ZnBeMgO thin films based UV Detectors by Spin Coating
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ZnBeMgO thin films based UV Detectors by Spin Coating Neeraj Panwar†, J. Liriano and Ram S. Katiyar† Department of Physics, University of Puerto Rico, San Juan, Puerto Rico-00931, USA ABSTRACT Be and Mg co-doped ZnO films Zn1-x-yBexMgyO (0 x 0.10, 0 y 0.20) have been deposited for the first time by novel chemical deposition or spin coating method. From the x-ray diffraction patterns it is noticed that the pristine ZnO film exhibits wurtzite polycrystalline structure, however, co-doped films are (0002) preferentially oriented. The (0002) peak intensity also increases with the corresponding increase in dopants concentrations. The systematic decrease in the c-axis lattice parameter value in the co-doped films as compared to pure ZnO film suggests the incorporation of smaller ions Be+2/Mg+2 at Zn+2 site. From the optical transmittance measurement it is noticed that all the films exhibited almost 80% transmittance in the visible region with sharp and single absorption edges in the UV region. The cut-off wavelength shifts from 375nm for the ZnO film to 330nm with 10 Be and 20% Mg co-doped film. The bandgap calculations revealed an increase in bandgap from 3.26eV (ZnO) to 3.60eV (Zn0.7Be0.1Mg0.2O) film. Such an increase in the co-doped films fabricated by a much simpler and cheaper process is very useful in the realization of UV radiation detection without having little interference from the visible light. †Corresponding authors: [email protected] , [email protected] 1. INTRODUCTION ZnO is a direct wide bandgap semiconductor (Eg = 3.37eV) suitable for applications in ultraviolet (UV) optoelectronic devices [1-2]. Other salient features of ZnO are high exciton binding energy (60meV), high radiation hardness, easily modulated bandgap and thin films of ZnO are easy to fabricate. The band gap modulation of ZnO films by alloying with a wider bandgap semiconductor MgO (7.8eV) has been widely investigated [3- 4]. However, there exists a miscibility gap in the ZnO-MgO binary system due to the structure difference and the large lattice mismatch between ZnO (wurtzite, 3.25Å) and MgO (rock salt, 4.22Å) [5-6]. To overcome this problem of phase separation another binary system Zn1-xBexO was suggested as both ZnO
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and BeO have the similar hexagonal wurtzite structure BeO possesses a much wider band gap of 10.8eV as compared to MgO [7]. However, the same problem of phase separation occurred in ZnBeO system also which was attributed to the large ionic radius difference between Be+2 (0.27 Å) and Zn+2 (0.60 Å) and produced a noticeable lattice mismatch when higher band gap was approached [8]. In order to obtain high quality crystalline ZnO films with bandgap modulated to solar blind region, incorporation of Be into ZnMgO matrix has been suggested [9]. The large lattice mismatches of ZnO/BeO and ZnO/MgO were expected to be counteracted by each other. Yang et al [9] deposited ZnBeMgO films on c-axis oriented sapphire substrates by pulsed laser deposition (PLD) and increased the bandgap from 3.7eV to 4.9eV with the incorpo
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