Low Energy Palladium Implantation in Silicon Carbide:
Solid State Gas Sensors
Claudiu I. Muntele, Daryush Ila, Robert L. Zimmerman
Center for Irradiation of Materials, Alabama A&M University, P. O. Box 1447, Normal, AL 35762-1447
Abstract. Silicon carbide is used in developing gas sensors for elevated temperature applications (400~ 800 °C), in
oxidizing environments. The option for this material was based on the cumulus of attractive properties that it offers, such
as stability up to 2700 °C, oxidation resistance up to 800 °C, and wide bandgap, which practically eliminates other
semiconductor materials. When hydrogen detection is desired, the most suitable combination appears to be palladium
either implanted into [1, 2] or deposited on the silicon carbide substrate [3, 4]. Here we present the methodology that we
developed for gas sensor fabrication using low energy palladium implantation followed by controlled surface sputtering.
The in-house built sensor testing facility is also described. Sensor parameters and performances, as well as behavior in
the presence of small concentrations of hydrogen into an inert gas (Ar) are also presented here.
improved, based on the zero diffusivity coefficient of
palladium in silicon carbide at temperatures up to 800
°C. Consequently, the region rich in palladium won’t
be destroyed, as in the case of classical sensors, where
palladium is deposited as a thin surface layer that
exfoliates at temperatures much lower than 800 ˚C,
damaging the contact to the silicon carbide material
causing the entire device to losing the efficiency in
detecting hydrogen. The study addressed problems like
final ion distribution and concentration vs. device
sensitivity, sensing mechanism, and lattice defect
evolution as a function of implantation parameters and
temperature.
INTRODUCTION
We investigated the approach of producing
hydrogen sensors using implantation of palladium ions
to create a p-n structure; several other research teams
have investigated the effects of ion implantation for
doping purposes in silicon carbide. The literature
published in the past few years contains several good
studies of damage evolution for light (hydrogen,
helium, oxygen) [5, 6, 7] to medium (germanium) [8]
ion implantation in silicon carbide. More recently, we
studied the effects of heavier ion implantation in this
material [9, 10].
A multi-year study of fabricating hydrogen sensors
using palladium implantation was performed at the
Center for Irradiation of Materials at Alabama A&M
University (CIM-AAMU). Palladium is a good
catalyst to be used for this type of sensor, to enable the
sensitivity of silicon carbide to hydrogen. Previous
studies were performed on palladium coatings on
semiconductors including silicon carbide, with a good
sensitivity to hydrogen at low temperatures, but not
withstanding high temperatures for times longer than a
few minutes. The leading hypothesis for our work was
that palladium ions implanted in the silicon carbide
host material would still have the catalytic effect on
the hydrogen, while the stability in time would be
EXPERIMENTAL WORK
Samples of 6H n-type silicon carbide were used as
a semiconductor substrate material for fabricating test
devices. For palladium implantation, we used the
Extrion low energy ion implanter and Ion-X tandem
ion accelerator facilities available within the Solid
State Division of Oak Ridge National Laboratory, as
well as the NEC Model 5SDH-2 tandem ion
accelerator at the Center for Irradiation of Materials of
Alabama A&M University (CIM-AAMU). Several
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electroplated with gold and the sample was secured on
the cooper holder with silver paste realizing a ohmic
contact (Figure 1). To provide a closed environment
with a limited volume for gas exchange and
temperature control over an extended range, the
sample holder and the electrical contacts were
suspended inside a vacuum-type cylindrical enclosure
terminated with conflat flanges equipped with
feedthroughs for gas and electrical connections. The
sample chamber was placed inside a PID controlled
furnace (Figure 2) allowing measurements at
temperatures between 20 and 500 °C. Several gas
species were available for flowing through the test
chamber: ultra-pure argon, forming gas (an argonhydrogen mixture containing 4% hydrogen), ultra-pure
nitrogen, and dry air. The response to hydrogen
(electrical current across the device) and the device
recovery in air (with no hydrogen flow) were
monitored as functions of time and temperature, for
bias voltages of either +1 or –1 V [12].
implantation fluences and ion energies were tried, in
order to achieve the best solution for the sensor
targeted. The most representative ones are grouped in
Table 1 under three distinctive sets. The implantation
conditions for the first set of samples were chosen
such that a large spectrum was covered with a
minimum of samples to be prepared. Once some
optimum conditions were identified, a second set of
samples were prepared and investigated. Finally, a
third set of samples was implanted at very specific
parameters, adding a post-implantation sputtering of
their damaged surface layers, for testing the
hypotheses formulated to explain the sensing
mechanism. To sputter the surface of this last set of
samples, we used a 175 mA argon ion beam of 700
eV. The experimentally found sputtering rate for these
conditions of the argon beam on silicon carbide was
54.3 Å/min. The thickness of the surface layer to be
sputtered from each sample of the third set was chosen
from SRIM [11] simulations for the final distributions
of palladium in the silicon carbide such that most
(95%) of the implanted palladium was preserved in the
substrate. Rutherford Backscattering Spectroscopy
(RBS) measurements on the samples before and after
the sputtering confirmed the SRIM predictions.
The methods of analysis used were current vs.
voltage and current vs. time (at constant voltage)
electrical measurements, RBS for in-depth distribution
of the implanted ions and layer thickness of the
deposited material, and confocal micro-Raman
spectroscopy (MR) for lattice damage assessment and
monitoring. There were two main types of electronic
devices fabricated and investigated: p-n junctions and
metal-semiconductor structures with rectifying
characteristics.
FIGURE 1. Schematic drawing of the I-V setup.
TABLE 1. Implantation Conditions for Selected
Samples.
Set 1
70
130
Energy (keV) 130
2e+15
3.2e+16
Fluence (cm-2) 3e+14
Set 2
160
160
Energy (keV) 80
1e+15
1.2e+16
Fluence (cm-2) 1e+15
Set 3
50
70
Energy (keV) 35
1e+15
1e+15
Fluence (cm-2) 1e+15
The electrical measurements were performed using
a Keithley low-current meter. The voltage was applied
on the sample holder (copper plate) and the current
was collected with a copper needle probe (ohmic
current drain) from the implanted side of the sample.
All copper parts in contact with the sample were
FIGURE 2. Picture of the I-V setup, with the mechanism
holding the test box outside the furnace. In the back are the
plastic tubes feeding various gases.
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The n-p-n structure formed here is similar to
considering two back-to-back diodes. Therefore, for
any positive or negative voltage applied, one diode
will always be in the forward bias mode (small
effective resistivity), and the other in reverse bias
mode (high effective resistivity). Whichever p-n
junction is reverse biased, it will set the current flow
through the entire device. The confirmation of this
working model was given by the electrical
measurements of the sputtered samples (the third set),
on which the top p-n junction was removed. Therefore,
the resulting device was giving response to hydrogen
just under positive applied voltage (Figure 4), and no
response under the applied negative voltage (Figure 5).
RESULTS; ELECTRICAL MODEL
From the electrical measurements on the first set of
samples [13] we established that the best results were
obtained for the samples implanted with 1×1015
ions/cm2. Lower values were not providing enough
palladium atoms for catalysis, while higher fluences
were damaging too much the crystalline lattice of
silicon carbide with negative effects on the electrical
carrier conduction. The second set of samples [12]
confirmed this observation, while also showing that a
device with a shallow palladium distribution (lower
implantation energy) gives a faster response to
hydrogen than one with a deeper distribution (higher
implantation energy). This phenomenon is directly
related with the path that the gas containing hydrogen
has to travel through the defective surface layer to
reach the palladium-rich region. Our proposed
explanation for the hydrogen sensing mechanism
displayed by the implanted samples is that after the
implantation of palladium into the n-type silicon
carbide, an n-p-n type of structure is obtained (Figure
3). Although no thermal annealing is involved after the
implantation, most of the palladium is activated
through a local annealing process (thermal spike)
taking place during the implantation. In a typical
sensor response, we could identify three stages: slow
response, rapid response, and saturation. The first
stage can be explained by the gas coming in contact
with the surface and starting to diffuse inside, reaching
first the region between the surface and the palladium
enriched layer, which has a low content of palladium,
resulting in a small amount of carriers injected in the
material. During the second stage, of moving deeper in
the material, the gas is dissociated and adsorbed by the
increased amount of palladium, process associated
with a large injection of carriers in the device. Finally,
when the palladium reached its absorbing capacity, the
sensor goes in the saturation mode and it needs
resetting in oxygen. During the sensor recovery (reset),
similar stages can be identified.
FIGURE 4. Response to hydrogen for U=+1V of the sample
implanted at 100 keV (Set 3) before and after sputtering.
FIGURE 3. Electrical model proposed for explaining the
sensing behavior of the silicon carbide device.
FIGURE 5: Response to hydrogen for U=−1V of the sample
implanted at 100 keV (Set 3) before and after sputtering.
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CONCLUSIONS
REFERENCES
The work presented here demonstrates the
possibility for fabricating hydrogen sensors using ion
implantation of palladium in silicon carbide. The
optimum implantation fluence was established to be in
the 1015 ions/cm2 region, as a good trade between
losing the sensing because of an excess of ion
implantation damages and losing it because of
insufficient palladium atoms in the device. For the
energy of ions, it was proven that the lower the energy
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as well. Above this temperature the oxidation of
silicon and carbon begins to take place, damaging the
device. Defect recovery by annealing, however, has a
very low yield even at temperatures above 1100 ˚C
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ACKNOWLEDGMENTS
This research was sponsored by the NASA Grant
No. NG3-2302, and partially by the Center for
Irradiation of Materials of Alabama A&M University
and U.S. Department of Energy under contract DEAC05-00OR22725 with the Oak Ridge National
Laboratory, managed by UT-Battelle, LLC.
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