Diamond Microelectronic Gas Sensor for Detection of Benzene and

Topic 3: Chemical and Gas Sensors
A solid-state hydrocarbon sensor for environmental applications
Yasar Gurbuz 1 , Weng P. Kang 2 , and Jimmy L. Davidson 2
1
Sabanci University, Faculty of Engineering and Natural Sciences, Orhanli, Tuzla, 34956, Istanbul, Turkey
Tel: +90 (216) 483 9533, Fax: +90 (216) 483 9550, email: yasar@sabanciuniv.edu
2 Vanderbilt University, Department of Engineering and Computer Engineering, Nashville, TN, 37235, USA
Tel: +1(615) 322-0952, Fax: +1(615) 343-6614, e-mail: wkang@vuse.vanderbilt.edu
Summary: We have applied CVD (chemical vapor deposition) diamond chemical sensor to detect benzene and toluene, volatile organic
compounds, possible indication of the existence of environmental hazardous and petroleum in soil/subsoil. Steady-state and transient
response of the sensor for both gases have been extensively studied. These studies have shown that the sensror shows a large sensitivity,
fast response, high selectivity, large dynamic range, repeatable/reproducible response for the detection of benzene and toluene gases. Gas
detection mechanisms of the sensor for both gases have been analyzed and models have been developed. Activation energy analysis of the
sensor for both gases resulted very small values, confirming fast response and high selectivity. Furthermore, the effect of structural
parameters of the sensor on gas sensing performance have been analyzed and optimized structures have been presented in this study.
Motivation: It has been demonstrated that the presence of benzene (C 6 H 6 ) and toluene (C 7 H 8 ) in the soil could be an indicator of oil
reservoirs as well as environmental contamination [1]. Researchers/engineers use miniature field portable gas chromatograph devices,
sensitive enough to detect benzene and toluene in the near surface layers of the earth, to predict the presence of oil reservoirs. But, with
these devices it takes minutes to detect tens of parts per billion of benzene and toluene in samples taken from soil. The long sample time
limits the area, surveyed per day. If the sample time can be shortened to a few seconds and the detection threshold can be reduced to a few
parts per billion, then much larger areas can be surveyed much more quickly for oil reservoir and environmental contamination.
We have applied CVD (chemical vapor deposition) diamond chemical sensor to detect benzene and toluene, volatile organic compounds,
possible indication of the existence of environmental hazardous and petroleum in soil/subsoil. These sensors were first invented, patented,
and demonstrated to detect very small quantities of hydrogen, oxygen, and carbon monoxide [2,3]. They have been produced using the
MIS (catalytic-Metal - Insulative-diamond - Semiconductor-diamond) architecture, as seen in Figure 1. The polycrystalline diamond retains
all of the superior material properties of single crystal diamond without the high cost. Some of those properties, also important to the
chemical sensor operations, are: excellent thermal conductivity allowing rapid change in temperature, very tight strong lattice that prevents
chemicals from dispersing into the diamond material, very low chemical reactivity, high temperature tolerance, and capability of being
fabricated as a semiconductor or insulator. We have already demonstrated the hydrogen sensitivity of this sensor at a temperature range of
23 - 300˚C with an extremely fast response, recovery and high selectivity performance [2,3]. Its physical operation as a hydrogen sensor
has been explained by Lundstrom, et. al. [4], and can briefly be summarized in a following sequence: a) hydrogen molecules outside the
device adsorbs on the palladium surface, b) hydrogen dissociates and diffuses through the palladium to the metal - insulator interface, c)
number of atomic hydrogen atoms adsorbs on the metal at the i-diamond interface creating a dipole layer, d) The dipole layer changes the
work function difference between the palladium and the i-diamond, e) the current-voltage of the MIS diode changes, and f) changes in the
current-voltage curve can be correlated to the hydrogen partial pressure outside the device.
Results: A detailed fabrication process of the sensors were already described in the past [2,3]. Using the steady-state and transient
characterization methods, detection characteristics and mechanisms, dynamic range, response and recovery times and activation energies of
the sensor for benzene and toluene hydrocarbon gases have been obtained, analyzed and determined over a temperature range of 25-200°C.
Figures 2 and 3 show current-voltage (I-V) characteristics of the sensor in different amounts of benzene (Fig. 2) and toluene (Fig. 3) at
200°C. A larger change in I-V characteristics upon exposure to larger gas amounts observed. Figure 4 and 5 show a family of steady-state
value of the current change (∆I), at a fixed biasing voltage, versus benzene and toluene partial pressure for several operating temperatures.
The curves show a rapid increase in ∆I with increasing gas amounts, followed by a saturation trend at higher amounts. The curves also
show that the sensitivity increases at higher temperature. Also, a real-time response of the sensor to subsequent increment of benzene and
toluene amounts is shown in Figs. 6 and 7, respectively. The device shows an immediate response to benzene and toluene amount
increment. Moreover, the sensor recovers as the gases pumped out, observed from the desorption curve. Figures 8 and 9 show the
activation energy plots of the sensor for benzene and toluene gases, respectively. The adsorption activation energy of the device was found
0.0017 eV in 6-torr benzene and 0.0022 eV in 6-torr toluene over the temperature range investigated. It is evidently true that toluene (C 7 H 8 )
is a more complex molecule than benzene (C 6 H 6 ). Hence, larger activation energy is required to activate toluene than benzene to some form
of activated complex or hydrocarbon radicals. Moreover, the low values of the adsorption activation energy confirm the observed high
sensitivity and fast response of the diamond-based MIS device for benzene and toluene gases.
Improvement of sensor sensitivity: The diamond-based hydrocarbon gas sensitive MIS (catalytic-Metal/ Intrinsic-diamond/ p + -diamond
Semiconductor) diode structure consists of three important layers: catalytic metal, i-diamond, and doped-diamond layers. I-diamond
thickness plays a crucial role in I-V characteristics of the sensor and, also found in this study, its gas sensing performance. Two different i-
diamond thicknesses (0.17µm and 0.25µm) were fabricated to evaluate this effect. Figure 10 shows a steady-state value of the current
change (∆I), at a fixed bias-voltage, versus benzene partial pressure, operating at 100°C, for i-diamond thickness of 0.17µm and 0.25µm. A
larger sensitivity and increased dynamic range of operation is observed in Fig. 10 for the sensor utilizing a thicker i-diamond. The effect of
catalytic metal and temperature on sensor performance have also been evaluated and will be presented in the final manuscript.
Detection Mechanism: Several physical theories have been considered to explain the operation of the MIS hydrocarbon sensor. These
include the following three concepts: a) dissociation of hydrocarbon molecules into individual atoms and subsequent detection of hydrogen
alone, b) dissociation of hydrocarbon molecules into smaller molecules that are able to diffuse through the palladium, adsorb onto the Pd
1

surface at the i-diamond interface causing a change in the work function and a subsequent change in the I-V relation, and c) movement of
the entire benzene or toluene molecule through the palladium to the i-diamond interface where it produces changes in the polarization of
the palladium surface. Each of these theories have been analyzed and results will be presented in the final manuscript.
References
[1] J. Getino, L. Ares, et. al., Environmental applications of gas sensor arrays:combustion atmospheres and contaminated soils, Sensors
and Actuators B 59 (1999) pp. 249-254
[2] W. P. Kang, Y. Gurbuz, J. L. Davidson, and D. V. Kerns, New Hydrogen Sensor Using a Polycrystalline Diamond-Based Schottky
Diodes, Journal of the Electrochem. Society, Vol. 141, No. 8, pp. 2231-2234, August 1994
[3] W. P. Kang et al. 1997, US Patent No: 5,656,827
[4] Lundstrom, K.I., M. S. Shivaraman and C. M. Svensson, “A Hydrogen-Sensitive Pd-Gate MOS Transistor,” J. Appl. Phys., Vol. 46, No.
9, (Sept. 1975) pp. 3876-3881
Figure 1: Diamond-based chemical sensor
Figure 2
Figure 3
Figure 4
Figure 5 Figure 6 Figure 7
Ln(dI/dt)
0.016
0.014
0.012
0.01
0.008
0.006
0.004
0.002
Ln(dI/dt) vs 1/T for Benzene
Ea=0.0017eV
Ln(dI/dt)
0.021
0.016
0.011
0.006
0.001
Ln(dI/dt) vs 1/T for Toluene
Ea=0.0022eV
Effect of i-diamond thickness on Benzene
sensitivity
0
0.0024 0.0026 0.0028 0.003 0.0032 0
0.0024 0.0026 0.0028 0.003 0.0032 -0.004
0 2 4 6 8 10 12
1/T
1/T
C 6H 6 Concentration(torr)
Figure 8 Figure 9 Figure 10
∆ I (mA)
4.5
4
3.5
3
2.5
2
1.5
1
0.5
i-diamond(0.25um)
i-diamond(0.17um)
2