Optical Properties Of A Few-layer WS₂ Film On SiC Substrate By PLD Method

In recent years,two-dimensional materials(2D),represented by graphene,have attracted much attention of investigation due to their unique physical,chemical,photoelectric properties and wide application prospects.Being of advantages in stackability in spatial structure,photoelectric conversion property and band-gap tunability,transition metal dichalcogenides(TMDs)have enhanced the application prospect of 2D TMDs,and have become one of the current research hot-spots in the field of thin film materials.However,the band gap engineering of TMDs is based on heterogeneous junction or by means of increasing or decreasing the thickness of the spatial structure of TMDs or doping them.Such band engineering technology,which cannot be changed again once processing fixed,cannot achieve continuous tuning of the band gap of TMDs.At present,the researches on the non-junction photovoltaic effect materials,such as ferroelectrics,piezoelectrics,gyrotropics crystals,and semiconductors,which lack a center of symmetry etc,,have shown the common drawback of low output current--the current of nA level(or below),which is far from realizing the practical application.Meanwhile,in the application field of portable electronic devices,the miniaturization of imaging lens and the requirement of optical zooming have strong demands for tunable optical refractive index materials.In this dissertation,tungsten disulfideWS2 was selected to investigate its film deposition technology,and photoelectric properties of this thin WS2 film were studied.The main research works and results are as follows:The influences on the qualities of WS2 film by different substrate(sapphire,ITO/silica,and 4H-SiC)were studied in this dissertation.At the first time,Being of the same hexagonal structure of 2H-WS2,4H-SiC was chosen as substrate for WS2 film deposition.After comparing the morphology of 3 PCS WS2 films on SiC formed by sputtering as well as PLD each,a pulsed laser deposition(PLD)system was selected to deposit a thin WS2 film with a controllable thickness on 4H-SiC substrate,according to the best parameters of PLD investigated.Raman,SEM,TEM,AFM,XRD,GIXRR and EDS were used to characterize the morphology,structure,thickness as well as compositions of the films.The study shows that 4H-SiC,because of the same crystalline structure as 2H-WS2 and a small lattice mismatch,is superior to sapphire and ITO/silica substrates in morphology of the few-layered WS2 film prepared by pulsed laser deposition(PLD).Other thin film deposition method,such as magnetron controlled sputtering,was discussed,we chose PLD as our WS2 film deposition method in this investigation.HRTEM was used to image the cross section of 15nm-thick WS2 film,and the layered structure of WS2 was determined and the film thickness was measured.At the same time,the film thickness was measured and compared by using the profilometer and AFM.the thin film thickness was measured by GIXRR either,and the measured results by 4 different method are consistent.In order to study the optical properties of few-layered WS2 film in a strong electric field,a WS2/SiC film stack structure is adopted to facilitate the formation of strong electric field,this is,pulse laser deposition(PLD)method is used to deposit thin WS2 films with film thickness of 15nm and 150nm on n-type SiC substrate.TheWS2film thickness 15nm was measured by TEM imaging of the film cross-section,both EDS and EDX mapping of the WS2film were also conducted to find the film compositions and correspondent distribution.WS2/SiC film stackwas constructed by using WS2 film-WS2film stacking.Using conductive SiC as electrodes to apply a low voltage to produce a strong electric field,the physical and optical properties of WS2/SiC film stack are studied:1)The influence of external dc electric field on WS2/SiC film stack band gap tuning;2)The influence of external dc electric field on the refractive index of WS2/SiC film stack;3)Effect of electric field applied to WS2/SiC film stackon photon wavelength blue shift;4)new photovoltaic characteristics of WS2/SiC film stack;5)Photovoltaic effect of single few-layered WS2/SiC film.The investigation results on the optical properties of few-layered WS2/SiC are summarized and listed as follows,1)Energy band engineering presents a new and an easy method to modulate the bandgap of a WS2/SiC film stack:External Electric Field method.This is a convienient way to modulate the bandgap.To accomplish this modulation.Both 15nm and 150nm thickness of WS2 films were deposited on a n-doped SiC substrate by pulsed laser deposition(PLD)method.Tungsten disulfide films were superimposed face to face to form a film stack,and the conducting silicon carbide was used as the electrode to apply an electric field ranging from 0V/nm to 0.18V/nm.The experimental results showed that bandgap were continuously tunable from 2.017eV to 1.507eV.In order to understand this phenomenon,the first principle calculation by using Quantum Espresso also was performed to simulate the band gap change with the increase of an external electric field.It was found thatthe band gap of WS2/SiC film changes from 1.973eV to 1.488eV as an electric field applied perpendicularly to the film ranging from 0V/nm to 0.18V/nm.The consistency of experimental results and the first principle calculation was found.2)A 15 nm-thick WS2 film was deposited on a n-doped SiC substrate(7.37×1019 cm-3)by PLD method.The same kind of WS2/SiC film stack(15nm+15nm case)was constructed by the preceding discussed structure.The optical refractive index of the WS2/SiC film stack was found to be electrically tunable.The value of the effective optical refractive index ranged from 4.03 to 5.84,which implies a 44.9%variation in the refractive index change,at a wavelength of 460 nm under an applied external DC voltage modulated in the range of 0 to 7 V.We also found experimentally that the focal length of the plane lens was tunable from 184 mm to 156 mm at a wavelength of 650 nm with the applied voltage ranging from 0 to 7 V.This investigation presented a new way to modulate the refractive index and made the compact focus tunable lens design convenient and practical.3)As to wavelength blue shift of photon and photovoltaic effect of WS2/SiC film stack,both 15nm and 150nm thickness of WS2 film were deposited on a n-doped SiC substrate(7.37×1019cm-3)by pulsed laser deposition(PLD)method.New optical properties of the WS2/SiC material were discovered.I)a new photovoltaic effect:1)there is a cutoff wavelength ?c(661nm)which means the wavelength of an incident monochromatic light must be less than ?c in order to have the photovoltaic effect,2)the incident light must be polarized.3)It was found that the maximum open circuit voltage output is 6.3V in a condition of 40mW@532nm ?)Wavelength Blueshift:when a laser of 532nm is used in the experiment to incident perpendicularly through the thin layer WS2/SiC film stack which is driven by an external electric field,it is found that the 532nm photons are blue shifted 1.33nm under a 30V(DC)voltage.We also find that the blue-shift of laser wavelength is tunable with the applied voltage.Inverse Compton scattering of the photon by both electron and hole is used to explain this blueshift,the consistency between experimental results and the theoretical calculation for the wavelength blueshift was found.4)The photovoltaic effect exists in WS2/SiC film stack(15nm+150nm),and the latest research results of volt-level open-circuit voltage and milliamp-level short-circuit current are obtained for the first time,which reveals that the photovoltaic effect is much related to the incident monochromatic light or laser wavelength,laser power,and the polarization of the laser.5)The mechanism of photovoltaic(PV)effect of even a few-layered WS2/SiC film(15nm)wasinvestigated.It is well known that photovoltaic effect(PVE)originates from the generation and spatial separation of positive and negative charges.However,the photoelectric conversion efficiency of a PV cell based on p-n junctions is confined by the Shockley-Queisser(S-Q)limit.Researchers have attempted to separate the charges by using PVE based on no junctions or a Flexo-PVE.However,the nA level of the short-circuit current(Isc)limits such applications in solar-energy harvesting.Here,we suggest and proof a different mechanism for PV charge separation in space:a linear polarized laser electric field(LPLEF)plus nonlinearity of the WS2 film..A 15-nm-thick WS2 film was deposited on an n-doped SiC substrate(7.37 × 1019 cm-3)by using the pulsed-laser-deposition method.The open circuit voltage was found to be Voc=1.503 V when a He-Ne laser with an average output power of 1.5mW was used to irradiate perpendicularly on the film.The corresponding Isc was measured to be 223.0 ?A.This work paves the way for the new mechanism to be used in broadband solar-energy harvesting.