Ž .Sensors and Actuators B 65 2000 154–156www.elsevier.nlrlocatersensorb

A study of silicon Schottky diode structures for NO gas detectionx

Wenyi Zhang a,), Elder A. de Vasconcelos a, H. Uchida a, T. Katsube a,T. Nakatsubo b, Y. Nishioka c

a Faculty of Engineering, Saitama UniÕersity, 255 Shimo-Okubo, Urawa 338, Japanb Texas Instruments Japan, Oyama Plant, 305 Kouyamacyou 410-13, Japan

c Texas Instruments Japan, Tsukuba R&D Center, 17 Miyukigaoka, Tsukuba 305, Japan

Received 30 July 1998; received in revised form 15 February 1999; accepted 11 May 1999

Abstract

A silicon Schottky diode structure was applied for detecting nitride oxide gases at room temperature. The Pt–PdrSirAl structure wasemployed successfully to detect NO gas concentration for as low as 6 ppm at room temperature. This sensor also showed useful2

response to NO gas, but the sensitivity was lower than its sensitivity to NO gas. Fabrication of the diode on a porous silicon surface2

enhances NO gas sensitivity, but the response time becomes longer. This structure provides a convenient technique to manufacture2

miniaturized and integrated sensors. q 2000 Elsevier Science S.A. All rights reserved.

Keywords: NO gas sensor; Silicon Schottky diode; Porous siliconx

1. Introduction

Recently, there has been increasing interest to detectnitrogen oxide gases since they are typical air pollutants.Many kinds of NO gas sensors have been investigated,x

w xfor example, metal oxide semiconductors 1 , organicw xsemiconductors or solid electrolytes 2 . Usually, these

sensors measure the electronic conductivity change in bulkor film structure. Semiconductor junction structures suchas p–n junctions and MIS structures have also been inves-

w xtigated for NO gas detection 3 . In this work, we investi-x

gated alternative silicon junction structures for NO gasx

detection. The sensor consists of a Pt–PdrSirAl structure.The advantage of the silicon junction structure is to pro-vide a convenient technique to make smaller and moreintelligent sensors using microelectronic technology.

2. Experimental

Two types of silicon Schottky diode sensors were fabri-Ž . Ž .cated: Pt–Pdr polished Si rAl and Pt–Pdr porous Si rAl

² : Žstructures. The substrate was 100 n-type silicon 5 V

) Corresponding author.

.cm . The Pt–Pd electrode was prepared by vacuum evapo-Ž . y6ration of an alloy Pt:Pds8:2 at a pressure of 10 Torr

to a thickness of 10–30 nm. Porous silicon layer wasprepared in a 46% HF solution by applying a positive biasvoltage on silicon substrate and a continuous current of 60mArcm2 for 5 min to make the silicon surface porous to adepth of 5–10 mm. The gas response of the sensors wasmeasured under repeated gas flow between diluted NO2

Ž .gas and standard air O : 20%, N : 80% , or diluted NO2 2

gas and standard air. The gas introduction and gas changeŽwas controlled by a gas flow controller Aera Japan,

.SG-7S1 and a multi-port valve. The gas response currentwas estimated by vertical direction current between Pt–Pdelectrode and Al ohmic contact.

3. Results and discussion

Fig. 1 shows the current–voltage characteristics of sili-con Schottky diode sensors with polished silicon andporous silicon surface. The diode fabricated on polishedsilicon showed characteristics close to the theoretical one;whereas, larger current was observed in the diode fabri-cated on porous silicon surface.

Fig. 2 shows the time response of polished siliconsensor to various NO gas concentrations, which was2

0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S0925-4005 99 00466-9

( )W. Zhang et al.rSensors and Actuators B 65 2000 154–156 155

Fig. 1. Current–voltage characteristics of Pt–PdrSirAl and Pt–Pdrpor-ous SirAl Schottky diode structure.

Fig. 2. NO gas response of Pt–PdrSirAl Schottky diode structure2

sensor.

measured at reverse bias voltage 10 V. For the repeatedmeasurements, the response returned to almost initial valueand the response time was within several minutes. Fig. 3

Fig. 3. NO gas response of Pt–PdrSirAl Schottky diode structuresensor.

Fig. 4. NO gas responses of Pt–Pdrporous SirAl Schottky diode2

structure sensor.

shows the time response to various NO gas concentrations.A distinct feature of the NO gas response is that, duringNO gas flow, the current decreases, whereas, it increasesto NO gas, and the sensitivity to NO gas is lower2

compared to NO gas. These results suggest that NO gas2

was adsorbed as an oxidizing gas, whereas NO gas was2

adsorbed as a reducing gas on n-type polished siliconSchottky diode structure sensor. An oxidizing gas is ad-sorbed as a negatively charged molecule so that it effec-tively increases negative bias voltage, leading to currentincrease. On the contrary, a reducing gas effectively de-creases the negative bias voltage, leading to the currentdecrease.

Fig. 4 shows the time response of porous silicon sensorfor various NO gas concentrations. Higher sensitivity for2

NO gas was obtained compared to the polished silicon2

sensor, although the recovery time became a little longer.Larger surface area of porous silicon may lead to thehigher sensitivity for NO gas. However, we could not2

Fig. 5. Comparison of normalized response current change of siliconSchottky diode structure sensors.

( )W. Zhang et al.rSensors and Actuators B 65 2000 154–156156

observe the sensitivity improvement to NO gas by theporous silicon surface structure.

Fig. 5 compares the sensitivity of polished silicon sen-sor and porous silicon layer sensor for various NO gasx

concentrations. For the NO gas detection, the sensitivity2

seems to reach the useful value for the practical use sincethe ACGIG value is 5 ppm. However, the sensitivity toNO gas should be improved. We are now investigating aheterojunction structure consisting of Pt–PdrWO rSirAl,3

in which the WO film works as a catalytic film and3w xsensitivity enhancement is expected 4 . Further details of

w xthese devices can be found in Ref. 5 .

4. Conclusion

We reported the application of silicon Schottky struc-Ž .ture for NO gas detection. The Pt–Pdr polished Si rAlx

structure was employed successfully to detect NO gas2

concentrations from 6 to 22 ppm and NO gas concentra-tions from 50 to 250 ppm, with a response roughly propor-

tional to the logarithm of gas concentration. It was nextshown that sensitivity enhancement for NO gas was2

attained by the use of porous silicon surface, although theresponse time became a little longer. It was not necessaryto heat these devices during the operation. Therefore, thesestructures may be considered for future development ofNO gas sensors at room temperature with miniaturization,x

integration and power saving features.

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