DESIGN AND FABRICATION OF A PORTABLE SPECTROMETER TO INVESTIGATE OPTICAL PROPERTIES OF THIN FILMS
BENARD RIRO MORUMBWA [B.Ed. (Sc)] I56/10991/2006
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A thesis submitted in partial fulfillment of the requirements for the award of the degree of Master of Science (Electronics and Instrumentation) in the School of Pure and Applied Sciences of Kenyatta University
June, 2013
ii
DECLARATION
This thesis is my original work and has not been presented for the award of a degree or any other award in any other University
Benard Riro Morumbwa …………………………. ………………………..
Department of Physics Signature Date Kenyatta University P.O. BOX 43844 - 00100 NAIROBI- KENYA
We confirm that the work reported in this thesis was carried out by the candidate under our supervision.
Dr. Patrick M. Karimi ………………………….. ………………………….
Department of Physics Signature Date
Kenyatta University P.O. BOX 43844 - 00100 NAIROBI- KENYA
Dr. Daniel M. Wamwangi ……… ……10.06.2013……
School of Physics Signature Date University of Witswatersrand PRIVATE BAG 3, 2050 JOHANNESBERG - SOUTH AFRICA
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DEDICATION
This thesis is dedicated to my wife Stellah Kemunto and my daughters, Brenda and Mitchell.
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ACKNOWLEDGEMENTS
I thank in a special way Dr. P.M. Karimi and Dr. D. Wamwangi for their invaluable support, guidance and advice which enabled successful completion of this research. I do appreciate the attention and timely response they gave whenever I was in need of help.
Their wide and in-depth knowledge in material science, electronics and instrumentation gave me the courage and ability I needed most in doing this work. I once more thank them for the great efforts and sacrifice they made to ensure that this work was a success.
I am grateful to Dr. Migwi, former and Dr. Njoroge the current head of physics department who gave conducive learning environment during my stay as a student at
Kenyatta University. I also acknowledge the physics laboratory technical staff, particularly the chief technician Mr. Simon Njuguna who made it possible for me to carry out thin film sample preparations in the laboratory. I will also not forget the encouragement and support I got from my colleagues Omayio, Agumba, Tuwei,
Namasaka and Ogaro to mention a few. Appreciation to my family for being patient and understanding during the time I was away because of my studies. Most importantly I thank almighty lord for making this work possible.
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TABLE OF CONTENTS
TITLE……………………………………………………………………………………..i
DECLARATION………………………………………………………………………...ii
DEDICATION…………………………………………………………………………..iii
ACKNOWLEDGEMENTS…………………………………………………………….iv
TABLE OF CONTENTS…………………………………………………………...... v
LIST OF TABLES……………………………………………………………………….x
LIST OF FIGURES……………………………………………………………………..xi
ABBREVIATIONS AND ACRONYMS……………………………………………...xiv
LIST OF SYMBOLS…………………………………………………………………...xv
ABSTRACT…………………………………………………………………………….xvi
CHAPTER 1……………………………………………………………………………...1
INTRODUCTION…………………………………………………………………...... 1
1.1 Background to the Study……………………………………………………………..1
1.2 Statement of Research Problem………………………………………………………2
1.3 Objectives of the Research Project…………………………………………………...3
1.3.1 Main Objective…………………………………………………………….3
1.3.2 Specific Objectives……………………………………….……………….3
1.4 Rationale………………………………………………………………....…………...4
CHAPTER 2……………………………………………………………………………...5
LITERATURE REVIEW……………………………………………………………….5
2.1 Introduction..……………………………………………………………………...... 5
2.2 Historical Background of Spectroscopy……………………………………………..5
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2.3 Development of Optical Spectrometers………………………………………………8
2.4 Types of Optical Spectrometers……………………………………………………....9
2.4.1 Ellipsometers………………………………………………………………9
2.4.2 Spectrophotometers……………………………………………………....10
CHAPTER 3…………………………………………………………………………….11
THEORY………………………………………………………………………………..11
3.1 Introduction………………………………………………………………………….11
3.2 Spectroscopy………………………………………………………………………...11
3.3 Optical Spectrometers……………………………………………………………….12
3.3.1 Ellipsometers……..………………………………………………………13
3.3.2 Visible Range Optical Spectrophotometers………………..…………….15
3.4 Spectrometer Automation…………………...………………………………………17
3.5 Enhanced Parallel Port (EPP)………………………………………………..……...18
3.6 Deposition of Thin Films……………..……………………………………………..20
3.6.1 Deposition of Thin Films by Thermal Evaporation…………..………….21
3.6.2 DC Magnetron Sputtering……………….……………………………….22
3.7 Optical Properties of Thin Films…………………..………………………………...24
3.7.1 Reflectance, (R)……………………...…………………………………..24
3.7.2 Real Refractive Index, (n)………………………………………………..25
3.7.3 Extinction Coefficient, (k)……………………………………………….26
3.7.4 Real Dielectric Constant, (ε1)…………….………………………………26
3.7.5 Imaginary Dielectric Constant, (ε2)…………….………………………..26
3.7.6 Absorption Coefficient, (α)……….……………………………………...27
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3.7.7 Determination of Thickness of a Thin Film, (x)……….………………...27
3.7.8 Transmittance, (T)………………………………………………………..28
3.7.9 Band Gap, (Eg)……………….…………………………………………..28
3.7.10 Direct Band Gap…………………………………………………………28
3.7.11 Indirect Band Gap………….…………………………………………….29
3.7.12 Band Gap of Amorphous Semiconductor Thin Films…………………...30
3.7.13 Energy Absorption Factor…………………….………………………….30
CHAPTER 4………………………………………………………………..…………...31
EXPERIMENTAL PROCEDURES………………………………………….……….31
4.1 Introduction………………………………..………………………………………....31
4.2 Hardware Design……………………………………...………………………...... 32
4.2.1 Powering System………………………………………………………...32
4.2.2 Diffraction System…………………………………………………...... 34
4.2.3 Stepper Motor……………………………………………………………35
4.2.4 Photo – detectors…………………………………………………………37
4.2.5 Analog to Digital Converter (ADC 0809CCN)………………………….38
4.3 Software Design……………………………………………………………………...40
4.3.1 Hardware Interfacing………………………….…………………………40
4.3.2 MSVC++ 6.0 Portable Spectrometer Driver Program Designed in this
Work……………………………………………………………………..42
4.4 Preparation of Sample Thin Films……………………………………..…………….45
4.4.1 Preparation of CuO Thin Film…………………………………………...45
4.4.2 Deposition of SnSe……………………………………………………....46
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4.5 Measurement Procedure of a Portable Spectrometer Designed in this Work………..48
CHAPTER 5………………………………………………...…………………………..50
RESULTS AND DISCUSSIONS……………………………...……………………….50
5.1 Introduction………………………...………………………………………………...50
5.2 Instrument Calibration………………………...………………………………...... 50
5.3 Optical Properties of Copper Oxide (CuO)………………………...………………..52
5.3.1 Reflectance, (R) of CuO…………………….………...…………………52
5.3.2 Phase Angle, (Φ) of CuO………………………………………………...53
5.3.3 Real Refractive Index, (n) of CuO…………………………………...…..54
5.3.4 Imaginary Refractive Index, (k) of CuO.……………………………..….55
5.3.5 Real Permittivity, (ε1) of CuO.…………………………………………..56
5.3.6 Imaginary Permittivity, (ε2) of CuO……………………………………..57
5.3.7 Energy Absorption Factor of CuO.………………………………………57
5.3.8 Absorption Coefficient, (α) of CuO…..……………………………….…58
5.3.9 Band Gap, (Eg) of CuO……………………………………...…………...59
5.3.10 Validation of CuO Results………………….……………………………59
5.4 Optical Properties of Tin Selenide (SnSe)………………………...………………....61
5.4.1 Reflectance, (R) of SnSe……………………………….………………...61
5.4.2 Phase Angle, (Φ) of SnSe………………………………………………..62
5.4.3 Real Refractive Index, (n) of SnSe………………………….…………...62
5.4.4 Imaginary Refractive Index, (k) of SnSe………………………….……..63
5.4.5 Real Permittivity, (ε1) of SnSe…………………………………………...63
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5.4.6 Imaginary Permittivity, (ε2) of SnSe…………………….……………….64
5.4.7 Energy Absorption Factor of SnSe……………………….……………...65
5.4.8 Absorption Coefficient, (α) of SnSe……………………………….…….65
5.4.9 Band Gap, (Eg) of SnSe……………………………………………...…..65
5.4.10 Validation of SnSe Results………………………………………………66
CHAPTER 6………………………………..…………………………………………...69
CONCLUSION AND RECOMMENDATIONS FOR FURTHER WORK………...69
6.1 Conclusion……………...……………………………………………………...... 69
6.2 Recommendations for Further Work…………………..………………………….....70
REFERENCES……………...…………………………..………………………………71
APPENDICES………………..…………………………………………………………74
Appendix I: How to Run MSVC++ 6.0 Portable Spectrometer Driver Program Designed
in this Work………………………………………………………………..74
Appendix II: Picture of Hardware Components of the Portable Spectrometer designed in
this Work………...... 78
Appendix III: Specification Chart for Stepper Motor used in the Portable Spectrometer
Designed in this Work…...... 79
Appendix IV: MSVC++ 6.0 Portable Spectrometer Driver Program………..…………..80
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LIST OF TABLES
Table 3.1: Pin Assignment for D-type 25 pin Female Connector of Enhanced Parallel Port
(EPP)………………………………………………………………………….19
Table 4.1: Unipolar Stepper Motor Half Stepping Mode Procedure….…………………36
Table 4.2: Input States for the Address lines (A, B and C) used to Select Analog Input
Channel into Analog to Digital Converter (ADC08090CCN)………………..40
Table 4.3: Binary Code generated by Half Stepping Mode of the Uni-polar Stepper
Motor………………………………………………………………………….45
Table 4.4: Deposition Parameters for CuO Thin Film used to test Portable Spectrometer
Designed in this Work..………………………………………………………46
Table 4.5: Deposition Parameters for SnSe Thin Film used to test Portable Spectrometer
Designed in this work………………………………………………………...47
Table 5.1: Comparison between Reflectance of CuO Using Portable Spectrometer
Designed in this Work and Spectrophotometer (SolidSpec-3700)…………...61
Table 5.2: Reflectance of SnSe using Portable spectrometer Designed in this Work and
Spectrophotometer (SolidSpec-3700)………………………………………...67
Table 5.3: Band Gap of SnSe using Portable spectrometer Designed in this Work and
Spectrophptometer (SolidSpec-3700)………………………………………...68
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LIST OF FIGURES
Figure 3.1: Schematic diagram of an ellipsometer showing phase change and polarization
state of incident light upon reflection by a thin film sample……………...…14
Figure 3.2: Schematic diagram showing components of an optical spectrophotometer…17
Figure 3.3: Pin out diagram of enhanced parallel port (EPP)………….……………...... 20
Figure 3.4: Diagram showing principal parts of a system used in thermal evaporation
deposition of thin films……………………………………………………....22
Figure 3.5: Schematic diagram showing essential components of a DC sputtering
system...... 23
Figure 4.1: Set up diagram showing hardware components of the portable spectrometer
designed in this work………………………………………………………..31
Figure 4.2: Powering system used to rectify a.c voltage for the portable spectrometer
designed in this work………………………………………………………..32
Figure 4.3: Picture of powering system of the portable spectrometer designed in this
work…………………………………………………………………………33
Figure 4.4: Picture of diffraction system of the portable spectrometer designed in this
work…………………………………………………………………………35
Figure 4.5: Diagram of circuit used to interface unipolar stepper motor to the enhanced
parallel port……………………………………………………...... 36
Figure 4.6: Circuit diagram of photo-detectors for sensing light intensity in the portable
spectrometer designed in this work………………………………………….37
Figure 4.7: ADC0809CCN pin out diagram……………………………………………..38
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Figure 4.8: Complete circuit diagram showing the diffraction system, photo detectors and
electronic circuitry of the portable spectrometer designed in this work……..39
Figure 4.9: Timing diagram for analog to digital converter (ADC0809CCN)…..………41
Figure 4.10: Hardware interfacing between the portable spectrometer designed in this
work and a personal computer using enhanced parallel port (EPP)………..42
Figure 4.11: Flow chart diagram for the portable spectrometer driver program……...... 43
Figure 5.1: Graph of reflectance against radiation energy for silver mirror using the
standard, SolidSpec-3700 and the portable spectrometer designed in this
work...... 51
Figure 5.2: Graph showing reflectance curves for CuO and SnSe thin films from data
obtained by SolidSpec-3700 and portable spectrometer designed in this
work………………………………………………………………………….52
Figure 5.3: Graph showing the phase angles for the CuO and SnSe thin films from data
obtained by SolidSpec-3700 and portable spectrometer designed in this
work…...... 54
Figure 5.4: Graph showing real and imaginary refractive index of CuO thin film from
data obtained by SolidSpec-3700 and portable spectrometer designed in this
work………………………………………………………………………….55
Figure 5.5: Graph showing real and imaginary permittivity of CuO thin film from data
obtained by SolidSpec-3700 and portable spectrometer designed in this
work…...... 56
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Figure 5.6: Graph showing the energy absorption factor for CuO and SnSe thin films
from data obtained by SolidSpec-3700 and portable spectrometer designed in
this work……………………………………………………………………...57
Figure 5.7: Graph showing the absorption coefficient for CuO and SnSe thin films from
data obtained by SolidSpec-3700 and portable spectrometer designed in this
work...... 58
Figure 5.8: Graph showing linear extrapolation to CuO curves used to determine band
gap………………………………………………………………………...... 60
Figure 5.9: Graph showing the real and imaginary refractive index for SnSe thin film
from data obtained by SolidSpec-3700 and portable spectrometer designed in
this work……………………………………………………………………...62
Figure 5.10: Graph showing the real and imaginary permittivity of SnSe thin film from
data obtained by SolidSpec-3700 and portable spectrometer designed in this
work………………………………………………………………………...64
Figure 5.11: Graph showing linear extrapolation to SnSe curves used to determine band
gap…………………………………………………………………………..66
xiv
ABBREVIATIONS AND ACRONYMS
IR Infra Red
VIS Visible Spectrum
UV Ultra Violet
ADC Analog to Digital Converter
R Reflectance
EPP Enhanced Parallel Port
ISA Industrial Standard Architecture
IBM International Business Machine
TTL Transistor to Transistor Logic
PC Personal Computer
Mbps Megabytes per second
DOS Disk Operating System
LPT Line Printer
CPU Central Processing Unit
LABVIEW Laboratory Virtual Engineering Workshop
IEEE Institute of Electrical and Electronic Engineering
xv
LIST OF SYMBOLS
ω Radiation frequency
ε Dielectric function
Γ Damping constant
CuO Copper oxide
SnSe Tin selenide
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ABSTRACT
Optical spectroscopy is a non-destructive and contactless method for characterizing optical and electrical properties of thin films; reflectance, transmittance, refractive index, permittivity and band gap. These properties determine the application of a thin film as a polarizer, laser, solar cell or photodiode among others. To measure accurately optical properties of thin films, the common types of optical spectrometers used are ellipsometers and spectrophotometers. However these instruments are expensive and there is need to device a cost effective optical spectrometer. In this study, a portable laboratory spectrometer that uses reflectance measurement to determine the optical and electrical properties of thin films was designed. The spectrometer consists of a fluorescent lamp (250V, 25W) as the source of visible light, a holographic grating (1200 lines/mm) which is placed in focus to the light generating electromagnetic spectrum in the 380 - 780 nm excitation range. A concave mirror of focal length 5 cm mounted on a shaft of a gear system (reduction ratio 50:1) driven by a stepper motor of resolution 3.75o focuses monochromatic beams onto thin film sample and to the first photo-detector, through computer controlled small angular rotations of the stepper motor. The reflected monochromatic beam from the probed thin film sample is directed to the second photo- detector and the intensities of both the incident and reflected beam are then converted to digital form by analog to digital converter (ADC0809CCN) connected to enhanced parallel port used in computer interfacing. Visual C++ programming language was used in developing and implementing the driver program that controls the designed instrument. The program enables the computer to record reflectance measurement of the probed sample in order to determine other optical properties. To test the accuracy and reliability of the portable spectrometer designed in this work, reflectance of semiconductor thin film samples of CuO and SnSe were measured using both the designed and a standard spectrometer (SolidSpec-3700). The reflectance measurement for CuO and SnSe had an error of ± 1.88 % and ± 2.17 % respectively, in reference to the standard spectrometer (SolidSpec-3700) reflectance spectra. This error margin is within tolerance level for reliable measurement. In addition the portable spectrometer designed in this work has a resolution of 2 nm ± 5%, cheaper compared to commercially available spectrometers measuring in the same range and portable.
xvii
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CHAPTER 1
INTRODUCTION
1.1 Background to the Study
Several approaches have been adapted for the characterization of the optical properties of thin films. The choice of the approaches taken is determined in part by the nature of excitation studied and on the cost benefit analysis of the method used. In optical characterization two approaches cannot escape unmentioned, namely Raman and Optical spectroscopy which have been widely used as complementary methods. In the study of lattice vibrations, Raman spectroscopy is a suitable choice, whereas optical spectroscopy both in the infra-red (IR), visible (VIS) and deep ultraviolet (UV) provides a dearth of information on the phonon modes, the Plasmon frequencies as well as the band gap of thin films. Among these two approaches their availability in the laboratories is governed by three factors namely cost, simplicity and excitations modes which will be investigated in this study.
Optical spectroscopy within the visible range (380 – 780 nm) is easy and does not involve complicated equipments like in Raman or Infrared spectroscopy. Most thin films experience high optical absorption in the visible light resulting in a negligible fraction of light being transmitted. Therefore measurements of reflectance are taken in this project.
(Frederick, 1972).
In optical characterization, spectroscopic ellipsometry and optical spectroscopy have found wide spread usage. The former technique relies on the change in the polarization
2 property of light after reflection by a thin film. The concept of measurement for this technique involves the computation of the amplitude ratio upon reflection and the phase difference between the incident and reflected light for determining the optical constants.
This approach to optical characterization is complicated and involves expensive sophisticated instruments whose maintenance costs are prohibitive in the long term.
However this technique has no match in terms of the speed of measurement and throughput (Sidney, 1993).
On the other hand, the commercially available optical spectrometers, which measure reflectance or transmittance, provide a suitable alternative for determining optical properties of thin film. Despite the wide scope of information acquired in optical analysis, this technique is costly and not sustainable for institutions with high financial constraints. Thus there is need to design a simple, fast, cheaper and portable optical spectrometer to determine the optical constants of diverse forms of samples
(Pankove, 1975).
1.2 Statement of Research Problem
The device applications and an in-depth understanding of material properties has driven the search of new materials or alloys to higher levels. This calls for accurate measurement techniques in characterization of physical properties of materials. In optical characterization of thin films, ellipsometers and spectrometers are used. Despite the high level of accuracy in measurement achieved by these optical measuring instruments, their
3 cost is prohibitive. Thus there is need to design a cheaper portable spectrometer which is affordable to most research institutions (Sternberg and James, 1969).
1.3 Objectives of the Research Project
1.3.1 Main Objective
The main objective of this research was to design and fabricate a portable spectrometer to investigate optical properties of thin films.
1.3.2 Specific Objectives
(i) To design and implement a diffraction system which separates visible light
into it is constituent monochromatic lights.
(ii) To develop electronic circuitry which controls a stepper motor, two photo-
detectors and analog to digital converter for purposes of data acquisition.
(iii) To align a concave mirror mounted on a shaft of a gear system driven by a
stepper motor as a screen of the electromagnetic spectrum produced and
through reflection ensure optical coupling to the photo-detectors with high
wavelength resolution.
(iv) To develop and implement VC + + portable spectrometer driver software.
(v) To test the applicability and accuracy of the designed portable spectrometer
by comparing reflectance measurement for copper oxide (CuO) and tin
selenide (SnSe) thin films using the portable spectrometer designed in this
work and the standard instrument (SolidSpec-3700).
(vi) To compare the cost of the portable spectrometer designed in this work in
relation to commercially available spectrometers.
4
1.4 Rationale
There is a growing need for new solar cell materials, efficient optical coupling devices and high quality electronic components in communication, control and instrumentation.
This demands the use of a characterization technique for thin films with high accuracy level. The common optical spectrometers employed for this purpose are ellipsometers and optical spectrophotometers. However these instruments are costly. There is need to design a cheaper portable spectrometer to enable institutions with financial constraints characterize thin films in search for better application materials in nanotechnology.
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CHAPTER 2
LITERATURE REVIEW
2.1 Introduction
In this chapter the historical background of spectroscopy, instruments used in optical spectroscopy of thin films and their limitations are discussed.
2.2 Historical Background of Spectroscopy
Although the spectral nature of light is present in the rainbow, it was beyond the ability of early man to recognize its significance. It was not until 1666 that Newton showed that the white light from the sun could be dispersed into a continuous series of colors. Newton introduced the word "spectrum" to describe this phenomenon. His instrument employed a small aperture to define a beam of light, a lens to collimate it, a glass prism to disperse it, and a screen to display the resulting spectrum. This first spectroscope was nearly in modern form. Newton's analysis of light was the beginning of the science of spectroscopy
(Tillmon and Aaron, 1951).
It gradually became clear that the sun's radiation has components outside the visible portion of the spectrum. W. Herschel (1800) demonstrated that the sun's radiation extended into the infrared, and J.W. Ritter in 1801 made similar observations in the ultraviolet (Walter, 1978). These studies were the precursors of radiometric and photographic measurements of light, respectively.
Joseph Fraunhofer in 1814, extended Newton's discovery by observing that the sun's spectrum, when sufficiently dispersed, was crossed by a large number of fine dark lines,
6 now known as Fraunhofer lines (Jackson, 2000). These were the first spectral lines ever observed, and Fraunhofer employed the most prominent of them as the first standards for comparing spectral lines obtained using prisms of different glasses. Fraunhofer also developed the diffraction grating, an array of slits, which disperses light in much the same way, as does a glass prism, but with important advantages. With a prism, the angle at which a spectral line is dispersed depends on the type of glass used. This makes it difficult to compare different spectral measurements, and absolute wavelength measurement is not possible. Gratings, which employ interference of light waves to produce diffraction, provide a means of directly measuring the wavelength of the diffracted beam. Earlier, Young Thomas (1804) had demonstrated that a light beam passing through a slit emerges in a pattern of bright and dark fringes. Fraunhofer extended these studies to the case of two, three and many closely spaced slits, and thus developed the transmission grating. With this, he was able to directly measure the wavelengths of spectral lines (Massachusetts Institute of Technology, 2012).
Later Kirchhoff established that each element and compound has its own unique spectrum, and that by studying the spectrum of an unknown source, one can determine its chemical composition. With these advancements, spectroscopy became a true scientific discipline.
In the early 1800's many workers, J.F.W. Herschel, W.H.F. Talbot, C. Wheatstone, A.J.
Angstrom, and D. Alter among them, studied spectra from terrestrial sources such as flames, arcs and sparks (Hockey, 2009). These sources were found to emit bright spectral lines, which were characteristic of the chemical elements in the flame. Foucault, the
French physicist, observed in 1848 that a flame containing sodium would absorb the
7 yellow light emitted by a strong arc placed behind it. This was the first demonstration of a laboratory absorption spectrum.
These facts were brought together in 1859 by G. Kirchhoff in his famous law, which states that the emitted power and absorbed power of light at a given wavelength are the same for all bodies at the same temperature. It follows that a gas, which radiates a line spectrum must, at the same temperature, absorb the spectral lines it radiates. From this,
Kirchhoff and R. Bunsen explained that the Fraunhofer lines in the sun's spectrum were due to absorption of the continuous spectrum emitted from the hot interior of the sun by elements at the cooler surface. Analysis of the sun's atmosphere thus became possible
(American Chemical Society, 1900).
By recognizing that each atom and molecule has its own characteristic spectrum,
Kirchhoff and Bunsen established spectroscopy as a scientific tool for probing atomic and molecular structure of materials. This technique is advantageous because it is contactless and hence used in remote sensing (Hockey, 2009).
In spectroscopy, analysis of substances can be done by projecting known electromagnetic spectrum to a sample and investigating the reflected, refracted or transmitted spectrum.
The resulting spectrum is directly related to interaction of incident spectrum and the sample. Spectroscopy therefore gives detailed information on internal structure and electronic configuration of a sample under scrutiny (Crouch, 2007).
8
2.3 Development of Optical Spectrometers
The earliest spectroscope was developed in the 18th century. It composed of a collimator with a source of light (gas burner), a slit and a lens to produce a parallel beam of light.
A prism separated the incoming parallel light into specific monochromatic sources, which were focused on a sample and the outcome spectrum analyzed. The gas burner as a source of light could not provide a wide spectrum thus limiting the number of samples to be investigated through this process. Weak output spectrum from a sample was another challenge; photo-detectors could hardly detect weak signals, reducing the accuracy in measurement. Light source with a wide spectrum and high strength was therefore most sought after (Dintenfass, 1971; Bourne, 2002; Hockey, 2009).
The earliest spectroscope utilized electromagnetic radiation within the visible range to investigate thin films. Spectroscopy gave an insight to the electronic configuration of materials and their optical properties. It is a contact-less, fast method of measurement and involves small sample volumes. The first optical spectrometer had a limitation – the glass prism could separate visible light into monochromatic sources but the rotation angle from one wavelength to another was not constant. The diffraction grating replaced the glass prism ensuring constant angle of rotation between monochromatic sources with the same difference in wavelength. Thereafter a set of lenses and parabolic reflectors were incorporated in optical spectrometers to increase the intensity of radiation. They also ensured axial focusing of light to the measured sample (Yamada and Kamioka, 2003;
Miller et al., 2006).
9
Laser light has nowadays replaced ordinary light source in optical spectrometer system enabling high sensitivity, fast measurement and use of small sample volumes. Recent progress has seen the inception of laser arrayed waveguide grating which allows miniaturization and high wavelength resolution of sub-nanometer order (Kodate and
Komai, 2008; Taguchi et al., 2008). In the same spirit of improvement silicon photomultipliers with a high gain (106) and operating at low voltages have also been exploited increasing sensitivity of photo-detectors. This has resulted in high precision in remote grading of minerals, characterization and production of high quality optical materials, signal transmission and environmental assessment (Clark and Roush, 1984;
U.S.A Geological Survey, 2002; Melles, 2002).
2.4 Types of Optical Spectrometers
There are two common types of optical spectrometers used in measurement of optical properties of thin films, ellipsometers and spectrophotometers. These two types of optical spectrometers are distinguished by the measurement techniques they use. Ellipsometers give high accuracy in measurement of optical properties of thin films but involve a complicated measurement procedure compared to that employed by spectrophotometers.
2.4.1 Ellipsometers
Ellipsometry is a versatile and powerful optical technique for the investigation of the dielectric properties (complex refractive index or dielectric function) of thin films. It is a very sensitive measurement technique and provides unequalled capabilities for thin film metrology. As an optical technique, ellipsometry is non-destructive and contactless. Upon
10 the analysis of the phase change and polarization state of light, which is reflected off a sample, ellipsometry can yield information about layers that are thinner than the wavelength of the probing light itself (Tompkins and McGahan, 1999). The complicated procedure it employs however makes ellipsometer expensive, prohibiting research institutions with weak financial base from using it.
2.4.2 Spectrophotometers
Optical spectrophotometers use a simple measurement procedure unlike the ellipsometers. The monochromatic source of light is incident normally on the probed sample and reflected or transmitted light is used to obtain optical constants of the material. Despite the simple measurement procedure utilized, optical spectrophotometers in the market are costly and there is need to device an optical spectrometer which is affordable to most research institutions (Manuel and Peter, 1996).
11
CHAPTER 3
THEORY
3.1 Introduction
This chapter briefly presents spectroscopy as a technique of remotely investigating materials, common types of optical spectrometers are discussed, their working principle and automation through computerization. The chapter also gives the theoretical background on deposition techniques of thin films and an insight to optical properties of thin films when they interact with light.
3.2 Spectroscopy
Spectroscopy is a scientific technique which uses absorption, emission or scattering of electromagnetic radiation by a material to investigate the physical properties. It is a scientific field employing remote sensing mechanism to study properties of materials, surfaces, planets, stars or chemical compounds of a substance (Crouch, 2007).
When light is incident on atoms or molecules it excites them to higher energy levels. The type of excitation depends on the wavelength of light. Electrons are promoted to higher orbitals by ultraviolet or visible light, vibrations are excited by infrared light, and rotations are excited by microwaves.
An absorption spectrum is the absorption of light by a material as a function of wavelength. The absorption spectrum depends on atomic and molecular structure of a substance. The excited electrons can equally fall back to lower energy levels emitting the excess energy through fluorescence or phosphorescence. Fluorescence occurs when the
12 transition is between states of the same spin while phosphorescence occurs if the transition is between states of different spin.
When electromagnetic radiation passes through matter, most of the radiation continues in its original direction (transmitted) but a small fraction is scattered in other directions.
Light that is scattered at the same wavelength as the incoming light undergo Rayleigh scattering. Light scattering in transparent solids due to vibrations (phonons) is called
Brillouin scattering. In Brillouin scattering light is typically shifted by 0.1 to 1 cm-1 from the incident light. Light that is scattered due to vibrations in molecules or optical phonons in solids experiences Raman scattering. Raman scattered light is shifted by as much as
4000 cm-1 from the incident light. Therefore to study lattice structure of a substance
Raman spectroscopy is used to probe a material sample.
Light which is not transmitted, absorbed or scattered is reflected. To obtain useful information about a material, it is important that a material is investigated using a wide spectrum of electromagnetic (E.M) radiation. Optical properties of materials can be comprehensively examined by optical spectroscopy while lattice structure is best investigated with Raman or x -ray spectroscopy.
3.3 Optical Spectrometers
Spectrometers are instruments used to measure reflection or transmission of incident light falling on a sample under investigation to determine physical properties of a material sample. Classification of spectrometers is based on measurement technique they use and the range of electromagnetic (E.M) spectrum utilized in measurement. Spectrometers can
13 therefore be classified as ellipsometers or spectrophotometers using the measurement technique criterion (James and Sternberg, 1969).
Visible range optical spectrometers investigate materials using visible range of electromagnetic (E.M) spectrum while Infra- red spectrometers work with infra-red radiation part of the electromagnetic (E.M) spectrum. Ellipsometers employ the analysis of phase change and polarization state of light reflected or transmitted by a sample to determine physical properties of a sample. Spectrophotometers on the other hand use normally incident light to the sample and the reflected or transmitted light from it to characterize a sample.
3.3.1 Ellipsometers
Ellipsometers employ the analysis of phase change and polarization state of light reflected or transmitted by a sample to determine physical properties of a sample. It is a very sensitive measurement technique and provides unequalled capabilities for thin film characterization. As an optical technique, ellipsometry is non-destructive and contactless.
It is commonly used to characterize film thickness for single layers or complex multilayer stacks ranging from a few angstroms or tenths of a nanometer to several micrometers with an excellent accuracy.
A typical ellipsometer consists of a source of light, two polarizers and compensators, and a photo-detector, as shown in figure 3.1.
14
Figure 3.1: Schematic diagram of an ellipsometer showing phase change and
polarization state of incident light upon reflection by a thin film sample
(Tompkins and McGahan, 1999).
Light from the source goes through the first polarizer which separates the parallel and perpendicular component of the light source. To ensure that the two polarized light components are linearly polarized, the light is passed through a phase modulator
(compensator), before being reflected by a sample to the second compensator (phase modulator) which modulates the phase angle of the reflected light to be linearly polarized. The second polarizer (analyzer) is rotated until the intensity of reflected light received by the photo - detector is minimum. Using the phase shift of the first polarizer and amplitude diminutions upon reflection, the thickness and optical properties of a thin film are calculated using an appropriate model (Tompkins and McGahan, 1999).
When ellipsometry is to be done on a wide spectrum then the laser source is replaced by a monochromator which selects a specific frequency light for probing a thin film sample.
This technique is insensitive to scatter and fluctuations, and requires no standard sample or reference beam.
15
Ellipsometry measures two of the four Stokes parameters: The wave amplitude, Ψ and phase shift, Δ to determine change in polarization. Alternatively, the polarization state of the light incident upon the sample is decomposed into s and p components (the s component oscillates perpendicular to the plane of incidence and parallel to the sample surface, and the p component oscillates parallel to the plane of incidence). The amplitudes of the s and p components, after reflection are normalized to their initial value, denoted by rs and rp, respectively to determine change in polarization, ρ as defined by the fundamental equation of ellipsometry, equation (3.1).
(3.1)
Where, tanΨ is the amplitude ratio upon reflection, Δ is the phase shift (difference) and ρ is the change in polarization. Since ellipsometry is measuring the ratio (or difference) of two values (rather than the absolute value of either), it is very robust, accurate, and reproducible.
3.3.2 Visible Range Optical Spectrophotometers
Visible range optical spectrophotometers use visible light to investigate physical properties of materials. They utilize a simple measurement procedure unlike the ellipsometer. The optical spectrophotometer consists of a broad band source of visible light, usually an incandescent lamp. To select the wavelength of electromagnetic radiation within the visible range (370 – 780 nm) to use in investigating a material, light is passed through a diffraction grating which separates it into monochromatic lights. The
16 angle between monochromatic lights can be determined using equation 3.2
(Lipson, 1996).
푑푠푖푛휃 = 푛휆 (3.2)
Where d is the grating constant, n is the order of the fringe, λ is the radiation wavelength and θ is the angle of inclination of a monochromatic light to the principal maxima
(fringe). Position of a given monochromatic light in the first order fringe on a screen is given by equation 3.3.
푦 = 퐷푡푎푛−1휃 (3.3)
Where, D is the separation distance between the diffraction grating and the screen and y the position of the monochromatic light from the central maxima.
A collimator (concave mirror) can be used to incident monochromatic light on an aperture (a narrow slit) in order to produce a monochromatic light used to study material samples. The collimator reflects the incident electromagnetic spectrum on an aperture by rotating. The reflected spectrum rotates through twice the angle the mirror rotates.
Therefore resolution wavelength of monochromatic light of spectrometer is determined by the angle at which the reflected spectrum rotates (twice the size of mirror rotation angle) and the accuracy level of the motor used.
17
Figure 3.2: Schematic diagram showing components of an optical spectrophotometer
(Allen, 2010).
Alternatively, using the adjustable aperture, the optical spectrophotometer selects the monochromatic visible light to be incident normally to the sample and the relative light intensity of transmitted beam before and after a test sample is inserted are measured using a photo-detector as shown in figure 3.2. The photo detector converts the light intensities into electrical signals which are amplified, computation of the optical parameters done by the microprocessor or a computer and output displayed (Allen, 2010).
The optical technique measures transmittance through a material sample and using Beer –
Lambert law, absorption coefficient and other optical constants are calculated and determined.
3.4 Spectrometer Automation
To automate spectrometers, several bus interfaces are used to connect the spectrometer to a computer for control, data acquisition, analysis and storage. These bus interfaces
18 include; RS -232 serial port, universal serial bus (USB), parallel printer port, general purpose interface bus (GPIB) or interface cards among others. The choice of the interface to be used is guided by data transmission speed, cost and availability. Enhanced parallel port interface (EPP) guarantees bidirectional high speed data transmission (2 Mbps), it’s cheap and common. These attributes make enhanced parallel port a good choice in interfacing spectrometers to computers.
3.5 Enhanced Parallel Port (EPP)
The EPP (enhanced parallel port) was originally developed by chip maker Intel, PC manufacturer Zenith and Xircom (a maker of parallel-port networking products). The data lines are bidirectional. An EPP can therefore read or write a byte of data in one cycle of the industrial standard architecture (ISA) expansion bus, or about 1 microsecond, including handshaking. An EPP can switch directions quickly, so it's very efficient when used with disk and tape drives and other devices that transfer data in both directions.
Table 3.1 shows the pin assignment for D-type 25 pin female connector of enhanced parallel port (EPP):
To access enhanced parallel port three registers are used; base register (378h), status register (379h) and the control register (37ah). The base register (data register) is used in data transfer; Status register (read only) shows the status and the control register is used by the computer to control the slave instrument.
19
Table 3.1: Pin assignment for D-Type 25 Pin female connector of Enhanced Parallel Port (EPP).
Pin No EPP Signal Direction Register Hardware (D-Type 25) In/out Inverted
1 Strobe In/Out Control Yes 2 Data 0 In/Out Data 3 Data 1 In/Out Data 4 Data 2 In/Out Data 5 Data 3 In/Out Data 6 Data 4 In/Out Data 7 Data 5 In/Out Data 8 Data 6 In/Out Data 9 Data 7 In/Out Data 10 Ack In Status 11 Busy In Status Yes 12 Paper-Out In Status Paper End 13 Select In Status 14 Outo Linefeed In/Out Control Yes 15 Error In Status 16 Initialize In/Out Control 17 Select-Printer In/Out Control Yes 18-25 Ground Gnd
Output signals of the parallel port are defined as typical transistor - transistor logic (TTL) level signals. High level is defined by voltage from 3.5V to 5.5V and low level from 0V to 0.4V. Different values of maximal load currents of the parallel port are done by physical realization of the port. Maximal current taken from pins should range from 4mA to 20mA. Therefore it's better to use a buffer between personal computer (PC) and peripheral device.
20
To prevent the enhanced parallel port from being damaged inputs should be brought only between 0V and 5 V. Figure 3.3 shows the pin out diagram of an enhanced parallel port.
In enhanced parallel port (EPP) mode read or write command initiates automatically handshake between the instrument and the computer, hence the port is bidirectional.
Figure 3.3: Pin-out diagram for an enhanced parallel port (EPP) (Jan, 1996).
3.6 Deposition of Thin Films
Thin film deposition is done in a reduced pressure environment. There are two common techniques used in deposition of thin films; sputtering and thermal evaporation.
Sputtering involves dislodging atoms from a target using ionized gas (argon) which are then accelerated by electric field to the substrate where they form a thin film.
Thermal evaporation technique entails heating a sample to evaporation which is followed by deposition of the gas on to the substrate to form a thin film.
21
3.6.1 Deposition of Thin Films by Thermal Evaporation
In evaporation method, the material sample is placed in a molybdenum boat, heated thermally to evaporation before deposition to the substrate. The rotating substrate holder ensures that the thin films formed are homogeneous and of constant thickness through constant angular revolutions during the deposition process. The partial vacuum created in the chamber during deposition improves the deposition process by reducing obstacles in the path of gas atoms as they move from the source to the substrate. The chamber is also securely closed to minimize leaking gas from the environment affecting the thin film formed at the substrate as an impurity.
The deposition process starts when the gas atoms are allowed into the substrate by the opening of the shutter, as shown in figure 3.4. The working gas (argon) ensures that the required operation pressure is maintained with the help of the vacuum pumping mechanism which reduces pressure when it increases beyond the required value.
Purity of the thin films to be formed is important and it affects their properties. To minimize chances of formation impure thin films, the evaporation chamber is cleaned and heated to higher temperatures (500 oC) to ensure that all reactive gases are removed from the chamber before the process of evaporation starts.
22
Substrate holder Working gas Cathode Substrate
Moveable shutter
Vacuum chamber
Vacuum pump
Molybdenum boat Material sample
Figure 3.4: Diagram showing principal parts of a system used in thermal evaporation
deposition of thin films (Ohring, 1992).
3.6.2 DC Magnetron Sputtering
In sputtering fast moving ions bombard a target cathode dislodging atoms from it’s surface. The dislodged atoms are then accelerated by anode voltage to the substrate where they are mounted forming a thin film coating. Argon gas is mostly used as a sputtering or working gas because it is an inert gas (Ohring, 1992). Sputtering is of four types; DC, RF, magnetron and reactive sputtering. RF sputtering is used in making dielectric films while
DC magnetron sputtering is used to make conducting and doped semiconductor thin films
(figure 3.5).
23
Insulation
Cathode (target)
Substrate
Anode
Vacuum pump
Working gas Insulation
Figure 3.5: Schematic diagram showing essential components of a DC sputtering system
(Ohring, 1992).
In DC magnetron sputtering, strong magnetic fields aligned parallel to the substrate is placed at the target to hold and whirl electron plasma close to the target (magnetron) in helical paths corresponding to magnetic field lines. This pool of electrons ionize the working gas (argon) to argon gas ions, Ar+ which are then accelerated to the highly negative target to dislodge atoms from it’s surface. The ejected atoms are then mounted onto the substrate, forming a thin film. When a compound thin film coating is required, a reactive gas like oxygen or nitrogen is used to bombard the dislodged target atoms.
24
3.7 Optical Properties of Thin Films
When light interacts with a thin film some light is reflected, transmitted, absorbed or scattered. The total incident light energy to a thin film is therefore given by equation
(3.4).
S + T + A + R = 1 (3.4)
Where S is the portion of incident light intensity which is scattered, T is part of light intensity transmitted, A is a fraction of light energy absorbed by a thin film and R is part of incident light reflected from the thin film (Frederick, 1972; Sidney, 1993).
Optical properties of thin films, refers to behavior of thin films as they interact with light.
These characteristics include reflectance, transmittance, refractive index, permittivity, absorption coefficient, band gap among others.
3.7.1 Reflectance, (R)
Reflectance refers to the ratio of incident light intensity which bounces back from a surface. At normal incidence of light to a thin film, reflectance, R is given by equation 3.5.
퐼푟 푅 = (3.5) 퐼0
Where 퐼푟 is intensity of reflected light and 퐼0 is the intensity of the incident light. From reflectance measurement of a thin film sample, other optical properties of the sample can be obtained as discussed in later sections.
25
3.7.2 Real Refractive Index, (n)
Real refractive index is the ratio of velocity of light in a vacuum to phase velocity of light in a material. It therefore indicates the phase velocity at which light traverses through a material. In normal dispersion, refractive index increases with frequency of incident radiation. At sharp absorptions of radiation energy by a material, real refractive index deviates from normal dispersion and experiences a reduction which is anomalous dispersion with increase in radiation frequency (Carisson, 2007).
Using reflectance measurement, R, in Kramers – Kronig transform (Shimadzu, 2008), the phase angle, ∅ upon reflection of light by a thin film is given by equation 3.6.
푙푛 푅 휔푗 4ℎ휔𝑔 ∅ 휔𝑔 = 푗 2 2 (3.6) 휋 휔푗 −휔𝑔
Where h = 휔j+1 - 휔j and if data interval g is an odd number then j = 2,4,6,…,g-1,g+1,…, while if g is an even number then j = 1,3,5,…,g-1,g+1,…, and 휔𝑔 is the radiation energy at which phase angle is determined.
For normally incident light coming from air (real refractive index, n = 1 and imaginary refractive index, k = 0) into a thin film sample, Fresnel reflectance, R and phase angle, ∅ can be used to calculate the real refractive index, n as shown by equation 3.7.
1−푅 푛 = (3.7) 1+푅−2 푅 푐표푠∅
26
3.7.3 Extinction Coefficient, (k)
Extinction coefficient, k is also known as the imaginary refractive index. It indicates the extent of radiation energy absorption by a material. Using the reflectance, R and phase change upon reflection ∅ for normally incident light from air to a material, extinction coefficient, k can be obtained by equation 3.8.
−2 푅 푠푖푛∅ 푘 = (3.8) 1+푅−2 푅 푐표푠∅
3.7.4 Real Dielectric Constant, (휺ퟏ)
The electromagnetic field of the incident radiation affects the orientation of positive and negative charges in a thin film. The electric current resulting from this field constitutes conduction and dipole displacement current. Real dielectric constant is also called real permittivity. Real permittivity indicates the radiation energy transmitted through a material medium.
Real permittivity, 휀1 is related to real refractive index, n and extinction coefficient, k by equation 3.9.
2 2 휀1 = 푛 − 푘 (3.9)
3.7.5 Imaginary Dielectric Constant, (휺ퟐ)
Imaginary dielectric constant is also known as imaginary permittivity, indicates the radiation energy absorbed by a material. The absorbed energy is typically taken up by electrons in the valence band to jump to conduction band and start conducting,
27 constituting electrical energy, scattered or used to heat a material. Imaginary dielectric constant, 휀2 can be calculated using real refractive index, n and extinction coefficient, k as indicated by equation 3.10.
휀2 = 2푛푘 (3.10)
3.7.6 Absorption Coefficient, (α)
When light traverses through a material, some energy is absorbed. To measure the extent of radiation energy absorption by a material, absorption coefficient, α is determined as shown in equation 3.11.
2휔푘 4휋푘 훼 = = (3.11) 푐 휆
Where c is the speed of light in a vacuum, ω is the cyclic frequency, λ is the radiation wavelength and k is the extinction coefficient.
3.7.7 Determination of Thickness of a Thin Film, (x)
Using reflectance, R thin film thickness, x can be approximated using equation 3.12.
푅 −푅 푙푛 푚푎푥 푚푖푛 푅−푅 푋 = 푚푖푛 (3.12) 2훼
28
Where x is the sample thickness, R is the reflectance for any intermediate photon energy,
푅푚푎푥 is the maximum reflectance and 푅푚푖푛 is the minimum reflectance (Kedar, 2006).
3.7.8 Transmittance, (T)
Transmittance refers to the ratio of incident light intensity which is transmitted through a material. It can be determined using Beer Lambert’s law as shown in equation 3.13.
푇 = 푒−∝푥 (3.13)
Where T is transmittance, α is the absorption coefficient and 푥 is the thickness of a thin film.
3.7.9 Band Gap, (Eg)
Semiconductor thin films can be crystalline or amorphous. Crystalline semiconductor thin films have regular arrangement of particles (molecules) in space to create regular shaped crystals. Crystalline semiconductor thin films can be of direct or indirect band gap.
3.7.10 Direct Band Gap
A direct band gap occurs when the valence band maximum and the conduction band minimum have the same momentum. The absorption coefficient, 훼 of a semiconductor with a direct band gap obeys equation (3.14) in the region near the band gap
(Kedar, 2006).
29
ħ휔−퐸𝑔 훼 ∝ (3.14) ħ휔
-34 Where ħ is reduced Planck’s constant = 1.0536 x 10 Js, Eg is the band gap and 휔 is the cyclic radiation frequency. Rearranging equation (3.14) we obtain equation (3.15).
2 훼ħ휔 = ħ휔 − 퐸𝑔 (3.15)
Plotting a graph of photon energy, ħ휔 against (훼ħ휔)2 and extrapolating the straight line portion near the edge of the fundamental transition to (훼ħ휔)2 = 0, the direct band gap is obtained by reading the intercept along the energy axis.
3.7.11 Indirect Band Gap
Indirect band gap is experienced when the valence band maximum and conduction band minimum are at different momentum or wave vector. When an electron in the valence band of an indirect band gap semiconductor gains energy above band gap it uses this energy to overcome the band gap and change it is momentum to reach the conduction band minimum where it can freely conduct.
The absorption coefficient, 훼 of indirect band gap semiconductor is given by equation
(3.16).
2 ħ휔−퐸𝑔 훼 ∝ (3.16) ħ휔
Rearranging equation (3.16) yields equation (3.17).
30
훼ħ휔 = ħ휔 − 퐸𝑔 (3.17)
Plotting radiation energy, ħ휔 against 훼ħ휔 and extrapolating the linear portion near the edge of the fundamental transition to 훼ħ휔 = 0, the intercept along the energy axis gives the value of indirect band gap to be determined.
3.7.12 Band Gap of Amorphous Semiconductor Thin Films
In amorphous semiconductor thin films, the band gap is blurred very much because there are many quantum mechanical energy levels outside the actual conduction and valence bands. The absorption coefficient is very small for photon energy below band gap and increases rapidly beyond 104 cm-1 for radiation energy above the band gap. Therefore the band gap of amorphous semiconductor thin film is defined as the energy at which the absorption coefficient, 훼 = 104 cm-1 (Grolik and Kopp, 2003).
3.7.13 Energy Absorption Factor
Energy absorption factor (E.A.F) is the ratio of radiation energy absorbed by a material to that which is transmitted. A material with a high E.A.F is good in absorbing radiation energy and poor in transmission. Energy absorption factor (E.A.F) is given by equation
3.18.
휀2 퐸. 퐴. 퐹 = (3.18) 휀1
Where, 휀1 is real permittivity and 휀2 is the imaginary permittivity.
31
CHAPTER 4
EXPERIMENTAL PROCEDURES
4.1 Introduction
In this chapter the hardware and software components of the designed optical spectrometer are discussed. To test the workability of the instrument CuO and SnSe thin film samples were used. In this section the preparation processes of these films are explained. The measurement procedure is finally highlighted and methodology used to determine optical properties of the thin films given.
The designed portable optical spectrometer set up diagram is as shown in figure 4.1. The hardware comprises of the powering system, diffraction system, stepper motor, photo- detectors and analog to digital converter as discussed in the next sections.
Lamp Convex lens Transmission grating Powering System 24 Vdc
Concave
5 Vdc mirror Thin film
Photo Slit Detector Stepper motor
Analog to digital Computer Converter
Figure 4.1: Set up diagram showing hardware components of the portable spectrometer
designed in this work.
32
4.2 Hardware Design
The designed portable spectrometer consists of the following components:
(i) Powering system;
(ii) Diffraction system;
(iii)Stepper motor;
(iv) Photo-detectors;
(v) Analog to digital converter.
4.2.1 Powering System
It consists of the power source (240 Va.c), 240 V to 48 V step down transformer, full wave rectifier, two capacitors, LM317 power regulator and two biasing resistors.
The power indicator and the lamp draw power directly from 240 Va.c. Stepper motor, the clock, analog to digital converter and other electronic components used to automate machine operations require small amount of direct current and voltages. A step down transformer (240V to 48V), full wave rectifier and power regulator were used to rectify alternating current voltage and stabilize it as shown in figure 4.2 and 4.3.
Figure 4.2: Powering system used to rectify a.c voltage for the portable spectrometer
designed in this work.
33
Fuse (250V, 2A) Rectifier diodes Power input
Transformer (240V to 48V)
Figure 4.3: A picture of powering system of the portable spectrometer designed in this
work.
In the circuit set up the maximum current required was about 1 A and to smoothen the rectified voltage, 680 µF filter capacitor was used, a value obtained by an approximation given by equation 4.1.
퐼푇 푐 = (4.1) 퐸
Where I = direct current, T = half cycle time, E = a.c voltage component in the rectified voltage, c = capacitance of the smoothening capacitor.
34
LM317T power regulator was used to stabilize the corrected voltage as to the power requirement of the electronic components in the control circuitry. The power output of the regulator was estimated closely using equation 4.2.
푅2 푉표 = 1.25푉퐷퐶 1 + (4.2) 푅1
Where Vo = stable output direct current voltage, 1.25 is the converting factor, R1 = 100Ω resistor, R2 = determinant resistor which regulates Vo.
4.2.2 Diffraction System
The diffraction system comprised of a fluorescent lamp (250V, 25W) as a source of a wide spectrum (370 – 780 nm). A convex lens (f=5 cm) which produced a parallel beam and focused it onto the diffraction grating to obtain the visible spectrum. The monochromatic lights obtained were separated from each other at an angular displacement determined by the wavelength of the monochromatic light and the grating constant (1200 lines/mm). This spectrum was then incident onto a concave mirror
(f=5 cm) which was controlled by a set of gears (reduction ratio 50:1) driven by a stepper motor, figure 4.1 and 4.4. In this work, first order fringes were used because of their high intensity as compared to the higher order fringes. This ensured that the incident and reflected light intensities were easily detectable by the photo-detectors.
35
Lamp (25W, 240V)
Diffraction Concave mirror grating Lens (f=5 cm) (f=5 cm) (1200lines/mm)
Figure 4.4: A picture of diffraction system of the portable spectrometer designed in this
work.
4.2.3 Stepper Motor
The focusing concave mirror was mounted to a unipolar stepper motor power rated
(500mA, 24V) which was used to control small angle rotations of the mirror. The
ULN2003 driver interfaced the stepper motor to the computer for control. Four control pins (C0, C1, C2, C3) of an enhanced parallel port were utilized in half stepping the motor, figure 4.5. The resolution of the motor used was 7.5o in single stepping mode as shown in specification chart in appendix III. Half stepping mode ensured that angle rotations of
3.75o were achieved. Half stepping mode of the rotor involved the mix of single stepping mode and high torque mode.
36
Figure 4.5: Diagram of circuit used to interface unipolar stepper motor to the enhanced
parallel port in the portable spectrometer designed in this work.
Table 4.1 summarizes the half stepping mode procedure using the four coils: A1; B1; A2 and B2 of the unipolar stepper motor (PM35S-048).
Table 4.1: Unipolar stepper motor half stepping mode procedure.
Pulse Coil A1(yellow) Coil B1(black) Coil A2(orange) Coil B2(brown)
1 ON
2 ON ON
3 ON
4 ON ON
5 ON
6 ON ON
7 ON
8 ON ON
37
4.2.4 Photo-detectors
An aperture allowed light to be incident on the first photo – detector and a thin film sample. Specific monochromatic light was focused at a time by a rotation of a concave mirror of focal length 5 cm through small angles (3.75o/50) using a set of gears and a stepper motor. Two silicon photo – detectors were used in this set up as shown in figure
4.6. The first one was placed at the same location as the thin film sample from the monochromatic source to record intensity of the incident light. The second was placed 3 cm from the sample along the optical path of the reflected monochromatic beam focused such that it trapped the reflected light intensity. The two photo - detectors then converted the light intensities to electrical signals as an input to ADC to produce digital output.
From these two readings, incident and reflected monochromatic light intensities, reflectance of the thin film samples were calculated at various specific radiation wavelengths within the visible range of E.M spectrum.
2.5 V
ADC0809CCN 26 25 IN0 ADD A 27 24 IN1 ADD B 28 23 IN2 ADD C 1 22 IN3 ALE 2 IN4 3 21 IN5 2 -1MSB 4 20 IN6 2 -2 5 19 IN7 2 -3 18 2 -4 6 8 22K 22K START 2 -5 7 15 EOC 2 -6 9 14 OUTPUT ENABLE 2 -7 10 17 CLOCK 2 -8LSB 11 13 VCC GND 12 VREF(+) VREF(-)
Figure 4.6: Circuit diagram of photo-detectors for sensing light intensity of the portable
spectrometer designed in this work.
38
4.2.5 Analog to Digital Converter (ADC0809CCN)
The analog to digital converter was powered on by sending a high logic “1” through pin 5 of the parallel port, which switched on the relay switch controlling the converter voltage.
Figure 4.7 shows the pin out diagram for ADC0809CCN.
Figure 4.7: ADC0809CCN Pin out diagram.
The ADC converted the variable analog voltages into digital form to be stored in a computer through the parallel port for analysis. The multiplexer option enabled selection of the analog input to be converted by setting address latch enable (ALE) high. In this set up two analog inputs were used hence all the address pins (ADD A, ADD B, ADD C) were tied together and connected to pin 2 ( data pin 0 ) of the parallel port, as illustrated in figure 4.8.
39
Figure 4.8: Complete circuit diagram showing the diffraction system, photo-detectors
and electronic circuitry of the portable spectrometer designed in this work.
Selecting incident analog input IN0, a low “0” was sent through pin 2. The reflected monochromatic light (input IN7) was selected by setting pin 2 high “1”, as shown in table
4.2. Start Conversion (SC) pin was set high and then low to sample analog input and start conversion.
40
Table 4.2: Input states for the address lines (A, B and C) used to select analog input
channel into analog to digital converter (ADC0809CCN).
SELECTED ANALOG ADDRESS LINE CHANNEL C B A IN0 L L L IN1 L L H IN2 L H L IN3 L H H IN4 H L L IN5 H L H IN6 H H L IN7 H H H
According to the manufacturer’s specification ADC requires a delay of 200 µs to convert analog signals to digital form before setting Output Enable (OE) high for reading by the computer port. This procedure was then repeated for all selected analog inputs. The timing diagram for the conversion process was as shown in figure 4.9.
4.3 Software Design
4.3.1 Hardware Interfacing
The enhanced parallel port was used to interface the portable spectrometer designed in this work with a computer for control and data acquisition. The dialogue between the spectrometer and the computer was initiated using enhanced parallel port (EPP) mode communication protocol (IEEE 1284-A). This allowed bidirectional data transfer between the computer and the spectrometer as shown in figure 4.8.
41
Figure 4.9: Timing diagram for analog to digital converter (ADC0809CCN).
42
Sliding window Spectrometer
Computer Enhanced parallel port cable Power cable
Figure 4.10: Hardware interfacing between the portable spectrometer designed in this work and a personal computer using enhanced parallel port (EPP).
4.3.2 MSVC++ 6.0 Portable Spectrometer Driver Program Designed in this Work
The portable spectrometer driver program was developed using Microsoft Foundation
Classes (MFC6.0) in VC++ programming environment. VC++ language has extended capabilities in programming and therefore it was chosen in the driver programming.
Figure 4.11 shows a flow chart diagram to summarize the steps which were followed logically during the execution of the portable spectrometer driver program:
43
Start
No Inpout 32.dll Is Inpout 32.dll loaded? not loaded!
Yes
Portable spectrometer No is not connected! Portable spectrometer connected?
Yes Initialize port registers & measure reflectance
Store data in optica4.txt file
Optica4.txt file
Stop
Figure 4.11: Flow chart diagram for the portable spectrometer driver program.
The program starts by checking whether the dynamic link library for accessing computer hardware (Enhanced parallel port), Inpout32.dll is installed, if not an error message
“Inpout32.dll not loaded” is displayed before it stops. When Inpout32.dll is installed the program checks whether the portable spectrometer designed is connected and it stops if the spectrometer is not connected, displaying “Portable spectrometer is not connected”.
When the portable spectrometer designed is connected, the program initializes enhanced
44 parallel port registers, measures reflectance, stores data in “optica4.txt” file and displays the data in MS DOS windows before stopping. To measure reflectance, four control pins of the parallel port were utilized to achieve the minimum angle of rotation
(7.5o ÷ 2=3.75o) in a half stepping mode for the stepper motor with step angle resolution of 7.5o. Half stepping mode involved a mix of single and high torque stepping modes.
Eight pulses therefore were required for half stepping mode and then the process was repeated to attain the required angle of rotation. Each pulse gave the stepper motor a rotation of 3.75o. The binary code generated takes into account the half stepping mode and the fact that all control pins except C2 are low enable. To rotate the stepper motor through 30o (8 steps) therefore a decimal code: 10, 14, 15, 13, 9, 1, 3, 2 in this order was written to the control port (890). To reset the rotation a decimal code: 3, 1, 9, 13, 15, 14,
10, 2 was written to the control port as shown in table 4.3. The analog to digital converter was controlled by the first three pins of the data port. All the data pins were buffered to enable the computer read data output, ensure unidirectional flow of data from the
ADC0809CCN and prevent code mixing in controlling the converter.
45
Table 4.3: Binary and decimal code generated by half stepping mode of the unipolar
stepper motor.
Pulse C3 C2 C1 C0 Decimal.
1 1 0 1 0 10
2 1 1 1 0 14
3 1 1 1 1 15
4 1 1 0 1 13
5 1 0 0 1 9
6 0 0 0 1 1
7 0 0 1 1 3
8 0 0 1 0 2
4.4 Preparation of Sample Thin Films
To test the workability of the portable spectrometer designed in this work, thin film samples (CuO and SnSe) were prepared and used to measure the accuracy of the instrument by comparing it’s results with the standard instrument (SolidSpec-3700).
4.4.1 Preparation of CuO Thin Film
CuO thin film was prepared by reactive magnetron sputtering using Edward AUTO 306.
The silicon glass slide was washed in clean water, rinsed in distilled water and methanol before being dried in the sun for 20 minutes. The glass slide was covered at the ends
46 using aluminium foil to prevent sputtering at the edges. It was then mounted to the substrate holder using clips. Copper target was mounted on the magnetron, maintaining a separation distance of 15 cm between the substrate and the target. The shutter and the chamber were then closed. Argon gas was used as the working gas at the flow rate of
20 sccm, oxygen was used as the reactive gas at a flow rate 5 sccm and other parameters were set as shown by table 4.4. Pre- sputtering was done for ten minutes to remove contaminated copper layer. The shutter was then open to allow dislodged copper atoms react with oxygen to form CuO which was then mounted on the substrate.
Table 4.4: Deposition parameters for CuO thin film used to test portable spectrometer
designed in this work.
Thin Film Argon Oxygen Base Sputtering Power Chamber
Flow Flow Pressure Pressure (W) Temperature
(sccm) (sccm) (mbars) (mbars) ( o C)
CuO 20 5 3 x 10-5 9 x 10-3 200 27
4.4.2 Deposition of SnSe
Deposition of tin selenide (SnSe) was carried out using evaporation technique by
Edwards AUTO 306. SnSe compound was prepared by mixing tin (Sn) and selenide (Se) in the ratio 1:1 by mass. This mixture was then put into a boiling tube and argon gas blown into the mixture to displace air from the tube while shaking the tube contents
47 during melting process to obtain a homogeneous melt. Deposition chamber was closed and baking was done for ten minutes by heating the boat at a temperature of 500 oC to remove air in the evaporation chamber.
Using a cleaned glass slide aluminium foiled at the edges as the substrate and the SnSe as the evaporated sample in the molybdenum boat thermal evaporation was carried out. The chamber was closed, pressure reduced to base pressure of 3 x 10-5 mbars, heating current set at 3.5 A and the boat temperature maintained at 750 oC, which was enough to melt the
SnSe compound. The deposition parameters are as shown in table 4.5.
Table 4.5: Deposition parameters for SnSe thin film used to test portable spectrometer
designed in this work.
Thin Film Tin (Sn) Selenide (Se) Deposition Base Pressure
Concentration Concentration Temperature (mbars)
by mass (%) by mass (%) (oC)
SnSe 50 50 750 3 x 10-5
The shutter was then open to allow SnSe vapour to the substrate, which on cooling formed SnSe thin film. Constant rotation of the substrate holder during deposition process ensured uniform composition and thickness of the thin film formed, this process took about 40 minutes. At the end of the process, heating current was switched off and the chamber opened after an hour to allow cooling which reduced chances of oxidation.
48
4.5 Measurement Procedure of a Portable Spectrometer Designed in this Work
The experiment was carried out in a black box to minimize the effects of ambient light as a source of secondary reflections in the visible range. The measurement procedure was performed in three steps as follows:
a) Calibration of the light source. This was a preliminary calibration to measure the intensity of light source. The significance of this calibration was to monitor the fluctuations of incident light due to aging as well as the accurate determination of the optical path. The stepper motor drove a set of gears with reduction ratio 50:1 on which a concave mirror was mounted. The gears further reduced the rotation angle of the motor by a scale of 50; (3.75o÷50) = 0.075o. The wavelength of the monochromatic light investigating a sample was selected by the concave mirror rotating the reflected spectrum through constant angle of (2x0.075o) = 0.15o in every step the motor rotated. Using equation 3.2 and 3.3, the resolution wavelength of monochromatic light incident on a sample was determined (2 nm ± 5%). The 5% error was the accuracy error of the stepper motor used. Silver mirror was used to calibrate the source of light since it has a known reflectance profile in the visible range of E.M spectrum. It was also used to check the alignment of optical components to determine whether they were in focus.
b) Measurement. Once the calibration had been done a semiconductor thin film was inserted into the sample stage, the mono-chromatic light was incident at near normal
(θ < 12o) on the first photo-detector and the thin film sample. The intensity of both
49 incident and reflected monochromatic light was then measured using the two photo- detectors and reflectance profile as a function of wavelength obtained.
c) Data acquisition and analysis. The data obtained was transferred and stored in a computer via analog to digital converter through enhanced parallel port (EPP). The portable spectrometer driver program developed in this work was then used to compute reflectance profile and other parameters of CuO and SnSe thin films.
50
CHAPTER 5
RESULTS AND DISCUSSIONS
5.1 Introduction
To test the workability, reliability and level of accuracy of the portable spectrometer designed in this work, calibration was done using silver mirror, semiconductor thin film samples (CuO and SnSe) were prepared and their optical properties determined using reflectance measurement. Finally comparison on the reflectance measurement and band gap determined using both the portable spectrometer designed in this work and
SolidSpec-3700 was done to ascertain the level of accuracy of the designed instrument.
5.2 Instrument Calibration
To ensure correct focus of monochromatic light to the thin film sample and the photo- sensors, silver mirror was used because it has a known reflectance profile in the visible spectrum. Silver has good reflectance in the visible spectrum with reflectance above 0.9 and falls significantly in the near ultraviolet range, 380 nm ( Nebojsa, 2001). This was used for initial focus setting of the concave mirror, thus controlling the incidence of monochromatic beam on the film sample and the photo - detectors. Calibration enables the user to detect the variation in intensity of the light source due to aging by observing the changes in the reflectance profile of silver associated with the aging source. The reflectance measurements obtained were compared with silver reflectance graph
(standard) from the literature (Nebojsa, 2001) and focus adjustments made until there was a match, an indication that the portable spectrometer designed in this work was now ready for use in measurement. Figure 5.1 shows a graph of reflectance of silver from
51 literature (Standard), SolidSpec-3700 and the portable spectrometer designed in this work.
1.0
0.9
0.8
0.7 Standard (Nebojsa, 2001). 0.6 SolidSpec-3700. Portable spectrometer designed in this work.
0.5 Reflectanceof Silver mirror, R 0.4
0.3 350 400 450 500 550 600 650 700 750 800 Radiation Wavelength, (nm)
Figure 5.1: Graph of reflectance against radiation energy for silver mirror using the standard, SolidSpec-3700 and the portable spectrometer designed in this work.
The reflectance from SolidSpec-3700 and the portable spectrometer matches well with the standard curve. The difference in reflectance between 450 – 650 nm radiation range is attributed to variation in thickness and surface quality of the silver mirrors which were used.
52
5.3 Optical Properties of Copper Oxide (CuO)
5.3.1 Reflectance, (R) of CuO
Figure 5.2 shows that the reflectance of CuO reduces with increasing radiation wavelength. The peak at 500 nm can be attributed to inter- band energy transition in
CuO. The reflectance measurement by the portable spectrometer designed in this work correlates well with those obtained by SolidSpec-3700. However from 380 – 425 nm it deviates in reflectance measurement from SolidSpec-3700, this may be attributed to reduced sensitivity of the silicon photodiode in the near ultra violet region.
1.0 R for CuO using the Portable spectrometer designed in this work. 0.9 R for CuO using SolidSpec-3700. R for SnSe using the Portable spectrometer designed in this work. R for SnSe using SolidSpec-3700. 0.8
0.7
0.6
0.5 Reflectance, R 0.4
0.3
0.2
350 400 450 500 550 600 650 700 750 800 Radiation Wavelength, (nm)
Figure 5.2: Graph showing reflectance curves for CuO and SnSe thin films from data obtained by SolidSpec-3700 and portable spectrometer designed in this work.
53
The difference in reflectance from 680 – 770 nm is attributed to step position error of the stepper motor used in rotating the concave mirror to focus monochromatic light in the portable spectrometer designed in this work.
5.3.2 Phase Angle, (Φ) of CuO
During reflection the path of light changes, this change in direction of light is presented by the phase angle, Φ. The phase angle depends on the incident radiation energy and the transitions it experiences with the material to give rise to a reflected wave. The nature of a material also affects the magnitude and direction the reflected light takes. Figure 5.3 shows that the phase angle for CuO increased with increase in wavelength of the radiation energy. This is attributed to reduced radiation energy below CuO band gap energy causing lattice vibrations which increase the scattering angle of the reflected radiation. The difference in phase angle obtained by SolidSpec-3700 and the portable spectrometer designed in this work ( ±0.05) at 675 nm is because of large scattering angle due to lattice vibrations in CuO which are out of phase with radiation energy.
54
0.2 for CuO using Portable spectrometer designed in this work. 0.1 for CuO using SolidSpec-3700. for SnSe using Portable spectrometer designed in this work. for SnSe using SolidSpec-3700. 0.0
-0.1 -0.2
-0.3
PhaseAngle, -0.4
-0.5
-0.6
-0.7 350 400 450 500 550 600 650 700 750 800 Radiation Wavelength, (nm)
Figure 5.3: Graph showing the phase angles for the CuO and SnSe thin films from data obtained by SolidSpec-3700 and portable spectrometer designed in this work.
5.3.3 Real Refractive Index, (n) of CuO
Real refractive index for CuO remained low in the entire visible range of electromagnetic spectrum (380 -780 nm) an indication that it remains transparent within this range of radiation energy. The values of real refractive index obtained by the designed portable spectrometer in this work related well with those by SolidSpec-3700 as shown in figure
5.4.
55
250 n for CuO using Portable spectrometer designed in this work. for CuO using Portable spectrometer designed in this work. n for CuO using SolidSpec-3700. 200 for CuO using SolidSpec-3700.
150
100 RefractiveIndex of CuO 50
0 350 400 450 500 550 600 650 700 750 800 Radiation Wavelength, (nm)
Figure 5.4: Graph showing real and imaginary refractive index of CuO thin film from data obtained by SolidSpec-3700 and portable spectrometer designed in this work.
5.3.4 Imaginary Refractive Index, (k) of CuO
Imaginary refractive index for CuO remained low in the visible range of electromagnetic spectrum. This means that the energy lost through scattering is minimal in this range for
CuO. The values for imaginary refractive index obtained by portable spectrometer designed in this work and SolidSpec-3700 matched well as shown in figure 5.4.
56
5.3.5 Real Permittivity, (ε1) of CuO
Real permittivity indicates the extent of absorption of radiation energy by a material.
for CuO using Portable spectrometer designed in this work. 4000 for CuO using Portable spectrometer designed in this work. for CuO using SolidSpec-3700. for CuO using SolidSpec-3700.
3000
2000
1000 Permittivityof CuO
0
400 450 500 550 600 650 700 750 800 Radiation Wavelength, (nm) Figure 5.5: Graph showing real and imaginary permittivity of CuO thin film from data obtained by SolidSpec-3700 and portable spectrometer designed in this work.
Figure 5.5 shows that CuO has a high absorption for radiation energy range 460 – 580 nm as indicated by the high real permittivity values in the range. The data values for real permittivity of CuO obtained by portable spectrometer designed in this work and
SolidSpec-3700 relate well. The difference in values obtained is attributed to the step
57 position error of the stepper motor and sensitivity of silicon photodiodes used in the portable spectrometer designed in this work.
5.3.6 Imaginary Permittivity, (ε2) of CuO
Imaginary permittivity indicate the extent of radiation energy scattering by a material.
The amount of scattered energy remained low in the entire visible part of electromagnetic spectrum as indicated by the low values of imaginary permittivity of CuO.
5.3.7 Energy Absorption Factor for CuO
for CuO using Portable spectrometer designed in this work. 7 for CuO using SolidSpec-3700. for SnSe using Portable spectrometer designed in this work. 6
) for SnSe using SolidSpec-3700.
( 5
4
3
2
1 Energy Absorption Factor,
0
400 450 500 550 600 650 700 750 800 Radiation Wavelength, (nm) Figure 5.6: Graph showing the energy absorption factor for CuO and SnSe thin films from data obtained by SolidSpec-3700 and portable spectrometer designed in this work.
58
Figure 5.6 shows that CuO absorbs more radiation energy than it losses from 430 – 580 nm radiation. It absorbs less and losses more in the radiation range 600 – 770 nm.
5.3.8 Absorption Coefficient, (α) of CuO
16
for CuO using Portable spectrometer designed in this work. 14 for CuO using SolidSpec-3700.
for SnSe using Portable spectrometer designed in this work. )
-1 for SnSe using SolidSpec-3700.
12
m 8
x 10 10
(
8
6
4
2 AbsorptionCoefficient,
0
400 450 500 550 600 650 700 750 800 Radiation Wavelength, (nm)
Figure 5.7: Graph showing the absorption coefficient for CuO and SnSe thin films from data obtained by SolidSpec-3700 and portable spectrometer designed in this work.
Figure 5.7 shows that the absorption coefficient for CuO increases with decrease of radiation wavelength. The peak at 530 nm is associated with the band gap of CuO. The
59 absorption coefficient values for CuO obtained using SolidSpec-3700 and the portable spectrometer designed in this work compare well with errors noted at the peak value and in radiation wavelengths below 450 nm. These are caused by the problems stated in section 5.3.5.
5.3.9 Band Gap, (Eg) of CuO
Figure 5.8 shows that a plotted graph of α2E2 versus radiation energy, E, was linear from radiation energy 2.2 eV, implying that CuO has a direct band gap. Extrapolating the linear region to where α2E2 = 0, the x- axis intercept to the line presents the direct band gap of CuO which equals to 2.19 eV ± 1% for both the portable spectrometer designed in this work and SolidSpec-3700 as shown in figure 5.8. To draw the linear extrapolation, three points in the linear region where used to minimize error. Accuracy error of the line drawn was estimated using the degree of accuracy used in the horizontal scale of the plotted graph. The horizontal scale used can enable reading up to 0.01 eV and this was the level of accuracy used in determining the band gap.
5.3.10 Validation of CuO Results
There was a maximum error in reflectance of 4.35 % in reflectance measurement from the portable spectrometer designed in this work to those from a standard spectrophotometer (SolidSpec-3700) as shown in table 5.1. The mean error was 1.88 % which is within the allowable limits of accuracy for reliable results. The reflectance measurement of CuO by the portable spectrometer designed in this work and SolidSpec-
60
3700 compared well, this is attributed to the high wavelength resolution achieved by small angular rotation of the focusing concave mirror (0.15o).
500
CuO curve for Portable spectrometer designed in this work. 450 CuO curve for SolidSpec-3700. Linear extrapolation to CuO curve for Portable spectrometer 400 designed in this work. Linear extrapolation to CuO curve for SolidSpec-3700.
350
-2 300
m
2 J
-21 250
x 10 200
2
E 2
150
100
50
0 1.6 1.8 2.0 2.2 2.4 Radiation Energy, E (eV)
Figure 5.8: Graph showing linear extrapolation to CuO curves used to determine band gap.
The error in reflectance measurement of CuO was caused by step position error of the stepper motor, low sensitivity of silicon photodiodes used in the region near ultra violet and thermal heating during measurement using the portable spectrometer designed in this work.
61
Table 5.1: Comparison between reflectance of CuO using portable spectrometer designed in this work and spectrophotometer (SolidSpec-3700).
Reflectance of CuO using Portable spectrometer Reflectance of CuO E(eV) designed in this work using SolidSpec-3700 (%) Error
773 0.2200 0.2300 4.35
688 0.5126 0.5118 0.15
619 0.5411 0.5337 1.42
563 0.6697 0.6553 2.17
516 0.7000 0.7000 0.00
476 0.6533 0.6767 3.45
442 0.5279 0.5191 1.69
413 0.7090 0.7000 1.28
387 0.7167 0.7000 2.38
Mean ± 1.88
5.4 Optical Properties of Tin Selenide (SnSe)
5.4.1 Reflectance, (R) of SnSe
Figure 5.2 shows that SnSe experiences interband energy transition near 650 nm radiation energy in which electrons from the valence band gain enough energy to move into the conduction band to start conducting, thus increasing reflection. The free electrons gain quantum energy to move into high allowable energy levels and emit this energy when they fall back to lower energy levels what causes a reflection peak near 475 nm.
62
5.4.2 Phase Angle, (Φ) of SnSe
Figure 5.3 shows that radiation scattering angle for SnSe increases with increase in radiation wavelength. Near 630 nm SnSe experiences secondary reflection
(reflection after absorprtion), the reflected wave is of a smaller energy than the radiation energy resulting in a large scattering angle.
5.4.3 Real Refractive Index, (n) of SnSe
13 12 n for SnSe using the Portable spectrometer designed in this work. for SnSe using the Portable spectrometer designed in this work. 11 n for SnSe using SolidSpec-3700. for SnSe using SolidSpec-3700. 10 9 8 7 6 5
4 RefractiveIndex 3 2 1 0
350 400 450 500 550 600 650 700 750 800 Radiation Wavelength, (nm)
Figure 5.9: Graph showing the real and imaginary refractive index for SnSe thin film from data obtained by SolidSpec-3700 and portable spectrometer designed in this work.
63
Figure 5.9 shows that the real refractive index for SnSe remained low in the visible range
(380 -780 nm) of the electromagnetic spectrum. SnSe thin film therefore is transparent in the visible range. The real refractive index data values correlated well for the portable spectrometer designed in this work and SolidSpec-3700. The difference seen from 450 nm to 370 nm radiation energy is attributed to reduced sensitivity of silicon photodiodes used in the portable spectrometer designed in this work.
5.4.4 Imaginary Refractive Index, (k) of SnSe
Imaginary refractive index indicates the extent radiation energy is scattered by a material.
The extent of radiation energy scattering by SnSe was moderate from 520 – 780 nm. The scattering increases to maximum at 470 nm. This is caused by the increased lattice oscillations due to high internal energy resulting from high energy radiation. The imaginary refractive index of SnSe obtained by the portable spectrometer designed in this work compared well with those of SolidSpec-3700 as shown in figure 5.9.
5.4.5 Real Permittivity, (ε1) of SnSe
The real permittivity indicates the extent of radiation energy absorbed by a material.
Figure 5.10 shows that the extent of radiation energy absorbed for SnSe is low from
680 -780 nm. Radiation energy absorption then increases in the remaining part of the visible spectrum. The sharp absorption of radiation energy at 470 nm by SnSe is attributed to intra-band energy transitions of electrons excited from lower to higher energy levels. The real permittivity data values obtained by SolidSpec-3700 and the portable spectrometer designed in this work related well. The difference in values from
64 radiation energy 380 – 470 nm is attributed to reduced sensitivity of silicon photodiodes used in the portable spectrometer designed in this work.
140 for SnSe using the Portable spectormeter designed in this work. for SnSe using the Portable spectrometer designed in this work. 120 for SnSe using SolidSpec-3700. for SnSe using SolidSpec-3700. 100
80
60
Permittivity 40
20
0
350 400 450 500 550 600 650 700 750 800 Radiation Wavelength, (nm)
Figure 5.10: Graph showing the real and imaginary permittivity of SnSe thin film from data obtained by SolidSpec-3700 and portable spectrometer designed in this work.
5.4.6 Imaginary Permittivity, (ε2) of SnSe
Figure 5.10 shows that the imaginary permittivity of SnSe increased with decreasing radiation wavelength. This indicates that the extent of radiation energy scattering by SnSe increased with increase in radiation energy. This is caused by random oscillations of
SnSe molecules due to increased internal energy. The imaginary permittivity values for
65
SolidSpec-3700 and the portable spectrometer designed in this work compared well. The difference in values from 380 – 470 nm is attributed to the problem stated in section
5.4.5.
5.4.7 Energy Absorption Factor of SnSe
Figure 5.6 shows that SnSe absorbs more radiation energy than it losses from 430 – 580 nm. SnSe losses more radiation energy compared to one absorbed in 600 – 770 nm range.
5.4.8 Absorption Coefficient, (α) of SnSe
The absorption of SnSe increased with increase in radiation energy (decrease in wavelength). The absorption of SnSe however is lower than that of CuO as shown in figure 5.7.
5.4.9 Band Gap, (Eg) of SnSe
Figure 5.11 shows a graph of 훼퐸 versus radiation energy for SnSe. The graph is linear near the edge of the fundamental transition implying that SnSe has indirect band gap. To determine the band gap linear extrapolation was done from the linear region to where
훼퐸 = 0. The x - intercept to the line indicates that the band gap of SnSe equals to
1.22 eV for the portable spectrometer designed in this work and 1.23 eV for
SolidSpec-3700 with an accuracy error of ± 1%.
66
12 SnSe curve for Portable spectrometer designed in this work. SnSe curve for SolidSpec-3700. Linear extrapolation to SnSe curve for Portable spectrometer 10 designed in this work. Linear extrapolation to SnSe curve for SolidSpec-3700.
8
-1/2
m
1/2 J
-6 6
x 10 1/2
E 4
1/2
2
0 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 Radiation Energy, E (eV)
Figure 5.11: Graph showing linear extrapolation to SnSe curves used to determine band gap.
5.4.10 Validation of SnSe Results
The maximum reflectance error for SnSe was 6.38 % when results of the portable spectrometer designed in this work were compared with those obtained by the standard
67 equipment (SolidSpec-3700). The mean error was 2.17 %. This is a tolerable range of error for reasonable level of accuracy in measurement as shown in table 5.2.
Table 5.2: Reflectance of SnSe using the portable spectrometer designed in this work and
SolidSpec-3700.
λ(nm) Reflectance of SnSe using Reflectance of SnSe (%)Error Portable spectrometer using SolidSpec-3700 designed in this work 773 0.2317 0.2389 3.01
688 0.5096 0.5159 1.22
619 0.5500 0.5472 0.51
563 0.2130 0.2022 5.35
516 0.7000 0.6941 0.84
476 0.8077 0.7983 1.17
442 0.5879 0.5816 1.07
413 0.3899 0.4165 6.38
387 0.7000 0.7000 0.00
Mean ± 2.17
The reflectance measurement of SnSe by the designed portable spectrometer and
SolidSpec-3700 correlated well. The mean error was caused by step position error of the stepper motor used in the portable spectrometer designed in this work and the reduced sensitivity of silicon photodiodes to monochromatic light near ultra violet. The measured band gap of SnSe using the portable spectrometer designed in this work compared well
68 with that obtained using standard instrument (SolidSpec-3700) with a marginal error of
± 0.81% as shown in table 5.3.
Table 5.3: Band gap of SnSe using portable spectrometer designed in this work and
SolidSpec-3700.
Film Band Gap
SolidSpec-3700 SnSe 1.23 eV
Designed SnSe 1.22 eV
Error 0.81%
The reflectance measurement of CuO and SnSe using the portable spectrometer designed in this work relate well to those of the standard equipment (SolidSpec-3700) with a mean error of ± 2.03 %. This gives the portable spectrometer designed in this work an average accuracy of 97.97 % in measurement.
69
CHAPTER 6
CONCLUSION AND RECOMMENDATIONS FOR FURTHER WORK
6.1 Conclusion
The designed portable spectrometer in this work measures 38 cm x 30 cm x 14 cm and has a mass of 5 kg. This enables easy movement of the instrument or portability. It consists of a lamp as a source of visible light, a diffraction grating, concave mirror, stepper motor, a set of gears, silicon photo-detectors and analog to digital converter as the main components. These components are common and do not cost much making the portable spectrometer designed in this work total cost to be Kshs. 40 000 which is half the cost of commercially available optical spectrometers measuring in the visible range of the electromagnetic spectrum.
The portable spectrometer used eight bit analog to digital converter (ADC0809CCN) with a resolution of 9.765625 mV for an analog input range of maximum 2.5 V. This sensitivity ensured that small changes in intensity of monochromatic light are detectable while locking out voltages resulting from background noise (temperature). The set of gears with reduction ratio 50:1 increased angular resolution of the stepper motor to 0.15o.
This improved monochromatic light resolution to 2 nm ± 5%. To ensure proper focusing of the monochromatic beam for the correct measurement of reflectance profile of thin film samples, silver mirror was used in calibrating the portable spectrometer designed in this work, increasing reliability and accuracy of the instrument to an average of 97.97 %.
70
The portable spectrometer designed in this work is recommended to institutions doing research in thin films because of it’s reliability, accuracy and cost as an alternative to commercially available ones measuring in the same visible range of electromagnetic spectrum.
6.2 Recommendations for Further Work
Despite the higher resolution in focusing monochromatic beam (0.15o) achieved in the portable spectrometer designed in this work, the step position error of the stepper motor used reduced the accuracy. There is need to use a closed loop control system to drive the stepper motor. This will ensure that there is feedback on the actual angular position of the stepper motor and hence improved accuracy in monochromatic beam focusing. To ensure real time graphical display of results during measurement, control using graphical programming (LABVIEW) is highly advised.
71
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APPENDICES
Appendix I
How to Run MSVC++ 6.0 Portable Spectrometer Driver Program Designed in this
Work.
The portable spectrometer driver program was developed using Microsoft Foundation
Classes (MFC6.0) in VC++ programming environment. VC++ language has extended capabilities in programming and therefore it was chosen in the driver programming. The program starts by displaying a dialogue window as shown in figure A;
Figure A: Dialogue window displayed when the portable spectrometer driver program
was started.
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To start the driver program running, the START button was pressed invoking the search for inpout32.dll, a dynamic link library file which allows the program access the enhanced parallel port. In the absence of inpout32.dll file, the program displayed an error message as indicated by figure B before terminating.
Figure B: MS DOS window error message displayed when inpout32.dll was not
loaded in the program file.
If the inpout32.dll was loaded to the program file, then it is now possible to access the enhanced parallel port. The status pin S6 ( pin 10 ) was used by the portable spectrometer to communicate the status of connection with a computer. Normally, when the enhanced parallel port is not connected to any peripheral device, the status register reads 120
(decimal). In this case the status pin 10 was set to ground when the spectrometer was
76 connected thus enabling the status register to output 56 (decimal), a condition used to ascertain connectivity of the portable spectrometer to a computer. After confirming that inpout32.dll was loaded, the program then searched whether the portable spectrometer was connected or not, if it was not, an error message indicating that the spectrometer was not connected was displayed, as shown in figure C;
Figure C: MS DOS window displayed when the portable spectrometer was not
connected.
When the portable spectrometer was connected, a message indicating that the spectrometer was connected was displayed. This was followed by initialization of the control and data registers of the enhanced parallel port to start reflectance measurement.
When reflectance measurement was complete the obtained data was stored in optica4.txt file for analysis and display on MS DOS window, as shown in figure D.
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The portable spectrometer driver program was made to run outside VC++ programming environment by attaching MFC42D.DLL, MSVCRTD.DLL, MFCO42D.DLL and
MSVCIRTD.DLL files to the executable driver program folder.
Figure D: MS DOS window display of measured optical properties of a thin film
sample under investigation.
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Appendix II
APicture of Hardware Components of Portable Spectrometer Designed in this Work
Aperture Stepper motor Photo-detectors
Analog inputs
Thin film slot
Enhanced parallel port ADC0809CCN
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Appendix III
Specifications Chart for Stepper Motor used in the Portable Spectrometer Designed in this Work (Minebea, 2010)
80
Appendix IV
MSVC++ 6.0 Portable Spectrometer Driver Program
// ben1Dlg.cpp : implementation file //
#include "stdafx.h" #include "ben1.h" #include "ben1Dlg.h"
#include
// DLL function signature typedef (_stdcall *out)( int, int); typedef (_stdcall *in)( int);
#ifdef _DEBUG #define new DEBUG_NEW #undef THIS_FILE static char THIS_FILE[] = __FILE__; #endif
///////////////////////////////////////////////////////////////////////////// // CAboutDlg dialog used for App About class CAboutDlg : public CDialog { public: CAboutDlg();
// Dialog Data //{{AFX_DATA(CAboutDlg) enum { IDD = IDD_ABOUTBOX }; //}}AFX_DATA
// ClassWizard generated virtual function overrides //{{AFX_VIRTUAL(CAboutDlg)
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protected: virtual void DoDataExchange(CDataExchange* pDX); // DDX/DDV support //}}AFX_VIRTUAL
// Implementation protected: //{{AFX_MSG(CAboutDlg) //}}AFX_MSG DECLARE_MESSAGE_MAP() };
CAboutDlg::CAboutDlg() : CDialog(CAboutDlg::IDD) { //{{AFX_DATA_INIT(CAboutDlg) //}}AFX_DATA_INIT } void CAboutDlg::DoDataExchange(CDataExchange* pDX) { CDialog::DoDataExchange(pDX); //{{AFX_DATA_MAP(CAboutDlg) //}}AFX_DATA_MAP }
BEGIN_MESSAGE_MAP(CAboutDlg, CDialog) //{{AFX_MSG_MAP(CAboutDlg) // No message handlers //}}AFX_MSG_MAP END_MESSAGE_MAP()
///////////////////////////////////////////////////////////////////////////// // CBen1Dlg dialog
CBen1Dlg::CBen1Dlg(CWnd* pParent /*=NULL*/) : CDialog(CBen1Dlg::IDD, pParent) { //{{AFX_DATA_INIT(CBen1Dlg) // NOTE: the ClassWizard will add member initialization here //}}AFX_DATA_INIT // Note that LoadIcon does not require a subsequent DestroyIcon in Win32 m_hIcon = AfxGetApp()->LoadIcon(IDR_MAINFRAME); } void CBen1Dlg::DoDataExchange(CDataExchange* pDX) {
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CDialog::DoDataExchange(pDX); //{{AFX_DATA_MAP(CBen1Dlg) // NOTE: the ClassWizard will add DDX and DDV calls here //}}AFX_DATA_MAP }
BEGIN_MESSAGE_MAP(CBen1Dlg, CDialog) //{{AFX_MSG_MAP(CBen1Dlg) ON_WM_SYSCOMMAND() ON_WM_PAINT() ON_WM_QUERYDRAGICON() //}}AFX_MSG_MAP END_MESSAGE_MAP()
///////////////////////////////////////////////////////////////////////////// // CBen1Dlg message handlers
BOOL CBen1Dlg::OnInitDialog() { CDialog::OnInitDialog();
// Add "About..." menu item to system menu.
// IDM_ABOUTBOX must be in the system command range. ASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX); ASSERT(IDM_ABOUTBOX < 0xF000);
CMenu* pSysMenu = GetSystemMenu(FALSE); if (pSysMenu != NULL) { CString strAboutMenu; strAboutMenu.LoadString(IDS_ABOUTBOX); if (!strAboutMenu.IsEmpty()) { pSysMenu->AppendMenu(MF_SEPARATOR); pSysMenu->AppendMenu(MF_STRING, IDM_ABOUTBOX, strAboutMenu); } }
// Set the icon for this dialog. The framework does this automatically // when the application's main window is not a dialog SetIcon(m_hIcon, TRUE); // Set big icon SetIcon(m_hIcon, FALSE); // Set small icon
// TODO: Add extra initialization here
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return TRUE; // return TRUE unless you set the focus to a control } void CBen1Dlg::OnSysCommand(UINT nID, LPARAM lParam) { if ((nID & 0xFFF0) == IDM_ABOUTBOX) { CAboutDlg dlgAbout; dlgAbout.DoModal(); } else { CDialog::OnSysCommand(nID, lParam); } }
// If you add a minimize button to your dialog, you will need the code below // to draw the icon. For MFC applications using the document/view model, // this is automatically done for you by the framework. void CBen1Dlg::OnPaint() { if (IsIconic()) { CPaintDC dc(this); // device context for painting
SendMessage(WM_ICONERASEBKGND, (WPARAM) dc.GetSafeHdc(), 0);
// Center icon in client rectangle int cxIcon = GetSystemMetrics(SM_CXICON); int cyIcon = GetSystemMetrics(SM_CYICON); CRect rect; GetClientRect(&rect); int x = (rect.Width() - cxIcon + 1) / 2; int y = (rect.Height() - cyIcon + 1) / 2;
// Draw the icon dc.DrawIcon(x, y, m_hIcon); } else { CDialog::OnPaint(); } }
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// The system calls this to obtain the cursor to display while the user drags // the minimized window. HCURSOR CBen1Dlg::OnQueryDragIcon() { return (HCURSOR) m_hIcon; } void CBen1Dlg::OnOK() { // TODO: Add extra validation here
out Out32; in Inp32; double a,b,c,d,e,f,g,h,i,j,k,l,m,n,p,q,r,s; void delay(int msecond); // Declaration of delay function, void because it does not return a value.
ofstream outfile("optica4.txt", ios::trunc);
// Load DLL file HINSTANCE hinstLib=LoadLibrary(TEXT("inpout32.dll")); if (hinstLib==NULL){ outfile< } // Get function pointer Out32=(out)GetProcAddress (hinstLib, "Out32"); if (Out32==NULL) { outfile<<"ERROR: Unable to find Out32 function\n"< } Inp32=(in)GetProcAddress (hinstLib, "Inp32"); if (Inp32==NULL) { outfile <<"ERROR: Unable to find Inp32 function\n"< } 85 // Reflectance measurement. else { Out32(888,8); delay(5000); if(Inp32(889)==120) { outfile< } else if(Inp32(889)==56) { outfile< { // Measurement starts. Out32(890,2); // first rotation of stepper motor 86 delay(3000); Out32(888,8); Out32(888,10); // latch analog input to the decoder Out32(888,8); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); c=Inp32(888)*0.009765625; // read output Out32(888,9); delay(300); Out32(888,11); // latch analog input to the decoder Out32(888,9); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); d=Inp32(888)*0.009765625; // read output Out32(890,10); // second rotation of stepper motor delay(3000); Out32(888,8); Out32(888,10); // latch analog input to the decoder Out32(888,8); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); e=Inp32(888)*0.009765625; // read output Out32(888,9); delay(300); Out32(888,11); // latch analog input to the decoder Out32(888,9); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); f=Inp32(888)*0.009765625; // read output Out32(890,14); // third rotation of stepper motor delay(3000); Out32(888,8); Out32(888,10); // latch analog input to the decoder Out32(888,8); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); g=Inp32(888)*0.009765625; // read output Out32(888,9); delay(300); Out32(888,11); // latch analog input to the decoder Out32(888,9); // start conversion delay(200); // allow conversion and EOC to set high 87 Out32(888,12); h=Inp32(888)*0.009765625; // read output Out32(890,15); // fourth rotation of stepper motor delay(3000); // allow time for the stepper to stabilize Out32(888,8); Out32(888,10); // latch analog input to the decoder Out32(888,8); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); i=Inp32(888)*0.009765625; // read output Out32(888,9); delay(300); Out32(888,11); // latch analog input to the decoder Out32(888,9); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); j=Inp32(888)*0.009765625; // read output Out32(890,13); // fifth rotation of stepper motor delay(3000); // allow time for the stepper to stabilize Out32(888,8); Out32(888,10); // latch analog input to the decoder Out32(888,8); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); k=Inp32(888)*0.009765625; // read output Out32(888,9); delay(300); Out32(888,11); // latch analog input to the decoder Out32(888,9); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); l=Inp32(888)*0.009765625; // read output Out32(890,9); // sixth rotation of stepper motor delay(3000); // allow time for the stepper to stabilize Out32(888,8); Out32(888,10); // latch analog input to the decoder Out32(888,8); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); m=Inp32(888)*0.009765625; // read output Out32(888,9); delay(300); Out32(888,11); // latch analog input to the decoder 88 Out32(888,9); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); n=Inp32(888)*0.009765625; // read output Out32(890,1); // seventh rotation of stepper motor delay(3000); // allow time for the stepper to stabilize Out32(888,8); Out32(888,10); // latch analog input to the decoder Out32(888,8); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); p=Inp32(888)*0.009765625; // read output Out32(888,9); delay(300); Out32(888,11); // latch analog input to the decoder Out32(888,9); // start conversion delay(200);// allow conversion and EOC to set high Out32(888,12); q=Inp32(888)*0.009765625; // read output Out32(890,3); // eighth rotation of stepper motor delay(3000); // allow time for the stepper to stabilize Out32(888,8); Out32(888,10); // latch analog input to the decoder Out32(888,8); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); r=Inp32(888)*0.009765625; // read output Out32(888,9); delay(300); Out32(888,11); // latch analog input to the decoder Out32(888,9); // start conversion delay(200); // allow conversion and EOC to set high Out32(888,12); s=Inp32(888)*0.009765625; // read output // Calculation of phase angle Q. double R1,R2,R3,R4,R5,R6,R7,R8,R9; double Q1,Q2,Q3,Q4,Q5,Q6,Q7,Q8,Q9; 89 double pie = 3.141592654; R1=i/j;R2=c/d;R3=r/s;R4=k/l;R5=p/q;R6=m/n;R7=e/f;R8=a/b;R9=g/h; Q1=(4*2.4*0.3/pie)*((log(sqrt(R2))/(pow(2.7,2)- pow(2.4,2)))+(log(sqrt(R4))/(pow(3.3,2)-pow(2.4,2)))+(log(sqrt(R6))/(pow(3.9,2)- pow(2.4,2)))+(log(sqrt(R8))/(pow(4.5,2)-pow(2.4,2)))); Q3=(4*3.0*0.3/pie)*((log(sqrt(R2))/(pow(3.0,2)- pow(2.7,2)))+(log(sqrt(R4))/(pow(3.3,2)-pow(3.0,2)))+(log(sqrt(R6))/(pow(3.9,2)- pow(3.0,2)))+(log(sqrt(R8))/(pow(4.5,2)-pow(3.0,2)))); Q5=(4*3.6*0.3/pie)*((log(sqrt(R2))/(pow(3.6,2)- pow(2.7,2)))+(log(sqrt(R4))/(pow(3.6,2)-pow(3.3,2)))+(log(sqrt(R6))/(pow(3.9,2)- pow(3.6,2)))+(log(sqrt(R8))/(pow(4.5,2)-pow(3.6,2)))); Q7=(4*4.2*0.3/pie)*((log(sqrt(R2))/(pow(4.2,2)- pow(2.7,2)))+(log(sqrt(R4))/(pow(4.2,2)-pow(3.3,2)))+(log(sqrt(R6))/(pow(4.2,2)- pow(3.9,2)))+(log(sqrt(R8))/(pow(4.5,2)-pow(4.2,2)))); Q9=(4*4.8*0.3/pie)*((log(sqrt(R2))/(pow(4.8,2)- pow(2.7,2)))+(log(sqrt(R4))/(pow(4.8,2)-pow(3.3,2)))+(log(sqrt(R6))/(pow(4.8,2)- pow(3.9,2)))+(log(sqrt(R8))/(pow(4.8,2)-pow(4.5,2)))); Q2=(4*2.7*0.3/pie)*((log(sqrt(R1))/(pow(2.7,2)- pow(2.4,2)))+(log(sqrt(R3))/(pow(3.0,2)-pow(2.7,2)))+(log(sqrt(R5))/(pow(3.6,2)- pow(2.7,2)))+(log(sqrt(R7))/(pow(4.2,2)-pow(2.7,2)))+(log(sqrt(R9))/(pow(4.8,2)- pow(2.7,2)))); Q4=(4*3.3*0.3/pie)*((log(sqrt(R1))/(pow(3.3,2)- pow(2.4,2)))+(log(sqrt(R3))/(pow(3.3,2)-pow(3.0,2)))+(log(sqrt(R5))/(pow(3.6,2)- pow(3.3,2)))+(log(sqrt(R7))/(pow(4.2,2)-pow(3.3,2)))+(log(sqrt(R9))/(pow(4.8,2)- pow(3.3,2)))); Q6=(4*3.9*0.3/pie)*((log(sqrt(R1))/(pow(3.9,2)- pow(2.4,2)))+(log(sqrt(R3))/(pow(3.9,2)-pow(3.0,2)))+(log(sqrt(R5))/(pow(3.9,2)- pow(3.6,2)))+(log(sqrt(R7))/(pow(4.2,2)-pow(3.9,2)))+(log(sqrt(R9))/(pow(4.8,2)- pow(3.9,2)))); Q8=(4*4.5*0.3/pie)*((log(sqrt(R1))/(pow(4.5,2)- pow(2.4,2)))+(log(sqrt(R3))/(pow(4.5,2)-pow(3.0,2)))+(log(sqrt(R5))/(pow(4.5,2)- pow(3.6,2)))+(log(sqrt(R7))/(pow(4.5,2)-pow(4.2,2)))+(log(sqrt(R9))/(pow(4.8,2)- pow(4.5,2)))); // Calculation of Real refractive index, n. 90 double n1,n2,n3,n4,n5,n6,n7,n8,n9; n1= fabs((1-R1)/(1+R1-2*sqrt(R1)*cos(Q1))); n2= fabs((1-R2)/(1+R2-2*sqrt(R2)*cos(Q2))); n3= fabs((1-R3)/(1+R3-2*sqrt(R3)*cos(Q3))); n4= fabs((1-R4)/(1+R4-2*sqrt(R4)*cos(Q4))); n5= fabs((1-R5)/(1+R5-2*sqrt(R5)*cos(Q5))); n6= fabs((1-R6)/(1+R6-2*sqrt(R6)*cos(Q6))); n7= fabs((1-R7)/(1+R7-2*sqrt(R7)*cos(Q7))); n8= fabs((1-R8)/(1+R8-2*sqrt(R8)*cos(Q8))); n9= fabs((1-R9)/(1+R9-2*sqrt(R9)*cos(Q9))); // Calculation of Extinction coefficient, k. double k1,k2,k3,k4,k5,k6,k7,k8,k9; k1= fabs((-2*sqrt(R1)*sin(Q1))/(1+R1-2*sqrt(R1)*cos(Q1))); k2= fabs((-2*sqrt(R2)*sin(Q2))/(1+R2-2*sqrt(R2)*cos(Q2))); k3= fabs((-2*sqrt(R3)*sin(Q3))/(1+R3-2*sqrt(R3)*cos(Q3))); k4= fabs((-2*sqrt(R4)*sin(Q4))/(1+R4-2*sqrt(R4)*cos(Q4))); k5= fabs((-2*sqrt(R5)*sin(Q5))/(1+R5-2*sqrt(R5)*cos(Q5))); k6= fabs((-2*sqrt(R6)*sin(Q6))/(1+R6-2*sqrt(R6)*cos(Q6))); k7= fabs((-2*sqrt(R7)*sin(Q7))/(1+R7-2*sqrt(R7)*cos(Q7))); k8= fabs((-2*sqrt(R8)*sin(Q8))/(1+R8-2*sqrt(R8)*cos(Q8))); k9= fabs((-2*sqrt(R9)*sin(Q9))/(1+R9-2*sqrt(R9)*cos(Q9))); // Calculation of Real Dielectric constant, D1. double D11,D12,D13,D14,D15,D16,D17,D18,D19; D11= fabs(pow(n1,2)-pow(k1,2)); D12= fabs(pow(n2,2)-pow(k2,2)); D13= fabs(pow(n3,2)-pow(k3,2)); D14= fabs(pow(n4,2)-pow(k4,2)); D15= fabs(pow(n5,2)-pow(k5,2)); D16= fabs(pow(n6,2)-pow(k6,2)); 91 D17= fabs(pow(n7,2)-pow(k7,2)); D18= fabs(pow(n8,2)-pow(k8,2)); D19= fabs(pow(n9,2)-pow(k9,2)); // Calculation of the Imaginary part of the Dielectric constant, D2. double D21,D22,D23,D24,D25,D26,D27,D28,D29; D21= fabs(2*n1*k1); D22= fabs(2*n2*k2); D23= fabs(2*n3*k3); D24= fabs(2*n4*k4); D25= fabs(2*n5*k5); D26= fabs(2*n6*k6); D27= fabs(2*n7*k7); D28= fabs(2*n8*k8); D29= fabs(2*n9*k9); // Absorption coefficient, A. double A1,A2,A3,A4,A5,A6,A7,A8,A9; A1=(4*pie*k1/0.0000007854); A2=(4*pie*k2/0.0000006981); A3=(4*pie*k3/0.0000006283); A4=(4*pie*k4/0.0000005712); A5=(4*pie*k5/0.0000005236); A6=(4*pie*k6/0.0000004833); A7=(4*pie*k7/0.0000004488); A8=(4*pie*k8/0.0000004189); A9=(4*pie*k9/0.0000003927); // Output data to "optica4.txt" file in the same folder or directory 92 outfile<<(ios::right,setprecision(4)); outfile<<"E(eV)"< outfile< } while (int M1=267; M1>0; M1- -) { // Reset stepper motor 93 Out32(890,1); delay(100); Out32(890,9); delay(100); Out32(890,13); delay(100); Out32(890,15); delay(100); Out32(890,14); delay(100); Out32(890,10); delay(100); Out32(890,2); delay(100); Out32(890,3); delay(100); } outfile<<"MEASUREMENT COMPLETED SUCCESSFULLY.\n"< } out32(888,0); delay(200); // Unload DLL file FreeLibrary(hinstLib); } system(" title PORTABLE OPTICAL SPECTROMETER < optica4.txt | type optica4.txt && pause"); // tunnelling the output through a pipe (file optica4.txt). CDialog::OnOK(); 94 } void delay(int mseconds) { clock_t goal= mseconds + clock(); while (goal>=clock()); }