Doped zinc oxide thin films for photodetectors devices

— In this work, structural, electrical properties of Li doped ZnO thin films, by chemical pulverization grown on glass substrates at different Li doping concentrations, were reported. Besides, The effects of lithium doping on structural properties of
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  Doped zinc oxide thin films for photodetectors devices Mohamed Salah 1 , Samir Azizi 1 , Abdelwaheb Boukhachem 2 , Chokri Khaldi 1 , Jilani Lamloumi 1   1 Equipe des Hydrures Métalliques, Laboratoire de Méca-nique, Matériaux et Procédés, Ecole Nationale Supérieure d'ingénieurs de Tunis, Université de Tunis. 5 Avenue Taha Hussein, 1008 Tunis, Tunisia  2 Unité de Physique des Dispositifs à Semi-Conducteurs, Faculté des Sciences de Tunis, Tunis El Manar University, 2092 Tunis, Tunisia  Abstract   — In this work, structural, electrical properties of Li doped ZnO thin films, by chemical pulverization grown on glass substrates at different Li doping concentrations, were reported. Besides, The effects of lithium doping on structural properties of ZnO: Li thin films were studied by means of XRD technique. This study demonstrated that all prepared thin films consisted of unique phase ZnO and were well crystallized in wurtzite struc-ture with the preferentially orientation (002) in the direction parallel to c-axis. The obtained results showed that these films have polycrystalline wurtzite-structure and high c-axis preferred orientation. Besides, the ZnO photodetectors revealed superior performance in the view of photocurrent.  Keywords— Li doped ZnO; Spray; Structure; photodetector. I.   I  NTRODUCTION  Doping metal oxides were used to improve and modify the characteristics of the oxides to enhance finally the perfor-mance of various electronic devices. Zinc oxide is a metal oxide that considered a one of the most important II-V semi-conductor materials as it is characterized by its wide band gap energy of 3.3 eV [1], a free-exciting binding energy of 60 meV at room temperature [2] and n-type conductivity. It is also in large quantities in nature and eco-friendly. Generally, more increased electron mobility of ZnO reduces the electron recombination loss by making injection of photo-excited elec-tron into the conduction band easier, improves the perfor-mance of solar cells. On the other hand, ZnO films having considerable transmittance in the visible region (400-800 nm) and little resistivity are adapted for use as transparent elec-trodes in optoelectronics devices. Lithium-doped zinc oxide thin films (ZnO:Li) were prepared by chemical pulverization technique [3] grown on glass substrate using the aqueous solu-tion of ZnCl 2  and Li 2 CO 3 . Indeed, the thin films were grown at substrate temperature 460   °C. II.E XPERIMENTS   ZnO: Li thin films were synthesized by the spraying tech-nique in the aqueous solution (chemical pulverization). The experimental device included a hot plate for the substrate and a nozzle fixed on a displacement table in three dimensions for spraying the entire isothermal zone containing the heated sub-strates. The precursor solution was prepared from 0.4 g of solid zinc chloride (ZnCl 2 ) in 0.3 L of water as a solvent. Li doping was achieved by the addition of lithium carbonate (Li 2 CO 3 ) as a lithium precursor to the initial solution. All along the deposit, the distance between the nozzle and the substrate and the speed of pulverization, equal to 27 cm and 4 ml/min respectively, remained constant [4]. in the pressure of 0.35 bar the nitrogen was used as a gas carrier through a 0.5 mm diameter nozzle. The electrical measurements were car-ried out by two electrodes, bonded on the extremities of the thin film using silver paste. [5]. X-ray diffraction measurements of ZnO: Li thin films were maked with a copper-source diffractometer with the Cu k  α  radiation ( λ   = 1.540 Å). The optical analyses, in the UV– Visible spectrum, were recorded using a Shimadzu UV 3100 double beam spectrophotometer within 300–1800 nm wave-length range. The current–voltage (I–V) characteristics of the  photoconductive visible detectors, created on MSM (Ag-ZnO-Ag) electrodes, were obtained using a Keithley electrometer model 6517A at room temperature. III.S TRUCTURAL ANALYSIS The structural study by using X-ray diffraction was real-ized out to identify the phases of the pure and ZnO: Li films and some other parameters. Fig. 1 shows the X-ray diffraction  patterns of undoped ZnO and all ZnO: Li thin films. The mod-el of Li-doped zinc oxide films shown a defined peaks of (103), (102), (101), (100) and (002) principal orientation, cor-responding to hexagonal wurtzite structure. Besides, The ori-entation of the peak, representing (002) plane, has a low inten-sity in the undoped ZnO film than in all ZnO: Li films. This indicates that lithium incorporation changed the crystallinity of the films. From the results presented in Table 1, It can be seen that the position of the peak (0 0 2) peak shifted gradual-ly to higher angles with the increase of lithium percentage (Fig.2). This slight peak shift was due to the insertion of the Lithium ions in ZnO matrix, which created the lattice strain and consequently modified the lattice parameters [6]. It was reported that Li atoms take interstitial sites rather than replace Zn sites. Also, they deform the lattice parameters [7]. The crystallite size was deduced from 2 θ  and the full width at half maximum (FWHM) of the (h k l) peaks using Debye–Scherer relation: 2016 7th International Conference on Sciences of Electronics, Technologies of Information and Telecommunications (SETIT)389 978-1-5090-4712-3/16/$31.00 ©2016 IEEE  12 k Dcos λ =β θ  (1) Where k = 0.90 is the Scherer constant, β 1/2  is the full width at half maximum of (002) peak and λ  = 1, 5406 Å is the wavelength of Cu Ka radiation. Thereby, the microstrain (  ), developed in these thin films, was estimated with this relation: 12 4tan βε =θ  (2) Also, the dislocation density (  ) was obtained by the rela-tion below: 2 1D δ =  (3) The microstrain decreased from 13, 50 10 -4 to 9, 55 10 -4 when Li concentration increased. Moreover, the dislocation density (  ) minimized from 13, 28 10 13  to 6,6510 13  lines/m 2  with the increase of lithium percentage. TABLE I. P OSITION OF (002)  PEAK  ,  VALUES OF DISLOCATION DENSITY ,  MICROSTRAIN AND GRAIN SIZE . Position (0 0 2) 2 θ  ( ◦ ) Grain size D (nm) Microstrain   (10 -4 ) Dislocation density   (10 13  Lines /m 2 ) ZnO: Li 0% 34.37 86.75 13.50 13.28 ZnO:Li 1% 34.46 124.20 9.42 6.48 ZnO: Li 3% 34.46 122.55 9.55 6.65 Fig. 1   X-ray diffraction patterns of Li-doped ZnO thin films with various Li doping concentration. Fig. 2   Shifting of the (002) peak position with Li doping. IV.   O PTICAL STUDY   The optical transmission T ( λ  ) spectra of the undoped and ZnO:Li films in 300-1800 nm range was shown in Fig. 3. These films are characterized by high transparency in the visi- ble domain with an average transmittance varying between 75% and 83%. The transmission is maximum for pure ZnO film. It maximizes as the lithium doping increases and mini-mizes by the dopant. From the transmission spectra we can determine the absorption coefficient ( α ) and thereby band gap energy. 11Ln()dT α =  (4) Furthermore, the absorption coefficient can be given by: 2g (h)A(hE) α υ = υ−  (5) Where A is a constant, h  υ  corresponds to the photon ener-gy and Eg represents the optical band gap energy. Obviously, the Eg decrease from 3.28 to 3.25 eV, respectively for 0% and 3% (Figs. 3). This result can be explained by ZnO crystal im- perfection which can be related to the influence of various factors, such as thickness, grain size, structural parameters, and lattice strain and carrier concentration. Fig. 3   Transmission spectra of sprayed ZnO:Li thin films. 2016 7th International Conference on Sciences of Electronics, Technologies of Information and Telecommunications (SETIT)390   Fig. 4   Plot of ( α h  υ ) 2 versus photon energy (h  υ ). V.   V ISIBLE P HOTODETECTOR    The measured dark and visible current–voltage (I–V) char-acteristics of the photoconductive visible detectors, based on MSM (Ag-ZnO-Ag) electrodes [8], are presented in Fig. 5, 6, and 7. (7 V applied bias and using a density of power 100 mW/cm 2 ). The linear I–V relations under both forward and reverse bias indicated good Ohmic contact behavior of the ZnO-based thin film/Ag electrode junctions [9]. The dark and the photocurrents for the ZnO:Li films were 77 µA and 0.6 mA, respectively. We note that dark current was much lower than the photocurrent, which approves the photodetector as- pect of the MSM (Ag-ZnO-Ag). The ZnO photodetectors re-vealed superior performance in the view of photocurrent. It is known that interaction of oxygen on the surface and the grain boundaries have an important role in the photo re-sponse. in the absence of light, oxygen molecules adsorbed on the surface of the sample as negatively charged ions by captur-ing free electrons from the n-type zinc oxide. This process led to the reduction of the carrier concentration in the thin films, creation of depletion layers with low conductivity, reduction of the remaining carrier’s mobility, and upward band bending near the surface. Upon visible light, at photon energies above semiconductor band gap, carrier density in the thin films in-creased. Obviously, the photo generated holes migrated to the surface and discharged the negatively charged adsorbed oxy-gen ions, and the oxygen was desorbed. It also resulted in a diminish in the width of the depletion layer and rise of the carrier mobility by lowering the barriers height. Thus, the  photocurrent of the MSM (Ag-ZnO-Ag) photo-detectors max-imized thanks to the visible light. Fig. 5   I–V characteristics of ZnO:Li 0% thin films visible photodetectors showing dark current and photocurrent. Fig. 6   I–V characteristics of ZnO:Li 1% thin films visible photodetectors showing dark current and photocurrent. Fig. 7   I–V characteristics of ZnO:Li 3% thin films visible photodetectors showing dark current and photocurrent. 2016 7th International Conference on Sciences of Electronics, Technologies of Information and Telecommunications (SETIT)391  VI.   C ONCLUSION  Li-doped ZnO thin films were synthesized with different Li doping percentage (0 at%, 1 at %, and 3 at %) on glass substrates using the chemical pulverization technique. In con-clusion, we have studied the effect of Li doping on structure, electrical proprieties. X-ray diffraction study shows that all  prepared ZnO: Li films are in polycrystalline hexagonal wurtzite phase (with preferential c-axis orientation). It is ob-tained that the (002) peak shifts slightly to higher Bragg angle, indicating the incorporation of lithium. The linear current–voltage (I–V) characteristics under for-ward and reverse bias exhibited Ohmic metal-semiconductor contacts. We showed, that by applying spray pyrolysis tech-nique, ZnO:Li thin films were simply and efficiently used to fabricate high-performance visible detectors. R  EFERENCES   [1]   C. Manoharan, G. Pavithra, S. Dhanapandian, P. Dhamodaran, B. Shanthi, Spectrochim. Acta, Part A 141 (2015) 292-299. [2]   R. Jayakrishnan, K. Mohanachandran, R. Sreekumar, C. Sudha Kartha, K.P. Vijayakumar, Mater. Sci. Semicond. Process. 16 (2013) 326-331. [3]   A. Amala Rani, Suhashini Ernest, Superlattices Microstruct. 75 (2014) 398-408. [4]   K.Boubaker, A.Chaouachi, M.Amlouk, H.Bouzouita, Eur. Phys. J. Appl. Phys. 37 (2007) 105–109. [5]   A. Purohit, S. Chander, A. Sharma, S.P. Nehra, M.S. Dhaka, Opt. Mater. 49 (2015) 51-58. [6]   Xiaolu Yan, Hu Dan, Hangshi Li, Linxiao Li, Xiaoyu Chong, Yude Wang, Physica B. 406 (2011) 3956–3962. [7]   Zeng Y-J, Ye Z-Z, Xu W-Z, Chen L-L, Li D-Y, Zhu L-P, Zhao B-H, Hu Y-L, J Cryst Growth 283 (2005)180-184. [8]   Jung Han Kim, Hee-Suk Chung, Kyu Hwan Oh, Tae-Sung Bae, Woong-Ki Hong, J. Alloys Compd. 681 (2016) 75-80. [9]   G. Yuan, Z. Ye, L. Zhu, J. Huang, Q. Qian, B. Zhao, J. Cryst. Growth 268 (2004) 169-173. 2016 7th International Conference on Sciences of Electronics, Technologies of Information and Telecommunications (SETIT)392 View publication statsView publication stats
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