A model of the vaporization and mass transport of mixed organometallics from a single source for thin film metalorganic chemical vapor deposition is presented. A stoichiometric gas phase can be obtained from a mixture of the organometallics in the desired mole ratios, in spite of differences in the volatilities of the individual compounds. Proper film composition and growth rates are obtained by controlling the velocity of a carriage containing the organometallics through the heating zone of a vaporizer

Levy, Moises

Meng, Guangyao

Sarma, Bimal K.

Schneider, Roger L.

Zhou, Gang

English

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NASA-CR-203704
Model for the vaporization of mixed organometallic compounds
in the metalorganic chemical vapor deposition of high
temperature superconducting films
Guangyao Meng, a) Gang Zhou, Roger L. Schneider, Bimal K. Sarma, and Moises Levy
Department of Physics, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201
(Received 4 March 1993; accepted for publication 25 July 1993)
A model of the vaporization and mass transport of mixed organometallics from a single source
for thin film metalorganic chemical vapor deposition is presented. A stoichiometric gas phase
can be obtained from a mixture of the organometallics in the desired mole ratios, in spite of
differences in the volatilities of the individual compounds. Proper film composition and growth
rates are obtained by controlling the velocity of a carriage containing the organometallics
through the heating zone of a vaporizer.
_' 5!: .... t
i
#"'_
,_>...? -./ ,-;_o ,2_.......
Metalorganic chemical vapor deposition (MOCVD)
systems have produced excellent high-T c thin films, t In the
conventional multisource MOCVD system, it is difficult to
control the gas phase composition and hence the film
stoichiometry. This is because the volatility and stability of
the organometallic compounds, which are individually va-
porized, then mixed, strongly depend on temperature, sys-
tem pressure, and carrier gas flow rate. Hiskes et al. 2 de-
veloped a single source MOCVD technique in which a
specially designed vaporizer was employed. This vaporizer
can be used to volatilize a stoichiometric mixture of
/_-diketonates of yttrium, barium, and copper to produce a
mixed vapor in a 1:2:3 ratio, respectively, of the organo-
metallics. This is accomplished even though the three com-
pounds have significantly different volatilities.
In this letter, we offer a model which provides insight
into the process of vaporizing mixed organometallic com-
pounds to produce high quality thin films of YBazCu307
(YBCO). The model clearly shows that under steady-state
conditions, the mixed organometallic vapor must have a
stoichiometric ratio of the individual organometallics iden-
tical to that in the solid mixture.
A simplified schematic diagram of the vaporizer,
which is part of our MOCVD system, is shown in'Fig. 1.
The organometallic compounds are finely ground and
mixed well, and then packed into a thin walled, slotted
glass tube. This slotted tube slowly traverses an abrupt
temperature gradient between the cooling jacket and the
heating lamp, which heats the mixed organometallics from
room temperature to above 300 "C, which is above the
highest vaporization temperature of the three
fl-diketonates) Steady-state vaporization of the powdered
mixture occurs within the sharp temperature gradient and
the vapors are swept from the vaporizer and transported to
the substrate by a carrier gas which flows in the same
direction as the slotted tube. This technique establishes a
stream of mixed organometallic vapors with the desired
composition.
In our model, for simplicity, we use an open channel
")On leave from Department of Materials Science and Engineering, Uni-
versity of Science and Technology of China, Hefei 230026, People's
Republic of China.
carriage as a container for the mixed organometallics,
rather than the circular cross-sectional slotted tube, actu-
ally used in our vaporizer. The rectangular cross-sectioned
carriage has an internal width, w, and height h, as shown in
Fig. 1.
We assume (1) the mixed powder contains three or-
ganometallic compounds A, B, and C in the volume con-
centrations qa, qB, and qc and that qA <qs<qc; (2) all of
the mixtures in any given layer are at the same tempera-
ture, and the surface area of the layer is constant until
vaporization commences; and (3) the surface vaporization
rates of the individual organometallic compounds, ra, r B,
and rc are only a function of temperature and are not
interdependent.
With r n < rA < rc, for example, a specific composition
profile of the powder will be established under steady-state
conditions within the heating zone, as is shown in Fig. 2.
The total vaporization rates of compounds A, B, and C in
units of, e.g. mol/s, are given by
c hvqi
Ri= J r,w dl=rlwli=r,w --=whvqi ,ri ( 1 )
where Ii is a length along the total length of the melt,
within the heating zone, in which component i is present,
and v is the velocity of the carriage containing the mixed
Ar Ar
water __ IH
organometallic IMover
charge cross-
section
_, J compacted, mixed
2_ organometallics
quartz-halogen
1 _Heater)
ent
of carriage
FIG. I. The mixed organometallic compounds vaporizer.
1981 Appl. Phys. Lett. 63 (14), 4 October 1993 0003-6951/93/63(14)/1981/3/$6.00 © 1993 American Institute of Physics 1981
https://ntrs.nasa.gov/search.jsp?R=19970017809 2020-06-16T01:55:22+00:00Z
(a)
..'4---- Heatingzone --
e _
_qB
/
(b)
IA 1c 18
Length of presence in heating zone
FIG. 2. Conditions within the heating zone (simple model). (a) Profile
of the mixed organometallics wedge. (b) Component concentrations and
temperature profile along the heating zone length.
organometallics, moving in the heating zone. The leading
edge of the melt wedge, depicted in Fig. 2(a), will consist
only of the least volatile component (B) with the quantity
of the other two more volatile components, A and C, in-
creasing proportionately with the height, h, or thickness of
the wedge. The equations summarize two important prop-
erties of the vaporization process: (a) The mass flow rate
of each component vaporized and, therefore, the film dep-
osition rate is simply determined by the cross-sectional
area of the organometallics and the velocity with which the
carriage is moved through the heating zone, and (b) the
ratio of the three compounds in the gas phase, and hence
the film's elemental composition, is the same as that in the
initial mixture, assuming the film composition is propor-
tional to that of the gas phase. This is a requirement for
mass continuity.
If the model is valid, the single source technique
should provide a simple means of controlling both the
film's composition and growth rate. In practice, .however,
the preceding assumptions (2) and (3) are not correct.
Actually, it is impossible to create a constant temperature
plateau, shown in Fig. 2(b), in so narrow a heating zone.
Because a very sharp temperature gradient exists, the va-
porization rate for each organometailic compound cannot
be invariant within the heated zone, because of the strong
dependence of volatility on temperature, a The vaporization
rate Will show an Arrhenius dependency on temperature of
the general form
ri=Kiexp (--RT]' (2)
Where AHi is the heat of vaporization for the species i, R
the gas constant, and K i is the frequency factor. The com-
position profiles of the three components, depicted in Fig.
2 (b), are no longer straight lines, but become curves which
are related to the temperature profile. Furthermore, the
vaporization rates of the organometallic compounds are
interdependent.
.c
o po_
Ib lX tX A
_£ __,___ (a)
n4--- A+B+C ----,IA+_ B l--
,,
=
•_ qc_/_ _,,__.___
_ qA (b)
1A 1c I a I c 1b 1_
Length of presence in heating zone
FIG. 3. Conditions within the heating zone (advanced model). (a) Pro-
file of mixed organometallics wedge, showing the mierostructure and non-
uniform vaporization surface. (b) Revised component concentrations and
temperature profile along heating zone length.
The coiligative properties of melting point depression
in the mechanical mixture of the organometallics and boil-
ing point elevation in the melt will effect changes in the
vaporization rate of individual components. However, the
differences in the individual volatilities of the organometal-
lics plays the major role in the interdependence of their
rates of vaporization.
From the melt wedge, the most volatile component
evaporates fastest and a thicker outer layer becomes the
vaporization surface. In this thicker outer layer, the less
volatile component becomes more concentrated. A rather
complicated microstructure profile develops along the
length of the melt within the heated zone, such as the one
shown schematically in Fig. 3(a). As the leading edge of
the wedge moves through the heating zone, the configura-
tion becomes more and more favorable for the vaporization
of the least volatile component because of the increase in
the percentage of the melt's total surface area it occupies.
Concomitantly, a lowering of the vaporization rates of the
more volatile components ensues due to the decrease in the
percentage of the melt's surface they occupy. The resulting
composition profile along the length of the wedge in the
heating zone is shown in Fig. 3(b). A desirable result of
this process is that the length (ln) for complete vaporiza-
tion of the least volatile component is shortened and the
lengths for the more volatile components (la and Ic) be-
comes longer. This means the gas phase in the heating zone
has a more uniform composition; but, is the gas phase
composition still controllable?
As was done earlier, we can derive an expression for
the total vaporization rate of each component in the com-
position profile, shown in Fig. 3(b). The vaporization rate
1982 Appl. Phys. Lett., Vol. 63, No. 14, 4 October 1993 Meng et aL 1982
of e_ch component ri at position l is given by
dqi dqi
ri=whV__l=hV--d_ ' (3)
and the total vaporization rate of each component R i in the
heated zone is then
Ri= f r,wdl= f dqihv --_ w dl=whv f dqi:whvqi,
(4)
which is identical to the expression in Eq. (1).
For a typical carrier gas flow rate of 200 sccm3/min at
a pressure of 5 Torr and an effective cross-sectional area of
1 cm 2, the linear gas flow velocity is approximately 5 m/s.
Normal values for Ii and the carriage velocity v are a few
mm and 0.5 mm/min, respectively. Therefore, it may be
assumed that the vaporized organometallics are swept
away immediately to reveal a fresh layer and promote va-
porization.
The gas phase concentration of each component, Ci is
give by
P , Pqi
C i _---R i _o =who -f-_o ' (5)
where f is the total gas flow rate (including oxygen) at the
inlet, P0 is the atmospheric pressure, and P the system
pressure. This simple expression shows the (absolute) gas
phase concentration is only determined by the organome-
tallics feed rate, the carrier gas flow rate, and the system's
dynamic pressure. This means the film deposition rate and
composition can be controlled by the CVD operation pa-
rameters, instead of by the volatility of the organometal-
lics, which is exactly what we want.
We have used a single source mixed organometallics
vaporizer to produce excellent high-To superconductive
thin films. 3 The shape and length of the melt wedge in the
heating zone is, as expected, a function of the carriage
velocity and the wattage of the heating lamp. For the Y,
Ba, Cu/3-diketonates mixture, the leading edge of the melt
is believed to be entirely composed of the barium diketo-
nate and the midsection composed of a mixture of yttrium
and barium diketonates. This distribution is consistent
with the literature 5-7 in which the Cu and Y fl-diketonates
are reported to have lower sublimation temperatures than
the Ba salt.
In summary, the single source mixed organometallic
vaporizer can provide under steady-state conditions a
stoichiometric ratio of organometallic vapors which is
identical to that in the solid mixture. The film deposition
rate is determined only by the carriage velocity and its
internal dimensions and the heater wattage. Neither the
film composition nor deposition rate are, under normal
conditions, influenced by the temperature profile of the
heating zone or the volatilities of the organometallics in the
mixture.
The model and attendant mathematical derivations
presented here are applicable to any multicomponent sin-
gle source vaporization system, once steady-state condi-
tions are achieved.
The research work was supported by NASA under
Contract No. NAG3-1274 and partially supported by the
Laboratory for Surface Studies.
t M. Schieber, S. C. Han, Y. Ariel, S. Chokron, T. Tsach, M. Maharizi,
C. Deutscher, D. Racah, A. Raizman, and S. Rotter, J. Cryst. Growth
115, 31 (1991).
2R. Hiskes, S. A. Dicarolis, J. L. Yong, S. S. Laderman, R. D. Jacowitz,
and R. C. Taber, Appl. Phys. Lett. 59, 606 (1992).
3G. Y. Meng, G. Zhou, R. L. Schneider, B. K. Sarma, and M. Levy,
Physica C 214 (1993).
4Z. H. Yuan, G. Y. Meng, and D. K. Peng, Thin Film Sci. Technol. (in
Chinese) 5, 46 (1991).
5H. Yu, D. K. Peng, and G. Y. Meng, J. China Univer. Sci. Technol. 16,
66 (1986).
6K. J. Eientraut and R. E. Sievers, J. Am. Chem. Soc. 87, 5254 (1965).
7E. W. Berg and N. M. Herrera, Anal. Chem. Acta 60, 117 (1972).
1983 Appl. Phys. Lett., Vol. 63, No. 14, 4 October 1993 Meng et aL 1983


Model for the Vaporization of Mixed Organometallic Compounds in the Metalorganic Chemical Vapor Deposition of High Temperature Superconducting Films

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