First-Principles Study On Defect Properties And Carrier Dynamics Of Novel Semiconductors

Defects and carriers are important factors that affect the performance of semiconductor devices.On one hand,ionization of intrinsic defects and impurities can provide equilibrium carriers,affecting the device’s electrical performance.On the other hand,deep-level defects can trap charge carriers and induce non-radiative recombination,reducing the lifetime of non-equilibrium carriers and severely limiting the efficiency and reliability enhancement of the device.Therefore,it is particularly important to gain a deep understanding of the defect properties and carrier dynamics in semiconductors.Owing to the development of theoretical calculations and experimental characterization techniques,there is currently a relatively clear understanding of the defect and carrier properties of traditional semiconductors such as Si,Ga As,and Ga N.However,due to the diversity of structures and compositions,the physical properties of defects and carriers in novel semiconductors,such as the multinary compound derived from traditional structure,layered-structure semiconductors and perovskite-structure semiconductors,still need to be investigated.Do defects and doping in novel semiconductors have different characteristics from traditional semiconductors?How can we manipulate the concentration of equilibrium carriers through defect?How can we accurately calculate the lifetime of non-equilibrium carriers?These questions urgently need to be addressed.Therefore,combining first-principles calculations and non-adiabatic molecular dynamics simulations,this dissertation will systematically investigate the defect and carrier dynamics properties in novel semiconductors.1.We study the defects in ternary II-IV-V₂ semiconductors derived from zinc-blende structure and find important anion-cation antisites that affect the conductivity of semiconductors.The ternary II-IV-V₂ compounds,derived from the traditional diamond structure and the zinc-blende structure,are novel semiconductors with unclear defect properties.Therefore,the first work focuses on the intrinsic defects in ternary semiconductors derived from the zinc-blende structure and their manipulation of carrier concentration.An important defect that has been overlooked in research,the cation-anion antisites,is discovered.This defect significantly affects the photoelectric properties of the material with high concentration.In Zn Ge P₂,this defect not only causes several absorption peaks within the bandgap,but also compensates each other and pins the Fermi level,which restricts the p-type conductivity.Additionally,it is prone to forming complex defects with H impurities.In Zn Sn P₂,high concentration of Sn _P defect with deep energy level affects the photovoltaic performance.Although the defect properties of Zn Ge As₂ are quite superior,due to the formation of a cliff-type conduction band offset and a high valence band maximum position with Cd S,the open-circuit voltage of the solar cell is reduced.Consequently,we propose the formation of Zn Ge（P,As）₂ alloy by mixing P elements to lower the valence band position while achieving good p-type conductivity.This work reveals the important role of previously neglected intrinsic defects in zinc-blende-derived ternary semiconductors.2.We identify the origin defects that lead to the intrinsic n-type conductivity of layered-structure binary semiconductors Sn Se₂ and screen the proper p-type dopant.In addition to the traditional zinc-blende structure,novel semiconductors also have various novel structures,and layered binary semiconductors are one representative material.The second work investigates the intrinsic defect properties and p-type doping characteristics of the heterojunction material Sn Se₂ in tunneling field-effect transistors（TFET）.Due to the intrinsic donor defects Sn_i,Sn Se₂ exhibits intrinsic n-type conductivity,with an electron concentration of up to 10¹⁷ cm^-3 under Sn rich condition.However,p-type conductivity is also demanding for application as a channel material in metal-oxide-semiconductor field-effect transistors（MOSFET）.Our calculation reveals that doping of Ga and In under Se rich condition contribute to p-type conductivity of the material without changing its band structure and electron properties.Furthermore,In has a higher doping threshold than Ga,indicating better p-type doping effects.This work emphasizes the effect of doping on the electrical properties of layered binary semiconductors and expands their application.3.We investigate the manipulation of strain on the doping limit of perovskite-structure multinary semiconductors and the origin of carrier density saturation.Perovskite-structure multinary semiconductors are novel semiconductors with different structure from traditional semiconductors.Although it can be doped n-type or p-type,their doping limit characteristics need further investigation.Based on this,we have carried out the third study.Firstly,we investigated the manipulation of strain on the doping limit of perovskites and found that applying pressure stress to perovskites results in the contraction of semiconductors,which is more beneficial to achieve p-type doping.However,strain has less effect on the doping limit of n-type conductivity.Additionally,the effect of strain on the doping limit follows certain trends with elements,which can be explained by the kinetic-induced band broadening and hybridization of orbitals of anions and cations.Secondly,unlike traditional semiconductors,we found that perovskites exhibit carrier concentration saturation that violates the doping limit rule when heavily doped with bismuth（Bi）.Calculations have discovered for the first time that Bi doping can form stable Bi-Bi dimer defects,accompanied with local structural distortions.The dimer defects introduce acceptor levels below the conduction band,pinning the Fermi level after Bi doping and restricting n-type doping.Furthermore,the calculated defect luminescence spectra indicates that the non-dimerized Bi impurities attribute to the infrared luminescence,but the dimer defects gradually increase with increasing Bi doping concentration,suppressing the luminescence of Bi impurities,consistent with the trend of decreasing luminescence intensity observed in experiments.This study provides theoretical guidance for strain-based doping limit control and highlights the importance of novel defect configurations.4.We develop the systematical theory and method of accurately calculating the effective non-equilibrium carrier lifetime of semiconductors.Defects affect device efficiency primarily through carriers,and the lifetime of non-equilibrium carrier is a key physical parameter for evaluating device performance.However,there is currently a significant discrepancy between experimental and theoretical values for carrier lifetime,especially for perovskites,where the values differ by several orders of magnitude.This indicates that there are still significant issues with current theoretical calculations of carrier lifetime.Therefore,focusing on the calculation method for non-equilibrium carrier lifetime,we have performed the fourth work.Analysis reveals that the current mainstream method of using non-adiabatic molecular dynamics simulation to obtain carrier lifetime has two problems:overestimation of carrier and defect concentration and incomplete consideration of recombination mechanisms.Based on this,we have developed a method for calculating the effective non-equilibrium carrier lifetime in real semiconductors,which can obtain lifetimes comparable to experimental values.It has been verified that this theoretical framework and calculation process can also be applied to traditional semiconductors such as Ga As and Cd Te.This work not only resolves the contradiction between experimental and theoretical results,but also provides a general method for accurately calculating non-equilibrium carrier lifetime in semiconductors.This dissertation investigates the defect properties and carrier dynamics in novel semiconductors from two aspects:manipulating the equilibrium carrier concentration through defect engineering,and the impact of defects on non-equilibrium carrier lifetime.Through a comparative study of defects and conductivity properties in ternary semiconductors derived from the zinc-blende structure,binary semiconductors with layered structures,and multinary semiconductors with perovskite structures,it is found that novel semiconductors have more complex and unique defect properties compared to traditional semiconductors.Furthermore,a method for precisely calculating non-equilibrium carrier lifetime is proposed,which provides a theoretical basis for further theoretical calculations,experimental characterization,and optimization of device performance.