| Transparent conducting materials(TCMs)are indispensable for the modern optical information industry and widely applied in solar cells,flat panel displays,photodetectors,and radio frequency devices,etc.Till now,the most popular TCMs in the industry are In-,Sn-,and Zn-oxides,which show outstanding n-type conductivities.Their p-type conductivities,however,are rather underdeveloped and cannot match with their n-type counterparts.This significant difference can be understood as follows.These oxides show n-type conducting characteristics due to the non-locality of the conduction band,resulting in small electron effective masses.Nevertheless,the O-2p orbitals of these oxides in the valence band tend to localize in small energy regions,thus giving rise to the heavy hole effective masses and poor p-type conductivities.The lack of high-performance p-type TCMs leads to the fact that only passive devices(based on n-type TCMs)can take advantage of TCMs.To develop active transparent electronic devices(e.g.,p-n junctions,transistors),it is particularly important to explore high-performance p-type or ambipolar TCMs.Due to the rapid development of machine learning and high-throughput computing in recent years,more and more new materials are coming into the researchers’ view.Researchers have found that some wide band gap alkaline earth metal sulfides possess non-local valence band edges and thus have small hole effective masses,which have great potential for application in the field of TCMs.In this paper,based on first-principles computational methods with the HSE06 hybrid functional,the electronic structure,optical properties,as well as the intrinsic and impurity defect behaviors of the wide band gap alkaline earth metal sulfides CaS and SrS,are investigated in depth.The calculated electronic structures indicate the band gap of CaS is 3.57 eV and the electron(hole)effective mass is 0.315(0.871)m0(m0 is the static electron mass).The light carrier effective masses(<1.0 m0)and large band gap(>3.1 eV)imply that the intrinsic properties of CaS satisfy the foundational requirements of high-performance TCMs.To further study the transparency and conductivity of CaS,the optical and defect properties are investigated.Optical properties show that the absorption coefficient and reflectivity are both low in the visible light region and for a 30.0 nm thickness CaS sample,the transmittance is above 80%,implying CaS is an excellent transparent material.Furthermore,the Al(Na)substituting Ca is a low formation energy and shallow ionization energy donor(acceptor)defect.The defect formation and ionization energies of AlCa(NaCa)are 0.85(1.40)and 0.15(0.14)eV,respectively.After rapid quenching from high growth temperatures to room temperature,the free electron(hole)density is 6.91 × 1017(1.23 × 1016)cm-3 for Al(Na)doped CaS.For intrinsic SrS,the electronic structures are calculated by using HSE06 hybrid functional.The indirect band gap is 3.48 eV and the electron(hole)effective mass is 0.316(0.874)m0,both of them are less than 1.0 m0.These properties indicate SrS is a potential TCM.Furthermore,we find Cl substituting S(Cls)is a low formation energy(0.43 eV)and shallow ionization energy(-0.02 eV)donor defect,while K substituting Sr(Ksr)is a low formation energy(1.39 eV)and shallow ionization energy(0.14 eV)acceptor defect.To enhance the carrier density at room temperature,the quench method is utilized.The electron(hole)density can reach 1.24×1019(1.23×1018)cm-3 at room temperature for Cl(K)doped SrS.Optical properties show that the absorptivity is less than 10%,and transmittance is large than 80%in the visible light region for a 30.0 nm thickness SrS sample,and the free carrier absorption is negligible.All in all,our studies indicate that wide band gap alkaline earth metal sulfides CaS and SrS possess high transparency in the visible light region,and high free carrier densities can be achieved in doped systems.These results imply that CaS and SrS are promising TCMs. |
