The Electronic Structure And Electrical Transport Properties Of Several Typical Wide Band Gap Oxide Semiconductors

Wide band gap oxide semiconductor is a kind of important functional material,which can be widely applied to different fields according to the band gap and electrical conductivity.The wide band gap oxide semiconductors mainly include the oxide semiconductors based on the IIIA group elements Ga and In,the oxide semiconductors based on the ⅡB group elements Zn and Cd,and the oxide semiconductor based on theⅣA group element Sn.In this dissertation,Ga₂O₃ and In₂O₃ wide band gap oxide semiconductor and amorphous InGaZnO（a-IGZO）oxide semiconductors are selected.The electronic structure and impurity level position of undoped,Si doped,and Mg dopedβ-Ga₂O₃ single crystals,the electrical transport properties of ultrathin Sn doped In₂O₃（ITO）films,the electronic structure and electron effective mass of a-IGZO films were systematically investigated.The defect levels in semiconductor are crucial to the performance of related devices.However,the positions of the defect levels in monoclinic β-Ga₂O₃,such as that for oxygen vacancy（VO）,Ga-O divacancy,and self-trapped hole（STH）,are still controversial.At the beginning of this dissertation,we systematically investigated the photoluminescence and photoluminescence excitation spectra of undoped,Si-doped,and Mg-doped β-Ga₂O₃ single crystals grown by the edge-defined film-fed growth method.It is found that the oxygen vacancies are deep donors and their levels are related to the coordinations of the vacancies.For each crystal,the recombination between VO and Ga-O divacancy levels is the origin of the blue emission,and the red emission originates from the transition from VO to STH levels.The location of STH level is also related to the geometrical position of oxygen.For the Si-doped crystal,no red emission is observed due to the low concentration and short lifetime of the STHs.While a Mg impurity level is observed at～1 eV above the valence band maximum,which causes an extra red emission in the spectra of Mg-doped crystal.Our results strongly support the recent theoretical results obtained via first-principle calculations and could be benefit for designing β-Ga₂O₃ related devices.Considering that the electrical transport properties of ultrathin ITO films in high temperature region are still not fully understood,we systematically studied the temperature dependence of the carrier concentration and electrical resistivity of a series of ultrathin ITO films from 150 to 395 K in the charpter 4 of this dissertation.For ITO films deposited on the[100]yttrium-stabilized ZrO₂（YSZ）single crystal substrates,when the thickness t is t（?）17.5 nm,the resistivity versus temperature can be described by the Bloch-Grüeisen formula,and the carrier concentration almost retains constant over the whole measured temperature range.For the t（?）5.3 nm（7.0 nm）film,the carrier concentration increases with increasing temperature above～300 K,and the resistivity initially increases with increasing temperature and then decreases with further increasing temperature above～330 K（～355 K）.By comparison of the transport properties of ITO films depositing on different substrates（[100]MgO、[0001]Al₂O₃,and[0001]SiO₂ single crystal）and exposing the films on different atmospheres,it is found that the enhancement of carrier concentration and reduction of resistivity at high temperature regime originate from the dissociative adsorption of water vapor on the surface of the ITO films.The a-IGZO film is easy to deviate from the stoichiometric ratio in the preparation process,which will significantly affect the electrical transport properties and electronic effective mass of the film.In the charpter 5 of this dissertation,the electrical and thermal transport properties,and optical transmittance and absorption spectra of the a-IGZO films with gallium and indium deficiencies were studied.It is found that for each film,the resistivity increases with increasing temperature above 150 K,while the carrier concentration almost keeps as a constant in this temperature regime.This indicates that the a-IGZO films possess degenerate semiconductor（or metal）characteristics in transport properties.The thermopower is negative and decreases linearly with decreasing temperature from 300 K down to liquid helium temperature,indicating the electron diffusion thermopower governs the thermal transport process in each film and the electrons in the conduction band have free-electron-like characteristics.Using free-electron-like model,we extracted the electron effective mass from the thermopower and the optical band gap measurements.The results obtained by the two methods are almost identical.The electron effective mass of the nonstoichiometric a-IGZO film is about three times as large as that of the stoichiometric one and increases with increasing carrier concentration.Finally,the schematic pseudo-band structure of the nonstoichiometric a-IGZO film is given.