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(1)MATERIAL PROPERTIES OF ZnO THIN FILMS PREPARED BY SPRAY PYROLYSIS by Johannes Lodewikus van Heerden. THESIS submitted in fulfilment of the requirements for the degree. PHILOSOPHIAE DOCTOR in. PHYSICS in the. FACULTY OF SCIENCE at the. RAND AFRIKAANS UNIVERSITY SUPERVISOR: Prof. R. Swanepoel. October 1996.

(2) Acknowledgements. Prof. R. Swanepoel, for his valuable advice, interest and guidance. My parents for their support and encouragement. My wife Liesl for her patients and support. The Foundation for Research Development for their financial support. I thank God for His grace..

(3) Abstract In the search to improve the conversion efficiency of solar cells such as cr-Si and CuInSe 2 cells, attention have recently been focused on the use of transparent conducting oxides (TCO's) as window layers and top electrodes in these cells. Materials such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) thin films were used due to their excellent electro-optical properties, but it was found that they were unstable when subjected to a hydrogen plasma (during the a-Si deposition) and that the materials reduced to their metallic forms, degrading their electrical and optical properties. Zinc oxide (ZnO), however, possess electrical and optical properties equal to ITO and FTO, but is stable in the presence of a hydrogen plasma. In this study a system for the deposition of ZnO thin films by spray pyrolysis was developed and the films successfully deposited. The films were also doped with A1C1 3 in an attempt to further improve the films' conductivities. The films were then characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical measurements (Hall and four-point probe measurements) and optical analyses of the films. The films were compared with films deposited by atomic layer epitaxy (ALE) and DC sputtering. It was found that the films were crystalline with a predominantly (002) preferred orientation. The addition of Al as dopant, however, resulted in the film structure deteriorating. The SEM micrographs obtained of the films indicated films with a close-packed structure, existing of small grains and the film surface having a textured appearance. It was further found that the deposition parameters of the films influenced both the structures of the films and the morphologies and the micrographs indicated that the addition of Al as dopant resulted in the film formation being inhibited and even resulting in no proper film being deposited. It was found that the as-deposited ZnO films were resistive and that the films had to be subjected to a post-deposition annealing to decrease the film resistivity. The annealing conditions were investigated and it was found that annealing the films in hydrogen at their deposition temperature for an hour resulted in the largest decrease in the films' resistivities, typically two orders of magnitude. Studies of the substrate temperature indicated that the films had to be deposited at between 350 and 420°C and that a reduction in the substrate temperature resulted in the film resistivity increasing. Contrary to literature, it was found that the addition of Al as dopant had no beneficial influence on the electrical properties of the films and that dopant concentrations exceeding 1.0 at.% resulted in the film resistivity increasing. The films were characterized optically by analysing the transmission spectra obtained of the films, using the envelope technique. It was found that the films had transmissions exceeding 95% and that the refractive indices and optical gaps centred around 1.99 and 3.3 eV respectively. Both properties were affected by the deposition parameters. The ZnO films deposited by spray pyrolysis compared excellently with the films prepared by ALE and DC sputtering in all aspects. It is hence clear that ZnO films, with characteristics suitable for solar cell application, can be deposited by the simple and inexpensive technique of spray pyrolysis..

(4) O psomming In 'n poging om die doeltreffendheid van sonselle soos a-Si en CuInSe 2 selle te verbeter, het deursigtige geleidende oksiede onlangs baie aandag ontvang vir gebruik as vensterlae en boonste elektrodes in die sonselle. Materiale soos indium tin oksied (ITO) en fluoor gedoteerde tin oksied (FTO) is vir hierdie doel gebruik, as gevolg van hul uitstekende elektro-optiese eienskappe, maar daar is gevind dat hulle onstabiel is in 'n waterstof plasma (gedurende die a-Si deposisie) en dat die materiale tot hul metaalforme reduseer wat tot 'n verslegting van hul elektriese en optiese eienskappe aanleiding gee. Sink oksied (ZnO) daarenteen beskik oor elektriese en optiese eienskappe gelykstaande aan die genoemde materiale, maar is ook stabiel in 'n waterstof plasma. In hierdie studie is 'n sisteem vir die deposisie van ZnO dun lagies deur sproei pirolise ontwikkel en die lagies suksesvol gedeponeer. Die lagies is verder met A1C1 3 gedoteer in 'n poging on the elektriese eienskappe van die lagies te verbeter. Die lagies is gekarakteriseer deur X-straal diffraksie (XRD), skandeer elektronmikroskopie (SEM), elektriese metings (Hall en vierpunt taster metings) en optiese analises van die lagies. Die lagies is ook vergelyk met lagies gedeponeer deur atoomlaag epitaksie (ALE) en gelykstroom verstuiwing. Daar is gevind dat die lagies kristallyn met 'n oorwegend (002) voorkeur orientasie is. Die byvoeging van Al as doteringsmiddel het egter veroorsaak dat die lagie se struktuur versleg. Die SEM foto's verkry van die lagies het 'n dig gepakte struktuur bestaande uit klein korrels getoon, met die lagie oppervlakte getekstureerd. Daar is verder gevind dat die bereidingstoestande beide die struktuur en die morfologie van die lagies beInvloed het, terwyl die SEM foto's verder getoon het dat dotering die lagie se groei vertraag het, tot so 'n mate dat daar selfs geen werklike lagie gedeponeer het nie. Elektries is gevind dat die soos-gedeponeerde lagies hoe weerstand het en dat dit nodig was om die lagies na deposisie uit te gloei om die weerstand te verlaag. Die uitgloeitoestande is ondersoek en daar is gevind dat deur die lagies in waterstof teen hul deposisie-temperatuur vir een uur uit te gloei, die grootste verlaging in weerstand te weeg gebring het, tipies twee grootteordes. 'n Studie van die substraattemperatuur het getoon dat die lagies tussen 350 en 420°C gedeponeer moes word en dat 'n verlaging in die temperatuur die lagie se weerstand verhoog het. Teenstrydig met die literatuur, is gevind dat die byvoeging van Al as doteringsmiddel geen positiewe invloed op die elektriese eienskappe van die lagies gehad het nie en dat doteringspersentasies groter as 1.0 at.% veroorsaak het dat die lagie se weerstand skerp toeneem. Die lagies is opties gekarakteriseer deur 'n analise van die transmissie-spektra deur middel van die "envelope" tegniek. Daar is gevind dat die lagies se transmissies beter as 95% was en dat die brekingsindekse en optiese gapings rondom 1.99 en 3.3 eV onderskeidelik gesentreer is. Beide eienskappe is beInvloed deur die bereidingstoestande. Die ZnO lagies gedeponeer deur sproei pirolise het uitstekend vergelyk met die lagies gedeponeer deur ALE en verstuiwing in alle opsigte. Dit is dus duidelik dat ZnO lagies, met eienskappe geskik vir sonsel toepassings, deur die eenvoudige en goedkoop tegniek van sproei pirolise gedeponeer kan word..

(5) Contents 1 Introduction 1.1 General Information 1.2 Aim of This Study 1.3 Structure of Thesis. 1 1 2 3. 2 Theory 2.1 Properties of ZnO 2.1.1 Introduction 2.1.2 Crystal Structure 2.1.3 Band Structure 2.2 Material Characteristics of ZnO: Spray Pyrolysis 2.2.1 Internal Structure: X-ray Diffraction (XRD) Results 2.2.2 Internal Structure: X-ray Photo-electron Spectroscopy (XPS) results 2.2.3 Morphology 2.2.4 Growth Rate 2.2.5 Film Thickness 2.2.6 Electrical Properties 2.2.7 Photo-electrochemical Results 2.2.8 Optical properties 2.2.9 Evaluation of ZnO Films 2.3 Chemical Deposition of Zinc Salts 2.3.1 Equilibrium thermodynamics of the Zn-O-CI and Al-CI systems 2.3.2 Thermal Analysis 2.4 RF and DC Sputtering RF Sputtering: Ar Atmosphere 2.4.1 Growth models 2.4.2 Composition, Structure and Morphology 2.4.3 Electrical Properties 2.4.4 Optical Properties 2.4.5 Evaluation of RF Sputtered Films RF Sputtering: Ar/0 2 and Ar/H 2 Atmospheres 2.4.6 Morphology and Structure 2.4.7 Deposition Rate 2.4.8 Grain Size 2.4.9 Internal Stress 2.4.10 Conductivity 2.4.11 Optical properties 2.4.12 Evaluation of Sputtered Films. 4 4 4 4 5 14 14 21 25 29 30 31 50 56 65 66 66 69 71 71 72 73 79 80 80 82 82 83 84 86 86.

(6) DC Sputtering: Ar and Ar/B 2 H6 Atmospheres 2.4.13 Grain Size 2.4.14 Structural Properties 2.4.15 Film thickness 2.4.16 Electrical Properties 2.4.17 Optical Properties DC Sputtering: Ar and 0 2 Atmospheres 2.4.18 Film Structure 2.4.19 Composition 2.4.20 Growth Rate 2.4.21 Position of Substrate 2.4.22 Electrical Properties 2.4.23 Optical Properties Ion-Beam-Assisted Reactive Deposition 2.4.24 Method 2.4.25 Structure 2.4.26 Electrical Properties 2.4.27 Optical Properties Dual-Ion-Beam Sputter Deposition 2.4.28 Growth Rate 2.4.29 Film Composition 2.4.30 Film Structure 2.4.31 Electrical Properties 2.4.32 Optical Properties 2.5 ZnO Prepared by Atomic Layer Epitaxy (ALE) 2.5.1 Growth Rate 2.5.2 Morphology and Structure 2.5.3 Electrical Properties 2.5.4 Optical Properties: Transmission 2.6 Chemical Vapour Deposition of ZnO 2.6.1 Film Growth Kinetics 2.6.2 Growth Rate 2.6.3 Structure and Morphology 2.6.4 Film Composition 2.6.5 Electrical Properties 2.6.6 Optical Properties 2.6.7 Film Roughness 2.6.8 Degradation 2.6.9 Evaluation of Films: Figure of Merit 2.7 Chemical Deposition of ZnO 2.7.1 Deposition by Immersion in Aqueous Solution 2.7.2 Electrodeposition from Aqueous Solutions II. 87 88 89 89 92 93 93 94 94 95 99 100 100 101 101 101 102 103 103 103 104 104 104 105 105 106 106 106 110 113 114 118 120 121 121 123 123 124.

(7) 2.8. 3. 127 127 132 134 137 138 138 141 143 148. Experimental. 149. 3.1 3.2. 149 149 149 150 151 154 155 155 156 156 156 156 157 159. 3.3 3.4 3.5 3.6 3.7 3.8 4. Application of ZnO Thin Films Application of ZnO: Spray Pyrolysis 2.8.1 Crystalline Si Solar Cells 2.8.2 CdS/CuInS 2 Solar Cells Application of ZnO: Sputtering RF-sputtering: Argon Atmosphere 2.8.3 a-Si:H Solar Cells 2.8.4 CIS/CdS/ZnO Solar Cells 2.8.5 ZnO/CdS/CuGaSe2 Single Crystal Solar Cells DC Sputtering: Ar and 0 2 Atmosphere 2.8.6 Single Crystalline Si Solar Cells Application of ZnO: Atomic Layer Epitaxy (ALE) 2.8.7 CuInSe2 (CIS) Solar Cells Application of ZnO: Chemical Vapour Deposition (CVD) 2.8.8 a-Si Solar Cells 2.8.9 CIS Solar Cells Introduction Film Deposition 3.2.1 Substrates 3.2.2 Precursor Solution 3.2.3 Deposition System and Procedure 3.2.4 DC Sputtering Film Thickness Annealing Structure Morphology Electrical Measurements 3.7.1 Four-Point Probe Measurements 3.7.2 Hall Effect Measurements Optical Measurements. Results and Discussion. 161. 4.1 4.2. 161 161 161 163 164 165 167 167 168 170. 4.3. Introduction Growth Rate 4.2.1 Solution Concentration 4.2.2 Substrate Temperature 4.2.3 Dopant Effect 4.2.4 Position of Substrate Film Structure 4.3.1 Film Thickness 4.3.2 Substrate Temperature 4.3.3 Dopant Influence. III.

(8) 5. 4.3.4 Solution Concentration 4.3.5 Substrate Influence 4.3.6 Annealing 4.3.7 Deposition Method 4.4 Surface Morphology 4.4.1 Film Thickness 4.4.2 Solution Concentration 4.4.3 Substrate Temperature 4.4.4 Dopant Influence 4.4.5 Post-Deposition Annealing 4.4.6 Substrate Influence 4.4.7 Deposition Method 4.5 Electrical Properties 4.5.1 Post-Deposition Annealing 4.5.2 Film Thickness 4.5.3 Substrate Temperature 4.5.4 Concentration of Precursor Solution 4.5.5 Al Dopant Influence 4.5.6 Substrate Influence 4.5.7 Ultraviolet Radiation 4.5.8 Deposition Method 4.6 Optical Characteristics 4.6.1 Transmission 4.6.2 Refractive Index 4.6.3 Optical Gap. 174 177 179 183 185 185 186 188 189 194 196 197 199 199 208 210 212 216 222 222 223 225 225 229 233. Conclusion 5.1 Material Properties 5.2 Recommendations for further research. 238 238 243. Publications and Presentations. 244. References. 245. IV.

(9) Chapter 1 Introduction 1.1 General Information At present, solar cell generated electricity is still more expensive than the electricity generated by fossil fuels (grid electricity). The cost of the electricity generated by solar cells (crystalline Si, a-Si or CuInSe 2 ) depends on the manufacturing cost and the energy conversion efficiency of the cell. Although there is still room in the manufacturing for new techniques and materials to decrease the production costs, the largest innovations are occurring in the increase of the cells' conversion efficiencies. One of these is the employment of transparent conducting oxides (TCO's) as top electrodes and window layers in the solar cells. Initially indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) thin films were used due to their excellent electro-optical properties. It was, however, found that the films reduced to their metallic forms of In and Sn when they were subjected to a hydrogen plasma during the deposition of a-Si cells. This resulted in the electrical and optical properties of the films degrading and together with the fact that the metals diffused into the a-Si, decreasing the cell's efficiency (Fukuda (1982) and Kitagawa (1983)). To overcome this problem, other materials were investigated. One of these is zinc oxide (ZnO). ZnO thin films are known to possess a c-axis oriented hexagonal wurtzite structure (Ghandhi (1980)) and is highly transparent and conductive as well as very stable when subjected to a hydrogen plasma. Additional advantages include the abundance of source material and a wide band gap with a cut-off at 3.3 eV (Goyal[3] (1992)). In order to deposit the ZnO films that will meet the requirements of having a stable, well oriented structure and exhibiting good electro-optical properties, many different techniques have been used. Some of these are:. Sputtering (RF or DC): Employing either the sputtering of a ceramic ZnO target in a pure Ar (Sato (1993)) or Ar-0 2 (Privato (1995)) atmospheres or the sputtering of a metallic Zn target in a Ar-0 2 (Igasaki (1991)) atmosphere. Advantages of these methods are the deposition of homogeneous films with low resistivities of 2-6x10 -4 1.

(10) 1.2 AIM OF THIS STUDY. 2. Skin, transmissions of 85%, fast growth rates and low deposition temperatures of about 140 to 250°C (Sato (1993) and Nakada (1994)). The low resistivities were obtained for Al-doped films. Disadvantages are the need for vacuum deposition, the possible damage of the substrate due to the sputtering as well as the high apparatus costs. Atomic Layer Epitaxy (ALE): The most controlled of the deposition methods, where the film is deposited atom layer by atom layer. The technique offers reproducible and uniform films by using dimethyl or diethyl zinc as zinc precursors and doping the films by adding trimethyl aluminium as dopant. Low resistivities around 8 x10 -4 Ocm are obtained with transmissions of 85%. The technique also allows for a low deposition temperature of 150 to 250°C and the deposition of large area films (Lujala (1994)). Disadvantages are the high apparatus cost and the toxicity of the precursor materials. Chemical Deposition: Deposition by immersion of the substrate in a solution (Ristov (1987) or by means of the cathodic electrodeposition from zinc solutions (Peulon (1995)). Both technique are uncomplicated and easily employed. An added advantage is the low substrate temperature of about 80°C. Films obtained by immersion were resistive, with transmissions exceeding 85%. Electrodeposition resulted in films with transmissions of about 75%. Disadvantages of these methods are the films' high resistivities and the fact that the electrodeposition can only take place if the substrate is conductive. Chemical Vapour Deposition (CVD): The deposition by means of diethyl zinc and using triethyl aluminium as dopant at either atmospheric pressure or under vacuum. The diethyl zinc/ethanol mixture is heated and the vapour allowed to drift over the substrate in the reactor (Hu (1992)). Advantages are the low resistivity 3-8x10 -4 ftcm, high transmissions (85%) and high growth rates. Disadvantages are the high deposition temperature (typically 400°C) and the toxicity of the precursor materials.. 1.2 Aim of This Study Taking into account the above information, it was decided that this study will focus on the deposition of ZnO thin films by means of spray pyrolysis. Spray pyrolysis is the name given to the method where a very fine mist is generated, eithei pneumatically (Guillemoles (1991)) or by ultrasonic vibration (Song (1994)), and the mist is then allowed to deposit onto a heated substrate. The mist is generated from a solution consisting typically of zinc acetate, an alcohol and de-ionized water. Al or In is usually employed as dopant. What made spray pyrolysis an attractive deposition method for the deposition of the films, was its simplicity, extreme cost effectiveness, suitability for the production of large area films and the fact that highly conductive and transparent films could be produced by this method. A disadvantage was the relatively high deposition temperature, typically 420 to 450°C (Pushparajah (1994) and Song (1994)). The aim of this study was hence to develop a system for the deposition of the ZnO films by the spray pyrolysis technique and to fully characterize the deposited films. During.

(11) 1.3 STRUCTURE OF THESIS. 3. the system development, attention was given to the materials used in the precursor solution, methods of generating the solution spray and the physical shape and dimensions of the reactor chamber and complete system setup. The films were characterized, structurally by means of X-ray diffraction (XRD), morphologically by scanning electron microscopy (SEM), electrically by four-point probe and Hall measurements and optically by an analysis of the film's transmission spectra obtained using a spectrophotometer, scanning in the wavelength range 300 to 900 nm. By optimizing the deposition parameters (the precursor solution concentration, the substrate temperature and the dopant concentration), it was attempted to make the films as conductive and transparent as possible, since the films were in a further study to be used as a buffer layer and top electrode on a-Si solar cells and top electrode on CuInSe 2 solar cells. Since the deposition temperature of the a-Si solar cells by means of glow discharge deposition is about 200-220°C, it was attempted to lower the known deposition temperature of the ZnO films.. 1.3 Structure of Thesis Chapter 2 presents the known material properties as well as information concerning the deposition and characterization of ZnO thin films by spray pyrolysis, RF and DC sputtering, chemical deposition, chemical vapour deposition (CVD) and atomic layer epitaxy (ALE). Attention is also given to applications of the ZnO films, deposited by the mentioned methods, on various types of solar cells. Chapter 3 provides information on the apparatus and method used in the deposition of the ZnO films, as well as information on the characterization techniques employed. Chapter 4 presents the results obtained in the characterization of the ZnO films, under the following headings: Growth Rate, Film Structure, Surface Morphology, Electrical Properties and Optical Characteristics. Original information obtained from this study include the complete characterization of the structural and morphological properties as functions of the deposition parameters, the optimization of the annealing conditions resulting in the largest decrease in the film resistivity as well as the influence of the deposition parameters on the refractive index of the ZnO films. A condensed version of the results as well as the conclusions reached from this study are provided in Chapter 5..

(12) Chapter 2 Theory 2.1 Properties of ZnO 2.1.1 Introduction In order to be effective as the top electrode on both a-Si and CIS solar cells, ZnO thin films have to meet certain requirements, as was discussed in the previous chapter. A detailed study of the known theory concerning ZnO is hence of the utmost importance, for it will serve as guidelines in the development of a method of deposition of such films that will indeed meet these requirements. Characteristics that will be discussed include not only those determined for films deposited by spray pyrolysis, but also those of films deposited by means of the other methods mentioned earlier. Attention will also be given to the application of the ZnO films, as well as some results achieved in using these films as TCO's on solar cells.. 2.1.2 Crystal Structure ZnO belongs to the Zincite group and is hexagonal, dihexagonal-pyramidal in structure (Palanche (1944) and Hurlbut (1959)), depicted in Figure 2.1(a,b,c).. (b). (a). (c. ). (a) Dihexagonal Pyramid, (b) Stereogram of Dihexagonal Pyramid and (c) Zincite structure (Hurlbut (1959)). Figure 2.1:. 4.

(13) 2.1 PROPERTIES OF ZNO. 5. ZnO further resembles the ZnS wurtzite structure (Bloss (1971)), depicted in Figure 2.2. The ZnO crystal has a a:c ratio of 1:1.5870 and a p o :ro ratio of 1.8325:1. The cell structure belongs to the C6mc space group. The ZnO crystal further contains Zn: 80.34 and 0: 19.66 per cent mass ratio.. Figure 2.2: Similarity of the wurtzite structure to two interpenetrating hexagonal closest-packing-like arrays, one of sulfur, the second of zinc atoms (Bloss (1971)).. As can be seen in Figure 2.2, the Zn atoms are nearly in the position of hexagonal close packing. Every oxygen atom lies within a tetrahedral group of 4 Zn atoms and these tetrahedra all point in the same direction along the hexagonal axis, giving the crystal its polar symmetry.. 2.1.3 Band Structure Experimental Band Structure of ZnO It is known that angle-resolved UV photo-electron spectroscopy (ARUPS) allows the experimental determination of the valence bands throughout the whole k-space, if the measurements are performed in connection with synchrotron radiation for the excitation (Himpsel (1983)). If a simple, nearly free electron-like final state and kconservation for the transition from the initial into the final states is assumed, the peaks in the observed spectra can be described quite accurately and the initial band states derived. Figure 2.3 represents the first Brillouin zone of bulk ZnO, including the conventional labeling of the symmetry elements..

(14) 2.1 PROPERTIES OF ZNO. 6. Schematic diagram of the first Brillouin zone for wurtzite ZnO (Zwicker (1985)).. Figure 2.3:. >. lines are perpendicular to the (1010) planes in real space. Hence, the spectra The taken at normal emission on the (1010) face by Zwicker (1985), probed transitions from initial states with k vectors, lying along the E line. In Figure 2.4, a representative set of ten spectra, obtained by Zwicker, is shown for which the photon energy has been varied. The initial state energy (E t ) can be divided into two regions, 0 eV to -6 eV and -6 eV to -9.5 eV. It is known (Ranke (1976) and Ley (1974)) that the valence bands appears between 0 eV and -6 eV. From calculations (Chelikowski (1977) and Ivanov (1981)) as well as from measurements of the cross-section for different parts of this region, it is known that the states at the bottom of the bands near -5 eV are derived from the Zn 4s states and those near the valence band maximum from 0 2p states. In this region several peaks and shoulders are recognized in Figure 2.4 (e.g. 5 for the 41.7 eV curve). Furthermore, most of the peaks show dispersion with energy. This dispersion is indicated in Figure 2.4 by the broken lines which connect peak positions, as evaluated by the peak resolving procedure. The large peak in the region -6 eV to -9.5 eV is due to the Zn 3d level. As seen, distinct structures can be discerned for this peak too. Two peaks, separated by about 0.8 eV, can be resolved. Some dispersion can also be recognized. So far, there has been an agreement among different groups that the Zn 3d states are not coupled to the valence states, i.e. they have been treated as non-itinerant core-like states (Rossler (1969) and Ivanov (1981)). This renders them suitable as an energy reference. There are time-dependent changes of work function and band bending for.

(15) 7. 2.1 PROPERTIES OF ZNO. IN TEN SI TY I ARB. UNITS I. the ZnO surfaces. Thus, beside the frequently repeated annealing cycles executed by Zwicker, the Zn 3d peak was used to correct for small band bending effects. This was feasible in spite of the structure in the 3d peak, by aligning the high- and low-energy flanks of the peak.. -12. -10. -8. -6. -4. -2. 0. E-Ievi. Angle-resolved UV photo-electron spectra for normal emission (9 = 00 ). The initial state energy Ei is referenced to the valence band maximum. The photon energy hw is as indicated (Zwicker (1985)). Figure 2.4:. For the evaluation of the band structure along the > line, the spectra in Figure 2.4, were deconvoluted into Gaussians. This was justified since the energy resolution is mainly due to that of the TGM. Up to five Gaussians were used for the deconvolution. The typical FWHM was about 1 eV. For the final state a free electron state was assumed with the amount of the wave vector k f: kf(a -1 ) = 0.512 ✓Ekin eVo where Eki n is the kinetic energy of the emitted photo-electrons in eV and Vo the inner potential of the electrons. With a V o of about 10 eV, the final states reached by the photon energies used are lying along F2M2F2, where the indices indicate the Brillouin zones in an extended scheme..

(16) 8. ENERGY I e Vl. 2.1 PROPERTIES OF ZNO. 1.0. 0.5. 0. r. M WAVE VECTOR W[AC I. Figure 2.5: Band structure for ZnO along the FEN' line. The energy is referenced to the valence band maximum. The dots connected by the broken lines are the experimental values, obtained by Zwicker (1985). The solid lines are from the theoretical calculations of Chelikowski (1977) (Zwicker (1985)).. The band structure for ZnO along the rEm line is presented in Figure 2.5. It is clear that five different bands are resolved experimentally in the FM direction. A sixth band, also indicated around -1 eV in Figure 2.5, may be hidden in the scattering of the points. It should, however, be pointed out that within the calculation, the uppermost two bands are split by only less than 400 meV throughout the whole Brillouin zone and therefore it may be difficult to resolve them. The failure to resolve these peaks may be due to reasons of preparation or to some lack in angular resolution. Since there were only a few experimental points near F, it was difficult to determine the top of the valence band. When Zwicker (1985) compared their experimental results with the calculations of Chelikowski (1977), it was seen that the experimental curves were generally not as.

(17) 2.1 PROPERTIES OF ZNO. 9. smooth as the calculated curves. The main difference was found in the two deepest bands, which seems to cross near k = 0.65A - I, giving rise to a hybridization gap. The reconstruction of the ZnO(1010) surface as it is deduced so far from the LEED work (Duke (1977)), consists of a vertical downward shift by 0.45A, of the surface Zn atoms whereas the 0 atoms remain nearly at their position anticipated from the bulk. This reconstruction makes it reasonable that the surface states are shifted downwards into states resonant with bulk states. This was also found in the wave-vector-resolved layer density of states calculation of Ivanov (1981). At the F point of the projected density of states, Ivanov found a resonance at -1 eV, overlapping the bulk emission. This emission also seems to overlap the bulk emission in the measurements of Zwicker. In the spectra around hw = 40 eV, a shoulder can be seen at -0.5 eV which does not shift downwards with hw. This shoulder may be due to the resonance from the 0 surface atoms. At photon energies around 25 eV the surface resonance overlaps with a peak from the bulk. This peak was sharpened by the annealing procedure of Zwicker, indicating again the surface resonance. A great deal of attention has been focused on the structure of the Zn 3d level. The spin orbit splitting for the atom is 0.34 eV (Morre (1958)). For Zn metal, Himpsel (1980) observed a split into three bands centered at 9.7, 10.3 and 10.6 eV below E f. The upper level showed a dispersion of 0.17 ± 0.05 eV. From these and other findings, Himpsel deduced evidence for the itinerant character of these levels. In the spectra obtained by Zwicker one peak and one shoulder are always visible and there are spectra with two separate peaks also visible. The levels are separated by about 0.8 eV when decomposed into Gaussians. A direct interaction between Zn atoms of the (0001) Zn planes is not likely, since the next neighbour distance is 3.25A in comparison to 2.76A in the metal. Crystal field splitting is not believed to amount to such a large level splitting (Ley (1974)). The structure in the Zn 3d emission is most likely due to an interaction via the 2p and eventually also the 2s levels of the oxygen neighbours. The interaction with the oxygen 2s levels is not as unlikely as it may be anticipated in view of its deep position at about -20 eV. It is interesting to note that it shows a dispersion of 0.6 eV in the calculations of Chelikowski (1977). Furthermore, the FWHM of the Zn 3d level is 0.8 eV in the case of ZnTe (Shevchik (1973)), and about 2 eV for ZnO. This indicates a stronger interaction between the Zn 3d and the 0 2p bands, than between the Zn 3d and Te 5p. This is reasonable, since the binding energy of 0 2p is larger than that of Te 5p. Vice versa, if an interaction via the oxygen neighbours takes place, the Zn 3d levels had to be taken into account also for the band calculation. Thus, no final conclusion about the structure in the Zn 3d level could be reached.. Theoretical Calculation of the ZnO Band Structure In order to verify the experimental results of Zwicker (1985), Yang (1993) calculated the ZnO band structure theoretically using the linear muffin-tin-orbital method, employing the atomic sphere approximation, or LMTO-ASA method (Andersen (1975).

(18) 10. 2.1 PROPERTIES OF ZNO. and Skriver (1984)), with the Zn 3d electrons included self-consistently with the other valence electrons. The LMTO-ASA method was first used by Svane and Antonchik (1986) to calculate the band structure of ZnO in the zinc-blende structure with the Zn 3d levels treated as frozen core states. Their results were used as a starting point by Yang (1993) in the search for the potential parameters for ZnO in the wurtzite structure. Two equivalent empty spaces centred at (a, a/V3, 3c/16) and (a, a/V3, 11c/16) were included in the unit cell to take care of the large open interatomic spaces in the lattice. The choice of atomic spheres radii is not unique in this problem and was discussed by Svane and Antonchik (1986). One approach is to choose the same radius for all the spheres which leads to a radius of 2.333 a.u. for the wurtzite structure and a radius of 2.125 a.u. for the zinc-blende structure of the same density. Another approach is to let the ratio of the radii approximately equal the ratio of the tetrahedral radii introduced by Phillips (1973). For ZnO this latter approach is not possible as the oxygen radius becomes too small. Yang chose a compromise by using the average radius of 2.333 a.u. for Zn and tried several smaller radii for oxygen near the value 2.125 a.u., used by Svane and Antonchik. It was found that the results seemed to be stable near a radius of 2.115 a.u.. The set of radii adopted by Yang is presented in Table 2.1, as well as the corresponding set of potential parameters that resulted in a fully self-consistent band structrure. Potential parameters calculated by Yang. C and V are the estimates of the positions of band center and square-well pseudo-potential, respectively, with respect to E. The parameter -y is a measure of the transform from the canonical bands to energy bands. Table 2.1:. Radius. Ey. C. V. a.u.. mRy. mRy. mRy. -837 -704 -1155 -1590 -624 -930 -879 -860 -850. -1993 -721 -1934 -2566 1300 2814 261 1589 3541. -2563 -2616 1478 -2830 -2512 -1760 -267 -365 -410. s Zn. 2.333. p. d s 0. 2.115. p. d s. Em. 2.517. p. d. 7 0.4148 0.1081 -0.004 0.3566 0.0543 0.0615 0.4481 0.1273 0.0690. -1/2 Ry 5.294 7.226 0.882 1.930 1.948 14.31 7.403 11.22 16.78. P. Figure 2.6 represents the calculated ZnO band structure, while in Figure 2.7 the upper valence bands from F to M are compared with Zwicker's ARPES results..

(19) 11. 2.1 PROPERTIES OF ZNO. 0. -1. -2. -4. -5. -6 A. R. LU. M. E. I' A A. S. H P K. k. 1. 0.8. 0.4 0.6 Wavevector k. 0.2. 0. Figure 2.6: Theoretical band structure of ZnO (Yang (1993)). Figure 2.7: Calculated upper valence band states (solid lines), compared with. ARPES results (dots) (Yang (1993)).. The conduction bands dispersion shown in Figure 2.6 is quite similar to those derived by Chelikowski (1977), but the band widths are generally lower in the calculation by Yang. The lower conduction bands have a width of about 5 eV compared to 6 eV (Chelikowski). The upper conduction bands start at about 10 eV compared to 12 eV. A fundamental gap of 2.54 eV was obtained by Yang, which is much smaller than the experimental value of 3.3 eV. The discrepancy arose because of the local density functional approximation (LDA) used in Yang's calculation. The dispersion of the upper valence bands shown in Figure 2.7 are in better agreement with the experimental results of Zwicker. The lowest two upper valence bands seem to exhibit a hybridization gap and the third band from the bottom showed a minimum. Because the overall width of calculated upper valence band states is about 1 eV less than the experimental width, it is not possible to make a comparison of the absolute position of the bands. However,.

(20) 2.1 PROPERTIES OF ZNO. 12. a semi quantitative comparison of the dispersion can be made if the calculated bands are shifted down by 0.5 eV. This adjustment is done in Figure 2.7 and the result shows that except for the fifth band, the shape of the bands are in general agreement with the experimental values.. '. 60. I. ,. .. .. S. -. _. d. _d. II II II 11 4. II. li. 40. rl I II. I. :I I. -10. 0 E(eV). 10. 111. I, 0. -10. 0 E(eV). 10. Figure 2.8: Partial wave density of states of 0 in ZnO (Yang (1993)). Figure 2.9: Partial wave density of states of Zn in ZnO (Yang (1993)).. From Figure 2.8 it can be seen that the upper valence bands consist mostly of 0 2p states. Figure 2.9, however, showed that near the bottom of these bands there is a significant contribution from Zn 4s states which is the source of the hybridization gap. The lowest band is the 0 2s band. In Figure 2.8 it is clear that this band is at -16 eV and has a width of 0.5 eV, while the position of this band was experimentally determined to be at about -20 eV. Svane and Antonchik obtained results similar to the above for this band in the other zinc chalcogenides and attributed the discrepancy with the experiments to the LDA approximation. Between the upper 0 2p valence bands and the lowest 0 2s band, is the Zn 3d band. The Zn 3d band is found at -9 eV (Figure 2.10) and has a width of about 1.5 eV. The band also splits into two peaks separated by 0.9 eV. The result is in good agreement with the experimental results of Zwicker. Examination of the oxygen density of states (Figure 2.8) near -9 eV, revealed that the lower part of the Zn 3d band mixed with the 0 2p states and the upper part of the band mixed with the 0 2s states, thus verifying Zwicker's proposed interaction between these states..

(21) 13. 2.1 PROPERTIES OF ZNO. 800. tn O o 400. 200. 0 -10. 0 E(eV). 10. Figure 2.10: Total density of states of ZnO (Yang (1993))..

(22) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 14. 2.2 Material Characteristics of ZnO: Spray Pyrolysis 2.2.1 Internal Structure: X-ray Diffraction (XRD) Results One of the most important tools in the structural analysis of ZnO thin films is that of X-ray diffraction or XRD. The X-ray diffractograms gives information concerning the crystalline nature of the films, as well as the influence of the various deposition methods and deposition parameters on the film structures. Of the utmost importance in the analysis of the X-ray diffractogram data is the Standard Powder Diffraction File data for ZnO, ASTM (1969). This standard reference data for ZnO powder diffraction is listed in Table 2.2.. Table 2.2:. Standard powder diffraction data for ZnO.. Diffraction peak Normalized peak intensity. (100) 0.71. (002) 0.56. (101) 1. (004) 0.03. Parameters Influencing the Internal Structure: Precursor Solution. Two samples, one deposited from a methanol solution of 0.014M zinc acetate and another from a solution of 0.067M zinc nitrate were prepared by Andres-Verges (1992). At a deposition temperature of 400°C, the film deposited from the zinc acetate solution exhibited a random orientation, while the zinc nitrate film had a (101) preferred orientation. Secondary peaks visible in both diffractograms were the (002) and (100) peaks. When the substrate temperature was increased to 600°C, both films exhibited a (101) preferential orientation and the sharper defined diffraction peaks indicated an increase in the crystallinity in both the films. A further temperature increase to 875°C resulted in almost identical diffractograms for the two films, each having three well defined peaks, (101), (002) and (100), with the (101) and (100) peaks having equally the highest intensity. Dopant Concentration: Aluminium. Goyal[1] (1992) deposited films from a mixture of zinc acetate (0.05M), de-ionized water and methanol on substrates at a temperature of 500°C, doped with aluminium. Figure 2.11 represents the diffractograms of four films, deposited with a varying amount of Al dopant. It can clearly be seen that all the films are polycrystalline in nature, with.

(23) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 15 a (002) preferred orientation. The weaker orientations observed are the (100), (101), (102) and (103) orientations. A comparison of the four films showed that an increase in the dopant concentration, resulted in the (002) peak becoming more pronounced, and hence the crystallites more oriented. Song (1994) too observed the dominance of the (002) peak, as well as the enhancement of the c-axis orientation with the Al dopant concentration increasing.. (004). (103). (101)(002)000). (102). IN TEN SITY(a r b• un its ). 1. (d). (c). 70. 65. I. 50. 45. 40. 35. 30. 2 A (DEGREES). Figure 2.11: X-ray diffractograms of Al-doped ZnO (AZO) films with dopant. concentrations (a) 0 at.%, (b) 0.6 at.%, (c) 1.0 at.% and (d) 1.4 at.% (Goyal[11 (1992)). Further information from the diffractograms can be gained from an analysis of the texture coefficient (TC), as defined by Barret (1980), as the extent of preferred orientation, compared to other observed orientations. The texture coefficient of a plane (hkl) is given by. TC(hkl) —. 1(hk1)110(hk1) 11N EN I(hk1)11.(hk1)'.

(24) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 16 where TC(hkl) is the texture coefficient of the (hkl) plane, I(hkl) is the measured relative intensity of the (hkl) plane, I0 (hkl) is the relative intensity of the corresponding plane as given in ASTM (1969) data and N the reflection number. Goyal[1] (1992) used the above formula of Barret and determined the TC as a function of the dopant concentration, shown in Figure 2.12. TC(002) showed a gradual increase with the dopant concentration, except for the higher dopant concentrations. Goyal found that higher doping resulted in an interstitial inclusion of dopant atoms which results in a poor structure of the films and hence a deterioration in the texture value. The same tendency was observed by Agashe (1988), for sprayed fluorine-doped tin oxide films. The same disordering of the crystalline structure for higher dopant concentrations, due to impurity scattering, was witnessed by Song (1994). 10. 8. 0. 0.2. 04. 3.6. 0.8. 1.0. 1.2. 1.4. al/ Zn (at. A). Figure 2.12: Variations of. TC(002) with the dopant concentration (Goyal[1]. (1992)). Dopant Concentration: Indium Tiburcio-Silver (1991) investigated the influence of using indium as dopant, on the structure of the ZnO films. The film deposited from a solution containing 2 at.% In, exhibited the preferred orientations (002) and (101). Major (1983) too observed the and (002) diffraction peaks for a film containing 2 at.% In, as well as the (103) and peaks. Comparing the undoped to the In-doped film, Major found that the (002).

(25) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 17 peak had become much smaller in intensity and the (101) peak had become enhanced, indicating a reorientation with In doping. The same behaviour with 2 at.% In dopant was observed by Abd Lefdil (1995). Goyal[2] (1992) found that the film deposited with a dopant concentration of 2 at.% In, showed a totally random orientation, which closely matched the powder pattern for ZnO, the (002) orientation having almost disappeared. Goyal[2] (1992) found that the crystallites in films deposited from a solution containing a higher In concentration (5 at.%), exhibited a preferential growth along the [100] direction. Although the (102), (103), (110) and (201) orientations were still observed, their intensities were weak and not influenced significantly by the increased amount of indium dopant. Goyal further presented the preferred orientation of the crystallites by the texture coefficient TC(hk1). The variation in the texture coefficients along the (002), (101) and (100) orientations were investigated with a variation in the dopant level. It could clearly be seen that TC(002) decreased continuously with an increase in the dopant, while TC(101) and TC(100) increased with doping. Goyal explained the changes in the X-ray diffractograms on the basis of c/a measurements. It is known that preferentially-oriented ZnO films have internal stresses (Miura (1982)), which are responsible for the shift in the angular positions of the reflection peaks in the X-ray diffractograms. The lattice constants and hence c/a were calculated using the angular positions of the reflection peaks. Goyal observed that with an increase in the dopant level the cla ratio of the films decreased towards the bulk value. The actual value of the c/a ratio calculated for the 2 at.% In-doped film was found to be 1.5982, which is very close to the 1.6024 value calculated from the powder pattern for ZnO. When the dopant concentration was increased, the c/a ratio increased slowly, which may have been due to an excess incorporation of indium which leads to a preferential orientation of the grains along the [100] direction. It was thus seen that indium incorporation during the growth of an indium zinc oxide (IZO) film initially reduced the internal stresses, so that the film possesses its random orientation. Excess incorporation of indium led to an increase in c/a and preferred growth along the [100] direction. When, however, Tiburcio-Silver (1991) increased the dopant concentration to 6 at.% In, the crystallites lost the above mentioned orientations and became strongly oriented along the [110] axis, parallel to the substrate. No lines corresponding to indium or its oxide could, however, be detected. Sreedhara Reddy (1987) deposited ZnO films from a solution containing ZnC1 2 and employed In as dopant by adding indium trichloride to the precursor solution. Two diffractograms were studied, belonging to a undoped and 1.5 at.% In-doped film. Both films were found to be polycrystalline with the wurtzite structure. The undoped ZnO films exhibited a preferred (002) orientation, indicating a orientation of crystallites perpendicular to the substrate surface. In the doped film, the (002) orientation had become strongly diminished, while the (101) orientation had become preferential..

(26) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 18 Dopant Concentration: Lithium Pushparajah (1994) as Sreedhara-Reddy, deposited ZnO films from a solution containing ZnC12 but doped the films by adding lithium chloride as dopant. These films, similarly to the ones discussed before, were found to be polycrystalline with a hexagonal structure and a preferred orientation with the c-axis perpendicular to the substrate, as was found by Ianno (1992) and Zhang (1992). Pushparajah studied the influence of the deposition temperature on the structure of three films, deposited at 380, 400 and 480°C. The only diffraction peak observed at 380°C was the (002) peak at 20 = 34.5°. When the substrate temperature was increased to 400°C, the (101) and (100) peaks were also observed at 20 = 36.2° and 31.7° respectively. It was further observed that the (002) peak decreased in intensity. When the substrate temperature was further raised to 480°C, the (002) peak diminished even further. The decrease in intensity was observed for the secondary peaks as well. Pushparajah further obtained a diffractogram of a film deposited at 400°C, but doped with 15% Li. The diffractogram showed no preferred orientation of crystallites, compared to the polycrystalline structure found for the undoped films. It was thus concluded that the addition of the Li to the precursor solution resulted in the film becoming amorphous. Dopant Concentration: Cobalt Bahadur (1992) deposited ZnO films from a 0.1M solution containing zinc nitrate and using cobalt nitrate as the dopant, at a substrate temperature of 400°C. The diffractograms obtained, showed both the undoped and the doped films to exhibit a polycrystalline nature, having a hexagonal wurtzite structure. It was observed that the diffractograms of the Co-doped films were identical to that of the undoped ZnO films, having no additional peak. This indicated the substitution of the dopant ions (Co 2+) inside the lattice and the absence of any free cobalt oxide. Unfortunately the diffraction peaks observed by Bahadur were not listed in the study. A further study by Bahadur (1994) on the doping of the ZnO films with Ni or Ni + Co, indicated the formation of a single phase and the spectra obtained were identical to that of the undoped ZnO films. It was concluded that the dopant ions were introduced inside the host matrix of ZnO. Dopant Concentration: Fluorine When NH 4 F was added to the starting solution, Sanchez-Juarez (1995) found that the films became randomly oriented and a further increase in the dopant resulted in the films having an almost amorphous structure. The effect the dopant had on the films can be attributed to two possibilities. One is that the fluorine atoms do not substitute.

(27) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 19 oxygen atoms, instead they occupy interstitial sites and generate a large number of dislocations. The other can be due to a possible ZnF 2 formation. Solution Concentration The structural properties of undoped, sprayed ZnO films, deposited from solutions containing varying amounts of zinc acetate in the precursor solution were investigated by Goyal[3] (1992). In this study the molarity of the starting solution was varied from 0.01 to 0.2M zinc acetate. The other process parameters were kept constant. Goyal obtained diffractograms for three films, deposited from solutions containing 0.02, 0.1 and 0.2M zinc acetate. The diffractograms clearly indicated that the films were polycrystalline with a (002) preferred orientation. The (101), (102), (103), (112) and (201) secondary orientations were also present. Further, it could clearly be seen that an increase in the zinc acetate concentration resulted in the intensity of the (002) peak becoming gradually larger. This result will be explained in terms of the texture coefficient. Goyal[3] (1992) investigated the variation of TC(101) and TC(002) with a variation in the molarity of the solution. The increase in the molarity, results in a continuous increase in TC(002), while TC(101) becomes attenuated, which indicates that the crystallites become increasingly preferentially oriented along [002]. Goyal suggested that a possible reason for the observed change could be that as the molarity of the solution is increased, more Zn is incorporated into the film. This incorporation, once sufficiently high, would make the Zn atoms occupy the interstitial sites in addition to the regular sites, which will change the growth pattern. A similar result was observed by Agashe (1991) for sprayed SnO 2 films. A further way Goyal (1992) analysed the growth mechanisms in the two prominent planes, was to analyse the standard deviation of all TC(hk1) values from the powder diffraction pattern, where the standard deviation (a), is defined as the standard deviation of the texture coefficient of various planes from their respective ASTM values, as defined by Kim (1986). Goyal investigated the effect of the variation in the molarity of the solution, on the standard deviation (a). The molarity effect, should be seen in the light of the following: If the nucleation on the substrate is followed by the growth of these nuclei due to surface diffusion of impinging flux, one should observe a deterioration in the preferred orientation and a should decrease. Additionally, if random nucleation takes place on the growing surface followed by their growth, it will also result in a decrease in a. On the other hand, if the nucleation is preferred in the initial stages and the overgrowth is oriented, a strict improvement in preferred growth is observed. Such a case will result in a continuous increase in a. The linear increase in a, with an increase in the solution concentration, is due to a.

(28) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 20 preferred nucleation as well as the oriented overgrowth. The slow increase in cr observed beyond 0.1M is because of the excessive incorporation of Zn into the growing film, which gives rise to the homogeneous nucleation during growth.. Substrate Temperature. Tiburcio-Silver (1991) investigated the influence of the substrate temperature on the structure of the ZnO films. It was found that the undoped films exhibited a preferential orientation along the [002] axis, which changed from parallel to perpendicular as the substrate temperature was changed from 400 to 550°C. Major (1983) and Goyal[2] (1992) observed the same (002) preferred orientation, although the (103), (102) and (101) secondary peaks were also visible, for the undoped film deposited from a solution containing 0.1M zinc acetate. ZnO thin films were prepared by Amlouk (1994), using a technique that employs an airless spray gun in the deposition of the films. The X-ray diffractograms of three films, deposited from a solution containing 0.1M zinc acetate, litm thick and deposited at 340, 380 and 420°C were investigated. When the three films were compared, it was seen that the orientation changed with a temperature increase. The films deposited at 340°C exhibited a (100) preferred orientation, whereas the preferred orientation changed to the (002) orientation at higher deposition temperatures. For the film deposited at 420°C, the (002) peak was higher in intensity and the (100) and (101) peaks almost disappeared, compared to the film deposited at 380°C. These results were similar to results achieved by Major (1983), for films deposited by means of a pneumatic spray method at 400°C. Sanchez-Juarez (1995) also investigated the effect of the deposition temperature on the film structure. Diffractograms obtained indicated that undoped films deposited at 300°C had a (100) preferential orientation. When the substrate temperature was increased to 400°C, the (100) peak decreased and the film exhibited a (002) preferred orientation. At a deposition temperature of 500°C, the intensity of the (002) peak increases further, with only the (101), (102) and (103) secondary peaks visible.. Annealing. Aktaruzzaman (1991) annealed the Al-doped zinc oxide (AZO) films in a hydrogen atmosphere for 1 hour, at a temperature of 400°C. It was found that after annealing, the (002) peak intensity remained the highest, as was previously observed for the asdeposited films, although the secondary (101), (103) and (100) peaks decreased. This suggested a preferential reorientation of grains, parallel to the film surface, due to the high temperature annealing in the hydrogen atmosphere..

(29) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 21 Film Thickness Song (1994) saw that the film thickness had a big effect on the crystallinity of the ZnO films. A film 0.36 pm thick, even though showing the dominant (002) peak, was found not to be very crystalline, exhibiting the (002), (101), (102) and (103) diffraction peaks. When the thickness of the film was increased to 0.64 pm, the secondary peaks disappeared and the (002) peak was enhanced. The [002] c-axis orientation became even more amplified for the thickest film of 1.2 pm. Hydrogen Plasma Exposure One of the solar cell applications of In-doped zinc oxide (IZO) films is as substrates for amorphous microcrystalline silicon solar cells. During the deposition of these solar cells, the transparent substrates are unavoidably exposed to the hydrogen plasma. Excellent transparent conductors such as fluorine-doped tin oxide and tin-doped indium oxide, are not sustained in hydrogen plasma and reduce to their respective metallic forms. The stability of the IZO films in a hydrogen plasma was investigated by Goyal[2] (1992). The X-ray diffractograms of two films were obtained, one before and one after hydrogen plasma treatment. It was seen that in both cases the ZnO films showed the same structure and composition. No traces of metallic zinc or indium were found in the films. It was thus clear that the IZO films were unaffected by the hydrogen plasma.. 2.2.2 Internal Structure: X-ray Photo-electron Spectroscopy (XPS) results Effect of Hydrogen Plasma Exposure The effect of hydrogen plasma treatment on indium tin oxide (ITO), fluorine-doped tin oxide (FTO) and IZO films, deposited by spray pyrolysis, was studied by means of XPS by Major (1986). All the films investigated had a thickness of about 0.5 pm and were exposed to a hydrogen plasma (pressure 100 mTorr, rf power 0.25 W/cm 2 and flow rate 10 sccm) at a temperature of 250°C for 30 min. Figure 2.13 shows the In(3d5/ 2 ) and O(ls) core levels of as-deposited, hydrogen plasma treated and air annealed films of ITO. At the surface of the as-deposited film, the In(3d5/ 2 ) level had a binding energy of 444.0 eV, which confirmed the presence of single phase In2 03 in these films. However, at the surface of the plasma treated film, the In(3d5 /2 ) level split into two components at 444.1 eV and 443.2 eV. The lower binding energy peak was due to the presence of metallic indium on the surface. This peak diminished as the film was etched and finally, for depths of about 500A, a single In(3d5 / 2 ) peak at 444.0 eV was obtained. The O(ls) level at the surface of the plasma treated film had a binding energy of 532.7 eV. The higher binding energy level of O(ls) in the range of 531.7-533.0 eV was attributed to chemisorbed or dissociated oxygen,.

(30) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 22 or OH type species on the surface of various oxides (Barr (1978)). Major attributed the shift in the O(ls) level to the presence of chemisorbed/dissociated oxygen on the surface of the reduced oxide. As the film was etched, the O(ls) peak at 529.7 eV due to oxygen in In2 03 appeared and increased with increasing depth and the higher binding energy 0(1s) peak diminished. Finally, at a sputter depth of about 1000A, only a single O(ls) peak at 529.7 eV was observed. It is clear that the plasma treatment of the ITO films resulted in the reduction of the oxide to metallic indium on the film surface. This conclusion is supported by the growth of metallic islands as observed by Auger chemical mapping. Consequently, the films exhibited a drastic reduction in the visible transmittance from about 85% to about 20-50%. Furthermore, the electrical conductivity was found to increase by a factor of 1.5. 4`.. (a). In(3d5/ 2 ) (b). 0(1s). 444.0 eV. 529.6 eV AIR ANNEALED 532.4eV 1000A 0 5004 SURFACE. PLASMA TREATED 443.2 eV. 1000A. 500. 4. SURFACE. AS DEPOSITED. SURFACE. 535. 531. 527 448. 441.. 440. BINDING ENERGY (eV) Figure 2.13: XPS spectra for (a) O(ls) and (b) In(3d 5 /2 ) core levels for as-. deposited, hydrogen treated and air annealed films of ITO. The In(3d 5 /2 ) peak position for metallic indium is at 443.2 eV. The sputter depths are indicated on each curve (Major (1986)).. When the plasma treated film was annealed in air at 400°C, the visible transmittance.

(31) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 23 was restored to 80-85%. The conductivity was slightly lower than the as-deposited value which may be due to some oxygen chemisorption at the grain boundaries. The XPS spectra of the air annealed film are also presented in Figure 2.13. Indium existed in the oxidized state (binding energy 444.0 eV) from immediately at the surface onwards. However, the O(ls) level had a higher binding energy (532.4 eV) at the surface. This peak diminished as the film was etched and finally, for depths of 1000A, only the 0(1s) peak, due to oxygen in the In 2 03 , was present. The presence of the higher binding energy in the surface region may be due to the non-removal of dissociated/chemisorbed oxygen during the oxidation of the plasma treated film. Figure 2.14 shows the Sn(3d5/2 ) and O(ls) core levels for the as-deposited, plasma treated and air annealed films of FTO. 0(1s) 529.8 eV. (a). Sn(3d5/2 ) (b) 486.2 eV. AIR ANNEALED 532.5 eV 700 A. 500 A. SURFAC E,/. w. 1. PLASMA TREATE0 484.4 eV 1000 A 500 A SURFAC 1 . AS DEPOSITED ....y.\.4 : SURFACE .. 535. t. ,. ..e'. .. 485 527 489 531 BINDING ENERGY ( eV ). 481. Figure 2.14: XPS spectra for (a) 0(1s) and (b) Sn(3d 5/2 ) core levels for asdeposited, hydrogen treated and air annealed films of FTO. The Sn(3d 5 /2 ) peak position for metallic tin is at 484.4 eV. The sputter depths are indicated on each curve (Major (1986))..

(32) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 24 The behaviour of these films was essentially similar to that of the ITO films in respect to both the Sn(3d 5/ 2 ) and 0(13) levels. At the surface of the plasma treated films, the Sn(3d5/2 ) level split into two components at 486.1 and 484.4 eV due to the presence of metallic tin (484.5 eV), as was reported by Schode (1984). The films also exhibited the presence of chemisorbed/dissociated oxygen in the surface region of the plasma treated and air annealed films. The changes in the electrical and optical properties of the films on plasma exposure, as well as on annealing, were similar to those in the case of the ITO films. In sharp contrast to ITO and FTO, the IZO films did not show any changes in the conductivity or optical properties in the visible and near infrared regions when exposed to a hydrogen plasma. o(is). Zn(L3M45M45). 53p.0 eV. (Q). ( b). PLASMA TREATED 532.2 eV 265.0 eV. 700. w 200. w z. A. SURFACE. AS DEPOSITED. 50. A. SURFACE. 535. 531. 527 273. 265. 257. BINDING ENERGY (eV Figure 2.15: XPS spectra for (a) 0(1s) and (b) Zn(L3M45M45) Auger transition for as-deposited and plasma treated films of IZO. The sputter depths are indicated on each curve (Major (1986)).. Figure 2.15 shows the Zn(L3M45M45) Auger transition and the O(ls) level for the asdeposited and plasma treated films. In both cases, the binding energy of Zn(L3M45M45) Auger transition was about 265 eV, which confirms that Zn exists only in the oxidized state even in the plasma treated films. It should be noted that the corresponding binding energy for Zn metal is 260.2 eV. However, the 0(1s) level showed markedly different behaviour. In the as-deposited IZO film, the O(ls) had a binding energy of.

(33) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 25 530.0 eV, which corresponded to the oxygen in ZnO. In the case of the plasma treated films, down to a depth of 50A, only the higher binding energy peak at 532.4 eV was observed. As the film was etched, the O(ls) level at 530 eV showed up and beyond 700A, the higher binding energy peak was absent. The above observations suggested that the IZO films were not reduced when exposed to a hydrogen plasma. It has been reported (Grunze (1981)) that slightly nonstoichiometric (zinc-rich) zinc oxide is stable towards thermal and photodecomposition. Further evidence of this stability comes from Gopel (1977), who observed that the partial pressure of oxygen over ZnO surface decreases with increasing metal excess. Furthermore, it has been observed that a prolonged reduction with CO of (001) and (101) ZnO surfaces, yielded a zinc-rich surface layer of ZnO which is nonreducible and stable (Hirschwald (1981)). It should also be noted that the absorption behaviour of hydrogen on ZnO surfaces is known to be quite complex (Hirschwald). Hydrogen absorbs on ZnO surfaces forming Zn-H and OH type species (Eischens (1962)). Furthermore, dissociatively absorbed hydrogen is also believed to be bonded in bridged structures between two Zn or 0 atoms (Zn-H-Zn and OH...0) (Boccuzzi (1978)). The formation of such surface species may lead to the passivation of the surface towards reduction by hydrogen plasma. The higher binding energy O(ls) peak observed in the case of plasma treated IZO films, may be attributed to OH and OH...O type of surface species. The results obtained by Major (1986) by a secondary ion mass spectrometry (SIMS) analysis also indicated that the plasma treated films have a greater abundance of H+ ions in the surface region (down to 100A) than the as-deposited films.. 2.2.3 Morphology Precursor Solution Electron micrographs of films, deposited from a 0.014M zinc acetate solution (1) and a 0.067M zinc nitrate solution (2), were obtained by Andres-Verges (1992). The initial particles observed by scanning electron microscopy (SEM), showed a spherical morphology in both cases, with an average size of 0.2 fern (1) and 0.7 prn (2), respectively. However, when film (1) was observed by TEM, a non-homogeneous arrangement of smaller particles, called primary particles (0.01-0.02 um) appeared. Therefore, the larger particles (secondary particles) were considered to be formed by an agglomeration of the primary particles. The following aspects of film (2) were noted: the primary particles were very scarce, being mostly integrated inside the secondary particles and these particles appeared to be more firmly packed than the particles obtained from the zinc acetate solution. When film (1) was heated above 600°C, most of the primary particles disappeared and the secondary particles appeared to be more closely and firmly packed. At 875°C, only one kind of particle was observed, with a particle size between 0.1 and 0.3 um, which could be explained through a sintering process from the primary particles. For film.

(34) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 26 (2) the evolution with temperature was rather different. SEM micrographs at 875°C showed a appreciable sintering between the secondary particles, increasing the average size from 0.7 to 1.0 pm. At higher temperatures, both sintering and agglomeration of the secondary particles took place, the resulting particles ranging in size between 0.1 and 0.8 pm, for film (1) at 1025°C and about 1.5 pm, for film (2) at 1150°C. 412 1. i 530464. 1. 44 0 , •. 530 49 SP:1300. .11/. 472 : :. \ 414 460 *,. 0. SP-1160. .0 0 .0. .ce. SP-1025...*. SP. \. SP_600. -3P:875. 700 (0). 500. (cm -1 ). 700. (b). 500. 300. (cm -1 ). Evolution of the infrared spectra of ZnO microcrystals obtained by spray pyrolysis. (a) Film deposited from zinc nitrate (2), (b) deposited from zinc acetate (1) (Andres-Verges (1992)). Figure 2.16:. Further conclusions on the morphology of the two films, was made by Andres-Verges (1992), from an analysis of the infrared spectra of the films, presented in Figure 2.16. The differences observed in the spectra of the two films, could only be attributed to morphological differences, because particle sizes below 10 pm, do not affect the infrared spectrum (Wadia (1968)) and the agglomeration state can shift one or more bands but cannot cause their splitting. The single band centred at 460 cm -1 , Figure 2.16(b) film (1), must be assigned to spherical particles, but when band splitting occurred, prolative or plate-like shapes were present. In film (2), the bands at 490 cm -1 and 442 cm -1 were assigned to the wT_L and wTII modes, respectively, therefore, for film (2) it could be concluded that slightly prolate particles were obtained. An initial sintering between the secondary particles, could probably explain the above occurrence. For the same temperature interval, film (1) showed one band shifted to different frequencies. As was pointed out,.

(35) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 27 this fact was successfully explained from variations in the state of agglomeration of the spherical morphology and the spectra of film (1), film (1) - 600°C and film (1) 875°C were well reproduced with filling factors of 0.5, 0.8 and 0.4 respectively. These results suggest that the initial sample, heated at 600°C, increases its agglomeration state from 0.5 to 0.8, which could be interpreted by TEM as being due to a sintering and growth of the primary particles. The reduction in agglomeration of the particles between 600 and 875°C was explained by a total sintering of the primary particles, leaving the secondary ones less agglomerated. At higher temperatures film(1) - 1025°C, band splitting due to prolate morphology is produced as a consequence of a new sintering process between the secondary particles. Therefore, from the results obtained for films (1) and (2), it was deduced that the splitting of the absorption band occurs only when sintering takes place between spherical particles which are above 0.3 pm in size. It was, however, also noted that film (1) presents a thermal behaviour (T < 1025°C) which resembles the formation of rod-like zinc oxide microcrystals in homogeneous solutions (Andres-Verges (1990)). In this case, the initial crystalline spherical particles increase in size up to 0.5-1.0 um. This size appeared to be critical because once reached, every pair of particles begins to engage by interaction through the c-axis, giving rise to embryonic rod-like microcrystals which later grow from the solution complexes by some kind of ripening mechanism. When film (1) is thermally treated, the very fine spherical particles increase in size up to 0.1-0.3 urn by a sintering process, maintaining their spherical morphology. From this size, the sintering process takes place through the interaction of the c-axis, giving nonhomogeneous particles, with an axial ratio c/a > 1. When the temperature increases to above 1025°C for film (1) and to between 875°C and 1150°C for film (2), dramatic changes were seen from the respective infrared spectra. Crossing of the bands is now produced (it is well observed in film (2) - 1150) in such a manner that the shoulder at 530 cm' is due to wTII mode, which is in agreement with a change in the axial ratio c/a to < 1. As mentioned earlier, Pushparajah (1994), deposited ZnO films from a solution containing ZnC1 2 . The sample composition was analysed using energy dispersive analysis of X-rays or (EDAX). The analysis confirmed that almost all the ZnC1 2 used had been converted to ZnO, since there was only a small Cl percentage obtained, 1.66%. A SEM micrograph obtained indicated that the film had a uniform grain size and that the grains were about 1 um in size. Substrate Temperature Bahadur (1986) investigated the morphology of films deposited at various temperatures (380, 430 and 480°C) and deposited for different deposition periods (10, 30 and 60 min). It was found at low temperatures that initially nucleation started around scattered centres with nearly spherical features with a crystallite size of ±5 urn. As the deposition.

(36) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 28 time increased, however, the film grew in ring forms, with some spherical features in the centres of the rings. Further spray material was found to deposit randomly on the ring without any continuous link, resulting in a nonuniform surface. As the substrate temperature increased, however, the film became more smooth with higher density, while the film prepared at 480°C, exhibited only circular features, the ring effect having almost totally disappeared. Bahadur further investigated the effect of annealing a film, deposited from a 0.1M zinc nitrate solution at 380°C, for 60 min, at a temperature of 400°C for 1 hour in hydrogen. No change in the SEM micrograph of the surface morphology of the film could be observed after annealing. Dopant Concentration: Aluminium Aktaruzzaman (1991) analysed the morphology of a ZnO film doped with 1.18 at.% Al by means of transmission electron microscopy (TEM). The micrograph obtained indicated that the average grain size of the AZO films were about 0.9 pm, which was half the size of the grains observed for undoped films. Aktaruzzaman concluded that the incorporation of Al into the host matrix inhibited the grain growth to a large extent. The corresponding electron diffraction pattern (EDP) obtained for the above mentioned Al-doped film indicated that since the (002) ring was absent in the EDP, the (002) planes were indeed preferentially oriented parallel to the film surface. XPS results obtained by Aktaruzzaman of plasma treated AZO films, revealed no traces of zinc in the films after exposure. This indicated that either the AZO films did not reduce in the plasma or that the reduced metallic zinc concentration in the films was very small and could not be detected. The variation in the grain size with Al doping was investigated by Goyal[1] (1992). Undoped films were found to possess grains of sizes in the order of 0.036 pm. When Al was added to the solution the grain sizes increased almost linearly with the doping concentration up to 0.32 at.% Al doping. The measured grain size at this point was about 0.043 pm. Further increase in the size was slow up to 0.8 at.%, 0.046 pm, and was followed by a saturation for higher doping levels. Dopant Concentration: Cobalt and Nickel The cobalt concentration in Co-doped ZnO films was determined by Bahadur (1992) by means of atomic absorption spectrophotometry. The values obtained were compared with the initial values in the starting solution. It is quite clear that the Co concentration in the films was found to be higher than in the solution in all the cases analysed. This indicates that the deposition rate of Co is larger than that of zinc. SEM micrographs were obtained of the surfaces of pure and 1 at.% Co-doped films. Little difference could be observed between the two micrographs. In both cases, instead of grains, Bahadur found interlinked rings with voids inside. Bahadur (1994) further, prepared ZnO films from a 0.1M zinc nitrate solution, doped with nickel (1 at.%) and nickel -I-.

(37) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 29 Co)cobalt (1 at.% -I- 1 at.%). SEM micrographs obtained from the Ni and (Ni doped films indicated a film texture of interlinked rings, full of patches on the upper surface, compared with the more continuous rings, without any patches, observed for the previously mentioned Co-doped films. The discontinuous, patched nature of the Ni and (Ni Co)-doped films resulted in the conductivity of these films being lower than the conductivity of the more Co-doped films, with the more continuous surface.. Dopant Concentration: Indium SEM micrographs of the surface morphology of undoped and indium-doped films obtained by Abd Lefdil (1995), indicated that both films had a textured and tetrapod-like morphology with a average grain size of about 1 ,um. Tiburcio-Silver (1991) deposited undoped and In-doped films from a 0.2M zinc acetate solution, by means of ultrasonic spray pyrolysis. It was found that the average grain size increased linearly with the substrate temperature for undoped ZnO films. For indium-doped films, the average grain size tended to saturate at the higher substrate temperatures. The indium contents in the solution and the content in the films, were also investigated as function of the substrate temperature and it was found that the indium content increased in the film, as the content in the solution was increased, but the indium content in the film, decreased as the substrate temperature increased. SIMS profiles obtained of indium-doped films, deposited at 525 and 550°C, indicated that Zn, 0 and In were uniformly distributed in the films. It was also clearly seen that an indium accumulation occurred at the film surface.. 2.2.4 Growth Rate Solution Concentration. Goyal[3] (1992) studied the influence of the concentration of the zinc acetate solution on the growth rate of undoped ZnO films deposited at a substrate temperature of 450°C. It was seen that the growth rate increased continuously as the molarity of the solution was increased. This continuous increase implied that the growth rate was governed by only one species, zinc acetate. In such a case the film grows by a continuous adsorption, followed by the reaction of Zn-containing species with the absorbed water molecules. This type of growth has been observed for chemical vapour deposited Sn0 2 films and is known as the Rideal-Eley mechanism (Ghostagore (1978)). Goyal found that the growth rate for a 0.1M solution was about 32 nm min -1 , which increased to approximately 57 nm min -1 as the solution concentration was increased to 0.2M..

(38) 2.2 MATERIAL CHARACTERISTICS OF ZNO: SPRAY PYROLYSIS 30 Film-Growth Activation Energy. Ambia (1994) investigated the growth rate of undoped ZnO films, deposited from a 0.4M solution, by examining the log of the film thickness, as a function of the inverse of the substrate temperature. Further, by considering the film growth process as a rate process, the activation energy for the film growth was determined from the slope of the graph and was found to be 0.22 eV in the temperature range of 270-420°C.. 2.2.5 Film Thickness Spray Rate. The effect of the spray rate on the film thickness, for films deposited at 360°C from solutions containing 0.4M zinc acetate and annealed in vacuum at 250°C, was investigated by Ambia (1994). The variation in the thickness was found to be non-linear. It was found that, initially, the thickness increased slowly with the spray rate, but that at higher spray rates, the thickness tended to saturate. At the lower spray rate, a smaller quantity of the aerosol reached the substrate, but at a higher spray rate the quantity of reacting aerosol reaching the substrate increased and the film thickness increased. However, a time is reached at a fixed temperature when the reaction rate became optimum and further reactant supply could not increase the rate of the film formation, rather, the excess reactant flowed outside, remaining unreacted. Hence, a saturation of film thickness was obtained. Ambia found the optimum spray rate to be about 0.78 ml min-1 .. Nozzle Distance. The effect of the substrate to nozzle distance on undoped ZnO films, deposited at 360°C, was investigated by Ambia (1994) on films annealed in vacuum at 250°C. It was observed that the thickness decreased with the increase in the distance. Ambia found that a distance of about 4 cm resulted in a film thickness of about 0.26 pm and that at higher distances, the decrease in the thickness became more pronounced. At small distances, the maximum amount of vapour molecules which emerge from the nozzle can strike the substrate directly before they become distributed in the reaction chamber. As the nozzle to substrate distance increases the vapour molecules have sufficient space to distribute laterally in the reaction chamber. As the result, a smaller quantity of the aerosol can reach the substrate and results in a decrease in the deposition rate. Ambia found that if the distance was sufficiently large (±30 cm), the substrate received no coating, even after a long spraying time. This was due to the evaporation of the aerosol before reaching the substrate..

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