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) x 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.

The multilayered optical coating whose structure consists of optical cavity and multiple layers of ultrathin phase change material (PCM) film is presented. The color changing is enabled via transition between amorphous and crystalline phases of PCM, which is accompanied by high optical contrast. The phase transition of each PCM layer, separated by ultrathin oxide barrier, can be realized through various crystallization temperatures with respect to the different thickness of ultrathin (<10 nm) PCM film. Accordingly, notable multicolor changing was achieved by thermally or electrically induced selective phase transition of double PCM layers, and its results were compared with transfer-matrix optical modeling. Further evaluation of optical simulations also demonstrated the possibility of increase in expressible colors with increasing number of layers.

A Schottky-type diode switch consisting of a Pt∕(In,Sn)2O3∕TiO2∕Pt stack was fabricated for applications to cross-bar type resistive-switching memory arrays. The high (0.55eV) and low potential barrier at the TiO2∕Pt and TiO2∕(In,Sn)2O3 junctions, respectively, constitute the rectifying properties of the stacked structure. The forward/reverse current ratio was as high as ∼1.6×104 at an applied voltage of ∼1V. When Pt∕TiO2∕Pt memory was connected to this diode in series, there was an insignificant interference on the memory function from the diode under the forward bias and virtually no resistive switching under a reverse bias.

An atomic layer deposition (ALD) process for SrTiO 3 (STO) thin film growth was developed using a newly designed and synthesized heteroleptic Sr-precursor, {Sr(demamp)(tmhd)} 2 (demampH = 1-{[2-(dimethylamino)ethyl](methyl)amino}-2-methylpropan-2-ol, tmhdH = 2,2,6,6-tetramethyl-3,5-heptanedione), which offered an intermediate reactivity toward oxygen between Sr(tmhd) 2 and Sr( i Pr 3 Cp) 2 . Because of the appropriate reactivity of {Sr(demamp)(tmhd)} 2 toward oxygen, the abnormal initial growth behavior (due to interaction between the Sr-precursor and active oxygen contained in the underlying oxidized Ru layer) became negligible during the growth of the SrO and STO films on the Ru electrode, which allowed the growth of the SrO and STO films to be highly controllable with a moderate growth rate. Using Ti(CpMe 5 )(OMe) 3 as the Ti-precursor and O 3 as the oxygen source in the TiO 2 ALD subcycle, the ALD process of the STO film revealed a growth rate of 0.05 nm/cycle and ∼85% of step coverage in terms of the thickness and cation composition on a capacitor hole structure with an aspect ratio of 10 (opening diameter of 100 nm and depth of 1 μm). The minimum achievable equivalent oxide thickness (t ox ) with a low leakage current (<10 −7 A/cm 2 at 0.8 V) was limited to 0.46 nm. The damage effect on the underlying Ru electrode by the prolonged ALD process time appears to affect the limited scalability of t ox . ■ INTRODUCTIONSrTiO 3 (STO) has been considered to be a promising candidate for a dielectric layer in the next-generation dynamic random access memory (DRAM) capacitors because of its high permittivity (∼300 in bulk material) compared with that of other dielectric materials, such as HfO 2 and ZrO 2 . Many studies have reported a high dielectric constant of >100 for metal− insulator−metal (MIM) capacitors that contain an STO insulator, in which the insulators are thinner than 20 nm. 1−6 Considering the extremely tiny three-dimensional (3D) structure of the DRAM capacitors, 7 atomic layer deposition (ALD) appears to be the only feasible thin film growth technique that can fulfill the stringent requirements of thickness and composition step coverage in the DRAM capacitors. Despite the acute requirement for a suitable ALD process of the STO films, the development has been hindered for two main reasons: first, the lack of a suitable Sr-precursor for feasible STO ALD, although that for Ti is abundant; second, because of the low temperature of the ALD, ensuring a suitable crystalline quality of the STO film is challenging in general. Because such problems and possible solutions have already been extensively reviewed in previous reports from the authors' group, 3,7,8 more recent reports that are directly related to the present work are described in this section.The first viable report in this field was published by the Helsinki group in the late 1990s. Vehkamaki et al. deposited STO films with Sr( i Pr 3 Cp) 2 (Pr and Cp are propyl and cyclopentadienyl group, respectively) and Ti(O i Pr) 4 [TTIP] as Sr-and Ti-precursors,...

This study concerned the effect of the substrate on the nucleation and growth behavior of Ge 2 Sb 2 Te 5 (GST) thin films deposited by a combination of plasma-enhanced chemical vapor deposition (for Sb and Te) and plasma-enhanced atomic layer deposition (for Ge) processes at wafer temperatures ranging from 100 to 200°C using Ge(i-C 4 H 9 ) 4 , Sb(i-C 3 H 7 ) 3 , and Te(i-C 3 H 7 ) 2 as the Ge, Sb, and Te precursors, respectively. Several oxide and nitride layers that were formed on the Si substrate were concerned as substrates. The nucleation of the GST films on the SiO 2 , Si 3 N 4 , and ZrO 2 substrates was seriously retarded (long incubation cycles) compared to those on the TiN and TiO 2 substrates, where smooth film growth with negligible incubation cycles was achieved. The GST film did not grow at all on the HfO 2 substrate. The reason for the enhanced nucleation and growth properties of GST on the TiO 2 and TiN (partially oxidized) substrates was related to the formation of a GeO 2 phase and the charge exchange effect of a partially reduced Ti oxide. On the other hand, the SiO 2 surface remained insulating during deposition, which inhibited GST nucleation. The different nucleation behaviors also influenced the crystallization behavior of the film, which in turn altered the saturated film growth rates. It is believed that the crystallized GST surface reduced the activation energy for the chemisorption of the precursors, which enhanced the saturated growth rate.