Study On Preparation And Properties Of MoSe_x And WS₂ Thin Films

Transition metal dichalcogenides?TMDs?are the photoelectric materials with graphene like structure,good optical activity,suitable band gap and relatively high mobility,which has great potential in the new generation of solar cells and field effect transistors?FET?.In general,p-type semiconductor is essential requirement for p-n junction solar cell and CMOS integrated circuit.Therefore,it is highly necessary to investigate the preparation and properties of p-type semiconductor.The preparation processing,structure and properties of MoSe_x and WS₂ films are studied systematically.Effects of Al doping on the structure and properties are investigated momentously,which attempts to provide the valuable supporting technology for the applications in the solar cell and FET.The main research contents are as following:MoSe_x and Al:MoSe_xO_y films have been fabricated by radio frequency?RF?magnetron sputtering and influences of substrate temperature and Al doping content on the structure and properties of the films are discussed.With the increment of substrate temperature,MoSe_x films exhibit apparent tendency of crystallization,and the carrier concentration increases.The MoSe_x film has the highest Hall mobility at 150oC?substrate temperature?,and MoSe_x FET fabricated by that film has higher I_on/I_off ratio and field-effect mobility.Study shows that the amount of Mo cation with higher valence and the optical band gap increase as increasing the Al doping content.P-type conductive Al:MoSe_xO_y FET can be obtained by Al doping and the electrical properties are improved by moderate Al doping concentration.The preparation processing(sputtering and H₂S/CS₂ vulcanization)of Al:WS₂films has been studied systematically,and influences of Al doping content on the structure,composition,optical and electrical properties of the films are studied.Structural analysis shows that Al:WS₂ films grow with a preferential c_?-orientation and the crystalline quality decreases after Al doping.XPS analysis shows that Al-doped WS₂films include W-S bond,Al-S bond,a small quantity of Al-O and W-O bond.Contrast two kinds of vulcanization processing at the same Al doping content,the concentration of O is less and the crystalline quality of Al:WS₂ films vulcanized by CS₂ is better,indicating that CS₂ vulcanization effect is more obvious.Optical and electrical properties analyses show that p-type conductive Al-doped WS₂ films can be obtained by Al doping.With the increment of the Al doping content,the resistivity increases gradually,Hall mobility decreases,optical band gap decreases first and then increases,carrier concentration increases first and then decreases.The absorption coefficients of the Al:WS₂ films in the range of visible light are at¹0⁵ cm^-1,and the absorption excitonic peaks are at¹.94 and².35 eV.The electrical properties of Al:WS₂ films vulcanized by CS₂ are better than those of H₂S vulcanization.Hall mobility of the un-doped WS₂ films vulcanized by CS₂ is 1.51×10¹ cm²V^-1s^-1.Al:WS₂ thin films have been fabricated by ALD and CS₂ vulcanization,and influences of Al doping content on the structure,composition,optical and electrical properties of the films are discussed.Structure analysis shows that Al:WS₂ films grow along the?002?direction,and the?002?peak intensity and crystalline quality decrease as increasing the Al doping content,which is similar to the sputtering and vulcanization processing.XPS analysis shows that the core-level binding energies?BEs?of W4f,S2p,O1s and Al2s decrease with increasing the Al doping concentration,indicating that the Fermi level is close to the valence band and p-type doping is obtained after Al doping.Optical and electrical properties analyses show that with the increment of Al doping content,the average transmittance and resistivity increase gradually,Hall mobility and absorption coefficient decrease,optical band gap decreases first and then increases,the carrier concentration increases.Hall measurement shows that p-type conduction can be achieved by Al dopant.Hall mobility of the pure WS₂ and 2.40%Al:WS₂ films are1.63×10¹and 9.71 cm²V^-1s^-1,respectively.Effects of thickness of the structure and property of Al:WS₂ films are investigated at the same Al doping content.After reducing the thickness of Al:WS₂ films,the grain size,crystalline quality and Hall mobility decrease,while the optical band gap and the resistivity increase.The crystal structure,band structure and density of states of un-doped,Al or O-doped and?Al,O?co-doped monolayer WS₂ are studied by using first-principles calculations.Study shows that the appropriate amount of Al can be more easily incorporated into the WS₂ monolayer under the conduction of O existence.The more the concentration of Al and O is,the harder the dopant is incorporated into the WS₂monolayer.Mulliken population value?0.41?of pure WS₂ monolayer shows that S-W bond has covalent character.W-S bonding of 2.08%?Al,O?co-doped WS₂ monolayer has higher covalent character and shorter bond length compared with other doping condition of this work.Pure and O-doped WS₂ monolayers have direct band gap,and p-type doping could be achieved by Al or?Al,O?doping monolayer WS₂.Then,Density of states and band structure of bulk WS₂ at different Al and O doping concentration have been calculated.Results show that the appropriate amount of Al can be more easily incorporated into the bulk WS₂ under the conduction of O existence.Bulk WS₂ is indirect band gap semiconductor.P-type conductive bulk WS₂ can be obtained by Al or?Al,O?doping.With the increment of Al doping concentration,the band gap decrease.With the augment of?Al,O?doping concentration,the band gap decrease first and then increase,which is consistent with experimental results.Moreover,deep energy level is generated at high Al doping concentration,which makes against the carrier transport.So it needs to control the doping concentration.