Epitaxy Synthesis of GaAs Nanowires on Silicon
Substrates
A study of the time-dependance of GaAs nanowire growth
Elsa De Geer, David Israel och Gustav Seemann
Supervisor: Rong Sun
December 17, 2014
1
Introduction
This project aimed to analyze epitaxial growth of gallium arsenide (GaAs) nanowires
on silicon (Si) substrates. The analysis was realized by changing growth time for different samples while keeping all other growth parameters constant. The samples were
characterized using a SEM.
2
Theory
For the laboratory part of the project, silicon wafers prepared with nanometer sized
gold (Au) seed particles were provided as substrates. The gold particles were deposited
onto the wafers by combined nanosphere lithography (NSL) and nanoimprint lithography
(NIL) techniques. These techniques enable controlled deposition of gold particles onto
large substrate areas. These nanosized gold particles acted as catalysts for nucleation of
GaAs nanowires.
The nanowire synthesis was carried out by a chemical vapor deposition method, more
specifically metalorganic vapour phase epitaxy (MOVPE). Epitaxy in general refers to
the formation of a crystalline material as a layer of adatoms adsorbed onto a crystalline
substrate. Epitaxial growth also means that the overlayer grows with a well-defined
crystal orientation with respect to the substrate material.
MOVPE is a specific epitaxy method. The process takes place in a sample chamber where
all growth affecting variables are precisely controlled. Pure gases, partly consisting of the
desired growth materials, are injected into the chamber. The reason metalorganic is part
of the method name is that one of these precursor gases is a metalorganic compound, i.e.
a metal atom with organic ligands. Therefore the by-product in this reaction is organic,
in many cases methane gas (CH4 ).
In this particular experiment, GaAs nanowires were to be grown. Hence, the precursors
were two compound gases containing the desired elements. More specifically arsine, AsH3 ,
and trimethylgallium (TMGa), Ga(CH3 )3 was used as precursors. Hydrogen gas was used
as the carrier gas.
As mentioned above, the sample in this project was coated with nanosized gold particles.
Since the formation of GaAs is chemically favoured at the interface between Si and Au, the
growth mainly took place where a gold seed particle was to be found. This resulting in the
vertical epitaxial formation of nanowires rather than a uniform crystalline overlayer.
3
Method
As the substrate for the growth process, a prepared silicon wafer with deposited Au
particles was used.
1
Before growing nanowires on the substrate, the accumulated oxide layer had to be etched
away. This was done by dipping the substrate into a HF 1:10 solution for approximately 1
min (or a little bit longer). Thereafter, the sample was quickly transported to the MOVPE
Aixtron. The transportation time was kept at a minimum (without compromising safety)
to avoid oxidation on the sample.
The nucleation of the sample was initiated by 20 s of TMGa flushing, after which the
AsH3 flow was opened. The sample was then nucleated at T = 550 ◦ C for 7 minutes.
After the nucleation, the nanowires were grown at T = 450 ◦ C for a variable time (1,2,3
and 4 minutes). After the growth phase, both the TMGa and AsH3 flow was cut off, and
the sample was removed when cooled below 200 ◦ C. For further details on the growth
phase, see the recipe attached to the report.
A V/III ratio of of the precursors was around 38 both during the nucleation and growth
phase.
After growing the nanowires, they were characterized using a SEM. Four or more images
of each sample were saved for further analysis. The number of nucleations, length and
diameter of the nanowires was measured for each sample.
4
Results
The pictures that have been analysed concerning nucleation, length and diameter were
taken with 25 thousand times magnification, an accelerating voltage of 15 kV and at a
30 degree tilt angle. Four pictures were taken on different locations on each sample to
get a better average value.
4.1
Number of nucleations
In order to count the number of nucleations in a reliable way, a grid with 5 x 3 rectangles
was applied to each picture, and the amount of nucleations in each rectangle was summed.
Only those seed-particles where a clear nucleation could be seen were counted. Since
there were many seed-particles and some wires became very long, in some cases, the
whole nucleation with both top and bottom could not be seen. As a rule of thumb, these
wires were not included, unless it was possible to tell it was a proper nucleation.
As we can see, the number of nucleations strongly increase as the growing time increases.
We have a more or less linear increase for the three first samples, then it seems that it
decreases for the fourth sample. This is probably not the case for the whole sample, but
is rather due to the fact that not enough data was imaged and analyzed.
4.2
Length and diameter
The length and diameter of five wires was measured in each picture in order to get a
good average. To avoid errors in the measuring, the longest and straightest wires were
2
Figure 1: The figure shows the samples with different growth times. (a) 1 minute, (b)
2 minutes, (c) 3 minutes and (d) 4 minutes
prioritized, and they had to have a diameter greater than 10 nanometers.
Since not all the wires in each picture have been measured one should be a little careful
in drawing conclusions concerning length and diameter. That being said, there seems to
be a linear dependence for both length and diameter as the growing time increases. For
the samples grown 1 and 2 minutes, it was in some cases hard even to find five decent
nanowires to measure. Therefore we can without doubt conclude that both the length and
diameter increases as the growing time is bigger, in what seems like a linear dependence.
This becomes even more clear if one looks at the average points for each plot.
5
Discussion
As can be seen in figure 1, the resulting nanowires are not growing in a uniform vertical
direction from the substrate surface, nor are they uniform in size. Several reasons for this
and other error sources will be discussed below.
3
Figure 2: The figure shows the number of nucleations for each sample. The black
crosses show the count for each of the 12 images, the red circles show the average count
for all the images of each sample.
Figure 3: The figure shows the length of the nanowires for each sample. The black
crosses show the average length of five wires for each image, the red circles show the
average length for all of the images for each sample.
5.1
Substrate
The etching solution used on the substrate to remove the oxide layer was HF in H2 O,
1:10. In [1], it is stated that a buffered oxide etch solution (BOE) consisting of six parts
40 % NH4 F and 1 part 49 % HF yielded the highest amount of vertical growth. Probable
explanations mentioned in the same article are that the BOE produces a flatter substrate
and improves the nucleation uniformity for vertical growth. A flat substrate surface with
few defects and impurities in the crystal lattice favour growth in uniform direction.
The substrates used in the experiment already had Au particles deposited on them since
a few months back. Firstly, there is limited knowledge on the seed deposition method.
However, the same substrates have been used for nanowire growth earlier, with successful
results. Secondly, the substrates had been stored for some time before our growth experiments. This probably does not affect the result as Au is an inert substance and the
oxidation on the Si substrate is, hopefully, completely etched away, but the long storage
time is still considered worth mentioning.
4
Figure 4: The figure shows the diameter for each sample. The black crosses show the
average diameter of five wires for each image, the red circles show the average diameter
for all of the images for each sample.
5.2
Growth
The growth temperature for our samples was 450 ◦ C. According to [1], the optimal growth
temperature is between 417 ◦ C and 455 ◦ C. Our samples are grown within the optimal
interval, and the temperature is therefore not considered one of the major reasons behind
the lack on uniformity in direction and length.
During the growth phase, a V/III ratio of 38 was used. In the reference article [1], it is
stated that a high V/III ratio is important, but that the curve levels out for values >25.
The ratio is thereby larger than necessary, but should not have had a negative effect on
the results.
Before the growth phase, a higher temperature of 550 ◦ C was used for nucleation of the
sample. In the reference article [1], there is no nucleation phase included. The temperature during the nucleation phase is higher than the recommended growth temperature. A
risk with too high growth temperature is non-epitaxial growth, which has not happened.
The article also states that high temperature growth can lead to non-vertical wire growth.
There is also a risk for growth directly on the substrate (not at the interface between Au
and Si). Whether there is a risk for these effects to occur in the nucleation phase or not
is not clear. The wire growth is, as seen in figure 1 and at the front page, not necessarily
vertical as desired. This might be because of the high nucleation temperature, or, as
previously mentioned, because of the substrate surface not being properly etched.
5.3
Characterization
A scanning electron miscope, SEM, was used for characterizing the samples. As earlier
mentioned, the images were taken with a magnification of 25 thousand times, an accelerating voltage of 15 kV and a 30 degree tilt angle. When analyzing the samples in the
SEM, it is crucial to find a good magnification. With a too high magnification it will be
very hard to get good resolution, and a too small sample area will be analyzed, which
won’t give trustworthy results. With a magnification that is too low it will of course be
5
hard to analyze the sample, in this case one might not be able to distinguish if a seed
particle has nucleated or not, or missjudge the length of the nanowires.
If the accelerating voltage is too large, it could give rise to a phenomenon known as
charging. Briefly, it means that the large amount of electrons affects the sample due to
their electric charge. If this happens the image will become whiter, and it will be hard
to analyze them.
The SEM produces a top-down view of the sample, which in our case where the nanowires
grow in different directions isn’t optimal. Especially when it comes to measuring length
and diameter of the wires. An easy way to get around this is tilting the sample. Our
sample was tilted to 30 degrees, which in reality means that the whole sample holder was
tilted. When the electrons hit the sample at an angle they scatter, and as the scattered
electrons hit a detector they produce an image where it is possible to view the sample
”sideways”.
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Conclusion
The experiments and theory study shows that the growth rate of nanowires seems to
be close to linear. It is difficult to conclude that it is a strictly linear dependence from
the experimental data, as a very small fraction of the surface areas were studied. When
analyzing the samples with the SEM at a lower magnification it also became clear that
the nucleation rate was far from uniform across the whole surface.
The nanowires produced in the experiments were not uniform in size and direction. Reasons for this might be the etching process of the sample, which might not have produced a
flat enough substrate surface, or the high initial nucleation temperature of 550 ◦ C.
6
References
[1] Darija Susac Jon Bratvold David P. R. Aplin Wei Wei Ching-Yang Chen Shadi A.
Dayeh Karen L. Kavanagh Xin-Yu Bao, Cesare Soci and Deli Wang. Heteroepitaxial
growth of vertical gaas nanowires on si (111) substrates by metal-organic chemical
vapor deposition.
7
GaAs on Si
read reactoropen_embedded.epi;
read basestate_embedded.epi;
variable growth_time_NWs = 600;
#
variable NW_nucleat_temp = 550;
variable NW_growth_temp = 450;
#
variable AsH3_1_growth = 20;
#
variable TMGa_growth = 10;
#
ReactorData =
Power ReactorTemp Position ReactorPress
DORPress dP_Filter;
layer {
basestate;
0:05
"Initialize", RunMO = 2000,
RunHydride = 2800,
RunDopant = 500,
MainRotation = 0,
SatRotation = 0,
AsH3_1.source = AsH3_1_growth,
AsH3_1.push = (500-AsH3_1_growth),
TMGa_1.source = TMGa_growth,
TMGa_1.push = (500-TMGa_growth),
DummyMO_1.source = 500,
DummyHyd_1.source = 500,
VentHydride = 150,
VentMO = 120,
LinerPurge = 1000,
WindowPurge = 500;
0:05
"switch to hydrogen", N2.run = close,
H2.run = open,
Heater = on;
3:00
"pump down", ReactorPress to 100,
LinerPurge to 7500,
Power to 95,
RunMO to 7000,
RunDopant to 1500,
RunHydride to 3500,
DummyMO_1.run = open,
DummyHyd_1.run = open,
MainRotation to 30,
SatRotation = 0,
"wait ", until ReactorTemp >> 250;
0:05
AsH3_1.line = open,
TMGa_1.line = open;
#------------------------------------------------------------nucleation @ 550°C------------------------------7:00
ReactorTemp = NW_nucleat_temp;
"wait for 550C", until ReactorTemp == NW_nucleat_temp;
0:20
Òflushing TMGaÓ,
TMGa_1.run = open,
DummyMO_1.run = close;
0:20
ÒGaAs nucleationÓ
DummyHyd_1.run = close,
AsH3_1.run = open;
0:03
"stop", TMGa_1.run = close,
DummyMO_1.run = open;
#-----------------------------------------------------Go to growth temp
-----------------------------------5:00
ReactorTemp = NW_growth_temp,
"wait for 450°C", until ReactorTemp == NW_growth_temp;
2:00
"Wait for the temperature to stabilize";
#-------------------------------------------GaAs NW growth
-----------------------------------[growth_time_NWs]
TMGa_1.run = open,
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0:03
GaAs on Si
DummyMO_1.run = close;
"stop growth",
TMGa_1.run = close,
DummyMO_1.run = open,
DummyHyd_1.run = open,
AsH3_1.run = close,
AsH3_1.line = close,
TMGa_1.line = close;
#----------------------------------------------------------------Go down
in temperature----------------------------------------------------10:00
ReactorTemp = 0, Heater = off;
"wait until T<300°C", until ReactorTemp << 300;
#----------------------------------------------------------------switch
from H2 to N2----------------------------------------------------0:05
H2.run = close, N2.run = open;
reactor_open;
}
#WININFO 4, 592, 71, 81
#COLINFO layer 7,3*0,3*70
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