Transcript Diamond Nucleation (Boris V. Spitsyn)

DIAMOND NUCLEATION
FROM AN ACTIVATED VAPOR
PHASE
Boris V. Spitsyn
Institute of Physical Chemistry RAS, 31 Leninsky Pr.,
119991 Moscow, RUSSIA
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CONTENTS
 Introduction
 Basic parameters
 The classical theory of nucleation
 Homogeneous nucleation
 Discovery the CVD heteronucleation. Synthesis of nanodiamond particles
 Modern research of diamond nucleation on carbide forming substrates
 Concentration carbon precursors and total pressure in gas phase
 Substrate temperature
 Topography of a surface
 Electrical bias
 Conclusion
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INTRODUCTION
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.
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Initial discovery of heterogeneous nucleation of
diamond on foreign substrates
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.
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Original chemical transport
reaction
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THE BASIC PARAMETERS
Probability of homogeneous nucleation at CVD of diamond to be
determined by following parameters of a gas phase:
• Chemical content, presence in it of ions,
• temperature,
• level of activation,
• gas flow through activation zone,
• total pressure,
At heterogeneous nucleation it is necessary to add:
• chemical, phase structure and topography of substrate,
• temperature of surface,
• electrical properties and charge of a substrate in relation to a gas phase,
• effective distance between activation zone and substrate surface.
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Nanometric diamond
Selfnucleation
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)
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Selectivity
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
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CVD diamond doping
B.V.Spitsyn, Dr.Sciences Thesis, Institute of Physical Chemistry, Moscow 1973.
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The CLASSICAL THEORY of NUCLEATION
the probability of homogeneous nucleation in monoatomic vapor phase is:
J = A exp (-Ga/kT) exp (-1623/3kT2)
(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
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Ways of the surface energy reduction
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.
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Experimental on 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
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Basic finding on heterogeneous nucleation
• The nucleation rate of diamond on nondiamond substrates changed from 103 up
to 108 cm-2 depending on conditions of synthesis, material of a substrate and its
preparation procedure: polishing, etching, annealing.
• The occurrence of diamond nuclea was observed mostly on defects, such as
scratches and grain boundaries.
• The nucleation rate of diamond in was several times lower on single crystalline
substrates in comparison with polycrystalline substrates of the same material and
after similar preliminary superficial processing.
• The nucleation of diamond on carbide forming substrates (silicon, tungsten,
molybdenum ) was on two and more orders of magnitude higher than on
noncarbide-forming substrates (copper, gold).
• At late stages of nucleation the increase of the sizes of the originally formed
germs and the frequency of occurrence of new germs on sites of the surfaces which
have been not covered with diamond, was reduced.
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Substrate nature
b)
a)
1000
2000
time [s]
Fig.1. Approximate temporary dependence for nucleation density
at initial stages of the diamond CVD on:
a) non-carbide forming substrates (like Cu, Au) and
b) carbide forming substrates (Si, Mo, W)
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Modern research of diamond nucleation on
carbideforming substrates
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.
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Enthalpy of carbide formation
• S.D. Wolter et al., J. Appl. Phys. 77, 5119 (1995).
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Carbon precursor concentration and total
pressure
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.
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Substrate temperature
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.
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Surface geometry
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 WentzelDerjaguin formula
Cos r = r Cos a
(2),
where r and a are 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.
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Wetting of rough surface
a) Wetting surface
Θ>90o, ΔΘ<0
Θsmooth
Θrought
b) Non-wetting surface
Θ>90o, ΔΘ<0
Θsmooth
Θrought
liquid
solid
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E.I.Givargizov et al., Inst. Of Crystallography RAS, Moscow
Electrical bias
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.
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Modern Diamond CVD
Isothermal CVD
Stationary content
of a gas phase
Thermal
activation
Variable content
of a gas phase
Activated chemical
crystallization of diamond
Electrical
activation
Chemical
activation
DC discharge
Chemical
transport reaction
Photochemical
activation
Acetylene-oxygen
torch
DC arc-jet
Heat transporting
gas apparatus
Hot filament
reactor
AC discharge
RF
mW
Nonequilibrium
oxidation chemical
reaction
ECR
Hybrid techniques
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Hybrid CVD
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.
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Hybrid CVD
SEM image of the surface morphology
of DF’s grown by HF technique,
at biases:
a)
# 14-80, 0 V,
b)
# 14-82, +70 V,
c)
# 14-81, +85 V.
Raman spectra of the DF
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Basic features of diamond CVD
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Basic features of diamond CVD
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Basic features of diamond CVD
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Conclusion
• The nucleation of diamond from activated gas phase allows to receive
both isolated free diamond nanopowders, and regrowing on surface of
nondiamond materials.
• There exist basic opportunity of nanodiamond synthesis with controllable
composition, structure and arrangement on a surface of substrates.
• Significant potential of the CVD nanodiamond in the favour of emergent
nanoscience and nanotechnology will be most comprehensive realized in
frameworks of interdisciplinary and international research and
development.
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Acknowledgements
• I would like to express my gratitude to
A.E.Alexenko, A.A.Botev. L.L.Bouilov and
G.A.Sokolina for many years cooperation.
• Slide preparation by A. Spitsyn, A.Velez, D.
Rangaraj, highly appreciated.
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