Three different novel dry-etching methods have been employed to fabricate nanophotonic devices upon a thin-film lithium niobate on insulator material platform. Different dry-etching processes and their advantages, drawbacks and applicable scenarios are systematically studied. Ultra-smooth etching surface with roughness of 0.46 nm (Rq), low-loss ridge waveguides with extracted propagation loss of 1.42 dB/cm, and microring resonators with high optical quality factors up to 1.4×105 are demonstrated using the optimized low-loss etching recipe. The low-loss etching technique lays a foundation for monolithic integration of passive optical components with quantum dots, on-chip broadband electro- optic modulators and wafer-scale lithium niobate integrated photonic circuits.
The Sn15Sb85 alloy is characterized by its rapid phase transition. However, its poor thermal stability hinders its application as phase change memory material. After nitrogen doping, the crystallization temperature and 10-year data retention temperature of Sn15Sb85 thin films even reach 235‡C and 173°C, respectively. Both the crystallization activation energy and the amorphous resistance of the thin films increase as well. As a result, the material thermal stability is significant improved. The surface roughness of the films is evaluated by atomic force microscope (AFM). The phase change speed of the thin films, measured by the picosecond laser technique, remains fast.
Environmental friendly Te-free phase change material of TixSb2.19Se was investigated for PCM application. As the important thermal properties, the crystallization temperature (Tc) and data retention for ten years for the best selected composition Ti0.34Sb2.19Se (TSS) are 234°C and 160‡C, respectively. Detection of the crystal structure of TSS by means of XRD, TEM and XPS reveals that the grains are more uniform compared with Ge2Sb2Te5 (GST). The Set and Reset operation voltages for TSS-based phase change memory devices are much lower than those of conventional GST-based ones. Remarkably, the TSS device exhibits an extremely high phase change speed of 10 ns.
Phase change random access memory (PCM) appears to be the strongest candidate for next-generation high density nonvolatile memory. The fabrication of ultrahigh density PCM depends heavily on the thin film growth technique for the phase changing chalcogenide material. In this study, TiSb2Te4 (TST) thin films were deposited by thermal atomic layer deposition (ALD) method using TiCl4, SbCl3, (Et3Si)2Te as precursors. The threshold voltage for the cell based on thermal ALD-deposited TST is about 2.0 V, which is much lower than that (3.5 V) of the device based on PVD-deposited Ge2Sb2Te5 (GST) with the identical cell architecture. Tests of TST-based PCM cells have demonstrated a fast switching rate of ~100 ns. Furthermore, because of the lower melting point and thermal conductivities of TST materials, TST-based PCM cells exhibit 19% reduction of pulse voltages for Reset operation compared with GST-based PCM cells. These results show that thermal ALD is an attractive method for the preparation of phase change materials.
Recently, carbon-doped Ge2Sb2Te5 (CGST) has been proved to be a high promising material for future phase change memory technology. In this article, reactive ion etching (RIE) of phase change material CGST films is studied using CF4/Ar gas mixture. The effects on gas-mixing ratio, RF power, gas pressure on the etch rate, etch profile and roughness of the CGST film are investigated. Conventional phase change material Ge2Sb2Te5 (GST) films are simultaneously studied for comparison. Compared with GST film, 10 % more CF4 is needed for high etch rate and 10% less CF4 for good anisotropy of CGST due to more fluorocarbon polymer deposition during CF4 etching. The trends of etch rates and roughness of CGST with varying RF power and chamber pressure are similar with those of GST. Furthermore, the etch rate of CGST are more easily to be saturated when higher RF power is applied.
Phase Change Memory (PCM) is regarded as one of the most promising candidates for the next-generation nonvolatile memory. Its storage medium, phase change material, has attracted continuous exploration. Sb2Te3 is a high-speed phase change material matrix with low crystallization temperature. Cr-doped Sb2Te3 (CST) films with suitable composition have been studied and proved to be a promising novel phase change material with high speed and good thermal stability. In this paper, detailed Rs-T characteristics and Hall characteristics of the CST films are studied. We find that, when more parts of the film crystallizes into the ordered structure, the activation energy for electrical conduction (Eσ) decreases, indicating that the semiconductor property is weakened. And with the increase of Cr-dopants, Eσ of the As-deposited (As-de) amorphous CST films decreases, thus the thermal stability of resistance is improved. Hall results show that Sb2Te3 and CST films are all in P-type. For As-de amorphous films, with the increase of Cr-dopants, the carrier mobility decreases all along, while the carrier density decreases at first and then increases. For the crystalline films, with the increase of Cr-dopants, the carrier mobility decreases, while the carrier density increases.
Phase Change Memory (PCM) has great potential for commercial applications of next generation non-volatile memory (NVM) due to its high operation speed, high endurance and low power consumption. Sb2Te (ST) is a common phase-change material and has fast crystallization speed, while thermal stability is relatively poor and its crystallization temperature is about 142°C. According to the Arrhenius law, the extrapolated failure temperature is about 55°C for ten years. When heated above the crystallization temperature while below the melting point, its structure can be transformed from amorphous phase to hexagonal phase. Due to the growth-dominated crystallization mechanism, the grain size of ST film is large and the diameter of about 300 nm is too large compared with Ge2Sb2Te5 (GST), which may deteriorate the device performance. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were employed to study the microstructures and the results indicate that the crystal plane is {110}. In addition, device cells were manufactured and their current–voltage (I–V) and resistance–voltage characteristics were tested, and the results reveal that the threshold voltage (Vth) of ST film is 0.87 V. By researching the basic properties of ST, we can understand its disadvantages and manage to improve its performance by doping or other proper methods. Finally, the improved ST can be a candidate for optical discs and PCM.
The reliability and operation speed have long been two great obstacles in phase change memory technology. Thus (SiC)0.85-Sb3Te alloy was proposed to be a new-type phase change material due to its high crystallization temperature (199.7°C) and good data retention ability (118.9°C for 10-year archival life) in this work. The stress accompanying the phase transition in (SiC)0.85-Sb3Te is smaller than those in pure Sb3Te and the traditional material, Ge2Sb2Te5. This is attributed to the fine crystal grain size due to SiC doping, which contributes to the ultrafast reversible operation (5 ns) and good endurance (2.3 × 104 cycles) of (SiC)0.85-Sb3Te based phase change memory cells.
The amorphous-to-crystalline transitions of N-doped GeTe films are investigated by in situ film resistance measurements. Both the crystallization temperature and resistance of the N-doped films increase. The analysis of X-ray diffraction (XRD) measurement indicates that the grain size of the films with more nitrogen content can be refined, leading to the improvement in the resistance and thermal stability of the phase change films. The N-doped GeTe films have higher activation energy for crystallization. The 10-year lifetime is raised from 90°C of undoped GeTe film to 138°C of the N-doped GeTe film. The better surface roughness is confirmed by atomic force microscopy. The phase change speed is evaluated by the picosecond laser pump-probe technology.
Phase change memory is regarded as one of the most promising candidates for the next-generation non-volatile memory. Zr9(Ge2Sb2Te5)91 film was investigated as storage material for phase-change memory application. The crystallization temperature (Tc) and 10 years data retention temperature of the Zr9(Ge2Sb2Te5)91 film are about 195 and 106.7°C, respectively, and both higher than that of Ge2Sb2Te5 (GST). The sheet resistance ratio between amorphous and crystalline states is up to four orders of magnitude. The crystalline resistance of Zr9(Ge2Sb2Te5)91 film is higher than GST for one order of magnitude, which contribute to reduce the power consumption for PCM device. Zr9(Ge2Sb2Te5)91 film exhibit larger optical band gap in comparison with GST. Zr9(Ge2Sb2Te5)91 is considered to be a promising material for phase change memory.
Sb-rich Sb-Te films with different composition were investigated by temperature-dependent resistance measurement, crystal structure characterization, and in situ crystallization behavior study. The results show that when the Sb content is more than 80 at.%, Sb-rich Sb-Te films cannot be used as phase change material due to their low crystallization temperature and small resistance contrast. Sb-rich Sb-Te films with the Sb content being between 80~67 at.% can be used as phase-change-material and they have similar properties because of their similar growth-dominant crystallization behaviors.
Crystallization behavior of the Ge2Te3-TiO2 films prepared by the co-sputtering using Ge2Te3 and TiO2 targets was investigated by in situ resistance-temperature measurement and transmission electron microscopy. The crystallization kinetic parameters including rate factor and kinetics exponent were obtained by the non-isothermal change in resistance using Kissinger’s plot and Ozawa’s method. The average kinetics exponent was estimated by the nonisothermal change in resistance. Compared with other studied compositions, the composition with TiO2 concentration of 5 at.% exhibited shorter crystallization time which was calculated by the Johnson-Mehl-Avrami equation. The crystallization behavior of Ge2Te3-TiO2 film was verified by the transition electron microscopy at different annealing temperature. With the short crystallization time and high crystallization temperature, the compositions with TiO2 concentration of 5-15 at.% may be one of the competing candidates for phase change memory application.
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