Comparative Study of As-Grown and Doped ZnS Thin Films for Optolectronic Device Applications

Abstract

Wide band gap semiconductors are the key components to make laser diodes and blue and green LEDS. They are being used in power switching and high temperature applications. The bandgap of ZnS is tuned by adding an impurity such as Mg that is useful for optoelectronic devices. Earlier reports indicate that there is slight blue shift in the band gap energy of Mg-doped ZnS but we have observed a reasonable red shift (3.48 eV to 2.58 eV) in ZnS band gap energy in Mg-doped ZnS structures. Theoretical model, based on first principle theory using local density approximation, has shown consistent results on Mg-doped ZnS structure. The present research work deals with as-grown and Mg-doped ZnS structures. A series of samples of ZnS with varying x content of Mg have been prepared with x= 0.0, 0.1, 0.2 and 0.3M by Sol-gel method using magnesium nitrate (Mg(NO3)2.6H2O), thiourea (SCN2H4) and zinc sulfate (ZnSO4.7H2O) for the sources of Mg2+, S2- and Zn2+ ions respectively. Ammonium hydroxide (NH3H2O) was used as a stabilizing agent and hydrazine hydrate (N2H4.H2O) was used as a complexing agent. The optical and structural properties were inspected as a function of Mg-concentration. The electrical, optical, morphological and structural properties were studied by different characterization techniques like XRD, SEM, FTIR, Hall measurements, EDX and UV-Vis Spectrometer. The cubic zinc blende crystal structure of ZnS:Mg was observed by the XRD. By SEM the crystallite size was found in micrometer (35nm for Mg=0.3). It has been concluded that with an increase of Mg-concentration in ZnS the crystallite size augments. The results of EDS show that the zinc and sulphur are major components in the composition of ZnS which confirms the purity of ZnS powder. In FTIR spectrum with range 500-4000cm-1, absorption peak was observed at 674cm-1 which confirms the ZnS synthesis. UV-Visible spectrophotometry results confirm the red shift in the bandgap energy of ZnS with Mg. The computational results performed by Quantum-ESPRESSO confirm the red shift band gap trend so as to support the experimentally determined results, only subject to the condition that Mg atoms substitute into ZnS structure. The additional related work deals with the sol-gel synthesis of BaTiO3 and Sr- doped BaTiO3 thin films having the composition BaTiO3+xSr (0 ≤ x ≤ 0.4) on Ti-substrate by spin coating. The barium acetate (Ba (C2H3O2)2), titanium (iv) n-butoxide (C16H36O4Ti), and strontium acetate (Sr (C2H3O2)2·2 H2O) were taken as preliminary materials. The starting materials were taken according to the stoichiometric proportions and mixed carefully in aqueous phases for uniform compositions. First of all barium acetate was dissolved in heated glacial acetic acid and then strontium acetate was added to this solution while stirring it so that all the particles were dissolved. After that a stoichiometric amount of titanium (iv) n-butoxide was dissolved in 2-methoxy-ethanol and added to the solution still under stirring. To stabilize the solution, ethylene glycol was added in proportion of 1:3 to acetic acid. The final solutions that were used for spin coating on titanium substrates have a concentration of 0.4 M. The structural, morphological and electrical characteristics of the Sr-doped BaTiO3 were studied. The X-ray diffraction results of BaTiO3 and Sr-doped BaTiO3 show the enhancement in the crystallinity of the films by increasing Sr-content. SEM measurements of doped and undoped BaTiO3 films reveal their uniform and cracks free nature. The P-E measurements of these thin films show the reduction in the ferroelectric properties with increasing the Sr content.