Ambipolar Memristive Phenomenon in Large‐Scale, Few‐Layered αMoO3 Recrystallized Films

Abstract

DOI: 10.1002/admi.201801591 in electrochromic devices,[10–12] organic photovoltaic devices,[13,14] organic light emission diodes,[15,16] gas or chemical sensors,[17] field effect transistors,[5] and nanophotonic waveguides.[18] More recently, there has been interest in using MoO3 to control the carrier concentration of other 2D materials such as molibdenum disulfide (MoS2) and tungsten diselenide (WSe2) by the creation of surface dipoles. For molybdenum oxide, each stoichiometry has very different chemical, electrical, and optical characteristics. αMoO3, one of many phases of molybdenum oxide, has very special electronic properties that make it of particular interest for electronic and optoelectronic devices. These include high electron mobility (≈1100 cm2 V −1 s −1),[5] a wide indirect bandgap (≈3.3 eV),[20] and high-relative dielectric constant (ε > 200),[5] making it a good candidate to replace GaN in many applications. For high-voltage devices in particular, semiconductor materials are often characterized by Baliga’s figure of merit (BFOM);[21] g 3 μεE , where μ is mobility in unit of cm2 V −1 s −1, ε is dielectric constant in unitless, and Eg is bandgap energy in unit of eV. For αMoO3, BFOM can be as high as 363.7, compared to 24.6 for GaN and 1 for Si. Furthermore, the wide bandgap properties of αMoO3 make possible optoelectronic devices in the ultraviolet by appropriate bandgap engineering. Adjusting oxygen and hydrogen intercalation allows bandgaps to be tuned in the range of 2.8–3.3eV.[20] Although the crystal structure of αMoO3 has been known for decades, synthesis of αMoO3 through techniques such as atomic layer deposition,[22] rapid thermal process,[23] and oxidation of molybdenum metal[24] have been largely unsuccessful because of poor control of growth conditions with a narrow band for stoichiometry, resulting in sparse αMoO3 crystal flakes entangled with alternate lattice planes. Consequently, device results to-date have been from exfoliated crystals of molybdite, a natural ore.[25] Practical device fabrication requires the ability to synthesize highly uniform polycrystalline αMoO3 in a manner similar to the chemical synthesis of other 2D materials such as CVD growth of graphene,[26] MoS2, and WS2. Here, we report for the first time large-scale growth of layered polycrystalline αMoO3 using a thermal phase transition from amorphous MoO3 which leads to high-quality crystallization. The resulting orthorhombic crystal structure in these films is verified and Studies of two-dimensional (2D) oxide materials are not common, primarily because of the difficulty in obtaining crystal sizes large enough to fabricate devices structures from exfoliation of bulk crystals. Among the layered oxide materials, alpha molybdenum trioxide (αMoO3) is of particular interest because of its wide bandgap and high hole mobility. Here the growth of highly uniform, large-scale, ambipolar, few-layered αMoO3 that is appropriate for nanofabrication is reported. Crystal grain sizes on the order of 5 μm are observed across samples as large as 10 × 10 mm2 with hexagonal grain boundaries and surface roughness of less than 500 pm rms. Exact [010] crystal orientation, characteristic of the layered atomic structure αMoO3, is observed. The measured bandgap energy is ≈2.8 eV. Carrier mobilities in polycrystalline films are and 2.28 cm2 V−1 s−1 (hole) and 3.18 cm2 V−1 s−1 (electron) at room temperature in air. Simple field-effect device structures are characterized by ambipolar carrier transport producing memristive device characteristics, which is attributed to a polarization field produced by the strong coupling between electron and phonons in these crystals.