DIAMOND NUCLEATION FROM AN ACTIVATED VAPOR PHASE. Boris V. Spitsyn Institute of Physical Chemistry RAS, 31 Leninsky Pr., 119991 Moscow, RUSSIA. CONTENTS. Introduction Basic parameters The classical theory of nucleation Homogeneous nucleation
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Boris V. Spitsyn
Institute of Physical Chemistry RAS, 31 Leninsky Pr.,
119991 Moscow, RUSSIA
Nucleation is one of the most important initial stages of synthesis of diamond from a gas phase. To no one problem in CVD of diamond does not devoted so significant number of the publications. At the same time many regularities of this major process not completely are found out. It and it is no wonder, as the process diamond nucleation depends on many conditions of experiment, which in a number of cases can vary in time. Now from knowledge and skill in nucleation area depends critically perfect thin film growth, adhesion, and progress in heteroepitaxy of diamond. Due to growing interest to nanomaterials CVD diamond at early stage of its emergence offer direct way to nanodiamond formation. Consideration of influence some, in our opinion, most important peculiarities of the diamond nucleation from activated gas phase this brief review is devoted.
In initial process of chemical transport reaction on copper and gold surface, and then and on carbideforming substrates, such as the silicon, tungsten, molybdenum was revealed in the first time nucleation and growth of microcrystals of diamond with characteristic cubo-octahedral habit. It is remarkable , that already in these first experiments and followed publications some of the most essential regularities of heterogeneous nucleation of diamond were established.
Probability of homogeneous nucleation at CVD of diamond to be determined by following parameters of a gas phase:
At heterogeneous nucleation it is necessary to add:
B.V.Spitsyn, L.L.Bouilov, and B.V.Derjaguin, Growth of diamond on diamond and another surfaces, J.Cryst. Growth 52, 219-226 (1981)
Y.Tzeng et al., ICNDST-4,Program and Abstrcts, Kobe, Japan, July 18-22, 1994, p. 63. Diamond microcrystal growth at ~ 1500 0C.
J.E.Butler, in Industrial Diamonds and Diamond Films, ed. by M.A.Prelas et al., NY, 1998.
Reversibility of C=C bond formation in presence of atomic hydrogen
B.V.Spitsyn, Dr.Sciences Thesis, Institute of Physical Chemistry, Moscow 1973.
the probability of homogeneous nucleation in monoatomic vapor phase is:
J = A exp (-Ga/kT) exp (-1623/3kT2) (1),
where J - number of germs arising in unit of time in unit of volume; - specific volume come on one particle in a crystal; A - preexponential term, A is proportional to density of these particles in parent phase; k – Boltzmann constant Больцмана; - specific free energy of crystal – environment boundary; Ga - free energy of activation of a new elementary particle addition to a germ; - difference of chemical potentials between initial and final phases.
A.A.Chernov, Crystallization, in: Annual Reviews of Materials Science, in: R.A.Huggins, et al., eds., 1973, vol.3, 397
The formula (1) testifies, that it, apparently, one of strongest dependence known in physics and physical chemistry, as the superficial energy, - enters in a degree 3 under exponent in numerator, and the natural logarithm of supersaturation, kT ln p/po, in a degree 2 is included into a denominator. Superficial energy of diamond - one of highest of known for solids. However it is essentially reduced, in ~ 10 times, at chemisorption of noncarbon atoms.
P.Demo et al., Diamond and Relat. Mater. 6 (1997).
The superficial energy of nonequilibrium surface at course of heterogeneous chemical reactions, can undergo additional reduction.]
A.A.Zhukhovitsky, V.A.Grigoryan, and E.Mikhalik, Dokl. Akad. Nauk SSSR 155, 392 (1964). By all by it can be caused the observable homogeneous nucleation of diamond.
By M.Frenklach, et.al., J.Appl. Phys. 66 (1989) 1247 it was revealed, that on exit from the activation zone from CH4 - H2- O2 initial gas mixture< converted in mW plasma can be found out as nanoparticles of cubic diamond, and diamond polytipes, as well. As demonstrate high-voltage TEM nanodiffraction most pronounced ids formation of polytypes, crystallochemically situated between sphaleritic (3C-) and wurtzitic,or lansdelite (2H-) structures. Particularely 6H- and 8H- polymorphs have been observed. It suggest that nucleation at extremely high nonequilibrium, at least in relation to diamond, took place. The nucleation frequency in conditions of the above mentioned work was about 103 cm-3 s-1
Research, carried out in 80-ties by Prof. B. Lux and co-authors have confirmed the regularities, originally found by the Russian researchers. Alongside with it, was shown, that nucleation of diamond on a number of metals, such as Mo, W, Nb, Ta, Fe, etc. demonstrate certain delay in the beginning of nucleation of diamond particles. It connected with various carbide forming ability of these materials, and also to various diffusion coefficient value. Really at use of substrates with high carbon in metal diffusion, the nucleation came with a delay in time. It was explained by the large time necessary for saturation by carbon of a material of a substrate and the greater thickness subsurface layers, subjected by carbonisation. Only after finishing of the process the nucleation centers can to emerge and to grow.
The increase of concentration carbon precursorsat preservation about the same level of activation of a gas phase will result in increase carbon precursors, CnHm , concentration
At the same time stationary concentration of atomic hydrogen falls at the presence of other radicals such, as methyl. Therefore working supersaturation, as was shown earlier, should grow considerably and to provide more intensive nucleation. The experimental data are in the good consent with such assumption, but only in the limited range of total pressure values. Recently was established, that density of nucleation in mW plasma can be very low near pressure ~ 1 Тorr and then, passing through a maximum, again is reduced. The experiments were carried out with application of electrical bias on a substrate. That’s why at low pressure action of ions with higher energy more pronounced. It could result in partial destruction of diamond nuclea.
New nucleation mechanism with basic precursor C2 molecule was discovered by Gruen by using fullerene–argon or methane–argon mixtures with very low content of hydrogen. It was revealed original path for new carbon atom addition through insertion of the C2 molecules in C-C and C-H bonds. Because much lower kinetic barriers of the reactions, nucleation density grow up 6 decimal orders of magnitude, what results to nano-DF formation with crystallite size 3 to 15 nm.
The significant role of substrate temperature in a stage of critical and even subcritical nucleation is played mostly by rate new atoms addition by diamond crystallization. Growth rate o macroscopic DF, after some maximum at temperature about 900 oC, is reduced. The similar character in nucleation frequency was observed in some circumstances. It finds an explanation in preferable destruction of sub critical diamond clusters under action of atomic hydrogen. So most probable nanodiamond heterogeneous formation at nearly 700oC takes place.
It must be stressed that the super-saturation, or chemical potential difference between activated vapor phase and diamond phase generally should be = kT ln p/po, where p and po are acting and equilibrium partial pressures, need special comments. The thermodynamic driving force, or super-saturation of condensed carbon formation by ACVD consists of two members. In the case, e.g., hot filament diamond CVD with stationary activated vapor phase CH3-H-H2 composition, we shall write down = kT ln P(CH3)/Po(CH3) - kT ln P(H)/Po(H), where P(CH3) and P(H) - working partial pressures of methyl radical and atomic hydrogen, and P(CH3) and Po(H) - their equilibrium values.
Surface geometry role in the diamond nucleation probability include several factors. First of all the sharp tips at the carbide – forming substrate surface should pass the carburizing/carbidization stage more early, and than be preferentially ready for the diamond nuclei formation (see Sect.3, Fig.2). Secondly, ‘wetting’ by diamond nucleus, due to very high surface energy of diamond, of convex parts of the surfaces much more pronounced, according to the Wentzel-Derjaguin formula
Cos r = r Cos a (2),
where r and aare wetting angles for the rough and flat surfaces, respectively; r = S r/ S r is ratio of the real and appearing (projection to the real) surfaces. Such opposite influence on wetting contact angle demonstrate next slide.
It phenomenon was discovered by S.Yugo et al. The nucleation enhancement started only at methane concentration in hydrogen more 5 %, and relative to smooth Si substrate bias, Ub –70 V. Most pronounced effect was achieved by 5 min action of seeding conditions at 40 % of methane in the incoming vapor phase and Ub = - 100 V. The procedure provide great nucleation promotion up to 1011 cm–2. Then usual growing conditions (methane concentration in hydrogen ~1%, no bias) provide submicrometer continuous DF regrowth on Si substrate. Authors explain influence of negative biasing of substrate by extra activation of precursors of diamond (or mixed sp3/ sp2 clusters) by impingement of arriving to the surface positive ions with some optimal energy. Also the effect just partially may originate from the enhancement of surface diffusion as well.
Stationary content of a gas phase
Variable content of a gas phase
Activated chemical crystallization of diamond
Chemical transport reaction
Nonequilibrium oxidation chemical reaction
Heat transporting gas apparatus
Charging of surface irrespective of a sighn of this extra charge can lower energy of formation of the transitive activated complex up to 20 kcal/mole, if this complex contains polar chemical bonds.
V.B.Kazansky and N.D.Ttshuvylkin, Dokl. Akad. Nauk SSSR 223, (1975) 910.
It provide remarkable gain in DF and diamond nuclea transfer through critical size by hybrid diamond CVD’es.
SEM image of the surface morphology of DF’s grown by HF technique, at biases:
Raman spectra of the DF