First-principles Study Of Defect Physics For Compound Semiconductors And Excited State Dynamics On Surface

The application of semiconductor promotes the development and innovation ofmicroelectronics, and changed the human life. The species of compound semiconductorsis extremely rich. The semiconductor property varies from compound to compound,which greatly meet the different kinds of applications in the semiconductor field.However, the diversity of compound semiconductors makes them difficult to investigate.Each compound semiconductor has its own core physical problem in the using process.Silicon dioxide (SiO2), Copper zinc tin sulfide (Cu2ZnSnS4), Born nitride (BN) andIndium arsenide (InAs), these four semiconductors relate to four key semiconductorissues of ground state properties and excited state dynamics, including dopingasymmetry in the wide bandgap semiconductors, critical defects in semiconductorswhich affect the device’s performance, defects’ behavior in semiconductors’ growth andsurface charge transport. Understand these physical issues could expand thesemiconductors’ application prospects in the related field. In this dissertation, using thefirst-principles calculation based on the density functional theory (DFT) and excited statedynamics based on the time-dependent density functional theory (TDDFT), we havesystematically studied these four compound semiconductors, and got four innovativeresults as following,1. Investigate the doping asymmetry in the wide bandgap semiconductor by using SiO2. In order to accommodate the application of microelectronic devices with the highperformance under extreme condition, people require the semiconductors with the widebandgap, high temperature and high mobility. However, dope in wide bandgapsemiconductor would meet the issue of doping asymmetry. Our systematic investigationof the doping property of SiO2reveals no obvious doping asymmetry in the donor andacceptor level position, and only modest asymmetry in the impurities’ formation energy.With an advanced approach of GGA+U, we achieve an accurate description on the localconfiguration of substituting impurities in SiO2, which is quite consistent with EPRspectroscopy. The SiO2can have both relatively shallow acceptor transition level anddonor transition level. Our predicted best candidates for p-type and n-type doping areAlSi(0.86eV above the VBM) and PSi(0.74eV below the CBM), respectively. Thefindings strongly proved the wide band gap semiconductor material may not introducethe issue of doping asymmetry.2. Investigate the native defects in the CZTS semiconductor. The CZTSsemiconductor is the perfect substitute of CuInSe2(CIS) in the application of solar cell.all the four constituent elements in CZTS are earth-abundant, cheap and nontoxic. CZTShas the potential to overcome the limitations of CIS in production capacity. In recentyears, the solar-energy conversion efficiency of CZTS has a huge raise, which acceleratesits application. For the quaternary compound semiconductor, it has a great number ofintrinsic defects. Therefore, in the experiment, it is very different to directly identify thenative defects, which significantly affect the properties. However, the correspondingdefect physics is not sufficient, the critical defects which affect the conductivity type anddevice performance is still under debate. Hybrid functional calculation reveals thephysical identity of native defects in CZTS. The SnZnantisite and the defect complex(Cu3)Snare found to be the main deep minority traps. Our results provide a microscopicunderstanding on the optimal growth condition of CZTS, e.g. the optimal Cu-poor andZn-rich growth condition suppress the formation of SnZnantisite and the defect complex(Cu3)Sn. Under the optimal growth condition, VCucould contribute the majority of thehole carriers, even though its concentration is much lower than another acceptor CuZn. In addition, no defects in our calculation show negative formation energy, indicating thatCZTS is a thermodynamically stable material.3． Investigate hydrogen behavior in the growth of BN semiconductor. BNincludes two common crystalline forms: hexagonal BN (hBN) and cubic BN (cBN).They have not only the excellent lubricating and mechanical properties, but also hightemperature, corrosion resistance and high insulation. Based on the broad and importantapplication prospect, lots of researchers over the world show a strong interest onstudying BN. The biggest problem in using BN is to synthesize the BN with single phase.A large number of hydrogens (H) exist during the BN growth and directly affect thequality of synthesizing BN material. But the H’s behavior during the BN growth is rarelydiscussed, which is worthy of further studying. We have systemically studied thethermodynamic and vibration properties of H in BN crystal. Essentially, H prefers toreside in hBN with forms of H2and H2**rather than in cBN. Unexpectedly, these H2andH2**can automatically gather to form clusters. In present of H, cBN phase tends to besuppressed. Therefore, Al-induced sp3nucleus in hBN is readily passivated. Thatexplains the puzzle why Al has no significant improvement to grow cBN from hBN.4. The study of quantum mechanical photo-Dember effect on InAs surface. TheTHz radiation will generate on the semiconductor by the femtosecond laser absorption.This phenomenon can be explained by the photo-Dember effect, but the photo-Dembereffect descripted by the semi-classical theory, has its innate limitations. For example, thedynamical relaxation and cool-down of the excited carriers in these states are governedby non-adiabatic processes involving strongly coupled carrier and ion motions. Therefore,it is very important to understand the surface charge transport, generating THz radiationfrom the perspective of quantum mechanics. First-principles MD simulation of iondynamics, coupled with TDDFT for electron dynamics, reveals the possible existence ofa non-classical pDe at semiconductor surface. Contrary to current understanding, surfaceexcitations with infinite mass can propagate into bulk with a speed as high as106cm/s.The propagation is caused by electron-phonon coupling. Because such a coupling is not reserved only to SSs, one can expect the quantum mechanical pDe to apply to any stateexcited at the surface, even when the semi-classical component dominates.Electron-phonon coupling usually slows down carrier diffusion via scattering and/orpolaron formation. Our findings thus represent a counter example where the couplingmay enable carrier diffusion.