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In this study, we propose a Se-incorporated Ge10Sb90 as a phase-change material for phase-change memory (PCM) with high reliability and low operation power. We investigated the effect of the Se concentration on the thermal and electrical properties of Se-doped Ge10Sb90 films by varying the Se concentration from 0 to 20 at.%. The crystallization temperature, crystallization activation energy, and maximum ten-year data retention temperature increased with the increasing Se, thus demonstrating the improved thermal stability of Se-doped Ge10Sb90 films with higher Se contents. More Se also increased the rate factor, band gap, threshold voltage, and load resistance. In addition, the crystallization speed, programming window, and resistances of both the amorphous and crystalline states increased with the increasing Se concentration. In contrast, the reset current decreased with the increasing Se concentration. These results demonstrate that Se-doped Ge10Sb90 is a highly promising material for PCM applications.
Post annealing of Hf-silicate thin film grown by ALD was done with different kind of nitrogen gas and order of annealing. Annealing conditions are as follows: (1) NO gas only, (2) NH3 gas only, (3) NO gas + NH3 gas, and (4) NH3 + NO gas. With these conditions, the physical and electrical properties of nitrided Hf-silicate films were analyzed. Content of nitrogen is decreased with post NO gas annealing. In case of NH3, content of nitrogen is much higher than NO case. Most nitrogen atoms were distributed between Si substrate and Hf-silicate film for NO gas annealing. However, with NH3 gas annealing, nitrogen atoms were distributed in the whole Hf-silicate film evenly. Leakage current was decreased with post NO gas annealing and flat band voltage was also decreased.
The silicidation reactions and thermal stability of Co silicide formed from Co-Ta/Si systems have been investigated. In case of Co-Ta alloy process, the formation of low resistive CoSi2phase is delayed to about 660°C, as compared to conventional Co/Si system. Moreover, the presence of Ta in Co-Ta alloy films reduces the silicidation reaction rate, resulting in the strong preferential orientation in CoSi2 films. Upon high temperature post annealing in the furnace, the sheet resistance of Co-silicide formed from Co/Si systems increases significantly, while that of Co-Ta/Si systems maintains low. This is due to the formation of TaSi2 at the grain boundaries and surface of Co-silicide films, which prevents the grain boundary migration thereby slowing the agglomeration. Therefore, from our research, increased thermal stability of Co-silicide films was successfully obtained from Co-Ta alloy process.
Poly Si1−xGex films have been suggested as a promising alternative to the currently employed poly-Si gate electrode for CMOS technology due to lower resistivity, less boron penetration, and less gate depletion effect than those of poly Si gates. We investigated the formation of poly Si1−xGex films grown by UHV CVD using Si2H6 and GeH4 gases, and studied their microstructures as well as their electrical characteristics. The Ge content of the Si1−xGex films increased linearly with the flux of the GeH4 gas up to x=0.3, and saturated above x=0.45. The deposition rate of the poly Si1−xGex films increased linearly with the flux of the GeH4 gas up to x=0.1, above which it is slightly changed. The resistivity of the Si1−xGex films decreased as the Ge content increased, and was about one half of that of poly-Si films at the Ge content of 45%. The C-V measurements of the MOSCAP structures with poly Si1−xGex gates demonstrated that the flat band voltage of the poly Si1−xGex films was lower than that of poly-Si films by 0.2V.
Elastic modulus and Poisson's ratio of diamond-like carbon (DLC) film was measured by a simple method using DLC bridges which are free from mechanical constraint of substrate. The DLC films were deposited on Si wafer by C6H6r.f. glow discharge at the deposition pressure 1.33 Pa. Because of the high residual compressive stress of the film, the bridge exhibited a sinusoidal displacement by removing the constraint of the substrate. By measuring the amplitude with known bridge length, we could determine the strain of the film required to adhere to the substrate. Combined with independent stress measurement by laser reflection method, this method allows calculation of the biaxial elastic modulus, E/(1–v), where E is the elastic modulus and v Poisson's ratio of the DLC film. By comparing the biaxial elastic modulus with plane-strain modulus, E/(1–v2), measured by nano-indentation, we could further determine the elastic modulus and Poisson's ratio, independently. The elastic modulus, E, increased from 87 to 133 GPa as the negative bias voltage increased from 400 to 550 V. Poisson's ratio was estimated to be about 0.20 in this bias voltage range. For the negative bias voltages less than 400 V, however, the present method resulted in negative Poisson's ratio which is physically impossible. The limitation of the present method was also discussed.
A simple method to measure the elastic modulus and Poisson's ratio of diamond-like carbon (DLC) films deposited on Si wafer was suggested. This method involved etching a side of Si substrate using the DLC film as an etching mask. The edge of DLC overhang free from constraint of Si substrate exhibited periodic sinusoidal shape. By measuring the amplitude and the wavelength of the sinusoidal edge, we can determine the strain of the film required to adhere to the substrate. Combined with independent stress measurement by laser reflection method, this method allows calculation of the biaxial elastic modulus, E/(1 − v), where E is the elastic modulus and v Poisson's ratio of the DLC films. By comparing the biaxial elastic modulus with plane-strain modulus, E/(1 −v2), measured by nano-indentation, we could further determine the elastic modulus and Poisson's ratio, independently. This method was employed to measure the mechanical properties of DLC films deposited by C6H6 r.f. glow discharge at the deposition pressure 1.33 Pa. The elastic modulus, E, increased from 94 to 128 GPa as the negative bias voltage increased from 400 to 550 V. Poisson's ratio was estimated to be about 0.22 in this bias voltage range. For the negative bias voltages less than 400 V, however, the present method resulted in negative Poisson's ratio. The limitation of the present method was discussed.
The amorphous phase formation and initial crystalline reactions at Pt/GaAs interfaces has been investigated via high-resolution electron microscopy (HREM), microdiffraction, and energy dispersive spectroscopy (EDS). A thin amorphous intermixed layer consisting of three elements, platinum, gallium, and arsenic was observed at Pt/GaAs interface in an as-deposited sample. This interlayer grew to 4.5nm in an amorphous state upon low temperature(e.g. 200°C) annealing by a solid-state amorphization reaction. Following the growth of the amorphous interlayer, subsequently, Pt3Ga and PtAs2 phases nucleated within the amorphous layer and grew at the Pt and GaAs sides, respectively. We also observed the same reaction processes with in-situ annealing HRTEM.
The thermal stability of PtAl thin films on GaAs substrates has been studied using transmission electron microscopy and Auger electron spectroscopy. The PtAl thin films were formed by sequential deposition of discrete Pt and Al layers on GaAs by e-beam evaporation followed by subsequent annealing processes. Interfacial reactions in the Al/Pt/GaAs system proceed in two stages. Upon low temperature annealing Pt and GaAs react to form PtGa and PtAs2. Further high temperature annealing causes PtGa, PtAs2 and Al to react together producing the desired PtAl on GaAs. We observed solid-phase epitaxial regrowth of GaAs during the second stage of reaction. The PtAl/GaAs interface is determined to be thermally stable during an 800°C/30 min. anneal, while remaining morphologically uniform on GaAs.
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