Demonstration of flexible thin film transistors with GaN channels

We report on the thin film transistors (TFTs) with Gallium Nitride (GaN) channels directly fabricated on flexible substrates. GaN thin films are grown by hollow cathode plasma assisted atomic layer deposition (HCPA-ALD) at 200 °C. TFTs exhibit 103 on-to-off current ratios and are shown to exhibit proper transistor saturation behavior in their output characteristics. Gate bias stress tests reveal that flexible GaN TFTs have extremely stable electrical characteristics. Overall fabrication thermal budget is below 200 °C, the lowest reported for the GaN based transistors so far. © 2016 Author(s

Plasmonically enhanced metal-insulator multistacked photodetectors with separate absorption and collection junctions for near-infrared applications

Plasmonically enhanced metal-insulator-metal (MIM) type structures are popular among perfect absorbers and photodetectors in which the field enhancement (for increased absorption) mechanism is directly coupled with collection (photocurrent) processes. In this work we propose a device structure that decouples absorption and collection parts for independent optimization. Double-stacked MIM (i.e. MIMIM) photodetectors operating in the near-infrared (NIR) spectrum up to 1200 nm wavelength are demonstrated. In the absorbing MIM (at the top side), we have used Silver nanoparticles resulting from dewetting, yielding a very low reflection of 10% for the most part of the 400 to 1000 nm wavelength range. An unconventional plasmonic material, Chromium, exhibits an absorption peak of over 80% at 1000 nm. The complete device has been fabricated and the photo-collection tunneling MIM (at the bottom) suppresses the leakage current by metal workfunction difference. An optimized stack consisting of Silver-Hafnium Oxide-Chromium-Aluminum Oxide-Silver nanoparticles (from bottom to top) yields a dark current of 7 nA and a photoresponsivity peak of 0.962 mA/W at 1000 nm and a full width at half maximum of 300 nm, while applied bias is 50 mV and device areas are 300 μm × 600 μm. © 2017 The Author(s)

LWIR all-atomic layer deposition ZnO bilayer microbolometer for thermal imaging

We propose an all-ZnO bilayer microbolometer, operating in the long-wave infrared regime that can be implemented by consecutive atomic layer deposition growth steps. Bilayer design of the bolometer provides very high absorption coefficients compared to the same thickness of a single ZnO layer. High absorptivity of the bilayer structure enables higher performance (lower noise equivalent temperature difference and time constant values) compared to single-layer structure. We observe these results computationally by conducting both optical and thermal simulations. © 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)

Atomic Layer Deposition for Vertically Integrated ZnO Thin Film Transistors: Toward 3D High Packing Density Thin Film Electronics

We report on the first demonstration of the atomic layer deposition (ALD) based three dimensional (3D) integrated ZnO thin film transistors (TFTs) on rigid substrates. Devices exhibit high on-off ratio (∼106) and high effective mobility (∼11.8 cm2 V−1 s−1). It has also been demonstrated that the steps of fabrication result in readily stable electrical characteristics in TFTs, eliminating the need for post-production steps. These results mark the potential of our fabrication method for the semiconducting metal oxide-based vertical-integrated circuits requiring high packing density and high functionality. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Collective behavior of boreholes and its optimization to maximize BTES performance

Borehole layout strongly affects the behavior of borehole heat exchangers (BHEs) and changes the performance of a borehole thermal energy storage (BTES). This study investigates the existence and importance of the optimum collective behavior of BHEs to maximize the performance of BTES. Charge benefit ratio, storage efficiency and configurational benefit factor are proposed as performance indicators for better and finer performance evaluations of BTES systems. A small-scale BTES consisting of ten boreholes arranged on a concentric double-ring layout is considered as an application. Performance variations with the inner and the outer radii of the borehole field are analyzed for the first five years of operation. The temperature fields of different configurations show the transition from collective to individual behavior of boreholes, which leads to an optimal radial configuration maximizing the performance indicators. It is seen that the indicators strongly depend on both inner and outer radii and they reach their maximums for the same distinct radial configuration. The optimum arrangement can almost double the thermal performance indicators. It is thus of great importance to optimize collective behavior to maximize the usage of stored thermal energy. The results are qualitatively general and represent the common behavioral patterns of BTES systems

Very high temperature BTES : A potential for operationally cost-free and emission-free heating

In cold climates, the heating load is much higher than the cooling load and borehole thermal energy storage (BTES) systems offer an opportunity for high -efficient heating. Heat energy from different sources is stored in the ground to use in wintertime. Higher storage temperatures lower the operating cost of heating and the size of the BTES field. BTES systems, up to 90°C, are studied in the literature. In this study, we consider the charge temperatures beyond 100°C to analyze the possibility of free -heating for Nordic countries. The system is called here very high -temperature BTES (VHT-BTES). A residence field of 25 houses, 125m2 each, is chosen as a mid -scale target and concentrated solar collectors (CSC) are used to charge VHT-BTES up to 140°C. A double -ring layout of ten boreholes is optimized to minimize the heat pump consumption. Free -heating and heat pump modes are activated for high and low borehole temperatures, respectively. The actual meteorological data for Uppsala/Sweden is used. The gradually increasing very high seasonal coefficient of performance values (SCOP), 8-23, are achieved using free -heating and heat pump modes together for the first four years. From the fifth year, all of the heating demand is basically met by the stored energy (free -heating). The results show that VHT-BTES provides practically operationally cost-free and emission -free heating even for a Nordic country. The return of investment is calculated as ten to fourteen years, depending on the cost of the additional land for CSC

Effect of layered geological structures on borehole heat transfer

Borehole heat exchangers, especially deep ones, are usually drilled through different geological layers havingvarying properties. Homogeneous and layered models can be used for borehole performance predictions. Thehomogeneous model considers all layers as a single layer having effective properties while the layered modelconsiders all layers separately and gives better accuracy, although it is more complicated and time consuming tocalculate. In this study, by considering real geological structures, thermal performance predictions of a deepborehole are made using both homogeneous and layered models and the results are compared to examine howpredictions differ from each other depending on the statistical characteristics of geological structures. Ananalytical expression is derived for the relation between statistical characteristics and deviations from the predictionsof the homogeneous model. The magnitudes of deviations are very small and essentially depend on thevariance of the difference for the thermal properties of the layers and a time decaying function. The results helpto understand how horizontally layered geological structures influence borehole performance and when we needa layered model

The effectiveness of a nail‐biting prevention program among primary school students

Purpose: This study aimed to demonstrate the effect of a program called Do Not Bite Your Nails, Cut Your Nails, which is based on a health promotion model to change the nail-biting habit among primary school students