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Chapter 1

Jignesh N. Panchal / Ph.D. (Electronics) Thesis/ Sardar Patel University /2014 01

CHAPTER – 1

Introduction

Chapter 1


Sr. No.

CONTENTS Page No.

1.1

1.2

1.3

1.4

1.5

1.6

1.7

Preamble

Sensor: Definition and Classification

Introduction to Volatile Organic Compounds (VOCs)

Semiconductor Gas/Vapour Sensors 1.4.1 Thin Film Sensors: Advantages and Drawbacks 1.4.2 Sensing Mechanism 1.4.3 Choice of Semiconducting material 1.4.4 Performance Parameters

(a) Improvement of the Performance (b) Solution to the Problem of Selectivity and Cross

Sensitivity: Array of Sensors and Electronic Nose

Importance of Indium Tin Oxide (ITO) Material

Reports on ITO based gas/vapour sensors

Aim of the Present Work

References

03

04

06

07

024

024

025

027

Chapter 1


1.1 Preamble

The understanding, imitation and extension of human sensing abilities have

been one of the prime pre-occupations of scientists and technocrats in the

field of electronics and instrumentation. This leads to a better, efficient

dealing of life with an enhanced standard of living; and, surely, towards a

more accurate understanding of the properties of Nature; - both of which are

the objectives laid for the applied scientific research.

Development of accurate, precise, sensitive, selective, stable sensor devices

with fast response and capability of interfacing with instrumentation system,

and which is reliable and viable, having low production cost for all different

variables in Nature is indeed a challenge for the scientific community.

After the successful breakthrough by audio and video sensing systems, and

thereby, the development of the modern communication system imitating

and extending our ‘ears’ ad ‘eyes’, the next task the scientists are looking

for is the development of electronic device to replicate the human ‘nose’,

or the olfactory system. Days are not far off when once such ‘e-nose’

devices are developed successfully, the complete ‘odour communication

system’, with both the transmitter and the receiver sections developed,

would facilitate the “smell” to be detected, transmitted, reproduced and

communicated. The odour sensor, in the form of “a Sensor detecting

Chapter 1


Volatile Organic Compounds”, would also facilitate in detecting (and

thereby controlling) the indoor-outdoor pollution, presence of explosives,

deterioration of food, and help medical diagnostics with non-invasive

detection. Semiconductor Vapour Sensor is a Chemical Sensor, which

fulfils the requirement of providing a reliable electronic nose system

when fabricated and connected as an Array driving a suitable

discriminative and/or quantifying pattern recognition system, using

statistical or artificial intelligence approaches.

1.2 Sensor: Definition and Classification

Sensor is an input interfacing device which detects qualitatively and/or

quantitatively the presence of a particular parameter under test. It is an input

transducer to any instrumentation system, which may or may not possess the

transduction function but basically it senses or detects or perceives the

change in the input variables, which can be physical, chemical or biological in

nature. A sensor along with a transducer is connected to the input processing

unit of any instrumentation or communication system or control unit.

Table 1.1 shows the classifications of sensors based upon different criteria.

Research and Development in Chemical sensors, which detect qualitatively

and quantitatively the chemical variables have been in great demand today,

Chapter 1


Table 1.1 Classifications of Sensors

Chapter 1


as, compared to the sensors detecting physical properties of matter, chemical

sensors, are less studied and hence lagging behind as far as the performance

criteria are concerned.

1.3 Introduction to Volatile Organic Compounds (VOCs)

Volatile organic compounds (VOCs) mean any compounds of carbon,

excluding carbon monoxide, carbon dioxide, carbonic acid, metallic

carbides or carbonates, and ammonium carbonate, which participate in

atmospheric photochemical reactions [1].

Volatile organic compounds (VOCs) are emitted as gases from certain solids

or liquids. VOCs include a variety of chemicals, some of which may have

short- and long-term adverse health effects. Concentrations of many VOCs

are consistently higher indoors (up to ten times higher) than outdoors

[2]. Formaldehyde, one of the best known VOCs, is one of the indoor air

pollutants that need to be measured due to its harmful effects.

Semiconductor gas sensors can be used for the detection of volatile organic

compounds (VOCs) as, vapours in the gaseous form respond to the oxide

semiconductors. VOCs are highly reactive and hence their oxidation

reduction process is very fast with oxide based semiconductor materials.

Gas sensing for detection and identification of odorants, vapours, or

Chapter 1


pollutant is of significant importance for many industries and

organizations, which include food industries, military and humanitarian

organizations, land mines, petro‐chemicals and manufacturing companies,

airport security and custom inspections agencies [3].

Challenges for VOCs sensors

VOCs have very low boiling point so their evaporation rate process is fast.

VOCs are an often mixed with interfering gases or vapours so it is difficult

to have their selective detection. As the production continues to increase

and as at times they emit such low levels of gas that it has proved difficult

to implement technically and economically viable VOC sensors. Lin et al [4]

reported long term study on industrial VOCs emissions of six typical

industries including emission characteristics, environmental impact and

health risk assessment, and control challenge analysis with the purpose to

explore industry VOCs emissions and offer some original baselines for

national control and management of industry VOC emissions as shown in

Table 1.2.

1.4 Semiconductor Gas/Vapour sensors

Semiconductor gas/vapour sensors are devices which allow determination

of the ambient gas atmosphere by exploiting the change in their surface

Chapter 1


Table 1.2. Summary of VOCs emission from typical industries [4]

Chapter 1


conductance in presence of oxidizing or reducing gases. They are fabricated

from the Oxide Semiconductor materials such as Tin Oxide, Indium Oxide,

Zinc Oxide etc.

In their simplest form, they are conductometric devices. Due to their

chemical composition and properties, semiconducting oxide gas sensors

are well-suited for a wide range of applications and for the detection of

many reactive gases and vapours. Over the past few decades, researchers

and engineers have dedicated their efforts to develop both materials and

devices with the characteristics of high sensitivity, good selectivity, and

reliability to detect and analyze the gaseous ambience.Depending upon the

material used and the gases/vapours that need to be detected, typical

operating temperatures of the semiconductor device ranges between

room temperature to higher Temperature [5]. Usually, these devices

follow empirical approach, and different materials have been tried to be

used for gas/vapour sensor application. Current research and development

efforts are focused on the synthesis of materials that have high relative

change in resistance and/or selectivity to vapours of interest,

improvements in sensor geometries and fabrication (especially in the area

of microstructure), and the applications of sensor analysis techniques for

solving complex, real time problems [6]. The oxide gas semiconductor can

Chapter 1


be distinguished by many parameters, the most common is given by, (i) the

classification by fabrication technology into ceramic, thick film and thin film

and (ii) by the device structure which makes disc or structured devices.

1.4.1 Thin Film Gas Sensors: Advantages and Drawbacks

Thin film semiconductor gas sensors are direct gas sensing device

which can be used for on-line and continuous monitoring of

gases/vapours.

Their advantages include: simplicity in design, low cost, low power

consumption, high selectivity, ease of miniaturization, large surface-

to-volume ratio, high sensitivity, fast response time, higher reliability,

safe evaporation, not requiring periodic calibration, reduced sensor

poisoning, improved Signal to Noise ratio, simultaneous detection of

multiple VOCs, broad dynamic range, ease of interface with

electronic system, possibility of developing array of sensors and

electronic nose.

The disadvantages of using thin film semiconductor sensors include

requiring special techniques, instruments for fabrication and trained

manpower to operate complex fabrication system, and the thin films

are not robust and need to be handled with care.

Chapter 1


1.4.2 Sensing Mechanism

The oxygen adsorbed in molecular form on the surface of the oxide

semiconductor can be ionised and get transformed into O2-, O-, O2-

depending upon the physic-sorption or chemi-sorption processes it

undergoes, and, the operating temperature of the device as shown

in Eqs. 1 to 4.[7-9]

( ) ( )2 2gas adsO O (1)

( ) ( )2 2ads adsO e O T < 100 °C (2)

( ) ( )2 2ads adsO e O

T =100- 300°C (3)

( ) ( )2

ads adsO e O T > 300 °C (4)

As shown in Fig.1.4.2 (i) the oxide ions form a surface depletion

region on the surface of the film, which creates the potential barrier.

When the sensor is exposed to a pollutant gas or vapours, the type

of oxygen formed at a certain temperature reacts with the gas, and

determines the maximum relative change in resistance by

undergoing the redox process on the surface [7-9].

Chapter 1


Fig 1.4.2(i) Potential barrier at grain boundary

The important processes responsible for the working of

semiconductor gas/vapour sensors are

a sensitive layer that is in chemical contact with the oxide

layers,

a change in the chemistry of the sensitive layer upon

chemisorption of test gas/vapour sensor,

oxidation-reduction process.

Principally, a semiconductor sensor consists of three different parts:

a receptor, a transducer, and a conditioning module defining the

operation model [10]. Figure 1.4.2(ii) shows that the receptor part

concerns with the ability of the sensor surface to interact with the

target gas/vapour to detect the presence, and to perceive and to

transform this chemical information into a form of energy. The

O2

Barrier potential

Depletion region

Chapter 1


transducer part concerns with the ability of the sensor to transform

the energy into useful electrical signal [11].

Fig.1.4.2 (ii) Schematic diagram of receptor and transducer functions of the semiconducting gas sensor [3]. L: thickness

of space charge layer, D: particle size

The receptor (the semiconductor surface) and transducer up on

contact with the reducing or oxidizing gases or vapours, alter the

surface conductivity of the material. The gas-induced changes at the

semiconductor surface are transduced by the transducer (through

the microstructure of the sensing material) into an electrical output

signal. In the case of a polycrystalline material, the grain size and

different grain intersections have a strong influence on the final

output signal [12].

Chapter 1


1.4.3 Choice of Semiconducting material

Semiconducting oxides are relatively wide band gap semiconductors

(Eg~3-4.5 eV). It is generally accepted that high conductivity of oxide

single crystal and polycrystalline thin films arises from the

stoichiometric deviation due to anion vacancies and/or interstitial

cations [11].

Brattain and Bardeen first reported gas sensitive effects with

Germanium since 1952 [13]. Later, in 1962, Seiyama et al [14]

demonstrated that thin films of ZnO, heated to 300°C in air, change

their conductivity in the presence of reactive gases in 1962. In the

same year, Taguchi [15] demonstrated similar properties for SnO2

with the greater stability.

A number of behavioural tendencies have been established for the

metal oxide sensors:

(i) Relative change in resistance to gases or vapours varies for

different concentrations and temperatures. Response times

are highly dependent on temperature (they are shorter at high

temperatures), and many times responses to gases or vapours

are nonlinear as a function of concentration [16].

Chapter 1


The temperature has an important effect on the relative

change in resistance of semiconductor gas/vapour sensors, as

it influences the physical properties of semiconductors

(change of the free carrier concentration, Debye length), and

also because every reaction taking place at the semiconductor

surface are temperature dependent [16]. Yamazoe et al [17]

have demonstrated that monitoring the operating

temperature, at which a semiconductor oxide sensor shows

maximum sensitivity, can be used to enhance the selectivity. It

is known that relative change in resistance of sensors,

operated in temperature range lower than 450°C, could be

controlled by either chemisorptions processes, or “redox”

processes [18-24].

(ii) A common feature is that water vapour affects both the

conductance in air and the sensitivity to other gases. The

solution is to find materials that are less dependent on

humidity values [16].

At room temperature, the water can be adsorbed in two states

on the surface of a semiconductor: molecular water (H2O –

physi-sorption), and hydroxyl groups (OH- - chemi-sorption).

Chapter 1


Absorption of water vapour always produces a large increase

in the conductivity of the gas sensor [16].

(iii) Gases/vapours with electron-accepting or electron-donating

abilities can be oxidized or reduced on a relative change in

resistance, generating ions or changing the electronic status of

oxide layers. [16].Fig.1.4.3 (i) shows the energy band diagram

for tin oxide with negatively charged adsorbed oxygen, where,

ES: potential barrier; EF: Fermi level; ED: donor level; EC: lowest

level of conduction band; EV: highest level of the valence band;

ECD: depth of donor level; ECV: energy gap between EC and EV.

The bending of band leads to the decrease of conductivity.The

sensors operate through a shift in the equilibrium of the

surface oxygen reaction by target gas or vapours [25].

Fig.1.4.3 (i) Energy diagram for SnO2 with Negatively charged adsorbed oxygen [25]

Chapter 1


The electrical conduction is controlled by potential barriers

associated with grain boundaries. And, the electrical

conductivity of the semiconductor oxide is extremely

sensitive to the composition of the surface, which also

varies reversibly to the surface reaction involving chemisorbed

oxygen and the gas/vapours mixture component. Since the

performance of sensor with respect to sensitivity, selectivity,

and response time, are dependent on the surface reaction

between the semiconductor oxide and gas or vapour

molecules in the ambient, the particle size, microstructure,

and doping level will have a large influence on the response of

the semiconductor sensors [26].

It is known that the smaller is the activation energy of

chemisorption and the higher is the activation energy of

desorption, the more is the gas-sensing effect of adsorption

type sensors [27-29].

1.4.4 Performance parameters

The kinetics of the processes, which take place in the receptor part,

determines the sensors parameters. The most common parameters

are, sensitivity, selectivity, stability, response times, recover time,

Chapter 1


reliability and working temperature [16]. They are commonly given

as three “S” factors and three “R” factors, as shown in Fig.1.4.4 (i).

The sensor parameters like sensitivity, selectivity, response time and

optimal working temperature depend on several factors:

Efficiency of receptor part,

Efficiency of transducer part,

Properties of sensors design.

Fig. 1.4.4(i) important parameters thin film gas/vapour sensors

And efficiencies of both receptor and transducer parts in turn,

depend upon:

Electronic structure of sensing material

Chapter 1


Density of surface state

Amount and quality of absorption centres

Microstructure of sensing material

Catalytic activity

Kind of detected gas or vapour.

(a) Improvement of the performance parameter

Use of new metal oxide materials that are more sensitive to

target gases/vapours and have less effect with temperature.

A better control of the thin films microstructure using nano-

metric oxide, in order to increase the active area.

Use new catalysts/dopants/promoter.

New technologies for material deposition and a different

structure.

The sensitivity and the optimum operating temperature of sensors

are correlated to the sensitive material and the material deposition

techniques. The amount of catalysts/doping/promoter on the

sensors surface also influences sensitivity. In general, sensitivity is

enhanced either by doping, which modifies the carrier concentration

and mobility, or by micro-structural changes such as reducing the

Chapter 1


oxide particle size to the nano-metric scale [30]. For example, most

commonly used, SnO2, In2O3, ITO, ZnO, can be sensitised to different

gas/vapours by selecting an optimal operating temperature for the

target gases/vapours, by making micro-structural modifications or by

using dopants and catalysts. Nano-structured materials present new

opportunities for enhancing the properties and performance of

gas/vapour sensors because of their surface-to-volume ratio.

(b) Solution to the problem of selectivity and cross sensitivity

To overcome the selectivity problem of sensors the most widely used

strategy has been to construct multisensory system, dopants, and

surface chemical modification systems [31-35]. The success of

artificial olfaction depends not only on the development of new

sensor technologies, but also on the availability of powerful pattern

recognition software.

Attempts to measure odours with electronic instruments were first

made as early as in 1950s [36], but the modern field of artificial

olfaction began in 1982 with the work of Persaud and Dodd [37],

who used a small array of gas-sensitive metal oxide devices to

classify odours. Since then, there has been a steady increase in the

number of systems using chemical sensor array. These systems are

Chapter 1


composed of several types of gas/vapour sensors, i.e. they use an

array of semiconductor sensors with partially overlapping

sensitivities, capable to obtain different response to the tested

odours [37].

The most common approach for fabricating an array of sensors is to

develop sensors with different dopants as sensing material [38]. The

literature available in the area of gas/vapours sensors on the

development of selective gas/vapours sensors system puts particular

emphasis on sensor arrays that use SnO2, In2O3, ITO, and ZnO in

association with various dopants [39]. A few examples for these

approaches are as follows:

Low concentration of NO2, CO, Ethanol, Methanol, Acetone, Benzene,

and Toluene have been detected by Cane et al. [40], with a micro

machined gas sensor array consisting of three devices working at

different temperatures. Yan et al. [41], designed a sensor array with

four specifically designed SnO2 and y-Fe2O3 sensors and used

improved back-propagation algorithm to accurately distinguish

ethanol and gasoline.

Many attempts have been made to develop electronic nose system

for applications in the fields of food, drinks, cosmetics,

Chapter 1


environmental monitoring, etc. [42]. Particular attention has been

paid to the cost, size and portability of these systems. Considerable

efforts have been made to study sensor arrays for the detection of

gases in a large variety of technological fields such as environmental

monitoring, food and drink analysis, medical appliances, and

industrial control systems [43-45].

Electronic nose

Gardner and Bartlett defined an electronic nose (e-nose) as [45]: “An

electronic nose is an instrument which comprises an array of

electronic chemical sensors with partial specificity and an

appropriate pattern recognition system, capable of recognizing

simple or complex odors" [45].

Fig 1.4.4 (ii) Electronic Nose system

Chapter 1


Fig.1.4.4 (ii) depicts the Mammalian and the Electronic nose

systems. Electronic nose is a new concept of semiconductor thin film

sensor application, which tries to mimic the human olfactory system

by using an array of electronic chemical sensors with partial

specificity and appropriate pattern-recognition electronics [46],

using artificial neural networks. Electronic noses can be applied

successfully to: -environmental monitoring; - medicine applications,

as an electronic nose can examine odors from human body and thus

can serve to recognize diseases; - Food industry, which constitutes

the largest market for electronic noses, whose applications include

quality assessment in food production and inspection of food quality

by odor. Qualitative analysis (identification of a certain gas from a

mixture of gases) and quantitative analysis (determination of each

gas concentration) can be usually achieved employing linear pattern

recognition methods such as: Principal Component Analysis (PCA)

and Discriminate Factor Analysis (DFA). Syeda Erfana Zohora et al

[47] reported detection of volatile organic compounds (VOCs) using

non-selective sensor requires an array of multiplexed sensors

followed by pattern recognition approach.

Chapter 1


1.5 Importance of Indium Tin Oxide ( ITO ) material

Indium tin oxide (In2O3:SnO2) is one of the most widely used transparent

conducting oxide. Its high electrical conductivity and high optical

transparency, as well as the ease with which it can be deposited as a thin

film are the attracting features which have drawn the attention of

researchers. As with all transparent conducting films, a compromise must

be made between conductivity and transparency, since increasing the

thickness and increasing the concentration of charge carriers will increase

the material's conductivity, but decrease its transparency. It has the

unusual property of remaining soft and workable at very low temperatures

[48].

1.6 Reports on ITO based gas/vapour sensors

There are various reports on the use of ITO as gas/vapour sensors. Patel et

al [49-50] have reported ITO thin film gas sensor for CO2 and CCl4 gases.

Galkidas et al [51] have reported ITO gas sensor for chlorine gas.

Sberveglieri et al [52] have reported RF Magnetron Sputtering ITO thin

films for NO2 gas sensors. Dibbren et al [53] have reported that the growth

parameter of ITO thin film strongly influences their response to gases.

Patel and Lashkari [54] have studied the effect of annealing on the

Chapter 1


properties of ITO film. Reactively sputtered ITO thin films have also

reported as NO and NO2 gas sensors [55]. ITO has proved to be successful

as a gas sensor and demands further exploration. V.S. Vaishnav et al have

reported the fabrication of Indium Tin Oxide thin film Gas Sensors for

Detection of Ethanol [56], and VOCs [57-58].

1.7 Aim of the Present work

The response of ITO to other gases is yet to be explored for which a

detailed study needs to be carried out and hence the present study has

been taken up on hand. A detailed investigation of the characterization of

ITO thin film, fabrication of ITO thin film sensor with promoting layers and

its application for detection of hazardous indoor pollutants like

formaldehyde, carbon tetrachloride, toluene, benzene and hydrogen

peroxide have been taken up.

Additionally, a preliminary study on the enhancement of sensitivity by the

using a special type of deposition technique, known as Chemical Vapour

Transport has been experimented and films of SnO2 with nano clusters

were deposited. A study on Flexible substrates for sensor devices is also

aimed.

Chapter 1


An Intelligent electronic nose, which consisted of an array of three

fabricated, selective sensors for the quantitative discrimination of benzene,

toluene and formaldehyde.

Chapter 1


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