Thermoelectric properties of Zn_5Sb_4In_(2-δ)(δ=0.15)

The polymorphic intermetallic compound Zn_5Sb_4In_(2−δ) (δ = 0.15(3)) shows promising thermoelectric properties at low temperatures, approaching a figure of merit ZT of 0.3 at 300 K. However, thermopower and electrical resistivity changes discontinuously at around 220 K. Measurement of the specific heat locates the previously unknown temperature of the order-disorder phase transition at around 180 K. Investigation of the charge carrier concentration and mobility by Hall measurements and infrared reflection spectroscopy indicate a mixed conduction behavior and the activation of charge carriers at temperatures above 220 K. Zn_5Sb_4In_(2−δ) has a low thermal stability, and at temperatures above 470 K samples decompose into a mixture of Zn, InSb, and Zn_4Sb_3

Integration of a visual tracking system into a four probe measuring system to evaluate the electrical sheet resistance of thin films

En los últimos años, las películas delgadas han sido ampliamente estudiadas debido a la amplia gama de aplicaciones técnicas que presentan, algunas de las cuales están determinadas por sus propiedades eléctricas tales como la resistividad. Generalmente, algunas propiedades medidas en la macroescala no siguen siendo válidas cuando el material es reducido a la nanoescala. Varios estudios demuestran que la resistividad en películas delgadas depende del espesor de la muestra. Por lo tanto, en la investigación y producción de películas delgadas para nuevas aplicaciones, es necesario un sistema eficaz y preciso para medir y caracterizar sus propiedades eléctricas. Con el fin de superar las limitaciones en la medición de la resistividad en películas delgadas, el objetivo de esta tesis es la de implementar un sistema de medición de la resistividad flexible implementado utilizando el software LabVIEW y conformado por instrumentos de medición Keithley y una cámara digital tipo microscopio. Este sistema presenta dos características principales: 1. Un sistema de seguimiento automático de posición (visual tracking) para determinar la ubicación de las puntas de medición sobre la muestra. Este sistema reduce los errores ocasionados por el desalineamiento de las puntas, proporciona una apropiada interfaz gráfica y es el primer paso para la automatización del sistema de medición. 2. El sistema es capaz de medir la resistividad utilizando cuatro métodos distintos (Van der Pauw, Linear Van der Pauw, y el método de las cuatro puntas lineal y cuadrado). Esta característica proporciona la posibilidad de medir una gama más amplia tanto de materiales como de dimensiones de las muestras. El desempeño del sistema desarrollado se válido midiendo muestras estándar de aluminio y tungsteno de diferentes espesores (100, 300 and 600 nm). Las películas se depositaron sobre sustrato de silicio mediante sputtering. La resistividad de las películas se midió aplicando los diferentes cuatro métodos y se obtuvo un error estándar menor a 1%. Con el _n de validar la eficacia del sistema de seguimiento visual (visual tracking), se analizó la influencia, tanto del desalineamiento como de la distribución de las puntas en la medición de la resistividad. Los resultados fueron validados por comparación con datos experimentales de la literatura y modelos teóricos de películas delgadas (Fuchs-Sondheimer, Mayadas-Shatzke y combinación de ambos modelos). Los resultados están en correlación con los datos experimentales y los modelos teóricos. Además, se confirmó la dependencia de la resistividad con el espesor. Asimismo, se demostró que el incremento de la resistividad eléctrica podrá explicarse por las contribuciones de los mecanismos de dispersión en los limites de grano y en la superficie de la película delgada.Tesi

Good practice guide on the electrical characterisation of graphene using contact methods

The electrical characterisation of graphene, either in plane sheets or in properly geometrised form can be approached using methods already employed for thin film materials. The extraordinary thinness (and, correspondingly, the volume) of graphene, however, makes the proper application of these methods difficult. The electrical properties of interest (sheet electrical resistivity/conductivity, concentration and mobility of charge carriers) must be indirectly derived from the measurement outcome by geometrical and electrical modelling; the assumptions behind such models (e.g., uniformity and isotropy, effective value of the applied fields, etc.) require careful consideration. The traceability of the measurement to the International System of units and a proper expression of measurement uncertainty is an issue. This guide focuses on contact methods, that is method where the graphene sample surface is physically contacted with metallic electrodes. A companion guide about non-contact and high-throughput methods is also available. The methods discussed are: the in-line four-point probe (4PP); the van der Pauw method (vdP) for sheet resistance measurement; the van der Pauw method for charge carrier mobility measurement; the electrical resistance tomography (ERT); the coplanar waveguide method (CPW). For each method, a corresponding measurement protocol is discussed, which describes: the measurement principle; sample requirements and preparation; a description of the measurement equipment / apparatus; calibration standards and ways to achieve a traceable measurement; environmental conditions to be considered; a detailed measurement procedure, with specific hints to achieve a reliable measurement; modeling and data analysis to determine the electrical property of interest; considerations about the expression of measurement uncertainty

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

Gated Hall and Field-Effect Transport Characterization of E-mode ZnO TFTs

Amorphous and nano-crystalline metal oxide semiconductors are an important class of materials under continuing investigation for emerging technologies. Accurate measurements of electron mobility in these materials is critical for furthering overall device development. This is complicated due to the fact that device measurements such as current response, transistor input and output characteristics, as well as mobility are affected by transport-limiting factors, such as charge trapping effects at the dielectric / active layer interface, and restriction of electronic transport across grain boundaries. In this work, we focus on the binary metal oxide thin film transistor (ZnO TFT), a normally-off (e-mode) transistor with a positive threshold voltage (Vth) and a large ( \u3e 10 MΩ)) sheet resistance at gate voltage below threshold (VG \u3c Vth), Field-effect and Hall (extrinsic and intrinsic mobilities) were measured on the same device at the same time (concurrent mobility measurements of our gated Hall system) at device relevant dimensions (25 nm Al2O3, gate dielectric, 50 nm ZnO active layer), at typical transistor gate and drain bias device operating conditions (VG \u3c 10 V and VD biased in the linear region), on the same device (100 × 100 µm Van der Pauw). The large sheet resistance (RS) of the material requires electrostatic doping (by gate bias) in order to modulate resistance and increase Hall test current. However, as VG interacts with VD and VS, resulting vertical electric fields EGS and EGD must remain below dielectric breakdown (EBD). The result of meeting these test requirements led to a fully automated gated Hall test system capable of making measurements and comparisons of mobility across the allowable test bias spectrum (VG and VD). A design of experiment in which test wafers were compared between in situ deposition and exposure to clean room ambient air between the dielectric and active layer depositions (by atomic layer deposition) was used to examine interface effects. Post temperature oven annealing was used to compare differences in grain boundary effects by increasing grain size. A simple model of two transport regimes was developed (localized and non-localized transport) to fit several contradictory trends observed in the measured data sets