Structural Stabilities And Electronic Properties Of Several Dopants And Surfaces Of Semiconductors

In the information age,multi-functional products have attracted much attention as the rapid development of science and technology.In order to obtain the products, people make great efforts to new feature optoelectronic semiconductors and Si electronic devices in nano-scale.However,a crucial problem of structure stability should be overcome before new optoelectronic devices and the nano-scale Si devices could potentially make inroads into the world.For a new optoelectronic semiconductor,wurtzite ZnO faces the difficulty and instability of p-type conductivity and the electrostatic instability of Zn and O polar surfaces.For the nano-scale Si devices,the geometrical and electronic structures of metal-clusters adsorbed on Si surfaces are subject to instability and unknown problems.Therefore,in the thesis we mainly studied the structural and electronic properties of isoelectronic Ga-N codoping of wurtzite ZnO,Zn and O polar surfaces of wurtzite ZnO,and self-assembled Zn nanoclusters on the Si(111)-(7×7)surfaces.Firstly,we demonstrated that the isoelectronic Ga-N complex in wurtzite ZnO can enhance the stability of the N_O acceptor and the N concentration by using the first-principles total energy calculations.As indicated by the calculated electronic structures,one of the isoelectronic Ga-N complexes can form a totally passive donor-acceptor complex by almost keeping on the basic electronic structure of undoped wurtzite ZnO,but only generates an additional fully occupied band above the top of the valence band.Then the ionization energy of the excess N_O acceptors will be reduced largely and the p-type conductivity will be enhanced significantly.Secondly,we calculated the geometrical and electronic structures of the Zn and O polar surfaces.For the Zn polar surface,steep surface states appear in the band gap of bulk ZnO and follow the bottom of bulk conduction band.Moreover,Fermi level shifts up into the conduction band,which leads to the n-type conduction behaviour. For the O polar surface,flat surface states emerge above the top of the valence band of bulk ZnO as Fermi level shifts down a little into the valence band.Thus,the O polar surface can be predicted to have the p-type conduction behaviour.STM measurements showed that the Zn polar surface can be stabilized by reducing the surface Zn/O stoichiometry with 0 atoms occupied at the edge of the triangular terraces.Different from the stabilization mechanism of the Zn polar surface,the O polar surface can be stabilized by transforming the surface charges and forming the p-type surface states.Finally,we successfully fabricated the identical-size Zn nanoclusters grown on Si(111)-(7×7)at room-temperature and demonstrated the atomic structure of the clusters by combining in situ STM and theoretical simulation.Due to the varying valence,Zn nanoclusters are favor in forming the Zn₇Si₃ geometrical structures with one characteristic Zn atom occupying the center and therefore distinguish this system from other nanoclusters(N=6).STS measurements indicate that the drastic depressions of the Zn₇Si₃ nanoclusters with respective to the corner Si adatoms at low bias voltages of±0.5V are attributed to the saturation of the metallic Si adatom dangling bond states at about-0.3 and +0.5V,which further reveal the semiconducting characteristics of the Zn₇Si+3 nanoclusters.Due to vanishing the Si adatom surface dangling bond state at about-0.3V,the closest edge Si adatoms in the nearest neighboring uncovered UHUCs are strongly influenced and thus almost darkened in the filled-state STM images,indicating the charge transfer from the closest edge Si adatoms to the Zn₇Si₃ nanocluster.We also demonstrated the Stranski-Krastanov(SK) growth mode of the multilayered Zn nanocluster arrays on the Si(111)-(7×7)surfaces. STM measurements showed that the interaction between the Zn atoms and the Si(111)-(7×7)surface become weak as the distance between the Zn atomlayer and the Si(111)-(7×7)surface increases,which will induce the transformation from the Frank-VanderMerwe(FV)mode to the Volmer-Weber(VW)mode,and further influence significantly the electronic properties of different Zn atomlayers.In-situ RHEED measurements suggest that the 1^stZn atomlayer is composed of the Zn₇Si₃ nanoclusters.The 2^ndand 3^rdZn atomlayers are made up of the Zn nanoclusters, which directly stack on the top of the Zn₇Si₃ nanoclusters with the hexagonal-closed-pack atomic geometry.And due to the specific atomic structure of Zn₇Si₃ nanoclusters,Zn(0001)and Si(111)are parallel with each other with an in-plane epitaxial relationship as Zn[112^-0]//Si[112^-].