INVESTIGATION OF OPTOELECTRONIC PROPERTIES OF ZNS AND SILVER DOPED ZNS USING DENSITY FUNCTIONAL THEORY AND CORRESPONDING DEVICE BUILD UP

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1 INVESTIGATION OF OPTOELECTRONIC PROPERTIES OF ZNS AND SILVER DOPED ZNS USING DENSITY FUNCTIONAL THEORY AND CORRESPONDING DEVICE BUILD UP Aakanksha Sud #,Ramesh K Sharma # # UCIM/CIL, Panjab University, Chandigarh 1 aakankshasood0001@gmail.com, 2 ramesh@pu.ac.in ABSTRACT-Recently efforts have been made for finding new materials for energy conversion. The rapid growing world demands speed along with technology and hence the need is to look forward for different semiconductor materials for these applications. Zinc sulfide (ZnS) is one of the first semiconductors discovered. It has traditionally shown great versatility and promise for novel fundamental and diverse applications. ZnS is a direct wide band gap compound semiconductor belonging to II- VI group with a high index of refraction and a high transmittance in the visible range and is one of the most important material in photonics research. It has a wide band gap ( ev) and it is used in various applications electronic devices, biomedical field, verity of sensors, etc.in the present study band structures and electronic structures for ZnS and silver doped zinc blende ZnS were calculated using ATK-VNL software based on density functional theory. The results show that doping with silver narrows the band gap and an acceptor level impurity is introduced because of the doping with silver.this property has been explored by constructing Ag doped ZnS diode using different percentages of Ag. Graphical Abstract KEYWORDS: Band Gap, ATK-VNL software, Zinc Sulphide, Semiconductor, Doping I. INTRODUCTION The nanoscale morphologies of ZnS are found to be one of the richest among all inorganic semiconductors. Its atomic structure and chemical properties are comparable to widely known and popular ZnO. However, certain properties of ZnS are unique and advantageous compared to ZnO. To name a few, ZnS has a larger bandgap of 3.72 ev and 3.77 ev (for cubic zinc blende (ZB) and hexagonal wurtzite (WZ) ZnS, respectively) as compared to ZnO ( 3.4 ev) and therefore it is more suitable for visible-blind ultraviolet (UV)-light based devices such as sensors/photodetectors[1-5].however the wavelength range which pure ZnS materials can stimulate is limited and so there is a need to modulate its band structure to meet specific functions. This can be achieved with the help of doping. Doping nanomaterials provides an easy way to tune to the properties of the materials while maintaining their high surface areas. The electronic, optical, photochemical photo electrochemical, photo catalytic and relaxation properties can be tuned towards the desired direction by doping different elements. The materials can be tailored towards specific applications through careful selection of the dopant atoms. By doping the band gap can be reduced and electronic structure improved which helps to enhance the response of visible light and photocatalytic activity[6]. It was found in the study that Zinc sulphide can be doped with transition elements or rare earth elements which provide conditions for modification of ZnS properties by doping. Transition metal doped semiconductors show higher photocatalytic activity and this has been the subject of recent studies. In this study ZnS doped with transition metal silver has been investigated in detail. Aakanksha Sud and Ramesh K Sharma ijesird, Vol. III, Issue III, September 2016/198

2 II. MATERIALS AND METHODS The DFT calculations were done using ATK-VNL software[7]. The thin film was simulated using 10 layers of (2x2x2) supercell using DFT calculator and (3x3x3) k-point sampling and local density approximation (LDA) exchange correlation.the corresponding values for ionization potential and electron affinity were taken as default values The muller sulphur and cerdia zinc were used.scf (self consistent field approximation) iterations were used. For doping the number of dopant atoms were calculated corresponding to 0.5%, 1%, 2% and 3% doping of ZnS and then corresponding Zinc atoms were substituted with silver atoms. For diode device to be built up half of the thin film was doped with silver and other half was undoped (corresponding to n-type behaviour of ZnS).The transmission spectra and I-V plot was found for the p-n diode and corresponding wavelength of light emitted was found. III. RESULTS AND DISCUSSIONS The band structure and electronic properties of ZnS thin film and that doped with silver was studied using density functional theory. Software analysis show that the effect of doping due to silver influences the band structure. It was found that in case of undoped ZnS thin film the Fermi level is near the conduction band with a band gap of 3.96 ev as shown in the figure 1 and the peak amplitude of density of states if found to be near the valence band however when it is doped with silver the Fermi level shifts and the shift is different for different percentage of dopant atoms. The results show that it is due to acceptor level impurity introduced. Fig 1: Band structure analyser of 5.41 nm ZnS thin film showing band gap of 3.96 ev Fig 2: Density of states of 5.41 nm ZnS thin film showing density of 1300 ev - 1 It was found that as compared to undoped ZnS thinfilm the corresponding bandgap in case of silver doped ZnS showed a marked decrease and that decrease occurred at very low concentrations of doping atoms of silver. The observed change in the band gap was from 3.13 to 2.54 ev for different concentrations of dopant atoms. Aakanksha Sud and Ramesh K Sharma ijesird, Vol. III, Issue III, September 2016/199

3 3(a) 3(d) Fig 3(a,b,c,d): Band structure analyser for 0.5%,1%,3%,5% Ag doped ZnS(bandgap 3.13 ev,3.40 ev,2.79 ev,2.54 ev respectively) Thus it can be seen that as the doping percentage changes the band gap changes and the band gap starts decreasing as the concentration is increased. Thus we can change the band gap by changing the doping percentage. Table 1: Bandgap of the Ag doped samples obtained by software simulation 3(b) Sr. No Sample Bandgap(eV) 1. ZnS:Ag 0.5% ZnS:Ag 1% ZnS:Ag 2% ZnS:Ag 3% 2.54 Since doping with Ag causes acceptor impurity as visible from band structure analysis. Thus we have used it for making pn diode using different concentrations of dopant atoms. The transmission spectra for different percentages of dopant atoms is shown. It was seen from the transmission spectra that the energy corresponded to wavelength in the UV region So this device could be used as a UV detector. 3(c) Aakanksha Sud and Ramesh K Sharma ijesird, Vol. III, Issue III, September 2016/200

4 4(a) 4(b) Fig 4(a,b): Transmission Spectra for 0.5%,3%,Ag doped ZnS(corresponding wavelength of 278 nm and 218nm,respectively) The corresponding I/V plot for the silver doped ZnS showed large rise in current for increasing concentrations of silver. Fig 5: I/V Plot for different concentrations of Ag IV. CONCLUSIONS The decrease in optical band gap as a result of doping with silver has numerous applications for use as a UV detector and leds. By varying the doping concentrations the band gap can be modified and can be tuned to obtain suitable material for fabrication of Led from UV range to visible range. The decrease in band gap due to doping with silver can be attributed to acceptor atom impurity. When ZnS is doped with silver it becomes a phosphor and from transmission spectra it can be proved that light emitted falls in the UV region so can be used as UV detector, a very versatile application. The computational analysis has been performed using local density approximation based on DFT theory. ACKNOWLEDGEMENTS Authors are thankful to Director NITTTR for extending lab facilities to carry out this work. REFERENCES 1. X.S. Fang, Y. Bando, U.K. Gautam, T.Y. Zhai, H.B. Zeng, X.J. Xu, et al. ZnO and ZnS nanostructures: ultravioletlight emitters, lasers and sensors Crit Rev Solid State Mat Sci, 34 (2009), pp Sidot, T. (1866). Sur les propriétés de la blende hexagonale Compt rend 63: Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. p ISBN W. Peng, G. Cong, S. Qu and Z. Wang, Optical Materials,29 (2006) 313. Aakanksha Sud and Ramesh K Sharma ijesird, Vol. III, Issue III, September 2016/201

5 5. A. Firdous, T. Rasool, G. Dar and M. Ahmad, Journal of Optoelectronics and Biomedical Materials, 2 ( 2010) X.S. Fang, Y. Bando, U.K. Gautam, C.H. Ye, D. Golberg, Inorganic semiconductor nanostructures and their field-emission applications,j Mater Chem, 18 (2008), pp Anurag Srivastava & R. Chandiramouli Band structure and transport studies on impurity substituted InSe nanosheet A first-principles investigation Superlattices and Microstructures Volume 79, March 2015, Pages Aakanksha Sud and Ramesh K Sharma ijesird, Vol. III, Issue III, September 2016/202