Preparation And Properties Of Cobalt Doping Chalcogenide Films By Pulsed Laser Deposition

As a group of traditional wide-bandgap semiconductors,II–VI metal chalcogenide semiconductor materials and the films have attracted considerable attention due to their potential applications in light-emitting devices,photodetectors,solar cells and infrared optical devices.Among all the II-VI semiconductors,Zn S and Zn Se are nontoxic and chemically more stable candidates suitable for actual manufacture.Researches show that transition metal(TM)ion doping is an efficient way to adjusting the structural,optical,electrical and magnetic properties of II-VI semiconductorsIn this paper,the Zn E:Co_x(x=0.1,0.3,0.5.E=S,Se)targets were prepared by means of solid state reaction.And then,the corresponding films were grown on sapphire(Al₂O₃)substrates with different Co doping concentration,various argon pressures and different substrate temperature by pulsed laser deposition.The microstructure,composition and optical properties of thin films were investigated by virous diagnostic instruments.1.First-principles studies on Zn E:Co_x(E=S,Se).The electronic structure and optical properties of Zn E:Co_x(E=S,Se)were calculated by using first-principles based on the density functional theory.The results indicate that the band gap decreased with Co doping.The analysis of the band structure,state density and partial state density showed that the hybridization of Co3d and Se4p(or S3p)generate a new level around the Femi-Level,so the top of valence band move to higher energy side and the band gap decreased.Furthermore,a new peak emerged at the Vis-NIR region of optical absorption with Co doping,which can be attributed to the electronic transition of ⁴A₂(4F)?⁴T₁(4P)in free ion Co²⁺.2.Effects of Co concentration on the properties of Zn E:Co_x films.Amorphous and crystalline Zn E:Co_x(x=0.1,0.3,0.5.E=S,Se)thin films were grown on sapphire substrates by pulsed laser deposition at substrate temperature of25°C and 800°C,respectively.The x-ray diffraction results show that the crystalline quality and lattice constant decreased with increasing Co doping concentration.The x-ray diffraction and x-ray photoelectron spectroscopy spectra reveal that the samples reached an overdoping state at Co doping concentration of x?0.5.The band gap decreases whereas the refractive index and dielectric constant increase with increasing doping concentration.The optical parameters and dispersion parameters can be adjusted by tuning the doping concentration,which is important for applications in designing the optoelectronic devices.3.Influence of Argon Pressure on Properties of Zn E:Co_0.1 Thin Films.Zn E:Co_0.1(E=S,Se)thin films were grown on sapphire substrates at various ambient pressure by pulsed laser deposition.The propagation of plasma plume,the structure and optical properties of films at pressure of 0.4 Pa to 10 Pa had been investigated.With the ambient pressure increased the angle of divergence increased and propagation distance of plasma plume decreased.The thickness,surface morphology,crystal structure and optical properties of the thin films were investigated by various diagnosis tools.With increasing ambient pressure,the collision probability between ablated species and argon molecule was increased,the transformation of deposition type from sputter deposition to adsorbent depositon was achieved with increasing pressure from 2 Pa to 4 Pa.In addition,it was found that these physical properties were correlated to the argon pressure and appeared a mutation between the pressure of 2 Pa and 4 Pa.4.Effects of substrate temperature on properties of Zn E:Co_0.1 filmsZn E:Co_0.1(E=S,Se)thin films were grown on sapphire substrates by pulsed laser deposition at different substrate temperature.Evolutions of the microstructure,morphology and optical properties of the thin films as a function of substrate temperature were analyzed.The results indicated that the crystalline quality was gradually promoted and the films demonstrated increasing(111)preferred orientation with increasing substrate temperature.The refractive index and dielectric constant analyzed by the transmission spectra increased with elevating substrate temperature,which may be due to the promotion of the film packing density.The band gap decreased when the substrate temperature was increased from 25°C to 600°C,and then increased with a further increase of substrate temperature to 800°C.The evolution of band gap associates with the quantum confinement effect was quantitative analyzed.