Cation Size Effects on the Electronic and Structural Properties of Solution‐Processed In–X–O Thin Films

Abstract

The nature of charge transport and local structure are investigated in amorphous indium oxide‐based thin films fabricated by spin‐coating. The In–X–O series where X = Sc, Y, or La is investigated to understand the effects of varying both the X cation ionic radius (0.89–1.17 Å) and the film processing temperature (250–300 °C). Larger cations in particular are found to be very effective amorphosizers and enable the study of high mobility (up to 9.7 cm2 V−1 s−1) amorphous oxide semiconductors without complex processing. Electron mobilities as a function of temperature and gate voltage are measured in thin‐film transistors, while X‐ray absorption spectroscopy and ab initio molecular dynamics simulations are used to probe local atomic structure. It is found that trap‐limited conduction and percolation‐type conduction mechanisms convincingly model transport for low‐ and high‐temperature processed films, respectively. Increased cation size leads to increased broadening of the tail states (10–23 meV) and increased percolation barrier heights (24–55 meV) in the two cases. For the first time in the amorphous In–X–O system, such effects can be explained by local structural changes in the films, including decreased In–O and In–M (M = In, X) coordination numbers, increased bond length disorder, and changes in the MO x polyhedra interconnectivity.

Recent Advances in Understanding the Structure and Properties of Amorphous Oxide Semiconductors

“…Most research has focused on quaternary oxides such as a-In-Ga-Zn-O or a-Zn-In-Sn-O given their technological appeal; [48,49,[51][52][53][54][55][56][57][62][63][64][65][66]68,69,73,74,76] only a few studies addressed the properties of ternary AOSs systematically. [67,85,86] Moreover, a comparison of the results available in the literature is likely to be inconclusive because the crystallization temperature depends strongly not only on the metal composition but also on growth conditions (such as oxygen partial pressure, postdeposition temperatures and times) as well as film thickness.…”

“…A systematic experimental and theoretical study of the amorphous indium oxide doped with group III metals, Sc, Y, and La, showed that the amorphization efficiency increases with the size of the ionic radius of the substituted cation. [67] The local structure comparison of the amorphous In-X-O with 20% substitution of X = Sc, Y, or La have revealed that scandium is too small to suppress the amount of InO 6 polyhedra and to prevent their clustering, whereas lanthanum is large enough to make the In-La-O samples fully X-ray amorphous-even with only 5% La doping in films processed at 300 °C. [67] While a theoretical calculation of the crystallization temperature in a-In-X-O is beyond the scope of this work, the above results of the MD liquid-quench simulations clearly illustrate that complex interplay between the local and medium-range structural preferences of both the host and the substitution metals must be taken into account in the search for efficient amorphizers.…”

“…Experimentally, the second and third shells cannot be fitted in this case; the corresponding structures are often called EXAFS-free amorphous. [67] Last but not least, an internal strain introduced by the presence of additional cations may also affect amorphization. A systematic experimental and theoretical study of the amorphous indium oxide doped with group III metals, Sc, Y, and La, showed that the amorphization efficiency increases with the size of the ionic radius of the substituted cation.…”

“…In addition, accurate characterization of the structural features beyond the first shell (i.e., beyond the nearest neighbors) is necessary in order to explain the high sensitivity of the AOSs properties to the deposition [84] and post-deposition conditions, [64] particularly oxygen environment, [85] as well as the chemical composition of the sample. [67,86] In this work, ab initio MD simulations and hybrid-functional DFT-based electronic structure calculations are performed i) to systematically study the local and medium-range structural characteristics of several prototype In-based oxides with different degree of amorphization, oxygen stoichiometry, cation composition, and/or lattice strain and ii) to connect the structural peculiarities to the resulting electronic, optical, or thermal properties of each material. Based on good agreement between the theoretically established trends and those observed experimentally, a unified model for prototype In-based AOS is proposed.…”

“…The research area of transparent conducting oxides (TCOs) dates back to 1907 when CdO was reported to combine both optical transparency in the visible range and good electrical of amorphous indium oxide appeared in 2009, [61] followed by models of electron transport in multi-cation AOSs, [62][63][64][65][66][67] DFT calculations of defect formation, [68][69][70][71][72][73][74][75] and statistical descriptions of amorphous network. [76][77][78] However, several key questions regarding the nanostructure and morphology, crystallization, carrier generation, and conductivity mechanisms in AOSs remain unanswered and require a unified theoretical framework capable of handling all these aspects in tandem.…”