Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

https://iopscience.iop.org/article/10.1088/1361-6463/50/4/043001/pdf

The 2017 terahertz science and technology roadmap

The 2016 terahertz science and technology roadmap

Van der Waals heterostructure based on layered two-dimensional (2D) materials offers unprecedented opportunities to create materials with atomic precision by design. By combining superior properties of each component, such heterostructure also provides possible solutions to address various challenges of the electronic devices, especially those with vertical multilayered structures. Here, we report the realization of robust memristors for the first time based on van der Waals heterostructure of fully layered 2D materials (graphene/MoS2-xOx/graphene) and demonstrate a good thermal stability lacking in traditional memristors. Such devices have shown excellent switching performance with endurance up to 107 and a record-high operating temperature up to 340oC. By combining in situ high-resolution TEM and STEM studies, we have shown that the MoS2-xOx switching layer, together with the graphene electrodes and their atomically sharp interfaces, are responsible for the observed thermal stability at elevated temperatures. A well-defined conduction channel and a switching mechanism based on the migration of oxygen ions were also revealed. In addition, the fully layered 2D materials offer a good mechanical flexibility for flexible electronic applications, manifested by our experimental demonstration of a good endurance against over 1000 bending cycles. Our results showcase a general and encouraging pathway toward engineering desired device properties by using 2D van der Waals heterostructures.

/pdf/robust-memristors-based-on-layered-two-dimensional-materials-1qbzmrrye0.pdf

Robust memristors based on layered two-dimensional materials

Black silicon (BSi) represents a very active research area in renewable energy materials. The rise of BSi as a focus of study for its fundamental properties and potentially lucrative practical applications is shown by several recent results ranging from solar cells and light-emitting devices to antibacterial coatings and gas-sensors. In this paper, the common BSi fabrication techniques are first reviewed, including electrochemical HF etching, stain etching, metal-assisted chemical etching, reactive ion etching, laser irradiation and the molten salt Fray-Farthing-Chen-Cambridge (FFC-Cambridge) process. The utilization of BSi as an anti-reflection coating in solar cells is then critically examined and appraised, based upon strategies towards higher efficiency renewable solar energy modules. Methods of incorporating BSi in advanced solar cell architectures and the production of ultra-thin and flexible BSi wafers are also surveyed. Particular attention is given to routes leading to passivated BSi surfaces, which are essential for improving the electrical properties of any devices incorporating BSi, with a special focus on atomic layer deposition of Al2O3. Finally, three potential research directions worth exploring for practical solar cell applications are highlighted, namely, encapsulation effects, the development of micro-nano dual-scale BSi, and the incorporation of BSi into thin solar cells. It is intended that this paper will serve as a useful introduction to this novel material and its properties, and provide a general overview of recent progress in research currently being undertaken for renewable energy applications.

/pdf/black-silicon-fabrication-methods-properties-and-solar-1gjw6owq4d.pdf

Black silicon: fabrication methods, properties and solar energy applications

Uncooled infrared bolometer arrays have become the technology of choice for low-cost infrared imaging systems used in applications such as thermography, firefighting, driver night vision, security and surveillance. Uncooled infrared bolometer arrays are reaching performance levels which previously only were possible with cooled infrared photon detectors. With a continuously increasing market volume (> 100 000 units per year to date), the cost for uncooled infrared imaging chips are decreasing accordingly. In this paper we give an overview of the historical development of uncooled infrared bolometer technology and present the most important bolometer performance parameters. The different technology concepts, bolometer design approaches and bolometer materials (including vanadium oxide, amorphous silicon, silicon diodes, silicon-germanium and metals) are discussed in detail. This is followed by an analysis of the current state-of-the-art infrared bolometer technologies, the status of the infrared industry and the latest technology trends.

MEMS-based uncooled infrared bolometer arrays: a review

In this work, we report the fabrication and characterization of bolometers based on amorphous silicon germanium alloys (a-Si 1 −  x Ge x :H,F). The fabrication of microbolometers with thermally isolated a-SiGe thin film as sensing layer, is described. Membrane-supported and bridge-supported detectors have been fabricated. Sensing layer is deposited by low frequency plasma enhanced chemical vapor deposition technique. Since this deposition is carried out at relative low temperature T D =573 K, the read-out IC fabricated on silicon substrate is not affected, providing compatibility with silicon IC technologies. Responsivity and detectivity were measured under illumination at different bias voltages. The thermal time constant, the thermal conductance and current noise are determined by means of the voltage–time curves. Responsivity and detectivity are observed to depend on the physical properties of the materials forming the device and on the device geometry. Obtained results of pixel resistance, thermal resistance and responsivity demonstrate a-SiGe:H,F like a material suitable for realizing high-performance microbolometers.

IR bolometers based on amorphous silicon germanium alloys

Fabrication and performance comparison of planar and sandwich structures of micro-bolometers with Ge thermo-sensing layer

A comparative study of wet and dry texturing processes of c-Si wafers for the fabrication of solar cells

The fabrication and characterization of an uncooled micro-bolometer, in which the sensing material is amorphous germanium (a-Ge:F) is presented. In order to obtain thermal isolation, the micro-bolometer was fabricated on a silicon dioxide membrane, which was made on a silicon wafer by micro-machining techniques. The sensing layer was deposited from GeF4 + H2 mixture by low frequency plasma-enhanced chemical vapor deposition. The a-Ge:F film had high thermal coefficient of resistance at room temperature α=0.04 and moderate conductivity σRT=2.5×10−3 Ω −1  cm−1. The micro-bolometer was characterized by the measurements of current–voltage characteristics in dark and under infrared illumination with a black body at temperature Tbb=1123 K. Thermal and noise characteristics were also measured. The micro-bolometer having the pixel area Ad=60×60 μm2 demonstrated thermal resistance Rth=5×106 K W−1, current responsivity R I =6.2 mA W−1, voltage responsivity R V =4.2×10 6 V W−1 and detectivity D ∗ =8×10 6 cm W Hz1/2.

Uncooled micro-bolometer based on amorphous germanium film

A study on the formation of Black Silicon on crystalline silicon surface using SF6/O2 and SF6/O2/CH4 based plasmas in a reactive ion etching (RIE) system is presented. The effect of the RF power, chamber pressure, process time, gas flow rates, and gas mixtures on the texture of silicon surface has been analyzed. Completely Black Silicon surfaces containing pyramid like structures have been obtained, using an optimized mask-free plasma process. Moreover, the Black Silicon surfaces have demonstrated average values of 1% and 4% for specular and diffuse reflectance respectively, feature that is suitable for the fabrication of low cost solar cells.

Alfonso Torres

Papers

IR bolometers based on amorphous silicon germanium alloys

Fabrication and performance comparison of planar and sandwich structures of micro-bolometers with Ge thermo-sensing layer

A comparative study of wet and dry texturing processes of c-Si wafers for the fabrication of solar cells

Uncooled micro-bolometer based on amorphous germanium film

Black Silicon formation using dry etching for solar cells applications