Optical Properties of Zinc Oxide Thin Films Using Two Dopant G T Yusuf, MA Raimi, O.E Alaje and AK Kazeem Abstract The undoped ZnO, Al doped ZnO and Mg doped ZnO films were deposited by a sol-gel spin coating method onto the glass substrates. 0.3M solution of zinc acetate dehydrates diluted in methanol and deionized water (3:1) was prepared. Equal quantity of Aluminum chloride and tin chloride were added to each solution to serve as dopants. The effect of Aluminum and Magnesium doping on the optical ZnO films was studied. The transparency properties of all thin films are more than 80 % at a visible wavelength of (300-800 nm). The optical band gap of pure ZnO thin film is 3.12ev while the band gap for Al-doped ZnO and Mg-doped films are 3.16eV and 3.26eV respectively. All film parameters changed with dopant types. The variation of optical band gap with doping is well described by Burstein–Moss effect. Keywords: Band gap; Doping; Films; Transmittance. Introduction In this Zinc oxide is an II-VI n-type semiconductor with band gap of approximately 3.3 eV at room temperature and a hexagonal wurtzite structure [1]. Recently, doped zinc oxide thin films have been widely studied for their application as conducting electrode materials in flat-panel displays or solar devices. Unlike the more commonly used indium tin oxide (ITO), zinc oxide is a non-toxic and inexpensive material [1]. Furthermore, pure zinc oxide films are highly transparent in the visible range (light wavelength of 400-700 nm) and have high electrical conductivity. However, non-stoichiometric or impurity (Group III elements or Group IV elements) doped zinc oxide films have electrical conductivities as well as high optical transparent. Non-stoichiometric zinc oxide films have unstable electrical properties at high temperature because the sheet resistance of ZnO thin films increases under either oxygen chemisorptions and desorption [9] or heat treatment in vacuum or in ambient oxygen pressure at 3000C-4000C [27]. Turning to impurity doped ZnO thin films, unlike non-stoichiometric ZnO thin films, impurity doped ZnO thin films possess stable electrical and optical properties. Among the zinc oxide films doped with group II elements such as barium, aluminum, gallium and indium, aluminum-doped zinc oxide (AZO) thin films show the lowest electrical resistivity [11]. Aluminum-doped zinc oxide (AZO) has a low resistivity of 2.4×10-4 Ω cm [11-13], which is quite similar to that of ITO films, which is about 1.2×10-4 Ω cm [14-16] and AZO also shows good optical transmission in the visible and near infrared (IR) regions. Thus, AZO films have been used as transparent conducting electrodes in solar cells [16, 8]. In addition to doping with Group III elements, doping ZnO with Group IV elements such as [9, 10] Ge, Sn, Ti, Si is also a good way to obtain low resistivity transparent materials in order to replace ITO because Ge, Ti, Zr could substitute on the Zn atom site. For example, Sn can serve as a doubly ionized donor with the incorporation of SnO2 as a solute in ZnO and, consequently, provide a high electron carrier concentration. It is, therefore, expected that the Sn doped ZnO (SZO) will have a higher electrical conductivity and better field emission properties compared with undoped ZnO [10]. A variety of techniques such as DC or RF magnetron sputtering [2], electron beam evaporation [19,20], pulsed laser deposition [21], spray pyrolysis [22,23], chemical vapor deposition [24] and sol–gel processing [25–34,5] have successfully been developed to prepare zinc oxide thin films. Among them, the sol–gel spin coating method is simpler and cost effective. Traditionally, AZO films prepared by this method follow the non-alkoxide route, using metal salts such as acetates, nitrates or chlorides as precursor and dopant, respectively. In addition, organic solvent, such as methanol [20,21], ethanol [16], isopropanol [14], methoxyethanol [11], ethyl glycol and glycerol [10] are widely employed by introducing monoethanolamine (MEA), diethanolamine (DEA) or tetramethyl ammonium hydroxide (TMAH) as stabilizer [10,11,30]. Recently, few studies had reported on the growth of the ZnO thin films with different dopants using sol gel spin coating technique. Therefore, the aim of this research works however is to study the optical and electrical properties of zinc oxide thin films using different dopants with locally fabricated sol gel spin coating technique. Experimental The films have been deposited onto the glass substrates at 400 °C substrate temperature. 0.3M solution of zinc acetate dehydrates diluted in methanol and deionized water (3:1) were prepared and divided into three portions. Aluminum chloride and tin chloride were added to each solution as dopants. A few drops of acetic acid were added to improve the clarity of solution. The concentration of dopants (aluminum chloride AlCl3·6H2O, magnesium nitrate hexahydrate [Mg (NO3)2.6H2O and was 3% and kept constant for all experiments. The starting solutions were mixed thoroughly with magnetic stirrer and filtered by WHATMAN filter paper. The solutions were then spin coated on glass substrates which have been procleaned with detergent and then in methanol and acetone for 10min each using ELA 110277248E/2510E-MT ultrasonic cleaner and then cleaned with de ionized water and heated on hot plate for 600C. The coating solutions were dropped onto the glass substrate which was rotated at 4000rpm 45 each by using Ws- 400 Bz – 6NPP/AS spin coater. After depositing by spin coating, the films were then dried at 3000C for 15minutes in a furnace to evapourate the solvent and remove organic residuals. The optical and electrical properties of the films at each time were investigated. The films were then inserted into a tube furnace and annealed in air at 7500C for 1 hour each. The optical transmission and reflectance of the films were examined by spectrophotometer ranging from 400 to 1000nm. The transmittance T and reflectance R data was used to calculate absorption coefficients of the AZO films at different wavelengths. The relationship between transmittance T, reflectance R, absorption coefficient, α, and thickness d of the film is given by equation (1). (1) The absorption coefficient data was used to determine energy band gap, Eg , using equation (2). (2) Where is the photon energy, A is a constant thus, a plot of against is a curve line whose intercept on the energy axis gives the energy gap. The band energy gap of the film was then determined by extrapolating the linear regions on the energy axis. The absorption coefficient,, associated with the strong absorption region of the film was calculated from absorbance A and the film thickness, t, using (3). (3) The extinction coefficient, k, was evaluated from (4) (4) Where the wavelength of the incident radiation and, t is, is the thickness of the film. The crystal phase of the films was determined by X-ray diffraction (XRD). The refractive index of the films was determined from the maxima and minima of the reflectance curve. (5) Where n is the refractive index, d is the film thickness (nm), is the wavelength (nm) of the incident light, and k is the interference order (an odd integer for maxima and even integer for minima). Results The crystal structure of ZnO films was investigated through X-ray diffraction (XRD). The X-ray diffraction spectrum of ZnO, Al-ZnO and Mg-ZnO film annealed at 7500C with prominent reflection planes is shown in figure 1.The peaks in the XRD spectrum correspond to those of the ZnO patterns from the JCPDS data (Powder Diffraction File, Card no: 36-1451) having hexagonal wurtzite structure with lattice constants a=3.24982Ã…, c=5.20661Ã….The presence of prominent peaks shows that the film is polycrystalline in nature. The lattice constants ‘a’ and ‘c’ of the Wurtzite structure of the films were calculated using the relations (6) and (7). a= √â…“.λ/sin θ(6) c= λ/sin θ(7) Figure 2 shows the optical transmittance spectra of ZnO, Al-ZnO and Mg-ZnO thin films in the wavelength range between 300 to 800 nm. The transparency properties of all thin films are more than 80 % at a visible wavelength of (300-800 nm). It is observed that the transmittance varies with dopant types i.e. aluminum and magnesium. The overall spectra shows an emission band with two obvious peaks, where the first peak, the UV peak which also called the emission or near band edge emission contributed to the free exciton recombination [18]. The second broad peak, also known as the green emission corresponds to the recombination of a photon generated hole with an electron in singly ionized [18]. Figure 1: X-ray diffraction patterns for ZnO thin film for aluminum and magnesium dopants The optical absorbance spectrum measured within the wavelength range of 300–800 nm using a Shimadzu Spectrophotometer is shown in figure 3. Figure 2: Optical Transmittance of the films for aluminum and magnesium dopants Approximately, the band gap alteration of the thin film can be deduced from Figure 3. Here, it evidently shows that changes in the absorption edges are in parallel with types of dopant in the thin film. In order to appropriately estimate the optical band gap equation (2) was used. The presence of a single slope in the plot suggests that the films have direct and allowed transition. It is also well known that ZnO is a direct band-gap material [1] and the energy gap (Eg) can thus be estimated by assuming direct transition between conduction band and valance bands. Theory of optical absorption gives the relationship between the absorption coefficients α and the photon energy hν for direct allowed transition as shown in (2) The direct band gap determined using this equation when linear portion of the (αhν)2 against hν plot is extrapolated to intersect the energy axis at α = 0. Plot of (αhν)2 against hν for undoped, Al-doped ZnO and Mg-doped films are shown in figure 3. The optical band of pure ZnO is 3.12ev while the band gap for Al-doped ZnO and Mg-doped films are 3.16eV and 3.26eV respectively. The variation of optical band gap with doping is well described by Burstein–Moss effect [2-5]. For AZO films, compared to pure ZnO films, the contribution from Al3+ ions on substitution sites of Zn2+ ions and Al interstitial atoms determines the widening of the band gap caused by increase in carrier concentration. This is the well-known Burstein–Moss effect and is due to the Fermi level moving into the conduction band. Since doping increases the carrier concentration in the conduction band, the optical band-gap energy increases [2]. Enhancement of band gap thus also ensures that doping was successfully carried out in the ZnO thin films. It is further observed in our present work that an increase in band gap occurs in Mg- doped film as compared with ZnO and Al-ZnO thin films. The absorption properties of the films in UV range are due to the behaviour of ZnO intrinsic optical band gap energy. An absorption coefficient in the UV region significantly varies with types of dopant used. The result suggests improvement in the optical absorption in the UV region with nature of dopant, which provides useful information especially in the optoelectronic devices and device fabrication. . Figure 3: Plot of (αhν)2 vs. photon energy (in eV) for aluminum and magnesium as dopants Conclusions Transparent conducting thin films (ZnO, Al-ZnO and Mg-ZnO) have been deposited by sol–gel spin coating technique. The optical properties of these films were systematically investigated. X-ray diffraction analysis shows that The peaks in the XRD spectrum correspond to those of the ZnO, Al-ZnO and Mg-ZnO structural patterns is that of hexagonal wurtzite structure with lattice constants a=3.24982Ã…, c=5.20661Ã…. The optical transmittance spectra in the wavelength range between 300 to 800 nm shows that all thin films are more than 80 % at a visible wavelength of (300-800 nm). It is observed that the transmittance varies with dopant types i.e. aluminum and magnesium. 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Implementing An Effective Food Safety Management System Management Essay
Regarding to the research hypothesis, the researcher concluded that there are two main critical factors were revealed through the data analysis that not only were considered as the most important factors in implementing an effective Food Safety Management System (ISO 22000), but also proved to be the most influential factors in the optimization of the degree of the effectiveness of FSMS (ISO 22000) for implementation in Egyptian food processing company. The management responsibility with regard to enhance communication between stuff, update FSMS, motivate stuff members and allocate resources for effective FSMS implementations and planning for realization of safe products implementation such as identifying potential hazards, update and review HACCP plan and planning for meeting FSMS requirements have a major and important impact on increasing the effective implementation of FSMS (ISO 22000). Regarding to availability of uncomplicated documentation and general managerial procedures requirements beside good resources management have a good impact on effective implementation of FSMS (ISO 22000). In other words, in order for the food companies to effectively implement FSMS system they should focus their efforts to strengthen their internal documentation system, management responsibility, resources management and planning and realization of safe products for enhancing improvement and effective implementation of FSMS (ISO22000). As a final conclusion, the food safety quality management system should be reviewed, evaluated and updated regularly to improve its effectiveness. An effective planning for producing a safe food product will participate in achieving a positive progress trend in FSMS effectiveness implementation. The researcher believes that HACCP planning has a big influence on the success of the effective implementation of a food safety quality management system in any food manufacturing organization, training courses, inductions and seminars should be regularly conducted for all levels of personnel to enhance their awareness of the importance of the food quality management system. 6.2 Recommendations for Future Work It is recommended to expand the scope of study to cover other sectors in the company food chain not only food manufacturing but also suppliers, warehouses, export and end user or any other sector, to have an overall evaluation of the food safety quality management system implemented in Dreem Company. This overall evaluation will help maintain and improve the food safety quality management system to be able to cope with the future developments and strengthen the ability of Dreem Company to produce safe products and compete in both local and international markets. 6.3 Limitations However, this study suffers from some limitations, which need corrective actions to help the system to be more effective, this limitation can be summarized as follows: Concerning sample size Due to the small sample size, these results cannot be generalized to the entire Egyptian food industry. Further work must incorporate a larger sample size and many companies interested in the field of food processing. Concerning Documentation and General Requirements 1. It can be noticed that there is a lot of paper work and documents that are used in monitoring FSMS, that consider being a hindrance of the constraints the application system effectively, redesign documentation system and eliminating all non value added paper work will support implementing FSMS effectively. 2. Documentation system needed to be updated periodically according to continuous updating of FSMS and to meet system and company modern applications. 3. Additional awareness and efforts is required to inform staff member with the importance of documents in monitoring FSMS and involve staff member in establishing documents that fit with the purpose of use. 4. Hazards that might affect products and food safety needed to be more defined and addressed in different functions to generate sharp focus considering potential hazards, only production, quality control and quality assurance functions have posted and addressed visual aids clarify hazards might affect the product, hazards that might affect Food Safety should be addressed in other functions like warehouses maintenance and logistics 5. It can be noticed that there is a lake of proper systems to drive the consistent implementation of the policy, since not all staff and employees understand the company policy and strategy of the company to achieve desired levels of food safety, awareness and inductions especially to the new comers or new entry employees needed to highlight policy and strategy. Concerning Management Responsibility 1. Top management should counter about motivating employee and inspiring them, high number of the staff feeling that top management focused only on quantities rather that quality, so employees and stuff needed to be motivated by top management and to align both of focusing on quantities with focus on product quality 2. Lack of defining responsibilities and authorities inter company, researcher noticed that many employees are not aware by the authority and responsibilities of middle management staff and team leaders and how the responsibility and authority can be used effectively in effective implementation of FSMS and overcome different potential obstacles, top management should prepare appropriate job descriptions and well defined rules for different employees in FSMS applications 3. Communication between top management and staff members should be developed to know the problems and listening to their suggestions for solving and removing obstacles that impede effective implementation of FSMS 4. At the same time top management should inform staff member with modification and updates in regulatory and statutory in regular manner through meeting with function headers and food safety team members. Concerning Resources Management 1. The researcher concluded that financial and physical resources should be ad equated provided to improve the effective implementation of FSMS, facilities and materials that enhance hygiene and GMPs should be available and provided in timely manner for different areas. 2. It is noticed that a lot of number of new employers are coming from different cultures and they have not previous experience about how to handle food products, hygiene requirements, GMPs and food manufacturing instructions, continuous inductions, awareness and training should be delivered to new comer employees and current employees 3. Level of current competences for staff members should be reviewed regularly to evaluate different training needs for staff member that enhance effective implementations of FSMS. Concerning Planning and realization for safe products 1. The researcher found that PRPs must gain more attention to be an effective tool in improving FSMS applications, procedures should be established to ensure the effectiveness of PRPs implementations and allocated resources for implementing PRPs programs. 2. To improve quality control level visual aids should be addressed and posted to communicate and inform staff members with quality parameters levels. Also to provide training for labors and quality control observers with the recent modification in FSMS and updated quality control parameters for different products. 3. Evaluating and monitoring CCPs results should be announced to staff member to participate in solving and remove potential hazards that might affect the products. 4. Corrective actions should be reviewed in timely regular manner. Remove all obstacles that might hinder follow up and implementation of corrective actions that researcher found that many corrective actions still opened and needed for effective actions to close them. Concerning Improve Food Safety Management System 1. It can be noticed that there is a lake of implementing evaluation and measurement for FSMS implementations before and after improvements or corrective actions to view the progress of applications. Analysis of performance is applied but not on wide scale in FSMS 2. Researcher found that there is shortage in validation of the effectiveness of implementing FSMS during regular period of times. Many methods and applications of FSMS needed to be verificated and validated, also all assignable root causes should be removed. 3. The FSMS could be more effective if the findings for internal and external audits are gained more attention for implementations and validations. References References: Adams, C.E. (1994), ¿½ISO 9000 and HACCP systems¿½, Food and Drug Law Journal., Vol. 49, pp. 603-7. Adams, C. (2000), ¿½HACCP applications in the foodservice industry¿½, Journal of the Association of Food and Drug Officials, Vol. 94 No. 4, pp. 22-5. Al-Nakeeb, A., Williams, T., Hibberd, P. and Gronow, S. (1998), ¿½Measuring the effectiveness of quality assurance systems in the construction industry¿½, Property Management, Vol. 16 No. 4, pp. 222-8. 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