Sensitivity characteristics of Ag doped BaTi03-CuO ... - IEEE Xplore

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rudra[email protected]. Palash Kumar Basu. Department of Electrical Communication. Engineering, IISc, Bangalore, [email protected].
IEEE CONECCT2014 1569822203

S ensitivity characteristic s of Ag doped B aTi03 -CuO mixed oxide as Carbon-dioxide S ensor S . B . Rudraswamy Department of Electrical Communication Engineering, IISc, Bangalore. Department of E&CE, SJCE, Mysore [email protected]

Palash Kumar B asu Department of Electrical Communication Engineering, IISc, Bangalore, [email protected]

Ag doped BaTi03-CuO mixed oxide thin films are evaluated for their carbon-dioxide sensing characteristics. The metal oxide films of different thicknesses are deposited on oxidized p type Si substrate by RF Sputtering. Sensing characteristics for different CO2 concentration, (300 ppm - 1000 ppm) are obtained for different operating temperatures, (100· C 400· C). Optimum temperature for maximum sensitivity is found to be 250· C. The effect of annealing on sensing properties is also evaluated. The unannealed films give better sensitivity than that of annealed films. Response time and recovery time are also calculated.

Abstract-

Navakanta Bhat Department of Electrical Communication Engineering, IISc, Bangalore. Center for Nano Science and Engineering, IISc, Bangalore. [email protected] II.

EXPERIMENTAL

A. Fabrication The BaTiOrCuO mixed oxide based gas sensor thin films were prepared by mixing the BaTi03 and CuO powders (99.9% purity) in 1 : 1 molar ratio. BaTi03-CuO with silver additions were prepared by adding silver powder (99.8% pure) to the mixture of BaTi03 and CuO in different ratios to obtain Ag content with the concentration of 1 , 2 and 5 wt%. The mixed materials were pressed to form the pellets of 3 inch in diameter and 4 mm in thickness. The pellets were then sintered at 1 1 000 C for 5hrs in air ambient to form the target. Then the targets are used to deposit films of different thicknesses on oxidized p type Si substrate by RF Sputtering. The samples were annealed at 5000 C and 6000 C temperatures in Nitrogen ambient for 20 mins to obtain the stable film. Finally the contacts were made of Ti/Pt (20nm/100nm thick respectively) using the Inter Digitated Capacitor (IDC) structure pattern.

Keywords- Carbon dioxide, Gas sensor, BaTi03-CuO

INTRODUCTION I. Carbon dioxide detection is very important in the current situation in many key fields ranging from indoor air quality monitoring, green house control to pollution monitoring. Carbon dioxide has been traditionally considered the most critical air pollutant. Most of the available carbon dioxide sensors based on optical principles are of high cost and bulky. Integration of these optical sensors with electronic circuits requires special consideration [1].

B. Characterization techniques

Material characterization such as, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Scanning Electron Microscopy (SEM) were performed on the deposited films to analyze the structure, the chemical compositions and the morphology of Ag doped BaTiOrCuO thin film deposited on oxidized silicon.

So an efficient and low cost Carbon dioxide sensor becomes very essential. Since, semiconducting sensors are a competitive low cost choice as compared to optical and electrochemical-based sensors, several materials have been tested and have shown diverse results [2-15]. A mixed BaTiOrCuO oxide has been shown to yield good CO2 sensing property. This is attributed to existence of distributed heterojunction between n-type BaTi03 and p-type CuO [2]. The depletion width is a function of buiIt-in-potential, which in turn depends on work function difference between n and p type materials. The resistivity in turn depends on the potential barrier caused by L'i

/kT). Thus the resistance can be approximated as R= Ro e (q &q>/kT). On exposure to CO2, the work function of BaTi03 changes, thus giving a signature in resistance change.

Electrical characterization is performed on the films in order to measure changes in film resistance at different working temperatures (room temperature to 4000 C). Sensing characterization is carried out inside a gas chamber with controlled substrate heating system at three different temperatures 100°C, 200°C and 300° C. The atmosphere inside the gas chamber is controlled by a MFC (Mass Flow Controller) based gas mixing system. Measurements are performed by Keithley SMU (Model 237). C.

Sensitivity Characteristics The measurements were made to evaluate the sensor response within a range of 300 ppm to 1000 ppm of carbon dioxide gas. The test was carried out by introducing synthetic air for 10 mins following carbon-dioxide gas for 10 mins. Sensitivity of the sensor response is measured as shown in equation (1).

In this work, we utilize Ag doping of BaTiOrCuO mixed oxide and evaluate the impact on CO2 sensing. We characterize the material properties and correlate it to the sensing properties. The response time and recovery time have also been calculated in this work.

1

s

=

Rg - R a Ra

x 1 00

Where Rg and Ra are respectively. III.

(1)

corresponding to Ba, Ti, Cu, 0 and Ag as well as considerable amount of carbon, which is the surface contaminant.

the resistance in CO2 gas and air

BaTi03

® -

SI

. 0 _

.=

=

RESULTS AND DISCUSSIONS

* -

CuO Ag

'-'

A. Film resistance characterization Ag doped BaTi03-CuO thin films have been tested in order to measure change in film resistance at different working temperatures. Film resistance decreases as the temperature increases from room temperature to elevated temperature and hence ensures the semiconducting behavior. The variation in film resistance with temperature is as shown in Fig 1 . It should be noted that silver doping reduces the resistance of the films .

20

40

60

28

9 1 0 r-------,

Figure 2. XRD pattern of B aTi03-CuO-Ag film

BaTi03 BaTiO:}...CuO_Ag(O%) BaTiO:}...CuO_Ag(l%) BaTIO:}...CuO_Ag(2%) BaTiO:}...CuO_Ag(5%) 50

1 00

150

200

250

(C)

300

Temperature

350

400

The Ti 2p region is shown in Fig 4(a) with the Ti 2P3/2 peak located at a binding energy of 458.3 eV and this corresponds to BaTi03 peak [ 1 6] . Fig 4(b) shows the Ba 3ds/2 peak with binding energy of 779.8 eV. The peak shape indicates that it corresponds to a single state of Ba on the film surface [ 1 7] . The 0 I s line shown in Fig 4(c) has two peaks, one at 526.7 e V corresponding to BaTi03 contribution and other at 528. 6 eV corresponding to CuO contribution [ 1 8, 19, 20] . The Cu 2p curve shown in Fig 4(d) located at binding energy 933 . 8 eV corresponds to CuO peak [21 ] . The Ag 3d region is shown in Fig 4(e) with the Ag 3ds/2 located at a binding energy of 368.4 e V corresponds to Ag peak [22] .

450

Figure 1. Film resistance variation with operating Temperature



Survey

B. Material Characterization

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