High Temperature Characterization of Ge2Sb2Te5Thin Films for Phase Change Memory Applications

The recent proliferation of portable communication devices or data storage equipment is strongly related to the development of memory technology. Non-volatile semiconductor solid-state memories are needed for high-capacity storage media, high-speed operation and low power consumption, with stringent requirements of retention and endurance. Phase change memory (PCM) is currently seen as one of the most promising candidates for a future storage-class memory with the potential to be close to dynamic random-access memory (DRAM) in speed but with much longer retention times and as dense as flash memory. PCM devices utilize chalcogenide materials (most commonly Ge2Sb2Te5 or GST) that can be switched rapidly and reversibly between amorphous and crystalline phases with orders of magnitude difference in electrical resistivity. Since PCM devices operate at elevated (current-induced) temperatures and are significantly impacted by thermoelectric effects it is very important to determine the high temperature material properties of GST. Resistivity, carrier mobility, and carrier concentration in semiconducting materials are three key parameters indispensable for device modeling. In this work two measurement setups for high temperature thin film characterizations were developed, a Seebeck setup and a Hall setup. The Seebeck coefficient measurement setup is fully automated and uses resistive and inductive heaters to control the temperature gradient and can reach temperatures up to ~650 °C. The Hall measurement setup, developed based on the van der Paw method for characterization of semiconducting thin films, can measure thin film samples of a wide resistivity range from room temperature to ~500 °C. The resistivity, carrier concentration, and Hall carrier mobility are calculated from I-V measurements and the constant magnetic field applied in ‘up’ and ‘down’ directions. Measurement results on GST thin films with different thicknesses revealed interesting correlations between S-T and ρ-T characteristics and showed that GST behaves as a unipolar p-type semiconducting material from room temperature up to melting. The thermoelectric properties of the GST films were also correlated to the average grain sizes obtained from in-situ XRD measurements during crystallization. These studies show that the activation energy of carriers in mixed phase amorphous-fcc GST is a linear function of the Peltier coefficient. From these results and the ρ-T characteristics, the expected Seebeck coefficient of single crystal fcc GST is obtained. Using the experimental results for resistivity and Seebeck coefficient, together with a phase separation model, the temperature-dependent thermal conductivity of the mixed phase GST is extracted

Atmospheric pressure microplasmas in ZnO nanoforests under high voltage stress

Atmospheric pressure ZnO microplasmas have been generated by high amplitude single pulses and DC voltages applied using micrometer-separated probes on ZnO nanoforests. The high voltage stress triggers plasma breakdown and breakdown in the surrounding air followed by sublimation of ZnO resulting in strong blue and white light emission with sharp spectral lines and non-linear current-voltage characteristics. The nanoforests are made of ZnO nanorods (NRs) grown on fluorine doped tin oxide (FTO) glass, poly-crystalline silicon and bulk p-type silicon substrates. The characteristics of the microplasmas depend strongly on the substrate and voltage parameters. Plasmas can be obtained with pulse durations as short as similar to 1 mu s for FTO glass substrate and similar to 100 ms for the silicon substrates. Besides enabling plasma generation with shorter pulses, NRs on FTO glass substrate also lead to better tunability of the operating gas temperature. Hot and cold ZnO microplasmas have been observed with these NRs on FTO glass substrate. Sputtering of nanomaterials during plasma generation in the regions surrounding the test area has also been noticed and result in interesting ZnO nanostructures ('nano-flowers' and 'nano-cauliflowers'). A practical way of generating atmospheric pressure ZnO microplasmas may lead to various lighting, biomedical and material processing applications. (C) 2015 Author(s)

Electrical Resistivity of Liquid Ge2Sb2Te5 Based on Thin-Film and Nanoscale Device Measurements

The electrical resistivity of liquid Ge2Sb2Te5 (GST) is obtained from dc-voltage measurements performed on thin GST films as well as from device-level microsecond-pulse voltage and current measurements performed on two arrays (thicknesses: 20 +/- 2 nm and 50 +/- 5 nm) of lithographically defined encapsulated GST nano-/microwires (length: 315 to 675 nm; width: 60 to 420 nm) with metal contacts. The thin-film measurements yield 1.26 +/- 0.15 m Omega.cm (thicknesses: 50, 100, and 200 nm); however, there is significant uncertainty regarding the integrity of the film in liquid state. The device-level measurements utilize the melting of the encapsulated structures by a single voltage pulse while monitoring the current through the wire. The melting is verified by the stabilization of the current during the pulse. The resistivity of liquid GST is extracted as 0.31 +/- 0.04 and 0.21 +/- 0.03 m Omega.cm from 20- and 50-nm-thick wire arrays