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Modulation Effect on Energy Gaps and Optical Properties of Transition Metal Doped Laser Crystals

Author: ZhangJiHua
Tutor: DingJianWen
School: Xiangtan University
Course: Condensed Matter Physics
Keywords: Laser crystals Density functional theory Electronic structure Optical property
CLC: O734
Type: PhD thesis
Year: 2013
Downloads: 72
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Abstract

It is well known that the laser crystals are fundamental matters for the laserdevices, which are formed by the laser hosts and the doped metal ions. The physicaland chemical properties of laser crystals depend mainly on laser hosts, while thespectral characteristics depend on energy levels of the metal ions. Although thedensity functional theory is not yet accurate to calculate the f electron of rare earthmetal ions, it has been developed to be suitable for transition metal doped laser hostssystem. Due to the fact that there are no shielding electrons outside the3d electrons oftransition metal, they are impacted by the lattice field directly. Thus, their leveldistribution properties are remarkably different from that of free ion. In this thesis, thegeometries, formation energy, electronic structures and optical properties of transitionmetal （TM） doped MgF₂, Al₂O₃, ZnS and diamond nanowires systems are studied byusing first-principles techniques.The main works of this thesis are organized asfollows:Firstly, based on the density functional theory, the geometries, electricalstructures and optical properties of Co²⁺-doped MgF₂system (Co²⁺:MgF₂) are studiedby using first-principles plane-wave approach. With increasing Co²⁺-doping, atetragonal-rhombic structural transition is obtained in Co²⁺:MgF₂. The band gapdecreases with increasing Co²⁺-doping, and a semiconductor-conductor transition isobserved. Also, the calculations show that the static dielectric constant and theabsorption coefficient can be remarkably modulated by Co²⁺-doping, indicating thepotential applications of Co²⁺: MgF₂optical system.Secondly, the geometric and electronic structures of Ti³⁺doped α-Al₂O₃system(Ti³⁺:α-Al₂O₃) are calculated by using the first-principles method. Due to Ti³⁺-doping,bond lengths and bond angles in the local geometries are both distorted, from which atrigonal-triclinic structural transition may be expected. With increasing Ti³⁺-doping,an insulator-semimetal transition is observed in the Ti³⁺:α-Al₂O₃system, mainly dueto a complete spin polarization of electrons at the Fermi level. The dielectric functionand absorption edge are further calculated. It is found that the absorption edge isdecreased and is much lower than that of pure Al₂O₃crystal, which is in goodagreement with the experiment. The results indicate the potential applications ofTi³⁺:α-Al₂O₃optical system. Moreover, the geometries, electrical structures, Milliken population and optical properties of Mg and Ti³⁺co-doped α-Al₂O₃system((Mg²⁺,Ti³⁺):α-Al₂O₃) are studied by using first-principles. The result shows that, withincreasing Mg and Ti³⁺co-doping, the lattice parameter increases, but the band gapdecreases. Also, the calculations show that the imaginary parts of dielectric functionand the absorption coefficient can be remarkably modulated by Mg and Ti co-doping,indicating the potential applications of (Mg²⁺,Ti³⁺):α-Al₂O₃optical system.Thirdly, the formation energies, electronic structures and optical properties ofTM:ZnS systems (TM=Cr²⁺, Mn²⁺, Fe²⁺, Co²⁺and Ni²⁺) are investigated by using firstprinciples method. It is found that the wurtzite and zinc blende structures almost havethe same stability and thus can coexist in the TM:ZnS system. From the wurtziteTM:ZnS, especially, a partially filled intermediate band （IB） is obtained whenTM=Cr²⁺, Ni²⁺and Fe²⁺, while it is absent for TM=Mn²⁺and Co²⁺. The additionalabsorptions are realized in infrared, visible and ultraviolet （UV） regions, due to thecompletely spin-polarized IB at Fermi level. The results are very useful for thedesigns and applications of TM:ZnS opto-electronics devices such as solar-cellprototype.Finally, by using the advanced Heyd-Scuseria-Ernzerhof hybridexchange-correlation and spin-polarization, we investigate the electronic structuresand optical properties of the hydrogenated diamond nanowires （DNW） systems withnitrogen-vacancy （NV） centers. It is shown that the lattice constants and band gaps ofthe hydrogenated DNW decrease with its cross-sectional area. In the presence of NVdoping, the strong localization of defect states results in spontaneous spin polarizationand local moment formation, which are mainly due to C2p orbitals with Hund energyclose to that of transition metal atoms. The spin polarized state of the doped systemshould be stable above room temperature, consistent with the previous experimentalreport. The refractive index and absorption coefficients are further calculated. It isfound that the refractive index for the smaller cross section areas is closer to1.0, thusmeaning the higher collection efficiency of single-photon emission in the thinnerDNW. A red-shift of optical adsorption edge is observed, basically due to theintraband transition of the spin polarized C2p orbitals’ valence bands and theconduction bands. In particular, a strong absorption in the visible-light region is found,suggesting that the NV-doped diamond nanowire systems could be a potentialcandidate for photoelectrochemical application.

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