Gas sensing behaviour of nano-grain tin oxide thin films

GAS SENSING BEHAVIOUR OF NANO-GRAIN TIN OXIDE THIN FILMS

by

OOMMAN K. VARGHESE Department of Physics

Submitted

n fulfillment of the requirements of the degree of Doctor of Phuosopny

to the

INDIAN INSTITUTE OF TECHNOLOGY, DELHI

September, 2000

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© Indian Institute of Technology New Delhi — 2001 All rights reserved

CERTIFICATE

We are satisfied that the thesis entitled

is worthy of consideration for the award of the degree of

record of the original bonafide research work carried out by him under our guidance and supervision and that the results contained in it have not been submitted in part or full to any other university or Institute for award of any degree/diploma.

Date:

cgfd-oVcsk.,

Dr. L. Malhotra Professor of Physics Thin Film Laboratory Department of Physics

Indian Institute of Technology New Delhi 110 016.

Dr. G.L. Sharma

Principal Scientific Officer Thin Film Laboratory Department of Physics,

Indian Institute of Technology

New Delhi — 110 016.

Dedicated to Daddy, Mummy, Helen, Milan

& Maggie

ACKNOWLEDGEMENTS

It is none other than God's abundance love and grace that helped me to complete this work. There is a long list of loving people who were instrumental in my achievement of this goal. Without the unconditional support and co-operation of them, to whom I have life long commitment, this achievement would have never been realised; the most important being the valuable suggestions, support and the facilities extended by my guide Prof. L.K. Maihotra. The stress he gives for perfection in work had a great impact on me. He has tried hard to mould me to perfection and particularly his motivation to adhere to the deadlines have helped me achieve this goal at least now. I was very fortunate to be guided by him in all ways. Dr. G.L. Sharma, who also guided me, has given valuable suggestions mainly in the experimental work. I am extremely thankful to him for the help he rendered especially in the fabrication of rf sputtering system.

No words can express my gratitude to Prof M.G. Krishnapillai, a selfless and student loving teacher, who took me as his research student and later gave necessary motivation for joining a place like IIT. Like many others, I was also inspired by the talks of the legendary personality Prof. K.L. Chopra. My sincere gratitude to Prof. Chopra. Prof. D.K. Pandya, Prof. S.C. Kasyap, Prof. V.D. Vankar, Prof. A.K.

Mukherjee, Dr. V. Dutta, Dr. G.B. Reddy, Dr. R.D. Tarey, Dr. B.R. Mehta and Dr.

M.C. Bhatnagar have shown special interest in my work and in one way or other gave crucial support in my work. The knowledge I have gained though the discussions with these faculty members is invaluable. Other faculty members in the department also extented their help which are thankfully remembered.

Dr. Rajan K. John, a great teacher, needs special mention because of the motivation he has given me to have a research oriented carrier, I am thankful to him for everything he has done for me. Dr Joby Joseph was there with me in difficult times for any help, which are always remembered.

ii

Life in la, chilled me to the present state. Nostalgia of TFL will remain forever.

Memories do come of saturday cleaning, discussions over a cup of tea, lively get together, etc. With people like Dr. Ashtlkar, Dr. Murthy, Dr. Kanwaljeet Singh, Dr.

Veer Singh, Dr. Sharat, Dr. Sujeet Choudhary, Dr. Senguttuvan, Dr. Paulson, Dr.

Jain, Dr. Barsania, Dr. Sonma Shah, Dr. Neeta and Dr. Rahul, TFL life was lively and discussions were knowledgeable. Among them Dr. T.D. Senguttuvan had taught me the sol - gel technique and Dr. M.K. ^Jain the fabrication of rf sputtering system. Dr.

Jain and I had many informative discussions and arguments which made me think better. The one and only one batchmate of mine Dr. Rajagolan always gave a company in sleepless nights and helped me in many ways. Labmates Gopal Mar, G.R. Pattanaik, Balamurugan, Vamsi, Babu Dayal, Somnath, Pushpender, Tarsairn Singh, Archana, Seema, Rishika, Prathista, Sheetal and Thirender were always very co-operative. The help rendered by Sheetal and Thirender while writing the thesis is thankfully acknowledged. Our lab technicians Khatriji, Mangalsinghji, and Ganeshji are acknowledged for their assistance. IDDC workshop persons were very helpful to me without them my fabrication work would not have progressed much.

Friends like Dr. Mohan & Ieenu , Sen & Nisha, Dr. Sakhti & Neena have always been a support for me. They gave me confidence when I was lacking it. My sincere thanks to them. Friends like DR. Anil Govindan & Reshmi, Dr. Joby & Ancy, Dr.

Potty & Usha, Dr. Sudheer & Shiny, Dr. S.K.P. Bhatt & Retna, Dr.P.D. Paulson &

Sangeetha, Dr.Reji Thomas & Cissy, Dr. Zachariah Alex & Liza and Dr. Unnikrishnan were very helpful and life in IIT would have been very boring without them.

If it were not my parents, sisters and my wife, nobody else would have the patience to wait without any grumbling, for these long years to complete my studies. Words are not enough to express my indebtedness to them.

62,

Oomman K. Varghese

iii

ABSTRACT

The recent interest in keeping the environment clean has brought gas sensor field into limelight. Gas sensing devices have application in almost all fields where gases or vapours are involved. Gas sensors based on metal oxide semiconductor materials like tin oxide, offer several advantages over its counterparts like simple and fast operation, low cost, capability to detect ppm concentrations of various gases and possibility of miniaturisation. The metal oxide semiconductors work on the principle that the interaction of these materials with gaseous medium can cause changes in their electrical resistance. Though, several hundreds of oxides have been studied for their gas sensing behaviour, tin oxide still remains the most favourite material for sensing reducing gases.

Tin oxide is a wide band gap (3.6eV) material that behaves as an insulator only if it is perfectly stoichiometric. In practice, tin oxide contains oxygen vacancies and hence is non-stnichiornetric, Though investigated extensively, gas sensors based on tin oxide still suffer from several disadvantages like insufficient sensitivity in several cases, lack of selectivity and long term stability. The development of tin oxide based gas sensors has reached a stage where a thorough understanding of the different mechanisms going on during interaction of the material with gas species is essential for its further improvement. Recent reports show that coupling nano-technology and thin film technology with gas sensor technology, can improve the sensing characteristics of tin oxide drastically. The blending of these technologies and the improvement of the sensing characteristics through a mechanistic understanding were the primary motives behind the present work.

Sol-gel non-alkoxide route and rf magnetron sputtering technique were used for depositing nano-grain tin oxide thin films. Dip coating technique was employed for depositing films using sol-gel. For this purpose, a vibration free dip coating system capable of pulling the substrate at a constant speed in the range 5-50 cm/minute was designed and fabricated. For depositing films using sputtering, a rf magnetron sputtering system capable of depositing the films in the sputter up mode

iv

was also designed and fabricated. The deposition conditions were optimised to get nano-grain films.

Glancing angle x-ray diffraction (GAXRD) patterns revealed the rutile phase of the films. As sputtered films were amorphous and became crystalline after annealing at 773K for 6 hours. The grain size of the films (after heat treatment at 773 K for 6 hours) deposited on float glass by both the techniques was calculated to be 45A. The grain size was found to be dependent on the substrate and was minimum for films deposited on float glass when compared to those on Corning 7059 and alumina.

No discernible microstructure in the surface of the films was seen through scanning electron microscope (SEM). But SEM micrographs showed cracks on the films deposited on alumina substrates. Atomic force microscope (AFM) pictures showed that the surfaces of the films deposited by sol-gel (after sintering at 773K) and that of the as-sputtered films were smooth with roughness being of the order of a few angstroms. The surface of the as-sputtered films showed a columnar structure after annealing them at 773 K for 6 hours.

Hall effect and resistivity measurements on the films revealed conduction to be n-type. It was also inferred that the films sputtered in the absence of oxygen were non-stoichiomettie where as the films (deposited by both sol-gel and sputtering) after heat treatment at 773 K were nearly stoichiometric. The influence of grain boundary scattering on carrier mobility was clearly evident from these measurements.

The films deposited on float glass substrates showed very high sensitivity (dc measurements) to ethyl alcohol vapour (at 698 K) compared to LPG and hydrogen.

The films exhibited very fast response and recovery times also. The sensitivity was linear in the low concentration range of ethanol. The films deposited on Corning 7059 glass and alumina substrates showed lower ethanol sensitivity. Poor sensitivity of the as-sputtered amorphous films proved the'need of having crystallinity in the films as a pre-requisite for high sensitivity. A phenomenon of charge retention of the films on float glass immediately after withdrawing the de bias, was observed during the gas sensitivity studies at high temperatures. The charge retention was found to vanish in

the presence of ethanol and was also negligible when the films were kept in vacuum.

No appreciable amount of charge retention was observed in films on other substrates.

A model was proposed for explaining the results. It was suggested that the chemisorbed oxygen ions, (02 at temperatures close to room temperature or CY at high temperatures) extract electrons from the film and deplete the nano-crystallites completely of electrons. In presence of applied electric field, they move towards the sample-electrode contact region and accumulate there. These processes reduce the conductivity of the films. Ethanol vapour removes the adsorbed as well as accumulated oxygen ions, thereby restoring the original conductivity. The high sensitivity of the films was partially attributed to the nano-grain size (less than twice the Debye length) and partially to the oxygen ion accumulation at the sample- electrode contact region.

More detailed study of the phenomenon of ion accumulation was done at different ambients and at different temperatures using impedance spectroscopy employing a lock-in amplifier in the frequency range 250 kHz to 10 MHz. Cole-Cole plots (impedance) were drawn and equivalent circuit models were suggested. Fitting of these models with experimental data was done using the approach of 'universal dielectric response' by Jonscher and the values of the individual circuit elements were found out. Impedance spectroscopic studies conducted at room temperature gave the information that though the films have considerable sensitivity to humidity at room temperature, the ions from the physisorbed water molecules accumulate at the sample- electrode contact region on the application of the low frequency signals, thereby inhibiting the humidity sensitivity. The measurements conducted at different temperatures revealed different transformations going on in the material as a result of adsorption or desorption of gases. This study along with that conducted in presence of ethanol, LPG and H2 at 698K gave confirmatory evidences of oxygen ion accumulation at the sample-electrode contact region. It was, therefore, suggested that high frequency signals should be used for using the film for humidity sensing at room temperatures where as low frequency signals or a de bias should be used for sensing ethanol vapour at high temperatures.

TABLE OF CONTENTS

CERTIFICATE

ACKNOWLEDGEMENTS

ABSTRACT iv

TABLE OF CONTENTS VII

LIST OF FIGURES xi

LIST OF TABLES xvi

CHAPTER 1 INTRODUCTION

1.1 Sensors : Definitions and classifications 1

1.2 Solid state gas sensors 3

1.2.1 Applications 3

1.2.2 Conventional gas analysis techniques 8

1.2.3 Types of solid state gas sensors 10

1.3 Metal Oxide Semiconductors as Gas Sensors 15

1.3.1 A brief history 15

1.3.2 n-type and p-type oxide semiconductors 16

1.3.3 Advantages 16

1.3.4 Selection of materials 16

1.3.5 Mechanism of gas sensing 18

1.3.5.1 Adsorption 18

1.3.5.2 Reduction, oxidation and dissociation of oxides 24 1.3.6 Sensitivity in single and polycrystalline oxides 25 1.3.7 Oxide semiconductor materials used as sensors 26

1.3.8 Fabrication technologies 32

1.3.8.1 Sintering technology 32

1.3.8.2 Thick film technology 33

1.3.8.3 Thin film technology 34

1.4 Tin Oxide 35

1.4.1 General properties 35

1.4.2 An overview of the sensing characteristics 36

1.4.2.1 Sensing mechanism 36

1.4.2.2 Effect of grain size on sensitivity of polycrystalline tin oxide 38

vii

1.4.2.3 Influence of humidity 41 1.4.2.4 Methods commonly used to improve the sensing 42

characteristics

1.4.2.5 Thin film tin oxide 46

1.5 Objectives of the present work 47

1.6 Thesis plan 48

CHAPTER 2 DEPOSITION AND CHARACTERISATION TECHNIQUES

2.1 Sol - Gel technique 51

2.1.1 Introduction 51

2.1.2 Principle 52

2.1.3 Thin film coating techniques 54

2.1.3.1 Dip coating 54

2.1.3.2 Spin coating 56

2.1.3.3 Spray coating 57

2.1.3.4 Electrophoresis 57

2.1.3.5 Thermophoresis 58

2.1.3.6 Sedimentation ( Settling ) 58

2.1.4 Fabrication of dip coating set up 58

2.1.5 Preparation, of the sol and optimisation of parameters 60

2.2 RF Magnetron Sputtering 62

2.2.1 Advantages and principles of operation 62 2.2.2 Fabrication of a RF magnetron sputtering system and 63

optimisation of parameters

2.3 Thickness Measurement 65

2.4 Characterisation techniques 66

2.4.1 Structural 66

2.4.1.1 Glancing angle x-ray diffraction 66

2.4.2 Surface Morphology 67

2.4.2.1 Scanning Electron Microscope ( SEM ) 67 2.4.2.2 Atomic Force Microscope ( AFM ) 68

2.4.3 Optical 71

2.4.3.1 UV - Vis - Near IR Spectroscopy 71 viii

2.4.4 Electrical 71 2.4.4.1 Electrical resistivity and Hall effect measurements 71 2.4.4.2 Gas sensitivity studies (dc electrical measurements) 72

2.4.4.3 Impedance Spectroscopy 74

CHAPTER 3 STRUCTURAL AND OPTICAL CHARACTERISATIONS

3.1 Introduction 77

3.2 Structural characterization using GAXRD 77

3.3 Surface morphology 83

3.4 Optical behaviour 86

3.4.1 Theory 86

3.4.2 Results of optical studies 89

3.5 Conclusions 94

CHAPTER 4 GAS SENSING BEHAVIOUR

4.1 Introduction 95

4.2 Electrical properties 95

4.3 V- I characteristics 97

4.4 Gas sensing characteristics 99

4.5 Observation of charging effect 105

4.6 A model for gas sensing behaviour 107

4.7 Conclusions 113

CHAPTER 5 IMPEDANCE SPECTROSCOPY

5.1 Introduction 115

5.2 Effect of humidity at room temperature 116

5.3 Effect of temperature 125

5.4 Effect of reducing gas environment 128

5.5 Conclusion 131

CHAPTER 6 SUMMARY OF RESULTS, CONCLUSIONS AND SCOPE FOR FUTURE WORK

6,1 Summary of results and conclusions 133

6.2 Scope for future work 137

REFERENCES 139

APPENDIX 1 157

ix

APPENDIX 2 158

LIST OF PUBLICATIONS 160

BIO-DATA 161