Today's pulsed THz sources enable us to excite, probe, and coherently control the vibrational or rotational dynamics of organic and inorganic materials on ultrafast time scales. Driven by standard laser sources THz electric field strengths of up to several MVm−1 have been reported and in order to reach even higher electric field strengths the use of dedicated electric field enhancement structures has been proposed. Here, we demonstrate resonant electric field enhancement structures, which concentrate the incident electric field in sub-diffraction size volumes and show an electric field enhancement as high as ~14,000 at 50 GHz. These values have been confirmed through a combination of near-field imaging experiments and electromagnetic simulations.
The formation of polycrystalline Si layers on flexible plastic substrates, through plasma enhanced chemical vapor deposition and excimer laser annealing, is investigated. Combining low-temperature ͑300°C͒ annealing with laser dehydrogenation/crystallization produces good-quality polycrystalline silicon with a reduced shot density. By using optimal crystallization conditions it is possible to achieve a superlateral growth crystallization regime, with a grain size up to 1 m, and void-free material, as confirmed by the presented structural analysis. The beneficial effect of the low-temperature thermal annealing has been related to the removal of nonbound hydrogen, as supported by the elastic recoil detection analysis and IR analysis of the samples. To validate the process, we fabricated non-self-aligned polysilicon thin-film transistors ͑TFTs͒ directly on spin-coated polyimide substrates, with a maximum processing temperature of 300°C and with a relatively low shot density ͑Ͻ10 shots/point͒. The TFTs presented good electrical characteristics with an on/off ratio Ͼ10 6 , a field-effect mobility up to 65 cm 2 /V s, and a threshold voltage of 7 V. These results confirmed that the developed crystallization process is suitable to fabricate polysilicon TFTs on polymeric substrates, allowing an increased process throughput.
).The remarkable electrical and optical properties of single-walled carbon nanotubes (SWNT) have allowed for engineering device prototypes showing great potential for applications such as photodectors and solar cells. However, any path towards industrial maturity requires a detailed understanding of the fundamental mechanisms governing the process of photocurrent generation. Here, we present scanning photocurrent microscopy measurements on a double-gated suspended semiconducting SWNT and show that both photovoltaic and photothermal mechanisms are relevant for the interpretation of the photocurrent. We find that the dominant or non-dominant character of one or the other processes depends on the doping profile, and that the magnitude of each contribution is strongly influenced by the series resistance from the band alignment with the metal contacts. These results provide new insight into the interpretation of features in scanning photocurrent microscopy and lay the foundation for the understanding of optoelectronic devices made from SWNTs. [6,7,8,9,10,11,12,13, 15] as well as 2D materials like graphene [16,17,18,19,20,21,22,23] and MoS 2 [24,25]. In such nanoscale systems, two mechanisms have been identified for the generation of photocurrent: i) photovoltaic processes where photo-excited carriers are separated by built-in electrical fields and ii) photothermal processes where thermoelectric forces drive carriers through light-induced thermal gradients.Single-walled carbon nanotubes (SWNT) are long, one dimensional conductors with a band structure ranging from quasimetallic (bandgap E g ∼ 30 meV) to semiconducting character (E g typically in the range of 0.1 to 1 eV) depending on chirality and diameter. These properties make SWNTs an ideal platform for exploring photocurrent generation with SPCM. In early single-walled nanotubes SPCM work the interpretation of photocurrent was mostly based on photovoltaic mechanisms [6,7,8,9,11].The importance of photothermal effects has been suggested in the context of measurements of bulk SWNT films [12,13,14] and very recently, SPCM work on graphene and individual metallic SWNTs has emphasized the importance of photothermal mechanisms in materials with no or small bandgaps [22,26].The question of the role of photothermal mechanisms in larger bandgap semiconducting nanotubes has been studied very recently in double-gated [26] and single-gated [27] suspended carbon nanotube devices.These two studies report contradictory results, leaving the understanding of fundamental mechanisms underlying photocurrent generation in semiconducting nanotubes unclear.Here we report on the study of a suspended semiconducting nanotube device where we show that both photovoltaic and photothermal mechanisms compete in the generation of photocurrent. In particular, we find that the dominant or non-dominant character of one or the other processes is a function of the doping profile and that the magnitude of each contribution is strongly influenced by the band alignment with the metal contacts throu...
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