Pergamon
NancStmctwed Materials. Vol. 10. No. 4. pp. 565-573. 1998
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PI1 SO9659773(98)00104-4
FEASIBIILITY STUDY OF NANOPARTICLE SYNTHESIS FROM
POWDERS OF COMPOUNDS WITH INCONGRUENT
SUBLIMATION BEHAVIOR BY THE EVAPORATION/
CONDENSATION METHOD
Knut Deppert, Kornelius Nielsch’, Martin H. Magnusson, F. Einar Kruis’,
and Heinz F&an*
Lund University, Solid State Physics, Box 118, S-221 00 Lund, Sweden
‘Gerhard-Mercator-University Duisburg, Process and Aerosol Measurement Technology,
D-47048 Duisburg, Germany
*present address: Max-Planck-Institut fur Mikrostrukturphysik, Weinberg 2,
D-06120 Halle, Germany
(Accepted May 14,1998)
Abstract -In this paper we investigate the feasibility of a fabrication route to produce
nanocrystals of compound material with incongruent sublimation behavior via the simple
evaporation ofthepowder of the compound. The generation of stoichiometricparticles would only
be possible if the particle formation occurs at temperatures below the incongruent sublimation
point. Our experiments,doneon three different III-Vcompounds, show fhaf fhe simple evaporation
of the powder of those materials to obtain stoichiometric particles is not possible. Particle
formation does not start at temperatures below the incongruent sublimation point. Particles
synthesizedconsistednotofthecompoundbutalmost
entirelyofthemorevolatilegroup-Velement,
leading to a ch’ange in the composition of the source material and thus to irreproducible behavior.
01998 Acta Metallurgica Inc.
INTRODUCTION
Semiconductor crystals with dimensions in the nanometer range are of substantial interest
due to their electronic quantum-size effects, but the fabrication of such structures remains difficult.
The demand for very narrow size distributions is strong since most quantum-size effects will
otherwise be averaged out. Several attempts have been made at producing nanocrystals of III-V
materials for electronic and optoelectronic application, e.g.,: gas-phase reaction in a vapor-phase
epitaxy reactor (l), pulverization of bulk crystal (2), spark processing (3), sol-gel processes (4-6),
organometallic vapor deposition into porous glass (7), solution phase synthesis (89), and by the
Stranski-Krastanow epitaxial growth mode (10). One reported attempt via the aerosol route uses
a rather complicated chemical reaction (11). These attempts are characterized by the fact that the
nanocrystals are bound in one way or the other to surrounding material, and most of these methods
result in nanocrystals with wide size distributions.
566
K DEPPERT. K NIELSCH, MH MAGNUSSON,FE KIWIS AND H FISSAN
A more promising method was introduced by Deppert and Samuelson (12) with the aerosol
formation of III-V semiconductor nanoparticles by controlled reaction of size-selected group-III
metal particles with group-V containing hydrides. An even easier way to produce GaAs and InP
nanocrystals would be the simple evaporation of powders of these materials as has already been
demonstrated for PbS (13). In this paper we investigate the feasibility to form III-V compound
nanoparticles, as examples of compounds with strong incongruent sublimation behavior, by
simple evaporation of the powders. Those materials will dissociate when heated above the
incongruent sublimation point.
EXPERIMENTAL
In the present study, the same aerosol generator set-up has been used as in the previous work
on the fabrication of GaAs nanocrystals (14) with the modification of using only one furnace and
one differential mobility analyzer (DMA). It is based on an evaporation/condensation aerosol
generator (15). The fabrication route utilizes the formation of an aerosol, formed by evaporating
the powder material in a tube furnace and subsequent cooling down of the vapor. The aerosol
particles are charged and size selection takes place in a differential mobility analyzer. This
instrument is a conventional tool in aerosol technology forboth the fabrication of size-selected test
aerosols and the measurement of aerosols (16). It size-selects by balancing the mobility of charged
particles in an electric field with the force of the gas used to flush unwanted particles. The size
distribution reached is mainly depending on the gas flows and can be very small, e.g. f5%.
In this study, the formation of nanoparticles was investigated for powdersof GaAs, InP, and
InSb. The powders were produced by grinding standard high-quality wafers of the respective
stoichiometric compound material with less than 1ppm impurities.The grinding process wasdone
in a cleaned marble pestle. A slight contamination of the material from the pestle can, however, not
be excluded. Powder of one material was placed in a boat and loaded into the furnace. The boat,
made of pyrolitic boron nitride, was cleaned by etching in nitric acid and baked out at 1200°C.
Palladium-purified hydrogen or clean nitrogen (purity > 99.995%) was used as a carrier gas. This
gas flow through the evaporation furnace was kept constant at 1.68 l/min. In order to charge the
aerosol particles, a diffusion charger, including a s5Kr source emitting P-radiation, is placed after
the furnace. The size selection takes place in a DMA, a home-built device of the Vienna type (17).
The DMA was operated at 10 l/min of N2 sheath gas. An electrometer was used to determine the
particle number concentration. At each temperature setting of the furnace, a mobility scan was
recorded of the aerosol particles leaving the evaporation furnace, which took about 15 minutes.
Samples have been prepared for high-resolution transmission electron microscopy by depositing
particles onto a standard grid for electron microscopy using an electrostatic precipitator. Energy
dispersive x-ray spectroscopy (EDS) data were acquired for a few samples.
RESULTS AND DISCUSSION
The present work focuses on the change in the aerosol particle characteristics with
temperature in the evaporation furnace. Increasing the temperature results in a change in mobility
size distribution. The characteristic log-normal shape of the size distribution, however, does not
FEASIBILIW
STUDYOFNANOPARTICLE
SYNTHESIS
BYTHEEVAPORATION/CONDENSATION
METHOD
567
InP powder
80-
1
.o.-Nitrogen
d
,O ‘Q
‘.b
*.
‘0
60 -
/yd'
F
I
/
d
d
40 -
20-
.d
,Q
.,.o.a
.Q
d.
6'
.'
- <j
-
OL”“““““““.““.““““““““”
400
500
600
700
800
900
1000
1100
1200
Furnace temperature [“Cl
Figure 1. Three typical plots of the mean mobility diameter of particles generated by evaporation of indium phosphide powder. The carrier gases used are indicated. Hydrogen I and
hydrogen II name two subsequent experiments with identical conditions.
change with temperature. Thus, the maximum of this curve is taken as a measure of the mean
particle diameter in this work.
Generation of particles from InP, GaAs and InSb powders was investigated with the
evaporation fulmace set at temperatures from room temperature up to 1300°C. The furnace
temperature wals increased step-wise and care has been taken to allow the system to equilibrate
after each temperature change. The process was tested with hydrogen as well as with nitrogen as
carrier gas.
Figures 11to 3 give examples of the changes of the mean diameter with increasing
temperature for powders of InP, GaAs and InSb, respectively. The signal obtained in the
electrometer was noisy and changed with time. Let us concentrate on figure 1, the curve for InP,
since this is a good example for all experiments made. The figure reveals several aspects. First, with
increasing temperature the size of the particles leaving the furnace increases as does the number
concentration, which is not shown in the figure. Second, the curve for evaporation under nitrogen
is different from the curves for evaporation under hydrogen. Third, the two curves for evaporation
under hydrogen, made consecutively, exchanging the source material in between, differ from one
another. FourthI, the curves are not smooth, exhibiting some peaks and valleys.
568
K DEPPEFIT,K NIELSCH,MH MAGNUSSON,
FE KRUISAND H FISSAN
60-
--+-
900
1000
“(‘I
1100
Hydrogen
“‘I”
1200
1300
Furnace temperature [%I
Figure 2. One typical plot of the mean mobility diameter of particles generated by evaporation
of gallium arsenide powder, with hydrogen as carrier gas.
The fact that the higher the temperature in the furnace, the more and larger particles are
generated, is nothing new and well documented in the literature (15). The difference between
nitrogen and hydrogen as carrier gas might be understood in terms of different heat conduction
behavior for the gases, andapossible malfunction of the DMA giving raise to uncalibrated readings
for hydrogen. This fact, however, as well as the observation of a non-smooth curve, will not be
discussed here. The differences between the two curves for evaporation under hydrogen can be
taken as an example of the problem we have to deal with: the process is not reproducible. Making
the same experiment a second time and under identical conditions will result in a different curve.
(One could argue here that taking exactly the same amount of material should lead to the same
result. Even in case one chooses the same amount, however, it is not possible to pile up the powder
material in exact the same way, thus differences can occur.) This behavior was found for all three
powder materials investigated and was independent of the carrier gas used.
Inspections of the source materials in the boat after the experiments showed that it had
undergone a melting and segregation process, even in cases where the furnace temperature had
never exceeded the melting temperature of the compound material. The material was no longer a
pure powder, but was a solidified drop with a metallic shine which included residues from the
powder. EDS analysis showed that the remaining source materials consisted of more than 90% of
FEABIBILIWSTUDYOF NANOPARTICLE
SYNTHESIS
BY THEEVAPORATIO&ONDEN~ATION
METHOO
100
569
““,““[““,““,““I”“I”“I””
InSb powder
_
0
80 -
60 -
40 -
20 -FG]
o-““‘.““““““““““““““““’
500
700
600
800
900
1000
1100
:
1200
1300
Furnace temperature [“Cl
Figure 3. One typical plot of the mean mobility diameter of particles generated by evaporation
of indium antimonide powder, with nitrogen as carrier gas.
the group-III element. This result was obtained for all three III-V compounds used in this study.
Particles, generated at different temperatures (450 and 770°C for InP, 940°C for GaAs, 650°C for
InSb), were deposited on standard grids for electron microscopy, and analyzed with EDS. The
analyses show that the particles consist of more than 99% of the group-V element. Again, this was
found for all thlree materials investigated. Furthermore, in the case of InP, there was clear evidence
that phosphorus had been evaporated from the source since almost all tubes were covered with a
pyrophoric deposit with a yellow-reddish color.
The observations that (a) the particles consist of the group-V element, (b) the remaining
source material consist of the group-III element, together with (c) irreproducible results for the
particle measurements, allow one conclusion only: particle formation is only measurable at
temperatures where incongruent evaporation occurs. This becomes even more clear if one looks
at the temperatures where the incongruent sublimation for the material starts. Table 1 lists the
incongruent sublimation points as found in literature together with the temperatures for which
particle generation was observed in our experiments, i.e., where particle concentration exceeds
500 cmJ. In all cases, particle generation was observed at temperatures higher than the incongruent sublimation point. This explains the h-reproducible results obtained: the material does not
evaporate in a congruent way, and thus the particle generation is dependent on the source
composition, which changes with time.
570
K DEPPERT,K NIELSCH,MH MAGNUSSON,
FE KRUISAND H FISSAN
TABLE 1
Comparison between the Temperatures for which Particle Generation was Observed and the
Temperatures for which Incongruent Sublimation has been Reported to Start
III-V compound
Particle generation
under hydrogen
Particle generation
under nitrogen
no data
2 420 “C
1595 “C
Incongruent
sublimation point
620 - 650 “C( 18-20)
355 “C( 19,21)
270 - 400 “C( 19,20)
Incongruent sublimation occurs for the III-V compounds according to the following scheme as
given by (22):
MS)
t) N)+$&g)+$g).
ill
where Aand B are the elements from the different groups of the periodic table, 4 is a composition
coefficient, and s, 1 and g describe the physical state. While the compound evaporates at low
temperatures in a congruent manner, i.e., as a compound, it will dissociate at temperatures above
the incongruent sublimation point. The more volatile group-v element evaporates from the source
as gaseous molecules, i.e., dimers or tetramers. The less volatile group-III element remains in the
source, the composition of which changes, eventually melting it. The gaseous molecules of the
group-V element will subsequently form particles, as is known from the evaporation/nucleation
aerosol generator for metals and salt. The particle generation is thus dependent on the source
composition and the behavior of dimers and tetramers in the gas phase. This explains why the
experimental results are u-reproducible.
In order to test if this occurs only for compounds with high tendency of incongruent
evaporation, e.g., III-V compounds, experiments were carried out with PbS powder as source
material. Kruis ef al. (23) showed the possibility to generate stoichiometric PbS particles by this
method. Lead sulfide is a IV-VI compound which has a much lower tendency to incongruent
sublimation than the III-V compounds. Incongruent sublimation occurs for the IV-VI compounds
according to the following scheme as given by (22):
~(s)ttAB(g)+A(g)+~~~(g).
PI
This means that incongruent sublimation may occur. Still, stoichiometric crystal layers of some
common IV-VI compounds, including PbS, PbSe and PbTe, have been prepared by evaporation
of the compounds. Incongruent sublimation is stated to occur at temperatures above the melting
point.
FEASIEILINSTUDYOF NANOPARTICLE
SYNTHESIS
BYTHEEVAPORATIO~JCONDEN~A~ON
METHOD
500
600
700
600
900
1000
1100
1200
571
1300
Furnace temperature [“Cl
Figure 4. Pl’ots of the mean mobility diameter of particles generated by evaporation of lead
sulfide powder. The carrier gases used are indicated.
Figure 4 shows the result of the tests on lead sulfide powders. Particle generation was
observed at kmperatures starting from 475°C under nitrogen and 500°C under hydrogen,
respectively. This process was stable and reproducible with nitrogen as carrier gas if the
temperature did not exceed 850°C. For hydrogen, however, it was only stable up to a temperature
of 650°C. At higher temperatures the process also had a tendency towards irreproducibility, as had
been the case for the III-V compounds. Furthermore, the material deposited on the walls of the
cooler end of the furnace tube consisted of PbS in the case of nitrogen and mainly of Pb in the case
of hydrogen as carrier gas. For hydrogen a possible reduction of PbS by the gas is reported for
temperatures above 400°C (24). Since Kruis et al. (23) used evaporation temperatures below
700°C and nitrogen as carrier gas, they were confronted neither with incongruent evaporation nor
with reduction reactions of the source material.
SUMMARY
We performed experiments to study the feasibility of generating stoichiometric particles of
III-V compotmds by plain evaporation of the powder. Three different compounds, GaAs, InPand
InSb, were tested as examples of this class of compounds. It was found that particle generation
starts only at temperatures above the incongruent sublimation point of the material. That means
572
K DEPPEAT,K NIELSCH,MH MAGNUSSON,
FE KRUISANDH FISSAN
that the nucleation rate at temperatures below the incongruent sublimation point is too low. This
results in irreproducible experiments with the formation of particles consisting mainly of the more
volatile group-V element at temperatures above the incongruent sublimation point. We can
conclude that this approach, plain evaporation of powder, is not suitable for the generation of
stoichiometric particles of such compounds, under the simple conditions tested. Applying much
more complicated methods, such as a higher system pressure, might result in a higher nucleation
rate at temperatures below the incongruent sublimation point.
ACKNOWLEDGMENT
This work was performed within the Nanometer Structure Consortium in Lund, and was supported
by grants from NUTEK, NFR, TFR and the ESF-NAN0 Programme. The authors wish to express
their thanks to J.-O. Malm for supplying the EDS data.
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