The electronic properties of β-Ga2O3 [beta-Ga2O3]

This thesis deals with experimental and theoretical investigations of the electronic structure of ß-Ga2O3. It is the transparent conducting oxide (TCO) which exhibits the widest band gap. The investigated ß-Ga2O3 single crystals were grown by chemical vapor transport (CVT) and the Czochralski method. The stoichiometry of ß-Ga2O3 crystals was checked by energy dispersive X-ray spectroscopy (EDX). The crystal structure and lattice parameters of the crystals were determined by X-ray diffraction. The surface morphology of (100)-planes were characterized by different techniques such as low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The STM pictures showed no surface imperfections. The carrier concentration, resistivity and mobility of the ß-Ga2O3 crystals were determined by the four contact, Van der Pauw method and by Hall effect. The electrical measurements confirmed that ß-Ga2O3 is a n-type semiconductor. In addition, optical measurements were performed for undoped and doped crystals at room temperature as well as low temperatures. The direct energy gaps were obtained and the band gaps decrease as temperature increases. In order to reveal the details of the electronic structure of ß-Ga2O3 the wavevector dependent experimental valence band structure of undoped and Sn-doped samples was determined by high-resolution angle-resolved photoelectron spectroscopy (ARPES) utilizing synchrotron radiation. Comparing ARPES results with the electronic structure determined by first-principles calculations based on density functional theory showed excellent agreement. By measuring ARPES on Sn-doped ß-Ga2O3 crystals enabled us to see an emission from the conduction band minimum and accordingly the absolute band gap was determined directly and compared to the optical results. In addition, the orbital character of the filled density of states (DOS) was determined from soft x-ray photoelectron spectroscopy (XPS). We have found three valence bands groups which agree well with the valence band density of states calculations. Furthermore, the XPS results showed that carbon impurities can attribute to the electrical properties. Also Schottky barrier formation has been studied at an Aun- type ß-Ga2O3 contact. The Schottky barrier height was found to be 1.01 eV which agrees totally with the determined value of 1.07 eV from current-voltage characteristics.