Fundamental Studies of Pulsed Laser Ablation

 

Frederik Claeyssens

 

A dissertation submitted to the University of Bristol in accordance with the requirements of the degree of Doctor of Philosophy in the Department of Chemistry December 2001.

Approximate number of words: 70.667

Contents Index


Abstract

 

The pulsed ultraviolet (l = 193, 248 nm) laser ablation of graphite, polycrystalline diamond and ZnO targets has been investigated. Characteristics of the resulting plumes of ablated material have been deduced using optical emission spectroscopy (wavelength dispersed spectra, together with spatially and temporally resolved measurements) and electrostatic probes. Deposited films were analysed using a variety of techniques, including scanning electron microscopy, UV-visible absorption and Raman spectroscopy, and X-Ray diffraction.

The study of graphite ablation focused mainly on the identification and characterisation of laser-plume interactions. Such processes are very important in the creation of the highly excited ablation plasma. Laser-plume interactions were probed directly, by monitoring the absorption of the laser light by the ablation plasma, and indirectly, using optical emission spectroscopy (OES) and a Faraday Cup. Laser-plume interactions are shown to be responsible for the observed asymmetry (with respect to the surface normal) of the emission from C+ species in the plume. Pulse duration effects were probed also, by comparing characteristics of the ablation process induced using short (~picosecond) and the more traditional (nanosecond) laser pulses. Raman spectroscopy on the deposited diamond-like-carbon films indicated that sp3 fractions >60% are achievable.

Investigations on the ablation of ZnO targets (with and without added Al and Ga dopant) have resulted in a coherent view of the resulting plume, which exhibits a multi-component structure correlated with different regimes of ablation, which are attributed to ejection from ZnO and ablation from a Zn melt. OES measurements show that the emitting Zn component within the plume accelerates during expansion in vacuum an observation attributable to the presence of hot, fast electrons in the plume. The same acceleration behaviour is observed in the case of Al atomic emissions resulting from ablation of an Al target in vacuum. Deposition conditions, substrate temperature and background gas pressure were all varied in a quest for optimally aligned, high quality ZnO thin films. Initial ab initio calculations were performed also, to aid in understanding the stability of these c-axis aligned films.

 

Author's Declaration

 

I declare that the work in this dissertation is carried out in accordance with the Regulations of the University of Bristol. This work is original except where indicated by special reference in the text and no part of the dissertation has been submitted for any other degree. Any views expressed in the dissertation are those of the author and in no way represent those of the University of Bristol. The dissertation has not been presented to any other University for examination either in the United Kingdom or overseas.

 

Frederik Claeyssens


Dedications

 

For my mum, dad and sister


Acknowledgements

 

There are a large number of people I need to thank for support, both academically and socially, during my Ph.D. First of all, I would like to thank Mike Ashfold for his help and support during my time in Bristol. Neil Allan was a great support for the theoretical work. Also Paul May, Andrew Orr-Ewing and Jeremy Harvey helped me a lot through fruitful discussions and Keith Rosser for his astounding technical support. Rob Lade was invaluable for his collaboration and his plain honesty in my first year. Special thanks also to James Smith for putting up with my complaints when the experiment did not work as it supposed to do. Andy Cheesman was definitely an interesting addition to the lab, and helped quite a bit during my last year. The people of the SEM suite, Sean and Pippa, and Sergei of the group of Professor Wang (Bath University) helped on various parts of the project. Thanks to Demetrios Anglos and Costas Fotakis I could stay in Crete (FO.R.T.H.) for a while in early summer, which was very pleasant, also because of the productive collaboration with Manolis and Carmen.

 

Additionally, many people helped out to keep me healthy and sane during the last three years: the Budgies, especially Neil S, James P, Kevin and Sean, and the people from the laser and theoretical group, Paul, Stu, Al, James F, Phil, John, Francesca, Marlies, Marko, Malcolm, Daniella, Eckart, Eloy. Jan, Fred, Steven and Tom kept me Belgian during these years. Wendy, thanks a lot for putting up with me.

 

For financial support I would like to thank the EPSRC.

 


 

We shall not cease from exploration

And the end of all our exploring

Will be to arrive where we started

And know the place for the first time

 

T.S. Eliot, "Four Quartets"


Index

 

Chapter 1

1. General introduction to deposition of thin films and laser

interaction with materials

1.1. Description of different deposition methods

1.1.1.      Description of Pulsed Laser Deposition

1.1.2.      1.1.2. Deposition Techniques

1.1.3.      1.1.3. Film Growth

1.1.3.1. Substrate temperature

1.1.3.2. Energy of the deposition flux

1.1.3.3. Deposition Rate, vacuum quality and background gas

1.1.4. Comparison of the different techniques for the different

fundamental film growth parameters

1.1.5. Practical deposition criteria

1.2. Production of thin films via laser ablation

1.3. New fields in the laser interaction with solids

1.3.1. Surface photochemistry

1.3.2. Matrix Assisted Laser Desorption and Ionisation

1.3.3. Microshaping of Photonic Components

1.3.4. Laser Ablation in space technology

1.4. Scope of this thesis

1.5. Conclusion

1.6. References

 

Chapter 2

2. Experimental Techniques

2.1. Lasers

2.2. Vacuum chamber

2.3. Target Materials and Holder

2.3.1. Target materials

2.3.2. Target holder

2.4. Plasma Detection Equipment

2.4.1. Optical Emission Spectroscopy

2.4.2. CCD camera imaging

2.4.3. Langmuir Probes

2.4.3.1. Theory of Langmuir Probes

2.4.3.2. Langmuir probes in laser ablation

2.4.3.3. Specifics of the Langmuir probe

2.4.4. Faraday Cup

2.4.5. Mass Spectrometer

2.4.6. Jacobian Transformation

2.4.7. Determination of the attenuation of the laser light by the ablation plasma

2.5. Film deposition

2.6. Film analysis techniques

2.6.1. Raman Spectrometry and Photoluminescence spectroscopy

2.6.2. Scanning and Transmission Electron Microscopy

2.6.3. Ultra-Violet and Visible Spectrometry

2.6.4. X-ray diffraction

2.6.5. Atomic Force Microscopy

2.6.6. Field emission testing

2.7. References

 

Chapter 3

3. General Theory

3.1. Laser Ablation

3.1.1. Laser interaction with solid material, and particle ejection

3.1.1.1. Nanosecond ablation

3.1.1.2. Femtosecond Ablation

3.1.2. Gas and plasma dynamics

3.1.3. Material deposition

3.2. Structure of the deposited materials

3.2.1. Diamond Like Carbon (DLC)

3.2.2. ZnO

3.3. Ab initio Theory

3.3.1. The Schrodinger equation and the Born-Oppenheimer approximation

3.3.2. HF Theory

3.3.3. DFT Theory

3.3.4. Properties calculated from the wavefunction

3.3.5. Plane Wave Approximation

3.3.6. Polar surfaces in solids

3.3.6.1. Criterion for surface polarity

3.3.6.2. Relevant processes for cancelling the polarity

3.3.6.3. Former studies on ZnO

3.4. References

 

Chapter 4

4. The UV pulsed laser ablation of graphite and the pulsed

laser deposition of Diamond Like Carbon

4.1. Introduction

4.2. Experimental

4.3. Direct measurement of the attenuation of the laser light by the plume

4.4. Optical emission of the ablation plume

4.4.1. Wavelength dispersed emission

4.4.2. Time resolved imaging of species specific emissions

4.5. Faraday Cup Measurements

4.6. Discussion of the results and model

4.6.1. Conclusions for the laser ablation plume characteristics

4.7. Deposition of Diamond Like Carbon films

4.7.1. Deposition of DLC films

4.7.2. Field emission characteristics

4.7.3. Deposition of C:S films

4.7.4. Conclusions for film deposition

4.8. References

 

Chapter 5

5. Nanosecond ArF Pulsed Laser Ablation of Zinc Oxide

and Pulsed Laser Deposition of Thin Zinc Oxide Films

5.1. Introduction

5.1.1. Applications of zinc oxide

5.1.2. Laser ablation of zinc oxide and deposition of thin films

5.2. Experimental

5.3. Pulsed laser ablation of zinc oxide

5.3.1. Time integrated optical emission spectroscopy in vacuum

5.3.2. Time differentiated optical emission spectroscopy in vacuumFluence

5.3.3. Langmuir Probe transients in vacuum

5.3.4. Target morphology

5.3.5. Optical emission and ion probe measurements in background gas

5.3.6. Conclusions from studies of the 193 nm pulsed laser ablation of zinc oxide

5.4. Pulsed laser deposition of zinc oxide and properties of the resulting films

5.4.1. Dependence of parameters on deposition conditions and film properties

5.4.1.1. UV-VIS

5.4.1.2. SEM

5.4.1.3. XRD

5.4.1.4. Planar TEM

5.4.1.5. AFM

5.4.1.6. Raman and Photoluminscence

5.4.2. Effect of Aluminium and Gallium doping

5.4.3. Conclusions and discussion of the deposition of ZnO films

5.5. Ab initio calculations

5.5.1. Bulk structure of the wurtzite structure

5.5.2. The non-polar (100) surface

5.5.3. The polar (002) surface

5.5.3.1. Two-bilayer slab

5.5.3.2. Four-bilayer slab

5.5.3.3. 2 2 reconstruction

5.5.4. Effect of bulk doping: Aluminium and Gallium

5.5.5. Conclusion regarding the ZnO doping from the ab initio calculations

5.6. General Conclusion

5.7. References

 

Chapter 6

6. Optical Emission Spectroscopy and imaging of ablation plumes

following ultraviolet KrF nanosecond, picosecond and femtosecond

excitation of graphite in vacuum.

6.1. Introduction

6.1.1. Picosecond and Femtosecond laser ablation

6.2. Experimental

6.3. Nanosecond laser ablation of graphite

6.3.1. Optical Emission Spectroscopy

6.3.2. i-CCD camera images

6.3.3. Results and Discussion

6.4. Picosecond and Femtosecond laser ablation of graphite

6.4.1. Optical Emission Spectroscopy

6.4.2. i-CCD camera images

6.4.3. Analysis of laser-irradiated region

6.4.4. Analysis of deposited films

6.5. 248 nm femtosecond laser ablation of Silicon(111)

6.5.1. Optical Emission Spectroscopy

6.5.2. i-CCD camera images

6.5.3. Analysis of laser irradiated target material

6.6. Results and Conclusion

6.6.1. Comparison of nanosecond, picosecond and femtosecond laser ablation of graphite

6.6.2. Comparison with the femtosecond ablation of Si(111)

6.6.3. General conclusion

6.7. References

 

Appendix 1

A1. Ablation plume properties arising from ArF PLD of

aluminium and copper

A1.1. Time integrated OES

A1.2. Faraday Cup measurements

A1.3. Time differentiated OES for aluminium

A1.4. Langmuir Probe transients for aluminium

A1.5. Conclusion

A1.6. References

 

Appendix 2

A2. Operation of the Quadrupole Mass Spectrometer

A2.1. Detailed Description of the Apparatus

A2.2. Specific usage of the EQP in Laser Ablation

A2.3. Experimental Determination of the System Performance

A2.3.1. RGA mode

A2.3.2. SIMS mode

A2.4. Modelling of the Transmission Function with SIMION

A2.5. Experimentally Determined Transmission Function

A2.6. Conclusion

A2.7. References

 

Appendix 3

A3. Linelist for neutral carbon atoms

 

Appendix 4

A4. Linelist for carbon cations

 

Appendix 5

A5. Linelist for carbon dications

 

Appendix 6

A6. Linelist for zinc atoms and cations


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