N2 input fractions and higher microwave powers than were probed experimentally, providing predictions for the gas-phase chemistry adjacent to the diamond surface and its variation across a wide range of conditions employed in practical diamond-growing CVD processes. The strongly bound N2 molecule is very resistant to dissociation at the input MW powers and pressures prevailing in typical diamond CVD reactors, but its chemical reactivity is boosted through energy pooling in its lowest-lying (metastable) triplet state and subsequent reactions with H atoms. For a CH4 input mole fraction of 4%, with N2 present at 1-6000 ppm, at pressure p = 150 Torr, and with applied microwave power P = 1.5 kW, the near-substrate gas-phase N atom concentration, [N]ns, scales linearly with the N2 input mole fraction and exceeds the concentrations [NH]ns, [NH2]ns, and [CN]ns of other reactive nitrogen-containing species by up to an order of magnitude. The ratio [N]ns/[CH3]ns scales proportionally with (but is 102-103 times smaller than) the ratio of the N2 to CH4 input mole fractions for the given values of p and P, but [N]ns/[CN]ns decreases (and thus the potential importance of CN in contributing to N-doped diamond growth increases) as p and P increase. Possible insights regarding the well-documented effects of trace N2 additions on the growth rates and morphologies of diamond films formed by CVD using MW-activated CH4/H2 gas mixtures are briefly considered.

Microwave Plasma-Activated Chemical Vapor Deposition of Nitrogen-Doped Diamond. I. N2/H2 and NH3/H2 Plasmas.

We report a combined experimental/modeling study of microwave activated dilute N2/H2 and NH3/H2 plasmas as a precursor to diagnosis of the CH4/N2/H2 plasmas used for the chemical vapor deposition (CVD) of N-doped diamond. Absolute column densities of H(n = 2) atoms and NH(X(3)Σ(-), v = 0) radicals have been determined by cavity ring down spectroscopy, as a function of height (z) above a molybdenum substrate and of the plasma process conditions, i.e., total gas pressure p, input power P, and the nitrogen/hydrogen atom ratio in the source gas. Optical emission spectroscopy has been used to investigate variations in the relative number densities of H(n = 3) atoms, NH(A(3)Π) radicals, and N2(C(3)Πu) molecules as functions of the same process conditions. These experimental data are complemented by 2-D (r, z) coupled kinetic and transport modeling for the same process conditions, which consider variations in both the overall chemistry and plasma parameters, including the electron (Te) and gas (T) temperatures, the electron density (ne), and the plasma power density (Q). Comparisons between experiment and theory allow refinement of prior understanding of N/H plasma-chemical reactivity, and its variation with process conditions and with location within the CVD reactor, and serve to highlight the essential role of metastable N2(A(3)Σ(+)u) molecules (formed by electron impact excitation) and their hitherto underappreciated reactivity with H atoms, in converting N2 process gas into reactive NHx (x = 0-3) radical species.

Exploring the plasma chemistry in microwave chemical vapor deposition of diamond from C/H/O gas mixtures.

Microwave (MW)-activated CH(4)/CO(2)/H(2) gas mixtures operating under conditions relevant to diamond chemical vapor deposition (i.e., X(C/Σ) = X(elem)(C)/(X(elem)(C) + X(elem)(O)) ≈ 0.5, H(2) mole fraction = 0.3, pressure, p = 150 Torr, and input power, P = 1 kW) have been explored in detail by a combination of spatially resolved absorption measurements (of CH, C(2)(a), and OH radicals and H(n = 2) atoms) within the hot plasma region and companion 2-dimensional modeling of the plasma. CO and H(2) are identified as the dominant species in the plasma core. The lower thermal conductivity of such a mixture (cf. the H(2)-rich plasmas used in most diamond chemical vapor deposition) accounts for the finding that CH(4)/CO(2)/H(2) plasmas can yield similar maximal gas temperatures and diamond growth rates at lower input powers than traditional CH(4)/H(2) plasmas. The plasma chemistry and composition is seen to switch upon changing from oxygen-rich (X(C/Σ) < 0.5) to carbon-rich (X(C/Σ) > 0.5) source gas mixtures and, by comparing CH(4)/CO(2)/H(2) (X(C/Σ) = 0.5) and CO/H(2) plasmas, to be sensitive to the choice of source gas (by virtue of the different prevailing gas activation mechanisms), in contrast to C/H process gas mixtures. CH(3) radicals are identified as the most abundant C(1)H(x) [x = 0-3] species near the growing diamond surface within the process window for successful diamond growth (X(C/Σ) ≈ 0.5-0.54) identified by Bachmann et al. (Diamond Relat. Mater.1991, 1, 1). This, and the findings of similar maximal gas temperatures (T(gas) ~2800-3000 K) and H atom mole fractions (X(H)~5-10%) to those found in MW-activated C/H plasmas, points to the prevalence of similar CH(3) radical based diamond growth mechanisms in both C/H and C/H/O plasmas.

Optical emission from microwave activated C/H/O gas mixtures for diamond chemical vapor deposition.

The spatial distributions and relative abundances of electronically excited H atoms, OH, CH, C(2) and C(3) radicals, and CO molecules in microwave (MW) activated CH(4)/CO(2)/H(2) and CO/H(2) gas mixtures operating under conditions appropriate for diamond growth by MW plasma enhanced chemical vapor deposition (CVD) have been investigated by optical emission spectroscopy (OES) as a function of process conditions (gas mixing ratio, incident MW power, and pressure) and rationalized by reference to extensive 2-dimensional plasma modeling. The OES measurements clearly reveal the switch in plasma chemistry and composition that occurs upon changing from oxygen-rich to carbon-rich source gas mixtures, complementing spatially resolved absorption measurements under identical plasma conditions (Kelly et al., companion article). Interpretation of OES data typically assumes that electron impact excitation (EIE) is the dominant route to forming the emitting species of interest. The present study identifies a number of factors that complicate the use of OES for monitoring C/H/O plasmas. The OH* emission from EIE of ground state OH(X) radicals can be enhanced by excitation energy transfer from metastable CO(a(3)Π) molecules. The CH* and C(2)* emissions can be boosted by chemiluminescent reactions between, for example, C(2)H radicals and O atoms, or C atoms and CH radicals. Additionally, the EIE efficiency of each of these radical species is sensitively dependent on any spatial mismatch between the regions of maximal radical and electron density, which itself is a sensitive function of elemental C/O ratio in the process gas mixture (particularly when close to 1:1, as required for diamond growth) and the H(2) mole fraction.