Interfacial Metal–Oxide Interactions in Resistive Switching Memories

Metal oxides are commonly used as electrolytes for redox-based resistive switching memories. In most cases, non-noble metals are directly deposited as ohmic electrodes. We demonstrate that irrespective of bulk thermodynamics predictions an intermediate oxide film a few nanometers in thickness is always formed at the metal/insulator interface, and this layer significantly contributes to the development of reliable switching characteristics. We have tested metal electrodes and metal oxides mostly used for memristive devices, that is, Ta, Hf, and Ti and Ta₂O₅, HfO₂, and SiO₂. Intermediate oxide layers are always formed at the interfaces, whereas only the rate of the electrode oxidation depends on the oxygen affinity of the metal and the chemical stability of the oxide matrix. Device failure is associated with complete transition of short-range order to a more disordered main matrix structure

Understanding the Role of Single Molecular ZnS Precursors in the Synthesis of In(Zn)P/ZnS Nanocrystals

Environmentally friendly nanocrystals (NCs) such as InP are in demand for various applications, such as biomedical labeling, solar cells, sensors, and light-emitting diodes (LEDs). To fulfill their potential applications, the synthesis of such high-quality “green” InP NCs required further improvement so as to achieve better stability, higher brightness NCs, and also to have a more robust synthesis route. The present study addresses our efforts on the synthesis of high-quality In­(Zn)­P/ZnS core–shell NCs using an air- and moisture-stable ZnS single molecular precursor (SMP) and In­(Zn)P cores. The SMP method has recently emerged as a promising route for the surface overcoating of NCs due to its simplicity, high reproducibility, low reaction temperature, and flexibility in controlling the reaction. The synthesis involved heating the In­(Zn)P core solution and Zn­(S₂CNR₂) (where R = methyl, ethyl, butyl, or benzyl and referred to as ZDMT, ZDET, ZDBT, or ZDBzT, respectively) in oleylamine (OLA) to 90–250 °C for 0.5–2.5 h. In this work, we systematically studied the influence of different SMP end groups, the complex formation and stability between the SMP and oleylamine (OLA), the reaction temperature, and the amount of SMP on the synthesis of high-quality In­(Zn)­P/ZnS NCs. We found that thiocarbamate end groups are an important factor contributing to the low-temperature growth of high-quality In­(Zn)­P/ZnS NCs, as the end groups affect the polarity of the molecules and result in a different steric arrangement. We found that use of SMP with bulky end groups (ZDBzT) results in nanocrystals with higher photoluminescence quantum yield (PL QY) and better dispersibility than those synthesized with SMPs with the shorter alkyl chain groups (ZDMT, ZDET, or ZDBT). At the optimal conditions, the PL QY of red emission In­(Zn)­P/ZnS NCs is 55 ± 4%, which is one of the highest values reported. On the basis of structural (XAS, XPS, XRD, TEM) and optical characterization, we propose a mechanism for the growth of a ZnS shell on an In­(Zn)­P core

Structural Analyses of Phase Stability in Amorphous and Partially Crystallized Ge-Rich GeTe Films Prepared by Atomic Layer Deposition

The local bonding structures of Ge<i>_x</i>Te_1–<i>x</i> (<i>x</i> = 0.5, 0.6, and 0.7) films prepared through atomic layer deposition (ALD) with Ge­(N­(Si­(CH₃)₃)₂)₂ and ((CH₃)₃Si)₂Te precursors were investigated using Ge K-edge X-ray absorption spectroscopy (XAS). The results of the X-ray absorption fine structure analyses show that for all of the compositions, the as-grown films were amorphous with a tetrahedral Ge coordination of a mixture of Ge–Te and Ge–Ge bonds but without any signature of Ge–GeTe decomposition. The compositional evolution in the valence band electronic structures probed through X-ray photoelectron spectroscopy suggests a substantial chemical influence of additional Ge on the nonstoichiometric GeTe. This implies that the ALD process can stabilize Ge-abundant bonding networks like −Te–Ge–Ge–Te– in amorphous GeTe. Meanwhile, the XAS results on the Ge-rich films that had undergone post-deposition annealing at 350 °C show that the parts of the crystalline Ge-rich GeTe became separated into Ge crystallites and rhombohedral GeTe in accordance with the bulk phase diagram, whereas the disordered GeTe domains still remained, consistent with the observations of transmission electron microscopy and Raman spectroscopy. Therefore, amorphousness in GeTe may be essential for the nonsegregated Ge-rich phases and the low growth temperature of the ALD enables the achievement of the structurally metastable phases

Conformal Formation of (GeTe₂)_{(1–<i>x</i>)}(Sb₂Te₃)_<i>x</i> Layers by Atomic Layer Deposition for Nanoscale Phase Change Memories

Phase change random access memory appears to be the strongest candidate for next-generation high density nonvolatile memory. The fabrication of ultrahigh density phase change memory (≫1 Gb) depends heavily on the thin film growth technique for the phase changing chalcogenide material, most typically containing Ge, Sb and Te (Ge–Sb–Te). Atomic layer deposition (ALD) at low temperatures is the most preferred growth method for depositing such complex materials over surfaces possessing extreme topology. In this study, [(CH₃)₃Si]₂Te and stable alkoxy-Ge (Ge­(OCH₃)₄) and alkoxy-Sb (Sb­(OC₂H₅)₃) metal–organic precursors were used to deposit various layers with compositions lying on the GeTe₂–Sb₂Te₃ tie lines at a substrate temperature as low as 70 °C using a thermal ALD process. The adsorption of Ge precursor was proven to be a physisorption type while other precursors showed a chemisorption behavior. However, the adsorption of Ge precursor was still self-regulated, and the facile ALD of the pseudobinary solid solutions with composition (GeTe₂)_(1‑x)(Sb₂Te₃)_<i>x</i> were achieved. This chemistry-specific ALD process was quite robust against process variations, allowing highly conformal, smooth, and reproducible film growth over a contact hole structure with an extreme geometry. The detailed ALD behavior of binary compounds and incorporation behaviors of the binary compounds in pseudobinary solid solutions were studied in detail. This new composition material showed reliable phase change and accompanying resistance switching behavior, which were slightly better than the standard Ge₂Sb₂Te₅ material in the nanoscale. The local chemical environment was similar to that of conventional Ge₂Sb₂Te₅ materials

Comparison of the Atomic Layer Deposition of Tantalum Oxide Thin Films Using Ta(N^<i>t</i>Bu)(NEt₂)₃, Ta(N^<i>t</i>Bu)(NEt₂)₂Cp, and H₂O

The growth characteristics of Ta₂O₅ thin films by atomic layer deposition (ALD) were examined using Ta­(N^<i>t</i>Bu)­(NEt₂)₃ (TBTDET) and Ta­(N^<i>t</i>Bu)­(NEt₂)₂Cp (TBDETCp) as Ta-precursors, where ^<i>t</i>Bu, Et, and Cp represent <i>tert</i>-butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown Ta₂O₅ films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of Ta₂O₅ films were 0.77 Å cycle^–1 at 250 °C and 0.67 Å cycle^–1 at 300 °C using TBTDET and TBDETCp precursors, respectively. The thermal decomposition of the amido ligand (NEt₂) limited the ALD process temperature below 275 °C for TBTDET precursor. However, the ALD temperature window could be extended up to 325 °C due to a strong Ta–Cp bond for the TBDETCp precursor. Because of the improved thermal stability of TBDETCp precursor, excellent nonuniformity of ∼2% in 200 mm wafer could be achieved with a step coverage of ∼90% in a deep hole structure (aspect ratio 5:1) which is promising for 3-dimensional architecture to form high density memories. Nonetheless, a rather high concentration (∼7 at. %) of carbon impurities was incorporated into the Ta₂O₅ film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of Ta₂O₅ film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the Ta₂O₅ film using TBDETCp showed a promising resistive switching performance with an endurance cycle as high as ∼17 500 for resistance switching random access memory application. The optical refractive index of the deposited Ta₂O₅ films was 2.1–2.2 at 632.8 nm using both the Ta-precursors, and indirect optical band gap was estimated to be ∼4.1 eV for both the cases

Giant Photoresponse in Quantized SrRuO₃ Monolayer at Oxide Interfaces

The photoelectric effect in semiconductors is the main mechanism for most modern optoelectronic devices, in which the adequate bandgap plays the key role for acquiring high photoresponse. Among numerous material categories applied in this field, the complex oxides exhibit great possibilities because they present a wide distribution of band gaps for absorbing light with any wavelength. Their physical properties and lattice structures are always strongly coupled and sensitive to light illumination. Moreover, the confinement of dimensionality of the complex oxides in the heterostructures can provide more diversities in designing and modulating the band structures. On the basis of this perspective, we have chosen itinerary ferromagnetic SrRuO₃ as the model material, and fabricated it in one-unit-cell thickness in order to open a small band gap for effective utilization of visible light. By inserting this SrRuO₃ monolayer at the interface of the well-developed two-dimensional electron gas system (LaAlO₃/SrTiO₃), the resistance of the monolayer can be further revealed. In addition, a giant enhancement (>300%) of photoresponse under illumination of visible light with power density of 500 mW/cm² is also observed. Such can be ascribed to the further modulation of band structure of the SrRuO₃ monolayer under the illumination, confirmed by cross-section scanning tunneling microscopy (XSTM). Therefore, this study demonstrates a simple route to design and explore the potential low dimensional oxide materials for future optoelectronic devices