Characterizations of p-Type Tin Monoxide Thin Films Deposited by a Co-sputtering Process

Abstract

SnO thin films were prepared by co-sputtering deposition technique with SnO and Sn targets under different Sn target power (PSn DC). The growth films annealed at a low temperature of 180 °C in the air afterward. The properties of as-deposited and annealed films systematically studied using various techniques for their application to p-type thin film transistor. Grazing-angle incident X-ray diffraction (GIXRD) pattern analysis showed that the as-deposited films showed a metallic Sn phase, while the annealed films indicated SnO phase irrespective of PSn DC, which could be of p-type nature. The phase transition at annealed films observed at 40 W of PSn DC from polycrystalline p-type SnO to a composite film containing both metallic Sn and p-type SnO phases with increasing the PSn DC. Field-emission scanning electron microscopy (FE-SEM) images showed the discernible ellipsoidal grains along the c-axis direction at the as-deposited film and the irregular protrusions with the tiny and indistinct hemispherical swellings at the surface of the annealed film with increasing the PSn DC. High-resolution transmission electron microscopy (HRTEM) images showed the metallic Sn phase at the as-deposited film, and the SnO phase in the protruded region and the metallic Sn phase in the uniform bulk region at the annealed film, which were a coincidence with the GIXRD results. The SnO phase had tetragonal structure and the lattice parameters a and c were extracted from the GIXRD results of the annealed films. It found that the unit cell volume (a2c) decreased with increasing the PSn DC. The SnO density and the Sn layer density of the films showed the increasing tendency with increasing the PSn DC. The residual stress tendency of the as-deposited films shows that the tensile to compressive stress characteristics change as the PSn DC increases. The residual stress of the annealed films indicated all compressive properties irrespective of PSn DC and its tendency was a similar trend of the lattice parameter c with increasing the PSn DC. X-ray photoelectron spectroscopy (XPS) results of the annealed films exhibited that the composition ratio of Sn atoms (Sn/O) increased with increasing the PSn DC, indicating the phases of changing from SnO to Sn-rich SnO. Hall measurement results showed p-type semiconducting behavior at 0 W ≤ PSn DC ≤ 40 W, but metallic behavior at 45 W ≤ PSn DC ≤ 70 W which could be ascribed to the dominant role of metallic Sn. The maximum Hall mobility of 8.8 cm2/V∙s obtained at 40 W of PSn DC, which originated from the well-controlled amount of metallic Sn in SnO structure, leading the increasing Hall mobility. The Hall measurements at various annealing and measurement temperatures performed for two samples with specific PSn DC conditions at 0 W representing the SnO phase as a reference and 40 W representing the Sn-rich SnO phase. The Sn-rich SnO sample showed the higher mobility, which indicated that the hole carrier conduction mechanisms and the microstructures of two samples could be different. This result is promising for fabricating high-performance p-type oxide thin film transistor.