Electronic Structures Of In₂O₃（ZnO）_m Compounds And Their Electronic Transport Properties In Nanostructures

Transparent conducting oxides with the common features of n-type conductivity and wide band gaps, play a unique role in realizing the transparent optoelectronic devices. Much attention and intensive study have been attracted to the homologous compounds InMO₃（ZnO）_m（M=In, Ga, m=integer）（IMZOm） due to their special optical and electronic properties and layered structure, such as the high mobility in the amorphous state, excellent thermal stability and good optical transparency in the visible and near UV regions. This material has been regarded as the potential candidate to be applied in the flexible and transparent optoelectronic and nano-optoelectronic devices. Taking In₂O₃（Zn O）m（IZOm） as the representative, we systematically investigated their crystal and electronic structures, and their electronic transport properties in one-dimensional nanostructures. Many important structure-related features for this material are revealed.A Zigzag model with an unique arrangement of the atoms is introduced as the ground-state structure for the IMZOm system. The formation mechanism of the atomic structure is revealed. The zigzag boundary can be clearly identified in our simulated high-resolution transmission electron microscopy（HRTEM） images, and almost all of the HRTEM experimental data can be well explained. Based on the first principles total-energy calculations, the stabilities of four controversial models（plane, Zigzag, DYW, and quasirandom models） and their transform mechanisms are clarified. The calculated results confirm that the models with the zigzag feature are more stable than the others and it is possible to form different zigzag configurations in the samples as observed in the experiments. The dynamic process of satisfying the coordination numbers and the requirements of maximizing the symmetry and the distances between the In atoms in the slabs can be regarded as the dominant rules to stabilize the system, but the statistical equilibrium processes have the chances to transform it from the ground state structures to the other model structures. The results confirm that IZOm have the polymorphous and polytypoid structures.An effective correction method is introduced to calculate the electronic structures of IZOm. The calculated results based on the plane and Zigzag models reveals that their band gaps and effective masses increase monotonically with m. The predicted range of the band gap is located in 2.59-3.18 e V（m=1-6）, which is consistent with the experimental results. The anisotropic feature of electron effective mass tensor exhibited in the plane model can be employed to explain the anisotropic conductivities observed in the experiments. The calculated results confirm the possibilities of the separation of conduction electrons and defects and the existence of the natural optimized transport channels in the layered structures, which demonstrate its advantage over doped Zn O to transport electrons and benefit its applications in the optoelectronic devices.The electronic transport properties of pure and donor doped IZOm nanobelts were investigated based on the directly probing technique under the metal-semiconductor-metal（MSM） structure and found that their I-V characteristics exhibit structure-related non-ohmic transport behaviors. A method is presented to calculate the I-V characteristics of semiconductor nanowires（nanobelts） under the MSM structure. The carrier concentration as an important parameter is introduced into the expression of the current. The subband structure of the nanowire has been considered for associating it with the position of the Fermi level and circumventing the uncertainties of the contact areas in the contacts. We find that the two barriers have different influences on the I-V characteristics of the MSM structure, one of which under the forward bias plays the role of threshold voltage if its barrier height is large and the applied voltage is small, and the other under the reverse bias controls the shapes of I-V curves. The calculated results show that the shapes of the I-V curves for the MSM structure are mainly determined by the barrier heights of the contacts and the carrier concentration. The nearly identical I-V characteristics can be obtained by using different values of the barrier heights and carrier concentration, which means that the contact type conversion can be ascribed not only to the changes of the barrier heights but also that of the carrier concentration. The ambiguity about the mechanism of the ohmic-Schottky conversions and the influences on the I-V characteristics derived from the barrier height and carrier concentration are clarified.After performing the simulation of the experimental results by using this method and fitting the data based on the space charge limited（SCL） transport theory, it is confirmed that their intrinsic non-ohmic transport behaviors are not caused by these mechanisms. A structure-related model based on the hopping assisted trap-state electrons transport process is introduced to calculate the nonlinear I-V characteristics and extract their electrical parameters. The understanding of these trap-state and MSM structure influenced carrier transport processes can advance the progress of nanomaterials applications for this system. The understanding of this trap-state influenced carrier transport can advance the progress of nanomaterials applications and facilitate us to distinguish their intrinsic transport behaviors from the contact effects. The results also exhibit good electrical properties for the material as a potential substitute for In₂O₃.