The nucleation behavior of Ru deposited by atomic layer deposition (ALD) using bis(ethylcyclopentadienyl)ruthenium precursor and O2 reactant is investigated as a function of the number of ALD cycles. The substrates are thermally grown SiO2, NH3 plasma-treated SiO2, and chemical vapor deposited SiNx. The nucleation of Ru strongly depends on the substrate and is much enhanced on the nitride substrates. Transmission electron microscopy analysis reveals that the maximum density of the nuclei is 5.7×1010cm−2 on the SiO2 surface at 500 ALD cycles, 1.2×1012cm−2 on SiNx at 160 ALD cycles, and 2.3×1012cm−2 on NH3 plasma-nitrided SiO2 at 110 ALD cycles. Although the kinetics of Ru nucleation is different on the various substrates, the overall nucleation process in each case consists of an initial slow nucleation stage and a subsequent fast nucleation stage before the coalescence of the nuclei occurs. Considering the adsorption of Ru precursor on the substrate and the surface diffusion of deposited Ru during an ALD cycle, we suggest a model for describing the nucleation of an ALD film at the initial stage with a low surface coverage based on the atomistic nucleation theory of a thin film. The proposed model shows that the density of the nuclei is proportional to the (i+2)th power of the number of ALD cycles and (i+1)th power of the density of atoms deposited per ALD cycle, where i is the critical nuclei size. By applying the proposed model to the experimental results, the critical nuclei size i is found to be 1. The amounts of Ru atoms deposited per ALD cycle on the NH3 plasma-nitrided SiO2 and SiNx are 70 and 24 times larger, respectively, than that on the SiO2 surface. This model quantitatively describes the nucleation kinetics in the ALD system and is verified by a comparison with the experimental results of Ru on various substrates.
The formation of Ru nanocrystals is demonstrated on a SiO2 substrate by plasma enhanced atomic layer deposition using diethylcyclopentadienyl ruthenium and NH3 plasma. The island growth of Ru was observed at the initial stages of the film formation up to a nominal thickness of 11.1nm. A maximum Ru nanocrystal spatial density of 9.7×1011∕cm2 was achieved with an average size of 3.5nm and standard deviation of the size of 20%. Electron charging/discharging effect in the Ru nanocrystals is demonstrated by measuring the flatband voltage shift in the capacitance-voltage measurement of metal-oxide-semiconductor memory capacitor structure.
We propose a deposition method capable of independently controlling the spatial density and average size of Ru nanocrystals by using both plasma-enhanced and thermal atomic layer deposition ͑ALD͒. Plasma-enhanced ALD is used to promote the nucleation of Ru nanocrystals, while thermal ALD is used to assist their growth. By the rigorous selection of each stage, we can demonstrate the formation of Ru nanocrystals with a density variation from 3.5 ϫ 10 11 to 8.4 ϫ 10 11 cm −2 and sizes from 2.2 to 5.1 nm, which is in the optimum density and size range of nanocrystal floating-gate memory application.Nanocrystal ͑NC͒ floating-gate memory ͑NFGM͒ devices have been extensively investigated for the replacement of current flash memory devices, because a discrete NC layer provides chargestorage sites which are immune to stress-induced leakage through the tunnel oxide; thus, a relatively thin oxide can be used for the low-voltage operation. 1 In this type of device, it has been reported that the density and size of the NCs, as well as material types of the NC and the surrounding dielectrics, strongly affect device performance such as the threshold voltage shift ͑⌬V th ͒, charging efficiency, and charge retention time. 2-4 Theoretically, it has been reported that lower density and larger size of the NCs are favorable in the aspects of charging efficiency and retention characteristics. 2 At the same time, however, a reasonably high density of the NCs ͑up to 1 ϫ 10 12 cm −2 ͒ is required in order to guarantee a sufficient memory window ͑conventionally 1-2 V difference of the ⌬V th before and after the NCs are charged͒. In addition, high density of the NCs is advantageous when the device size is as low as a few tens of nanometers because the deviation in the number of NCs per device can be statistically reduced.Therefore, the density and size of the NCs should be rigorously controlled in order to obtain the optimum NFGM performance. However, independent control of the density and size is a very difficult task by employing any type of deposition processes such as physical vapor deposition ͑PVD͒ followed by thermal treatment, 5,6 chemical vapor deposition ͑CVD͒, 7-9 and atomic layer deposition ͑ALD͒. 10 For instance, in the case of a PVD-based process, in which many of the processes commonly have utilized thin-film agglomeration, density and size of the NCs are determined simultaneously by the nominal thickness of a starting film and the subsequent annealing temperature. In the case of CVD and ALD processes, the origin of these difficulties comes from the fact that the nucleation and growth occur simultaneously during the formation of the NCs. Thus, the density and size of NCs are determined only by deposition time at the given deposition conditions, including precursor injection time, partial pressure, and the type of substrate.Certainly, one of the most promising ways to control density and size of NCs is to independently control the nucleation and growth stage during deposition. Ideally, it is hoped that the density of NCs is de...
We report separate domain formation in cosputtered Ge2Sb2Te5–SiOx mixed layer, with SiOx amount less than 10mol%. As-prepared Ge2Sb2Te5–SiOx layer exhibits amorphous phase with separate domains smaller than 20nm. The separation maintains after thermal annealing, which results in crystallization into fcc phase. The crystallization activation energies of Ge2Sb2Te5–SiOx are obtained as 4.99 and 6.44eV for mixed layers containing 5.3 and 8.4mol% SiOx, respectively. Those are larger than 2.75eV of pure Ge2Sb2Te5. Furthermore, the mixed layer exhibits sublimation at increased temperature. These are interpreted as formation of Ge2Sb2Te5-rich domains separated from each other by SiOx-rich domains.
Amorphous Ge 2 Sb 2 Te 5 clusters with a size of 20 nm, self-enclosed by a thin layer of TiO x , were obtained by cosputtering Ge 2 Sb 2 Te 5 and TiO 2 targets at room temperature with the aim of reducing the reset current for phase change random access memory applications. Eutectic decomposition during the deposition caused a phase separation of Ge 2 Sb 2 Te 5 and TiO x . The temperature-dependent resistance change results showed that the activation energy for crystallization increased from 2.44 Ϯ 0.76 to 3.84 Ϯ 1.43 eV in the Ge 2 Sb 2 Te 5 film. The set resistance can be tuned within an acceptable range, and the reliability of this microstructure during repetitive laser melt-quenching cycles was tested.
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