CHAPTER 1 GENERAL INTRODUCTION OF ZINC OXIDE AND IT S PROPERTIES. In recent years, scientists have made rapid and significant advances in the field of

Transcription

1 CHAPTER 1 GENERAL INTRODUCTION OF ZINC OXIDE AND IT S PROPERTIES 1.1 Introduction In recent years, scientists have made rapid and significant advances in the field of materials science, especially in semiconductor physics. One of the most important fields of current interest in materials science is the development of fundamental aspects and applications of transparent conducting oxide thin films (TCO). The characteristic properties of such coatings are low electrical resistivity and high transparency in the visible region. First semi-transparent and electrically conducting CdO film was reported as early as in 1907 [1]. The early works on these films were performed out of purely scientific interest but substantial technological advances in such films were observed only after Interest in the study of transparent semiconducting films has been generated mainly by their potential applications in industries that are related to optoelectronic and photovoltaic device fabrication. Such films have demonstrated their utility as it has led to a production of transparent electrical heaters for windscreens in the aircraft industry. However, during the last decade, these conducting transparent films have been widely used in a variety of other interesting applications such as gas sensors, solar cells, heat reflectors, protective coatings, light transparent electrodes and laser damage resistant coatings in high power laser technology. Just a few materials dominate the current TCO industry and the two dominant markets for TCO s are architectural applications and flat panel displays. The architectural use of TCO is for energy efficient window application. Fluorine doped tin oxide, deposited through pyrolysis process, is the TCO that is most often used for this purpose. SnO2 can also be used as coating for windows, which are efficient in preventing radiative heat loss due to low emissivity (0.16). 1

2 Pyrolitic tin oxide is used in PV modules, touch screens and plasma displays. However indium tin oxide (ITO) is the TCO used most often in a majority of flat panel display (FPD) applications. In FPDs, the basic function of ITO is transparent electrodes. The volume of FPDs produced and hence the volume of ITO coatings continues to grow rapidly. But the enormously high cost of indium and the scarcity of this material pose difficulties in obtaining low cost TCOs. Hence search for other alternative TCO materials has been a topic of research for the last few decades. It includes some binary materials like ZnO, SnO2, CdO and ternary materials like Zn2SnO4, CdSb2O6:Y, ZnSO3, GaInO3 etc. The introduction of multicomponent oxide materials has resulted in the design of TCO films suitable for specialized applications. This is mainly because one can control their electrical, optical, chemical and physical properties by altering the chemical compositions. But the major advantages of using binary materials are their chemical compositions and deposition conditions that can be controlled easily. Even though CdO:In films has been prepared with a resistivity of the order of 10-5 ohm cm making them quite useful for flat panel displays and solar cells but currently these are not of much importance because of the toxicity of Cd. ZnO developed in 1980s, uses Zn, an abundant, inexpensive and nontoxic material. Resistivity of this material though not very low, can be reduced further by doping it with group-iii elements like In, Al or Ga or with F. Hence there is a renewed interest in ZnO as an alternative of ITO. ZnO materials received a lot of attention many years ago when it had already featured as the subject of numerous research papers as early as 1935 [2]. It has been useful in many industries such as paints, cosmetics, pharmaceuticals, plastic and rubber manufacturing and electronics as a result of its unique properties. ZnO has become valuable material due to the magnificent properties such as ultra violet absorbance, electric conductivity, transparency, piezoelectricity and luminescence [3]. 2

3 In electronics, ZnO plays a vital role because of its semiconducting properties which make it a prospective candidate for optoelectronic device fabrication. Structure of ZnO thin films exhibits high conductivity, high carrier mobility, chemical stability and transparency in the visible range [4 6]. It is also non-toxic and sustainable and could be prepared using low cost materials [7]. In light emitting technology, although gallium nitride (GaN) based materials have dominated in green, blue, white and ultra violet light emitting devices, ZnO enters the arena with several advantages. They are high exciton binding energy, ability to grow high quality single crystal substrate with low cost and simple crystal growth technology [8]. In solar cell technology, owing to its properties such as high transparency and conductivity, ZnO materials based dye sensitized solar cell has been developed and it has achieved efficiency as high as 5% [9, 10]. The ZnO based solar cells are expected to be an alternative candidate for Si based solar cell as a replacement due to their chemical and thermal stability including the stability against photo corrosion [11]. ZnO is also used as different kind of sensors based on its properties. Pyroelectric, piezoelectric and other sensing properties enable ZnO to be used as thermal, pressure and gaseous sensors [12, 13]. 1.2 General properties of zinc oxide Zinc oxide (ZnO) is II VI compound semiconductor which has direct band gap energy of approximately 3.2 to 3.4 ev at room temperature [14]. It has a melting point of 1,975ᵒC that suggest a strong bonding and implies that ZnO is thermal and chemical resistance material [15]. It is not completely soluble in water but could be dissolved in alkalis and acids. ZnO is known as zinc white and is commonly available as a white powder. Although under certain conditions growth of ZnO leading to p-type conductivity, basically ZnO exhibits dominant n-type conductivity. The n-type conductivity might be caused by intrinsic defects such as zinc interstitials and oxygen vacancies [16]. 3

4 The n-type conductivity of ZnO could be enhanced by introducing elements such as boron, aluminium, gallium and indium as dopant materials [17 19]. Important parameters related to the physical properties of ZnO are tabulated in Table 1.1 [20]. It should be noted that still there exists uncertainty in some of these values like hole mobility, thermal conductivity etc.. Table 1.1 Different parameters of physical properties of ZnO at 300 K. Property Value a 0 c nm nm a 0 /c u Density g/cm 3 Stable phase Wurtzite Melting point 1975 C Thermal conductivity Linear expansion coefficient (/C) a 0 : 6.5x10-6 c 0 : 3.0x10-6 Static dielectric constant Refractive index Energy band gap 3.4 ev, direct Exciton binding Energy 60 mev Electron effective mass 0.24 Electron Hall mobility 200 cm 2 /Vs Hole effective mass 0.59 Hole Hall Mobility 5-50 cm 2 /Vs 1.3 Crystal structure and lattice parameters At ambient pressure and temperature, ZnO crystallizes in the wurtzite (B4 type) structure, as shown in Fig.1.1. This is a hexagonal lattice, belonging to the space group 4

5 P63mc and is characterized by two interconnecting sublattices of Zn 2+ and O 2, such that each Zn ion is surrounded by a tetrahedral of O ions and vice-versa. This tetrahedral coordination gives rise to polar symmetry along the hexagonal axis. Fig.1.1 The hexagonal wurtzite structure of ZnO. O atoms are shown as large white spheres, Zn atoms as smaller black spheres. One unit cell is outlined for clarity. This polarity is responsible for a number of properties of ZnO, including its piezoelectricity and spontaneous polarization. It is also a key factor in crystal growth, etching and defect generation. The four most common face terminations of wurtzite ZnO are the polar Zn terminated (0001), O terminated (0001) faces (c-axis oriented), the non-polar (1120) (a-axis) and (1010) faces both of which contain an equal number of Zn and O atoms. The polar faces are known to possess different chemical and physical properties and the O terminated face possesses an electronic structure which is different from the other three faces. Additionally, the polar surfaces and the (1010) surface are found to be stable. However the 5

6 (1120) face is less stable and generally has a higher level of surface roughness than its counterparts. The (0001) plane is also basal. Aside from causing the inherent polarity in the ZnO crystal, the tetrahedral coordination of this compound is also a common indicator of sp 3 covalent bonding. However, the Zn O bond also possesses very strong ionic character and thus ZnO lies on the borderline between being classed as a covalent and ionic compound, with an ionicity of fi = on the Phillips iconicity scale [21]. The lattice parameters of the hexagonal unit cell are a = b= Å, c = Å and the density is gcm 3 [22]. In a wurtzite structure, the axial ratio c/a and the u parameter (which is a measure of the amount by which each atom is displaced with respect to the next along the c-axis) are correlated by the relationship uc/a = (3/8) 1/2, where c/a = (8/3) 1/2 and u = 3/8 for an ideal crystal. ZnO crystals deviate from this ideal arrangement by changing both of these values. This deviation occurs such that the tetrahedral distances are kept roughly constant in the lattice. Experimentally, for wurtzite ZnO, the real values of u and c/a were determined in the range u = and c/a = [23 25]. In Addition to the wurtzite phase, ZnO is also known to crystallize in the cubic zincblende and rocksalt (NaCl) structures, which are illustrated in Fig.1.2. Fig.1.2 The rock salt (left) and zincblende (right) phases of ZnO. O atoms are shown as white spheres, Zn atoms as black spheres. Only one unit cell is illustrated for clarity. 6

7 Zinc blende ZnO is stable only when it is grown on cubic structures [8, 26, 27], whilst the rock salt structure is a high-pressure metastable phase forming at 10 GPa and cannot be epitaxially stabilized [28]. Theoretical calculations indicate that it is possible to have a fourth phase, cubic caesium chloride at extremely high temperatures, however, this phase is yet to be experimentally observed [29]. 1.4 Electrical properties In electronic devices including piezoelectric transducers, thin film transistors (TFTs), cross-bar memories and transparent conductive oxides (TCOs), where zinc oxide can be used, one requires stable and controllable doping in the wide range of values of electrical parameters. This problem still remains a vital one for ZnO [30]. The free carriers concentration can be quite easily regulated in this compound, for instance, by obtaining asgrown ZnO thin films and crystals with substantially reduced level of electrically active unintentional defects, which introduce donor levels under the conduction band and therefore increase n-type conductivity. These defects can either be native (e.g., oxygen vacancies, zinc interstitials [31]) or incorporated ones (such as hydrogen atoms [32]). The low carrier concentration (n) desired for many ZnO applications, can be achieved also by different ways of post-growth treatment, such as annealing at high temperatures [33]. This is unfortunately excluded when ZnO is predicted for an active element in cross-bar memories [34] or deposited onto temperature sensitive organic substrates [35]. For ZnO based Schottky junctions, high carriers mobility is desired simultaneously with possibly low level of n. Fulfilling these two requirements enables one to obtain a high forward current (due to high mobility) and a low reverse current (as a result of a low n concentration) [36]. However, obtaining a satisfactory level of mobility in ZnO requires more effort than it was in the case of n concentration. 7

8 The mobility values achieved in many growth methods are very diverse (particularly, for ZnO thin films, where they are utmost in the level of 50 cm 2 /Vs or lower, as in the case of the PLD technique) and strongly dependent on the substrates and growth conditions [37]. Thus, in order to ensure the control of concentration and mobility, the deposited layer should have both appropriate chemical composition [38] and highly ordered crystalline structure as scattering processes (e.g on grain boundaries) which are often regarded to be the key factor limiting Hall mobility [39]. 1.5 Optical properties Semiconductor compounds have drawn much attention during the last few years because of their novel optical and transport properties which have great potential for many optoelectronic applications. ZnO is a wide band gap semiconductor that displays high optical transparency and luminescent properties in the near ultra violet and the visible regions. Due to these properties ZnO is a promising material for electronic and optoelectronic applications such as solar cells, anti-reflecting coating and transparent conducting materials. The high exciton binding energy of ZnO (~ 60 mev) would allow for excitonic transitions even at room temperature, which could mean high radiative recombination efficiency for spontaneous emission as well as a lower threshold voltage for laser emission. It paves the way for an intense near-band-edge excitonic emission at room and even higher temperatures, because this value is 2.4 times the room-temperature (RT) thermal energy (kbt ¼ = 25 mev). Therefore, laser operation based on excitonic transitions, as opposed to electron hole plasma is expected. In this respect, there have also been a number of reports on laser emission from ZnO based structures at room temperature and beyond. Studies have been carried out to fine tune the properties of ZnO to adopt it for different applications; for example, the band gap of ZnO is modified to use as UV detectors and emitters [40]. 8

9 Optical transitions in ZnO have been studied by a variety of experimental techniques such as optical absorption, transmission, reflection, photoreflection, spectroscopic ellipsometry, photoluminescence, cathodoluminescence, calorimetric spectroscopy and so on. Refractive index is one of the fundamental properties for an optical material, because it is closely related to the electronic polarizability of ions and the local field inside materials. The evaluation of refractive indices of optical materials is considerably important for the applications in integrated optic devices. Optical properties and processes in ZnO as well as its refractive index were extensively studied many decades ago [41]. Compendiums dealing with optical properties of ZnO and to some extent its alloys from far infrared to vacuum ultraviolet including phonons, plasmons, dielectric constant and refractive indices are available in literatures [42, 43]. The renewed interest in ZnO is fuelled and fanned by prospects of its applications in optoelectronics owing to its direct wide bandgap (Eg = 3.3 ev at 300 K), large exciton binding energy (60 mev, Refs [44, 45]) and efficient radiative recombination. 1.6 Doping mechanism The addition of a very small amount of a foreign substance (impurity) to a very pure substance is known as 'doping' (one dopant atom per 100 million atoms). The addition of these impurities serves different purposes under different needs. For example, the addition of nitrogen or any other metal as an impurity to pure ZnO may affect its physical and chemical properties. In particular, doping to ZnO allows researchers to tailor its structural, morphological, optical, magnetic and electrical properties. Each type of dopant has unique impact on crystal lattice of metal oxide. For example, Cationic doping leads to localised d- states in the band gap of ZnO, which usually acts as a recombination centre for photo excited electrons and holes leading to lower photocatalytic activity [46]. It might also unfavourably shift the conduction band below the redox potential of adsorbents, rendering the material inactive for photocatalysis. In contrast, anionic doping 9

10 results in p-states near the valence band similar to other deep donor levels in the semiconductor [47]. Nearly 5% of the incoming solar energy is the UV part of the solar spectrum, while the rest is visible light [48]. It is therefore of great significance to develop a material that can absorb both UV irradiation and visible light to widen the range of a photocatalyst. In almost all photocatalytic materials, the band gap of the semiconductor metal oxide plays a pivotal role in triggering the photoreaction after UV-induced electron-hole pair [49]. In undoped ZnO, the energy associated with visible light is not enough to initiate the photoreaction. Many studies have been cited showing that doping of a metal or transition element can cause a hyper-chromic shift in the optical absorption of semiconductor metal oxide [50]. Usually doping is done to create tail states within the band gap [51]; to increase surface defects [52] which ultimately increase the surface area (a mandatory aspect if there is to be a significant increase in photocatalytic activity); and/or to alter the electrical properties of the semiconductor metal oxide. The doped metal or transition element causes the following changes, Incorporation of localised dopant levels near the valence band and the conduction band Band gap narrowing resulting from the broadening of the valance band Localised dopant levels and electronic changes to the conduction band Electronic transition from localised levels near the valence band to their corresponding excited states. Another aspect of doping to semiconductor metal oxides is the thermal instability and its tendency towards charge-carrier recombination centres [53] that could reduce the overall UV-induced electron and/or hole taken up by their respective accepting species. Overall, the dopant is favourable in imparting and tailoring the electrical changes, narrowing of band gap, significant impact on crystallinity with possible increased optical adsorption range. 10

11 1.7 Codoping mechanism In order to develop such optoelectronic devices based on wide-band-gap semiconductors, one important issue that must be resolved is the fabrication of low-resistivity p and n-type materials. A codoping method of using acceptors and donors simultaneously as a solution to the crucial doping problem or unipolarity of wide-band-gap semiconductors which exhibit an asymmetry in their ability to be doped as n-type or p-type. The deliberate codoping of donors is essential for the enhancement of acceptor incorporation, decrease of the binding energy of the acceptor impurity and stabilization of the ionic charge distributions in p-type highly doped semiconductors. The donor is not the p-type killer but a good byplayer that activates acceptors, i.e it acts a reactive codopant. Confirmation of the applicability of the codoping method to produce low-resistivity p-type GaN, ZnO and ZnS was sought experimentally. The theoretical prediction for the realization of p-type ZnO by codoping N acceptors and Ga donors could be in the ratio N:Ga = 2:1 [54 57]. We note that the formation of a cluster, constituting acceptor-donor-acceptor that occupy the nearest neighbour sites or second-nearest ones, is energetically favourable due to the strong attractive interaction between the dopants and codopants at high doping levels. Subsequent confirmation of the applicability of this codoping in producing p-type ZnO was conducted by Joseph et al. [58]. The basic concepts of codoping methods are summarized from the given literatures [59 61]. The codoping method using acceptors (A) and donors (D) as a reactive codopant, in a ratio of A:D = 2:1, contributes (i) to enhancing the incorporation of the acceptors because the strong attractive interactions between the acceptor and donor dopants overcome the repulsive interactions between the acceptors, where the driving force is the electrostatic energy gain and (ii) to lowering the energy levels of the acceptors and raising those of the 11

12 donors in the band gap due to the strong attractive interactions between the acceptor and donor reactive codopants, as shown in Fig Fig.1.3 Schematic energy diagram for p-type codoped semiconductors. The acceptor (A) level is lowered and the donor (D) level is raised with the formation of acceptor donor acceptor complexes upon codoping. The first mechanism above requires a high affinity between the acceptors and the reactive codopants. For example, considering the fact that the formation of enthalpy is kj mol -1 [62] for ZnO is significantly lower than that of -20 kjmol -1 for Zn3N2 [63] which could be present as an actual separate phase in the material. The formation of Zn O bonds is energetically favourable compared to that of Zn N bonds. This suggests a low solubility of N in the ZnO matrix. Among the Zn species, tetrahedral complexes are the most common type which are formed with a variety of O-donor ligands and more stable ones are formed with N-donor ligands. The cohesive energies per bond for AlN and GaN, which are wurtzite structures like ZnO, are 2.88 and 2.24 ev [64], respectively. They are higher than the value of ZnO (

13 ev). This means there is a higher affinity between Al or Ga and the N species. Thus, both Ga and Al species are eminently suitable for use in codoping with N acceptors. Moreover, we note that the Al to N or Ga to N bond distance is similar to the Zn O bond distance. This reduces the elastic contribution to the energy of formation of the N atoms at O (NO2) site acceptors. Nakahara et al. [65] has reported that the presence of Ga clearly enhances the incorporation of N into ZnO, at the basis of the analysis of the data obtained by secondary ion mass spectroscopy (SIMS) measurements of ZnO co-doped with Ga and N (ZnO:(Ga, N)). The second mechanism leads to enhanced impurity ionization. A non-random configuration, such as in A-D-A trimers or trimer like complexes, is postulated to produce the required reduced ionization energy of the non-compensated acceptor impurities for materials doped with acceptors alone. The postulation will be satisfied by the appropriate choice of the codopant pair under the condition discussed in relation to the first mechanism and the control of the partial pressure for each atom. The estimated acceptor or donor binding energy of the semiconductor based on the Bohr Theory of the hydrogen atom. In order to enhance the Bohr radius of the acceptor, hybridization between p states at acceptor sites and s states at codopant donor sites with a larger Bohr radius and high solubility is very effective. In p-type codoped materials, therefore, acceptor orbitals can overlap sufficiently for good conduction to occur. Below, a change from localized impurity states for ZnO doped with N alone to delocalized ones for p-type codoped materials. The net doping effect of the majority impurity, which is related to free carrier concentrations, appears to decrease as the codoping method is applied. Noting that the effective mass of holes decreases as the acceptor impurity band width increases, it is very easy to understand the reduced acceptor binding energy due to codoping. As a result, codoping increases the net doping effect of the majority impurity. Thus, a deliberate codoping of the donors is essential for the enhancement of the solubility of 13

14 acceptors with the stabilization of the ionic charge distributions and the delocalization of the impurity states at the acceptors, which lead to a reduced acceptor binding energy, in p-type highly doped semiconductors 1.8 Literature Review Undoped ZnO and II, III, IV, V, rare earth and F doped ZnO had been studied extensively for a number of decades with reviews of the structural, electrical and optical properties. Literature review reported in this chapter is mainly on group I element doped ZnO films. In the 60s and 70s many different ways, such as doping in the melt, in the vapor phase, by diffusion and implantation were tested to bring impurities in a controlled way into ZnO [66]. The focus was on group-i elements [67], Li and Na may induce shallow acceptor states as in other II VI semiconductors such as ZnTe and ZnSe. However, Li doping has always been unpredictable since the Li ion is very mobile and might also be located at interstitial sites thereby acting as a shallow donor and compensating for the acceptor action (self-compensation). Besides self-compensation, limitations in solubility may prohibit p-type conduction. Another important aspect is the magnitude of the acceptor binding energy. One has also consider the size mismatch (e.g. As on O-site or K on Zn-site) as impurities which may not be incorporated into the preferred lattice sites. Intrinsic defects such as the cation and anion vacancies which are double acceptors and donors in ZnO, respectively, may act as compensating centres either isolated or as complexes with the dopant atoms. In 1960 Lander [68] reported on the donor and acceptor properties of Li and gave the first indications about the possible defect structure. He proposed the interplay between interstitial Li donors and substitutional LiZn acceptors. 14

15 In 1963 Schneider et al. [69] reported the electron paramagnetic resonance (EPR) properties of ZnO:Li. In the EPR investigations reported by Schirmer in 1968 [70] the g-anisotropy and the hyperfine interactions of the LiZn acceptor were analyzed. The interpretation of the hyperfine data showed that the hole was primarily located on one neighboring oxygen atom along the c-axis since Li prefers to be in the 1s 2 configuration. The localization of the hole in the framework of small polaron coupling, causes the large distortion and the substantial change in bond length an increase of approx. 30%. This is the reason why Li Zn is a deep acceptor with a binding energy of 800 mev. Na Zn behaves similarly [71, 72] and the binding energy is 600 mev. Both defects are responsible for broad strong phonon coupled luminescence bands in the visible spectral range with maxima at 2.1 ev (Li) and 2.17 ev (Na). The acceptors show up in shallow donor to deep acceptor recombination as demonstrated by optically detected magnetic resonance experiments [73, 74]. These experimental findings are in conflict with the calculations based on first principles calculations, which predict shallow levels for the group-i elements Li: 0.09 ev and Na: 0.17 ev [75]. There is no relaxation around the LiZn acceptor with a small outward relaxation for the surrounding oxygen atoms in the case of Na Zn. Group-I elements such as Li, Na, K and Ag are always used as dopants for substituting Zn sites in ZnO and introducing acceptor energy level. Recently, Lee and coworkers [76], fabricated p-type ZnO films on a (0001) Al2O3 substrate using Ag2O as a silver dopant by pulsed laser deposition. The structural property of those films was systematically characterized by observing the shift of (0002) peak to investigate the substitution of Ag + for Zn +. Narrow window of deposition temperature existed for films doped with Ag, in which p- type ZnO films could be obtained and with the hole concentration of 4.9 x x cm 3. 15

16 A neutral acceptor bound exciton had been observed by photoluminescence emitted at ev in Ag doped p-type ZnO thin films. Ye and co-workers [77, 78] fabricated Li doped p-type ZnO films by pulsed laser deposition and using dc reactive magnetron sputtering. The results demonstrated reproducible growth of Li doped p-type ZnO thin films by dc reactive magnetron sputtering. A shallow acceptor level of 150 mev was identified from free-toneutral-acceptor transitions which is assigned to the LiZn acceptor. Another deeper acceptor level of 250 mev was tentatively assigned to the Li related complexes, emerges with the increase of Li concentration. While the realization of Li doped p-type ZnO films by pulsed laser deposition could be controlled by adjusting the growth conditions for the amount of Li introduced into ZnO and the relative concentrations of such defects as Li substitutions and interstitials could play an important role in determining the conductivity of films. Majumdar and Banerji [79] reported the structural and optical properties of the as-prepared and Li doped ZnO at different percentages of Li incorporation in ZnO by sol gel method. They found that the crystalline structure of the films has wurtzite structure of the lattice. According to this report, the ZnO films had 3.36 ev band gap value. Xiao et al. [80] deposited Li-doped ZnO thin films by using pulsed laser deposition. They reported that the lowest electrical resistivity, Hall mobility and hole concentration. They were found to be 34 ohm cm, cm 2 /Vs and cm 3 at 450 C. According to this report, the films grown at different substrate temperatures had the preferred (0 0 2) growth orientation and high transmittance (about 90%) in the visible region. Mohamed et al. [81] deposited thin films of zinc oxide doped with Lithium (Zn1 xlixo) by DC magnetron sputtering method on sapphire, MgO and quartz substrates. From the observations of transmission and reflection spectra they determined the absorption coefficient and optical band gap of the films at room temperature. 16

17 The films showed the direct allowed optical transitions with Eg values of 3.38, 3.43 and 3.29 ev for films deposited on sapphire, MgO and quartz substrates, respectively. Wardle et al [82] suggested that p-type doping may be limited by the formation of complexes, such as LiZn Lii, LiZn H, and LiZn AX. As far as we know, Li doping typically increases the resistivity of otherwise n-type ZnO [83]. Jianguo Lu et al [84] investigated undoped and Na doped ZnO thin films by the sol gel method. X-ray diffractometry, atomic force microscopy and a fluorescence spectrometer were used to investigate the influence of Na concentration on microstructure, surface topography and optical property of the thin films. The experimental results of crystalline structure, residual stress, surface morphology and photoluminescence (PL) of Na doped ZnO thin films with different heat treatment process had been investigated [85]. Pure and Na doped ZnO thin films were prepared by sol gel method. The effect of the Na/Zn ratio on the surface topography and optical properties such as optical band gap, refractive index and extinction coefficient were studied [86]. The influence of different annealing conditions on the microstructures and optical properties of Na doped ZnO a sol-gel thin film was studied by Wang et al [87]. Less attention has been paid to another important element K in group-i and its related alloys [88], which behave analogously to Na. The fabrication and properties of K doped Zn1 xmgxo (Zn1 xmgxo:k) alloy thin films were analysed by Zhang et al [89] using pulsed laser deposition method. When Mg is introduced into ZnO, the band gap gets wider than the pure ZnO, and the conduction level is enhanced, giving rise to deeper native defect energy levels [90, 91]. Moreover element K is large in size, the forming of interstitials is suppressed and it has a relative low acceptor level, which benefits the p-type conversion as well. The fabricating of Zn1 xmgxo:k alloy thin film method achieves not only p-type conversion but also band gap engineering simultaneously without disturbing the wurtzite structure. 17

18 In experimental research respect, both Wu and Yang [92] and Tay et al. [93] obtained stable p-type K doped ZnO thin films. The above results are important for us to understand deeply the p-type doping mechanism for K element in ZnO. Group-I elements, potassium (K) and propose a method for fabricating p-zno thin films deposited on (0001) Al2O3 substrates by radio frequency (rf) magnetron sputtering technique. The electrical properties, crystalline structure and composition of as-grow films had been investigated [94]. Synthesis of potassium doped zinc oxide thin films by chemical bath deposition technique and the studies carried out on the effect of K doping on the surface morphology and optical properties such as optical energy band gap, refractive index, extinction coefficient, dielectric constant, absorption coefficient and photoluminescence of ZnO thin films [95]. Thangavel et al [96] reported on the deposition and characterization of cesium doped ZnO thin films on sapphire by sol gel spin coating method and also on the effect of doping concentration on the structure and optical properties of the films studied. 1.9 Aim of the Study Owing to the immense use of ZnO and doped ZnO thin films in the application of optoelectronic and photovoltaic devices, the current study is carved to improve the performances carried out earlier. The above literature survey revealed the performance of doped ZnO and there were difficulties in making p-type ZnO under ambient conditions. Hence an earnest attempt has been proposed to improve the p-type conductivity through codoping mechanism. The proposed work is planned to use group-i and rare earth elements for codoping mechanism. Also our interest lies in finding the magnetic properties of the co doped materials as it involves the use of rare earth elements. Several studies relating to magnetic properties of ZnO with transition elements were available and a viable comparison if any can be made with the present study. 18

19 Our interest is also to study the luminescence behaviour as it would be helpful in identifying the use of these materials in optoelectronic and photovoltaic applications For example, in Er 3+ ion doped ZnO, PL studies indicate a strong interaction between electron hole pairs of the host with 4-f electrons of the rare earth elements. This leads to an increase in the electrical conductivity and hence it has optoelectronic device applications. We propose to undertake Raman studies to ascertain the incorporation of dopants especially rare earth elements in the environment of ZnO. 19