NANO STRUCTURED THIN FILMS FOR SOLAR CELLS

Report of the work done on the Minor project

NANO STRUCTURED THIN FILMS FOR SOLAR CELLS

Submitted by

Dr Sindu A. Kartha

Assistant professor

Department of physics

H.H.M.S.P.B NSS College for Women

Neeramankara, Thiruvananthapuram,

Kerala, 695 040

To

University Grants Commission

South Western Regional Office,

Bangalore-560 009


Acknowledgements

It is a matter of joy for me to present this report and I wish to express my grateful appreciation to all who have helped me to accomplish this piece of work.

With immense pleasure I express my sincere thanks and indebtedness to our principals, Dr. Vijaya Kumar, Dr. G Sudheesh, Dr. Ajitha Bai and Dr.Sandhya Gopinath for providing the basic facilities to carry out this work. Their wholehearted cooperation will be remembered forever. I am indebted to UGC for the financial assistance. The help rendered by the School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam and Department of Opto Electronics, Kerala University is also gratefully acknowledged.

I express my heartfelt thanks to Dr. B. Ambika and Dr. S Prasanna, the former heads of the department of Physics and all my colleagues, Smt Sreelatha. K, Dr Sutheertha S. Nair, Dr Bijini B.R , Dr Parvathi. M.R, Dr. Kavitha V.T and Dr. Smitha. S.L. I am grateful to all my colleagues in various departments in H.H M.S.P.B NSS College for women, Thiruvananthapuram. I express my deep felt thanks to all staffs in the college.

I express my sincere gratitude to all my family members for their co-operation without which the work would not have been possible.

Finally and above all I thank the Almighty for all the blessings showered upon me.

Preface

  1. General Introduction
    • Introduction
    • Thin films-An introduction
  • Thin film growth process
    • Basics of Chemical bath deposition(CBD)
    • Factors affecting CBD
  • Measurement of thin film thickness
  • Characterization of thin films
  • Solar cell materials
  • Scope of the present work
  • References
  1. Experimental Techniques
  • Introduction
  • Preparation of thin films
    • Substrate cleaning
    • Mechanism of film formation
  • Characterization Techniques
    • . UV-Vis-NIR spectroscopy
    • Scanning Electron Microscopy (SEM)
      • Principle of SEM
    • Energy Dispersive X-ray Spectroscopy (EDX)
    • X-ray diffraction (XRD)
  • Structural, chemicall and Optical Analysis of CdSe thin films prepared by Chemical bath deposition (CBD)
    • Introduction
    • Characterization of CdSe thin films prepared by C
    • Structural analysis
    • Morphological analysis
    • Elemental analysis
    • Optical Studies
      • Theory
      • Experimental
      • Result and Discussions
  1. General Conclusion

PREFACE

Thin film technology plays a key role in many segments of industry today. The properties of thin films may be quite different from those of the bulk, particularly if the thickness is very small. These anomalous properties are due to particular structure of the film, and this structure is dictated by the processes which occur during film formation.

Photovoltaic cells, which converts solar energy directly to electricity is a major research area for the last few decades. The technology was based on p-n junction that enables the use of the photovoltaic characteristic of the suitable semiconductor.

The first generation solar cells are based on Si wafers. Deposition and characterization of group II-VI semiconductor compounds such as ZnSe, CdO, CdS, CdTe, CdSe etc.in thin film forms are of great interest due to their potential applications in various optoelectronic devices.

CdSe film has a band gap of 1.73eV, which makes it a best candidate for top layer of tandem solar cell. It is also a direct band gap semiconductor with a very high absorption coefficient. Therefore CdSe solar cells need only a very thin film to absorb sunlight.

This report is divided into four chapters. Chapter1gives an overview of thin films, growth processes giving more importance to chemical bath deposition (CBD). An introduction to solar cell materials and the scope of the present work is also included.

Chapter 2 gives the details of various experimental techniques used in the present study. The preparation of thin films by CBD and mechanism of film formation is also included.

The third chapter deals with the structural,morphological chemical analysis of CdSe thin film prepared by CBD. Structural analysis was done by XRD. The morphology of the film was analyzed by SEM. Chemical analysis was done by EDX. Optical studies done by UV –Vis spectroscopy was described in this chapter. The determination of optical band gap and other optical parameters were also described.

The forth chapter, included as a conclusion is a correlation between the results obtained in various studies. The increase in grain size with thickness was identified from XRD spectra. Annealing has also improved the crystallinity of the sample. These results were confirmed by SEM analysis. Chemical composition determined by EDX showed the required stoichiometry of the thin film. Optical band gap of the film also has shown the dependence on thickness.


  1. General Introduction
    • Introduction

The supply and demand of energy determine the symbol of global development. Sufficient supply of clean energy is very important for economic prosperity, global stability and quality of life. Finding alternate energy sources for perishing fossil fuels is a major challenge for the next decade.

Our primary source of clean energy is the SUN. It is an abundant nonperishable source which will exist as long as our planet. The main steps to utilize solar energy are the capture, conversion and storage of sun’s radiation.

The energy of this radiation must be captured as excited electron hole pair in a semiconductor or a dye or as heat in a thermal storage medium. Excited electrons and holes can be tapped off for immediate conversion to electrical power, or transferred to biological or chemical molecules for conversion to fuel.

Although many routes use solar energy to produce electricity, fuel and heat, none are currently competitive with fossil fuels for a combination of cost, reliability and performance. While solar energy has enormous promise as a clean, abundant and economical source of energy, it presents formidable basic research challenges in designing materials and in understanding the electronic and molecular basis of capture, conversion and storage before its promise can be realized.

1.2. Thin films-An introduction

Thin film technology plays a key role in many areas of industry today [1]. The properties of thin films may be quite different from those of the bulk, particularly if the film thickness is very small. In the case of thin films the two surfaces are very close to each other that they can have a decisive influence on the internal physical properties and processes of the substance which differ in a profound way from those of the bulk. The reduction of one dimension of a material to an order only of several atomic layers creates an intermediate system between macro systems and molecular systems

There has been increasing interest in group II-VI semiconductor compound thin films because of their wide application in various fields of optoelectronic technology [, 2, 3, 4, 5].

  • Thin film growth process

Three major steps that constitute a typical thin film deposition process are i) production of appropriate atomic, molecular or ionic species, ii) transport of these species to the substrate iii) condensation on the substrate, either directly or via a chemical/or electrochemical reaction to form solid deposit [1]. Thin film deposition techniques can be broadly classified into two viz. physical and chemical [6].

The physical deposition techniques include vacuum evaporation, laser ablation, molecular beam epitaxy and sputtering. The chemical method comprises gas phase deposition and solution techniques.

The chemical methods are economical and easier than that of the physical methods. But there is no ideal method to prepare thin films which satisfy all possible requirements. Among the chemical methods, Chemical bath deposition (CBD) is most popular today, because large number of conducting and semiconducting thin films can be prepared by this technique. It is also popular due to simplicity and low cost. Also thin films can be deposited on different substrates like glass, ceramics etc. Large area of high quality adherent films of uniform thickness can be prepared for different types of materials including superconductors, metal oxides binary and tertiary chacogenides etc. by CBD.

1.2.1.1. Basics of Chemical bath deposition (CBD)

Films can be grown on either metallic or nonmetallic substrates by dipping them in appropriate solutions of salts without the application of electric field. It the simplest of the chemical methods and has many advantages such as

  1. It is simple and doesn’t require any sophisticated instrumentation which makes it most economic.
  2. Ideally suited for large area depositions on substrates of any nature.
  • The deposition is usually at low temperature and avoids oxidation or corrosion of the metallic substrates.
  1. It is possible to obtain uniform and large area semiconductor deposits on a variety of substrate materials (most important for the present work)
  2. Film thickness can be controlled by the variation in the preparative parameters.

1.2. Factors affecting CBD

It is the most suited method for large area deposition of thin films. Howeverthegrowth of the film is found to be governed by the various factors such as bath composition, the pHand deposition time and temperature.

  1. The bath concentration:

The growth rate and quality of the deposited films were greatly influenced by the concentration of the reacting species. The films deposited using low concentration are thin and non-uniform. This observation can be related to the insufficient supply of ionic species at such low concentration. When the concentration of the species is increased, the quality and uniformity of the films goes on increasing and the films were thick. This is true up to a certain level of concentration and then saturation in growth process was observed.

  1. The pH of the solution

It is the most important factor in CBD which controls the film properties. The desired films are obtained on the substrate surface by optimizing the pH value of its bath solution.

  1. Deposition time:

Deposition time has great effect on the thickness of the deposited film.

  1. Bath temperature:

The rate of deposition increases with bath temperature resulting into formation of the fine grained structure.

  • Measurement of thin film thickness

Thickness is one of the most important thin film parameters since it largely determines the properties of thin films. Almost all properties of thin films depend on the thickness and can therefore used for the thickness measurement.

Methods of monitoring thickness can be divided into several groups, including gravimetric method, optical and electrical methods ,methods based on emission and absorption of radiation ,chemical analysis etc. In the present work scanning electron microscopy is the method used for the thickness determination

  • Characterization of thin films

Detailed investigation of the structure of thin film and theprocess involved in their formation has been made possible by two physical methods, electron microscopy and electron/X-ray diffraction. In electron/Xray diffraction, the waves are diffracted while passing through a crystal and cancel or reinforce each other, depending on the direction of propagation in such a way that after the impact of electrons upon a screen they give rise to a number of light and dark spots. It is possible to find from the positions and intensities of these spots whether a substance is amorphous, polycrystalline or monocrystalline and what kind of lattice it has and how it is oriented.

A combination of optical and spectroscopic technique has been found to be useful for the determination of composition chemical stability etc.

The different methods used for thin film evaluation are

Transmission electron microscopy

Scanning electron microscopy

Tunnel emission and field ionization microscopy

X-ray Spectroscopy

FTIR and Raman spectroscopy etc.

Complete evaluation will also involve various electrical measurements such as resistivity, dielectric constant, surface charge densities etc.

 

1.3 Solar cell materials

Photovoltaic cells, which converts solar energy directly to electricity is a major research area for the last few decades. The technology was based on p-n junction that enables the use of the photovoltaic characteristic of the suitable semiconductor. The first generation solar cells are based on Si wafers [7, 8].

Deposition and characterization of group II-VI semiconductor compounds such as ZnSe, CdO, CdS, CdTe, CdSe etc.in thin film forms are of great interest due to their potential applications in various optoelectronic devices [9, 10, 11, 12].

T.Minemoto et.al. proposed CBD method as the novel deposition method of anti-reflection (AR) coating for spherical solar cell, in which CdS was utilized as AR coating[13]

Musale et.al has reported the preparation of thin films of CdS by CBD using alkaline solution and their FTIR and XRD studies [14].Structural and optical characterization of CdSe thin films deposited by CBD was reported by K.Girija et.al.[15] P.K Nair et.al has presented a perspective on chemically deposited thin films of various groupII-VI compounds,post deposition processing, the use of precipitate and an over viewof the applications related to solar energy utilization [16]

1.4. Scope of the present work

Thin films of group II-VI semiconductor compounds have many applications, which can be utilized for the preparation of solar cells. Before attempting to make a thin film solar cell or any opto electronic applications, studies are required to examine the properties of materials in thin film form. CdSe film has a band gap of 1.73eV, which makes it a best candidate for top layer of tandem solar cell. It is also a direct band gap semiconductor with a very high absorption coefficient. Therefore CdSe solar cells need only a very thin film to absorb sunlight.

The purpose of the present work is to identify the optimum conditions required for the preparation of CdSe thin films by CBD and to improve basic understanding of film growth. Thin films of CdSe of two different thicknesses were prepared and one of the films was annealed at a temperature of 350 °C. The structural analysis was done by X-Ray diffraction spectroscopy. Field emission SEM was used for surface imaging. Quantitative analysis of the film was done by EDX. To find out the optical band gap UV-Vis spectroscopy was used. The value of band gap was used to determine the refractive index of the films.

1.5. References

  1. Ludmila Eekartova, Physics of thin films, Plenum Press, New York (1977)
  2. P.Hankare, V.M. Bhuse, K.M.Garadkar, S.D Delekarand I.S.Mulla, Semicond.Sci.Technol.19(2004) 70
  3. . K.R.Murali,V Swaminathanand D.C Trivedi, Energy Mater.Sol.Cells 81 (2004) 113
  4. S.Khomane, P. P. Hankare, J.Alloys Compd,489(2010) 605
  5. A Van Calster, A.Vervaet, i.De Rycke, J De Baets, J.Vanfleteren. J Cryst.Growth 86 (1989) 924
  6. Joy George, Preparation of Thin Films, Mared Dekkis Inc., New York,(1992)
  7. Bisconti, h.Ossenbrink, Sol. Energy Mater. Sol. Cells 48 (1997) 1.
  8. Minemoto, C. Okamoto, S.Omae, mMurozono, H.Takakura, Y.Hamakawa, Jpn.J.Appl. Phys.44 (2005) 4820
  9. N Romeo, A. Bosio, R Tedeschi, Romeo and V. Canevari, Sol. Energy Mater. Sol. Cells) ,58(1999) 209
  10. Wu and P Sheldon, Proceedings of 16th European Photovoltaic Solar Energy Conference and exhibition ,U.K 2000
  11. Kaur, D.Kpandya, and K.LChopra, J Solid State Sci. Tech 128 (4) (1980 ) 943
  12. B. Kale, C.D. Lokhande, Appl.Surf.Sci.223 (2004)345
  13. Minemoto,H Takakura, Y Hamakawa Sol. Energy Mater. Sol. Cells 90 (2006) 3576
  14. N Musale, S. K Devade, Int.J. Basic Appl. Res. Special issue (NCRTP 2012) 141
  15. Girija, S Thirumalairajan, S.M Mohan, J Chandrasekaran Iraqi J.Appl Phys. Lett. Vol2 ,4 (2009) 21
  16. K. Nair, M.T.S Nair, V.M Garcia,O.L. Arenas et.al, Sol. Energy Mater. Sol. Cells 52 (1998) 313.

Chapter II

  1. Experimental Techniques
    • Introduction

This report includes the preparation of CdSe thin films by CBD and investigation of their structural and optical properties. Structural and chemical characterizations are important in the development of exotic materials. There are different techniques for structural and chemical characterization. Preparation of thin films and different experimental techniques employed in the study are explained in this section with relevant principles. Different analytical techniques used to characterize our thin film include X-ray diffraction, SEM, EDAX and UV-Vis near IR spectroscopy.

  • . Preparation of thin films

Thin films of CdSe are prepared by CBD, a simple low cost, large area deposition technique which does not require any sophisticated instrument[1,2] Film thickness and stoichiometry can be controlled by adjusting various deposition conditions.

Thin films of different thickness are prepared from appropriate amount of AR grade cadmium chloride, Selenium powder and sodium sulphate. Sodium selenosulphate solution was prepared using sodium sulphate and selenium powder. Stoichiometric amount of the chemicals were heated with 50ml of distilled water for three hours at 90°C by constant stirring using a magnetic stirrer adjusted at a rotational speed of 300rpm. Excess selenium powder was filtered out. The 20ml of 0.1 M cadmium chloride was taken, to which added 15ml of 30% NH3 was added to adjust the pH to about 12. Few drops of tri ethanol amine (TEA) were added as complexing agent. This mixture was kept in a constant temperature water bath at 70°C and freshly prepared Sodium selenosulphate solution was added to the mixture by constant stirring.

The following conditions are found to be very important in the successful deposition of thin films.

The pH of the solution should be 11-12range

The presence of complexing agent TEA is necessary for the thin film formation

Sodium selenosulphate solution should be freshly prepared.

  • Substrate cleaning

Before any substrate can be used for the film preparation, it must be adequately cleaned for the growth of film to start. Cleaning involves the breaking of adsorption bands between the substrate and the contaminants, without damaging the substrate surface itself. Glass slides, used as substrates, were cleaned using detergent solution and distilled water. The substrate was suspended in an ultrasonically agitated detergent solution. In this method ultrasonic waves were transmitted to the solvent from a transducer and cause cavitation or bubble collapse with corresponding local surges of hydrostatic pressure. The stress created when a bubble cavity implodes on the surface of the substrate allows the solvent to penetrate between the contaminant and the substrate. Then they were rinsed in acetone, cleaned again in distilled water and finally dried by blowing hot air over them. Cleaned glass plates were arranged vertically in the solution as shown in the figure1.

The colourless bath turned orange in colour and then to orange red as time progressed. (Figure2-5) After deposition the glass plates were rinsed in distilled water and then dried in air. Figure 6 shows the film formed in our work.

Films of different thickness were prepared by adjusting the deposition time. The dried film were annealed at 150°C and 350°C for 30minutes in a digital muffle furnace which can be adjusted up to 25°C accuracy.

  • Mechanism of film formation

The mechanism of film formation is based on the slow release of Cd2+ ions and Se2- ions and their condensation on the glass substrate. Precipitation of the solid phase occurs due to the super saturation in the reaction bath. For a given temperature when ionic product of reactants exceeds the solubility product, precipitation takes place. But if the ionic product is less, then the solid product produced will dissolve back to the solution.

Hydrolysis of sodium selenite gives Se2- ions according to the following reaction.

Na2SeSO3+ OH- →Na2SO4+HSe-

HSe- + OH- → H2O+ Se2-

The slow release of Cd2+ ions is achieved by the dissociation equilibrium of a complex species of Cd(TEA) 2+ ,where TEA is used as a complexing agent.

Cd (TEA)2+ → Cd2++TEA

When ammonia is added to the Cd2+ salt solution Cd (OH)2 starts precipitating.

Cd2++2OH- →Cd (OH)2

When ammonia is added to the solution it forms a complex cadmium tetra amine [Cd(NH3)4 ]2+ as

Cd2+ +4NH3 → [Cd (NH3)4 ]2+

[Cd (NH3)4 ]2++ Se2- →CdSe+4NH3

TEA and ammonia are used to adjust the pH of the reaction mixture and to increase the adherence of the film on the substrate.

  • .Characterization Techniques

There are different techniques for the analysis of prepared thin films. The methods used in this work include, SEM, EDX, XRD and UV-Vis-NIR spectroscopy. Principle and theory of these methods are explained here.

  • UV-Vis-NIR spectroscopy

Spectrophotometers are optical instruments that measure the intensity of light transmitted or absorbed by objects as a function of wavelength. Light from a lamp enters the monochromator, which disperses the light and selects the particular wavelength. The light beam of selected wavelength is divided into two the sample and the reference beam. The reference and sample light beams pass through the cell compartment consisting of a reference space and a sample space. The two beams converge on the detector.

A diagram of the components of a typical spectrometer is shown. The beam of light from a visible/a UV light source is separated into its component wavelengths by a prism or a diffraction grating. Each monochromatic beam in turn is split into two equal intensity beams by a half mirrored device, one beam the sample beam passes through a small transparent container (cuvette) containing the thin film sample being studied and the reference beam passes through the glass plate which is used as the substrate. The intensities of these beams are then measured by electronic detectors and compared over a period of time. The spectrometer automatically scans all the component wavelengths in this maner. Absorption may be presentedas transmittance T=I/I0 or absorbance A=log I0/I. If no absorption has occurred T=1.0 and A=0.

 

  • Scanning Electron Microscopy (SEM)

SEM is the most widely used electron microscopic technique [3]. Modern advanced SEMs utilize field emission sources. When an electron beam interacts with matter several processes occur which include elastic and inelastic scattering and the emission of secondary electrons and Auger electrons. The different particle or photon emission processes are summarized in the figure (fig 2).

Primary electron beam
Back scattered electrons
Ions/atoms
Characteristic X-rays
Auger electrons
Continuous X-rays
Secondary electrons

Figure1.2. Electron beam induced processes in the sample

2.4.2 1. Principle of SEM

When a well-focused mono-energetic beam is incident on a solid surface various signals can be produced as mentioned above. Back scattered electrons and secondary electrons are particularly useful for SEM application as their intensity depends on the atomic structure of the host atoms. These electrons are collected, amplified and utilized to control the brightness of the spot on a cathode ray tube. (C.R.T). To obtain signals from an area, the electron beam is scanned over the specimen surface by two pairs of electro-magnetic deflection coils and so is the CRT beam in synchronization with this. The signals are transferred from point to point and signal map of the scanned are is displayed on a long persistent phosphor CRT screen. Change in brightness represents change of a particular property within the scanned area of the specimen.

  • Energy Dispersive X-ray Spectroscopy (EDX)

The elemental and chemical nature of the films has been analyzed by SEM having the attached detector for energy Dispersive X-ray spectroscopy. It relies on the investigation of a sample through interactions between electromagnetic radiation and matter analyzing X-rays emitted by the matter in response being hit with charged particles. The fundamental principle is that, each element has a unique atomic structure allowing X-rays that are characteristic of an element’s atomic structure to be identified uniquely from one another. To stimulate the mission of characteristic X-rays from a specimen, a high energy beam of charged particle such as electrons or protons or a beam of x-rays, is focused into the sample being studied.

At rest an atom within the sample contains ground state electrons in discrete enrgy levels or electron shell bound to the nucleus. The incident beam may excite an electron in an inner shell while creating a vacancy –hole- where the electron was. An electron from an outer, higher energy shell then fills the hole and the excess energy may be released in the form of X-ray. The number of photons and the energy of the x-rays emitted from the specimen can be measured by an energy dispersive spectrometer. As the energy of X-rays is characteristic of the difference in energy between the two shells, and of the atomic structure of the element from which they are emitted, this allows the respective ratio of elemental composition of the specimen to be measured.

  • X-ray diffraction (XRD)

XRD is a powerful technique that can used for quantitative as well as qualitative analysis of all crystalline solids including ceramics metals, insulators, thin films, powders etc. It can be used for the determination of crystalline structure and lattice parameters.

The basic principle of determination of crystal structure using X-rays is based on the diffraction phenomenon of electromagnetic wave. Diffraction in general occurs only when the wave length of radiation used is of the same order of magnitude of the repeated distance between the scattering centres. X-rays corresponds to electromagnetic radiation in the wavelength range of 1Å. When matter is irradiated with a beam of X-ray photons, the interaction of photon with the bounded electron mostly result in a coherent scattering of these photons which can be detected using electromagnetic photon detectors. The condition for diffraction is given by Bragg’s law

2dsinθ=nλ

d =the inter planar distance

θ= diffraction angle

λ= wavelength of x-ray

n= order of diffraction

in this technique the diffracted radiation is detected by the counter tube, which moves along the angular range of reflections. The intensities are recorded on a computer system. The ‘d’ values are calculated using the relation 1. for known values of θ, λ and n. The X-ray diffraction data thus obtained is printed in tabular form on paper and compared with JCPDS data to identify the unknown material. The sample used may be powder, single crystal or thin film.

Reference.

  1. I.Y Tok, f. Y. C. Boey,X. L. Zhao, J. Mater. Process. Tech.178(2006) 270
  2. Hyun-Suk Kim, Chang Sool Kim, Sun-Geon Kim, of Non- Crystalline Solids 352 (2006) 2204.
  3. Nano:The essentials, T.Pradeep, McGraw hills Education Privite Limited.

ChapterIII

Structural and chemical Analysis of CdSe thin films prepared by Chemical bath deposition (CBD)

  • Introduction

Among group II-VI compounds CdSe is considered to be a promising material for various applications in the field of optoelectronic technology, because of high efficiency of radiative recombination, high absorption coefficient, direct band gap corresponding to a wide spectrum of wavelengths from UV to IR regions[1].

A variety of techniques have been used for the deposition of thin films such as vacuum evaporation, electro deposition, molecular beam epitaxy, spray pyrolysis, successive ionic layer adsorption and reaction (SILAR) method and chemical bath deposition (CBD)[2-6]. CBD is being largely used as convenient and low cost technique for producing large area thin films of semiconducting materials. Advantages of this method includes use of very simple instrument, does not require high pressure or temperature, high reproducibility, environment friendly, and scope of control of film thickness or stoichiometry by the optimization of various deposition conditions.

Preparation of Cadmium selenide (CdSe) thin films onto precleaned glass substrate using ammonia free Cd and Se precursor solutions employing solution growth technique (SGT) at 80 °C bath temperature was reported by H.K Sadekar et.al[7]. The as-deposited film present excellent adherence, uniform deposition, smooth morphological and nanocrystalline properties, confirmed by SEM, AFM and XRD analysis

The synthesis of CdSe thin films by CBD using tri sodium citrate as capping agent was reported by A.K Singh et.al[8]. Characterization of films was done by XRD,SEM edax and optical absorption studies. All the results indicated that the CdSe nanocrystalline film could be obtained by CBD technique

Cadmium selenide (CdSe) thin films have been deposited onto well cleaned glass substrates at different substrate temperatures by spray pyrolysis by A.A Yadav et.al[9]. Aqueous solutions containing precursors of Cd and Se have been used to obtain good quality films. The as-deposited films were characterized for structural, morphological, optical, electrical and thermo electrical properties.

H.M. Pathan et.al has used the successive ionic layer adsorption and reaction (SILAR) method for the first time to deposit nanocrystalline CdSe thin film onto glass substrates [10]. The SILAR method is a modified version of chemical bath deposition (CBD) method in a way that substrates are immersed in cations and anions alternatively and film growth takes place on the substrates. Characterization of the film was done by XRD , HRTEM ,EDAX optical absorption studies and electrical resistivity measurement The films are Nano crystalline with hexagonal structure.

I.A Kariper has studied the effect of pH on the deposition of CdSe thin films by CBD and has been concluded that researchers can produce CdSe thin films only at pH: 8 and 9. and CdSe cannot be formed at lower pH levels whereas at higher pH levels CdSeO4 was formed instead of CdSe [11] .

This chapter reports the structural analysis of CdSe thin films prepared by CBD. The preparation of thin films is explained in the previous section. Thin films of different thickness are prepared by changing the time of deposition, keeping all other conditions same. Thickness of the film was measured from FESEM by taking the micrograph of the side view of the film. Sample S1 has a thickness of about 700nm and S2 is about 300nm thick. Sample S2 was annealed at 350°C using a muffle furnace

  • Characterization of CdSe thin films prepared by CBD

Characterizations of the films were done by FESEM, XRD analysis and EDX.

  • Structural analysis

The structural elucidation of the prepared CdSe thin film was done by XRD analysis using Bruker AXX D 5005, Germany X ray diffractometer with Cu Kα radiation.

CdSe crystallizes in hexagonal and cubic crystal structures. Cubic structure of CdSe, a low temperature phase changes to hexagonal phase at high temperature. Thus deposition condition has a vital role in the formation of CdSe in various phases.

Figure 1 and 2 show the XRD pattern of S1 and S2 film respectively. The peak observed at 25.27 or 25.73corresponds to 111 plane of cubic phase. The peaks are broad due to the small size of grains [12]. Large FWHM of the peaks gives the indication of very small scale domain.

For annealed film S2 at 350°C, the broadness of the peak has reduced (Figure 3) indicating the improvement in the grain size. Table 1 shows the prominent peaks and their standard value obtained from JCPDS data.

Table1: Prominent peaks and their standard values obtained from JCPDS data

Ssample hkl d space inÅ Cell parameter(a)
Standard observed Standard calculated
S1

S2

S1 annealed

111

111

111

220

3.510

3.510

3.510

2.149

3.746

3.573

3.5187

2.1473

 

6.077

 

5.978

6.488

6.188

6.091

6.0726

The lattice parameter ‘a’ for the cubic structure was determined using the relation

(1)

Where d is the spacing between the planes in the atomic lattice and h,k,l are the Miller indices.

The average grain size ‘D’ for the thin film was obtained by Schrrer formula,

(2)

Where θ is the diffraction angle, β is the full width at half maximum (in radian) and λ is the wave length of X ray radiation (1.5406Å here) used. ‘k’ is a constant known as the shape factor, taken as 0.94 by assuming the shape factor, taken as 0.94 by assuming the crystallite to be spherical in shape. The dislocation density

, shows the amount of defects in the film. (3)

The micro strain (4)

All these parameters are calculated and presented in Table2.

Table2: diffraction angle, FWHM, grain size, dislocation density and micro strain

Sample FWHM Grain size Dislocation density ×1015 Micro strain

× 10-3

S1

S2

Annealed

25.27

25.73

25.82

42.0439

3.84

4.608

1.5360

1.1520

2.2155

1.847

5.539

6.7266

203.7

293.1

32.59

22.1

16.3

19.3

6.53

4.69

Both the films have same crystal structure but the intensities and full width at half maximum value of these peaks changed with thickness. These changes may be attributed to the improvement of crystallinity with thickness. The broad hump in the range 20-30° is due to the amorphous glass substrate. These observations are in accordance with the reported work [13, 14]. Y. Akaltun et al has reported the improvement of crystallinity of CdSe thin films with thickness [15]. A.K Singh etal. has also reported the cubic nature of chemically deposited CdSe thin films [8]. Annealing the film at 350°C has improved its crystalline nature. The grain size in the thin film S2 is found to the least. As the thickness increased, the grain size also has increased in S1film. Annealed film has got the highest value for grain size. Such observations are reported earlier. S. R. Deo et.al reported the increase in the grain size of CdZnSe thin films after annealing [16]. Dislocation density and micro strain were also found to be reduced after annealing.

  • Morphological analysis

The surface properties of films influence their optical and electrical properties. They are the factors which determine their uses in optoelectronic applications. So it is very important to study the surface morphology of thin films. SEM has been proved to be a brilliant, convenient and versatile method for the study of surface morphology of thin films. Many researchers have done the morphological analysis of CdSe thin films prepared by various methods.

SEM analysis of CdSe nanocrystalline thin films prepared using tartaric acid as the complexing agent and cadmium acetate as the source of cadmium was reported by Fekadu Gashaw Hone[17]. The SEM analysis showed that the film surface was composed of spherically shaped grains over the entire substrate surface.

S.Mathuri et.al has reported the SEM analysis of CdSe thin films deposited by electron beam evaporation technique for different distances of source and substrates [18]. The analysis showed that the surface of the film contained particles of various sizes and shapes.

The influence of film thickness on the structural, morphological optical and electrical properties of thin films of CdSe prepared by SILAR method was studied by Y. Akaltum et.al [15]. The surface properties were found to be changed significantly with film thickness.

The influence of annealing temperature on the properties of Nano crystalline CdO thin films synthesized by thermal oxidation process was studied by Vijay B. Sanap and B. H. pawar. [20]. SEM analysis showed the presence of nano walls in the thin film structure.

Microstructural study of chemically deposited Nano crystalline CdZnSe thin films by SEM was reported by S.R. Deo et.al [16]. They have reported that during annealing the recrystallization process densified and the distribution of nanoparticles became more ordered as shown by the SEM micrograph.

A.J Singh etal also used SEM to study the surface morphology of CdSe nanocrystalline thin films [8]. A.A.Yadav et.al also noticed the spherically shaped grains in the SEM micrograph of CdSe thin films prepared by spray pyrolysis [20]

Figure 1 shows the SEM micrograph of film S2 (thickness300nm). Films are without any void or crack and they covered the substrate well. Further magnification (figure 2) shows that the film consists of a network of small very thin tubes which forms a homogenous background over which very small grains are distributed. Figure 1 is the side view of the S2 film, which was used to measure the thickness. The thickness of the film is found to be approximately uniform. The film appeared to be porous with the network of thin tubes

For thick film S1 (thickness 705nm) the number of grains has found to be increased (figure3).

Figure 4 shows the SEM image of the thin film S2 annealed at 350°C.

It can be seen that annealing has improved the structure by increasing the size and number of grains. This can be explained as during annealing the recrystallization process has occurred which made the film denser and fewer defects [17].

The surface morphology of the film has changed significantly with thickness and also by annealing. The increase in grain size with thickness was reported earlier [15]. It was reported that generally semiconductor films with porous structures showed improved performance of solar cells [21, 22]. The porous nature of the surface is also important for various other applications such as optical band pass filters and high sensitive chemical sensors [23, 24]

  • Elemental analysis

Elemental analysis of CdSe thin films prepared by various methods was reported earlier. S Mathuri et. Al has reported the elemental analysis of CdSe thin films prepared by electron beam evaporation technique [18]. In the characterization of CdSe thin films synthesized by CBD, F.G Hone et.al has used EDX to measure the composition [17].

In order to verify the elemental composition of deposited CdSe thin films the EDX spectra of the films were taken

Figure 1and 2 show the EDX spectra of the samples S1 and S2 It confirms the presence of Cd and Selenium in the sample .The atomic percentage of Cd:Se is 50.14: 49.86 and 50.58and 49.42 in thick and thin film respectively, which is very close to 1: ration indicating that the films were in the required stoichiometry.. Presence of Si in the film is due to the silicon content of the glass and since Na2SeSO3 was used as the source of Se, a small amount of sodium was also detected.

 

  • Optical Studies

Optical absorption studies near the fundamental absorption edge provide a productive method to understand the energy gap and band structure of both crystalline and amorphous nonmetallic materials. Analysis of absorption spectra in the low energy region gives information about atomic vibration and that at high energy region provides information about electronic state in materials. Optical properties of materials are controlled by their microscopic, structural compositional and chemical properties.

Sarika Singh et.al has developed CdSe nano particles by CBD. The optical band gap of the sample was found to be 1.7eV [25]. R.B .Kale has reported the effect of annealing on the optical, structural and electrical properties of CdSe thin films prepared by CBD. the optical band gap has found to decrease up to 0.6eV depending upon temperature[26].

Here the study of optical properties of CdSe thin film prepared by CBD is reported. From the absorbance spectra, the band gap energy of the thin films is calculated for two different thicknesses. Effect of annealing the thin film is also investigated.

  • Theory

The optical absorption spectra of semiconductors generally exhibit a sharp rise at a certain value of the incident photon energy, which can be attributed to the excitation of electron from valance band to conduction band. It may also include acceptor or donor impurity levels, traps etc. The conservation of energy and momentum must be satisfied in optical absorption process. Basically there are two types of optical transition-direct and indirect- that can occur at the fundamental edge of crystalline semiconductor. The direct interband optical transition involves vertical transition of electron from the valance band to conduction band such that there is no change in momentum of the electron and energy is conserved. But in indirect transitions the required change in momentum ћk (where k is the wave vector) needs co-operation from a phonon .ie. momentum change may be taken or given up by a phonon.

When the energy of incident photons lies in the range comparable to the energy gap of the material, (with the energy of the incident photon is greater than the optical band gap) the optical absorption shows a power law dependence on photon energy [27] (5)

where Eg is the separation between bottom of the conduction band and top of the valance band, hν is the photon energy and n is a constant that can take values 2, 3, 1/2 ,3/2 for indirect allowed, indirect forbidden, direct allowed and direct forbidden transitions respectively. α0 is a constant depending upon the transition probability for direct transition. If the plot of (αhν) 2 against hν is linear then the transition is direct allowed. The band gap energy Eg is determined by extrapolating the linear portion of the curve to the energy axis at hν = 0.

The refractive index (n) of the thin film is related to band gap Eg by the Moss relation [28,29]

(6)

Where k is constant with a value of 108eV

A different relation between the refractive index and band gap energy is presented by Ravindra as (7)

The dielectric behavior of solids is also an important property. The high frequency dielectric constant εα is related to refractive index as[15]

(8)

The static dielectric constant ε0 of the film is related to energy band gap

(9) Energy band gap and electron effective mass for semiconductor compounds are related as

(10)

  • Experimental

The optical absorption spectra of the prepared samples S1 and S2 and the annealed film were recorded using Sssss spectrophotometer within 200-800nm. Figure 1a,1b and csre the recorded spectra of the samples S1,S2 and annealed thin film respectively. The absorbance spectra show that

  • Result and Discussions

From the absorbance measurement absorption coefficient (α) of thin films was calculated. The variation of (αhν)2 with hν for the thin films are shown in figure 2a,2b and c for S1, S2 and annealed S2 film.

The straight line graphs indicate the direct type of transition. The intercept of these plots on the energy axis give the band gap. The values of band gap energy for the samples are given in table 2. The refractive index of the films was calculated using the Moss’s formula (6) and Ravidra’s formula (7). These values, the values of static and high frequency dielectric constants calculated using equations (8) and (9) and the electron effective mass from equation (10) are also shown in the table

Table 2: the band gap, refractive index, optical static dielectric constant, and high frequency dielectric constant and conduction band edge electron effective mass of various samples

sample Eg(eV) n(Moss’s relation ) n (Ravindra’s relation ) εα ε0 me*/m0
S1

S2

S2 annealed

1.499

1.85

1.58

 

2.913

2.764

2.875

2.885

2.587

2.817

8.485

7.639

8.265

9.913

8.447

9.677

0.2024

0.1472

0.1722