APPLIED PHYSICS LETTERS 91, 122110 共2007兲
Ammonia sensitivity of amorphous carbon film/silicon heterojunctions
Xili Gao, Qingzhong Xue,a兲 Lanzhong Hao, Qingbin Zheng, and Qun Li
College of Physics Science and Technology, China University of Petroleum, Dongying, Shandong 257061,
People’s Republic of China
共Received 20 May 2007; accepted 6 September 2007; published online 21 September 2007兲
The amorphous carbon film/n-Si 共a-C / Si兲 junctions have been fabricated by magnetron sputtering.
The results show that these junctions have good rectifying properties and high ammonia 共NH3兲 gas
sensitivity. For a given reverse bias voltage, the resistance of the junction can increase by 100 times
rapidly when exposed to NH3 gas. This phenomenon may be attributed to the change of the space
charge width of the junction, which is caused by the adsorption of NH3 gas molecules. This study
shows that these a-C / Si junctions have potential application as NH3 gas detect sensors. © 2007
American Institute of Physics. 关DOI: 10.1063/1.2790371兴
Gas sensors are important in many areas, including industrial, medical, and living environments. In order to get
safe, highly sensitive, and inexpensive gas sensors, various
materials have been studied and developed. In the past, gas
sensors have been developed using semiconducting metal
oxides and conducting polymers.1–4 However, most of these
materials are required to operate at high temperatures for
enhancing their reactivity with respect to gaseous molecules.
This major fault leads traditional gas sensors to a limitation
for many fields. Therefore, new sensing materials are expected for gas detection at room temperature. Recently, many
researchers have focused on nanotube, nanoparticle, nanobelt, and nanowire materials, and the results show that these
materials have high sensitivity and good recovery speed for
their large surface area and hollow geometry.5–10 However, it
is very difficult to develop these materials as sensors due to
the complicated production process and microfabrication
techniques.
In this letter, we deposited amorphous carbon 共a-C兲
films on n-Si substrates by magnetron sputtering at room
temperature. It is found that when the reverse voltage is
larger than a threshold, the leak current begins to increase
abruptly with increasing reverse voltage and it can be repeated. The most interesting phenomenon is that NH3 gas
has a large effect on the current-voltage 共I-V兲 characteristics
of these a-C / Si junctions at room temperature. For the reverse bias voltages, the resistance of the a-C / Si junction
increases rapidly and significantly when exposed to NH3 gas.
Moreover, repeated experiments indicate that these junctions
have immediate response, high sensitivity, and good reproducibility.
The a-C films were deposited on n-Si 共100兲 substrates
using direct current magnetron sputtering from a graphite
target. The target is a cold-pressed graphite disk and the purity of the graphite is better than 99.9%. The silicon substrates are n-type materials with resistivity in the range of
2 – 5 ⍀ cm. Before deposition, the Si substrates were etched
in HF solution for 3 min, and then ultrasonically cleaned in
ethanol and acetone. The deposition took place inside a
chamber where the argon pressure was kept at 2 Pa and the
Si substrates were kept at room temperature. The thickness
of the a-C films is about 100 nm. Hall measurements of the
a-C films have been done, and the results indicate that these
films are just like insulators, the carrier concentrations of
which are very small so that it cannot be obtained by current
measurement. NH3 gas detection experiments were carried
out by a simple conical flask system. These I-V characteristics of the a-C / Si junction were measured by using twoprobe method with a Keithley 2400 sourcemeter.
Figure 1 shows the Raman spectrum of the a-C film
deposited at room temperature. It is well known that the
Raman spectrum of carbon materials can be fitted to the D
band at 1350 cm−1 and the G band at 1580 cm−1. The variation of the D and G peaks and the ratio of their intensities
provide information on sp2 / sp3 and the sp2 cluster size in the
films. The spectrum shows that the film is a disordered diamondlike carbon system, which contains much more
sp3-bonded carbon clusters. The scanning electron microscopy image of the surface morphology of the a-C film is
shown in Fig. 2, which indicates that the surface of the a-C
film is rough and the average grain size is about 40 nm.
Figure 3共a兲 shows the anomalous I-V characteristics of
the a-C / Si junction at various temperatures. It can be seen
that the a-C / Si junction has a good rectifying behavior during the temperature range of 220– 300 K. The inset shows
the schematic illustration of electrical measurement. In order
to understand the I-V properties more clearly, the relation
between log共I兲 and V of the junction is shown in Fig. 3共b兲.
We can find that the junction has a good rectifying behavior
in the voltage range besieged in the ellipse 关as drawn in Fig.
3共b兲兴. When the value of the reverse voltage is larger than a
a兲
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FIG. 1. Raman spectrum of the a-C film deposited on n-Si substrate at
300 K. Fitting of Raman D and G bands with two Gaussians is also shown.
0003-6951/2007/91共12兲/122110/3/$23.00
91, 122110-1
© 2007 American Institute of Physics
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122110-2
Gao et al.
FIG. 2. Scanning electron micrograph image of the a-C film deposited on
n-Si substrate at 300 K.
threshold, the leak current begins to increase abruptly with
increasing reverse voltage.
Figure 4 shows the effect of NH3 gas on the I-V characteristic of the a-C / Si junction at room temperature. When
the junction is transferred from air to a conical flask which
contains a few drops of ammonia, we find that the forward
current increases lightly; however, the reverse current decreases significantly when the reverse voltage is larger than a
threshold, which indicates that the breakdown voltage of the
a-C / Si junction shifts to a larger value.
The NH3 gas detection may be explained by the adsorption process of the a-C films. It has been reported that NH3
gas molecules can be considered as electron donators, which
can donate electrons to carbon nanomateials during adsorption progress.11,12 When the a-C / Si junction is exposed to a
conical flask which contains a few drops of ammonia, NH3
gas molecules will interact with the a-C film by replacing
preadsorbed gas molecules due to the different adsorption
energies of the molecules 共NH3, O2, N2, and CO2兲 interacting with the a-C film.11 In this work, the indium electrode,
a-C film, and n-Si substrate can be considered as a metalinsulator-semiconductor structure. When a forward bias voltage 共positive voltage兲 is applied to the indium electrode
above the a-C film, the conduction and valence band energies of the n-Si substrate are bent downward, which induces
lots of electrons gathered at the n-Si surface adjacent to the
a-C film.13 When the device is exposed to NH3 gas, many
Appl. Phys. Lett. 91, 122110 共2007兲
FIG. 4. 共a兲 The effect of NH3 gas on the I-V characteristic of the a-C / Si
junction at room temperature. 共b兲 Replots of 共a兲 with log共I兲 and V.
electrons are donated from NH3 gas molecules to the a-C
film, which increases the negative space charge region at the
a-C / n-Si interface. The result implies that the electrical field
between the indium electrode and n-Si substrate increases.
Therefore, the carriers can pass through the a-C film easier,
which induces the forward current to increase 共Fig. 4兲. When
a reverse bias voltage 共negative voltage兲 is applied to the
indium electrode above the a-C film, the conduction and
valence band energies of the Si substrate are bent upward
and its intrinsic Fermi level can move above the Fermi level
with increasing reverse bias voltage. When applying a sufficiently large negative voltage to the indium electrode above
the a-C film, the n-Si surface adjacent to the a-C film is
inverted from n type to p type. An inversion layer of holes
has been induced at the n-Si surface adjacent to the a-C
film.13 When the device is exposed to NH3 gas, many electrons are donated from NH3 gas molecules to the a-C film,
which decreases the positive space charge region at the
a-C / n-Si interface. The result implies that the electrical field
between the indium electrode and n-Si substrate decreases.
Therefore, the carriers pass through the a-C film with more
difficulty, which induces the reverse current to decrease, and
the breakdown voltage to shift to a higher value. When the
junction is exposed to air again, NH3 gas molecules are desorbed from the a-C film, and the electrons are released from
the a-C film, so that the I-V characteristics of the device
recover to the original.
As noted, upon exposure to ammonia, firstly, a slow resistance increase occurs and, during the followed recovery
共or response兲 periods, the resistance decreases 共or increases兲
quickly. The response and recovery of the junction may underlie complicated physical/chemistry processes, including
surface adsorption/desorption and gas diffusion inside the
junction.
The grown a-C film/Si junction was not given any treatment prior to employing as a sensor, defects must exist in the
interfaces in the a-C film, which could produce dangling
bonds on these sites. When the junction was firstly exposed
to ammonia, some ammonia molecules should chemically
bond with the defects in the a-C film, and then the other
ammonia molecules can diffuse into the interface of the
a-C / Si junction. In other words, the first response period
includes chemical adsorption process and physical diffusion
process, which need more time than the followed response or
FIG. 3. 共a兲 I-V characteristics of the a-C / Si junction measured at various
temperatures. 共b兲 Replots of 共a兲 with log共I兲 and V. The inset shows the
schematic illustration of the electrical measurement.
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122110-3
Appl. Phys. Lett. 91, 122110 共2007兲
Gao et al.
for NH3 gas detection. For a given reverse bias voltage, the
resistance of the junction can increase by 100 times rapidly
when exposed to NH3 gas. This phenomenon may be attributed to the change of the space charge width of the junction,
which is caused by the adsorption of NH3 gas molecules.
This study shows that these a-C / Si junctions have potential
application as NH3 gas detect sensors.
This work was supported by the Key Project of the Chinese Ministry of Education under Contract No. 106036, and
the Shandong Natural Science Foundation Contract Nos.
Y2005A10 and Q2006A09.
FIG. 5. The electrical resistance of the a-C / Si junction response to air and
NH3 gas at a given reverse bias voltage of −16 V.
recovery periods, because the followed periods are mainly
controlled by the physical diffusion process.
In order to further confirm the influence of NH3 gas on
the resistance of the a-C / Si junction, this experiment was
repeated several times. Figure 5 shows the electrical resistance of the a-C / Si junction response to air and NH3 gas. It
can be seen that when the junction is transferred form air to
a conical flask which contains a few drops of ammonia, the
initial resistance of the junction increases rapidly from
103 to 105 ⍀ at a given reverse bias voltage of −16 V. Moreover, the resistance of the junction recovers rapidly when the
junction is transferred from NH3 gas to air. The results show
that these junctions have immediate response, high sensitivity, and good reproducibility for NH3 gas detection.
In summary, an approach to detect NH3 gas at room
temperature was demonstrated by using simple a-C / Si junctions. The results show that the a-C / Si junctions have immediate response, high sensitivity, and good reproducibility
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