JSSSJournal of Sensors and Sensor SystemsJSSSJ. Sens. Sens. Syst.2194-878XCopernicus GmbHGöttingen, Germany10.5194/jsss-4-1-2015Catalytic metal-gate field effect transistors based on SiC for indoor
air quality controlPuglisiD.donatella.puglisi@liu.seErikssonJ.BurC.SchuetzeA.Lloyd SpetzA.AnderssonM.Department of Physics, Chemistry and Biology, Applied Sensor Science,
Linköping University, 58183 Linköping, SwedenDepartment of Physics and Mechatronics Engineering, Lab for Measurement
Technology, Saarland University, 66123 Saarbruecken, GermanyD. Puglisi (donatella.puglisi@liu.se)6January2015411827August201423October201429November2014This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://jsss.copernicus.org/articles/4/1/2015/jsss-4-1-2015.htmlThe full text article is available as a PDF file from https://jsss.copernicus.org/articles/4/1/2015/jsss-4-1-2015.pdf
High-temperature iridium-gated field effect transistors based on silicon
carbide have been used for sensitive detection of specific volatile organic
compounds (VOCs) in concentrations of health concern, for indoor air quality
monitoring and control. Formaldehyde, naphthalene, and benzene were studied
as hazardous VOCs at parts per billion (ppb) down to sub-ppb levels. The
sensor performance and characteristics were investigated at a constant
temperature of 330 ∘C and at different levels of relative humidity up
to 60 %, showing good stability and repeatability of the sensor response,
and excellent detection limits in the sub-ppb range.
Introduction
Common living environments such as homes, schools, or workplaces, where the
exposure to indoor air pollutants is continuous or prolonged, have become
dangerous sites of health problems related to bad air quality. Symptoms
include headache, dizziness, respiratory problems like asthma, skin
irritation, hypersensitivity to odors and tastes, but also acute effects
related to personality change or cancer, depending upon toxicological
characteristics of the harmful substances, duration or frequency of exposure,
people's age, and other related factors (Ashmore and Dimitroulopoulou, 2009;
Salonen et al., 2009). Recently, the World Health Organization (WHO, 2010)
released guidelines for a range of hazardous chemical substances belonging to
the wide family of volatile organic compounds (VOCs) such as formaldehyde,
naphthalene, and benzene, which are often found in indoor environments in
concentrations of health concern.
For indoor air quality, all organic chemical compounds with the potential to
evaporate under normal indoor atmospheric conditions (i.e., in the range of
temperature and pressure usually found in buildings occupied by people) are
defined as VOCs. A VOC is also defined as an organic compound having an
initial boiling point less than or equal to 250 ∘C at a pressure of
1 atm. The higher the compound's volatility is (the lower the boiling point), the higher its tendency to be
emitted from a product or surface into the air is (EPA, 2012).
The Total Exposure Assessment Methodology (TEAM) study by the United States
Environmental Protection Agency (EPA)'s Office of Research and Development
found already in 1985 levels of a dozen common organic pollutants to be 2 to
5 times higher indoors than outdoors, regardless of whether the homes were
located in rural or industrial areas (Wallace, 1987). During and for several
hours after certain activities, such as paint stripping, levels can reach
1000 times background outdoor levels. A proper reduction of VOCs is required
for decreasing the atmospheric levels of air pollutants and reaching
effective health protection measures.
Heating, ventilating, and air-conditioning (HVAC) systems are usually used to
reduce exposure to VOCs, but they can result in considerable energy
consumption, emissions, and cost. It has been estimated that HVAC systems
account for 39 % of the energy used in commercial buildings in the United
States (Granham, 2009). In the last years, several initiatives and research
projects have been supported by the European Union for the development of
cost-effective sensors and sensor systems for monitoring and measurement of
the indoor environmental quality. To reach this goal, different sensor
technologies have been proposed, such as field effect gas-sensitive devices.
Field effect transistor devices based on silicon carbide (SiC-FETs) have been
extensively studied in the last 15 years as high-performance, low-cost gas
sensors for room- and high-temperature applications, such as emission
monitoring, combustion control and exhaust after-treatment (Lloyd
Spetz et al.,
2013a, b; Andersson et al., 2004, 2013), and, more recently, indoor air
quality applications (Puglisi et al., 2014; Bur et al., 2012). Due to the
chemical inertness and wide bandgap of SiC (3.26 eV for the 4H-SiC
polytype), gas sensors based on this semiconductor material have the
potential to work efficiently in harsh environmental conditions, like
corrosive atmospheres and high temperatures, with notable advantages in terms
of stability during long-term operation and the possibility of direct online
control. Such properties are suitable also for indoor air quality
applications, where the environment is at room temperature, but the sensors
must be operated at high temperature to allow sensitive and selective
detection of certain gas species.
In this work, we study a sensor technology based on gas-sensitive SiC-FETs.
Iridium (Ir) has been used as a sensing layer for the gate contact, whereby
gas molecules may dissociate and react on the catalytic gate surface. This
interaction charges the gate area and thereby changes the
drain-to-source voltage, VDS, as the current through the
transistor is kept constant. VDS is utilized to measure the
response to the target gas (Lloyd Spetz et al., 2013b). The study,
evaluation, and choice of the catalytic material and its support (the gate
dielectric) are important because the electrical performance of FET sensor
devices as well as the chemical reactions responsible for the gas response
depend on the type and nanostructure of the sensing layer processed onto the
device (Lloyd Spetz et al., 2004), in conjunction with the nature and quality
of the gate insulator (Schalwig et al., 2002; Eriksson et al., 2005).
The sensor response is highly temperature dependent. This is an advantage of
gas-sensitive SiC-FET sensors for this kind of application because it is
possible to obtain additional information about the presence of particular
gases using the gas sensors under temperature-cycled operating conditions
over a wide range of temperatures (Bur et al., 2012).
High-precision sensor performance tests have been carried out with specially
developed instrumentation under controlled lab conditions. Tests included
electrical characterization of the sensors, nanoscale structural and
electrical characterization of the surface morphology and surface potential
before and after gas exposure to VOCs to study potential gate degradation,
and gas tests including variations of the target gas concentration and the
humidity level.
We have already studied the temperature dependence on such Ir-gate SiC-FETs
demonstrating that the best operating temperature is around 330 ∘C
(Puglisi et al., 2014), and we have quantitatively investigated Pt-gate
SiC-FETs under dynamic operation, demonstrating that temperature cycling is a
powerful approach to increasing the selectivity of the gas sensors allowing
discrimination of the three studied VOCs (Bur et al., 2014).
Here we have performed a systematic study on the influence of water on the
Ir-gate SiC-FETs during highly sensitive VOC detection.
Physical and chemical properties
Formaldehyde is one of the best known VOCs, and a very common and hazardous
indoor air pollutant. It is extensively used in the production of resins for
use as adhesives and binders for wood products, paper or pulp. Formaldehyde
is also contained in many construction materials, tobacco smoke, foods and
cooking, paints, varnishes, floor finishes, and sanitary paper products. The
eyes are most sensitive to formaldehyde exposure. The lowest level at which
many people can smell formaldehyde is about 50 ppb. Acute toxicity to humans
has been widely demonstrated, ranging from irritation of eyes and mucous
membranes at concentrations of about 100 ppb to more severe respiratory
problems, nasal obstruction, pulmonary edema, choking, dyspnea, and chest
tightness at higher concentrations of a few parts per million (ppm) after
1 week of exposure, as reported by the Air Toxicology and Epidemiology Branch
of the Environmental Health Hazard Assessment Office of EPA (ATEB-EPA, 2008).
A case study by Rumchev et al. (2002) on 6 month to 3 year old children
demonstrated that children in homes with formaldehyde levels greater than
49 ppb had a 39 % higher risk of asthma than children exposed to less
than 8 ppb. The International Agency for Research on Cancer (IARC) of the
WHO concluded that formaldehyde is carcinogenic to humans. EPA considers
formaldehyde a probable human carcinogen. WHO recommends an exposure limit of
81 ppb for a short-term (30 min) exposure time.
Naphthalene is a combustion product when organic materials are burned.
Tobacco smoke, concrete and plasterboard, dyes, household fumigant, some air
fresheners, cooking, and moth repellents may all be sources of naphthalene
indoors (Agency for Toxic Substances and Disease Registry, 2010). Inhalation
of naphthalene vapor has been associated with headaches, nausea, vomiting,
confusion, and dizziness. Other health effects include damage or destruction
of red blood cells, fatigue, lack of appetite, restlessness, pale skin,
diarrhea, blood in the urine, jaundice, and haemolytic anaemia (in children).
The characteristic naphthalene's strong odor of coal tar is detectable by
humans at concentrations as low as 80 ppb. There is still inadequate
evidence to evaluate the carcinogenicity of naphthalene to humans, but there
is sufficient evidence in animals to conclude that naphthalene is
carcinogenic (Gervais et al., 2010). IARC and EPA have classified naphthalene
as a possible human carcinogen. The WHO recommends an exposure limit of
1.9 ppb (annual average).
Main characteristics of formaldehyde, naphthalene, and benzene.
PropertyFormaldehydeNaphthaleneBenzeneMolecular formulaCH2OC10H8C6H6Molecular weight30.03 g mol-1128.19 g mol-178.11 g mol-1Boiling point @ 1 atm-19.5 ∘C218 ∘C80.1 ∘CAppearance @ room temperatureColorless gasWhite solid crystal or powderColorless liquidOdor @ room temperaturePungent, irritatingStrong odor of coal tarAromatic (sweet), gasoline-likeOdor threshold*0.83 ppm0.084 ppm1.5 ppmConversion factor(in air, at 25 ∘C)*1 ppm = 1.23 mg m-31 ppm = 5.24 mg m-31 ppm = 3.19 mg m-3Main hazard*Probable human carcinogenPossible human carcinogenKnown human carcinogenHealth effectsEye and nasal irritation, asthma, damage to pulmonary function, reproductive problems in women, allergies, dermatitis, leukemia, cancerNausea, vomiting, dizziness, fatigue, confusion, lack of appetite, pale skin, diarrhea, blood in the urine, damage or destruction of red blood cellsEye, nose, and throat irritation, depression, bone marrow failure, leukemia, cancer (targets: liver, kidney, lung, heart, and brain)Indoor sourcesTobacco smoke, construction materials, pressed-wood products, carpeting, paints, varnishes, floor finishes, sanitary paper productsTobacco smoke, concrete and plasterboard, dyes, household fumigant, some air fresheners, cooking, moth repellentsTobacco smoke, dyes, detergents, glues, paints, furniture wax; from derivatives: plastics, resins, adhesives, nylon, lubricantsOutdoor sourcesAutomobile exhaust, wild fires, components for the transmission, electrical system, engine block, door panels, axles, and brake shoesCoal tar, rubbers, tanningagents in leather industry,dispersant for pesticides,pyrotechnic special effectsAutomobile service stations, gasoline additive, exhaust from motor vehicles, pesticides, explosives, rubbers, wood smoke, volcanic eruptionsRecommended exposure limit (WHO, 2010)81 ppb (30 min exposure)1.9 ppb (annual average)No safe level of exposure
* Data from the United States Environmental Protection
Agency (EPA).
Benzene exists mostly in the vapor phase, and it is reactive with
photochemically produced hydroxyl radicals with a calculated half-life of
13.4 days. In atmospheres polluted with NOx or SO2, its
half-life can be as short as 4–6 h (ATEB-EPA, 2008). Tobacco smoke, dyes,
detergents, glues, paints, and furniture wax may all be sources of benzene
indoors. Inhalation exposure to benzene may lead to eye, nose, and throat
irritation, central nervous system depression in humans, bone marrow failure,
leukaemia, and cancer. The IARC and EPA have classified benzene as a known
human carcinogen for all routes of exposure. According to WHO, there is no
safe level of exposure to benzene. However, the French Decree
no. 2011-1727 (2011) has established an exposure limit of 0.6 ppb by 2016
for public buildings.
The main characteristics of the three VOCs studied in this work are reported
in Table 1.
(a) Sensor chip mounted on a 16-pin TO8 header and glued on
a ceramic heater together with a Pt100 temperature sensor.
(b) Cross-sectional view of the SiC based field effect transistor
used in this work.
ExperimentalDevice fabrication
Metal insulator semiconductor field effect transistors (MISFETs) with
catalytic metal-gate contacts were fabricated on top of 4 inch n-type 4H-SiC
wafers by SenSiC AB, Sweden
. A p-type buffer layer of 1 µm
thickness and 1 × 1017 cm-3 doping concentration, an
n-type active layer of 400 nm thickness and
3 × 1016 cm-3 doping concentration and an n-type contact
layer of 300 nm thickness and about 1 × 1020 cm-3 doping
concentration were epitaxially grown on top of the 4H-SiC substrate. The
highly doped drain and source regions were subsequently created by etch-back.
The ohmic contacts (bonding pads) to source, drain, and the substrate were
formed by rapid thermal annealing of 50 nm nickel (Ni) at 950 ∘C in
an argon (Ar) atmosphere and sputter deposition of 10 nm titanium (Ti) plus
400 nm platinum (Pt) on top of the Ni layer. The Pt layer works as an oxygen
diffusion barrier as well as bonding pad material. A porous iridium (Ir) gate
contact was deposited by dc magnetron sputtering at an Ar pressure of
50 mTorr to a total thickness of 30 nm. The gate width is 300 µm
and the corresponding gate length is 10 µm, with a separation
between the gate and the source-to-drain contacts of 5 µm
(Andersson et al., 2013). A cross-sectional view of the SiC based MISFET used
in this work is shown in Fig. 1a. The sensor chip, which is
2 mm × 2 mm and contains four SiC-FET devices, is attached to a
heater substrate (Heraeus PT 6.8/1020) together with a Pt100 temperature
sensor, using high temperature, and non-conducting ceramic die (Fig. 1b). The
electrical contacts of the heater substrate and the Pt100 temperature sensor
were established by spot welding to two pairs of pins of the gold-plated
16-pin TO8 header. Electrical connections to the SiC-FET devices were made
using gold wire bonding.
Current-voltage characteristics of an Ir-gate SiC-FET at
300 ∘C with zero, 2 V, and 3 V applied gate bias, VGS, after
exposure to VOCs.
Electrical characterization
Before testing as highly sensitive gas sensors, several SiC-FETs were
characterized by means of current-voltage (I–V) measurements at 100,
200, and 300 ∘C. A source meter Keithley 2601 was used to operate the
devices sweeping the voltage over the drain-to-source contacts,
VDS, from 0 to 5 V at a rate of 0.1 V s-1, and measuring
the drain current, ID. Separate gate voltages, VGS,
up to 5 V were applied using a stabilized voltage source. I–V
measurements were carried out in synthetic air (81 %
N2/ 19 % O2, at a flow rate of 100 mL min-1) for
all temperatures and VGS values. The electrical characterization was
repeated after exposure to VOCs. Figure 2 shows the drain currents measured
at 300 ∘C on an Ir-gate SiC-FET biasing the gate contact at zero, 2,
and 3 V after exposure to VOCs.
The saturation currents, IDsat, measured on six Ir-gate SiC-FETs
at 300 ∘C and 2 V applied VGS before and after exposure
to VOCs are shown in Fig. 3. In all cases, a variation of the drain current
(the saturation current in Fig. 3 is the drain current at a drain-to-source
voltage of 5 V) has been measured, even if it is not possible to define a
net tendency of the behavior of the SiC-FETs due to exposure to VOCs. The
current's variation could be related to built-in defects present in the
device or to the gate oxide on the interface. In terms of operating time
under exposure to VOCs, SiC-FETs 1 and 2 were operated for 20 h, SiC-FET 3
for 21 h, SiC-FET 4 for 66 h, SiC-FET 5 for 131 h, and SiC-FET 6 for
218 h.
Gas tests
Iridium-gate SiC-FET sensor devices were operated at a constant temperature
between 300 and 330 ∘C, in dry air and under different levels of
relative humidity (RH) from 10 to 60 %. The response characteristics of
all Ir-gate FET sensors to various concentrations of formaldehyde,
naphthalene, and benzene at different temperatures and humidity levels were
obtained operating the devices at a constant drain current and gate bias,
adjusted in the ranges 15 to 25 µA and 2.0 to 2.8 V, respectively,
so as to keep the drain-to-source voltage close to the saturation voltage,
VDS,sat (at the onset of saturation), corresponding to an initial
VDS of around 0.8–0.9 V. The voltage drop between drain and
source, VDS, was the sensor signal. Different concentrations of
formaldehyde (CH2O), naphthalene (C10H8), and benzene
(C6H6) were used to study the sensor's performance and
characteristics. Sensor response, detection limit, sensitivity, response and
recovery times, and repeatability of the sensor response were studied in dry
air as well as in a humid atmosphere.
Saturation currents measured on six Ir-gate SiC-FETs at
300 ∘C and 2 V applied gate bias, VGS, before and after exposure
to VOCs.
The VOC gases were supplied by using an advanced gas mixing system consisting
of two permeation ovens for supplying ultra-low concentrations of benzene and
naphthalene, and a gas dilution section containing a gas bottle of
formaldehyde (Helwig et al., 2014). Synthetic air, humidified by a water
bubbler temperature stabilized at 20 ∘C, was used as a carrier gas in
the permeation ovens as well as in the gas dilution section. The main
advantage of using the same carrier gas in the whole system is to keep
constant the contamination levels contained in the carrier gas and to
establish a constant background not affecting the sensor response (Bur et
al., 2014).
The gas mixing system is controlled by a LabVIEW program to keep the total
flow over the sensor at a constant flow rate of 200 mL min-1. The
temperature of the ovens was adjusted to reach the lowest VOC concentrations,
and kept constant during measurements. Naphthalene was supplied from 50 ppb
down to 0.5 ppb, formaldehyde from 1 ppm down to 0.1 ppb, and benzene from
7 ppb down to 0.1 ppb. The gas specifications are summarized in Table 2.
Gas specifications.
Min.Max.VOCconcentrationconcentrationGas sourceFormaldehyde0.1 ppb1 ppmGas bottleBenzene0.1 ppb7 ppbPermeation oven at 30 ∘CNaphthalene0.5 ppb25 ppbPermeation oven at 60.7 ∘CNaphthalene2 ppb50 ppbPermeation oven at 70 ∘CResults
Figure 4 shows the sensor response to naphthalene at 300 ∘C in dry
air and under 20 %RH. The
sensor signals shown in the figure are smoothed using an adjacent-average
filter to reduce the electronic noise (100 pts smooth is equivalent to a
sampling time of 100 ms). During the same gas test, the sensor was exposed
twice to the same concentrations of C10H8, but for two different
durations, of 1 h, and 15 min. The results showed good repeatability within
an error ranging from 1.6 % at 50 ppb to 7 % at 2 ppb in dry air,
and from 8 to 11 % at the same concentrations under 20 %RH. The
dependence of sensor response on VOC concentration in dry air and at
20 %RH is given in Fig. 5. The corresponding sensitivity, defined as the
change in response magnitude for a certain change in gas concentration, is
7.5 mV ppb-1 in dry air, and 4.5 mV ppb-1 under 20 %RH at
2 ppb. The effect of humidity is significantly less evident at lower
concentrations, decreasing the sensor response by a factor of 5.1 at 50 ppb,
but only by a factor of 1.6 at 2 ppb. These results are in agreement with
those previously obtained (Puglisi et al., 2014). The relative response, S,
defined as
S=VDS(air)-VDS(VOC)VDS(air)×100,
where VDS(air) and VDS(VOC) are the sensor responses
in background gas (synthetic air) and under exposure to the test gas (VOC),
respectively, 9.7 % at 50 ppb, and 2.5 % at 2 ppb under 20 %RH.
Sensor response to different concentrations of naphthalene
(C10H8) from 50 to 2 ppb at 300 ∘C in dry air and
under 20 % relative humidity (RH). The sensor signals are filtered to
reduce the electronic noise. The value 100 pts smooth is equivalent to a
sampling time of 100 ms.
From these results, the detection limit is expected to be less than 2 ppb.
By varying the temperature of the permeation oven from 70 to 60.7 ∘C,
it was possible to supply lower concentrations down to 0.5 ppb. Measurements
were carried out at 330 ∘C from 20 to
60 %RH, revealing a
detection limit below 0.5 ppb at any humidity level.
Effect of relative humidity on the sensor response to naphthalene
(C10H8) from 50 to 2 ppb at 300 ∘C.
Figure 6 shows the sensor response to naphthalene at 330 ∘C, and
20 %RH. The sensor signal has very low electronic noise (the sensor
signal in the figure is not filtered) and shows a superb sensitivity to
naphthalene in the sub-ppb range. The sensor response is 32 mV at 0.5 ppb,
corresponding to a relative response of 3.4 %. At high humidity levels,
the relative response at 0.5 ppb is 2.7 % under 40 %RH (response
time about 5 min, recovery time about 10 min), and 1.5 % under
60 %RH (response time about 3 min, recovery time about 6 min). Such
good results were possible also due to a significant reduction of the
background electronic noise (0.6 mV standard deviation).
Figure 7 shows the sensor response to formaldehyde at 330 ∘C in dry
air. The relative response ranges from 13.8 % at 1 ppm to 1.2 % at
the lowest tested concentration of 0.2 ppb. The sensor is extremely
sensitive to CH2O, showing a superb detection limit below 0.2 ppb in
dry air, but the effect of RH seems to be critical below 10 ppb already
under 10 %RH, probably due to the hydrophilic nature of the molecule.
Under the effect of 10 %RH, the relative response ranges from 6.9 %
at 1 ppm to 0.9 % at 10 ppb. The dependence of sensor response on VOC
concentration in dry air and at 10 %RH is given in Fig. 8. The effect of
humidity is more evident at lower concentrations, decreasing the sensor
response by a factor of 2.5 at 1 ppm, and by a factor of 4.2 at 10 ppb.
This is opposite to the influence of humidity on the response to naphthalene.
Sensor response to different concentrations of naphthalene
(C10H8) from 10 to 0.5 ppb at 330 ∘C, and 20 %
relative humidity (RH). The sensor signal (not filtered in the figure) has
very low electronic noise and shows superb sensitivity to naphthalene in the
sub-ppb range.
Sensor response to different concentrations of formaldehyde
(CH2O) from 1 ppm to 0.2 ppb at 330 ∘C in dry air. The
sensor signal is filtered to reduce the electronic noise. The value 100 pts
smooth is equivalent to a sampling time of 100 ms.
Also in the case of formaldehyde, measurements were carried out at high
humidity levels in order to investigate, in particular, the detection limit
of the sensor device. The Ir-gate SiC-FET revealed a detection limit of
1 ppb under 20 %RH, 5 ppb under 40 %RH, and 10 ppb under
60 %RH. The relative response is 1.2 % at 5 ppb and 40 %RH
(response time about 18 min, recovery time about 16 min), and 0.6 % at
10 ppb and 60 %RH (response time about 1.5 min, recovery time about
4 min).
In the case of benzene, a significant improvement of the sensor sensitivity
was reached compared to previous results (Puglisi et al., 2014), reducing the
detection limit to the very low value of 0.2 ppb under 20 %RH.
Effect of relative humidity on the sensor response to formaldehyde
(CH2O) from 1 ppm to 0.2 ppb at 330 ∘C.
Sensor response to low concentrations of benzene (C6H6)
from 7 to 3 ppb at 330 ∘C in dry air and under 10 %
relative humidity (RH). The sensor signal is filtered to reduce the
electronic noise. The value 100 pts smooth is equivalent to a sampling time
of 100 ms.
During a first gas test carried out in dry air and under 10 %RH, the
relative response is 3.9 % at 7 ppb (sensitivity 3.1 mV ppb-1),
and 1.8 % at 3 ppb (sensitivity 3.3 mV ppb-1) in dry air. Under
the effect of 10 %RH, the relative response is 1.5 % at 7 ppb
(sensitivity 1 mV ppb-1), and 1.1 % at 3 ppb (sensitivity
1.7 mV ppb-1). The effect of humidity decreases the sensor response by
a factor of 3.1 at 7 ppb, and by a factor of 2 at 3 ppb (Fig. 9).
The test was repeated under 20 %RH at lower concentrations down to
0.1 ppb (Fig. 10), showing a relative response of 1.6 % at 3 ppb
(sensitivity 5 mV ppb-1), and of 0.5 % at 0.2 ppb (sensitivity
25 mV ppb-1).
At high humidity levels, a detection limit of 1–3 ppb was measured up to
60 %RH with a relative response of 1 % at 3 ppb (sensitivity
2.3 mV ppb-1), and 0.7 % at 1 ppb (sensitivity 4 mV ppb-1)
within a response/recovery time of a few minutes.
Sensor response to very low concentrations of benzene
(C6H6) from 3 to 0.2 ppb at 330 ∘C and 20 %RH. The sensor signal is not filtered.
The detection limits of the studied VOCs as a function of relative humidity
are shown in Fig. 11. In the case of naphthalene, it is only possible to say
that the detection limit is below 0.5 ppb, since our gas mixing system does
not supply C10H8 concentrations below 0.5 ppb.
The Ir-gate SiC-FETs were studied also by means of nanoscale structural and
electrical characterization of the surface morphology and surface potential
of the gate before and after exposure to VOCs (Fig. 12). The analysis has not
revealed any significant degradation of the gate due to gas exposure. This
means that the sensing layer is not degraded upon long-term exposure to
elevated temperatures (330 ∘C) and repeated VOC
adsorption/desorption. This is worth pointing out, as the target application
(air quality control) will require long-term stability of the devices under
such conditions. Other metals, commonly used as gate material, such as Pt,
have been found to degrade (by delamination and restructuring by
agglomeration, forming particles) upon long-term operation at similar or
lower temperatures (Andersson et al., 2013).
Detection limit for formaldehyde (CH2O), benzene
(C6H6), and naphthalene (C10H8) as a function of
relative humidity. In the case of C10H8, it is only possible to
say that the detection limit is below 0.5 ppb, since our gas mixing system
does not supply C10H8 concentrations below 0.5 ppb.
Surface morphology and surface potential of the Ir gate before (top
figures) and after (bottom figures) 2 week exposure to VOCs at high
temperature. It is worth pointing out that the sensing layer is not degraded
upon long-term exposure to elevated temperatures and repeated VOC
adsorption/desorption, which is extremely important for our target
application (air quality control).
Discussion and conclusions
In this work, we tested gas-sensitive Ir-gate SiC-FETs at constant
temperature, in dry air and under different levels of relative humidity (RH)
from 10 to 60 %, demonstrating high performance of the sensor devices to
be used for highly sensitive detection of specific volatile organic compounds
(VOCs), in agreement with current legal requirements, especially in terms of
detection limits. Good stability and repeatability of sensor response during
2 week operation was confirmed. Excellent detection limits of 1 ppb for
formaldehyde, 0.2 ppb for benzene, and below 0.5 ppb for naphthalene were
measured under 20 %RH with a relative response of 0.4 % for
formaldehyde and benzene, and 3.4 % for naphthalene. At high humidity
levels, the sensors' performance and characteristics remained good, showing a
detection limit of 10 ppb for formaldehyde, about 1 ppb for benzene, and
below 0.5 ppb for naphthalene with a relative response of 0.6 % for
formaldehyde, 0.7 % for benzene, and 1.5 % for naphthalene at
60 %RH. These results are very encouraging for indoor air quality
control, being below the threshold limits recommended by WHO guidelines.
Further investigation will include the use of temperature and bias cycling
and smart data evaluation to study the selectivity of the sensors and achieve
a quantitative discrimination of mixtures of the three studied compounds. VOC
detection will be done in a more complex environment, changing the background
by using typical interfering gases, such as ethanol. Other catalytic metals
or metal oxides will be used as gate material and tested for comparison.
Moreover, interpretations of gas interaction on Ir / SiC will be
important for future studies on developing field effect based sensors for VOC
detection.
Acknowledgements
The authors would like to thank P. Möller for his technical support,
M. Bastuck for his contribution to the characterization of the sensors,
SenSiC AB, Sweden, for supplying the sensors, and 3S-Sensors, Signal
Processing, System GmbH, Germany, for supplying the hardware for sensor
operation and read-out. This project has received funding from the European
Union's Seventh Programme for research, technological development and
demonstration, under grant agreement no. 604311 (SENSIndoor). The authors
wish to dedicate this work to the memory of M. Cravino Vasta. Edited by: M. Penza Reviewed by: two anonymous
referees
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