First-Principles Calculation Study Of Novel Optoelectronic Semiconductor Materials

Traditional optoelectronic semiconductor materials,such as Si,CdTe,GaN and Cu?In,Ga?Se_R2R,already have a huge number of applications in our daily life.These ap-plications include photovoltaic devices,light-emitting diodes?LEDs?,laser,detector and so on.However,the increasing demands of optoelectronic devices and the defi-ciency of traditional optoelectronic materials make the research and development?R&D?of new optoelectronic materials essential.Among so many methods,first-prin-ciples calculation is an effective way of R&D.Hence,using the first-principles calcu-lation method,following the line of design?study fundamental properties?including crystal structure,stability,electronic structure and optical properties??study defect properties?formation and function of defects?,we made a systematic study of the novel optoelectronic semiconductor materials.This paper has six main chapters.The first chapter introduces the evolution of the optoelectronic semiconductor devices,including traditional optoelectronic semicon-ductor materials and their deficiencies,novel optoelectronic semiconductor materials and their advantages,and how to design and study the novel optoelectronic semicon-ductor materials theoretically.The second chapter contains all the methods used in this paper,such as cation-mutation,first-principles calculation?these two methods are used for the design and study of fundamental properties?,supercell method and Time-de-pendent Density Functional Theory?these two methods are used for the study of defect properties?.In the third chapter,using the cation-mutation,we designed a series of quaternary nitride compounds I-III-Ge_R2RN_R4R?I=Cu,Ag,Li,Na,K;III=Al,Ga,In?following the GaN?ZnGeN₂?I-III-Ge₂N₄ mutation,and studied their fundamental properties.These quaternary nitrides crystallize in a wurtzite-derived structure as their ground state.The thermodynamic stability analysis shows that while most of them are not stable with respect to phase separation there are two key exceptions:LiAlGe₂N₄ and LiGaGe₂N₄,which are stable and can be synthesized without any secondary phases.Interestingly,they are both lattice-matched to GaN and ZnO,and their band gaps are direct and larger than that of GaN.They have valence band edges as low as ZnO and conduction band edges as high as that of GaN.We predict flexible and efficient band structure engineer-ing can be achieved through forming GaN/LiAlGe₂N₄/LiGaGe₂N₄ heterostructures,which have tremendous potential for ultraviolet optoelectronics.Then,in the fourth chapter,similarly,we used cation mutation and designed 18unconventional quaternary chalcogenide semiconductors I₂-II-SnS₄?I=Cu,Ag;II=Zn,Cd,Hg,Mg,Ca,Sr,Ba?and Cu₂-II-SnSe₄?II=Mg,Ca,Sr,Ba?.Using the basic structure unit?BSU?,the connections have been established between conventional?such as kesterite or stannite structure?and unconventional structures?such as trigonal or orthorhombic structure?.Through analyzing the tendency of the stability,we find that the ground state structure of these quaternary chalcogenide compounds changes from conventional to unconventional structures with the increase of the atomic size of group II cations.The calculation results of electronic and optical properties show that most of these quaternary chalcogenides crystallizing in unconventional structures have excellent band gap values?1.1-1.6 eV?and strong absorption of visible light.This indi-cates that they have huge potential in the photovoltaic applications.In the fifth chapter,defect properties of binary Sb₂S₃ are investigated using the supercell method.From our calculated results,we find that intrinsic Sb₂S₃ shows weak n-type conductivity,which is caused by the comparable concentration of the dominant ionized donor V_S and acceptor Sb_S and V_Sb.This weak n-type conductivity can explain the relatively poor performance of Sb₂S₃ based solar cells?compared with commercial solar cells,such as Si-based solar cells?,which is thought to be caused by the high resistivity of Sb₂S₃ as claimed in the experiment.For extrinsic doping,Pb and Sn can only make Sb₂S₃ weak p-type,while Cl can make Sb₂S₃ itself much better in n-type conductivity.So the solar cells using Cl-doped Sb₂S₃ to act as n-type carrier transport layer may witness a breakthrough in the efficiency.In the sixth chapter,in order to study the complex dynamic process of forming defect,we developed a new real-time time-dependent density functional theory?rt-TDDFT?.Using the dynamic process of molecular dissociation under the electron beam illumination as an example?because this process is relatively simple,and it also con-tains the formation and breakdown of atomic bonds,which is similar with the formation of defect?,we tested the newly developed rt-TDDFT and find that:?1?To the relatively shallow ionizations,large atomic kinetic energies released by hot hole?ionization caused by electron beam illumination?cooling act as the dominated driving force in the dissociation processes.These processes are very quick,taking tens to hundreds of femtoseconds;?2?If the ionization occurs at the very deep energy levels,Auger process can be more significant than hot hole cooling and makes the C₂H₆O₂ molecule doubly ionized.Interestingly,this doubly ionized molecule will dissociate automatically.Two possible dissociation channels are both exothermal,which could result from the Cou-lomb explosion;?3?Our simulations indicate that C₂H₆O⁺_2PP?CH₃O_PP⁺+CH₃O is the dominated dissociation channel which agrees very well with the highest intensities of CH₃O_PP⁺fragment in the experimental mass spectra.Meanwhile,the unobserved CH₅O⁺_PPP may originate from ground state thermal activation process due to the high energy bar-rier of its dissociation channel.This process needs to take a relatively long time;?4?Through the analysis of the bond order,the weak and easily broken C-C bond in C₂H₆O⁺_2PPcould be the reason for the low intensities of C₂H₃O⁺_PP,C₂H₄O⁺_PP and C₂H₅O_P⁺_Pfragments in the experimental mass spectra.This work paves the way for exploring the dynamic process of forming defect in the novel optoelectronic semiconductor materials.