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We investigated the impact of incorporating 2% oxygen during intrinsic ZnO sputtering on the efficiency of Cu(In,Ga)Se2 solar cells. The added oxygen not only reduced the optical absorption loss of the Al-doped ZnO overlaying layer but also improved the electronic properties of the underlying CdS/Cu(In,Ga)Se2 by increasing carrier density, lowering defect level, and increasing diffusion length, eventually enhancing JSC, VOC, and fill factor. It was found that the Na doping concentration was significantly increased around the CdS/Cu(In,Ga)Se2 junction due to the plasmaactivated oxygen. The improved electronic properties are better explained by the increased Na concentration than simply the oxygen-related defect passivation. [ABSTRACT FROM AUTHOR]

In this study, we investigated the structural and optical properties of Ge-doped SbTe (Ge-ST) thin films with three differing compositions: Ge0.06Sb0.77Te0.17 (Ge-STH), Ge0.05Sb0.70Te0.25 (Ge-STM), and Ge0.05Sb0.64Te0.31 (Ge-STL), grown on Si substrate by radio-frequency sputtering method. The films were annealed at 250°C for crystallization and their crystal structures were examined by X-ray diffraction. Compared to the X-ray diffraction spectra of the undoped SbTe, the Ge-ST thin films had a hexagonal structure with large stacking periods. Using Raman spectroscopy, we investigated the shift of the phonon mode frequencies (A1g and Eg) of the films with varying Sb:Te ratios. We compared the dependence of the phonon frequencies of Ge-ST on the Sb content to those of the corresponding undoped SbTe. The composition dependence of the A1g phonon frequency could be explained in terms of the linear extrapolation of Sb and Sb2Te3 crystals. By using spectroscopic ellipsometry, we measured the dielectric function of the thin films in the near-IR, visible, and ultraviolet spectral regions. The optical energy gaps and bandgaps of the amorphous and crystalline phases, respectively, were determined using linear extrapolation of the absorption coefficient. The optical gap energies of the amorphous Ge-ST films were determined to be about 0.5-0.6 eV, whereas the indirect bandgap energies of the crystalline films shrank substantially to about 0.15-0.2 eV. [ABSTRACT FROM AUTHOR]

For potential use in phase-change memory (PCM) devices, a Ge-doped SbTe (Ge-ST) was investigated with respect to both material properties and PCM device characteristics. Compared with Ge2Sb2Te5 (GST), the used Ge-ST (Ge5.3Sb77.7Te17) was characterized to have a lower melting temperature (814 K), a higher crystallization temperature (436 K), and a lower electrical resistivity in both amorphous and crystalline states. The Ge-ST devices have a much faster operation speed by about I order and consume less RESET power compared with the GST devices. Nevertheless, a higher RESET-programming current was required due to a lower dynamic resistance. Device reliability properties such as write/erase cycling endurance and data retention properties were also examined and found to be comparable to those of the GST devices, suggesting that the Ge-ST is a promising phasechange material for high speed PCMs. [ABSTRACT FROM AUTHOR]

Laterally driven microresonators were used to estimate the temperature-dependent elastic modulus of single-crystalline Si for microelectromechanical systems (MEMS). The resonators were fabricated through surface micromachining from silicon-on-glass wafers. They were moved laterally by alternating electrostatic force at a series of frequencies, and then a resonance frequency was determined, under temperature cycling in the range of 25 °C to 600 °C, by detecting the maximum displacement. The elastic modulus was obtained at the temperature range by Rayleigh's energy method from the detected resonance frequency. At this time, the temperature dependency of elastic modulus was affected by surface oxidation as well as its intrinsic variation: a temperature cycle permanently reduces the resonance frequency. The effect of Si oxidation was analyzed for thermal cycling by applying a simple composite model to the measured frequency data; here the oxide thickness was estimated from the difference in the resonance frequency before and after the temperature cycle, and was confirmed by field-emission scanning electron microscopy. Finally, the temperature coefficient of the elastic modulus of Si in the «110» direction was determined as -64 × 10[sup -6][°C[sup -1]]. This value was quite comparable to those reported in previous literatures, and much more so if the specimen temperature is calibrated more exactly. [ABSTRACT FROM AUTHOR]

Many thermomechanical reliability studies on microelectronics and microsystems have relied upon computational analysis, since experimental work is rather difficult and very time-consuming. For computational analysis, it is essential to use as input accurate material properties; if not, the results of a reliability analysis may be very inaccurate. However, it is still quite difficult to arrive at unified material properties for modeling microelectronic assemblies because of the absence of standards for micro-material characterization, the difference between bulk and in-situ material properties, and so forth. The goal of this study was to determine the uniaxial stress-strain curve of a solder in a flip-chip assembly, using experimental measurements and finite-element analysis (FEA) of the solder's thermal deformation characteristics with increasing temperature. The thermal deformation of flip-chip solder joints was measured by electronic speckle pattern interferometry (ESPI). For the scale of evaluation required, the measurement magnification was modified to allow its application to micromaterials by using a long-working-distance microscope, iris and zoom lens. Local deformation of solder balls could be measured at submicrometer scale, and stress-strain curves could be determined using the measured thermal deformation as input data for finite-element analysis. The procedure was applied to an Sn-36Pb-2Ag flip-chip solder joint. [ABSTRACT FROM AUTHOR]

Ge-doped SbTe (Ge–ST) was compared with Ge2Sb2Te5 (GST) for its potential use in the phase-change memory with improved stability at varying ambient temperature (TA). Device characteristics such as RESET current, RESET resistance, and SET resistance of Ge–ST devices were found to vary significantly less with TA than those of GST devices. From measured carrier density, mobility, and optical band gaps, these findings are interpreted to derive from a metallic nature of the crystalline Ge–ST in contrast with a semiconducting nature of the crystalline GST as well as a relatively weaker covalent bonding in amorphous Ge–ST. [ABSTRACT FROM AUTHOR]

We investigated the temperature dependence of the threshold switching characteristics of a memory-type chalcogenide material, Ge2Sb2Te5. We found that the threshold voltage (Vth) decreased linearly with temperature, implying the existence of a critical conductivity of Ge2Sb2Te5 for its threshold switching. In addition, we investigated the effect of bias voltage and temperature on the delay time (tdel) of the threshold switching of Ge2Sb2Te5 and described the measured relationship by an analytic expression which we derived based on a physical model where thermally activated hopping is a dominant transport mechanism in the material. [ABSTRACT FROM AUTHOR]

We investigated the temperature dependence of characteristics of phase change memory devices composed of Ge2Sb2Te5 (GST). We found that the RESET resistance (RR), SET resistance (RS), and SET time (tSET) decreased with increasing ambient temperature (Tamb.) while RESET current (IR) increased with Tamb. These results were explained in terms of the enhanced thermal activation of electrical conduction in GST and the increased thermal conductivity of the bottom electrode with Tamb.. Besides, threshold voltage (Vth) was found to decrease linearly with Tamb., seemingly in support of the relaxation semiconductor model and hopping-dominated transport model both of which predicted the existence of critical conductivity for threshold switching in chalcogenide glasses. [ABSTRACT FROM AUTHOR]

Using a phase-change memory (PCM) device com- posed of Ge2Sb2Te5 (GST), we studied the mechanism of the SET-stuck failure (SSF), a constantly low-resistance state during write/erase (W/E) cycling. The SSF state was characterized with increased RESET current and decreased threshold voltage, which were thought to be due to depletion of Ge and enrichment of Sb inside the active volume of GST. Moreover, we found that device characteristics of an SSF-PCM could be recovered by reversing bias polarity and the repaired device could endure many W/E cycles, implying that field-induced ion migration was the major cause of the SSF of a PCM. [ABSTRACT FROM AUTHOR]

An experimental investigation was carried out on the kinetic nature of the set process in a phase change memory device by combined analyses of set voltage wave forms and time-resolved low-field resistances. As it turned out, the progress of a set process may be measured in terms of three characteristic times in sequence i.e., threshold switching time tth, incubation time for crystallization tinc, and complete set time tset. These characteristic times are supposed to demarcate, in some measure, different stages of crystallization in the memory material during a set process. Each of these times has a strong dependence on input pulse voltage and particularly threshold switching time tth was found to have an exponentially decaying dependence. The latter may be related to the decreasing capacitance of an amorphous phase-change material with approaching threshold switching. [ABSTRACT FROM AUTHOR]

A nonvolatile memory technology utilizing reversible changes between fcc and hcp crystalline phases is proposed. In this new type of phase-change memory, data are stored in different forms of crystalline phases of (Ge1 Sb2 Te4)0.8 (Sn1Bi2Te4)0.2 chalcogenide alloy. RESET operation produces the less conductive metastable fcc phase via melt-quenching from the more conduce tin stable hcp phase and SET operation involves a phase change from fcc directly to hcp. Both RESET and SET operations can be completed as fast as 70 ns with large changes in cell resistance. [ABSTRACT FROM AUTHOR]

Current efforts of guided growth of carbon nanotubes alone cannot make large-scale and directed assembly of them. The nanopelleting concept overcomes this limitation by embedding carbon nanotubes in microscale pellets that can be transplanted readily. This technique includes vertical growth of carbon nanotubes, pellet casting, planarization, pellet separation, and transplantation. A specific manufacturing process is developed and tested with favorable results. This technology will enable directed assembly of carbon nanotubes in a long-range order. [ABSTRACT FROM AUTHOR]

This paper reports the fabrication of crystalline silicon passivated emitter rear totally diffused (c-Si PERT) solar cells with ultra-thin p-type wafers 50 µm in thickness. Co-diffusion of boron and phosphorus in a single rapid thermal processing cycle, and an Al spin-on glass post-curing process were developed to remove the boron rich layer which is detrimental to c-Si solar cells. Co-diffusion was carried out with spin-on diffusion sources using boric acid and a P spin on dopant for simple and cost-effective emitter and back surface field (BSF) formation processes. The fabricated ultra-thin c-Si PERT cell featured an open circuit voltage (Voc) of 0.575 V, a short circuit current density (Jsc) of 35.8 mA cm−2, a fill factor of 0.725, and a power conversion efficiency of 15.0%. The efficiency has improved by 2% compared with the standard structure cell with Al-BSF using thin evaporated Al 2 µm in thickness. Along with cell output parameters, the flexural strength and critical bending radius were measured by a four point bending test, and the results showed that the solar cells with thinner rear Al electrodes are more applicable for a flexible solar cell device. [ABSTRACT FROM AUTHOR]