X-ray absorption spectroscopy elucidates the impact of structural disorder on electron mobility in amorphous zinc-tin-oxide thin films

Abstract

We investigate the correlation between the atomic structures of amorphous zinc-tin-oxide (a-ZTO) thin films grown by atomic layer deposition (ALD) and their electronic transport properties. We perform synchrotron-based X-ray absorption spectroscopy at the K-edges of Zn and Sn with varying [Zn]/[Sn] compositions in a-ZTO thin films. In extended X-ray absorption fine structure (EXAFS) measurements, signal attenuation from higher-order shells confirms the amorphous structure of a-ZTO thin films. Both quantitative EXAFS modeling and X-ray absorption near edge spectroscopy (XANES) reveal that structural disorder around Zn atoms increases with increasing [Sn]. Field-and Hall-effect mobilities are observed to decrease with increasing structural disorder around Zn atoms, suggesting that the degradation in electron mobility may be correlated with structural changes. V C 2014 AIP Publishing LLC. The high mobility and amorphous structure of AMOS enable a faster operation of the devices and elimination of grain-boundary related defects, respectively.2,3 The high electron mobility in the amorphous state originates from the conduction band, which mainly consists of unoccupied Ns (N ! 5) orbitals of metal atoms such as In-5s in a-IGZO. [4][5][6] The spherical symmetry of the s orbitals makes them less prone to form traps and scattering centers when the metal-oxygen-metal bonds are distorted by rotation of metal-oxygen polyhedrons.Amorphous zinc-tin-oxide (a-ZTO) is recently highlighted as a promising candidate material for cost-effective applications, as it is free from toxic and expensive elements (e.g., indium and gallium). Sn] ratio in a-ZTO thin films on device performance suggests a strong correlation between the atomic structure and electronic transport properties of the films. Because amorphous materials can exhibit a continuum of structures, 18 a probe of local order is needed. In this work, we investigate the effect of atomic structure in a-ZTO on electron transport properties by using synchrotron-based X-ray absorption spectroscopy (XAS). We perform EXAFS at the Zn and Sn K-edges to probe the local chemical neighborhoods of Zn and Sn atoms in a-ZTO films, respectively. We find that the Debye-Waller factor, which gives a measure of structural disorder, 19 at the Zn K-edge increases with increasing [Sn] in the films. The structural disorder is further investigated by XANES analysis, which indicates the amorphization around Zn atoms with increasing [Sn] in the films. Disorder as measured by EXAFS and XANES coincides with a decrease in measured electron mobility, suggesting that the degradation in electron mobility may be correlated with structural changes for Zn rich films ([Zn] > 0.5), whereas for Sn rich regions, larger ionic size of Sn dominates the electron mobility.A set of a-ZTO thin films is grown on quartz substrates by ALD at a growth temperature of 120 C. Diethylzinc (DEZ) and a cyclic tin (II) amide ((1,3-bis(1,1-dimethylethyl)-4,5-dimethyl-(4R,5R)-1,3,2-diazastannolidin-2-ylidene)Sn(II)) 20 are used as th...

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