Abstract
We investigated the characteristics of carrier transport and crystallographic orientation distribution in 500-nm-thick Al-doped ZnO (AZO) polycrystalline films to achieve high-Hall-mobility AZO films. The AZO films were deposited on glass substrates at 200 °C by direct-current, radio-frequency, or radio-frequency-superimposed direct-current magnetron sputtering at various power ratios. We used sintered AZO targets with an Al2O3 content of 2.0 wt. %. The analysis of the data obtained by X-ray diffraction, Hall-effect, and optical measurements of AZO films at various power ratios showed that the complex orientation texture depending on the growth process enhanced the contribution of grain boundary scattering to carrier transport and of carrier sinks on net carrier concentration, resulting in the reduction in the Hall mobility of polycrystalline AZO films.
Highlights
Polycrystalline materials consist of grains of finite size
In our previous work [15], for 500-nm-thick Al-doped ZnO (AZO) films grown by various types of magnetron sputtering with an AZO target, we obtained the following findings: AZO films deposited by DC magnetron sputtering had high optical mobility, corresponding to high in-grain carrier mobility, high carrier concentration, and a large contribution of grain boundary scattering to carrier transport compared with AZO films deposited by RF magnetron sputtering; the Hall mobility was reduced owing to the above-mentioned large contribution of grain boundary scattering to carrier transport for DC-magnetron-sputtered AZO films, whereas AZO films deposited by RF
Due to the disordered nature of the grain boundaries of the films showing a texture with poor alignment between columnar grains and the discontinuities that they introduce into the periodic structure of the grains of the films having a texture with a mix crystallographic orientation, can act as barriers for transport, resulting the creation of a potential barrier for free electrons, and can have a large impact on carrier transport together with N
Summary
Polycrystalline materials consist of grains of finite size. The boundary between two grains is a lattice defect, across which the orientation of a crystal changes. We must take into account the effect of the grain boundaries on doping using external dopant atoms, that is, the change in the number of active n-type dopant atoms, Al donors, and free carriers in crystallites by acting as sites for dopant segregation and carrier trapping [14] For such polycrystalline films, an approach to investigating the factors limiting carrier transport from a viewpoint different from those of conventional studies of single crystals is required. AZO films having a strongly fiber-textured polycrystalline structure, which had most of the grains with the c-axis orientation aligned within a fraction of a degree of the film normal. We found that 500-nm-thick AZO films deposited by DC magnetron sputtering showed poor c-axis alignment between columnar grains, having textures with mixed orientations of the atomically closely packed (0001) and (1011) planes. We attempted to more accurately determine the crystallographic orientation distribution in AZO films deposited by DC magnetron sputtering or RF magnetron sputtering together with RF/DC magnetron sputtering at various power ratios, to investigate the relationship between the crystallographic orientation distribution and carrier transport for the determination of factors except for carrier concentration limiting the carrier transport of polycrystalline AZO films [23,24]
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