Study of wide bandgap oxide semiconductors, SnO2 and ZnGa2O4, using thin film transistors

Abstract

This dissertation focused on the study of wide bandgap oxide semiconductors, SnO2 and ZnGa2O4, using thin film transistors. To investigate the material properties and potential to visible/UV transparent and high power devices, thin films and thin film transistors were exploited. Especially, using thin film transistors has the advantages of not only demonstrating device performance but also investigating material electric transport properties by modulating carriers with the electric field. Semiconductor materials which have larger bandgap than conventional semiconductors, such as Si and GaAs (bandgap, Eg: 1 ~ 1.5 eV), are called as wide bandgap semiconductors. With the larger bandgap than the visible range of 3.1 eV, the semiconductor materials are transparent in the visible range. The materials are transparent in ultraviolet A (UVA: 3.1 ~ 3.94 eV) and ultraviolet B (UVB: 3.94 ~ 4.43 eV) with the bandgap over 3.94 eV and 4.43 eV, respectively. Moreover, the wider bandgap materials have a higher breakdown field strength because the electric field to generate and accelerate carriers become large as increasing bandgap. With these properties, wide bandgap semiconductors are particularly important for display/optoelectronics and high power devices due to their transparency and high breakdown field. The known wide bandgap semiconductors are IGZO (Eg = 3.0 eV), BaSnO3 (Eg = 3.1 eV), ZnO (Eg = 3.3 eV), SiC (Eg = 3.3 eV), SnO2 (Eg = 3.6 eV), SrSnO3 (Eg = 4.6 eV), β-Ga2O3 (Eg = 4.8 ~ 4.9 eV), ZnGa2O4 (Eg = 4.6 ~ 5.2 eV), AlxGa1-xN (Eg = 3.4 ~ 6.0 eV), and diamond (Eg = 5.5 eV). SnO2 is a transparent semiconductor with a wide bandgap of 3.6 eV. Despite the difficulty of transparency and conduction coexistence, SnO2 shows optical transparency and conductive electric properties at the same time. SnO2 exhibits the 97 % transmittance in the visible range. Depending on crystallinity and doping concentration, SnO2 has a resistivity of 10-4 ~ 106 Ω∙cm (most semiconductor 10-3 ~ 109 Ω∙cm) and carrier concentration of 1017 ~ 1020 cm-3 with high mobility. From these properties, SnO2 can be used not only for a transparent conductive oxide (TCO) but also for a transparent oxide semiconductor (TOS). As a TCO, SnO2 is widely used to solar cell and flat panel display in itself or alloy with In2O¬3. Due to its electrical transport properties and transparency along with thermal and chemical stability, SnO2 is one of the promising TOS candidates especially for replacing IGZO at the display industry. The transparent thin film transistors (TFTs) based on polycrystalline SnO2 and epitaxial SnO2 were fabricated and compared. Reactive sputtering methods and subsequent annealing process were used for polycrystalline SnO2 deposition on the glass. Both top and bottom gate geometry TFTs of polycrystalline SnO2 showed high mobility of 145.7 cm2/Vs and 160.0 cm2/Vs, respectively. However, the polycrystalline SnO2 TFTs exhibited the non-ideal characteristics in output and transfer characteristics; a large hysteresis along with voltage dependence. The probable origin of these behaviors is the barrier formation across polycrystalline SnO2 grains. To confirm this, epitaxial SnO2 TFTs were fabricated on r plane sapphire (r-Al2O3) by a pulsed laser deposition method. Although the mobility of epitaxial SnO2 TFT was not as high as that of polycrystalline SnO2 TFT, non-ideal behaviors disappeared. By comparing TFTs characteristics and structural properties, it was confirmed that grain boundaries of polycrystalline SnO2 cause the unstable TFT characteristics and high density of threading dislocations and antiphase boundaries are the origins of the low mobility of epitaxial SnO2 TFT. The use of polycrystalline SnO2-x TFTs will require a thorough understanding of grain boundaries. Normal spinel oxide ZnGa2O4 (ZGO) is known as having an ultra-wide bandgap of 4.6 ~ 5.2 eV and transparency in UV region. ZGO has two cations of Zn2+ at the tetrahedral site and Ga3+ at the octahedral site along with the GaO6 octahedra network as β-Ga2O3. ZGO is expected to possess strengths over β-Ga2O3. From a normal cubic spinel structure, ZGO shows isotropic properties and stable phase. ZGO has higher conductivity and doping possibility than β-Ga2O3 due to its normal spinel phase with two cation sites. Based on these strengths, ZGO is spotlighted as a high power device and UV transparent device candidate. The coherent epitaxial ZnGa2O4(ZGO) layers were grown on MgAl2O4 and MgO substrates by pulsed laser deposition. Using X-ray diffraction and transmission electron microscopy, it was confirmed that the ZGO is a spinel structure without any threading/misfit dislocations. Depending on the strains (compressive or tensile strain) by substrates and cations (Zn2+, Ga3+) compositional ratio, the ZGO thin films and TFTs exhibited different structural and electric transport properties. When the Zn/Ga ratio is slightly lower than the ideal value of 0.5 with tensile strain, the ZGO TFT showed the highest mobility of 5.4 cm2/Vs, a large ION/IOFF ratio of 4.5×108, and small subthreshold swing value of 0.19 V/dec. From the structural and electrical characteristics of the ZGO thin films and ZGO TFTs, Zn vacancies and antisite defects of Ga located at Zn site seem to be the dominant defects of the ZGO. Further understanding of ZGO defects and strain will improve TFTs performances.