Sensors and Actuators B 65 Ž2000. 122–124 www.elsevier.nlrlocatersensorb
Highly sensitive semiconductor NO x gas sensor operating at room temperature A. Kunimoto ) , N. Abe, H. Uchida, T. Katsube Faculty of Engineering, Saitama UniÕersity, 225 Shimo-Okubo, Urawa, Saitama 338, Japan Received 30 July 1998; received in revised form 22 February 1999; accepted 11 May 1999
Abstract The novel semiconductor NO x gas sensor based on a heterojunction structure workable at room temperature has been investigated. The sensor consists of PtrTin-dioxideŽSnO 2 .rn-Sirpq-SirAl in which vertical direction current between Pt and Al electrode was measured with applying reverse direction bias voltage on Pt electrode. All the films including SnO 2 with the thickness of 50–200 nm were deposited on an epitaxial layer of n-Si over pq-Si substrate by RF sputtering method. As a result, the current in the sensor decreased when the gas flow was switched from dry air to mixed gas of air and NO x . Clear response was obtained at the NO x gas concentration as low as 1 ppm at room temperature, while the almost no response was observed for the n-Si, p-Si, and p-Sirnq-Si substrate. The generation of large change in current for the sensor was considered that the barrier height change or conductivity change of SnO 2 gas sensitive layer may cause the modulation of the depletion layer at the nrpq-junction of Si substrate. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Semiconductor; NO x gas sensor; Room temperature
Large amount of research and investigations were tried to obtain the gas sensor operating at room temperature with miniaturizing sensor size. For this purpose, Si is the most suitable material for the sensor substrate, because of its easy machinability with micro processing and barrier formation with nrp interface. To obtain a Si gas sensor having high sensitivity to NO x in atmosphere, multi-layered Si gas sensor with Pt and WO 3 films as the gas-sensitive layer has been investigated w1x. Also, negative-resistance switching response to hydrogen was found with MISŽMetal-Insulator-Semiconductor. type Si gas sensor w2,3x. On the other hand, SnO 2 is the most popular for a gas sensitive material. We have reported the gas sensitivity is extremely depends on the morphology of SnO 2 thin film, which is well controlled by RF sputtering conditions w4x. This paper presents characteristics of new sensor with heterostructure for detecting NO x in concentration less than 1 ppm, consisting of PtrSnO 2rn-Sirpq-SirAl,with a porous SnO 2 as a gas-sensitive layer.
The sensor structure mainly used in this study is shown in Fig. 1. Boron-doped pq-Si substrate Ž0.5 mm thick. equipped with phosphorous-doped epitaxial n-Si layer Ž5–7 mm thick. was employed in this study, having square size of 5 = 5 mm. SnO 2 thin films of 50–200 nm in thickness were deposited onto the epitaxial n-Si layer of Si substrate by conventional RF sputtering with 3–30 mTorr of Ar and O 2 mixture, and RF power of 1.27 Wrcm2 . Consequently, Pt thin film as a electrode of 50 nm in thickness was deposited onto the SnO 2 layer by sputtering with 10 mTorr
Fig. 1. Cross-section view of the new NO x gas sensor with heterostructure.
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 4 4 0 - 2
A. Kunimoto et al.r Sensors and Actuators B 65 (2000) 122–124
Table 1 Resistivity of epitaxial layer and substrates Subtrate
Resistivity of epitaxial layer
Resistivity of substrate
n-Sirpq-Si p-Sirnq-Si n-Si p-Si
2.9–5.4 V cm 3.2 V cm
- 0.02 V cm - 0.01 V cm 2–5 V cm 6–12 V cm
of pure Ar, and the same RF power as SnO 2 deposition. Al thin film Ž150 nm thick. was also sputtered onto the other side of the pq-Si substrate to make a ohmic contact. Both of the electrodes were 2.6 mm in diameter, patterned by stainless metal mask during the sputtering process. The sensor samples were measured in a silicon rubber tube ŽB8 mm. under dry air or sample gas flow Ž500 ccrmin.. without any heating. The sample gases introduced were NO Ž1–50 ppm., NO 2 Ž1–50 ppm., CO Ž100 ppm., and H 2 Ž100 ppm.. DC power of 10 V was applied between Pt electrode and Al electrode in reverse bias direction as shown in Fig. 1. To clarify the role of the n-Sirpq-Si structure, and the interface between SnO 2 layer and n-Si layer, the n-Sirpq-Si substrate was replaced to n-Si, p-Si, and p-Sirnq-Si substrate. The resistivity of Si layers or substrates employed is shown in Table 1.
3. Results and discussion The influence of sensor structures including Si substrate, film thickness of SnO 2 gas-sensitive layer, and micro-structureŽsputtering gas pressure. of SnO 2 layer on gas sensitivity defined by R gasrR air is shown in Table 2, where R air is impedance of sensor device in air and R gas is that in sample gases. The most outstanding influence was the structure of Si substrate, indicating almost no gas sensitivity except n-Sirpq-Si substrate. As for n-Sirpq-Si structure, SnO 2 layer and Pt electrode, deposited onto the epitaxial n-Si layer, are necessary for gas detection and sufficient conditions. Moreover, high sensitivity obtained was limited to the reverse bias direction. The sensor
Fig. 2. Typical I – V characteristics of PtrSnO 2 rn-Sirpq-SirAl structure.
currents of the reverse bias in dry air were about 200–600 mA. In the sample gas such as NO or NO 2 , the sensor current decreased down to 20–50 mA, as shown in Fig. 2, with dependency on the applied voltage of 0 to 10 V. In the case of PtrSnO 2rn-Sirpq-SirAl structure, the gas sensitivity depends extremely on the thickness of SnO 2 layer and also the sputtering gas pressure, mostly affecting the porosity of SnO 2 thin films. It has been reported the sputtering gas pressure of 3 mTorr causes a quite fine and continuous film, while the pressure of 30 mTorr makes the film very porousw4x. On the contrary, as shown in Fig. 3, gas response characteristics, including response time, recovery time and also gas sensitivity, depend heavily on the sputtering gas pressures. As a result, the very porous SnO 2 layer produced high sensitivity, and rapid response and good recovery. The thickness of SnO 2 layer may cause the gas response change as well as the film porosity. The gas sensitivity to NO 2 increased as the thickness of SnO 2 layer decreased ŽTable.2.. The exact mechanism of large sensitivity in PtrSnO 2rn-Sirpq-SirAl is still not clear. However, the Pt layer works not only as a electron collector but also as
Table 2 Influence of sensor structures on gas sensitivity of NO, NO 2 at room temperature Sensor structure
Gas sensitivity Ž R gasrR air . in reverse bias
NO 2r30 ppm
50 nm Ž10 mTorr. 50 nm Ž10 mTorr. 50 nm Ž10 mTorr. 50 nm Ž10 mTorr. 50 nm Ž10 mTorr. Aluminum Ž50 nm. 50 nm Ž10 mTorr. 50 nm Ž10 mTorr. 50 nm Ž10 mTorr.
100 nm Ž3 mTorr. 50 nm Ž10 mTorr. 100 nm Ž10 mTorr. 200 nm Ž10 mTorr. 100 nm Ž30 mTorr. 100 nm Ž10 mTorr. 100 nm Ž10 mTorr. 100 nm Ž10 mTorr. 100 nm Ž10 mTorr.
n-Sirpq-Si n-Sirpq-Si n-Sirpq-Si n-Sirpq-Si n-Sirpq-Si n-Sirpq-Si p-Sirnq-Si n-Si p-Si
2.94 16.7 7.14 2.44 6.25 1.14 1.89 1.04 2.13
Was measured with different samples.
NO 2r1 ppm
A. Kunimoto et al.r Sensors and Actuators B 65 (2000) 122–124
duced under low NO x gas concentration, due to the amplification effects of junction structure combined with the gas reaction. Thus, it may be reasonable that the gas sensitivity may strongly depend on the gas adsorptive material over the n-Si epitaxial layer and also morphology of the adsorptive layer.
Fig. 3. Influence of micro-structure of SnO 2 film on response curves in NO 2 r30 ppm.
NO x gas adsorptive or catalyst, which may be originated from synergistic effect with SnO 2 layer. Focusing on the interface between SnO 2 layer and n-Si epitaxial layer, or on SnO 2 layer itself, change of potential barrier at the interface or conductivity change inside the SnO 2 layer may occur. If both of these effects works for expanding the depletion layer at the nrpq-junction of the Si substrate, the sensor impedance increases with decreasing the sensor current. Therefore, the gas sensitivity can easily be pro-
It has been found that the novel Si device with multilayered Ptr SnO 2r n-Sirpq-SirAl is highly sensitive to NO x of low concentration less than 1 ppm. This large sensitivity may be originated from the barrier height change or conductivity change of SnO 2 gas sensitive layer, which leads to modulation of the depletion layer at the nrpqjunction of Si substrate.
References w1x W. Zhang, H. Uchida, T. Katsube, T. Nakatsubo, Y. Nishioka; Sens. Actuators, B, in print. w2x K. Kawamura, Yamamoto, IEEE Electron Device Lett. EDL-4 Ž1983. 88–89. w3x M. Ogita, D.B. Ye, K. Kawamura, T. Yamamoto, Sens. Actuators 9 Ž1986. 157–164. w4x A. Kunimoto, H. Tamura, T. Katsube, Trans. IEEE Jpn., 118-E Ž2. Ž1998. 141–146.